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Review ArticlePhotocatalytic Based Degradation Processesof Lignin Derivatives
Colin Awungacha Lekelefac1 Nadine Busse1 Michael Herrenbauer2 and Peter Czermak134
1 Institute of Bioprocess Engineering and Pharmaceutical Technology University of Applied Sciences Mittelhessen35390 Giessen Germany2Media University Packaging Technology 70569 Stuttgart Germany3Department of Chemical Engineering Faculty of Engineering Kansas State University Manhattan KS 66506 USA4Faculty of Biology and Chemistry Justus-Liebig-University Giessen 35392 Giessen Germany
Correspondence should be addressed to Peter Czermak peterczermakkmubthmde
Received 8 August 2014 Accepted 13 October 2014
Academic Editor Elisa Isabel Garcia-Lopez
Copyright copy 2015 Colin Awungacha Lekelefac et al This is an open access article distributed under the Creative CommonsAttribution License which permits unrestricted use distribution and reproduction in any medium provided the original work isproperly cited
Photocatalysis belonging to the advanced oxidation processes (AOPs) is a potential new transformation technology for ligninderivatives to value added products (eg phenol benzene toluene and xylene) Moreover lignin represents the only viable sourceto produce aromatic compounds as fossil fuel alternative This review covers recent advancement made in the photochemicaltransformation of industrial lignins It starts with the photochemical reaction principle followed by results obtained by varyingprocess parameters In this context influences of photocatalysts metal ions additives lignin concentration and illuminationintensity and the influence of pH are presented and discussed Furthermore an overview is given on several used process analyticalmethods describing the results obtained from the degradation of lignin derivatives Finally a promising concept by couplingphotocatalysis with a consecutive biocatalytic process was briefly reviewed
1 Introduction
In October 2014 the price of crude oil was 85 dollars perbarrel and the forecast for next year is 98 dollars per barrel [1]This is a symbolic indicator for the decreasing availability ofconventional nonrenewable energy sources due to the globaleconomy growth coupled with frequent political instabilityAccording to the World Energy Technology and ClimatePolicy Outlook of the European Commission [2] the worldtotal energy consumption levels will rise from 121 times 109 tonsoil equivalent (toe) (2010) to 145 times 109 toe (2020) to 171 times109 toe by 2030As a result theworld carbondioxide emissionfrom the combustion of fossil fuels will increase from 293 times109 tons (2010) to 367 times109 tons (2020) to 445 times 109tons (2030) That means the world carbon dioxide emissionwill be almost doubled by 2030 Also less than 1 of the300 times 106 tons of plastic produced per year are natural
polymers [3] Thus there is a need for the developmentof biobased macromolecular materials which would reducethe consumption of fossil resources and hence reduce CO
2
emissionThe major option is a gradual replacement of these fossil
resources by renewable alternatives for example wind sunwater and biomass Ligneous biomass also known as ligno-cellulosic biomass is of great interest for industries (chem-istry biotechnology and fuel) and biorefineries convertingsustainable materials [5] This is due to the biomassrsquos highvalue-added compounds cellulose (40ndash50) hemicellulose(24ndash35) and lignin (18ndash35) [6] Furthermore biomassis inexpensive and available in large amounts [7] as wellas being CO
2neutral [8] Nevertheless just 3ndash35 of the
yearly produced biomass (170ndash200 times 109 tons) is utilized bynonfood applications [5] because of reasons related to thelignocellulosic structure per se and its processability
Hindawi Publishing CorporationInternational Journal of PhotoenergyVolume 2015 Article ID 137634 18 pageshttpdxdoiorg1011552015137634
2 International Journal of Photoenergy
Cellulosehemicellulose
lignin
Biomass
Pretreatment
Pyrolysis andgasification
Platformchemicals
Step 1
1
2
3
Step 2catalysis
Catalysis
Syngas
Furtherprocessing
catalysis
New technology
New technology
Current technology
Lignin
Cellulose
Hemicellulose
Fuelsbulk and fine
chemicals
PhenolBenzene Toluene Xylene
ProteinsLipidsAsh
SoilSaltsWater
KraftSulfiteOrganosolvPyrolysisSteamExplosionAFEXHot waterOther
For example Fischer-Tropsch methanol synthesis
COH2
OH
OH
OH
OH
OH
R R
ROH
OH
OH
OH
OH
OH
OH
O
OOO
HO
HO
O O
O
Figure 1 Lignocellulosic biorefinery scheme with particular emphasis on the lignin stream reprinted with permission from Zakzeski et al[4] copyright (2010) American Chemical Society
Figure 1 depicts an exemplary lignocellulosic biorefineryscheme with emphasis on the lignin stream For separa-tion purposes several pretreatment procedures are currentlyapplied in order to generate lignin derivatives (modificationin lignin structure) which can be differentiated on thebasis of their isolation method and their origin since theirphysical and chemical properties differ [9ndash11] The mostindustrialtechnical lignins (gt70 million tons per year) areobtained as waste material by the pulp and paper industry[12] mainly from the kraft process as noted by Kamm etal [5] 98 is burnt for energy recovery in the paper mills[13] while less than 2 is sold primarily in formulationof dispersants adhesives and surfactants [14] Lignin is anaromatic biopolymer of high potential fission products fora wide range of sectors and it is thus gaining attention forexample for the production of platform chemicals (Figure 1)Nonetheless lignin is still underutilized and fundamentalresearch and development are needed [5]This is explained byligninrsquos complex nature its recalcitrance to degradation andthe difficulty to analyze its numerous degradation productsTherefore intensive research goes on both sides processengineering and development (homogeneous heterogeneouscatalysis thermal electrochemical andor hybrid proce-dures) and process analytics
Photocatalysis is an advanced oxidation process (AOPs)[15] with the potential to transform lignin to value addedproducts such as phenol benzene toluene and xylene [4]In this context it can be applicable as stand-alone unit or it
can be coupledwith otherAOPs (eg Fentonrsquos reagent ozoneelectrochemical oxidation) as well as biocatalysis [15] (eghydrolysis through ligninolytic enzymes)
This paper is aimed at reviewing photocatalysis of indus-trial lignins in connection with evaluating the effects ofbasic operating parameters such as catalyst loading illu-mination intensity integration of metal ions and otheradditives pH and initial lignin concentration Furthermoreanalytical techniques used by different researchers are dis-cussed Finally a brief overview concerning the suitabilityof photocatalysis as a pretreatment method for subsequentbiocatalysis by ligninolytic systems for example fungal hemeperoxidases andor laccases is highlighted
2 Lignin as Raw MaterialChemical Structure and Sources
Lignin is the only naturally synthesized aromatic biopolymer[16] and after cellulose themost abundant renewable carbonsource on earth [17] In addition native lignin is a poly-disperse 3D macromolecule with an undefined molecularmass The biopolymer is made up of randomly arrangedphenylpropane units 119901-coumaryl alcohol coniferyl alcoholand sinapyl alcohol as depicted in Figure 2 contributing to anirregular structure [11]
An overview of the common interunit linkages and itsestimated proportions are shown in Table 1 For a better
International Journal of Photoenergy 3
1
2
3
45
120572120573
120574OH
OH
p-Coumaryl alcohol
OCH3
OH
OH
Coniferyl alcohol Sinapyl alcohol
OCH3H3CO
OH
OH
Figure 2 Monomer structures of lignin [18]
Table 1 Overview of most frequent bond types found in lignin
Model linkagea W G Glasser and H R Glasser [19] Erickson et al [20] Nimz [21]120573-Carbon-oxygen-4-aromatic carbon 55 49ndash51 65120572-Carbon-oxygen-4-aromatic carbon mdash 6ndash8 mdash120573-Carbon-5-aromatic carbon 16 9ndash15 6120573-Carbon-1-aromatic carbon 9 2 155-Aromatic-carbon-5-aromatic carbon 9 95 234-Aromatic-carbon-5-aromatic carbon 3 35 15120573-Carbon-120573-carbon 2 2 55120573-Carbon-120573-carbon forming furanic structure mdash mdash 2120572-Carbon-120574-carbon-oxygen-120574-carbon 10 mdash mdasha of total phenylpropane units
illustration Figure 3 shows the structure of a softwood ligninfragment containing all prominent linkage types
Native lignin cannot yet be isolated Contrary manymodified lignins of great variety are available throughbiomass transformation technologies [17] Depending onthe used isolation method lignosulfonate alkali lignin sul-fate lignin hydrolytic lignin steam exploded lignin andorganosolv lignins can be derived [31] The kraft and sodapulping processes generate liquors referred to as black liquorcontaining alkali lignin also called kraft or sulfate lignin [31]Although the kraft process is the dominant process (89)lignosulfonates attract more attention for the application oflignin as industrial products This is probably because ofadvantages such as its solubility properties and relativelylower mass range Moreover lignosulfonates are the onlycommercial lignins which are water soluble Their averagemolecular weight varies from 400Da up to 150 kDa or even17000 kDa (taken from Goring [9]) determined by processconditions and applied analytical methods [11]
3 Photocatalysis of Lignin
In this section the reaction principle for the photocatalyticdegradation of lignin is described After that themost impor-tant operating parameters and their impacts are discussed
31 On the Reaction Pathways for Photocatalytic Lignin Degra-dation Photocatalysis is the acceleration of a photoreactionin the presence of a catalyst In other words it involves theinitial absorption of photons by a molecule or substrate toproduce highly reactive electronically excited states
Lignin degradation is generally in the range of lowerenergy (between 300 and 400 nm) because of its multifunc-tional character [32ndash34]This region falls within the UV-lightregion TiO
2is the most applied photocatalyst and its energy
band gap is approximately 32 eV [35]In photogenerated catalysis the photocatalytic activity
depends on the ability of the catalyst to create electron-hole pair which generates free radicals (eg hydroxyl radi-cals ∙OH) enabling secondary reactions [36] Other aspectsinclude the rate of electron transfer the rate of chargerecombination crystal structure surface area of catalystporosity and surface hydroxyl group density [37]
In what follows equations summarizing the formationof radical species under photocatalytic conditions shall bedescribed S stands for the lignin substrate while TiO
2(h+VB)
and TiO2(eminusCB) represent the electron deficient (valence
band) and electron-rich (conduction band) parts in thestructure of TiO
2 respectively
The initial photocatalytic process involves the generationof electron-hole pair in the semiconductor particles as a result
4 International Journal of Photoenergy
Spirodienone
O
OO
O
O
OHO
OO
O
O
O
O
OHO
O
O
O
O
O
O
OO O
O
O
O
O
O
O
O
OH
OO
OO
O
O
OO
p-Coumarylalcohol fragment
Coniferylalcohol fragment
Branching caused bydibenzodioxin linkage Phenylcoumaran
HO
OAr
HO
OH
OHOH
OH
HO
HO
OH
HO
OH
HO
OH
HO
OH
HO
OH
HO
OH
HO
OH
HO
OH
HO4-O-5
OH
HOOH
120573-120573
5-5998400
120573-O-4
120573-1
120573-1
O
Figure 3 Structure of a softwood lignin fragment showing the prominent linkage types reprinted with permission from Zakzeski et al [4]and Evtuguin et al [22]
O2∙minus + H+ HO2
∙
2HO2∙
HO∙ + OHminus
∙OH
S + HO∙ S∙+ + OHminus
S∙+ + O2SO2
∙+
O2∙minus
∙O2minus
∙OH (hydroxyl radical)
(a) Reduction of O2H2O2
O2 + eminus
H2O2 + O2
H2O2 + eminus
(b) Oxidation of substrate
h+ + OHminus
Electron capture
Recombination
eminus
h+
Conduction band
Valence band
Electron release
eminus
O2
(super oxide)
Light
OHminus
Figure 4 Photocatalysis principle adapted from Linsebigler et al [23]
of UV radiation [38 39] Figure 4 shows the excitation ofan electron from the valence band to the conduction bandinitiated by light absorption with energy equal to or greaterthan the band gap of the semiconductor This is expressed by(1)
Upon excitation the fate of the separated electron andhole can follow several pathways Electron or holes canthen react with hydroxyl ions (OHminus) or H
2O producing
hydroxyl radicals (∙OH) as shown in (2) Jaeger and Bard[40] Matthews [41] andMachado et al [42] report that ∙OHis the main oxidizing agent in the photocatalytic oxidationbecause of the unpaired electrons Therefore it can react fastand unspecifically with almost all organic compounds (S)
[43] abstracting an electron with the formation of a radicalorganic species as shown in (3) [44]
Formation of singlet oxygen hydroxyl and superoxideradicals as principal reactive species in a photocatalyticprocess [38 39] is as follows
TiO2
ℎV997888997888rarr TiO
2(
eminusCBh+VB) (1)
TiO2(h+) +H
2O 997888rarr TiO
2+HO∙ +H+ (2)
S +HO∙ 997888rarr S∙+ +OHminus (3)
International Journal of Photoenergy 5
MeO MeOMeOMeO
∙OO
O
O ∙
OH
HO+ +
Figure 5 Formation of phenoxyl radicals by intermolecular abstraction of phenolic hydrogen by carbonyl groups
RH
bond cleavage
Vanillin
O
OO
O
OO O
O
O
O
O
O
HO HO
HOHO
HO
HO
HO HO
OH OH
HO
OCH3
OCH3
OCH3
OCH3
OCH3 OCH3
OCH3
OCH3 OCH3
OCH3
h minusH∙OH∙∙
O2TiO2
HminusOH∙
C120572-C120573
∙
Figure 6 Supposed lignin degradation scheme by autoxidation induced by TiO2poly (ethylene oxide) [24]
S∙+ +O2997888rarr SO
2
∙+ (4)
TiO2(h+) + S 997888rarr TiO
2+∙S+ (5)
TiO2(eminus) +O
2997888rarr TiO
2+∙O2
minus (6)
HO∙ + ∙O2
minus
997888rarr OHminus + 1O2
(7)
TiO2(eminus) + TiO
2(h+) 997888rarr heat (8)
The organic radicals and radical cations can for examplereact with molecular oxygen to form organic peroxy radicalsand peroxy radical cations respectively (4) The holes canoxidize organic compounds by electron abstraction to formorganic cationic radicals (5) [42] Superoxides can be formedby the reaction of electrons with electron acceptors suchas O2(6) Meanwhile the formation of singlet oxygen can
be from the reaction of hydroxyl radical and superoxide(7) [42] Moreover there is a possibility that electrons andholes recombine if electron acceptors are limited In thiscase recombination can take place in the volume of thesemiconductor particle When recombination takes placeradiation energy is lost or converted into heat (8) [45]
From investigations caried out by Mazellier et al [46](photochemistry of 26-dimethylphenol) it was postulatedthat hydrogen can be abstracted by 120572-carbonyl groups In thesame context lignin derivatives having similar functionalitycan follow a similar pathway In addition oxidative chainreactions with the participation of ground-state oxygencan be initiated leading to fragmentation and combinationreactions and thus the formation of new dimers or oligomers(Figure 5)
Miyata et al [24] proposed a cleavage mechanism forthe C120572ndashC120573 bonds which leads to the formation of smallfragments such as vanillin as shown in Figure 6
Figure 7 [25] illustrates the formation of a radical cationformed as a result of enzyme (lipase) mediated reactionof adlerol Adlerol is characterized by a C120573ndashOndash4 bondand considered to be a lignin model compound With theformation of the radical species subsequent nonenzymaticreactions such as radical reactions can take place generatinga wide variety of products and complex compounds
It is widely assumed that the photocatalytic degradationof lignin follows a radical reaction pathwaywhich is similar tothat considered in thermal electrochemical and biochemical
6 International Journal of Photoenergy
Nonenzymatic subsequent reactions
Adlerol
Veratryl alcohol radical
Enzyme1
23
4
4
1
2
3
5
5
6
6
Guaiacol
Veratraldehyde
Ketone
O
OO
O
O O
O
O
O
HO
HOHO
HO
OCH3
OCH3
OCH3
OCH3
OCH3
OCH3
120572120573
120574
minusH+
∙+
∙
∙+
c120572 oxidation H2O
O2
120572 120573
120574
HOHOHO
OCH3
OCH3OCH3
OCH3 OCH3
OCH3
OCH3
120572120572
120573 120573
120573
120574 120574
HO
HO
120574
OCH3
120573
HO
HO
120574
HO
OCH3
OCH3
OCH3
120572120573
120574
Figure 7 Proposed radical reaction scheme initiated by enzyme lignolytic heme peroxidase for the conversion of adlerol possessing C120573ndashOndash4 bonds into smaller units summarized by Busse et al [25] and abstracted from Tien and Kirk [26] Kirk et al [27] Lundell et al [28]Schoemaker et al [29] and Palmer et al [30]
processes However reporting on the degradation pathway oflignin derivatives and even that of lignin model compoundsis still a major challenge This is probably due to the complexnature and variety of possible degradation products Indeedthemechanism is far more complex considering other factorssuch as type of lignin type of catalyst pH illuminationsource and additives
32 Influence of Process Parameters in Lignin DegradationVarying process parameter could either have a positive ornegative impact on the photocatalytic efficiency The basicprocess parameters such as catalyst concentration [4 4856] substrate concentration [48 51] addition of metal ion
to TiO2catalyst [17 50 56ndash58] pH [47 48] illumination
[33 48 49 59] and their influence shall be discussed in thissubchapter Table 2 gives an overview about starting reactionconditions and catalyst applied by some work groups whileTable 3 portrays parameters analytical methods and resultsobtained It is worthwhile noting that comparing the differentphotochemical processes poses a big challenge because ofthe wide variables involvedThese discrepancies start alreadyfrom the source and type of lignin followed by the differencesin reactor design illumination source intensity of radiationand different types of TiO
2catalyst such as Fischer scientific
rutile TiO2[49] TiO
2(TiO2mdashTP-2 of Fujititan) just to name
a few
International Journal of Photoenergy 7
Table2Summaryof
startingcond
ition
sfor
thep
hotocatalytic
degradationof
lignin
Reference
Lign
insource
Catalyst
Reactio
ncond
ition
s
MacHadoetal[42]
Peroxyform
icacid
lignins
from
eucalyptus
grandisw
ood(EL1)
EL1+
sodium
borohydride
TiO
2andH
2O2
Lign
inconcentration
025
mgmL119879=25∘C
pH11U
VVisradiatio
n120582gt300nm
40
0Wmercury
lampcylin
dricalPy
rexglassreactorcon
stanto
xygenbu
bblin
g
Ksibietal[47]
Water
solubleligninob
tained
from
blackliq
uor
TiO
2-P2
5(D
egussa)
Lign
inconcentration
90mgL119879=20∘C
pH82UV-radiation120582gt290nm
pyrex
reactoro
pento
airPh
ilips
HPK
125W
lamp
Kansaletal[48]
Lign
infro
mwheatstr
awkraft
digestion
TiO
2-P2
5(D
egussa)a
ndZn
O
Lign
inconcentration
10ndash100
mgLin
100m
LZn
Ocatalystdo
se(05ndash20gL)pH
ofthes
olution(pH3ndash11)solarillu
mination
oxidantcon
centratio
n306times10minus6M
to153times10minus6Mthinbedfilm
slurrypo
ndreactoroxidantsodium
hypo
chlorite
solutio
n(4availableC
l 2)
Dahm
andLu
cia[
49]
Whitewater
from
indu
strialprocess
water
thatexits
inpaperm
achines
FischerS
cientifi
crutile
TiO
2
Lign
inconcentration
40mgLin
500m
Lbatchreactor119879=21∘Cndash
42∘C
Rayonet
photochemicalcham
ber16
VWR8-W
blacklightpho
spho
r(350-nm
)lam
ps
constant
oxygen
bubb
lingpo
wer
ofillum
ination
128to
64Wlight
intensity
223ndash44
5mWcm
3
Portjanskajaand
Preis[50]
Lign
inpurchased
from
Aldric
hTiO
2P2
5-N(D
egussa)
Lign
inconcentration
100m
gLpH
8batchreactorsystemreactor
open
toair
PhillipsT
LD15W05low-pressurelum
inescent
mercury
UV-lampUV-radiation
120582gt360nm
pow
erdensity
ofirr
adiatio
n=07m
Wcm
2 visib
lelight
source
Tanaka
etal[51]
Lign
infro
mconiferous
woo
dTiO
2(TiO
2mdashTP
-2of
Fujititan)
Lign
inconcentration
0003to
003U
V-radiation120582gt310nm
cylindrical
reactio
nvessel
Tonu
ccietal[33]
Ca2+andNH4
+ligninderiv
atives
TiO
2as
Degussa
P25
+po
lyoxom
etalates
(POM)H
2O2
Openqu
artztubes(20
mL)
reactor119879=20∘C
1atm
multirayso
ften
UVlamps
of15W
power
eachU
V-radiation120582gt254nm
Miyatae
tal[24]
Piceag
lehn
iiwoo
dflo
urTiO
2po
lyethylene
oxide(PE
O)
Reactorop
enpetrid
ish119879=30∘C119905=48h40
0Wmercury
lamp
Shende
etal[17]
Kraft
lignin
TiO
2-Zn
O-ZrO
2Lign
inconcentration
1mgmLin
10mL250W
Xeno
nlampandAM
15Glamp
filterpo
wer
density
ofirr
adiatio
n100m
Wcm
2 solarlam
psim
ulator
Tian
etal[52]
andPanetal[53]
Kraft
ligninfro
mblackliq
uor
Ta2O
5-IrO
2andPb
O2thin
film
TiO
2nano
tubePbO
2
Lign
inconcentration
30(w
w)UV-radiation120582gt365nm
119905=10minpow
erdensity
ofirr
adiatio
n20
mWcm
2 blue
waveT
M50
ASUVspot
lampEG
ampG2273
potentiosta
tgalvano
stattoapplycurrentTiTiO
2NTPb
O2ele
ctrode
asworking
electrode
andPt
coilas
outere
lectrode
andAgAgC
lasreference
electrode
Awun
gachaL
ekele
fac
etal[54][55]
Lign
insulfo
natefro
mpaperw
aste
water
TiO
2as
Degussa
P25
TiO
2fro
msol-g
elprocesso
fTiOSO
4TT
IP
Lign
inconcentration
500m
gLin
200m
LOsram
Planon
light
sourceirradiance
30ndash4
0Wm
2 UV-radiation120582280ndash4
20nm
119905=20hreactoro
pento
air
recirculationsyste
mflow
rate225mLmin
8 International Journal of Photoenergy
Table3Parametersanalyticalmetho
dsand
results
from
different
workgrou
ps
Reference
Parameter
studied
Analytic
sRe
sult
MacHado
etal[42]
Roleof
hydroxylradicals
irradiatio
nof
ligninin
the
absencea
ndpresence
ofph
otocatalystT
iO2andH
2O2
Ultraviolet-v
isible(UV-Vis)spectro
scop
yionizatio
nabsorptio
nspectro
scop
y(IA
S)
sizee
xclusio
nchromatograph
y(SEC
)
Asharpdecrease
inthep
heno
liccontento
bservedforreactions
involvingdirect
photolysis
SEC
areductio
nof
almost50
inthea
verage
molecular
weighto
fligninequalto
14kD
after
90min
ofirr
adiatio
n
Ksibietal[47]
Irradiationof
ligninin
the
absencea
ndpresence
ofph
otocatalystT
iO2-P2
5
UV-Visspectroscop
y13C-
nucle
armagnetic
resonance(NMR)
solid
state
totalion
gasc
hrom
atograph
y(TIC)
indu
ctioncoup
lingplasma(
ICP)
chem
icaloxygen
demand(C
OD)
56degradationratewith
TiO
2catalystaft
er420m
inreactiontim
eethylacetate-extractableprod
uctsshow
edvanillin
vanillica
cidpalm
itica
cid
biph
enylstructuresand
345-trim
etho
xybenzaldehyde
presence
ofmagnesiu
mandcalcium
ions
CODremovalishigh
erforthe
initiallylowconcentrations
ofligninsolutio
n
Kansaletal[48]
Catalystdo
sepHoxidant
concentration
initialsubstrate
concentration
ZnOcatalystin
slurryandim
mob
ilizedmod
e
UV-Visspectroscop
yCO
D
Optim
umcatalystdo
seis1gL
optim
umoxidantcon
centratio
n2times10minus6M
graduald
ecreaseo
fabsorptionpeak
indicatin
gdecompo
sitionof
organics
CODremovalishigh
erforthe
initiallylowconcentrations
ofligninsolutio
n
Dahm
and
Lucia[
49]
Catalystdo
seillu
mination
intensity
UV-Visspectroscop
ytotalorganiccarbon
(TOC)
capillaryionelectro
phoresis
analysis(C
IA)
Gradu
aldecrease
inabsorptio
npeak
indicatin
gdecompo
sitionof
organics
optim
alcatalystdo
seof
10mgm
high
erillum
inationintensities
correlated
well
with
high
erinitialdegradationrate
74disapp
earanceo
fTOC
Portjanskajaand
Preis[50]
Influ
ence
offerrou
sion
sN-dop
edcatalysteffect
sprayedcatalyston
supp
ortand
subm
ersedcatalyst
CODU
V-Visspectroscop
ybiochemical
oxygen
demand(BOD)colorim
etric
measurementat570
nm
Additio
nof
Fe2+upto
28m
gLleadsto25increase
inph
otocatalyticeffi
ciency
sprayedcatalystexhibited15
times
high
ereffi
ciency
than
theo
neattached
bysubm
ersio
nnegligibleeffecto
fN-dop
edcatalyst
increase
ofaldehyde
concentrationover
reactio
ntim
eneutralm
ediawas
mostb
eneficialforb
iodegradability
80of
freep
heno
lsremoved
undern
eutralcond
ition
s
Tanaka
etal[51]Ca
talystloadinglignin
concentration
illum
inationtim
e
UV-Visspectroscop
yTO
Cgel
perm
eatio
nchromatograph
y(G
PC)
1 HNMR
Fouriertransform
ationinfrared
(FTIR)
spectro
scop
y
FTIR
measurementrevealedafasttransform
ationof
arom
aticmoietyp
resent
inlignin
characteris
ticband
sofaromaticrin
gsm
etho
xyand
aliphatic
sidec
hains
decrease
inTO
Cvalues
over
time
decrease
indegradationratewith
increase
catalystdo
sageB
utaft
ercatalystthreshold
valueisa
ttainedcatalystincreasec
ausesa
decrease
indegradationrates
FTIR
peaksa
reshifted
towards
lower
molecular
weightregionaft
erph
otocatalysis
Tonu
cci
etal[33]
Testof
catalytic
syste
mstoob
tain
fractio
nswith
redu
ceddegreeso
fpo
lymerization
comparis
onof
thermalandph
otochemical
reactio
ns
1 HNMR
gasc
hrom
atograph
y-mass
spectro
scop
y(G
C-MS)
POMsa
relessselectivew
henused
asph
otocatalystsandno
appreciableb
leaching
ofthes
olutionwas
seen
whenPO
Mwas
used
asthermalcatalyst
deriv
edchem
icalsfrom
experim
entvanillin
hydroxylmetho
xy-acetoph
enon
econiferylalcoh
olcon
iferylaldehydemethano
lform
icacidacetic
acidand
sometim
essm
allamou
ntso
fC-2
andC-
3alcoho
ls
Miyatae
tal[24]
Exam
inationof
cellwall
structureo
flignin(w
oodflo
ur)
before
andaft
erph
otocatalysis
GC-
MSscanning
electronmicroscop
y(SEM
)1 H
NMR
Highdelignificationactiv
itydelignificationconfi
nedto
thes
urface
oflignin
deriv
edchem
icalse
xperim
entvanillin
Shende
etal[17]Testof
combinedactio
nof
bio-
andph
otocatalyticsyste
ms
UV-Visspectroscop
yGC-
MSX-
ray
dispersiv
eenergyspectro
scop
yandX-
ray
diffractio
nanalysis(EDX
XRD)SE
M
Detectio
nof
follo
wingchem
icalsacetylguaiacol4-ethoxym
ethyl-2
-metho
xyph
enol
metho
xyph
enyloxim
eguaiacolsuccinica
cidacetylguaiacolvanillicacidand
vanillin
International Journal of Photoenergy 9
Table3Con
tinued
Reference
Parameter
studied
Analytic
sRe
sult
Tian
etal[52]
andPanetal
[53]
Testof
combinedactio
nof
electro-and
photocatalytic
syste
ms
UV-Visspectroscop
yFT
IRSEM
X-ray
(EDX)
highperfo
rmance
liquid
chromatograph
y(H
PLC)
COD
Detectio
nof
thefollowingchem
icalscarbon
ylfunctio
nalityvanillin
andvanillica
cid
Awun
gacha
Lekeleface
tal
[54][55]
Com
paris
onof
degradationrates
bydifferent
catalyst
HPL
Cflu
orescencea
ndUV-Vis
spectro
scop
ySE
MT
OC
UV-Visresultsrevealfaste
rdegradatio
nof
thea
liphatic
moietycomparedto
the
arom
aticmoietyof
ligninsulfonateob
tained
from
paperw
astewaterPeaks
observed
durin
gHPL
Canalysis
Someo
fthe
peaksp
rodu
cedaft
erph
otocatalysishad
fluorescences
ignals
Thissuggeststhep
rodu
ctionof
newsubstances
andflu
orop
hores
Coatin
gsprod
uced
throug
hSol-G
elprocedures
arestablea
ndcanbe
used
manytim
es
10 International Journal of Photoenergy
321 Influence of Catalyst One aspect to successfully imple-ment photocatalysis is the choice of an appropriate photo-catalyst The majority of catalysts used are based on TiO
2as
summarized in Table 2 ZnO has also been applied either assingle catalyst [48] or in a combination with other catalystsuch as TiO
2[54] de Lasa et al [60] described ZnO as
being less active than TiO2 They add that the use of ZnO
is particularly relevant when the oxidative degradation ratebecomes limited This is opposite to what Kansal et al [48]reported by saying that ZnO is more reactive than TiO
2
However TiO2(mainly anatase) remains the most used
catalyst as can be depicted fromTable 2 TiO2is reported to be
favored because of its nontoxic property cost efficiency andchemical and biological inertness Moreover TiO
2possesses
the most efficient photoactivity and the highest stability thusmaking it suitable for industrial use [61]
ZnO has been described to degrade lignin under visiblelight sources [48 62] whereas TiO
2is mostly applied in
connection to UV-light sources as highlighted in Table 2However both ZnO and TiO
2possess energy band gap
energy of 32 eV [23]Additives such as SiO
2[63] polyethylene oxide (PEO)
[24] and polyethylene glycol [64] have been added to TiO2
catalyst particularly when applying immobilized catalystAddamo et al [63] noted a high adhesion of TiO
2to glass
support material when precoating was done with SiO2
Moreover the precoating might have other advantages suchas a hindering diffusion of Na+ ions from the glass materialinto the nascent TiO
2film during heat treatment processes
Analogous to the addition of SiO2 polyethylene glycol
(PEG) has also been introduced to mitigate catalyst surfaceactivity modify surface hydrophobicity and also reduceagglomeration tendency of the TiO
2gel or TiO
2particles
in the suspensions [64] By a proper surface modificationinteraction between catalyst and substrate can be enhanced[55 63]
Kansal et al [48] varied catalyst (ZnO) dose from 05 gLto 20 gL for 01 gL kraft lignin solutions and found out thatthere was an optimum catalyst threshold value at 1 gL whichgives a catalyst to substrate ratio of 1 10
Dahm and Lucia [49] examined catalyst dose from 2 sdot10minus3 gL to 12sdot10minus2 gL for lignin solutions (fromwhite water
liner mill) of 4 sdot 10minus2 gL (catalyst to lignin ratio 5 sdot 10minus3ndash3 sdot 10minus2) at pH 8 and obtained best energy efficiency valuesand lignin degradation rates with a catalyst loading of 10 sdot10minus2 gLIn contrast Ma et al [56] applied far higher catalyst
concentration compared to Dahm and Lucia [49] Catalystconcentration was varied between 1 gL and 10 gL PtTiO
2
Best catalysis dose with respect to reaction turnover wasobtained at 5 gL PtTiO
2With the increase of catalyst dose at
pH 7 the reaction rate increased from 61 times 10minus3minminus1 (1 gLTiO2) to 71 sdot 10minus3minminus1 (5 gL TiO
2) and 99 sdot 10minus3minminus1
(10 gL TiO2)
Catalyst effect has been explained on the basis thatoptimum catalyst loading is dependent on the initial soluteconcentration An increase in catalyst dosage leads to a cor-responding increase of total active surface area for reactions
[65] To that at higher TiO2concentrations the photon flux is
more easily intercepted by the catalyst before penetrating intothe bulk of the system At the same time due to an increase inturbidity of the suspension with high dose of photocatalystthere is a decrease in penetration of UV light and hencephotoactivated volume of suspension or solution decreases[66]
In summary authors have obtained best catalyst to ligninrelations for different reaction designs and thus a generalrecommendation on catalyst dose is not possible Howeverwhen lignin solution is treated with increasing catalyst loadsa corresponding increase in degradation rate is observed untila threshold value is reached [48ndash50 56]
322 Influence of Metal Ion Addition (Doping) and AdditivesThe purpose of adding metal ion to photocatalyst is to miti-gate band gap energy through the introduction of intrabandgap states and as a consequence produce a bathochromicshift in the absorption spectrum [37] Altering the absorptionspectral range gives the possibility to exploit both the visiblelight spectrum and UV light sources Metal ion doping isalso introduced to serve as electron or hole traps in orderto minimize recombination between generated electron-holepairs [37]
Portjanskaja and Preis [50] studied the addition of Fe2+ions to an acidic lignin solution and found an increasein photocatalytic oxidation (PCO) efficiency The optimumFe2+ ions quantity was 28mgL while using 100mgL ligninsolution Upon further elevation of Fe2+ ions concentrationa corresponding reduction of the photocatalytic oxidationefficiency of lignin was noted Likewise Ohnishi et al [57]made a comparative study by doping platinum (Pt) silver(Ag) and gold (Au) ions to TiO
2 In these reactions 50mg of
catalyst (TiO2) was used with the addition of an equiva1ent
15 wt (based on TiO2) metal ion The addition of noble
metals brought about a faster decolorization of lignin Aushowed better results than Ag followed by Pt In the samecontext adding sodium hypochlorite as oxidant to PtTiO
2
catalyst an additional fivefold degradation rate was observedcompared to that without doping [56] Contradictory to theresults described above negligible effect of photocatalyticefficiency due to doping has been reported as well Awun-gacha Lekelefac et al [54] obtained little or no change indegradation rate by doping TiO
2-P25-SiO
2catalyst with Pt
ions (1 wt relative to TiO2catalyst) Likewise Portjanskaja
and Preis [50] noted a negligible change of photocatalyticefficiency of TiO
2when doped with nitrogen
Sarkanen et al [67] and Gellerstedt and Lindfors [68]reported the bias of peroxides to oxidation with reagentssuch as permanganate to favor aromatic moieties Oxidationagents like permanganate oxidizes predominantly aliphaticchains in alkaline and neutral media However by theapplication of H
2O2(Fenton system) lignin disappeared
completely [33] Tonucci et al [33] conclude that in orderto satisfactorily conserve the organic material the best com-promise appears to be the TiO
2photosystem which shows
low carbon consumption good preservation of the aromaticrings and greatly reduced mineralization
International Journal of Photoenergy 11
In summary different results have been obtained con-cerning the influence of noble metal ion addition Whilesome authors report an improvement in the photocatalyticefficiency upon their addition others report their addition ashaving no considerable influence However for reactions inwhich an improvement in the photocatalytic efficiency wasnoticed there was a threshold value to be considered Whenthe concentration of dopant surpasses this threshold valueelectron-hole recombination is favored and this has a negativeimpact to photocatalysis In such a case the space-chargelayer gets narrower and 119901-type dopants attract electrons andby virtue become negative They would now then act ashole acceptor attracting holes On the other hand 119899-typedopants which act as electron donor centers and possessexcess electrons attract holes as well [37]
323 Influence of Lignin Concentration Once the initiallignin concentration becomes higher exceeding a thresholdvalue an inhibitory effect on the photodegradationwas noted[47ndash49] This threshold value varies and depends on thereaction system and reaction parameters such as opticaldensity catalyst concentration and reaction volume Fromthe literature different authors have implemented varyinglignin concentrations probably to suit their reaction designFor example Ksibi et al [47] use 90mgL and AwungachaLekelefac et al [54] use 500mgL while Kansal et al [48]applied 10mgndash100mgL
Explanations arising from the findings are as followsat low lignin concentrations the incidental photonic fluxirradiated interacts with the catalyst generating radicals (eghydroxyl radicals (OH∙)) which allow a faster degradation[69] On the other hand high initial lignin concentrationsmay lead to tight adsorption which can suppress CO
2
evolution [51] and hence maintain chemical oxygen demand(COD) values Moreover low delignification yields maybe due to an inhibitory effect because of autoxidation bylow molecular weight lignin degradation products formed[24] Also due to the polymer structure of lignin which iscross-linked this makes it difficult for radical species acidand the aldehyde compounds produced to spread into theinner region of the substrate hence limiting autoxidationAs a worst case this might be the rate-determining step ofdelignification which is hindered [70ndash72]
In summary it can be concluded that the time taken forcomplete degradation depends on the initial concentration oflignin and faster degradation occurs at low lignin concentra-tions
324 Influence of pH Varying pH entails an alteration inthe properties of semiconductor-liquid interface [73] mainlyrelated to the acid-base equilibrium of the adsorbed hydroxylgroup [39] Furthermore pH also impacts lignin degradationrates [47 48 57] In this context several studies were carriedout with partly contradictory outcomes
Kansal et al [48] made pH investigations 3ndash11 undersolar light illumination using ZnO as catalyst Maximumdegradation was reached in alkaline conditions (pH 11)This is supported by Villasenor and Mansilla [74] reporting
an almost complete decolorization of kraft black liquorfrom pine wood at pH value of 116 in combination withZnO catalyst Similar results were achieved by Ohnishi etal [57] with TiO
2and ZnO being catalyst for bleaching
alkaline lignin in aqueous solution with TiO2and ZnO being
catalyst High activities at neutral pH were also reportedby Ohnishi et al [57] In contrast Ma et al [56] observedhigher reaction rates and rapid degradation of a syntheticlignin wastewater (prepared by dissolving commercial ligninpowder in aqueous solution pH 11) in acidic solution (pH 3)than in alkaline solutions at pH 11 for either TiO
2or PtTiO
2
catalystsReconsidering the photocatalytic principle the formed
superoxide anion radicals (∙O2
minus) are in a pH-dependentequilibriumwith perhydroxyl radicals (HO
2
∙) as follows [75]
HO2
∙999445999468 H+ + ∙O
2
minus pKa sim 48 (9)
See [76]∙O2
minus undergoes dismutation reaction resulting in H2O2
and O2competing to any other ∙O
2
minus triggered reaction Incase of low pH operation conditions in aqueous solutionsHO2
∙ becomes dominant whose reactivity is considerablyhigher compared to ∙O
2
minus [77] Subsequently HO2
∙ initiatessubstrate (S) oxidation to the radical cation (S+∙) and is itselfreduced to H
2O2[78] Thus increased degradation rates can
be reasonably expected supporting the results made by Ma etal [56] in an acidic environment ∙O
2
minus is extremely reactive inorganic solvents [77] Another aspect is the solubility of kraftlignin (soluble at pH gt 105) which reduces with decreasingpH whereas lignosulfonate should remain unaffected by pHin aqueous solution Moreover 120573ndashOndash4 bonds have beendescribed to be stable at acidic pH [11] In fact this couldadditionally explain the elevated degradation of kraft ligninmade by Kansal et al [48] and Villasenor and Mansilla [74]Nevertheless the contradictory results gained by Ma et al[56] still exist under the assumption that kraft ligninwas used(which would be supported by the high pH of 11 obviouslynecessary for dissolving the lignin powder) Although mostphotocatalytic reactions described in the literature are in anaqueous milieu lignin raw material and its fission productsmay however vary considerably Therefore the optimal pHis most likely to be reaction specific and has to be evaluatedexperimentally in principle
325 Influence of Illumination Many of the studies foundin the literature so far have not dealt on this subject per seWhat is found is the use of different illumination sourceseach having a specified power and lamp type However alltend to emitUV-light between the range 280ndash420 nm Table 2depicts this in detail Other illumination sources include thevisible light spectrum
In general terms illumination influences in that it ini-tiates photocatalysis by generating electron-hole pair in thesemiconductor particles [38 39] Dahm and Lucia [49]altered illumination intensity while observing lignin degra-dation In this study 004 gL lignin was used and lightintensity was varied 223ndash445mWcm3 It was found out thathigher illumination intensities correlated well with higher
12 International Journal of Photoenergy
initial degradation rates and hence total lignin degradation[49] Neppolian et al [69] report degradation to be propor-tional to radiation intensity and best results are achieved forlow lignin concentrations because of enhanced interactionbetween catalyst and incidental photonic flux
In summary high illumination power causes a corre-sponding high initial degradation rate at low lignin concen-trations becausemaximum light penetration into the reactionmedium is favored
33 Process Analytical Methods Various analytical tech-niques have been used to monitor lignin degradation Atthe beginning of this subchapter analytics revealing com-pounds formed from lignin degradation are treated Thisincludes for example gas chromatography (GC) and 1HNMR (nuclear magnetic resonance) This is then followed byresults qualitative analytic measurements such as ultraviolet-visible (UV-Vis) spectroscopy and dissolved carbon (DC)A list of authors analytical techniques applied and resultsachieved are outlined in Table 3
Portjanskaja and Preis [50] studied lignin degradationby measuring the removal of phenols through colorimetricmeasurements As a result of 24 h photocatalytic oxidationunder neutral media conditions 80 of free phenols wereremoved Gas chromatography (GC) result from Ksibi et al[47] attested vanillin vanillic acid palmitic acid biphenyland 345-trimethoxy benzaldehyde structures after the pho-tocatalysis of lignin from black liquor This is in accordancewith the findings of Tonucci et al [33] reporting the formationof vanillin hydroxyl methoxy-acetophenone coniferyl alco-hol coniferyl aldehyde methanol formic acid acetic acidand small amounts of C-2 and C-3 alcohols as degradationproducts1H NMR spectral analysis of lignin before illumination
and after 24 h of illumination showing characteristic bandsof aromatic rings methoxy and aliphatic side chains wascompared with each other Results revealed that the aro-matic ring degraded faster than the aliphatic chain [51]Fourier transformation infrared spectroscopy (FTIR) showedbands corresponding to CH
3 CH2 and CH which remained
unchanged after illumination while bands corresponding toaromatic rings disappeared as a result of illumination [51 53]
Results obtained from the combination of photochemicaland electrochemical oxidation [52 53] were similar to thoseof Tanaka et al [51] Here 13C-NMR confirmed the presenceof the carbonyl functionality and the presence of vanillinand vanillic acid after 12 h photochemical-electrochemicaloxidation These results showed that the combination of aphotocatalytic and an electrochemical oxidation significantlyenhanced the efficiency of lignin degradationThis is becausethe applied anodic potential bias greatly suppressed therecombination of photogenerated electrons and holes [53]
Ultraviolet spectrophotometry offers a convenientmethod to qualitatively and quantitatively analyze ligninin solution [79] This is reflected by the large number ofpublications using this technique [17 33 47ndash51 56 58 80]This is most likely due to its simplicity to interpret lignindegradation Lignins absorb UV light with high molar
200 220 240 260 280 300 320 340 360
00
05
10
15
20
25
Abso
rban
ce
Wavelength (nm)0min
30min
60min
120min
180min
300min
360min
420min
1200min
Figure 8 Time dependent UV-Vis absorption spectra of aqueouslignin solution from waste paper water irradiated with UV light(280ndash420 nm) for different time intervals The spectra are obtainedfor sol-gel derived TiO
2nanocrystalline coating (TiO
2-P25-SiO
2)
[54]
extinction coefficients because of the several methox-ylated phenylpropane units of which they are composed [33]Figure 8 depicts a series of photometric scans of ligninsul-fonate from paper waste water showing a gradual reductionof absorbance during photocatalytic treatment [54] Herethe absorption peaks are around 210 nm and 280 nmAbsorbance decreases with time implying the decompositionof lignin and the deterioration of chromophore groups [54]
Peaks at 210 nmcorrespond to portions of the unsaturatedchains while those at 280 nm correspond to unconjugatedphenolic hydroxyl groups [17] and the aromatic moiety [57]of the lignin molecule The absorption tailing to the longwavelength region arises from the color of lignin [57] Lignindegradation has been reported either at wavelength around280 nm corresponding to unconjugated phenolic hydroxylgroups [17 48 51 58] or for both wavelengths (210 nm and280 nm) [33 54 57] Kobayakawa et al [81] noted some otherabsorbance at wavelengths lower than 250 nm and pointedout that this could be due to the modification of ligninfragmentations leading to the formation of transient specieslikemethanol ethanol formaldehyde formic acid and oxalicacid among others
Analyticalmethods to effectively quantify lignin degrada-tion by calculating the oxygen demand by organic substancesand remaining organic carbon before and after photocatalysishave been studied Amongst the methods are dissolvedcarbon (DC) [49 51 58] chemical oxygen demand (COD)[47 48 50 57 80] biochemical oxygen demand (BOC) [50]dissolved organic carbon (DOC) [56] and American dyemanufacture institute value (ADMI) [56] COD and BODdescribe the oxygen demand by organic substances to be
International Journal of Photoenergy 13
Figure 9 Gradual change from the characteristic yellow ligninsulfonate to a colorless liquid after a period of 20 h Catalyst TiO
2-
P25 (Degussa) +TEOSUV-light 25∘C lignin concentration 05 gL[54]
converted to CO and CO2and H
2O and NH
3 Total organic
carbon (TOC) describes the amount of carbon bound inan organic compound while DOC describes the dissolvedfraction of organic carbon ADMI measures the amount ofdyestuff in water
Decolorization of lignin solution has been reported tobe another parameter observed during photocatalytic degra-dation Color is an indirect indicator of the lignin amountThe higher the color intensity of the solution is the greaterthe lignin content is (high concentrated lignin solutions egblack liquor appear dark brown) [82] Thus color changescan be interpreted as conversion of lignin to transient speciesor conversion to CO
2and H
2O Awungacha Lekelefac et al
[54] observed a gradual change from the characteristic yellowlignin (when highly diluted) to a colorless liquid after a periodof 20 h with sol-gel derived TiO
2nanocrystalline coatings on
sintered borosilicate glass as depicted in Figure 9 A corre-sponding decrease in DC values close to 82 was observedfor TiO
2-P25-SiO
2catalyst confirming degradation This is
shown in Figure 10These findings are analogous to that of Ohnishi et al
[57] who reported the bleaching of lignin when illuminatedcontinuously and that the solution becomes colorless Tothat the chemical oxygen demand (COD) value decreasesgenerating carbon dioxide and a small amount of carbonmonoxide as the main gaseous products COD removalwas reported to be effective at low lignin concentrations ascompared to high lignin concentrations [48]
Another applied analytical technique is fluorescencedetection directly coupled to a high performance liquidchromatography (HPLC) as a means to identify nonaliphaticcomponent in the complex mixture of lignin degradationproducts [54] Fluorescence emission in lignin is attributedto aromatic structures such as conjugated carbonyl biphenylphenylcoumarone and stilbene groups [83 84] AwungachaLekelefac et al [54] observed peaks on both HPLC and
0 200 400 600 800 1000 12000
50
100
150
200
250
Diss
olve
d ca
rbon
(ppm
)
t (min)
TiO2-P25-SiO2
TiO2-P25-SiO2 + PtTiOSO4minus30 6wt
ZnO + TiO2-P25-SiO2
No coating UV light
Figure 10 Variation of DC with time of aqueous lignin solutionfrom waste paper water irradiated with UV light (280ndash420 nm) fordifferent time intervals The spectra are obtained for sol-gel derivedTiO2nanocrystalline coatings (TiO
2-P25-SiO
2+Pt TiO
2-P25-SiO
2
TiOSO4minus306 wt ZnO + TiO
2-P25-SiO
2) [54]
fluorescence chromatograms suggesting the production ofnew substances and fluorophores
Despite developed analytical technologies analyzinglignin degradation products remains challenging Proofssuch as mass spectroscopy (MS) HPLC 13C or 1H-NMRspectra from photocatalytic lignin degradations are not yetestablished The setback to qualitatively and quantitativelyanalyze lignin and its degradation products starts from thenative lignin polymer itself with its indefinite polymericstructure and multiple bond types Also the influence ofdifferent pretreatments additives and the wide variety ofcompounds obtainable from its degradation makes ligninanalysis challenging [85] Moreover lignin streams couldcontain proteins inorganic salts and other potential poisonsthat generally complicate catalysis [4]
The challenge to identify and separate the productsstreams derived from the photocatalytic degradation ligninis also worth noting Lignin product stream is highly func-tionalized and conventional techniques such as gas chro-matography have the disadvantage of requiring a time-consuming derivatization step Also because of high boilingpoint of substances arising from lignin degradation it is noteasily applicable High performance liquid chromatography(HPLC) seems to be the remedy because analysis can becarried out without derivatization but the exact identificationof the separated substances is difficult because of the numer-ous peaks arising from such a chromatogram [87] Unfortu-nately well-established databases such as that of the nationalinstitute of standard and technology (NIST) [88] cannot giveinformation onHPLC-MS chromatogramsThis is because ofthe ionization sources such as electrospray ionization (ESI)
14 International Journal of Photoenergy
Table 4 Main advantages and disadvantages photocatalysis versus enzymatic biocatalysis
Lignin degradation process Advantages Disadvantages
Photocatalysis(single stage)
(i) Relatively fast degradation of complex as wellas nonbiodegradable organic (macro)molecules(ii) Stable catalysts(iii) Moderate reaction conditions
(i) Not selective(ii) Limited selection of operating conditions(iii) Energy costs
Enzymatic conversion(single stage)
(i) More selective(ii) Moderate reaction conditions
(i) High enzyme loads required otherwise ligninhydrolysis is quite slow [16](ii) Enzymes are less stable1(iii) cost-intensive (POXs)2
1For example POXs are sensitive to high H2O2 concentrations2According to Torres and Ayala [86]
and atmospheric pressure chemical ionization (APCI) forLC-MS while GC-MS is based on electron ionization Aremedy to this can be the development of a database ofmodel lignin compounds based on a unanimous HPLC-MSmeasuring procedure This would mean much time and costexpensive investments for adequate personnel and material
Though product identification based on peak superpo-sition gives information on a possible product this can-not always be true for lignin degradation because of thelarge number of possible degradation products In order toproperly identify products from lignin degradation productisolation techniques are to be implemented and further ana-lytic methods such as tandem mass spectroscopy (MSMS)and other advanced structure enhancing research techniquessuch as 1H-NMR 13C-NMR have to be studied extensively
4 Photocatalysis and Its Suitability asIntegrated Technology in MultistageConcepts with Biocatalysis
Depending on lignin treatment specification photocatalysiswill find application for complete lignin mineralization toCO2and H
2O (single stage) or as integrated technology in
a multistage system aiming at partial conversion of ligninmacromolecules to organic low molecular weight intermedi-ates for a consecutive biological oxidation finally generatingvalue added end-products [15]
For biological oxidation of lignin or lignin deriva-tives H
2O2-dependent ligninolytic heme peroxidases (POXs
including lignin peroxidase (LiP EC 111114) manganeseperoxidase (MnP EC 111113) versatile peroxidase (VPEC 111116) O
2-dependent laccases (Lac EC 11032) and
extracellular enzymes from Basidiomycetous white-rot fungiare the most efficient lignin degraders in nature [89] Thussuch enzymatic systems especially POXs (with high redoxpotentials) have attractedmuch interest as industrial biocata-lyst [86 90]The enzyme degradationmechanism (in nature)is facilitated by nonenzymatic processes mainly throughfree ∙OH radicals also generated by the fungus (Fenton-type reaction) Those ∙OH radicals enable the requiredphysical contact between the enzymes and structural units ofthe lignin molecule due to numerous nonspecific oxidative
reactions [91] Both the photocatalytic (equations (2)ndash(7))and the POX reactionmechanism recently reviewed by Busseat al [25] are quite similar Consequently a combination ofphotocatalysis followed by an enzymatic oxidation maybe apromising concept utilizing lignin derivatives for exampleoriginated from industrial effluents [15 92] The advantagesforming biobased products are as follows see also Table 4 inthis context A reduction of the lignin polymerization degreevia photocatalysis at best in more biodegradable interme-diates and a simultaneous detoxification causes savings inenzyme costs since it will be expected that less enzyme loadsare necessary for sufficiently rapid reaction rates [15 91] As aresult hydraulic retention times in bioreactor systems shouldbe diminished Moreover the delignification is expectedto be enhanced simultaneously which was in recent yearsshown in a dual system by Kamwilaisak and Wright [93]using TiO
2H2O2UV for photocatalytic pretreatment and
Lac (from Trametes versicolor) in the subsequent biocatalyticstep Within 24 h they obtained in their dual system anelevation in delignification of 20 (without H
2O2) up to at
least 50 when H2O2was present
In previous studies the treatment of lignin (from thepulp and paper industry) containing effluents by fungus asa biological system (excreting appropriate lignin degradingenzyme cocktails) were more focused for posttreatmentexclusively (Duran et al [92] Reyes et al [94] and Gonzalezet al [95]) Shende et al [17] even examined ligninolyticbacteria with photooxidized kraft lignin as substrate
Although POXs are potential industrial biocatalysts [86]no application studies were found for the direct use in suchdual systems as described above The major reason may bethe complexity of the reaction mechanism (inclusive ligninderivatives as substrate and their analysis) per se on the onehand slowing down research and development processes Onthe other hand POXs are sensitive to their cosubstrate H
2O2
once it is supplied in excess causing considerable inactivation(for details refer to Busse et al [25]) At the present severalstudies are carried out modifying these enzymes regardingenhanced stability activity and selectivity as well Henceit can be expected that their technological applicability willbe raised significantly right after successful modification isreached [86]
International Journal of Photoenergy 15
5 Concluding Remarks
It is widely assumed that the photocatalytic degradation oflignin follows a radical reaction pathway which is similar tothat considered in thermal electrochemical and biochemicalprocesses However reporting on the degradation pathway oflignin derivatives and even that of lignin model compoundsare still amajor challengeThis is probably due to the complexnature and variety of possible degradation products Indeedthemechanism is far more complex considering other factorssuch as type of lignin type of catalyst pH illuminationsource and additives
Generally comparing the different photochemical pro-cesses poses a big challenge because of the wide variablesinvolved These discrepancies start from the source and typeof lignin followed by differences in reactor design illumi-nation source intensity of radiation and different types ofcatalyst An ideawould be to have a specific reference reactionwith well-defined starting parameters which include lignintype source and purity catalyst specifications illuminationsource and intensity so as to ease comparison of results
Basic process parameters such as catalyst concentrationsubstrate concentration addition of metal ion to catalyst pHand illumination have been discussed
Despite developed analytical technologies analyzinglignin degradation products remains challenging Proofssuch as mass spectroscopy (MS) HPLC 13C or 1H-NMRspectra from photocatalytic lignin degradations are not yetestablished
In order to properly identify products from lignin degra-dation product isolation techniques are to be implementedand further analytic methods such as MSMS and otheradvanced structure enhancing research techniques such as1H-NMR and 13C-NMR have to be studied extensively Aremedy to this can be the development of a databank ofmodel lignin compounds based on a unanimous HPLC-MSmeasuring procedure This would mean much time and costexpensive investments for adequate personnel and material
Photocatalysis is denoted as the most popular ligninpretreatment technology besides ozonation [96] Photo-catalyzed lignin may be an appropriate substrate for aconsecutive biocatalytic process using ligninolytic enzymes(POX andor Lac) as supported by experimental results ofKamwilaisak and Wright [93] Combining the advantages ofboth catalytic processes savings in the overall process costswill be expected in addition to elevated lignin conversionNonetheless extensive research work including POX modi-fications is still required
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Authorsrsquo Contribution
Colin Awungacha Lekelefac and Nadine Busse contributedequally to this work
Acknowledgments
The authors gratefully thank the Federal Ministry of Educa-tion and Research (BMBF) for funding (FKZ17N0310) Theresearchers also thank the Hessen State Ministry of HigherEducation Research andArts for the financial support withinthe Hessen initiative for scientific and economic excellence(LOEWE)
References
[1] Crude Oil and Commodity Prices httpwwwoil-pricenet[2] Research Directorate-General for World Energy Technology
and Climate Policy Outlook (WETO) Luxembourg Office forOfficial Publications of the European Communities 2003
[3] Green and Natural Polymers Are on the Rise 2014 httpwwwpolymersolutionscombloggreen-and-natural-polymers-on-the-rise
[4] J Zakzeski P C Bruijnincx A L Jongerius and B M Weck-huysen ldquoThe catalytic valorization of lignin for the productionof renewable chemicalsrdquo Chemical Reviews vol 110 no 6 pp3552ndash3599 2010
[5] B KammM Kamm P R Gruber and S KromusBiorefineries-Industrial Processes and Products Status Quo and Future Direc-tions vol 1 Wiley-VCH 2006
[6] R L Howard E Abotsi E L J van Rensburg and S HowardldquoLignocellulose biotechnology issues of bioconversion andenzyme productionrdquoAfrican Journal of Biotechnology vol 2 no12 pp 602ndash619 2003
[7] M Stocker ldquoBio- und BTL-Kraftstoffe in der Bioraffineriekatalytische Umwandlung Lignocellulose-reicher Biomasse mitporosen Stoffenrdquo Angewandte Chemie vol 120 no 48 pp9340ndash9351 2008
[8] J-P Lange ldquoLignocellulose conversion an introduction tochemistry process and economicsrdquo Biofuels Bioproducts andBiorefining vol 1 no 1 pp 39ndash48 2007
[9] D A I Goring ldquoThe physical chemistry of ligninrdquo Pure andApplied Chemistry vol 5 no 1-2 pp 233ndash310 1962
[10] M Ek G Gellerstedt and G HenrikssonWood Chemistry andBiotechnology Pulp and Paper Chemistry and Technology vol 1Walter de Gruyter GmbH amp Co KG Berlin Germany 2009
[11] B Saake and R Lehnen ldquoLigninrdquo in Ullmannrsquos Encyclopedia ofIndustrial Chemistry Wiley-VCH Verlag GmbH amp Co KGaAWeinheim Germany 2012
[12] M N S Kumar A K Mohanty L Erickson and M MisraldquoLignin and its applications with polymersrdquo Journal of BiobasedMaterials and Bioenergy vol 3 no 1 pp 1ndash24 2009
[13] R J A Gosselink E de Jong B Guran and A AbacherlildquoCo-ordination network for ligninmdashstandardisation produc-tion and applications adapted to market requirements(EUROLIGNIN)rdquo Industrial Crops and Products vol 20 no 2pp 121ndash129 2004
[14] D Fengel and G Wegener Wood Chemistry UltrastructureReactions Verlag Kessel 1984
[15] D Mantzavinos and E Psillakis ldquoEnhancement of biodegrad-ability of industrial wastewaters by chemical oxidation pre-treatmentrdquo Journal of Chemical Technology and Biotechnologyvol 79 no 5 pp 431ndash454 2004
16 International Journal of Photoenergy
[16] M Dashtban H Schraft and W Qin ldquoFungal bioconversionof lignocellulosic residues Opportunities amp perspectivesrdquo Inter-national Journal of Biological Sciences vol 5 no 6 pp 578ndash5952009
[17] A Shende R Jaswal D Harder-Heinz A Menan and RShende ldquoIntergrated photocatalytic and microbial degradationof kraft ligninrdquo Cleantech pp 120ndash123 2012
[18] F S Chakar and A J Ragauskas ldquoReview of current and futuresoftwood kraft lignin process chemistryrdquo Industrial Crops andProducts vol 20 no 2 pp 131ndash141 2004
[19] W G Glasser and H R Glasser ldquoEvaluation of ligninrsquoschemical structure by experimental and computer simulationtechniquesrdquo Paperi ja Puu vol 63 pp 71ndash83 1981
[20] M Erickson S Larsson and G E Miksche ldquoZur Struktur desLignins der FichterdquoActaChemica Scandinavica vol 27 pp 903ndash914 1973
[21] H Nimz ldquoDas Lignin der BuchemdashEntwurf eines Konstitution-sschemasrdquo Angewandte ChemiemdashInternational Edition vol 86pp 336ndash344 1974
[22] D V Evtuguin C P Neto J Rocha and J D P de JesusldquoOxidative delignification in the presence of molybdovana-dophosphate heteropolyanions mechanism and kinetic stud-iesrdquo Applied Catalysis A General vol 167 no 1 pp 123ndash1391998
[23] A L Linsebigler G Lu and J T Yates Jr ldquoPhotocatalysis onTiO2surfaces Principles mechanisms and selected resultsrdquo
Chemical Reviews vol 95 no 3 pp 735ndash758 1995[24] Y Miyata K Miyazaki M Miura Y Shimotori M Aoyama
and H Nakatani ldquoSolventless delignification of wood flourwith TiO
2poly(ethylene oxide) photocatalyst systemrdquo Journal
of Polymers and the Environment vol 21 no 1 pp 115ndash121 2013[25] N Busse D Wagner M Kraume and P Czermak ldquoReaction
kinetics of versatile peroxidase for the degradation of lignincompoundsrdquoTheAmerican Journal of Biochemistry andBiotech-nology vol 9 no 4 pp 365ndash394 2013
[26] M Tien and T K Kirk ldquoLignin-degrading enzyme fromphanerochaete chrysosporium Purification characterizationand catalytic properties of a uniqueH
2O2-requiring oxygenaserdquo
Proceedings of the National Academy of Sciences vol 81 pp2280ndash2284 1984
[27] T K Kirk M Tien P J Kersten M D Mozuch and BKalyanaraman ldquoLigninase of Phanerochaete chrysosporiumMechanism of its degradation of the non-phenolic arylglycerol120573-aryl ether substructure of ligninrdquo Biochemical Journal vol236 no 1 pp 279ndash287 1986
[28] T Lundell RWever R Floris et al ldquoLignin peroxidase L3 fromPhlebia radiata pre-steady-state and steady-state studies withveratryl alcohol and a non-phenolic lignin model compound1-(34-dimethoxyphenyl)-2-(2-methoxyphenoxy)propane-13-diolrdquo European Journal of Biochemistry vol 211 no 3 pp391ndash402 1993
[29] H E Schoemaker T K Lundell A I Hatakka and K PiontekldquoThe oxidation of veratryl alcohol dimeric lignin models andlignin by lignin peroxidase the redox cycle revisitedrdquo FEMSMicrobiology Reviews vol 13 no 2-3 pp 321ndash331 1994
[30] J M Palmer P J Harvey and H E Schoemaker ldquoThe role ofperoxidases radical cations and oxygen in the degradation oflignin [and discussion]rdquo Philosophical Transactions of the RoyalSociety of London A vol 321 no 1561 pp 495ndash505 1987
[31] C Hill Wood Modification Chemical Thermal and OtherProcesses John Wiley amp Sons Chichester UK 2006
[32] Y-H Xu H-R Chen Z-X Zeng and B Lei ldquoInvestigation onmechanism of photocatalytic activity enhancement of nanome-ter cerium-doped titaniardquo Applied Surface Science vol 252 no24 pp 8565ndash8570 2006
[33] L Tonucci F Coccia M Bressan and N DrsquoAlessandro ldquoMildphotocatalysed and catalysed green oxidation of lignin auseful pathway to low-molecular-weight derivativesrdquoWaste andBiomass Valorization vol 3 no 2 pp 165ndash174 2012
[34] A Castellan N Colombo C Vanucci P Fornier de HViolet and H Bouas-Laurent ldquoPhotodegradation of ligninA photochemical study of an O-methylated 120572-carbonyl 120573-1lignin model dimer 12-di(3101584041015840-dimethoxyphenyl) ethanone(deoxyveratroin)rdquo Journal of Photochemistry and PhotobiologyA vol 51 pp 451ndash467 1990
[35] A Fujishima X Zhang and D A Tryk ldquoTiO2photocatalysis
and related surface phenomenardquo Surface Science Reports vol63 no 12 pp 515ndash582 2008
[36] C S Turchi and D F Ollis ldquoPhotocatalytic reactor designan example of mass-transfer limitations with an immobilizedcatalystrdquo The Journal of Physical Chemistry vol 92 no 23 pp6852ndash6853 1988
[37] R Subasri M Tripathi K Murugan J Revathi G V N Raoand T N Rao ldquoInvestigations on the photocatalytic activity ofsolndashgel derived plain and Fe3+Nb5+ doped titania coatings onglass substratesrdquo Materials Chemistry and Physics vol 124 pp63ndash68 2010
[38] N Serpone ldquoRelative photonic efficiencies and quantum yieldsin heterogeneous photocatalysisrdquo Journal of Photochemistry andPhotobiology A Chemistry vol 104 no 1ndash3 pp 1ndash12 1997
[39] M R Hoffmann S T Martin W Choi and D W BahnemannldquoEnvironmental applications of semiconductor photocatalysisrdquoChemical Reviews vol 95 no 1 pp 69ndash96 1995
[40] C D Jaeger and A J Bard ldquoSpin trapping and electron spinresonance detection of radical intermediates in the photode-composition of water at titanium dioxide particulate systemsrdquoThe Journal of Physical Chemistry vol 83 no 24 pp 3146ndash31521979
[41] R W Matthews ldquoHydroxylation reactions induced by near-ultraviolet photolysis of aqueous titaniumdioxide suspensionsrdquoJournal of the Chemical Society Faraday Transactions vol 80no 2 pp 457ndash471 1984
[42] A E H Machado A M Furuyama S Z Falone R RuggieroD D S Perez and A Castellan ldquoPhotocatalytic degradation oflignin and lignin models using titanium dioxide the role of thehydroxyl radicalrdquoChemosphere vol 40 no 1 pp 115ndash124 2000
[43] W Sigg and L Stumm Aquatische Chemie Stuttgart 2 AuflageBG Teubner Stuttgart Germany 1991
[44] O Legrini E Oliveros and A M Braun ldquoPhotochemicalprocesses for water treatmentrdquo Chemical Reviews vol 93 no2 pp 671ndash698 1993
[45] G Rothenberger J Moser M Gratzel N Serpone and DSharma ldquoCharge carrier trapping and recombination dynamicsin small semiconductor particlesrdquo Journal of the AmericanChemical Society vol 107 no 26 pp 8054ndash8059 1985
[46] P Mazellier M Sarakha A Rossi and M Bolte ldquoThe aqueousphotochemistry of 26-dimethylphenol Evidence for the frag-mentation of the 120572 C-C bondrdquo Journal of Photochemistry andPhotobiology A vol 115 no 2 pp 117ndash121 1998
[47] M Ksibi S B Amor S Cherif E Elaloui A Houas and MElaloui ldquoPhotodegradation of lignin from black liquor using aUVTiO
2systemrdquo Journal of Photochemistry and Photobiology
A vol 154 no 2-3 pp 211ndash218 2003
International Journal of Photoenergy 17
[48] S K Kansal M Singh and D Sud ldquoStudies on TiO2ZnO
photocatalysed degradation of ligninrdquo Journal of HazardousMaterials vol 153 no 1-2 pp 412ndash417 2008
[49] A Dahm and L A Lucia ldquoTitanium dioxide catalyzed pho-todegradation of lignin in industrial effluentsrdquo Industrial andEngineering Chemistry Research vol 43 no 25 pp 7996ndash80002004
[50] E Portjanskaja and S Preis ldquoAqueous photocatalytic oxidationof lignin the influence of mineral admixturesrdquo InternationalJournal of Photoenergy vol 2007Article ID76730 7 pages 2007
[51] K Tanaka R C R Calanag and T Hisanaga ldquoPhotocatalyzeddegradation of lignin on TiO
2rdquo Journal of Molecular Catalysis
A Chemical vol 138 no 2-3 pp 287ndash294 1999[52] M Tian J Wen D MacDonald R M Asmussen and A Chen
ldquoA novel approach for lignin modification and degradationrdquoElectrochemistry Communications vol 12 no 4 pp 527ndash5302010
[53] K PanM Tian Z-H Jiang B Kjartanson and A Chen ldquoElec-trochemical oxidation of lignin at lead dioxide nanoparticlesphotoelectrodeposited on TiO
2nanotube arraysrdquo Electrochim-
ica Acta vol 60 pp 147ndash153 2012[54] C Awungacha Lekelefac J Hild P Czermak and M Herren-
bauer ldquoPhotocatalytic active coatings for lignin degradationin a continuous packed bed reactorrdquo International Journal ofPhotoenergy vol 2014 Article ID 502326 10 pages 2014
[55] C Awungacha Lekelefac P Czermak and M HerrenbauerldquoEvaluation of photocatalytic active coatings on sintered glasstubes by methylene bluerdquo International Journal of Photoenergyvol 2013 Article ID 614567 9 pages 2013
[56] Y-S Ma C-N Chang Y-P Chiang H-F Sung and A CChao ldquoPhotocatalytic degradation of lignin using PtTiO
2as
the catalystrdquo Chemosphere vol 71 no 5 pp 998ndash1004 2008[57] H Ohnishi M Matsumura H Tsubomura and M Iwasaki
ldquoBleaching of lignin solution by a photocatalyzed reactionon semiconductor photocatalystsrdquo Industrial and EngineeringChemistry Research vol 28 no 6 pp 719ndash724 1989
[58] C A K Gouvea F Wypych S G Moraes N Duran and PPeralta-Zamora ldquoSemiconductor-assisted photodegradation oflignin dye and kraft effluent by Ag-doped ZnOrdquo Chemospherevol 40 no 4 pp 427ndash432 2000
[59] A V Vahatalo K Salonen M Salkinoja-Salonen and AHatakka ldquoPhotochemical mineralization of synthetic lignin inlake water indicates enhanced turnover of aromatic organicmatter under solar radiationrdquo Biodegradation vol 10 no 6 pp415ndash420 1999
[60] H de Lasa B Serrano and M Salaices Photocatalytic ReactionEngineering Springer New York NY USA 2005
[61] K Hashimoto H Irie and A Fujishima ldquoTiO 2 photocatalysisA historical overview and future prospectsrdquo Japanese Journal ofApplied Physics vol 44 no 12 pp 8269ndash8285 2005
[62] M A Behnajady N Modirshahla and R Hamzavi ldquoKineticstudy on photocatalytic degradation of CI Acid Yellow 23 byZnOphotocatalystrdquo Journal ofHazardousMaterials vol 133 no1ndash3 pp 226ndash232 2006
[63] M Addamo V Augugliaro A di Paola et al ldquoPhotocatalyticthin films of TiO
2formed by a sol-gel process using titanium
tetraisopropoxide as the precursorrdquo Thin Solid Films vol 516no 12 pp 3802ndash3807 2008
[64] N Negishi K Takeuchi and T Ibusuki ldquoPreparation of theTiO2thin filmphotocatalyst by the dip-coating processrdquo Journal
of Sol-Gel Science and Technology vol 13 no 1ndash3 pp 691ndash6941998
[65] L Rideh A Wehrer D Ronze and A Zoulalian ldquoPhotocat-alytic degradation of 2-chlorophenol in TiO
2aqueous suspen-
sion modeling of reaction raterdquo Industrial and EngineeringChemistry Research vol 36 no 11 pp 4712ndash4718 1997
[66] R-ADoongC-HChen RAMaithreepala and S-MChangldquoThe influence of pHand cadmiumsulfide on the photocatalyticdegradation of 2-chlorophenol in titanium dioxide suspen-sionsrdquoWater Research vol 35 no 12 pp 2873ndash2880 2001
[67] K V Sarkanen A Islam and C D Anderson ldquoOzonationrdquo inMethods in Lignin Chemistry S Y Lin and C W Dence EdsSpringer Series inWood Science pp 387ndash406 Springer BerlinGermany 1992
[68] G Gellerstedt and E-L Lindfors ldquoStructural changes in ligninduring kraft pulpingrdquo Holzforschung vol 38 no 3 pp 151ndash1581984
[69] B Neppolian H C Choi M V Shankar B Arabindoo andV Murugesan in Proceedings of the International Symposiumon Environmental Pollution Control and Waste Management(EPCOWM 02) p 647 2002
[70] C Pouteau P Dole B Cathala L Averous and N BoquillonldquoAntioxidant properties of lignin in polypropylenerdquo PolymerDegradation and Stability vol 81 no 1 pp 9ndash18 2003
[71] J Hafren T Fujino and T Itoh ldquoChanges in cell wall archi-tecture of differentiating tracheids of Pinus thunbergii duringlignificationrdquo Plant and Cell Physiology vol 40 no 5 pp 532ndash541 1999
[72] T Dizhbite G Telysheva V Jurkjane and U Viesturs ldquoChar-acterization of the radical scavenging activity of lignins naturalantioxidantsrdquo Bioresource Technology vol 95 no 3 pp 309ndash3172004
[73] K Hofstadler R Bauer S Novalic and S G Heisier ldquoNew reac-tor design for photocatalytic wastewater treatment with TiO
2
immobilized on fused-silica glass fibers photomineralization of4-chlorophenolrdquo Environmental Science amp Technology vol 28no 4 pp 670ndash674 1994
[74] J Villasenor and H D Mansilla ldquoEffect of temperature onkraft black liquor degradation by ZnO-photoassisted catalysisrdquoJournal of Photochemistry and Photobiology A Chemistry vol93 no 2-3 pp 205ndash209 1996
[75] B H Bielski D E Cabelli L A Ravindra and A B RossldquoReactivity of HO
2Ominus2Radicals in aqueous solutionrdquo Journal
of Physical Chemistry vol 14 pp 1041ndash1100 1985[76] B H J Bielski and A O Allen ldquoMechanism of the dispropor-
tionation of superoxide radicalsrdquo Journal of Physical Chemistryvol 81 no 11 pp 1048ndash1050 1977
[77] B Halliwell and J M C Gutteridge ldquoThe importance of freeradicals and catalytic metal ions in human diseasesrdquoMolecularAspects of Medicine vol 8 no 2 pp 89ndash193 1985
[78] J M Palmer P J Harvey and H E Schoemaker ldquoThe role ofperoxidases radical cations and oxygen in the degradation oflignin [and discussion]rdquo Philosophical Transaction of the RoyalSociety A vol 321 no 1561 pp 495ndash505 1987
[79] S Y Yin and C W Dense Methods in Lignin ChemistrySpringer New York NY USA 1992
[80] P Kumar S Kumar and N K Bhardwaj ldquoPhotocatalyticoxidation of elemental chlorine free bleaching effluent withUVTiO
2rdquo in Proceedings of the 2nd International Conference on
Environmental Science and Technology (ICEST rsquo11) SingaporeFebruary 2011
[81] K Kobayakawa Y Sato S Nakamura and A FujishimaldquoPhotodecomposition of Kraft lignin catalyzed by titanium
18 International Journal of Photoenergy
dioxiderdquo Bulletin of the Chemical Society of Japan vol 62 no11 pp 3433ndash3436 1989
[82] R B Kinstre ldquoAn overview of strategies for reducing theenvironmental impact of bleach-plant effluentsrdquo Tappi Journalvol 76 no 5 pp 105ndash113 1993
[83] A Castellan H Choudhury R Stephen Davidson and SGrelier ldquoComparative study of stone-ground wood pulp andnative wood 3 Application of fluorescence spectroscopy to astudy of the weathering of stone-ground pulp and native woodrdquoJournal of Photochemistry and Photobiology A Chemistry vol81 no 2 pp 123ndash130 1994
[84] B Albinsson S Li K Lundquist and R Stomberg ldquoThe originof lignin fluorescencerdquo Journal of Molecular Structure vol 508no 1ndash3 pp 19ndash27 1999
[85] S Baumberger A Abaecherli M Fasching et al ldquoMolar massdetermination of lignins by size-exclusion chromatographytowards standardisation of the methodrdquo Holzforschung vol 61no 4 pp 459ndash468 2007
[86] E Torres andM Ayala Biocatalysis Based on Heme PeroxidasesSpringer New York NY USA 1st edition 2010
[87] R Pecina P Burtscher G Bonn and O Bobleter ldquoGC-MSand HPLC analyses of lignin degradation products in biomasshydrolyzatesrdquo Fresenius Zeitschrift fur Analytische Chemie vol325 no 5 pp 461ndash465 1986
[88] NIST NIST Standard Reference Database The National Insti-tute of Standards and Technology 2014 httpwwwnistgovsrdnist1acfm
[89] T K Kirk and R L Farrell ldquoEnzymatic ldquocombustionrdquo themicrobial degradation of ligninrdquo Annual Review of Microbiol-ogy vol 41 pp 465ndash501 1987
[90] A TMartinez ldquoHigh redox potential peroxidasesrdquo in IndustrialEnzymes Structure Function and Applications K-B BeckerEd pp 477ndash488 Springer Amsterdam The Netherlands 1stedition 2007
[91] M Dashtban H Schraft T A Syed and W Qin ldquoFungalbiodegradation and enzymatic modification of ligninrdquo Interna-tional Journal of Biochemistry and Molecular Biology vol 1 no1 pp 36ndash50 2010
[92] N Duran E Esposito L H Innocentini-Mei and V P CanhosldquoA new alternative process for Kraft E1 effluent treatmentrdquoBiodegradation vol 5 no 1 pp 13ndash19 1994
[93] K Kamwilaisak and P C Wright ldquoInvestigating laccase andtitanium dioxide for lignin degradationrdquo Energy and Fuels vol26 no 4 pp 2400ndash2406 2012
[94] J Reyes M Dezotti H Mansilla J Villasenor E Espositoand N Duran ldquoBiomass photochemistry-XXII combined pho-tochemical and biological process for treatment of Kraft E1effluentrdquoApplied Catalysis B Environmental vol 15 no 3-4 pp211ndash219 1998
[95] L F Gonzalez V Sarria and O F Sanchez ldquoDegradationof chlorophenols by sequential biological-advanced oxidativeprocess using Trametespubescens and TiO
2UVrdquo Bioresource
Technology vol 101 no 10 pp 3493ndash3499 2010[96] J H Lora and W G Glasser ldquoRecent industrial applications
of lignin a sustainable alternative to nonrenewable materialsrdquoJournal of Polymers and the Environment vol 10 no 1-2 pp 39ndash48 2002
Submit your manuscripts athttpwwwhindawicom
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Inorganic ChemistryInternational Journal of
Hindawi Publishing Corporation httpwwwhindawicom Volume 2014
International Journal ofPhotoenergy
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Carbohydrate Chemistry
International Journal of
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Journal of
Chemistry
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Advances in
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Analytical Methods in Chemistry
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Bioinorganic Chemistry and ApplicationsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
SpectroscopyInternational Journal of
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The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Medicinal ChemistryInternational Journal of
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Chromatography Research International
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Applied ChemistryJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
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Analytical ChemistryInternational Journal of
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Quantum Chemistry
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Organic Chemistry International
ElectrochemistryInternational Journal of
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Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CatalystsJournal of
2 International Journal of Photoenergy
Cellulosehemicellulose
lignin
Biomass
Pretreatment
Pyrolysis andgasification
Platformchemicals
Step 1
1
2
3
Step 2catalysis
Catalysis
Syngas
Furtherprocessing
catalysis
New technology
New technology
Current technology
Lignin
Cellulose
Hemicellulose
Fuelsbulk and fine
chemicals
PhenolBenzene Toluene Xylene
ProteinsLipidsAsh
SoilSaltsWater
KraftSulfiteOrganosolvPyrolysisSteamExplosionAFEXHot waterOther
For example Fischer-Tropsch methanol synthesis
COH2
OH
OH
OH
OH
OH
R R
ROH
OH
OH
OH
OH
OH
OH
O
OOO
HO
HO
O O
O
Figure 1 Lignocellulosic biorefinery scheme with particular emphasis on the lignin stream reprinted with permission from Zakzeski et al[4] copyright (2010) American Chemical Society
Figure 1 depicts an exemplary lignocellulosic biorefineryscheme with emphasis on the lignin stream For separa-tion purposes several pretreatment procedures are currentlyapplied in order to generate lignin derivatives (modificationin lignin structure) which can be differentiated on thebasis of their isolation method and their origin since theirphysical and chemical properties differ [9ndash11] The mostindustrialtechnical lignins (gt70 million tons per year) areobtained as waste material by the pulp and paper industry[12] mainly from the kraft process as noted by Kamm etal [5] 98 is burnt for energy recovery in the paper mills[13] while less than 2 is sold primarily in formulationof dispersants adhesives and surfactants [14] Lignin is anaromatic biopolymer of high potential fission products fora wide range of sectors and it is thus gaining attention forexample for the production of platform chemicals (Figure 1)Nonetheless lignin is still underutilized and fundamentalresearch and development are needed [5]This is explained byligninrsquos complex nature its recalcitrance to degradation andthe difficulty to analyze its numerous degradation productsTherefore intensive research goes on both sides processengineering and development (homogeneous heterogeneouscatalysis thermal electrochemical andor hybrid proce-dures) and process analytics
Photocatalysis is an advanced oxidation process (AOPs)[15] with the potential to transform lignin to value addedproducts such as phenol benzene toluene and xylene [4]In this context it can be applicable as stand-alone unit or it
can be coupledwith otherAOPs (eg Fentonrsquos reagent ozoneelectrochemical oxidation) as well as biocatalysis [15] (eghydrolysis through ligninolytic enzymes)
This paper is aimed at reviewing photocatalysis of indus-trial lignins in connection with evaluating the effects ofbasic operating parameters such as catalyst loading illu-mination intensity integration of metal ions and otheradditives pH and initial lignin concentration Furthermoreanalytical techniques used by different researchers are dis-cussed Finally a brief overview concerning the suitabilityof photocatalysis as a pretreatment method for subsequentbiocatalysis by ligninolytic systems for example fungal hemeperoxidases andor laccases is highlighted
2 Lignin as Raw MaterialChemical Structure and Sources
Lignin is the only naturally synthesized aromatic biopolymer[16] and after cellulose themost abundant renewable carbonsource on earth [17] In addition native lignin is a poly-disperse 3D macromolecule with an undefined molecularmass The biopolymer is made up of randomly arrangedphenylpropane units 119901-coumaryl alcohol coniferyl alcoholand sinapyl alcohol as depicted in Figure 2 contributing to anirregular structure [11]
An overview of the common interunit linkages and itsestimated proportions are shown in Table 1 For a better
International Journal of Photoenergy 3
1
2
3
45
120572120573
120574OH
OH
p-Coumaryl alcohol
OCH3
OH
OH
Coniferyl alcohol Sinapyl alcohol
OCH3H3CO
OH
OH
Figure 2 Monomer structures of lignin [18]
Table 1 Overview of most frequent bond types found in lignin
Model linkagea W G Glasser and H R Glasser [19] Erickson et al [20] Nimz [21]120573-Carbon-oxygen-4-aromatic carbon 55 49ndash51 65120572-Carbon-oxygen-4-aromatic carbon mdash 6ndash8 mdash120573-Carbon-5-aromatic carbon 16 9ndash15 6120573-Carbon-1-aromatic carbon 9 2 155-Aromatic-carbon-5-aromatic carbon 9 95 234-Aromatic-carbon-5-aromatic carbon 3 35 15120573-Carbon-120573-carbon 2 2 55120573-Carbon-120573-carbon forming furanic structure mdash mdash 2120572-Carbon-120574-carbon-oxygen-120574-carbon 10 mdash mdasha of total phenylpropane units
illustration Figure 3 shows the structure of a softwood ligninfragment containing all prominent linkage types
Native lignin cannot yet be isolated Contrary manymodified lignins of great variety are available throughbiomass transformation technologies [17] Depending onthe used isolation method lignosulfonate alkali lignin sul-fate lignin hydrolytic lignin steam exploded lignin andorganosolv lignins can be derived [31] The kraft and sodapulping processes generate liquors referred to as black liquorcontaining alkali lignin also called kraft or sulfate lignin [31]Although the kraft process is the dominant process (89)lignosulfonates attract more attention for the application oflignin as industrial products This is probably because ofadvantages such as its solubility properties and relativelylower mass range Moreover lignosulfonates are the onlycommercial lignins which are water soluble Their averagemolecular weight varies from 400Da up to 150 kDa or even17000 kDa (taken from Goring [9]) determined by processconditions and applied analytical methods [11]
3 Photocatalysis of Lignin
In this section the reaction principle for the photocatalyticdegradation of lignin is described After that themost impor-tant operating parameters and their impacts are discussed
31 On the Reaction Pathways for Photocatalytic Lignin Degra-dation Photocatalysis is the acceleration of a photoreactionin the presence of a catalyst In other words it involves theinitial absorption of photons by a molecule or substrate toproduce highly reactive electronically excited states
Lignin degradation is generally in the range of lowerenergy (between 300 and 400 nm) because of its multifunc-tional character [32ndash34]This region falls within the UV-lightregion TiO
2is the most applied photocatalyst and its energy
band gap is approximately 32 eV [35]In photogenerated catalysis the photocatalytic activity
depends on the ability of the catalyst to create electron-hole pair which generates free radicals (eg hydroxyl radi-cals ∙OH) enabling secondary reactions [36] Other aspectsinclude the rate of electron transfer the rate of chargerecombination crystal structure surface area of catalystporosity and surface hydroxyl group density [37]
In what follows equations summarizing the formationof radical species under photocatalytic conditions shall bedescribed S stands for the lignin substrate while TiO
2(h+VB)
and TiO2(eminusCB) represent the electron deficient (valence
band) and electron-rich (conduction band) parts in thestructure of TiO
2 respectively
The initial photocatalytic process involves the generationof electron-hole pair in the semiconductor particles as a result
4 International Journal of Photoenergy
Spirodienone
O
OO
O
O
OHO
OO
O
O
O
O
OHO
O
O
O
O
O
O
OO O
O
O
O
O
O
O
O
OH
OO
OO
O
O
OO
p-Coumarylalcohol fragment
Coniferylalcohol fragment
Branching caused bydibenzodioxin linkage Phenylcoumaran
HO
OAr
HO
OH
OHOH
OH
HO
HO
OH
HO
OH
HO
OH
HO
OH
HO
OH
HO
OH
HO
OH
HO
OH
HO4-O-5
OH
HOOH
120573-120573
5-5998400
120573-O-4
120573-1
120573-1
O
Figure 3 Structure of a softwood lignin fragment showing the prominent linkage types reprinted with permission from Zakzeski et al [4]and Evtuguin et al [22]
O2∙minus + H+ HO2
∙
2HO2∙
HO∙ + OHminus
∙OH
S + HO∙ S∙+ + OHminus
S∙+ + O2SO2
∙+
O2∙minus
∙O2minus
∙OH (hydroxyl radical)
(a) Reduction of O2H2O2
O2 + eminus
H2O2 + O2
H2O2 + eminus
(b) Oxidation of substrate
h+ + OHminus
Electron capture
Recombination
eminus
h+
Conduction band
Valence band
Electron release
eminus
O2
(super oxide)
Light
OHminus
Figure 4 Photocatalysis principle adapted from Linsebigler et al [23]
of UV radiation [38 39] Figure 4 shows the excitation ofan electron from the valence band to the conduction bandinitiated by light absorption with energy equal to or greaterthan the band gap of the semiconductor This is expressed by(1)
Upon excitation the fate of the separated electron andhole can follow several pathways Electron or holes canthen react with hydroxyl ions (OHminus) or H
2O producing
hydroxyl radicals (∙OH) as shown in (2) Jaeger and Bard[40] Matthews [41] andMachado et al [42] report that ∙OHis the main oxidizing agent in the photocatalytic oxidationbecause of the unpaired electrons Therefore it can react fastand unspecifically with almost all organic compounds (S)
[43] abstracting an electron with the formation of a radicalorganic species as shown in (3) [44]
Formation of singlet oxygen hydroxyl and superoxideradicals as principal reactive species in a photocatalyticprocess [38 39] is as follows
TiO2
ℎV997888997888rarr TiO
2(
eminusCBh+VB) (1)
TiO2(h+) +H
2O 997888rarr TiO
2+HO∙ +H+ (2)
S +HO∙ 997888rarr S∙+ +OHminus (3)
International Journal of Photoenergy 5
MeO MeOMeOMeO
∙OO
O
O ∙
OH
HO+ +
Figure 5 Formation of phenoxyl radicals by intermolecular abstraction of phenolic hydrogen by carbonyl groups
RH
bond cleavage
Vanillin
O
OO
O
OO O
O
O
O
O
O
HO HO
HOHO
HO
HO
HO HO
OH OH
HO
OCH3
OCH3
OCH3
OCH3
OCH3 OCH3
OCH3
OCH3 OCH3
OCH3
h minusH∙OH∙∙
O2TiO2
HminusOH∙
C120572-C120573
∙
Figure 6 Supposed lignin degradation scheme by autoxidation induced by TiO2poly (ethylene oxide) [24]
S∙+ +O2997888rarr SO
2
∙+ (4)
TiO2(h+) + S 997888rarr TiO
2+∙S+ (5)
TiO2(eminus) +O
2997888rarr TiO
2+∙O2
minus (6)
HO∙ + ∙O2
minus
997888rarr OHminus + 1O2
(7)
TiO2(eminus) + TiO
2(h+) 997888rarr heat (8)
The organic radicals and radical cations can for examplereact with molecular oxygen to form organic peroxy radicalsand peroxy radical cations respectively (4) The holes canoxidize organic compounds by electron abstraction to formorganic cationic radicals (5) [42] Superoxides can be formedby the reaction of electrons with electron acceptors suchas O2(6) Meanwhile the formation of singlet oxygen can
be from the reaction of hydroxyl radical and superoxide(7) [42] Moreover there is a possibility that electrons andholes recombine if electron acceptors are limited In thiscase recombination can take place in the volume of thesemiconductor particle When recombination takes placeradiation energy is lost or converted into heat (8) [45]
From investigations caried out by Mazellier et al [46](photochemistry of 26-dimethylphenol) it was postulatedthat hydrogen can be abstracted by 120572-carbonyl groups In thesame context lignin derivatives having similar functionalitycan follow a similar pathway In addition oxidative chainreactions with the participation of ground-state oxygencan be initiated leading to fragmentation and combinationreactions and thus the formation of new dimers or oligomers(Figure 5)
Miyata et al [24] proposed a cleavage mechanism forthe C120572ndashC120573 bonds which leads to the formation of smallfragments such as vanillin as shown in Figure 6
Figure 7 [25] illustrates the formation of a radical cationformed as a result of enzyme (lipase) mediated reactionof adlerol Adlerol is characterized by a C120573ndashOndash4 bondand considered to be a lignin model compound With theformation of the radical species subsequent nonenzymaticreactions such as radical reactions can take place generatinga wide variety of products and complex compounds
It is widely assumed that the photocatalytic degradationof lignin follows a radical reaction pathwaywhich is similar tothat considered in thermal electrochemical and biochemical
6 International Journal of Photoenergy
Nonenzymatic subsequent reactions
Adlerol
Veratryl alcohol radical
Enzyme1
23
4
4
1
2
3
5
5
6
6
Guaiacol
Veratraldehyde
Ketone
O
OO
O
O O
O
O
O
HO
HOHO
HO
OCH3
OCH3
OCH3
OCH3
OCH3
OCH3
120572120573
120574
minusH+
∙+
∙
∙+
c120572 oxidation H2O
O2
120572 120573
120574
HOHOHO
OCH3
OCH3OCH3
OCH3 OCH3
OCH3
OCH3
120572120572
120573 120573
120573
120574 120574
HO
HO
120574
OCH3
120573
HO
HO
120574
HO
OCH3
OCH3
OCH3
120572120573
120574
Figure 7 Proposed radical reaction scheme initiated by enzyme lignolytic heme peroxidase for the conversion of adlerol possessing C120573ndashOndash4 bonds into smaller units summarized by Busse et al [25] and abstracted from Tien and Kirk [26] Kirk et al [27] Lundell et al [28]Schoemaker et al [29] and Palmer et al [30]
processes However reporting on the degradation pathway oflignin derivatives and even that of lignin model compoundsis still a major challenge This is probably due to the complexnature and variety of possible degradation products Indeedthemechanism is far more complex considering other factorssuch as type of lignin type of catalyst pH illuminationsource and additives
32 Influence of Process Parameters in Lignin DegradationVarying process parameter could either have a positive ornegative impact on the photocatalytic efficiency The basicprocess parameters such as catalyst concentration [4 4856] substrate concentration [48 51] addition of metal ion
to TiO2catalyst [17 50 56ndash58] pH [47 48] illumination
[33 48 49 59] and their influence shall be discussed in thissubchapter Table 2 gives an overview about starting reactionconditions and catalyst applied by some work groups whileTable 3 portrays parameters analytical methods and resultsobtained It is worthwhile noting that comparing the differentphotochemical processes poses a big challenge because ofthe wide variables involvedThese discrepancies start alreadyfrom the source and type of lignin followed by the differencesin reactor design illumination source intensity of radiationand different types of TiO
2catalyst such as Fischer scientific
rutile TiO2[49] TiO
2(TiO2mdashTP-2 of Fujititan) just to name
a few
International Journal of Photoenergy 7
Table2Summaryof
startingcond
ition
sfor
thep
hotocatalytic
degradationof
lignin
Reference
Lign
insource
Catalyst
Reactio
ncond
ition
s
MacHadoetal[42]
Peroxyform
icacid
lignins
from
eucalyptus
grandisw
ood(EL1)
EL1+
sodium
borohydride
TiO
2andH
2O2
Lign
inconcentration
025
mgmL119879=25∘C
pH11U
VVisradiatio
n120582gt300nm
40
0Wmercury
lampcylin
dricalPy
rexglassreactorcon
stanto
xygenbu
bblin
g
Ksibietal[47]
Water
solubleligninob
tained
from
blackliq
uor
TiO
2-P2
5(D
egussa)
Lign
inconcentration
90mgL119879=20∘C
pH82UV-radiation120582gt290nm
pyrex
reactoro
pento
airPh
ilips
HPK
125W
lamp
Kansaletal[48]
Lign
infro
mwheatstr
awkraft
digestion
TiO
2-P2
5(D
egussa)a
ndZn
O
Lign
inconcentration
10ndash100
mgLin
100m
LZn
Ocatalystdo
se(05ndash20gL)pH
ofthes
olution(pH3ndash11)solarillu
mination
oxidantcon
centratio
n306times10minus6M
to153times10minus6Mthinbedfilm
slurrypo
ndreactoroxidantsodium
hypo
chlorite
solutio
n(4availableC
l 2)
Dahm
andLu
cia[
49]
Whitewater
from
indu
strialprocess
water
thatexits
inpaperm
achines
FischerS
cientifi
crutile
TiO
2
Lign
inconcentration
40mgLin
500m
Lbatchreactor119879=21∘Cndash
42∘C
Rayonet
photochemicalcham
ber16
VWR8-W
blacklightpho
spho
r(350-nm
)lam
ps
constant
oxygen
bubb
lingpo
wer
ofillum
ination
128to
64Wlight
intensity
223ndash44
5mWcm
3
Portjanskajaand
Preis[50]
Lign
inpurchased
from
Aldric
hTiO
2P2
5-N(D
egussa)
Lign
inconcentration
100m
gLpH
8batchreactorsystemreactor
open
toair
PhillipsT
LD15W05low-pressurelum
inescent
mercury
UV-lampUV-radiation
120582gt360nm
pow
erdensity
ofirr
adiatio
n=07m
Wcm
2 visib
lelight
source
Tanaka
etal[51]
Lign
infro
mconiferous
woo
dTiO
2(TiO
2mdashTP
-2of
Fujititan)
Lign
inconcentration
0003to
003U
V-radiation120582gt310nm
cylindrical
reactio
nvessel
Tonu
ccietal[33]
Ca2+andNH4
+ligninderiv
atives
TiO
2as
Degussa
P25
+po
lyoxom
etalates
(POM)H
2O2
Openqu
artztubes(20
mL)
reactor119879=20∘C
1atm
multirayso
ften
UVlamps
of15W
power
eachU
V-radiation120582gt254nm
Miyatae
tal[24]
Piceag
lehn
iiwoo
dflo
urTiO
2po
lyethylene
oxide(PE
O)
Reactorop
enpetrid
ish119879=30∘C119905=48h40
0Wmercury
lamp
Shende
etal[17]
Kraft
lignin
TiO
2-Zn
O-ZrO
2Lign
inconcentration
1mgmLin
10mL250W
Xeno
nlampandAM
15Glamp
filterpo
wer
density
ofirr
adiatio
n100m
Wcm
2 solarlam
psim
ulator
Tian
etal[52]
andPanetal[53]
Kraft
ligninfro
mblackliq
uor
Ta2O
5-IrO
2andPb
O2thin
film
TiO
2nano
tubePbO
2
Lign
inconcentration
30(w
w)UV-radiation120582gt365nm
119905=10minpow
erdensity
ofirr
adiatio
n20
mWcm
2 blue
waveT
M50
ASUVspot
lampEG
ampG2273
potentiosta
tgalvano
stattoapplycurrentTiTiO
2NTPb
O2ele
ctrode
asworking
electrode
andPt
coilas
outere
lectrode
andAgAgC
lasreference
electrode
Awun
gachaL
ekele
fac
etal[54][55]
Lign
insulfo
natefro
mpaperw
aste
water
TiO
2as
Degussa
P25
TiO
2fro
msol-g
elprocesso
fTiOSO
4TT
IP
Lign
inconcentration
500m
gLin
200m
LOsram
Planon
light
sourceirradiance
30ndash4
0Wm
2 UV-radiation120582280ndash4
20nm
119905=20hreactoro
pento
air
recirculationsyste
mflow
rate225mLmin
8 International Journal of Photoenergy
Table3Parametersanalyticalmetho
dsand
results
from
different
workgrou
ps
Reference
Parameter
studied
Analytic
sRe
sult
MacHado
etal[42]
Roleof
hydroxylradicals
irradiatio
nof
ligninin
the
absencea
ndpresence
ofph
otocatalystT
iO2andH
2O2
Ultraviolet-v
isible(UV-Vis)spectro
scop
yionizatio
nabsorptio
nspectro
scop
y(IA
S)
sizee
xclusio
nchromatograph
y(SEC
)
Asharpdecrease
inthep
heno
liccontento
bservedforreactions
involvingdirect
photolysis
SEC
areductio
nof
almost50
inthea
verage
molecular
weighto
fligninequalto
14kD
after
90min
ofirr
adiatio
n
Ksibietal[47]
Irradiationof
ligninin
the
absencea
ndpresence
ofph
otocatalystT
iO2-P2
5
UV-Visspectroscop
y13C-
nucle
armagnetic
resonance(NMR)
solid
state
totalion
gasc
hrom
atograph
y(TIC)
indu
ctioncoup
lingplasma(
ICP)
chem
icaloxygen
demand(C
OD)
56degradationratewith
TiO
2catalystaft
er420m
inreactiontim
eethylacetate-extractableprod
uctsshow
edvanillin
vanillica
cidpalm
itica
cid
biph
enylstructuresand
345-trim
etho
xybenzaldehyde
presence
ofmagnesiu
mandcalcium
ions
CODremovalishigh
erforthe
initiallylowconcentrations
ofligninsolutio
n
Kansaletal[48]
Catalystdo
sepHoxidant
concentration
initialsubstrate
concentration
ZnOcatalystin
slurryandim
mob
ilizedmod
e
UV-Visspectroscop
yCO
D
Optim
umcatalystdo
seis1gL
optim
umoxidantcon
centratio
n2times10minus6M
graduald
ecreaseo
fabsorptionpeak
indicatin
gdecompo
sitionof
organics
CODremovalishigh
erforthe
initiallylowconcentrations
ofligninsolutio
n
Dahm
and
Lucia[
49]
Catalystdo
seillu
mination
intensity
UV-Visspectroscop
ytotalorganiccarbon
(TOC)
capillaryionelectro
phoresis
analysis(C
IA)
Gradu
aldecrease
inabsorptio
npeak
indicatin
gdecompo
sitionof
organics
optim
alcatalystdo
seof
10mgm
high
erillum
inationintensities
correlated
well
with
high
erinitialdegradationrate
74disapp
earanceo
fTOC
Portjanskajaand
Preis[50]
Influ
ence
offerrou
sion
sN-dop
edcatalysteffect
sprayedcatalyston
supp
ortand
subm
ersedcatalyst
CODU
V-Visspectroscop
ybiochemical
oxygen
demand(BOD)colorim
etric
measurementat570
nm
Additio
nof
Fe2+upto
28m
gLleadsto25increase
inph
otocatalyticeffi
ciency
sprayedcatalystexhibited15
times
high
ereffi
ciency
than
theo
neattached
bysubm
ersio
nnegligibleeffecto
fN-dop
edcatalyst
increase
ofaldehyde
concentrationover
reactio
ntim
eneutralm
ediawas
mostb
eneficialforb
iodegradability
80of
freep
heno
lsremoved
undern
eutralcond
ition
s
Tanaka
etal[51]Ca
talystloadinglignin
concentration
illum
inationtim
e
UV-Visspectroscop
yTO
Cgel
perm
eatio
nchromatograph
y(G
PC)
1 HNMR
Fouriertransform
ationinfrared
(FTIR)
spectro
scop
y
FTIR
measurementrevealedafasttransform
ationof
arom
aticmoietyp
resent
inlignin
characteris
ticband
sofaromaticrin
gsm
etho
xyand
aliphatic
sidec
hains
decrease
inTO
Cvalues
over
time
decrease
indegradationratewith
increase
catalystdo
sageB
utaft
ercatalystthreshold
valueisa
ttainedcatalystincreasec
ausesa
decrease
indegradationrates
FTIR
peaksa
reshifted
towards
lower
molecular
weightregionaft
erph
otocatalysis
Tonu
cci
etal[33]
Testof
catalytic
syste
mstoob
tain
fractio
nswith
redu
ceddegreeso
fpo
lymerization
comparis
onof
thermalandph
otochemical
reactio
ns
1 HNMR
gasc
hrom
atograph
y-mass
spectro
scop
y(G
C-MS)
POMsa
relessselectivew
henused
asph
otocatalystsandno
appreciableb
leaching
ofthes
olutionwas
seen
whenPO
Mwas
used
asthermalcatalyst
deriv
edchem
icalsfrom
experim
entvanillin
hydroxylmetho
xy-acetoph
enon
econiferylalcoh
olcon
iferylaldehydemethano
lform
icacidacetic
acidand
sometim
essm
allamou
ntso
fC-2
andC-
3alcoho
ls
Miyatae
tal[24]
Exam
inationof
cellwall
structureo
flignin(w
oodflo
ur)
before
andaft
erph
otocatalysis
GC-
MSscanning
electronmicroscop
y(SEM
)1 H
NMR
Highdelignificationactiv
itydelignificationconfi
nedto
thes
urface
oflignin
deriv
edchem
icalse
xperim
entvanillin
Shende
etal[17]Testof
combinedactio
nof
bio-
andph
otocatalyticsyste
ms
UV-Visspectroscop
yGC-
MSX-
ray
dispersiv
eenergyspectro
scop
yandX-
ray
diffractio
nanalysis(EDX
XRD)SE
M
Detectio
nof
follo
wingchem
icalsacetylguaiacol4-ethoxym
ethyl-2
-metho
xyph
enol
metho
xyph
enyloxim
eguaiacolsuccinica
cidacetylguaiacolvanillicacidand
vanillin
International Journal of Photoenergy 9
Table3Con
tinued
Reference
Parameter
studied
Analytic
sRe
sult
Tian
etal[52]
andPanetal
[53]
Testof
combinedactio
nof
electro-and
photocatalytic
syste
ms
UV-Visspectroscop
yFT
IRSEM
X-ray
(EDX)
highperfo
rmance
liquid
chromatograph
y(H
PLC)
COD
Detectio
nof
thefollowingchem
icalscarbon
ylfunctio
nalityvanillin
andvanillica
cid
Awun
gacha
Lekeleface
tal
[54][55]
Com
paris
onof
degradationrates
bydifferent
catalyst
HPL
Cflu
orescencea
ndUV-Vis
spectro
scop
ySE
MT
OC
UV-Visresultsrevealfaste
rdegradatio
nof
thea
liphatic
moietycomparedto
the
arom
aticmoietyof
ligninsulfonateob
tained
from
paperw
astewaterPeaks
observed
durin
gHPL
Canalysis
Someo
fthe
peaksp
rodu
cedaft
erph
otocatalysishad
fluorescences
ignals
Thissuggeststhep
rodu
ctionof
newsubstances
andflu
orop
hores
Coatin
gsprod
uced
throug
hSol-G
elprocedures
arestablea
ndcanbe
used
manytim
es
10 International Journal of Photoenergy
321 Influence of Catalyst One aspect to successfully imple-ment photocatalysis is the choice of an appropriate photo-catalyst The majority of catalysts used are based on TiO
2as
summarized in Table 2 ZnO has also been applied either assingle catalyst [48] or in a combination with other catalystsuch as TiO
2[54] de Lasa et al [60] described ZnO as
being less active than TiO2 They add that the use of ZnO
is particularly relevant when the oxidative degradation ratebecomes limited This is opposite to what Kansal et al [48]reported by saying that ZnO is more reactive than TiO
2
However TiO2(mainly anatase) remains the most used
catalyst as can be depicted fromTable 2 TiO2is reported to be
favored because of its nontoxic property cost efficiency andchemical and biological inertness Moreover TiO
2possesses
the most efficient photoactivity and the highest stability thusmaking it suitable for industrial use [61]
ZnO has been described to degrade lignin under visiblelight sources [48 62] whereas TiO
2is mostly applied in
connection to UV-light sources as highlighted in Table 2However both ZnO and TiO
2possess energy band gap
energy of 32 eV [23]Additives such as SiO
2[63] polyethylene oxide (PEO)
[24] and polyethylene glycol [64] have been added to TiO2
catalyst particularly when applying immobilized catalystAddamo et al [63] noted a high adhesion of TiO
2to glass
support material when precoating was done with SiO2
Moreover the precoating might have other advantages suchas a hindering diffusion of Na+ ions from the glass materialinto the nascent TiO
2film during heat treatment processes
Analogous to the addition of SiO2 polyethylene glycol
(PEG) has also been introduced to mitigate catalyst surfaceactivity modify surface hydrophobicity and also reduceagglomeration tendency of the TiO
2gel or TiO
2particles
in the suspensions [64] By a proper surface modificationinteraction between catalyst and substrate can be enhanced[55 63]
Kansal et al [48] varied catalyst (ZnO) dose from 05 gLto 20 gL for 01 gL kraft lignin solutions and found out thatthere was an optimum catalyst threshold value at 1 gL whichgives a catalyst to substrate ratio of 1 10
Dahm and Lucia [49] examined catalyst dose from 2 sdot10minus3 gL to 12sdot10minus2 gL for lignin solutions (fromwhite water
liner mill) of 4 sdot 10minus2 gL (catalyst to lignin ratio 5 sdot 10minus3ndash3 sdot 10minus2) at pH 8 and obtained best energy efficiency valuesand lignin degradation rates with a catalyst loading of 10 sdot10minus2 gLIn contrast Ma et al [56] applied far higher catalyst
concentration compared to Dahm and Lucia [49] Catalystconcentration was varied between 1 gL and 10 gL PtTiO
2
Best catalysis dose with respect to reaction turnover wasobtained at 5 gL PtTiO
2With the increase of catalyst dose at
pH 7 the reaction rate increased from 61 times 10minus3minminus1 (1 gLTiO2) to 71 sdot 10minus3minminus1 (5 gL TiO
2) and 99 sdot 10minus3minminus1
(10 gL TiO2)
Catalyst effect has been explained on the basis thatoptimum catalyst loading is dependent on the initial soluteconcentration An increase in catalyst dosage leads to a cor-responding increase of total active surface area for reactions
[65] To that at higher TiO2concentrations the photon flux is
more easily intercepted by the catalyst before penetrating intothe bulk of the system At the same time due to an increase inturbidity of the suspension with high dose of photocatalystthere is a decrease in penetration of UV light and hencephotoactivated volume of suspension or solution decreases[66]
