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TiO2-assisted photocatalytic degradation of azo dyes in aqueous solution: kinetic and
mechanistic investigations: A review
Ioannis K Konstantinou , , Triantafyllos A Albanis
Department of Chemistry, Laboratory of Environmental Technology, University of Ioannina,Ioannina 45110, Greece
Received 5 July 2003; revised 24 November 2003; Accepted 24 November 2003. Available
online 7 February 2004.
Abstract
The photocatalytic degradation of azo dyes containing different functionalities has been
reviewed using TiO2 as photocatalyst in aqueous solution under solar and UV irradiation. Themechanism of the photodegradation depends on the radiation used. Charge injection
mechanism takes place under visible radiation whereas charge separation occurred under UV
light radiation. The process is monitored by following either the decolorization rate and the
formation of its end-products. Kinetic analyses indicate that the photodegradation rates of azo
dyes can usually be approximated as pseudo-first-order kinetics for both degradation
mechanisms, according to the LangmuirHinshelwood model. The degradation of dyes depend
on several parameters such as pH, catalyst concentration, substrate concentration and the
presence of electron acceptors such as hydrogen peroxide and ammonium persulphate
besides molecular oxygen. The presence of other substances such as inorganic ions, humic
acids and solvents commonly found in textile effluents is also discussed. The photocatalyzed
degradation of pesticides does not occur instantaneously to form carbon dioxide, but through
the formation of long-lived intermediate species. Thus, the study focuses also on the
determination of the nature of the principal organic intermediates and the evolution of the
mineralization as well as on the degradation pathways followed during the process. Major
identified intermediates are hydroxylated derivatives, aromatic amines, naphthoquinone,
phenolic compounds and several organic acids. By-products evaluation and toxicity
measurements are the key-actions in order to assess the overall process.
Keywords: Azo dyes; Photocatalytic degradation processes; Operational parameters;
Transformation products
Article Outline
1. Introduction
2. Experimental procedures
2.1. Photocatalytic degradation mechanisms
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2.1.1. Photocatalytic oxidation
2.1.2. Photosensitized oxidation
2.2. Primary substrate disappearance
2.3. Factors influencing the photocatalytic degradation
2.3.1. Effect of initial dye concentration
2.3.2. Effect of TiO2 loading
2.3.3. Effect of pH
2.3.4. Effect of light intensity and irradiation time
2.3.5. Effect of oxidants
2.3.6. Effect of humic acids, natural occurring ions and solvents
2.4. Photocatalytic mineralization of dyes
2.4.1. Analysis of the end products
2.4.2. Nature and evolution of organic intermediates
2.4.2.1. Monoazo dyes
2.4.2.2. Di- and triazo dyes
2.4.2.3. Triazine containing azo dyes
3. Conclusions
References
1. Introduction
Textile dyes and other industrial dyestuffs constitute one of the largest group of organic
compounds that represent an increasing environmental danger. About 120% of the total
world production of dyes is lost during the dyeing process and is released in the textile
effluents [1], [2], [3] and [4]. The release of those colored waste waters in the environment is a
considerable source of non aesthetic pollution and eutrophication and can originate dangerous
byproducts through oxidation, hydrolysis, or other chemical reactions taking place in the
wastewater phase [5], [6], [7] and [8].
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Decolorization of dye effluents has therefore received increasing attention. For the removal of
dye pollutants, traditional physical techniques (adsorption on activated carbon, ultrafiltration,
reverse osmosis, coagulation by chemical agents, ion exchange on synthetic adsorbent resins,
etc.) can generally be used efficiently [9], [10], [11] and [12]. Nevertheless, they are non-
destructive, since they just transfer organic compounds from water to another phase, thus
causing secondary pollution. Consequently, regeneration of the adsorbent materials and post-
treatment of solid-wastes, which are expensive operations, are needed [12] and [13]. Due to
the large degree of aromatics present in dye molecules and the stability of modern dyes,
conventional biological treatment methods are ineffective for decolorization and degradation
[14], [15], [16] and [17]. Furthermore, the majority of dyes is only adsorbed on the sludge and
is not degraded [18]. Chlorination and ozonation are also being used for the removal of certain
dyes but at slower rates as they have often high operating costs and limited effect on carbon
content [12], [19], [20], [21] and [22].
