www.elsevier.com/locate/apcatb
Applied Catalysis B: Environmental 70 (2007) 479–487
Electrochemical removal of antibiotics from wastewaters
C. Carlesi Jara, D. Fino *, V. Specchia, G. Saracco, P. Spinelli
Department of Materials Science and Chemical Engineering, Politecnico di Torino, Cso. Duca degli Abruzzi, 24-10129 Torino, Italy
Available online 23 June 2006
Abstract
Electro-oxidation tests with different anodes (Ti/Pt, DSA1 type, graphite and three-dimensional (3D) electrode made of a fixed bed of activated
carbon pellets) were performed on aqueous solutions containing the antibiotics Ofloxacin and Lincomycin. The effectiveness of the treatment of
wastewater containing pharmaceuticals was assessed, as well as the electro-oxidation mechanism.
The use of high electrode potentials (>2.8 V versus NHE) ensured either significant anodic surface activation or minimization of fouling by in
situ generated polymeric material. The use of a membrane-divided cell showed positive aspects in terms of molecule demolition, and average
power consumption. The electro-oxidation was found to occur with first order kinetics mainly at anode surface when using Na2SO4 at low
concentration (0.02N). Under these conditions, Ofloxacin is efficiently oxidized over all tested anodes (e.g. 50 mgcm�2 A�1 h�1 for the bi-
dimensional Ti/Pt electrode), whereas Lincomycin is oxidized with slow overall kinetics mainly due to difficult deprotonation, a step that precedes
the primary electron transfer stage of the oxidation process. The three-dimensional electrode would be the most appropriate for continuous
industrial-scale process. However, at the used potential, unacceptable corrosion of the carbon based electrode was noticed.
# 2006 Elsevier B.V. All rights reserved.
Keywords: Electrochemical oxidation; Antibiotics; Ofloxacin; Lincomycin; Three-dimensional electrodes; Wastewater treatment
1. Introduction
The presence of antibiotic compounds in surface waters is an
emerging environmental issue. Pharmaceuticals industries,
health attention centers (especially hospitals) or simple civil
buildings represent important points of antibiotic discharge into
the environment and produce a non negligible effect on the
physical, chemical and biological composition of receptor
water bodies. Hospital effluents, in particular, proved to entail
an important effect on the development of resistant bacterial
strains [1]. Sewage treatment plants (STP) are also recognized
as important discharge point of these residuals substances that
become partially excreted with urine or feces. A monitoring
campaign on STP effluents was carried out in four European
countries (Italy, France, Greece and Sweden), in which more
than 20 individual pharmaceuticals belonging to different
therapeutic classes were found [2,3].
Many of these substances are not biodegradable, toxic and
capable of accumulating in single aquatic organisms (algae).
* Corresponding author. Tel.: +39 011 5644710; fax: +39 011 5644699.
E-mail address: [email protected] (D. Fino).
0926-3373/$ – see front matter # 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcatb.2005.11.035
Their presence in the influents of municipal wastewater
treatment plants may, on the one hand, cause adverse effects
to sensitive biological processes, such as nitrification, while, on
the other hand, they may pass throughout the activated sludge
process unoxidized [4]. The electro-oxidation method is here
devised to transform the Ofloxacin and Lincomycin antibiotics at
least into biodegradable sub-products. These drugs are inhibitory
for biomass growth and their treatment cannot be accomplished
via classical biological processes. Therefore, specific treatment
routes (chemical or photochemical oxidation, selective adsorp-
tion, etc.) are required. The Ofloxacin antibiotic (Fig. 1a) belongs
to a class of drugs called fluoroquinolones (fluorinated
carboxyquinolone). It is used to treat various bacterial infections,
such as bronchitis, pneumonia, chlamydia, gonorrhoea, skin
infections, urinary tract infections and infections of the prostate
[5]. Lincomycin (Fig. 1b) is an amino-glycoside antibiotic
generated by the Streptomyces lincolnesis. Its structure is similar
to aminoglycosides by exhibiting substituted glucose rings with a
nitrogen-containing substituent on C-6. It is widely used in
human and veterinary medicine and is particularly active against
anaerobic bacteria [6]. Both molecules have the potential to form
stable coordination compounds with many metal ions (chelation)
[6,7].
C. Carlesi Jara et al. / Applied Catalysis B: Environmental 70 (2007) 479–487480
Nomenclature
List of Symbols
A Anodic surface area (m2)
C Organic pollutant concentration (mg l�1)
C1 Anodic compartment inlet concentration of
organic pollutant (mg l�1)
C2 Anodic compartment outlet concentration of
organic pollutant (mg l�1)
E Electrode potential (V)
F Faraday constant (C mol�1)
I Electrical current (A)
i Current density (A m�2)
ilim Diffusive limit current density (A m�2)
KM Transport coefficient in the electrochemical reac-
tor (m s�1)
MOX Metal oxide
Q Recycled flow between anodic compartment and
mixing tank (m3 s�1)
qe Electro-generated heat (removal from mixing
tank by heat exchange) (J)
R Cell electrical resistance (V)
r Electrochemical reaction rate (s�1)
t Time (s)
VM Mixing tank volume (m3)
z Number of exchanged electrons per mol of
organic matters
a Model parameter defined in Table 1
D Constant calculated from Eq. (6)
h Current efficiency
Fig. 1. The Ofloxacin (a) and Lincomycin (b) molecules.