In summary authors have obtained best catalyst to ligninrelations for different reaction designs and thus a generalrecommendation on catalyst dose is not possible Howeverwhen lignin solution is treated with increasing catalyst loadsa corresponding increase in degradation rate is observed untila threshold value is reached [48ndash50 56]
322 Influence of Metal Ion Addition (Doping) and AdditivesThe purpose of adding metal ion to photocatalyst is to miti-gate band gap energy through the introduction of intrabandgap states and as a consequence produce a bathochromicshift in the absorption spectrum [37] Altering the absorptionspectral range gives the possibility to exploit both the visiblelight spectrum and UV light sources Metal ion doping isalso introduced to serve as electron or hole traps in orderto minimize recombination between generated electron-holepairs [37]
Portjanskaja and Preis [50] studied the addition of Fe2+ions to an acidic lignin solution and found an increasein photocatalytic oxidation (PCO) efficiency The optimumFe2+ ions quantity was 28mgL while using 100mgL ligninsolution Upon further elevation of Fe2+ ions concentrationa corresponding reduction of the photocatalytic oxidationefficiency of lignin was noted Likewise Ohnishi et al [57]made a comparative study by doping platinum (Pt) silver(Ag) and gold (Au) ions to TiO
2 In these reactions 50mg of
catalyst (TiO2) was used with the addition of an equiva1ent
15 wt (based on TiO2) metal ion The addition of noble
metals brought about a faster decolorization of lignin Aushowed better results than Ag followed by Pt In the samecontext adding sodium hypochlorite as oxidant to PtTiO
2
catalyst an additional fivefold degradation rate was observedcompared to that without doping [56] Contradictory to theresults described above negligible effect of photocatalyticefficiency due to doping has been reported as well Awun-gacha Lekelefac et al [54] obtained little or no change indegradation rate by doping TiO
2-P25-SiO
2catalyst with Pt
ions (1 wt relative to TiO2catalyst) Likewise Portjanskaja
and Preis [50] noted a negligible change of photocatalyticefficiency of TiO
2when doped with nitrogen
Sarkanen et al [67] and Gellerstedt and Lindfors [68]reported the bias of peroxides to oxidation with reagentssuch as permanganate to favor aromatic moieties Oxidationagents like permanganate oxidizes predominantly aliphaticchains in alkaline and neutral media However by theapplication of H
2O2(Fenton system) lignin disappeared
completely [33] Tonucci et al [33] conclude that in orderto satisfactorily conserve the organic material the best com-promise appears to be the TiO
2photosystem which shows
low carbon consumption good preservation of the aromaticrings and greatly reduced mineralization
International Journal of Photoenergy 11
In summary different results have been obtained con-cerning the influence of noble metal ion addition Whilesome authors report an improvement in the photocatalyticefficiency upon their addition others report their addition ashaving no considerable influence However for reactions inwhich an improvement in the photocatalytic efficiency wasnoticed there was a threshold value to be considered Whenthe concentration of dopant surpasses this threshold valueelectron-hole recombination is favored and this has a negativeimpact to photocatalysis In such a case the space-chargelayer gets narrower and 119901-type dopants attract electrons andby virtue become negative They would now then act ashole acceptor attracting holes On the other hand 119899-typedopants which act as electron donor centers and possessexcess electrons attract holes as well [37]
323 Influence of Lignin Concentration Once the initiallignin concentration becomes higher exceeding a thresholdvalue an inhibitory effect on the photodegradationwas noted[47ndash49] This threshold value varies and depends on thereaction system and reaction parameters such as opticaldensity catalyst concentration and reaction volume Fromthe literature different authors have implemented varyinglignin concentrations probably to suit their reaction designFor example Ksibi et al [47] use 90mgL and AwungachaLekelefac et al [54] use 500mgL while Kansal et al [48]applied 10mgndash100mgL
Explanations arising from the findings are as followsat low lignin concentrations the incidental photonic fluxirradiated interacts with the catalyst generating radicals (eghydroxyl radicals (OH∙)) which allow a faster degradation[69] On the other hand high initial lignin concentrationsmay lead to tight adsorption which can suppress CO
2
evolution [51] and hence maintain chemical oxygen demand(COD) values Moreover low delignification yields maybe due to an inhibitory effect because of autoxidation bylow molecular weight lignin degradation products formed[24] Also due to the polymer structure of lignin which iscross-linked this makes it difficult for radical species acidand the aldehyde compounds produced to spread into theinner region of the substrate hence limiting autoxidationAs a worst case this might be the rate-determining step ofdelignification which is hindered [70ndash72]
In summary it can be concluded that the time taken forcomplete degradation depends on the initial concentration oflignin and faster degradation occurs at low lignin concentra-tions
324 Influence of pH Varying pH entails an alteration inthe properties of semiconductor-liquid interface [73] mainlyrelated to the acid-base equilibrium of the adsorbed hydroxylgroup [39] Furthermore pH also impacts lignin degradationrates [47 48 57] In this context several studies were carriedout with partly contradictory outcomes
Kansal et al [48] made pH investigations 3ndash11 undersolar light illumination using ZnO as catalyst Maximumdegradation was reached in alkaline conditions (pH 11)This is supported by Villasenor and Mansilla [74] reporting
an almost complete decolorization of kraft black liquorfrom pine wood at pH value of 116 in combination withZnO catalyst Similar results were achieved by Ohnishi etal [57] with TiO
2and ZnO being catalyst for bleaching
alkaline lignin in aqueous solution with TiO2and ZnO being
catalyst High activities at neutral pH were also reportedby Ohnishi et al [57] In contrast Ma et al [56] observedhigher reaction rates and rapid degradation of a syntheticlignin wastewater (prepared by dissolving commercial ligninpowder in aqueous solution pH 11) in acidic solution (pH 3)than in alkaline solutions at pH 11 for either TiO
2or PtTiO
2
catalystsReconsidering the photocatalytic principle the formed
superoxide anion radicals (∙O2
minus) are in a pH-dependentequilibriumwith perhydroxyl radicals (HO
2
∙) as follows [75]
HO2
∙999445999468 H+ + ∙O
2
minus pKa sim 48 (9)
See [76]∙O2
minus undergoes dismutation reaction resulting in H2O2
and O2competing to any other ∙O
2
minus triggered reaction Incase of low pH operation conditions in aqueous solutionsHO2
∙ becomes dominant whose reactivity is considerablyhigher compared to ∙O
2
minus [77] Subsequently HO2
∙ initiatessubstrate (S) oxidation to the radical cation (S+∙) and is itselfreduced to H
2O2[78] Thus increased degradation rates can
be reasonably expected supporting the results made by Ma etal [56] in an acidic environment ∙O
2
minus is extremely reactive inorganic solvents [77] Another aspect is the solubility of kraftlignin (soluble at pH gt 105) which reduces with decreasingpH whereas lignosulfonate should remain unaffected by pHin aqueous solution Moreover 120573ndashOndash4 bonds have beendescribed to be stable at acidic pH [11] In fact this couldadditionally explain the elevated degradation of kraft ligninmade by Kansal et al [48] and Villasenor and Mansilla [74]Nevertheless the contradictory results gained by Ma et al[56] still exist under the assumption that kraft ligninwas used(which would be supported by the high pH of 11 obviouslynecessary for dissolving the lignin powder) Although mostphotocatalytic reactions described in the literature are in anaqueous milieu lignin raw material and its fission productsmay however vary considerably Therefore the optimal pHis most likely to be reaction specific and has to be evaluatedexperimentally in principle
325 Influence of Illumination Many of the studies foundin the literature so far have not dealt on this subject per seWhat is found is the use of different illumination sourceseach having a specified power and lamp type However alltend to emitUV-light between the range 280ndash420 nm Table 2depicts this in detail Other illumination sources include thevisible light spectrum
In general terms illumination influences in that it ini-tiates photocatalysis by generating electron-hole pair in thesemiconductor particles [38 39] Dahm and Lucia [49]altered illumination intensity while observing lignin degra-dation In this study 004 gL lignin was used and lightintensity was varied 223ndash445mWcm3 It was found out thathigher illumination intensities correlated well with higher
12 International Journal of Photoenergy
initial degradation rates and hence total lignin degradation[49] Neppolian et al [69] report degradation to be propor-tional to radiation intensity and best results are achieved forlow lignin concentrations because of enhanced interactionbetween catalyst and incidental photonic flux
In summary high illumination power causes a corre-sponding high initial degradation rate at low lignin concen-trations becausemaximum light penetration into the reactionmedium is favored
33 Process Analytical Methods Various analytical tech-niques have been used to monitor lignin degradation Atthe beginning of this subchapter analytics revealing com-pounds formed from lignin degradation are treated Thisincludes for example gas chromatography (GC) and 1HNMR (nuclear magnetic resonance) This is then followed byresults qualitative analytic measurements such as ultraviolet-visible (UV-Vis) spectroscopy and dissolved carbon (DC)A list of authors analytical techniques applied and resultsachieved are outlined in Table 3
Portjanskaja and Preis [50] studied lignin degradationby measuring the removal of phenols through colorimetricmeasurements As a result of 24 h photocatalytic oxidationunder neutral media conditions 80 of free phenols wereremoved Gas chromatography (GC) result from Ksibi et al[47] attested vanillin vanillic acid palmitic acid biphenyland 345-trimethoxy benzaldehyde structures after the pho-tocatalysis of lignin from black liquor This is in accordancewith the findings of Tonucci et al [33] reporting the formationof vanillin hydroxyl methoxy-acetophenone coniferyl alco-hol coniferyl aldehyde methanol formic acid acetic acidand small amounts of C-2 and C-3 alcohols as degradationproducts1H NMR spectral analysis of lignin before illumination
and after 24 h of illumination showing characteristic bandsof aromatic rings methoxy and aliphatic side chains wascompared with each other Results revealed that the aro-matic ring degraded faster than the aliphatic chain [51]Fourier transformation infrared spectroscopy (FTIR) showedbands corresponding to CH
3 CH2 and CH which remained
unchanged after illumination while bands corresponding toaromatic rings disappeared as a result of illumination [51 53]
Results obtained from the combination of photochemicaland electrochemical oxidation [52 53] were similar to thoseof Tanaka et al [51] Here 13C-NMR confirmed the presenceof the carbonyl functionality and the presence of vanillinand vanillic acid after 12 h photochemical-electrochemicaloxidation These results showed that the combination of aphotocatalytic and an electrochemical oxidation significantlyenhanced the efficiency of lignin degradationThis is becausethe applied anodic potential bias greatly suppressed therecombination of photogenerated electrons and holes [53]
Ultraviolet spectrophotometry offers a convenientmethod to qualitatively and quantitatively analyze ligninin solution [79] This is reflected by the large number ofpublications using this technique [17 33 47ndash51 56 58 80]This is most likely due to its simplicity to interpret lignindegradation Lignins absorb UV light with high molar
200 220 240 260 280 300 320 340 360
00
05
10
15
20
25
Abso
rban
ce
Wavelength (nm)0min
30min
60min
120min
180min
300min
360min
420min
1200min
Figure 8 Time dependent UV-Vis absorption spectra of aqueouslignin solution from waste paper water irradiated with UV light(280ndash420 nm) for different time intervals The spectra are obtainedfor sol-gel derived TiO
2nanocrystalline coating (TiO
2-P25-SiO
2)
[54]
extinction coefficients because of the several methox-ylated phenylpropane units of which they are composed [33]Figure 8 depicts a series of photometric scans of ligninsul-fonate from paper waste water showing a gradual reductionof absorbance during photocatalytic treatment [54] Herethe absorption peaks are around 210 nm and 280 nmAbsorbance decreases with time implying the decompositionof lignin and the deterioration of chromophore groups [54]
Peaks at 210 nmcorrespond to portions of the unsaturatedchains while those at 280 nm correspond to unconjugatedphenolic hydroxyl groups [17] and the aromatic moiety [57]of the lignin molecule The absorption tailing to the longwavelength region arises from the color of lignin [57] Lignindegradation has been reported either at wavelength around280 nm corresponding to unconjugated phenolic hydroxylgroups [17 48 51 58] or for both wavelengths (210 nm and280 nm) [33 54 57] Kobayakawa et al [81] noted some otherabsorbance at wavelengths lower than 250 nm and pointedout that this could be due to the modification of ligninfragmentations leading to the formation of transient specieslikemethanol ethanol formaldehyde formic acid and oxalicacid among others
Analyticalmethods to effectively quantify lignin degrada-tion by calculating the oxygen demand by organic substancesand remaining organic carbon before and after photocatalysishave been studied Amongst the methods are dissolvedcarbon (DC) [49 51 58] chemical oxygen demand (COD)[47 48 50 57 80] biochemical oxygen demand (BOC) [50]dissolved organic carbon (DOC) [56] and American dyemanufacture institute value (ADMI) [56] COD and BODdescribe the oxygen demand by organic substances to be
International Journal of Photoenergy 13
Figure 9 Gradual change from the characteristic yellow ligninsulfonate to a colorless liquid after a period of 20 h Catalyst TiO
2-
P25 (Degussa) +TEOSUV-light 25∘C lignin concentration 05 gL[54]
converted to CO and CO2and H
2O and NH
3 Total organic
carbon (TOC) describes the amount of carbon bound inan organic compound while DOC describes the dissolvedfraction of organic carbon ADMI measures the amount ofdyestuff in water
Decolorization of lignin solution has been reported tobe another parameter observed during photocatalytic degra-dation Color is an indirect indicator of the lignin amountThe higher the color intensity of the solution is the greaterthe lignin content is (high concentrated lignin solutions egblack liquor appear dark brown) [82] Thus color changescan be interpreted as conversion of lignin to transient speciesor conversion to CO
2and H
2O Awungacha Lekelefac et al
[54] observed a gradual change from the characteristic yellowlignin (when highly diluted) to a colorless liquid after a periodof 20 h with sol-gel derived TiO
2nanocrystalline coatings on
sintered borosilicate glass as depicted in Figure 9 A corre-sponding decrease in DC values close to 82 was observedfor TiO
2-P25-SiO
2catalyst confirming degradation This is
shown in Figure 10These findings are analogous to that of Ohnishi et al
[57] who reported the bleaching of lignin when illuminatedcontinuously and that the solution becomes colorless Tothat the chemical oxygen demand (COD) value decreasesgenerating carbon dioxide and a small amount of carbonmonoxide as the main gaseous products COD removalwas reported to be effective at low lignin concentrations ascompared to high lignin concentrations [48]
Another applied analytical technique is fluorescencedetection directly coupled to a high performance liquidchromatography (HPLC) as a means to identify nonaliphaticcomponent in the complex mixture of lignin degradationproducts [54] Fluorescence emission in lignin is attributedto aromatic structures such as conjugated carbonyl biphenylphenylcoumarone and stilbene groups [83 84] AwungachaLekelefac et al [54] observed peaks on both HPLC and
0 200 400 600 800 1000 12000
50
100
150
200
250
Diss
olve
d ca
rbon
(ppm
)
t (min)
TiO2-P25-SiO2
TiO2-P25-SiO2 + PtTiOSO4minus30 6wt
ZnO + TiO2-P25-SiO2
No coating UV light
Figure 10 Variation of DC with time of aqueous lignin solutionfrom waste paper water irradiated with UV light (280ndash420 nm) fordifferent time intervals The spectra are obtained for sol-gel derivedTiO2nanocrystalline coatings (TiO
2-P25-SiO
2+Pt TiO
2-P25-SiO
2
TiOSO4minus306 wt ZnO + TiO
2-P25-SiO
2) [54]
fluorescence chromatograms suggesting the production ofnew substances and fluorophores
Despite developed analytical technologies analyzinglignin degradation products remains challenging Proofssuch as mass spectroscopy (MS) HPLC 13C or 1H-NMRspectra from photocatalytic lignin degradations are not yetestablished The setback to qualitatively and quantitativelyanalyze lignin and its degradation products starts from thenative lignin polymer itself with its indefinite polymericstructure and multiple bond types Also the influence ofdifferent pretreatments additives and the wide variety ofcompounds obtainable from its degradation makes ligninanalysis challenging [85] Moreover lignin streams couldcontain proteins inorganic salts and other potential poisonsthat generally complicate catalysis [4]
The challenge to identify and separate the productsstreams derived from the photocatalytic degradation ligninis also worth noting Lignin product stream is highly func-tionalized and conventional techniques such as gas chro-matography have the disadvantage of requiring a time-consuming derivatization step Also because of high boilingpoint of substances arising from lignin degradation it is noteasily applicable High performance liquid chromatography(HPLC) seems to be the remedy because analysis can becarried out without derivatization but the exact identificationof the separated substances is difficult because of the numer-ous peaks arising from such a chromatogram [87] Unfortu-nately well-established databases such as that of the nationalinstitute of standard and technology (NIST) [88] cannot giveinformation onHPLC-MS chromatogramsThis is because ofthe ionization sources such as electrospray ionization (ESI)
14 International Journal of Photoenergy
Table 4 Main advantages and disadvantages photocatalysis versus enzymatic biocatalysis
Lignin degradation process Advantages Disadvantages
Photocatalysis(single stage)
(i) Relatively fast degradation of complex as wellas nonbiodegradable organic (macro)molecules(ii) Stable catalysts(iii) Moderate reaction conditions
(i) Not selective(ii) Limited selection of operating conditions(iii) Energy costs
Enzymatic conversion(single stage)
(i) More selective(ii) Moderate reaction conditions
(i) High enzyme loads required otherwise ligninhydrolysis is quite slow [16](ii) Enzymes are less stable1(iii) cost-intensive (POXs)2
1For example POXs are sensitive to high H2O2 concentrations2According to Torres and Ayala [86]
and atmospheric pressure chemical ionization (APCI) forLC-MS while GC-MS is based on electron ionization Aremedy to this can be the development of a database ofmodel lignin compounds based on a unanimous HPLC-MSmeasuring procedure This would mean much time and costexpensive investments for adequate personnel and material
Though product identification based on peak superpo-sition gives information on a possible product this can-not always be true for lignin degradation because of thelarge number of possible degradation products In order toproperly identify products from lignin degradation productisolation techniques are to be implemented and further ana-lytic methods such as tandem mass spectroscopy (MSMS)and other advanced structure enhancing research techniquessuch as 1H-NMR 13C-NMR have to be studied extensively
4 Photocatalysis and Its Suitability asIntegrated Technology in MultistageConcepts with Biocatalysis
Depending on lignin treatment specification photocatalysiswill find application for complete lignin mineralization toCO2and H
2O (single stage) or as integrated technology in
a multistage system aiming at partial conversion of ligninmacromolecules to organic low molecular weight intermedi-ates for a consecutive biological oxidation finally generatingvalue added end-products [15]
For biological oxidation of lignin or lignin deriva-tives H
2O2-dependent ligninolytic heme peroxidases (POXs
including lignin peroxidase (LiP EC 111114) manganeseperoxidase (MnP EC 111113) versatile peroxidase (VPEC 111116) O
2-dependent laccases (Lac EC 11032) and
extracellular enzymes from Basidiomycetous white-rot fungiare the most efficient lignin degraders in nature [89] Thussuch enzymatic systems especially POXs (with high redoxpotentials) have attractedmuch interest as industrial biocata-lyst [86 90]The enzyme degradationmechanism (in nature)is facilitated by nonenzymatic processes mainly throughfree ∙OH radicals also generated by the fungus (Fenton-type reaction) Those ∙OH radicals enable the requiredphysical contact between the enzymes and structural units ofthe lignin molecule due to numerous nonspecific oxidative
reactions [91] Both the photocatalytic (equations (2)ndash(7))and the POX reactionmechanism recently reviewed by Busseat al [25] are quite similar Consequently a combination ofphotocatalysis followed by an enzymatic oxidation maybe apromising concept utilizing lignin derivatives for exampleoriginated from industrial effluents [15 92] The advantagesforming biobased products are as follows see also Table 4 inthis context A reduction of the lignin polymerization degreevia photocatalysis at best in more biodegradable interme-diates and a simultaneous detoxification causes savings inenzyme costs since it will be expected that less enzyme loadsare necessary for sufficiently rapid reaction rates [15 91] As aresult hydraulic retention times in bioreactor systems shouldbe diminished Moreover the delignification is expectedto be enhanced simultaneously which was in recent yearsshown in a dual system by Kamwilaisak and Wright [93]using TiO
2H2O2UV for photocatalytic pretreatment and
Lac (from Trametes versicolor) in the subsequent biocatalyticstep Within 24 h they obtained in their dual system anelevation in delignification of 20 (without H
2O2) up to at
least 50 when H2O2was present
In previous studies the treatment of lignin (from thepulp and paper industry) containing effluents by fungus asa biological system (excreting appropriate lignin degradingenzyme cocktails) were more focused for posttreatmentexclusively (Duran et al [92] Reyes et al [94] and Gonzalezet al [95]) Shende et al [17] even examined ligninolyticbacteria with photooxidized kraft lignin as substrate
Although POXs are potential industrial biocatalysts [86]no application studies were found for the direct use in suchdual systems as described above The major reason may bethe complexity of the reaction mechanism (inclusive ligninderivatives as substrate and their analysis) per se on the onehand slowing down research and development processes Onthe other hand POXs are sensitive to their cosubstrate H
2O2
once it is supplied in excess causing considerable inactivation(for details refer to Busse et al [25]) At the present severalstudies are carried out modifying these enzymes regardingenhanced stability activity and selectivity as well Henceit can be expected that their technological applicability willbe raised significantly right after successful modification isreached [86]
International Journal of Photoenergy 15
5 Concluding Remarks
It is widely assumed that the photocatalytic degradation oflignin follows a radical reaction pathway which is similar tothat considered in thermal electrochemical and biochemicalprocesses However reporting on the degradation pathway oflignin derivatives and even that of lignin model compoundsare still amajor challengeThis is probably due to the complexnature and variety of possible degradation products Indeedthemechanism is far more complex considering other factorssuch as type of lignin type of catalyst pH illuminationsource and additives
Generally comparing the different photochemical pro-cesses poses a big challenge because of the wide variablesinvolved These discrepancies start from the source and typeof lignin followed by differences in reactor design illumi-nation source intensity of radiation and different types ofcatalyst An ideawould be to have a specific reference reactionwith well-defined starting parameters which include lignintype source and purity catalyst specifications illuminationsource and intensity so as to ease comparison of results
Basic process parameters such as catalyst concentrationsubstrate concentration addition of metal ion to catalyst pHand illumination have been discussed
Despite developed analytical technologies analyzinglignin degradation products remains challenging Proofssuch as mass spectroscopy (MS) HPLC 13C or 1H-NMRspectra from photocatalytic lignin degradations are not yetestablished
In order to properly identify products from lignin degra-dation product isolation techniques are to be implementedand further analytic methods such as MSMS and otheradvanced structure enhancing research techniques such as1H-NMR and 13C-NMR have to be studied extensively Aremedy to this can be the development of a databank ofmodel lignin compounds based on a unanimous HPLC-MSmeasuring procedure This would mean much time and costexpensive investments for adequate personnel and material
Photocatalysis is denoted as the most popular ligninpretreatment technology besides ozonation [96] Photo-catalyzed lignin may be an appropriate substrate for aconsecutive biocatalytic process using ligninolytic enzymes(POX andor Lac) as supported by experimental results ofKamwilaisak and Wright [93] Combining the advantages ofboth catalytic processes savings in the overall process costswill be expected in addition to elevated lignin conversionNonetheless extensive research work including POX modi-fications is still required
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Authorsrsquo Contribution
Colin Awungacha Lekelefac and Nadine Busse contributedequally to this work
Acknowledgments
The authors gratefully thank the Federal Ministry of Educa-tion and Research (BMBF) for funding (FKZ17N0310) Theresearchers also thank the Hessen State Ministry of HigherEducation Research andArts for the financial support withinthe Hessen initiative for scientific and economic excellence(LOEWE)
References
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[3] Green and Natural Polymers Are on the Rise 2014 httpwwwpolymersolutionscombloggreen-and-natural-polymers-on-the-rise
[4] J Zakzeski P C Bruijnincx A L Jongerius and B M Weck-huysen ldquoThe catalytic valorization of lignin for the productionof renewable chemicalsrdquo Chemical Reviews vol 110 no 6 pp3552ndash3599 2010
[5] B KammM Kamm P R Gruber and S KromusBiorefineries-Industrial Processes and Products Status Quo and Future Direc-tions vol 1 Wiley-VCH 2006
[6] R L Howard E Abotsi E L J van Rensburg and S HowardldquoLignocellulose biotechnology issues of bioconversion andenzyme productionrdquoAfrican Journal of Biotechnology vol 2 no12 pp 602ndash619 2003
[7] M Stocker ldquoBio- und BTL-Kraftstoffe in der Bioraffineriekatalytische Umwandlung Lignocellulose-reicher Biomasse mitporosen Stoffenrdquo Angewandte Chemie vol 120 no 48 pp9340ndash9351 2008
[8] J-P Lange ldquoLignocellulose conversion an introduction tochemistry process and economicsrdquo Biofuels Bioproducts andBiorefining vol 1 no 1 pp 39ndash48 2007
[9] D A I Goring ldquoThe physical chemistry of ligninrdquo Pure andApplied Chemistry vol 5 no 1-2 pp 233ndash310 1962
[10] M Ek G Gellerstedt and G HenrikssonWood Chemistry andBiotechnology Pulp and Paper Chemistry and Technology vol 1Walter de Gruyter GmbH amp Co KG Berlin Germany 2009
[11] B Saake and R Lehnen ldquoLigninrdquo in Ullmannrsquos Encyclopedia ofIndustrial Chemistry Wiley-VCH Verlag GmbH amp Co KGaAWeinheim Germany 2012
[12] M N S Kumar A K Mohanty L Erickson and M MisraldquoLignin and its applications with polymersrdquo Journal of BiobasedMaterials and Bioenergy vol 3 no 1 pp 1ndash24 2009
[13] R J A Gosselink E de Jong B Guran and A AbacherlildquoCo-ordination network for ligninmdashstandardisation produc-tion and applications adapted to market requirements(EUROLIGNIN)rdquo Industrial Crops and Products vol 20 no 2pp 121ndash129 2004
[14] D Fengel and G Wegener Wood Chemistry UltrastructureReactions Verlag Kessel 1984
[15] D Mantzavinos and E Psillakis ldquoEnhancement of biodegrad-ability of industrial wastewaters by chemical oxidation pre-treatmentrdquo Journal of Chemical Technology and Biotechnologyvol 79 no 5 pp 431ndash454 2004
16 International Journal of Photoenergy
[16] M Dashtban H Schraft and W Qin ldquoFungal bioconversionof lignocellulosic residues Opportunities amp perspectivesrdquo Inter-national Journal of Biological Sciences vol 5 no 6 pp 578ndash5952009
[17] A Shende R Jaswal D Harder-Heinz A Menan and RShende ldquoIntergrated photocatalytic and microbial degradationof kraft ligninrdquo Cleantech pp 120ndash123 2012
[18] F S Chakar and A J Ragauskas ldquoReview of current and futuresoftwood kraft lignin process chemistryrdquo Industrial Crops andProducts vol 20 no 2 pp 131ndash141 2004
[19] W G Glasser and H R Glasser ldquoEvaluation of ligninrsquoschemical structure by experimental and computer simulationtechniquesrdquo Paperi ja Puu vol 63 pp 71ndash83 1981
[20] M Erickson S Larsson and G E Miksche ldquoZur Struktur desLignins der FichterdquoActaChemica Scandinavica vol 27 pp 903ndash914 1973
[21] H Nimz ldquoDas Lignin der BuchemdashEntwurf eines Konstitution-sschemasrdquo Angewandte ChemiemdashInternational Edition vol 86pp 336ndash344 1974
[22] D V Evtuguin C P Neto J Rocha and J D P de JesusldquoOxidative delignification in the presence of molybdovana-dophosphate heteropolyanions mechanism and kinetic stud-iesrdquo Applied Catalysis A General vol 167 no 1 pp 123ndash1391998
[23] A L Linsebigler G Lu and J T Yates Jr ldquoPhotocatalysis onTiO2surfaces Principles mechanisms and selected resultsrdquo
Chemical Reviews vol 95 no 3 pp 735ndash758 1995[24] Y Miyata K Miyazaki M Miura Y Shimotori M Aoyama
and H Nakatani ldquoSolventless delignification of wood flourwith TiO
2poly(ethylene oxide) photocatalyst systemrdquo Journal
of Polymers and the Environment vol 21 no 1 pp 115ndash121 2013[25] N Busse D Wagner M Kraume and P Czermak ldquoReaction
kinetics of versatile peroxidase for the degradation of lignincompoundsrdquoTheAmerican Journal of Biochemistry andBiotech-nology vol 9 no 4 pp 365ndash394 2013
[26] M Tien and T K Kirk ldquoLignin-degrading enzyme fromphanerochaete chrysosporium Purification characterizationand catalytic properties of a uniqueH
2O2-requiring oxygenaserdquo
Proceedings of the National Academy of Sciences vol 81 pp2280ndash2284 1984
[27] T K Kirk M Tien P J Kersten M D Mozuch and BKalyanaraman ldquoLigninase of Phanerochaete chrysosporiumMechanism of its degradation of the non-phenolic arylglycerol120573-aryl ether substructure of ligninrdquo Biochemical Journal vol236 no 1 pp 279ndash287 1986
[28] T Lundell RWever R Floris et al ldquoLignin peroxidase L3 fromPhlebia radiata pre-steady-state and steady-state studies withveratryl alcohol and a non-phenolic lignin model compound1-(34-dimethoxyphenyl)-2-(2-methoxyphenoxy)propane-13-diolrdquo European Journal of Biochemistry vol 211 no 3 pp391ndash402 1993
[29] H E Schoemaker T K Lundell A I Hatakka and K PiontekldquoThe oxidation of veratryl alcohol dimeric lignin models andlignin by lignin peroxidase the redox cycle revisitedrdquo FEMSMicrobiology Reviews vol 13 no 2-3 pp 321ndash331 1994
[30] J M Palmer P J Harvey and H E Schoemaker ldquoThe role ofperoxidases radical cations and oxygen in the degradation oflignin [and discussion]rdquo Philosophical Transactions of the RoyalSociety of London A vol 321 no 1561 pp 495ndash505 1987
[31] C Hill Wood Modification Chemical Thermal and OtherProcesses John Wiley amp Sons Chichester UK 2006
[32] Y-H Xu H-R Chen Z-X Zeng and B Lei ldquoInvestigation onmechanism of photocatalytic activity enhancement of nanome-ter cerium-doped titaniardquo Applied Surface Science vol 252 no24 pp 8565ndash8570 2006
[33] L Tonucci F Coccia M Bressan and N DrsquoAlessandro ldquoMildphotocatalysed and catalysed green oxidation of lignin auseful pathway to low-molecular-weight derivativesrdquoWaste andBiomass Valorization vol 3 no 2 pp 165ndash174 2012
[34] A Castellan N Colombo C Vanucci P Fornier de HViolet and H Bouas-Laurent ldquoPhotodegradation of ligninA photochemical study of an O-methylated 120572-carbonyl 120573-1lignin model dimer 12-di(3101584041015840-dimethoxyphenyl) ethanone(deoxyveratroin)rdquo Journal of Photochemistry and PhotobiologyA vol 51 pp 451ndash467 1990
[35] A Fujishima X Zhang and D A Tryk ldquoTiO2photocatalysis
and related surface phenomenardquo Surface Science Reports vol63 no 12 pp 515ndash582 2008
[36] C S Turchi and D F Ollis ldquoPhotocatalytic reactor designan example of mass-transfer limitations with an immobilizedcatalystrdquo The Journal of Physical Chemistry vol 92 no 23 pp6852ndash6853 1988
[37] R Subasri M Tripathi K Murugan J Revathi G V N Raoand T N Rao ldquoInvestigations on the photocatalytic activity ofsolndashgel derived plain and Fe3+Nb5+ doped titania coatings onglass substratesrdquo Materials Chemistry and Physics vol 124 pp63ndash68 2010
[38] N Serpone ldquoRelative photonic efficiencies and quantum yieldsin heterogeneous photocatalysisrdquo Journal of Photochemistry andPhotobiology A Chemistry vol 104 no 1ndash3 pp 1ndash12 1997
[39] M R Hoffmann S T Martin W Choi and D W BahnemannldquoEnvironmental applications of semiconductor photocatalysisrdquoChemical Reviews vol 95 no 1 pp 69ndash96 1995
[40] C D Jaeger and A J Bard ldquoSpin trapping and electron spinresonance detection of radical intermediates in the photode-composition of water at titanium dioxide particulate systemsrdquoThe Journal of Physical Chemistry vol 83 no 24 pp 3146ndash31521979
[41] R W Matthews ldquoHydroxylation reactions induced by near-ultraviolet photolysis of aqueous titaniumdioxide suspensionsrdquoJournal of the Chemical Society Faraday Transactions vol 80no 2 pp 457ndash471 1984
[42] A E H Machado A M Furuyama S Z Falone R RuggieroD D S Perez and A Castellan ldquoPhotocatalytic degradation oflignin and lignin models using titanium dioxide the role of thehydroxyl radicalrdquoChemosphere vol 40 no 1 pp 115ndash124 2000
[43] W Sigg and L Stumm Aquatische Chemie Stuttgart 2 AuflageBG Teubner Stuttgart Germany 1991
[44] O Legrini E Oliveros and A M Braun ldquoPhotochemicalprocesses for water treatmentrdquo Chemical Reviews vol 93 no2 pp 671ndash698 1993
[45] G Rothenberger J Moser M Gratzel N Serpone and DSharma ldquoCharge carrier trapping and recombination dynamicsin small semiconductor particlesrdquo Journal of the AmericanChemical Society vol 107 no 26 pp 8054ndash8059 1985
[46] P Mazellier M Sarakha A Rossi and M Bolte ldquoThe aqueousphotochemistry of 26-dimethylphenol Evidence for the frag-mentation of the 120572 C-C bondrdquo Journal of Photochemistry andPhotobiology A vol 115 no 2 pp 117ndash121 1998
[47] M Ksibi S B Amor S Cherif E Elaloui A Houas and MElaloui ldquoPhotodegradation of lignin from black liquor using aUVTiO
2systemrdquo Journal of Photochemistry and Photobiology
A vol 154 no 2-3 pp 211ndash218 2003
International Journal of Photoenergy 17
[48] S K Kansal M Singh and D Sud ldquoStudies on TiO2ZnO
photocatalysed degradation of ligninrdquo Journal of HazardousMaterials vol 153 no 1-2 pp 412ndash417 2008
[49] A Dahm and L A Lucia ldquoTitanium dioxide catalyzed pho-todegradation of lignin in industrial effluentsrdquo Industrial andEngineering Chemistry Research vol 43 no 25 pp 7996ndash80002004
[50] E Portjanskaja and S Preis ldquoAqueous photocatalytic oxidationof lignin the influence of mineral admixturesrdquo InternationalJournal of Photoenergy vol 2007Article ID76730 7 pages 2007
[51] K Tanaka R C R Calanag and T Hisanaga ldquoPhotocatalyzeddegradation of lignin on TiO
2rdquo Journal of Molecular Catalysis
A Chemical vol 138 no 2-3 pp 287ndash294 1999[52] M Tian J Wen D MacDonald R M Asmussen and A Chen
ldquoA novel approach for lignin modification and degradationrdquoElectrochemistry Communications vol 12 no 4 pp 527ndash5302010
[53] K PanM Tian Z-H Jiang B Kjartanson and A Chen ldquoElec-trochemical oxidation of lignin at lead dioxide nanoparticlesphotoelectrodeposited on TiO
2nanotube arraysrdquo Electrochim-
ica Acta vol 60 pp 147ndash153 2012[54] C Awungacha Lekelefac J Hild P Czermak and M Herren-
bauer ldquoPhotocatalytic active coatings for lignin degradationin a continuous packed bed reactorrdquo International Journal ofPhotoenergy vol 2014 Article ID 502326 10 pages 2014
[55] C Awungacha Lekelefac P Czermak and M HerrenbauerldquoEvaluation of photocatalytic active coatings on sintered glasstubes by methylene bluerdquo International Journal of Photoenergyvol 2013 Article ID 614567 9 pages 2013
[56] Y-S Ma C-N Chang Y-P Chiang H-F Sung and A CChao ldquoPhotocatalytic degradation of lignin using PtTiO
2as
the catalystrdquo Chemosphere vol 71 no 5 pp 998ndash1004 2008[57] H Ohnishi M Matsumura H Tsubomura and M Iwasaki
ldquoBleaching of lignin solution by a photocatalyzed reactionon semiconductor photocatalystsrdquo Industrial and EngineeringChemistry Research vol 28 no 6 pp 719ndash724 1989
[58] C A K Gouvea F Wypych S G Moraes N Duran and PPeralta-Zamora ldquoSemiconductor-assisted photodegradation oflignin dye and kraft effluent by Ag-doped ZnOrdquo Chemospherevol 40 no 4 pp 427ndash432 2000
[59] A V Vahatalo K Salonen M Salkinoja-Salonen and AHatakka ldquoPhotochemical mineralization of synthetic lignin inlake water indicates enhanced turnover of aromatic organicmatter under solar radiationrdquo Biodegradation vol 10 no 6 pp415ndash420 1999
[60] H de Lasa B Serrano and M Salaices Photocatalytic ReactionEngineering Springer New York NY USA 2005
[61] K Hashimoto H Irie and A Fujishima ldquoTiO 2 photocatalysisA historical overview and future prospectsrdquo Japanese Journal ofApplied Physics vol 44 no 12 pp 8269ndash8285 2005
[62] M A Behnajady N Modirshahla and R Hamzavi ldquoKineticstudy on photocatalytic degradation of CI Acid Yellow 23 byZnOphotocatalystrdquo Journal ofHazardousMaterials vol 133 no1ndash3 pp 226ndash232 2006
[63] M Addamo V Augugliaro A di Paola et al ldquoPhotocatalyticthin films of TiO
2formed by a sol-gel process using titanium
tetraisopropoxide as the precursorrdquo Thin Solid Films vol 516no 12 pp 3802ndash3807 2008
[64] N Negishi K Takeuchi and T Ibusuki ldquoPreparation of theTiO2thin filmphotocatalyst by the dip-coating processrdquo Journal
of Sol-Gel Science and Technology vol 13 no 1ndash3 pp 691ndash6941998
[65] L Rideh A Wehrer D Ronze and A Zoulalian ldquoPhotocat-alytic degradation of 2-chlorophenol in TiO
2aqueous suspen-
sion modeling of reaction raterdquo Industrial and EngineeringChemistry Research vol 36 no 11 pp 4712ndash4718 1997
[66] R-ADoongC-HChen RAMaithreepala and S-MChangldquoThe influence of pHand cadmiumsulfide on the photocatalyticdegradation of 2-chlorophenol in titanium dioxide suspen-sionsrdquoWater Research vol 35 no 12 pp 2873ndash2880 2001
[67] K V Sarkanen A Islam and C D Anderson ldquoOzonationrdquo inMethods in Lignin Chemistry S Y Lin and C W Dence EdsSpringer Series inWood Science pp 387ndash406 Springer BerlinGermany 1992
[68] G Gellerstedt and E-L Lindfors ldquoStructural changes in ligninduring kraft pulpingrdquo Holzforschung vol 38 no 3 pp 151ndash1581984
[69] B Neppolian H C Choi M V Shankar B Arabindoo andV Murugesan in Proceedings of the International Symposiumon Environmental Pollution Control and Waste Management(EPCOWM 02) p 647 2002
[70] C Pouteau P Dole B Cathala L Averous and N BoquillonldquoAntioxidant properties of lignin in polypropylenerdquo PolymerDegradation and Stability vol 81 no 1 pp 9ndash18 2003
[71] J Hafren T Fujino and T Itoh ldquoChanges in cell wall archi-tecture of differentiating tracheids of Pinus thunbergii duringlignificationrdquo Plant and Cell Physiology vol 40 no 5 pp 532ndash541 1999
[72] T Dizhbite G Telysheva V Jurkjane and U Viesturs ldquoChar-acterization of the radical scavenging activity of lignins naturalantioxidantsrdquo Bioresource Technology vol 95 no 3 pp 309ndash3172004
[73] K Hofstadler R Bauer S Novalic and S G Heisier ldquoNew reac-tor design for photocatalytic wastewater treatment with TiO
2
immobilized on fused-silica glass fibers photomineralization of4-chlorophenolrdquo Environmental Science amp Technology vol 28no 4 pp 670ndash674 1994
[74] J Villasenor and H D Mansilla ldquoEffect of temperature onkraft black liquor degradation by ZnO-photoassisted catalysisrdquoJournal of Photochemistry and Photobiology A Chemistry vol93 no 2-3 pp 205ndash209 1996
[75] B H Bielski D E Cabelli L A Ravindra and A B RossldquoReactivity of HO
2Ominus2Radicals in aqueous solutionrdquo Journal
of Physical Chemistry vol 14 pp 1041ndash1100 1985[76] B H J Bielski and A O Allen ldquoMechanism of the dispropor-
tionation of superoxide radicalsrdquo Journal of Physical Chemistryvol 81 no 11 pp 1048ndash1050 1977
[77] B Halliwell and J M C Gutteridge ldquoThe importance of freeradicals and catalytic metal ions in human diseasesrdquoMolecularAspects of Medicine vol 8 no 2 pp 89ndash193 1985
[78] J M Palmer P J Harvey and H E Schoemaker ldquoThe role ofperoxidases radical cations and oxygen in the degradation oflignin [and discussion]rdquo Philosophical Transaction of the RoyalSociety A vol 321 no 1561 pp 495ndash505 1987
[79] S Y Yin and C W Dense Methods in Lignin ChemistrySpringer New York NY USA 1992
[80] P Kumar S Kumar and N K Bhardwaj ldquoPhotocatalyticoxidation of elemental chlorine free bleaching effluent withUVTiO
2rdquo in Proceedings of the 2nd International Conference on
Environmental Science and Technology (ICEST rsquo11) SingaporeFebruary 2011
[81] K Kobayakawa Y Sato S Nakamura and A FujishimaldquoPhotodecomposition of Kraft lignin catalyzed by titanium
18 International Journal of Photoenergy
dioxiderdquo Bulletin of the Chemical Society of Japan vol 62 no11 pp 3433ndash3436 1989
[82] R B Kinstre ldquoAn overview of strategies for reducing theenvironmental impact of bleach-plant effluentsrdquo Tappi Journalvol 76 no 5 pp 105ndash113 1993
[83] A Castellan H Choudhury R Stephen Davidson and SGrelier ldquoComparative study of stone-ground wood pulp andnative wood 3 Application of fluorescence spectroscopy to astudy of the weathering of stone-ground pulp and native woodrdquoJournal of Photochemistry and Photobiology A Chemistry vol81 no 2 pp 123ndash130 1994
[84] B Albinsson S Li K Lundquist and R Stomberg ldquoThe originof lignin fluorescencerdquo Journal of Molecular Structure vol 508no 1ndash3 pp 19ndash27 1999
[85] S Baumberger A Abaecherli M Fasching et al ldquoMolar massdetermination of lignins by size-exclusion chromatographytowards standardisation of the methodrdquo Holzforschung vol 61no 4 pp 459ndash468 2007
[86] E Torres andM Ayala Biocatalysis Based on Heme PeroxidasesSpringer New York NY USA 1st edition 2010
[87] R Pecina P Burtscher G Bonn and O Bobleter ldquoGC-MSand HPLC analyses of lignin degradation products in biomasshydrolyzatesrdquo Fresenius Zeitschrift fur Analytische Chemie vol325 no 5 pp 461ndash465 1986
[88] NIST NIST Standard Reference Database The National Insti-tute of Standards and Technology 2014 httpwwwnistgovsrdnist1acfm
[89] T K Kirk and R L Farrell ldquoEnzymatic ldquocombustionrdquo themicrobial degradation of ligninrdquo Annual Review of Microbiol-ogy vol 41 pp 465ndash501 1987
[90] A TMartinez ldquoHigh redox potential peroxidasesrdquo in IndustrialEnzymes Structure Function and Applications K-B BeckerEd pp 477ndash488 Springer Amsterdam The Netherlands 1stedition 2007
[91] M Dashtban H Schraft T A Syed and W Qin ldquoFungalbiodegradation and enzymatic modification of ligninrdquo Interna-tional Journal of Biochemistry and Molecular Biology vol 1 no1 pp 36ndash50 2010
[92] N Duran E Esposito L H Innocentini-Mei and V P CanhosldquoA new alternative process for Kraft E1 effluent treatmentrdquoBiodegradation vol 5 no 1 pp 13ndash19 1994
[93] K Kamwilaisak and P C Wright ldquoInvestigating laccase andtitanium dioxide for lignin degradationrdquo Energy and Fuels vol26 no 4 pp 2400ndash2406 2012
[94] J Reyes M Dezotti H Mansilla J Villasenor E Espositoand N Duran ldquoBiomass photochemistry-XXII combined pho-tochemical and biological process for treatment of Kraft E1effluentrdquoApplied Catalysis B Environmental vol 15 no 3-4 pp211ndash219 1998
[95] L F Gonzalez V Sarria and O F Sanchez ldquoDegradationof chlorophenols by sequential biological-advanced oxidativeprocess using Trametespubescens and TiO
2UVrdquo Bioresource
Technology vol 101 no 10 pp 3493ndash3499 2010[96] J H Lora and W G Glasser ldquoRecent industrial applications
of lignin a sustainable alternative to nonrenewable materialsrdquoJournal of Polymers and the Environment vol 10 no 1-2 pp 39ndash48 2002
Submit your manuscripts athttpwwwhindawicom
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Inorganic ChemistryInternational Journal of
Hindawi Publishing Corporation httpwwwhindawicom Volume 2014
International Journal ofPhotoenergy
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Carbohydrate Chemistry
International Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
Chemistry
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Advances in
Physical Chemistry
Hindawi Publishing Corporationhttpwwwhindawicom
Analytical Methods in Chemistry
Journal of
Volume 2014
Bioinorganic Chemistry and ApplicationsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
SpectroscopyInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Medicinal ChemistryInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Chromatography Research International
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Applied ChemistryJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Theoretical ChemistryJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
Spectroscopy
Analytical ChemistryInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Quantum Chemistry
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Organic Chemistry International
ElectrochemistryInternational Journal of
Hindawi Publishing Corporation httpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CatalystsJournal of
International Journal of Photoenergy 3
1
2
3
45
120572120573
120574OH
OH
p-Coumaryl alcohol
OCH3
OH
OH
Coniferyl alcohol Sinapyl alcohol
OCH3H3CO
OH
OH
Figure 2 Monomer structures of lignin [18]
Table 1 Overview of most frequent bond types found in lignin
Model linkagea W G Glasser and H R Glasser [19] Erickson et al [20] Nimz [21]120573-Carbon-oxygen-4-aromatic carbon 55 49ndash51 65120572-Carbon-oxygen-4-aromatic carbon mdash 6ndash8 mdash120573-Carbon-5-aromatic carbon 16 9ndash15 6120573-Carbon-1-aromatic carbon 9 2 155-Aromatic-carbon-5-aromatic carbon 9 95 234-Aromatic-carbon-5-aromatic carbon 3 35 15120573-Carbon-120573-carbon 2 2 55120573-Carbon-120573-carbon forming furanic structure mdash mdash 2120572-Carbon-120574-carbon-oxygen-120574-carbon 10 mdash mdasha of total phenylpropane units
illustration Figure 3 shows the structure of a softwood ligninfragment containing all prominent linkage types
Native lignin cannot yet be isolated Contrary manymodified lignins of great variety are available throughbiomass transformation technologies [17] Depending onthe used isolation method lignosulfonate alkali lignin sul-fate lignin hydrolytic lignin steam exploded lignin andorganosolv lignins can be derived [31] The kraft and sodapulping processes generate liquors referred to as black liquorcontaining alkali lignin also called kraft or sulfate lignin [31]Although the kraft process is the dominant process (89)lignosulfonates attract more attention for the application oflignin as industrial products This is probably because ofadvantages such as its solubility properties and relativelylower mass range Moreover lignosulfonates are the onlycommercial lignins which are water soluble Their averagemolecular weight varies from 400Da up to 150 kDa or even17000 kDa (taken from Goring [9]) determined by processconditions and applied analytical methods [11]
3 Photocatalysis of Lignin
In this section the reaction principle for the photocatalyticdegradation of lignin is described After that themost impor-tant operating parameters and their impacts are discussed
31 On the Reaction Pathways for Photocatalytic Lignin Degra-dation Photocatalysis is the acceleration of a photoreactionin the presence of a catalyst In other words it involves theinitial absorption of photons by a molecule or substrate toproduce highly reactive electronically excited states
Lignin degradation is generally in the range of lowerenergy (between 300 and 400 nm) because of its multifunc-tional character [32ndash34]This region falls within the UV-lightregion TiO
2is the most applied photocatalyst and its energy
band gap is approximately 32 eV [35]In photogenerated catalysis the photocatalytic activity
depends on the ability of the catalyst to create electron-hole pair which generates free radicals (eg hydroxyl radi-cals ∙OH) enabling secondary reactions [36] Other aspectsinclude the rate of electron transfer the rate of chargerecombination crystal structure surface area of catalystporosity and surface hydroxyl group density [37]
In what follows equations summarizing the formationof radical species under photocatalytic conditions shall bedescribed S stands for the lignin substrate while TiO
2(h+VB)
and TiO2(eminusCB) represent the electron deficient (valence
band) and electron-rich (conduction band) parts in thestructure of TiO
2 respectively
The initial photocatalytic process involves the generationof electron-hole pair in the semiconductor particles as a result
4 International Journal of Photoenergy
Spirodienone
O
OO
O
O
OHO
OO
O
O
O
O
OHO
O
O
O
O
O
O
OO O
O
O
O
O
O
O
O
OH
OO
OO
O
O
OO
p-Coumarylalcohol fragment
Coniferylalcohol fragment
Branching caused bydibenzodioxin linkage Phenylcoumaran
HO
OAr
HO
OH
OHOH
OH
HO
HO
OH
HO
OH
HO
OH
HO
OH
HO
OH
HO
OH
HO
OH
HO
OH
HO4-O-5
OH
HOOH
120573-120573
5-5998400
120573-O-4
120573-1
120573-1
O
Figure 3 Structure of a softwood lignin fragment showing the prominent linkage types reprinted with permission from Zakzeski et al [4]and Evtuguin et al [22]
O2∙minus + H+ HO2
∙
2HO2∙
HO∙ + OHminus
∙OH
S + HO∙ S∙+ + OHminus
S∙+ + O2SO2
∙+
O2∙minus
∙O2minus
∙OH (hydroxyl radical)
(a) Reduction of O2H2O2
O2 + eminus
H2O2 + O2
H2O2 + eminus
(b) Oxidation of substrate
h+ + OHminus
Electron capture
Recombination
eminus
h+
Conduction band
Valence band
Electron release
eminus
O2
(super oxide)
Light
OHminus
Figure 4 Photocatalysis principle adapted from Linsebigler et al [23]
of UV radiation [38 39] Figure 4 shows the excitation ofan electron from the valence band to the conduction bandinitiated by light absorption with energy equal to or greaterthan the band gap of the semiconductor This is expressed by(1)
Upon excitation the fate of the separated electron andhole can follow several pathways Electron or holes canthen react with hydroxyl ions (OHminus) or H
2O producing
hydroxyl radicals (∙OH) as shown in (2) Jaeger and Bard[40] Matthews [41] andMachado et al [42] report that ∙OHis the main oxidizing agent in the photocatalytic oxidationbecause of the unpaired electrons Therefore it can react fastand unspecifically with almost all organic compounds (S)
[43] abstracting an electron with the formation of a radicalorganic species as shown in (3) [44]
Formation of singlet oxygen hydroxyl and superoxideradicals as principal reactive species in a photocatalyticprocess [38 39] is as follows
TiO2
ℎV997888997888rarr TiO
2(
eminusCBh+VB) (1)
TiO2(h+) +H
2O 997888rarr TiO
2+HO∙ +H+ (2)
S +HO∙ 997888rarr S∙+ +OHminus (3)
International Journal of Photoenergy 5
MeO MeOMeOMeO
∙OO
O
O ∙
OH
HO+ +
Figure 5 Formation of phenoxyl radicals by intermolecular abstraction of phenolic hydrogen by carbonyl groups
RH
bond cleavage
Vanillin
O
OO
O
OO O
O
O
O
O
O
HO HO
HOHO
HO
HO
HO HO
OH OH
HO
OCH3
OCH3
OCH3
OCH3
OCH3 OCH3
OCH3
OCH3 OCH3
OCH3
h minusH∙OH∙∙
O2TiO2
HminusOH∙
C120572-C120573
∙
Figure 6 Supposed lignin degradation scheme by autoxidation induced by TiO2poly (ethylene oxide) [24]
S∙+ +O2997888rarr SO
2
∙+ (4)
TiO2(h+) + S 997888rarr TiO
2+∙S+ (5)
TiO2(eminus) +O
2997888rarr TiO
2+∙O2
minus (6)
HO∙ + ∙O2
minus
997888rarr OHminus + 1O2
(7)
TiO2(eminus) + TiO
2(h+) 997888rarr heat (8)
The organic radicals and radical cations can for examplereact with molecular oxygen to form organic peroxy radicalsand peroxy radical cations respectively (4) The holes canoxidize organic compounds by electron abstraction to formorganic cationic radicals (5) [42] Superoxides can be formedby the reaction of electrons with electron acceptors suchas O2(6) Meanwhile the formation of singlet oxygen can
be from the reaction of hydroxyl radical and superoxide(7) [42] Moreover there is a possibility that electrons andholes recombine if electron acceptors are limited In thiscase recombination can take place in the volume of thesemiconductor particle When recombination takes placeradiation energy is lost or converted into heat (8) [45]
From investigations caried out by Mazellier et al [46](photochemistry of 26-dimethylphenol) it was postulatedthat hydrogen can be abstracted by 120572-carbonyl groups In thesame context lignin derivatives having similar functionalitycan follow a similar pathway In addition oxidative chainreactions with the participation of ground-state oxygencan be initiated leading to fragmentation and combinationreactions and thus the formation of new dimers or oligomers(Figure 5)
Miyata et al [24] proposed a cleavage mechanism forthe C120572ndashC120573 bonds which leads to the formation of smallfragments such as vanillin as shown in Figure 6
Figure 7 [25] illustrates the formation of a radical cationformed as a result of enzyme (lipase) mediated reactionof adlerol Adlerol is characterized by a C120573ndashOndash4 bondand considered to be a lignin model compound With theformation of the radical species subsequent nonenzymaticreactions such as radical reactions can take place generatinga wide variety of products and complex compounds
It is widely assumed that the photocatalytic degradationof lignin follows a radical reaction pathwaywhich is similar tothat considered in thermal electrochemical and biochemical
6 International Journal of Photoenergy
Nonenzymatic subsequent reactions
Adlerol
Veratryl alcohol radical
Enzyme1
23
4
4
1
2
3
5
5
6
6
Guaiacol
Veratraldehyde
Ketone
O
OO
O
O O
O
O
O
HO
HOHO
HO
OCH3
OCH3
OCH3
OCH3
OCH3
OCH3
120572120573
120574
minusH+
∙+
∙
∙+
c120572 oxidation H2O
O2
120572 120573
120574
HOHOHO
OCH3
OCH3OCH3
OCH3 OCH3
OCH3
OCH3
120572120572
120573 120573
120573
120574 120574
HO
HO
120574
OCH3
120573
HO
HO
120574
HO
OCH3
OCH3
OCH3
120572120573
120574
Figure 7 Proposed radical reaction scheme initiated by enzyme lignolytic heme peroxidase for the conversion of adlerol possessing C120573ndashOndash4 bonds into smaller units summarized by Busse et al [25] and abstracted from Tien and Kirk [26] Kirk et al [27] Lundell et al [28]Schoemaker et al [29] and Palmer et al [30]
processes However reporting on the degradation pathway oflignin derivatives and even that of lignin model compoundsis still a major challenge This is probably due to the complexnature and variety of possible degradation products Indeedthemechanism is far more complex considering other factorssuch as type of lignin type of catalyst pH illuminationsource and additives
32 Influence of Process Parameters in Lignin DegradationVarying process parameter could either have a positive ornegative impact on the photocatalytic efficiency The basicprocess parameters such as catalyst concentration [4 4856] substrate concentration [48 51] addition of metal ion
to TiO2catalyst [17 50 56ndash58] pH [47 48] illumination
[33 48 49 59] and their influence shall be discussed in thissubchapter Table 2 gives an overview about starting reactionconditions and catalyst applied by some work groups whileTable 3 portrays parameters analytical methods and resultsobtained It is worthwhile noting that comparing the differentphotochemical processes poses a big challenge because ofthe wide variables involvedThese discrepancies start alreadyfrom the source and type of lignin followed by the differencesin reactor design illumination source intensity of radiationand different types of TiO
2catalyst such as Fischer scientific
rutile TiO2[49] TiO
2(TiO2mdashTP-2 of Fujititan) just to name
a few
International Journal of Photoenergy 7
Table2Summaryof
startingcond
ition
sfor
thep
hotocatalytic
degradationof
lignin
Reference
Lign
insource
Catalyst
Reactio
ncond
ition
s
MacHadoetal[42]
Peroxyform
icacid
lignins
from
eucalyptus
grandisw
ood(EL1)
EL1+
sodium
borohydride
TiO
2andH
2O2
Lign
inconcentration
025
mgmL119879=25∘C
pH11U
VVisradiatio
n120582gt300nm
40
0Wmercury
lampcylin
dricalPy
rexglassreactorcon
stanto
xygenbu
bblin
g
Ksibietal[47]
Water
solubleligninob
tained
from
blackliq
uor
TiO
2-P2
5(D
egussa)
Lign
inconcentration
90mgL119879=20∘C
pH82UV-radiation120582gt290nm
pyrex
reactoro
pento
airPh
ilips
HPK
125W
lamp
Kansaletal[48]
Lign
infro
mwheatstr
awkraft
digestion
TiO
2-P2
5(D
egussa)a
ndZn
O
Lign
inconcentration
10ndash100
mgLin
100m
LZn
Ocatalystdo
se(05ndash20gL)pH
ofthes
olution(pH3ndash11)solarillu
mination
oxidantcon
centratio
n306times10minus6M
to153times10minus6Mthinbedfilm
slurrypo
ndreactoroxidantsodium
hypo
chlorite
solutio
n(4availableC
l 2)
Dahm
andLu
cia[
49]
Whitewater
from
indu
strialprocess
water
thatexits
inpaperm
achines
FischerS
cientifi
crutile
TiO
2
Lign
inconcentration
40mgLin
500m
Lbatchreactor119879=21∘Cndash
42∘C
Rayonet
photochemicalcham
ber16
VWR8-W
blacklightpho
spho
r(350-nm
)lam
ps
constant
oxygen
bubb
lingpo
wer
ofillum
ination
128to
64Wlight
intensity
223ndash44
5mWcm
3
Portjanskajaand
Preis[50]
Lign
inpurchased
from
Aldric
hTiO
2P2
5-N(D
egussa)
Lign
inconcentration
100m
gLpH
8batchreactorsystemreactor
open
toair
PhillipsT
LD15W05low-pressurelum
inescent
mercury
UV-lampUV-radiation
120582gt360nm
pow
erdensity
ofirr
adiatio
n=07m
Wcm
2 visib
lelight
source
Tanaka
etal[51]
Lign
infro
mconiferous
woo
dTiO
2(TiO
2mdashTP
-2of
Fujititan)
Lign
inconcentration
0003to
003U
V-radiation120582gt310nm
cylindrical
reactio
nvessel
Tonu
ccietal[33]
Ca2+andNH4
+ligninderiv
atives
TiO
2as
Degussa
P25
+po
lyoxom
etalates
(POM)H
2O2
Openqu
artztubes(20
mL)
reactor119879=20∘C
1atm
multirayso
ften
UVlamps
of15W
power
eachU
V-radiation120582gt254nm
Miyatae
tal[24]
Piceag
lehn
iiwoo
dflo
urTiO
2po
lyethylene
oxide(PE
O)
Reactorop
enpetrid
ish119879=30∘C119905=48h40
0Wmercury
lamp
Shende
etal[17]
Kraft
lignin
TiO
2-Zn
O-ZrO
2Lign
inconcentration
1mgmLin
10mL250W
Xeno
nlampandAM
15Glamp
filterpo
wer
density
ofirr
adiatio
n100m
Wcm
2 solarlam
psim
ulator
Tian
etal[52]
andPanetal[53]
Kraft
ligninfro
mblackliq
uor
Ta2O
5-IrO
2andPb
O2thin
film
TiO
2nano
tubePbO
2
Lign
inconcentration
30(w
w)UV-radiation120582gt365nm
119905=10minpow
erdensity
ofirr
adiatio
n20
mWcm
2 blue
waveT
M50
ASUVspot
lampEG
ampG2273
potentiosta
tgalvano
stattoapplycurrentTiTiO
2NTPb
O2ele
ctrode
asworking
electrode
andPt
coilas
outere
lectrode
andAgAgC
lasreference
electrode
Awun
gachaL
ekele
fac
etal[54][55]
Lign
insulfo
natefro
mpaperw
aste
water
TiO
2as
Degussa
P25
TiO
2fro
msol-g
elprocesso
fTiOSO
4TT
IP
Lign
inconcentration
500m
gLin
200m
LOsram
Planon
light
sourceirradiance
30ndash4
0Wm
2 UV-radiation120582280ndash4
20nm
119905=20hreactoro
pento
air
recirculationsyste
mflow
rate225mLmin
8 International Journal of Photoenergy
Table3Parametersanalyticalmetho
dsand
results
from
different
workgrou
ps
Reference
Parameter
studied
Analytic
sRe
sult
MacHado
etal[42]
Roleof
hydroxylradicals
irradiatio
nof
ligninin
the
absencea
ndpresence
ofph
otocatalystT
iO2andH
2O2
Ultraviolet-v
isible(UV-Vis)spectro
scop
yionizatio
nabsorptio
nspectro
scop
y(IA
S)
sizee
xclusio
nchromatograph
y(SEC
)
Asharpdecrease
inthep
heno
liccontento
bservedforreactions
involvingdirect
photolysis
SEC
areductio
nof
almost50
inthea
verage
molecular
weighto
fligninequalto
14kD
after
90min
ofirr
adiatio
n
Ksibietal[47]
Irradiationof
ligninin
the
absencea
ndpresence
ofph
otocatalystT
iO2-P2
5
UV-Visspectroscop
y13C-
nucle
armagnetic
resonance(NMR)
solid
state
totalion
gasc
hrom
atograph
y(TIC)
indu
ctioncoup
lingplasma(
ICP)
chem
icaloxygen
demand(C
OD)
56degradationratewith
TiO
2catalystaft
er420m
inreactiontim
eethylacetate-extractableprod
uctsshow
edvanillin
vanillica
cidpalm
itica
cid
biph
enylstructuresand
345-trim
etho
xybenzaldehyde
presence
ofmagnesiu
mandcalcium
ions
CODremovalishigh
erforthe
initiallylowconcentrations
ofligninsolutio
n
Kansaletal[48]
Catalystdo
sepHoxidant
concentration
initialsubstrate
concentration
ZnOcatalystin
slurryandim
mob
ilizedmod
e
UV-Visspectroscop
yCO
D
Optim
umcatalystdo
seis1gL
optim
umoxidantcon
centratio
n2times10minus6M
graduald
ecreaseo
fabsorptionpeak
indicatin
gdecompo
sitionof
organics
CODremovalishigh
erforthe
initiallylowconcentrations
ofligninsolutio
n
Dahm
and
Lucia[
49]
Catalystdo
seillu
mination
intensity
UV-Visspectroscop
ytotalorganiccarbon
(TOC)
capillaryionelectro
phoresis
analysis(C
IA)
Gradu
aldecrease
inabsorptio
npeak
indicatin
gdecompo
sitionof
organics
optim
alcatalystdo
seof
10mgm
high
erillum
inationintensities
correlated
well
with
high
erinitialdegradationrate
74disapp
earanceo
fTOC
Portjanskajaand
Preis[50]
Influ
ence
offerrou
sion
sN-dop
edcatalysteffect
sprayedcatalyston
supp
ortand
subm
ersedcatalyst
CODU
V-Visspectroscop
ybiochemical
oxygen
demand(BOD)colorim
etric
measurementat570
nm
Additio
nof
Fe2+upto
28m
gLleadsto25increase
inph
otocatalyticeffi
ciency
sprayedcatalystexhibited15
times
high
ereffi
ciency
than
theo
neattached
bysubm
ersio
nnegligibleeffecto
fN-dop
edcatalyst
increase
ofaldehyde
concentrationover
reactio
ntim
eneutralm
ediawas
mostb
eneficialforb
iodegradability
80of
freep
heno
lsremoved
undern
eutralcond
ition
s
Tanaka
etal[51]Ca
talystloadinglignin
concentration
illum
inationtim
e
UV-Visspectroscop
yTO
Cgel
perm
eatio
nchromatograph
y(G
PC)
1 HNMR
Fouriertransform
ationinfrared
(FTIR)
spectro
scop
y
FTIR
measurementrevealedafasttransform
ationof
arom
aticmoietyp
resent
inlignin
characteris
ticband
sofaromaticrin
gsm
etho
xyand
aliphatic
sidec
hains
decrease
inTO
Cvalues
over
time
decrease
indegradationratewith
increase
catalystdo
sageB
utaft
ercatalystthreshold
valueisa
ttainedcatalystincreasec
ausesa
decrease
indegradationrates
FTIR
peaksa
reshifted
towards
lower
molecular
weightregionaft
erph
otocatalysis
Tonu
cci
etal[33]
Testof
catalytic
syste
mstoob
tain
fractio
nswith
redu
ceddegreeso
fpo
lymerization
comparis
onof
thermalandph
otochemical
reactio
ns
1 HNMR
gasc
hrom
atograph
y-mass
spectro
scop
y(G
C-MS)
POMsa
relessselectivew
henused
asph
otocatalystsandno
appreciableb
leaching
ofthes
olutionwas
seen
whenPO
Mwas
used
asthermalcatalyst
deriv
edchem
icalsfrom
experim
entvanillin
hydroxylmetho
xy-acetoph
enon
econiferylalcoh
olcon
iferylaldehydemethano
lform
icacidacetic
acidand
sometim
essm
allamou
ntso
fC-2
andC-
3alcoho
ls
Miyatae
tal[24]
Exam
inationof
cellwall
structureo
flignin(w
oodflo
ur)
before
andaft
erph
otocatalysis
GC-
MSscanning
electronmicroscop
y(SEM
)1 H
NMR
Highdelignificationactiv
itydelignificationconfi
nedto
thes
urface
oflignin
deriv
edchem
icalse
xperim
entvanillin
Shende
etal[17]Testof
combinedactio
nof
bio-
andph
otocatalyticsyste
ms
UV-Visspectroscop
yGC-
MSX-
ray
dispersiv
eenergyspectro
scop
yandX-
ray
diffractio
nanalysis(EDX
XRD)SE
M
Detectio
nof
follo
wingchem
icalsacetylguaiacol4-ethoxym
ethyl-2
-metho
xyph
enol
metho
xyph
enyloxim
eguaiacolsuccinica
cidacetylguaiacolvanillicacidand
vanillin
International Journal of Photoenergy 9
Table3Con
tinued
Reference
Parameter
studied
Analytic
sRe
sult
Tian
etal[52]
andPanetal
[53]
Testof
combinedactio
nof
electro-and
photocatalytic
syste
ms
UV-Visspectroscop
yFT
IRSEM
X-ray
(EDX)
highperfo
rmance
liquid
chromatograph
y(H
PLC)
COD
Detectio
nof
thefollowingchem
icalscarbon
ylfunctio
nalityvanillin
andvanillica
cid
Awun
gacha
Lekeleface
tal
[54][55]
Com
paris
onof
degradationrates
bydifferent
catalyst
HPL
Cflu
orescencea
ndUV-Vis
spectro
scop
ySE
MT
OC
UV-Visresultsrevealfaste
rdegradatio
nof
thea
liphatic
moietycomparedto
the
arom
aticmoietyof
ligninsulfonateob
tained
from
paperw
astewaterPeaks
observed
durin
gHPL
Canalysis
Someo
fthe
peaksp
rodu
cedaft
erph
otocatalysishad
fluorescences
ignals
Thissuggeststhep
rodu
ctionof
newsubstances
andflu
orop
hores
Coatin
gsprod
uced
throug
hSol-G
elprocedures
arestablea
ndcanbe
used
manytim
es
10 International Journal of Photoenergy
321 Influence of Catalyst One aspect to successfully imple-ment photocatalysis is the choice of an appropriate photo-catalyst The majority of catalysts used are based on TiO
2as
summarized in Table 2 ZnO has also been applied either assingle catalyst [48] or in a combination with other catalystsuch as TiO
2[54] de Lasa et al [60] described ZnO as
being less active than TiO2 They add that the use of ZnO
is particularly relevant when the oxidative degradation ratebecomes limited This is opposite to what Kansal et al [48]reported by saying that ZnO is more reactive than TiO
2
However TiO2(mainly anatase) remains the most used
catalyst as can be depicted fromTable 2 TiO2is reported to be
favored because of its nontoxic property cost efficiency andchemical and biological inertness Moreover TiO
2possesses
the most efficient photoactivity and the highest stability thusmaking it suitable for industrial use [61]
ZnO has been described to degrade lignin under visiblelight sources [48 62] whereas TiO
2is mostly applied in
connection to UV-light sources as highlighted in Table 2However both ZnO and TiO
2possess energy band gap
energy of 32 eV [23]Additives such as SiO
2[63] polyethylene oxide (PEO)
[24] and polyethylene glycol [64] have been added to TiO2
catalyst particularly when applying immobilized catalystAddamo et al [63] noted a high adhesion of TiO
2to glass
support material when precoating was done with SiO2
Moreover the precoating might have other advantages suchas a hindering diffusion of Na+ ions from the glass materialinto the nascent TiO
2film during heat treatment processes
Analogous to the addition of SiO2 polyethylene glycol
(PEG) has also been introduced to mitigate catalyst surfaceactivity modify surface hydrophobicity and also reduceagglomeration tendency of the TiO
2gel or TiO
2particles
in the suspensions [64] By a proper surface modificationinteraction between catalyst and substrate can be enhanced[55 63]
Kansal et al [48] varied catalyst (ZnO) dose from 05 gLto 20 gL for 01 gL kraft lignin solutions and found out thatthere was an optimum catalyst threshold value at 1 gL whichgives a catalyst to substrate ratio of 1 10
Dahm and Lucia [49] examined catalyst dose from 2 sdot10minus3 gL to 12sdot10minus2 gL for lignin solutions (fromwhite water
liner mill) of 4 sdot 10minus2 gL (catalyst to lignin ratio 5 sdot 10minus3ndash3 sdot 10minus2) at pH 8 and obtained best energy efficiency valuesand lignin degradation rates with a catalyst loading of 10 sdot10minus2 gLIn contrast Ma et al [56] applied far higher catalyst
concentration compared to Dahm and Lucia [49] Catalystconcentration was varied between 1 gL and 10 gL PtTiO
2
Best catalysis dose with respect to reaction turnover wasobtained at 5 gL PtTiO
2With the increase of catalyst dose at
pH 7 the reaction rate increased from 61 times 10minus3minminus1 (1 gLTiO2) to 71 sdot 10minus3minminus1 (5 gL TiO
2) and 99 sdot 10minus3minminus1
(10 gL TiO2)
Catalyst effect has been explained on the basis thatoptimum catalyst loading is dependent on the initial soluteconcentration An increase in catalyst dosage leads to a cor-responding increase of total active surface area for reactions
[65] To that at higher TiO2concentrations the photon flux is
more easily intercepted by the catalyst before penetrating intothe bulk of the system At the same time due to an increase inturbidity of the suspension with high dose of photocatalystthere is a decrease in penetration of UV light and hencephotoactivated volume of suspension or solution decreases[66]
In summary authors have obtained best catalyst to ligninrelations for different reaction designs and thus a generalrecommendation on catalyst dose is not possible Howeverwhen lignin solution is treated with increasing catalyst loadsa corresponding increase in degradation rate is observed untila threshold value is reached [48ndash50 56]
322 Influence of Metal Ion Addition (Doping) and AdditivesThe purpose of adding metal ion to photocatalyst is to miti-gate band gap energy through the introduction of intrabandgap states and as a consequence produce a bathochromicshift in the absorption spectrum [37] Altering the absorptionspectral range gives the possibility to exploit both the visiblelight spectrum and UV light sources Metal ion doping isalso introduced to serve as electron or hole traps in orderto minimize recombination between generated electron-holepairs [37]
Portjanskaja and Preis [50] studied the addition of Fe2+ions to an acidic lignin solution and found an increasein photocatalytic oxidation (PCO) efficiency The optimumFe2+ ions quantity was 28mgL while using 100mgL ligninsolution Upon further elevation of Fe2+ ions concentrationa corresponding reduction of the photocatalytic oxidationefficiency of lignin was noted Likewise Ohnishi et al [57]made a comparative study by doping platinum (Pt) silver(Ag) and gold (Au) ions to TiO
2 In these reactions 50mg of
catalyst (TiO2) was used with the addition of an equiva1ent
15 wt (based on TiO2) metal ion The addition of noble
metals brought about a faster decolorization of lignin Aushowed better results than Ag followed by Pt In the samecontext adding sodium hypochlorite as oxidant to PtTiO
2
catalyst an additional fivefold degradation rate was observedcompared to that without doping [56] Contradictory to theresults described above negligible effect of photocatalyticefficiency due to doping has been reported as well Awun-gacha Lekelefac et al [54] obtained little or no change indegradation rate by doping TiO
2-P25-SiO
2catalyst with Pt
ions (1 wt relative to TiO2catalyst) Likewise Portjanskaja
and Preis [50] noted a negligible change of photocatalyticefficiency of TiO
2when doped with nitrogen
Sarkanen et al [67] and Gellerstedt and Lindfors [68]reported the bias of peroxides to oxidation with reagentssuch as permanganate to favor aromatic moieties Oxidationagents like permanganate oxidizes predominantly aliphaticchains in alkaline and neutral media However by theapplication of H
2O2(Fenton system) lignin disappeared
completely [33] Tonucci et al [33] conclude that in orderto satisfactorily conserve the organic material the best com-promise appears to be the TiO
2photosystem which shows
low carbon consumption good preservation of the aromaticrings and greatly reduced mineralization
International Journal of Photoenergy 11
In summary different results have been obtained con-cerning the influence of noble metal ion addition Whilesome authors report an improvement in the photocatalyticefficiency upon their addition others report their addition ashaving no considerable influence However for reactions inwhich an improvement in the photocatalytic efficiency wasnoticed there was a threshold value to be considered Whenthe concentration of dopant surpasses this threshold valueelectron-hole recombination is favored and this has a negativeimpact to photocatalysis In such a case the space-chargelayer gets narrower and 119901-type dopants attract electrons andby virtue become negative They would now then act ashole acceptor attracting holes On the other hand 119899-typedopants which act as electron donor centers and possessexcess electrons attract holes as well [37]
323 Influence of Lignin Concentration Once the initiallignin concentration becomes higher exceeding a thresholdvalue an inhibitory effect on the photodegradationwas noted[47ndash49] This threshold value varies and depends on thereaction system and reaction parameters such as opticaldensity catalyst concentration and reaction volume Fromthe literature different authors have implemented varyinglignin concentrations probably to suit their reaction designFor example Ksibi et al [47] use 90mgL and AwungachaLekelefac et al [54] use 500mgL while Kansal et al [48]applied 10mgndash100mgL
Explanations arising from the findings are as followsat low lignin concentrations the incidental photonic fluxirradiated interacts with the catalyst generating radicals (eghydroxyl radicals (OH∙)) which allow a faster degradation[69] On the other hand high initial lignin concentrationsmay lead to tight adsorption which can suppress CO
2
evolution [51] and hence maintain chemical oxygen demand(COD) values Moreover low delignification yields maybe due to an inhibitory effect because of autoxidation bylow molecular weight lignin degradation products formed[24] Also due to the polymer structure of lignin which iscross-linked this makes it difficult for radical species acidand the aldehyde compounds produced to spread into theinner region of the substrate hence limiting autoxidationAs a worst case this might be the rate-determining step ofdelignification which is hindered [70ndash72]
In summary it can be concluded that the time taken forcomplete degradation depends on the initial concentration oflignin and faster degradation occurs at low lignin concentra-tions
324 Influence of pH Varying pH entails an alteration inthe properties of semiconductor-liquid interface [73] mainlyrelated to the acid-base equilibrium of the adsorbed hydroxylgroup [39] Furthermore pH also impacts lignin degradationrates [47 48 57] In this context several studies were carriedout with partly contradictory outcomes
Kansal et al [48] made pH investigations 3ndash11 undersolar light illumination using ZnO as catalyst Maximumdegradation was reached in alkaline conditions (pH 11)This is supported by Villasenor and Mansilla [74] reporting
an almost complete decolorization of kraft black liquorfrom pine wood at pH value of 116 in combination withZnO catalyst Similar results were achieved by Ohnishi etal [57] with TiO
2and ZnO being catalyst for bleaching
alkaline lignin in aqueous solution with TiO2and ZnO being
catalyst High activities at neutral pH were also reportedby Ohnishi et al [57] In contrast Ma et al [56] observedhigher reaction rates and rapid degradation of a syntheticlignin wastewater (prepared by dissolving commercial ligninpowder in aqueous solution pH 11) in acidic solution (pH 3)than in alkaline solutions at pH 11 for either TiO
2or PtTiO
2
catalystsReconsidering the photocatalytic principle the formed
superoxide anion radicals (∙O2
minus) are in a pH-dependentequilibriumwith perhydroxyl radicals (HO
2
∙) as follows [75]
HO2
∙999445999468 H+ + ∙O
2
minus pKa sim 48 (9)
See [76]∙O2
minus undergoes dismutation reaction resulting in H2O2
and O2competing to any other ∙O
2
minus triggered reaction Incase of low pH operation conditions in aqueous solutionsHO2
∙ becomes dominant whose reactivity is considerablyhigher compared to ∙O
2
minus [77] Subsequently HO2
∙ initiatessubstrate (S) oxidation to the radical cation (S+∙) and is itselfreduced to H
2O2[78] Thus increased degradation rates can
be reasonably expected supporting the results made by Ma etal [56] in an acidic environment ∙O
2
minus is extremely reactive inorganic solvents [77] Another aspect is the solubility of kraftlignin (soluble at pH gt 105) which reduces with decreasingpH whereas lignosulfonate should remain unaffected by pHin aqueous solution Moreover 120573ndashOndash4 bonds have beendescribed to be stable at acidic pH [11] In fact this couldadditionally explain the elevated degradation of kraft ligninmade by Kansal et al [48] and Villasenor and Mansilla [74]Nevertheless the contradictory results gained by Ma et al[56] still exist under the assumption that kraft ligninwas used(which would be supported by the high pH of 11 obviouslynecessary for dissolving the lignin powder) Although mostphotocatalytic reactions described in the literature are in anaqueous milieu lignin raw material and its fission productsmay however vary considerably Therefore the optimal pHis most likely to be reaction specific and has to be evaluatedexperimentally in principle
325 Influence of Illumination Many of the studies foundin the literature so far have not dealt on this subject per seWhat is found is the use of different illumination sourceseach having a specified power and lamp type However alltend to emitUV-light between the range 280ndash420 nm Table 2depicts this in detail Other illumination sources include thevisible light spectrum
In general terms illumination influences in that it ini-tiates photocatalysis by generating electron-hole pair in thesemiconductor particles [38 39] Dahm and Lucia [49]altered illumination intensity while observing lignin degra-dation In this study 004 gL lignin was used and lightintensity was varied 223ndash445mWcm3 It was found out thathigher illumination intensities correlated well with higher
12 International Journal of Photoenergy
initial degradation rates and hence total lignin degradation[49] Neppolian et al [69] report degradation to be propor-tional to radiation intensity and best results are achieved forlow lignin concentrations because of enhanced interactionbetween catalyst and incidental photonic flux
In summary high illumination power causes a corre-sponding high initial degradation rate at low lignin concen-trations becausemaximum light penetration into the reactionmedium is favored
33 Process Analytical Methods Various analytical tech-niques have been used to monitor lignin degradation Atthe beginning of this subchapter analytics revealing com-pounds formed from lignin degradation are treated Thisincludes for example gas chromatography (GC) and 1HNMR (nuclear magnetic resonance) This is then followed byresults qualitative analytic measurements such as ultraviolet-visible (UV-Vis) spectroscopy and dissolved carbon (DC)A list of authors analytical techniques applied and resultsachieved are outlined in Table 3
Portjanskaja and Preis [50] studied lignin degradationby measuring the removal of phenols through colorimetricmeasurements As a result of 24 h photocatalytic oxidationunder neutral media conditions 80 of free phenols wereremoved Gas chromatography (GC) result from Ksibi et al[47] attested vanillin vanillic acid palmitic acid biphenyland 345-trimethoxy benzaldehyde structures after the pho-tocatalysis of lignin from black liquor This is in accordancewith the findings of Tonucci et al [33] reporting the formationof vanillin hydroxyl methoxy-acetophenone coniferyl alco-hol coniferyl aldehyde methanol formic acid acetic acidand small amounts of C-2 and C-3 alcohols as degradationproducts1H NMR spectral analysis of lignin before illumination
and after 24 h of illumination showing characteristic bandsof aromatic rings methoxy and aliphatic side chains wascompared with each other Results revealed that the aro-matic ring degraded faster than the aliphatic chain [51]Fourier transformation infrared spectroscopy (FTIR) showedbands corresponding to CH
3 CH2 and CH which remained
unchanged after illumination while bands corresponding toaromatic rings disappeared as a result of illumination [51 53]
Results obtained from the combination of photochemicaland electrochemical oxidation [52 53] were similar to thoseof Tanaka et al [51] Here 13C-NMR confirmed the presenceof the carbonyl functionality and the presence of vanillinand vanillic acid after 12 h photochemical-electrochemicaloxidation These results showed that the combination of aphotocatalytic and an electrochemical oxidation significantlyenhanced the efficiency of lignin degradationThis is becausethe applied anodic potential bias greatly suppressed therecombination of photogenerated electrons and holes [53]
Ultraviolet spectrophotometry offers a convenientmethod to qualitatively and quantitatively analyze ligninin solution [79] This is reflected by the large number ofpublications using this technique [17 33 47ndash51 56 58 80]This is most likely due to its simplicity to interpret lignindegradation Lignins absorb UV light with high molar
200 220 240 260 280 300 320 340 360
00
05
10
15
20
25
Abso
rban
ce
Wavelength (nm)0min
30min
60min
120min
180min
300min
360min
420min
1200min
Figure 8 Time dependent UV-Vis absorption spectra of aqueouslignin solution from waste paper water irradiated with UV light(280ndash420 nm) for different time intervals The spectra are obtainedfor sol-gel derived TiO
2nanocrystalline coating (TiO
2-P25-SiO
2)
[54]
extinction coefficients because of the several methox-ylated phenylpropane units of which they are composed [33]Figure 8 depicts a series of photometric scans of ligninsul-fonate from paper waste water showing a gradual reductionof absorbance during photocatalytic treatment [54] Herethe absorption peaks are around 210 nm and 280 nmAbsorbance decreases with time implying the decompositionof lignin and the deterioration of chromophore groups [54]
Peaks at 210 nmcorrespond to portions of the unsaturatedchains while those at 280 nm correspond to unconjugatedphenolic hydroxyl groups [17] and the aromatic moiety [57]of the lignin molecule The absorption tailing to the longwavelength region arises from the color of lignin [57] Lignindegradation has been reported either at wavelength around280 nm corresponding to unconjugated phenolic hydroxylgroups [17 48 51 58] or for both wavelengths (210 nm and280 nm) [33 54 57] Kobayakawa et al [81] noted some otherabsorbance at wavelengths lower than 250 nm and pointedout that this could be due to the modification of ligninfragmentations leading to the formation of transient specieslikemethanol ethanol formaldehyde formic acid and oxalicacid among others
Analyticalmethods to effectively quantify lignin degrada-tion by calculating the oxygen demand by organic substancesand remaining organic carbon before and after photocatalysishave been studied Amongst the methods are dissolvedcarbon (DC) [49 51 58] chemical oxygen demand (COD)[47 48 50 57 80] biochemical oxygen demand (BOC) [50]dissolved organic carbon (DOC) [56] and American dyemanufacture institute value (ADMI) [56] COD and BODdescribe the oxygen demand by organic substances to be
International Journal of Photoenergy 13
Figure 9 Gradual change from the characteristic yellow ligninsulfonate to a colorless liquid after a period of 20 h Catalyst TiO
2-
P25 (Degussa) +TEOSUV-light 25∘C lignin concentration 05 gL[54]
converted to CO and CO2and H
2O and NH
3 Total organic
carbon (TOC) describes the amount of carbon bound inan organic compound while DOC describes the dissolvedfraction of organic carbon ADMI measures the amount ofdyestuff in water
Decolorization of lignin solution has been reported tobe another parameter observed during photocatalytic degra-dation Color is an indirect indicator of the lignin amountThe higher the color intensity of the solution is the greaterthe lignin content is (high concentrated lignin solutions egblack liquor appear dark brown) [82] Thus color changescan be interpreted as conversion of lignin to transient speciesor conversion to CO
2and H
2O Awungacha Lekelefac et al
[54] observed a gradual change from the characteristic yellowlignin (when highly diluted) to a colorless liquid after a periodof 20 h with sol-gel derived TiO
2nanocrystalline coatings on
sintered borosilicate glass as depicted in Figure 9 A corre-sponding decrease in DC values close to 82 was observedfor TiO
2-P25-SiO
2catalyst confirming degradation This is
shown in Figure 10These findings are analogous to that of Ohnishi et al
[57] who reported the bleaching of lignin when illuminatedcontinuously and that the solution becomes colorless Tothat the chemical oxygen demand (COD) value decreasesgenerating carbon dioxide and a small amount of carbonmonoxide as the main gaseous products COD removalwas reported to be effective at low lignin concentrations ascompared to high lignin concentrations [48]
Another applied analytical technique is fluorescencedetection directly coupled to a high performance liquidchromatography (HPLC) as a means to identify nonaliphaticcomponent in the complex mixture of lignin degradationproducts [54] Fluorescence emission in lignin is attributedto aromatic structures such as conjugated carbonyl biphenylphenylcoumarone and stilbene groups [83 84] AwungachaLekelefac et al [54] observed peaks on both HPLC and
0 200 400 600 800 1000 12000
50
100
150
200
250
Diss
olve
d ca
rbon
(ppm
)
t (min)
TiO2-P25-SiO2
TiO2-P25-SiO2 + PtTiOSO4minus30 6wt
ZnO + TiO2-P25-SiO2
No coating UV light
Figure 10 Variation of DC with time of aqueous lignin solutionfrom waste paper water irradiated with UV light (280ndash420 nm) fordifferent time intervals The spectra are obtained for sol-gel derivedTiO2nanocrystalline coatings (TiO
2-P25-SiO
2+Pt TiO
2-P25-SiO
2
TiOSO4minus306 wt ZnO + TiO
2-P25-SiO
2) [54]
fluorescence chromatograms suggesting the production ofnew substances and fluorophores
Despite developed analytical technologies analyzinglignin degradation products remains challenging Proofssuch as mass spectroscopy (MS) HPLC 13C or 1H-NMRspectra from photocatalytic lignin degradations are not yetestablished The setback to qualitatively and quantitativelyanalyze lignin and its degradation products starts from thenative lignin polymer itself with its indefinite polymericstructure and multiple bond types Also the influence ofdifferent pretreatments additives and the wide variety ofcompounds obtainable from its degradation makes ligninanalysis challenging [85] Moreover lignin streams couldcontain proteins inorganic salts and other potential poisonsthat generally complicate catalysis [4]
The challenge to identify and separate the productsstreams derived from the photocatalytic degradation ligninis also worth noting Lignin product stream is highly func-tionalized and conventional techniques such as gas chro-matography have the disadvantage of requiring a time-consuming derivatization step Also because of high boilingpoint of substances arising from lignin degradation it is noteasily applicable High performance liquid chromatography(HPLC) seems to be the remedy because analysis can becarried out without derivatization but the exact identificationof the separated substances is difficult because of the numer-ous peaks arising from such a chromatogram [87] Unfortu-nately well-established databases such as that of the nationalinstitute of standard and technology (NIST) [88] cannot giveinformation onHPLC-MS chromatogramsThis is because ofthe ionization sources such as electrospray ionization (ESI)
14 International Journal of Photoenergy
Table 4 Main advantages and disadvantages photocatalysis versus enzymatic biocatalysis
Lignin degradation process Advantages Disadvantages
Photocatalysis(single stage)
(i) Relatively fast degradation of complex as wellas nonbiodegradable organic (macro)molecules(ii) Stable catalysts(iii) Moderate reaction conditions
(i) Not selective(ii) Limited selection of operating conditions(iii) Energy costs
Enzymatic conversion(single stage)
(i) More selective(ii) Moderate reaction conditions
(i) High enzyme loads required otherwise ligninhydrolysis is quite slow [16](ii) Enzymes are less stable1(iii) cost-intensive (POXs)2
1For example POXs are sensitive to high H2O2 concentrations2According to Torres and Ayala [86]
and atmospheric pressure chemical ionization (APCI) forLC-MS while GC-MS is based on electron ionization Aremedy to this can be the development of a database ofmodel lignin compounds based on a unanimous HPLC-MSmeasuring procedure This would mean much time and costexpensive investments for adequate personnel and material
Though product identification based on peak superpo-sition gives information on a possible product this can-not always be true for lignin degradation because of thelarge number of possible degradation products In order toproperly identify products from lignin degradation productisolation techniques are to be implemented and further ana-lytic methods such as tandem mass spectroscopy (MSMS)and other advanced structure enhancing research techniquessuch as 1H-NMR 13C-NMR have to be studied extensively
4 Photocatalysis and Its Suitability asIntegrated Technology in MultistageConcepts with Biocatalysis
Depending on lignin treatment specification photocatalysiswill find application for complete lignin mineralization toCO2and H
2O (single stage) or as integrated technology in
a multistage system aiming at partial conversion of ligninmacromolecules to organic low molecular weight intermedi-ates for a consecutive biological oxidation finally generatingvalue added end-products [15]
For biological oxidation of lignin or lignin deriva-tives H
2O2-dependent ligninolytic heme peroxidases (POXs
including lignin peroxidase (LiP EC 111114) manganeseperoxidase (MnP EC 111113) versatile peroxidase (VPEC 111116) O
2-dependent laccases (Lac EC 11032) and
extracellular enzymes from Basidiomycetous white-rot fungiare the most efficient lignin degraders in nature [89] Thussuch enzymatic systems especially POXs (with high redoxpotentials) have attractedmuch interest as industrial biocata-lyst [86 90]The enzyme degradationmechanism (in nature)is facilitated by nonenzymatic processes mainly throughfree ∙OH radicals also generated by the fungus (Fenton-type reaction) Those ∙OH radicals enable the requiredphysical contact between the enzymes and structural units ofthe lignin molecule due to numerous nonspecific oxidative
reactions [91] Both the photocatalytic (equations (2)ndash(7))and the POX reactionmechanism recently reviewed by Busseat al [25] are quite similar Consequently a combination ofphotocatalysis followed by an enzymatic oxidation maybe apromising concept utilizing lignin derivatives for exampleoriginated from industrial effluents [15 92] The advantagesforming biobased products are as follows see also Table 4 inthis context A reduction of the lignin polymerization degreevia photocatalysis at best in more biodegradable interme-diates and a simultaneous detoxification causes savings inenzyme costs since it will be expected that less enzyme loadsare necessary for sufficiently rapid reaction rates [15 91] As aresult hydraulic retention times in bioreactor systems shouldbe diminished Moreover the delignification is expectedto be enhanced simultaneously which was in recent yearsshown in a dual system by Kamwilaisak and Wright [93]using TiO
2H2O2UV for photocatalytic pretreatment and
Lac (from Trametes versicolor) in the subsequent biocatalyticstep Within 24 h they obtained in their dual system anelevation in delignification of 20 (without H
2O2) up to at
least 50 when H2O2was present
In previous studies the treatment of lignin (from thepulp and paper industry) containing effluents by fungus asa biological system (excreting appropriate lignin degradingenzyme cocktails) were more focused for posttreatmentexclusively (Duran et al [92] Reyes et al [94] and Gonzalezet al [95]) Shende et al [17] even examined ligninolyticbacteria with photooxidized kraft lignin as substrate
Although POXs are potential industrial biocatalysts [86]no application studies were found for the direct use in suchdual systems as described above The major reason may bethe complexity of the reaction mechanism (inclusive ligninderivatives as substrate and their analysis) per se on the onehand slowing down research and development processes Onthe other hand POXs are sensitive to their cosubstrate H
2O2
once it is supplied in excess causing considerable inactivation(for details refer to Busse et al [25]) At the present severalstudies are carried out modifying these enzymes regardingenhanced stability activity and selectivity as well Henceit can be expected that their technological applicability willbe raised significantly right after successful modification isreached [86]
International Journal of Photoenergy 15
5 Concluding Remarks
It is widely assumed that the photocatalytic degradation oflignin follows a radical reaction pathway which is similar tothat considered in thermal electrochemical and biochemicalprocesses However reporting on the degradation pathway oflignin derivatives and even that of lignin model compoundsare still amajor challengeThis is probably due to the complexnature and variety of possible degradation products Indeedthemechanism is far more complex considering other factorssuch as type of lignin type of catalyst pH illuminationsource and additives
Generally comparing the different photochemical pro-cesses poses a big challenge because of the wide variablesinvolved These discrepancies start from the source and typeof lignin followed by differences in reactor design illumi-nation source intensity of radiation and different types ofcatalyst An ideawould be to have a specific reference reactionwith well-defined starting parameters which include lignintype source and purity catalyst specifications illuminationsource and intensity so as to ease comparison of results
Basic process parameters such as catalyst concentrationsubstrate concentration addition of metal ion to catalyst pHand illumination have been discussed
Despite developed analytical technologies analyzinglignin degradation products remains challenging Proofssuch as mass spectroscopy (MS) HPLC 13C or 1H-NMRspectra from photocatalytic lignin degradations are not yetestablished
In order to properly identify products from lignin degra-dation product isolation techniques are to be implementedand further analytic methods such as MSMS and otheradvanced structure enhancing research techniques such as1H-NMR and 13C-NMR have to be studied extensively Aremedy to this can be the development of a databank ofmodel lignin compounds based on a unanimous HPLC-MSmeasuring procedure This would mean much time and costexpensive investments for adequate personnel and material
Photocatalysis is denoted as the most popular ligninpretreatment technology besides ozonation [96] Photo-catalyzed lignin may be an appropriate substrate for aconsecutive biocatalytic process using ligninolytic enzymes(POX andor Lac) as supported by experimental results ofKamwilaisak and Wright [93] Combining the advantages ofboth catalytic processes savings in the overall process costswill be expected in addition to elevated lignin conversionNonetheless extensive research work including POX modi-fications is still required
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Authorsrsquo Contribution
Colin Awungacha Lekelefac and Nadine Busse contributedequally to this work
Acknowledgments
The authors gratefully thank the Federal Ministry of Educa-tion and Research (BMBF) for funding (FKZ17N0310) Theresearchers also thank the Hessen State Ministry of HigherEducation Research andArts for the financial support withinthe Hessen initiative for scientific and economic excellence(LOEWE)
References
[1] Crude Oil and Commodity Prices httpwwwoil-pricenet[2] Research Directorate-General for World Energy Technology
and Climate Policy Outlook (WETO) Luxembourg Office forOfficial Publications of the European Communities 2003
[3] Green and Natural Polymers Are on the Rise 2014 httpwwwpolymersolutionscombloggreen-and-natural-polymers-on-the-rise
[4] J Zakzeski P C Bruijnincx A L Jongerius and B M Weck-huysen ldquoThe catalytic valorization of lignin for the productionof renewable chemicalsrdquo Chemical Reviews vol 110 no 6 pp3552ndash3599 2010
[5] B KammM Kamm P R Gruber and S KromusBiorefineries-Industrial Processes and Products Status Quo and Future Direc-tions vol 1 Wiley-VCH 2006
[6] R L Howard E Abotsi E L J van Rensburg and S HowardldquoLignocellulose biotechnology issues of bioconversion andenzyme productionrdquoAfrican Journal of Biotechnology vol 2 no12 pp 602ndash619 2003
[7] M Stocker ldquoBio- und BTL-Kraftstoffe in der Bioraffineriekatalytische Umwandlung Lignocellulose-reicher Biomasse mitporosen Stoffenrdquo Angewandte Chemie vol 120 no 48 pp9340ndash9351 2008
[8] J-P Lange ldquoLignocellulose conversion an introduction tochemistry process and economicsrdquo Biofuels Bioproducts andBiorefining vol 1 no 1 pp 39ndash48 2007
[9] D A I Goring ldquoThe physical chemistry of ligninrdquo Pure andApplied Chemistry vol 5 no 1-2 pp 233ndash310 1962
[10] M Ek G Gellerstedt and G HenrikssonWood Chemistry andBiotechnology Pulp and Paper Chemistry and Technology vol 1Walter de Gruyter GmbH amp Co KG Berlin Germany 2009
[11] B Saake and R Lehnen ldquoLigninrdquo in Ullmannrsquos Encyclopedia ofIndustrial Chemistry Wiley-VCH Verlag GmbH amp Co KGaAWeinheim Germany 2012
[12] M N S Kumar A K Mohanty L Erickson and M MisraldquoLignin and its applications with polymersrdquo Journal of BiobasedMaterials and Bioenergy vol 3 no 1 pp 1ndash24 2009
[13] R J A Gosselink E de Jong B Guran and A AbacherlildquoCo-ordination network for ligninmdashstandardisation produc-tion and applications adapted to market requirements(EUROLIGNIN)rdquo Industrial Crops and Products vol 20 no 2pp 121ndash129 2004
[14] D Fengel and G Wegener Wood Chemistry UltrastructureReactions Verlag Kessel 1984
[15] D Mantzavinos and E Psillakis ldquoEnhancement of biodegrad-ability of industrial wastewaters by chemical oxidation pre-treatmentrdquo Journal of Chemical Technology and Biotechnologyvol 79 no 5 pp 431ndash454 2004
16 International Journal of Photoenergy
[16] M Dashtban H Schraft and W Qin ldquoFungal bioconversionof lignocellulosic residues Opportunities amp perspectivesrdquo Inter-national Journal of Biological Sciences vol 5 no 6 pp 578ndash5952009
[17] A Shende R Jaswal D Harder-Heinz A Menan and RShende ldquoIntergrated photocatalytic and microbial degradationof kraft ligninrdquo Cleantech pp 120ndash123 2012
[18] F S Chakar and A J Ragauskas ldquoReview of current and futuresoftwood kraft lignin process chemistryrdquo Industrial Crops andProducts vol 20 no 2 pp 131ndash141 2004
[19] W G Glasser and H R Glasser ldquoEvaluation of ligninrsquoschemical structure by experimental and computer simulationtechniquesrdquo Paperi ja Puu vol 63 pp 71ndash83 1981
[20] M Erickson S Larsson and G E Miksche ldquoZur Struktur desLignins der FichterdquoActaChemica Scandinavica vol 27 pp 903ndash914 1973
[21] H Nimz ldquoDas Lignin der BuchemdashEntwurf eines Konstitution-sschemasrdquo Angewandte ChemiemdashInternational Edition vol 86pp 336ndash344 1974
[22] D V Evtuguin C P Neto J Rocha and J D P de JesusldquoOxidative delignification in the presence of molybdovana-dophosphate heteropolyanions mechanism and kinetic stud-iesrdquo Applied Catalysis A General vol 167 no 1 pp 123ndash1391998
[23] A L Linsebigler G Lu and J T Yates Jr ldquoPhotocatalysis onTiO2surfaces Principles mechanisms and selected resultsrdquo
Chemical Reviews vol 95 no 3 pp 735ndash758 1995[24] Y Miyata K Miyazaki M Miura Y Shimotori M Aoyama
and H Nakatani ldquoSolventless delignification of wood flourwith TiO
2poly(ethylene oxide) photocatalyst systemrdquo Journal
of Polymers and the Environment vol 21 no 1 pp 115ndash121 2013[25] N Busse D Wagner M Kraume and P Czermak ldquoReaction
kinetics of versatile peroxidase for the degradation of lignincompoundsrdquoTheAmerican Journal of Biochemistry andBiotech-nology vol 9 no 4 pp 365ndash394 2013
[26] M Tien and T K Kirk ldquoLignin-degrading enzyme fromphanerochaete chrysosporium Purification characterizationand catalytic properties of a uniqueH
2O2-requiring oxygenaserdquo
Proceedings of the National Academy of Sciences vol 81 pp2280ndash2284 1984
[27] T K Kirk M Tien P J Kersten M D Mozuch and BKalyanaraman ldquoLigninase of Phanerochaete chrysosporiumMechanism of its degradation of the non-phenolic arylglycerol120573-aryl ether substructure of ligninrdquo Biochemical Journal vol236 no 1 pp 279ndash287 1986
[28] T Lundell RWever R Floris et al ldquoLignin peroxidase L3 fromPhlebia radiata pre-steady-state and steady-state studies withveratryl alcohol and a non-phenolic lignin model compound1-(34-dimethoxyphenyl)-2-(2-methoxyphenoxy)propane-13-diolrdquo European Journal of Biochemistry vol 211 no 3 pp391ndash402 1993
[29] H E Schoemaker T K Lundell A I Hatakka and K PiontekldquoThe oxidation of veratryl alcohol dimeric lignin models andlignin by lignin peroxidase the redox cycle revisitedrdquo FEMSMicrobiology Reviews vol 13 no 2-3 pp 321ndash331 1994
[30] J M Palmer P J Harvey and H E Schoemaker ldquoThe role ofperoxidases radical cations and oxygen in the degradation oflignin [and discussion]rdquo Philosophical Transactions of the RoyalSociety of London A vol 321 no 1561 pp 495ndash505 1987
[31] C Hill Wood Modification Chemical Thermal and OtherProcesses John Wiley amp Sons Chichester UK 2006
[32] Y-H Xu H-R Chen Z-X Zeng and B Lei ldquoInvestigation onmechanism of photocatalytic activity enhancement of nanome-ter cerium-doped titaniardquo Applied Surface Science vol 252 no24 pp 8565ndash8570 2006
[33] L Tonucci F Coccia M Bressan and N DrsquoAlessandro ldquoMildphotocatalysed and catalysed green oxidation of lignin auseful pathway to low-molecular-weight derivativesrdquoWaste andBiomass Valorization vol 3 no 2 pp 165ndash174 2012
[34] A Castellan N Colombo C Vanucci P Fornier de HViolet and H Bouas-Laurent ldquoPhotodegradation of ligninA photochemical study of an O-methylated 120572-carbonyl 120573-1lignin model dimer 12-di(3101584041015840-dimethoxyphenyl) ethanone(deoxyveratroin)rdquo Journal of Photochemistry and PhotobiologyA vol 51 pp 451ndash467 1990