These are the reasons why advanced oxidation processes (AOPs) have been growing during the
last decade since they are able to deal with the problem of dye destruction in aqueous
systems. AOPs were based on the generation of very reactive species such as hydroxy radicals (
OH) that oxidize a broad range of pollutants quickly and non selectively. AOPs such as Fenton
and photo-Fenton catalytic reactions [23], [24], [25], [26] and [27], H2O2/UV processes [28]
and [29] and TiO2 mediated photo-catalysis [11], [30], [31], [32] and [33] have been studied
under a broad range of experimental conditions in order to reduce the color and organic load
of dye containing effluent waste waters.
Among AOPs, heterogeneous photocatalysis using TiO2 as photo-catalyst appears as the most
emerging destructive technology [4], [34], [35], [36], [37], [38] and [39]. The key advantage of
the former is its inherent destructive nature: it does not involve mass transfer; it can be carried
out under ambient conditions (atmospheric oxygen is used as oxidant) and may lead to
complete mineralization of organic carbon into CO2. Moreover, TiO2 photocatalyst is largely
available, inexpensive, non-toxic and show relatively high chemical stability. Finally, TiO2
photocatalytic process is receiving increasing attention because of its low cost when using
sunlight as the source of irradiation. The utilization of combined photocatalysis and solar
technologies may be developed to a useful process for the reduction of water pollution by
dying compounds because of the mild conditions required and their efficiency in the
mineralization [7], [40], [41], [42], [43] and [44].
The application of photocatalytic procedures for remediation of textile wastewater is rather
limited to few investigations [45], [46], [47], [48] and [49]. There are many studies dealing with
the photocatalytic decolorization of specific textile dyes from different chemical categories,
and most of them including a detailed examination of the so-called primary process under
different working conditions [7], [11], [32], [41], [42], [50], [51], [52], [53], [54], [55], [56] and
[57]. On the contrary, little information is available on the reaction mechanisms involved in the
photocatalytic degradation of dyes and on the identification of major transient intermediates
which have been more recently recognized as very important aspects of these processes,
especially in view of their practical applications [6], [34], [35], [44] and [58]. Thus, information
about real mineralization of the dye or decreases in toxicity are scarce and therefore ourattention has been also focused on the reaction types and mechanisms, based on the
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identification of the transformation products. Moreover, the effect of common dyebath
constituents on the photocatalytic treatment efficiency is also discussed in order to examine
the application of the photocatalytic degradation on real wastewater effluents.
Of the dyes available on the market today, approximately 5070% are azo compounds
followed by the anthraquinone group. Azo dyes can be divided into monoazo, diazo, triazoclasses according to the presence of one or more azo bonds ( N N ) and are found in various
categories, i.e. acid, basic, direct, disperse, azoic and pigments [3] and [59]. Some azo dyes and
their dye precursors have been shown to be or are suspected to be human carcinogens as they
form toxic aromatic amines [44], [60] and [61]. Therefore azo dyes are pollutants of high
environmental impact and were selected as the most relevant group of dyes concerning their
degradation using TiO2 assisted photocatalysis.
To our knowledge there is not a review dealing with the photocatalytic degradation of dyes
although that there are some reviews concerning the photocatalytic degradation of other
pollutants such as pesticides [37], [62], [63] and [64]. This review intend to assist workersinvolved in azo dyes photocatalytic treatment using TiO2 by: (a) compiling data on the degree
and on the factors influencing dye photodegradation, and (b) Summarizing and discussing data
on the mineralization degree, the intermediates and reaction mechanisms followed during the
process. The azo dyes were classified in terms of the characteristic structural groups.
2. Experimental procedures
2.1. Photocatalytic degradation mechanisms
2.1.1. Photocatalytic oxidation
The detailed mechanism of the process has been discussed previously in the literature [4], [6],
[58], [65], [66], [67] and [68] and will be only briefly summarized here. It is well established
that conduction band electrons (e) and valence band holes (h+) are generated when aqueous
TiO2 suspension is irradiated with light energy greater than its band gap energy (Eg, 3.2 eV).