[35] A Fujishima X Zhang and D A Tryk ldquoTiO2photocatalysis
and related surface phenomenardquo Surface Science Reports vol63 no 12 pp 515ndash582 2008
[36] C S Turchi and D F Ollis ldquoPhotocatalytic reactor designan example of mass-transfer limitations with an immobilizedcatalystrdquo The Journal of Physical Chemistry vol 92 no 23 pp6852ndash6853 1988
[37] R Subasri M Tripathi K Murugan J Revathi G V N Raoand T N Rao ldquoInvestigations on the photocatalytic activity ofsolndashgel derived plain and Fe3+Nb5+ doped titania coatings onglass substratesrdquo Materials Chemistry and Physics vol 124 pp63ndash68 2010
[38] N Serpone ldquoRelative photonic efficiencies and quantum yieldsin heterogeneous photocatalysisrdquo Journal of Photochemistry andPhotobiology A Chemistry vol 104 no 1ndash3 pp 1ndash12 1997
[39] M R Hoffmann S T Martin W Choi and D W BahnemannldquoEnvironmental applications of semiconductor photocatalysisrdquoChemical Reviews vol 95 no 1 pp 69ndash96 1995
[40] C D Jaeger and A J Bard ldquoSpin trapping and electron spinresonance detection of radical intermediates in the photode-composition of water at titanium dioxide particulate systemsrdquoThe Journal of Physical Chemistry vol 83 no 24 pp 3146ndash31521979
[41] R W Matthews ldquoHydroxylation reactions induced by near-ultraviolet photolysis of aqueous titaniumdioxide suspensionsrdquoJournal of the Chemical Society Faraday Transactions vol 80no 2 pp 457ndash471 1984
[42] A E H Machado A M Furuyama S Z Falone R RuggieroD D S Perez and A Castellan ldquoPhotocatalytic degradation oflignin and lignin models using titanium dioxide the role of thehydroxyl radicalrdquoChemosphere vol 40 no 1 pp 115ndash124 2000
[43] W Sigg and L Stumm Aquatische Chemie Stuttgart 2 AuflageBG Teubner Stuttgart Germany 1991
[44] O Legrini E Oliveros and A M Braun ldquoPhotochemicalprocesses for water treatmentrdquo Chemical Reviews vol 93 no2 pp 671ndash698 1993
[45] G Rothenberger J Moser M Gratzel N Serpone and DSharma ldquoCharge carrier trapping and recombination dynamicsin small semiconductor particlesrdquo Journal of the AmericanChemical Society vol 107 no 26 pp 8054ndash8059 1985
[46] P Mazellier M Sarakha A Rossi and M Bolte ldquoThe aqueousphotochemistry of 26-dimethylphenol Evidence for the frag-mentation of the 120572 C-C bondrdquo Journal of Photochemistry andPhotobiology A vol 115 no 2 pp 117ndash121 1998
[47] M Ksibi S B Amor S Cherif E Elaloui A Houas and MElaloui ldquoPhotodegradation of lignin from black liquor using aUVTiO
2systemrdquo Journal of Photochemistry and Photobiology
A vol 154 no 2-3 pp 211ndash218 2003
International Journal of Photoenergy 17
[48] S K Kansal M Singh and D Sud ldquoStudies on TiO2ZnO
photocatalysed degradation of ligninrdquo Journal of HazardousMaterials vol 153 no 1-2 pp 412ndash417 2008
[49] A Dahm and L A Lucia ldquoTitanium dioxide catalyzed pho-todegradation of lignin in industrial effluentsrdquo Industrial andEngineering Chemistry Research vol 43 no 25 pp 7996ndash80002004
[50] E Portjanskaja and S Preis ldquoAqueous photocatalytic oxidationof lignin the influence of mineral admixturesrdquo InternationalJournal of Photoenergy vol 2007Article ID76730 7 pages 2007
[51] K Tanaka R C R Calanag and T Hisanaga ldquoPhotocatalyzeddegradation of lignin on TiO
2rdquo Journal of Molecular Catalysis
A Chemical vol 138 no 2-3 pp 287ndash294 1999[52] M Tian J Wen D MacDonald R M Asmussen and A Chen
ldquoA novel approach for lignin modification and degradationrdquoElectrochemistry Communications vol 12 no 4 pp 527ndash5302010
[53] K PanM Tian Z-H Jiang B Kjartanson and A Chen ldquoElec-trochemical oxidation of lignin at lead dioxide nanoparticlesphotoelectrodeposited on TiO
2nanotube arraysrdquo Electrochim-
ica Acta vol 60 pp 147ndash153 2012[54] C Awungacha Lekelefac J Hild P Czermak and M Herren-
bauer ldquoPhotocatalytic active coatings for lignin degradationin a continuous packed bed reactorrdquo International Journal ofPhotoenergy vol 2014 Article ID 502326 10 pages 2014
[55] C Awungacha Lekelefac P Czermak and M HerrenbauerldquoEvaluation of photocatalytic active coatings on sintered glasstubes by methylene bluerdquo International Journal of Photoenergyvol 2013 Article ID 614567 9 pages 2013
[56] Y-S Ma C-N Chang Y-P Chiang H-F Sung and A CChao ldquoPhotocatalytic degradation of lignin using PtTiO
2as
the catalystrdquo Chemosphere vol 71 no 5 pp 998ndash1004 2008[57] H Ohnishi M Matsumura H Tsubomura and M Iwasaki
ldquoBleaching of lignin solution by a photocatalyzed reactionon semiconductor photocatalystsrdquo Industrial and EngineeringChemistry Research vol 28 no 6 pp 719ndash724 1989
[58] C A K Gouvea F Wypych S G Moraes N Duran and PPeralta-Zamora ldquoSemiconductor-assisted photodegradation oflignin dye and kraft effluent by Ag-doped ZnOrdquo Chemospherevol 40 no 4 pp 427ndash432 2000
[59] A V Vahatalo K Salonen M Salkinoja-Salonen and AHatakka ldquoPhotochemical mineralization of synthetic lignin inlake water indicates enhanced turnover of aromatic organicmatter under solar radiationrdquo Biodegradation vol 10 no 6 pp415ndash420 1999
[60] H de Lasa B Serrano and M Salaices Photocatalytic ReactionEngineering Springer New York NY USA 2005
[61] K Hashimoto H Irie and A Fujishima ldquoTiO 2 photocatalysisA historical overview and future prospectsrdquo Japanese Journal ofApplied Physics vol 44 no 12 pp 8269ndash8285 2005
[62] M A Behnajady N Modirshahla and R Hamzavi ldquoKineticstudy on photocatalytic degradation of CI Acid Yellow 23 byZnOphotocatalystrdquo Journal ofHazardousMaterials vol 133 no1ndash3 pp 226ndash232 2006
[63] M Addamo V Augugliaro A di Paola et al ldquoPhotocatalyticthin films of TiO
2formed by a sol-gel process using titanium
tetraisopropoxide as the precursorrdquo Thin Solid Films vol 516no 12 pp 3802ndash3807 2008
[64] N Negishi K Takeuchi and T Ibusuki ldquoPreparation of theTiO2thin filmphotocatalyst by the dip-coating processrdquo Journal
of Sol-Gel Science and Technology vol 13 no 1ndash3 pp 691ndash6941998
[65] L Rideh A Wehrer D Ronze and A Zoulalian ldquoPhotocat-alytic degradation of 2-chlorophenol in TiO
2aqueous suspen-
sion modeling of reaction raterdquo Industrial and EngineeringChemistry Research vol 36 no 11 pp 4712ndash4718 1997
[66] R-ADoongC-HChen RAMaithreepala and S-MChangldquoThe influence of pHand cadmiumsulfide on the photocatalyticdegradation of 2-chlorophenol in titanium dioxide suspen-sionsrdquoWater Research vol 35 no 12 pp 2873ndash2880 2001
[67] K V Sarkanen A Islam and C D Anderson ldquoOzonationrdquo inMethods in Lignin Chemistry S Y Lin and C W Dence EdsSpringer Series inWood Science pp 387ndash406 Springer BerlinGermany 1992
[68] G Gellerstedt and E-L Lindfors ldquoStructural changes in ligninduring kraft pulpingrdquo Holzforschung vol 38 no 3 pp 151ndash1581984
[69] B Neppolian H C Choi M V Shankar B Arabindoo andV Murugesan in Proceedings of the International Symposiumon Environmental Pollution Control and Waste Management(EPCOWM 02) p 647 2002
[70] C Pouteau P Dole B Cathala L Averous and N BoquillonldquoAntioxidant properties of lignin in polypropylenerdquo PolymerDegradation and Stability vol 81 no 1 pp 9ndash18 2003
[71] J Hafren T Fujino and T Itoh ldquoChanges in cell wall archi-tecture of differentiating tracheids of Pinus thunbergii duringlignificationrdquo Plant and Cell Physiology vol 40 no 5 pp 532ndash541 1999
[72] T Dizhbite G Telysheva V Jurkjane and U Viesturs ldquoChar-acterization of the radical scavenging activity of lignins naturalantioxidantsrdquo Bioresource Technology vol 95 no 3 pp 309ndash3172004
[73] K Hofstadler R Bauer S Novalic and S G Heisier ldquoNew reac-tor design for photocatalytic wastewater treatment with TiO
2
immobilized on fused-silica glass fibers photomineralization of4-chlorophenolrdquo Environmental Science amp Technology vol 28no 4 pp 670ndash674 1994
[74] J Villasenor and H D Mansilla ldquoEffect of temperature onkraft black liquor degradation by ZnO-photoassisted catalysisrdquoJournal of Photochemistry and Photobiology A Chemistry vol93 no 2-3 pp 205ndash209 1996
[75] B H Bielski D E Cabelli L A Ravindra and A B RossldquoReactivity of HO
2Ominus2Radicals in aqueous solutionrdquo Journal
of Physical Chemistry vol 14 pp 1041ndash1100 1985[76] B H J Bielski and A O Allen ldquoMechanism of the dispropor-
tionation of superoxide radicalsrdquo Journal of Physical Chemistryvol 81 no 11 pp 1048ndash1050 1977
[77] B Halliwell and J M C Gutteridge ldquoThe importance of freeradicals and catalytic metal ions in human diseasesrdquoMolecularAspects of Medicine vol 8 no 2 pp 89ndash193 1985
[78] J M Palmer P J Harvey and H E Schoemaker ldquoThe role ofperoxidases radical cations and oxygen in the degradation oflignin [and discussion]rdquo Philosophical Transaction of the RoyalSociety A vol 321 no 1561 pp 495ndash505 1987
[79] S Y Yin and C W Dense Methods in Lignin ChemistrySpringer New York NY USA 1992
[80] P Kumar S Kumar and N K Bhardwaj ldquoPhotocatalyticoxidation of elemental chlorine free bleaching effluent withUVTiO
2rdquo in Proceedings of the 2nd International Conference on
Environmental Science and Technology (ICEST rsquo11) SingaporeFebruary 2011
[81] K Kobayakawa Y Sato S Nakamura and A FujishimaldquoPhotodecomposition of Kraft lignin catalyzed by titanium
18 International Journal of Photoenergy
dioxiderdquo Bulletin of the Chemical Society of Japan vol 62 no11 pp 3433ndash3436 1989
[82] R B Kinstre ldquoAn overview of strategies for reducing theenvironmental impact of bleach-plant effluentsrdquo Tappi Journalvol 76 no 5 pp 105ndash113 1993
[83] A Castellan H Choudhury R Stephen Davidson and SGrelier ldquoComparative study of stone-ground wood pulp andnative wood 3 Application of fluorescence spectroscopy to astudy of the weathering of stone-ground pulp and native woodrdquoJournal of Photochemistry and Photobiology A Chemistry vol81 no 2 pp 123ndash130 1994
[84] B Albinsson S Li K Lundquist and R Stomberg ldquoThe originof lignin fluorescencerdquo Journal of Molecular Structure vol 508no 1ndash3 pp 19ndash27 1999
[85] S Baumberger A Abaecherli M Fasching et al ldquoMolar massdetermination of lignins by size-exclusion chromatographytowards standardisation of the methodrdquo Holzforschung vol 61no 4 pp 459ndash468 2007
[86] E Torres andM Ayala Biocatalysis Based on Heme PeroxidasesSpringer New York NY USA 1st edition 2010
[87] R Pecina P Burtscher G Bonn and O Bobleter ldquoGC-MSand HPLC analyses of lignin degradation products in biomasshydrolyzatesrdquo Fresenius Zeitschrift fur Analytische Chemie vol325 no 5 pp 461ndash465 1986
[88] NIST NIST Standard Reference Database The National Insti-tute of Standards and Technology 2014 httpwwwnistgovsrdnist1acfm
[89] T K Kirk and R L Farrell ldquoEnzymatic ldquocombustionrdquo themicrobial degradation of ligninrdquo Annual Review of Microbiol-ogy vol 41 pp 465ndash501 1987
[90] A TMartinez ldquoHigh redox potential peroxidasesrdquo in IndustrialEnzymes Structure Function and Applications K-B BeckerEd pp 477ndash488 Springer Amsterdam The Netherlands 1stedition 2007
[91] M Dashtban H Schraft T A Syed and W Qin ldquoFungalbiodegradation and enzymatic modification of ligninrdquo Interna-tional Journal of Biochemistry and Molecular Biology vol 1 no1 pp 36ndash50 2010
[92] N Duran E Esposito L H Innocentini-Mei and V P CanhosldquoA new alternative process for Kraft E1 effluent treatmentrdquoBiodegradation vol 5 no 1 pp 13ndash19 1994
[93] K Kamwilaisak and P C Wright ldquoInvestigating laccase andtitanium dioxide for lignin degradationrdquo Energy and Fuels vol26 no 4 pp 2400ndash2406 2012
[94] J Reyes M Dezotti H Mansilla J Villasenor E Espositoand N Duran ldquoBiomass photochemistry-XXII combined pho-tochemical and biological process for treatment of Kraft E1effluentrdquoApplied Catalysis B Environmental vol 15 no 3-4 pp211ndash219 1998
[95] L F Gonzalez V Sarria and O F Sanchez ldquoDegradationof chlorophenols by sequential biological-advanced oxidativeprocess using Trametespubescens and TiO
2UVrdquo Bioresource
Technology vol 101 no 10 pp 3493ndash3499 2010[96] J H Lora and W G Glasser ldquoRecent industrial applications
of lignin a sustainable alternative to nonrenewable materialsrdquoJournal of Polymers and the Environment vol 10 no 1-2 pp 39ndash48 2002
Submit your manuscripts athttpwwwhindawicom
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Inorganic ChemistryInternational Journal of
Hindawi Publishing Corporation httpwwwhindawicom Volume 2014
International Journal ofPhotoenergy
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Carbohydrate Chemistry
International Journal of
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Journal of
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Bioinorganic Chemistry and ApplicationsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
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Chromatography Research International
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Organic Chemistry International
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CatalystsJournal of
4 International Journal of Photoenergy
Spirodienone
O
OO
O
O
OHO
OO
O
O
O
O
OHO
O
O
O
O
O
O
OO O
O
O
O
O
O
O
O
OH
OO
OO
O
O
OO
p-Coumarylalcohol fragment
Coniferylalcohol fragment
Branching caused bydibenzodioxin linkage Phenylcoumaran
HO
OAr
HO
OH
OHOH
OH
HO
HO
OH
HO
OH
HO
OH
HO
OH
HO
OH
HO
OH
HO
OH
HO
OH
HO4-O-5
OH
HOOH
120573-120573
5-5998400
120573-O-4
120573-1
120573-1
O
Figure 3 Structure of a softwood lignin fragment showing the prominent linkage types reprinted with permission from Zakzeski et al [4]and Evtuguin et al [22]
O2∙minus + H+ HO2
∙
2HO2∙
HO∙ + OHminus
∙OH
S + HO∙ S∙+ + OHminus
S∙+ + O2SO2
∙+
O2∙minus
∙O2minus
∙OH (hydroxyl radical)
(a) Reduction of O2H2O2
O2 + eminus
H2O2 + O2
H2O2 + eminus
(b) Oxidation of substrate
h+ + OHminus
Electron capture
Recombination
eminus
h+
Conduction band
Valence band
Electron release
eminus
O2
(super oxide)
Light
OHminus
Figure 4 Photocatalysis principle adapted from Linsebigler et al [23]
of UV radiation [38 39] Figure 4 shows the excitation ofan electron from the valence band to the conduction bandinitiated by light absorption with energy equal to or greaterthan the band gap of the semiconductor This is expressed by(1)
Upon excitation the fate of the separated electron andhole can follow several pathways Electron or holes canthen react with hydroxyl ions (OHminus) or H
2O producing
hydroxyl radicals (∙OH) as shown in (2) Jaeger and Bard[40] Matthews [41] andMachado et al [42] report that ∙OHis the main oxidizing agent in the photocatalytic oxidationbecause of the unpaired electrons Therefore it can react fastand unspecifically with almost all organic compounds (S)
[43] abstracting an electron with the formation of a radicalorganic species as shown in (3) [44]
Formation of singlet oxygen hydroxyl and superoxideradicals as principal reactive species in a photocatalyticprocess [38 39] is as follows
TiO2
ℎV997888997888rarr TiO
2(
eminusCBh+VB) (1)
TiO2(h+) +H
2O 997888rarr TiO
2+HO∙ +H+ (2)
S +HO∙ 997888rarr S∙+ +OHminus (3)
International Journal of Photoenergy 5
MeO MeOMeOMeO
∙OO
O
O ∙
OH
HO+ +
Figure 5 Formation of phenoxyl radicals by intermolecular abstraction of phenolic hydrogen by carbonyl groups
RH
bond cleavage
Vanillin
O
OO
O
OO O
O
O
O
O
O
HO HO
HOHO
HO
HO
HO HO
OH OH
HO
OCH3
OCH3
OCH3
OCH3
OCH3 OCH3
OCH3
OCH3 OCH3
OCH3
h minusH∙OH∙∙
O2TiO2
HminusOH∙
C120572-C120573
∙
Figure 6 Supposed lignin degradation scheme by autoxidation induced by TiO2poly (ethylene oxide) [24]
S∙+ +O2997888rarr SO
2
∙+ (4)
TiO2(h+) + S 997888rarr TiO
2+∙S+ (5)
TiO2(eminus) +O
2997888rarr TiO
2+∙O2
minus (6)
HO∙ + ∙O2
minus
997888rarr OHminus + 1O2
(7)
TiO2(eminus) + TiO
2(h+) 997888rarr heat (8)
The organic radicals and radical cations can for examplereact with molecular oxygen to form organic peroxy radicalsand peroxy radical cations respectively (4) The holes canoxidize organic compounds by electron abstraction to formorganic cationic radicals (5) [42] Superoxides can be formedby the reaction of electrons with electron acceptors suchas O2(6) Meanwhile the formation of singlet oxygen can
be from the reaction of hydroxyl radical and superoxide(7) [42] Moreover there is a possibility that electrons andholes recombine if electron acceptors are limited In thiscase recombination can take place in the volume of thesemiconductor particle When recombination takes placeradiation energy is lost or converted into heat (8) [45]
From investigations caried out by Mazellier et al [46](photochemistry of 26-dimethylphenol) it was postulatedthat hydrogen can be abstracted by 120572-carbonyl groups In thesame context lignin derivatives having similar functionalitycan follow a similar pathway In addition oxidative chainreactions with the participation of ground-state oxygencan be initiated leading to fragmentation and combinationreactions and thus the formation of new dimers or oligomers(Figure 5)
Miyata et al [24] proposed a cleavage mechanism forthe C120572ndashC120573 bonds which leads to the formation of smallfragments such as vanillin as shown in Figure 6
Figure 7 [25] illustrates the formation of a radical cationformed as a result of enzyme (lipase) mediated reactionof adlerol Adlerol is characterized by a C120573ndashOndash4 bondand considered to be a lignin model compound With theformation of the radical species subsequent nonenzymaticreactions such as radical reactions can take place generatinga wide variety of products and complex compounds
It is widely assumed that the photocatalytic degradationof lignin follows a radical reaction pathwaywhich is similar tothat considered in thermal electrochemical and biochemical
6 International Journal of Photoenergy
Nonenzymatic subsequent reactions
Adlerol
Veratryl alcohol radical
Enzyme1
23
4
4
1
2
3
5
5
6
6
Guaiacol
Veratraldehyde
Ketone
O
OO
O
O O
O
O
O
HO
HOHO
HO
OCH3
OCH3
OCH3
OCH3
OCH3
OCH3
120572120573
120574
minusH+
∙+
∙
∙+
c120572 oxidation H2O
O2
120572 120573
120574
HOHOHO
OCH3
OCH3OCH3
OCH3 OCH3
OCH3
OCH3
120572120572
120573 120573
120573
120574 120574
HO
HO
120574
OCH3
120573
HO
HO
120574
HO
OCH3
OCH3
OCH3
120572120573
120574
Figure 7 Proposed radical reaction scheme initiated by enzyme lignolytic heme peroxidase for the conversion of adlerol possessing C120573ndashOndash4 bonds into smaller units summarized by Busse et al [25] and abstracted from Tien and Kirk [26] Kirk et al [27] Lundell et al [28]Schoemaker et al [29] and Palmer et al [30]
processes However reporting on the degradation pathway oflignin derivatives and even that of lignin model compoundsis still a major challenge This is probably due to the complexnature and variety of possible degradation products Indeedthemechanism is far more complex considering other factorssuch as type of lignin type of catalyst pH illuminationsource and additives
32 Influence of Process Parameters in Lignin DegradationVarying process parameter could either have a positive ornegative impact on the photocatalytic efficiency The basicprocess parameters such as catalyst concentration [4 4856] substrate concentration [48 51] addition of metal ion
to TiO2catalyst [17 50 56ndash58] pH [47 48] illumination
[33 48 49 59] and their influence shall be discussed in thissubchapter Table 2 gives an overview about starting reactionconditions and catalyst applied by some work groups whileTable 3 portrays parameters analytical methods and resultsobtained It is worthwhile noting that comparing the differentphotochemical processes poses a big challenge because ofthe wide variables involvedThese discrepancies start alreadyfrom the source and type of lignin followed by the differencesin reactor design illumination source intensity of radiationand different types of TiO
2catalyst such as Fischer scientific
rutile TiO2[49] TiO
2(TiO2mdashTP-2 of Fujititan) just to name
a few
International Journal of Photoenergy 7
Table2Summaryof
startingcond
ition
sfor
thep
hotocatalytic
degradationof
lignin
Reference
Lign
insource
Catalyst
Reactio
ncond
ition
s
MacHadoetal[42]
Peroxyform
icacid
lignins
from
eucalyptus
grandisw
ood(EL1)
EL1+
sodium
borohydride
TiO
2andH
2O2
Lign
inconcentration
025
mgmL119879=25∘C
pH11U
VVisradiatio
n120582gt300nm
40
0Wmercury
lampcylin
dricalPy
rexglassreactorcon
stanto
xygenbu
bblin
g
Ksibietal[47]
Water
solubleligninob
tained
from
blackliq
uor
TiO
2-P2
5(D
egussa)
Lign
inconcentration
90mgL119879=20∘C
pH82UV-radiation120582gt290nm
pyrex
reactoro
pento
airPh
ilips
HPK
125W
lamp
Kansaletal[48]
Lign
infro
mwheatstr
awkraft
digestion
TiO
2-P2
5(D
egussa)a
ndZn
O
Lign
inconcentration
10ndash100
mgLin
100m
LZn
Ocatalystdo
se(05ndash20gL)pH
ofthes
olution(pH3ndash11)solarillu
mination
oxidantcon
centratio
n306times10minus6M
to153times10minus6Mthinbedfilm
slurrypo
ndreactoroxidantsodium
hypo
chlorite
solutio
n(4availableC
l 2)
Dahm
andLu
cia[
49]
Whitewater
from
indu
strialprocess
water
thatexits
inpaperm
achines
FischerS
cientifi
crutile
TiO
2
Lign
inconcentration
40mgLin
500m
Lbatchreactor119879=21∘Cndash
42∘C
Rayonet
photochemicalcham
ber16
VWR8-W
blacklightpho
spho
r(350-nm
)lam
ps
constant
oxygen
bubb
lingpo
wer
ofillum
ination
128to
64Wlight
intensity
223ndash44
5mWcm
3
Portjanskajaand
Preis[50]
Lign
inpurchased
from
Aldric
hTiO
2P2
5-N(D
egussa)
Lign
inconcentration
100m
gLpH
8batchreactorsystemreactor
open
toair
PhillipsT
LD15W05low-pressurelum
inescent
mercury
UV-lampUV-radiation
120582gt360nm
pow
erdensity
ofirr
adiatio
n=07m
Wcm
2 visib
lelight
source
Tanaka
etal[51]
Lign
infro
mconiferous
woo
dTiO
2(TiO
2mdashTP
-2of
Fujititan)
Lign
inconcentration
0003to
003U
V-radiation120582gt310nm
cylindrical
reactio
nvessel
Tonu
ccietal[33]
Ca2+andNH4
+ligninderiv
atives
TiO
2as
Degussa
P25
+po
lyoxom
etalates
(POM)H
2O2
Openqu
artztubes(20
mL)
reactor119879=20∘C
1atm
multirayso
ften
UVlamps
of15W
power
eachU
V-radiation120582gt254nm
Miyatae
tal[24]
Piceag
lehn
iiwoo
dflo
urTiO
2po
lyethylene
oxide(PE
O)
Reactorop
enpetrid
ish119879=30∘C119905=48h40
0Wmercury
lamp
Shende
etal[17]
Kraft
lignin
TiO
2-Zn
O-ZrO
2Lign
inconcentration
1mgmLin
10mL250W
Xeno
nlampandAM
15Glamp
filterpo
wer
density
ofirr
adiatio
n100m
Wcm
2 solarlam
psim
ulator
Tian
etal[52]
andPanetal[53]
Kraft
ligninfro
mblackliq
uor
Ta2O
5-IrO
2andPb
O2thin
film
TiO
2nano
tubePbO
2
Lign
inconcentration
30(w
w)UV-radiation120582gt365nm
119905=10minpow
erdensity
ofirr
adiatio
n20
mWcm
2 blue
waveT
M50
ASUVspot
lampEG
ampG2273
potentiosta
tgalvano
stattoapplycurrentTiTiO
2NTPb
O2ele
ctrode
asworking
electrode
andPt
coilas
outere
lectrode
andAgAgC
lasreference
electrode
Awun
gachaL
ekele
fac
etal[54][55]
Lign
insulfo
natefro
mpaperw
aste
water
TiO
2as
Degussa
P25
TiO
2fro
msol-g
elprocesso
fTiOSO
4TT
IP
Lign
inconcentration
500m
gLin
200m
LOsram
Planon
light
sourceirradiance
30ndash4
0Wm
2 UV-radiation120582280ndash4
20nm
119905=20hreactoro
pento
air
recirculationsyste
mflow
rate225mLmin
8 International Journal of Photoenergy
Table3Parametersanalyticalmetho
dsand
results
from
different
workgrou
ps
Reference
Parameter
studied
Analytic
sRe
sult
MacHado
etal[42]
Roleof
hydroxylradicals
irradiatio
nof
ligninin
the
absencea
ndpresence
ofph
otocatalystT
iO2andH
2O2
Ultraviolet-v
isible(UV-Vis)spectro
scop
yionizatio
nabsorptio
nspectro
scop
y(IA
S)
sizee
xclusio
nchromatograph
y(SEC
)
Asharpdecrease
inthep
heno
liccontento
bservedforreactions
involvingdirect
photolysis
SEC
areductio
nof
almost50
inthea
verage
molecular
weighto
fligninequalto
14kD
after
90min
ofirr
adiatio
n
Ksibietal[47]
Irradiationof
ligninin
the
absencea
ndpresence
ofph
otocatalystT
iO2-P2
5
UV-Visspectroscop
y13C-
nucle
armagnetic
resonance(NMR)
solid
state
totalion
gasc
hrom
atograph
y(TIC)
indu
ctioncoup
lingplasma(
ICP)
chem
icaloxygen
demand(C
OD)
56degradationratewith
TiO
2catalystaft
er420m
inreactiontim
eethylacetate-extractableprod
uctsshow
edvanillin
vanillica
cidpalm
itica
cid
biph
enylstructuresand
345-trim
etho
xybenzaldehyde
presence
ofmagnesiu
mandcalcium
ions
CODremovalishigh
erforthe
initiallylowconcentrations
ofligninsolutio
n
Kansaletal[48]
Catalystdo
sepHoxidant
concentration
initialsubstrate
concentration
ZnOcatalystin
slurryandim
mob
ilizedmod
e
UV-Visspectroscop
yCO
D
Optim
umcatalystdo
seis1gL
optim
umoxidantcon
centratio
n2times10minus6M
graduald
ecreaseo
fabsorptionpeak
indicatin
gdecompo
sitionof
organics
CODremovalishigh
erforthe
initiallylowconcentrations
ofligninsolutio
n
Dahm
and
Lucia[
49]
Catalystdo
seillu
mination
intensity
UV-Visspectroscop
ytotalorganiccarbon
(TOC)
capillaryionelectro
phoresis
analysis(C
IA)
Gradu
aldecrease
inabsorptio
npeak
indicatin
gdecompo
sitionof
organics
optim
alcatalystdo
seof
10mgm
high
erillum
inationintensities
correlated
well
with
high
erinitialdegradationrate
74disapp
earanceo
fTOC
Portjanskajaand
Preis[50]
Influ
ence
offerrou
sion
sN-dop
edcatalysteffect
sprayedcatalyston
supp
ortand
subm
ersedcatalyst
CODU
V-Visspectroscop
ybiochemical
oxygen
demand(BOD)colorim
etric
measurementat570
nm
Additio
nof
Fe2+upto
28m
gLleadsto25increase
inph
otocatalyticeffi
ciency
sprayedcatalystexhibited15
times
high
ereffi
ciency
than
theo
neattached
bysubm
ersio
nnegligibleeffecto
fN-dop
edcatalyst
increase
ofaldehyde
concentrationover
reactio
ntim
eneutralm
ediawas
mostb
eneficialforb
iodegradability
80of
freep
heno
lsremoved
undern
eutralcond
ition
s
Tanaka
etal[51]Ca
talystloadinglignin
concentration
illum
inationtim
e
UV-Visspectroscop
yTO
Cgel
perm
eatio
nchromatograph
y(G
PC)
1 HNMR
Fouriertransform
ationinfrared
(FTIR)
spectro
scop
y
FTIR
measurementrevealedafasttransform
ationof
arom
aticmoietyp
resent
inlignin
characteris
ticband
sofaromaticrin
gsm
etho
xyand
aliphatic
sidec
hains
decrease
inTO
Cvalues
over
time
decrease
indegradationratewith
increase
catalystdo
sageB
utaft
ercatalystthreshold
valueisa
ttainedcatalystincreasec
ausesa
decrease
indegradationrates
FTIR
peaksa
reshifted
towards
lower
molecular
weightregionaft
erph
otocatalysis
Tonu
cci
etal[33]
Testof
catalytic
syste
mstoob
tain
fractio
nswith
redu
ceddegreeso
fpo
lymerization
comparis
onof
thermalandph
otochemical
reactio
ns
1 HNMR
gasc
hrom
atograph
y-mass
spectro
scop
y(G
C-MS)
POMsa
relessselectivew
henused
asph
otocatalystsandno
appreciableb
leaching
ofthes
olutionwas
seen
whenPO
Mwas
used
asthermalcatalyst
deriv
edchem
icalsfrom
experim
entvanillin
hydroxylmetho
xy-acetoph
enon
econiferylalcoh
olcon
iferylaldehydemethano
lform
icacidacetic
acidand
sometim
essm
allamou
ntso
fC-2
andC-
3alcoho
ls
Miyatae
tal[24]
Exam
inationof
cellwall
structureo
flignin(w
oodflo
ur)
before
andaft
erph
otocatalysis
GC-
MSscanning
electronmicroscop
y(SEM
)1 H
NMR
Highdelignificationactiv
itydelignificationconfi
nedto
thes
urface
oflignin
deriv
edchem
icalse
xperim
entvanillin
Shende
etal[17]Testof
combinedactio
nof
bio-
andph
otocatalyticsyste
ms
UV-Visspectroscop
yGC-
MSX-
ray
dispersiv
eenergyspectro
scop
yandX-
ray
diffractio
nanalysis(EDX
XRD)SE
M
Detectio
nof
follo
wingchem
icalsacetylguaiacol4-ethoxym
ethyl-2
-metho
xyph
enol
metho
xyph
enyloxim
eguaiacolsuccinica
cidacetylguaiacolvanillicacidand
vanillin
International Journal of Photoenergy 9
Table3Con
tinued
Reference
Parameter
studied
Analytic
sRe
sult
Tian
etal[52]
andPanetal
[53]
Testof
combinedactio
nof
electro-and
photocatalytic
syste
ms
UV-Visspectroscop
yFT
IRSEM
X-ray
(EDX)
highperfo
rmance
liquid
chromatograph
y(H
PLC)
COD
Detectio
nof
thefollowingchem
icalscarbon
ylfunctio
nalityvanillin
andvanillica
cid
Awun
gacha
Lekeleface
tal
[54][55]
Com
paris
onof
degradationrates
bydifferent
catalyst
HPL
Cflu
orescencea
ndUV-Vis
spectro
scop
ySE
MT
OC
UV-Visresultsrevealfaste
rdegradatio
nof
thea
liphatic
moietycomparedto
the
arom
aticmoietyof
ligninsulfonateob
tained
from
paperw
astewaterPeaks
observed
durin
gHPL
Canalysis
Someo
fthe
peaksp
rodu
cedaft
erph
otocatalysishad
fluorescences
ignals
Thissuggeststhep
rodu
ctionof
newsubstances
andflu
orop
hores
Coatin
gsprod
uced
throug
hSol-G
elprocedures
arestablea
ndcanbe
used
manytim
es
10 International Journal of Photoenergy
321 Influence of Catalyst One aspect to successfully imple-ment photocatalysis is the choice of an appropriate photo-catalyst The majority of catalysts used are based on TiO
2as
summarized in Table 2 ZnO has also been applied either assingle catalyst [48] or in a combination with other catalystsuch as TiO
2[54] de Lasa et al [60] described ZnO as
being less active than TiO2 They add that the use of ZnO
is particularly relevant when the oxidative degradation ratebecomes limited This is opposite to what Kansal et al [48]reported by saying that ZnO is more reactive than TiO
2
However TiO2(mainly anatase) remains the most used
catalyst as can be depicted fromTable 2 TiO2is reported to be
favored because of its nontoxic property cost efficiency andchemical and biological inertness Moreover TiO
2possesses
the most efficient photoactivity and the highest stability thusmaking it suitable for industrial use [61]
ZnO has been described to degrade lignin under visiblelight sources [48 62] whereas TiO
2is mostly applied in
connection to UV-light sources as highlighted in Table 2However both ZnO and TiO
2possess energy band gap
energy of 32 eV [23]Additives such as SiO
2[63] polyethylene oxide (PEO)
[24] and polyethylene glycol [64] have been added to TiO2
catalyst particularly when applying immobilized catalystAddamo et al [63] noted a high adhesion of TiO
2to glass
support material when precoating was done with SiO2
Moreover the precoating might have other advantages suchas a hindering diffusion of Na+ ions from the glass materialinto the nascent TiO
2film during heat treatment processes
Analogous to the addition of SiO2 polyethylene glycol
(PEG) has also been introduced to mitigate catalyst surfaceactivity modify surface hydrophobicity and also reduceagglomeration tendency of the TiO
2gel or TiO
2particles
in the suspensions [64] By a proper surface modificationinteraction between catalyst and substrate can be enhanced[55 63]
Kansal et al [48] varied catalyst (ZnO) dose from 05 gLto 20 gL for 01 gL kraft lignin solutions and found out thatthere was an optimum catalyst threshold value at 1 gL whichgives a catalyst to substrate ratio of 1 10
Dahm and Lucia [49] examined catalyst dose from 2 sdot10minus3 gL to 12sdot10minus2 gL for lignin solutions (fromwhite water
liner mill) of 4 sdot 10minus2 gL (catalyst to lignin ratio 5 sdot 10minus3ndash3 sdot 10minus2) at pH 8 and obtained best energy efficiency valuesand lignin degradation rates with a catalyst loading of 10 sdot10minus2 gLIn contrast Ma et al [56] applied far higher catalyst
concentration compared to Dahm and Lucia [49] Catalystconcentration was varied between 1 gL and 10 gL PtTiO
2
Best catalysis dose with respect to reaction turnover wasobtained at 5 gL PtTiO
2With the increase of catalyst dose at
pH 7 the reaction rate increased from 61 times 10minus3minminus1 (1 gLTiO2) to 71 sdot 10minus3minminus1 (5 gL TiO
2) and 99 sdot 10minus3minminus1
(10 gL TiO2)
Catalyst effect has been explained on the basis thatoptimum catalyst loading is dependent on the initial soluteconcentration An increase in catalyst dosage leads to a cor-responding increase of total active surface area for reactions
[65] To that at higher TiO2concentrations the photon flux is
more easily intercepted by the catalyst before penetrating intothe bulk of the system At the same time due to an increase inturbidity of the suspension with high dose of photocatalystthere is a decrease in penetration of UV light and hencephotoactivated volume of suspension or solution decreases[66]
In summary authors have obtained best catalyst to ligninrelations for different reaction designs and thus a generalrecommendation on catalyst dose is not possible Howeverwhen lignin solution is treated with increasing catalyst loadsa corresponding increase in degradation rate is observed untila threshold value is reached [48ndash50 56]
322 Influence of Metal Ion Addition (Doping) and AdditivesThe purpose of adding metal ion to photocatalyst is to miti-gate band gap energy through the introduction of intrabandgap states and as a consequence produce a bathochromicshift in the absorption spectrum [37] Altering the absorptionspectral range gives the possibility to exploit both the visiblelight spectrum and UV light sources Metal ion doping isalso introduced to serve as electron or hole traps in orderto minimize recombination between generated electron-holepairs [37]
Portjanskaja and Preis [50] studied the addition of Fe2+ions to an acidic lignin solution and found an increasein photocatalytic oxidation (PCO) efficiency The optimumFe2+ ions quantity was 28mgL while using 100mgL ligninsolution Upon further elevation of Fe2+ ions concentrationa corresponding reduction of the photocatalytic oxidationefficiency of lignin was noted Likewise Ohnishi et al [57]made a comparative study by doping platinum (Pt) silver(Ag) and gold (Au) ions to TiO
2 In these reactions 50mg of
catalyst (TiO2) was used with the addition of an equiva1ent
15 wt (based on TiO2) metal ion The addition of noble
metals brought about a faster decolorization of lignin Aushowed better results than Ag followed by Pt In the samecontext adding sodium hypochlorite as oxidant to PtTiO
2
catalyst an additional fivefold degradation rate was observedcompared to that without doping [56] Contradictory to theresults described above negligible effect of photocatalyticefficiency due to doping has been reported as well Awun-gacha Lekelefac et al [54] obtained little or no change indegradation rate by doping TiO
2-P25-SiO
2catalyst with Pt
ions (1 wt relative to TiO2catalyst) Likewise Portjanskaja
and Preis [50] noted a negligible change of photocatalyticefficiency of TiO
2when doped with nitrogen
Sarkanen et al [67] and Gellerstedt and Lindfors [68]reported the bias of peroxides to oxidation with reagentssuch as permanganate to favor aromatic moieties Oxidationagents like permanganate oxidizes predominantly aliphaticchains in alkaline and neutral media However by theapplication of H
2O2(Fenton system) lignin disappeared
completely [33] Tonucci et al [33] conclude that in orderto satisfactorily conserve the organic material the best com-promise appears to be the TiO
2photosystem which shows
low carbon consumption good preservation of the aromaticrings and greatly reduced mineralization
International Journal of Photoenergy 11
In summary different results have been obtained con-cerning the influence of noble metal ion addition Whilesome authors report an improvement in the photocatalyticefficiency upon their addition others report their addition ashaving no considerable influence However for reactions inwhich an improvement in the photocatalytic efficiency wasnoticed there was a threshold value to be considered Whenthe concentration of dopant surpasses this threshold valueelectron-hole recombination is favored and this has a negativeimpact to photocatalysis In such a case the space-chargelayer gets narrower and 119901-type dopants attract electrons andby virtue become negative They would now then act ashole acceptor attracting holes On the other hand 119899-typedopants which act as electron donor centers and possessexcess electrons attract holes as well [37]
323 Influence of Lignin Concentration Once the initiallignin concentration becomes higher exceeding a thresholdvalue an inhibitory effect on the photodegradationwas noted[47ndash49] This threshold value varies and depends on thereaction system and reaction parameters such as opticaldensity catalyst concentration and reaction volume Fromthe literature different authors have implemented varyinglignin concentrations probably to suit their reaction designFor example Ksibi et al [47] use 90mgL and AwungachaLekelefac et al [54] use 500mgL while Kansal et al [48]applied 10mgndash100mgL
Explanations arising from the findings are as followsat low lignin concentrations the incidental photonic fluxirradiated interacts with the catalyst generating radicals (eghydroxyl radicals (OH∙)) which allow a faster degradation[69] On the other hand high initial lignin concentrationsmay lead to tight adsorption which can suppress CO
2
evolution [51] and hence maintain chemical oxygen demand(COD) values Moreover low delignification yields maybe due to an inhibitory effect because of autoxidation bylow molecular weight lignin degradation products formed[24] Also due to the polymer structure of lignin which iscross-linked this makes it difficult for radical species acidand the aldehyde compounds produced to spread into theinner region of the substrate hence limiting autoxidationAs a worst case this might be the rate-determining step ofdelignification which is hindered [70ndash72]
In summary it can be concluded that the time taken forcomplete degradation depends on the initial concentration oflignin and faster degradation occurs at low lignin concentra-tions
324 Influence of pH Varying pH entails an alteration inthe properties of semiconductor-liquid interface [73] mainlyrelated to the acid-base equilibrium of the adsorbed hydroxylgroup [39] Furthermore pH also impacts lignin degradationrates [47 48 57] In this context several studies were carriedout with partly contradictory outcomes
Kansal et al [48] made pH investigations 3ndash11 undersolar light illumination using ZnO as catalyst Maximumdegradation was reached in alkaline conditions (pH 11)This is supported by Villasenor and Mansilla [74] reporting
an almost complete decolorization of kraft black liquorfrom pine wood at pH value of 116 in combination withZnO catalyst Similar results were achieved by Ohnishi etal [57] with TiO
2and ZnO being catalyst for bleaching
alkaline lignin in aqueous solution with TiO2and ZnO being
catalyst High activities at neutral pH were also reportedby Ohnishi et al [57] In contrast Ma et al [56] observedhigher reaction rates and rapid degradation of a syntheticlignin wastewater (prepared by dissolving commercial ligninpowder in aqueous solution pH 11) in acidic solution (pH 3)than in alkaline solutions at pH 11 for either TiO
2or PtTiO
2
catalystsReconsidering the photocatalytic principle the formed
superoxide anion radicals (∙O2
minus) are in a pH-dependentequilibriumwith perhydroxyl radicals (HO
2
∙) as follows [75]
HO2
∙999445999468 H+ + ∙O
2
minus pKa sim 48 (9)
See [76]∙O2
minus undergoes dismutation reaction resulting in H2O2
and O2competing to any other ∙O
2
minus triggered reaction Incase of low pH operation conditions in aqueous solutionsHO2
∙ becomes dominant whose reactivity is considerablyhigher compared to ∙O
2
minus [77] Subsequently HO2
∙ initiatessubstrate (S) oxidation to the radical cation (S+∙) and is itselfreduced to H
2O2[78] Thus increased degradation rates can
be reasonably expected supporting the results made by Ma etal [56] in an acidic environment ∙O
2
minus is extremely reactive inorganic solvents [77] Another aspect is the solubility of kraftlignin (soluble at pH gt 105) which reduces with decreasingpH whereas lignosulfonate should remain unaffected by pHin aqueous solution Moreover 120573ndashOndash4 bonds have beendescribed to be stable at acidic pH [11] In fact this couldadditionally explain the elevated degradation of kraft ligninmade by Kansal et al [48] and Villasenor and Mansilla [74]Nevertheless the contradictory results gained by Ma et al[56] still exist under the assumption that kraft ligninwas used(which would be supported by the high pH of 11 obviouslynecessary for dissolving the lignin powder) Although mostphotocatalytic reactions described in the literature are in anaqueous milieu lignin raw material and its fission productsmay however vary considerably Therefore the optimal pHis most likely to be reaction specific and has to be evaluatedexperimentally in principle
325 Influence of Illumination Many of the studies foundin the literature so far have not dealt on this subject per seWhat is found is the use of different illumination sourceseach having a specified power and lamp type However alltend to emitUV-light between the range 280ndash420 nm Table 2depicts this in detail Other illumination sources include thevisible light spectrum
In general terms illumination influences in that it ini-tiates photocatalysis by generating electron-hole pair in thesemiconductor particles [38 39] Dahm and Lucia [49]altered illumination intensity while observing lignin degra-dation In this study 004 gL lignin was used and lightintensity was varied 223ndash445mWcm3 It was found out thathigher illumination intensities correlated well with higher
12 International Journal of Photoenergy
initial degradation rates and hence total lignin degradation[49] Neppolian et al [69] report degradation to be propor-tional to radiation intensity and best results are achieved forlow lignin concentrations because of enhanced interactionbetween catalyst and incidental photonic flux
In summary high illumination power causes a corre-sponding high initial degradation rate at low lignin concen-trations becausemaximum light penetration into the reactionmedium is favored
33 Process Analytical Methods Various analytical tech-niques have been used to monitor lignin degradation Atthe beginning of this subchapter analytics revealing com-pounds formed from lignin degradation are treated Thisincludes for example gas chromatography (GC) and 1HNMR (nuclear magnetic resonance) This is then followed byresults qualitative analytic measurements such as ultraviolet-visible (UV-Vis) spectroscopy and dissolved carbon (DC)A list of authors analytical techniques applied and resultsachieved are outlined in Table 3
Portjanskaja and Preis [50] studied lignin degradationby measuring the removal of phenols through colorimetricmeasurements As a result of 24 h photocatalytic oxidationunder neutral media conditions 80 of free phenols wereremoved Gas chromatography (GC) result from Ksibi et al[47] attested vanillin vanillic acid palmitic acid biphenyland 345-trimethoxy benzaldehyde structures after the pho-tocatalysis of lignin from black liquor This is in accordancewith the findings of Tonucci et al [33] reporting the formationof vanillin hydroxyl methoxy-acetophenone coniferyl alco-hol coniferyl aldehyde methanol formic acid acetic acidand small amounts of C-2 and C-3 alcohols as degradationproducts1H NMR spectral analysis of lignin before illumination
and after 24 h of illumination showing characteristic bandsof aromatic rings methoxy and aliphatic side chains wascompared with each other Results revealed that the aro-matic ring degraded faster than the aliphatic chain [51]Fourier transformation infrared spectroscopy (FTIR) showedbands corresponding to CH
3 CH2 and CH which remained
unchanged after illumination while bands corresponding toaromatic rings disappeared as a result of illumination [51 53]
Results obtained from the combination of photochemicaland electrochemical oxidation [52 53] were similar to thoseof Tanaka et al [51] Here 13C-NMR confirmed the presenceof the carbonyl functionality and the presence of vanillinand vanillic acid after 12 h photochemical-electrochemicaloxidation These results showed that the combination of aphotocatalytic and an electrochemical oxidation significantlyenhanced the efficiency of lignin degradationThis is becausethe applied anodic potential bias greatly suppressed therecombination of photogenerated electrons and holes [53]
Ultraviolet spectrophotometry offers a convenientmethod to qualitatively and quantitatively analyze ligninin solution [79] This is reflected by the large number ofpublications using this technique [17 33 47ndash51 56 58 80]This is most likely due to its simplicity to interpret lignindegradation Lignins absorb UV light with high molar
200 220 240 260 280 300 320 340 360
00
05
10
15
20
25
Abso
rban
ce
Wavelength (nm)0min
30min
60min
120min
180min
300min
360min
420min
1200min
Figure 8 Time dependent UV-Vis absorption spectra of aqueouslignin solution from waste paper water irradiated with UV light(280ndash420 nm) for different time intervals The spectra are obtainedfor sol-gel derived TiO
2nanocrystalline coating (TiO
2-P25-SiO
2)
[54]
extinction coefficients because of the several methox-ylated phenylpropane units of which they are composed [33]Figure 8 depicts a series of photometric scans of ligninsul-fonate from paper waste water showing a gradual reductionof absorbance during photocatalytic treatment [54] Herethe absorption peaks are around 210 nm and 280 nmAbsorbance decreases with time implying the decompositionof lignin and the deterioration of chromophore groups [54]
Peaks at 210 nmcorrespond to portions of the unsaturatedchains while those at 280 nm correspond to unconjugatedphenolic hydroxyl groups [17] and the aromatic moiety [57]of the lignin molecule The absorption tailing to the longwavelength region arises from the color of lignin [57] Lignindegradation has been reported either at wavelength around280 nm corresponding to unconjugated phenolic hydroxylgroups [17 48 51 58] or for both wavelengths (210 nm and280 nm) [33 54 57] Kobayakawa et al [81] noted some otherabsorbance at wavelengths lower than 250 nm and pointedout that this could be due to the modification of ligninfragmentations leading to the formation of transient specieslikemethanol ethanol formaldehyde formic acid and oxalicacid among others
Analyticalmethods to effectively quantify lignin degrada-tion by calculating the oxygen demand by organic substancesand remaining organic carbon before and after photocatalysishave been studied Amongst the methods are dissolvedcarbon (DC) [49 51 58] chemical oxygen demand (COD)[47 48 50 57 80] biochemical oxygen demand (BOC) [50]dissolved organic carbon (DOC) [56] and American dyemanufacture institute value (ADMI) [56] COD and BODdescribe the oxygen demand by organic substances to be
International Journal of Photoenergy 13
Figure 9 Gradual change from the characteristic yellow ligninsulfonate to a colorless liquid after a period of 20 h Catalyst TiO
2-
P25 (Degussa) +TEOSUV-light 25∘C lignin concentration 05 gL[54]
converted to CO and CO2and H
2O and NH
3 Total organic
carbon (TOC) describes the amount of carbon bound inan organic compound while DOC describes the dissolvedfraction of organic carbon ADMI measures the amount ofdyestuff in water
Decolorization of lignin solution has been reported tobe another parameter observed during photocatalytic degra-dation Color is an indirect indicator of the lignin amountThe higher the color intensity of the solution is the greaterthe lignin content is (high concentrated lignin solutions egblack liquor appear dark brown) [82] Thus color changescan be interpreted as conversion of lignin to transient speciesor conversion to CO
2and H
2O Awungacha Lekelefac et al
[54] observed a gradual change from the characteristic yellowlignin (when highly diluted) to a colorless liquid after a periodof 20 h with sol-gel derived TiO
2nanocrystalline coatings on
sintered borosilicate glass as depicted in Figure 9 A corre-sponding decrease in DC values close to 82 was observedfor TiO
2-P25-SiO
2catalyst confirming degradation This is
shown in Figure 10These findings are analogous to that of Ohnishi et al
[57] who reported the bleaching of lignin when illuminatedcontinuously and that the solution becomes colorless Tothat the chemical oxygen demand (COD) value decreasesgenerating carbon dioxide and a small amount of carbonmonoxide as the main gaseous products COD removalwas reported to be effective at low lignin concentrations ascompared to high lignin concentrations [48]
Another applied analytical technique is fluorescencedetection directly coupled to a high performance liquidchromatography (HPLC) as a means to identify nonaliphaticcomponent in the complex mixture of lignin degradationproducts [54] Fluorescence emission in lignin is attributedto aromatic structures such as conjugated carbonyl biphenylphenylcoumarone and stilbene groups [83 84] AwungachaLekelefac et al [54] observed peaks on both HPLC and
0 200 400 600 800 1000 12000
50
100
150
200
250
Diss
olve
d ca
rbon
(ppm
)
t (min)
TiO2-P25-SiO2
TiO2-P25-SiO2 + PtTiOSO4minus30 6wt
ZnO + TiO2-P25-SiO2
No coating UV light
Figure 10 Variation of DC with time of aqueous lignin solutionfrom waste paper water irradiated with UV light (280ndash420 nm) fordifferent time intervals The spectra are obtained for sol-gel derivedTiO2nanocrystalline coatings (TiO
2-P25-SiO
2+Pt TiO
2-P25-SiO
2
TiOSO4minus306 wt ZnO + TiO
2-P25-SiO
2) [54]
fluorescence chromatograms suggesting the production ofnew substances and fluorophores
Despite developed analytical technologies analyzinglignin degradation products remains challenging Proofssuch as mass spectroscopy (MS) HPLC 13C or 1H-NMRspectra from photocatalytic lignin degradations are not yetestablished The setback to qualitatively and quantitativelyanalyze lignin and its degradation products starts from thenative lignin polymer itself with its indefinite polymericstructure and multiple bond types Also the influence ofdifferent pretreatments additives and the wide variety ofcompounds obtainable from its degradation makes ligninanalysis challenging [85] Moreover lignin streams couldcontain proteins inorganic salts and other potential poisonsthat generally complicate catalysis [4]
The challenge to identify and separate the productsstreams derived from the photocatalytic degradation ligninis also worth noting Lignin product stream is highly func-tionalized and conventional techniques such as gas chro-matography have the disadvantage of requiring a time-consuming derivatization step Also because of high boilingpoint of substances arising from lignin degradation it is noteasily applicable High performance liquid chromatography(HPLC) seems to be the remedy because analysis can becarried out without derivatization but the exact identificationof the separated substances is difficult because of the numer-ous peaks arising from such a chromatogram [87] Unfortu-nately well-established databases such as that of the nationalinstitute of standard and technology (NIST) [88] cannot giveinformation onHPLC-MS chromatogramsThis is because ofthe ionization sources such as electrospray ionization (ESI)
14 International Journal of Photoenergy
Table 4 Main advantages and disadvantages photocatalysis versus enzymatic biocatalysis
Lignin degradation process Advantages Disadvantages
Photocatalysis(single stage)
(i) Relatively fast degradation of complex as wellas nonbiodegradable organic (macro)molecules(ii) Stable catalysts(iii) Moderate reaction conditions
(i) Not selective(ii) Limited selection of operating conditions(iii) Energy costs
Enzymatic conversion(single stage)
(i) More selective(ii) Moderate reaction conditions
(i) High enzyme loads required otherwise ligninhydrolysis is quite slow [16](ii) Enzymes are less stable1(iii) cost-intensive (POXs)2
1For example POXs are sensitive to high H2O2 concentrations2According to Torres and Ayala [86]
and atmospheric pressure chemical ionization (APCI) forLC-MS while GC-MS is based on electron ionization Aremedy to this can be the development of a database ofmodel lignin compounds based on a unanimous HPLC-MSmeasuring procedure This would mean much time and costexpensive investments for adequate personnel and material
Though product identification based on peak superpo-sition gives information on a possible product this can-not always be true for lignin degradation because of thelarge number of possible degradation products In order toproperly identify products from lignin degradation productisolation techniques are to be implemented and further ana-lytic methods such as tandem mass spectroscopy (MSMS)and other advanced structure enhancing research techniquessuch as 1H-NMR 13C-NMR have to be studied extensively
4 Photocatalysis and Its Suitability asIntegrated Technology in MultistageConcepts with Biocatalysis
Depending on lignin treatment specification photocatalysiswill find application for complete lignin mineralization toCO2and H
2O (single stage) or as integrated technology in
a multistage system aiming at partial conversion of ligninmacromolecules to organic low molecular weight intermedi-ates for a consecutive biological oxidation finally generatingvalue added end-products [15]
For biological oxidation of lignin or lignin deriva-tives H
2O2-dependent ligninolytic heme peroxidases (POXs
including lignin peroxidase (LiP EC 111114) manganeseperoxidase (MnP EC 111113) versatile peroxidase (VPEC 111116) O
2-dependent laccases (Lac EC 11032) and
extracellular enzymes from Basidiomycetous white-rot fungiare the most efficient lignin degraders in nature [89] Thussuch enzymatic systems especially POXs (with high redoxpotentials) have attractedmuch interest as industrial biocata-lyst [86 90]The enzyme degradationmechanism (in nature)is facilitated by nonenzymatic processes mainly throughfree ∙OH radicals also generated by the fungus (Fenton-type reaction) Those ∙OH radicals enable the requiredphysical contact between the enzymes and structural units ofthe lignin molecule due to numerous nonspecific oxidative
reactions [91] Both the photocatalytic (equations (2)ndash(7))and the POX reactionmechanism recently reviewed by Busseat al [25] are quite similar Consequently a combination ofphotocatalysis followed by an enzymatic oxidation maybe apromising concept utilizing lignin derivatives for exampleoriginated from industrial effluents [15 92] The advantagesforming biobased products are as follows see also Table 4 inthis context A reduction of the lignin polymerization degreevia photocatalysis at best in more biodegradable interme-diates and a simultaneous detoxification causes savings inenzyme costs since it will be expected that less enzyme loadsare necessary for sufficiently rapid reaction rates [15 91] As aresult hydraulic retention times in bioreactor systems shouldbe diminished Moreover the delignification is expectedto be enhanced simultaneously which was in recent yearsshown in a dual system by Kamwilaisak and Wright [93]using TiO
2H2O2UV for photocatalytic pretreatment and
Lac (from Trametes versicolor) in the subsequent biocatalyticstep Within 24 h they obtained in their dual system anelevation in delignification of 20 (without H
2O2) up to at
least 50 when H2O2was present
In previous studies the treatment of lignin (from thepulp and paper industry) containing effluents by fungus asa biological system (excreting appropriate lignin degradingenzyme cocktails) were more focused for posttreatmentexclusively (Duran et al [92] Reyes et al [94] and Gonzalezet al [95]) Shende et al [17] even examined ligninolyticbacteria with photooxidized kraft lignin as substrate
Although POXs are potential industrial biocatalysts [86]no application studies were found for the direct use in suchdual systems as described above The major reason may bethe complexity of the reaction mechanism (inclusive ligninderivatives as substrate and their analysis) per se on the onehand slowing down research and development processes Onthe other hand POXs are sensitive to their cosubstrate H
2O2
once it is supplied in excess causing considerable inactivation(for details refer to Busse et al [25]) At the present severalstudies are carried out modifying these enzymes regardingenhanced stability activity and selectivity as well Henceit can be expected that their technological applicability willbe raised significantly right after successful modification isreached [86]
International Journal of Photoenergy 15
5 Concluding Remarks
It is widely assumed that the photocatalytic degradation oflignin follows a radical reaction pathway which is similar tothat considered in thermal electrochemical and biochemicalprocesses However reporting on the degradation pathway oflignin derivatives and even that of lignin model compoundsare still amajor challengeThis is probably due to the complexnature and variety of possible degradation products Indeedthemechanism is far more complex considering other factorssuch as type of lignin type of catalyst pH illuminationsource and additives
Generally comparing the different photochemical pro-cesses poses a big challenge because of the wide variablesinvolved These discrepancies start from the source and typeof lignin followed by differences in reactor design illumi-nation source intensity of radiation and different types ofcatalyst An ideawould be to have a specific reference reactionwith well-defined starting parameters which include lignintype source and purity catalyst specifications illuminationsource and intensity so as to ease comparison of results
Basic process parameters such as catalyst concentrationsubstrate concentration addition of metal ion to catalyst pHand illumination have been discussed
Despite developed analytical technologies analyzinglignin degradation products remains challenging Proofssuch as mass spectroscopy (MS) HPLC 13C or 1H-NMRspectra from photocatalytic lignin degradations are not yetestablished
In order to properly identify products from lignin degra-dation product isolation techniques are to be implementedand further analytic methods such as MSMS and otheradvanced structure enhancing research techniques such as1H-NMR and 13C-NMR have to be studied extensively Aremedy to this can be the development of a databank ofmodel lignin compounds based on a unanimous HPLC-MSmeasuring procedure This would mean much time and costexpensive investments for adequate personnel and material
Photocatalysis is denoted as the most popular ligninpretreatment technology besides ozonation [96] Photo-catalyzed lignin may be an appropriate substrate for aconsecutive biocatalytic process using ligninolytic enzymes(POX andor Lac) as supported by experimental results ofKamwilaisak and Wright [93] Combining the advantages ofboth catalytic processes savings in the overall process costswill be expected in addition to elevated lignin conversionNonetheless extensive research work including POX modi-fications is still required
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Authorsrsquo Contribution
Colin Awungacha Lekelefac and Nadine Busse contributedequally to this work
Acknowledgments
The authors gratefully thank the Federal Ministry of Educa-tion and Research (BMBF) for funding (FKZ17N0310) Theresearchers also thank the Hessen State Ministry of HigherEducation Research andArts for the financial support withinthe Hessen initiative for scientific and economic excellence(LOEWE)
References
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[3] Green and Natural Polymers Are on the Rise 2014 httpwwwpolymersolutionscombloggreen-and-natural-polymers-on-the-rise
[4] J Zakzeski P C Bruijnincx A L Jongerius and B M Weck-huysen ldquoThe catalytic valorization of lignin for the productionof renewable chemicalsrdquo Chemical Reviews vol 110 no 6 pp3552ndash3599 2010
[5] B KammM Kamm P R Gruber and S KromusBiorefineries-Industrial Processes and Products Status Quo and Future Direc-tions vol 1 Wiley-VCH 2006
[6] R L Howard E Abotsi E L J van Rensburg and S HowardldquoLignocellulose biotechnology issues of bioconversion andenzyme productionrdquoAfrican Journal of Biotechnology vol 2 no12 pp 602ndash619 2003
[7] M Stocker ldquoBio- und BTL-Kraftstoffe in der Bioraffineriekatalytische Umwandlung Lignocellulose-reicher Biomasse mitporosen Stoffenrdquo Angewandte Chemie vol 120 no 48 pp9340ndash9351 2008
[8] J-P Lange ldquoLignocellulose conversion an introduction tochemistry process and economicsrdquo Biofuels Bioproducts andBiorefining vol 1 no 1 pp 39ndash48 2007
[9] D A I Goring ldquoThe physical chemistry of ligninrdquo Pure andApplied Chemistry vol 5 no 1-2 pp 233ndash310 1962
[10] M Ek G Gellerstedt and G HenrikssonWood Chemistry andBiotechnology Pulp and Paper Chemistry and Technology vol 1Walter de Gruyter GmbH amp Co KG Berlin Germany 2009
[11] B Saake and R Lehnen ldquoLigninrdquo in Ullmannrsquos Encyclopedia ofIndustrial Chemistry Wiley-VCH Verlag GmbH amp Co KGaAWeinheim Germany 2012
[12] M N S Kumar A K Mohanty L Erickson and M MisraldquoLignin and its applications with polymersrdquo Journal of BiobasedMaterials and Bioenergy vol 3 no 1 pp 1ndash24 2009
[13] R J A Gosselink E de Jong B Guran and A AbacherlildquoCo-ordination network for ligninmdashstandardisation produc-tion and applications adapted to market requirements(EUROLIGNIN)rdquo Industrial Crops and Products vol 20 no 2pp 121ndash129 2004
[14] D Fengel and G Wegener Wood Chemistry UltrastructureReactions Verlag Kessel 1984
[15] D Mantzavinos and E Psillakis ldquoEnhancement of biodegrad-ability of industrial wastewaters by chemical oxidation pre-treatmentrdquo Journal of Chemical Technology and Biotechnologyvol 79 no 5 pp 431ndash454 2004
16 International Journal of Photoenergy
[16] M Dashtban H Schraft and W Qin ldquoFungal bioconversionof lignocellulosic residues Opportunities amp perspectivesrdquo Inter-national Journal of Biological Sciences vol 5 no 6 pp 578ndash5952009
[17] A Shende R Jaswal D Harder-Heinz A Menan and RShende ldquoIntergrated photocatalytic and microbial degradationof kraft ligninrdquo Cleantech pp 120ndash123 2012
[18] F S Chakar and A J Ragauskas ldquoReview of current and futuresoftwood kraft lignin process chemistryrdquo Industrial Crops andProducts vol 20 no 2 pp 131ndash141 2004
[19] W G Glasser and H R Glasser ldquoEvaluation of ligninrsquoschemical structure by experimental and computer simulationtechniquesrdquo Paperi ja Puu vol 63 pp 71ndash83 1981
[20] M Erickson S Larsson and G E Miksche ldquoZur Struktur desLignins der FichterdquoActaChemica Scandinavica vol 27 pp 903ndash914 1973
[21] H Nimz ldquoDas Lignin der BuchemdashEntwurf eines Konstitution-sschemasrdquo Angewandte ChemiemdashInternational Edition vol 86pp 336ndash344 1974
[22] D V Evtuguin C P Neto J Rocha and J D P de JesusldquoOxidative delignification in the presence of molybdovana-dophosphate heteropolyanions mechanism and kinetic stud-iesrdquo Applied Catalysis A General vol 167 no 1 pp 123ndash1391998
[23] A L Linsebigler G Lu and J T Yates Jr ldquoPhotocatalysis onTiO2surfaces Principles mechanisms and selected resultsrdquo
Chemical Reviews vol 95 no 3 pp 735ndash758 1995[24] Y Miyata K Miyazaki M Miura Y Shimotori M Aoyama
and H Nakatani ldquoSolventless delignification of wood flourwith TiO
2poly(ethylene oxide) photocatalyst systemrdquo Journal
of Polymers and the Environment vol 21 no 1 pp 115ndash121 2013[25] N Busse D Wagner M Kraume and P Czermak ldquoReaction
kinetics of versatile peroxidase for the degradation of lignincompoundsrdquoTheAmerican Journal of Biochemistry andBiotech-nology vol 9 no 4 pp 365ndash394 2013
[26] M Tien and T K Kirk ldquoLignin-degrading enzyme fromphanerochaete chrysosporium Purification characterizationand catalytic properties of a uniqueH
2O2-requiring oxygenaserdquo
Proceedings of the National Academy of Sciences vol 81 pp2280ndash2284 1984
[27] T K Kirk M Tien P J Kersten M D Mozuch and BKalyanaraman ldquoLigninase of Phanerochaete chrysosporiumMechanism of its degradation of the non-phenolic arylglycerol120573-aryl ether substructure of ligninrdquo Biochemical Journal vol236 no 1 pp 279ndash287 1986
[28] T Lundell RWever R Floris et al ldquoLignin peroxidase L3 fromPhlebia radiata pre-steady-state and steady-state studies withveratryl alcohol and a non-phenolic lignin model compound1-(34-dimethoxyphenyl)-2-(2-methoxyphenoxy)propane-13-diolrdquo European Journal of Biochemistry vol 211 no 3 pp391ndash402 1993
[29] H E Schoemaker T K Lundell A I Hatakka and K PiontekldquoThe oxidation of veratryl alcohol dimeric lignin models andlignin by lignin peroxidase the redox cycle revisitedrdquo FEMSMicrobiology Reviews vol 13 no 2-3 pp 321ndash331 1994
[30] J M Palmer P J Harvey and H E Schoemaker ldquoThe role ofperoxidases radical cations and oxygen in the degradation oflignin [and discussion]rdquo Philosophical Transactions of the RoyalSociety of London A vol 321 no 1561 pp 495ndash505 1987
[31] C Hill Wood Modification Chemical Thermal and OtherProcesses John Wiley amp Sons Chichester UK 2006
[32] Y-H Xu H-R Chen Z-X Zeng and B Lei ldquoInvestigation onmechanism of photocatalytic activity enhancement of nanome-ter cerium-doped titaniardquo Applied Surface Science vol 252 no24 pp 8565ndash8570 2006
[33] L Tonucci F Coccia M Bressan and N DrsquoAlessandro ldquoMildphotocatalysed and catalysed green oxidation of lignin auseful pathway to low-molecular-weight derivativesrdquoWaste andBiomass Valorization vol 3 no 2 pp 165ndash174 2012
[34] A Castellan N Colombo C Vanucci P Fornier de HViolet and H Bouas-Laurent ldquoPhotodegradation of ligninA photochemical study of an O-methylated 120572-carbonyl 120573-1lignin model dimer 12-di(3101584041015840-dimethoxyphenyl) ethanone(deoxyveratroin)rdquo Journal of Photochemistry and PhotobiologyA vol 51 pp 451ndash467 1990
[35] A Fujishima X Zhang and D A Tryk ldquoTiO2photocatalysis
and related surface phenomenardquo Surface Science Reports vol63 no 12 pp 515ndash582 2008
[36] C S Turchi and D F Ollis ldquoPhotocatalytic reactor designan example of mass-transfer limitations with an immobilizedcatalystrdquo The Journal of Physical Chemistry vol 92 no 23 pp6852ndash6853 1988
[37] R Subasri M Tripathi K Murugan J Revathi G V N Raoand T N Rao ldquoInvestigations on the photocatalytic activity ofsolndashgel derived plain and Fe3+Nb5+ doped titania coatings onglass substratesrdquo Materials Chemistry and Physics vol 124 pp63ndash68 2010
[38] N Serpone ldquoRelative photonic efficiencies and quantum yieldsin heterogeneous photocatalysisrdquo Journal of Photochemistry andPhotobiology A Chemistry vol 104 no 1ndash3 pp 1ndash12 1997
[39] M R Hoffmann S T Martin W Choi and D W BahnemannldquoEnvironmental applications of semiconductor photocatalysisrdquoChemical Reviews vol 95 no 1 pp 69ndash96 1995
[40] C D Jaeger and A J Bard ldquoSpin trapping and electron spinresonance detection of radical intermediates in the photode-composition of water at titanium dioxide particulate systemsrdquoThe Journal of Physical Chemistry vol 83 no 24 pp 3146ndash31521979
[41] R W Matthews ldquoHydroxylation reactions induced by near-ultraviolet photolysis of aqueous titaniumdioxide suspensionsrdquoJournal of the Chemical Society Faraday Transactions vol 80no 2 pp 457ndash471 1984
[42] A E H Machado A M Furuyama S Z Falone R RuggieroD D S Perez and A Castellan ldquoPhotocatalytic degradation oflignin and lignin models using titanium dioxide the role of thehydroxyl radicalrdquoChemosphere vol 40 no 1 pp 115ndash124 2000
[43] W Sigg and L Stumm Aquatische Chemie Stuttgart 2 AuflageBG Teubner Stuttgart Germany 1991
[44] O Legrini E Oliveros and A M Braun ldquoPhotochemicalprocesses for water treatmentrdquo Chemical Reviews vol 93 no2 pp 671ndash698 1993
[45] G Rothenberger J Moser M Gratzel N Serpone and DSharma ldquoCharge carrier trapping and recombination dynamicsin small semiconductor particlesrdquo Journal of the AmericanChemical Society vol 107 no 26 pp 8054ndash8059 1985
[46] P Mazellier M Sarakha A Rossi and M Bolte ldquoThe aqueousphotochemistry of 26-dimethylphenol Evidence for the frag-mentation of the 120572 C-C bondrdquo Journal of Photochemistry andPhotobiology A vol 115 no 2 pp 117ndash121 1998
[47] M Ksibi S B Amor S Cherif E Elaloui A Houas and MElaloui ldquoPhotodegradation of lignin from black liquor using aUVTiO
2systemrdquo Journal of Photochemistry and Photobiology
A vol 154 no 2-3 pp 211ndash218 2003
International Journal of Photoenergy 17
[48] S K Kansal M Singh and D Sud ldquoStudies on TiO2ZnO
photocatalysed degradation of ligninrdquo Journal of HazardousMaterials vol 153 no 1-2 pp 412ndash417 2008
[49] A Dahm and L A Lucia ldquoTitanium dioxide catalyzed pho-todegradation of lignin in industrial effluentsrdquo Industrial andEngineering Chemistry Research vol 43 no 25 pp 7996ndash80002004
[50] E Portjanskaja and S Preis ldquoAqueous photocatalytic oxidationof lignin the influence of mineral admixturesrdquo InternationalJournal of Photoenergy vol 2007Article ID76730 7 pages 2007
[51] K Tanaka R C R Calanag and T Hisanaga ldquoPhotocatalyzeddegradation of lignin on TiO
2rdquo Journal of Molecular Catalysis
A Chemical vol 138 no 2-3 pp 287ndash294 1999[52] M Tian J Wen D MacDonald R M Asmussen and A Chen
ldquoA novel approach for lignin modification and degradationrdquoElectrochemistry Communications vol 12 no 4 pp 527ndash5302010
[53] K PanM Tian Z-H Jiang B Kjartanson and A Chen ldquoElec-trochemical oxidation of lignin at lead dioxide nanoparticlesphotoelectrodeposited on TiO
2nanotube arraysrdquo Electrochim-
ica Acta vol 60 pp 147ndash153 2012[54] C Awungacha Lekelefac J Hild P Czermak and M Herren-
bauer ldquoPhotocatalytic active coatings for lignin degradationin a continuous packed bed reactorrdquo International Journal ofPhotoenergy vol 2014 Article ID 502326 10 pages 2014
[55] C Awungacha Lekelefac P Czermak and M HerrenbauerldquoEvaluation of photocatalytic active coatings on sintered glasstubes by methylene bluerdquo International Journal of Photoenergyvol 2013 Article ID 614567 9 pages 2013
[56] Y-S Ma C-N Chang Y-P Chiang H-F Sung and A CChao ldquoPhotocatalytic degradation of lignin using PtTiO
2as
the catalystrdquo Chemosphere vol 71 no 5 pp 998ndash1004 2008[57] H Ohnishi M Matsumura H Tsubomura and M Iwasaki
ldquoBleaching of lignin solution by a photocatalyzed reactionon semiconductor photocatalystsrdquo Industrial and EngineeringChemistry Research vol 28 no 6 pp 719ndash724 1989
[58] C A K Gouvea F Wypych S G Moraes N Duran and PPeralta-Zamora ldquoSemiconductor-assisted photodegradation oflignin dye and kraft effluent by Ag-doped ZnOrdquo Chemospherevol 40 no 4 pp 427ndash432 2000
[59] A V Vahatalo K Salonen M Salkinoja-Salonen and AHatakka ldquoPhotochemical mineralization of synthetic lignin inlake water indicates enhanced turnover of aromatic organicmatter under solar radiationrdquo Biodegradation vol 10 no 6 pp415ndash420 1999
[60] H de Lasa B Serrano and M Salaices Photocatalytic ReactionEngineering Springer New York NY USA 2005
[61] K Hashimoto H Irie and A Fujishima ldquoTiO 2 photocatalysisA historical overview and future prospectsrdquo Japanese Journal ofApplied Physics vol 44 no 12 pp 8269ndash8285 2005
[62] M A Behnajady N Modirshahla and R Hamzavi ldquoKineticstudy on photocatalytic degradation of CI Acid Yellow 23 byZnOphotocatalystrdquo Journal ofHazardousMaterials vol 133 no1ndash3 pp 226ndash232 2006
[63] M Addamo V Augugliaro A di Paola et al ldquoPhotocatalyticthin films of TiO
2formed by a sol-gel process using titanium
tetraisopropoxide as the precursorrdquo Thin Solid Films vol 516no 12 pp 3802ndash3807 2008
[64] N Negishi K Takeuchi and T Ibusuki ldquoPreparation of theTiO2thin filmphotocatalyst by the dip-coating processrdquo Journal
of Sol-Gel Science and Technology vol 13 no 1ndash3 pp 691ndash6941998
[65] L Rideh A Wehrer D Ronze and A Zoulalian ldquoPhotocat-alytic degradation of 2-chlorophenol in TiO
2aqueous suspen-
sion modeling of reaction raterdquo Industrial and EngineeringChemistry Research vol 36 no 11 pp 4712ndash4718 1997
[66] R-ADoongC-HChen RAMaithreepala and S-MChangldquoThe influence of pHand cadmiumsulfide on the photocatalyticdegradation of 2-chlorophenol in titanium dioxide suspen-sionsrdquoWater Research vol 35 no 12 pp 2873ndash2880 2001
[67] K V Sarkanen A Islam and C D Anderson ldquoOzonationrdquo inMethods in Lignin Chemistry S Y Lin and C W Dence EdsSpringer Series inWood Science pp 387ndash406 Springer BerlinGermany 1992
[68] G Gellerstedt and E-L Lindfors ldquoStructural changes in ligninduring kraft pulpingrdquo Holzforschung vol 38 no 3 pp 151ndash1581984
[69] B Neppolian H C Choi M V Shankar B Arabindoo andV Murugesan in Proceedings of the International Symposiumon Environmental Pollution Control and Waste Management(EPCOWM 02) p 647 2002
[70] C Pouteau P Dole B Cathala L Averous and N BoquillonldquoAntioxidant properties of lignin in polypropylenerdquo PolymerDegradation and Stability vol 81 no 1 pp 9ndash18 2003
[71] J Hafren T Fujino and T Itoh ldquoChanges in cell wall archi-tecture of differentiating tracheids of Pinus thunbergii duringlignificationrdquo Plant and Cell Physiology vol 40 no 5 pp 532ndash541 1999
[72] T Dizhbite G Telysheva V Jurkjane and U Viesturs ldquoChar-acterization of the radical scavenging activity of lignins naturalantioxidantsrdquo Bioresource Technology vol 95 no 3 pp 309ndash3172004
[73] K Hofstadler R Bauer S Novalic and S G Heisier ldquoNew reac-tor design for photocatalytic wastewater treatment with TiO
2
immobilized on fused-silica glass fibers photomineralization of4-chlorophenolrdquo Environmental Science amp Technology vol 28no 4 pp 670ndash674 1994
[74] J Villasenor and H D Mansilla ldquoEffect of temperature onkraft black liquor degradation by ZnO-photoassisted catalysisrdquoJournal of Photochemistry and Photobiology A Chemistry vol93 no 2-3 pp 205ndash209 1996
[75] B H Bielski D E Cabelli L A Ravindra and A B RossldquoReactivity of HO
2Ominus2Radicals in aqueous solutionrdquo Journal
of Physical Chemistry vol 14 pp 1041ndash1100 1985[76] B H J Bielski and A O Allen ldquoMechanism of the dispropor-
tionation of superoxide radicalsrdquo Journal of Physical Chemistryvol 81 no 11 pp 1048ndash1050 1977
[77] B Halliwell and J M C Gutteridge ldquoThe importance of freeradicals and catalytic metal ions in human diseasesrdquoMolecularAspects of Medicine vol 8 no 2 pp 89ndash193 1985
[78] J M Palmer P J Harvey and H E Schoemaker ldquoThe role ofperoxidases radical cations and oxygen in the degradation oflignin [and discussion]rdquo Philosophical Transaction of the RoyalSociety A vol 321 no 1561 pp 495ndash505 1987
[79] S Y Yin and C W Dense Methods in Lignin ChemistrySpringer New York NY USA 1992
[80] P Kumar S Kumar and N K Bhardwaj ldquoPhotocatalyticoxidation of elemental chlorine free bleaching effluent withUVTiO
2rdquo in Proceedings of the 2nd International Conference on
Environmental Science and Technology (ICEST rsquo11) SingaporeFebruary 2011
[81] K Kobayakawa Y Sato S Nakamura and A FujishimaldquoPhotodecomposition of Kraft lignin catalyzed by titanium
18 International Journal of Photoenergy
dioxiderdquo Bulletin of the Chemical Society of Japan vol 62 no11 pp 3433ndash3436 1989
[82] R B Kinstre ldquoAn overview of strategies for reducing theenvironmental impact of bleach-plant effluentsrdquo Tappi Journalvol 76 no 5 pp 105ndash113 1993
[83] A Castellan H Choudhury R Stephen Davidson and SGrelier ldquoComparative study of stone-ground wood pulp andnative wood 3 Application of fluorescence spectroscopy to astudy of the weathering of stone-ground pulp and native woodrdquoJournal of Photochemistry and Photobiology A Chemistry vol81 no 2 pp 123ndash130 1994
[84] B Albinsson S Li K Lundquist and R Stomberg ldquoThe originof lignin fluorescencerdquo Journal of Molecular Structure vol 508no 1ndash3 pp 19ndash27 1999
[85] S Baumberger A Abaecherli M Fasching et al ldquoMolar massdetermination of lignins by size-exclusion chromatographytowards standardisation of the methodrdquo Holzforschung vol 61no 4 pp 459ndash468 2007
[86] E Torres andM Ayala Biocatalysis Based on Heme PeroxidasesSpringer New York NY USA 1st edition 2010
[87] R Pecina P Burtscher G Bonn and O Bobleter ldquoGC-MSand HPLC analyses of lignin degradation products in biomasshydrolyzatesrdquo Fresenius Zeitschrift fur Analytische Chemie vol325 no 5 pp 461ndash465 1986
[88] NIST NIST Standard Reference Database The National Insti-tute of Standards and Technology 2014 httpwwwnistgovsrdnist1acfm
[89] T K Kirk and R L Farrell ldquoEnzymatic ldquocombustionrdquo themicrobial degradation of ligninrdquo Annual Review of Microbiol-ogy vol 41 pp 465ndash501 1987
[90] A TMartinez ldquoHigh redox potential peroxidasesrdquo in IndustrialEnzymes Structure Function and Applications K-B BeckerEd pp 477ndash488 Springer Amsterdam The Netherlands 1stedition 2007
[91] M Dashtban H Schraft T A Syed and W Qin ldquoFungalbiodegradation and enzymatic modification of ligninrdquo Interna-tional Journal of Biochemistry and Molecular Biology vol 1 no1 pp 36ndash50 2010
[92] N Duran E Esposito L H Innocentini-Mei and V P CanhosldquoA new alternative process for Kraft E1 effluent treatmentrdquoBiodegradation vol 5 no 1 pp 13ndash19 1994
[93] K Kamwilaisak and P C Wright ldquoInvestigating laccase andtitanium dioxide for lignin degradationrdquo Energy and Fuels vol26 no 4 pp 2400ndash2406 2012
[94] J Reyes M Dezotti H Mansilla J Villasenor E Espositoand N Duran ldquoBiomass photochemistry-XXII combined pho-tochemical and biological process for treatment of Kraft E1effluentrdquoApplied Catalysis B Environmental vol 15 no 3-4 pp211ndash219 1998
[95] L F Gonzalez V Sarria and O F Sanchez ldquoDegradationof chlorophenols by sequential biological-advanced oxidativeprocess using Trametespubescens and TiO
2UVrdquo Bioresource
Technology vol 101 no 10 pp 3493ndash3499 2010[96] J H Lora and W G Glasser ldquoRecent industrial applications
of lignin a sustainable alternative to nonrenewable materialsrdquoJournal of Polymers and the Environment vol 10 no 1-2 pp 39ndash48 2002
Submit your manuscripts athttpwwwhindawicom
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Inorganic ChemistryInternational Journal of
Hindawi Publishing Corporation httpwwwhindawicom Volume 2014
International Journal ofPhotoenergy
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Carbohydrate Chemistry
International Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
Chemistry
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Advances in
Physical Chemistry
Hindawi Publishing Corporationhttpwwwhindawicom
Analytical Methods in Chemistry
Journal of
Volume 2014
Bioinorganic Chemistry and ApplicationsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
SpectroscopyInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Medicinal ChemistryInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Chromatography Research International
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Applied ChemistryJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Theoretical ChemistryJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
Spectroscopy
Analytical ChemistryInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Quantum Chemistry
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Organic Chemistry International
ElectrochemistryInternational Journal of
Hindawi Publishing Corporation httpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CatalystsJournal of
International Journal of Photoenergy 5
MeO MeOMeOMeO
∙OO
O
O ∙
OH
HO+ +
Figure 5 Formation of phenoxyl radicals by intermolecular abstraction of phenolic hydrogen by carbonyl groups
RH
bond cleavage
Vanillin
O
OO
O
OO O
O
O
O
O
O
HO HO
HOHO
HO
HO
HO HO
OH OH
HO
OCH3
OCH3
OCH3
OCH3
OCH3 OCH3
OCH3
OCH3 OCH3
OCH3
h minusH∙OH∙∙
O2TiO2
HminusOH∙
C120572-C120573
∙
Figure 6 Supposed lignin degradation scheme by autoxidation induced by TiO2poly (ethylene oxide) [24]
S∙+ +O2997888rarr SO
2
∙+ (4)
TiO2(h+) + S 997888rarr TiO
2+∙S+ (5)
TiO2(eminus) +O
2997888rarr TiO
2+∙O2
minus (6)
HO∙ + ∙O2
minus
997888rarr OHminus + 1O2
(7)
TiO2(eminus) + TiO
2(h+) 997888rarr heat (8)
The organic radicals and radical cations can for examplereact with molecular oxygen to form organic peroxy radicalsand peroxy radical cations respectively (4) The holes canoxidize organic compounds by electron abstraction to formorganic cationic radicals (5) [42] Superoxides can be formedby the reaction of electrons with electron acceptors suchas O2(6) Meanwhile the formation of singlet oxygen can
be from the reaction of hydroxyl radical and superoxide(7) [42] Moreover there is a possibility that electrons andholes recombine if electron acceptors are limited In thiscase recombination can take place in the volume of thesemiconductor particle When recombination takes placeradiation energy is lost or converted into heat (8) [45]
From investigations caried out by Mazellier et al [46](photochemistry of 26-dimethylphenol) it was postulatedthat hydrogen can be abstracted by 120572-carbonyl groups In thesame context lignin derivatives having similar functionalitycan follow a similar pathway In addition oxidative chainreactions with the participation of ground-state oxygencan be initiated leading to fragmentation and combinationreactions and thus the formation of new dimers or oligomers(Figure 5)
Miyata et al [24] proposed a cleavage mechanism forthe C120572ndashC120573 bonds which leads to the formation of smallfragments such as vanillin as shown in Figure 6
Figure 7 [25] illustrates the formation of a radical cationformed as a result of enzyme (lipase) mediated reactionof adlerol Adlerol is characterized by a C120573ndashOndash4 bondand considered to be a lignin model compound With theformation of the radical species subsequent nonenzymaticreactions such as radical reactions can take place generatinga wide variety of products and complex compounds
It is widely assumed that the photocatalytic degradationof lignin follows a radical reaction pathwaywhich is similar tothat considered in thermal electrochemical and biochemical
6 International Journal of Photoenergy
Nonenzymatic subsequent reactions
Adlerol
Veratryl alcohol radical
Enzyme1
23
4
4
1
2
3
5
5
6
6
Guaiacol
Veratraldehyde
Ketone
O
OO
O
O O
O
O
O
HO
HOHO
HO
OCH3
OCH3
OCH3
OCH3
OCH3
OCH3
120572120573
120574
minusH+
∙+
∙
∙+
c120572 oxidation H2O
O2
120572 120573
120574
HOHOHO
OCH3
OCH3OCH3
OCH3 OCH3
OCH3
OCH3
120572120572
120573 120573
120573
120574 120574
HO
HO
120574
OCH3
120573
HO
HO
120574
HO
OCH3
OCH3
OCH3
120572120573
120574
Figure 7 Proposed radical reaction scheme initiated by enzyme lignolytic heme peroxidase for the conversion of adlerol possessing C120573ndashOndash4 bonds into smaller units summarized by Busse et al [25] and abstracted from Tien and Kirk [26] Kirk et al [27] Lundell et al [28]Schoemaker et al [29] and Palmer et al [30]
processes However reporting on the degradation pathway oflignin derivatives and even that of lignin model compoundsis still a major challenge This is probably due to the complexnature and variety of possible degradation products Indeedthemechanism is far more complex considering other factorssuch as type of lignin type of catalyst pH illuminationsource and additives
32 Influence of Process Parameters in Lignin DegradationVarying process parameter could either have a positive ornegative impact on the photocatalytic efficiency The basicprocess parameters such as catalyst concentration [4 4856] substrate concentration [48 51] addition of metal ion
to TiO2catalyst [17 50 56ndash58] pH [47 48] illumination
[33 48 49 59] and their influence shall be discussed in thissubchapter Table 2 gives an overview about starting reactionconditions and catalyst applied by some work groups whileTable 3 portrays parameters analytical methods and resultsobtained It is worthwhile noting that comparing the differentphotochemical processes poses a big challenge because ofthe wide variables involvedThese discrepancies start alreadyfrom the source and type of lignin followed by the differencesin reactor design illumination source intensity of radiationand different types of TiO
2catalyst such as Fischer scientific
rutile TiO2[49] TiO
2(TiO2mdashTP-2 of Fujititan) just to name
a few
International Journal of Photoenergy 7
Table2Summaryof
startingcond
ition
sfor
thep
hotocatalytic
degradationof
lignin
Reference
Lign
insource
Catalyst
Reactio
ncond
ition
s
MacHadoetal[42]
Peroxyform
icacid
lignins
from
eucalyptus
grandisw
ood(EL1)
EL1+
sodium
borohydride
TiO
2andH
2O2
Lign
inconcentration
025
mgmL119879=25∘C
pH11U
VVisradiatio
n120582gt300nm
40
0Wmercury
lampcylin
dricalPy
rexglassreactorcon
stanto
xygenbu
bblin
g
Ksibietal[47]
Water
solubleligninob
tained
from
blackliq
uor
TiO
2-P2
5(D
egussa)
Lign
inconcentration
90mgL119879=20∘C
pH82UV-radiation120582gt290nm
pyrex
reactoro
pento
airPh
ilips
HPK
125W
lamp
Kansaletal[48]
Lign
infro
mwheatstr
awkraft
digestion
TiO
2-P2
5(D
egussa)a
ndZn
O
Lign
inconcentration
10ndash100
mgLin
100m
LZn
Ocatalystdo
se(05ndash20gL)pH
ofthes
olution(pH3ndash11)solarillu
mination
oxidantcon
centratio
n306times10minus6M
to153times10minus6Mthinbedfilm
slurrypo
ndreactoroxidantsodium
hypo
chlorite
solutio
n(4availableC
l 2)
Dahm
andLu
cia[
49]
Whitewater
from
indu
strialprocess
water
thatexits
inpaperm
achines
FischerS
cientifi
crutile
TiO
2
Lign
inconcentration
40mgLin
500m
Lbatchreactor119879=21∘Cndash
42∘C
Rayonet
photochemicalcham
ber16
VWR8-W
blacklightpho
spho
r(350-nm
)lam
ps
constant
oxygen
bubb
lingpo
wer
ofillum
ination
128to
64Wlight
intensity
223ndash44
5mWcm
3
Portjanskajaand
Preis[50]
Lign
inpurchased
from
Aldric
hTiO
2P2
5-N(D
egussa)
Lign
inconcentration
100m
gLpH
8batchreactorsystemreactor
open
toair
PhillipsT
LD15W05low-pressurelum
inescent
mercury
UV-lampUV-radiation
120582gt360nm
pow
erdensity
ofirr
adiatio
n=07m
Wcm
2 visib
lelight
source
Tanaka
etal[51]
Lign
infro
mconiferous
woo
dTiO
2(TiO
2mdashTP
-2of
Fujititan)
Lign
inconcentration
0003to
003U
V-radiation120582gt310nm
cylindrical
reactio
nvessel
Tonu
ccietal[33]
Ca2+andNH4
+ligninderiv
atives
TiO
2as
Degussa
P25
+po
lyoxom
etalates
(POM)H
2O2
Openqu
artztubes(20
mL)
reactor119879=20∘C
1atm
multirayso
ften
UVlamps
of15W
power
eachU
V-radiation120582gt254nm
Miyatae
tal[24]
Piceag
lehn
iiwoo
dflo
urTiO
2po
lyethylene
oxide(PE
O)
Reactorop
enpetrid
ish119879=30∘C119905=48h40
0Wmercury
lamp
Shende
etal[17]
Kraft
lignin
TiO
2-Zn
O-ZrO
2Lign
inconcentration
1mgmLin
10mL250W
Xeno
nlampandAM
15Glamp
filterpo
wer
density
ofirr
adiatio
n100m
Wcm
2 solarlam
psim
ulator
Tian
etal[52]
andPanetal[53]
Kraft
ligninfro
mblackliq
uor
Ta2O
5-IrO
2andPb
O2thin
film
TiO
2nano
tubePbO
2
Lign
inconcentration
30(w
w)UV-radiation120582gt365nm
119905=10minpow
erdensity
ofirr
adiatio
n20
mWcm
2 blue
waveT
M50
ASUVspot
lampEG
ampG2273
potentiosta
tgalvano
stattoapplycurrentTiTiO
2NTPb
O2ele
ctrode
asworking
electrode
andPt
coilas
outere
lectrode
andAgAgC
lasreference
electrode
Awun
gachaL
ekele
fac
etal[54][55]
Lign
insulfo
natefro
mpaperw
aste
water
TiO
2as
Degussa
P25
TiO
2fro
msol-g
elprocesso
fTiOSO
4TT
IP
Lign
inconcentration
500m
gLin
200m
LOsram
Planon
light
sourceirradiance
30ndash4
0Wm
2 UV-radiation120582280ndash4
20nm
119905=20hreactoro
pento
air
recirculationsyste
mflow
rate225mLmin
8 International Journal of Photoenergy
Table3Parametersanalyticalmetho
dsand
results
from
different
workgrou
ps
Reference
Parameter
studied
Analytic
sRe
sult
MacHado
etal[42]
Roleof
hydroxylradicals
irradiatio
nof
ligninin
the
absencea
ndpresence
ofph
otocatalystT
iO2andH
2O2
Ultraviolet-v
isible(UV-Vis)spectro
scop
yionizatio
nabsorptio
nspectro
scop
y(IA
S)
sizee
xclusio
nchromatograph
y(SEC
)
Asharpdecrease
inthep
heno
liccontento
bservedforreactions
involvingdirect
photolysis
SEC
areductio
nof
almost50
inthea
verage
molecular
weighto
fligninequalto
14kD
after
90min
ofirr
adiatio
n
Ksibietal[47]
Irradiationof
ligninin
the
absencea
ndpresence
ofph
otocatalystT
iO2-P2
5
UV-Visspectroscop
y13C-
nucle
armagnetic
resonance(NMR)
solid
state
totalion
gasc
hrom
atograph
y(TIC)
indu
ctioncoup
lingplasma(
ICP)
chem
icaloxygen
demand(C
OD)
56degradationratewith
TiO
2catalystaft
er420m
inreactiontim
eethylacetate-extractableprod
uctsshow
edvanillin
vanillica
cidpalm
itica
cid
biph
enylstructuresand
345-trim
etho
xybenzaldehyde
presence
ofmagnesiu
mandcalcium
ions
CODremovalishigh
erforthe
initiallylowconcentrations
ofligninsolutio
n
Kansaletal[48]
Catalystdo
sepHoxidant
concentration
initialsubstrate
concentration
ZnOcatalystin
slurryandim
mob
ilizedmod
e
UV-Visspectroscop
yCO
D
Optim
umcatalystdo
seis1gL
optim
umoxidantcon
centratio
n2times10minus6M
graduald
ecreaseo
fabsorptionpeak
indicatin
gdecompo
sitionof
organics
CODremovalishigh
erforthe
initiallylowconcentrations
ofligninsolutio
n
Dahm
and
Lucia[
49]
Catalystdo
seillu
mination
intensity
UV-Visspectroscop
ytotalorganiccarbon
(TOC)
capillaryionelectro
phoresis
analysis(C
IA)
Gradu
aldecrease
inabsorptio
npeak
indicatin
gdecompo
sitionof
organics
optim
alcatalystdo
seof
10mgm
high
erillum
inationintensities
correlated
well
with
high
erinitialdegradationrate
74disapp
earanceo
fTOC
Portjanskajaand
Preis[50]
Influ
ence
offerrou
sion
sN-dop
edcatalysteffect
sprayedcatalyston
supp
ortand
subm
ersedcatalyst
CODU
V-Visspectroscop
ybiochemical
oxygen
demand(BOD)colorim
etric
measurementat570
nm
Additio
nof
Fe2+upto
28m
gLleadsto25increase
inph
otocatalyticeffi
ciency
sprayedcatalystexhibited15
times
high
ereffi
ciency
than
theo
neattached
bysubm
ersio
nnegligibleeffecto
fN-dop
edcatalyst
increase
ofaldehyde
concentrationover
reactio
ntim
eneutralm
ediawas
mostb
eneficialforb
iodegradability
80of
freep
heno
lsremoved
undern
eutralcond
ition
s
Tanaka
etal[51]Ca
talystloadinglignin
concentration
illum
inationtim
e
UV-Visspectroscop
yTO
Cgel
perm
eatio
nchromatograph
y(G
PC)
1 HNMR
Fouriertransform
ationinfrared
(FTIR)
spectro
scop
y
FTIR
measurementrevealedafasttransform
ationof
arom
aticmoietyp
resent
inlignin
characteris
ticband
sofaromaticrin
gsm
etho
xyand
aliphatic
sidec
hains
decrease
inTO
Cvalues
over
time
decrease
indegradationratewith
increase
catalystdo
sageB
utaft
ercatalystthreshold
valueisa
ttainedcatalystincreasec
ausesa
decrease
indegradationrates
FTIR
peaksa
reshifted
towards
lower
molecular
weightregionaft
erph
otocatalysis
Tonu
cci
etal[33]
Testof
catalytic
syste
mstoob
tain
fractio
nswith
redu
ceddegreeso
fpo
lymerization
comparis
onof
thermalandph
otochemical
reactio
ns
1 HNMR
gasc
hrom
atograph
y-mass
spectro
scop
y(G
C-MS)
POMsa
relessselectivew
henused
asph
otocatalystsandno
appreciableb
leaching
ofthes
olutionwas
seen
whenPO
Mwas
used
asthermalcatalyst
deriv
edchem
icalsfrom
experim
entvanillin
hydroxylmetho
xy-acetoph
enon
econiferylalcoh
olcon
iferylaldehydemethano
lform
icacidacetic
acidand
sometim
essm
allamou
ntso
fC-2
andC-
3alcoho
ls
Miyatae
tal[24]
Exam
inationof
cellwall
structureo
flignin(w
oodflo
ur)
before
andaft
erph
otocatalysis
GC-
MSscanning
electronmicroscop
y(SEM
)1 H
NMR
Highdelignificationactiv
itydelignificationconfi
nedto
thes
urface
oflignin
deriv
edchem
icalse
xperim
entvanillin
Shende
etal[17]Testof
combinedactio
nof
bio-
andph
otocatalyticsyste
ms
UV-Visspectroscop
yGC-
MSX-
ray
dispersiv
eenergyspectro
scop
yandX-
ray
diffractio
nanalysis(EDX
XRD)SE
M
Detectio
nof
follo
wingchem
icalsacetylguaiacol4-ethoxym
ethyl-2
-metho
xyph
enol
metho
xyph
enyloxim
eguaiacolsuccinica
cidacetylguaiacolvanillicacidand
vanillin
International Journal of Photoenergy 9
Table3Con
tinued
Reference
Parameter
studied
Analytic
sRe
sult
Tian
etal[52]
andPanetal
[53]
Testof
combinedactio
nof
electro-and
photocatalytic
syste
ms
UV-Visspectroscop
yFT
IRSEM
X-ray
(EDX)
highperfo
rmance
liquid
chromatograph
y(H
PLC)
COD
Detectio
nof
thefollowingchem
icalscarbon
ylfunctio
nalityvanillin
andvanillica
cid
Awun
gacha
Lekeleface
tal
[54][55]
Com
paris
onof
degradationrates
bydifferent
catalyst
HPL
Cflu
orescencea
ndUV-Vis
spectro
scop
ySE
MT
OC
UV-Visresultsrevealfaste
rdegradatio
nof
thea
liphatic
moietycomparedto
the
arom
aticmoietyof
ligninsulfonateob
tained
from
paperw
astewaterPeaks
observed
durin
gHPL
Canalysis
Someo
fthe
peaksp
rodu
cedaft
erph
otocatalysishad
fluorescences
ignals
Thissuggeststhep
rodu
ctionof
newsubstances
andflu
orop
hores
Coatin
gsprod
uced
throug
hSol-G
elprocedures
arestablea
ndcanbe
used
manytim
es
10 International Journal of Photoenergy
321 Influence of Catalyst One aspect to successfully imple-ment photocatalysis is the choice of an appropriate photo-catalyst The majority of catalysts used are based on TiO
2as
summarized in Table 2 ZnO has also been applied either assingle catalyst [48] or in a combination with other catalystsuch as TiO
2[54] de Lasa et al [60] described ZnO as
being less active than TiO2 They add that the use of ZnO
is particularly relevant when the oxidative degradation ratebecomes limited This is opposite to what Kansal et al [48]reported by saying that ZnO is more reactive than TiO
2
However TiO2(mainly anatase) remains the most used
catalyst as can be depicted fromTable 2 TiO2is reported to be
favored because of its nontoxic property cost efficiency andchemical and biological inertness Moreover TiO
2possesses
the most efficient photoactivity and the highest stability thusmaking it suitable for industrial use [61]
ZnO has been described to degrade lignin under visiblelight sources [48 62] whereas TiO
2is mostly applied in
connection to UV-light sources as highlighted in Table 2However both ZnO and TiO
2possess energy band gap
energy of 32 eV [23]Additives such as SiO
2[63] polyethylene oxide (PEO)
[24] and polyethylene glycol [64] have been added to TiO2
catalyst particularly when applying immobilized catalystAddamo et al [63] noted a high adhesion of TiO
2to glass
support material when precoating was done with SiO2
Moreover the precoating might have other advantages suchas a hindering diffusion of Na+ ions from the glass materialinto the nascent TiO
2film during heat treatment processes
Analogous to the addition of SiO2 polyethylene glycol
(PEG) has also been introduced to mitigate catalyst surfaceactivity modify surface hydrophobicity and also reduceagglomeration tendency of the TiO
2gel or TiO
2particles
in the suspensions [64] By a proper surface modificationinteraction between catalyst and substrate can be enhanced[55 63]
Kansal et al [48] varied catalyst (ZnO) dose from 05 gLto 20 gL for 01 gL kraft lignin solutions and found out thatthere was an optimum catalyst threshold value at 1 gL whichgives a catalyst to substrate ratio of 1 10
Dahm and Lucia [49] examined catalyst dose from 2 sdot10minus3 gL to 12sdot10minus2 gL for lignin solutions (fromwhite water
liner mill) of 4 sdot 10minus2 gL (catalyst to lignin ratio 5 sdot 10minus3ndash3 sdot 10minus2) at pH 8 and obtained best energy efficiency valuesand lignin degradation rates with a catalyst loading of 10 sdot10minus2 gLIn contrast Ma et al [56] applied far higher catalyst
concentration compared to Dahm and Lucia [49] Catalystconcentration was varied between 1 gL and 10 gL PtTiO
2
Best catalysis dose with respect to reaction turnover wasobtained at 5 gL PtTiO
2With the increase of catalyst dose at
pH 7 the reaction rate increased from 61 times 10minus3minminus1 (1 gLTiO2) to 71 sdot 10minus3minminus1 (5 gL TiO
2) and 99 sdot 10minus3minminus1
(10 gL TiO2)
Catalyst effect has been explained on the basis thatoptimum catalyst loading is dependent on the initial soluteconcentration An increase in catalyst dosage leads to a cor-responding increase of total active surface area for reactions
[65] To that at higher TiO2concentrations the photon flux is
more easily intercepted by the catalyst before penetrating intothe bulk of the system At the same time due to an increase inturbidity of the suspension with high dose of photocatalystthere is a decrease in penetration of UV light and hencephotoactivated volume of suspension or solution decreases[66]
In summary authors have obtained best catalyst to ligninrelations for different reaction designs and thus a generalrecommendation on catalyst dose is not possible Howeverwhen lignin solution is treated with increasing catalyst loadsa corresponding increase in degradation rate is observed untila threshold value is reached [48ndash50 56]
322 Influence of Metal Ion Addition (Doping) and AdditivesThe purpose of adding metal ion to photocatalyst is to miti-gate band gap energy through the introduction of intrabandgap states and as a consequence produce a bathochromicshift in the absorption spectrum [37] Altering the absorptionspectral range gives the possibility to exploit both the visiblelight spectrum and UV light sources Metal ion doping isalso introduced to serve as electron or hole traps in orderto minimize recombination between generated electron-holepairs [37]
Portjanskaja and Preis [50] studied the addition of Fe2+ions to an acidic lignin solution and found an increasein photocatalytic oxidation (PCO) efficiency The optimumFe2+ ions quantity was 28mgL while using 100mgL ligninsolution Upon further elevation of Fe2+ ions concentrationa corresponding reduction of the photocatalytic oxidationefficiency of lignin was noted Likewise Ohnishi et al [57]made a comparative study by doping platinum (Pt) silver(Ag) and gold (Au) ions to TiO
2 In these reactions 50mg of
catalyst (TiO2) was used with the addition of an equiva1ent
15 wt (based on TiO2) metal ion The addition of noble
metals brought about a faster decolorization of lignin Aushowed better results than Ag followed by Pt In the samecontext adding sodium hypochlorite as oxidant to PtTiO
2
catalyst an additional fivefold degradation rate was observedcompared to that without doping [56] Contradictory to theresults described above negligible effect of photocatalyticefficiency due to doping has been reported as well Awun-gacha Lekelefac et al [54] obtained little or no change indegradation rate by doping TiO
2-P25-SiO
2catalyst with Pt
ions (1 wt relative to TiO2catalyst) Likewise Portjanskaja
and Preis [50] noted a negligible change of photocatalyticefficiency of TiO
2when doped with nitrogen
Sarkanen et al [67] and Gellerstedt and Lindfors [68]reported the bias of peroxides to oxidation with reagentssuch as permanganate to favor aromatic moieties Oxidationagents like permanganate oxidizes predominantly aliphaticchains in alkaline and neutral media However by theapplication of H
2O2(Fenton system) lignin disappeared
completely [33] Tonucci et al [33] conclude that in orderto satisfactorily conserve the organic material the best com-promise appears to be the TiO
2photosystem which shows
low carbon consumption good preservation of the aromaticrings and greatly reduced mineralization
International Journal of Photoenergy 11
In summary different results have been obtained con-cerning the influence of noble metal ion addition Whilesome authors report an improvement in the photocatalyticefficiency upon their addition others report their addition ashaving no considerable influence However for reactions inwhich an improvement in the photocatalytic efficiency wasnoticed there was a threshold value to be considered Whenthe concentration of dopant surpasses this threshold valueelectron-hole recombination is favored and this has a negativeimpact to photocatalysis In such a case the space-chargelayer gets narrower and 119901-type dopants attract electrons andby virtue become negative They would now then act ashole acceptor attracting holes On the other hand 119899-typedopants which act as electron donor centers and possessexcess electrons attract holes as well [37]
323 Influence of Lignin Concentration Once the initiallignin concentration becomes higher exceeding a thresholdvalue an inhibitory effect on the photodegradationwas noted[47ndash49] This threshold value varies and depends on thereaction system and reaction parameters such as opticaldensity catalyst concentration and reaction volume Fromthe literature different authors have implemented varyinglignin concentrations probably to suit their reaction designFor example Ksibi et al [47] use 90mgL and AwungachaLekelefac et al [54] use 500mgL while Kansal et al [48]applied 10mgndash100mgL
Explanations arising from the findings are as followsat low lignin concentrations the incidental photonic fluxirradiated interacts with the catalyst generating radicals (eghydroxyl radicals (OH∙)) which allow a faster degradation[69] On the other hand high initial lignin concentrationsmay lead to tight adsorption which can suppress CO
2
evolution [51] and hence maintain chemical oxygen demand(COD) values Moreover low delignification yields maybe due to an inhibitory effect because of autoxidation bylow molecular weight lignin degradation products formed[24] Also due to the polymer structure of lignin which iscross-linked this makes it difficult for radical species acidand the aldehyde compounds produced to spread into theinner region of the substrate hence limiting autoxidationAs a worst case this might be the rate-determining step ofdelignification which is hindered [70ndash72]
In summary it can be concluded that the time taken forcomplete degradation depends on the initial concentration oflignin and faster degradation occurs at low lignin concentra-tions
324 Influence of pH Varying pH entails an alteration inthe properties of semiconductor-liquid interface [73] mainlyrelated to the acid-base equilibrium of the adsorbed hydroxylgroup [39] Furthermore pH also impacts lignin degradationrates [47 48 57] In this context several studies were carriedout with partly contradictory outcomes
Kansal et al [48] made pH investigations 3ndash11 undersolar light illumination using ZnO as catalyst Maximumdegradation was reached in alkaline conditions (pH 11)This is supported by Villasenor and Mansilla [74] reporting
an almost complete decolorization of kraft black liquorfrom pine wood at pH value of 116 in combination withZnO catalyst Similar results were achieved by Ohnishi etal [57] with TiO
2and ZnO being catalyst for bleaching
alkaline lignin in aqueous solution with TiO2and ZnO being
catalyst High activities at neutral pH were also reportedby Ohnishi et al [57] In contrast Ma et al [56] observedhigher reaction rates and rapid degradation of a syntheticlignin wastewater (prepared by dissolving commercial ligninpowder in aqueous solution pH 11) in acidic solution (pH 3)than in alkaline solutions at pH 11 for either TiO
2or PtTiO
2
catalystsReconsidering the photocatalytic principle the formed
superoxide anion radicals (∙O2
minus) are in a pH-dependentequilibriumwith perhydroxyl radicals (HO
2
∙) as follows [75]
HO2
∙999445999468 H+ + ∙O
2
minus pKa sim 48 (9)
See [76]∙O2
minus undergoes dismutation reaction resulting in H2O2
and O2competing to any other ∙O
2
minus triggered reaction Incase of low pH operation conditions in aqueous solutionsHO2
∙ becomes dominant whose reactivity is considerablyhigher compared to ∙O
2
minus [77] Subsequently HO2
∙ initiatessubstrate (S) oxidation to the radical cation (S+∙) and is itselfreduced to H
2O2[78] Thus increased degradation rates can
be reasonably expected supporting the results made by Ma etal [56] in an acidic environment ∙O
2