The photogenerated electrons could reduce the dye or react with electron acceptors such as
O2 adsorbed on the Ti(III)-surface or dissolved in water, reducing it to superoxide radical anion
O2 . The photogenerated holes can oxidize the organic molecule to form R+, or react with
OH or H2O oxidizing them into OH radicals. Together with other highly oxidant species
(peroxide radicals) they are reported to be responsible for the heterogeneous TiO2
photodecomposition of organic substrates as dyes. According to this, the relevant reactions at
the semiconductor surface causing the degradation of dyes can be expressed as follows:
(1)
(2)
(3)
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(4)
(5)
(6)
(7)
(8)
The resulting OH radical, being a very strong oxidizing agent (standard redox potential +2.8 V)
can oxidize most of azo dyes to the mineral end-products. Substrates not reactive toward
hydroxyl radicals are degraded employing TiO2 photocatalysis with rates of decay highly
influenced by the semiconductor valence band edge position [69]. The role of reductive
pathways (Eq. (8)) in heterogeneous photocatalysis has been envisaged also in the degradation
of several dyes but in a minor extent than oxidation [58] and [70].
2.1.2. Photosensitized oxidation
The mechanism of photosensitized oxidation (called also photo-assisted degradation) by visible
radiation (>420 nm) is different from the pathway implicated under UV light radiation. In the
former case the mechanism suggests that excitation of the adsorbed dye takes place by visible
light to appropriate singlet or triplet states, subsequently followed by electron injection from
the excited dye molecule onto the conduction band of the TiO2 particles, whereas the dye is
converted to the cationic dye radicals (Dye +) that undergoes degradation to yield products as
follows [32], [34], [50], [51], [67], [68], [70], [71] and [72]:
(9)
(10)
(11)
(12)
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The cationic dye radicals readily reacts with hydroxyl ions undergoing oxidation via reactions
13 and 14 or interacts effectively with O2 , HO2 or HO species to generate intermediates
that ultimately lead to CO2 ((15), (16), (17), (18) and (19)).
(13)
(14)
(15)
(16)
(17)
(18)
(19)
In experiments that are carried out using sunlight or simulated sunlight (laboratory
experiments) it is suggested that both photooxidation or photosensitizing mechanism occurred
during the irradiation and both TiO2 and the light source are necessary for the reaction to
occur. In the photocatalytic oxidation, TiO2 has to be irradiated and excited in a near-UV
energy to induce charge separation. On the other hand, dyes rather TiO2 are excited by visible
light followed by electron injection onto TiO2 conduction band, which leads to photosensitizedoxidation. It is difficult to conclude whether the photocatalytic oxidation is superior to the
photosensitizing oxidation mechanism, but the photosensitizing mechanism will help to
improve the overall efficiency and make the photobleaching of dyes using solar light more
feasible [51].
2.2. Primary substrate disappearance
Several experimental results indicated that the destruction rates of photocatalytic oxidation of
various dyes over illuminated TiO2 fitted the LangmuirHinshelwood (LH) kinetics model [4],
[9], [65], [73], [74] and [75]:
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(20)
where r is the oxidation rate of the reactant (mg/l min), C the concentration of the reactant
(mg/l), t the illumination time, k the reaction rate constant (mg/l min), and K is the adsorption
coefficient of the reactant (l/mg).
When the chemical concentration Co is a millimolar solution (Co small) the equation can be
simplified to an apparent first-order equation [4], [9] and [37]:
(21)
A plot of ln Co/C versus time represents a straight line, the slope of which upon linear
regression equals the apparent first-order rate constant kapp.
Generally first-order kinetics are appropriate for the entire concentration range up to few ppm
and several studies were reasonably well fitted by this kinetic model [8], [38], [43], [44], [51],
[54], [55], [58], [60] and [76]. The LH model was established to describe the dependence of
the observed reaction rate on the initial solute concentrations.
It has been agreed, with minor doubts that the expression for the rate of photomineralization
of organic substrates such dyes with irradiated TiO2 follows the LangmuirHinshelwood (LH)
law for the four possible situations; (a) the reaction takes place between two adsorbed
substances, (b) the reaction occurs between a radical in solution and an adsorbed substrate
molecule, (c) the reaction takes place between a radical linked to the surface and a substrate
molecule in solution, and (d) the reaction occurs with the both of species being in solution. In
all cases, the expression for the rate equation is similar to that derived from the LH model,
which has been useful in modeling the process, although it is not possible to find out whether
the process takes place on the surface in the solution or at the interface [6].
It is likely that sorption of the dye is an important parameter in determining photocatalytic
degradation rates. All isotherms showed L-shape curves according to the classification of Giles
et al. [77] that means there is no strong competition between the water and the dye molecules
to occupy the TiO2 surface sites. The adsorption isotherms fit well to Langmuirian type
implying a monolayer adsorption model [4], [73], [75] and [76].
The color removal of the dye solution was determined usually with the absorbance value at
the maximum of the absorption spectrum for every dye by monitoring UV-Vis spectrum in
200800 nm zone using a spectrophotometer [41], [51], [72] and [78]. Alternatively, the
disappearance of dye was monitored by high performance liquid chromatography equipped
with a UV diode array detector [42], [58], [79] and [80] or MS detector [3].
2.3. Factors influencing the photocatalytic degradation
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2.3.1. Effect of initial dye concentration
It is important both from a mechanistic and from an application point of view to study the
dependence of the photocatalytic reaction rate on the substrate concentration. It is generally
noted that the degradation rate increases with the increase in dye concentration to a certain
level and a further increase in dye concentration leads to decrease the degradation rate of thedye [8] and [42]. The rate of degradation relates to the probability of OH radicals formation
on the catalyst surface and to the probability of OH radicals reacting with dye molecules. As
the initial concentrations of the dye increase the probability of reaction between dye
molecules and oxidizing species also increases, leading to an enhancement in the
decolorization rate. On the contrary, the degradation efficiency of the dye decreases as the
dye concentration increases further. The presumed reason is that at high dye concentrations
the generation of OH radicals on the surface of catalyst is reduced since the active sites are
covered by dye ions. Another possible cause for such results is the UV-screening effect of the
dye itself. At a high dye concentration, a significant amount of UV may be absorbed by the dye
molecules rather than the TiO2 particles and that reduces the efficiency of the catalytic
reaction because the concentrations of OH and O2 decrease [9], [52], [53], [68], [74], [81]
and [82].
The major portion of degradation occurs in the region near to the irradiated side (termed as
reaction zone) where the irradiation intensity is much higher than in the other side [83]. Thus
at higher dye concentration, degradation decreases at sufficiently long distances from the light
source or the reaction zone due to the retardation in the penetration of light. Hence, it is
concluded that as initial concentration of the dye increases, the requirement of catalyst
surface needed for the degradation also increases [7].
2.3.2. Effect of TiO2 loading
Whether in static, slurry or dynamic flow reactors the initial reaction rates were found to be
directly proportional to catalyst concentration indicating the heterogeneous regime. However,
it was observed that above a certain level of concentration the reaction rate even decreases
and becomes independent of the catalyst concentration. Most of studies reported enhanced
degradation rates for catalyst loading up to 400500 mg/l [8], [42], [52], [53], [55], [84] and
[85]. Only a slight enhancement or decrease was observed when TiO2 concentration further
increased up to 2000 mg/l. This can be rationalized in terms of availability of active sites on
TiO2 surface and the light penetration of photoactivating light into the suspension. Theavailability of active sites increases with the suspension of catalyst loading, but the light
penetration, and hence, the photoactivated volume of the suspension shrinks. Moreover, the
decrease in the percentage of degradation at higher catalyst loading may be due to
deactivation of activated molecules by collision with ground state molecules [7].
Agglomeration and sedimentation of the TiO2 particles were observed elsewhere when 2000
mg/l of TiO2 was added to the dye solution [52]. In such a condition, part of the catalyst
surface probably became unavailable for photon absorption and dye adsorption, thus bringing
little stimulation to the catalytic reaction. On the contrary, continuous increase of the
photocatalytic degradation rate of Reactive Black 5 was found up to 3500 mg/l TiO2[74]. The
crucial concentration depends on the geometry, the working conditions of the photoreactor
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and the type of UV-lamp (power, wavelength). The optimum amount of TiO2 has to be added
in order to avoid unnecessary excess catalyst and also to ensure total absorption of light
photons for efficient photomineralization. This optimum loading of photocatalyst is found to
be dependent on the initial solute concentration [62].