minus is extremely reactive inorganic solvents [77] Another aspect is the solubility of kraftlignin (soluble at pH gt 105) which reduces with decreasingpH whereas lignosulfonate should remain unaffected by pHin aqueous solution Moreover 120573ndashOndash4 bonds have beendescribed to be stable at acidic pH [11] In fact this couldadditionally explain the elevated degradation of kraft ligninmade by Kansal et al [48] and Villasenor and Mansilla [74]Nevertheless the contradictory results gained by Ma et al[56] still exist under the assumption that kraft ligninwas used(which would be supported by the high pH of 11 obviouslynecessary for dissolving the lignin powder) Although mostphotocatalytic reactions described in the literature are in anaqueous milieu lignin raw material and its fission productsmay however vary considerably Therefore the optimal pHis most likely to be reaction specific and has to be evaluatedexperimentally in principle
325 Influence of Illumination Many of the studies foundin the literature so far have not dealt on this subject per seWhat is found is the use of different illumination sourceseach having a specified power and lamp type However alltend to emitUV-light between the range 280ndash420 nm Table 2depicts this in detail Other illumination sources include thevisible light spectrum
In general terms illumination influences in that it ini-tiates photocatalysis by generating electron-hole pair in thesemiconductor particles [38 39] Dahm and Lucia [49]altered illumination intensity while observing lignin degra-dation In this study 004 gL lignin was used and lightintensity was varied 223ndash445mWcm3 It was found out thathigher illumination intensities correlated well with higher
12 International Journal of Photoenergy
initial degradation rates and hence total lignin degradation[49] Neppolian et al [69] report degradation to be propor-tional to radiation intensity and best results are achieved forlow lignin concentrations because of enhanced interactionbetween catalyst and incidental photonic flux
In summary high illumination power causes a corre-sponding high initial degradation rate at low lignin concen-trations becausemaximum light penetration into the reactionmedium is favored
33 Process Analytical Methods Various analytical tech-niques have been used to monitor lignin degradation Atthe beginning of this subchapter analytics revealing com-pounds formed from lignin degradation are treated Thisincludes for example gas chromatography (GC) and 1HNMR (nuclear magnetic resonance) This is then followed byresults qualitative analytic measurements such as ultraviolet-visible (UV-Vis) spectroscopy and dissolved carbon (DC)A list of authors analytical techniques applied and resultsachieved are outlined in Table 3
Portjanskaja and Preis [50] studied lignin degradationby measuring the removal of phenols through colorimetricmeasurements As a result of 24 h photocatalytic oxidationunder neutral media conditions 80 of free phenols wereremoved Gas chromatography (GC) result from Ksibi et al[47] attested vanillin vanillic acid palmitic acid biphenyland 345-trimethoxy benzaldehyde structures after the pho-tocatalysis of lignin from black liquor This is in accordancewith the findings of Tonucci et al [33] reporting the formationof vanillin hydroxyl methoxy-acetophenone coniferyl alco-hol coniferyl aldehyde methanol formic acid acetic acidand small amounts of C-2 and C-3 alcohols as degradationproducts1H NMR spectral analysis of lignin before illumination
and after 24 h of illumination showing characteristic bandsof aromatic rings methoxy and aliphatic side chains wascompared with each other Results revealed that the aro-matic ring degraded faster than the aliphatic chain [51]Fourier transformation infrared spectroscopy (FTIR) showedbands corresponding to CH
3 CH2 and CH which remained
unchanged after illumination while bands corresponding toaromatic rings disappeared as a result of illumination [51 53]
Results obtained from the combination of photochemicaland electrochemical oxidation [52 53] were similar to thoseof Tanaka et al [51] Here 13C-NMR confirmed the presenceof the carbonyl functionality and the presence of vanillinand vanillic acid after 12 h photochemical-electrochemicaloxidation These results showed that the combination of aphotocatalytic and an electrochemical oxidation significantlyenhanced the efficiency of lignin degradationThis is becausethe applied anodic potential bias greatly suppressed therecombination of photogenerated electrons and holes [53]
Ultraviolet spectrophotometry offers a convenientmethod to qualitatively and quantitatively analyze ligninin solution [79] This is reflected by the large number ofpublications using this technique [17 33 47ndash51 56 58 80]This is most likely due to its simplicity to interpret lignindegradation Lignins absorb UV light with high molar
200 220 240 260 280 300 320 340 360
00
05
10
15
20
25
Abso
rban
ce
Wavelength (nm)0min
30min
60min
120min
180min
300min
360min
420min
1200min
Figure 8 Time dependent UV-Vis absorption spectra of aqueouslignin solution from waste paper water irradiated with UV light(280ndash420 nm) for different time intervals The spectra are obtainedfor sol-gel derived TiO
2nanocrystalline coating (TiO
2-P25-SiO
2)
[54]
extinction coefficients because of the several methox-ylated phenylpropane units of which they are composed [33]Figure 8 depicts a series of photometric scans of ligninsul-fonate from paper waste water showing a gradual reductionof absorbance during photocatalytic treatment [54] Herethe absorption peaks are around 210 nm and 280 nmAbsorbance decreases with time implying the decompositionof lignin and the deterioration of chromophore groups [54]
Peaks at 210 nmcorrespond to portions of the unsaturatedchains while those at 280 nm correspond to unconjugatedphenolic hydroxyl groups [17] and the aromatic moiety [57]of the lignin molecule The absorption tailing to the longwavelength region arises from the color of lignin [57] Lignindegradation has been reported either at wavelength around280 nm corresponding to unconjugated phenolic hydroxylgroups [17 48 51 58] or for both wavelengths (210 nm and280 nm) [33 54 57] Kobayakawa et al [81] noted some otherabsorbance at wavelengths lower than 250 nm and pointedout that this could be due to the modification of ligninfragmentations leading to the formation of transient specieslikemethanol ethanol formaldehyde formic acid and oxalicacid among others
Analyticalmethods to effectively quantify lignin degrada-tion by calculating the oxygen demand by organic substancesand remaining organic carbon before and after photocatalysishave been studied Amongst the methods are dissolvedcarbon (DC) [49 51 58] chemical oxygen demand (COD)[47 48 50 57 80] biochemical oxygen demand (BOC) [50]dissolved organic carbon (DOC) [56] and American dyemanufacture institute value (ADMI) [56] COD and BODdescribe the oxygen demand by organic substances to be
International Journal of Photoenergy 13
Figure 9 Gradual change from the characteristic yellow ligninsulfonate to a colorless liquid after a period of 20 h Catalyst TiO
2-
P25 (Degussa) +TEOSUV-light 25∘C lignin concentration 05 gL[54]
converted to CO and CO2and H
2O and NH
3 Total organic
carbon (TOC) describes the amount of carbon bound inan organic compound while DOC describes the dissolvedfraction of organic carbon ADMI measures the amount ofdyestuff in water
Decolorization of lignin solution has been reported tobe another parameter observed during photocatalytic degra-dation Color is an indirect indicator of the lignin amountThe higher the color intensity of the solution is the greaterthe lignin content is (high concentrated lignin solutions egblack liquor appear dark brown) [82] Thus color changescan be interpreted as conversion of lignin to transient speciesor conversion to CO
2and H
2O Awungacha Lekelefac et al
[54] observed a gradual change from the characteristic yellowlignin (when highly diluted) to a colorless liquid after a periodof 20 h with sol-gel derived TiO
2nanocrystalline coatings on
sintered borosilicate glass as depicted in Figure 9 A corre-sponding decrease in DC values close to 82 was observedfor TiO
2-P25-SiO
2catalyst confirming degradation This is
shown in Figure 10These findings are analogous to that of Ohnishi et al
[57] who reported the bleaching of lignin when illuminatedcontinuously and that the solution becomes colorless Tothat the chemical oxygen demand (COD) value decreasesgenerating carbon dioxide and a small amount of carbonmonoxide as the main gaseous products COD removalwas reported to be effective at low lignin concentrations ascompared to high lignin concentrations [48]
Another applied analytical technique is fluorescencedetection directly coupled to a high performance liquidchromatography (HPLC) as a means to identify nonaliphaticcomponent in the complex mixture of lignin degradationproducts [54] Fluorescence emission in lignin is attributedto aromatic structures such as conjugated carbonyl biphenylphenylcoumarone and stilbene groups [83 84] AwungachaLekelefac et al [54] observed peaks on both HPLC and
0 200 400 600 800 1000 12000
50
100
150
200
250
Diss
olve
d ca
rbon
(ppm
)
t (min)
TiO2-P25-SiO2
TiO2-P25-SiO2 + PtTiOSO4minus30 6wt
ZnO + TiO2-P25-SiO2
No coating UV light
Figure 10 Variation of DC with time of aqueous lignin solutionfrom waste paper water irradiated with UV light (280ndash420 nm) fordifferent time intervals The spectra are obtained for sol-gel derivedTiO2nanocrystalline coatings (TiO
2-P25-SiO
2+Pt TiO
2-P25-SiO
2
TiOSO4minus306 wt ZnO + TiO
2-P25-SiO
2) [54]
fluorescence chromatograms suggesting the production ofnew substances and fluorophores
Despite developed analytical technologies analyzinglignin degradation products remains challenging Proofssuch as mass spectroscopy (MS) HPLC 13C or 1H-NMRspectra from photocatalytic lignin degradations are not yetestablished The setback to qualitatively and quantitativelyanalyze lignin and its degradation products starts from thenative lignin polymer itself with its indefinite polymericstructure and multiple bond types Also the influence ofdifferent pretreatments additives and the wide variety ofcompounds obtainable from its degradation makes ligninanalysis challenging [85] Moreover lignin streams couldcontain proteins inorganic salts and other potential poisonsthat generally complicate catalysis [4]
The challenge to identify and separate the productsstreams derived from the photocatalytic degradation ligninis also worth noting Lignin product stream is highly func-tionalized and conventional techniques such as gas chro-matography have the disadvantage of requiring a time-consuming derivatization step Also because of high boilingpoint of substances arising from lignin degradation it is noteasily applicable High performance liquid chromatography(HPLC) seems to be the remedy because analysis can becarried out without derivatization but the exact identificationof the separated substances is difficult because of the numer-ous peaks arising from such a chromatogram [87] Unfortu-nately well-established databases such as that of the nationalinstitute of standard and technology (NIST) [88] cannot giveinformation onHPLC-MS chromatogramsThis is because ofthe ionization sources such as electrospray ionization (ESI)
14 International Journal of Photoenergy
Table 4 Main advantages and disadvantages photocatalysis versus enzymatic biocatalysis
Lignin degradation process Advantages Disadvantages
Photocatalysis(single stage)
(i) Relatively fast degradation of complex as wellas nonbiodegradable organic (macro)molecules(ii) Stable catalysts(iii) Moderate reaction conditions
(i) Not selective(ii) Limited selection of operating conditions(iii) Energy costs
Enzymatic conversion(single stage)
(i) More selective(ii) Moderate reaction conditions
(i) High enzyme loads required otherwise ligninhydrolysis is quite slow [16](ii) Enzymes are less stable1(iii) cost-intensive (POXs)2
1For example POXs are sensitive to high H2O2 concentrations2According to Torres and Ayala [86]
and atmospheric pressure chemical ionization (APCI) forLC-MS while GC-MS is based on electron ionization Aremedy to this can be the development of a database ofmodel lignin compounds based on a unanimous HPLC-MSmeasuring procedure This would mean much time and costexpensive investments for adequate personnel and material
Though product identification based on peak superpo-sition gives information on a possible product this can-not always be true for lignin degradation because of thelarge number of possible degradation products In order toproperly identify products from lignin degradation productisolation techniques are to be implemented and further ana-lytic methods such as tandem mass spectroscopy (MSMS)and other advanced structure enhancing research techniquessuch as 1H-NMR 13C-NMR have to be studied extensively
4 Photocatalysis and Its Suitability asIntegrated Technology in MultistageConcepts with Biocatalysis
Depending on lignin treatment specification photocatalysiswill find application for complete lignin mineralization toCO2and H
2O (single stage) or as integrated technology in
a multistage system aiming at partial conversion of ligninmacromolecules to organic low molecular weight intermedi-ates for a consecutive biological oxidation finally generatingvalue added end-products [15]
For biological oxidation of lignin or lignin deriva-tives H
2O2-dependent ligninolytic heme peroxidases (POXs
including lignin peroxidase (LiP EC 111114) manganeseperoxidase (MnP EC 111113) versatile peroxidase (VPEC 111116) O
2-dependent laccases (Lac EC 11032) and
extracellular enzymes from Basidiomycetous white-rot fungiare the most efficient lignin degraders in nature [89] Thussuch enzymatic systems especially POXs (with high redoxpotentials) have attractedmuch interest as industrial biocata-lyst [86 90]The enzyme degradationmechanism (in nature)is facilitated by nonenzymatic processes mainly throughfree ∙OH radicals also generated by the fungus (Fenton-type reaction) Those ∙OH radicals enable the requiredphysical contact between the enzymes and structural units ofthe lignin molecule due to numerous nonspecific oxidative
reactions [91] Both the photocatalytic (equations (2)ndash(7))and the POX reactionmechanism recently reviewed by Busseat al [25] are quite similar Consequently a combination ofphotocatalysis followed by an enzymatic oxidation maybe apromising concept utilizing lignin derivatives for exampleoriginated from industrial effluents [15 92] The advantagesforming biobased products are as follows see also Table 4 inthis context A reduction of the lignin polymerization degreevia photocatalysis at best in more biodegradable interme-diates and a simultaneous detoxification causes savings inenzyme costs since it will be expected that less enzyme loadsare necessary for sufficiently rapid reaction rates [15 91] As aresult hydraulic retention times in bioreactor systems shouldbe diminished Moreover the delignification is expectedto be enhanced simultaneously which was in recent yearsshown in a dual system by Kamwilaisak and Wright [93]using TiO
2H2O2UV for photocatalytic pretreatment and
Lac (from Trametes versicolor) in the subsequent biocatalyticstep Within 24 h they obtained in their dual system anelevation in delignification of 20 (without H
2O2) up to at
least 50 when H2O2was present
In previous studies the treatment of lignin (from thepulp and paper industry) containing effluents by fungus asa biological system (excreting appropriate lignin degradingenzyme cocktails) were more focused for posttreatmentexclusively (Duran et al [92] Reyes et al [94] and Gonzalezet al [95]) Shende et al [17] even examined ligninolyticbacteria with photooxidized kraft lignin as substrate
Although POXs are potential industrial biocatalysts [86]no application studies were found for the direct use in suchdual systems as described above The major reason may bethe complexity of the reaction mechanism (inclusive ligninderivatives as substrate and their analysis) per se on the onehand slowing down research and development processes Onthe other hand POXs are sensitive to their cosubstrate H
2O2
once it is supplied in excess causing considerable inactivation(for details refer to Busse et al [25]) At the present severalstudies are carried out modifying these enzymes regardingenhanced stability activity and selectivity as well Henceit can be expected that their technological applicability willbe raised significantly right after successful modification isreached [86]
International Journal of Photoenergy 15
5 Concluding Remarks
It is widely assumed that the photocatalytic degradation oflignin follows a radical reaction pathway which is similar tothat considered in thermal electrochemical and biochemicalprocesses However reporting on the degradation pathway oflignin derivatives and even that of lignin model compoundsare still amajor challengeThis is probably due to the complexnature and variety of possible degradation products Indeedthemechanism is far more complex considering other factorssuch as type of lignin type of catalyst pH illuminationsource and additives
Generally comparing the different photochemical pro-cesses poses a big challenge because of the wide variablesinvolved These discrepancies start from the source and typeof lignin followed by differences in reactor design illumi-nation source intensity of radiation and different types ofcatalyst An ideawould be to have a specific reference reactionwith well-defined starting parameters which include lignintype source and purity catalyst specifications illuminationsource and intensity so as to ease comparison of results
Basic process parameters such as catalyst concentrationsubstrate concentration addition of metal ion to catalyst pHand illumination have been discussed
Despite developed analytical technologies analyzinglignin degradation products remains challenging Proofssuch as mass spectroscopy (MS) HPLC 13C or 1H-NMRspectra from photocatalytic lignin degradations are not yetestablished
In order to properly identify products from lignin degra-dation product isolation techniques are to be implementedand further analytic methods such as MSMS and otheradvanced structure enhancing research techniques such as1H-NMR and 13C-NMR have to be studied extensively Aremedy to this can be the development of a databank ofmodel lignin compounds based on a unanimous HPLC-MSmeasuring procedure This would mean much time and costexpensive investments for adequate personnel and material
Photocatalysis is denoted as the most popular ligninpretreatment technology besides ozonation [96] Photo-catalyzed lignin may be an appropriate substrate for aconsecutive biocatalytic process using ligninolytic enzymes(POX andor Lac) as supported by experimental results ofKamwilaisak and Wright [93] Combining the advantages ofboth catalytic processes savings in the overall process costswill be expected in addition to elevated lignin conversionNonetheless extensive research work including POX modi-fications is still required
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Authorsrsquo Contribution
Colin Awungacha Lekelefac and Nadine Busse contributedequally to this work
Acknowledgments
The authors gratefully thank the Federal Ministry of Educa-tion and Research (BMBF) for funding (FKZ17N0310) Theresearchers also thank the Hessen State Ministry of HigherEducation Research andArts for the financial support withinthe Hessen initiative for scientific and economic excellence(LOEWE)
References
[1] Crude Oil and Commodity Prices httpwwwoil-pricenet[2] Research Directorate-General for World Energy Technology
and Climate Policy Outlook (WETO) Luxembourg Office forOfficial Publications of the European Communities 2003
[3] Green and Natural Polymers Are on the Rise 2014 httpwwwpolymersolutionscombloggreen-and-natural-polymers-on-the-rise
[4] J Zakzeski P C Bruijnincx A L Jongerius and B M Weck-huysen ldquoThe catalytic valorization of lignin for the productionof renewable chemicalsrdquo Chemical Reviews vol 110 no 6 pp3552ndash3599 2010
[5] B KammM Kamm P R Gruber and S KromusBiorefineries-Industrial Processes and Products Status Quo and Future Direc-tions vol 1 Wiley-VCH 2006
[6] R L Howard E Abotsi E L J van Rensburg and S HowardldquoLignocellulose biotechnology issues of bioconversion andenzyme productionrdquoAfrican Journal of Biotechnology vol 2 no12 pp 602ndash619 2003
[7] M Stocker ldquoBio- und BTL-Kraftstoffe in der Bioraffineriekatalytische Umwandlung Lignocellulose-reicher Biomasse mitporosen Stoffenrdquo Angewandte Chemie vol 120 no 48 pp9340ndash9351 2008
[8] J-P Lange ldquoLignocellulose conversion an introduction tochemistry process and economicsrdquo Biofuels Bioproducts andBiorefining vol 1 no 1 pp 39ndash48 2007
[9] D A I Goring ldquoThe physical chemistry of ligninrdquo Pure andApplied Chemistry vol 5 no 1-2 pp 233ndash310 1962
[10] M Ek G Gellerstedt and G HenrikssonWood Chemistry andBiotechnology Pulp and Paper Chemistry and Technology vol 1Walter de Gruyter GmbH amp Co KG Berlin Germany 2009
[11] B Saake and R Lehnen ldquoLigninrdquo in Ullmannrsquos Encyclopedia ofIndustrial Chemistry Wiley-VCH Verlag GmbH amp Co KGaAWeinheim Germany 2012
[12] M N S Kumar A K Mohanty L Erickson and M MisraldquoLignin and its applications with polymersrdquo Journal of BiobasedMaterials and Bioenergy vol 3 no 1 pp 1ndash24 2009
[13] R J A Gosselink E de Jong B Guran and A AbacherlildquoCo-ordination network for ligninmdashstandardisation produc-tion and applications adapted to market requirements(EUROLIGNIN)rdquo Industrial Crops and Products vol 20 no 2pp 121ndash129 2004
[14] D Fengel and G Wegener Wood Chemistry UltrastructureReactions Verlag Kessel 1984
[15] D Mantzavinos and E Psillakis ldquoEnhancement of biodegrad-ability of industrial wastewaters by chemical oxidation pre-treatmentrdquo Journal of Chemical Technology and Biotechnologyvol 79 no 5 pp 431ndash454 2004
16 International Journal of Photoenergy
[16] M Dashtban H Schraft and W Qin ldquoFungal bioconversionof lignocellulosic residues Opportunities amp perspectivesrdquo Inter-national Journal of Biological Sciences vol 5 no 6 pp 578ndash5952009
[17] A Shende R Jaswal D Harder-Heinz A Menan and RShende ldquoIntergrated photocatalytic and microbial degradationof kraft ligninrdquo Cleantech pp 120ndash123 2012
[18] F S Chakar and A J Ragauskas ldquoReview of current and futuresoftwood kraft lignin process chemistryrdquo Industrial Crops andProducts vol 20 no 2 pp 131ndash141 2004
[19] W G Glasser and H R Glasser ldquoEvaluation of ligninrsquoschemical structure by experimental and computer simulationtechniquesrdquo Paperi ja Puu vol 63 pp 71ndash83 1981
[20] M Erickson S Larsson and G E Miksche ldquoZur Struktur desLignins der FichterdquoActaChemica Scandinavica vol 27 pp 903ndash914 1973
[21] H Nimz ldquoDas Lignin der BuchemdashEntwurf eines Konstitution-sschemasrdquo Angewandte ChemiemdashInternational Edition vol 86pp 336ndash344 1974
[22] D V Evtuguin C P Neto J Rocha and J D P de JesusldquoOxidative delignification in the presence of molybdovana-dophosphate heteropolyanions mechanism and kinetic stud-iesrdquo Applied Catalysis A General vol 167 no 1 pp 123ndash1391998
[23] A L Linsebigler G Lu and J T Yates Jr ldquoPhotocatalysis onTiO2surfaces Principles mechanisms and selected resultsrdquo
Chemical Reviews vol 95 no 3 pp 735ndash758 1995[24] Y Miyata K Miyazaki M Miura Y Shimotori M Aoyama
and H Nakatani ldquoSolventless delignification of wood flourwith TiO
2poly(ethylene oxide) photocatalyst systemrdquo Journal
of Polymers and the Environment vol 21 no 1 pp 115ndash121 2013[25] N Busse D Wagner M Kraume and P Czermak ldquoReaction
kinetics of versatile peroxidase for the degradation of lignincompoundsrdquoTheAmerican Journal of Biochemistry andBiotech-nology vol 9 no 4 pp 365ndash394 2013
[26] M Tien and T K Kirk ldquoLignin-degrading enzyme fromphanerochaete chrysosporium Purification characterizationand catalytic properties of a uniqueH
2O2-requiring oxygenaserdquo
Proceedings of the National Academy of Sciences vol 81 pp2280ndash2284 1984
[27] T K Kirk M Tien P J Kersten M D Mozuch and BKalyanaraman ldquoLigninase of Phanerochaete chrysosporiumMechanism of its degradation of the non-phenolic arylglycerol120573-aryl ether substructure of ligninrdquo Biochemical Journal vol236 no 1 pp 279ndash287 1986
[28] T Lundell RWever R Floris et al ldquoLignin peroxidase L3 fromPhlebia radiata pre-steady-state and steady-state studies withveratryl alcohol and a non-phenolic lignin model compound1-(34-dimethoxyphenyl)-2-(2-methoxyphenoxy)propane-13-diolrdquo European Journal of Biochemistry vol 211 no 3 pp391ndash402 1993
[29] H E Schoemaker T K Lundell A I Hatakka and K PiontekldquoThe oxidation of veratryl alcohol dimeric lignin models andlignin by lignin peroxidase the redox cycle revisitedrdquo FEMSMicrobiology Reviews vol 13 no 2-3 pp 321ndash331 1994
[30] J M Palmer P J Harvey and H E Schoemaker ldquoThe role ofperoxidases radical cations and oxygen in the degradation oflignin [and discussion]rdquo Philosophical Transactions of the RoyalSociety of London A vol 321 no 1561 pp 495ndash505 1987
[31] C Hill Wood Modification Chemical Thermal and OtherProcesses John Wiley amp Sons Chichester UK 2006
[32] Y-H Xu H-R Chen Z-X Zeng and B Lei ldquoInvestigation onmechanism of photocatalytic activity enhancement of nanome-ter cerium-doped titaniardquo Applied Surface Science vol 252 no24 pp 8565ndash8570 2006
[33] L Tonucci F Coccia M Bressan and N DrsquoAlessandro ldquoMildphotocatalysed and catalysed green oxidation of lignin auseful pathway to low-molecular-weight derivativesrdquoWaste andBiomass Valorization vol 3 no 2 pp 165ndash174 2012
[34] A Castellan N Colombo C Vanucci P Fornier de HViolet and H Bouas-Laurent ldquoPhotodegradation of ligninA photochemical study of an O-methylated 120572-carbonyl 120573-1lignin model dimer 12-di(3101584041015840-dimethoxyphenyl) ethanone(deoxyveratroin)rdquo Journal of Photochemistry and PhotobiologyA vol 51 pp 451ndash467 1990
[35] A Fujishima X Zhang and D A Tryk ldquoTiO2photocatalysis
and related surface phenomenardquo Surface Science Reports vol63 no 12 pp 515ndash582 2008
[36] C S Turchi and D F Ollis ldquoPhotocatalytic reactor designan example of mass-transfer limitations with an immobilizedcatalystrdquo The Journal of Physical Chemistry vol 92 no 23 pp6852ndash6853 1988
[37] R Subasri M Tripathi K Murugan J Revathi G V N Raoand T N Rao ldquoInvestigations on the photocatalytic activity ofsolndashgel derived plain and Fe3+Nb5+ doped titania coatings onglass substratesrdquo Materials Chemistry and Physics vol 124 pp63ndash68 2010
[38] N Serpone ldquoRelative photonic efficiencies and quantum yieldsin heterogeneous photocatalysisrdquo Journal of Photochemistry andPhotobiology A Chemistry vol 104 no 1ndash3 pp 1ndash12 1997
[39] M R Hoffmann S T Martin W Choi and D W BahnemannldquoEnvironmental applications of semiconductor photocatalysisrdquoChemical Reviews vol 95 no 1 pp 69ndash96 1995
[40] C D Jaeger and A J Bard ldquoSpin trapping and electron spinresonance detection of radical intermediates in the photode-composition of water at titanium dioxide particulate systemsrdquoThe Journal of Physical Chemistry vol 83 no 24 pp 3146ndash31521979
[41] R W Matthews ldquoHydroxylation reactions induced by near-ultraviolet photolysis of aqueous titaniumdioxide suspensionsrdquoJournal of the Chemical Society Faraday Transactions vol 80no 2 pp 457ndash471 1984
[42] A E H Machado A M Furuyama S Z Falone R RuggieroD D S Perez and A Castellan ldquoPhotocatalytic degradation oflignin and lignin models using titanium dioxide the role of thehydroxyl radicalrdquoChemosphere vol 40 no 1 pp 115ndash124 2000
[43] W Sigg and L Stumm Aquatische Chemie Stuttgart 2 AuflageBG Teubner Stuttgart Germany 1991
[44] O Legrini E Oliveros and A M Braun ldquoPhotochemicalprocesses for water treatmentrdquo Chemical Reviews vol 93 no2 pp 671ndash698 1993
[45] G Rothenberger J Moser M Gratzel N Serpone and DSharma ldquoCharge carrier trapping and recombination dynamicsin small semiconductor particlesrdquo Journal of the AmericanChemical Society vol 107 no 26 pp 8054ndash8059 1985
[46] P Mazellier M Sarakha A Rossi and M Bolte ldquoThe aqueousphotochemistry of 26-dimethylphenol Evidence for the frag-mentation of the 120572 C-C bondrdquo Journal of Photochemistry andPhotobiology A vol 115 no 2 pp 117ndash121 1998
[47] M Ksibi S B Amor S Cherif E Elaloui A Houas and MElaloui ldquoPhotodegradation of lignin from black liquor using aUVTiO
2systemrdquo Journal of Photochemistry and Photobiology
A vol 154 no 2-3 pp 211ndash218 2003
International Journal of Photoenergy 17
[48] S K Kansal M Singh and D Sud ldquoStudies on TiO2ZnO
photocatalysed degradation of ligninrdquo Journal of HazardousMaterials vol 153 no 1-2 pp 412ndash417 2008
[49] A Dahm and L A Lucia ldquoTitanium dioxide catalyzed pho-todegradation of lignin in industrial effluentsrdquo Industrial andEngineering Chemistry Research vol 43 no 25 pp 7996ndash80002004
[50] E Portjanskaja and S Preis ldquoAqueous photocatalytic oxidationof lignin the influence of mineral admixturesrdquo InternationalJournal of Photoenergy vol 2007Article ID76730 7 pages 2007
[51] K Tanaka R C R Calanag and T Hisanaga ldquoPhotocatalyzeddegradation of lignin on TiO
2rdquo Journal of Molecular Catalysis
A Chemical vol 138 no 2-3 pp 287ndash294 1999[52] M Tian J Wen D MacDonald R M Asmussen and A Chen
ldquoA novel approach for lignin modification and degradationrdquoElectrochemistry Communications vol 12 no 4 pp 527ndash5302010
[53] K PanM Tian Z-H Jiang B Kjartanson and A Chen ldquoElec-trochemical oxidation of lignin at lead dioxide nanoparticlesphotoelectrodeposited on TiO
2nanotube arraysrdquo Electrochim-
ica Acta vol 60 pp 147ndash153 2012[54] C Awungacha Lekelefac J Hild P Czermak and M Herren-
bauer ldquoPhotocatalytic active coatings for lignin degradationin a continuous packed bed reactorrdquo International Journal ofPhotoenergy vol 2014 Article ID 502326 10 pages 2014
[55] C Awungacha Lekelefac P Czermak and M HerrenbauerldquoEvaluation of photocatalytic active coatings on sintered glasstubes by methylene bluerdquo International Journal of Photoenergyvol 2013 Article ID 614567 9 pages 2013
[56] Y-S Ma C-N Chang Y-P Chiang H-F Sung and A CChao ldquoPhotocatalytic degradation of lignin using PtTiO
2as
the catalystrdquo Chemosphere vol 71 no 5 pp 998ndash1004 2008[57] H Ohnishi M Matsumura H Tsubomura and M Iwasaki
ldquoBleaching of lignin solution by a photocatalyzed reactionon semiconductor photocatalystsrdquo Industrial and EngineeringChemistry Research vol 28 no 6 pp 719ndash724 1989
[58] C A K Gouvea F Wypych S G Moraes N Duran and PPeralta-Zamora ldquoSemiconductor-assisted photodegradation oflignin dye and kraft effluent by Ag-doped ZnOrdquo Chemospherevol 40 no 4 pp 427ndash432 2000
[59] A V Vahatalo K Salonen M Salkinoja-Salonen and AHatakka ldquoPhotochemical mineralization of synthetic lignin inlake water indicates enhanced turnover of aromatic organicmatter under solar radiationrdquo Biodegradation vol 10 no 6 pp415ndash420 1999
[60] H de Lasa B Serrano and M Salaices Photocatalytic ReactionEngineering Springer New York NY USA 2005
[61] K Hashimoto H Irie and A Fujishima ldquoTiO 2 photocatalysisA historical overview and future prospectsrdquo Japanese Journal ofApplied Physics vol 44 no 12 pp 8269ndash8285 2005
[62] M A Behnajady N Modirshahla and R Hamzavi ldquoKineticstudy on photocatalytic degradation of CI Acid Yellow 23 byZnOphotocatalystrdquo Journal ofHazardousMaterials vol 133 no1ndash3 pp 226ndash232 2006
[63] M Addamo V Augugliaro A di Paola et al ldquoPhotocatalyticthin films of TiO
2formed by a sol-gel process using titanium
tetraisopropoxide as the precursorrdquo Thin Solid Films vol 516no 12 pp 3802ndash3807 2008
[64] N Negishi K Takeuchi and T Ibusuki ldquoPreparation of theTiO2thin filmphotocatalyst by the dip-coating processrdquo Journal
of Sol-Gel Science and Technology vol 13 no 1ndash3 pp 691ndash6941998
[65] L Rideh A Wehrer D Ronze and A Zoulalian ldquoPhotocat-alytic degradation of 2-chlorophenol in TiO
2aqueous suspen-
sion modeling of reaction raterdquo Industrial and EngineeringChemistry Research vol 36 no 11 pp 4712ndash4718 1997
[66] R-ADoongC-HChen RAMaithreepala and S-MChangldquoThe influence of pHand cadmiumsulfide on the photocatalyticdegradation of 2-chlorophenol in titanium dioxide suspen-sionsrdquoWater Research vol 35 no 12 pp 2873ndash2880 2001
[67] K V Sarkanen A Islam and C D Anderson ldquoOzonationrdquo inMethods in Lignin Chemistry S Y Lin and C W Dence EdsSpringer Series inWood Science pp 387ndash406 Springer BerlinGermany 1992
[68] G Gellerstedt and E-L Lindfors ldquoStructural changes in ligninduring kraft pulpingrdquo Holzforschung vol 38 no 3 pp 151ndash1581984
[69] B Neppolian H C Choi M V Shankar B Arabindoo andV Murugesan in Proceedings of the International Symposiumon Environmental Pollution Control and Waste Management(EPCOWM 02) p 647 2002
[70] C Pouteau P Dole B Cathala L Averous and N BoquillonldquoAntioxidant properties of lignin in polypropylenerdquo PolymerDegradation and Stability vol 81 no 1 pp 9ndash18 2003
[71] J Hafren T Fujino and T Itoh ldquoChanges in cell wall archi-tecture of differentiating tracheids of Pinus thunbergii duringlignificationrdquo Plant and Cell Physiology vol 40 no 5 pp 532ndash541 1999
[72] T Dizhbite G Telysheva V Jurkjane and U Viesturs ldquoChar-acterization of the radical scavenging activity of lignins naturalantioxidantsrdquo Bioresource Technology vol 95 no 3 pp 309ndash3172004
[73] K Hofstadler R Bauer S Novalic and S G Heisier ldquoNew reac-tor design for photocatalytic wastewater treatment with TiO
2
immobilized on fused-silica glass fibers photomineralization of4-chlorophenolrdquo Environmental Science amp Technology vol 28no 4 pp 670ndash674 1994
[74] J Villasenor and H D Mansilla ldquoEffect of temperature onkraft black liquor degradation by ZnO-photoassisted catalysisrdquoJournal of Photochemistry and Photobiology A Chemistry vol93 no 2-3 pp 205ndash209 1996
[75] B H Bielski D E Cabelli L A Ravindra and A B RossldquoReactivity of HO
2Ominus2Radicals in aqueous solutionrdquo Journal
of Physical Chemistry vol 14 pp 1041ndash1100 1985[76] B H J Bielski and A O Allen ldquoMechanism of the dispropor-
tionation of superoxide radicalsrdquo Journal of Physical Chemistryvol 81 no 11 pp 1048ndash1050 1977
[77] B Halliwell and J M C Gutteridge ldquoThe importance of freeradicals and catalytic metal ions in human diseasesrdquoMolecularAspects of Medicine vol 8 no 2 pp 89ndash193 1985
[78] J M Palmer P J Harvey and H E Schoemaker ldquoThe role ofperoxidases radical cations and oxygen in the degradation oflignin [and discussion]rdquo Philosophical Transaction of the RoyalSociety A vol 321 no 1561 pp 495ndash505 1987
[79] S Y Yin and C W Dense Methods in Lignin ChemistrySpringer New York NY USA 1992
[80] P Kumar S Kumar and N K Bhardwaj ldquoPhotocatalyticoxidation of elemental chlorine free bleaching effluent withUVTiO
2rdquo in Proceedings of the 2nd International Conference on
Environmental Science and Technology (ICEST rsquo11) SingaporeFebruary 2011
[81] K Kobayakawa Y Sato S Nakamura and A FujishimaldquoPhotodecomposition of Kraft lignin catalyzed by titanium
18 International Journal of Photoenergy
dioxiderdquo Bulletin of the Chemical Society of Japan vol 62 no11 pp 3433ndash3436 1989
[82] R B Kinstre ldquoAn overview of strategies for reducing theenvironmental impact of bleach-plant effluentsrdquo Tappi Journalvol 76 no 5 pp 105ndash113 1993
[83] A Castellan H Choudhury R Stephen Davidson and SGrelier ldquoComparative study of stone-ground wood pulp andnative wood 3 Application of fluorescence spectroscopy to astudy of the weathering of stone-ground pulp and native woodrdquoJournal of Photochemistry and Photobiology A Chemistry vol81 no 2 pp 123ndash130 1994
[84] B Albinsson S Li K Lundquist and R Stomberg ldquoThe originof lignin fluorescencerdquo Journal of Molecular Structure vol 508no 1ndash3 pp 19ndash27 1999
[85] S Baumberger A Abaecherli M Fasching et al ldquoMolar massdetermination of lignins by size-exclusion chromatographytowards standardisation of the methodrdquo Holzforschung vol 61no 4 pp 459ndash468 2007
[86] E Torres andM Ayala Biocatalysis Based on Heme PeroxidasesSpringer New York NY USA 1st edition 2010
[87] R Pecina P Burtscher G Bonn and O Bobleter ldquoGC-MSand HPLC analyses of lignin degradation products in biomasshydrolyzatesrdquo Fresenius Zeitschrift fur Analytische Chemie vol325 no 5 pp 461ndash465 1986
[88] NIST NIST Standard Reference Database The National Insti-tute of Standards and Technology 2014 httpwwwnistgovsrdnist1acfm
[89] T K Kirk and R L Farrell ldquoEnzymatic ldquocombustionrdquo themicrobial degradation of ligninrdquo Annual Review of Microbiol-ogy vol 41 pp 465ndash501 1987
[90] A TMartinez ldquoHigh redox potential peroxidasesrdquo in IndustrialEnzymes Structure Function and Applications K-B BeckerEd pp 477ndash488 Springer Amsterdam The Netherlands 1stedition 2007
[91] M Dashtban H Schraft T A Syed and W Qin ldquoFungalbiodegradation and enzymatic modification of ligninrdquo Interna-tional Journal of Biochemistry and Molecular Biology vol 1 no1 pp 36ndash50 2010
[92] N Duran E Esposito L H Innocentini-Mei and V P CanhosldquoA new alternative process for Kraft E1 effluent treatmentrdquoBiodegradation vol 5 no 1 pp 13ndash19 1994
[93] K Kamwilaisak and P C Wright ldquoInvestigating laccase andtitanium dioxide for lignin degradationrdquo Energy and Fuels vol26 no 4 pp 2400ndash2406 2012
[94] J Reyes M Dezotti H Mansilla J Villasenor E Espositoand N Duran ldquoBiomass photochemistry-XXII combined pho-tochemical and biological process for treatment of Kraft E1effluentrdquoApplied Catalysis B Environmental vol 15 no 3-4 pp211ndash219 1998
[95] L F Gonzalez V Sarria and O F Sanchez ldquoDegradationof chlorophenols by sequential biological-advanced oxidativeprocess using Trametespubescens and TiO
2UVrdquo Bioresource
Technology vol 101 no 10 pp 3493ndash3499 2010[96] J H Lora and W G Glasser ldquoRecent industrial applications
of lignin a sustainable alternative to nonrenewable materialsrdquoJournal of Polymers and the Environment vol 10 no 1-2 pp 39ndash48 2002
Submit your manuscripts athttpwwwhindawicom
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Inorganic ChemistryInternational Journal of
Hindawi Publishing Corporation httpwwwhindawicom Volume 2014
International Journal ofPhotoenergy
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Carbohydrate Chemistry
International Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
Chemistry
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Advances in
Physical Chemistry
Hindawi Publishing Corporationhttpwwwhindawicom
Analytical Methods in Chemistry
Journal of
Volume 2014
Bioinorganic Chemistry and ApplicationsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
SpectroscopyInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Medicinal ChemistryInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Chromatography Research International
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Applied ChemistryJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Theoretical ChemistryJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
Spectroscopy
Analytical ChemistryInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Quantum Chemistry
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Organic Chemistry International
ElectrochemistryInternational Journal of
Hindawi Publishing Corporation httpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CatalystsJournal of
6 International Journal of Photoenergy
Nonenzymatic subsequent reactions
Adlerol
Veratryl alcohol radical
Enzyme1
23
4
4
1
2
3
5
5
6
6
Guaiacol
Veratraldehyde
Ketone
O
OO
O
O O
O
O
O
HO
HOHO
HO
OCH3
OCH3
OCH3
OCH3
OCH3
OCH3
120572120573
120574
minusH+
∙+
∙
∙+
c120572 oxidation H2O
O2
120572 120573
120574
HOHOHO
OCH3
OCH3OCH3
OCH3 OCH3
OCH3
OCH3
120572120572
120573 120573
120573
120574 120574
HO
HO
120574
OCH3
120573
HO
HO
120574
HO
OCH3
OCH3
OCH3
120572120573
120574
Figure 7 Proposed radical reaction scheme initiated by enzyme lignolytic heme peroxidase for the conversion of adlerol possessing C120573ndashOndash4 bonds into smaller units summarized by Busse et al [25] and abstracted from Tien and Kirk [26] Kirk et al [27] Lundell et al [28]Schoemaker et al [29] and Palmer et al [30]
processes However reporting on the degradation pathway oflignin derivatives and even that of lignin model compoundsis still a major challenge This is probably due to the complexnature and variety of possible degradation products Indeedthemechanism is far more complex considering other factorssuch as type of lignin type of catalyst pH illuminationsource and additives
32 Influence of Process Parameters in Lignin DegradationVarying process parameter could either have a positive ornegative impact on the photocatalytic efficiency The basicprocess parameters such as catalyst concentration [4 4856] substrate concentration [48 51] addition of metal ion
to TiO2catalyst [17 50 56ndash58] pH [47 48] illumination
[33 48 49 59] and their influence shall be discussed in thissubchapter Table 2 gives an overview about starting reactionconditions and catalyst applied by some work groups whileTable 3 portrays parameters analytical methods and resultsobtained It is worthwhile noting that comparing the differentphotochemical processes poses a big challenge because ofthe wide variables involvedThese discrepancies start alreadyfrom the source and type of lignin followed by the differencesin reactor design illumination source intensity of radiationand different types of TiO
2catalyst such as Fischer scientific
rutile TiO2[49] TiO
2(TiO2mdashTP-2 of Fujititan) just to name
a few
International Journal of Photoenergy 7
Table2Summaryof
startingcond
ition
sfor
thep
hotocatalytic
degradationof
lignin
Reference
Lign
insource
Catalyst
Reactio
ncond
ition
s
MacHadoetal[42]
Peroxyform
icacid
lignins
from
eucalyptus
grandisw
ood(EL1)
EL1+
sodium
borohydride
TiO
2andH
2O2
Lign
inconcentration
025
mgmL119879=25∘C
pH11U
VVisradiatio
n120582gt300nm
40
0Wmercury
lampcylin
dricalPy
rexglassreactorcon
stanto
xygenbu
bblin
g
Ksibietal[47]
Water
solubleligninob
tained
from
blackliq
uor
TiO
2-P2
5(D
egussa)
Lign
inconcentration
90mgL119879=20∘C
pH82UV-radiation120582gt290nm
pyrex
reactoro
pento
airPh
ilips
HPK
125W
lamp
Kansaletal[48]
Lign
infro
mwheatstr
awkraft
digestion
TiO
2-P2
5(D
egussa)a
ndZn
O
Lign
inconcentration
10ndash100
mgLin
100m
LZn
Ocatalystdo
se(05ndash20gL)pH
ofthes
olution(pH3ndash11)solarillu
mination
oxidantcon
centratio
n306times10minus6M
to153times10minus6Mthinbedfilm
slurrypo
ndreactoroxidantsodium
hypo
chlorite
solutio
n(4availableC
l 2)
Dahm
andLu
cia[
49]
Whitewater
from
indu
strialprocess
water
thatexits
inpaperm
achines
FischerS
cientifi
crutile
TiO
2
Lign
inconcentration
40mgLin
500m
Lbatchreactor119879=21∘Cndash
42∘C
Rayonet
photochemicalcham
ber16
VWR8-W
blacklightpho
spho
r(350-nm
)lam
ps
constant
oxygen
bubb
lingpo
wer
ofillum
ination
128to
64Wlight
intensity
223ndash44
5mWcm
3
Portjanskajaand
Preis[50]
Lign
inpurchased
from
Aldric
hTiO
2P2
5-N(D
egussa)
Lign
inconcentration
100m
gLpH
8batchreactorsystemreactor
open
toair
PhillipsT
LD15W05low-pressurelum
inescent
mercury
UV-lampUV-radiation
120582gt360nm
pow
erdensity
ofirr
adiatio
n=07m
Wcm
2 visib
lelight
source
Tanaka
etal[51]
Lign
infro
mconiferous
woo
dTiO
2(TiO
2mdashTP
-2of
Fujititan)
Lign
inconcentration
0003to
003U
V-radiation120582gt310nm
cylindrical
reactio
nvessel
Tonu
ccietal[33]
Ca2+andNH4
+ligninderiv
atives
TiO
2as
Degussa
P25
+po
lyoxom
etalates
(POM)H
2O2
Openqu
artztubes(20
mL)
reactor119879=20∘C
1atm
multirayso
ften
UVlamps
of15W
power
eachU
V-radiation120582gt254nm
Miyatae
tal[24]
Piceag
lehn
iiwoo
dflo
urTiO
2po
lyethylene
oxide(PE
O)
Reactorop
enpetrid
ish119879=30∘C119905=48h40
0Wmercury
lamp
Shende
etal[17]
Kraft
lignin
TiO
2-Zn
O-ZrO
2Lign
inconcentration
1mgmLin
10mL250W
Xeno
nlampandAM
15Glamp
filterpo
wer
density
ofirr
adiatio
n100m
Wcm
2 solarlam
psim
ulator
Tian
etal[52]
andPanetal[53]
Kraft
ligninfro
mblackliq
uor
Ta2O
5-IrO
2andPb
O2thin
film
TiO
2nano
tubePbO
2
Lign
inconcentration
30(w
w)UV-radiation120582gt365nm
119905=10minpow
erdensity
ofirr
adiatio
n20
mWcm
2 blue
waveT
M50
ASUVspot
lampEG
ampG2273
potentiosta
tgalvano
stattoapplycurrentTiTiO
2NTPb
O2ele
ctrode
asworking
electrode
andPt
coilas
outere
lectrode
andAgAgC
lasreference
electrode
Awun
gachaL
ekele
fac
etal[54][55]
Lign
insulfo
natefro
mpaperw
aste
water
TiO
2as
Degussa
P25
TiO
2fro
msol-g
elprocesso
fTiOSO
4TT
IP
Lign
inconcentration
500m
gLin
200m
LOsram
Planon
light
sourceirradiance
30ndash4
0Wm
2 UV-radiation120582280ndash4
20nm
119905=20hreactoro
pento
air
recirculationsyste
mflow
rate225mLmin
8 International Journal of Photoenergy
Table3Parametersanalyticalmetho
dsand
results
from
different
workgrou
ps
Reference
Parameter
studied
Analytic
sRe
sult
MacHado
etal[42]
Roleof
hydroxylradicals
irradiatio
nof
ligninin
the
absencea
ndpresence
ofph
otocatalystT
iO2andH
2O2
Ultraviolet-v
isible(UV-Vis)spectro
scop
yionizatio
nabsorptio
nspectro
scop
y(IA
S)
sizee
xclusio
nchromatograph
y(SEC
)
Asharpdecrease
inthep
heno
liccontento
bservedforreactions
involvingdirect
photolysis
SEC
areductio
nof
almost50
inthea
verage
molecular
weighto
fligninequalto
14kD
after
90min
ofirr
adiatio
n
Ksibietal[47]
Irradiationof
ligninin
the
absencea
ndpresence
ofph
otocatalystT
iO2-P2
5
UV-Visspectroscop
y13C-
nucle
armagnetic
resonance(NMR)
solid
state
totalion
gasc
hrom
atograph
y(TIC)
indu
ctioncoup
lingplasma(
ICP)
chem
icaloxygen
demand(C
OD)
56degradationratewith
TiO
2catalystaft
er420m
inreactiontim
eethylacetate-extractableprod
uctsshow
edvanillin
vanillica
cidpalm
itica
cid
biph
enylstructuresand
345-trim
etho
xybenzaldehyde
presence
ofmagnesiu
mandcalcium
ions
CODremovalishigh
erforthe
initiallylowconcentrations
ofligninsolutio
n
Kansaletal[48]
Catalystdo
sepHoxidant
concentration
initialsubstrate
concentration
ZnOcatalystin
slurryandim
mob
ilizedmod
e
UV-Visspectroscop
yCO
D
Optim
umcatalystdo
seis1gL
optim
umoxidantcon
centratio
n2times10minus6M
graduald
ecreaseo
fabsorptionpeak
indicatin
gdecompo
sitionof
organics
CODremovalishigh
erforthe
initiallylowconcentrations
ofligninsolutio
n
Dahm
and
Lucia[
49]
Catalystdo
seillu
mination
intensity
UV-Visspectroscop
ytotalorganiccarbon
(TOC)
capillaryionelectro
phoresis
analysis(C
IA)
Gradu
aldecrease
inabsorptio
npeak
indicatin
gdecompo
sitionof
organics
optim
alcatalystdo
seof
10mgm
high
erillum
inationintensities
correlated
well
with
high
erinitialdegradationrate
74disapp
earanceo
fTOC
Portjanskajaand
Preis[50]
Influ
ence
offerrou
sion
sN-dop
edcatalysteffect
sprayedcatalyston
supp
ortand
subm
ersedcatalyst
CODU
V-Visspectroscop
ybiochemical
oxygen
demand(BOD)colorim
etric
measurementat570
nm
Additio
nof
Fe2+upto
28m
gLleadsto25increase
inph
otocatalyticeffi
ciency
sprayedcatalystexhibited15
times
high
ereffi
ciency
than
theo
neattached
bysubm
ersio
nnegligibleeffecto
fN-dop
edcatalyst
increase
ofaldehyde
concentrationover
reactio
ntim
eneutralm
ediawas
mostb
eneficialforb
iodegradability
80of
freep
heno
lsremoved
undern
eutralcond
ition
s
Tanaka
etal[51]Ca
talystloadinglignin
concentration
illum
inationtim
e
UV-Visspectroscop
yTO
Cgel
perm
eatio
nchromatograph
y(G
PC)
1 HNMR
Fouriertransform
ationinfrared
(FTIR)
spectro
scop
y
FTIR
measurementrevealedafasttransform
ationof
arom
aticmoietyp
resent
inlignin
characteris
ticband
sofaromaticrin
gsm
etho
xyand
aliphatic
sidec
hains
decrease
inTO
Cvalues
over
time
decrease
indegradationratewith
increase
catalystdo
sageB
utaft
ercatalystthreshold
valueisa
ttainedcatalystincreasec
ausesa
decrease
indegradationrates
FTIR
peaksa
reshifted
towards
lower
molecular
weightregionaft
erph
otocatalysis
Tonu
cci
etal[33]
Testof
catalytic
syste
mstoob
tain
fractio
nswith
redu
ceddegreeso
fpo
lymerization
comparis
onof
thermalandph
otochemical
reactio
ns
1 HNMR
gasc
hrom
atograph
y-mass
spectro
scop
y(G
C-MS)
POMsa
relessselectivew
henused
asph
otocatalystsandno
appreciableb
leaching
ofthes
olutionwas
seen
whenPO
Mwas
used
asthermalcatalyst
deriv
edchem
icalsfrom
experim
entvanillin
hydroxylmetho
xy-acetoph
enon
econiferylalcoh
olcon
iferylaldehydemethano
lform
icacidacetic
acidand
sometim
essm
allamou
ntso
fC-2
andC-
3alcoho
ls
Miyatae
tal[24]
Exam
inationof
cellwall
structureo
flignin(w
oodflo
ur)
before
andaft
erph
otocatalysis
GC-
MSscanning
electronmicroscop
y(SEM
)1 H
NMR
Highdelignificationactiv
itydelignificationconfi
nedto
thes
urface
oflignin
deriv
edchem
icalse
xperim
entvanillin
Shende
etal[17]Testof
combinedactio
nof
bio-
andph
otocatalyticsyste
ms
UV-Visspectroscop
yGC-
MSX-
ray
dispersiv
eenergyspectro
scop
yandX-
ray
diffractio
nanalysis(EDX
XRD)SE
M
Detectio
nof
follo
wingchem
icalsacetylguaiacol4-ethoxym
ethyl-2
-metho
xyph
enol
metho
xyph
enyloxim
eguaiacolsuccinica
cidacetylguaiacolvanillicacidand
vanillin
International Journal of Photoenergy 9
Table3Con
tinued
Reference
Parameter
studied
Analytic
sRe
sult
Tian
etal[52]
andPanetal
[53]
Testof
combinedactio
nof
electro-and
photocatalytic
syste
ms
UV-Visspectroscop
yFT
IRSEM
X-ray
(EDX)
highperfo
rmance
liquid
chromatograph
y(H
PLC)
COD
Detectio
nof
thefollowingchem
icalscarbon
ylfunctio
nalityvanillin
andvanillica
cid
Awun
gacha
Lekeleface
tal
[54][55]
Com
paris
onof
degradationrates
bydifferent
catalyst
HPL
Cflu
orescencea
ndUV-Vis
spectro
scop
ySE
MT
OC
UV-Visresultsrevealfaste
rdegradatio
nof
thea
liphatic
moietycomparedto
the
arom
aticmoietyof
ligninsulfonateob
tained
from
paperw
astewaterPeaks
observed
durin
gHPL
Canalysis
Someo
fthe
peaksp
rodu
cedaft
erph
otocatalysishad
fluorescences
ignals
Thissuggeststhep
rodu
ctionof
newsubstances
andflu
orop
hores
Coatin
gsprod
uced
throug
hSol-G
elprocedures
arestablea
ndcanbe
used
manytim
es
10 International Journal of Photoenergy
321 Influence of Catalyst One aspect to successfully imple-ment photocatalysis is the choice of an appropriate photo-catalyst The majority of catalysts used are based on TiO
2as
summarized in Table 2 ZnO has also been applied either assingle catalyst [48] or in a combination with other catalystsuch as TiO
2[54] de Lasa et al [60] described ZnO as
being less active than TiO2 They add that the use of ZnO
is particularly relevant when the oxidative degradation ratebecomes limited This is opposite to what Kansal et al [48]reported by saying that ZnO is more reactive than TiO
2
However TiO2(mainly anatase) remains the most used
catalyst as can be depicted fromTable 2 TiO2is reported to be
favored because of its nontoxic property cost efficiency andchemical and biological inertness Moreover TiO
2possesses
the most efficient photoactivity and the highest stability thusmaking it suitable for industrial use [61]
ZnO has been described to degrade lignin under visiblelight sources [48 62] whereas TiO
2is mostly applied in
connection to UV-light sources as highlighted in Table 2However both ZnO and TiO
2possess energy band gap
energy of 32 eV [23]Additives such as SiO
2[63] polyethylene oxide (PEO)
[24] and polyethylene glycol [64] have been added to TiO2
catalyst particularly when applying immobilized catalystAddamo et al [63] noted a high adhesion of TiO
2to glass
support material when precoating was done with SiO2
Moreover the precoating might have other advantages suchas a hindering diffusion of Na+ ions from the glass materialinto the nascent TiO
2film during heat treatment processes
Analogous to the addition of SiO2 polyethylene glycol
(PEG) has also been introduced to mitigate catalyst surfaceactivity modify surface hydrophobicity and also reduceagglomeration tendency of the TiO
2gel or TiO
2particles
in the suspensions [64] By a proper surface modificationinteraction between catalyst and substrate can be enhanced[55 63]
Kansal et al [48] varied catalyst (ZnO) dose from 05 gLto 20 gL for 01 gL kraft lignin solutions and found out thatthere was an optimum catalyst threshold value at 1 gL whichgives a catalyst to substrate ratio of 1 10
Dahm and Lucia [49] examined catalyst dose from 2 sdot10minus3 gL to 12sdot10minus2 gL for lignin solutions (fromwhite water
liner mill) of 4 sdot 10minus2 gL (catalyst to lignin ratio 5 sdot 10minus3ndash3 sdot 10minus2) at pH 8 and obtained best energy efficiency valuesand lignin degradation rates with a catalyst loading of 10 sdot10minus2 gLIn contrast Ma et al [56] applied far higher catalyst
concentration compared to Dahm and Lucia [49] Catalystconcentration was varied between 1 gL and 10 gL PtTiO
2
Best catalysis dose with respect to reaction turnover wasobtained at 5 gL PtTiO
2With the increase of catalyst dose at
pH 7 the reaction rate increased from 61 times 10minus3minminus1 (1 gLTiO2) to 71 sdot 10minus3minminus1 (5 gL TiO
2) and 99 sdot 10minus3minminus1
(10 gL TiO2)
Catalyst effect has been explained on the basis thatoptimum catalyst loading is dependent on the initial soluteconcentration An increase in catalyst dosage leads to a cor-responding increase of total active surface area for reactions
[65] To that at higher TiO2concentrations the photon flux is
more easily intercepted by the catalyst before penetrating intothe bulk of the system At the same time due to an increase inturbidity of the suspension with high dose of photocatalystthere is a decrease in penetration of UV light and hencephotoactivated volume of suspension or solution decreases[66]
In summary authors have obtained best catalyst to ligninrelations for different reaction designs and thus a generalrecommendation on catalyst dose is not possible Howeverwhen lignin solution is treated with increasing catalyst loadsa corresponding increase in degradation rate is observed untila threshold value is reached [48ndash50 56]
322 Influence of Metal Ion Addition (Doping) and AdditivesThe purpose of adding metal ion to photocatalyst is to miti-gate band gap energy through the introduction of intrabandgap states and as a consequence produce a bathochromicshift in the absorption spectrum [37] Altering the absorptionspectral range gives the possibility to exploit both the visiblelight spectrum and UV light sources Metal ion doping isalso introduced to serve as electron or hole traps in orderto minimize recombination between generated electron-holepairs [37]
Portjanskaja and Preis [50] studied the addition of Fe2+ions to an acidic lignin solution and found an increasein photocatalytic oxidation (PCO) efficiency The optimumFe2+ ions quantity was 28mgL while using 100mgL ligninsolution Upon further elevation of Fe2+ ions concentrationa corresponding reduction of the photocatalytic oxidationefficiency of lignin was noted Likewise Ohnishi et al [57]made a comparative study by doping platinum (Pt) silver(Ag) and gold (Au) ions to TiO
2 In these reactions 50mg of
catalyst (TiO2) was used with the addition of an equiva1ent
15 wt (based on TiO2) metal ion The addition of noble
metals brought about a faster decolorization of lignin Aushowed better results than Ag followed by Pt In the samecontext adding sodium hypochlorite as oxidant to PtTiO
2
catalyst an additional fivefold degradation rate was observedcompared to that without doping [56] Contradictory to theresults described above negligible effect of photocatalyticefficiency due to doping has been reported as well Awun-gacha Lekelefac et al [54] obtained little or no change indegradation rate by doping TiO
2-P25-SiO
2catalyst with Pt
ions (1 wt relative to TiO2catalyst) Likewise Portjanskaja
and Preis [50] noted a negligible change of photocatalyticefficiency of TiO
2when doped with nitrogen
Sarkanen et al [67] and Gellerstedt and Lindfors [68]reported the bias of peroxides to oxidation with reagentssuch as permanganate to favor aromatic moieties Oxidationagents like permanganate oxidizes predominantly aliphaticchains in alkaline and neutral media However by theapplication of H
2O2(Fenton system) lignin disappeared
completely [33] Tonucci et al [33] conclude that in orderto satisfactorily conserve the organic material the best com-promise appears to be the TiO
2photosystem which shows
low carbon consumption good preservation of the aromaticrings and greatly reduced mineralization
International Journal of Photoenergy 11
In summary different results have been obtained con-cerning the influence of noble metal ion addition Whilesome authors report an improvement in the photocatalyticefficiency upon their addition others report their addition ashaving no considerable influence However for reactions inwhich an improvement in the photocatalytic efficiency wasnoticed there was a threshold value to be considered Whenthe concentration of dopant surpasses this threshold valueelectron-hole recombination is favored and this has a negativeimpact to photocatalysis In such a case the space-chargelayer gets narrower and 119901-type dopants attract electrons andby virtue become negative They would now then act ashole acceptor attracting holes On the other hand 119899-typedopants which act as electron donor centers and possessexcess electrons attract holes as well [37]
323 Influence of Lignin Concentration Once the initiallignin concentration becomes higher exceeding a thresholdvalue an inhibitory effect on the photodegradationwas noted[47ndash49] This threshold value varies and depends on thereaction system and reaction parameters such as opticaldensity catalyst concentration and reaction volume Fromthe literature different authors have implemented varyinglignin concentrations probably to suit their reaction designFor example Ksibi et al [47] use 90mgL and AwungachaLekelefac et al [54] use 500mgL while Kansal et al [48]applied 10mgndash100mgL
Explanations arising from the findings are as followsat low lignin concentrations the incidental photonic fluxirradiated interacts with the catalyst generating radicals (eghydroxyl radicals (OH∙)) which allow a faster degradation[69] On the other hand high initial lignin concentrationsmay lead to tight adsorption which can suppress CO
2
evolution [51] and hence maintain chemical oxygen demand(COD) values Moreover low delignification yields maybe due to an inhibitory effect because of autoxidation bylow molecular weight lignin degradation products formed[24] Also due to the polymer structure of lignin which iscross-linked this makes it difficult for radical species acidand the aldehyde compounds produced to spread into theinner region of the substrate hence limiting autoxidationAs a worst case this might be the rate-determining step ofdelignification which is hindered [70ndash72]
In summary it can be concluded that the time taken forcomplete degradation depends on the initial concentration oflignin and faster degradation occurs at low lignin concentra-tions
324 Influence of pH Varying pH entails an alteration inthe properties of semiconductor-liquid interface [73] mainlyrelated to the acid-base equilibrium of the adsorbed hydroxylgroup [39] Furthermore pH also impacts lignin degradationrates [47 48 57] In this context several studies were carriedout with partly contradictory outcomes
Kansal et al [48] made pH investigations 3ndash11 undersolar light illumination using ZnO as catalyst Maximumdegradation was reached in alkaline conditions (pH 11)This is supported by Villasenor and Mansilla [74] reporting
an almost complete decolorization of kraft black liquorfrom pine wood at pH value of 116 in combination withZnO catalyst Similar results were achieved by Ohnishi etal [57] with TiO
2and ZnO being catalyst for bleaching
alkaline lignin in aqueous solution with TiO2and ZnO being
catalyst High activities at neutral pH were also reportedby Ohnishi et al [57] In contrast Ma et al [56] observedhigher reaction rates and rapid degradation of a syntheticlignin wastewater (prepared by dissolving commercial ligninpowder in aqueous solution pH 11) in acidic solution (pH 3)than in alkaline solutions at pH 11 for either TiO
2or PtTiO
2
catalystsReconsidering the photocatalytic principle the formed
superoxide anion radicals (∙O2
minus) are in a pH-dependentequilibriumwith perhydroxyl radicals (HO
2
∙) as follows [75]
HO2
∙999445999468 H+ + ∙O
2
minus pKa sim 48 (9)
See [76]∙O2
minus undergoes dismutation reaction resulting in H2O2
and O2competing to any other ∙O
2
minus triggered reaction Incase of low pH operation conditions in aqueous solutionsHO2
∙ becomes dominant whose reactivity is considerablyhigher compared to ∙O
2
minus [77] Subsequently HO2
∙ initiatessubstrate (S) oxidation to the radical cation (S+∙) and is itselfreduced to H
2O2[78] Thus increased degradation rates can
be reasonably expected supporting the results made by Ma etal [56] in an acidic environment ∙O
2
minus is extremely reactive inorganic solvents [77] Another aspect is the solubility of kraftlignin (soluble at pH gt 105) which reduces with decreasingpH whereas lignosulfonate should remain unaffected by pHin aqueous solution Moreover 120573ndashOndash4 bonds have beendescribed to be stable at acidic pH [11] In fact this couldadditionally explain the elevated degradation of kraft ligninmade by Kansal et al [48] and Villasenor and Mansilla [74]Nevertheless the contradictory results gained by Ma et al[56] still exist under the assumption that kraft ligninwas used(which would be supported by the high pH of 11 obviouslynecessary for dissolving the lignin powder) Although mostphotocatalytic reactions described in the literature are in anaqueous milieu lignin raw material and its fission productsmay however vary considerably Therefore the optimal pHis most likely to be reaction specific and has to be evaluatedexperimentally in principle
325 Influence of Illumination Many of the studies foundin the literature so far have not dealt on this subject per seWhat is found is the use of different illumination sourceseach having a specified power and lamp type However alltend to emitUV-light between the range 280ndash420 nm Table 2depicts this in detail Other illumination sources include thevisible light spectrum
In general terms illumination influences in that it ini-tiates photocatalysis by generating electron-hole pair in thesemiconductor particles [38 39] Dahm and Lucia [49]altered illumination intensity while observing lignin degra-dation In this study 004 gL lignin was used and lightintensity was varied 223ndash445mWcm3 It was found out thathigher illumination intensities correlated well with higher
12 International Journal of Photoenergy
initial degradation rates and hence total lignin degradation[49] Neppolian et al [69] report degradation to be propor-tional to radiation intensity and best results are achieved forlow lignin concentrations because of enhanced interactionbetween catalyst and incidental photonic flux
In summary high illumination power causes a corre-sponding high initial degradation rate at low lignin concen-trations becausemaximum light penetration into the reactionmedium is favored
33 Process Analytical Methods Various analytical tech-niques have been used to monitor lignin degradation Atthe beginning of this subchapter analytics revealing com-pounds formed from lignin degradation are treated Thisincludes for example gas chromatography (GC) and 1HNMR (nuclear magnetic resonance) This is then followed byresults qualitative analytic measurements such as ultraviolet-visible (UV-Vis) spectroscopy and dissolved carbon (DC)A list of authors analytical techniques applied and resultsachieved are outlined in Table 3
Portjanskaja and Preis [50] studied lignin degradationby measuring the removal of phenols through colorimetricmeasurements As a result of 24 h photocatalytic oxidationunder neutral media conditions 80 of free phenols wereremoved Gas chromatography (GC) result from Ksibi et al[47] attested vanillin vanillic acid palmitic acid biphenyland 345-trimethoxy benzaldehyde structures after the pho-tocatalysis of lignin from black liquor This is in accordancewith the findings of Tonucci et al [33] reporting the formationof vanillin hydroxyl methoxy-acetophenone coniferyl alco-hol coniferyl aldehyde methanol formic acid acetic acidand small amounts of C-2 and C-3 alcohols as degradationproducts1H NMR spectral analysis of lignin before illumination
and after 24 h of illumination showing characteristic bandsof aromatic rings methoxy and aliphatic side chains wascompared with each other Results revealed that the aro-matic ring degraded faster than the aliphatic chain [51]Fourier transformation infrared spectroscopy (FTIR) showedbands corresponding to CH
3 CH2 and CH which remained
unchanged after illumination while bands corresponding toaromatic rings disappeared as a result of illumination [51 53]
Results obtained from the combination of photochemicaland electrochemical oxidation [52 53] were similar to thoseof Tanaka et al [51] Here 13C-NMR confirmed the presenceof the carbonyl functionality and the presence of vanillinand vanillic acid after 12 h photochemical-electrochemicaloxidation These results showed that the combination of aphotocatalytic and an electrochemical oxidation significantlyenhanced the efficiency of lignin degradationThis is becausethe applied anodic potential bias greatly suppressed therecombination of photogenerated electrons and holes [53]
Ultraviolet spectrophotometry offers a convenientmethod to qualitatively and quantitatively analyze ligninin solution [79] This is reflected by the large number ofpublications using this technique [17 33 47ndash51 56 58 80]This is most likely due to its simplicity to interpret lignindegradation Lignins absorb UV light with high molar
200 220 240 260 280 300 320 340 360
00
05
10
15
20
25
Abso
rban
ce
Wavelength (nm)0min
30min
60min
120min
180min
300min
360min
420min
1200min
Figure 8 Time dependent UV-Vis absorption spectra of aqueouslignin solution from waste paper water irradiated with UV light(280ndash420 nm) for different time intervals The spectra are obtainedfor sol-gel derived TiO
2nanocrystalline coating (TiO
2-P25-SiO
2)
[54]
extinction coefficients because of the several methox-ylated phenylpropane units of which they are composed [33]Figure 8 depicts a series of photometric scans of ligninsul-fonate from paper waste water showing a gradual reductionof absorbance during photocatalytic treatment [54] Herethe absorption peaks are around 210 nm and 280 nmAbsorbance decreases with time implying the decompositionof lignin and the deterioration of chromophore groups [54]
Peaks at 210 nmcorrespond to portions of the unsaturatedchains while those at 280 nm correspond to unconjugatedphenolic hydroxyl groups [17] and the aromatic moiety [57]of the lignin molecule The absorption tailing to the longwavelength region arises from the color of lignin [57] Lignindegradation has been reported either at wavelength around280 nm corresponding to unconjugated phenolic hydroxylgroups [17 48 51 58] or for both wavelengths (210 nm and280 nm) [33 54 57] Kobayakawa et al [81] noted some otherabsorbance at wavelengths lower than 250 nm and pointedout that this could be due to the modification of ligninfragmentations leading to the formation of transient specieslikemethanol ethanol formaldehyde formic acid and oxalicacid among others
Analyticalmethods to effectively quantify lignin degrada-tion by calculating the oxygen demand by organic substancesand remaining organic carbon before and after photocatalysishave been studied Amongst the methods are dissolvedcarbon (DC) [49 51 58] chemical oxygen demand (COD)[47 48 50 57 80] biochemical oxygen demand (BOC) [50]dissolved organic carbon (DOC) [56] and American dyemanufacture institute value (ADMI) [56] COD and BODdescribe the oxygen demand by organic substances to be
International Journal of Photoenergy 13
Figure 9 Gradual change from the characteristic yellow ligninsulfonate to a colorless liquid after a period of 20 h Catalyst TiO
2-
P25 (Degussa) +TEOSUV-light 25∘C lignin concentration 05 gL[54]
converted to CO and CO2and H
2O and NH
3 Total organic
carbon (TOC) describes the amount of carbon bound inan organic compound while DOC describes the dissolvedfraction of organic carbon ADMI measures the amount ofdyestuff in water
Decolorization of lignin solution has been reported tobe another parameter observed during photocatalytic degra-dation Color is an indirect indicator of the lignin amountThe higher the color intensity of the solution is the greaterthe lignin content is (high concentrated lignin solutions egblack liquor appear dark brown) [82] Thus color changescan be interpreted as conversion of lignin to transient speciesor conversion to CO
2and H
2O Awungacha Lekelefac et al
[54] observed a gradual change from the characteristic yellowlignin (when highly diluted) to a colorless liquid after a periodof 20 h with sol-gel derived TiO
2nanocrystalline coatings on
sintered borosilicate glass as depicted in Figure 9 A corre-sponding decrease in DC values close to 82 was observedfor TiO
2-P25-SiO
2catalyst confirming degradation This is
shown in Figure 10These findings are analogous to that of Ohnishi et al
[57] who reported the bleaching of lignin when illuminatedcontinuously and that the solution becomes colorless Tothat the chemical oxygen demand (COD) value decreasesgenerating carbon dioxide and a small amount of carbonmonoxide as the main gaseous products COD removalwas reported to be effective at low lignin concentrations ascompared to high lignin concentrations [48]
Another applied analytical technique is fluorescencedetection directly coupled to a high performance liquidchromatography (HPLC) as a means to identify nonaliphaticcomponent in the complex mixture of lignin degradationproducts [54] Fluorescence emission in lignin is attributedto aromatic structures such as conjugated carbonyl biphenylphenylcoumarone and stilbene groups [83 84] AwungachaLekelefac et al [54] observed peaks on both HPLC and
0 200 400 600 800 1000 12000
50
100
150
200
250
Diss
olve
d ca
rbon
(ppm
)
t (min)
TiO2-P25-SiO2
TiO2-P25-SiO2 + PtTiOSO4minus30 6wt
ZnO + TiO2-P25-SiO2
No coating UV light
Figure 10 Variation of DC with time of aqueous lignin solutionfrom waste paper water irradiated with UV light (280ndash420 nm) fordifferent time intervals The spectra are obtained for sol-gel derivedTiO2nanocrystalline coatings (TiO
2-P25-SiO
2+Pt TiO
2-P25-SiO
2
TiOSO4minus306 wt ZnO + TiO
2-P25-SiO
2) [54]
fluorescence chromatograms suggesting the production ofnew substances and fluorophores
Despite developed analytical technologies analyzinglignin degradation products remains challenging Proofssuch as mass spectroscopy (MS) HPLC 13C or 1H-NMRspectra from photocatalytic lignin degradations are not yetestablished The setback to qualitatively and quantitativelyanalyze lignin and its degradation products starts from thenative lignin polymer itself with its indefinite polymericstructure and multiple bond types Also the influence ofdifferent pretreatments additives and the wide variety ofcompounds obtainable from its degradation makes ligninanalysis challenging [85] Moreover lignin streams couldcontain proteins inorganic salts and other potential poisonsthat generally complicate catalysis [4]
The challenge to identify and separate the productsstreams derived from the photocatalytic degradation ligninis also worth noting Lignin product stream is highly func-tionalized and conventional techniques such as gas chro-matography have the disadvantage of requiring a time-consuming derivatization step Also because of high boilingpoint of substances arising from lignin degradation it is noteasily applicable High performance liquid chromatography(HPLC) seems to be the remedy because analysis can becarried out without derivatization but the exact identificationof the separated substances is difficult because of the numer-ous peaks arising from such a chromatogram [87] Unfortu-nately well-established databases such as that of the nationalinstitute of standard and technology (NIST) [88] cannot giveinformation onHPLC-MS chromatogramsThis is because ofthe ionization sources such as electrospray ionization (ESI)
14 International Journal of Photoenergy
Table 4 Main advantages and disadvantages photocatalysis versus enzymatic biocatalysis
Lignin degradation process Advantages Disadvantages
Photocatalysis(single stage)
(i) Relatively fast degradation of complex as wellas nonbiodegradable organic (macro)molecules(ii) Stable catalysts(iii) Moderate reaction conditions
(i) Not selective(ii) Limited selection of operating conditions(iii) Energy costs
Enzymatic conversion(single stage)
(i) More selective(ii) Moderate reaction conditions
(i) High enzyme loads required otherwise ligninhydrolysis is quite slow [16](ii) Enzymes are less stable1(iii) cost-intensive (POXs)2
1For example POXs are sensitive to high H2O2 concentrations2According to Torres and Ayala [86]
and atmospheric pressure chemical ionization (APCI) forLC-MS while GC-MS is based on electron ionization Aremedy to this can be the development of a database ofmodel lignin compounds based on a unanimous HPLC-MSmeasuring procedure This would mean much time and costexpensive investments for adequate personnel and material
Though product identification based on peak superpo-sition gives information on a possible product this can-not always be true for lignin degradation because of thelarge number of possible degradation products In order toproperly identify products from lignin degradation productisolation techniques are to be implemented and further ana-lytic methods such as tandem mass spectroscopy (MSMS)and other advanced structure enhancing research techniquessuch as 1H-NMR 13C-NMR have to be studied extensively
4 Photocatalysis and Its Suitability asIntegrated Technology in MultistageConcepts with Biocatalysis
Depending on lignin treatment specification photocatalysiswill find application for complete lignin mineralization toCO2and H
2O (single stage) or as integrated technology in
a multistage system aiming at partial conversion of ligninmacromolecules to organic low molecular weight intermedi-ates for a consecutive biological oxidation finally generatingvalue added end-products [15]
For biological oxidation of lignin or lignin deriva-tives H
2O2-dependent ligninolytic heme peroxidases (POXs
including lignin peroxidase (LiP EC 111114) manganeseperoxidase (MnP EC 111113) versatile peroxidase (VPEC 111116) O
2-dependent laccases (Lac EC 11032) and
extracellular enzymes from Basidiomycetous white-rot fungiare the most efficient lignin degraders in nature [89] Thussuch enzymatic systems especially POXs (with high redoxpotentials) have attractedmuch interest as industrial biocata-lyst [86 90]The enzyme degradationmechanism (in nature)is facilitated by nonenzymatic processes mainly throughfree ∙OH radicals also generated by the fungus (Fenton-type reaction) Those ∙OH radicals enable the requiredphysical contact between the enzymes and structural units ofthe lignin molecule due to numerous nonspecific oxidative
reactions [91] Both the photocatalytic (equations (2)ndash(7))and the POX reactionmechanism recently reviewed by Busseat al [25] are quite similar Consequently a combination ofphotocatalysis followed by an enzymatic oxidation maybe apromising concept utilizing lignin derivatives for exampleoriginated from industrial effluents [15 92] The advantagesforming biobased products are as follows see also Table 4 inthis context A reduction of the lignin polymerization degreevia photocatalysis at best in more biodegradable interme-diates and a simultaneous detoxification causes savings inenzyme costs since it will be expected that less enzyme loadsare necessary for sufficiently rapid reaction rates [15 91] As aresult hydraulic retention times in bioreactor systems shouldbe diminished Moreover the delignification is expectedto be enhanced simultaneously which was in recent yearsshown in a dual system by Kamwilaisak and Wright [93]using TiO
2H2O2UV for photocatalytic pretreatment and
Lac (from Trametes versicolor) in the subsequent biocatalyticstep Within 24 h they obtained in their dual system anelevation in delignification of 20 (without H
2O2) up to at
least 50 when H2O2was present
In previous studies the treatment of lignin (from thepulp and paper industry) containing effluents by fungus asa biological system (excreting appropriate lignin degradingenzyme cocktails) were more focused for posttreatmentexclusively (Duran et al [92] Reyes et al [94] and Gonzalezet al [95]) Shende et al [17] even examined ligninolyticbacteria with photooxidized kraft lignin as substrate
Although POXs are potential industrial biocatalysts [86]no application studies were found for the direct use in suchdual systems as described above The major reason may bethe complexity of the reaction mechanism (inclusive ligninderivatives as substrate and their analysis) per se on the onehand slowing down research and development processes Onthe other hand POXs are sensitive to their cosubstrate H
2O2
once it is supplied in excess causing considerable inactivation(for details refer to Busse et al [25]) At the present severalstudies are carried out modifying these enzymes regardingenhanced stability activity and selectivity as well Henceit can be expected that their technological applicability willbe raised significantly right after successful modification isreached [86]
International Journal of Photoenergy 15
5 Concluding Remarks
It is widely assumed that the photocatalytic degradation oflignin follows a radical reaction pathway which is similar tothat considered in thermal electrochemical and biochemicalprocesses However reporting on the degradation pathway oflignin derivatives and even that of lignin model compoundsare still amajor challengeThis is probably due to the complexnature and variety of possible degradation products Indeedthemechanism is far more complex considering other factorssuch as type of lignin type of catalyst pH illuminationsource and additives
Generally comparing the different photochemical pro-cesses poses a big challenge because of the wide variablesinvolved These discrepancies start from the source and typeof lignin followed by differences in reactor design illumi-nation source intensity of radiation and different types ofcatalyst An ideawould be to have a specific reference reactionwith well-defined starting parameters which include lignintype source and purity catalyst specifications illuminationsource and intensity so as to ease comparison of results
Basic process parameters such as catalyst concentrationsubstrate concentration addition of metal ion to catalyst pHand illumination have been discussed
Despite developed analytical technologies analyzinglignin degradation products remains challenging Proofssuch as mass spectroscopy (MS) HPLC 13C or 1H-NMRspectra from photocatalytic lignin degradations are not yetestablished
In order to properly identify products from lignin degra-dation product isolation techniques are to be implementedand further analytic methods such as MSMS and otheradvanced structure enhancing research techniques such as1H-NMR and 13C-NMR have to be studied extensively Aremedy to this can be the development of a databank ofmodel lignin compounds based on a unanimous HPLC-MSmeasuring procedure This would mean much time and costexpensive investments for adequate personnel and material
Photocatalysis is denoted as the most popular ligninpretreatment technology besides ozonation [96] Photo-catalyzed lignin may be an appropriate substrate for aconsecutive biocatalytic process using ligninolytic enzymes(POX andor Lac) as supported by experimental results ofKamwilaisak and Wright [93] Combining the advantages ofboth catalytic processes savings in the overall process costswill be expected in addition to elevated lignin conversionNonetheless extensive research work including POX modi-fications is still required
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Authorsrsquo Contribution
Colin Awungacha Lekelefac and Nadine Busse contributedequally to this work
Acknowledgments
The authors gratefully thank the Federal Ministry of Educa-tion and Research (BMBF) for funding (FKZ17N0310) Theresearchers also thank the Hessen State Ministry of HigherEducation Research andArts for the financial support withinthe Hessen initiative for scientific and economic excellence(LOEWE)
References
[1] Crude Oil and Commodity Prices httpwwwoil-pricenet[2] Research Directorate-General for World Energy Technology
and Climate Policy Outlook (WETO) Luxembourg Office forOfficial Publications of the European Communities 2003
[3] Green and Natural Polymers Are on the Rise 2014 httpwwwpolymersolutionscombloggreen-and-natural-polymers-on-the-rise
[4] J Zakzeski P C Bruijnincx A L Jongerius and B M Weck-huysen ldquoThe catalytic valorization of lignin for the productionof renewable chemicalsrdquo Chemical Reviews vol 110 no 6 pp3552ndash3599 2010
[5] B KammM Kamm P R Gruber and S KromusBiorefineries-Industrial Processes and Products Status Quo and Future Direc-tions vol 1 Wiley-VCH 2006
[6] R L Howard E Abotsi E L J van Rensburg and S HowardldquoLignocellulose biotechnology issues of bioconversion andenzyme productionrdquoAfrican Journal of Biotechnology vol 2 no12 pp 602ndash619 2003
[7] M Stocker ldquoBio- und BTL-Kraftstoffe in der Bioraffineriekatalytische Umwandlung Lignocellulose-reicher Biomasse mitporosen Stoffenrdquo Angewandte Chemie vol 120 no 48 pp9340ndash9351 2008
[8] J-P Lange ldquoLignocellulose conversion an introduction tochemistry process and economicsrdquo Biofuels Bioproducts andBiorefining vol 1 no 1 pp 39ndash48 2007
[9] D A I Goring ldquoThe physical chemistry of ligninrdquo Pure andApplied Chemistry vol 5 no 1-2 pp 233ndash310 1962
[10] M Ek G Gellerstedt and G HenrikssonWood Chemistry andBiotechnology Pulp and Paper Chemistry and Technology vol 1Walter de Gruyter GmbH amp Co KG Berlin Germany 2009
[11] B Saake and R Lehnen ldquoLigninrdquo in Ullmannrsquos Encyclopedia ofIndustrial Chemistry Wiley-VCH Verlag GmbH amp Co KGaAWeinheim Germany 2012
[12] M N S Kumar A K Mohanty L Erickson and M MisraldquoLignin and its applications with polymersrdquo Journal of BiobasedMaterials and Bioenergy vol 3 no 1 pp 1ndash24 2009
[13] R J A Gosselink E de Jong B Guran and A AbacherlildquoCo-ordination network for ligninmdashstandardisation produc-tion and applications adapted to market requirements(EUROLIGNIN)rdquo Industrial Crops and Products vol 20 no 2pp 121ndash129 2004
[14] D Fengel and G Wegener Wood Chemistry UltrastructureReactions Verlag Kessel 1984
[15] D Mantzavinos and E Psillakis ldquoEnhancement of biodegrad-ability of industrial wastewaters by chemical oxidation pre-treatmentrdquo Journal of Chemical Technology and Biotechnologyvol 79 no 5 pp 431ndash454 2004
16 International Journal of Photoenergy
[16] M Dashtban H Schraft and W Qin ldquoFungal bioconversionof lignocellulosic residues Opportunities amp perspectivesrdquo Inter-national Journal of Biological Sciences vol 5 no 6 pp 578ndash5952009
[17] A Shende R Jaswal D Harder-Heinz A Menan and RShende ldquoIntergrated photocatalytic and microbial degradationof kraft ligninrdquo Cleantech pp 120ndash123 2012
[18] F S Chakar and A J Ragauskas ldquoReview of current and futuresoftwood kraft lignin process chemistryrdquo Industrial Crops andProducts vol 20 no 2 pp 131ndash141 2004
[19] W G Glasser and H R Glasser ldquoEvaluation of ligninrsquoschemical structure by experimental and computer simulationtechniquesrdquo Paperi ja Puu vol 63 pp 71ndash83 1981
[20] M Erickson S Larsson and G E Miksche ldquoZur Struktur desLignins der FichterdquoActaChemica Scandinavica vol 27 pp 903ndash914 1973
[21] H Nimz ldquoDas Lignin der BuchemdashEntwurf eines Konstitution-sschemasrdquo Angewandte ChemiemdashInternational Edition vol 86pp 336ndash344 1974
[22] D V Evtuguin C P Neto J Rocha and J D P de JesusldquoOxidative delignification in the presence of molybdovana-dophosphate heteropolyanions mechanism and kinetic stud-iesrdquo Applied Catalysis A General vol 167 no 1 pp 123ndash1391998
[23] A L Linsebigler G Lu and J T Yates Jr ldquoPhotocatalysis onTiO2surfaces Principles mechanisms and selected resultsrdquo
Chemical Reviews vol 95 no 3 pp 735ndash758 1995[24] Y Miyata K Miyazaki M Miura Y Shimotori M Aoyama
and H Nakatani ldquoSolventless delignification of wood flourwith TiO
2poly(ethylene oxide) photocatalyst systemrdquo Journal
of Polymers and the Environment vol 21 no 1 pp 115ndash121 2013[25] N Busse D Wagner M Kraume and P Czermak ldquoReaction
kinetics of versatile peroxidase for the degradation of lignincompoundsrdquoTheAmerican Journal of Biochemistry andBiotech-nology vol 9 no 4 pp 365ndash394 2013
[26] M Tien and T K Kirk ldquoLignin-degrading enzyme fromphanerochaete chrysosporium Purification characterizationand catalytic properties of a uniqueH
2O2-requiring oxygenaserdquo
Proceedings of the National Academy of Sciences vol 81 pp2280ndash2284 1984
[27] T K Kirk M Tien P J Kersten M D Mozuch and BKalyanaraman ldquoLigninase of Phanerochaete chrysosporiumMechanism of its degradation of the non-phenolic arylglycerol120573-aryl ether substructure of ligninrdquo Biochemical Journal vol236 no 1 pp 279ndash287 1986
[28] T Lundell RWever R Floris et al ldquoLignin peroxidase L3 fromPhlebia radiata pre-steady-state and steady-state studies withveratryl alcohol and a non-phenolic lignin model compound1-(34-dimethoxyphenyl)-2-(2-methoxyphenoxy)propane-13-diolrdquo European Journal of Biochemistry vol 211 no 3 pp391ndash402 1993
[29] H E Schoemaker T K Lundell A I Hatakka and K PiontekldquoThe oxidation of veratryl alcohol dimeric lignin models andlignin by lignin peroxidase the redox cycle revisitedrdquo FEMSMicrobiology Reviews vol 13 no 2-3 pp 321ndash331 1994
[30] J M Palmer P J Harvey and H E Schoemaker ldquoThe role ofperoxidases radical cations and oxygen in the degradation oflignin [and discussion]rdquo Philosophical Transactions of the RoyalSociety of London A vol 321 no 1561 pp 495ndash505 1987
[31] C Hill Wood Modification Chemical Thermal and OtherProcesses John Wiley amp Sons Chichester UK 2006
[32] Y-H Xu H-R Chen Z-X Zeng and B Lei ldquoInvestigation onmechanism of photocatalytic activity enhancement of nanome-ter cerium-doped titaniardquo Applied Surface Science vol 252 no24 pp 8565ndash8570 2006
[33] L Tonucci F Coccia M Bressan and N DrsquoAlessandro ldquoMildphotocatalysed and catalysed green oxidation of lignin auseful pathway to low-molecular-weight derivativesrdquoWaste andBiomass Valorization vol 3 no 2 pp 165ndash174 2012
[34] A Castellan N Colombo C Vanucci P Fornier de HViolet and H Bouas-Laurent ldquoPhotodegradation of ligninA photochemical study of an O-methylated 120572-carbonyl 120573-1lignin model dimer 12-di(3101584041015840-dimethoxyphenyl) ethanone(deoxyveratroin)rdquo Journal of Photochemistry and PhotobiologyA vol 51 pp 451ndash467 1990
[35] A Fujishima X Zhang and D A Tryk ldquoTiO2photocatalysis
and related surface phenomenardquo Surface Science Reports vol63 no 12 pp 515ndash582 2008
[36] C S Turchi and D F Ollis ldquoPhotocatalytic reactor designan example of mass-transfer limitations with an immobilizedcatalystrdquo The Journal of Physical Chemistry vol 92 no 23 pp6852ndash6853 1988
[37] R Subasri M Tripathi K Murugan J Revathi G V N Raoand T N Rao ldquoInvestigations on the photocatalytic activity ofsolndashgel derived plain and Fe3+Nb5+ doped titania coatings onglass substratesrdquo Materials Chemistry and Physics vol 124 pp63ndash68 2010
[38] N Serpone ldquoRelative photonic efficiencies and quantum yieldsin heterogeneous photocatalysisrdquo Journal of Photochemistry andPhotobiology A Chemistry vol 104 no 1ndash3 pp 1ndash12 1997
[39] M R Hoffmann S T Martin W Choi and D W BahnemannldquoEnvironmental applications of semiconductor photocatalysisrdquoChemical Reviews vol 95 no 1 pp 69ndash96 1995
[40] C D Jaeger and A J Bard ldquoSpin trapping and electron spinresonance detection of radical intermediates in the photode-composition of water at titanium dioxide particulate systemsrdquoThe Journal of Physical Chemistry vol 83 no 24 pp 3146ndash31521979
[41] R W Matthews ldquoHydroxylation reactions induced by near-ultraviolet photolysis of aqueous titaniumdioxide suspensionsrdquoJournal of the Chemical Society Faraday Transactions vol 80no 2 pp 457ndash471 1984
[42] A E H Machado A M Furuyama S Z Falone R RuggieroD D S Perez and A Castellan ldquoPhotocatalytic degradation oflignin and lignin models using titanium dioxide the role of thehydroxyl radicalrdquoChemosphere vol 40 no 1 pp 115ndash124 2000
[43] W Sigg and L Stumm Aquatische Chemie Stuttgart 2 AuflageBG Teubner Stuttgart Germany 1991
[44] O Legrini E Oliveros and A M Braun ldquoPhotochemicalprocesses for water treatmentrdquo Chemical Reviews vol 93 no2 pp 671ndash698 1993
[45] G Rothenberger J Moser M Gratzel N Serpone and DSharma ldquoCharge carrier trapping and recombination dynamicsin small semiconductor particlesrdquo Journal of the AmericanChemical Society vol 107 no 26 pp 8054ndash8059 1985
[46] P Mazellier M Sarakha A Rossi and M Bolte ldquoThe aqueousphotochemistry of 26-dimethylphenol Evidence for the frag-mentation of the 120572 C-C bondrdquo Journal of Photochemistry andPhotobiology A vol 115 no 2 pp 117ndash121 1998
[47] M Ksibi S B Amor S Cherif E Elaloui A Houas and MElaloui ldquoPhotodegradation of lignin from black liquor using aUVTiO
2systemrdquo Journal of Photochemistry and Photobiology
A vol 154 no 2-3 pp 211ndash218 2003
International Journal of Photoenergy 17
[48] S K Kansal M Singh and D Sud ldquoStudies on TiO2ZnO
photocatalysed degradation of ligninrdquo Journal of HazardousMaterials vol 153 no 1-2 pp 412ndash417 2008
[49] A Dahm and L A Lucia ldquoTitanium dioxide catalyzed pho-todegradation of lignin in industrial effluentsrdquo Industrial andEngineering Chemistry Research vol 43 no 25 pp 7996ndash80002004
[50] E Portjanskaja and S Preis ldquoAqueous photocatalytic oxidationof lignin the influence of mineral admixturesrdquo InternationalJournal of Photoenergy vol 2007Article ID76730 7 pages 2007
[51] K Tanaka R C R Calanag and T Hisanaga ldquoPhotocatalyzeddegradation of lignin on TiO
2rdquo Journal of Molecular Catalysis
A Chemical vol 138 no 2-3 pp 287ndash294 1999[52] M Tian J Wen D MacDonald R M Asmussen and A Chen
ldquoA novel approach for lignin modification and degradationrdquoElectrochemistry Communications vol 12 no 4 pp 527ndash5302010
[53] K PanM Tian Z-H Jiang B Kjartanson and A Chen ldquoElec-trochemical oxidation of lignin at lead dioxide nanoparticlesphotoelectrodeposited on TiO
2nanotube arraysrdquo Electrochim-
ica Acta vol 60 pp 147ndash153 2012[54] C Awungacha Lekelefac J Hild P Czermak and M Herren-
bauer ldquoPhotocatalytic active coatings for lignin degradationin a continuous packed bed reactorrdquo International Journal ofPhotoenergy vol 2014 Article ID 502326 10 pages 2014
[55] C Awungacha Lekelefac P Czermak and M HerrenbauerldquoEvaluation of photocatalytic active coatings on sintered glasstubes by methylene bluerdquo International Journal of Photoenergyvol 2013 Article ID 614567 9 pages 2013
[56] Y-S Ma C-N Chang Y-P Chiang H-F Sung and A CChao ldquoPhotocatalytic degradation of lignin using PtTiO
2as
the catalystrdquo Chemosphere vol 71 no 5 pp 998ndash1004 2008[57] H Ohnishi M Matsumura H Tsubomura and M Iwasaki
ldquoBleaching of lignin solution by a photocatalyzed reactionon semiconductor photocatalystsrdquo Industrial and EngineeringChemistry Research vol 28 no 6 pp 719ndash724 1989
[58] C A K Gouvea F Wypych S G Moraes N Duran and PPeralta-Zamora ldquoSemiconductor-assisted photodegradation oflignin dye and kraft effluent by Ag-doped ZnOrdquo Chemospherevol 40 no 4 pp 427ndash432 2000
[59] A V Vahatalo K Salonen M Salkinoja-Salonen and AHatakka ldquoPhotochemical mineralization of synthetic lignin inlake water indicates enhanced turnover of aromatic organicmatter under solar radiationrdquo Biodegradation vol 10 no 6 pp415ndash420 1999
[60] H de Lasa B Serrano and M Salaices Photocatalytic ReactionEngineering Springer New York NY USA 2005
[61] K Hashimoto H Irie and A Fujishima ldquoTiO 2 photocatalysisA historical overview and future prospectsrdquo Japanese Journal ofApplied Physics vol 44 no 12 pp 8269ndash8285 2005
[62] M A Behnajady N Modirshahla and R Hamzavi ldquoKineticstudy on photocatalytic degradation of CI Acid Yellow 23 byZnOphotocatalystrdquo Journal ofHazardousMaterials vol 133 no1ndash3 pp 226ndash232 2006
[63] M Addamo V Augugliaro A di Paola et al ldquoPhotocatalyticthin films of TiO
2formed by a sol-gel process using titanium
tetraisopropoxide as the precursorrdquo Thin Solid Films vol 516no 12 pp 3802ndash3807 2008
[64] N Negishi K Takeuchi and T Ibusuki ldquoPreparation of theTiO2thin filmphotocatalyst by the dip-coating processrdquo Journal
of Sol-Gel Science and Technology vol 13 no 1ndash3 pp 691ndash6941998
[65] L Rideh A Wehrer D Ronze and A Zoulalian ldquoPhotocat-alytic degradation of 2-chlorophenol in TiO
2aqueous suspen-
sion modeling of reaction raterdquo Industrial and EngineeringChemistry Research vol 36 no 11 pp 4712ndash4718 1997
[66] R-ADoongC-HChen RAMaithreepala and S-MChangldquoThe influence of pHand cadmiumsulfide on the photocatalyticdegradation of 2-chlorophenol in titanium dioxide suspen-sionsrdquoWater Research vol 35 no 12 pp 2873ndash2880 2001
[67] K V Sarkanen A Islam and C D Anderson ldquoOzonationrdquo inMethods in Lignin Chemistry S Y Lin and C W Dence EdsSpringer Series inWood Science pp 387ndash406 Springer BerlinGermany 1992
[68] G Gellerstedt and E-L Lindfors ldquoStructural changes in ligninduring kraft pulpingrdquo Holzforschung vol 38 no 3 pp 151ndash1581984
[69] B Neppolian H C Choi M V Shankar B Arabindoo andV Murugesan in Proceedings of the International Symposiumon Environmental Pollution Control and Waste Management(EPCOWM 02) p 647 2002
[70] C Pouteau P Dole B Cathala L Averous and N BoquillonldquoAntioxidant properties of lignin in polypropylenerdquo PolymerDegradation and Stability vol 81 no 1 pp 9ndash18 2003
[71] J Hafren T Fujino and T Itoh ldquoChanges in cell wall archi-tecture of differentiating tracheids of Pinus thunbergii duringlignificationrdquo Plant and Cell Physiology vol 40 no 5 pp 532ndash541 1999
[72] T Dizhbite G Telysheva V Jurkjane and U Viesturs ldquoChar-acterization of the radical scavenging activity of lignins naturalantioxidantsrdquo Bioresource Technology vol 95 no 3 pp 309ndash3172004
[73] K Hofstadler R Bauer S Novalic and S G Heisier ldquoNew reac-tor design for photocatalytic wastewater treatment with TiO
2
immobilized on fused-silica glass fibers photomineralization of4-chlorophenolrdquo Environmental Science amp Technology vol 28no 4 pp 670ndash674 1994
[74] J Villasenor and H D Mansilla ldquoEffect of temperature onkraft black liquor degradation by ZnO-photoassisted catalysisrdquoJournal of Photochemistry and Photobiology A Chemistry vol93 no 2-3 pp 205ndash209 1996
[75] B H Bielski D E Cabelli L A Ravindra and A B RossldquoReactivity of HO
2Ominus2Radicals in aqueous solutionrdquo Journal
of Physical Chemistry vol 14 pp 1041ndash1100 1985[76] B H J Bielski and A O Allen ldquoMechanism of the dispropor-
tionation of superoxide radicalsrdquo Journal of Physical Chemistryvol 81 no 11 pp 1048ndash1050 1977
[77] B Halliwell and J M C Gutteridge ldquoThe importance of freeradicals and catalytic metal ions in human diseasesrdquoMolecularAspects of Medicine vol 8 no 2 pp 89ndash193 1985
[78] J M Palmer P J Harvey and H E Schoemaker ldquoThe role ofperoxidases radical cations and oxygen in the degradation oflignin [and discussion]rdquo Philosophical Transaction of the RoyalSociety A vol 321 no 1561 pp 495ndash505 1987
[79] S Y Yin and C W Dense Methods in Lignin ChemistrySpringer New York NY USA 1992
[80] P Kumar S Kumar and N K Bhardwaj ldquoPhotocatalyticoxidation of elemental chlorine free bleaching effluent withUVTiO
2rdquo in Proceedings of the 2nd International Conference on
Environmental Science and Technology (ICEST rsquo11) SingaporeFebruary 2011
[81] K Kobayakawa Y Sato S Nakamura and A FujishimaldquoPhotodecomposition of Kraft lignin catalyzed by titanium
18 International Journal of Photoenergy
dioxiderdquo Bulletin of the Chemical Society of Japan vol 62 no11 pp 3433ndash3436 1989
[82] R B Kinstre ldquoAn overview of strategies for reducing theenvironmental impact of bleach-plant effluentsrdquo Tappi Journalvol 76 no 5 pp 105ndash113 1993
[83] A Castellan H Choudhury R Stephen Davidson and SGrelier ldquoComparative study of stone-ground wood pulp andnative wood 3 Application of fluorescence spectroscopy to astudy of the weathering of stone-ground pulp and native woodrdquoJournal of Photochemistry and Photobiology A Chemistry vol81 no 2 pp 123ndash130 1994
[84] B Albinsson S Li K Lundquist and R Stomberg ldquoThe originof lignin fluorescencerdquo Journal of Molecular Structure vol 508no 1ndash3 pp 19ndash27 1999
[85] S Baumberger A Abaecherli M Fasching et al ldquoMolar massdetermination of lignins by size-exclusion chromatographytowards standardisation of the methodrdquo Holzforschung vol 61no 4 pp 459ndash468 2007
[86] E Torres andM Ayala Biocatalysis Based on Heme PeroxidasesSpringer New York NY USA 1st edition 2010
[87] R Pecina P Burtscher G Bonn and O Bobleter ldquoGC-MSand HPLC analyses of lignin degradation products in biomasshydrolyzatesrdquo Fresenius Zeitschrift fur Analytische Chemie vol325 no 5 pp 461ndash465 1986
[88] NIST NIST Standard Reference Database The National Insti-tute of Standards and Technology 2014 httpwwwnistgovsrdnist1acfm
[89] T K Kirk and R L Farrell ldquoEnzymatic ldquocombustionrdquo themicrobial degradation of ligninrdquo Annual Review of Microbiol-ogy vol 41 pp 465ndash501 1987
[90] A TMartinez ldquoHigh redox potential peroxidasesrdquo in IndustrialEnzymes Structure Function and Applications K-B BeckerEd pp 477ndash488 Springer Amsterdam The Netherlands 1stedition 2007
[91] M Dashtban H Schraft T A Syed and W Qin ldquoFungalbiodegradation and enzymatic modification of ligninrdquo Interna-tional Journal of Biochemistry and Molecular Biology vol 1 no1 pp 36ndash50 2010
[92] N Duran E Esposito L H Innocentini-Mei and V P CanhosldquoA new alternative process for Kraft E1 effluent treatmentrdquoBiodegradation vol 5 no 1 pp 13ndash19 1994
[93] K Kamwilaisak and P C Wright ldquoInvestigating laccase andtitanium dioxide for lignin degradationrdquo Energy and Fuels vol26 no 4 pp 2400ndash2406 2012
[94] J Reyes M Dezotti H Mansilla J Villasenor E Espositoand N Duran ldquoBiomass photochemistry-XXII combined pho-tochemical and biological process for treatment of Kraft E1effluentrdquoApplied Catalysis B Environmental vol 15 no 3-4 pp211ndash219 1998
[95] L F Gonzalez V Sarria and O F Sanchez ldquoDegradationof chlorophenols by sequential biological-advanced oxidativeprocess using Trametespubescens and TiO
2UVrdquo Bioresource
Technology vol 101 no 10 pp 3493ndash3499 2010[96] J H Lora and W G Glasser ldquoRecent industrial applications
of lignin a sustainable alternative to nonrenewable materialsrdquoJournal of Polymers and the Environment vol 10 no 1-2 pp 39ndash48 2002
Submit your manuscripts athttpwwwhindawicom
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Inorganic ChemistryInternational Journal of
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International Journal ofPhotoenergy
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Carbohydrate Chemistry
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CatalystsJournal of
International Journal of Photoenergy 7
Table2Summaryof
startingcond
ition
sfor
thep
hotocatalytic
degradationof
lignin
Reference
Lign
insource
Catalyst
Reactio
ncond
ition
s
MacHadoetal[42]
Peroxyform
icacid
lignins
from
eucalyptus
grandisw
ood(EL1)
EL1+
sodium
borohydride
TiO
2andH
2O2
Lign
inconcentration
025
mgmL119879=25∘C
pH11U
VVisradiatio
n120582gt300nm
40
0Wmercury
lampcylin
dricalPy
rexglassreactorcon
stanto
xygenbu
bblin
g
Ksibietal[47]
Water
solubleligninob
tained
from
blackliq
uor
TiO
2-P2
5(D
egussa)
Lign
inconcentration
90mgL119879=20∘C
pH82UV-radiation120582gt290nm
pyrex
reactoro
pento
airPh
ilips
HPK
125W
lamp
Kansaletal[48]
Lign
infro
mwheatstr
awkraft
digestion
TiO
2-P2
5(D
egussa)a
ndZn
O
Lign
inconcentration
10ndash100
mgLin
100m
LZn
Ocatalystdo
se(05ndash20gL)pH
ofthes
olution(pH3ndash11)solarillu
mination
oxidantcon
centratio
n306times10minus6M
to153times10minus6Mthinbedfilm
slurrypo
ndreactoroxidantsodium
hypo
chlorite
solutio
n(4availableC
l 2)
Dahm
andLu
cia[
49]
Whitewater
from
indu
strialprocess
water
thatexits
inpaperm
achines
FischerS
cientifi
crutile
TiO
2
Lign
inconcentration
40mgLin
500m
Lbatchreactor119879=21∘Cndash
42∘C
Rayonet
photochemicalcham
ber16
VWR8-W
blacklightpho
spho
r(350-nm
)lam
ps
constant
oxygen
bubb
lingpo
wer
ofillum
ination
128to
64Wlight
intensity
223ndash44
5mWcm
3
Portjanskajaand
Preis[50]
Lign
inpurchased
from
Aldric
hTiO
2P2
5-N(D
egussa)
Lign
inconcentration
100m
gLpH
8batchreactorsystemreactor
open
toair
PhillipsT
LD15W05low-pressurelum
inescent
mercury
UV-lampUV-radiation
120582gt360nm
pow
erdensity
ofirr
adiatio
n=07m
Wcm
2 visib
lelight
source
Tanaka
etal[51]
Lign
infro
mconiferous
woo
dTiO
2(TiO
2mdashTP
-2of
Fujititan)
Lign
inconcentration
0003to
003U
V-radiation120582gt310nm
cylindrical
reactio
nvessel
Tonu
ccietal[33]
Ca2+andNH4
+ligninderiv
atives
TiO
2as
Degussa
P25
+po
lyoxom
etalates
(POM)H
2O2
Openqu
artztubes(20
mL)
reactor119879=20∘C
1atm
multirayso
ften
UVlamps
of15W
power
eachU
V-radiation120582gt254nm
Miyatae
tal[24]
Piceag
lehn
iiwoo
dflo
urTiO
2po
lyethylene
oxide(PE
O)
Reactorop
enpetrid
ish119879=30∘C119905=48h40
0Wmercury
lamp
Shende
etal[17]
Kraft
lignin
TiO
2-Zn
O-ZrO
2Lign
inconcentration
1mgmLin
10mL250W
Xeno
nlampandAM
15Glamp
filterpo
wer
density
ofirr
adiatio
n100m
Wcm
2 solarlam
psim
ulator
Tian
etal[52]
andPanetal[53]
Kraft
ligninfro
mblackliq
uor
Ta2O
5-IrO
2andPb
O2thin
film
TiO
2nano
tubePbO
2
Lign
inconcentration
30(w
w)UV-radiation120582gt365nm
119905=10minpow
erdensity
ofirr
adiatio
n20
mWcm
2 blue
waveT
M50
ASUVspot
lampEG
ampG2273