2.3.3. Effect of pH
The interpretation of pH effects on the efficiency of dye photodegradation process is a very
difficult task because of its multiple roles. First, is related to the ionization state of the surface
according to the following reactions,
(22)
(23)
as well as to that of reactant dyes and products such as acids and amines. pH changes can thus
influence the adsorption of dye molecules onto the TiO2 surfaces, an important step for the
photocatalytic oxidation to take place [86]. Bahnemann et al. [87] have already reviewed that
acid-base properties of the metal oxide surfaces can have considerable implications upon their
photocatalytic activity. The point of zero charge (pzc) of the TiO2 (Degussa P25) is at pH 6.8
[88]. Thus, the TiO2 surface is positively charged in acidic media (pH6.8).
Second, hydroxyl radicals can be formed by the reaction between hydroxide ions and positive
holes. The positive holes are considered as the major oxidation species at low pH whereas
hydroxyl radicals are considered as the predominant species at neutral or high pH levels [78]
and [89]. It was stated that in alkaline solution OH are easier to be generated by oxidizing
more hydroxide ions available on TiO2 surface, thus the efficiency of the process is logically
enhanced [54], [55], [65], [90] and [91]. Similar results are reported in the photocatalysed
degradation of acidic azo dyes and triazine containing azo dyes [7], [9], [52], [74], [92] and [93],
although it should be noted that in alkaline solution there is a Coulombic repulsion between
the negative charged surface of photocatalyst and the hydroxide anions. This fact could
prevent the formation of OH and thus decrease the photoxidation. Very high pHs have beenfound favorable even when anionic azo dyes should hamper adsorption on the negatively
charged surface [81]. At low pH, reduction by electrons in conduction band may play a very
important role in the degradation of dyes due to the reductive cleavage of azo bonds.
Third, the TiO2 particles tend to agglomerate under acidic condition and the surface area
available for dye adsorption and photon absorption would be reduced [86]. Hence, pH plays an
important role both in the characteristics of textile waters and in the reaction mechanisms that
can contribute to dye degradation, namely, hydroxyl radical attack, direct oxidation by the
positive hole and direct reduction by the electron in the conducting band.
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The degradation rate of some azo dyes increases with decrease in pH as reported elsewhere
[42], [67], [94] and [95]. At pH6.8 as dye molecules are negatively charged in alkaline media, their adsorption is also
expected to be affected by an increase in the density of TiO groups on the semiconductor
surface. Thus, due to Coulombic repulsion the dyes are scarcely adsorbed [44], [65] and [76].
For the above reasons the photocatalytic activity of anionic dyes (mainly sulphonated dyes)
reached a maximum in acidic conditions followed by a decrease in the pH range 711 [42],
[53], [58], [68], [75] and [76]. Moreover, the higher degradation rate at acid pH is seen also for
Vis/TiO2 experiments due to the efficient electron-transfer process due to strong surface
complex bond formation. This effect is less marked in neutral/basic pH solutions [67].
On the contrary, different optimal pHs (67) have been observed for the photocatalytic
degradation of other azo dyes, and a decrease of degradation in both acidic and alkaline pH
was reported [82] and [96]. The inhibitory effect seems to be more pronounced in the alkaline
range (pH=1113). At high pH values the hydroxyl radicals are rapidly scavenged and they do
not have the opportunity to react with dyes [97]. An additional explanation for the pH effects
can be related with changes in the specification of the dye. That is, protonation or
deprotonation of the dye can change its adsorption characteristics and redox activity [7].
Since the influence of the pH is dependent on dye type and on properties of TiO2 surface his
effect on the photocatalytic efficiency must be accurately checked before any application.
2.3.4. Effect of light intensity and irradiation time
Ollis et al. [98] reviewed the studies reported for the effect of light intensity on the kinetics ofthe photocatalysis process and stated that (i) at low light intensities (020 mW/cm2), the rate
would increase linearly with increasing light intensity (first order), (ii) at intermediate light
intensities beyond a certain value (approximately 25 mW/cm2) [62], the rate would depend on
the square root of the light intensity (half order), and (iii) at high light intensities the rate is
independent of light intensity. This is likely because at low light intensity reactions involving
electronhole formation are predominant and electronhole recombination is negligible.
However, at increased light intensity electronhole pair separation competes with
recombination, thereby causing lower effect on the reaction rate. In the studies reviewed
here, the enhancement of the rate of decolorization as the light intensity increased was also
observed [7], [42], [52], [74] and [75].
It is evident that the percentage of decolorization and photodegradation increases with
increase in irradiation time. The reaction rate decreases with irradiation time since it follows
apparent first-order kinetics and additionally a competition for degradation may occur
between the reactant and the intermediate products. The slow kinetics of dyes degradation
after certain time limit is due to: (a) the difficulty in converting the N-atoms of dye into
oxidized nitrogen compounds [99], (b) the slow reaction of short chain aliphatics with OH
radicals [100], and (c) the short life-time of photocatalyst because of active sites deactivation
by strong by-products deposition (carbon etc.).
2.3.5. Effect of oxidants
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It was observed that H2O2 and S2O82 addition was beneficial for the photoxidation of the
dyes of different chemical groups included azo dyes [6], [8], [43] and [52]. The reactive radical
intermediates ( SO4 and OH) formed from these oxidants by reactions with the
photogenerated electrons can exert a dual function: as strong oxidant themselves and as
electron scavengers, thus inhibiting the electronhole recombination at the semiconductor
surface [101] according to the following equations:
(24)
(25)
(26)
(27)
(28)
Moreover, the solution phase may at times be oxygen starved, because of either oxygen
consumption or slow oxygen mass transfer. Peroxide addition thereby increases the rate
towards what it would have been an adequate oxygen supply. The presence of S2O82
positively influences the mineralization rate, despite the decreasing of pH as the oxidant
properties of the system probably prevail on the effect of pH reduction. On the contrary, as far
as the substrate is concerned, the faster degradation rate can be due to both the decrease of
the pH and the oxidant action of S2O82[43].
However, H2O2 can also become a scavenger of valence band holes and OH, when present at
high concentration, [68], [102], [103] and [104]:
(29)
(30)
(31)
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As both hVB+ and OH are strong oxidants for dyes, the photocatalytic oxidation will be
inhibited when H2O2 level gets too high. Furthermore, H2O2 can be adsorbed onto TiO2
particles to modify their surfaces and subsequently decrease its catalytic activity.
Since the influence of the above additives, in particular H2O2, has been in some cases
controversial and it appeared dependent on the substrate type and on various experimentalparameters [105] the usefulness of which must be accurately checked before their application
[43].
2.3.6. Effect of humic acids, natural occurring ions and solvents
The occurrence of dissolved inorganic ions is rather common in dye-containing industrial
wastewater. Often, wastewater contains a mixture of pollutants, organic solvents as well as
dissolved organic matter and humic substances, if mixed with other waste streams. These
substances may compete for the active sites on the TiO2 surface or deactivate the
photocatalyst and, subsequently, decrease the degradation rate of the target dyes.Alternatively, they may act as light screens, thus reducing the photon receiving efficiency.
The Vis/TiO2 photocatalytic degradation of different classes of dyes is reported to be retarded
by many commonly used industrial solvents and acids, as well as by many naturally abundant
mineral species and dissolved organic matter [99]. The retardation by humic substances may
be by the combination effects of light attenuation, competition for active sites and surface
deactivation [106], [107] and [108]. Finally, various solvents such as acetonitrile and ethanol
were found to have a significant retardation effect on the photobleaching of dyes even at low
concentrations [68] and [106] as it is also stated for phenols and aromatic products [109]. Of
the anionic species studied (HCl, NaCl, NaNO3, HNO3, H3PO4 and NaHCO3), HCl exhibited thestrongest inhibition effect followed by H3PO4[68] and [106]. Inhibition effects of anions can be
explained as the reaction of positive holes and hydroxyl radical with anions, that behaved as
h+ and OH scavengers ((32), (33), (34), (35), (36) and (37)) resulting prolonged color removal.
Probably the adsorbed anions compete with dye for the photo-oxidizing species on the surface
and preventing the photocatalytic degradation of the dyes [87], [93] and [110]. Formation of
inorganic radical anions (e.g. Cl , NO3 ) under these circumstances is possible to occur [111].