potentiosta
tgalvano
stattoapplycurrentTiTiO
2NTPb
O2ele
ctrode
asworking
electrode
andPt
coilas
outere
lectrode
andAgAgC
lasreference
electrode
Awun
gachaL
ekele
fac
etal[54][55]
Lign
insulfo
natefro
mpaperw
aste
water
TiO
2as
Degussa
P25
TiO
2fro
msol-g
elprocesso
fTiOSO
4TT
IP
Lign
inconcentration
500m
gLin
200m
LOsram
Planon
light
sourceirradiance
30ndash4
0Wm
2 UV-radiation120582280ndash4
20nm
119905=20hreactoro
pento
air
recirculationsyste
mflow
rate225mLmin
8 International Journal of Photoenergy
Table3Parametersanalyticalmetho
dsand
results
from
different
workgrou
ps
Reference
Parameter
studied
Analytic
sRe
sult
MacHado
etal[42]
Roleof
hydroxylradicals
irradiatio
nof
ligninin
the
absencea
ndpresence
ofph
otocatalystT
iO2andH
2O2
Ultraviolet-v
isible(UV-Vis)spectro
scop
yionizatio
nabsorptio
nspectro
scop
y(IA
S)
sizee
xclusio
nchromatograph
y(SEC
)
Asharpdecrease
inthep
heno
liccontento
bservedforreactions
involvingdirect
photolysis
SEC
areductio
nof
almost50
inthea
verage
molecular
weighto
fligninequalto
14kD
after
90min
ofirr
adiatio
n
Ksibietal[47]
Irradiationof
ligninin
the
absencea
ndpresence
ofph
otocatalystT
iO2-P2
5
UV-Visspectroscop
y13C-
nucle
armagnetic
resonance(NMR)
solid
state
totalion
gasc
hrom
atograph
y(TIC)
indu
ctioncoup
lingplasma(
ICP)
chem
icaloxygen
demand(C
OD)
56degradationratewith
TiO
2catalystaft
er420m
inreactiontim
eethylacetate-extractableprod
uctsshow
edvanillin
vanillica
cidpalm
itica
cid
biph
enylstructuresand
345-trim
etho
xybenzaldehyde
presence
ofmagnesiu
mandcalcium
ions
CODremovalishigh
erforthe
initiallylowconcentrations
ofligninsolutio
n
Kansaletal[48]
Catalystdo
sepHoxidant
concentration
initialsubstrate
concentration
ZnOcatalystin
slurryandim
mob
ilizedmod
e
UV-Visspectroscop
yCO
D
Optim
umcatalystdo
seis1gL
optim
umoxidantcon
centratio
n2times10minus6M
graduald
ecreaseo
fabsorptionpeak
indicatin
gdecompo
sitionof
organics
CODremovalishigh
erforthe
initiallylowconcentrations
ofligninsolutio
n
Dahm
and
Lucia[
49]
Catalystdo
seillu
mination
intensity
UV-Visspectroscop
ytotalorganiccarbon
(TOC)
capillaryionelectro
phoresis
analysis(C
IA)
Gradu
aldecrease
inabsorptio
npeak
indicatin
gdecompo
sitionof
organics
optim
alcatalystdo
seof
10mgm
high
erillum
inationintensities
correlated
well
with
high
erinitialdegradationrate
74disapp
earanceo
fTOC
Portjanskajaand
Preis[50]
Influ
ence
offerrou
sion
sN-dop
edcatalysteffect
sprayedcatalyston
supp
ortand
subm
ersedcatalyst
CODU
V-Visspectroscop
ybiochemical
oxygen
demand(BOD)colorim
etric
measurementat570
nm
Additio
nof
Fe2+upto
28m
gLleadsto25increase
inph
otocatalyticeffi
ciency
sprayedcatalystexhibited15
times
high
ereffi
ciency
than
theo
neattached
bysubm
ersio
nnegligibleeffecto
fN-dop
edcatalyst
increase
ofaldehyde
concentrationover
reactio
ntim
eneutralm
ediawas
mostb
eneficialforb
iodegradability
80of
freep
heno
lsremoved
undern
eutralcond
ition
s
Tanaka
etal[51]Ca
talystloadinglignin
concentration
illum
inationtim
e
UV-Visspectroscop
yTO
Cgel
perm
eatio
nchromatograph
y(G
PC)
1 HNMR
Fouriertransform
ationinfrared
(FTIR)
spectro
scop
y
FTIR
measurementrevealedafasttransform
ationof
arom
aticmoietyp
resent
inlignin
characteris
ticband
sofaromaticrin
gsm
etho
xyand
aliphatic
sidec
hains
decrease
inTO
Cvalues
over
time
decrease
indegradationratewith
increase
catalystdo
sageB
utaft
ercatalystthreshold
valueisa
ttainedcatalystincreasec
ausesa
decrease
indegradationrates
FTIR
peaksa
reshifted
towards
lower
molecular
weightregionaft
erph
otocatalysis
Tonu
cci
etal[33]
Testof
catalytic
syste
mstoob
tain
fractio
nswith
redu
ceddegreeso
fpo
lymerization
comparis
onof
thermalandph
otochemical
reactio
ns
1 HNMR
gasc
hrom
atograph
y-mass
spectro
scop
y(G
C-MS)
POMsa
relessselectivew
henused
asph
otocatalystsandno
appreciableb
leaching
ofthes
olutionwas
seen
whenPO
Mwas
used
asthermalcatalyst
deriv
edchem
icalsfrom
experim
entvanillin
hydroxylmetho
xy-acetoph
enon
econiferylalcoh
olcon
iferylaldehydemethano
lform
icacidacetic
acidand
sometim
essm
allamou
ntso
fC-2
andC-
3alcoho
ls
Miyatae
tal[24]
Exam
inationof
cellwall
structureo
flignin(w
oodflo
ur)
before
andaft
erph
otocatalysis
GC-
MSscanning
electronmicroscop
y(SEM
)1 H
NMR
Highdelignificationactiv
itydelignificationconfi
nedto
thes
urface
oflignin
deriv
edchem
icalse
xperim
entvanillin
Shende
etal[17]Testof
combinedactio
nof
bio-
andph
otocatalyticsyste
ms
UV-Visspectroscop
yGC-
MSX-
ray
dispersiv
eenergyspectro
scop
yandX-
ray
diffractio
nanalysis(EDX
XRD)SE
M
Detectio
nof
follo
wingchem
icalsacetylguaiacol4-ethoxym
ethyl-2
-metho
xyph
enol
metho
xyph
enyloxim
eguaiacolsuccinica
cidacetylguaiacolvanillicacidand
vanillin
International Journal of Photoenergy 9
Table3Con
tinued
Reference
Parameter
studied
Analytic
sRe
sult
Tian
etal[52]
andPanetal
[53]
Testof
combinedactio
nof
electro-and
photocatalytic
syste
ms
UV-Visspectroscop
yFT
IRSEM
X-ray
(EDX)
highperfo
rmance
liquid
chromatograph
y(H
PLC)
COD
Detectio
nof
thefollowingchem
icalscarbon
ylfunctio
nalityvanillin
andvanillica
cid
Awun
gacha
Lekeleface
tal
[54][55]
Com
paris
onof
degradationrates
bydifferent
catalyst
HPL
Cflu
orescencea
ndUV-Vis
spectro
scop
ySE
MT
OC
UV-Visresultsrevealfaste
rdegradatio
nof
thea
liphatic
moietycomparedto
the
arom
aticmoietyof
ligninsulfonateob
tained
from
paperw
astewaterPeaks
observed
durin
gHPL
Canalysis
Someo
fthe
peaksp
rodu
cedaft
erph
otocatalysishad
fluorescences
ignals
Thissuggeststhep
rodu
ctionof
newsubstances
andflu
orop
hores
Coatin
gsprod
uced
throug
hSol-G
elprocedures
arestablea
ndcanbe
used
manytim
es
10 International Journal of Photoenergy
321 Influence of Catalyst One aspect to successfully imple-ment photocatalysis is the choice of an appropriate photo-catalyst The majority of catalysts used are based on TiO
2as
summarized in Table 2 ZnO has also been applied either assingle catalyst [48] or in a combination with other catalystsuch as TiO
2[54] de Lasa et al [60] described ZnO as
being less active than TiO2 They add that the use of ZnO
is particularly relevant when the oxidative degradation ratebecomes limited This is opposite to what Kansal et al [48]reported by saying that ZnO is more reactive than TiO
2
However TiO2(mainly anatase) remains the most used
catalyst as can be depicted fromTable 2 TiO2is reported to be
favored because of its nontoxic property cost efficiency andchemical and biological inertness Moreover TiO
2possesses
the most efficient photoactivity and the highest stability thusmaking it suitable for industrial use [61]
ZnO has been described to degrade lignin under visiblelight sources [48 62] whereas TiO
2is mostly applied in
connection to UV-light sources as highlighted in Table 2However both ZnO and TiO
2possess energy band gap
energy of 32 eV [23]Additives such as SiO
2[63] polyethylene oxide (PEO)
[24] and polyethylene glycol [64] have been added to TiO2
catalyst particularly when applying immobilized catalystAddamo et al [63] noted a high adhesion of TiO
2to glass
support material when precoating was done with SiO2
Moreover the precoating might have other advantages suchas a hindering diffusion of Na+ ions from the glass materialinto the nascent TiO
2film during heat treatment processes
Analogous to the addition of SiO2 polyethylene glycol
(PEG) has also been introduced to mitigate catalyst surfaceactivity modify surface hydrophobicity and also reduceagglomeration tendency of the TiO
2gel or TiO
2particles
in the suspensions [64] By a proper surface modificationinteraction between catalyst and substrate can be enhanced[55 63]
Kansal et al [48] varied catalyst (ZnO) dose from 05 gLto 20 gL for 01 gL kraft lignin solutions and found out thatthere was an optimum catalyst threshold value at 1 gL whichgives a catalyst to substrate ratio of 1 10
Dahm and Lucia [49] examined catalyst dose from 2 sdot10minus3 gL to 12sdot10minus2 gL for lignin solutions (fromwhite water
liner mill) of 4 sdot 10minus2 gL (catalyst to lignin ratio 5 sdot 10minus3ndash3 sdot 10minus2) at pH 8 and obtained best energy efficiency valuesand lignin degradation rates with a catalyst loading of 10 sdot10minus2 gLIn contrast Ma et al [56] applied far higher catalyst
concentration compared to Dahm and Lucia [49] Catalystconcentration was varied between 1 gL and 10 gL PtTiO
2
Best catalysis dose with respect to reaction turnover wasobtained at 5 gL PtTiO
2With the increase of catalyst dose at
pH 7 the reaction rate increased from 61 times 10minus3minminus1 (1 gLTiO2) to 71 sdot 10minus3minminus1 (5 gL TiO
2) and 99 sdot 10minus3minminus1
(10 gL TiO2)
Catalyst effect has been explained on the basis thatoptimum catalyst loading is dependent on the initial soluteconcentration An increase in catalyst dosage leads to a cor-responding increase of total active surface area for reactions
[65] To that at higher TiO2concentrations the photon flux is
more easily intercepted by the catalyst before penetrating intothe bulk of the system At the same time due to an increase inturbidity of the suspension with high dose of photocatalystthere is a decrease in penetration of UV light and hencephotoactivated volume of suspension or solution decreases[66]
In summary authors have obtained best catalyst to ligninrelations for different reaction designs and thus a generalrecommendation on catalyst dose is not possible Howeverwhen lignin solution is treated with increasing catalyst loadsa corresponding increase in degradation rate is observed untila threshold value is reached [48ndash50 56]
322 Influence of Metal Ion Addition (Doping) and AdditivesThe purpose of adding metal ion to photocatalyst is to miti-gate band gap energy through the introduction of intrabandgap states and as a consequence produce a bathochromicshift in the absorption spectrum [37] Altering the absorptionspectral range gives the possibility to exploit both the visiblelight spectrum and UV light sources Metal ion doping isalso introduced to serve as electron or hole traps in orderto minimize recombination between generated electron-holepairs [37]
Portjanskaja and Preis [50] studied the addition of Fe2+ions to an acidic lignin solution and found an increasein photocatalytic oxidation (PCO) efficiency The optimumFe2+ ions quantity was 28mgL while using 100mgL ligninsolution Upon further elevation of Fe2+ ions concentrationa corresponding reduction of the photocatalytic oxidationefficiency of lignin was noted Likewise Ohnishi et al [57]made a comparative study by doping platinum (Pt) silver(Ag) and gold (Au) ions to TiO
2 In these reactions 50mg of
catalyst (TiO2) was used with the addition of an equiva1ent
15 wt (based on TiO2) metal ion The addition of noble
metals brought about a faster decolorization of lignin Aushowed better results than Ag followed by Pt In the samecontext adding sodium hypochlorite as oxidant to PtTiO
2
catalyst an additional fivefold degradation rate was observedcompared to that without doping [56] Contradictory to theresults described above negligible effect of photocatalyticefficiency due to doping has been reported as well Awun-gacha Lekelefac et al [54] obtained little or no change indegradation rate by doping TiO
2-P25-SiO
2catalyst with Pt
ions (1 wt relative to TiO2catalyst) Likewise Portjanskaja
and Preis [50] noted a negligible change of photocatalyticefficiency of TiO
2when doped with nitrogen
Sarkanen et al [67] and Gellerstedt and Lindfors [68]reported the bias of peroxides to oxidation with reagentssuch as permanganate to favor aromatic moieties Oxidationagents like permanganate oxidizes predominantly aliphaticchains in alkaline and neutral media However by theapplication of H
2O2(Fenton system) lignin disappeared
completely [33] Tonucci et al [33] conclude that in orderto satisfactorily conserve the organic material the best com-promise appears to be the TiO
2photosystem which shows
low carbon consumption good preservation of the aromaticrings and greatly reduced mineralization
International Journal of Photoenergy 11
In summary different results have been obtained con-cerning the influence of noble metal ion addition Whilesome authors report an improvement in the photocatalyticefficiency upon their addition others report their addition ashaving no considerable influence However for reactions inwhich an improvement in the photocatalytic efficiency wasnoticed there was a threshold value to be considered Whenthe concentration of dopant surpasses this threshold valueelectron-hole recombination is favored and this has a negativeimpact to photocatalysis In such a case the space-chargelayer gets narrower and 119901-type dopants attract electrons andby virtue become negative They would now then act ashole acceptor attracting holes On the other hand 119899-typedopants which act as electron donor centers and possessexcess electrons attract holes as well [37]
323 Influence of Lignin Concentration Once the initiallignin concentration becomes higher exceeding a thresholdvalue an inhibitory effect on the photodegradationwas noted[47ndash49] This threshold value varies and depends on thereaction system and reaction parameters such as opticaldensity catalyst concentration and reaction volume Fromthe literature different authors have implemented varyinglignin concentrations probably to suit their reaction designFor example Ksibi et al [47] use 90mgL and AwungachaLekelefac et al [54] use 500mgL while Kansal et al [48]applied 10mgndash100mgL
Explanations arising from the findings are as followsat low lignin concentrations the incidental photonic fluxirradiated interacts with the catalyst generating radicals (eghydroxyl radicals (OH∙)) which allow a faster degradation[69] On the other hand high initial lignin concentrationsmay lead to tight adsorption which can suppress CO
2
evolution [51] and hence maintain chemical oxygen demand(COD) values Moreover low delignification yields maybe due to an inhibitory effect because of autoxidation bylow molecular weight lignin degradation products formed[24] Also due to the polymer structure of lignin which iscross-linked this makes it difficult for radical species acidand the aldehyde compounds produced to spread into theinner region of the substrate hence limiting autoxidationAs a worst case this might be the rate-determining step ofdelignification which is hindered [70ndash72]
In summary it can be concluded that the time taken forcomplete degradation depends on the initial concentration oflignin and faster degradation occurs at low lignin concentra-tions
324 Influence of pH Varying pH entails an alteration inthe properties of semiconductor-liquid interface [73] mainlyrelated to the acid-base equilibrium of the adsorbed hydroxylgroup [39] Furthermore pH also impacts lignin degradationrates [47 48 57] In this context several studies were carriedout with partly contradictory outcomes
Kansal et al [48] made pH investigations 3ndash11 undersolar light illumination using ZnO as catalyst Maximumdegradation was reached in alkaline conditions (pH 11)This is supported by Villasenor and Mansilla [74] reporting
an almost complete decolorization of kraft black liquorfrom pine wood at pH value of 116 in combination withZnO catalyst Similar results were achieved by Ohnishi etal [57] with TiO
2and ZnO being catalyst for bleaching
alkaline lignin in aqueous solution with TiO2and ZnO being
catalyst High activities at neutral pH were also reportedby Ohnishi et al [57] In contrast Ma et al [56] observedhigher reaction rates and rapid degradation of a syntheticlignin wastewater (prepared by dissolving commercial ligninpowder in aqueous solution pH 11) in acidic solution (pH 3)than in alkaline solutions at pH 11 for either TiO
2or PtTiO
2
catalystsReconsidering the photocatalytic principle the formed
superoxide anion radicals (∙O2
minus) are in a pH-dependentequilibriumwith perhydroxyl radicals (HO
2
∙) as follows [75]
HO2
∙999445999468 H+ + ∙O
2
minus pKa sim 48 (9)
See [76]∙O2
minus undergoes dismutation reaction resulting in H2O2
and O2competing to any other ∙O
2
minus triggered reaction Incase of low pH operation conditions in aqueous solutionsHO2
∙ becomes dominant whose reactivity is considerablyhigher compared to ∙O
2
minus [77] Subsequently HO2
∙ initiatessubstrate (S) oxidation to the radical cation (S+∙) and is itselfreduced to H
2O2[78] Thus increased degradation rates can
be reasonably expected supporting the results made by Ma etal [56] in an acidic environment ∙O
2
minus is extremely reactive inorganic solvents [77] Another aspect is the solubility of kraftlignin (soluble at pH gt 105) which reduces with decreasingpH whereas lignosulfonate should remain unaffected by pHin aqueous solution Moreover 120573ndashOndash4 bonds have beendescribed to be stable at acidic pH [11] In fact this couldadditionally explain the elevated degradation of kraft ligninmade by Kansal et al [48] and Villasenor and Mansilla [74]Nevertheless the contradictory results gained by Ma et al[56] still exist under the assumption that kraft ligninwas used(which would be supported by the high pH of 11 obviouslynecessary for dissolving the lignin powder) Although mostphotocatalytic reactions described in the literature are in anaqueous milieu lignin raw material and its fission productsmay however vary considerably Therefore the optimal pHis most likely to be reaction specific and has to be evaluatedexperimentally in principle
325 Influence of Illumination Many of the studies foundin the literature so far have not dealt on this subject per seWhat is found is the use of different illumination sourceseach having a specified power and lamp type However alltend to emitUV-light between the range 280ndash420 nm Table 2depicts this in detail Other illumination sources include thevisible light spectrum
In general terms illumination influences in that it ini-tiates photocatalysis by generating electron-hole pair in thesemiconductor particles [38 39] Dahm and Lucia [49]altered illumination intensity while observing lignin degra-dation In this study 004 gL lignin was used and lightintensity was varied 223ndash445mWcm3 It was found out thathigher illumination intensities correlated well with higher
12 International Journal of Photoenergy
initial degradation rates and hence total lignin degradation[49] Neppolian et al [69] report degradation to be propor-tional to radiation intensity and best results are achieved forlow lignin concentrations because of enhanced interactionbetween catalyst and incidental photonic flux
In summary high illumination power causes a corre-sponding high initial degradation rate at low lignin concen-trations becausemaximum light penetration into the reactionmedium is favored
33 Process Analytical Methods Various analytical tech-niques have been used to monitor lignin degradation Atthe beginning of this subchapter analytics revealing com-pounds formed from lignin degradation are treated Thisincludes for example gas chromatography (GC) and 1HNMR (nuclear magnetic resonance) This is then followed byresults qualitative analytic measurements such as ultraviolet-visible (UV-Vis) spectroscopy and dissolved carbon (DC)A list of authors analytical techniques applied and resultsachieved are outlined in Table 3
Portjanskaja and Preis [50] studied lignin degradationby measuring the removal of phenols through colorimetricmeasurements As a result of 24 h photocatalytic oxidationunder neutral media conditions 80 of free phenols wereremoved Gas chromatography (GC) result from Ksibi et al[47] attested vanillin vanillic acid palmitic acid biphenyland 345-trimethoxy benzaldehyde structures after the pho-tocatalysis of lignin from black liquor This is in accordancewith the findings of Tonucci et al [33] reporting the formationof vanillin hydroxyl methoxy-acetophenone coniferyl alco-hol coniferyl aldehyde methanol formic acid acetic acidand small amounts of C-2 and C-3 alcohols as degradationproducts1H NMR spectral analysis of lignin before illumination
and after 24 h of illumination showing characteristic bandsof aromatic rings methoxy and aliphatic side chains wascompared with each other Results revealed that the aro-matic ring degraded faster than the aliphatic chain [51]Fourier transformation infrared spectroscopy (FTIR) showedbands corresponding to CH
3 CH2 and CH which remained
unchanged after illumination while bands corresponding toaromatic rings disappeared as a result of illumination [51 53]
Results obtained from the combination of photochemicaland electrochemical oxidation [52 53] were similar to thoseof Tanaka et al [51] Here 13C-NMR confirmed the presenceof the carbonyl functionality and the presence of vanillinand vanillic acid after 12 h photochemical-electrochemicaloxidation These results showed that the combination of aphotocatalytic and an electrochemical oxidation significantlyenhanced the efficiency of lignin degradationThis is becausethe applied anodic potential bias greatly suppressed therecombination of photogenerated electrons and holes [53]
Ultraviolet spectrophotometry offers a convenientmethod to qualitatively and quantitatively analyze ligninin solution [79] This is reflected by the large number ofpublications using this technique [17 33 47ndash51 56 58 80]This is most likely due to its simplicity to interpret lignindegradation Lignins absorb UV light with high molar
200 220 240 260 280 300 320 340 360
00
05
10
15
20
25
Abso
rban
ce
Wavelength (nm)0min
30min
60min
120min
180min
300min
360min
420min
1200min
Figure 8 Time dependent UV-Vis absorption spectra of aqueouslignin solution from waste paper water irradiated with UV light(280ndash420 nm) for different time intervals The spectra are obtainedfor sol-gel derived TiO
2nanocrystalline coating (TiO
2-P25-SiO
2)
[54]
extinction coefficients because of the several methox-ylated phenylpropane units of which they are composed [33]Figure 8 depicts a series of photometric scans of ligninsul-fonate from paper waste water showing a gradual reductionof absorbance during photocatalytic treatment [54] Herethe absorption peaks are around 210 nm and 280 nmAbsorbance decreases with time implying the decompositionof lignin and the deterioration of chromophore groups [54]
Peaks at 210 nmcorrespond to portions of the unsaturatedchains while those at 280 nm correspond to unconjugatedphenolic hydroxyl groups [17] and the aromatic moiety [57]of the lignin molecule The absorption tailing to the longwavelength region arises from the color of lignin [57] Lignindegradation has been reported either at wavelength around280 nm corresponding to unconjugated phenolic hydroxylgroups [17 48 51 58] or for both wavelengths (210 nm and280 nm) [33 54 57] Kobayakawa et al [81] noted some otherabsorbance at wavelengths lower than 250 nm and pointedout that this could be due to the modification of ligninfragmentations leading to the formation of transient specieslikemethanol ethanol formaldehyde formic acid and oxalicacid among others
Analyticalmethods to effectively quantify lignin degrada-tion by calculating the oxygen demand by organic substancesand remaining organic carbon before and after photocatalysishave been studied Amongst the methods are dissolvedcarbon (DC) [49 51 58] chemical oxygen demand (COD)[47 48 50 57 80] biochemical oxygen demand (BOC) [50]dissolved organic carbon (DOC) [56] and American dyemanufacture institute value (ADMI) [56] COD and BODdescribe the oxygen demand by organic substances to be
International Journal of Photoenergy 13
Figure 9 Gradual change from the characteristic yellow ligninsulfonate to a colorless liquid after a period of 20 h Catalyst TiO
2-
P25 (Degussa) +TEOSUV-light 25∘C lignin concentration 05 gL[54]
converted to CO and CO2and H
2O and NH
3 Total organic
carbon (TOC) describes the amount of carbon bound inan organic compound while DOC describes the dissolvedfraction of organic carbon ADMI measures the amount ofdyestuff in water
Decolorization of lignin solution has been reported tobe another parameter observed during photocatalytic degra-dation Color is an indirect indicator of the lignin amountThe higher the color intensity of the solution is the greaterthe lignin content is (high concentrated lignin solutions egblack liquor appear dark brown) [82] Thus color changescan be interpreted as conversion of lignin to transient speciesor conversion to CO
2and H
2O Awungacha Lekelefac et al
[54] observed a gradual change from the characteristic yellowlignin (when highly diluted) to a colorless liquid after a periodof 20 h with sol-gel derived TiO
2nanocrystalline coatings on
sintered borosilicate glass as depicted in Figure 9 A corre-sponding decrease in DC values close to 82 was observedfor TiO
2-P25-SiO
2catalyst confirming degradation This is
shown in Figure 10These findings are analogous to that of Ohnishi et al
[57] who reported the bleaching of lignin when illuminatedcontinuously and that the solution becomes colorless Tothat the chemical oxygen demand (COD) value decreasesgenerating carbon dioxide and a small amount of carbonmonoxide as the main gaseous products COD removalwas reported to be effective at low lignin concentrations ascompared to high lignin concentrations [48]
Another applied analytical technique is fluorescencedetection directly coupled to a high performance liquidchromatography (HPLC) as a means to identify nonaliphaticcomponent in the complex mixture of lignin degradationproducts [54] Fluorescence emission in lignin is attributedto aromatic structures such as conjugated carbonyl biphenylphenylcoumarone and stilbene groups [83 84] AwungachaLekelefac et al [54] observed peaks on both HPLC and
0 200 400 600 800 1000 12000
50
100
150
200
250
Diss
olve
d ca
rbon
(ppm
)
t (min)
TiO2-P25-SiO2
TiO2-P25-SiO2 + PtTiOSO4minus30 6wt
ZnO + TiO2-P25-SiO2
No coating UV light
Figure 10 Variation of DC with time of aqueous lignin solutionfrom waste paper water irradiated with UV light (280ndash420 nm) fordifferent time intervals The spectra are obtained for sol-gel derivedTiO2nanocrystalline coatings (TiO
2-P25-SiO
2+Pt TiO
2-P25-SiO
2
TiOSO4minus306 wt ZnO + TiO
2-P25-SiO
2) [54]
fluorescence chromatograms suggesting the production ofnew substances and fluorophores
Despite developed analytical technologies analyzinglignin degradation products remains challenging Proofssuch as mass spectroscopy (MS) HPLC 13C or 1H-NMRspectra from photocatalytic lignin degradations are not yetestablished The setback to qualitatively and quantitativelyanalyze lignin and its degradation products starts from thenative lignin polymer itself with its indefinite polymericstructure and multiple bond types Also the influence ofdifferent pretreatments additives and the wide variety ofcompounds obtainable from its degradation makes ligninanalysis challenging [85] Moreover lignin streams couldcontain proteins inorganic salts and other potential poisonsthat generally complicate catalysis [4]
The challenge to identify and separate the productsstreams derived from the photocatalytic degradation ligninis also worth noting Lignin product stream is highly func-tionalized and conventional techniques such as gas chro-matography have the disadvantage of requiring a time-consuming derivatization step Also because of high boilingpoint of substances arising from lignin degradation it is noteasily applicable High performance liquid chromatography(HPLC) seems to be the remedy because analysis can becarried out without derivatization but the exact identificationof the separated substances is difficult because of the numer-ous peaks arising from such a chromatogram [87] Unfortu-nately well-established databases such as that of the nationalinstitute of standard and technology (NIST) [88] cannot giveinformation onHPLC-MS chromatogramsThis is because ofthe ionization sources such as electrospray ionization (ESI)
14 International Journal of Photoenergy
Table 4 Main advantages and disadvantages photocatalysis versus enzymatic biocatalysis
Lignin degradation process Advantages Disadvantages
Photocatalysis(single stage)
(i) Relatively fast degradation of complex as wellas nonbiodegradable organic (macro)molecules(ii) Stable catalysts(iii) Moderate reaction conditions
(i) Not selective(ii) Limited selection of operating conditions(iii) Energy costs
Enzymatic conversion(single stage)
(i) More selective(ii) Moderate reaction conditions
(i) High enzyme loads required otherwise ligninhydrolysis is quite slow [16](ii) Enzymes are less stable1(iii) cost-intensive (POXs)2
1For example POXs are sensitive to high H2O2 concentrations2According to Torres and Ayala [86]
and atmospheric pressure chemical ionization (APCI) forLC-MS while GC-MS is based on electron ionization Aremedy to this can be the development of a database ofmodel lignin compounds based on a unanimous HPLC-MSmeasuring procedure This would mean much time and costexpensive investments for adequate personnel and material
Though product identification based on peak superpo-sition gives information on a possible product this can-not always be true for lignin degradation because of thelarge number of possible degradation products In order toproperly identify products from lignin degradation productisolation techniques are to be implemented and further ana-lytic methods such as tandem mass spectroscopy (MSMS)and other advanced structure enhancing research techniquessuch as 1H-NMR 13C-NMR have to be studied extensively
4 Photocatalysis and Its Suitability asIntegrated Technology in MultistageConcepts with Biocatalysis
Depending on lignin treatment specification photocatalysiswill find application for complete lignin mineralization toCO2and H
2O (single stage) or as integrated technology in
a multistage system aiming at partial conversion of ligninmacromolecules to organic low molecular weight intermedi-ates for a consecutive biological oxidation finally generatingvalue added end-products [15]
For biological oxidation of lignin or lignin deriva-tives H
2O2-dependent ligninolytic heme peroxidases (POXs
including lignin peroxidase (LiP EC 111114) manganeseperoxidase (MnP EC 111113) versatile peroxidase (VPEC 111116) O
2-dependent laccases (Lac EC 11032) and
extracellular enzymes from Basidiomycetous white-rot fungiare the most efficient lignin degraders in nature [89] Thussuch enzymatic systems especially POXs (with high redoxpotentials) have attractedmuch interest as industrial biocata-lyst [86 90]The enzyme degradationmechanism (in nature)is facilitated by nonenzymatic processes mainly throughfree ∙OH radicals also generated by the fungus (Fenton-type reaction) Those ∙OH radicals enable the requiredphysical contact between the enzymes and structural units ofthe lignin molecule due to numerous nonspecific oxidative
reactions [91] Both the photocatalytic (equations (2)ndash(7))and the POX reactionmechanism recently reviewed by Busseat al [25] are quite similar Consequently a combination ofphotocatalysis followed by an enzymatic oxidation maybe apromising concept utilizing lignin derivatives for exampleoriginated from industrial effluents [15 92] The advantagesforming biobased products are as follows see also Table 4 inthis context A reduction of the lignin polymerization degreevia photocatalysis at best in more biodegradable interme-diates and a simultaneous detoxification causes savings inenzyme costs since it will be expected that less enzyme loadsare necessary for sufficiently rapid reaction rates [15 91] As aresult hydraulic retention times in bioreactor systems shouldbe diminished Moreover the delignification is expectedto be enhanced simultaneously which was in recent yearsshown in a dual system by Kamwilaisak and Wright [93]using TiO
2H2O2UV for photocatalytic pretreatment and
Lac (from Trametes versicolor) in the subsequent biocatalyticstep Within 24 h they obtained in their dual system anelevation in delignification of 20 (without H
2O2) up to at
least 50 when H2O2was present
In previous studies the treatment of lignin (from thepulp and paper industry) containing effluents by fungus asa biological system (excreting appropriate lignin degradingenzyme cocktails) were more focused for posttreatmentexclusively (Duran et al [92] Reyes et al [94] and Gonzalezet al [95]) Shende et al [17] even examined ligninolyticbacteria with photooxidized kraft lignin as substrate
Although POXs are potential industrial biocatalysts [86]no application studies were found for the direct use in suchdual systems as described above The major reason may bethe complexity of the reaction mechanism (inclusive ligninderivatives as substrate and their analysis) per se on the onehand slowing down research and development processes Onthe other hand POXs are sensitive to their cosubstrate H
2O2
once it is supplied in excess causing considerable inactivation(for details refer to Busse et al [25]) At the present severalstudies are carried out modifying these enzymes regardingenhanced stability activity and selectivity as well Henceit can be expected that their technological applicability willbe raised significantly right after successful modification isreached [86]
International Journal of Photoenergy 15
5 Concluding Remarks
It is widely assumed that the photocatalytic degradation oflignin follows a radical reaction pathway which is similar tothat considered in thermal electrochemical and biochemicalprocesses However reporting on the degradation pathway oflignin derivatives and even that of lignin model compoundsare still amajor challengeThis is probably due to the complexnature and variety of possible degradation products Indeedthemechanism is far more complex considering other factorssuch as type of lignin type of catalyst pH illuminationsource and additives
Generally comparing the different photochemical pro-cesses poses a big challenge because of the wide variablesinvolved These discrepancies start from the source and typeof lignin followed by differences in reactor design illumi-nation source intensity of radiation and different types ofcatalyst An ideawould be to have a specific reference reactionwith well-defined starting parameters which include lignintype source and purity catalyst specifications illuminationsource and intensity so as to ease comparison of results
Basic process parameters such as catalyst concentrationsubstrate concentration addition of metal ion to catalyst pHand illumination have been discussed
Despite developed analytical technologies analyzinglignin degradation products remains challenging Proofssuch as mass spectroscopy (MS) HPLC 13C or 1H-NMRspectra from photocatalytic lignin degradations are not yetestablished
In order to properly identify products from lignin degra-dation product isolation techniques are to be implementedand further analytic methods such as MSMS and otheradvanced structure enhancing research techniques such as1H-NMR and 13C-NMR have to be studied extensively Aremedy to this can be the development of a databank ofmodel lignin compounds based on a unanimous HPLC-MSmeasuring procedure This would mean much time and costexpensive investments for adequate personnel and material
Photocatalysis is denoted as the most popular ligninpretreatment technology besides ozonation [96] Photo-catalyzed lignin may be an appropriate substrate for aconsecutive biocatalytic process using ligninolytic enzymes(POX andor Lac) as supported by experimental results ofKamwilaisak and Wright [93] Combining the advantages ofboth catalytic processes savings in the overall process costswill be expected in addition to elevated lignin conversionNonetheless extensive research work including POX modi-fications is still required
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Authorsrsquo Contribution
Colin Awungacha Lekelefac and Nadine Busse contributedequally to this work
Acknowledgments
The authors gratefully thank the Federal Ministry of Educa-tion and Research (BMBF) for funding (FKZ17N0310) Theresearchers also thank the Hessen State Ministry of HigherEducation Research andArts for the financial support withinthe Hessen initiative for scientific and economic excellence(LOEWE)
References
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and Climate Policy Outlook (WETO) Luxembourg Office forOfficial Publications of the European Communities 2003
[3] Green and Natural Polymers Are on the Rise 2014 httpwwwpolymersolutionscombloggreen-and-natural-polymers-on-the-rise
[4] J Zakzeski P C Bruijnincx A L Jongerius and B M Weck-huysen ldquoThe catalytic valorization of lignin for the productionof renewable chemicalsrdquo Chemical Reviews vol 110 no 6 pp3552ndash3599 2010
[5] B KammM Kamm P R Gruber and S KromusBiorefineries-Industrial Processes and Products Status Quo and Future Direc-tions vol 1 Wiley-VCH 2006
[6] R L Howard E Abotsi E L J van Rensburg and S HowardldquoLignocellulose biotechnology issues of bioconversion andenzyme productionrdquoAfrican Journal of Biotechnology vol 2 no12 pp 602ndash619 2003
[7] M Stocker ldquoBio- und BTL-Kraftstoffe in der Bioraffineriekatalytische Umwandlung Lignocellulose-reicher Biomasse mitporosen Stoffenrdquo Angewandte Chemie vol 120 no 48 pp9340ndash9351 2008
[8] J-P Lange ldquoLignocellulose conversion an introduction tochemistry process and economicsrdquo Biofuels Bioproducts andBiorefining vol 1 no 1 pp 39ndash48 2007
[9] D A I Goring ldquoThe physical chemistry of ligninrdquo Pure andApplied Chemistry vol 5 no 1-2 pp 233ndash310 1962
[10] M Ek G Gellerstedt and G HenrikssonWood Chemistry andBiotechnology Pulp and Paper Chemistry and Technology vol 1Walter de Gruyter GmbH amp Co KG Berlin Germany 2009
[11] B Saake and R Lehnen ldquoLigninrdquo in Ullmannrsquos Encyclopedia ofIndustrial Chemistry Wiley-VCH Verlag GmbH amp Co KGaAWeinheim Germany 2012
[12] M N S Kumar A K Mohanty L Erickson and M MisraldquoLignin and its applications with polymersrdquo Journal of BiobasedMaterials and Bioenergy vol 3 no 1 pp 1ndash24 2009
[13] R J A Gosselink E de Jong B Guran and A AbacherlildquoCo-ordination network for ligninmdashstandardisation produc-tion and applications adapted to market requirements(EUROLIGNIN)rdquo Industrial Crops and Products vol 20 no 2pp 121ndash129 2004
[14] D Fengel and G Wegener Wood Chemistry UltrastructureReactions Verlag Kessel 1984
[15] D Mantzavinos and E Psillakis ldquoEnhancement of biodegrad-ability of industrial wastewaters by chemical oxidation pre-treatmentrdquo Journal of Chemical Technology and Biotechnologyvol 79 no 5 pp 431ndash454 2004
16 International Journal of Photoenergy
[16] M Dashtban H Schraft and W Qin ldquoFungal bioconversionof lignocellulosic residues Opportunities amp perspectivesrdquo Inter-national Journal of Biological Sciences vol 5 no 6 pp 578ndash5952009
[17] A Shende R Jaswal D Harder-Heinz A Menan and RShende ldquoIntergrated photocatalytic and microbial degradationof kraft ligninrdquo Cleantech pp 120ndash123 2012
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[19] W G Glasser and H R Glasser ldquoEvaluation of ligninrsquoschemical structure by experimental and computer simulationtechniquesrdquo Paperi ja Puu vol 63 pp 71ndash83 1981
[20] M Erickson S Larsson and G E Miksche ldquoZur Struktur desLignins der FichterdquoActaChemica Scandinavica vol 27 pp 903ndash914 1973
[21] H Nimz ldquoDas Lignin der BuchemdashEntwurf eines Konstitution-sschemasrdquo Angewandte ChemiemdashInternational Edition vol 86pp 336ndash344 1974
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Chemical Reviews vol 95 no 3 pp 735ndash758 1995[24] Y Miyata K Miyazaki M Miura Y Shimotori M Aoyama
and H Nakatani ldquoSolventless delignification of wood flourwith TiO
2poly(ethylene oxide) photocatalyst systemrdquo Journal
of Polymers and the Environment vol 21 no 1 pp 115ndash121 2013[25] N Busse D Wagner M Kraume and P Czermak ldquoReaction
kinetics of versatile peroxidase for the degradation of lignincompoundsrdquoTheAmerican Journal of Biochemistry andBiotech-nology vol 9 no 4 pp 365ndash394 2013
[26] M Tien and T K Kirk ldquoLignin-degrading enzyme fromphanerochaete chrysosporium Purification characterizationand catalytic properties of a uniqueH
2O2-requiring oxygenaserdquo
Proceedings of the National Academy of Sciences vol 81 pp2280ndash2284 1984
[27] T K Kirk M Tien P J Kersten M D Mozuch and BKalyanaraman ldquoLigninase of Phanerochaete chrysosporiumMechanism of its degradation of the non-phenolic arylglycerol120573-aryl ether substructure of ligninrdquo Biochemical Journal vol236 no 1 pp 279ndash287 1986
[28] T Lundell RWever R Floris et al ldquoLignin peroxidase L3 fromPhlebia radiata pre-steady-state and steady-state studies withveratryl alcohol and a non-phenolic lignin model compound1-(34-dimethoxyphenyl)-2-(2-methoxyphenoxy)propane-13-diolrdquo European Journal of Biochemistry vol 211 no 3 pp391ndash402 1993
[29] H E Schoemaker T K Lundell A I Hatakka and K PiontekldquoThe oxidation of veratryl alcohol dimeric lignin models andlignin by lignin peroxidase the redox cycle revisitedrdquo FEMSMicrobiology Reviews vol 13 no 2-3 pp 321ndash331 1994
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[32] Y-H Xu H-R Chen Z-X Zeng and B Lei ldquoInvestigation onmechanism of photocatalytic activity enhancement of nanome-ter cerium-doped titaniardquo Applied Surface Science vol 252 no24 pp 8565ndash8570 2006
[33] L Tonucci F Coccia M Bressan and N DrsquoAlessandro ldquoMildphotocatalysed and catalysed green oxidation of lignin auseful pathway to low-molecular-weight derivativesrdquoWaste andBiomass Valorization vol 3 no 2 pp 165ndash174 2012
[34] A Castellan N Colombo C Vanucci P Fornier de HViolet and H Bouas-Laurent ldquoPhotodegradation of ligninA photochemical study of an O-methylated 120572-carbonyl 120573-1lignin model dimer 12-di(3101584041015840-dimethoxyphenyl) ethanone(deoxyveratroin)rdquo Journal of Photochemistry and PhotobiologyA vol 51 pp 451ndash467 1990
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and related surface phenomenardquo Surface Science Reports vol63 no 12 pp 515ndash582 2008
[36] C S Turchi and D F Ollis ldquoPhotocatalytic reactor designan example of mass-transfer limitations with an immobilizedcatalystrdquo The Journal of Physical Chemistry vol 92 no 23 pp6852ndash6853 1988
[37] R Subasri M Tripathi K Murugan J Revathi G V N Raoand T N Rao ldquoInvestigations on the photocatalytic activity ofsolndashgel derived plain and Fe3+Nb5+ doped titania coatings onglass substratesrdquo Materials Chemistry and Physics vol 124 pp63ndash68 2010
[38] N Serpone ldquoRelative photonic efficiencies and quantum yieldsin heterogeneous photocatalysisrdquo Journal of Photochemistry andPhotobiology A Chemistry vol 104 no 1ndash3 pp 1ndash12 1997
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[40] C D Jaeger and A J Bard ldquoSpin trapping and electron spinresonance detection of radical intermediates in the photode-composition of water at titanium dioxide particulate systemsrdquoThe Journal of Physical Chemistry vol 83 no 24 pp 3146ndash31521979
[41] R W Matthews ldquoHydroxylation reactions induced by near-ultraviolet photolysis of aqueous titaniumdioxide suspensionsrdquoJournal of the Chemical Society Faraday Transactions vol 80no 2 pp 457ndash471 1984
[42] A E H Machado A M Furuyama S Z Falone R RuggieroD D S Perez and A Castellan ldquoPhotocatalytic degradation oflignin and lignin models using titanium dioxide the role of thehydroxyl radicalrdquoChemosphere vol 40 no 1 pp 115ndash124 2000
[43] W Sigg and L Stumm Aquatische Chemie Stuttgart 2 AuflageBG Teubner Stuttgart Germany 1991
[44] O Legrini E Oliveros and A M Braun ldquoPhotochemicalprocesses for water treatmentrdquo Chemical Reviews vol 93 no2 pp 671ndash698 1993
[45] G Rothenberger J Moser M Gratzel N Serpone and DSharma ldquoCharge carrier trapping and recombination dynamicsin small semiconductor particlesrdquo Journal of the AmericanChemical Society vol 107 no 26 pp 8054ndash8059 1985
[46] P Mazellier M Sarakha A Rossi and M Bolte ldquoThe aqueousphotochemistry of 26-dimethylphenol Evidence for the frag-mentation of the 120572 C-C bondrdquo Journal of Photochemistry andPhotobiology A vol 115 no 2 pp 117ndash121 1998
[47] M Ksibi S B Amor S Cherif E Elaloui A Houas and MElaloui ldquoPhotodegradation of lignin from black liquor using aUVTiO
2systemrdquo Journal of Photochemistry and Photobiology
A vol 154 no 2-3 pp 211ndash218 2003
International Journal of Photoenergy 17
[48] S K Kansal M Singh and D Sud ldquoStudies on TiO2ZnO
photocatalysed degradation of ligninrdquo Journal of HazardousMaterials vol 153 no 1-2 pp 412ndash417 2008
[49] A Dahm and L A Lucia ldquoTitanium dioxide catalyzed pho-todegradation of lignin in industrial effluentsrdquo Industrial andEngineering Chemistry Research vol 43 no 25 pp 7996ndash80002004
[50] E Portjanskaja and S Preis ldquoAqueous photocatalytic oxidationof lignin the influence of mineral admixturesrdquo InternationalJournal of Photoenergy vol 2007Article ID76730 7 pages 2007
[51] K Tanaka R C R Calanag and T Hisanaga ldquoPhotocatalyzeddegradation of lignin on TiO
2rdquo Journal of Molecular Catalysis
A Chemical vol 138 no 2-3 pp 287ndash294 1999[52] M Tian J Wen D MacDonald R M Asmussen and A Chen
ldquoA novel approach for lignin modification and degradationrdquoElectrochemistry Communications vol 12 no 4 pp 527ndash5302010
[53] K PanM Tian Z-H Jiang B Kjartanson and A Chen ldquoElec-trochemical oxidation of lignin at lead dioxide nanoparticlesphotoelectrodeposited on TiO
2nanotube arraysrdquo Electrochim-
ica Acta vol 60 pp 147ndash153 2012[54] C Awungacha Lekelefac J Hild P Czermak and M Herren-
bauer ldquoPhotocatalytic active coatings for lignin degradationin a continuous packed bed reactorrdquo International Journal ofPhotoenergy vol 2014 Article ID 502326 10 pages 2014
[55] C Awungacha Lekelefac P Czermak and M HerrenbauerldquoEvaluation of photocatalytic active coatings on sintered glasstubes by methylene bluerdquo International Journal of Photoenergyvol 2013 Article ID 614567 9 pages 2013
[56] Y-S Ma C-N Chang Y-P Chiang H-F Sung and A CChao ldquoPhotocatalytic degradation of lignin using PtTiO
2as
the catalystrdquo Chemosphere vol 71 no 5 pp 998ndash1004 2008[57] H Ohnishi M Matsumura H Tsubomura and M Iwasaki
ldquoBleaching of lignin solution by a photocatalyzed reactionon semiconductor photocatalystsrdquo Industrial and EngineeringChemistry Research vol 28 no 6 pp 719ndash724 1989
[58] C A K Gouvea F Wypych S G Moraes N Duran and PPeralta-Zamora ldquoSemiconductor-assisted photodegradation oflignin dye and kraft effluent by Ag-doped ZnOrdquo Chemospherevol 40 no 4 pp 427ndash432 2000
[59] A V Vahatalo K Salonen M Salkinoja-Salonen and AHatakka ldquoPhotochemical mineralization of synthetic lignin inlake water indicates enhanced turnover of aromatic organicmatter under solar radiationrdquo Biodegradation vol 10 no 6 pp415ndash420 1999
[60] H de Lasa B Serrano and M Salaices Photocatalytic ReactionEngineering Springer New York NY USA 2005
[61] K Hashimoto H Irie and A Fujishima ldquoTiO 2 photocatalysisA historical overview and future prospectsrdquo Japanese Journal ofApplied Physics vol 44 no 12 pp 8269ndash8285 2005
[62] M A Behnajady N Modirshahla and R Hamzavi ldquoKineticstudy on photocatalytic degradation of CI Acid Yellow 23 byZnOphotocatalystrdquo Journal ofHazardousMaterials vol 133 no1ndash3 pp 226ndash232 2006
[63] M Addamo V Augugliaro A di Paola et al ldquoPhotocatalyticthin films of TiO
2formed by a sol-gel process using titanium
tetraisopropoxide as the precursorrdquo Thin Solid Films vol 516no 12 pp 3802ndash3807 2008
[64] N Negishi K Takeuchi and T Ibusuki ldquoPreparation of theTiO2thin filmphotocatalyst by the dip-coating processrdquo Journal
of Sol-Gel Science and Technology vol 13 no 1ndash3 pp 691ndash6941998
[65] L Rideh A Wehrer D Ronze and A Zoulalian ldquoPhotocat-alytic degradation of 2-chlorophenol in TiO
2aqueous suspen-
sion modeling of reaction raterdquo Industrial and EngineeringChemistry Research vol 36 no 11 pp 4712ndash4718 1997
[66] R-ADoongC-HChen RAMaithreepala and S-MChangldquoThe influence of pHand cadmiumsulfide on the photocatalyticdegradation of 2-chlorophenol in titanium dioxide suspen-sionsrdquoWater Research vol 35 no 12 pp 2873ndash2880 2001
[67] K V Sarkanen A Islam and C D Anderson ldquoOzonationrdquo inMethods in Lignin Chemistry S Y Lin and C W Dence EdsSpringer Series inWood Science pp 387ndash406 Springer BerlinGermany 1992
[68] G Gellerstedt and E-L Lindfors ldquoStructural changes in ligninduring kraft pulpingrdquo Holzforschung vol 38 no 3 pp 151ndash1581984
[69] B Neppolian H C Choi M V Shankar B Arabindoo andV Murugesan in Proceedings of the International Symposiumon Environmental Pollution Control and Waste Management(EPCOWM 02) p 647 2002
[70] C Pouteau P Dole B Cathala L Averous and N BoquillonldquoAntioxidant properties of lignin in polypropylenerdquo PolymerDegradation and Stability vol 81 no 1 pp 9ndash18 2003
[71] J Hafren T Fujino and T Itoh ldquoChanges in cell wall archi-tecture of differentiating tracheids of Pinus thunbergii duringlignificationrdquo Plant and Cell Physiology vol 40 no 5 pp 532ndash541 1999
[72] T Dizhbite G Telysheva V Jurkjane and U Viesturs ldquoChar-acterization of the radical scavenging activity of lignins naturalantioxidantsrdquo Bioresource Technology vol 95 no 3 pp 309ndash3172004
[73] K Hofstadler R Bauer S Novalic and S G Heisier ldquoNew reac-tor design for photocatalytic wastewater treatment with TiO
2
immobilized on fused-silica glass fibers photomineralization of4-chlorophenolrdquo Environmental Science amp Technology vol 28no 4 pp 670ndash674 1994
[74] J Villasenor and H D Mansilla ldquoEffect of temperature onkraft black liquor degradation by ZnO-photoassisted catalysisrdquoJournal of Photochemistry and Photobiology A Chemistry vol93 no 2-3 pp 205ndash209 1996
[75] B H Bielski D E Cabelli L A Ravindra and A B RossldquoReactivity of HO
2Ominus2Radicals in aqueous solutionrdquo Journal
of Physical Chemistry vol 14 pp 1041ndash1100 1985[76] B H J Bielski and A O Allen ldquoMechanism of the dispropor-
tionation of superoxide radicalsrdquo Journal of Physical Chemistryvol 81 no 11 pp 1048ndash1050 1977
[77] B Halliwell and J M C Gutteridge ldquoThe importance of freeradicals and catalytic metal ions in human diseasesrdquoMolecularAspects of Medicine vol 8 no 2 pp 89ndash193 1985
[78] J M Palmer P J Harvey and H E Schoemaker ldquoThe role ofperoxidases radical cations and oxygen in the degradation oflignin [and discussion]rdquo Philosophical Transaction of the RoyalSociety A vol 321 no 1561 pp 495ndash505 1987
[79] S Y Yin and C W Dense Methods in Lignin ChemistrySpringer New York NY USA 1992
[80] P Kumar S Kumar and N K Bhardwaj ldquoPhotocatalyticoxidation of elemental chlorine free bleaching effluent withUVTiO
2rdquo in Proceedings of the 2nd International Conference on
Environmental Science and Technology (ICEST rsquo11) SingaporeFebruary 2011
[81] K Kobayakawa Y Sato S Nakamura and A FujishimaldquoPhotodecomposition of Kraft lignin catalyzed by titanium
18 International Journal of Photoenergy
dioxiderdquo Bulletin of the Chemical Society of Japan vol 62 no11 pp 3433ndash3436 1989
[82] R B Kinstre ldquoAn overview of strategies for reducing theenvironmental impact of bleach-plant effluentsrdquo Tappi Journalvol 76 no 5 pp 105ndash113 1993
[83] A Castellan H Choudhury R Stephen Davidson and SGrelier ldquoComparative study of stone-ground wood pulp andnative wood 3 Application of fluorescence spectroscopy to astudy of the weathering of stone-ground pulp and native woodrdquoJournal of Photochemistry and Photobiology A Chemistry vol81 no 2 pp 123ndash130 1994
[84] B Albinsson S Li K Lundquist and R Stomberg ldquoThe originof lignin fluorescencerdquo Journal of Molecular Structure vol 508no 1ndash3 pp 19ndash27 1999
[85] S Baumberger A Abaecherli M Fasching et al ldquoMolar massdetermination of lignins by size-exclusion chromatographytowards standardisation of the methodrdquo Holzforschung vol 61no 4 pp 459ndash468 2007
[86] E Torres andM Ayala Biocatalysis Based on Heme PeroxidasesSpringer New York NY USA 1st edition 2010
[87] R Pecina P Burtscher G Bonn and O Bobleter ldquoGC-MSand HPLC analyses of lignin degradation products in biomasshydrolyzatesrdquo Fresenius Zeitschrift fur Analytische Chemie vol325 no 5 pp 461ndash465 1986
[88] NIST NIST Standard Reference Database The National Insti-tute of Standards and Technology 2014 httpwwwnistgovsrdnist1acfm
[89] T K Kirk and R L Farrell ldquoEnzymatic ldquocombustionrdquo themicrobial degradation of ligninrdquo Annual Review of Microbiol-ogy vol 41 pp 465ndash501 1987
[90] A TMartinez ldquoHigh redox potential peroxidasesrdquo in IndustrialEnzymes Structure Function and Applications K-B BeckerEd pp 477ndash488 Springer Amsterdam The Netherlands 1stedition 2007
[91] M Dashtban H Schraft T A Syed and W Qin ldquoFungalbiodegradation and enzymatic modification of ligninrdquo Interna-tional Journal of Biochemistry and Molecular Biology vol 1 no1 pp 36ndash50 2010
[92] N Duran E Esposito L H Innocentini-Mei and V P CanhosldquoA new alternative process for Kraft E1 effluent treatmentrdquoBiodegradation vol 5 no 1 pp 13ndash19 1994
[93] K Kamwilaisak and P C Wright ldquoInvestigating laccase andtitanium dioxide for lignin degradationrdquo Energy and Fuels vol26 no 4 pp 2400ndash2406 2012
[94] J Reyes M Dezotti H Mansilla J Villasenor E Espositoand N Duran ldquoBiomass photochemistry-XXII combined pho-tochemical and biological process for treatment of Kraft E1effluentrdquoApplied Catalysis B Environmental vol 15 no 3-4 pp211ndash219 1998
[95] L F Gonzalez V Sarria and O F Sanchez ldquoDegradationof chlorophenols by sequential biological-advanced oxidativeprocess using Trametespubescens and TiO
2UVrdquo Bioresource
Technology vol 101 no 10 pp 3493ndash3499 2010[96] J H Lora and W G Glasser ldquoRecent industrial applications
of lignin a sustainable alternative to nonrenewable materialsrdquoJournal of Polymers and the Environment vol 10 no 1-2 pp 39ndash48 2002
Submit your manuscripts athttpwwwhindawicom
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Inorganic ChemistryInternational Journal of
Hindawi Publishing Corporation httpwwwhindawicom Volume 2014
International Journal ofPhotoenergy
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Carbohydrate Chemistry
International Journal of
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Journal of
Chemistry
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Bioinorganic Chemistry and ApplicationsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
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CatalystsJournal of
8 International Journal of Photoenergy
Table3Parametersanalyticalmetho
dsand
results
from
different
workgrou
ps
Reference
Parameter
studied
Analytic
sRe
sult
MacHado
etal[42]
Roleof
hydroxylradicals
irradiatio
nof
ligninin
the
absencea
ndpresence
ofph
otocatalystT
iO2andH
2O2
Ultraviolet-v
isible(UV-Vis)spectro
scop
yionizatio
nabsorptio
nspectro
scop
y(IA
S)
sizee
xclusio
nchromatograph
y(SEC
)
Asharpdecrease
inthep
heno
liccontento
bservedforreactions
involvingdirect
photolysis
SEC
areductio
nof
almost50
inthea
verage
molecular
weighto
fligninequalto
14kD
after
90min
ofirr
adiatio
n
Ksibietal[47]
Irradiationof
ligninin
the
absencea
ndpresence
ofph
otocatalystT
iO2-P2
5
UV-Visspectroscop
y13C-
nucle
armagnetic
resonance(NMR)
solid
state
totalion
gasc
hrom
atograph
y(TIC)
indu
ctioncoup
lingplasma(
ICP)
chem
icaloxygen
demand(C
OD)
56degradationratewith
TiO
2catalystaft
er420m
inreactiontim
eethylacetate-extractableprod
uctsshow
edvanillin
vanillica
cidpalm
itica
cid
biph
enylstructuresand
345-trim
etho
xybenzaldehyde
presence
ofmagnesiu
mandcalcium
ions
CODremovalishigh
erforthe
initiallylowconcentrations
ofligninsolutio
n
Kansaletal[48]
Catalystdo
sepHoxidant
concentration
initialsubstrate
concentration
ZnOcatalystin
slurryandim
mob
ilizedmod
e
UV-Visspectroscop
yCO
D
Optim
umcatalystdo
seis1gL
optim
umoxidantcon
centratio
n2times10minus6M
graduald
ecreaseo
fabsorptionpeak
indicatin
gdecompo
sitionof
organics
CODremovalishigh
erforthe
initiallylowconcentrations
ofligninsolutio
n
Dahm
and
Lucia[
49]
Catalystdo
seillu
mination
intensity
UV-Visspectroscop
ytotalorganiccarbon
(TOC)
capillaryionelectro
phoresis
analysis(C
IA)
Gradu
aldecrease
inabsorptio
npeak
indicatin
gdecompo
sitionof
organics
optim
alcatalystdo
seof
10mgm
high
erillum
inationintensities
correlated
well
with
high
erinitialdegradationrate
74disapp
earanceo
fTOC
Portjanskajaand
Preis[50]
Influ
ence
offerrou
sion
sN-dop
edcatalysteffect
sprayedcatalyston
supp
ortand
subm
ersedcatalyst
CODU
V-Visspectroscop
ybiochemical
oxygen
demand(BOD)colorim
etric
measurementat570
nm
Additio
nof
Fe2+upto
28m
gLleadsto25increase
inph
otocatalyticeffi
ciency
sprayedcatalystexhibited15
times
high
ereffi
ciency
than
theo
neattached
bysubm
ersio
nnegligibleeffecto
fN-dop
edcatalyst
increase
ofaldehyde
concentrationover
reactio
ntim
eneutralm
ediawas
mostb
eneficialforb
iodegradability
80of
freep
heno
lsremoved
undern
eutralcond
ition
s
Tanaka
etal[51]Ca
talystloadinglignin
concentration
illum
inationtim
e
UV-Visspectroscop
yTO
Cgel
perm
eatio
nchromatograph
y(G
PC)
1 HNMR
Fouriertransform
ationinfrared
(FTIR)
spectro
scop
y
FTIR
measurementrevealedafasttransform
ationof
arom
aticmoietyp
resent
inlignin
characteris
ticband
sofaromaticrin
gsm
etho
xyand
aliphatic
sidec
hains
decrease
inTO
Cvalues
over
time
decrease
indegradationratewith
increase
catalystdo
sageB
utaft
ercatalystthreshold
valueisa
ttainedcatalystincreasec
ausesa
decrease
indegradationrates
FTIR
peaksa
reshifted
towards
lower
molecular
weightregionaft
erph
otocatalysis
Tonu
cci
etal[33]
Testof
catalytic
syste
mstoob
tain
fractio
nswith
redu
ceddegreeso
fpo
lymerization
comparis
onof
thermalandph
otochemical
reactio
ns
1 HNMR
gasc
hrom
atograph
y-mass
spectro
scop
y(G
C-MS)
POMsa
relessselectivew
henused
asph
otocatalystsandno
appreciableb
leaching
ofthes
olutionwas
seen
whenPO
Mwas
used
asthermalcatalyst
deriv
edchem
icalsfrom
experim
entvanillin
hydroxylmetho
xy-acetoph
enon
econiferylalcoh
olcon
iferylaldehydemethano
lform
icacidacetic
acidand
sometim
essm
allamou
ntso
fC-2
andC-
3alcoho
ls
Miyatae
tal[24]
Exam
inationof
cellwall
structureo
flignin(w
oodflo
ur)
before
andaft
erph
otocatalysis
GC-
MSscanning
electronmicroscop
y(SEM
)1 H
NMR
Highdelignificationactiv
itydelignificationconfi
nedto
thes
urface
oflignin
deriv
edchem
icalse
xperim
entvanillin
Shende
etal[17]Testof
combinedactio
nof
bio-
andph
otocatalyticsyste
ms
UV-Visspectroscop
yGC-
MSX-
ray
dispersiv
eenergyspectro
scop
yandX-
ray
diffractio
nanalysis(EDX
XRD)SE
M
Detectio
nof
follo
wingchem
icalsacetylguaiacol4-ethoxym
ethyl-2
-metho
xyph
enol
metho
xyph
enyloxim
eguaiacolsuccinica
cidacetylguaiacolvanillicacidand
vanillin
International Journal of Photoenergy 9
Table3Con
tinued
Reference
Parameter
studied
Analytic
sRe
sult
Tian
etal[52]
andPanetal
[53]
Testof
combinedactio
nof
electro-and
photocatalytic
syste
ms
UV-Visspectroscop
yFT
IRSEM
X-ray
(EDX)
highperfo
rmance
liquid
chromatograph
y(H
PLC)
COD
Detectio
nof
thefollowingchem
icalscarbon
ylfunctio
nalityvanillin
andvanillica
cid
Awun
gacha
Lekeleface
tal
[54][55]
Com
paris
onof
degradationrates
bydifferent
catalyst
HPL
Cflu
orescencea
ndUV-Vis
spectro
scop
ySE
MT
OC
UV-Visresultsrevealfaste
rdegradatio
nof
thea
liphatic
moietycomparedto
the
arom
aticmoietyof
ligninsulfonateob
tained
from
paperw
astewaterPeaks
observed
durin
gHPL
Canalysis
Someo
fthe
peaksp
rodu
cedaft
erph
otocatalysishad
fluorescences
ignals
Thissuggeststhep
rodu
ctionof
newsubstances
andflu
orop
hores
Coatin
gsprod
uced
throug
hSol-G
elprocedures
arestablea
ndcanbe
used
manytim
es
10 International Journal of Photoenergy
321 Influence of Catalyst One aspect to successfully imple-ment photocatalysis is the choice of an appropriate photo-catalyst The majority of catalysts used are based on TiO
2as
summarized in Table 2 ZnO has also been applied either assingle catalyst [48] or in a combination with other catalystsuch as TiO
2[54] de Lasa et al [60] described ZnO as
being less active than TiO2 They add that the use of ZnO
is particularly relevant when the oxidative degradation ratebecomes limited This is opposite to what Kansal et al [48]reported by saying that ZnO is more reactive than TiO
2
However TiO2(mainly anatase) remains the most used
catalyst as can be depicted fromTable 2 TiO2is reported to be
favored because of its nontoxic property cost efficiency andchemical and biological inertness Moreover TiO
2possesses
the most efficient photoactivity and the highest stability thusmaking it suitable for industrial use [61]
ZnO has been described to degrade lignin under visiblelight sources [48 62] whereas TiO
2is mostly applied in
connection to UV-light sources as highlighted in Table 2However both ZnO and TiO
2possess energy band gap
energy of 32 eV [23]Additives such as SiO
2[63] polyethylene oxide (PEO)
[24] and polyethylene glycol [64] have been added to TiO2
catalyst particularly when applying immobilized catalystAddamo et al [63] noted a high adhesion of TiO
2to glass
support material when precoating was done with SiO2
Moreover the precoating might have other advantages suchas a hindering diffusion of Na+ ions from the glass materialinto the nascent TiO
2film during heat treatment processes
Analogous to the addition of SiO2 polyethylene glycol
(PEG) has also been introduced to mitigate catalyst surfaceactivity modify surface hydrophobicity and also reduceagglomeration tendency of the TiO
2gel or TiO
2particles
in the suspensions [64] By a proper surface modificationinteraction between catalyst and substrate can be enhanced[55 63]
Kansal et al [48] varied catalyst (ZnO) dose from 05 gLto 20 gL for 01 gL kraft lignin solutions and found out thatthere was an optimum catalyst threshold value at 1 gL whichgives a catalyst to substrate ratio of 1 10
Dahm and Lucia [49] examined catalyst dose from 2 sdot10minus3 gL to 12sdot10minus2 gL for lignin solutions (fromwhite water
liner mill) of 4 sdot 10minus2 gL (catalyst to lignin ratio 5 sdot 10minus3ndash3 sdot 10minus2) at pH 8 and obtained best energy efficiency valuesand lignin degradation rates with a catalyst loading of 10 sdot10minus2 gLIn contrast Ma et al [56] applied far higher catalyst
concentration compared to Dahm and Lucia [49] Catalystconcentration was varied between 1 gL and 10 gL PtTiO
2
Best catalysis dose with respect to reaction turnover wasobtained at 5 gL PtTiO
2With the increase of catalyst dose at
pH 7 the reaction rate increased from 61 times 10minus3minminus1 (1 gLTiO2) to 71 sdot 10minus3minminus1 (5 gL TiO
2) and 99 sdot 10minus3minminus1
(10 gL TiO2)
Catalyst effect has been explained on the basis thatoptimum catalyst loading is dependent on the initial soluteconcentration An increase in catalyst dosage leads to a cor-responding increase of total active surface area for reactions
[65] To that at higher TiO2concentrations the photon flux is
more easily intercepted by the catalyst before penetrating intothe bulk of the system At the same time due to an increase inturbidity of the suspension with high dose of photocatalystthere is a decrease in penetration of UV light and hencephotoactivated volume of suspension or solution decreases[66]
In summary authors have obtained best catalyst to ligninrelations for different reaction designs and thus a generalrecommendation on catalyst dose is not possible Howeverwhen lignin solution is treated with increasing catalyst loadsa corresponding increase in degradation rate is observed untila threshold value is reached [48ndash50 56]
322 Influence of Metal Ion Addition (Doping) and AdditivesThe purpose of adding metal ion to photocatalyst is to miti-gate band gap energy through the introduction of intrabandgap states and as a consequence produce a bathochromicshift in the absorption spectrum [37] Altering the absorptionspectral range gives the possibility to exploit both the visiblelight spectrum and UV light sources Metal ion doping isalso introduced to serve as electron or hole traps in orderto minimize recombination between generated electron-holepairs [37]
Portjanskaja and Preis [50] studied the addition of Fe2+ions to an acidic lignin solution and found an increasein photocatalytic oxidation (PCO) efficiency The optimumFe2+ ions quantity was 28mgL while using 100mgL ligninsolution Upon further elevation of Fe2+ ions concentrationa corresponding reduction of the photocatalytic oxidationefficiency of lignin was noted Likewise Ohnishi et al [57]made a comparative study by doping platinum (Pt) silver(Ag) and gold (Au) ions to TiO
2 In these reactions 50mg of
catalyst (TiO2) was used with the addition of an equiva1ent
15 wt (based on TiO2) metal ion The addition of noble
metals brought about a faster decolorization of lignin Aushowed better results than Ag followed by Pt In the samecontext adding sodium hypochlorite as oxidant to PtTiO
2
catalyst an additional fivefold degradation rate was observedcompared to that without doping [56] Contradictory to theresults described above negligible effect of photocatalyticefficiency due to doping has been reported as well Awun-gacha Lekelefac et al [54] obtained little or no change indegradation rate by doping TiO
2-P25-SiO
2catalyst with Pt
ions (1 wt relative to TiO2catalyst) Likewise Portjanskaja
and Preis [50] noted a negligible change of photocatalyticefficiency of TiO
2when doped with nitrogen
Sarkanen et al [67] and Gellerstedt and Lindfors [68]reported the bias of peroxides to oxidation with reagentssuch as permanganate to favor aromatic moieties Oxidationagents like permanganate oxidizes predominantly aliphaticchains in alkaline and neutral media However by theapplication of H
2O2(Fenton system) lignin disappeared
completely [33] Tonucci et al [33] conclude that in orderto satisfactorily conserve the organic material the best com-promise appears to be the TiO
2photosystem which shows
low carbon consumption good preservation of the aromaticrings and greatly reduced mineralization
International Journal of Photoenergy 11
In summary different results have been obtained con-cerning the influence of noble metal ion addition Whilesome authors report an improvement in the photocatalyticefficiency upon their addition others report their addition ashaving no considerable influence However for reactions inwhich an improvement in the photocatalytic efficiency wasnoticed there was a threshold value to be considered Whenthe concentration of dopant surpasses this threshold valueelectron-hole recombination is favored and this has a negativeimpact to photocatalysis In such a case the space-chargelayer gets narrower and 119901-type dopants attract electrons andby virtue become negative They would now then act ashole acceptor attracting holes On the other hand 119899-typedopants which act as electron donor centers and possessexcess electrons attract holes as well [37]
323 Influence of Lignin Concentration Once the initiallignin concentration becomes higher exceeding a thresholdvalue an inhibitory effect on the photodegradationwas noted[47ndash49] This threshold value varies and depends on thereaction system and reaction parameters such as opticaldensity catalyst concentration and reaction volume Fromthe literature different authors have implemented varyinglignin concentrations probably to suit their reaction designFor example Ksibi et al [47] use 90mgL and AwungachaLekelefac et al [54] use 500mgL while Kansal et al [48]applied 10mgndash100mgL
Explanations arising from the findings are as followsat low lignin concentrations the incidental photonic fluxirradiated interacts with the catalyst generating radicals (eghydroxyl radicals (OH∙)) which allow a faster degradation[69] On the other hand high initial lignin concentrationsmay lead to tight adsorption which can suppress CO
2
evolution [51] and hence maintain chemical oxygen demand(COD) values Moreover low delignification yields maybe due to an inhibitory effect because of autoxidation bylow molecular weight lignin degradation products formed[24] Also due to the polymer structure of lignin which iscross-linked this makes it difficult for radical species acidand the aldehyde compounds produced to spread into theinner region of the substrate hence limiting autoxidationAs a worst case this might be the rate-determining step ofdelignification which is hindered [70ndash72]
In summary it can be concluded that the time taken forcomplete degradation depends on the initial concentration oflignin and faster degradation occurs at low lignin concentra-tions
324 Influence of pH Varying pH entails an alteration inthe properties of semiconductor-liquid interface [73] mainlyrelated to the acid-base equilibrium of the adsorbed hydroxylgroup [39] Furthermore pH also impacts lignin degradationrates [47 48 57] In this context several studies were carriedout with partly contradictory outcomes
Kansal et al [48] made pH investigations 3ndash11 undersolar light illumination using ZnO as catalyst Maximumdegradation was reached in alkaline conditions (pH 11)This is supported by Villasenor and Mansilla [74] reporting
an almost complete decolorization of kraft black liquorfrom pine wood at pH value of 116 in combination withZnO catalyst Similar results were achieved by Ohnishi etal [57] with TiO
2and ZnO being catalyst for bleaching
alkaline lignin in aqueous solution with TiO2and ZnO being
catalyst High activities at neutral pH were also reportedby Ohnishi et al [57] In contrast Ma et al [56] observedhigher reaction rates and rapid degradation of a syntheticlignin wastewater (prepared by dissolving commercial ligninpowder in aqueous solution pH 11) in acidic solution (pH 3)than in alkaline solutions at pH 11 for either TiO
2or PtTiO
2
catalystsReconsidering the photocatalytic principle the formed
superoxide anion radicals (∙O2
minus) are in a pH-dependentequilibriumwith perhydroxyl radicals (HO
2
∙) as follows [75]
HO2
∙999445999468 H+ + ∙O
2
minus pKa sim 48 (9)
See [76]∙O2
minus undergoes dismutation reaction resulting in H2O2
and O2competing to any other ∙O
2
minus triggered reaction Incase of low pH operation conditions in aqueous solutionsHO2
∙ becomes dominant whose reactivity is considerablyhigher compared to ∙O
2
minus [77] Subsequently HO2
∙ initiatessubstrate (S) oxidation to the radical cation (S+∙) and is itselfreduced to H
2O2[78] Thus increased degradation rates can
be reasonably expected supporting the results made by Ma etal [56] in an acidic environment ∙O
2
minus is extremely reactive inorganic solvents [77] Another aspect is the solubility of kraftlignin (soluble at pH gt 105) which reduces with decreasingpH whereas lignosulfonate should remain unaffected by pHin aqueous solution Moreover 120573ndashOndash4 bonds have beendescribed to be stable at acidic pH [11] In fact this couldadditionally explain the elevated degradation of kraft ligninmade by Kansal et al [48] and Villasenor and Mansilla [74]Nevertheless the contradictory results gained by Ma et al[56] still exist under the assumption that kraft ligninwas used(which would be supported by the high pH of 11 obviouslynecessary for dissolving the lignin powder) Although mostphotocatalytic reactions described in the literature are in anaqueous milieu lignin raw material and its fission productsmay however vary considerably Therefore the optimal pHis most likely to be reaction specific and has to be evaluatedexperimentally in principle
325 Influence of Illumination Many of the studies foundin the literature so far have not dealt on this subject per seWhat is found is the use of different illumination sourceseach having a specified power and lamp type However alltend to emitUV-light between the range 280ndash420 nm Table 2depicts this in detail Other illumination sources include thevisible light spectrum
In general terms illumination influences in that it ini-tiates photocatalysis by generating electron-hole pair in thesemiconductor particles [38 39] Dahm and Lucia [49]altered illumination intensity while observing lignin degra-dation In this study 004 gL lignin was used and lightintensity was varied 223ndash445mWcm3 It was found out thathigher illumination intensities correlated well with higher
12 International Journal of Photoenergy
initial degradation rates and hence total lignin degradation[49] Neppolian et al [69] report degradation to be propor-tional to radiation intensity and best results are achieved forlow lignin concentrations because of enhanced interactionbetween catalyst and incidental photonic flux
In summary high illumination power causes a corre-sponding high initial degradation rate at low lignin concen-trations becausemaximum light penetration into the reactionmedium is favored
33 Process Analytical Methods Various analytical tech-niques have been used to monitor lignin degradation Atthe beginning of this subchapter analytics revealing com-pounds formed from lignin degradation are treated Thisincludes for example gas chromatography (GC) and 1HNMR (nuclear magnetic resonance) This is then followed byresults qualitative analytic measurements such as ultraviolet-visible (UV-Vis) spectroscopy and dissolved carbon (DC)A list of authors analytical techniques applied and resultsachieved are outlined in Table 3
Portjanskaja and Preis [50] studied lignin degradationby measuring the removal of phenols through colorimetricmeasurements As a result of 24 h photocatalytic oxidationunder neutral media conditions 80 of free phenols wereremoved Gas chromatography (GC) result from Ksibi et al[47] attested vanillin vanillic acid palmitic acid biphenyland 345-trimethoxy benzaldehyde structures after the pho-tocatalysis of lignin from black liquor This is in accordancewith the findings of Tonucci et al [33] reporting the formationof vanillin hydroxyl methoxy-acetophenone coniferyl alco-hol coniferyl aldehyde methanol formic acid acetic acidand small amounts of C-2 and C-3 alcohols as degradationproducts1H NMR spectral analysis of lignin before illumination
and after 24 h of illumination showing characteristic bandsof aromatic rings methoxy and aliphatic side chains wascompared with each other Results revealed that the aro-matic ring degraded faster than the aliphatic chain [51]Fourier transformation infrared spectroscopy (FTIR) showedbands corresponding to CH
3 CH2 and CH which remained
unchanged after illumination while bands corresponding toaromatic rings disappeared as a result of illumination [51 53]
Results obtained from the combination of photochemicaland electrochemical oxidation [52 53] were similar to thoseof Tanaka et al [51] Here 13C-NMR confirmed the presenceof the carbonyl functionality and the presence of vanillinand vanillic acid after 12 h photochemical-electrochemicaloxidation These results showed that the combination of aphotocatalytic and an electrochemical oxidation significantlyenhanced the efficiency of lignin degradationThis is becausethe applied anodic potential bias greatly suppressed therecombination of photogenerated electrons and holes [53]
Ultraviolet spectrophotometry offers a convenientmethod to qualitatively and quantitatively analyze ligninin solution [79] This is reflected by the large number ofpublications using this technique [17 33 47ndash51 56 58 80]This is most likely due to its simplicity to interpret lignindegradation Lignins absorb UV light with high molar
200 220 240 260 280 300 320 340 360
00
05
10
15
20
25
Abso
rban
ce
Wavelength (nm)0min
30min
60min
120min
180min
300min
360min
420min
1200min
Figure 8 Time dependent UV-Vis absorption spectra of aqueouslignin solution from waste paper water irradiated with UV light(280ndash420 nm) for different time intervals The spectra are obtainedfor sol-gel derived TiO
2nanocrystalline coating (TiO
2-P25-SiO
2)
[54]
extinction coefficients because of the several methox-ylated phenylpropane units of which they are composed [33]Figure 8 depicts a series of photometric scans of ligninsul-fonate from paper waste water showing a gradual reductionof absorbance during photocatalytic treatment [54] Herethe absorption peaks are around 210 nm and 280 nmAbsorbance decreases with time implying the decompositionof lignin and the deterioration of chromophore groups [54]
Peaks at 210 nmcorrespond to portions of the unsaturatedchains while those at 280 nm correspond to unconjugatedphenolic hydroxyl groups [17] and the aromatic moiety [57]of the lignin molecule The absorption tailing to the longwavelength region arises from the color of lignin [57] Lignindegradation has been reported either at wavelength around280 nm corresponding to unconjugated phenolic hydroxylgroups [17 48 51 58] or for both wavelengths (210 nm and280 nm) [33 54 57] Kobayakawa et al [81] noted some otherabsorbance at wavelengths lower than 250 nm and pointedout that this could be due to the modification of ligninfragmentations leading to the formation of transient specieslikemethanol ethanol formaldehyde formic acid and oxalicacid among others
Analyticalmethods to effectively quantify lignin degrada-tion by calculating the oxygen demand by organic substancesand remaining organic carbon before and after photocatalysishave been studied Amongst the methods are dissolvedcarbon (DC) [49 51 58] chemical oxygen demand (COD)[47 48 50 57 80] biochemical oxygen demand (BOC) [50]dissolved organic carbon (DOC) [56] and American dyemanufacture institute value (ADMI) [56] COD and BODdescribe the oxygen demand by organic substances to be
International Journal of Photoenergy 13
Figure 9 Gradual change from the characteristic yellow ligninsulfonate to a colorless liquid after a period of 20 h Catalyst TiO
2-
P25 (Degussa) +TEOSUV-light 25∘C lignin concentration 05 gL[54]
converted to CO and CO2and H
2O and NH
3 Total organic
carbon (TOC) describes the amount of carbon bound inan organic compound while DOC describes the dissolvedfraction of organic carbon ADMI measures the amount ofdyestuff in water
Decolorization of lignin solution has been reported tobe another parameter observed during photocatalytic degra-dation Color is an indirect indicator of the lignin amountThe higher the color intensity of the solution is the greaterthe lignin content is (high concentrated lignin solutions egblack liquor appear dark brown) [82] Thus color changescan be interpreted as conversion of lignin to transient speciesor conversion to CO
2and H
2O Awungacha Lekelefac et al
[54] observed a gradual change from the characteristic yellowlignin (when highly diluted) to a colorless liquid after a periodof 20 h with sol-gel derived TiO
2nanocrystalline coatings on
sintered borosilicate glass as depicted in Figure 9 A corre-sponding decrease in DC values close to 82 was observedfor TiO
2-P25-SiO
2catalyst confirming degradation This is
shown in Figure 10These findings are analogous to that of Ohnishi et al
[57] who reported the bleaching of lignin when illuminatedcontinuously and that the solution becomes colorless Tothat the chemical oxygen demand (COD) value decreasesgenerating carbon dioxide and a small amount of carbonmonoxide as the main gaseous products COD removalwas reported to be effective at low lignin concentrations ascompared to high lignin concentrations [48]
Another applied analytical technique is fluorescencedetection directly coupled to a high performance liquidchromatography (HPLC) as a means to identify nonaliphaticcomponent in the complex mixture of lignin degradationproducts [54] Fluorescence emission in lignin is attributedto aromatic structures such as conjugated carbonyl biphenylphenylcoumarone and stilbene groups [83 84] AwungachaLekelefac et al [54] observed peaks on both HPLC and
0 200 400 600 800 1000 12000
50
100
150
200
250
Diss
olve
d ca
rbon
(ppm
)
t (min)
TiO2-P25-SiO2
TiO2-P25-SiO2 + PtTiOSO4minus30 6wt
ZnO + TiO2-P25-SiO2
No coating UV light
Figure 10 Variation of DC with time of aqueous lignin solutionfrom waste paper water irradiated with UV light (280ndash420 nm) fordifferent time intervals The spectra are obtained for sol-gel derivedTiO2nanocrystalline coatings (TiO
2-P25-SiO
2+Pt TiO
2-P25-SiO
2
TiOSO4minus306 wt ZnO + TiO
2-P25-SiO
2) [54]
fluorescence chromatograms suggesting the production ofnew substances and fluorophores
Despite developed analytical technologies analyzinglignin degradation products remains challenging Proofssuch as mass spectroscopy (MS) HPLC 13C or 1H-NMRspectra from photocatalytic lignin degradations are not yetestablished The setback to qualitatively and quantitativelyanalyze lignin and its degradation products starts from thenative lignin polymer itself with its indefinite polymericstructure and multiple bond types Also the influence ofdifferent pretreatments additives and the wide variety ofcompounds obtainable from its degradation makes ligninanalysis challenging [85] Moreover lignin streams couldcontain proteins inorganic salts and other potential poisonsthat generally complicate catalysis [4]
The challenge to identify and separate the productsstreams derived from the photocatalytic degradation ligninis also worth noting Lignin product stream is highly func-tionalized and conventional techniques such as gas chro-matography have the disadvantage of requiring a time-consuming derivatization step Also because of high boilingpoint of substances arising from lignin degradation it is noteasily applicable High performance liquid chromatography(HPLC) seems to be the remedy because analysis can becarried out without derivatization but the exact identificationof the separated substances is difficult because of the numer-ous peaks arising from such a chromatogram [87] Unfortu-nately well-established databases such as that of the nationalinstitute of standard and technology (NIST) [88] cannot giveinformation onHPLC-MS chromatogramsThis is because ofthe ionization sources such as electrospray ionization (ESI)
14 International Journal of Photoenergy
Table 4 Main advantages and disadvantages photocatalysis versus enzymatic biocatalysis
Lignin degradation process Advantages Disadvantages
Photocatalysis(single stage)
(i) Relatively fast degradation of complex as wellas nonbiodegradable organic (macro)molecules(ii) Stable catalysts(iii) Moderate reaction conditions
(i) Not selective(ii) Limited selection of operating conditions(iii) Energy costs
Enzymatic conversion(single stage)
(i) More selective(ii) Moderate reaction conditions
(i) High enzyme loads required otherwise ligninhydrolysis is quite slow [16](ii) Enzymes are less stable1(iii) cost-intensive (POXs)2
1For example POXs are sensitive to high H2O2 concentrations2According to Torres and Ayala [86]
and atmospheric pressure chemical ionization (APCI) forLC-MS while GC-MS is based on electron ionization Aremedy to this can be the development of a database ofmodel lignin compounds based on a unanimous HPLC-MSmeasuring procedure This would mean much time and costexpensive investments for adequate personnel and material
Though product identification based on peak superpo-sition gives information on a possible product this can-not always be true for lignin degradation because of thelarge number of possible degradation products In order toproperly identify products from lignin degradation productisolation techniques are to be implemented and further ana-lytic methods such as tandem mass spectroscopy (MSMS)and other advanced structure enhancing research techniquessuch as 1H-NMR 13C-NMR have to be studied extensively
4 Photocatalysis and Its Suitability asIntegrated Technology in MultistageConcepts with Biocatalysis
Depending on lignin treatment specification photocatalysiswill find application for complete lignin mineralization toCO2and H
2O (single stage) or as integrated technology in
a multistage system aiming at partial conversion of ligninmacromolecules to organic low molecular weight intermedi-ates for a consecutive biological oxidation finally generatingvalue added end-products [15]
For biological oxidation of lignin or lignin deriva-tives H
2O2-dependent ligninolytic heme peroxidases (POXs
including lignin peroxidase (LiP EC 111114) manganeseperoxidase (MnP EC 111113) versatile peroxidase (VPEC 111116) O
2-dependent laccases (Lac EC 11032) and
extracellular enzymes from Basidiomycetous white-rot fungiare the most efficient lignin degraders in nature [89] Thussuch enzymatic systems especially POXs (with high redoxpotentials) have attractedmuch interest as industrial biocata-lyst [86 90]The enzyme degradationmechanism (in nature)is facilitated by nonenzymatic processes mainly throughfree ∙OH radicals also generated by the fungus (Fenton-type reaction) Those ∙OH radicals enable the requiredphysical contact between the enzymes and structural units ofthe lignin molecule due to numerous nonspecific oxidative
reactions [91] Both the photocatalytic (equations (2)ndash(7))and the POX reactionmechanism recently reviewed by Busseat al [25] are quite similar Consequently a combination ofphotocatalysis followed by an enzymatic oxidation maybe apromising concept utilizing lignin derivatives for exampleoriginated from industrial effluents [15 92] The advantagesforming biobased products are as follows see also Table 4 inthis context A reduction of the lignin polymerization degreevia photocatalysis at best in more biodegradable interme-diates and a simultaneous detoxification causes savings inenzyme costs since it will be expected that less enzyme loadsare necessary for sufficiently rapid reaction rates [15 91] As aresult hydraulic retention times in bioreactor systems shouldbe diminished Moreover the delignification is expectedto be enhanced simultaneously which was in recent yearsshown in a dual system by Kamwilaisak and Wright [93]using TiO
2H2O2UV for photocatalytic pretreatment and
Lac (from Trametes versicolor) in the subsequent biocatalyticstep Within 24 h they obtained in their dual system anelevation in delignification of 20 (without H
2O2) up to at
least 50 when H2O2was present
In previous studies the treatment of lignin (from thepulp and paper industry) containing effluents by fungus asa biological system (excreting appropriate lignin degradingenzyme cocktails) were more focused for posttreatmentexclusively (Duran et al [92] Reyes et al [94] and Gonzalezet al [95]) Shende et al [17] even examined ligninolyticbacteria with photooxidized kraft lignin as substrate
Although POXs are potential industrial biocatalysts [86]no application studies were found for the direct use in suchdual systems as described above The major reason may bethe complexity of the reaction mechanism (inclusive ligninderivatives as substrate and their analysis) per se on the onehand slowing down research and development processes Onthe other hand POXs are sensitive to their cosubstrate H
2O2
once it is supplied in excess causing considerable inactivation(for details refer to Busse et al [25]) At the present severalstudies are carried out modifying these enzymes regardingenhanced stability activity and selectivity as well Henceit can be expected that their technological applicability willbe raised significantly right after successful modification isreached [86]
International Journal of Photoenergy 15
5 Concluding Remarks
It is widely assumed that the photocatalytic degradation oflignin follows a radical reaction pathway which is similar tothat considered in thermal electrochemical and biochemicalprocesses However reporting on the degradation pathway oflignin derivatives and even that of lignin model compoundsare still amajor challengeThis is probably due to the complexnature and variety of possible degradation products Indeedthemechanism is far more complex considering other factorssuch as type of lignin type of catalyst pH illuminationsource and additives
Generally comparing the different photochemical pro-cesses poses a big challenge because of the wide variablesinvolved These discrepancies start from the source and typeof lignin followed by differences in reactor design illumi-nation source intensity of radiation and different types ofcatalyst An ideawould be to have a specific reference reactionwith well-defined starting parameters which include lignintype source and purity catalyst specifications illuminationsource and intensity so as to ease comparison of results
Basic process parameters such as catalyst concentrationsubstrate concentration addition of metal ion to catalyst pHand illumination have been discussed
Despite developed analytical technologies analyzinglignin degradation products remains challenging Proofssuch as mass spectroscopy (MS) HPLC 13C or 1H-NMRspectra from photocatalytic lignin degradations are not yetestablished
In order to properly identify products from lignin degra-dation product isolation techniques are to be implementedand further analytic methods such as MSMS and otheradvanced structure enhancing research techniques such as1H-NMR and 13C-NMR have to be studied extensively Aremedy to this can be the development of a databank ofmodel lignin compounds based on a unanimous HPLC-MSmeasuring procedure This would mean much time and costexpensive investments for adequate personnel and material
Photocatalysis is denoted as the most popular ligninpretreatment technology besides ozonation [96] Photo-catalyzed lignin may be an appropriate substrate for aconsecutive biocatalytic process using ligninolytic enzymes(POX andor Lac) as supported by experimental results ofKamwilaisak and Wright [93] Combining the advantages ofboth catalytic processes savings in the overall process costswill be expected in addition to elevated lignin conversionNonetheless extensive research work including POX modi-fications is still required
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Authorsrsquo Contribution
Colin Awungacha Lekelefac and Nadine Busse contributedequally to this work
Acknowledgments
The authors gratefully thank the Federal Ministry of Educa-tion and Research (BMBF) for funding (FKZ17N0310) Theresearchers also thank the Hessen State Ministry of HigherEducation Research andArts for the financial support withinthe Hessen initiative for scientific and economic excellence(LOEWE)
References
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and Climate Policy Outlook (WETO) Luxembourg Office forOfficial Publications of the European Communities 2003
[3] Green and Natural Polymers Are on the Rise 2014 httpwwwpolymersolutionscombloggreen-and-natural-polymers-on-the-rise
[4] J Zakzeski P C Bruijnincx A L Jongerius and B M Weck-huysen ldquoThe catalytic valorization of lignin for the productionof renewable chemicalsrdquo Chemical Reviews vol 110 no 6 pp3552ndash3599 2010
[5] B KammM Kamm P R Gruber and S KromusBiorefineries-Industrial Processes and Products Status Quo and Future Direc-tions vol 1 Wiley-VCH 2006
[6] R L Howard E Abotsi E L J van Rensburg and S HowardldquoLignocellulose biotechnology issues of bioconversion andenzyme productionrdquoAfrican Journal of Biotechnology vol 2 no12 pp 602ndash619 2003
[7] M Stocker ldquoBio- und BTL-Kraftstoffe in der Bioraffineriekatalytische Umwandlung Lignocellulose-reicher Biomasse mitporosen Stoffenrdquo Angewandte Chemie vol 120 no 48 pp9340ndash9351 2008
[8] J-P Lange ldquoLignocellulose conversion an introduction tochemistry process and economicsrdquo Biofuels Bioproducts andBiorefining vol 1 no 1 pp 39ndash48 2007
[9] D A I Goring ldquoThe physical chemistry of ligninrdquo Pure andApplied Chemistry vol 5 no 1-2 pp 233ndash310 1962
[10] M Ek G Gellerstedt and G HenrikssonWood Chemistry andBiotechnology Pulp and Paper Chemistry and Technology vol 1Walter de Gruyter GmbH amp Co KG Berlin Germany 2009
[11] B Saake and R Lehnen ldquoLigninrdquo in Ullmannrsquos Encyclopedia ofIndustrial Chemistry Wiley-VCH Verlag GmbH amp Co KGaAWeinheim Germany 2012
[12] M N S Kumar A K Mohanty L Erickson and M MisraldquoLignin and its applications with polymersrdquo Journal of BiobasedMaterials and Bioenergy vol 3 no 1 pp 1ndash24 2009
[13] R J A Gosselink E de Jong B Guran and A AbacherlildquoCo-ordination network for ligninmdashstandardisation produc-tion and applications adapted to market requirements(EUROLIGNIN)rdquo Industrial Crops and Products vol 20 no 2pp 121ndash129 2004
[14] D Fengel and G Wegener Wood Chemistry UltrastructureReactions Verlag Kessel 1984
[15] D Mantzavinos and E Psillakis ldquoEnhancement of biodegrad-ability of industrial wastewaters by chemical oxidation pre-treatmentrdquo Journal of Chemical Technology and Biotechnologyvol 79 no 5 pp 431ndash454 2004
16 International Journal of Photoenergy
[16] M Dashtban H Schraft and W Qin ldquoFungal bioconversionof lignocellulosic residues Opportunities amp perspectivesrdquo Inter-national Journal of Biological Sciences vol 5 no 6 pp 578ndash5952009
[17] A Shende R Jaswal D Harder-Heinz A Menan and RShende ldquoIntergrated photocatalytic and microbial degradationof kraft ligninrdquo Cleantech pp 120ndash123 2012
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[20] M Erickson S Larsson and G E Miksche ldquoZur Struktur desLignins der FichterdquoActaChemica Scandinavica vol 27 pp 903ndash914 1973
[21] H Nimz ldquoDas Lignin der BuchemdashEntwurf eines Konstitution-sschemasrdquo Angewandte ChemiemdashInternational Edition vol 86pp 336ndash344 1974
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and H Nakatani ldquoSolventless delignification of wood flourwith TiO
2poly(ethylene oxide) photocatalyst systemrdquo Journal
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kinetics of versatile peroxidase for the degradation of lignincompoundsrdquoTheAmerican Journal of Biochemistry andBiotech-nology vol 9 no 4 pp 365ndash394 2013
[26] M Tien and T K Kirk ldquoLignin-degrading enzyme fromphanerochaete chrysosporium Purification characterizationand catalytic properties of a uniqueH
2O2-requiring oxygenaserdquo
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[27] T K Kirk M Tien P J Kersten M D Mozuch and BKalyanaraman ldquoLigninase of Phanerochaete chrysosporiumMechanism of its degradation of the non-phenolic arylglycerol120573-aryl ether substructure of ligninrdquo Biochemical Journal vol236 no 1 pp 279ndash287 1986
[28] T Lundell RWever R Floris et al ldquoLignin peroxidase L3 fromPhlebia radiata pre-steady-state and steady-state studies withveratryl alcohol and a non-phenolic lignin model compound1-(34-dimethoxyphenyl)-2-(2-methoxyphenoxy)propane-13-diolrdquo European Journal of Biochemistry vol 211 no 3 pp391ndash402 1993
[29] H E Schoemaker T K Lundell A I Hatakka and K PiontekldquoThe oxidation of veratryl alcohol dimeric lignin models andlignin by lignin peroxidase the redox cycle revisitedrdquo FEMSMicrobiology Reviews vol 13 no 2-3 pp 321ndash331 1994
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[33] L Tonucci F Coccia M Bressan and N DrsquoAlessandro ldquoMildphotocatalysed and catalysed green oxidation of lignin auseful pathway to low-molecular-weight derivativesrdquoWaste andBiomass Valorization vol 3 no 2 pp 165ndash174 2012
[34] A Castellan N Colombo C Vanucci P Fornier de HViolet and H Bouas-Laurent ldquoPhotodegradation of ligninA photochemical study of an O-methylated 120572-carbonyl 120573-1lignin model dimer 12-di(3101584041015840-dimethoxyphenyl) ethanone(deoxyveratroin)rdquo Journal of Photochemistry and PhotobiologyA vol 51 pp 451ndash467 1990
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and related surface phenomenardquo Surface Science Reports vol63 no 12 pp 515ndash582 2008
[36] C S Turchi and D F Ollis ldquoPhotocatalytic reactor designan example of mass-transfer limitations with an immobilizedcatalystrdquo The Journal of Physical Chemistry vol 92 no 23 pp6852ndash6853 1988
[37] R Subasri M Tripathi K Murugan J Revathi G V N Raoand T N Rao ldquoInvestigations on the photocatalytic activity ofsolndashgel derived plain and Fe3+Nb5+ doped titania coatings onglass substratesrdquo Materials Chemistry and Physics vol 124 pp63ndash68 2010
[38] N Serpone ldquoRelative photonic efficiencies and quantum yieldsin heterogeneous photocatalysisrdquo Journal of Photochemistry andPhotobiology A Chemistry vol 104 no 1ndash3 pp 1ndash12 1997
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[40] C D Jaeger and A J Bard ldquoSpin trapping and electron spinresonance detection of radical intermediates in the photode-composition of water at titanium dioxide particulate systemsrdquoThe Journal of Physical Chemistry vol 83 no 24 pp 3146ndash31521979
[41] R W Matthews ldquoHydroxylation reactions induced by near-ultraviolet photolysis of aqueous titaniumdioxide suspensionsrdquoJournal of the Chemical Society Faraday Transactions vol 80no 2 pp 457ndash471 1984
[42] A E H Machado A M Furuyama S Z Falone R RuggieroD D S Perez and A Castellan ldquoPhotocatalytic degradation oflignin and lignin models using titanium dioxide the role of thehydroxyl radicalrdquoChemosphere vol 40 no 1 pp 115ndash124 2000
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[44] O Legrini E Oliveros and A M Braun ldquoPhotochemicalprocesses for water treatmentrdquo Chemical Reviews vol 93 no2 pp 671ndash698 1993
[45] G Rothenberger J Moser M Gratzel N Serpone and DSharma ldquoCharge carrier trapping and recombination dynamicsin small semiconductor particlesrdquo Journal of the AmericanChemical Society vol 107 no 26 pp 8054ndash8059 1985
[46] P Mazellier M Sarakha A Rossi and M Bolte ldquoThe aqueousphotochemistry of 26-dimethylphenol Evidence for the frag-mentation of the 120572 C-C bondrdquo Journal of Photochemistry andPhotobiology A vol 115 no 2 pp 117ndash121 1998
[47] M Ksibi S B Amor S Cherif E Elaloui A Houas and MElaloui ldquoPhotodegradation of lignin from black liquor using aUVTiO
2systemrdquo Journal of Photochemistry and Photobiology
A vol 154 no 2-3 pp 211ndash218 2003
International Journal of Photoenergy 17
[48] S K Kansal M Singh and D Sud ldquoStudies on TiO2ZnO
photocatalysed degradation of ligninrdquo Journal of HazardousMaterials vol 153 no 1-2 pp 412ndash417 2008
[49] A Dahm and L A Lucia ldquoTitanium dioxide catalyzed pho-todegradation of lignin in industrial effluentsrdquo Industrial andEngineering Chemistry Research vol 43 no 25 pp 7996ndash80002004
[50] E Portjanskaja and S Preis ldquoAqueous photocatalytic oxidationof lignin the influence of mineral admixturesrdquo InternationalJournal of Photoenergy vol 2007Article ID76730 7 pages 2007
[51] K Tanaka R C R Calanag and T Hisanaga ldquoPhotocatalyzeddegradation of lignin on TiO
2rdquo Journal of Molecular Catalysis
A Chemical vol 138 no 2-3 pp 287ndash294 1999[52] M Tian J Wen D MacDonald R M Asmussen and A Chen
ldquoA novel approach for lignin modification and degradationrdquoElectrochemistry Communications vol 12 no 4 pp 527ndash5302010
[53] K PanM Tian Z-H Jiang B Kjartanson and A Chen ldquoElec-trochemical oxidation of lignin at lead dioxide nanoparticlesphotoelectrodeposited on TiO
2nanotube arraysrdquo Electrochim-
ica Acta vol 60 pp 147ndash153 2012[54] C Awungacha Lekelefac J Hild P Czermak and M Herren-
bauer ldquoPhotocatalytic active coatings for lignin degradationin a continuous packed bed reactorrdquo International Journal ofPhotoenergy vol 2014 Article ID 502326 10 pages 2014
[55] C Awungacha Lekelefac P Czermak and M HerrenbauerldquoEvaluation of photocatalytic active coatings on sintered glasstubes by methylene bluerdquo International Journal of Photoenergyvol 2013 Article ID 614567 9 pages 2013
[56] Y-S Ma C-N Chang Y-P Chiang H-F Sung and A CChao ldquoPhotocatalytic degradation of lignin using PtTiO
2as
the catalystrdquo Chemosphere vol 71 no 5 pp 998ndash1004 2008[57] H Ohnishi M Matsumura H Tsubomura and M Iwasaki
ldquoBleaching of lignin solution by a photocatalyzed reactionon semiconductor photocatalystsrdquo Industrial and EngineeringChemistry Research vol 28 no 6 pp 719ndash724 1989
[58] C A K Gouvea F Wypych S G Moraes N Duran and PPeralta-Zamora ldquoSemiconductor-assisted photodegradation oflignin dye and kraft effluent by Ag-doped ZnOrdquo Chemospherevol 40 no 4 pp 427ndash432 2000
[59] A V Vahatalo K Salonen M Salkinoja-Salonen and AHatakka ldquoPhotochemical mineralization of synthetic lignin inlake water indicates enhanced turnover of aromatic organicmatter under solar radiationrdquo Biodegradation vol 10 no 6 pp415ndash420 1999
[60] H de Lasa B Serrano and M Salaices Photocatalytic ReactionEngineering Springer New York NY USA 2005
[61] K Hashimoto H Irie and A Fujishima ldquoTiO 2 photocatalysisA historical overview and future prospectsrdquo Japanese Journal ofApplied Physics vol 44 no 12 pp 8269ndash8285 2005
[62] M A Behnajady N Modirshahla and R Hamzavi ldquoKineticstudy on photocatalytic degradation of CI Acid Yellow 23 byZnOphotocatalystrdquo Journal ofHazardousMaterials vol 133 no1ndash3 pp 226ndash232 2006
[63] M Addamo V Augugliaro A di Paola et al ldquoPhotocatalyticthin films of TiO
2formed by a sol-gel process using titanium
tetraisopropoxide as the precursorrdquo Thin Solid Films vol 516no 12 pp 3802ndash3807 2008
[64] N Negishi K Takeuchi and T Ibusuki ldquoPreparation of theTiO2thin filmphotocatalyst by the dip-coating processrdquo Journal
of Sol-Gel Science and Technology vol 13 no 1ndash3 pp 691ndash6941998
[65] L Rideh A Wehrer D Ronze and A Zoulalian ldquoPhotocat-alytic degradation of 2-chlorophenol in TiO
2aqueous suspen-
sion modeling of reaction raterdquo Industrial and EngineeringChemistry Research vol 36 no 11 pp 4712ndash4718 1997
[66] R-ADoongC-HChen RAMaithreepala and S-MChangldquoThe influence of pHand cadmiumsulfide on the photocatalyticdegradation of 2-chlorophenol in titanium dioxide suspen-sionsrdquoWater Research vol 35 no 12 pp 2873ndash2880 2001
[67] K V Sarkanen A Islam and C D Anderson ldquoOzonationrdquo inMethods in Lignin Chemistry S Y Lin and C W Dence EdsSpringer Series inWood Science pp 387ndash406 Springer BerlinGermany 1992
[68] G Gellerstedt and E-L Lindfors ldquoStructural changes in ligninduring kraft pulpingrdquo Holzforschung vol 38 no 3 pp 151ndash1581984
[69] B Neppolian H C Choi M V Shankar B Arabindoo andV Murugesan in Proceedings of the International Symposiumon Environmental Pollution Control and Waste Management(EPCOWM 02) p 647 2002
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[71] J Hafren T Fujino and T Itoh ldquoChanges in cell wall archi-tecture of differentiating tracheids of Pinus thunbergii duringlignificationrdquo Plant and Cell Physiology vol 40 no 5 pp 532ndash541 1999
[72] T Dizhbite G Telysheva V Jurkjane and U Viesturs ldquoChar-acterization of the radical scavenging activity of lignins naturalantioxidantsrdquo Bioresource Technology vol 95 no 3 pp 309ndash3172004
[73] K Hofstadler R Bauer S Novalic and S G Heisier ldquoNew reac-tor design for photocatalytic wastewater treatment with TiO
2
immobilized on fused-silica glass fibers photomineralization of4-chlorophenolrdquo Environmental Science amp Technology vol 28no 4 pp 670ndash674 1994
[74] J Villasenor and H D Mansilla ldquoEffect of temperature onkraft black liquor degradation by ZnO-photoassisted catalysisrdquoJournal of Photochemistry and Photobiology A Chemistry vol93 no 2-3 pp 205ndash209 1996
[75] B H Bielski D E Cabelli L A Ravindra and A B RossldquoReactivity of HO
2Ominus2Radicals in aqueous solutionrdquo Journal
of Physical Chemistry vol 14 pp 1041ndash1100 1985[76] B H J Bielski and A O Allen ldquoMechanism of the dispropor-
tionation of superoxide radicalsrdquo Journal of Physical Chemistryvol 81 no 11 pp 1048ndash1050 1977
[77] B Halliwell and J M C Gutteridge ldquoThe importance of freeradicals and catalytic metal ions in human diseasesrdquoMolecularAspects of Medicine vol 8 no 2 pp 89ndash193 1985
[78] J M Palmer P J Harvey and H E Schoemaker ldquoThe role ofperoxidases radical cations and oxygen in the degradation oflignin [and discussion]rdquo Philosophical Transaction of the RoyalSociety A vol 321 no 1561 pp 495ndash505 1987
[79] S Y Yin and C W Dense Methods in Lignin ChemistrySpringer New York NY USA 1992
[80] P Kumar S Kumar and N K Bhardwaj ldquoPhotocatalyticoxidation of elemental chlorine free bleaching effluent withUVTiO
2rdquo in Proceedings of the 2nd International Conference on
Environmental Science and Technology (ICEST rsquo11) SingaporeFebruary 2011
[81] K Kobayakawa Y Sato S Nakamura and A FujishimaldquoPhotodecomposition of Kraft lignin catalyzed by titanium
18 International Journal of Photoenergy
dioxiderdquo Bulletin of the Chemical Society of Japan vol 62 no11 pp 3433ndash3436 1989
[82] R B Kinstre ldquoAn overview of strategies for reducing theenvironmental impact of bleach-plant effluentsrdquo Tappi Journalvol 76 no 5 pp 105ndash113 1993
[83] A Castellan H Choudhury R Stephen Davidson and SGrelier ldquoComparative study of stone-ground wood pulp andnative wood 3 Application of fluorescence spectroscopy to astudy of the weathering of stone-ground pulp and native woodrdquoJournal of Photochemistry and Photobiology A Chemistry vol81 no 2 pp 123ndash130 1994
[84] B Albinsson S Li K Lundquist and R Stomberg ldquoThe originof lignin fluorescencerdquo Journal of Molecular Structure vol 508no 1ndash3 pp 19ndash27 1999
[85] S Baumberger A Abaecherli M Fasching et al ldquoMolar massdetermination of lignins by size-exclusion chromatographytowards standardisation of the methodrdquo Holzforschung vol 61no 4 pp 459ndash468 2007
[86] E Torres andM Ayala Biocatalysis Based on Heme PeroxidasesSpringer New York NY USA 1st edition 2010
[87] R Pecina P Burtscher G Bonn and O Bobleter ldquoGC-MSand HPLC analyses of lignin degradation products in biomasshydrolyzatesrdquo Fresenius Zeitschrift fur Analytische Chemie vol325 no 5 pp 461ndash465 1986
[88] NIST NIST Standard Reference Database The National Insti-tute of Standards and Technology 2014 httpwwwnistgovsrdnist1acfm
[89] T K Kirk and R L Farrell ldquoEnzymatic ldquocombustionrdquo themicrobial degradation of ligninrdquo Annual Review of Microbiol-ogy vol 41 pp 465ndash501 1987
[90] A TMartinez ldquoHigh redox potential peroxidasesrdquo in IndustrialEnzymes Structure Function and Applications K-B BeckerEd pp 477ndash488 Springer Amsterdam The Netherlands 1stedition 2007
[91] M Dashtban H Schraft T A Syed and W Qin ldquoFungalbiodegradation and enzymatic modification of ligninrdquo Interna-tional Journal of Biochemistry and Molecular Biology vol 1 no1 pp 36ndash50 2010
[92] N Duran E Esposito L H Innocentini-Mei and V P CanhosldquoA new alternative process for Kraft E1 effluent treatmentrdquoBiodegradation vol 5 no 1 pp 13ndash19 1994
[93] K Kamwilaisak and P C Wright ldquoInvestigating laccase andtitanium dioxide for lignin degradationrdquo Energy and Fuels vol26 no 4 pp 2400ndash2406 2012
[94] J Reyes M Dezotti H Mansilla J Villasenor E Espositoand N Duran ldquoBiomass photochemistry-XXII combined pho-tochemical and biological process for treatment of Kraft E1effluentrdquoApplied Catalysis B Environmental vol 15 no 3-4 pp211ndash219 1998
[95] L F Gonzalez V Sarria and O F Sanchez ldquoDegradationof chlorophenols by sequential biological-advanced oxidativeprocess using Trametespubescens and TiO
2UVrdquo Bioresource
Technology vol 101 no 10 pp 3493ndash3499 2010[96] J H Lora and W G Glasser ldquoRecent industrial applications
of lignin a sustainable alternative to nonrenewable materialsrdquoJournal of Polymers and the Environment vol 10 no 1-2 pp 39ndash48 2002
Submit your manuscripts athttpwwwhindawicom
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Inorganic ChemistryInternational Journal of
Hindawi Publishing Corporation httpwwwhindawicom Volume 2014
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CatalystsJournal of
International Journal of Photoenergy 9
Table3Con
tinued
Reference
Parameter
studied
Analytic
sRe
sult
Tian
etal[52]
andPanetal
[53]
Testof
combinedactio
nof
electro-and
photocatalytic
syste
ms
UV-Visspectroscop
yFT
IRSEM
X-ray
(EDX)
highperfo
rmance
liquid
chromatograph
y(H
PLC)
COD
Detectio
nof
thefollowingchem
icalscarbon
ylfunctio
nalityvanillin
andvanillica
cid
Awun
gacha
Lekeleface
tal
[54][55]
Com
paris
onof
degradationrates
bydifferent
catalyst
HPL
Cflu
orescencea
ndUV-Vis
spectro
scop
ySE
MT
OC
UV-Visresultsrevealfaste
rdegradatio
nof
thea
liphatic
moietycomparedto
the
arom
aticmoietyof
ligninsulfonateob
tained
from
paperw
astewaterPeaks
observed
durin
gHPL
Canalysis
Someo
fthe
peaksp
rodu
cedaft
erph
otocatalysishad
fluorescences
ignals
Thissuggeststhep
rodu
ctionof
newsubstances
andflu
orop
hores
Coatin
gsprod
uced
throug
hSol-G
elprocedures
arestablea
ndcanbe
used
manytim
es
10 International Journal of Photoenergy
321 Influence of Catalyst One aspect to successfully imple-ment photocatalysis is the choice of an appropriate photo-catalyst The majority of catalysts used are based on TiO
2as
summarized in Table 2 ZnO has also been applied either assingle catalyst [48] or in a combination with other catalystsuch as TiO
2[54] de Lasa et al [60] described ZnO as
being less active than TiO2 They add that the use of ZnO
is particularly relevant when the oxidative degradation ratebecomes limited This is opposite to what Kansal et al [48]reported by saying that ZnO is more reactive than TiO
2
However TiO2(mainly anatase) remains the most used
catalyst as can be depicted fromTable 2 TiO2is reported to be
favored because of its nontoxic property cost efficiency andchemical and biological inertness Moreover TiO
2possesses
the most efficient photoactivity and the highest stability thusmaking it suitable for industrial use [61]
ZnO has been described to degrade lignin under visiblelight sources [48 62] whereas TiO
2is mostly applied in
connection to UV-light sources as highlighted in Table 2However both ZnO and TiO
2possess energy band gap
energy of 32 eV [23]Additives such as SiO
2[63] polyethylene oxide (PEO)
[24] and polyethylene glycol [64] have been added to TiO2
catalyst particularly when applying immobilized catalystAddamo et al [63] noted a high adhesion of TiO
2to glass
support material when precoating was done with SiO2
Moreover the precoating might have other advantages suchas a hindering diffusion of Na+ ions from the glass materialinto the nascent TiO
2film during heat treatment processes
Analogous to the addition of SiO2 polyethylene glycol
(PEG) has also been introduced to mitigate catalyst surfaceactivity modify surface hydrophobicity and also reduceagglomeration tendency of the TiO
2gel or TiO
2particles
in the suspensions [64] By a proper surface modificationinteraction between catalyst and substrate can be enhanced[55 63]
Kansal et al [48] varied catalyst (ZnO) dose from 05 gLto 20 gL for 01 gL kraft lignin solutions and found out thatthere was an optimum catalyst threshold value at 1 gL whichgives a catalyst to substrate ratio of 1 10
Dahm and Lucia [49] examined catalyst dose from 2 sdot10minus3 gL to 12sdot10minus2 gL for lignin solutions (fromwhite water
liner mill) of 4 sdot 10minus2 gL (catalyst to lignin ratio 5 sdot 10minus3ndash3 sdot 10minus2) at pH 8 and obtained best energy efficiency valuesand lignin degradation rates with a catalyst loading of 10 sdot10minus2 gLIn contrast Ma et al [56] applied far higher catalyst
concentration compared to Dahm and Lucia [49] Catalystconcentration was varied between 1 gL and 10 gL PtTiO
2
Best catalysis dose with respect to reaction turnover wasobtained at 5 gL PtTiO
2With the increase of catalyst dose at
pH 7 the reaction rate increased from 61 times 10minus3minminus1 (1 gLTiO2) to 71 sdot 10minus3minminus1 (5 gL TiO
2) and 99 sdot 10minus3minminus1
(10 gL TiO2)
Catalyst effect has been explained on the basis thatoptimum catalyst loading is dependent on the initial soluteconcentration An increase in catalyst dosage leads to a cor-responding increase of total active surface area for reactions
[65] To that at higher TiO2concentrations the photon flux is
more easily intercepted by the catalyst before penetrating intothe bulk of the system At the same time due to an increase inturbidity of the suspension with high dose of photocatalystthere is a decrease in penetration of UV light and hencephotoactivated volume of suspension or solution decreases[66]
In summary authors have obtained best catalyst to ligninrelations for different reaction designs and thus a generalrecommendation on catalyst dose is not possible Howeverwhen lignin solution is treated with increasing catalyst loadsa corresponding increase in degradation rate is observed untila threshold value is reached [48ndash50 56]
322 Influence of Metal Ion Addition (Doping) and AdditivesThe purpose of adding metal ion to photocatalyst is to miti-gate band gap energy through the introduction of intrabandgap states and as a consequence produce a bathochromicshift in the absorption spectrum [37] Altering the absorptionspectral range gives the possibility to exploit both the visiblelight spectrum and UV light sources Metal ion doping isalso introduced to serve as electron or hole traps in orderto minimize recombination between generated electron-holepairs [37]
Portjanskaja and Preis [50] studied the addition of Fe2+ions to an acidic lignin solution and found an increasein photocatalytic oxidation (PCO) efficiency The optimumFe2+ ions quantity was 28mgL while using 100mgL ligninsolution Upon further elevation of Fe2+ ions concentrationa corresponding reduction of the photocatalytic oxidationefficiency of lignin was noted Likewise Ohnishi et al [57]made a comparative study by doping platinum (Pt) silver(Ag) and gold (Au) ions to TiO
2 In these reactions 50mg of
catalyst (TiO2) was used with the addition of an equiva1ent
15 wt (based on TiO2) metal ion The addition of noble
metals brought about a faster decolorization of lignin Aushowed better results than Ag followed by Pt In the samecontext adding sodium hypochlorite as oxidant to PtTiO
2
catalyst an additional fivefold degradation rate was observedcompared to that without doping [56] Contradictory to theresults described above negligible effect of photocatalyticefficiency due to doping has been reported as well Awun-gacha Lekelefac et al [54] obtained little or no change indegradation rate by doping TiO
2-P25-SiO
2catalyst with Pt
ions (1 wt relative to TiO2catalyst) Likewise Portjanskaja
and Preis [50] noted a negligible change of photocatalyticefficiency of TiO
2when doped with nitrogen
Sarkanen et al [67] and Gellerstedt and Lindfors [68]reported the bias of peroxides to oxidation with reagentssuch as permanganate to favor aromatic moieties Oxidationagents like permanganate oxidizes predominantly aliphaticchains in alkaline and neutral media However by theapplication of H
2O2(Fenton system) lignin disappeared
completely [33] Tonucci et al [33] conclude that in orderto satisfactorily conserve the organic material the best com-promise appears to be the TiO
2photosystem which shows
low carbon consumption good preservation of the aromaticrings and greatly reduced mineralization
International Journal of Photoenergy 11
In summary different results have been obtained con-cerning the influence of noble metal ion addition Whilesome authors report an improvement in the photocatalyticefficiency upon their addition others report their addition ashaving no considerable influence However for reactions inwhich an improvement in the photocatalytic efficiency wasnoticed there was a threshold value to be considered Whenthe concentration of dopant surpasses this threshold valueelectron-hole recombination is favored and this has a negativeimpact to photocatalysis In such a case the space-chargelayer gets narrower and 119901-type dopants attract electrons andby virtue become negative They would now then act ashole acceptor attracting holes On the other hand 119899-typedopants which act as electron donor centers and possessexcess electrons attract holes as well [37]
323 Influence of Lignin Concentration Once the initiallignin concentration becomes higher exceeding a thresholdvalue an inhibitory effect on the photodegradationwas noted[47ndash49] This threshold value varies and depends on thereaction system and reaction parameters such as opticaldensity catalyst concentration and reaction volume Fromthe literature different authors have implemented varyinglignin concentrations probably to suit their reaction designFor example Ksibi et al [47] use 90mgL and AwungachaLekelefac et al [54] use 500mgL while Kansal et al [48]applied 10mgndash100mgL
Explanations arising from the findings are as followsat low lignin concentrations the incidental photonic fluxirradiated interacts with the catalyst generating radicals (eghydroxyl radicals (OH∙)) which allow a faster degradation[69] On the other hand high initial lignin concentrationsmay lead to tight adsorption which can suppress CO
2
evolution [51] and hence maintain chemical oxygen demand(COD) values Moreover low delignification yields maybe due to an inhibitory effect because of autoxidation bylow molecular weight lignin degradation products formed[24] Also due to the polymer structure of lignin which iscross-linked this makes it difficult for radical species acidand the aldehyde compounds produced to spread into theinner region of the substrate hence limiting autoxidationAs a worst case this might be the rate-determining step ofdelignification which is hindered [70ndash72]
In summary it can be concluded that the time taken forcomplete degradation depends on the initial concentration oflignin and faster degradation occurs at low lignin concentra-tions
324 Influence of pH Varying pH entails an alteration inthe properties of semiconductor-liquid interface [73] mainlyrelated to the acid-base equilibrium of the adsorbed hydroxylgroup [39] Furthermore pH also impacts lignin degradationrates [47 48 57] In this context several studies were carriedout with partly contradictory outcomes
Kansal et al [48] made pH investigations 3ndash11 undersolar light illumination using ZnO as catalyst Maximumdegradation was reached in alkaline conditions (pH 11)This is supported by Villasenor and Mansilla [74] reporting
an almost complete decolorization of kraft black liquorfrom pine wood at pH value of 116 in combination withZnO catalyst Similar results were achieved by Ohnishi etal [57] with TiO
2and ZnO being catalyst for bleaching
alkaline lignin in aqueous solution with TiO2and ZnO being
catalyst High activities at neutral pH were also reportedby Ohnishi et al [57] In contrast Ma et al [56] observedhigher reaction rates and rapid degradation of a syntheticlignin wastewater (prepared by dissolving commercial ligninpowder in aqueous solution pH 11) in acidic solution (pH 3)than in alkaline solutions at pH 11 for either TiO
2or PtTiO
2
catalystsReconsidering the photocatalytic principle the formed
superoxide anion radicals (∙O2
minus) are in a pH-dependentequilibriumwith perhydroxyl radicals (HO
2
∙) as follows [75]
HO2
∙999445999468 H+ + ∙O
2
minus pKa sim 48 (9)
See [76]∙O2
minus undergoes dismutation reaction resulting in H2O2
and O2competing to any other ∙O
2
minus triggered reaction Incase of low pH operation conditions in aqueous solutionsHO2
∙ becomes dominant whose reactivity is considerablyhigher compared to ∙O
2
minus [77] Subsequently HO2
∙ initiatessubstrate (S) oxidation to the radical cation (S+∙) and is itselfreduced to H
2O2[78] Thus increased degradation rates can
be reasonably expected supporting the results made by Ma etal [56] in an acidic environment ∙O
2
minus is extremely reactive inorganic solvents [77] Another aspect is the solubility of kraftlignin (soluble at pH gt 105) which reduces with decreasingpH whereas lignosulfonate should remain unaffected by pHin aqueous solution Moreover 120573ndashOndash4 bonds have beendescribed to be stable at acidic pH [11] In fact this couldadditionally explain the elevated degradation of kraft ligninmade by Kansal et al [48] and Villasenor and Mansilla [74]Nevertheless the contradictory results gained by Ma et al[56] still exist under the assumption that kraft ligninwas used(which would be supported by the high pH of 11 obviouslynecessary for dissolving the lignin powder) Although mostphotocatalytic reactions described in the literature are in anaqueous milieu lignin raw material and its fission productsmay however vary considerably Therefore the optimal pHis most likely to be reaction specific and has to be evaluatedexperimentally in principle
325 Influence of Illumination Many of the studies foundin the literature so far have not dealt on this subject per seWhat is found is the use of different illumination sourceseach having a specified power and lamp type However alltend to emitUV-light between the range 280ndash420 nm Table 2depicts this in detail Other illumination sources include thevisible light spectrum
In general terms illumination influences in that it ini-tiates photocatalysis by generating electron-hole pair in thesemiconductor particles [38 39] Dahm and Lucia [49]altered illumination intensity while observing lignin degra-dation In this study 004 gL lignin was used and lightintensity was varied 223ndash445mWcm3 It was found out thathigher illumination intensities correlated well with higher
12 International Journal of Photoenergy
initial degradation rates and hence total lignin degradation[49] Neppolian et al [69] report degradation to be propor-tional to radiation intensity and best results are achieved forlow lignin concentrations because of enhanced interactionbetween catalyst and incidental photonic flux
In summary high illumination power causes a corre-sponding high initial degradation rate at low lignin concen-trations becausemaximum light penetration into the reactionmedium is favored
33 Process Analytical Methods Various analytical tech-niques have been used to monitor lignin degradation Atthe beginning of this subchapter analytics revealing com-pounds formed from lignin degradation are treated Thisincludes for example gas chromatography (GC) and 1HNMR (nuclear magnetic resonance) This is then followed byresults qualitative analytic measurements such as ultraviolet-visible (UV-Vis) spectroscopy and dissolved carbon (DC)A list of authors analytical techniques applied and resultsachieved are outlined in Table 3
Portjanskaja and Preis [50] studied lignin degradationby measuring the removal of phenols through colorimetricmeasurements As a result of 24 h photocatalytic oxidationunder neutral media conditions 80 of free phenols wereremoved Gas chromatography (GC) result from Ksibi et al[47] attested vanillin vanillic acid palmitic acid biphenyland 345-trimethoxy benzaldehyde structures after the pho-tocatalysis of lignin from black liquor This is in accordancewith the findings of Tonucci et al [33] reporting the formationof vanillin hydroxyl methoxy-acetophenone coniferyl alco-hol coniferyl aldehyde methanol formic acid acetic acidand small amounts of C-2 and C-3 alcohols as degradationproducts1H NMR spectral analysis of lignin before illumination
and after 24 h of illumination showing characteristic bandsof aromatic rings methoxy and aliphatic side chains wascompared with each other Results revealed that the aro-matic ring degraded faster than the aliphatic chain [51]Fourier transformation infrared spectroscopy (FTIR) showedbands corresponding to CH
3 CH2 and CH which remained
unchanged after illumination while bands corresponding toaromatic rings disappeared as a result of illumination [51 53]
Results obtained from the combination of photochemicaland electrochemical oxidation [52 53] were similar to thoseof Tanaka et al [51] Here 13C-NMR confirmed the presenceof the carbonyl functionality and the presence of vanillinand vanillic acid after 12 h photochemical-electrochemicaloxidation These results showed that the combination of aphotocatalytic and an electrochemical oxidation significantlyenhanced the efficiency of lignin degradationThis is becausethe applied anodic potential bias greatly suppressed therecombination of photogenerated electrons and holes [53]
Ultraviolet spectrophotometry offers a convenientmethod to qualitatively and quantitatively analyze ligninin solution [79] This is reflected by the large number ofpublications using this technique [17 33 47ndash51 56 58 80]This is most likely due to its simplicity to interpret lignindegradation Lignins absorb UV light with high molar
200 220 240 260 280 300 320 340 360
00
05
10
15
20
25
Abso
rban
ce
Wavelength (nm)0min
30min
60min
120min
180min
300min
360min
420min
1200min
Figure 8 Time dependent UV-Vis absorption spectra of aqueouslignin solution from waste paper water irradiated with UV light(280ndash420 nm) for different time intervals The spectra are obtainedfor sol-gel derived TiO
2nanocrystalline coating (TiO
2-P25-SiO
2)
[54]
extinction coefficients because of the several methox-ylated phenylpropane units of which they are composed [33]Figure 8 depicts a series of photometric scans of ligninsul-fonate from paper waste water showing a gradual reductionof absorbance during photocatalytic treatment [54] Herethe absorption peaks are around 210 nm and 280 nmAbsorbance decreases with time implying the decompositionof lignin and the deterioration of chromophore groups [54]
Peaks at 210 nmcorrespond to portions of the unsaturatedchains while those at 280 nm correspond to unconjugatedphenolic hydroxyl groups [17] and the aromatic moiety [57]of the lignin molecule The absorption tailing to the longwavelength region arises from the color of lignin [57] Lignindegradation has been reported either at wavelength around280 nm corresponding to unconjugated phenolic hydroxylgroups [17 48 51 58] or for both wavelengths (210 nm and280 nm) [33 54 57] Kobayakawa et al [81] noted some otherabsorbance at wavelengths lower than 250 nm and pointedout that this could be due to the modification of ligninfragmentations leading to the formation of transient specieslikemethanol ethanol formaldehyde formic acid and oxalicacid among others
Analyticalmethods to effectively quantify lignin degrada-tion by calculating the oxygen demand by organic substancesand remaining organic carbon before and after photocatalysishave been studied Amongst the methods are dissolvedcarbon (DC) [49 51 58] chemical oxygen demand (COD)[47 48 50 57 80] biochemical oxygen demand (BOC) [50]dissolved organic carbon (DOC) [56] and American dyemanufacture institute value (ADMI) [56] COD and BODdescribe the oxygen demand by organic substances to be
International Journal of Photoenergy 13
Figure 9 Gradual change from the characteristic yellow ligninsulfonate to a colorless liquid after a period of 20 h Catalyst TiO
2-
P25 (Degussa) +TEOSUV-light 25∘C lignin concentration 05 gL[54]
converted to CO and CO2and H
2O and NH
3 Total organic
carbon (TOC) describes the amount of carbon bound inan organic compound while DOC describes the dissolvedfraction of organic carbon ADMI measures the amount ofdyestuff in water
Decolorization of lignin solution has been reported tobe another parameter observed during photocatalytic degra-dation Color is an indirect indicator of the lignin amountThe higher the color intensity of the solution is the greaterthe lignin content is (high concentrated lignin solutions egblack liquor appear dark brown) [82] Thus color changescan be interpreted as conversion of lignin to transient speciesor conversion to CO
2and H
2O Awungacha Lekelefac et al
[54] observed a gradual change from the characteristic yellowlignin (when highly diluted) to a colorless liquid after a periodof 20 h with sol-gel derived TiO
2nanocrystalline coatings on
sintered borosilicate glass as depicted in Figure 9 A corre-sponding decrease in DC values close to 82 was observedfor TiO
2-P25-SiO
2catalyst confirming degradation This is
shown in Figure 10These findings are analogous to that of Ohnishi et al
[57] who reported the bleaching of lignin when illuminatedcontinuously and that the solution becomes colorless Tothat the chemical oxygen demand (COD) value decreasesgenerating carbon dioxide and a small amount of carbonmonoxide as the main gaseous products COD removalwas reported to be effective at low lignin concentrations ascompared to high lignin concentrations [48]
Another applied analytical technique is fluorescencedetection directly coupled to a high performance liquidchromatography (HPLC) as a means to identify nonaliphaticcomponent in the complex mixture of lignin degradationproducts [54] Fluorescence emission in lignin is attributedto aromatic structures such as conjugated carbonyl biphenylphenylcoumarone and stilbene groups [83 84] AwungachaLekelefac et al [54] observed peaks on both HPLC and
0 200 400 600 800 1000 12000
50
100
150
200
250
Diss
olve
d ca
rbon
(ppm
)
t (min)
TiO2-P25-SiO2
TiO2-P25-SiO2 + PtTiOSO4minus30 6wt
ZnO + TiO2-P25-SiO2
No coating UV light
Figure 10 Variation of DC with time of aqueous lignin solutionfrom waste paper water irradiated with UV light (280ndash420 nm) fordifferent time intervals The spectra are obtained for sol-gel derivedTiO2nanocrystalline coatings (TiO
2-P25-SiO
2+Pt TiO
2-P25-SiO
2
TiOSO4minus306 wt ZnO + TiO
2-P25-SiO
2) [54]
fluorescence chromatograms suggesting the production ofnew substances and fluorophores
Despite developed analytical technologies analyzinglignin degradation products remains challenging Proofssuch as mass spectroscopy (MS) HPLC 13C or 1H-NMRspectra from photocatalytic lignin degradations are not yetestablished The setback to qualitatively and quantitativelyanalyze lignin and its degradation products starts from thenative lignin polymer itself with its indefinite polymericstructure and multiple bond types Also the influence ofdifferent pretreatments additives and the wide variety ofcompounds obtainable from its degradation makes ligninanalysis challenging [85] Moreover lignin streams couldcontain proteins inorganic salts and other potential poisonsthat generally complicate catalysis [4]
The challenge to identify and separate the productsstreams derived from the photocatalytic degradation ligninis also worth noting Lignin product stream is highly func-tionalized and conventional techniques such as gas chro-matography have the disadvantage of requiring a time-consuming derivatization step Also because of high boilingpoint of substances arising from lignin degradation it is noteasily applicable High performance liquid chromatography(HPLC) seems to be the remedy because analysis can becarried out without derivatization but the exact identificationof the separated substances is difficult because of the numer-ous peaks arising from such a chromatogram [87] Unfortu-nately well-established databases such as that of the nationalinstitute of standard and technology (NIST) [88] cannot giveinformation onHPLC-MS chromatogramsThis is because ofthe ionization sources such as electrospray ionization (ESI)
14 International Journal of Photoenergy
Table 4 Main advantages and disadvantages photocatalysis versus enzymatic biocatalysis
Lignin degradation process Advantages Disadvantages
Photocatalysis(single stage)
(i) Relatively fast degradation of complex as wellas nonbiodegradable organic (macro)molecules(ii) Stable catalysts(iii) Moderate reaction conditions
(i) Not selective(ii) Limited selection of operating conditions(iii) Energy costs
Enzymatic conversion(single stage)
(i) More selective(ii) Moderate reaction conditions
(i) High enzyme loads required otherwise ligninhydrolysis is quite slow [16](ii) Enzymes are less stable1(iii) cost-intensive (POXs)2
1For example POXs are sensitive to high H2O2 concentrations2According to Torres and Ayala [86]
and atmospheric pressure chemical ionization (APCI) forLC-MS while GC-MS is based on electron ionization Aremedy to this can be the development of a database ofmodel lignin compounds based on a unanimous HPLC-MSmeasuring procedure This would mean much time and costexpensive investments for adequate personnel and material
Though product identification based on peak superpo-sition gives information on a possible product this can-not always be true for lignin degradation because of thelarge number of possible degradation products In order toproperly identify products from lignin degradation productisolation techniques are to be implemented and further ana-lytic methods such as tandem mass spectroscopy (MSMS)and other advanced structure enhancing research techniquessuch as 1H-NMR 13C-NMR have to be studied extensively
4 Photocatalysis and Its Suitability asIntegrated Technology in MultistageConcepts with Biocatalysis
Depending on lignin treatment specification photocatalysiswill find application for complete lignin mineralization toCO2and H
2O (single stage) or as integrated technology in
a multistage system aiming at partial conversion of ligninmacromolecules to organic low molecular weight intermedi-ates for a consecutive biological oxidation finally generatingvalue added end-products [15]
For biological oxidation of lignin or lignin deriva-tives H
2O2-dependent ligninolytic heme peroxidases (POXs
including lignin peroxidase (LiP EC 111114) manganeseperoxidase (MnP EC 111113) versatile peroxidase (VPEC 111116) O
2-dependent laccases (Lac EC 11032) and
extracellular enzymes from Basidiomycetous white-rot fungiare the most efficient lignin degraders in nature [89] Thussuch enzymatic systems especially POXs (with high redoxpotentials) have attractedmuch interest as industrial biocata-lyst [86 90]The enzyme degradationmechanism (in nature)is facilitated by nonenzymatic processes mainly throughfree ∙OH radicals also generated by the fungus (Fenton-type reaction) Those ∙OH radicals enable the requiredphysical contact between the enzymes and structural units ofthe lignin molecule due to numerous nonspecific oxidative
reactions [91] Both the photocatalytic (equations (2)ndash(7))and the POX reactionmechanism recently reviewed by Busseat al [25] are quite similar Consequently a combination ofphotocatalysis followed by an enzymatic oxidation maybe apromising concept utilizing lignin derivatives for exampleoriginated from industrial effluents [15 92] The advantagesforming biobased products are as follows see also Table 4 inthis context A reduction of the lignin polymerization degreevia photocatalysis at best in more biodegradable interme-diates and a simultaneous detoxification causes savings inenzyme costs since it will be expected that less enzyme loadsare necessary for sufficiently rapid reaction rates [15 91] As aresult hydraulic retention times in bioreactor systems shouldbe diminished Moreover the delignification is expectedto be enhanced simultaneously which was in recent yearsshown in a dual system by Kamwilaisak and Wright [93]using TiO
2H2O2UV for photocatalytic pretreatment and
Lac (from Trametes versicolor) in the subsequent biocatalyticstep Within 24 h they obtained in their dual system anelevation in delignification of 20 (without H
2O2) up to at
least 50 when H2O2was present
In previous studies the treatment of lignin (from thepulp and paper industry) containing effluents by fungus asa biological system (excreting appropriate lignin degradingenzyme cocktails) were more focused for posttreatmentexclusively (Duran et al [92] Reyes et al [94] and Gonzalezet al [95]) Shende et al [17] even examined ligninolyticbacteria with photooxidized kraft lignin as substrate
Although POXs are potential industrial biocatalysts [86]no application studies were found for the direct use in suchdual systems as described above The major reason may bethe complexity of the reaction mechanism (inclusive ligninderivatives as substrate and their analysis) per se on the onehand slowing down research and development processes Onthe other hand POXs are sensitive to their cosubstrate H
2O2
once it is supplied in excess causing considerable inactivation(for details refer to Busse et al [25]) At the present severalstudies are carried out modifying these enzymes regardingenhanced stability activity and selectivity as well Henceit can be expected that their technological applicability willbe raised significantly right after successful modification isreached [86]
International Journal of Photoenergy 15
5 Concluding Remarks
It is widely assumed that the photocatalytic degradation oflignin follows a radical reaction pathway which is similar tothat considered in thermal electrochemical and biochemicalprocesses However reporting on the degradation pathway oflignin derivatives and even that of lignin model compoundsare still amajor challengeThis is probably due to the complexnature and variety of possible degradation products Indeedthemechanism is far more complex considering other factorssuch as type of lignin type of catalyst pH illuminationsource and additives
Generally comparing the different photochemical pro-cesses poses a big challenge because of the wide variablesinvolved These discrepancies start from the source and typeof lignin followed by differences in reactor design illumi-nation source intensity of radiation and different types ofcatalyst An ideawould be to have a specific reference reactionwith well-defined starting parameters which include lignintype source and purity catalyst specifications illuminationsource and intensity so as to ease comparison of results
Basic process parameters such as catalyst concentrationsubstrate concentration addition of metal ion to catalyst pHand illumination have been discussed
Despite developed analytical technologies analyzinglignin degradation products remains challenging Proofssuch as mass spectroscopy (MS) HPLC 13C or 1H-NMRspectra from photocatalytic lignin degradations are not yetestablished
In order to properly identify products from lignin degra-dation product isolation techniques are to be implementedand further analytic methods such as MSMS and otheradvanced structure enhancing research techniques such as1H-NMR and 13C-NMR have to be studied extensively Aremedy to this can be the development of a databank ofmodel lignin compounds based on a unanimous HPLC-MSmeasuring procedure This would mean much time and costexpensive investments for adequate personnel and material
Photocatalysis is denoted as the most popular ligninpretreatment technology besides ozonation [96] Photo-catalyzed lignin may be an appropriate substrate for aconsecutive biocatalytic process using ligninolytic enzymes(POX andor Lac) as supported by experimental results ofKamwilaisak and Wright [93] Combining the advantages ofboth catalytic processes savings in the overall process costswill be expected in addition to elevated lignin conversionNonetheless extensive research work including POX modi-fications is still required
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Authorsrsquo Contribution
Colin Awungacha Lekelefac and Nadine Busse contributedequally to this work
Acknowledgments
The authors gratefully thank the Federal Ministry of Educa-tion and Research (BMBF) for funding (FKZ17N0310) Theresearchers also thank the Hessen State Ministry of HigherEducation Research andArts for the financial support withinthe Hessen initiative for scientific and economic excellence(LOEWE)
References
[1] Crude Oil and Commodity Prices httpwwwoil-pricenet[2] Research Directorate-General for World Energy Technology
and Climate Policy Outlook (WETO) Luxembourg Office forOfficial Publications of the European Communities 2003
[3] Green and Natural Polymers Are on the Rise 2014 httpwwwpolymersolutionscombloggreen-and-natural-polymers-on-the-rise
[4] J Zakzeski P C Bruijnincx A L Jongerius and B M Weck-huysen ldquoThe catalytic valorization of lignin for the productionof renewable chemicalsrdquo Chemical Reviews vol 110 no 6 pp3552ndash3599 2010
[5] B KammM Kamm P R Gruber and S KromusBiorefineries-Industrial Processes and Products Status Quo and Future Direc-tions vol 1 Wiley-VCH 2006
[6] R L Howard E Abotsi E L J van Rensburg and S HowardldquoLignocellulose biotechnology issues of bioconversion andenzyme productionrdquoAfrican Journal of Biotechnology vol 2 no12 pp 602ndash619 2003
[7] M Stocker ldquoBio- und BTL-Kraftstoffe in der Bioraffineriekatalytische Umwandlung Lignocellulose-reicher Biomasse mitporosen Stoffenrdquo Angewandte Chemie vol 120 no 48 pp9340ndash9351 2008
[8] J-P Lange ldquoLignocellulose conversion an introduction tochemistry process and economicsrdquo Biofuels Bioproducts andBiorefining vol 1 no 1 pp 39ndash48 2007
[9] D A I Goring ldquoThe physical chemistry of ligninrdquo Pure andApplied Chemistry vol 5 no 1-2 pp 233ndash310 1962
[10] M Ek G Gellerstedt and G HenrikssonWood Chemistry andBiotechnology Pulp and Paper Chemistry and Technology vol 1Walter de Gruyter GmbH amp Co KG Berlin Germany 2009
[11] B Saake and R Lehnen ldquoLigninrdquo in Ullmannrsquos Encyclopedia ofIndustrial Chemistry Wiley-VCH Verlag GmbH amp Co KGaAWeinheim Germany 2012
[12] M N S Kumar A K Mohanty L Erickson and M MisraldquoLignin and its applications with polymersrdquo Journal of BiobasedMaterials and Bioenergy vol 3 no 1 pp 1ndash24 2009
[13] R J A Gosselink E de Jong B Guran and A AbacherlildquoCo-ordination network for ligninmdashstandardisation produc-tion and applications adapted to market requirements(EUROLIGNIN)rdquo Industrial Crops and Products vol 20 no 2pp 121ndash129 2004
[14] D Fengel and G Wegener Wood Chemistry UltrastructureReactions Verlag Kessel 1984
[15] D Mantzavinos and E Psillakis ldquoEnhancement of biodegrad-ability of industrial wastewaters by chemical oxidation pre-treatmentrdquo Journal of Chemical Technology and Biotechnologyvol 79 no 5 pp 431ndash454 2004
16 International Journal of Photoenergy
[16] M Dashtban H Schraft and W Qin ldquoFungal bioconversionof lignocellulosic residues Opportunities amp perspectivesrdquo Inter-national Journal of Biological Sciences vol 5 no 6 pp 578ndash5952009
[17] A Shende R Jaswal D Harder-Heinz A Menan and RShende ldquoIntergrated photocatalytic and microbial degradationof kraft ligninrdquo Cleantech pp 120ndash123 2012
[18] F S Chakar and A J Ragauskas ldquoReview of current and futuresoftwood kraft lignin process chemistryrdquo Industrial Crops andProducts vol 20 no 2 pp 131ndash141 2004
[19] W G Glasser and H R Glasser ldquoEvaluation of ligninrsquoschemical structure by experimental and computer simulationtechniquesrdquo Paperi ja Puu vol 63 pp 71ndash83 1981
[20] M Erickson S Larsson and G E Miksche ldquoZur Struktur desLignins der FichterdquoActaChemica Scandinavica vol 27 pp 903ndash914 1973
[21] H Nimz ldquoDas Lignin der BuchemdashEntwurf eines Konstitution-sschemasrdquo Angewandte ChemiemdashInternational Edition vol 86pp 336ndash344 1974
[22] D V Evtuguin C P Neto J Rocha and J D P de JesusldquoOxidative delignification in the presence of molybdovana-dophosphate heteropolyanions mechanism and kinetic stud-iesrdquo Applied Catalysis A General vol 167 no 1 pp 123ndash1391998
[23] A L Linsebigler G Lu and J T Yates Jr ldquoPhotocatalysis onTiO2surfaces Principles mechanisms and selected resultsrdquo
Chemical Reviews vol 95 no 3 pp 735ndash758 1995[24] Y Miyata K Miyazaki M Miura Y Shimotori M Aoyama
and H Nakatani ldquoSolventless delignification of wood flourwith TiO
2poly(ethylene oxide) photocatalyst systemrdquo Journal
of Polymers and the Environment vol 21 no 1 pp 115ndash121 2013[25] N Busse D Wagner M Kraume and P Czermak ldquoReaction
kinetics of versatile peroxidase for the degradation of lignincompoundsrdquoTheAmerican Journal of Biochemistry andBiotech-nology vol 9 no 4 pp 365ndash394 2013
[26] M Tien and T K Kirk ldquoLignin-degrading enzyme fromphanerochaete chrysosporium Purification characterizationand catalytic properties of a uniqueH
2O2-requiring oxygenaserdquo
Proceedings of the National Academy of Sciences vol 81 pp2280ndash2284 1984
[27] T K Kirk M Tien P J Kersten M D Mozuch and BKalyanaraman ldquoLigninase of Phanerochaete chrysosporiumMechanism of its degradation of the non-phenolic arylglycerol120573-aryl ether substructure of ligninrdquo Biochemical Journal vol236 no 1 pp 279ndash287 1986
[28] T Lundell RWever R Floris et al ldquoLignin peroxidase L3 fromPhlebia radiata pre-steady-state and steady-state studies withveratryl alcohol and a non-phenolic lignin model compound1-(34-dimethoxyphenyl)-2-(2-methoxyphenoxy)propane-13-diolrdquo European Journal of Biochemistry vol 211 no 3 pp391ndash402 1993
[29] H E Schoemaker T K Lundell A I Hatakka and K PiontekldquoThe oxidation of veratryl alcohol dimeric lignin models andlignin by lignin peroxidase the redox cycle revisitedrdquo FEMSMicrobiology Reviews vol 13 no 2-3 pp 321ndash331 1994
[30] J M Palmer P J Harvey and H E Schoemaker ldquoThe role ofperoxidases radical cations and oxygen in the degradation oflignin [and discussion]rdquo Philosophical Transactions of the RoyalSociety of London A vol 321 no 1561 pp 495ndash505 1987
[31] C Hill Wood Modification Chemical Thermal and OtherProcesses John Wiley amp Sons Chichester UK 2006
[32] Y-H Xu H-R Chen Z-X Zeng and B Lei ldquoInvestigation onmechanism of photocatalytic activity enhancement of nanome-ter cerium-doped titaniardquo Applied Surface Science vol 252 no24 pp 8565ndash8570 2006
[33] L Tonucci F Coccia M Bressan and N DrsquoAlessandro ldquoMildphotocatalysed and catalysed green oxidation of lignin auseful pathway to low-molecular-weight derivativesrdquoWaste andBiomass Valorization vol 3 no 2 pp 165ndash174 2012
[34] A Castellan N Colombo C Vanucci P Fornier de HViolet and H Bouas-Laurent ldquoPhotodegradation of ligninA photochemical study of an O-methylated 120572-carbonyl 120573-1lignin model dimer 12-di(3101584041015840-dimethoxyphenyl) ethanone(deoxyveratroin)rdquo Journal of Photochemistry and PhotobiologyA vol 51 pp 451ndash467 1990
[35] A Fujishima X Zhang and D A Tryk ldquoTiO2photocatalysis
and related surface phenomenardquo Surface Science Reports vol63 no 12 pp 515ndash582 2008
[36] C S Turchi and D F Ollis ldquoPhotocatalytic reactor designan example of mass-transfer limitations with an immobilizedcatalystrdquo The Journal of Physical Chemistry vol 92 no 23 pp6852ndash6853 1988
[37] R Subasri M Tripathi K Murugan J Revathi G V N Raoand T N Rao ldquoInvestigations on the photocatalytic activity ofsolndashgel derived plain and Fe3+Nb5+ doped titania coatings onglass substratesrdquo Materials Chemistry and Physics vol 124 pp63ndash68 2010
[38] N Serpone ldquoRelative photonic efficiencies and quantum yieldsin heterogeneous photocatalysisrdquo Journal of Photochemistry andPhotobiology A Chemistry vol 104 no 1ndash3 pp 1ndash12 1997
[39] M R Hoffmann S T Martin W Choi and D W BahnemannldquoEnvironmental applications of semiconductor photocatalysisrdquoChemical Reviews vol 95 no 1 pp 69ndash96 1995
[40] C D Jaeger and A J Bard ldquoSpin trapping and electron spinresonance detection of radical intermediates in the photode-composition of water at titanium dioxide particulate systemsrdquoThe Journal of Physical Chemistry vol 83 no 24 pp 3146ndash31521979
[41] R W Matthews ldquoHydroxylation reactions induced by near-ultraviolet photolysis of aqueous titaniumdioxide suspensionsrdquoJournal of the Chemical Society Faraday Transactions vol 80no 2 pp 457ndash471 1984
[42] A E H Machado A M Furuyama S Z Falone R RuggieroD D S Perez and A Castellan ldquoPhotocatalytic degradation oflignin and lignin models using titanium dioxide the role of thehydroxyl radicalrdquoChemosphere vol 40 no 1 pp 115ndash124 2000
[43] W Sigg and L Stumm Aquatische Chemie Stuttgart 2 AuflageBG Teubner Stuttgart Germany 1991
[44] O Legrini E Oliveros and A M Braun ldquoPhotochemicalprocesses for water treatmentrdquo Chemical Reviews vol 93 no2 pp 671ndash698 1993
[45] G Rothenberger J Moser M Gratzel N Serpone and DSharma ldquoCharge carrier trapping and recombination dynamicsin small semiconductor particlesrdquo Journal of the AmericanChemical Society vol 107 no 26 pp 8054ndash8059 1985
[46] P Mazellier M Sarakha A Rossi and M Bolte ldquoThe aqueousphotochemistry of 26-dimethylphenol Evidence for the frag-mentation of the 120572 C-C bondrdquo Journal of Photochemistry andPhotobiology A vol 115 no 2 pp 117ndash121 1998
[47] M Ksibi S B Amor S Cherif E Elaloui A Houas and MElaloui ldquoPhotodegradation of lignin from black liquor using aUVTiO
2systemrdquo Journal of Photochemistry and Photobiology
A vol 154 no 2-3 pp 211ndash218 2003
International Journal of Photoenergy 17
[48] S K Kansal M Singh and D Sud ldquoStudies on TiO2ZnO
photocatalysed degradation of ligninrdquo Journal of HazardousMaterials vol 153 no 1-2 pp 412ndash417 2008
[49] A Dahm and L A Lucia ldquoTitanium dioxide catalyzed pho-todegradation of lignin in industrial effluentsrdquo Industrial andEngineering Chemistry Research vol 43 no 25 pp 7996ndash80002004
[50] E Portjanskaja and S Preis ldquoAqueous photocatalytic oxidationof lignin the influence of mineral admixturesrdquo InternationalJournal of Photoenergy vol 2007Article ID76730 7 pages 2007
[51] K Tanaka R C R Calanag and T Hisanaga ldquoPhotocatalyzeddegradation of lignin on TiO
2rdquo Journal of Molecular Catalysis
A Chemical vol 138 no 2-3 pp 287ndash294 1999[52] M Tian J Wen D MacDonald R M Asmussen and A Chen
ldquoA novel approach for lignin modification and degradationrdquoElectrochemistry Communications vol 12 no 4 pp 527ndash5302010
[53] K PanM Tian Z-H Jiang B Kjartanson and A Chen ldquoElec-trochemical oxidation of lignin at lead dioxide nanoparticlesphotoelectrodeposited on TiO
2nanotube arraysrdquo Electrochim-
ica Acta vol 60 pp 147ndash153 2012[54] C Awungacha Lekelefac J Hild P Czermak and M Herren-
bauer ldquoPhotocatalytic active coatings for lignin degradationin a continuous packed bed reactorrdquo International Journal ofPhotoenergy vol 2014 Article ID 502326 10 pages 2014
[55] C Awungacha Lekelefac P Czermak and M HerrenbauerldquoEvaluation of photocatalytic active coatings on sintered glasstubes by methylene bluerdquo International Journal of Photoenergyvol 2013 Article ID 614567 9 pages 2013
[56] Y-S Ma C-N Chang Y-P Chiang H-F Sung and A CChao ldquoPhotocatalytic degradation of lignin using PtTiO
2as
the catalystrdquo Chemosphere vol 71 no 5 pp 998ndash1004 2008[57] H Ohnishi M Matsumura H Tsubomura and M Iwasaki
ldquoBleaching of lignin solution by a photocatalyzed reactionon semiconductor photocatalystsrdquo Industrial and EngineeringChemistry Research vol 28 no 6 pp 719ndash724 1989
[58] C A K Gouvea F Wypych S G Moraes N Duran and PPeralta-Zamora ldquoSemiconductor-assisted photodegradation oflignin dye and kraft effluent by Ag-doped ZnOrdquo Chemospherevol 40 no 4 pp 427ndash432 2000
[59] A V Vahatalo K Salonen M Salkinoja-Salonen and AHatakka ldquoPhotochemical mineralization of synthetic lignin inlake water indicates enhanced turnover of aromatic organicmatter under solar radiationrdquo Biodegradation vol 10 no 6 pp415ndash420 1999
[60] H de Lasa B Serrano and M Salaices Photocatalytic ReactionEngineering Springer New York NY USA 2005
[61] K Hashimoto H Irie and A Fujishima ldquoTiO 2 photocatalysisA historical overview and future prospectsrdquo Japanese Journal ofApplied Physics vol 44 no 12 pp 8269ndash8285 2005
[62] M A Behnajady N Modirshahla and R Hamzavi ldquoKineticstudy on photocatalytic degradation of CI Acid Yellow 23 byZnOphotocatalystrdquo Journal ofHazardousMaterials vol 133 no1ndash3 pp 226ndash232 2006
[63] M Addamo V Augugliaro A di Paola et al ldquoPhotocatalyticthin films of TiO
2formed by a sol-gel process using titanium
tetraisopropoxide as the precursorrdquo Thin Solid Films vol 516no 12 pp 3802ndash3807 2008
[64] N Negishi K Takeuchi and T Ibusuki ldquoPreparation of theTiO2thin filmphotocatalyst by the dip-coating processrdquo Journal
of Sol-Gel Science and Technology vol 13 no 1ndash3 pp 691ndash6941998
[65] L Rideh A Wehrer D Ronze and A Zoulalian ldquoPhotocat-alytic degradation of 2-chlorophenol in TiO
2aqueous suspen-
sion modeling of reaction raterdquo Industrial and EngineeringChemistry Research vol 36 no 11 pp 4712ndash4718 1997
[66] R-ADoongC-HChen RAMaithreepala and S-MChangldquoThe influence of pHand cadmiumsulfide on the photocatalyticdegradation of 2-chlorophenol in titanium dioxide suspen-sionsrdquoWater Research vol 35 no 12 pp 2873ndash2880 2001
[67] K V Sarkanen A Islam and C D Anderson ldquoOzonationrdquo inMethods in Lignin Chemistry S Y Lin and C W Dence EdsSpringer Series inWood Science pp 387ndash406 Springer BerlinGermany 1992
[68] G Gellerstedt and E-L Lindfors ldquoStructural changes in ligninduring kraft pulpingrdquo Holzforschung vol 38 no 3 pp 151ndash1581984
[69] B Neppolian H C Choi M V Shankar B Arabindoo andV Murugesan in Proceedings of the International Symposiumon Environmental Pollution Control and Waste Management(EPCOWM 02) p 647 2002
[70] C Pouteau P Dole B Cathala L Averous and N BoquillonldquoAntioxidant properties of lignin in polypropylenerdquo PolymerDegradation and Stability vol 81 no 1 pp 9ndash18 2003
[71] J Hafren T Fujino and T Itoh ldquoChanges in cell wall archi-tecture of differentiating tracheids of Pinus thunbergii duringlignificationrdquo Plant and Cell Physiology vol 40 no 5 pp 532ndash541 1999
[72] T Dizhbite G Telysheva V Jurkjane and U Viesturs ldquoChar-acterization of the radical scavenging activity of lignins naturalantioxidantsrdquo Bioresource Technology vol 95 no 3 pp 309ndash3172004
[73] K Hofstadler R Bauer S Novalic and S G Heisier ldquoNew reac-tor design for photocatalytic wastewater treatment with TiO
2
immobilized on fused-silica glass fibers photomineralization of4-chlorophenolrdquo Environmental Science amp Technology vol 28no 4 pp 670ndash674 1994
[74] J Villasenor and H D Mansilla ldquoEffect of temperature onkraft black liquor degradation by ZnO-photoassisted catalysisrdquoJournal of Photochemistry and Photobiology A Chemistry vol93 no 2-3 pp 205ndash209 1996
[75] B H Bielski D E Cabelli L A Ravindra and A B RossldquoReactivity of HO
2Ominus2Radicals in aqueous solutionrdquo Journal
of Physical Chemistry vol 14 pp 1041ndash1100 1985[76] B H J Bielski and A O Allen ldquoMechanism of the dispropor-
tionation of superoxide radicalsrdquo Journal of Physical Chemistryvol 81 no 11 pp 1048ndash1050 1977
[77] B Halliwell and J M C Gutteridge ldquoThe importance of freeradicals and catalytic metal ions in human diseasesrdquoMolecularAspects of Medicine vol 8 no 2 pp 89ndash193 1985
[78] J M Palmer P J Harvey and H E Schoemaker ldquoThe role ofperoxidases radical cations and oxygen in the degradation oflignin [and discussion]rdquo Philosophical Transaction of the RoyalSociety A vol 321 no 1561 pp 495ndash505 1987
[79] S Y Yin and C W Dense Methods in Lignin ChemistrySpringer New York NY USA 1992
[80] P Kumar S Kumar and N K Bhardwaj ldquoPhotocatalyticoxidation of elemental chlorine free bleaching effluent withUVTiO
2rdquo in Proceedings of the 2nd International Conference on
Environmental Science and Technology (ICEST rsquo11) SingaporeFebruary 2011
[81] K Kobayakawa Y Sato S Nakamura and A FujishimaldquoPhotodecomposition of Kraft lignin catalyzed by titanium
18 International Journal of Photoenergy
dioxiderdquo Bulletin of the Chemical Society of Japan vol 62 no11 pp 3433ndash3436 1989
[82] R B Kinstre ldquoAn overview of strategies for reducing theenvironmental impact of bleach-plant effluentsrdquo Tappi Journalvol 76 no 5 pp 105ndash113 1993
[83] A Castellan H Choudhury R Stephen Davidson and SGrelier ldquoComparative study of stone-ground wood pulp andnative wood 3 Application of fluorescence spectroscopy to astudy of the weathering of stone-ground pulp and native woodrdquoJournal of Photochemistry and Photobiology A Chemistry vol81 no 2 pp 123ndash130 1994
[84] B Albinsson S Li K Lundquist and R Stomberg ldquoThe originof lignin fluorescencerdquo Journal of Molecular Structure vol 508no 1ndash3 pp 19ndash27 1999
[85] S Baumberger A Abaecherli M Fasching et al ldquoMolar massdetermination of lignins by size-exclusion chromatographytowards standardisation of the methodrdquo Holzforschung vol 61no 4 pp 459ndash468 2007
[86] E Torres andM Ayala Biocatalysis Based on Heme PeroxidasesSpringer New York NY USA 1st edition 2010
[87] R Pecina P Burtscher G Bonn and O Bobleter ldquoGC-MSand HPLC analyses of lignin degradation products in biomasshydrolyzatesrdquo Fresenius Zeitschrift fur Analytische Chemie vol325 no 5 pp 461ndash465 1986
[88] NIST NIST Standard Reference Database The National Insti-tute of Standards and Technology 2014 httpwwwnistgovsrdnist1acfm
[89] T K Kirk and R L Farrell ldquoEnzymatic ldquocombustionrdquo themicrobial degradation of ligninrdquo Annual Review of Microbiol-ogy vol 41 pp 465ndash501 1987
[90] A TMartinez ldquoHigh redox potential peroxidasesrdquo in IndustrialEnzymes Structure Function and Applications K-B BeckerEd pp 477ndash488 Springer Amsterdam The Netherlands 1stedition 2007
[91] M Dashtban H Schraft T A Syed and W Qin ldquoFungalbiodegradation and enzymatic modification of ligninrdquo Interna-tional Journal of Biochemistry and Molecular Biology vol 1 no1 pp 36ndash50 2010
[92] N Duran E Esposito L H Innocentini-Mei and V P CanhosldquoA new alternative process for Kraft E1 effluent treatmentrdquoBiodegradation vol 5 no 1 pp 13ndash19 1994
[93] K Kamwilaisak and P C Wright ldquoInvestigating laccase andtitanium dioxide for lignin degradationrdquo Energy and Fuels vol26 no 4 pp 2400ndash2406 2012
[94] J Reyes M Dezotti H Mansilla J Villasenor E Espositoand N Duran ldquoBiomass photochemistry-XXII combined pho-tochemical and biological process for treatment of Kraft E1effluentrdquoApplied Catalysis B Environmental vol 15 no 3-4 pp211ndash219 1998
[95] L F Gonzalez V Sarria and O F Sanchez ldquoDegradationof chlorophenols by sequential biological-advanced oxidativeprocess using Trametespubescens and TiO
2UVrdquo Bioresource
Technology vol 101 no 10 pp 3493ndash3499 2010[96] J H Lora and W G Glasser ldquoRecent industrial applications
of lignin a sustainable alternative to nonrenewable materialsrdquoJournal of Polymers and the Environment vol 10 no 1-2 pp 39ndash48 2002
Submit your manuscripts athttpwwwhindawicom
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Inorganic ChemistryInternational Journal of
Hindawi Publishing Corporation httpwwwhindawicom Volume 2014
International Journal ofPhotoenergy
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Carbohydrate Chemistry
International Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
Chemistry
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Advances in
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Hindawi Publishing Corporationhttpwwwhindawicom
Analytical Methods in Chemistry
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Volume 2014
Bioinorganic Chemistry and ApplicationsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
SpectroscopyInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Medicinal ChemistryInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Chromatography Research International
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Applied ChemistryJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
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Journal of
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Analytical ChemistryInternational Journal of
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Quantum Chemistry
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Organic Chemistry International
ElectrochemistryInternational Journal of
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CatalystsJournal of
10 International Journal of Photoenergy
321 Influence of Catalyst One aspect to successfully imple-ment photocatalysis is the choice of an appropriate photo-catalyst The majority of catalysts used are based on TiO
2as
summarized in Table 2 ZnO has also been applied either assingle catalyst [48] or in a combination with other catalystsuch as TiO
2[54] de Lasa et al [60] described ZnO as
being less active than TiO2 They add that the use of ZnO
is particularly relevant when the oxidative degradation ratebecomes limited This is opposite to what Kansal et al [48]reported by saying that ZnO is more reactive than TiO
2
However TiO2(mainly anatase) remains the most used
catalyst as can be depicted fromTable 2 TiO2is reported to be
favored because of its nontoxic property cost efficiency andchemical and biological inertness Moreover TiO
2possesses
the most efficient photoactivity and the highest stability thusmaking it suitable for industrial use [61]
ZnO has been described to degrade lignin under visiblelight sources [48 62] whereas TiO
2is mostly applied in
connection to UV-light sources as highlighted in Table 2However both ZnO and TiO
2possess energy band gap
energy of 32 eV [23]Additives such as SiO
2[63] polyethylene oxide (PEO)
[24] and polyethylene glycol [64] have been added to TiO2
catalyst particularly when applying immobilized catalystAddamo et al [63] noted a high adhesion of TiO
2to glass
support material when precoating was done with SiO2
Moreover the precoating might have other advantages suchas a hindering diffusion of Na+ ions from the glass materialinto the nascent TiO
2film during heat treatment processes
Analogous to the addition of SiO2 polyethylene glycol
(PEG) has also been introduced to mitigate catalyst surfaceactivity modify surface hydrophobicity and also reduceagglomeration tendency of the TiO
2gel or TiO
2particles
in the suspensions [64] By a proper surface modificationinteraction between catalyst and substrate can be enhanced[55 63]
Kansal et al [48] varied catalyst (ZnO) dose from 05 gLto 20 gL for 01 gL kraft lignin solutions and found out thatthere was an optimum catalyst threshold value at 1 gL whichgives a catalyst to substrate ratio of 1 10
Dahm and Lucia [49] examined catalyst dose from 2 sdot10minus3 gL to 12sdot10minus2 gL for lignin solutions (fromwhite water
liner mill) of 4 sdot 10minus2 gL (catalyst to lignin ratio 5 sdot 10minus3ndash3 sdot 10minus2) at pH 8 and obtained best energy efficiency valuesand lignin degradation rates with a catalyst loading of 10 sdot10minus2 gLIn contrast Ma et al [56] applied far higher catalyst
concentration compared to Dahm and Lucia [49] Catalystconcentration was varied between 1 gL and 10 gL PtTiO
2
Best catalysis dose with respect to reaction turnover wasobtained at 5 gL PtTiO
2With the increase of catalyst dose at
pH 7 the reaction rate increased from 61 times 10minus3minminus1 (1 gLTiO2) to 71 sdot 10minus3minminus1 (5 gL TiO
2) and 99 sdot 10minus3minminus1
(10 gL TiO2)
Catalyst effect has been explained on the basis thatoptimum catalyst loading is dependent on the initial soluteconcentration An increase in catalyst dosage leads to a cor-responding increase of total active surface area for reactions
[65] To that at higher TiO2concentrations the photon flux is
more easily intercepted by the catalyst before penetrating intothe bulk of the system At the same time due to an increase inturbidity of the suspension with high dose of photocatalystthere is a decrease in penetration of UV light and hencephotoactivated volume of suspension or solution decreases[66]
In summary authors have obtained best catalyst to ligninrelations for different reaction designs and thus a generalrecommendation on catalyst dose is not possible Howeverwhen lignin solution is treated with increasing catalyst loadsa corresponding increase in degradation rate is observed untila threshold value is reached [48ndash50 56]
322 Influence of Metal Ion Addition (Doping) and AdditivesThe purpose of adding metal ion to photocatalyst is to miti-gate band gap energy through the introduction of intrabandgap states and as a consequence produce a bathochromicshift in the absorption spectrum [37] Altering the absorptionspectral range gives the possibility to exploit both the visiblelight spectrum and UV light sources Metal ion doping isalso introduced to serve as electron or hole traps in orderto minimize recombination between generated electron-holepairs [37]
Portjanskaja and Preis [50] studied the addition of Fe2+ions to an acidic lignin solution and found an increasein photocatalytic oxidation (PCO) efficiency The optimumFe2+ ions quantity was 28mgL while using 100mgL ligninsolution Upon further elevation of Fe2+ ions concentrationa corresponding reduction of the photocatalytic oxidationefficiency of lignin was noted Likewise Ohnishi et al [57]made a comparative study by doping platinum (Pt) silver(Ag) and gold (Au) ions to TiO
2 In these reactions 50mg of
catalyst (TiO2) was used with the addition of an equiva1ent
15 wt (based on TiO2) metal ion The addition of noble
metals brought about a faster decolorization of lignin Aushowed better results than Ag followed by Pt In the samecontext adding sodium hypochlorite as oxidant to PtTiO
2
catalyst an additional fivefold degradation rate was observedcompared to that without doping [56] Contradictory to theresults described above negligible effect of photocatalyticefficiency due to doping has been reported as well Awun-gacha Lekelefac et al [54] obtained little or no change indegradation rate by doping TiO
2-P25-SiO
2catalyst with Pt
ions (1 wt relative to TiO2catalyst) Likewise Portjanskaja
and Preis [50] noted a negligible change of photocatalyticefficiency of TiO
2when doped with nitrogen
Sarkanen et al [67] and Gellerstedt and Lindfors [68]reported the bias of peroxides to oxidation with reagentssuch as permanganate to favor aromatic moieties Oxidationagents like permanganate oxidizes predominantly aliphaticchains in alkaline and neutral media However by theapplication of H
2O2(Fenton system) lignin disappeared
completely [33] Tonucci et al [33] conclude that in orderto satisfactorily conserve the organic material the best com-promise appears to be the TiO
2photosystem which shows
low carbon consumption good preservation of the aromaticrings and greatly reduced mineralization
International Journal of Photoenergy 11
In summary different results have been obtained con-cerning the influence of noble metal ion addition Whilesome authors report an improvement in the photocatalyticefficiency upon their addition others report their addition ashaving no considerable influence However for reactions inwhich an improvement in the photocatalytic efficiency wasnoticed there was a threshold value to be considered Whenthe concentration of dopant surpasses this threshold valueelectron-hole recombination is favored and this has a negativeimpact to photocatalysis In such a case the space-chargelayer gets narrower and 119901-type dopants attract electrons andby virtue become negative They would now then act ashole acceptor attracting holes On the other hand 119899-typedopants which act as electron donor centers and possessexcess electrons attract holes as well [37]
323 Influence of Lignin Concentration Once the initiallignin concentration becomes higher exceeding a thresholdvalue an inhibitory effect on the photodegradationwas noted[47ndash49] This threshold value varies and depends on thereaction system and reaction parameters such as opticaldensity catalyst concentration and reaction volume Fromthe literature different authors have implemented varyinglignin concentrations probably to suit their reaction designFor example Ksibi et al [47] use 90mgL and AwungachaLekelefac et al [54] use 500mgL while Kansal et al [48]applied 10mgndash100mgL
Explanations arising from the findings are as followsat low lignin concentrations the incidental photonic fluxirradiated interacts with the catalyst generating radicals (eghydroxyl radicals (OH∙)) which allow a faster degradation[69] On the other hand high initial lignin concentrationsmay lead to tight adsorption which can suppress CO
2
evolution [51] and hence maintain chemical oxygen demand(COD) values Moreover low delignification yields maybe due to an inhibitory effect because of autoxidation bylow molecular weight lignin degradation products formed[24] Also due to the polymer structure of lignin which iscross-linked this makes it difficult for radical species acidand the aldehyde compounds produced to spread into theinner region of the substrate hence limiting autoxidationAs a worst case this might be the rate-determining step ofdelignification which is hindered [70ndash72]
In summary it can be concluded that the time taken forcomplete degradation depends on the initial concentration oflignin and faster degradation occurs at low lignin concentra-tions
324 Influence of pH Varying pH entails an alteration inthe properties of semiconductor-liquid interface [73] mainlyrelated to the acid-base equilibrium of the adsorbed hydroxylgroup [39] Furthermore pH also impacts lignin degradationrates [47 48 57] In this context several studies were carriedout with partly contradictory outcomes
Kansal et al [48] made pH investigations 3ndash11 undersolar light illumination using ZnO as catalyst Maximumdegradation was reached in alkaline conditions (pH 11)This is supported by Villasenor and Mansilla [74] reporting
an almost complete decolorization of kraft black liquorfrom pine wood at pH value of 116 in combination withZnO catalyst Similar results were achieved by Ohnishi etal [57] with TiO
2and ZnO being catalyst for bleaching
alkaline lignin in aqueous solution with TiO2and ZnO being
catalyst High activities at neutral pH were also reportedby Ohnishi et al [57] In contrast Ma et al [56] observedhigher reaction rates and rapid degradation of a syntheticlignin wastewater (prepared by dissolving commercial ligninpowder in aqueous solution pH 11) in acidic solution (pH 3)than in alkaline solutions at pH 11 for either TiO
2or PtTiO
2
catalystsReconsidering the photocatalytic principle the formed
superoxide anion radicals (∙O2
minus) are in a pH-dependentequilibriumwith perhydroxyl radicals (HO
2
∙) as follows [75]
HO2
∙999445999468 H+ + ∙O
2
minus pKa sim 48 (9)
See [76]∙O2
minus undergoes dismutation reaction resulting in H2O2
and O2competing to any other ∙O
2
minus triggered reaction Incase of low pH operation conditions in aqueous solutionsHO2
∙ becomes dominant whose reactivity is considerablyhigher compared to ∙O
2
minus [77] Subsequently HO2
∙ initiatessubstrate (S) oxidation to the radical cation (S+∙) and is itselfreduced to H
2O2[78] Thus increased degradation rates can
be reasonably expected supporting the results made by Ma etal [56] in an acidic environment ∙O
2
minus is extremely reactive inorganic solvents [77] Another aspect is the solubility of kraftlignin (soluble at pH gt 105) which reduces with decreasingpH whereas lignosulfonate should remain unaffected by pHin aqueous solution Moreover 120573ndashOndash4 bonds have beendescribed to be stable at acidic pH [11] In fact this couldadditionally explain the elevated degradation of kraft ligninmade by Kansal et al [48] and Villasenor and Mansilla [74]Nevertheless the contradictory results gained by Ma et al[56] still exist under the assumption that kraft ligninwas used(which would be supported by the high pH of 11 obviouslynecessary for dissolving the lignin powder) Although mostphotocatalytic reactions described in the literature are in anaqueous milieu lignin raw material and its fission productsmay however vary considerably Therefore the optimal pHis most likely to be reaction specific and has to be evaluatedexperimentally in principle
325 Influence of Illumination Many of the studies foundin the literature so far have not dealt on this subject per seWhat is found is the use of different illumination sourceseach having a specified power and lamp type However alltend to emitUV-light between the range 280ndash420 nm Table 2depicts this in detail Other illumination sources include thevisible light spectrum
In general terms illumination influences in that it ini-tiates photocatalysis by generating electron-hole pair in thesemiconductor particles [38 39] Dahm and Lucia [49]altered illumination intensity while observing lignin degra-dation In this study 004 gL lignin was used and lightintensity was varied 223ndash445mWcm3 It was found out thathigher illumination intensities correlated well with higher
12 International Journal of Photoenergy
initial degradation rates and hence total lignin degradation[49] Neppolian et al [69] report degradation to be propor-tional to radiation intensity and best results are achieved forlow lignin concentrations because of enhanced interactionbetween catalyst and incidental photonic flux
In summary high illumination power causes a corre-sponding high initial degradation rate at low lignin concen-trations becausemaximum light penetration into the reactionmedium is favored
33 Process Analytical Methods Various analytical tech-niques have been used to monitor lignin degradation Atthe beginning of this subchapter analytics revealing com-pounds formed from lignin degradation are treated Thisincludes for example gas chromatography (GC) and 1HNMR (nuclear magnetic resonance) This is then followed byresults qualitative analytic measurements such as ultraviolet-visible (UV-Vis) spectroscopy and dissolved carbon (DC)A list of authors analytical techniques applied and resultsachieved are outlined in Table 3
Portjanskaja and Preis [50] studied lignin degradationby measuring the removal of phenols through colorimetricmeasurements As a result of 24 h photocatalytic oxidationunder neutral media conditions 80 of free phenols wereremoved Gas chromatography (GC) result from Ksibi et al[47] attested vanillin vanillic acid palmitic acid biphenyland 345-trimethoxy benzaldehyde structures after the pho-tocatalysis of lignin from black liquor This is in accordancewith the findings of Tonucci et al [33] reporting the formationof vanillin hydroxyl methoxy-acetophenone coniferyl alco-hol coniferyl aldehyde methanol formic acid acetic acidand small amounts of C-2 and C-3 alcohols as degradationproducts1H NMR spectral analysis of lignin before illumination
and after 24 h of illumination showing characteristic bandsof aromatic rings methoxy and aliphatic side chains wascompared with each other Results revealed that the aro-matic ring degraded faster than the aliphatic chain [51]Fourier transformation infrared spectroscopy (FTIR) showedbands corresponding to CH
3 CH2 and CH which remained
unchanged after illumination while bands corresponding toaromatic rings disappeared as a result of illumination [51 53]
Results obtained from the combination of photochemicaland electrochemical oxidation [52 53] were similar to thoseof Tanaka et al [51] Here 13C-NMR confirmed the presenceof the carbonyl functionality and the presence of vanillinand vanillic acid after 12 h photochemical-electrochemicaloxidation These results showed that the combination of aphotocatalytic and an electrochemical oxidation significantlyenhanced the efficiency of lignin degradationThis is becausethe applied anodic potential bias greatly suppressed therecombination of photogenerated electrons and holes [53]
Ultraviolet spectrophotometry offers a convenientmethod to qualitatively and quantitatively analyze ligninin solution [79] This is reflected by the large number ofpublications using this technique [17 33 47ndash51 56 58 80]This is most likely due to its simplicity to interpret lignindegradation Lignins absorb UV light with high molar
200 220 240 260 280 300 320 340 360
00
05
10
15
20
25
Abso
rban
ce
Wavelength (nm)0min
30min
60min
120min
180min
300min
360min
420min
1200min
Figure 8 Time dependent UV-Vis absorption spectra of aqueouslignin solution from waste paper water irradiated with UV light(280ndash420 nm) for different time intervals The spectra are obtainedfor sol-gel derived TiO
2nanocrystalline coating (TiO
2-P25-SiO
2)
[54]
extinction coefficients because of the several methox-ylated phenylpropane units of which they are composed [33]Figure 8 depicts a series of photometric scans of ligninsul-fonate from paper waste water showing a gradual reductionof absorbance during photocatalytic treatment [54] Herethe absorption peaks are around 210 nm and 280 nmAbsorbance decreases with time implying the decompositionof lignin and the deterioration of chromophore groups [54]
Peaks at 210 nmcorrespond to portions of the unsaturatedchains while those at 280 nm correspond to unconjugatedphenolic hydroxyl groups [17] and the aromatic moiety [57]of the lignin molecule The absorption tailing to the longwavelength region arises from the color of lignin [57] Lignindegradation has been reported either at wavelength around280 nm corresponding to unconjugated phenolic hydroxylgroups [17 48 51 58] or for both wavelengths (210 nm and280 nm) [33 54 57] Kobayakawa et al [81] noted some otherabsorbance at wavelengths lower than 250 nm and pointedout that this could be due to the modification of ligninfragmentations leading to the formation of transient specieslikemethanol ethanol formaldehyde formic acid and oxalicacid among others
Analyticalmethods to effectively quantify lignin degrada-tion by calculating the oxygen demand by organic substancesand remaining organic carbon before and after photocatalysishave been studied Amongst the methods are dissolvedcarbon (DC) [49 51 58] chemical oxygen demand (COD)[47 48 50 57 80] biochemical oxygen demand (BOC) [50]dissolved organic carbon (DOC) [56] and American dyemanufacture institute value (ADMI) [56] COD and BODdescribe the oxygen demand by organic substances to be
International Journal of Photoenergy 13
Figure 9 Gradual change from the characteristic yellow ligninsulfonate to a colorless liquid after a period of 20 h Catalyst TiO
2-
P25 (Degussa) +TEOSUV-light 25∘C lignin concentration 05 gL[54]
converted to CO and CO2and H
2O and NH
3 Total organic
carbon (TOC) describes the amount of carbon bound inan organic compound while DOC describes the dissolvedfraction of organic carbon ADMI measures the amount ofdyestuff in water
Decolorization of lignin solution has been reported tobe another parameter observed during photocatalytic degra-dation Color is an indirect indicator of the lignin amountThe higher the color intensity of the solution is the greaterthe lignin content is (high concentrated lignin solutions egblack liquor appear dark brown) [82] Thus color changescan be interpreted as conversion of lignin to transient speciesor conversion to CO
2and H
2O Awungacha Lekelefac et al
[54] observed a gradual change from the characteristic yellowlignin (when highly diluted) to a colorless liquid after a periodof 20 h with sol-gel derived TiO
2nanocrystalline coatings on
sintered borosilicate glass as depicted in Figure 9 A corre-sponding decrease in DC values close to 82 was observedfor TiO
2-P25-SiO
2catalyst confirming degradation This is
shown in Figure 10These findings are analogous to that of Ohnishi et al
[57] who reported the bleaching of lignin when illuminatedcontinuously and that the solution becomes colorless Tothat the chemical oxygen demand (COD) value decreasesgenerating carbon dioxide and a small amount of carbonmonoxide as the main gaseous products COD removalwas reported to be effective at low lignin concentrations ascompared to high lignin concentrations [48]
Another applied analytical technique is fluorescencedetection directly coupled to a high performance liquidchromatography (HPLC) as a means to identify nonaliphaticcomponent in the complex mixture of lignin degradationproducts [54] Fluorescence emission in lignin is attributedto aromatic structures such as conjugated carbonyl biphenylphenylcoumarone and stilbene groups [83 84] AwungachaLekelefac et al [54] observed peaks on both HPLC and
0 200 400 600 800 1000 12000
50
100
150
200
250
Diss
olve
d ca
rbon
(ppm
)
t (min)
TiO2-P25-SiO2
TiO2-P25-SiO2 + PtTiOSO4minus30 6wt
ZnO + TiO2-P25-SiO2
No coating UV light
Figure 10 Variation of DC with time of aqueous lignin solutionfrom waste paper water irradiated with UV light (280ndash420 nm) fordifferent time intervals The spectra are obtained for sol-gel derivedTiO2nanocrystalline coatings (TiO
2-P25-SiO
2+Pt TiO
2-P25-SiO
2
TiOSO4minus306 wt ZnO + TiO
2-P25-SiO
2) [54]
fluorescence chromatograms suggesting the production ofnew substances and fluorophores
Despite developed analytical technologies analyzinglignin degradation products remains challenging Proofssuch as mass spectroscopy (MS) HPLC 13C or 1H-NMRspectra from photocatalytic lignin degradations are not yetestablished The setback to qualitatively and quantitativelyanalyze lignin and its degradation products starts from thenative lignin polymer itself with its indefinite polymericstructure and multiple bond types Also the influence ofdifferent pretreatments additives and the wide variety ofcompounds obtainable from its degradation makes ligninanalysis challenging [85] Moreover lignin streams couldcontain proteins inorganic salts and other potential poisonsthat generally complicate catalysis [4]
The challenge to identify and separate the productsstreams derived from the photocatalytic degradation ligninis also worth noting Lignin product stream is highly func-tionalized and conventional techniques such as gas chro-matography have the disadvantage of requiring a time-consuming derivatization step Also because of high boilingpoint of substances arising from lignin degradation it is noteasily applicable High performance liquid chromatography(HPLC) seems to be the remedy because analysis can becarried out without derivatization but the exact identificationof the separated substances is difficult because of the numer-ous peaks arising from such a chromatogram [87] Unfortu-nately well-established databases such as that of the nationalinstitute of standard and technology (NIST) [88] cannot giveinformation onHPLC-MS chromatogramsThis is because ofthe ionization sources such as electrospray ionization (ESI)
14 International Journal of Photoenergy
Table 4 Main advantages and disadvantages photocatalysis versus enzymatic biocatalysis
Lignin degradation process Advantages Disadvantages
Photocatalysis(single stage)
(i) Relatively fast degradation of complex as wellas nonbiodegradable organic (macro)molecules(ii) Stable catalysts(iii) Moderate reaction conditions
(i) Not selective(ii) Limited selection of operating conditions(iii) Energy costs
Enzymatic conversion(single stage)
(i) More selective(ii) Moderate reaction conditions
(i) High enzyme loads required otherwise ligninhydrolysis is quite slow [16](ii) Enzymes are less stable1(iii) cost-intensive (POXs)2
1For example POXs are sensitive to high H2O2 concentrations2According to Torres and Ayala [86]
and atmospheric pressure chemical ionization (APCI) forLC-MS while GC-MS is based on electron ionization Aremedy to this can be the development of a database ofmodel lignin compounds based on a unanimous HPLC-MSmeasuring procedure This would mean much time and costexpensive investments for adequate personnel and material
Though product identification based on peak superpo-sition gives information on a possible product this can-not always be true for lignin degradation because of thelarge number of possible degradation products In order toproperly identify products from lignin degradation productisolation techniques are to be implemented and further ana-lytic methods such as tandem mass spectroscopy (MSMS)and other advanced structure enhancing research techniquessuch as 1H-NMR 13C-NMR have to be studied extensively
4 Photocatalysis and Its Suitability asIntegrated Technology in MultistageConcepts with Biocatalysis
Depending on lignin treatment specification photocatalysiswill find application for complete lignin mineralization toCO2and H
2O (single stage) or as integrated technology in
a multistage system aiming at partial conversion of ligninmacromolecules to organic low molecular weight intermedi-ates for a consecutive biological oxidation finally generatingvalue added end-products [15]
For biological oxidation of lignin or lignin deriva-tives H
2O2-dependent ligninolytic heme peroxidases (POXs
including lignin peroxidase (LiP EC 111114) manganeseperoxidase (MnP EC 111113) versatile peroxidase (VPEC 111116) O
2-dependent laccases (Lac EC 11032) and
extracellular enzymes from Basidiomycetous white-rot fungiare the most efficient lignin degraders in nature [89] Thussuch enzymatic systems especially POXs (with high redoxpotentials) have attractedmuch interest as industrial biocata-lyst [86 90]The enzyme degradationmechanism (in nature)is facilitated by nonenzymatic processes mainly throughfree ∙OH radicals also generated by the fungus (Fenton-type reaction) Those ∙OH radicals enable the requiredphysical contact between the enzymes and structural units ofthe lignin molecule due to numerous nonspecific oxidative
reactions [91] Both the photocatalytic (equations (2)ndash(7))and the POX reactionmechanism recently reviewed by Busseat al [25] are quite similar Consequently a combination ofphotocatalysis followed by an enzymatic oxidation maybe apromising concept utilizing lignin derivatives for exampleoriginated from industrial effluents [15 92] The advantagesforming biobased products are as follows see also Table 4 inthis context A reduction of the lignin polymerization degreevia photocatalysis at best in more biodegradable interme-diates and a simultaneous detoxification causes savings inenzyme costs since it will be expected that less enzyme loadsare necessary for sufficiently rapid reaction rates [15 91] As aresult hydraulic retention times in bioreactor systems shouldbe diminished Moreover the delignification is expectedto be enhanced simultaneously which was in recent yearsshown in a dual system by Kamwilaisak and Wright [93]using TiO
2H2O2UV for photocatalytic pretreatment and
Lac (from Trametes versicolor) in the subsequent biocatalyticstep Within 24 h they obtained in their dual system anelevation in delignification of 20 (without H
2O2) up to at
least 50 when H2O2was present
In previous studies the treatment of lignin (from thepulp and paper industry) containing effluents by fungus asa biological system (excreting appropriate lignin degradingenzyme cocktails) were more focused for posttreatmentexclusively (Duran et al [92] Reyes et al [94] and Gonzalezet al [95]) Shende et al [17] even examined ligninolyticbacteria with photooxidized kraft lignin as substrate
Although POXs are potential industrial biocatalysts [86]no application studies were found for the direct use in suchdual systems as described above The major reason may bethe complexity of the reaction mechanism (inclusive ligninderivatives as substrate and their analysis) per se on the onehand slowing down research and development processes Onthe other hand POXs are sensitive to their cosubstrate H
2O2
once it is supplied in excess causing considerable inactivation(for details refer to Busse et al [25]) At the present severalstudies are carried out modifying these enzymes regardingenhanced stability activity and selectivity as well Henceit can be expected that their technological applicability willbe raised significantly right after successful modification isreached [86]
International Journal of Photoenergy 15
5 Concluding Remarks
It is widely assumed that the photocatalytic degradation oflignin follows a radical reaction pathway which is similar tothat considered in thermal electrochemical and biochemicalprocesses However reporting on the degradation pathway oflignin derivatives and even that of lignin model compoundsare still amajor challengeThis is probably due to the complexnature and variety of possible degradation products Indeedthemechanism is far more complex considering other factorssuch as type of lignin type of catalyst pH illuminationsource and additives
Generally comparing the different photochemical pro-cesses poses a big challenge because of the wide variablesinvolved These discrepancies start from the source and typeof lignin followed by differences in reactor design illumi-nation source intensity of radiation and different types ofcatalyst An ideawould be to have a specific reference reactionwith well-defined starting parameters which include lignintype source and purity catalyst specifications illuminationsource and intensity so as to ease comparison of results
Basic process parameters such as catalyst concentrationsubstrate concentration addition of metal ion to catalyst pHand illumination have been discussed
Despite developed analytical technologies analyzinglignin degradation products remains challenging Proofssuch as mass spectroscopy (MS) HPLC 13C or 1H-NMRspectra from photocatalytic lignin degradations are not yetestablished
In order to properly identify products from lignin degra-dation product isolation techniques are to be implementedand further analytic methods such as MSMS and otheradvanced structure enhancing research techniques such as1H-NMR and 13C-NMR have to be studied extensively Aremedy to this can be the development of a databank ofmodel lignin compounds based on a unanimous HPLC-MSmeasuring procedure This would mean much time and costexpensive investments for adequate personnel and material
Photocatalysis is denoted as the most popular ligninpretreatment technology besides ozonation [96] Photo-catalyzed lignin may be an appropriate substrate for aconsecutive biocatalytic process using ligninolytic enzymes(POX andor Lac) as supported by experimental results ofKamwilaisak and Wright [93] Combining the advantages ofboth catalytic processes savings in the overall process costswill be expected in addition to elevated lignin conversionNonetheless extensive research work including POX modi-fications is still required
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Authorsrsquo Contribution
Colin Awungacha Lekelefac and Nadine Busse contributedequally to this work
Acknowledgments
The authors gratefully thank the Federal Ministry of Educa-tion and Research (BMBF) for funding (FKZ17N0310) Theresearchers also thank the Hessen State Ministry of HigherEducation Research andArts for the financial support withinthe Hessen initiative for scientific and economic excellence(LOEWE)
References
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[4] J Zakzeski P C Bruijnincx A L Jongerius and B M Weck-huysen ldquoThe catalytic valorization of lignin for the productionof renewable chemicalsrdquo Chemical Reviews vol 110 no 6 pp3552ndash3599 2010
[5] B KammM Kamm P R Gruber and S KromusBiorefineries-Industrial Processes and Products Status Quo and Future Direc-tions vol 1 Wiley-VCH 2006
[6] R L Howard E Abotsi E L J van Rensburg and S HowardldquoLignocellulose biotechnology issues of bioconversion andenzyme productionrdquoAfrican Journal of Biotechnology vol 2 no12 pp 602ndash619 2003
[7] M Stocker ldquoBio- und BTL-Kraftstoffe in der Bioraffineriekatalytische Umwandlung Lignocellulose-reicher Biomasse mitporosen Stoffenrdquo Angewandte Chemie vol 120 no 48 pp9340ndash9351 2008
[8] J-P Lange ldquoLignocellulose conversion an introduction tochemistry process and economicsrdquo Biofuels Bioproducts andBiorefining vol 1 no 1 pp 39ndash48 2007
[9] D A I Goring ldquoThe physical chemistry of ligninrdquo Pure andApplied Chemistry vol 5 no 1-2 pp 233ndash310 1962
[10] M Ek G Gellerstedt and G HenrikssonWood Chemistry andBiotechnology Pulp and Paper Chemistry and Technology vol 1Walter de Gruyter GmbH amp Co KG Berlin Germany 2009
[11] B Saake and R Lehnen ldquoLigninrdquo in Ullmannrsquos Encyclopedia ofIndustrial Chemistry Wiley-VCH Verlag GmbH amp Co KGaAWeinheim Germany 2012
[12] M N S Kumar A K Mohanty L Erickson and M MisraldquoLignin and its applications with polymersrdquo Journal of BiobasedMaterials and Bioenergy vol 3 no 1 pp 1ndash24 2009
[13] R J A Gosselink E de Jong B Guran and A AbacherlildquoCo-ordination network for ligninmdashstandardisation produc-tion and applications adapted to market requirements(EUROLIGNIN)rdquo Industrial Crops and Products vol 20 no 2pp 121ndash129 2004
[14] D Fengel and G Wegener Wood Chemistry UltrastructureReactions Verlag Kessel 1984
[15] D Mantzavinos and E Psillakis ldquoEnhancement of biodegrad-ability of industrial wastewaters by chemical oxidation pre-treatmentrdquo Journal of Chemical Technology and Biotechnologyvol 79 no 5 pp 431ndash454 2004
16 International Journal of Photoenergy
[16] M Dashtban H Schraft and W Qin ldquoFungal bioconversionof lignocellulosic residues Opportunities amp perspectivesrdquo Inter-national Journal of Biological Sciences vol 5 no 6 pp 578ndash5952009
[17] A Shende R Jaswal D Harder-Heinz A Menan and RShende ldquoIntergrated photocatalytic and microbial degradationof kraft ligninrdquo Cleantech pp 120ndash123 2012
[18] F S Chakar and A J Ragauskas ldquoReview of current and futuresoftwood kraft lignin process chemistryrdquo Industrial Crops andProducts vol 20 no 2 pp 131ndash141 2004
[19] W G Glasser and H R Glasser ldquoEvaluation of ligninrsquoschemical structure by experimental and computer simulationtechniquesrdquo Paperi ja Puu vol 63 pp 71ndash83 1981
[20] M Erickson S Larsson and G E Miksche ldquoZur Struktur desLignins der FichterdquoActaChemica Scandinavica vol 27 pp 903ndash914 1973
[21] H Nimz ldquoDas Lignin der BuchemdashEntwurf eines Konstitution-sschemasrdquo Angewandte ChemiemdashInternational Edition vol 86pp 336ndash344 1974
[22] D V Evtuguin C P Neto J Rocha and J D P de JesusldquoOxidative delignification in the presence of molybdovana-dophosphate heteropolyanions mechanism and kinetic stud-iesrdquo Applied Catalysis A General vol 167 no 1 pp 123ndash1391998
[23] A L Linsebigler G Lu and J T Yates Jr ldquoPhotocatalysis onTiO2surfaces Principles mechanisms and selected resultsrdquo
Chemical Reviews vol 95 no 3 pp 735ndash758 1995[24] Y Miyata K Miyazaki M Miura Y Shimotori M Aoyama
and H Nakatani ldquoSolventless delignification of wood flourwith TiO
2poly(ethylene oxide) photocatalyst systemrdquo Journal
of Polymers and the Environment vol 21 no 1 pp 115ndash121 2013[25] N Busse D Wagner M Kraume and P Czermak ldquoReaction
kinetics of versatile peroxidase for the degradation of lignincompoundsrdquoTheAmerican Journal of Biochemistry andBiotech-nology vol 9 no 4 pp 365ndash394 2013
[26] M Tien and T K Kirk ldquoLignin-degrading enzyme fromphanerochaete chrysosporium Purification characterizationand catalytic properties of a uniqueH
2O2-requiring oxygenaserdquo
Proceedings of the National Academy of Sciences vol 81 pp2280ndash2284 1984
[27] T K Kirk M Tien P J Kersten M D Mozuch and BKalyanaraman ldquoLigninase of Phanerochaete chrysosporiumMechanism of its degradation of the non-phenolic arylglycerol120573-aryl ether substructure of ligninrdquo Biochemical Journal vol236 no 1 pp 279ndash287 1986
[28] T Lundell RWever R Floris et al ldquoLignin peroxidase L3 fromPhlebia radiata pre-steady-state and steady-state studies withveratryl alcohol and a non-phenolic lignin model compound1-(34-dimethoxyphenyl)-2-(2-methoxyphenoxy)propane-13-diolrdquo European Journal of Biochemistry vol 211 no 3 pp391ndash402 1993
[29] H E Schoemaker T K Lundell A I Hatakka and K PiontekldquoThe oxidation of veratryl alcohol dimeric lignin models andlignin by lignin peroxidase the redox cycle revisitedrdquo FEMSMicrobiology Reviews vol 13 no 2-3 pp 321ndash331 1994
[30] J M Palmer P J Harvey and H E Schoemaker ldquoThe role ofperoxidases radical cations and oxygen in the degradation oflignin [and discussion]rdquo Philosophical Transactions of the RoyalSociety of London A vol 321 no 1561 pp 495ndash505 1987
[31] C Hill Wood Modification Chemical Thermal and OtherProcesses John Wiley amp Sons Chichester UK 2006
[32] Y-H Xu H-R Chen Z-X Zeng and B Lei ldquoInvestigation onmechanism of photocatalytic activity enhancement of nanome-ter cerium-doped titaniardquo Applied Surface Science vol 252 no24 pp 8565ndash8570 2006
[33] L Tonucci F Coccia M Bressan and N DrsquoAlessandro ldquoMildphotocatalysed and catalysed green oxidation of lignin auseful pathway to low-molecular-weight derivativesrdquoWaste andBiomass Valorization vol 3 no 2 pp 165ndash174 2012
[34] A Castellan N Colombo C Vanucci P Fornier de HViolet and H Bouas-Laurent ldquoPhotodegradation of ligninA photochemical study of an O-methylated 120572-carbonyl 120573-1lignin model dimer 12-di(3101584041015840-dimethoxyphenyl) ethanone(deoxyveratroin)rdquo Journal of Photochemistry and PhotobiologyA vol 51 pp 451ndash467 1990
[35] A Fujishima X Zhang and D A Tryk ldquoTiO2photocatalysis
and related surface phenomenardquo Surface Science Reports vol63 no 12 pp 515ndash582 2008
[36] C S Turchi and D F Ollis ldquoPhotocatalytic reactor designan example of mass-transfer limitations with an immobilizedcatalystrdquo The Journal of Physical Chemistry vol 92 no 23 pp6852ndash6853 1988
[37] R Subasri M Tripathi K Murugan J Revathi G V N Raoand T N Rao ldquoInvestigations on the photocatalytic activity ofsolndashgel derived plain and Fe3+Nb5+ doped titania coatings onglass substratesrdquo Materials Chemistry and Physics vol 124 pp63ndash68 2010
[38] N Serpone ldquoRelative photonic efficiencies and quantum yieldsin heterogeneous photocatalysisrdquo Journal of Photochemistry andPhotobiology A Chemistry vol 104 no 1ndash3 pp 1ndash12 1997
[39] M R Hoffmann S T Martin W Choi and D W BahnemannldquoEnvironmental applications of semiconductor photocatalysisrdquoChemical Reviews vol 95 no 1 pp 69ndash96 1995
[40] C D Jaeger and A J Bard ldquoSpin trapping and electron spinresonance detection of radical intermediates in the photode-composition of water at titanium dioxide particulate systemsrdquoThe Journal of Physical Chemistry vol 83 no 24 pp 3146ndash31521979
[41] R W Matthews ldquoHydroxylation reactions induced by near-ultraviolet photolysis of aqueous titaniumdioxide suspensionsrdquoJournal of the Chemical Society Faraday Transactions vol 80no 2 pp 457ndash471 1984
[42] A E H Machado A M Furuyama S Z Falone R RuggieroD D S Perez and A Castellan ldquoPhotocatalytic degradation oflignin and lignin models using titanium dioxide the role of thehydroxyl radicalrdquoChemosphere vol 40 no 1 pp 115ndash124 2000
[43] W Sigg and L Stumm Aquatische Chemie Stuttgart 2 AuflageBG Teubner Stuttgart Germany 1991
[44] O Legrini E Oliveros and A M Braun ldquoPhotochemicalprocesses for water treatmentrdquo Chemical Reviews vol 93 no2 pp 671ndash698 1993
[45] G Rothenberger J Moser M Gratzel N Serpone and DSharma ldquoCharge carrier trapping and recombination dynamicsin small semiconductor particlesrdquo Journal of the AmericanChemical Society vol 107 no 26 pp 8054ndash8059 1985
[46] P Mazellier M Sarakha A Rossi and M Bolte ldquoThe aqueousphotochemistry of 26-dimethylphenol Evidence for the frag-mentation of the 120572 C-C bondrdquo Journal of Photochemistry andPhotobiology A vol 115 no 2 pp 117ndash121 1998
[47] M Ksibi S B Amor S Cherif E Elaloui A Houas and MElaloui ldquoPhotodegradation of lignin from black liquor using aUVTiO
2systemrdquo Journal of Photochemistry and Photobiology
A vol 154 no 2-3 pp 211ndash218 2003
International Journal of Photoenergy 17
[48] S K Kansal M Singh and D Sud ldquoStudies on TiO2ZnO
photocatalysed degradation of ligninrdquo Journal of HazardousMaterials vol 153 no 1-2 pp 412ndash417 2008
[49] A Dahm and L A Lucia ldquoTitanium dioxide catalyzed pho-todegradation of lignin in industrial effluentsrdquo Industrial andEngineering Chemistry Research vol 43 no 25 pp 7996ndash80002004
[50] E Portjanskaja and S Preis ldquoAqueous photocatalytic oxidationof lignin the influence of mineral admixturesrdquo InternationalJournal of Photoenergy vol 2007Article ID76730 7 pages 2007
[51] K Tanaka R C R Calanag and T Hisanaga ldquoPhotocatalyzeddegradation of lignin on TiO
2rdquo Journal of Molecular Catalysis
A Chemical vol 138 no 2-3 pp 287ndash294 1999[52] M Tian J Wen D MacDonald R M Asmussen and A Chen
ldquoA novel approach for lignin modification and degradationrdquoElectrochemistry Communications vol 12 no 4 pp 527ndash5302010
[53] K PanM Tian Z-H Jiang B Kjartanson and A Chen ldquoElec-trochemical oxidation of lignin at lead dioxide nanoparticlesphotoelectrodeposited on TiO
2nanotube arraysrdquo Electrochim-
ica Acta vol 60 pp 147ndash153 2012[54] C Awungacha Lekelefac J Hild P Czermak and M Herren-
bauer ldquoPhotocatalytic active coatings for lignin degradationin a continuous packed bed reactorrdquo International Journal ofPhotoenergy vol 2014 Article ID 502326 10 pages 2014
[55] C Awungacha Lekelefac P Czermak and M HerrenbauerldquoEvaluation of photocatalytic active coatings on sintered glasstubes by methylene bluerdquo International Journal of Photoenergyvol 2013 Article ID 614567 9 pages 2013
[56] Y-S Ma C-N Chang Y-P Chiang H-F Sung and A CChao ldquoPhotocatalytic degradation of lignin using PtTiO
2as
the catalystrdquo Chemosphere vol 71 no 5 pp 998ndash1004 2008[57] H Ohnishi M Matsumura H Tsubomura and M Iwasaki
ldquoBleaching of lignin solution by a photocatalyzed reactionon semiconductor photocatalystsrdquo Industrial and EngineeringChemistry Research vol 28 no 6 pp 719ndash724 1989
[58] C A K Gouvea F Wypych S G Moraes N Duran and PPeralta-Zamora ldquoSemiconductor-assisted photodegradation oflignin dye and kraft effluent by Ag-doped ZnOrdquo Chemospherevol 40 no 4 pp 427ndash432 2000
[59] A V Vahatalo K Salonen M Salkinoja-Salonen and AHatakka ldquoPhotochemical mineralization of synthetic lignin inlake water indicates enhanced turnover of aromatic organicmatter under solar radiationrdquo Biodegradation vol 10 no 6 pp415ndash420 1999
[60] H de Lasa B Serrano and M Salaices Photocatalytic ReactionEngineering Springer New York NY USA 2005
[61] K Hashimoto H Irie and A Fujishima ldquoTiO 2 photocatalysisA historical overview and future prospectsrdquo Japanese Journal ofApplied Physics vol 44 no 12 pp 8269ndash8285 2005
[62] M A Behnajady N Modirshahla and R Hamzavi ldquoKineticstudy on photocatalytic degradation of CI Acid Yellow 23 byZnOphotocatalystrdquo Journal ofHazardousMaterials vol 133 no1ndash3 pp 226ndash232 2006
[63] M Addamo V Augugliaro A di Paola et al ldquoPhotocatalyticthin films of TiO
2formed by a sol-gel process using titanium
tetraisopropoxide as the precursorrdquo Thin Solid Films vol 516no 12 pp 3802ndash3807 2008
[64] N Negishi K Takeuchi and T Ibusuki ldquoPreparation of theTiO2thin filmphotocatalyst by the dip-coating processrdquo Journal
of Sol-Gel Science and Technology vol 13 no 1ndash3 pp 691ndash6941998
[65] L Rideh A Wehrer D Ronze and A Zoulalian ldquoPhotocat-alytic degradation of 2-chlorophenol in TiO
2aqueous suspen-
sion modeling of reaction raterdquo Industrial and EngineeringChemistry Research vol 36 no 11 pp 4712ndash4718 1997
[66] R-ADoongC-HChen RAMaithreepala and S-MChangldquoThe influence of pHand cadmiumsulfide on the photocatalyticdegradation of 2-chlorophenol in titanium dioxide suspen-sionsrdquoWater Research vol 35 no 12 pp 2873ndash2880 2001
[67] K V Sarkanen A Islam and C D Anderson ldquoOzonationrdquo inMethods in Lignin Chemistry S Y Lin and C W Dence EdsSpringer Series inWood Science pp 387ndash406 Springer BerlinGermany 1992
[68] G Gellerstedt and E-L Lindfors ldquoStructural changes in ligninduring kraft pulpingrdquo Holzforschung vol 38 no 3 pp 151ndash1581984
[69] B Neppolian H C Choi M V Shankar B Arabindoo andV Murugesan in Proceedings of the International Symposiumon Environmental Pollution Control and Waste Management(EPCOWM 02) p 647 2002
[70] C Pouteau P Dole B Cathala L Averous and N BoquillonldquoAntioxidant properties of lignin in polypropylenerdquo PolymerDegradation and Stability vol 81 no 1 pp 9ndash18 2003
[71] J Hafren T Fujino and T Itoh ldquoChanges in cell wall archi-tecture of differentiating tracheids of Pinus thunbergii duringlignificationrdquo Plant and Cell Physiology vol 40 no 5 pp 532ndash541 1999
[72] T Dizhbite G Telysheva V Jurkjane and U Viesturs ldquoChar-acterization of the radical scavenging activity of lignins naturalantioxidantsrdquo Bioresource Technology vol 95 no 3 pp 309ndash3172004
[73] K Hofstadler R Bauer S Novalic and S G Heisier ldquoNew reac-tor design for photocatalytic wastewater treatment with TiO
2
immobilized on fused-silica glass fibers photomineralization of4-chlorophenolrdquo Environmental Science amp Technology vol 28no 4 pp 670ndash674 1994
[74] J Villasenor and H D Mansilla ldquoEffect of temperature onkraft black liquor degradation by ZnO-photoassisted catalysisrdquoJournal of Photochemistry and Photobiology A Chemistry vol93 no 2-3 pp 205ndash209 1996
[75] B H Bielski D E Cabelli L A Ravindra and A B RossldquoReactivity of HO
2Ominus2Radicals in aqueous solutionrdquo Journal
of Physical Chemistry vol 14 pp 1041ndash1100 1985[76] B H J Bielski and A O Allen ldquoMechanism of the dispropor-
tionation of superoxide radicalsrdquo Journal of Physical Chemistryvol 81 no 11 pp 1048ndash1050 1977
[77] B Halliwell and J M C Gutteridge ldquoThe importance of freeradicals and catalytic metal ions in human diseasesrdquoMolecularAspects of Medicine vol 8 no 2 pp 89ndash193 1985