(32)
(33)
(34)
(35)
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(36)
(37)
Although the reactivity of these radicals may be considered, they are not as reactive as h+ and
OH [112] and thus, the observed retardation effect is still thought to be the strong adsorption
of the anions on the TiO2 surface [110].
The effect of several types of metal ions (Cu2+, Zn2+, Fe3+, Al3+ and Cd2+) on the
photodegradation of non-azo dyes in TiO2 aqueous dispersions under visible light illumination,
has been investigated by Chen et al. [113]. They have concluded that Cu2+ and Fe3+ ions have
a strong suppressing effect on the photodegradation of all three dyes examined, by altering
the interfacial electron-transfer pathway under visible light irradiation. They noted that the
addition of Cu2+ and Fe3+ decreases the reduction of O2 by the conduction electrons,
subsequently blocks the formation of reactive oxygen species (O2 / OOH, OH) and hence
suppresses the photodegradation of dyes under visible irradiation. However, other metal ions
such as Zn2+, Cd2+ and Al3+ affect the photoreaction only slightly through an alteration of the
adsorption of dyes.
On the basis of hydroxyl radical formation through photocatalytic reactions of Fe3+ ions and
the products of their hydrolysis in aqueous solutions [114] is assumed that, the presence of
Fe3+ in the reaction environment, together with TiO2, should increase the rate of the
photocatalytic processes. An increased degradation rate was observed in the photocatalytic
degradation of azo dye acid red 1 in TiO2 suspensions containing Fe(III) aquo ions (105 to
104 M)[115]. This beneficial behavior was attributed to the increased amount of dye
adsorbed on the iron(III)-modified TiO2 surface and this was further confirmed by the fact that
iron species such as Fe2+ not adsorbed on the semiconductor had no kinetic effects. The
beneficial effect of Fe3+ ions was also found on the photocatalytic degradation of rhodamine B
in aqueous TiO2 suspensions [33].
Baran et al. [116] studied the photocatalytic degradation of several anionic and cationic azo
dyes in the presence of TiO2 and FeCl3. They have found that Fe3+ ions have a catalytic
influence on the decolorization of the studied anionic dyes but an inhibiting influence on the
decolorization of the cationic dyes. In conclusion, the role of Fe3+ ions on the photocatalytic
degradation of several dyes shows a controversial behavior depending on the physico-
chemical properties of dyes. Thus, in the presence of these ions the specific azo dye
degradation should be considered in order to determine the treatment efficiency.
The photocatalytic decolorization of the triazine azo dye MX-5B was reported to increase
slightly in the presence of 1 M of Cu2+ and Ni2+ at pH=2.4 [112]. Their reduced forms could
trap holes and that explains the decrease of the e/h+ recombination rate and a higher
production of OH. Excess of Cu2+ and Ni2+ led to short-circuiting reactions, which created a
cyclic process without generating active OH and retarded the reaction. However, at pH 10.8
the photodegradation of MX-5B was completely inhibited by the trace quantities of Cu2+ and
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Ni2+. The deposition of NiO2 on the surface of TiO2 was found to deactivate the photocatalyst
[112].
An understanding of the retardation effects not only aids in assessing the feasibility of using
photocatalytic oxidations to treat wastewater, but also allows a thoughtful photocatalytic
oxidation design.
2.4. Photocatalytic mineralization of dyes
2.4.1. Analysis of the end products
In order to assess the degree of mineralization reached during the photocatalytic treatment
the formation of CO2 and inorganic ions [6], [32], [34], [44], [58], [91] and [117], is generally
determined. However, in the presence of real wastewaters the monitoring of inorganic ions
and CO2 gives only a global estimation on the well functioning of the treatment, but does not
provide information on the real decay of the contaminant. In such cases the determination of
total organic carbon (TOC) and/or the measurement of the chemical oxygen demand (COD) or
the biological oxygen demand (BOD) of the irradiated solution is generally used for monitoring
the mineralization of the dye [7], [32], [42], [52], [58], [76], [80] and [118]. In general, at low
reactant levels or for compounds which do not form important intermediates, complete
mineralization and reactant disappearance proceed with similar half lives, but at higher
reactant levels where important intermediates occur, mineralization is slower than the
degradation of the parent compound. Until now, total mineralization has been observed for
the photacatalytic degradation of most of the azo dyes even at longer irradiation periods [42],
[44], [58], [74], [76] and [119]. Only in the case of triazine containing dyes, the mineralization
was not complete due to the high stability of triazine nucleus and the stable cyanuric acid wasformed, as in the case of s-triazine herbicides [120], which fortunately is not toxic [41], [52],
[80] and [121].