[78] J M Palmer P J Harvey and H E Schoemaker ldquoThe role ofperoxidases radical cations and oxygen in the degradation oflignin [and discussion]rdquo Philosophical Transaction of the RoyalSociety A vol 321 no 1561 pp 495ndash505 1987
[79] S Y Yin and C W Dense Methods in Lignin ChemistrySpringer New York NY USA 1992
[80] P Kumar S Kumar and N K Bhardwaj ldquoPhotocatalyticoxidation of elemental chlorine free bleaching effluent withUVTiO
2rdquo in Proceedings of the 2nd International Conference on
Environmental Science and Technology (ICEST rsquo11) SingaporeFebruary 2011
[81] K Kobayakawa Y Sato S Nakamura and A FujishimaldquoPhotodecomposition of Kraft lignin catalyzed by titanium
18 International Journal of Photoenergy
dioxiderdquo Bulletin of the Chemical Society of Japan vol 62 no11 pp 3433ndash3436 1989
[82] R B Kinstre ldquoAn overview of strategies for reducing theenvironmental impact of bleach-plant effluentsrdquo Tappi Journalvol 76 no 5 pp 105ndash113 1993
[83] A Castellan H Choudhury R Stephen Davidson and SGrelier ldquoComparative study of stone-ground wood pulp andnative wood 3 Application of fluorescence spectroscopy to astudy of the weathering of stone-ground pulp and native woodrdquoJournal of Photochemistry and Photobiology A Chemistry vol81 no 2 pp 123ndash130 1994
[84] B Albinsson S Li K Lundquist and R Stomberg ldquoThe originof lignin fluorescencerdquo Journal of Molecular Structure vol 508no 1ndash3 pp 19ndash27 1999
[85] S Baumberger A Abaecherli M Fasching et al ldquoMolar massdetermination of lignins by size-exclusion chromatographytowards standardisation of the methodrdquo Holzforschung vol 61no 4 pp 459ndash468 2007
[86] E Torres andM Ayala Biocatalysis Based on Heme PeroxidasesSpringer New York NY USA 1st edition 2010
[87] R Pecina P Burtscher G Bonn and O Bobleter ldquoGC-MSand HPLC analyses of lignin degradation products in biomasshydrolyzatesrdquo Fresenius Zeitschrift fur Analytische Chemie vol325 no 5 pp 461ndash465 1986
[88] NIST NIST Standard Reference Database The National Insti-tute of Standards and Technology 2014 httpwwwnistgovsrdnist1acfm
[89] T K Kirk and R L Farrell ldquoEnzymatic ldquocombustionrdquo themicrobial degradation of ligninrdquo Annual Review of Microbiol-ogy vol 41 pp 465ndash501 1987
[90] A TMartinez ldquoHigh redox potential peroxidasesrdquo in IndustrialEnzymes Structure Function and Applications K-B BeckerEd pp 477ndash488 Springer Amsterdam The Netherlands 1stedition 2007
[91] M Dashtban H Schraft T A Syed and W Qin ldquoFungalbiodegradation and enzymatic modification of ligninrdquo Interna-tional Journal of Biochemistry and Molecular Biology vol 1 no1 pp 36ndash50 2010
[92] N Duran E Esposito L H Innocentini-Mei and V P CanhosldquoA new alternative process for Kraft E1 effluent treatmentrdquoBiodegradation vol 5 no 1 pp 13ndash19 1994
[93] K Kamwilaisak and P C Wright ldquoInvestigating laccase andtitanium dioxide for lignin degradationrdquo Energy and Fuels vol26 no 4 pp 2400ndash2406 2012
[94] J Reyes M Dezotti H Mansilla J Villasenor E Espositoand N Duran ldquoBiomass photochemistry-XXII combined pho-tochemical and biological process for treatment of Kraft E1effluentrdquoApplied Catalysis B Environmental vol 15 no 3-4 pp211ndash219 1998
[95] L F Gonzalez V Sarria and O F Sanchez ldquoDegradationof chlorophenols by sequential biological-advanced oxidativeprocess using Trametespubescens and TiO
2UVrdquo Bioresource
Technology vol 101 no 10 pp 3493ndash3499 2010[96] J H Lora and W G Glasser ldquoRecent industrial applications
of lignin a sustainable alternative to nonrenewable materialsrdquoJournal of Polymers and the Environment vol 10 no 1-2 pp 39ndash48 2002
Submit your manuscripts athttpwwwhindawicom
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Inorganic ChemistryInternational Journal of
Hindawi Publishing Corporation httpwwwhindawicom Volume 2014
International Journal ofPhotoenergy
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Carbohydrate Chemistry
International Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
Chemistry
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Advances in
Physical Chemistry
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Analytical Methods in Chemistry
Journal of
Volume 2014
Bioinorganic Chemistry and ApplicationsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
SpectroscopyInternational Journal of
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The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Medicinal ChemistryInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Chromatography Research International
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Applied ChemistryJournal of
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Theoretical ChemistryJournal of
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Journal of
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Quantum Chemistry
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CatalystsJournal of
International Journal of Photoenergy 11
In summary different results have been obtained con-cerning the influence of noble metal ion addition Whilesome authors report an improvement in the photocatalyticefficiency upon their addition others report their addition ashaving no considerable influence However for reactions inwhich an improvement in the photocatalytic efficiency wasnoticed there was a threshold value to be considered Whenthe concentration of dopant surpasses this threshold valueelectron-hole recombination is favored and this has a negativeimpact to photocatalysis In such a case the space-chargelayer gets narrower and 119901-type dopants attract electrons andby virtue become negative They would now then act ashole acceptor attracting holes On the other hand 119899-typedopants which act as electron donor centers and possessexcess electrons attract holes as well [37]
323 Influence of Lignin Concentration Once the initiallignin concentration becomes higher exceeding a thresholdvalue an inhibitory effect on the photodegradationwas noted[47ndash49] This threshold value varies and depends on thereaction system and reaction parameters such as opticaldensity catalyst concentration and reaction volume Fromthe literature different authors have implemented varyinglignin concentrations probably to suit their reaction designFor example Ksibi et al [47] use 90mgL and AwungachaLekelefac et al [54] use 500mgL while Kansal et al [48]applied 10mgndash100mgL
Explanations arising from the findings are as followsat low lignin concentrations the incidental photonic fluxirradiated interacts with the catalyst generating radicals (eghydroxyl radicals (OH∙)) which allow a faster degradation[69] On the other hand high initial lignin concentrationsmay lead to tight adsorption which can suppress CO
2
evolution [51] and hence maintain chemical oxygen demand(COD) values Moreover low delignification yields maybe due to an inhibitory effect because of autoxidation bylow molecular weight lignin degradation products formed[24] Also due to the polymer structure of lignin which iscross-linked this makes it difficult for radical species acidand the aldehyde compounds produced to spread into theinner region of the substrate hence limiting autoxidationAs a worst case this might be the rate-determining step ofdelignification which is hindered [70ndash72]
In summary it can be concluded that the time taken forcomplete degradation depends on the initial concentration oflignin and faster degradation occurs at low lignin concentra-tions
324 Influence of pH Varying pH entails an alteration inthe properties of semiconductor-liquid interface [73] mainlyrelated to the acid-base equilibrium of the adsorbed hydroxylgroup [39] Furthermore pH also impacts lignin degradationrates [47 48 57] In this context several studies were carriedout with partly contradictory outcomes
Kansal et al [48] made pH investigations 3ndash11 undersolar light illumination using ZnO as catalyst Maximumdegradation was reached in alkaline conditions (pH 11)This is supported by Villasenor and Mansilla [74] reporting
an almost complete decolorization of kraft black liquorfrom pine wood at pH value of 116 in combination withZnO catalyst Similar results were achieved by Ohnishi etal [57] with TiO
2and ZnO being catalyst for bleaching
alkaline lignin in aqueous solution with TiO2and ZnO being
catalyst High activities at neutral pH were also reportedby Ohnishi et al [57] In contrast Ma et al [56] observedhigher reaction rates and rapid degradation of a syntheticlignin wastewater (prepared by dissolving commercial ligninpowder in aqueous solution pH 11) in acidic solution (pH 3)than in alkaline solutions at pH 11 for either TiO
2or PtTiO
2
catalystsReconsidering the photocatalytic principle the formed
superoxide anion radicals (∙O2
minus) are in a pH-dependentequilibriumwith perhydroxyl radicals (HO
2
∙) as follows [75]
HO2
∙999445999468 H+ + ∙O
2
minus pKa sim 48 (9)
See [76]∙O2
minus undergoes dismutation reaction resulting in H2O2
and O2competing to any other ∙O
2
minus triggered reaction Incase of low pH operation conditions in aqueous solutionsHO2
∙ becomes dominant whose reactivity is considerablyhigher compared to ∙O
2
minus [77] Subsequently HO2
∙ initiatessubstrate (S) oxidation to the radical cation (S+∙) and is itselfreduced to H
2O2[78] Thus increased degradation rates can
be reasonably expected supporting the results made by Ma etal [56] in an acidic environment ∙O
2
minus is extremely reactive inorganic solvents [77] Another aspect is the solubility of kraftlignin (soluble at pH gt 105) which reduces with decreasingpH whereas lignosulfonate should remain unaffected by pHin aqueous solution Moreover 120573ndashOndash4 bonds have beendescribed to be stable at acidic pH [11] In fact this couldadditionally explain the elevated degradation of kraft ligninmade by Kansal et al [48] and Villasenor and Mansilla [74]Nevertheless the contradictory results gained by Ma et al[56] still exist under the assumption that kraft ligninwas used(which would be supported by the high pH of 11 obviouslynecessary for dissolving the lignin powder) Although mostphotocatalytic reactions described in the literature are in anaqueous milieu lignin raw material and its fission productsmay however vary considerably Therefore the optimal pHis most likely to be reaction specific and has to be evaluatedexperimentally in principle
325 Influence of Illumination Many of the studies foundin the literature so far have not dealt on this subject per seWhat is found is the use of different illumination sourceseach having a specified power and lamp type However alltend to emitUV-light between the range 280ndash420 nm Table 2depicts this in detail Other illumination sources include thevisible light spectrum
In general terms illumination influences in that it ini-tiates photocatalysis by generating electron-hole pair in thesemiconductor particles [38 39] Dahm and Lucia [49]altered illumination intensity while observing lignin degra-dation In this study 004 gL lignin was used and lightintensity was varied 223ndash445mWcm3 It was found out thathigher illumination intensities correlated well with higher
12 International Journal of Photoenergy
initial degradation rates and hence total lignin degradation[49] Neppolian et al [69] report degradation to be propor-tional to radiation intensity and best results are achieved forlow lignin concentrations because of enhanced interactionbetween catalyst and incidental photonic flux
In summary high illumination power causes a corre-sponding high initial degradation rate at low lignin concen-trations becausemaximum light penetration into the reactionmedium is favored
33 Process Analytical Methods Various analytical tech-niques have been used to monitor lignin degradation Atthe beginning of this subchapter analytics revealing com-pounds formed from lignin degradation are treated Thisincludes for example gas chromatography (GC) and 1HNMR (nuclear magnetic resonance) This is then followed byresults qualitative analytic measurements such as ultraviolet-visible (UV-Vis) spectroscopy and dissolved carbon (DC)A list of authors analytical techniques applied and resultsachieved are outlined in Table 3
Portjanskaja and Preis [50] studied lignin degradationby measuring the removal of phenols through colorimetricmeasurements As a result of 24 h photocatalytic oxidationunder neutral media conditions 80 of free phenols wereremoved Gas chromatography (GC) result from Ksibi et al[47] attested vanillin vanillic acid palmitic acid biphenyland 345-trimethoxy benzaldehyde structures after the pho-tocatalysis of lignin from black liquor This is in accordancewith the findings of Tonucci et al [33] reporting the formationof vanillin hydroxyl methoxy-acetophenone coniferyl alco-hol coniferyl aldehyde methanol formic acid acetic acidand small amounts of C-2 and C-3 alcohols as degradationproducts1H NMR spectral analysis of lignin before illumination
and after 24 h of illumination showing characteristic bandsof aromatic rings methoxy and aliphatic side chains wascompared with each other Results revealed that the aro-matic ring degraded faster than the aliphatic chain [51]Fourier transformation infrared spectroscopy (FTIR) showedbands corresponding to CH
3 CH2 and CH which remained
unchanged after illumination while bands corresponding toaromatic rings disappeared as a result of illumination [51 53]
Results obtained from the combination of photochemicaland electrochemical oxidation [52 53] were similar to thoseof Tanaka et al [51] Here 13C-NMR confirmed the presenceof the carbonyl functionality and the presence of vanillinand vanillic acid after 12 h photochemical-electrochemicaloxidation These results showed that the combination of aphotocatalytic and an electrochemical oxidation significantlyenhanced the efficiency of lignin degradationThis is becausethe applied anodic potential bias greatly suppressed therecombination of photogenerated electrons and holes [53]
Ultraviolet spectrophotometry offers a convenientmethod to qualitatively and quantitatively analyze ligninin solution [79] This is reflected by the large number ofpublications using this technique [17 33 47ndash51 56 58 80]This is most likely due to its simplicity to interpret lignindegradation Lignins absorb UV light with high molar
200 220 240 260 280 300 320 340 360
00
05
10
15
20
25
Abso
rban
ce
Wavelength (nm)0min
30min
60min
120min
180min
300min
360min
420min
1200min
Figure 8 Time dependent UV-Vis absorption spectra of aqueouslignin solution from waste paper water irradiated with UV light(280ndash420 nm) for different time intervals The spectra are obtainedfor sol-gel derived TiO
2nanocrystalline coating (TiO
2-P25-SiO
2)
[54]
extinction coefficients because of the several methox-ylated phenylpropane units of which they are composed [33]Figure 8 depicts a series of photometric scans of ligninsul-fonate from paper waste water showing a gradual reductionof absorbance during photocatalytic treatment [54] Herethe absorption peaks are around 210 nm and 280 nmAbsorbance decreases with time implying the decompositionof lignin and the deterioration of chromophore groups [54]
Peaks at 210 nmcorrespond to portions of the unsaturatedchains while those at 280 nm correspond to unconjugatedphenolic hydroxyl groups [17] and the aromatic moiety [57]of the lignin molecule The absorption tailing to the longwavelength region arises from the color of lignin [57] Lignindegradation has been reported either at wavelength around280 nm corresponding to unconjugated phenolic hydroxylgroups [17 48 51 58] or for both wavelengths (210 nm and280 nm) [33 54 57] Kobayakawa et al [81] noted some otherabsorbance at wavelengths lower than 250 nm and pointedout that this could be due to the modification of ligninfragmentations leading to the formation of transient specieslikemethanol ethanol formaldehyde formic acid and oxalicacid among others
Analyticalmethods to effectively quantify lignin degrada-tion by calculating the oxygen demand by organic substancesand remaining organic carbon before and after photocatalysishave been studied Amongst the methods are dissolvedcarbon (DC) [49 51 58] chemical oxygen demand (COD)[47 48 50 57 80] biochemical oxygen demand (BOC) [50]dissolved organic carbon (DOC) [56] and American dyemanufacture institute value (ADMI) [56] COD and BODdescribe the oxygen demand by organic substances to be
International Journal of Photoenergy 13
Figure 9 Gradual change from the characteristic yellow ligninsulfonate to a colorless liquid after a period of 20 h Catalyst TiO
2-
P25 (Degussa) +TEOSUV-light 25∘C lignin concentration 05 gL[54]
converted to CO and CO2and H
2O and NH
3 Total organic
carbon (TOC) describes the amount of carbon bound inan organic compound while DOC describes the dissolvedfraction of organic carbon ADMI measures the amount ofdyestuff in water
Decolorization of lignin solution has been reported tobe another parameter observed during photocatalytic degra-dation Color is an indirect indicator of the lignin amountThe higher the color intensity of the solution is the greaterthe lignin content is (high concentrated lignin solutions egblack liquor appear dark brown) [82] Thus color changescan be interpreted as conversion of lignin to transient speciesor conversion to CO
2and H
2O Awungacha Lekelefac et al
[54] observed a gradual change from the characteristic yellowlignin (when highly diluted) to a colorless liquid after a periodof 20 h with sol-gel derived TiO
2nanocrystalline coatings on
sintered borosilicate glass as depicted in Figure 9 A corre-sponding decrease in DC values close to 82 was observedfor TiO
2-P25-SiO
2catalyst confirming degradation This is
shown in Figure 10These findings are analogous to that of Ohnishi et al
[57] who reported the bleaching of lignin when illuminatedcontinuously and that the solution becomes colorless Tothat the chemical oxygen demand (COD) value decreasesgenerating carbon dioxide and a small amount of carbonmonoxide as the main gaseous products COD removalwas reported to be effective at low lignin concentrations ascompared to high lignin concentrations [48]
Another applied analytical technique is fluorescencedetection directly coupled to a high performance liquidchromatography (HPLC) as a means to identify nonaliphaticcomponent in the complex mixture of lignin degradationproducts [54] Fluorescence emission in lignin is attributedto aromatic structures such as conjugated carbonyl biphenylphenylcoumarone and stilbene groups [83 84] AwungachaLekelefac et al [54] observed peaks on both HPLC and
0 200 400 600 800 1000 12000
50
100
150
200
250
Diss
olve
d ca
rbon
(ppm
)
t (min)
TiO2-P25-SiO2
TiO2-P25-SiO2 + PtTiOSO4minus30 6wt
ZnO + TiO2-P25-SiO2
No coating UV light
Figure 10 Variation of DC with time of aqueous lignin solutionfrom waste paper water irradiated with UV light (280ndash420 nm) fordifferent time intervals The spectra are obtained for sol-gel derivedTiO2nanocrystalline coatings (TiO
2-P25-SiO
2+Pt TiO
2-P25-SiO
2
TiOSO4minus306 wt ZnO + TiO
2-P25-SiO
2) [54]
fluorescence chromatograms suggesting the production ofnew substances and fluorophores
Despite developed analytical technologies analyzinglignin degradation products remains challenging Proofssuch as mass spectroscopy (MS) HPLC 13C or 1H-NMRspectra from photocatalytic lignin degradations are not yetestablished The setback to qualitatively and quantitativelyanalyze lignin and its degradation products starts from thenative lignin polymer itself with its indefinite polymericstructure and multiple bond types Also the influence ofdifferent pretreatments additives and the wide variety ofcompounds obtainable from its degradation makes ligninanalysis challenging [85] Moreover lignin streams couldcontain proteins inorganic salts and other potential poisonsthat generally complicate catalysis [4]
The challenge to identify and separate the productsstreams derived from the photocatalytic degradation ligninis also worth noting Lignin product stream is highly func-tionalized and conventional techniques such as gas chro-matography have the disadvantage of requiring a time-consuming derivatization step Also because of high boilingpoint of substances arising from lignin degradation it is noteasily applicable High performance liquid chromatography(HPLC) seems to be the remedy because analysis can becarried out without derivatization but the exact identificationof the separated substances is difficult because of the numer-ous peaks arising from such a chromatogram [87] Unfortu-nately well-established databases such as that of the nationalinstitute of standard and technology (NIST) [88] cannot giveinformation onHPLC-MS chromatogramsThis is because ofthe ionization sources such as electrospray ionization (ESI)
14 International Journal of Photoenergy
Table 4 Main advantages and disadvantages photocatalysis versus enzymatic biocatalysis
Lignin degradation process Advantages Disadvantages
Photocatalysis(single stage)
(i) Relatively fast degradation of complex as wellas nonbiodegradable organic (macro)molecules(ii) Stable catalysts(iii) Moderate reaction conditions
(i) Not selective(ii) Limited selection of operating conditions(iii) Energy costs
Enzymatic conversion(single stage)
(i) More selective(ii) Moderate reaction conditions
(i) High enzyme loads required otherwise ligninhydrolysis is quite slow [16](ii) Enzymes are less stable1(iii) cost-intensive (POXs)2
1For example POXs are sensitive to high H2O2 concentrations2According to Torres and Ayala [86]
and atmospheric pressure chemical ionization (APCI) forLC-MS while GC-MS is based on electron ionization Aremedy to this can be the development of a database ofmodel lignin compounds based on a unanimous HPLC-MSmeasuring procedure This would mean much time and costexpensive investments for adequate personnel and material
Though product identification based on peak superpo-sition gives information on a possible product this can-not always be true for lignin degradation because of thelarge number of possible degradation products In order toproperly identify products from lignin degradation productisolation techniques are to be implemented and further ana-lytic methods such as tandem mass spectroscopy (MSMS)and other advanced structure enhancing research techniquessuch as 1H-NMR 13C-NMR have to be studied extensively
4 Photocatalysis and Its Suitability asIntegrated Technology in MultistageConcepts with Biocatalysis
Depending on lignin treatment specification photocatalysiswill find application for complete lignin mineralization toCO2and H
2O (single stage) or as integrated technology in
a multistage system aiming at partial conversion of ligninmacromolecules to organic low molecular weight intermedi-ates for a consecutive biological oxidation finally generatingvalue added end-products [15]
For biological oxidation of lignin or lignin deriva-tives H
2O2-dependent ligninolytic heme peroxidases (POXs
including lignin peroxidase (LiP EC 111114) manganeseperoxidase (MnP EC 111113) versatile peroxidase (VPEC 111116) O
2-dependent laccases (Lac EC 11032) and
extracellular enzymes from Basidiomycetous white-rot fungiare the most efficient lignin degraders in nature [89] Thussuch enzymatic systems especially POXs (with high redoxpotentials) have attractedmuch interest as industrial biocata-lyst [86 90]The enzyme degradationmechanism (in nature)is facilitated by nonenzymatic processes mainly throughfree ∙OH radicals also generated by the fungus (Fenton-type reaction) Those ∙OH radicals enable the requiredphysical contact between the enzymes and structural units ofthe lignin molecule due to numerous nonspecific oxidative
reactions [91] Both the photocatalytic (equations (2)ndash(7))and the POX reactionmechanism recently reviewed by Busseat al [25] are quite similar Consequently a combination ofphotocatalysis followed by an enzymatic oxidation maybe apromising concept utilizing lignin derivatives for exampleoriginated from industrial effluents [15 92] The advantagesforming biobased products are as follows see also Table 4 inthis context A reduction of the lignin polymerization degreevia photocatalysis at best in more biodegradable interme-diates and a simultaneous detoxification causes savings inenzyme costs since it will be expected that less enzyme loadsare necessary for sufficiently rapid reaction rates [15 91] As aresult hydraulic retention times in bioreactor systems shouldbe diminished Moreover the delignification is expectedto be enhanced simultaneously which was in recent yearsshown in a dual system by Kamwilaisak and Wright [93]using TiO
2H2O2UV for photocatalytic pretreatment and
Lac (from Trametes versicolor) in the subsequent biocatalyticstep Within 24 h they obtained in their dual system anelevation in delignification of 20 (without H
2O2) up to at
least 50 when H2O2was present
In previous studies the treatment of lignin (from thepulp and paper industry) containing effluents by fungus asa biological system (excreting appropriate lignin degradingenzyme cocktails) were more focused for posttreatmentexclusively (Duran et al [92] Reyes et al [94] and Gonzalezet al [95]) Shende et al [17] even examined ligninolyticbacteria with photooxidized kraft lignin as substrate
Although POXs are potential industrial biocatalysts [86]no application studies were found for the direct use in suchdual systems as described above The major reason may bethe complexity of the reaction mechanism (inclusive ligninderivatives as substrate and their analysis) per se on the onehand slowing down research and development processes Onthe other hand POXs are sensitive to their cosubstrate H
2O2
once it is supplied in excess causing considerable inactivation(for details refer to Busse et al [25]) At the present severalstudies are carried out modifying these enzymes regardingenhanced stability activity and selectivity as well Henceit can be expected that their technological applicability willbe raised significantly right after successful modification isreached [86]
International Journal of Photoenergy 15
5 Concluding Remarks
It is widely assumed that the photocatalytic degradation oflignin follows a radical reaction pathway which is similar tothat considered in thermal electrochemical and biochemicalprocesses However reporting on the degradation pathway oflignin derivatives and even that of lignin model compoundsare still amajor challengeThis is probably due to the complexnature and variety of possible degradation products Indeedthemechanism is far more complex considering other factorssuch as type of lignin type of catalyst pH illuminationsource and additives
Generally comparing the different photochemical pro-cesses poses a big challenge because of the wide variablesinvolved These discrepancies start from the source and typeof lignin followed by differences in reactor design illumi-nation source intensity of radiation and different types ofcatalyst An ideawould be to have a specific reference reactionwith well-defined starting parameters which include lignintype source and purity catalyst specifications illuminationsource and intensity so as to ease comparison of results
Basic process parameters such as catalyst concentrationsubstrate concentration addition of metal ion to catalyst pHand illumination have been discussed
Despite developed analytical technologies analyzinglignin degradation products remains challenging Proofssuch as mass spectroscopy (MS) HPLC 13C or 1H-NMRspectra from photocatalytic lignin degradations are not yetestablished
In order to properly identify products from lignin degra-dation product isolation techniques are to be implementedand further analytic methods such as MSMS and otheradvanced structure enhancing research techniques such as1H-NMR and 13C-NMR have to be studied extensively Aremedy to this can be the development of a databank ofmodel lignin compounds based on a unanimous HPLC-MSmeasuring procedure This would mean much time and costexpensive investments for adequate personnel and material
Photocatalysis is denoted as the most popular ligninpretreatment technology besides ozonation [96] Photo-catalyzed lignin may be an appropriate substrate for aconsecutive biocatalytic process using ligninolytic enzymes(POX andor Lac) as supported by experimental results ofKamwilaisak and Wright [93] Combining the advantages ofboth catalytic processes savings in the overall process costswill be expected in addition to elevated lignin conversionNonetheless extensive research work including POX modi-fications is still required
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Authorsrsquo Contribution
Colin Awungacha Lekelefac and Nadine Busse contributedequally to this work
Acknowledgments
The authors gratefully thank the Federal Ministry of Educa-tion and Research (BMBF) for funding (FKZ17N0310) Theresearchers also thank the Hessen State Ministry of HigherEducation Research andArts for the financial support withinthe Hessen initiative for scientific and economic excellence(LOEWE)
References
[1] Crude Oil and Commodity Prices httpwwwoil-pricenet[2] Research Directorate-General for World Energy Technology
and Climate Policy Outlook (WETO) Luxembourg Office forOfficial Publications of the European Communities 2003
[3] Green and Natural Polymers Are on the Rise 2014 httpwwwpolymersolutionscombloggreen-and-natural-polymers-on-the-rise
[4] J Zakzeski P C Bruijnincx A L Jongerius and B M Weck-huysen ldquoThe catalytic valorization of lignin for the productionof renewable chemicalsrdquo Chemical Reviews vol 110 no 6 pp3552ndash3599 2010
[5] B KammM Kamm P R Gruber and S KromusBiorefineries-Industrial Processes and Products Status Quo and Future Direc-tions vol 1 Wiley-VCH 2006
[6] R L Howard E Abotsi E L J van Rensburg and S HowardldquoLignocellulose biotechnology issues of bioconversion andenzyme productionrdquoAfrican Journal of Biotechnology vol 2 no12 pp 602ndash619 2003
[7] M Stocker ldquoBio- und BTL-Kraftstoffe in der Bioraffineriekatalytische Umwandlung Lignocellulose-reicher Biomasse mitporosen Stoffenrdquo Angewandte Chemie vol 120 no 48 pp9340ndash9351 2008
[8] J-P Lange ldquoLignocellulose conversion an introduction tochemistry process and economicsrdquo Biofuels Bioproducts andBiorefining vol 1 no 1 pp 39ndash48 2007
[9] D A I Goring ldquoThe physical chemistry of ligninrdquo Pure andApplied Chemistry vol 5 no 1-2 pp 233ndash310 1962
[10] M Ek G Gellerstedt and G HenrikssonWood Chemistry andBiotechnology Pulp and Paper Chemistry and Technology vol 1Walter de Gruyter GmbH amp Co KG Berlin Germany 2009
[11] B Saake and R Lehnen ldquoLigninrdquo in Ullmannrsquos Encyclopedia ofIndustrial Chemistry Wiley-VCH Verlag GmbH amp Co KGaAWeinheim Germany 2012
[12] M N S Kumar A K Mohanty L Erickson and M MisraldquoLignin and its applications with polymersrdquo Journal of BiobasedMaterials and Bioenergy vol 3 no 1 pp 1ndash24 2009
[13] R J A Gosselink E de Jong B Guran and A AbacherlildquoCo-ordination network for ligninmdashstandardisation produc-tion and applications adapted to market requirements(EUROLIGNIN)rdquo Industrial Crops and Products vol 20 no 2pp 121ndash129 2004
[14] D Fengel and G Wegener Wood Chemistry UltrastructureReactions Verlag Kessel 1984
[15] D Mantzavinos and E Psillakis ldquoEnhancement of biodegrad-ability of industrial wastewaters by chemical oxidation pre-treatmentrdquo Journal of Chemical Technology and Biotechnologyvol 79 no 5 pp 431ndash454 2004
16 International Journal of Photoenergy
[16] M Dashtban H Schraft and W Qin ldquoFungal bioconversionof lignocellulosic residues Opportunities amp perspectivesrdquo Inter-national Journal of Biological Sciences vol 5 no 6 pp 578ndash5952009
[17] A Shende R Jaswal D Harder-Heinz A Menan and RShende ldquoIntergrated photocatalytic and microbial degradationof kraft ligninrdquo Cleantech pp 120ndash123 2012
[18] F S Chakar and A J Ragauskas ldquoReview of current and futuresoftwood kraft lignin process chemistryrdquo Industrial Crops andProducts vol 20 no 2 pp 131ndash141 2004
[19] W G Glasser and H R Glasser ldquoEvaluation of ligninrsquoschemical structure by experimental and computer simulationtechniquesrdquo Paperi ja Puu vol 63 pp 71ndash83 1981
[20] M Erickson S Larsson and G E Miksche ldquoZur Struktur desLignins der FichterdquoActaChemica Scandinavica vol 27 pp 903ndash914 1973
[21] H Nimz ldquoDas Lignin der BuchemdashEntwurf eines Konstitution-sschemasrdquo Angewandte ChemiemdashInternational Edition vol 86pp 336ndash344 1974
[22] D V Evtuguin C P Neto J Rocha and J D P de JesusldquoOxidative delignification in the presence of molybdovana-dophosphate heteropolyanions mechanism and kinetic stud-iesrdquo Applied Catalysis A General vol 167 no 1 pp 123ndash1391998
[23] A L Linsebigler G Lu and J T Yates Jr ldquoPhotocatalysis onTiO2surfaces Principles mechanisms and selected resultsrdquo
Chemical Reviews vol 95 no 3 pp 735ndash758 1995[24] Y Miyata K Miyazaki M Miura Y Shimotori M Aoyama
and H Nakatani ldquoSolventless delignification of wood flourwith TiO
2poly(ethylene oxide) photocatalyst systemrdquo Journal
of Polymers and the Environment vol 21 no 1 pp 115ndash121 2013[25] N Busse D Wagner M Kraume and P Czermak ldquoReaction
kinetics of versatile peroxidase for the degradation of lignincompoundsrdquoTheAmerican Journal of Biochemistry andBiotech-nology vol 9 no 4 pp 365ndash394 2013
[26] M Tien and T K Kirk ldquoLignin-degrading enzyme fromphanerochaete chrysosporium Purification characterizationand catalytic properties of a uniqueH
2O2-requiring oxygenaserdquo
Proceedings of the National Academy of Sciences vol 81 pp2280ndash2284 1984
[27] T K Kirk M Tien P J Kersten M D Mozuch and BKalyanaraman ldquoLigninase of Phanerochaete chrysosporiumMechanism of its degradation of the non-phenolic arylglycerol120573-aryl ether substructure of ligninrdquo Biochemical Journal vol236 no 1 pp 279ndash287 1986
[28] T Lundell RWever R Floris et al ldquoLignin peroxidase L3 fromPhlebia radiata pre-steady-state and steady-state studies withveratryl alcohol and a non-phenolic lignin model compound1-(34-dimethoxyphenyl)-2-(2-methoxyphenoxy)propane-13-diolrdquo European Journal of Biochemistry vol 211 no 3 pp391ndash402 1993
[29] H E Schoemaker T K Lundell A I Hatakka and K PiontekldquoThe oxidation of veratryl alcohol dimeric lignin models andlignin by lignin peroxidase the redox cycle revisitedrdquo FEMSMicrobiology Reviews vol 13 no 2-3 pp 321ndash331 1994
[30] J M Palmer P J Harvey and H E Schoemaker ldquoThe role ofperoxidases radical cations and oxygen in the degradation oflignin [and discussion]rdquo Philosophical Transactions of the RoyalSociety of London A vol 321 no 1561 pp 495ndash505 1987
[31] C Hill Wood Modification Chemical Thermal and OtherProcesses John Wiley amp Sons Chichester UK 2006
[32] Y-H Xu H-R Chen Z-X Zeng and B Lei ldquoInvestigation onmechanism of photocatalytic activity enhancement of nanome-ter cerium-doped titaniardquo Applied Surface Science vol 252 no24 pp 8565ndash8570 2006
[33] L Tonucci F Coccia M Bressan and N DrsquoAlessandro ldquoMildphotocatalysed and catalysed green oxidation of lignin auseful pathway to low-molecular-weight derivativesrdquoWaste andBiomass Valorization vol 3 no 2 pp 165ndash174 2012
[34] A Castellan N Colombo C Vanucci P Fornier de HViolet and H Bouas-Laurent ldquoPhotodegradation of ligninA photochemical study of an O-methylated 120572-carbonyl 120573-1lignin model dimer 12-di(3101584041015840-dimethoxyphenyl) ethanone(deoxyveratroin)rdquo Journal of Photochemistry and PhotobiologyA vol 51 pp 451ndash467 1990
[35] A Fujishima X Zhang and D A Tryk ldquoTiO2photocatalysis
and related surface phenomenardquo Surface Science Reports vol63 no 12 pp 515ndash582 2008
[36] C S Turchi and D F Ollis ldquoPhotocatalytic reactor designan example of mass-transfer limitations with an immobilizedcatalystrdquo The Journal of Physical Chemistry vol 92 no 23 pp6852ndash6853 1988
[37] R Subasri M Tripathi K Murugan J Revathi G V N Raoand T N Rao ldquoInvestigations on the photocatalytic activity ofsolndashgel derived plain and Fe3+Nb5+ doped titania coatings onglass substratesrdquo Materials Chemistry and Physics vol 124 pp63ndash68 2010
[38] N Serpone ldquoRelative photonic efficiencies and quantum yieldsin heterogeneous photocatalysisrdquo Journal of Photochemistry andPhotobiology A Chemistry vol 104 no 1ndash3 pp 1ndash12 1997
[39] M R Hoffmann S T Martin W Choi and D W BahnemannldquoEnvironmental applications of semiconductor photocatalysisrdquoChemical Reviews vol 95 no 1 pp 69ndash96 1995
[40] C D Jaeger and A J Bard ldquoSpin trapping and electron spinresonance detection of radical intermediates in the photode-composition of water at titanium dioxide particulate systemsrdquoThe Journal of Physical Chemistry vol 83 no 24 pp 3146ndash31521979
[41] R W Matthews ldquoHydroxylation reactions induced by near-ultraviolet photolysis of aqueous titaniumdioxide suspensionsrdquoJournal of the Chemical Society Faraday Transactions vol 80no 2 pp 457ndash471 1984
[42] A E H Machado A M Furuyama S Z Falone R RuggieroD D S Perez and A Castellan ldquoPhotocatalytic degradation oflignin and lignin models using titanium dioxide the role of thehydroxyl radicalrdquoChemosphere vol 40 no 1 pp 115ndash124 2000
[43] W Sigg and L Stumm Aquatische Chemie Stuttgart 2 AuflageBG Teubner Stuttgart Germany 1991
[44] O Legrini E Oliveros and A M Braun ldquoPhotochemicalprocesses for water treatmentrdquo Chemical Reviews vol 93 no2 pp 671ndash698 1993
[45] G Rothenberger J Moser M Gratzel N Serpone and DSharma ldquoCharge carrier trapping and recombination dynamicsin small semiconductor particlesrdquo Journal of the AmericanChemical Society vol 107 no 26 pp 8054ndash8059 1985
[46] P Mazellier M Sarakha A Rossi and M Bolte ldquoThe aqueousphotochemistry of 26-dimethylphenol Evidence for the frag-mentation of the 120572 C-C bondrdquo Journal of Photochemistry andPhotobiology A vol 115 no 2 pp 117ndash121 1998
[47] M Ksibi S B Amor S Cherif E Elaloui A Houas and MElaloui ldquoPhotodegradation of lignin from black liquor using aUVTiO
2systemrdquo Journal of Photochemistry and Photobiology
A vol 154 no 2-3 pp 211ndash218 2003
International Journal of Photoenergy 17
[48] S K Kansal M Singh and D Sud ldquoStudies on TiO2ZnO
photocatalysed degradation of ligninrdquo Journal of HazardousMaterials vol 153 no 1-2 pp 412ndash417 2008
[49] A Dahm and L A Lucia ldquoTitanium dioxide catalyzed pho-todegradation of lignin in industrial effluentsrdquo Industrial andEngineering Chemistry Research vol 43 no 25 pp 7996ndash80002004
[50] E Portjanskaja and S Preis ldquoAqueous photocatalytic oxidationof lignin the influence of mineral admixturesrdquo InternationalJournal of Photoenergy vol 2007Article ID76730 7 pages 2007
[51] K Tanaka R C R Calanag and T Hisanaga ldquoPhotocatalyzeddegradation of lignin on TiO
2rdquo Journal of Molecular Catalysis
A Chemical vol 138 no 2-3 pp 287ndash294 1999[52] M Tian J Wen D MacDonald R M Asmussen and A Chen
ldquoA novel approach for lignin modification and degradationrdquoElectrochemistry Communications vol 12 no 4 pp 527ndash5302010
[53] K PanM Tian Z-H Jiang B Kjartanson and A Chen ldquoElec-trochemical oxidation of lignin at lead dioxide nanoparticlesphotoelectrodeposited on TiO
2nanotube arraysrdquo Electrochim-
ica Acta vol 60 pp 147ndash153 2012[54] C Awungacha Lekelefac J Hild P Czermak and M Herren-
bauer ldquoPhotocatalytic active coatings for lignin degradationin a continuous packed bed reactorrdquo International Journal ofPhotoenergy vol 2014 Article ID 502326 10 pages 2014
[55] C Awungacha Lekelefac P Czermak and M HerrenbauerldquoEvaluation of photocatalytic active coatings on sintered glasstubes by methylene bluerdquo International Journal of Photoenergyvol 2013 Article ID 614567 9 pages 2013
[56] Y-S Ma C-N Chang Y-P Chiang H-F Sung and A CChao ldquoPhotocatalytic degradation of lignin using PtTiO
2as
the catalystrdquo Chemosphere vol 71 no 5 pp 998ndash1004 2008[57] H Ohnishi M Matsumura H Tsubomura and M Iwasaki
ldquoBleaching of lignin solution by a photocatalyzed reactionon semiconductor photocatalystsrdquo Industrial and EngineeringChemistry Research vol 28 no 6 pp 719ndash724 1989
[58] C A K Gouvea F Wypych S G Moraes N Duran and PPeralta-Zamora ldquoSemiconductor-assisted photodegradation oflignin dye and kraft effluent by Ag-doped ZnOrdquo Chemospherevol 40 no 4 pp 427ndash432 2000
[59] A V Vahatalo K Salonen M Salkinoja-Salonen and AHatakka ldquoPhotochemical mineralization of synthetic lignin inlake water indicates enhanced turnover of aromatic organicmatter under solar radiationrdquo Biodegradation vol 10 no 6 pp415ndash420 1999
[60] H de Lasa B Serrano and M Salaices Photocatalytic ReactionEngineering Springer New York NY USA 2005
[61] K Hashimoto H Irie and A Fujishima ldquoTiO 2 photocatalysisA historical overview and future prospectsrdquo Japanese Journal ofApplied Physics vol 44 no 12 pp 8269ndash8285 2005
[62] M A Behnajady N Modirshahla and R Hamzavi ldquoKineticstudy on photocatalytic degradation of CI Acid Yellow 23 byZnOphotocatalystrdquo Journal ofHazardousMaterials vol 133 no1ndash3 pp 226ndash232 2006
[63] M Addamo V Augugliaro A di Paola et al ldquoPhotocatalyticthin films of TiO
2formed by a sol-gel process using titanium
tetraisopropoxide as the precursorrdquo Thin Solid Films vol 516no 12 pp 3802ndash3807 2008
[64] N Negishi K Takeuchi and T Ibusuki ldquoPreparation of theTiO2thin filmphotocatalyst by the dip-coating processrdquo Journal
of Sol-Gel Science and Technology vol 13 no 1ndash3 pp 691ndash6941998
[65] L Rideh A Wehrer D Ronze and A Zoulalian ldquoPhotocat-alytic degradation of 2-chlorophenol in TiO
2aqueous suspen-
sion modeling of reaction raterdquo Industrial and EngineeringChemistry Research vol 36 no 11 pp 4712ndash4718 1997
[66] R-ADoongC-HChen RAMaithreepala and S-MChangldquoThe influence of pHand cadmiumsulfide on the photocatalyticdegradation of 2-chlorophenol in titanium dioxide suspen-sionsrdquoWater Research vol 35 no 12 pp 2873ndash2880 2001
[67] K V Sarkanen A Islam and C D Anderson ldquoOzonationrdquo inMethods in Lignin Chemistry S Y Lin and C W Dence EdsSpringer Series inWood Science pp 387ndash406 Springer BerlinGermany 1992
[68] G Gellerstedt and E-L Lindfors ldquoStructural changes in ligninduring kraft pulpingrdquo Holzforschung vol 38 no 3 pp 151ndash1581984
[69] B Neppolian H C Choi M V Shankar B Arabindoo andV Murugesan in Proceedings of the International Symposiumon Environmental Pollution Control and Waste Management(EPCOWM 02) p 647 2002
[70] C Pouteau P Dole B Cathala L Averous and N BoquillonldquoAntioxidant properties of lignin in polypropylenerdquo PolymerDegradation and Stability vol 81 no 1 pp 9ndash18 2003
[71] J Hafren T Fujino and T Itoh ldquoChanges in cell wall archi-tecture of differentiating tracheids of Pinus thunbergii duringlignificationrdquo Plant and Cell Physiology vol 40 no 5 pp 532ndash541 1999
[72] T Dizhbite G Telysheva V Jurkjane and U Viesturs ldquoChar-acterization of the radical scavenging activity of lignins naturalantioxidantsrdquo Bioresource Technology vol 95 no 3 pp 309ndash3172004
[73] K Hofstadler R Bauer S Novalic and S G Heisier ldquoNew reac-tor design for photocatalytic wastewater treatment with TiO
2
immobilized on fused-silica glass fibers photomineralization of4-chlorophenolrdquo Environmental Science amp Technology vol 28no 4 pp 670ndash674 1994
[74] J Villasenor and H D Mansilla ldquoEffect of temperature onkraft black liquor degradation by ZnO-photoassisted catalysisrdquoJournal of Photochemistry and Photobiology A Chemistry vol93 no 2-3 pp 205ndash209 1996
[75] B H Bielski D E Cabelli L A Ravindra and A B RossldquoReactivity of HO
2Ominus2Radicals in aqueous solutionrdquo Journal
of Physical Chemistry vol 14 pp 1041ndash1100 1985[76] B H J Bielski and A O Allen ldquoMechanism of the dispropor-
tionation of superoxide radicalsrdquo Journal of Physical Chemistryvol 81 no 11 pp 1048ndash1050 1977
[77] B Halliwell and J M C Gutteridge ldquoThe importance of freeradicals and catalytic metal ions in human diseasesrdquoMolecularAspects of Medicine vol 8 no 2 pp 89ndash193 1985
[78] J M Palmer P J Harvey and H E Schoemaker ldquoThe role ofperoxidases radical cations and oxygen in the degradation oflignin [and discussion]rdquo Philosophical Transaction of the RoyalSociety A vol 321 no 1561 pp 495ndash505 1987
[79] S Y Yin and C W Dense Methods in Lignin ChemistrySpringer New York NY USA 1992
[80] P Kumar S Kumar and N K Bhardwaj ldquoPhotocatalyticoxidation of elemental chlorine free bleaching effluent withUVTiO
2rdquo in Proceedings of the 2nd International Conference on
Environmental Science and Technology (ICEST rsquo11) SingaporeFebruary 2011
[81] K Kobayakawa Y Sato S Nakamura and A FujishimaldquoPhotodecomposition of Kraft lignin catalyzed by titanium
18 International Journal of Photoenergy
dioxiderdquo Bulletin of the Chemical Society of Japan vol 62 no11 pp 3433ndash3436 1989
[82] R B Kinstre ldquoAn overview of strategies for reducing theenvironmental impact of bleach-plant effluentsrdquo Tappi Journalvol 76 no 5 pp 105ndash113 1993
[83] A Castellan H Choudhury R Stephen Davidson and SGrelier ldquoComparative study of stone-ground wood pulp andnative wood 3 Application of fluorescence spectroscopy to astudy of the weathering of stone-ground pulp and native woodrdquoJournal of Photochemistry and Photobiology A Chemistry vol81 no 2 pp 123ndash130 1994
[84] B Albinsson S Li K Lundquist and R Stomberg ldquoThe originof lignin fluorescencerdquo Journal of Molecular Structure vol 508no 1ndash3 pp 19ndash27 1999
[85] S Baumberger A Abaecherli M Fasching et al ldquoMolar massdetermination of lignins by size-exclusion chromatographytowards standardisation of the methodrdquo Holzforschung vol 61no 4 pp 459ndash468 2007
[86] E Torres andM Ayala Biocatalysis Based on Heme PeroxidasesSpringer New York NY USA 1st edition 2010
[87] R Pecina P Burtscher G Bonn and O Bobleter ldquoGC-MSand HPLC analyses of lignin degradation products in biomasshydrolyzatesrdquo Fresenius Zeitschrift fur Analytische Chemie vol325 no 5 pp 461ndash465 1986
[88] NIST NIST Standard Reference Database The National Insti-tute of Standards and Technology 2014 httpwwwnistgovsrdnist1acfm
[89] T K Kirk and R L Farrell ldquoEnzymatic ldquocombustionrdquo themicrobial degradation of ligninrdquo Annual Review of Microbiol-ogy vol 41 pp 465ndash501 1987
[90] A TMartinez ldquoHigh redox potential peroxidasesrdquo in IndustrialEnzymes Structure Function and Applications K-B BeckerEd pp 477ndash488 Springer Amsterdam The Netherlands 1stedition 2007
[91] M Dashtban H Schraft T A Syed and W Qin ldquoFungalbiodegradation and enzymatic modification of ligninrdquo Interna-tional Journal of Biochemistry and Molecular Biology vol 1 no1 pp 36ndash50 2010
[92] N Duran E Esposito L H Innocentini-Mei and V P CanhosldquoA new alternative process for Kraft E1 effluent treatmentrdquoBiodegradation vol 5 no 1 pp 13ndash19 1994
[93] K Kamwilaisak and P C Wright ldquoInvestigating laccase andtitanium dioxide for lignin degradationrdquo Energy and Fuels vol26 no 4 pp 2400ndash2406 2012
[94] J Reyes M Dezotti H Mansilla J Villasenor E Espositoand N Duran ldquoBiomass photochemistry-XXII combined pho-tochemical and biological process for treatment of Kraft E1effluentrdquoApplied Catalysis B Environmental vol 15 no 3-4 pp211ndash219 1998
[95] L F Gonzalez V Sarria and O F Sanchez ldquoDegradationof chlorophenols by sequential biological-advanced oxidativeprocess using Trametespubescens and TiO
2UVrdquo Bioresource
Technology vol 101 no 10 pp 3493ndash3499 2010[96] J H Lora and W G Glasser ldquoRecent industrial applications
of lignin a sustainable alternative to nonrenewable materialsrdquoJournal of Polymers and the Environment vol 10 no 1-2 pp 39ndash48 2002
Submit your manuscripts athttpwwwhindawicom
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Inorganic ChemistryInternational Journal of
Hindawi Publishing Corporation httpwwwhindawicom Volume 2014
International Journal ofPhotoenergy
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Carbohydrate Chemistry
International Journal of
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Journal of
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Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Advances in
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Analytical Methods in Chemistry
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Bioinorganic Chemistry and ApplicationsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
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The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014
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Chromatography Research International
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
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Quantum Chemistry
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CatalystsJournal of
12 International Journal of Photoenergy
initial degradation rates and hence total lignin degradation[49] Neppolian et al [69] report degradation to be propor-tional to radiation intensity and best results are achieved forlow lignin concentrations because of enhanced interactionbetween catalyst and incidental photonic flux
In summary high illumination power causes a corre-sponding high initial degradation rate at low lignin concen-trations becausemaximum light penetration into the reactionmedium is favored
33 Process Analytical Methods Various analytical tech-niques have been used to monitor lignin degradation Atthe beginning of this subchapter analytics revealing com-pounds formed from lignin degradation are treated Thisincludes for example gas chromatography (GC) and 1HNMR (nuclear magnetic resonance) This is then followed byresults qualitative analytic measurements such as ultraviolet-visible (UV-Vis) spectroscopy and dissolved carbon (DC)A list of authors analytical techniques applied and resultsachieved are outlined in Table 3
Portjanskaja and Preis [50] studied lignin degradationby measuring the removal of phenols through colorimetricmeasurements As a result of 24 h photocatalytic oxidationunder neutral media conditions 80 of free phenols wereremoved Gas chromatography (GC) result from Ksibi et al[47] attested vanillin vanillic acid palmitic acid biphenyland 345-trimethoxy benzaldehyde structures after the pho-tocatalysis of lignin from black liquor This is in accordancewith the findings of Tonucci et al [33] reporting the formationof vanillin hydroxyl methoxy-acetophenone coniferyl alco-hol coniferyl aldehyde methanol formic acid acetic acidand small amounts of C-2 and C-3 alcohols as degradationproducts1H NMR spectral analysis of lignin before illumination
and after 24 h of illumination showing characteristic bandsof aromatic rings methoxy and aliphatic side chains wascompared with each other Results revealed that the aro-matic ring degraded faster than the aliphatic chain [51]Fourier transformation infrared spectroscopy (FTIR) showedbands corresponding to CH
3 CH2 and CH which remained
unchanged after illumination while bands corresponding toaromatic rings disappeared as a result of illumination [51 53]
Results obtained from the combination of photochemicaland electrochemical oxidation [52 53] were similar to thoseof Tanaka et al [51] Here 13C-NMR confirmed the presenceof the carbonyl functionality and the presence of vanillinand vanillic acid after 12 h photochemical-electrochemicaloxidation These results showed that the combination of aphotocatalytic and an electrochemical oxidation significantlyenhanced the efficiency of lignin degradationThis is becausethe applied anodic potential bias greatly suppressed therecombination of photogenerated electrons and holes [53]
Ultraviolet spectrophotometry offers a convenientmethod to qualitatively and quantitatively analyze ligninin solution [79] This is reflected by the large number ofpublications using this technique [17 33 47ndash51 56 58 80]This is most likely due to its simplicity to interpret lignindegradation Lignins absorb UV light with high molar
200 220 240 260 280 300 320 340 360
00
05
10
15
20
25
Abso
rban
ce
Wavelength (nm)0min
30min
60min
120min
180min
300min
360min
420min
1200min
Figure 8 Time dependent UV-Vis absorption spectra of aqueouslignin solution from waste paper water irradiated with UV light(280ndash420 nm) for different time intervals The spectra are obtainedfor sol-gel derived TiO
2nanocrystalline coating (TiO
2-P25-SiO
2)
[54]
extinction coefficients because of the several methox-ylated phenylpropane units of which they are composed [33]Figure 8 depicts a series of photometric scans of ligninsul-fonate from paper waste water showing a gradual reductionof absorbance during photocatalytic treatment [54] Herethe absorption peaks are around 210 nm and 280 nmAbsorbance decreases with time implying the decompositionof lignin and the deterioration of chromophore groups [54]
Peaks at 210 nmcorrespond to portions of the unsaturatedchains while those at 280 nm correspond to unconjugatedphenolic hydroxyl groups [17] and the aromatic moiety [57]of the lignin molecule The absorption tailing to the longwavelength region arises from the color of lignin [57] Lignindegradation has been reported either at wavelength around280 nm corresponding to unconjugated phenolic hydroxylgroups [17 48 51 58] or for both wavelengths (210 nm and280 nm) [33 54 57] Kobayakawa et al [81] noted some otherabsorbance at wavelengths lower than 250 nm and pointedout that this could be due to the modification of ligninfragmentations leading to the formation of transient specieslikemethanol ethanol formaldehyde formic acid and oxalicacid among others
Analyticalmethods to effectively quantify lignin degrada-tion by calculating the oxygen demand by organic substancesand remaining organic carbon before and after photocatalysishave been studied Amongst the methods are dissolvedcarbon (DC) [49 51 58] chemical oxygen demand (COD)[47 48 50 57 80] biochemical oxygen demand (BOC) [50]dissolved organic carbon (DOC) [56] and American dyemanufacture institute value (ADMI) [56] COD and BODdescribe the oxygen demand by organic substances to be
International Journal of Photoenergy 13
Figure 9 Gradual change from the characteristic yellow ligninsulfonate to a colorless liquid after a period of 20 h Catalyst TiO
2-
P25 (Degussa) +TEOSUV-light 25∘C lignin concentration 05 gL[54]
converted to CO and CO2and H
2O and NH
3 Total organic
carbon (TOC) describes the amount of carbon bound inan organic compound while DOC describes the dissolvedfraction of organic carbon ADMI measures the amount ofdyestuff in water
Decolorization of lignin solution has been reported tobe another parameter observed during photocatalytic degra-dation Color is an indirect indicator of the lignin amountThe higher the color intensity of the solution is the greaterthe lignin content is (high concentrated lignin solutions egblack liquor appear dark brown) [82] Thus color changescan be interpreted as conversion of lignin to transient speciesor conversion to CO
2and H
2O Awungacha Lekelefac et al
[54] observed a gradual change from the characteristic yellowlignin (when highly diluted) to a colorless liquid after a periodof 20 h with sol-gel derived TiO
2nanocrystalline coatings on
sintered borosilicate glass as depicted in Figure 9 A corre-sponding decrease in DC values close to 82 was observedfor TiO
2-P25-SiO
2catalyst confirming degradation This is
shown in Figure 10These findings are analogous to that of Ohnishi et al
[57] who reported the bleaching of lignin when illuminatedcontinuously and that the solution becomes colorless Tothat the chemical oxygen demand (COD) value decreasesgenerating carbon dioxide and a small amount of carbonmonoxide as the main gaseous products COD removalwas reported to be effective at low lignin concentrations ascompared to high lignin concentrations [48]
Another applied analytical technique is fluorescencedetection directly coupled to a high performance liquidchromatography (HPLC) as a means to identify nonaliphaticcomponent in the complex mixture of lignin degradationproducts [54] Fluorescence emission in lignin is attributedto aromatic structures such as conjugated carbonyl biphenylphenylcoumarone and stilbene groups [83 84] AwungachaLekelefac et al [54] observed peaks on both HPLC and
0 200 400 600 800 1000 12000
50
100
150
200
250
Diss
olve
d ca
rbon
(ppm
)
t (min)
TiO2-P25-SiO2
TiO2-P25-SiO2 + PtTiOSO4minus30 6wt
ZnO + TiO2-P25-SiO2
No coating UV light
Figure 10 Variation of DC with time of aqueous lignin solutionfrom waste paper water irradiated with UV light (280ndash420 nm) fordifferent time intervals The spectra are obtained for sol-gel derivedTiO2nanocrystalline coatings (TiO
2-P25-SiO
2+Pt TiO
2-P25-SiO
2
TiOSO4minus306 wt ZnO + TiO
2-P25-SiO
2) [54]
fluorescence chromatograms suggesting the production ofnew substances and fluorophores
Despite developed analytical technologies analyzinglignin degradation products remains challenging Proofssuch as mass spectroscopy (MS) HPLC 13C or 1H-NMRspectra from photocatalytic lignin degradations are not yetestablished The setback to qualitatively and quantitativelyanalyze lignin and its degradation products starts from thenative lignin polymer itself with its indefinite polymericstructure and multiple bond types Also the influence ofdifferent pretreatments additives and the wide variety ofcompounds obtainable from its degradation makes ligninanalysis challenging [85] Moreover lignin streams couldcontain proteins inorganic salts and other potential poisonsthat generally complicate catalysis [4]
The challenge to identify and separate the productsstreams derived from the photocatalytic degradation ligninis also worth noting Lignin product stream is highly func-tionalized and conventional techniques such as gas chro-matography have the disadvantage of requiring a time-consuming derivatization step Also because of high boilingpoint of substances arising from lignin degradation it is noteasily applicable High performance liquid chromatography(HPLC) seems to be the remedy because analysis can becarried out without derivatization but the exact identificationof the separated substances is difficult because of the numer-ous peaks arising from such a chromatogram [87] Unfortu-nately well-established databases such as that of the nationalinstitute of standard and technology (NIST) [88] cannot giveinformation onHPLC-MS chromatogramsThis is because ofthe ionization sources such as electrospray ionization (ESI)
14 International Journal of Photoenergy
Table 4 Main advantages and disadvantages photocatalysis versus enzymatic biocatalysis
Lignin degradation process Advantages Disadvantages
Photocatalysis(single stage)
(i) Relatively fast degradation of complex as wellas nonbiodegradable organic (macro)molecules(ii) Stable catalysts(iii) Moderate reaction conditions
(i) Not selective(ii) Limited selection of operating conditions(iii) Energy costs
Enzymatic conversion(single stage)
(i) More selective(ii) Moderate reaction conditions
(i) High enzyme loads required otherwise ligninhydrolysis is quite slow [16](ii) Enzymes are less stable1(iii) cost-intensive (POXs)2
1For example POXs are sensitive to high H2O2 concentrations2According to Torres and Ayala [86]
and atmospheric pressure chemical ionization (APCI) forLC-MS while GC-MS is based on electron ionization Aremedy to this can be the development of a database ofmodel lignin compounds based on a unanimous HPLC-MSmeasuring procedure This would mean much time and costexpensive investments for adequate personnel and material
Though product identification based on peak superpo-sition gives information on a possible product this can-not always be true for lignin degradation because of thelarge number of possible degradation products In order toproperly identify products from lignin degradation productisolation techniques are to be implemented and further ana-lytic methods such as tandem mass spectroscopy (MSMS)and other advanced structure enhancing research techniquessuch as 1H-NMR 13C-NMR have to be studied extensively
4 Photocatalysis and Its Suitability asIntegrated Technology in MultistageConcepts with Biocatalysis
Depending on lignin treatment specification photocatalysiswill find application for complete lignin mineralization toCO2and H
2O (single stage) or as integrated technology in
a multistage system aiming at partial conversion of ligninmacromolecules to organic low molecular weight intermedi-ates for a consecutive biological oxidation finally generatingvalue added end-products [15]
For biological oxidation of lignin or lignin deriva-tives H
2O2-dependent ligninolytic heme peroxidases (POXs
including lignin peroxidase (LiP EC 111114) manganeseperoxidase (MnP EC 111113) versatile peroxidase (VPEC 111116) O
2-dependent laccases (Lac EC 11032) and
extracellular enzymes from Basidiomycetous white-rot fungiare the most efficient lignin degraders in nature [89] Thussuch enzymatic systems especially POXs (with high redoxpotentials) have attractedmuch interest as industrial biocata-lyst [86 90]The enzyme degradationmechanism (in nature)is facilitated by nonenzymatic processes mainly throughfree ∙OH radicals also generated by the fungus (Fenton-type reaction) Those ∙OH radicals enable the requiredphysical contact between the enzymes and structural units ofthe lignin molecule due to numerous nonspecific oxidative
reactions [91] Both the photocatalytic (equations (2)ndash(7))and the POX reactionmechanism recently reviewed by Busseat al [25] are quite similar Consequently a combination ofphotocatalysis followed by an enzymatic oxidation maybe apromising concept utilizing lignin derivatives for exampleoriginated from industrial effluents [15 92] The advantagesforming biobased products are as follows see also Table 4 inthis context A reduction of the lignin polymerization degreevia photocatalysis at best in more biodegradable interme-diates and a simultaneous detoxification causes savings inenzyme costs since it will be expected that less enzyme loadsare necessary for sufficiently rapid reaction rates [15 91] As aresult hydraulic retention times in bioreactor systems shouldbe diminished Moreover the delignification is expectedto be enhanced simultaneously which was in recent yearsshown in a dual system by Kamwilaisak and Wright [93]using TiO
2H2O2UV for photocatalytic pretreatment and
Lac (from Trametes versicolor) in the subsequent biocatalyticstep Within 24 h they obtained in their dual system anelevation in delignification of 20 (without H
2O2) up to at
least 50 when H2O2was present
In previous studies the treatment of lignin (from thepulp and paper industry) containing effluents by fungus asa biological system (excreting appropriate lignin degradingenzyme cocktails) were more focused for posttreatmentexclusively (Duran et al [92] Reyes et al [94] and Gonzalezet al [95]) Shende et al [17] even examined ligninolyticbacteria with photooxidized kraft lignin as substrate
Although POXs are potential industrial biocatalysts [86]no application studies were found for the direct use in suchdual systems as described above The major reason may bethe complexity of the reaction mechanism (inclusive ligninderivatives as substrate and their analysis) per se on the onehand slowing down research and development processes Onthe other hand POXs are sensitive to their cosubstrate H
2O2
once it is supplied in excess causing considerable inactivation(for details refer to Busse et al [25]) At the present severalstudies are carried out modifying these enzymes regardingenhanced stability activity and selectivity as well Henceit can be expected that their technological applicability willbe raised significantly right after successful modification isreached [86]
International Journal of Photoenergy 15
5 Concluding Remarks
It is widely assumed that the photocatalytic degradation oflignin follows a radical reaction pathway which is similar tothat considered in thermal electrochemical and biochemicalprocesses However reporting on the degradation pathway oflignin derivatives and even that of lignin model compoundsare still amajor challengeThis is probably due to the complexnature and variety of possible degradation products Indeedthemechanism is far more complex considering other factorssuch as type of lignin type of catalyst pH illuminationsource and additives
Generally comparing the different photochemical pro-cesses poses a big challenge because of the wide variablesinvolved These discrepancies start from the source and typeof lignin followed by differences in reactor design illumi-nation source intensity of radiation and different types ofcatalyst An ideawould be to have a specific reference reactionwith well-defined starting parameters which include lignintype source and purity catalyst specifications illuminationsource and intensity so as to ease comparison of results
Basic process parameters such as catalyst concentrationsubstrate concentration addition of metal ion to catalyst pHand illumination have been discussed
Despite developed analytical technologies analyzinglignin degradation products remains challenging Proofssuch as mass spectroscopy (MS) HPLC 13C or 1H-NMRspectra from photocatalytic lignin degradations are not yetestablished
In order to properly identify products from lignin degra-dation product isolation techniques are to be implementedand further analytic methods such as MSMS and otheradvanced structure enhancing research techniques such as1H-NMR and 13C-NMR have to be studied extensively Aremedy to this can be the development of a databank ofmodel lignin compounds based on a unanimous HPLC-MSmeasuring procedure This would mean much time and costexpensive investments for adequate personnel and material
Photocatalysis is denoted as the most popular ligninpretreatment technology besides ozonation [96] Photo-catalyzed lignin may be an appropriate substrate for aconsecutive biocatalytic process using ligninolytic enzymes(POX andor Lac) as supported by experimental results ofKamwilaisak and Wright [93] Combining the advantages ofboth catalytic processes savings in the overall process costswill be expected in addition to elevated lignin conversionNonetheless extensive research work including POX modi-fications is still required
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Authorsrsquo Contribution
Colin Awungacha Lekelefac and Nadine Busse contributedequally to this work
Acknowledgments
The authors gratefully thank the Federal Ministry of Educa-tion and Research (BMBF) for funding (FKZ17N0310) Theresearchers also thank the Hessen State Ministry of HigherEducation Research andArts for the financial support withinthe Hessen initiative for scientific and economic excellence(LOEWE)
References
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and Climate Policy Outlook (WETO) Luxembourg Office forOfficial Publications of the European Communities 2003
[3] Green and Natural Polymers Are on the Rise 2014 httpwwwpolymersolutionscombloggreen-and-natural-polymers-on-the-rise
[4] J Zakzeski P C Bruijnincx A L Jongerius and B M Weck-huysen ldquoThe catalytic valorization of lignin for the productionof renewable chemicalsrdquo Chemical Reviews vol 110 no 6 pp3552ndash3599 2010
[5] B KammM Kamm P R Gruber and S KromusBiorefineries-Industrial Processes and Products Status Quo and Future Direc-tions vol 1 Wiley-VCH 2006
[6] R L Howard E Abotsi E L J van Rensburg and S HowardldquoLignocellulose biotechnology issues of bioconversion andenzyme productionrdquoAfrican Journal of Biotechnology vol 2 no12 pp 602ndash619 2003
[7] M Stocker ldquoBio- und BTL-Kraftstoffe in der Bioraffineriekatalytische Umwandlung Lignocellulose-reicher Biomasse mitporosen Stoffenrdquo Angewandte Chemie vol 120 no 48 pp9340ndash9351 2008
[8] J-P Lange ldquoLignocellulose conversion an introduction tochemistry process and economicsrdquo Biofuels Bioproducts andBiorefining vol 1 no 1 pp 39ndash48 2007
[9] D A I Goring ldquoThe physical chemistry of ligninrdquo Pure andApplied Chemistry vol 5 no 1-2 pp 233ndash310 1962
[10] M Ek G Gellerstedt and G HenrikssonWood Chemistry andBiotechnology Pulp and Paper Chemistry and Technology vol 1Walter de Gruyter GmbH amp Co KG Berlin Germany 2009
[11] B Saake and R Lehnen ldquoLigninrdquo in Ullmannrsquos Encyclopedia ofIndustrial Chemistry Wiley-VCH Verlag GmbH amp Co KGaAWeinheim Germany 2012
[12] M N S Kumar A K Mohanty L Erickson and M MisraldquoLignin and its applications with polymersrdquo Journal of BiobasedMaterials and Bioenergy vol 3 no 1 pp 1ndash24 2009
[13] R J A Gosselink E de Jong B Guran and A AbacherlildquoCo-ordination network for ligninmdashstandardisation produc-tion and applications adapted to market requirements(EUROLIGNIN)rdquo Industrial Crops and Products vol 20 no 2pp 121ndash129 2004
[14] D Fengel and G Wegener Wood Chemistry UltrastructureReactions Verlag Kessel 1984
[15] D Mantzavinos and E Psillakis ldquoEnhancement of biodegrad-ability of industrial wastewaters by chemical oxidation pre-treatmentrdquo Journal of Chemical Technology and Biotechnologyvol 79 no 5 pp 431ndash454 2004
16 International Journal of Photoenergy
[16] M Dashtban H Schraft and W Qin ldquoFungal bioconversionof lignocellulosic residues Opportunities amp perspectivesrdquo Inter-national Journal of Biological Sciences vol 5 no 6 pp 578ndash5952009
[17] A Shende R Jaswal D Harder-Heinz A Menan and RShende ldquoIntergrated photocatalytic and microbial degradationof kraft ligninrdquo Cleantech pp 120ndash123 2012
[18] F S Chakar and A J Ragauskas ldquoReview of current and futuresoftwood kraft lignin process chemistryrdquo Industrial Crops andProducts vol 20 no 2 pp 131ndash141 2004
[19] W G Glasser and H R Glasser ldquoEvaluation of ligninrsquoschemical structure by experimental and computer simulationtechniquesrdquo Paperi ja Puu vol 63 pp 71ndash83 1981
[20] M Erickson S Larsson and G E Miksche ldquoZur Struktur desLignins der FichterdquoActaChemica Scandinavica vol 27 pp 903ndash914 1973
[21] H Nimz ldquoDas Lignin der BuchemdashEntwurf eines Konstitution-sschemasrdquo Angewandte ChemiemdashInternational Edition vol 86pp 336ndash344 1974
[22] D V Evtuguin C P Neto J Rocha and J D P de JesusldquoOxidative delignification in the presence of molybdovana-dophosphate heteropolyanions mechanism and kinetic stud-iesrdquo Applied Catalysis A General vol 167 no 1 pp 123ndash1391998
[23] A L Linsebigler G Lu and J T Yates Jr ldquoPhotocatalysis onTiO2surfaces Principles mechanisms and selected resultsrdquo
Chemical Reviews vol 95 no 3 pp 735ndash758 1995[24] Y Miyata K Miyazaki M Miura Y Shimotori M Aoyama
and H Nakatani ldquoSolventless delignification of wood flourwith TiO
2poly(ethylene oxide) photocatalyst systemrdquo Journal
of Polymers and the Environment vol 21 no 1 pp 115ndash121 2013[25] N Busse D Wagner M Kraume and P Czermak ldquoReaction
kinetics of versatile peroxidase for the degradation of lignincompoundsrdquoTheAmerican Journal of Biochemistry andBiotech-nology vol 9 no 4 pp 365ndash394 2013
[26] M Tien and T K Kirk ldquoLignin-degrading enzyme fromphanerochaete chrysosporium Purification characterizationand catalytic properties of a uniqueH
2O2-requiring oxygenaserdquo
Proceedings of the National Academy of Sciences vol 81 pp2280ndash2284 1984
[27] T K Kirk M Tien P J Kersten M D Mozuch and BKalyanaraman ldquoLigninase of Phanerochaete chrysosporiumMechanism of its degradation of the non-phenolic arylglycerol120573-aryl ether substructure of ligninrdquo Biochemical Journal vol236 no 1 pp 279ndash287 1986
[28] T Lundell RWever R Floris et al ldquoLignin peroxidase L3 fromPhlebia radiata pre-steady-state and steady-state studies withveratryl alcohol and a non-phenolic lignin model compound1-(34-dimethoxyphenyl)-2-(2-methoxyphenoxy)propane-13-diolrdquo European Journal of Biochemistry vol 211 no 3 pp391ndash402 1993
[29] H E Schoemaker T K Lundell A I Hatakka and K PiontekldquoThe oxidation of veratryl alcohol dimeric lignin models andlignin by lignin peroxidase the redox cycle revisitedrdquo FEMSMicrobiology Reviews vol 13 no 2-3 pp 321ndash331 1994
[30] J M Palmer P J Harvey and H E Schoemaker ldquoThe role ofperoxidases radical cations and oxygen in the degradation oflignin [and discussion]rdquo Philosophical Transactions of the RoyalSociety of London A vol 321 no 1561 pp 495ndash505 1987
[31] C Hill Wood Modification Chemical Thermal and OtherProcesses John Wiley amp Sons Chichester UK 2006
[32] Y-H Xu H-R Chen Z-X Zeng and B Lei ldquoInvestigation onmechanism of photocatalytic activity enhancement of nanome-ter cerium-doped titaniardquo Applied Surface Science vol 252 no24 pp 8565ndash8570 2006
[33] L Tonucci F Coccia M Bressan and N DrsquoAlessandro ldquoMildphotocatalysed and catalysed green oxidation of lignin auseful pathway to low-molecular-weight derivativesrdquoWaste andBiomass Valorization vol 3 no 2 pp 165ndash174 2012
[34] A Castellan N Colombo C Vanucci P Fornier de HViolet and H Bouas-Laurent ldquoPhotodegradation of ligninA photochemical study of an O-methylated 120572-carbonyl 120573-1lignin model dimer 12-di(3101584041015840-dimethoxyphenyl) ethanone(deoxyveratroin)rdquo Journal of Photochemistry and PhotobiologyA vol 51 pp 451ndash467 1990
[35] A Fujishima X Zhang and D A Tryk ldquoTiO2photocatalysis
and related surface phenomenardquo Surface Science Reports vol63 no 12 pp 515ndash582 2008
[36] C S Turchi and D F Ollis ldquoPhotocatalytic reactor designan example of mass-transfer limitations with an immobilizedcatalystrdquo The Journal of Physical Chemistry vol 92 no 23 pp6852ndash6853 1988
[37] R Subasri M Tripathi K Murugan J Revathi G V N Raoand T N Rao ldquoInvestigations on the photocatalytic activity ofsolndashgel derived plain and Fe3+Nb5+ doped titania coatings onglass substratesrdquo Materials Chemistry and Physics vol 124 pp63ndash68 2010
[38] N Serpone ldquoRelative photonic efficiencies and quantum yieldsin heterogeneous photocatalysisrdquo Journal of Photochemistry andPhotobiology A Chemistry vol 104 no 1ndash3 pp 1ndash12 1997
[39] M R Hoffmann S T Martin W Choi and D W BahnemannldquoEnvironmental applications of semiconductor photocatalysisrdquoChemical Reviews vol 95 no 1 pp 69ndash96 1995
[40] C D Jaeger and A J Bard ldquoSpin trapping and electron spinresonance detection of radical intermediates in the photode-composition of water at titanium dioxide particulate systemsrdquoThe Journal of Physical Chemistry vol 83 no 24 pp 3146ndash31521979
[41] R W Matthews ldquoHydroxylation reactions induced by near-ultraviolet photolysis of aqueous titaniumdioxide suspensionsrdquoJournal of the Chemical Society Faraday Transactions vol 80no 2 pp 457ndash471 1984
[42] A E H Machado A M Furuyama S Z Falone R RuggieroD D S Perez and A Castellan ldquoPhotocatalytic degradation oflignin and lignin models using titanium dioxide the role of thehydroxyl radicalrdquoChemosphere vol 40 no 1 pp 115ndash124 2000
[43] W Sigg and L Stumm Aquatische Chemie Stuttgart 2 AuflageBG Teubner Stuttgart Germany 1991
[44] O Legrini E Oliveros and A M Braun ldquoPhotochemicalprocesses for water treatmentrdquo Chemical Reviews vol 93 no2 pp 671ndash698 1993
[45] G Rothenberger J Moser M Gratzel N Serpone and DSharma ldquoCharge carrier trapping and recombination dynamicsin small semiconductor particlesrdquo Journal of the AmericanChemical Society vol 107 no 26 pp 8054ndash8059 1985
[46] P Mazellier M Sarakha A Rossi and M Bolte ldquoThe aqueousphotochemistry of 26-dimethylphenol Evidence for the frag-mentation of the 120572 C-C bondrdquo Journal of Photochemistry andPhotobiology A vol 115 no 2 pp 117ndash121 1998
[47] M Ksibi S B Amor S Cherif E Elaloui A Houas and MElaloui ldquoPhotodegradation of lignin from black liquor using aUVTiO
2systemrdquo Journal of Photochemistry and Photobiology
A vol 154 no 2-3 pp 211ndash218 2003
International Journal of Photoenergy 17
[48] S K Kansal M Singh and D Sud ldquoStudies on TiO2ZnO
photocatalysed degradation of ligninrdquo Journal of HazardousMaterials vol 153 no 1-2 pp 412ndash417 2008
[49] A Dahm and L A Lucia ldquoTitanium dioxide catalyzed pho-todegradation of lignin in industrial effluentsrdquo Industrial andEngineering Chemistry Research vol 43 no 25 pp 7996ndash80002004
[50] E Portjanskaja and S Preis ldquoAqueous photocatalytic oxidationof lignin the influence of mineral admixturesrdquo InternationalJournal of Photoenergy vol 2007Article ID76730 7 pages 2007
[51] K Tanaka R C R Calanag and T Hisanaga ldquoPhotocatalyzeddegradation of lignin on TiO
2rdquo Journal of Molecular Catalysis
A Chemical vol 138 no 2-3 pp 287ndash294 1999[52] M Tian J Wen D MacDonald R M Asmussen and A Chen
ldquoA novel approach for lignin modification and degradationrdquoElectrochemistry Communications vol 12 no 4 pp 527ndash5302010
[53] K PanM Tian Z-H Jiang B Kjartanson and A Chen ldquoElec-trochemical oxidation of lignin at lead dioxide nanoparticlesphotoelectrodeposited on TiO
2nanotube arraysrdquo Electrochim-
ica Acta vol 60 pp 147ndash153 2012[54] C Awungacha Lekelefac J Hild P Czermak and M Herren-
bauer ldquoPhotocatalytic active coatings for lignin degradationin a continuous packed bed reactorrdquo International Journal ofPhotoenergy vol 2014 Article ID 502326 10 pages 2014
[55] C Awungacha Lekelefac P Czermak and M HerrenbauerldquoEvaluation of photocatalytic active coatings on sintered glasstubes by methylene bluerdquo International Journal of Photoenergyvol 2013 Article ID 614567 9 pages 2013
[56] Y-S Ma C-N Chang Y-P Chiang H-F Sung and A CChao ldquoPhotocatalytic degradation of lignin using PtTiO
2as
the catalystrdquo Chemosphere vol 71 no 5 pp 998ndash1004 2008[57] H Ohnishi M Matsumura H Tsubomura and M Iwasaki
ldquoBleaching of lignin solution by a photocatalyzed reactionon semiconductor photocatalystsrdquo Industrial and EngineeringChemistry Research vol 28 no 6 pp 719ndash724 1989
[58] C A K Gouvea F Wypych S G Moraes N Duran and PPeralta-Zamora ldquoSemiconductor-assisted photodegradation oflignin dye and kraft effluent by Ag-doped ZnOrdquo Chemospherevol 40 no 4 pp 427ndash432 2000
[59] A V Vahatalo K Salonen M Salkinoja-Salonen and AHatakka ldquoPhotochemical mineralization of synthetic lignin inlake water indicates enhanced turnover of aromatic organicmatter under solar radiationrdquo Biodegradation vol 10 no 6 pp415ndash420 1999
[60] H de Lasa B Serrano and M Salaices Photocatalytic ReactionEngineering Springer New York NY USA 2005
[61] K Hashimoto H Irie and A Fujishima ldquoTiO 2 photocatalysisA historical overview and future prospectsrdquo Japanese Journal ofApplied Physics vol 44 no 12 pp 8269ndash8285 2005
[62] M A Behnajady N Modirshahla and R Hamzavi ldquoKineticstudy on photocatalytic degradation of CI Acid Yellow 23 byZnOphotocatalystrdquo Journal ofHazardousMaterials vol 133 no1ndash3 pp 226ndash232 2006
[63] M Addamo V Augugliaro A di Paola et al ldquoPhotocatalyticthin films of TiO
2formed by a sol-gel process using titanium
tetraisopropoxide as the precursorrdquo Thin Solid Films vol 516no 12 pp 3802ndash3807 2008
[64] N Negishi K Takeuchi and T Ibusuki ldquoPreparation of theTiO2thin filmphotocatalyst by the dip-coating processrdquo Journal
of Sol-Gel Science and Technology vol 13 no 1ndash3 pp 691ndash6941998
[65] L Rideh A Wehrer D Ronze and A Zoulalian ldquoPhotocat-alytic degradation of 2-chlorophenol in TiO
2aqueous suspen-
sion modeling of reaction raterdquo Industrial and EngineeringChemistry Research vol 36 no 11 pp 4712ndash4718 1997
[66] R-ADoongC-HChen RAMaithreepala and S-MChangldquoThe influence of pHand cadmiumsulfide on the photocatalyticdegradation of 2-chlorophenol in titanium dioxide suspen-sionsrdquoWater Research vol 35 no 12 pp 2873ndash2880 2001
[67] K V Sarkanen A Islam and C D Anderson ldquoOzonationrdquo inMethods in Lignin Chemistry S Y Lin and C W Dence EdsSpringer Series inWood Science pp 387ndash406 Springer BerlinGermany 1992
[68] G Gellerstedt and E-L Lindfors ldquoStructural changes in ligninduring kraft pulpingrdquo Holzforschung vol 38 no 3 pp 151ndash1581984
[69] B Neppolian H C Choi M V Shankar B Arabindoo andV Murugesan in Proceedings of the International Symposiumon Environmental Pollution Control and Waste Management(EPCOWM 02) p 647 2002
[70] C Pouteau P Dole B Cathala L Averous and N BoquillonldquoAntioxidant properties of lignin in polypropylenerdquo PolymerDegradation and Stability vol 81 no 1 pp 9ndash18 2003
[71] J Hafren T Fujino and T Itoh ldquoChanges in cell wall archi-tecture of differentiating tracheids of Pinus thunbergii duringlignificationrdquo Plant and Cell Physiology vol 40 no 5 pp 532ndash541 1999
[72] T Dizhbite G Telysheva V Jurkjane and U Viesturs ldquoChar-acterization of the radical scavenging activity of lignins naturalantioxidantsrdquo Bioresource Technology vol 95 no 3 pp 309ndash3172004
[73] K Hofstadler R Bauer S Novalic and S G Heisier ldquoNew reac-tor design for photocatalytic wastewater treatment with TiO
2
immobilized on fused-silica glass fibers photomineralization of4-chlorophenolrdquo Environmental Science amp Technology vol 28no 4 pp 670ndash674 1994
[74] J Villasenor and H D Mansilla ldquoEffect of temperature onkraft black liquor degradation by ZnO-photoassisted catalysisrdquoJournal of Photochemistry and Photobiology A Chemistry vol93 no 2-3 pp 205ndash209 1996
[75] B H Bielski D E Cabelli L A Ravindra and A B RossldquoReactivity of HO
2Ominus2Radicals in aqueous solutionrdquo Journal
of Physical Chemistry vol 14 pp 1041ndash1100 1985[76] B H J Bielski and A O Allen ldquoMechanism of the dispropor-
tionation of superoxide radicalsrdquo Journal of Physical Chemistryvol 81 no 11 pp 1048ndash1050 1977
[77] B Halliwell and J M C Gutteridge ldquoThe importance of freeradicals and catalytic metal ions in human diseasesrdquoMolecularAspects of Medicine vol 8 no 2 pp 89ndash193 1985
[78] J M Palmer P J Harvey and H E Schoemaker ldquoThe role ofperoxidases radical cations and oxygen in the degradation oflignin [and discussion]rdquo Philosophical Transaction of the RoyalSociety A vol 321 no 1561 pp 495ndash505 1987
[79] S Y Yin and C W Dense Methods in Lignin ChemistrySpringer New York NY USA 1992
[80] P Kumar S Kumar and N K Bhardwaj ldquoPhotocatalyticoxidation of elemental chlorine free bleaching effluent withUVTiO
2rdquo in Proceedings of the 2nd International Conference on
Environmental Science and Technology (ICEST rsquo11) SingaporeFebruary 2011
[81] K Kobayakawa Y Sato S Nakamura and A FujishimaldquoPhotodecomposition of Kraft lignin catalyzed by titanium
18 International Journal of Photoenergy
dioxiderdquo Bulletin of the Chemical Society of Japan vol 62 no11 pp 3433ndash3436 1989
[82] R B Kinstre ldquoAn overview of strategies for reducing theenvironmental impact of bleach-plant effluentsrdquo Tappi Journalvol 76 no 5 pp 105ndash113 1993
[83] A Castellan H Choudhury R Stephen Davidson and SGrelier ldquoComparative study of stone-ground wood pulp andnative wood 3 Application of fluorescence spectroscopy to astudy of the weathering of stone-ground pulp and native woodrdquoJournal of Photochemistry and Photobiology A Chemistry vol81 no 2 pp 123ndash130 1994
[84] B Albinsson S Li K Lundquist and R Stomberg ldquoThe originof lignin fluorescencerdquo Journal of Molecular Structure vol 508no 1ndash3 pp 19ndash27 1999
[85] S Baumberger A Abaecherli M Fasching et al ldquoMolar massdetermination of lignins by size-exclusion chromatographytowards standardisation of the methodrdquo Holzforschung vol 61no 4 pp 459ndash468 2007
[86] E Torres andM Ayala Biocatalysis Based on Heme PeroxidasesSpringer New York NY USA 1st edition 2010
[87] R Pecina P Burtscher G Bonn and O Bobleter ldquoGC-MSand HPLC analyses of lignin degradation products in biomasshydrolyzatesrdquo Fresenius Zeitschrift fur Analytische Chemie vol325 no 5 pp 461ndash465 1986
[88] NIST NIST Standard Reference Database The National Insti-tute of Standards and Technology 2014 httpwwwnistgovsrdnist1acfm
[89] T K Kirk and R L Farrell ldquoEnzymatic ldquocombustionrdquo themicrobial degradation of ligninrdquo Annual Review of Microbiol-ogy vol 41 pp 465ndash501 1987
[90] A TMartinez ldquoHigh redox potential peroxidasesrdquo in IndustrialEnzymes Structure Function and Applications K-B BeckerEd pp 477ndash488 Springer Amsterdam The Netherlands 1stedition 2007
[91] M Dashtban H Schraft T A Syed and W Qin ldquoFungalbiodegradation and enzymatic modification of ligninrdquo Interna-tional Journal of Biochemistry and Molecular Biology vol 1 no1 pp 36ndash50 2010
[92] N Duran E Esposito L H Innocentini-Mei and V P CanhosldquoA new alternative process for Kraft E1 effluent treatmentrdquoBiodegradation vol 5 no 1 pp 13ndash19 1994
[93] K Kamwilaisak and P C Wright ldquoInvestigating laccase andtitanium dioxide for lignin degradationrdquo Energy and Fuels vol26 no 4 pp 2400ndash2406 2012
[94] J Reyes M Dezotti H Mansilla J Villasenor E Espositoand N Duran ldquoBiomass photochemistry-XXII combined pho-tochemical and biological process for treatment of Kraft E1effluentrdquoApplied Catalysis B Environmental vol 15 no 3-4 pp211ndash219 1998
[95] L F Gonzalez V Sarria and O F Sanchez ldquoDegradationof chlorophenols by sequential biological-advanced oxidativeprocess using Trametespubescens and TiO
2UVrdquo Bioresource
Technology vol 101 no 10 pp 3493ndash3499 2010[96] J H Lora and W G Glasser ldquoRecent industrial applications
of lignin a sustainable alternative to nonrenewable materialsrdquoJournal of Polymers and the Environment vol 10 no 1-2 pp 39ndash48 2002
Submit your manuscripts athttpwwwhindawicom
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
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International Journal ofPhotoenergy
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International Journal of Photoenergy 13
Figure 9 Gradual change from the characteristic yellow ligninsulfonate to a colorless liquid after a period of 20 h Catalyst TiO
2-
P25 (Degussa) +TEOSUV-light 25∘C lignin concentration 05 gL[54]
converted to CO and CO2and H
2O and NH
3 Total organic
carbon (TOC) describes the amount of carbon bound inan organic compound while DOC describes the dissolvedfraction of organic carbon ADMI measures the amount ofdyestuff in water
Decolorization of lignin solution has been reported tobe another parameter observed during photocatalytic degra-dation Color is an indirect indicator of the lignin amountThe higher the color intensity of the solution is the greaterthe lignin content is (high concentrated lignin solutions egblack liquor appear dark brown) [82] Thus color changescan be interpreted as conversion of lignin to transient speciesor conversion to CO
2and H
2O Awungacha Lekelefac et al
[54] observed a gradual change from the characteristic yellowlignin (when highly diluted) to a colorless liquid after a periodof 20 h with sol-gel derived TiO
2nanocrystalline coatings on
sintered borosilicate glass as depicted in Figure 9 A corre-sponding decrease in DC values close to 82 was observedfor TiO
2-P25-SiO
2catalyst confirming degradation This is
shown in Figure 10These findings are analogous to that of Ohnishi et al
[57] who reported the bleaching of lignin when illuminatedcontinuously and that the solution becomes colorless Tothat the chemical oxygen demand (COD) value decreasesgenerating carbon dioxide and a small amount of carbonmonoxide as the main gaseous products COD removalwas reported to be effective at low lignin concentrations ascompared to high lignin concentrations [48]
Another applied analytical technique is fluorescencedetection directly coupled to a high performance liquidchromatography (HPLC) as a means to identify nonaliphaticcomponent in the complex mixture of lignin degradationproducts [54] Fluorescence emission in lignin is attributedto aromatic structures such as conjugated carbonyl biphenylphenylcoumarone and stilbene groups [83 84] AwungachaLekelefac et al [54] observed peaks on both HPLC and
0 200 400 600 800 1000 12000
50
100
150
200
250
Diss
olve
d ca
rbon
(ppm
)
t (min)
TiO2-P25-SiO2
TiO2-P25-SiO2 + PtTiOSO4minus30 6wt
ZnO + TiO2-P25-SiO2
No coating UV light
Figure 10 Variation of DC with time of aqueous lignin solutionfrom waste paper water irradiated with UV light (280ndash420 nm) fordifferent time intervals The spectra are obtained for sol-gel derivedTiO2nanocrystalline coatings (TiO
2-P25-SiO
2+Pt TiO
2-P25-SiO
2
TiOSO4minus306 wt ZnO + TiO
2-P25-SiO
2) [54]
fluorescence chromatograms suggesting the production ofnew substances and fluorophores
Despite developed analytical technologies analyzinglignin degradation products remains challenging Proofssuch as mass spectroscopy (MS) HPLC 13C or 1H-NMRspectra from photocatalytic lignin degradations are not yetestablished The setback to qualitatively and quantitativelyanalyze lignin and its degradation products starts from thenative lignin polymer itself with its indefinite polymericstructure and multiple bond types Also the influence ofdifferent pretreatments additives and the wide variety ofcompounds obtainable from its degradation makes ligninanalysis challenging [85] Moreover lignin streams couldcontain proteins inorganic salts and other potential poisonsthat generally complicate catalysis [4]
The challenge to identify and separate the productsstreams derived from the photocatalytic degradation ligninis also worth noting Lignin product stream is highly func-tionalized and conventional techniques such as gas chro-matography have the disadvantage of requiring a time-consuming derivatization step Also because of high boilingpoint of substances arising from lignin degradation it is noteasily applicable High performance liquid chromatography(HPLC) seems to be the remedy because analysis can becarried out without derivatization but the exact identificationof the separated substances is difficult because of the numer-ous peaks arising from such a chromatogram [87] Unfortu-nately well-established databases such as that of the nationalinstitute of standard and technology (NIST) [88] cannot giveinformation onHPLC-MS chromatogramsThis is because ofthe ionization sources such as electrospray ionization (ESI)
14 International Journal of Photoenergy
Table 4 Main advantages and disadvantages photocatalysis versus enzymatic biocatalysis
Lignin degradation process Advantages Disadvantages
Photocatalysis(single stage)
(i) Relatively fast degradation of complex as wellas nonbiodegradable organic (macro)molecules(ii) Stable catalysts(iii) Moderate reaction conditions
(i) Not selective(ii) Limited selection of operating conditions(iii) Energy costs
Enzymatic conversion(single stage)
(i) More selective(ii) Moderate reaction conditions
(i) High enzyme loads required otherwise ligninhydrolysis is quite slow [16](ii) Enzymes are less stable1(iii) cost-intensive (POXs)2
1For example POXs are sensitive to high H2O2 concentrations2According to Torres and Ayala [86]
and atmospheric pressure chemical ionization (APCI) forLC-MS while GC-MS is based on electron ionization Aremedy to this can be the development of a database ofmodel lignin compounds based on a unanimous HPLC-MSmeasuring procedure This would mean much time and costexpensive investments for adequate personnel and material
Though product identification based on peak superpo-sition gives information on a possible product this can-not always be true for lignin degradation because of thelarge number of possible degradation products In order toproperly identify products from lignin degradation productisolation techniques are to be implemented and further ana-lytic methods such as tandem mass spectroscopy (MSMS)and other advanced structure enhancing research techniquessuch as 1H-NMR 13C-NMR have to be studied extensively
4 Photocatalysis and Its Suitability asIntegrated Technology in MultistageConcepts with Biocatalysis
Depending on lignin treatment specification photocatalysiswill find application for complete lignin mineralization toCO2and H
2O (single stage) or as integrated technology in
a multistage system aiming at partial conversion of ligninmacromolecules to organic low molecular weight intermedi-ates for a consecutive biological oxidation finally generatingvalue added end-products [15]
For biological oxidation of lignin or lignin deriva-tives H
2O2-dependent ligninolytic heme peroxidases (POXs
including lignin peroxidase (LiP EC 111114) manganeseperoxidase (MnP EC 111113) versatile peroxidase (VPEC 111116) O
2-dependent laccases (Lac EC 11032) and
extracellular enzymes from Basidiomycetous white-rot fungiare the most efficient lignin degraders in nature [89] Thussuch enzymatic systems especially POXs (with high redoxpotentials) have attractedmuch interest as industrial biocata-lyst [86 90]The enzyme degradationmechanism (in nature)is facilitated by nonenzymatic processes mainly throughfree ∙OH radicals also generated by the fungus (Fenton-type reaction) Those ∙OH radicals enable the requiredphysical contact between the enzymes and structural units ofthe lignin molecule due to numerous nonspecific oxidative
reactions [91] Both the photocatalytic (equations (2)ndash(7))and the POX reactionmechanism recently reviewed by Busseat al [25] are quite similar Consequently a combination ofphotocatalysis followed by an enzymatic oxidation maybe apromising concept utilizing lignin derivatives for exampleoriginated from industrial effluents [15 92] The advantagesforming biobased products are as follows see also Table 4 inthis context A reduction of the lignin polymerization degreevia photocatalysis at best in more biodegradable interme-diates and a simultaneous detoxification causes savings inenzyme costs since it will be expected that less enzyme loadsare necessary for sufficiently rapid reaction rates [15 91] As aresult hydraulic retention times in bioreactor systems shouldbe diminished Moreover the delignification is expectedto be enhanced simultaneously which was in recent yearsshown in a dual system by Kamwilaisak and Wright [93]using TiO
2H2O2UV for photocatalytic pretreatment and
Lac (from Trametes versicolor) in the subsequent biocatalyticstep Within 24 h they obtained in their dual system anelevation in delignification of 20 (without H
2O2) up to at
least 50 when H2O2was present
In previous studies the treatment of lignin (from thepulp and paper industry) containing effluents by fungus asa biological system (excreting appropriate lignin degradingenzyme cocktails) were more focused for posttreatmentexclusively (Duran et al [92] Reyes et al [94] and Gonzalezet al [95]) Shende et al [17] even examined ligninolyticbacteria with photooxidized kraft lignin as substrate
Although POXs are potential industrial biocatalysts [86]no application studies were found for the direct use in suchdual systems as described above The major reason may bethe complexity of the reaction mechanism (inclusive ligninderivatives as substrate and their analysis) per se on the onehand slowing down research and development processes Onthe other hand POXs are sensitive to their cosubstrate H
2O2
once it is supplied in excess causing considerable inactivation(for details refer to Busse et al [25]) At the present severalstudies are carried out modifying these enzymes regardingenhanced stability activity and selectivity as well Henceit can be expected that their technological applicability willbe raised significantly right after successful modification isreached [86]
International Journal of Photoenergy 15
5 Concluding Remarks
It is widely assumed that the photocatalytic degradation oflignin follows a radical reaction pathway which is similar tothat considered in thermal electrochemical and biochemicalprocesses However reporting on the degradation pathway oflignin derivatives and even that of lignin model compoundsare still amajor challengeThis is probably due to the complexnature and variety of possible degradation products Indeedthemechanism is far more complex considering other factorssuch as type of lignin type of catalyst pH illuminationsource and additives
Generally comparing the different photochemical pro-cesses poses a big challenge because of the wide variablesinvolved These discrepancies start from the source and typeof lignin followed by differences in reactor design illumi-nation source intensity of radiation and different types ofcatalyst An ideawould be to have a specific reference reactionwith well-defined starting parameters which include lignintype source and purity catalyst specifications illuminationsource and intensity so as to ease comparison of results
Basic process parameters such as catalyst concentrationsubstrate concentration addition of metal ion to catalyst pHand illumination have been discussed
Despite developed analytical technologies analyzinglignin degradation products remains challenging Proofssuch as mass spectroscopy (MS) HPLC 13C or 1H-NMRspectra from photocatalytic lignin degradations are not yetestablished
In order to properly identify products from lignin degra-dation product isolation techniques are to be implementedand further analytic methods such as MSMS and otheradvanced structure enhancing research techniques such as1H-NMR and 13C-NMR have to be studied extensively Aremedy to this can be the development of a databank ofmodel lignin compounds based on a unanimous HPLC-MSmeasuring procedure This would mean much time and costexpensive investments for adequate personnel and material
Photocatalysis is denoted as the most popular ligninpretreatment technology besides ozonation [96] Photo-catalyzed lignin may be an appropriate substrate for aconsecutive biocatalytic process using ligninolytic enzymes(POX andor Lac) as supported by experimental results ofKamwilaisak and Wright [93] Combining the advantages ofboth catalytic processes savings in the overall process costswill be expected in addition to elevated lignin conversionNonetheless extensive research work including POX modi-fications is still required
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Authorsrsquo Contribution
Colin Awungacha Lekelefac and Nadine Busse contributedequally to this work
Acknowledgments
The authors gratefully thank the Federal Ministry of Educa-tion and Research (BMBF) for funding (FKZ17N0310) Theresearchers also thank the Hessen State Ministry of HigherEducation Research andArts for the financial support withinthe Hessen initiative for scientific and economic excellence(LOEWE)
References
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and Climate Policy Outlook (WETO) Luxembourg Office forOfficial Publications of the European Communities 2003
[3] Green and Natural Polymers Are on the Rise 2014 httpwwwpolymersolutionscombloggreen-and-natural-polymers-on-the-rise
[4] J Zakzeski P C Bruijnincx A L Jongerius and B M Weck-huysen ldquoThe catalytic valorization of lignin for the productionof renewable chemicalsrdquo Chemical Reviews vol 110 no 6 pp3552ndash3599 2010
[5] B KammM Kamm P R Gruber and S KromusBiorefineries-Industrial Processes and Products Status Quo and Future Direc-tions vol 1 Wiley-VCH 2006
[6] R L Howard E Abotsi E L J van Rensburg and S HowardldquoLignocellulose biotechnology issues of bioconversion andenzyme productionrdquoAfrican Journal of Biotechnology vol 2 no12 pp 602ndash619 2003
[7] M Stocker ldquoBio- und BTL-Kraftstoffe in der Bioraffineriekatalytische Umwandlung Lignocellulose-reicher Biomasse mitporosen Stoffenrdquo Angewandte Chemie vol 120 no 48 pp9340ndash9351 2008
[8] J-P Lange ldquoLignocellulose conversion an introduction tochemistry process and economicsrdquo Biofuels Bioproducts andBiorefining vol 1 no 1 pp 39ndash48 2007
[9] D A I Goring ldquoThe physical chemistry of ligninrdquo Pure andApplied Chemistry vol 5 no 1-2 pp 233ndash310 1962
[10] M Ek G Gellerstedt and G HenrikssonWood Chemistry andBiotechnology Pulp and Paper Chemistry and Technology vol 1Walter de Gruyter GmbH amp Co KG Berlin Germany 2009
[11] B Saake and R Lehnen ldquoLigninrdquo in Ullmannrsquos Encyclopedia ofIndustrial Chemistry Wiley-VCH Verlag GmbH amp Co KGaAWeinheim Germany 2012
[12] M N S Kumar A K Mohanty L Erickson and M MisraldquoLignin and its applications with polymersrdquo Journal of BiobasedMaterials and Bioenergy vol 3 no 1 pp 1ndash24 2009
[13] R J A Gosselink E de Jong B Guran and A AbacherlildquoCo-ordination network for ligninmdashstandardisation produc-tion and applications adapted to market requirements(EUROLIGNIN)rdquo Industrial Crops and Products vol 20 no 2pp 121ndash129 2004
[14] D Fengel and G Wegener Wood Chemistry UltrastructureReactions Verlag Kessel 1984
[15] D Mantzavinos and E Psillakis ldquoEnhancement of biodegrad-ability of industrial wastewaters by chemical oxidation pre-treatmentrdquo Journal of Chemical Technology and Biotechnologyvol 79 no 5 pp 431ndash454 2004
16 International Journal of Photoenergy
[16] M Dashtban H Schraft and W Qin ldquoFungal bioconversionof lignocellulosic residues Opportunities amp perspectivesrdquo Inter-national Journal of Biological Sciences vol 5 no 6 pp 578ndash5952009
[17] A Shende R Jaswal D Harder-Heinz A Menan and RShende ldquoIntergrated photocatalytic and microbial degradationof kraft ligninrdquo Cleantech pp 120ndash123 2012
[18] F S Chakar and A J Ragauskas ldquoReview of current and futuresoftwood kraft lignin process chemistryrdquo Industrial Crops andProducts vol 20 no 2 pp 131ndash141 2004
[19] W G Glasser and H R Glasser ldquoEvaluation of ligninrsquoschemical structure by experimental and computer simulationtechniquesrdquo Paperi ja Puu vol 63 pp 71ndash83 1981
[20] M Erickson S Larsson and G E Miksche ldquoZur Struktur desLignins der FichterdquoActaChemica Scandinavica vol 27 pp 903ndash914 1973
[21] H Nimz ldquoDas Lignin der BuchemdashEntwurf eines Konstitution-sschemasrdquo Angewandte ChemiemdashInternational Edition vol 86pp 336ndash344 1974
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[23] A L Linsebigler G Lu and J T Yates Jr ldquoPhotocatalysis onTiO2surfaces Principles mechanisms and selected resultsrdquo
Chemical Reviews vol 95 no 3 pp 735ndash758 1995[24] Y Miyata K Miyazaki M Miura Y Shimotori M Aoyama
and H Nakatani ldquoSolventless delignification of wood flourwith TiO