Usually COD or TOC values decrease with increase in irradiation time whereas the amount of
NH4+ and NO3 ions increase with increase in irradiation time. However, the formation of Cl
and SO42 increases initially and subsequently remains unchanged. COD or TOC curves have
an exponential or sigmoidal shape. The sigma-shaped curves indicating that is related to the
formation of relative tolerant by-products [44], [52] and [118]. This pattern means that during
the first steps of the process where the solution is still colored there is only a small decrease of
the parameter measured (TOC or COD or BOD) due to the fact that dye molecules aredecomposed to lower molecular weight compounds and the resulting intermediates still
contribute to the COD of the solution. After the decolorization of the solution the COD
decreases sharply (the linear segment of the S shaped curve) reaching a plateau that
corresponds to the oxidation of most stable compounds indicating that almost complete
mineralization of intermediates has occurred.
For chlorinated dye molecules, Cl ions are easily released in the solution and are the first of
the ions appearing during the photocatalytic degradation [42], [43] and [80]. This could be
interesting in a process, where photocatalysis would be associated with a biological treatment
which is generally not efficient for chlorinated compounds. Nitrogen is mineralized into NH4+,NO3 and N2. The proportion depends mainly on the initial oxidation degree of nitrogen, the
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926337303005411#ref_BIB1127/31/2019 TiO2-degradacin de grupos azo
15/33
substrate structure and on irradiation time [122], [123] and [124]. By comparing the initial
rates, NH4+ appears as the primary product with respect to NO3 in the case of amine
compounds. The nitrogen atoms in the amino-groups of the dyes can lead to NH4+ ions by
successive attacks by hydrogen species
(38)
(39)
The total amount of nitrogen-containing ions present in the solution at the end of the
experiments is usually lower than that expected from stoichiometry indicating that N-
containing species remain adsorbed in the photocatalyst surface or most probably, that
significant quantities of N2 and/or NH3 have been produced and transferred to the gas-phase.[42], [44] and [76]. The formation of N2 in azo dyes can be accounted for by the same
processes responsible for NH4+ formation:
(40)
(41)
When nitrogen is present in the 3 state as in amino groups or in pyrazoline ring, it
spontaneously evolves as NH4+ cations with the same oxidation degree, before being
subsequently and slowly oxidized into nitrate [58]. In the azo bonds each nitrogen atom is in its
+1 oxidation degree. This oxidation degree favors the evolution of gaseous dinitrogen by the
two step reduction process expressed previously. N2 evolution constitutes the ideal case for a
decontamination reaction involving totally innocuous nitrogen-containing final product.
The dyes containing sulfur atoms are mineralized into sulfate ions [43] and [80]. In all the
studies the formation of SO42 was always observed and in most cases its stoichiometric
formation was found in the final steps of the photoreaction when organic intermediates stillwere present [43], [52] and [80]. The reported initial slopes are positive indicating that SO42
ions are initial products, directly resulting from the initial attack on the sulfonyl group. Release
of sulphate ions upon dye degradation was a little slower than decolorization but much faster
than TOC loss. Non-stoichiometric formation of sulphate ions is usually explained by a strong
adsorption on the photocatalyst surface [44], [125] and [126]. This strong adsorption could
partially inhibit the reaction rate which, however, remains acceptable [111] and [127].
Generally, it is found that nitrate anions have little effect on the kinetics of reaction whereas
sulfate, chloride and phosphate ions, especially at concentrations of greater than 103 mol
dm3, can reduce the rate by 2070% due to the competitive adsorption at the photoactivated
reaction sites [111]. The release of SO42 can be accounted by an initial attack by a photo-induced OH radical:
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