2poly(ethylene oxide) photocatalyst systemrdquo Journal
of Polymers and the Environment vol 21 no 1 pp 115ndash121 2013[25] N Busse D Wagner M Kraume and P Czermak ldquoReaction
kinetics of versatile peroxidase for the degradation of lignincompoundsrdquoTheAmerican Journal of Biochemistry andBiotech-nology vol 9 no 4 pp 365ndash394 2013
[26] M Tien and T K Kirk ldquoLignin-degrading enzyme fromphanerochaete chrysosporium Purification characterizationand catalytic properties of a uniqueH
2O2-requiring oxygenaserdquo
Proceedings of the National Academy of Sciences vol 81 pp2280ndash2284 1984
[27] T K Kirk M Tien P J Kersten M D Mozuch and BKalyanaraman ldquoLigninase of Phanerochaete chrysosporiumMechanism of its degradation of the non-phenolic arylglycerol120573-aryl ether substructure of ligninrdquo Biochemical Journal vol236 no 1 pp 279ndash287 1986
[28] T Lundell RWever R Floris et al ldquoLignin peroxidase L3 fromPhlebia radiata pre-steady-state and steady-state studies withveratryl alcohol and a non-phenolic lignin model compound1-(34-dimethoxyphenyl)-2-(2-methoxyphenoxy)propane-13-diolrdquo European Journal of Biochemistry vol 211 no 3 pp391ndash402 1993
[29] H E Schoemaker T K Lundell A I Hatakka and K PiontekldquoThe oxidation of veratryl alcohol dimeric lignin models andlignin by lignin peroxidase the redox cycle revisitedrdquo FEMSMicrobiology Reviews vol 13 no 2-3 pp 321ndash331 1994
[30] J M Palmer P J Harvey and H E Schoemaker ldquoThe role ofperoxidases radical cations and oxygen in the degradation oflignin [and discussion]rdquo Philosophical Transactions of the RoyalSociety of London A vol 321 no 1561 pp 495ndash505 1987
[31] C Hill Wood Modification Chemical Thermal and OtherProcesses John Wiley amp Sons Chichester UK 2006
[32] Y-H Xu H-R Chen Z-X Zeng and B Lei ldquoInvestigation onmechanism of photocatalytic activity enhancement of nanome-ter cerium-doped titaniardquo Applied Surface Science vol 252 no24 pp 8565ndash8570 2006
[33] L Tonucci F Coccia M Bressan and N DrsquoAlessandro ldquoMildphotocatalysed and catalysed green oxidation of lignin auseful pathway to low-molecular-weight derivativesrdquoWaste andBiomass Valorization vol 3 no 2 pp 165ndash174 2012
[34] A Castellan N Colombo C Vanucci P Fornier de HViolet and H Bouas-Laurent ldquoPhotodegradation of ligninA photochemical study of an O-methylated 120572-carbonyl 120573-1lignin model dimer 12-di(3101584041015840-dimethoxyphenyl) ethanone(deoxyveratroin)rdquo Journal of Photochemistry and PhotobiologyA vol 51 pp 451ndash467 1990
[35] A Fujishima X Zhang and D A Tryk ldquoTiO2photocatalysis
and related surface phenomenardquo Surface Science Reports vol63 no 12 pp 515ndash582 2008
[36] C S Turchi and D F Ollis ldquoPhotocatalytic reactor designan example of mass-transfer limitations with an immobilizedcatalystrdquo The Journal of Physical Chemistry vol 92 no 23 pp6852ndash6853 1988
[37] R Subasri M Tripathi K Murugan J Revathi G V N Raoand T N Rao ldquoInvestigations on the photocatalytic activity ofsolndashgel derived plain and Fe3+Nb5+ doped titania coatings onglass substratesrdquo Materials Chemistry and Physics vol 124 pp63ndash68 2010
[38] N Serpone ldquoRelative photonic efficiencies and quantum yieldsin heterogeneous photocatalysisrdquo Journal of Photochemistry andPhotobiology A Chemistry vol 104 no 1ndash3 pp 1ndash12 1997
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[40] C D Jaeger and A J Bard ldquoSpin trapping and electron spinresonance detection of radical intermediates in the photode-composition of water at titanium dioxide particulate systemsrdquoThe Journal of Physical Chemistry vol 83 no 24 pp 3146ndash31521979
[41] R W Matthews ldquoHydroxylation reactions induced by near-ultraviolet photolysis of aqueous titaniumdioxide suspensionsrdquoJournal of the Chemical Society Faraday Transactions vol 80no 2 pp 457ndash471 1984
[42] A E H Machado A M Furuyama S Z Falone R RuggieroD D S Perez and A Castellan ldquoPhotocatalytic degradation oflignin and lignin models using titanium dioxide the role of thehydroxyl radicalrdquoChemosphere vol 40 no 1 pp 115ndash124 2000
[43] W Sigg and L Stumm Aquatische Chemie Stuttgart 2 AuflageBG Teubner Stuttgart Germany 1991
[44] O Legrini E Oliveros and A M Braun ldquoPhotochemicalprocesses for water treatmentrdquo Chemical Reviews vol 93 no2 pp 671ndash698 1993
[45] G Rothenberger J Moser M Gratzel N Serpone and DSharma ldquoCharge carrier trapping and recombination dynamicsin small semiconductor particlesrdquo Journal of the AmericanChemical Society vol 107 no 26 pp 8054ndash8059 1985
[46] P Mazellier M Sarakha A Rossi and M Bolte ldquoThe aqueousphotochemistry of 26-dimethylphenol Evidence for the frag-mentation of the 120572 C-C bondrdquo Journal of Photochemistry andPhotobiology A vol 115 no 2 pp 117ndash121 1998
[47] M Ksibi S B Amor S Cherif E Elaloui A Houas and MElaloui ldquoPhotodegradation of lignin from black liquor using aUVTiO
2systemrdquo Journal of Photochemistry and Photobiology
A vol 154 no 2-3 pp 211ndash218 2003
International Journal of Photoenergy 17
[48] S K Kansal M Singh and D Sud ldquoStudies on TiO2ZnO
photocatalysed degradation of ligninrdquo Journal of HazardousMaterials vol 153 no 1-2 pp 412ndash417 2008
[49] A Dahm and L A Lucia ldquoTitanium dioxide catalyzed pho-todegradation of lignin in industrial effluentsrdquo Industrial andEngineering Chemistry Research vol 43 no 25 pp 7996ndash80002004
[50] E Portjanskaja and S Preis ldquoAqueous photocatalytic oxidationof lignin the influence of mineral admixturesrdquo InternationalJournal of Photoenergy vol 2007Article ID76730 7 pages 2007
[51] K Tanaka R C R Calanag and T Hisanaga ldquoPhotocatalyzeddegradation of lignin on TiO
2rdquo Journal of Molecular Catalysis
A Chemical vol 138 no 2-3 pp 287ndash294 1999[52] M Tian J Wen D MacDonald R M Asmussen and A Chen
ldquoA novel approach for lignin modification and degradationrdquoElectrochemistry Communications vol 12 no 4 pp 527ndash5302010
[53] K PanM Tian Z-H Jiang B Kjartanson and A Chen ldquoElec-trochemical oxidation of lignin at lead dioxide nanoparticlesphotoelectrodeposited on TiO
2nanotube arraysrdquo Electrochim-
ica Acta vol 60 pp 147ndash153 2012[54] C Awungacha Lekelefac J Hild P Czermak and M Herren-
bauer ldquoPhotocatalytic active coatings for lignin degradationin a continuous packed bed reactorrdquo International Journal ofPhotoenergy vol 2014 Article ID 502326 10 pages 2014
[55] C Awungacha Lekelefac P Czermak and M HerrenbauerldquoEvaluation of photocatalytic active coatings on sintered glasstubes by methylene bluerdquo International Journal of Photoenergyvol 2013 Article ID 614567 9 pages 2013
[56] Y-S Ma C-N Chang Y-P Chiang H-F Sung and A CChao ldquoPhotocatalytic degradation of lignin using PtTiO
2as
the catalystrdquo Chemosphere vol 71 no 5 pp 998ndash1004 2008[57] H Ohnishi M Matsumura H Tsubomura and M Iwasaki
ldquoBleaching of lignin solution by a photocatalyzed reactionon semiconductor photocatalystsrdquo Industrial and EngineeringChemistry Research vol 28 no 6 pp 719ndash724 1989
[58] C A K Gouvea F Wypych S G Moraes N Duran and PPeralta-Zamora ldquoSemiconductor-assisted photodegradation oflignin dye and kraft effluent by Ag-doped ZnOrdquo Chemospherevol 40 no 4 pp 427ndash432 2000
[59] A V Vahatalo K Salonen M Salkinoja-Salonen and AHatakka ldquoPhotochemical mineralization of synthetic lignin inlake water indicates enhanced turnover of aromatic organicmatter under solar radiationrdquo Biodegradation vol 10 no 6 pp415ndash420 1999
[60] H de Lasa B Serrano and M Salaices Photocatalytic ReactionEngineering Springer New York NY USA 2005
[61] K Hashimoto H Irie and A Fujishima ldquoTiO 2 photocatalysisA historical overview and future prospectsrdquo Japanese Journal ofApplied Physics vol 44 no 12 pp 8269ndash8285 2005
[62] M A Behnajady N Modirshahla and R Hamzavi ldquoKineticstudy on photocatalytic degradation of CI Acid Yellow 23 byZnOphotocatalystrdquo Journal ofHazardousMaterials vol 133 no1ndash3 pp 226ndash232 2006
[63] M Addamo V Augugliaro A di Paola et al ldquoPhotocatalyticthin films of TiO
2formed by a sol-gel process using titanium
tetraisopropoxide as the precursorrdquo Thin Solid Films vol 516no 12 pp 3802ndash3807 2008
[64] N Negishi K Takeuchi and T Ibusuki ldquoPreparation of theTiO2thin filmphotocatalyst by the dip-coating processrdquo Journal
of Sol-Gel Science and Technology vol 13 no 1ndash3 pp 691ndash6941998
[65] L Rideh A Wehrer D Ronze and A Zoulalian ldquoPhotocat-alytic degradation of 2-chlorophenol in TiO
2aqueous suspen-
sion modeling of reaction raterdquo Industrial and EngineeringChemistry Research vol 36 no 11 pp 4712ndash4718 1997
[66] R-ADoongC-HChen RAMaithreepala and S-MChangldquoThe influence of pHand cadmiumsulfide on the photocatalyticdegradation of 2-chlorophenol in titanium dioxide suspen-sionsrdquoWater Research vol 35 no 12 pp 2873ndash2880 2001
[67] K V Sarkanen A Islam and C D Anderson ldquoOzonationrdquo inMethods in Lignin Chemistry S Y Lin and C W Dence EdsSpringer Series inWood Science pp 387ndash406 Springer BerlinGermany 1992
[68] G Gellerstedt and E-L Lindfors ldquoStructural changes in ligninduring kraft pulpingrdquo Holzforschung vol 38 no 3 pp 151ndash1581984
[69] B Neppolian H C Choi M V Shankar B Arabindoo andV Murugesan in Proceedings of the International Symposiumon Environmental Pollution Control and Waste Management(EPCOWM 02) p 647 2002
[70] C Pouteau P Dole B Cathala L Averous and N BoquillonldquoAntioxidant properties of lignin in polypropylenerdquo PolymerDegradation and Stability vol 81 no 1 pp 9ndash18 2003
[71] J Hafren T Fujino and T Itoh ldquoChanges in cell wall archi-tecture of differentiating tracheids of Pinus thunbergii duringlignificationrdquo Plant and Cell Physiology vol 40 no 5 pp 532ndash541 1999
[72] T Dizhbite G Telysheva V Jurkjane and U Viesturs ldquoChar-acterization of the radical scavenging activity of lignins naturalantioxidantsrdquo Bioresource Technology vol 95 no 3 pp 309ndash3172004
[73] K Hofstadler R Bauer S Novalic and S G Heisier ldquoNew reac-tor design for photocatalytic wastewater treatment with TiO
2
immobilized on fused-silica glass fibers photomineralization of4-chlorophenolrdquo Environmental Science amp Technology vol 28no 4 pp 670ndash674 1994
[74] J Villasenor and H D Mansilla ldquoEffect of temperature onkraft black liquor degradation by ZnO-photoassisted catalysisrdquoJournal of Photochemistry and Photobiology A Chemistry vol93 no 2-3 pp 205ndash209 1996
[75] B H Bielski D E Cabelli L A Ravindra and A B RossldquoReactivity of HO
2Ominus2Radicals in aqueous solutionrdquo Journal
of Physical Chemistry vol 14 pp 1041ndash1100 1985[76] B H J Bielski and A O Allen ldquoMechanism of the dispropor-
tionation of superoxide radicalsrdquo Journal of Physical Chemistryvol 81 no 11 pp 1048ndash1050 1977
[77] B Halliwell and J M C Gutteridge ldquoThe importance of freeradicals and catalytic metal ions in human diseasesrdquoMolecularAspects of Medicine vol 8 no 2 pp 89ndash193 1985
[78] J M Palmer P J Harvey and H E Schoemaker ldquoThe role ofperoxidases radical cations and oxygen in the degradation oflignin [and discussion]rdquo Philosophical Transaction of the RoyalSociety A vol 321 no 1561 pp 495ndash505 1987
[79] S Y Yin and C W Dense Methods in Lignin ChemistrySpringer New York NY USA 1992
[80] P Kumar S Kumar and N K Bhardwaj ldquoPhotocatalyticoxidation of elemental chlorine free bleaching effluent withUVTiO
2rdquo in Proceedings of the 2nd International Conference on
Environmental Science and Technology (ICEST rsquo11) SingaporeFebruary 2011
[81] K Kobayakawa Y Sato S Nakamura and A FujishimaldquoPhotodecomposition of Kraft lignin catalyzed by titanium
18 International Journal of Photoenergy
dioxiderdquo Bulletin of the Chemical Society of Japan vol 62 no11 pp 3433ndash3436 1989
[82] R B Kinstre ldquoAn overview of strategies for reducing theenvironmental impact of bleach-plant effluentsrdquo Tappi Journalvol 76 no 5 pp 105ndash113 1993
[83] A Castellan H Choudhury R Stephen Davidson and SGrelier ldquoComparative study of stone-ground wood pulp andnative wood 3 Application of fluorescence spectroscopy to astudy of the weathering of stone-ground pulp and native woodrdquoJournal of Photochemistry and Photobiology A Chemistry vol81 no 2 pp 123ndash130 1994
[84] B Albinsson S Li K Lundquist and R Stomberg ldquoThe originof lignin fluorescencerdquo Journal of Molecular Structure vol 508no 1ndash3 pp 19ndash27 1999
[85] S Baumberger A Abaecherli M Fasching et al ldquoMolar massdetermination of lignins by size-exclusion chromatographytowards standardisation of the methodrdquo Holzforschung vol 61no 4 pp 459ndash468 2007
[86] E Torres andM Ayala Biocatalysis Based on Heme PeroxidasesSpringer New York NY USA 1st edition 2010
[87] R Pecina P Burtscher G Bonn and O Bobleter ldquoGC-MSand HPLC analyses of lignin degradation products in biomasshydrolyzatesrdquo Fresenius Zeitschrift fur Analytische Chemie vol325 no 5 pp 461ndash465 1986
[88] NIST NIST Standard Reference Database The National Insti-tute of Standards and Technology 2014 httpwwwnistgovsrdnist1acfm
[89] T K Kirk and R L Farrell ldquoEnzymatic ldquocombustionrdquo themicrobial degradation of ligninrdquo Annual Review of Microbiol-ogy vol 41 pp 465ndash501 1987
[90] A TMartinez ldquoHigh redox potential peroxidasesrdquo in IndustrialEnzymes Structure Function and Applications K-B BeckerEd pp 477ndash488 Springer Amsterdam The Netherlands 1stedition 2007
[91] M Dashtban H Schraft T A Syed and W Qin ldquoFungalbiodegradation and enzymatic modification of ligninrdquo Interna-tional Journal of Biochemistry and Molecular Biology vol 1 no1 pp 36ndash50 2010
[92] N Duran E Esposito L H Innocentini-Mei and V P CanhosldquoA new alternative process for Kraft E1 effluent treatmentrdquoBiodegradation vol 5 no 1 pp 13ndash19 1994
[93] K Kamwilaisak and P C Wright ldquoInvestigating laccase andtitanium dioxide for lignin degradationrdquo Energy and Fuels vol26 no 4 pp 2400ndash2406 2012
[94] J Reyes M Dezotti H Mansilla J Villasenor E Espositoand N Duran ldquoBiomass photochemistry-XXII combined pho-tochemical and biological process for treatment of Kraft E1effluentrdquoApplied Catalysis B Environmental vol 15 no 3-4 pp211ndash219 1998
[95] L F Gonzalez V Sarria and O F Sanchez ldquoDegradationof chlorophenols by sequential biological-advanced oxidativeprocess using Trametespubescens and TiO
2UVrdquo Bioresource
Technology vol 101 no 10 pp 3493ndash3499 2010[96] J H Lora and W G Glasser ldquoRecent industrial applications
of lignin a sustainable alternative to nonrenewable materialsrdquoJournal of Polymers and the Environment vol 10 no 1-2 pp 39ndash48 2002
Submit your manuscripts athttpwwwhindawicom
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Inorganic ChemistryInternational Journal of
Hindawi Publishing Corporation httpwwwhindawicom Volume 2014
International Journal ofPhotoenergy
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Carbohydrate Chemistry
International Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
Chemistry
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Advances in
Physical Chemistry
Hindawi Publishing Corporationhttpwwwhindawicom
Analytical Methods in Chemistry
Journal of
Volume 2014
Bioinorganic Chemistry and ApplicationsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
SpectroscopyInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Medicinal ChemistryInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Chromatography Research International
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Applied ChemistryJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Theoretical ChemistryJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
Spectroscopy
Analytical ChemistryInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
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Quantum Chemistry
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Organic Chemistry International
ElectrochemistryInternational Journal of
Hindawi Publishing Corporation httpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CatalystsJournal of
14 International Journal of Photoenergy
Table 4 Main advantages and disadvantages photocatalysis versus enzymatic biocatalysis
Lignin degradation process Advantages Disadvantages
Photocatalysis(single stage)
(i) Relatively fast degradation of complex as wellas nonbiodegradable organic (macro)molecules(ii) Stable catalysts(iii) Moderate reaction conditions
(i) Not selective(ii) Limited selection of operating conditions(iii) Energy costs
Enzymatic conversion(single stage)
(i) More selective(ii) Moderate reaction conditions
(i) High enzyme loads required otherwise ligninhydrolysis is quite slow [16](ii) Enzymes are less stable1(iii) cost-intensive (POXs)2
1For example POXs are sensitive to high H2O2 concentrations2According to Torres and Ayala [86]
and atmospheric pressure chemical ionization (APCI) forLC-MS while GC-MS is based on electron ionization Aremedy to this can be the development of a database ofmodel lignin compounds based on a unanimous HPLC-MSmeasuring procedure This would mean much time and costexpensive investments for adequate personnel and material
Though product identification based on peak superpo-sition gives information on a possible product this can-not always be true for lignin degradation because of thelarge number of possible degradation products In order toproperly identify products from lignin degradation productisolation techniques are to be implemented and further ana-lytic methods such as tandem mass spectroscopy (MSMS)and other advanced structure enhancing research techniquessuch as 1H-NMR 13C-NMR have to be studied extensively
4 Photocatalysis and Its Suitability asIntegrated Technology in MultistageConcepts with Biocatalysis
Depending on lignin treatment specification photocatalysiswill find application for complete lignin mineralization toCO2and H
2O (single stage) or as integrated technology in
a multistage system aiming at partial conversion of ligninmacromolecules to organic low molecular weight intermedi-ates for a consecutive biological oxidation finally generatingvalue added end-products [15]
For biological oxidation of lignin or lignin deriva-tives H
2O2-dependent ligninolytic heme peroxidases (POXs
including lignin peroxidase (LiP EC 111114) manganeseperoxidase (MnP EC 111113) versatile peroxidase (VPEC 111116) O
2-dependent laccases (Lac EC 11032) and
extracellular enzymes from Basidiomycetous white-rot fungiare the most efficient lignin degraders in nature [89] Thussuch enzymatic systems especially POXs (with high redoxpotentials) have attractedmuch interest as industrial biocata-lyst [86 90]The enzyme degradationmechanism (in nature)is facilitated by nonenzymatic processes mainly throughfree ∙OH radicals also generated by the fungus (Fenton-type reaction) Those ∙OH radicals enable the requiredphysical contact between the enzymes and structural units ofthe lignin molecule due to numerous nonspecific oxidative
reactions [91] Both the photocatalytic (equations (2)ndash(7))and the POX reactionmechanism recently reviewed by Busseat al [25] are quite similar Consequently a combination ofphotocatalysis followed by an enzymatic oxidation maybe apromising concept utilizing lignin derivatives for exampleoriginated from industrial effluents [15 92] The advantagesforming biobased products are as follows see also Table 4 inthis context A reduction of the lignin polymerization degreevia photocatalysis at best in more biodegradable interme-diates and a simultaneous detoxification causes savings inenzyme costs since it will be expected that less enzyme loadsare necessary for sufficiently rapid reaction rates [15 91] As aresult hydraulic retention times in bioreactor systems shouldbe diminished Moreover the delignification is expectedto be enhanced simultaneously which was in recent yearsshown in a dual system by Kamwilaisak and Wright [93]using TiO
2H2O2UV for photocatalytic pretreatment and
Lac (from Trametes versicolor) in the subsequent biocatalyticstep Within 24 h they obtained in their dual system anelevation in delignification of 20 (without H
2O2) up to at
least 50 when H2O2was present
In previous studies the treatment of lignin (from thepulp and paper industry) containing effluents by fungus asa biological system (excreting appropriate lignin degradingenzyme cocktails) were more focused for posttreatmentexclusively (Duran et al [92] Reyes et al [94] and Gonzalezet al [95]) Shende et al [17] even examined ligninolyticbacteria with photooxidized kraft lignin as substrate
Although POXs are potential industrial biocatalysts [86]no application studies were found for the direct use in suchdual systems as described above The major reason may bethe complexity of the reaction mechanism (inclusive ligninderivatives as substrate and their analysis) per se on the onehand slowing down research and development processes Onthe other hand POXs are sensitive to their cosubstrate H
2O2
once it is supplied in excess causing considerable inactivation(for details refer to Busse et al [25]) At the present severalstudies are carried out modifying these enzymes regardingenhanced stability activity and selectivity as well Henceit can be expected that their technological applicability willbe raised significantly right after successful modification isreached [86]
International Journal of Photoenergy 15
5 Concluding Remarks
It is widely assumed that the photocatalytic degradation oflignin follows a radical reaction pathway which is similar tothat considered in thermal electrochemical and biochemicalprocesses However reporting on the degradation pathway oflignin derivatives and even that of lignin model compoundsare still amajor challengeThis is probably due to the complexnature and variety of possible degradation products Indeedthemechanism is far more complex considering other factorssuch as type of lignin type of catalyst pH illuminationsource and additives
Generally comparing the different photochemical pro-cesses poses a big challenge because of the wide variablesinvolved These discrepancies start from the source and typeof lignin followed by differences in reactor design illumi-nation source intensity of radiation and different types ofcatalyst An ideawould be to have a specific reference reactionwith well-defined starting parameters which include lignintype source and purity catalyst specifications illuminationsource and intensity so as to ease comparison of results
Basic process parameters such as catalyst concentrationsubstrate concentration addition of metal ion to catalyst pHand illumination have been discussed
Despite developed analytical technologies analyzinglignin degradation products remains challenging Proofssuch as mass spectroscopy (MS) HPLC 13C or 1H-NMRspectra from photocatalytic lignin degradations are not yetestablished
In order to properly identify products from lignin degra-dation product isolation techniques are to be implementedand further analytic methods such as MSMS and otheradvanced structure enhancing research techniques such as1H-NMR and 13C-NMR have to be studied extensively Aremedy to this can be the development of a databank ofmodel lignin compounds based on a unanimous HPLC-MSmeasuring procedure This would mean much time and costexpensive investments for adequate personnel and material
Photocatalysis is denoted as the most popular ligninpretreatment technology besides ozonation [96] Photo-catalyzed lignin may be an appropriate substrate for aconsecutive biocatalytic process using ligninolytic enzymes(POX andor Lac) as supported by experimental results ofKamwilaisak and Wright [93] Combining the advantages ofboth catalytic processes savings in the overall process costswill be expected in addition to elevated lignin conversionNonetheless extensive research work including POX modi-fications is still required
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Authorsrsquo Contribution
Colin Awungacha Lekelefac and Nadine Busse contributedequally to this work
Acknowledgments
The authors gratefully thank the Federal Ministry of Educa-tion and Research (BMBF) for funding (FKZ17N0310) Theresearchers also thank the Hessen State Ministry of HigherEducation Research andArts for the financial support withinthe Hessen initiative for scientific and economic excellence(LOEWE)
References
[1] Crude Oil and Commodity Prices httpwwwoil-pricenet[2] Research Directorate-General for World Energy Technology
and Climate Policy Outlook (WETO) Luxembourg Office forOfficial Publications of the European Communities 2003
[3] Green and Natural Polymers Are on the Rise 2014 httpwwwpolymersolutionscombloggreen-and-natural-polymers-on-the-rise
[4] J Zakzeski P C Bruijnincx A L Jongerius and B M Weck-huysen ldquoThe catalytic valorization of lignin for the productionof renewable chemicalsrdquo Chemical Reviews vol 110 no 6 pp3552ndash3599 2010
[5] B KammM Kamm P R Gruber and S KromusBiorefineries-Industrial Processes and Products Status Quo and Future Direc-tions vol 1 Wiley-VCH 2006
[6] R L Howard E Abotsi E L J van Rensburg and S HowardldquoLignocellulose biotechnology issues of bioconversion andenzyme productionrdquoAfrican Journal of Biotechnology vol 2 no12 pp 602ndash619 2003
[7] M Stocker ldquoBio- und BTL-Kraftstoffe in der Bioraffineriekatalytische Umwandlung Lignocellulose-reicher Biomasse mitporosen Stoffenrdquo Angewandte Chemie vol 120 no 48 pp9340ndash9351 2008
[8] J-P Lange ldquoLignocellulose conversion an introduction tochemistry process and economicsrdquo Biofuels Bioproducts andBiorefining vol 1 no 1 pp 39ndash48 2007
[9] D A I Goring ldquoThe physical chemistry of ligninrdquo Pure andApplied Chemistry vol 5 no 1-2 pp 233ndash310 1962
[10] M Ek G Gellerstedt and G HenrikssonWood Chemistry andBiotechnology Pulp and Paper Chemistry and Technology vol 1Walter de Gruyter GmbH amp Co KG Berlin Germany 2009
[11] B Saake and R Lehnen ldquoLigninrdquo in Ullmannrsquos Encyclopedia ofIndustrial Chemistry Wiley-VCH Verlag GmbH amp Co KGaAWeinheim Germany 2012
[12] M N S Kumar A K Mohanty L Erickson and M MisraldquoLignin and its applications with polymersrdquo Journal of BiobasedMaterials and Bioenergy vol 3 no 1 pp 1ndash24 2009
[13] R J A Gosselink E de Jong B Guran and A AbacherlildquoCo-ordination network for ligninmdashstandardisation produc-tion and applications adapted to market requirements(EUROLIGNIN)rdquo Industrial Crops and Products vol 20 no 2pp 121ndash129 2004
[14] D Fengel and G Wegener Wood Chemistry UltrastructureReactions Verlag Kessel 1984
[15] D Mantzavinos and E Psillakis ldquoEnhancement of biodegrad-ability of industrial wastewaters by chemical oxidation pre-treatmentrdquo Journal of Chemical Technology and Biotechnologyvol 79 no 5 pp 431ndash454 2004
16 International Journal of Photoenergy
[16] M Dashtban H Schraft and W Qin ldquoFungal bioconversionof lignocellulosic residues Opportunities amp perspectivesrdquo Inter-national Journal of Biological Sciences vol 5 no 6 pp 578ndash5952009
[17] A Shende R Jaswal D Harder-Heinz A Menan and RShende ldquoIntergrated photocatalytic and microbial degradationof kraft ligninrdquo Cleantech pp 120ndash123 2012
[18] F S Chakar and A J Ragauskas ldquoReview of current and futuresoftwood kraft lignin process chemistryrdquo Industrial Crops andProducts vol 20 no 2 pp 131ndash141 2004
[19] W G Glasser and H R Glasser ldquoEvaluation of ligninrsquoschemical structure by experimental and computer simulationtechniquesrdquo Paperi ja Puu vol 63 pp 71ndash83 1981
[20] M Erickson S Larsson and G E Miksche ldquoZur Struktur desLignins der FichterdquoActaChemica Scandinavica vol 27 pp 903ndash914 1973
[21] H Nimz ldquoDas Lignin der BuchemdashEntwurf eines Konstitution-sschemasrdquo Angewandte ChemiemdashInternational Edition vol 86pp 336ndash344 1974
[22] D V Evtuguin C P Neto J Rocha and J D P de JesusldquoOxidative delignification in the presence of molybdovana-dophosphate heteropolyanions mechanism and kinetic stud-iesrdquo Applied Catalysis A General vol 167 no 1 pp 123ndash1391998
[23] A L Linsebigler G Lu and J T Yates Jr ldquoPhotocatalysis onTiO2surfaces Principles mechanisms and selected resultsrdquo
Chemical Reviews vol 95 no 3 pp 735ndash758 1995[24] Y Miyata K Miyazaki M Miura Y Shimotori M Aoyama
and H Nakatani ldquoSolventless delignification of wood flourwith TiO
2poly(ethylene oxide) photocatalyst systemrdquo Journal
of Polymers and the Environment vol 21 no 1 pp 115ndash121 2013[25] N Busse D Wagner M Kraume and P Czermak ldquoReaction
kinetics of versatile peroxidase for the degradation of lignincompoundsrdquoTheAmerican Journal of Biochemistry andBiotech-nology vol 9 no 4 pp 365ndash394 2013
[26] M Tien and T K Kirk ldquoLignin-degrading enzyme fromphanerochaete chrysosporium Purification characterizationand catalytic properties of a uniqueH
2O2-requiring oxygenaserdquo
Proceedings of the National Academy of Sciences vol 81 pp2280ndash2284 1984
[27] T K Kirk M Tien P J Kersten M D Mozuch and BKalyanaraman ldquoLigninase of Phanerochaete chrysosporiumMechanism of its degradation of the non-phenolic arylglycerol120573-aryl ether substructure of ligninrdquo Biochemical Journal vol236 no 1 pp 279ndash287 1986
[28] T Lundell RWever R Floris et al ldquoLignin peroxidase L3 fromPhlebia radiata pre-steady-state and steady-state studies withveratryl alcohol and a non-phenolic lignin model compound1-(34-dimethoxyphenyl)-2-(2-methoxyphenoxy)propane-13-diolrdquo European Journal of Biochemistry vol 211 no 3 pp391ndash402 1993
[29] H E Schoemaker T K Lundell A I Hatakka and K PiontekldquoThe oxidation of veratryl alcohol dimeric lignin models andlignin by lignin peroxidase the redox cycle revisitedrdquo FEMSMicrobiology Reviews vol 13 no 2-3 pp 321ndash331 1994
[30] J M Palmer P J Harvey and H E Schoemaker ldquoThe role ofperoxidases radical cations and oxygen in the degradation oflignin [and discussion]rdquo Philosophical Transactions of the RoyalSociety of London A vol 321 no 1561 pp 495ndash505 1987
[31] C Hill Wood Modification Chemical Thermal and OtherProcesses John Wiley amp Sons Chichester UK 2006
[32] Y-H Xu H-R Chen Z-X Zeng and B Lei ldquoInvestigation onmechanism of photocatalytic activity enhancement of nanome-ter cerium-doped titaniardquo Applied Surface Science vol 252 no24 pp 8565ndash8570 2006
[33] L Tonucci F Coccia M Bressan and N DrsquoAlessandro ldquoMildphotocatalysed and catalysed green oxidation of lignin auseful pathway to low-molecular-weight derivativesrdquoWaste andBiomass Valorization vol 3 no 2 pp 165ndash174 2012
[34] A Castellan N Colombo C Vanucci P Fornier de HViolet and H Bouas-Laurent ldquoPhotodegradation of ligninA photochemical study of an O-methylated 120572-carbonyl 120573-1lignin model dimer 12-di(3101584041015840-dimethoxyphenyl) ethanone(deoxyveratroin)rdquo Journal of Photochemistry and PhotobiologyA vol 51 pp 451ndash467 1990
[35] A Fujishima X Zhang and D A Tryk ldquoTiO2photocatalysis
and related surface phenomenardquo Surface Science Reports vol63 no 12 pp 515ndash582 2008
[36] C S Turchi and D F Ollis ldquoPhotocatalytic reactor designan example of mass-transfer limitations with an immobilizedcatalystrdquo The Journal of Physical Chemistry vol 92 no 23 pp6852ndash6853 1988
[37] R Subasri M Tripathi K Murugan J Revathi G V N Raoand T N Rao ldquoInvestigations on the photocatalytic activity ofsolndashgel derived plain and Fe3+Nb5+ doped titania coatings onglass substratesrdquo Materials Chemistry and Physics vol 124 pp63ndash68 2010
[38] N Serpone ldquoRelative photonic efficiencies and quantum yieldsin heterogeneous photocatalysisrdquo Journal of Photochemistry andPhotobiology A Chemistry vol 104 no 1ndash3 pp 1ndash12 1997
[39] M R Hoffmann S T Martin W Choi and D W BahnemannldquoEnvironmental applications of semiconductor photocatalysisrdquoChemical Reviews vol 95 no 1 pp 69ndash96 1995
[40] C D Jaeger and A J Bard ldquoSpin trapping and electron spinresonance detection of radical intermediates in the photode-composition of water at titanium dioxide particulate systemsrdquoThe Journal of Physical Chemistry vol 83 no 24 pp 3146ndash31521979
[41] R W Matthews ldquoHydroxylation reactions induced by near-ultraviolet photolysis of aqueous titaniumdioxide suspensionsrdquoJournal of the Chemical Society Faraday Transactions vol 80no 2 pp 457ndash471 1984
[42] A E H Machado A M Furuyama S Z Falone R RuggieroD D S Perez and A Castellan ldquoPhotocatalytic degradation oflignin and lignin models using titanium dioxide the role of thehydroxyl radicalrdquoChemosphere vol 40 no 1 pp 115ndash124 2000
[43] W Sigg and L Stumm Aquatische Chemie Stuttgart 2 AuflageBG Teubner Stuttgart Germany 1991
[44] O Legrini E Oliveros and A M Braun ldquoPhotochemicalprocesses for water treatmentrdquo Chemical Reviews vol 93 no2 pp 671ndash698 1993
[45] G Rothenberger J Moser M Gratzel N Serpone and DSharma ldquoCharge carrier trapping and recombination dynamicsin small semiconductor particlesrdquo Journal of the AmericanChemical Society vol 107 no 26 pp 8054ndash8059 1985
[46] P Mazellier M Sarakha A Rossi and M Bolte ldquoThe aqueousphotochemistry of 26-dimethylphenol Evidence for the frag-mentation of the 120572 C-C bondrdquo Journal of Photochemistry andPhotobiology A vol 115 no 2 pp 117ndash121 1998
[47] M Ksibi S B Amor S Cherif E Elaloui A Houas and MElaloui ldquoPhotodegradation of lignin from black liquor using aUVTiO
2systemrdquo Journal of Photochemistry and Photobiology
A vol 154 no 2-3 pp 211ndash218 2003
International Journal of Photoenergy 17
[48] S K Kansal M Singh and D Sud ldquoStudies on TiO2ZnO
photocatalysed degradation of ligninrdquo Journal of HazardousMaterials vol 153 no 1-2 pp 412ndash417 2008
[49] A Dahm and L A Lucia ldquoTitanium dioxide catalyzed pho-todegradation of lignin in industrial effluentsrdquo Industrial andEngineering Chemistry Research vol 43 no 25 pp 7996ndash80002004
[50] E Portjanskaja and S Preis ldquoAqueous photocatalytic oxidationof lignin the influence of mineral admixturesrdquo InternationalJournal of Photoenergy vol 2007Article ID76730 7 pages 2007
[51] K Tanaka R C R Calanag and T Hisanaga ldquoPhotocatalyzeddegradation of lignin on TiO
2rdquo Journal of Molecular Catalysis
A Chemical vol 138 no 2-3 pp 287ndash294 1999[52] M Tian J Wen D MacDonald R M Asmussen and A Chen
ldquoA novel approach for lignin modification and degradationrdquoElectrochemistry Communications vol 12 no 4 pp 527ndash5302010
[53] K PanM Tian Z-H Jiang B Kjartanson and A Chen ldquoElec-trochemical oxidation of lignin at lead dioxide nanoparticlesphotoelectrodeposited on TiO
2nanotube arraysrdquo Electrochim-
ica Acta vol 60 pp 147ndash153 2012[54] C Awungacha Lekelefac J Hild P Czermak and M Herren-
bauer ldquoPhotocatalytic active coatings for lignin degradationin a continuous packed bed reactorrdquo International Journal ofPhotoenergy vol 2014 Article ID 502326 10 pages 2014
[55] C Awungacha Lekelefac P Czermak and M HerrenbauerldquoEvaluation of photocatalytic active coatings on sintered glasstubes by methylene bluerdquo International Journal of Photoenergyvol 2013 Article ID 614567 9 pages 2013
[56] Y-S Ma C-N Chang Y-P Chiang H-F Sung and A CChao ldquoPhotocatalytic degradation of lignin using PtTiO
2as
the catalystrdquo Chemosphere vol 71 no 5 pp 998ndash1004 2008[57] H Ohnishi M Matsumura H Tsubomura and M Iwasaki
ldquoBleaching of lignin solution by a photocatalyzed reactionon semiconductor photocatalystsrdquo Industrial and EngineeringChemistry Research vol 28 no 6 pp 719ndash724 1989
[58] C A K Gouvea F Wypych S G Moraes N Duran and PPeralta-Zamora ldquoSemiconductor-assisted photodegradation oflignin dye and kraft effluent by Ag-doped ZnOrdquo Chemospherevol 40 no 4 pp 427ndash432 2000
[59] A V Vahatalo K Salonen M Salkinoja-Salonen and AHatakka ldquoPhotochemical mineralization of synthetic lignin inlake water indicates enhanced turnover of aromatic organicmatter under solar radiationrdquo Biodegradation vol 10 no 6 pp415ndash420 1999
[60] H de Lasa B Serrano and M Salaices Photocatalytic ReactionEngineering Springer New York NY USA 2005
[61] K Hashimoto H Irie and A Fujishima ldquoTiO 2 photocatalysisA historical overview and future prospectsrdquo Japanese Journal ofApplied Physics vol 44 no 12 pp 8269ndash8285 2005
[62] M A Behnajady N Modirshahla and R Hamzavi ldquoKineticstudy on photocatalytic degradation of CI Acid Yellow 23 byZnOphotocatalystrdquo Journal ofHazardousMaterials vol 133 no1ndash3 pp 226ndash232 2006
[63] M Addamo V Augugliaro A di Paola et al ldquoPhotocatalyticthin films of TiO
2formed by a sol-gel process using titanium
tetraisopropoxide as the precursorrdquo Thin Solid Films vol 516no 12 pp 3802ndash3807 2008
[64] N Negishi K Takeuchi and T Ibusuki ldquoPreparation of theTiO2thin filmphotocatalyst by the dip-coating processrdquo Journal
of Sol-Gel Science and Technology vol 13 no 1ndash3 pp 691ndash6941998
[65] L Rideh A Wehrer D Ronze and A Zoulalian ldquoPhotocat-alytic degradation of 2-chlorophenol in TiO
2aqueous suspen-
sion modeling of reaction raterdquo Industrial and EngineeringChemistry Research vol 36 no 11 pp 4712ndash4718 1997
[66] R-ADoongC-HChen RAMaithreepala and S-MChangldquoThe influence of pHand cadmiumsulfide on the photocatalyticdegradation of 2-chlorophenol in titanium dioxide suspen-sionsrdquoWater Research vol 35 no 12 pp 2873ndash2880 2001
[67] K V Sarkanen A Islam and C D Anderson ldquoOzonationrdquo inMethods in Lignin Chemistry S Y Lin and C W Dence EdsSpringer Series inWood Science pp 387ndash406 Springer BerlinGermany 1992
[68] G Gellerstedt and E-L Lindfors ldquoStructural changes in ligninduring kraft pulpingrdquo Holzforschung vol 38 no 3 pp 151ndash1581984
[69] B Neppolian H C Choi M V Shankar B Arabindoo andV Murugesan in Proceedings of the International Symposiumon Environmental Pollution Control and Waste Management(EPCOWM 02) p 647 2002
[70] C Pouteau P Dole B Cathala L Averous and N BoquillonldquoAntioxidant properties of lignin in polypropylenerdquo PolymerDegradation and Stability vol 81 no 1 pp 9ndash18 2003
[71] J Hafren T Fujino and T Itoh ldquoChanges in cell wall archi-tecture of differentiating tracheids of Pinus thunbergii duringlignificationrdquo Plant and Cell Physiology vol 40 no 5 pp 532ndash541 1999
[72] T Dizhbite G Telysheva V Jurkjane and U Viesturs ldquoChar-acterization of the radical scavenging activity of lignins naturalantioxidantsrdquo Bioresource Technology vol 95 no 3 pp 309ndash3172004
[73] K Hofstadler R Bauer S Novalic and S G Heisier ldquoNew reac-tor design for photocatalytic wastewater treatment with TiO
2
immobilized on fused-silica glass fibers photomineralization of4-chlorophenolrdquo Environmental Science amp Technology vol 28no 4 pp 670ndash674 1994
[74] J Villasenor and H D Mansilla ldquoEffect of temperature onkraft black liquor degradation by ZnO-photoassisted catalysisrdquoJournal of Photochemistry and Photobiology A Chemistry vol93 no 2-3 pp 205ndash209 1996
[75] B H Bielski D E Cabelli L A Ravindra and A B RossldquoReactivity of HO
2Ominus2Radicals in aqueous solutionrdquo Journal
of Physical Chemistry vol 14 pp 1041ndash1100 1985[76] B H J Bielski and A O Allen ldquoMechanism of the dispropor-
tionation of superoxide radicalsrdquo Journal of Physical Chemistryvol 81 no 11 pp 1048ndash1050 1977
[77] B Halliwell and J M C Gutteridge ldquoThe importance of freeradicals and catalytic metal ions in human diseasesrdquoMolecularAspects of Medicine vol 8 no 2 pp 89ndash193 1985
[78] J M Palmer P J Harvey and H E Schoemaker ldquoThe role ofperoxidases radical cations and oxygen in the degradation oflignin [and discussion]rdquo Philosophical Transaction of the RoyalSociety A vol 321 no 1561 pp 495ndash505 1987
[79] S Y Yin and C W Dense Methods in Lignin ChemistrySpringer New York NY USA 1992
[80] P Kumar S Kumar and N K Bhardwaj ldquoPhotocatalyticoxidation of elemental chlorine free bleaching effluent withUVTiO
2rdquo in Proceedings of the 2nd International Conference on
Environmental Science and Technology (ICEST rsquo11) SingaporeFebruary 2011
[81] K Kobayakawa Y Sato S Nakamura and A FujishimaldquoPhotodecomposition of Kraft lignin catalyzed by titanium
18 International Journal of Photoenergy
dioxiderdquo Bulletin of the Chemical Society of Japan vol 62 no11 pp 3433ndash3436 1989
[82] R B Kinstre ldquoAn overview of strategies for reducing theenvironmental impact of bleach-plant effluentsrdquo Tappi Journalvol 76 no 5 pp 105ndash113 1993
[83] A Castellan H Choudhury R Stephen Davidson and SGrelier ldquoComparative study of stone-ground wood pulp andnative wood 3 Application of fluorescence spectroscopy to astudy of the weathering of stone-ground pulp and native woodrdquoJournal of Photochemistry and Photobiology A Chemistry vol81 no 2 pp 123ndash130 1994
[84] B Albinsson S Li K Lundquist and R Stomberg ldquoThe originof lignin fluorescencerdquo Journal of Molecular Structure vol 508no 1ndash3 pp 19ndash27 1999
[85] S Baumberger A Abaecherli M Fasching et al ldquoMolar massdetermination of lignins by size-exclusion chromatographytowards standardisation of the methodrdquo Holzforschung vol 61no 4 pp 459ndash468 2007
[86] E Torres andM Ayala Biocatalysis Based on Heme PeroxidasesSpringer New York NY USA 1st edition 2010
[87] R Pecina P Burtscher G Bonn and O Bobleter ldquoGC-MSand HPLC analyses of lignin degradation products in biomasshydrolyzatesrdquo Fresenius Zeitschrift fur Analytische Chemie vol325 no 5 pp 461ndash465 1986
[88] NIST NIST Standard Reference Database The National Insti-tute of Standards and Technology 2014 httpwwwnistgovsrdnist1acfm
[89] T K Kirk and R L Farrell ldquoEnzymatic ldquocombustionrdquo themicrobial degradation of ligninrdquo Annual Review of Microbiol-ogy vol 41 pp 465ndash501 1987
[90] A TMartinez ldquoHigh redox potential peroxidasesrdquo in IndustrialEnzymes Structure Function and Applications K-B BeckerEd pp 477ndash488 Springer Amsterdam The Netherlands 1stedition 2007
[91] M Dashtban H Schraft T A Syed and W Qin ldquoFungalbiodegradation and enzymatic modification of ligninrdquo Interna-tional Journal of Biochemistry and Molecular Biology vol 1 no1 pp 36ndash50 2010
[92] N Duran E Esposito L H Innocentini-Mei and V P CanhosldquoA new alternative process for Kraft E1 effluent treatmentrdquoBiodegradation vol 5 no 1 pp 13ndash19 1994
[93] K Kamwilaisak and P C Wright ldquoInvestigating laccase andtitanium dioxide for lignin degradationrdquo Energy and Fuels vol26 no 4 pp 2400ndash2406 2012
[94] J Reyes M Dezotti H Mansilla J Villasenor E Espositoand N Duran ldquoBiomass photochemistry-XXII combined pho-tochemical and biological process for treatment of Kraft E1effluentrdquoApplied Catalysis B Environmental vol 15 no 3-4 pp211ndash219 1998
[95] L F Gonzalez V Sarria and O F Sanchez ldquoDegradationof chlorophenols by sequential biological-advanced oxidativeprocess using Trametespubescens and TiO
2UVrdquo Bioresource
Technology vol 101 no 10 pp 3493ndash3499 2010[96] J H Lora and W G Glasser ldquoRecent industrial applications
of lignin a sustainable alternative to nonrenewable materialsrdquoJournal of Polymers and the Environment vol 10 no 1-2 pp 39ndash48 2002
Submit your manuscripts athttpwwwhindawicom
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Inorganic ChemistryInternational Journal of
Hindawi Publishing Corporation httpwwwhindawicom Volume 2014
International Journal ofPhotoenergy
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Carbohydrate Chemistry
International Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
Chemistry
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Advances in
Physical Chemistry
Hindawi Publishing Corporationhttpwwwhindawicom
Analytical Methods in Chemistry
Journal of
Volume 2014
Bioinorganic Chemistry and ApplicationsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
SpectroscopyInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Medicinal ChemistryInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Chromatography Research International
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Applied ChemistryJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Theoretical ChemistryJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
Spectroscopy
Analytical ChemistryInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Quantum Chemistry
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Organic Chemistry International
ElectrochemistryInternational Journal of
Hindawi Publishing Corporation httpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CatalystsJournal of
International Journal of Photoenergy 15
5 Concluding Remarks
It is widely assumed that the photocatalytic degradation oflignin follows a radical reaction pathway which is similar tothat considered in thermal electrochemical and biochemicalprocesses However reporting on the degradation pathway oflignin derivatives and even that of lignin model compoundsare still amajor challengeThis is probably due to the complexnature and variety of possible degradation products Indeedthemechanism is far more complex considering other factorssuch as type of lignin type of catalyst pH illuminationsource and additives
Generally comparing the different photochemical pro-cesses poses a big challenge because of the wide variablesinvolved These discrepancies start from the source and typeof lignin followed by differences in reactor design illumi-nation source intensity of radiation and different types ofcatalyst An ideawould be to have a specific reference reactionwith well-defined starting parameters which include lignintype source and purity catalyst specifications illuminationsource and intensity so as to ease comparison of results
Basic process parameters such as catalyst concentrationsubstrate concentration addition of metal ion to catalyst pHand illumination have been discussed
Despite developed analytical technologies analyzinglignin degradation products remains challenging Proofssuch as mass spectroscopy (MS) HPLC 13C or 1H-NMRspectra from photocatalytic lignin degradations are not yetestablished
In order to properly identify products from lignin degra-dation product isolation techniques are to be implementedand further analytic methods such as MSMS and otheradvanced structure enhancing research techniques such as1H-NMR and 13C-NMR have to be studied extensively Aremedy to this can be the development of a databank ofmodel lignin compounds based on a unanimous HPLC-MSmeasuring procedure This would mean much time and costexpensive investments for adequate personnel and material
Photocatalysis is denoted as the most popular ligninpretreatment technology besides ozonation [96] Photo-catalyzed lignin may be an appropriate substrate for aconsecutive biocatalytic process using ligninolytic enzymes(POX andor Lac) as supported by experimental results ofKamwilaisak and Wright [93] Combining the advantages ofboth catalytic processes savings in the overall process costswill be expected in addition to elevated lignin conversionNonetheless extensive research work including POX modi-fications is still required
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Authorsrsquo Contribution
Colin Awungacha Lekelefac and Nadine Busse contributedequally to this work
Acknowledgments
The authors gratefully thank the Federal Ministry of Educa-tion and Research (BMBF) for funding (FKZ17N0310) Theresearchers also thank the Hessen State Ministry of HigherEducation Research andArts for the financial support withinthe Hessen initiative for scientific and economic excellence(LOEWE)
References
[1] Crude Oil and Commodity Prices httpwwwoil-pricenet[2] Research Directorate-General for World Energy Technology
and Climate Policy Outlook (WETO) Luxembourg Office forOfficial Publications of the European Communities 2003
[3] Green and Natural Polymers Are on the Rise 2014 httpwwwpolymersolutionscombloggreen-and-natural-polymers-on-the-rise
[4] J Zakzeski P C Bruijnincx A L Jongerius and B M Weck-huysen ldquoThe catalytic valorization of lignin for the productionof renewable chemicalsrdquo Chemical Reviews vol 110 no 6 pp3552ndash3599 2010
[5] B KammM Kamm P R Gruber and S KromusBiorefineries-Industrial Processes and Products Status Quo and Future Direc-tions vol 1 Wiley-VCH 2006
[6] R L Howard E Abotsi E L J van Rensburg and S HowardldquoLignocellulose biotechnology issues of bioconversion andenzyme productionrdquoAfrican Journal of Biotechnology vol 2 no12 pp 602ndash619 2003
[7] M Stocker ldquoBio- und BTL-Kraftstoffe in der Bioraffineriekatalytische Umwandlung Lignocellulose-reicher Biomasse mitporosen Stoffenrdquo Angewandte Chemie vol 120 no 48 pp9340ndash9351 2008
[8] J-P Lange ldquoLignocellulose conversion an introduction tochemistry process and economicsrdquo Biofuels Bioproducts andBiorefining vol 1 no 1 pp 39ndash48 2007
[9] D A I Goring ldquoThe physical chemistry of ligninrdquo Pure andApplied Chemistry vol 5 no 1-2 pp 233ndash310 1962
[10] M Ek G Gellerstedt and G HenrikssonWood Chemistry andBiotechnology Pulp and Paper Chemistry and Technology vol 1Walter de Gruyter GmbH amp Co KG Berlin Germany 2009
[11] B Saake and R Lehnen ldquoLigninrdquo in Ullmannrsquos Encyclopedia ofIndustrial Chemistry Wiley-VCH Verlag GmbH amp Co KGaAWeinheim Germany 2012
[12] M N S Kumar A K Mohanty L Erickson and M MisraldquoLignin and its applications with polymersrdquo Journal of BiobasedMaterials and Bioenergy vol 3 no 1 pp 1ndash24 2009
[13] R J A Gosselink E de Jong B Guran and A AbacherlildquoCo-ordination network for ligninmdashstandardisation produc-tion and applications adapted to market requirements(EUROLIGNIN)rdquo Industrial Crops and Products vol 20 no 2pp 121ndash129 2004
[14] D Fengel and G Wegener Wood Chemistry UltrastructureReactions Verlag Kessel 1984
[15] D Mantzavinos and E Psillakis ldquoEnhancement of biodegrad-ability of industrial wastewaters by chemical oxidation pre-treatmentrdquo Journal of Chemical Technology and Biotechnologyvol 79 no 5 pp 431ndash454 2004
16 International Journal of Photoenergy
[16] M Dashtban H Schraft and W Qin ldquoFungal bioconversionof lignocellulosic residues Opportunities amp perspectivesrdquo Inter-national Journal of Biological Sciences vol 5 no 6 pp 578ndash5952009
[17] A Shende R Jaswal D Harder-Heinz A Menan and RShende ldquoIntergrated photocatalytic and microbial degradationof kraft ligninrdquo Cleantech pp 120ndash123 2012
[18] F S Chakar and A J Ragauskas ldquoReview of current and futuresoftwood kraft lignin process chemistryrdquo Industrial Crops andProducts vol 20 no 2 pp 131ndash141 2004
[19] W G Glasser and H R Glasser ldquoEvaluation of ligninrsquoschemical structure by experimental and computer simulationtechniquesrdquo Paperi ja Puu vol 63 pp 71ndash83 1981
[20] M Erickson S Larsson and G E Miksche ldquoZur Struktur desLignins der FichterdquoActaChemica Scandinavica vol 27 pp 903ndash914 1973
[21] H Nimz ldquoDas Lignin der BuchemdashEntwurf eines Konstitution-sschemasrdquo Angewandte ChemiemdashInternational Edition vol 86pp 336ndash344 1974
[22] D V Evtuguin C P Neto J Rocha and J D P de JesusldquoOxidative delignification in the presence of molybdovana-dophosphate heteropolyanions mechanism and kinetic stud-iesrdquo Applied Catalysis A General vol 167 no 1 pp 123ndash1391998
[23] A L Linsebigler G Lu and J T Yates Jr ldquoPhotocatalysis onTiO2surfaces Principles mechanisms and selected resultsrdquo
Chemical Reviews vol 95 no 3 pp 735ndash758 1995[24] Y Miyata K Miyazaki M Miura Y Shimotori M Aoyama
and H Nakatani ldquoSolventless delignification of wood flourwith TiO
2poly(ethylene oxide) photocatalyst systemrdquo Journal
of Polymers and the Environment vol 21 no 1 pp 115ndash121 2013[25] N Busse D Wagner M Kraume and P Czermak ldquoReaction
kinetics of versatile peroxidase for the degradation of lignincompoundsrdquoTheAmerican Journal of Biochemistry andBiotech-nology vol 9 no 4 pp 365ndash394 2013
[26] M Tien and T K Kirk ldquoLignin-degrading enzyme fromphanerochaete chrysosporium Purification characterizationand catalytic properties of a uniqueH
2O2-requiring oxygenaserdquo
Proceedings of the National Academy of Sciences vol 81 pp2280ndash2284 1984
[27] T K Kirk M Tien P J Kersten M D Mozuch and BKalyanaraman ldquoLigninase of Phanerochaete chrysosporiumMechanism of its degradation of the non-phenolic arylglycerol120573-aryl ether substructure of ligninrdquo Biochemical Journal vol236 no 1 pp 279ndash287 1986
[28] T Lundell RWever R Floris et al ldquoLignin peroxidase L3 fromPhlebia radiata pre-steady-state and steady-state studies withveratryl alcohol and a non-phenolic lignin model compound1-(34-dimethoxyphenyl)-2-(2-methoxyphenoxy)propane-13-diolrdquo European Journal of Biochemistry vol 211 no 3 pp391ndash402 1993
[29] H E Schoemaker T K Lundell A I Hatakka and K PiontekldquoThe oxidation of veratryl alcohol dimeric lignin models andlignin by lignin peroxidase the redox cycle revisitedrdquo FEMSMicrobiology Reviews vol 13 no 2-3 pp 321ndash331 1994
[30] J M Palmer P J Harvey and H E Schoemaker ldquoThe role ofperoxidases radical cations and oxygen in the degradation oflignin [and discussion]rdquo Philosophical Transactions of the RoyalSociety of London A vol 321 no 1561 pp 495ndash505 1987
[31] C Hill Wood Modification Chemical Thermal and OtherProcesses John Wiley amp Sons Chichester UK 2006
[32] Y-H Xu H-R Chen Z-X Zeng and B Lei ldquoInvestigation onmechanism of photocatalytic activity enhancement of nanome-ter cerium-doped titaniardquo Applied Surface Science vol 252 no24 pp 8565ndash8570 2006
[33] L Tonucci F Coccia M Bressan and N DrsquoAlessandro ldquoMildphotocatalysed and catalysed green oxidation of lignin auseful pathway to low-molecular-weight derivativesrdquoWaste andBiomass Valorization vol 3 no 2 pp 165ndash174 2012
[34] A Castellan N Colombo C Vanucci P Fornier de HViolet and H Bouas-Laurent ldquoPhotodegradation of ligninA photochemical study of an O-methylated 120572-carbonyl 120573-1lignin model dimer 12-di(3101584041015840-dimethoxyphenyl) ethanone(deoxyveratroin)rdquo Journal of Photochemistry and PhotobiologyA vol 51 pp 451ndash467 1990
[35] A Fujishima X Zhang and D A Tryk ldquoTiO2photocatalysis
and related surface phenomenardquo Surface Science Reports vol63 no 12 pp 515ndash582 2008
[36] C S Turchi and D F Ollis ldquoPhotocatalytic reactor designan example of mass-transfer limitations with an immobilizedcatalystrdquo The Journal of Physical Chemistry vol 92 no 23 pp6852ndash6853 1988
[37] R Subasri M Tripathi K Murugan J Revathi G V N Raoand T N Rao ldquoInvestigations on the photocatalytic activity ofsolndashgel derived plain and Fe3+Nb5+ doped titania coatings onglass substratesrdquo Materials Chemistry and Physics vol 124 pp63ndash68 2010
[38] N Serpone ldquoRelative photonic efficiencies and quantum yieldsin heterogeneous photocatalysisrdquo Journal of Photochemistry andPhotobiology A Chemistry vol 104 no 1ndash3 pp 1ndash12 1997
[39] M R Hoffmann S T Martin W Choi and D W BahnemannldquoEnvironmental applications of semiconductor photocatalysisrdquoChemical Reviews vol 95 no 1 pp 69ndash96 1995
[40] C D Jaeger and A J Bard ldquoSpin trapping and electron spinresonance detection of radical intermediates in the photode-composition of water at titanium dioxide particulate systemsrdquoThe Journal of Physical Chemistry vol 83 no 24 pp 3146ndash31521979
[41] R W Matthews ldquoHydroxylation reactions induced by near-ultraviolet photolysis of aqueous titaniumdioxide suspensionsrdquoJournal of the Chemical Society Faraday Transactions vol 80no 2 pp 457ndash471 1984
[42] A E H Machado A M Furuyama S Z Falone R RuggieroD D S Perez and A Castellan ldquoPhotocatalytic degradation oflignin and lignin models using titanium dioxide the role of thehydroxyl radicalrdquoChemosphere vol 40 no 1 pp 115ndash124 2000
[43] W Sigg and L Stumm Aquatische Chemie Stuttgart 2 AuflageBG Teubner Stuttgart Germany 1991
[44] O Legrini E Oliveros and A M Braun ldquoPhotochemicalprocesses for water treatmentrdquo Chemical Reviews vol 93 no2 pp 671ndash698 1993
[45] G Rothenberger J Moser M Gratzel N Serpone and DSharma ldquoCharge carrier trapping and recombination dynamicsin small semiconductor particlesrdquo Journal of the AmericanChemical Society vol 107 no 26 pp 8054ndash8059 1985
[46] P Mazellier M Sarakha A Rossi and M Bolte ldquoThe aqueousphotochemistry of 26-dimethylphenol Evidence for the frag-mentation of the 120572 C-C bondrdquo Journal of Photochemistry andPhotobiology A vol 115 no 2 pp 117ndash121 1998
[47] M Ksibi S B Amor S Cherif E Elaloui A Houas and MElaloui ldquoPhotodegradation of lignin from black liquor using aUVTiO
2systemrdquo Journal of Photochemistry and Photobiology
A vol 154 no 2-3 pp 211ndash218 2003
International Journal of Photoenergy 17
[48] S K Kansal M Singh and D Sud ldquoStudies on TiO2ZnO
photocatalysed degradation of ligninrdquo Journal of HazardousMaterials vol 153 no 1-2 pp 412ndash417 2008
[49] A Dahm and L A Lucia ldquoTitanium dioxide catalyzed pho-todegradation of lignin in industrial effluentsrdquo Industrial andEngineering Chemistry Research vol 43 no 25 pp 7996ndash80002004
[50] E Portjanskaja and S Preis ldquoAqueous photocatalytic oxidationof lignin the influence of mineral admixturesrdquo InternationalJournal of Photoenergy vol 2007Article ID76730 7 pages 2007
[51] K Tanaka R C R Calanag and T Hisanaga ldquoPhotocatalyzeddegradation of lignin on TiO
2rdquo Journal of Molecular Catalysis
A Chemical vol 138 no 2-3 pp 287ndash294 1999[52] M Tian J Wen D MacDonald R M Asmussen and A Chen
ldquoA novel approach for lignin modification and degradationrdquoElectrochemistry Communications vol 12 no 4 pp 527ndash5302010
[53] K PanM Tian Z-H Jiang B Kjartanson and A Chen ldquoElec-trochemical oxidation of lignin at lead dioxide nanoparticlesphotoelectrodeposited on TiO
2nanotube arraysrdquo Electrochim-
ica Acta vol 60 pp 147ndash153 2012[54] C Awungacha Lekelefac J Hild P Czermak and M Herren-
bauer ldquoPhotocatalytic active coatings for lignin degradationin a continuous packed bed reactorrdquo International Journal ofPhotoenergy vol 2014 Article ID 502326 10 pages 2014
[55] C Awungacha Lekelefac P Czermak and M HerrenbauerldquoEvaluation of photocatalytic active coatings on sintered glasstubes by methylene bluerdquo International Journal of Photoenergyvol 2013 Article ID 614567 9 pages 2013
[56] Y-S Ma C-N Chang Y-P Chiang H-F Sung and A CChao ldquoPhotocatalytic degradation of lignin using PtTiO
2as
the catalystrdquo Chemosphere vol 71 no 5 pp 998ndash1004 2008[57] H Ohnishi M Matsumura H Tsubomura and M Iwasaki
ldquoBleaching of lignin solution by a photocatalyzed reactionon semiconductor photocatalystsrdquo Industrial and EngineeringChemistry Research vol 28 no 6 pp 719ndash724 1989
[58] C A K Gouvea F Wypych S G Moraes N Duran and PPeralta-Zamora ldquoSemiconductor-assisted photodegradation oflignin dye and kraft effluent by Ag-doped ZnOrdquo Chemospherevol 40 no 4 pp 427ndash432 2000
[59] A V Vahatalo K Salonen M Salkinoja-Salonen and AHatakka ldquoPhotochemical mineralization of synthetic lignin inlake water indicates enhanced turnover of aromatic organicmatter under solar radiationrdquo Biodegradation vol 10 no 6 pp415ndash420 1999
[60] H de Lasa B Serrano and M Salaices Photocatalytic ReactionEngineering Springer New York NY USA 2005
[61] K Hashimoto H Irie and A Fujishima ldquoTiO 2 photocatalysisA historical overview and future prospectsrdquo Japanese Journal ofApplied Physics vol 44 no 12 pp 8269ndash8285 2005
[62] M A Behnajady N Modirshahla and R Hamzavi ldquoKineticstudy on photocatalytic degradation of CI Acid Yellow 23 byZnOphotocatalystrdquo Journal ofHazardousMaterials vol 133 no1ndash3 pp 226ndash232 2006
[63] M Addamo V Augugliaro A di Paola et al ldquoPhotocatalyticthin films of TiO
2formed by a sol-gel process using titanium
tetraisopropoxide as the precursorrdquo Thin Solid Films vol 516no 12 pp 3802ndash3807 2008
[64] N Negishi K Takeuchi and T Ibusuki ldquoPreparation of theTiO2thin filmphotocatalyst by the dip-coating processrdquo Journal
of Sol-Gel Science and Technology vol 13 no 1ndash3 pp 691ndash6941998
[65] L Rideh A Wehrer D Ronze and A Zoulalian ldquoPhotocat-alytic degradation of 2-chlorophenol in TiO
2aqueous suspen-
sion modeling of reaction raterdquo Industrial and EngineeringChemistry Research vol 36 no 11 pp 4712ndash4718 1997
[66] R-ADoongC-HChen RAMaithreepala and S-MChangldquoThe influence of pHand cadmiumsulfide on the photocatalyticdegradation of 2-chlorophenol in titanium dioxide suspen-sionsrdquoWater Research vol 35 no 12 pp 2873ndash2880 2001
[67] K V Sarkanen A Islam and C D Anderson ldquoOzonationrdquo inMethods in Lignin Chemistry S Y Lin and C W Dence EdsSpringer Series inWood Science pp 387ndash406 Springer BerlinGermany 1992
[68] G Gellerstedt and E-L Lindfors ldquoStructural changes in ligninduring kraft pulpingrdquo Holzforschung vol 38 no 3 pp 151ndash1581984
[69] B Neppolian H C Choi M V Shankar B Arabindoo andV Murugesan in Proceedings of the International Symposiumon Environmental Pollution Control and Waste Management(EPCOWM 02) p 647 2002
[70] C Pouteau P Dole B Cathala L Averous and N BoquillonldquoAntioxidant properties of lignin in polypropylenerdquo PolymerDegradation and Stability vol 81 no 1 pp 9ndash18 2003
[71] J Hafren T Fujino and T Itoh ldquoChanges in cell wall archi-tecture of differentiating tracheids of Pinus thunbergii duringlignificationrdquo Plant and Cell Physiology vol 40 no 5 pp 532ndash541 1999
[72] T Dizhbite G Telysheva V Jurkjane and U Viesturs ldquoChar-acterization of the radical scavenging activity of lignins naturalantioxidantsrdquo Bioresource Technology vol 95 no 3 pp 309ndash3172004
[73] K Hofstadler R Bauer S Novalic and S G Heisier ldquoNew reac-tor design for photocatalytic wastewater treatment with TiO
2
immobilized on fused-silica glass fibers photomineralization of4-chlorophenolrdquo Environmental Science amp Technology vol 28no 4 pp 670ndash674 1994
[74] J Villasenor and H D Mansilla ldquoEffect of temperature onkraft black liquor degradation by ZnO-photoassisted catalysisrdquoJournal of Photochemistry and Photobiology A Chemistry vol93 no 2-3 pp 205ndash209 1996
[75] B H Bielski D E Cabelli L A Ravindra and A B RossldquoReactivity of HO
2Ominus2Radicals in aqueous solutionrdquo Journal
of Physical Chemistry vol 14 pp 1041ndash1100 1985[76] B H J Bielski and A O Allen ldquoMechanism of the dispropor-
tionation of superoxide radicalsrdquo Journal of Physical Chemistryvol 81 no 11 pp 1048ndash1050 1977
[77] B Halliwell and J M C Gutteridge ldquoThe importance of freeradicals and catalytic metal ions in human diseasesrdquoMolecularAspects of Medicine vol 8 no 2 pp 89ndash193 1985
[78] J M Palmer P J Harvey and H E Schoemaker ldquoThe role ofperoxidases radical cations and oxygen in the degradation oflignin [and discussion]rdquo Philosophical Transaction of the RoyalSociety A vol 321 no 1561 pp 495ndash505 1987
[79] S Y Yin and C W Dense Methods in Lignin ChemistrySpringer New York NY USA 1992
[80] P Kumar S Kumar and N K Bhardwaj ldquoPhotocatalyticoxidation of elemental chlorine free bleaching effluent withUVTiO
2rdquo in Proceedings of the 2nd International Conference on
Environmental Science and Technology (ICEST rsquo11) SingaporeFebruary 2011
[81] K Kobayakawa Y Sato S Nakamura and A FujishimaldquoPhotodecomposition of Kraft lignin catalyzed by titanium
18 International Journal of Photoenergy
dioxiderdquo Bulletin of the Chemical Society of Japan vol 62 no11 pp 3433ndash3436 1989
[82] R B Kinstre ldquoAn overview of strategies for reducing theenvironmental impact of bleach-plant effluentsrdquo Tappi Journalvol 76 no 5 pp 105ndash113 1993
[83] A Castellan H Choudhury R Stephen Davidson and SGrelier ldquoComparative study of stone-ground wood pulp andnative wood 3 Application of fluorescence spectroscopy to astudy of the weathering of stone-ground pulp and native woodrdquoJournal of Photochemistry and Photobiology A Chemistry vol81 no 2 pp 123ndash130 1994
[84] B Albinsson S Li K Lundquist and R Stomberg ldquoThe originof lignin fluorescencerdquo Journal of Molecular Structure vol 508no 1ndash3 pp 19ndash27 1999
[85] S Baumberger A Abaecherli M Fasching et al ldquoMolar massdetermination of lignins by size-exclusion chromatographytowards standardisation of the methodrdquo Holzforschung vol 61no 4 pp 459ndash468 2007
[86] E Torres andM Ayala Biocatalysis Based on Heme PeroxidasesSpringer New York NY USA 1st edition 2010
[87] R Pecina P Burtscher G Bonn and O Bobleter ldquoGC-MSand HPLC analyses of lignin degradation products in biomasshydrolyzatesrdquo Fresenius Zeitschrift fur Analytische Chemie vol325 no 5 pp 461ndash465 1986
[88] NIST NIST Standard Reference Database The National Insti-tute of Standards and Technology 2014 httpwwwnistgovsrdnist1acfm
[89] T K Kirk and R L Farrell ldquoEnzymatic ldquocombustionrdquo themicrobial degradation of ligninrdquo Annual Review of Microbiol-ogy vol 41 pp 465ndash501 1987
[90] A TMartinez ldquoHigh redox potential peroxidasesrdquo in IndustrialEnzymes Structure Function and Applications K-B BeckerEd pp 477ndash488 Springer Amsterdam The Netherlands 1stedition 2007
[91] M Dashtban H Schraft T A Syed and W Qin ldquoFungalbiodegradation and enzymatic modification of ligninrdquo Interna-tional Journal of Biochemistry and Molecular Biology vol 1 no1 pp 36ndash50 2010
[92] N Duran E Esposito L H Innocentini-Mei and V P CanhosldquoA new alternative process for Kraft E1 effluent treatmentrdquoBiodegradation vol 5 no 1 pp 13ndash19 1994
[93] K Kamwilaisak and P C Wright ldquoInvestigating laccase andtitanium dioxide for lignin degradationrdquo Energy and Fuels vol26 no 4 pp 2400ndash2406 2012
[94] J Reyes M Dezotti H Mansilla J Villasenor E Espositoand N Duran ldquoBiomass photochemistry-XXII combined pho-tochemical and biological process for treatment of Kraft E1effluentrdquoApplied Catalysis B Environmental vol 15 no 3-4 pp211ndash219 1998
[95] L F Gonzalez V Sarria and O F Sanchez ldquoDegradationof chlorophenols by sequential biological-advanced oxidativeprocess using Trametespubescens and TiO
2UVrdquo Bioresource
Technology vol 101 no 10 pp 3493ndash3499 2010[96] J H Lora and W G Glasser ldquoRecent industrial applications
of lignin a sustainable alternative to nonrenewable materialsrdquoJournal of Polymers and the Environment vol 10 no 1-2 pp 39ndash48 2002
Submit your manuscripts athttpwwwhindawicom
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Inorganic ChemistryInternational Journal of
Hindawi Publishing Corporation httpwwwhindawicom Volume 2014
International Journal ofPhotoenergy
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Carbohydrate Chemistry
International Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
Chemistry
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Advances in
Physical Chemistry
Hindawi Publishing Corporationhttpwwwhindawicom
Analytical Methods in Chemistry
Journal of
Volume 2014
Bioinorganic Chemistry and ApplicationsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
SpectroscopyInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Medicinal ChemistryInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Chromatography Research International
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Applied ChemistryJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Theoretical ChemistryJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
Spectroscopy
Analytical ChemistryInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Quantum Chemistry
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Organic Chemistry International
ElectrochemistryInternational Journal of
Hindawi Publishing Corporation httpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CatalystsJournal of
16 International Journal of Photoenergy
[16] M Dashtban H Schraft and W Qin ldquoFungal bioconversionof lignocellulosic residues Opportunities amp perspectivesrdquo Inter-national Journal of Biological Sciences vol 5 no 6 pp 578ndash5952009
[17] A Shende R Jaswal D Harder-Heinz A Menan and RShende ldquoIntergrated photocatalytic and microbial degradationof kraft ligninrdquo Cleantech pp 120ndash123 2012
[18] F S Chakar and A J Ragauskas ldquoReview of current and futuresoftwood kraft lignin process chemistryrdquo Industrial Crops andProducts vol 20 no 2 pp 131ndash141 2004
[19] W G Glasser and H R Glasser ldquoEvaluation of ligninrsquoschemical structure by experimental and computer simulationtechniquesrdquo Paperi ja Puu vol 63 pp 71ndash83 1981
[20] M Erickson S Larsson and G E Miksche ldquoZur Struktur desLignins der FichterdquoActaChemica Scandinavica vol 27 pp 903ndash914 1973
[21] H Nimz ldquoDas Lignin der BuchemdashEntwurf eines Konstitution-sschemasrdquo Angewandte ChemiemdashInternational Edition vol 86pp 336ndash344 1974
[22] D V Evtuguin C P Neto J Rocha and J D P de JesusldquoOxidative delignification in the presence of molybdovana-dophosphate heteropolyanions mechanism and kinetic stud-iesrdquo Applied Catalysis A General vol 167 no 1 pp 123ndash1391998
[23] A L Linsebigler G Lu and J T Yates Jr ldquoPhotocatalysis onTiO2surfaces Principles mechanisms and selected resultsrdquo
Chemical Reviews vol 95 no 3 pp 735ndash758 1995[24] Y Miyata K Miyazaki M Miura Y Shimotori M Aoyama
and H Nakatani ldquoSolventless delignification of wood flourwith TiO
2poly(ethylene oxide) photocatalyst systemrdquo Journal
of Polymers and the Environment vol 21 no 1 pp 115ndash121 2013[25] N Busse D Wagner M Kraume and P Czermak ldquoReaction
kinetics of versatile peroxidase for the degradation of lignincompoundsrdquoTheAmerican Journal of Biochemistry andBiotech-nology vol 9 no 4 pp 365ndash394 2013
[26] M Tien and T K Kirk ldquoLignin-degrading enzyme fromphanerochaete chrysosporium Purification characterizationand catalytic properties of a uniqueH
2O2-requiring oxygenaserdquo
Proceedings of the National Academy of Sciences vol 81 pp2280ndash2284 1984
[27] T K Kirk M Tien P J Kersten M D Mozuch and BKalyanaraman ldquoLigninase of Phanerochaete chrysosporiumMechanism of its degradation of the non-phenolic arylglycerol120573-aryl ether substructure of ligninrdquo Biochemical Journal vol236 no 1 pp 279ndash287 1986
[28] T Lundell RWever R Floris et al ldquoLignin peroxidase L3 fromPhlebia radiata pre-steady-state and steady-state studies withveratryl alcohol and a non-phenolic lignin model compound1-(34-dimethoxyphenyl)-2-(2-methoxyphenoxy)propane-13-diolrdquo European Journal of Biochemistry vol 211 no 3 pp391ndash402 1993
[29] H E Schoemaker T K Lundell A I Hatakka and K PiontekldquoThe oxidation of veratryl alcohol dimeric lignin models andlignin by lignin peroxidase the redox cycle revisitedrdquo FEMSMicrobiology Reviews vol 13 no 2-3 pp 321ndash331 1994
[30] J M Palmer P J Harvey and H E Schoemaker ldquoThe role ofperoxidases radical cations and oxygen in the degradation oflignin [and discussion]rdquo Philosophical Transactions of the RoyalSociety of London A vol 321 no 1561 pp 495ndash505 1987
[31] C Hill Wood Modification Chemical Thermal and OtherProcesses John Wiley amp Sons Chichester UK 2006
[32] Y-H Xu H-R Chen Z-X Zeng and B Lei ldquoInvestigation onmechanism of photocatalytic activity enhancement of nanome-ter cerium-doped titaniardquo Applied Surface Science vol 252 no24 pp 8565ndash8570 2006
[33] L Tonucci F Coccia M Bressan and N DrsquoAlessandro ldquoMildphotocatalysed and catalysed green oxidation of lignin auseful pathway to low-molecular-weight derivativesrdquoWaste andBiomass Valorization vol 3 no 2 pp 165ndash174 2012
[34] A Castellan N Colombo C Vanucci P Fornier de HViolet and H Bouas-Laurent ldquoPhotodegradation of ligninA photochemical study of an O-methylated 120572-carbonyl 120573-1lignin model dimer 12-di(3101584041015840-dimethoxyphenyl) ethanone(deoxyveratroin)rdquo Journal of Photochemistry and PhotobiologyA vol 51 pp 451ndash467 1990
[35] A Fujishima X Zhang and D A Tryk ldquoTiO2photocatalysis
and related surface phenomenardquo Surface Science Reports vol63 no 12 pp 515ndash582 2008
[36] C S Turchi and D F Ollis ldquoPhotocatalytic reactor designan example of mass-transfer limitations with an immobilizedcatalystrdquo The Journal of Physical Chemistry vol 92 no 23 pp6852ndash6853 1988
[37] R Subasri M Tripathi K Murugan J Revathi G V N Raoand T N Rao ldquoInvestigations on the photocatalytic activity ofsolndashgel derived plain and Fe3+Nb5+ doped titania coatings onglass substratesrdquo Materials Chemistry and Physics vol 124 pp63ndash68 2010
[38] N Serpone ldquoRelative photonic efficiencies and quantum yieldsin heterogeneous photocatalysisrdquo Journal of Photochemistry andPhotobiology A Chemistry vol 104 no 1ndash3 pp 1ndash12 1997
[39] M R Hoffmann S T Martin W Choi and D W BahnemannldquoEnvironmental applications of semiconductor photocatalysisrdquoChemical Reviews vol 95 no 1 pp 69ndash96 1995
[40] C D Jaeger and A J Bard ldquoSpin trapping and electron spinresonance detection of radical intermediates in the photode-composition of water at titanium dioxide particulate systemsrdquoThe Journal of Physical Chemistry vol 83 no 24 pp 3146ndash31521979
[41] R W Matthews ldquoHydroxylation reactions induced by near-ultraviolet photolysis of aqueous titaniumdioxide suspensionsrdquoJournal of the Chemical Society Faraday Transactions vol 80no 2 pp 457ndash471 1984
[42] A E H Machado A M Furuyama S Z Falone R RuggieroD D S Perez and A Castellan ldquoPhotocatalytic degradation oflignin and lignin models using titanium dioxide the role of thehydroxyl radicalrdquoChemosphere vol 40 no 1 pp 115ndash124 2000
[43] W Sigg and L Stumm Aquatische Chemie Stuttgart 2 AuflageBG Teubner Stuttgart Germany 1991
[44] O Legrini E Oliveros and A M Braun ldquoPhotochemicalprocesses for water treatmentrdquo Chemical Reviews vol 93 no2 pp 671ndash698 1993
[45] G Rothenberger J Moser M Gratzel N Serpone and DSharma ldquoCharge carrier trapping and recombination dynamicsin small semiconductor particlesrdquo Journal of the AmericanChemical Society vol 107 no 26 pp 8054ndash8059 1985
[46] P Mazellier M Sarakha A Rossi and M Bolte ldquoThe aqueousphotochemistry of 26-dimethylphenol Evidence for the frag-mentation of the 120572 C-C bondrdquo Journal of Photochemistry andPhotobiology A vol 115 no 2 pp 117ndash121 1998
[47] M Ksibi S B Amor S Cherif E Elaloui A Houas and MElaloui ldquoPhotodegradation of lignin from black liquor using aUVTiO
2systemrdquo Journal of Photochemistry and Photobiology
A vol 154 no 2-3 pp 211ndash218 2003
International Journal of Photoenergy 17
[48] S K Kansal M Singh and D Sud ldquoStudies on TiO2ZnO
photocatalysed degradation of ligninrdquo Journal of HazardousMaterials vol 153 no 1-2 pp 412ndash417 2008
[49] A Dahm and L A Lucia ldquoTitanium dioxide catalyzed pho-todegradation of lignin in industrial effluentsrdquo Industrial andEngineering Chemistry Research vol 43 no 25 pp 7996ndash80002004
[50] E Portjanskaja and S Preis ldquoAqueous photocatalytic oxidationof lignin the influence of mineral admixturesrdquo InternationalJournal of Photoenergy vol 2007Article ID76730 7 pages 2007
[51] K Tanaka R C R Calanag and T Hisanaga ldquoPhotocatalyzeddegradation of lignin on TiO
2rdquo Journal of Molecular Catalysis
A Chemical vol 138 no 2-3 pp 287ndash294 1999[52] M Tian J Wen D MacDonald R M Asmussen and A Chen
ldquoA novel approach for lignin modification and degradationrdquoElectrochemistry Communications vol 12 no 4 pp 527ndash5302010
[53] K PanM Tian Z-H Jiang B Kjartanson and A Chen ldquoElec-trochemical oxidation of lignin at lead dioxide nanoparticlesphotoelectrodeposited on TiO
2nanotube arraysrdquo Electrochim-
ica Acta vol 60 pp 147ndash153 2012[54] C Awungacha Lekelefac J Hild P Czermak and M Herren-
bauer ldquoPhotocatalytic active coatings for lignin degradationin a continuous packed bed reactorrdquo International Journal ofPhotoenergy vol 2014 Article ID 502326 10 pages 2014
[55] C Awungacha Lekelefac P Czermak and M HerrenbauerldquoEvaluation of photocatalytic active coatings on sintered glasstubes by methylene bluerdquo International Journal of Photoenergyvol 2013 Article ID 614567 9 pages 2013
[56] Y-S Ma C-N Chang Y-P Chiang H-F Sung and A CChao ldquoPhotocatalytic degradation of lignin using PtTiO
2as
the catalystrdquo Chemosphere vol 71 no 5 pp 998ndash1004 2008[57] H Ohnishi M Matsumura H Tsubomura and M Iwasaki
ldquoBleaching of lignin solution by a photocatalyzed reactionon semiconductor photocatalystsrdquo Industrial and EngineeringChemistry Research vol 28 no 6 pp 719ndash724 1989
[58] C A K Gouvea F Wypych S G Moraes N Duran and PPeralta-Zamora ldquoSemiconductor-assisted photodegradation oflignin dye and kraft effluent by Ag-doped ZnOrdquo Chemospherevol 40 no 4 pp 427ndash432 2000
[59] A V Vahatalo K Salonen M Salkinoja-Salonen and AHatakka ldquoPhotochemical mineralization of synthetic lignin inlake water indicates enhanced turnover of aromatic organicmatter under solar radiationrdquo Biodegradation vol 10 no 6 pp415ndash420 1999
[60] H de Lasa B Serrano and M Salaices Photocatalytic ReactionEngineering Springer New York NY USA 2005
[61] K Hashimoto H Irie and A Fujishima ldquoTiO 2 photocatalysisA historical overview and future prospectsrdquo Japanese Journal ofApplied Physics vol 44 no 12 pp 8269ndash8285 2005
[62] M A Behnajady N Modirshahla and R Hamzavi ldquoKineticstudy on photocatalytic degradation of CI Acid Yellow 23 byZnOphotocatalystrdquo Journal ofHazardousMaterials vol 133 no1ndash3 pp 226ndash232 2006
[63] M Addamo V Augugliaro A di Paola et al ldquoPhotocatalyticthin films of TiO
2formed by a sol-gel process using titanium
tetraisopropoxide as the precursorrdquo Thin Solid Films vol 516no 12 pp 3802ndash3807 2008
[64] N Negishi K Takeuchi and T Ibusuki ldquoPreparation of theTiO2thin filmphotocatalyst by the dip-coating processrdquo Journal
of Sol-Gel Science and Technology vol 13 no 1ndash3 pp 691ndash6941998
[65] L Rideh A Wehrer D Ronze and A Zoulalian ldquoPhotocat-alytic degradation of 2-chlorophenol in TiO
2aqueous suspen-
sion modeling of reaction raterdquo Industrial and EngineeringChemistry Research vol 36 no 11 pp 4712ndash4718 1997
[66] R-ADoongC-HChen RAMaithreepala and S-MChangldquoThe influence of pHand cadmiumsulfide on the photocatalyticdegradation of 2-chlorophenol in titanium dioxide suspen-sionsrdquoWater Research vol 35 no 12 pp 2873ndash2880 2001
[67] K V Sarkanen A Islam and C D Anderson ldquoOzonationrdquo inMethods in Lignin Chemistry S Y Lin and C W Dence EdsSpringer Series inWood Science pp 387ndash406 Springer BerlinGermany 1992
[68] G Gellerstedt and E-L Lindfors ldquoStructural changes in ligninduring kraft pulpingrdquo Holzforschung vol 38 no 3 pp 151ndash1581984
[69] B Neppolian H C Choi M V Shankar B Arabindoo andV Murugesan in Proceedings of the International Symposiumon Environmental Pollution Control and Waste Management(EPCOWM 02) p 647 2002
[70] C Pouteau P Dole B Cathala L Averous and N BoquillonldquoAntioxidant properties of lignin in polypropylenerdquo PolymerDegradation and Stability vol 81 no 1 pp 9ndash18 2003
[71] J Hafren T Fujino and T Itoh ldquoChanges in cell wall archi-tecture of differentiating tracheids of Pinus thunbergii duringlignificationrdquo Plant and Cell Physiology vol 40 no 5 pp 532ndash541 1999
[72] T Dizhbite G Telysheva V Jurkjane and U Viesturs ldquoChar-acterization of the radical scavenging activity of lignins naturalantioxidantsrdquo Bioresource Technology vol 95 no 3 pp 309ndash3172004
[73] K Hofstadler R Bauer S Novalic and S G Heisier ldquoNew reac-tor design for photocatalytic wastewater treatment with TiO
2
immobilized on fused-silica glass fibers photomineralization of4-chlorophenolrdquo Environmental Science amp Technology vol 28no 4 pp 670ndash674 1994
[74] J Villasenor and H D Mansilla ldquoEffect of temperature onkraft black liquor degradation by ZnO-photoassisted catalysisrdquoJournal of Photochemistry and Photobiology A Chemistry vol93 no 2-3 pp 205ndash209 1996
[75] B H Bielski D E Cabelli L A Ravindra and A B RossldquoReactivity of HO
2Ominus2Radicals in aqueous solutionrdquo Journal
of Physical Chemistry vol 14 pp 1041ndash1100 1985[76] B H J Bielski and A O Allen ldquoMechanism of the dispropor-
tionation of superoxide radicalsrdquo Journal of Physical Chemistryvol 81 no 11 pp 1048ndash1050 1977
[77] B Halliwell and J M C Gutteridge ldquoThe importance of freeradicals and catalytic metal ions in human diseasesrdquoMolecularAspects of Medicine vol 8 no 2 pp 89ndash193 1985
[78] J M Palmer P J Harvey and H E Schoemaker ldquoThe role ofperoxidases radical cations and oxygen in the degradation oflignin [and discussion]rdquo Philosophical Transaction of the RoyalSociety A vol 321 no 1561 pp 495ndash505 1987
[79] S Y Yin and C W Dense Methods in Lignin ChemistrySpringer New York NY USA 1992
[80] P Kumar S Kumar and N K Bhardwaj ldquoPhotocatalyticoxidation of elemental chlorine free bleaching effluent withUVTiO
2rdquo in Proceedings of the 2nd International Conference on
Environmental Science and Technology (ICEST rsquo11) SingaporeFebruary 2011
[81] K Kobayakawa Y Sato S Nakamura and A FujishimaldquoPhotodecomposition of Kraft lignin catalyzed by titanium
18 International Journal of Photoenergy
dioxiderdquo Bulletin of the Chemical Society of Japan vol 62 no11 pp 3433ndash3436 1989
[82] R B Kinstre ldquoAn overview of strategies for reducing theenvironmental impact of bleach-plant effluentsrdquo Tappi Journalvol 76 no 5 pp 105ndash113 1993
[83] A Castellan H Choudhury R Stephen Davidson and SGrelier ldquoComparative study of stone-ground wood pulp andnative wood 3 Application of fluorescence spectroscopy to astudy of the weathering of stone-ground pulp and native woodrdquoJournal of Photochemistry and Photobiology A Chemistry vol81 no 2 pp 123ndash130 1994
[84] B Albinsson S Li K Lundquist and R Stomberg ldquoThe originof lignin fluorescencerdquo Journal of Molecular Structure vol 508no 1ndash3 pp 19ndash27 1999
[85] S Baumberger A Abaecherli M Fasching et al ldquoMolar massdetermination of lignins by size-exclusion chromatographytowards standardisation of the methodrdquo Holzforschung vol 61no 4 pp 459ndash468 2007
[86] E Torres andM Ayala Biocatalysis Based on Heme PeroxidasesSpringer New York NY USA 1st edition 2010
[87] R Pecina P Burtscher G Bonn and O Bobleter ldquoGC-MSand HPLC analyses of lignin degradation products in biomasshydrolyzatesrdquo Fresenius Zeitschrift fur Analytische Chemie vol325 no 5 pp 461ndash465 1986
[88] NIST NIST Standard Reference Database The National Insti-tute of Standards and Technology 2014 httpwwwnistgovsrdnist1acfm
[89] T K Kirk and R L Farrell ldquoEnzymatic ldquocombustionrdquo themicrobial degradation of ligninrdquo Annual Review of Microbiol-ogy vol 41 pp 465ndash501 1987
[90] A TMartinez ldquoHigh redox potential peroxidasesrdquo in IndustrialEnzymes Structure Function and Applications K-B BeckerEd pp 477ndash488 Springer Amsterdam The Netherlands 1stedition 2007
[91] M Dashtban H Schraft T A Syed and W Qin ldquoFungalbiodegradation and enzymatic modification of ligninrdquo Interna-tional Journal of Biochemistry and Molecular Biology vol 1 no1 pp 36ndash50 2010
[92] N Duran E Esposito L H Innocentini-Mei and V P CanhosldquoA new alternative process for Kraft E1 effluent treatmentrdquoBiodegradation vol 5 no 1 pp 13ndash19 1994
[93] K Kamwilaisak and P C Wright ldquoInvestigating laccase andtitanium dioxide for lignin degradationrdquo Energy and Fuels vol26 no 4 pp 2400ndash2406 2012
[94] J Reyes M Dezotti H Mansilla J Villasenor E Espositoand N Duran ldquoBiomass photochemistry-XXII combined pho-tochemical and biological process for treatment of Kraft E1effluentrdquoApplied Catalysis B Environmental vol 15 no 3-4 pp211ndash219 1998
[95] L F Gonzalez V Sarria and O F Sanchez ldquoDegradationof chlorophenols by sequential biological-advanced oxidativeprocess using Trametespubescens and TiO
2UVrdquo Bioresource
Technology vol 101 no 10 pp 3493ndash3499 2010[96] J H Lora and W G Glasser ldquoRecent industrial applications
of lignin a sustainable alternative to nonrenewable materialsrdquoJournal of Polymers and the Environment vol 10 no 1-2 pp 39ndash48 2002
Submit your manuscripts athttpwwwhindawicom
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Inorganic ChemistryInternational Journal of
Hindawi Publishing Corporation httpwwwhindawicom Volume 2014
International Journal ofPhotoenergy
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Carbohydrate Chemistry
International Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
Chemistry
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Advances in
Physical Chemistry
Hindawi Publishing Corporationhttpwwwhindawicom
Analytical Methods in Chemistry
Journal of
Volume 2014
Bioinorganic Chemistry and ApplicationsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
SpectroscopyInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Medicinal ChemistryInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Chromatography Research International
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Applied ChemistryJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Theoretical ChemistryJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
Spectroscopy
Analytical ChemistryInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Quantum Chemistry
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Organic Chemistry International
ElectrochemistryInternational Journal of
Hindawi Publishing Corporation httpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CatalystsJournal of
International Journal of Photoenergy 17
[48] S K Kansal M Singh and D Sud ldquoStudies on TiO2ZnO
photocatalysed degradation of ligninrdquo Journal of HazardousMaterials vol 153 no 1-2 pp 412ndash417 2008
[49] A Dahm and L A Lucia ldquoTitanium dioxide catalyzed pho-todegradation of lignin in industrial effluentsrdquo Industrial andEngineering Chemistry Research vol 43 no 25 pp 7996ndash80002004
[50] E Portjanskaja and S Preis ldquoAqueous photocatalytic oxidationof lignin the influence of mineral admixturesrdquo InternationalJournal of Photoenergy vol 2007Article ID76730 7 pages 2007
[51] K Tanaka R C R Calanag and T Hisanaga ldquoPhotocatalyzeddegradation of lignin on TiO
2rdquo Journal of Molecular Catalysis
A Chemical vol 138 no 2-3 pp 287ndash294 1999[52] M Tian J Wen D MacDonald R M Asmussen and A Chen
ldquoA novel approach for lignin modification and degradationrdquoElectrochemistry Communications vol 12 no 4 pp 527ndash5302010
[53] K PanM Tian Z-H Jiang B Kjartanson and A Chen ldquoElec-trochemical oxidation of lignin at lead dioxide nanoparticlesphotoelectrodeposited on TiO
2nanotube arraysrdquo Electrochim-
ica Acta vol 60 pp 147ndash153 2012[54] C Awungacha Lekelefac J Hild P Czermak and M Herren-
bauer ldquoPhotocatalytic active coatings for lignin degradationin a continuous packed bed reactorrdquo International Journal ofPhotoenergy vol 2014 Article ID 502326 10 pages 2014
[55] C Awungacha Lekelefac P Czermak and M HerrenbauerldquoEvaluation of photocatalytic active coatings on sintered glasstubes by methylene bluerdquo International Journal of Photoenergyvol 2013 Article ID 614567 9 pages 2013
[56] Y-S Ma C-N Chang Y-P Chiang H-F Sung and A CChao ldquoPhotocatalytic degradation of lignin using PtTiO
2as
the catalystrdquo Chemosphere vol 71 no 5 pp 998ndash1004 2008[57] H Ohnishi M Matsumura H Tsubomura and M Iwasaki
ldquoBleaching of lignin solution by a photocatalyzed reactionon semiconductor photocatalystsrdquo Industrial and EngineeringChemistry Research vol 28 no 6 pp 719ndash724 1989
[58] C A K Gouvea F Wypych S G Moraes N Duran and PPeralta-Zamora ldquoSemiconductor-assisted photodegradation oflignin dye and kraft effluent by Ag-doped ZnOrdquo Chemospherevol 40 no 4 pp 427ndash432 2000
[59] A V Vahatalo K Salonen M Salkinoja-Salonen and AHatakka ldquoPhotochemical mineralization of synthetic lignin inlake water indicates enhanced turnover of aromatic organicmatter under solar radiationrdquo Biodegradation vol 10 no 6 pp415ndash420 1999
[60] H de Lasa B Serrano and M Salaices Photocatalytic ReactionEngineering Springer New York NY USA 2005
[61] K Hashimoto H Irie and A Fujishima ldquoTiO 2 photocatalysisA historical overview and future prospectsrdquo Japanese Journal ofApplied Physics vol 44 no 12 pp 8269ndash8285 2005
[62] M A Behnajady N Modirshahla and R Hamzavi ldquoKineticstudy on photocatalytic degradation of CI Acid Yellow 23 byZnOphotocatalystrdquo Journal ofHazardousMaterials vol 133 no1ndash3 pp 226ndash232 2006
[63] M Addamo V Augugliaro A di Paola et al ldquoPhotocatalyticthin films of TiO
2formed by a sol-gel process using titanium
tetraisopropoxide as the precursorrdquo Thin Solid Films vol 516no 12 pp 3802ndash3807 2008
[64] N Negishi K Takeuchi and T Ibusuki ldquoPreparation of theTiO2thin filmphotocatalyst by the dip-coating processrdquo Journal
of Sol-Gel Science and Technology vol 13 no 1ndash3 pp 691ndash6941998
[65] L Rideh A Wehrer D Ronze and A Zoulalian ldquoPhotocat-alytic degradation of 2-chlorophenol in TiO
2aqueous suspen-
sion modeling of reaction raterdquo Industrial and EngineeringChemistry Research vol 36 no 11 pp 4712ndash4718 1997
[66] R-ADoongC-HChen RAMaithreepala and S-MChangldquoThe influence of pHand cadmiumsulfide on the photocatalyticdegradation of 2-chlorophenol in titanium dioxide suspen-sionsrdquoWater Research vol 35 no 12 pp 2873ndash2880 2001
[67] K V Sarkanen A Islam and C D Anderson ldquoOzonationrdquo inMethods in Lignin Chemistry S Y Lin and C W Dence EdsSpringer Series inWood Science pp 387ndash406 Springer BerlinGermany 1992
[68] G Gellerstedt and E-L Lindfors ldquoStructural changes in ligninduring kraft pulpingrdquo Holzforschung vol 38 no 3 pp 151ndash1581984
[69] B Neppolian H C Choi M V Shankar B Arabindoo andV Murugesan in Proceedings of the International Symposiumon Environmental Pollution Control and Waste Management(EPCOWM 02) p 647 2002
[70] C Pouteau P Dole B Cathala L Averous and N BoquillonldquoAntioxidant properties of lignin in polypropylenerdquo PolymerDegradation and Stability vol 81 no 1 pp 9ndash18 2003
[71] J Hafren T Fujino and T Itoh ldquoChanges in cell wall archi-tecture of differentiating tracheids of Pinus thunbergii duringlignificationrdquo Plant and Cell Physiology vol 40 no 5 pp 532ndash541 1999
[72] T Dizhbite G Telysheva V Jurkjane and U Viesturs ldquoChar-acterization of the radical scavenging activity of lignins naturalantioxidantsrdquo Bioresource Technology vol 95 no 3 pp 309ndash3172004
[73] K Hofstadler R Bauer S Novalic and S G Heisier ldquoNew reac-tor design for photocatalytic wastewater treatment with TiO
2
immobilized on fused-silica glass fibers photomineralization of4-chlorophenolrdquo Environmental Science amp Technology vol 28no 4 pp 670ndash674 1994
[74] J Villasenor and H D Mansilla ldquoEffect of temperature onkraft black liquor degradation by ZnO-photoassisted catalysisrdquoJournal of Photochemistry and Photobiology A Chemistry vol93 no 2-3 pp 205ndash209 1996
[75] B H Bielski D E Cabelli L A Ravindra and A B RossldquoReactivity of HO
2Ominus2Radicals in aqueous solutionrdquo Journal
of Physical Chemistry vol 14 pp 1041ndash1100 1985[76] B H J Bielski and A O Allen ldquoMechanism of the dispropor-
tionation of superoxide radicalsrdquo Journal of Physical Chemistryvol 81 no 11 pp 1048ndash1050 1977
[77] B Halliwell and J M C Gutteridge ldquoThe importance of freeradicals and catalytic metal ions in human diseasesrdquoMolecularAspects of Medicine vol 8 no 2 pp 89ndash193 1985
[78] J M Palmer P J Harvey and H E Schoemaker ldquoThe role ofperoxidases radical cations and oxygen in the degradation oflignin [and discussion]rdquo Philosophical Transaction of the RoyalSociety A vol 321 no 1561 pp 495ndash505 1987
[79] S Y Yin and C W Dense Methods in Lignin ChemistrySpringer New York NY USA 1992
[80] P Kumar S Kumar and N K Bhardwaj ldquoPhotocatalyticoxidation of elemental chlorine free bleaching effluent withUVTiO
2rdquo in Proceedings of the 2nd International Conference on
Environmental Science and Technology (ICEST rsquo11) SingaporeFebruary 2011
[81] K Kobayakawa Y Sato S Nakamura and A FujishimaldquoPhotodecomposition of Kraft lignin catalyzed by titanium
18 International Journal of Photoenergy
dioxiderdquo Bulletin of the Chemical Society of Japan vol 62 no11 pp 3433ndash3436 1989
[82] R B Kinstre ldquoAn overview of strategies for reducing theenvironmental impact of bleach-plant effluentsrdquo Tappi Journalvol 76 no 5 pp 105ndash113 1993
[83] A Castellan H Choudhury R Stephen Davidson and SGrelier ldquoComparative study of stone-ground wood pulp andnative wood 3 Application of fluorescence spectroscopy to astudy of the weathering of stone-ground pulp and native woodrdquoJournal of Photochemistry and Photobiology A Chemistry vol81 no 2 pp 123ndash130 1994
[84] B Albinsson S Li K Lundquist and R Stomberg ldquoThe originof lignin fluorescencerdquo Journal of Molecular Structure vol 508no 1ndash3 pp 19ndash27 1999
[85] S Baumberger A Abaecherli M Fasching et al ldquoMolar massdetermination of lignins by size-exclusion chromatographytowards standardisation of the methodrdquo Holzforschung vol 61no 4 pp 459ndash468 2007
[86] E Torres andM Ayala Biocatalysis Based on Heme PeroxidasesSpringer New York NY USA 1st edition 2010
[87] R Pecina P Burtscher G Bonn and O Bobleter ldquoGC-MSand HPLC analyses of lignin degradation products in biomasshydrolyzatesrdquo Fresenius Zeitschrift fur Analytische Chemie vol325 no 5 pp 461ndash465 1986
[88] NIST NIST Standard Reference Database The National Insti-tute of Standards and Technology 2014 httpwwwnistgovsrdnist1acfm
[89] T K Kirk and R L Farrell ldquoEnzymatic ldquocombustionrdquo themicrobial degradation of ligninrdquo Annual Review of Microbiol-ogy vol 41 pp 465ndash501 1987
[90] A TMartinez ldquoHigh redox potential peroxidasesrdquo in IndustrialEnzymes Structure Function and Applications K-B BeckerEd pp 477ndash488 Springer Amsterdam The Netherlands 1stedition 2007
[91] M Dashtban H Schraft T A Syed and W Qin ldquoFungalbiodegradation and enzymatic modification of ligninrdquo Interna-tional Journal of Biochemistry and Molecular Biology vol 1 no1 pp 36ndash50 2010
[92] N Duran E Esposito L H Innocentini-Mei and V P CanhosldquoA new alternative process for Kraft E1 effluent treatmentrdquoBiodegradation vol 5 no 1 pp 13ndash19 1994
[93] K Kamwilaisak and P C Wright ldquoInvestigating laccase andtitanium dioxide for lignin degradationrdquo Energy and Fuels vol26 no 4 pp 2400ndash2406 2012
[94] J Reyes M Dezotti H Mansilla J Villasenor E Espositoand N Duran ldquoBiomass photochemistry-XXII combined pho-tochemical and biological process for treatment of Kraft E1effluentrdquoApplied Catalysis B Environmental vol 15 no 3-4 pp211ndash219 1998
[95] L F Gonzalez V Sarria and O F Sanchez ldquoDegradationof chlorophenols by sequential biological-advanced oxidativeprocess using Trametespubescens and TiO
2UVrdquo Bioresource
Technology vol 101 no 10 pp 3493ndash3499 2010[96] J H Lora and W G Glasser ldquoRecent industrial applications
of lignin a sustainable alternative to nonrenewable materialsrdquoJournal of Polymers and the Environment vol 10 no 1-2 pp 39ndash48 2002
Submit your manuscripts athttpwwwhindawicom
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Inorganic ChemistryInternational Journal of
Hindawi Publishing Corporation httpwwwhindawicom Volume 2014
International Journal ofPhotoenergy
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Carbohydrate Chemistry
International Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
Chemistry
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Advances in
Physical Chemistry
Hindawi Publishing Corporationhttpwwwhindawicom
Analytical Methods in Chemistry
Journal of
Volume 2014
Bioinorganic Chemistry and ApplicationsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
SpectroscopyInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Medicinal ChemistryInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Chromatography Research International
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Applied ChemistryJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Theoretical ChemistryJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
Spectroscopy
Analytical ChemistryInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Quantum Chemistry
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Organic Chemistry International
ElectrochemistryInternational Journal of
Hindawi Publishing Corporation httpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CatalystsJournal of
18 International Journal of Photoenergy
dioxiderdquo Bulletin of the Chemical Society of Japan vol 62 no11 pp 3433ndash3436 1989
[82] R B Kinstre ldquoAn overview of strategies for reducing theenvironmental impact of bleach-plant effluentsrdquo Tappi Journalvol 76 no 5 pp 105ndash113 1993
[83] A Castellan H Choudhury R Stephen Davidson and SGrelier ldquoComparative study of stone-ground wood pulp andnative wood 3 Application of fluorescence spectroscopy to astudy of the weathering of stone-ground pulp and native woodrdquoJournal of Photochemistry and Photobiology A Chemistry vol81 no 2 pp 123ndash130 1994
[84] B Albinsson S Li K Lundquist and R Stomberg ldquoThe originof lignin fluorescencerdquo Journal of Molecular Structure vol 508no 1ndash3 pp 19ndash27 1999
[85] S Baumberger A Abaecherli M Fasching et al ldquoMolar massdetermination of lignins by size-exclusion chromatographytowards standardisation of the methodrdquo Holzforschung vol 61no 4 pp 459ndash468 2007
[86] E Torres andM Ayala Biocatalysis Based on Heme PeroxidasesSpringer New York NY USA 1st edition 2010
[87] R Pecina P Burtscher G Bonn and O Bobleter ldquoGC-MSand HPLC analyses of lignin degradation products in biomasshydrolyzatesrdquo Fresenius Zeitschrift fur Analytische Chemie vol325 no 5 pp 461ndash465 1986
[88] NIST NIST Standard Reference Database The National Insti-tute of Standards and Technology 2014 httpwwwnistgovsrdnist1acfm
[89] T K Kirk and R L Farrell ldquoEnzymatic ldquocombustionrdquo themicrobial degradation of ligninrdquo Annual Review of Microbiol-ogy vol 41 pp 465ndash501 1987
[90] A TMartinez ldquoHigh redox potential peroxidasesrdquo in IndustrialEnzymes Structure Function and Applications K-B BeckerEd pp 477ndash488 Springer Amsterdam The Netherlands 1stedition 2007
[91] M Dashtban H Schraft T A Syed and W Qin ldquoFungalbiodegradation and enzymatic modification of ligninrdquo Interna-tional Journal of Biochemistry and Molecular Biology vol 1 no1 pp 36ndash50 2010
[92] N Duran E Esposito L H Innocentini-Mei and V P CanhosldquoA new alternative process for Kraft E1 effluent treatmentrdquoBiodegradation vol 5 no 1 pp 13ndash19 1994
[93] K Kamwilaisak and P C Wright ldquoInvestigating laccase andtitanium dioxide for lignin degradationrdquo Energy and Fuels vol26 no 4 pp 2400ndash2406 2012
[94] J Reyes M Dezotti H Mansilla J Villasenor E Espositoand N Duran ldquoBiomass photochemistry-XXII combined pho-tochemical and biological process for treatment of Kraft E1effluentrdquoApplied Catalysis B Environmental vol 15 no 3-4 pp211ndash219 1998
[95] L F Gonzalez V Sarria and O F Sanchez ldquoDegradationof chlorophenols by sequential biological-advanced oxidativeprocess using Trametespubescens and TiO
2UVrdquo Bioresource
Technology vol 101 no 10 pp 3493ndash3499 2010[96] J H Lora and W G Glasser ldquoRecent industrial applications
of lignin a sustainable alternative to nonrenewable materialsrdquoJournal of Polymers and the Environment vol 10 no 1-2 pp 39ndash48 2002
Submit your manuscripts athttpwwwhindawicom
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Inorganic ChemistryInternational Journal of
Hindawi Publishing Corporation httpwwwhindawicom Volume 2014
International Journal ofPhotoenergy
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Bioinorganic Chemistry and ApplicationsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
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Medicinal ChemistryInternational Journal of
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Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
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Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
Spectroscopy
Analytical ChemistryInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
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Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Quantum Chemistry
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Organic Chemistry International
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Hindawi Publishing Corporation httpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
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Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Inorganic ChemistryInternational Journal of
Hindawi Publishing Corporation httpwwwhindawicom Volume 2014
International Journal ofPhotoenergy
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Carbohydrate Chemistry
International Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
Chemistry
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Advances in
Physical Chemistry
Hindawi Publishing Corporationhttpwwwhindawicom
Analytical Methods in Chemistry
Journal of
Volume 2014
Bioinorganic Chemistry and ApplicationsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
SpectroscopyInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Medicinal ChemistryInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Chromatography Research International
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Applied ChemistryJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Theoretical ChemistryJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
Spectroscopy
Analytical ChemistryInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Quantum Chemistry
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Organic Chemistry International
ElectrochemistryInternational Journal of
Hindawi Publishing Corporation httpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CatalystsJournal of