The electrochemical treatment is an interesting process for
toxic organic abatement since clean reagents are used: the
electrons. An effective control of electron transfer rate and the
reaction conditions (current density and electrode potential)
can be easily accomplished. Moreover, ambient temperature
and pressure can be employed for this process [8].
The organic molecules react directly at the anode surface with
in situ formed higher oxides or with adsorbed hydroxyl radicals.
Conversion tends to be mainly controlled by mass transfer,
whereas the main factor that decreases the current efficiency is
the simultaneous evolution of oxygen at the anode itself [9].
The direct electro-oxidation rates of organic pollutants
depend on the catalytic activity of the anode, on the diffusion
rates of the organics compounds towards the active sites of the
anode and on the applied current density. Indirect electro-
oxidation may also occur, when working at high electrode
potentials, as a consequence of the generation of secondary
bulk oxidants. Its rate is related to the diffusion rate of
secondary oxidants (hydrogen peroxide, persulphates, chlorine
species,. . .) into the solutions, the temperature and the pH
values [10].
The different and specific nature of the pollutants and their
reaction intermediates may lead to an ad hoc specific
optimization of the cell geometry, the electrode material and
the operative conditions. In the present investigation, the
performance of the electrochemical oxidation of the mentioned
antibiotics at high electrode potential (>2.8 V versus NHE) is
assessed in view of practical application to wastewater
treatment.
2. Experimental
2.1. Materials
Synthetic solutions were prepared by using pure grade
Ofloxacin (Aldrich) and Lincomycin hydrochloride (Fluka)
with initial concentration of organics in the range 25–50 mg l�1
in distilled water. Sodium sulphate or chloride (Fluka) were
added as electrolytes, at the low electrolyte concentrations
(0.02N) permitted by current legislation at the discharge point
in rivers or surface basins (e.g. law 152/99 for Italy).
2.2. Pilot plant
Electrochemical oxidation experiments were carried out in a
divided cell employing a stainless steel plate as the cathode and
various anode materials:
� p
latinised titanium (Tokuyama Soda, Japan);� r
igid graphite (purity 99.5% Good Fellow, England);� T
i/IrO2/Ta2O5 (DSA1 type anodes, DeNora, Italy) and� 3
D GAC: three-dimensional anode consisting of a fixed bed(60 g dry) of activated carbon pellets (Camel Envirotech-
Multisorb MM 450) positioned on the Ti/Pt anode so as to
assure a good electric contact and negligible pressure drop.
The divided cell employed an anionic membrane (Neosepta
AFN by Tokuyama Soda Co. Japan [10,11]) to separate the
anodic and the cathodic compartments. The ratio between the
C. Carlesi Jara et al. / Applied Catalysis B: Environmental 70 (2007) 479–487 481
Fig. 2. The electrolytic divided cell used and a cross section of the anodic/cathodic chamber.
electrode surface area and the anodic compartment volume
was equal to 2 � 10�2 m�1; the electrodes surface was
2 � 10�2 m2. Details of cell structure are provided in Fig. 2,
where the tortuous pattern followed by the electrolyte solutions
at the anodic cell and cathodic compartments is also shown, as
induced by means of turbulence promoters (calculated
Reynolds number � 5700). Conversely, Fig. 3 shows the loop
flushed by the solution treated at the anode side, which also
includes an anodic solution recirculation tank.
The cell was connected with a simple current rectifier.
The applied current was varied in the range 1.5–400 A m�2,
at an operating temperature of 30 � 2 8C controlled by
means of a water cooling system placed within the mixing
tank.
Before the tests with the 3D electrodes, the GAC was
saturated with the molecule under investigation. An increase of
the concentration was observed in the first minutes of any run
due to release of the molecule from saturated pellets; after that
the typical abatement trend (Eq. (1)) is restored.
Fig. 3. Scheme of the closed recirculating loop of electrolyte solution contain-
ing the organic pollutants throughout the anodic compartment and its mixing
tank.
2.3. Analytical methods
The residual concentration of organics was analyzed on 10
cc samples of anodic solution (periodically withdrawn) by
means of U.V. spectrophotometry (CARY 500 Scan single ray
spectrophotometer). On these samples iodometric titration of
H2O2, NaOCl or equivalent bulk oxidants as well as C.O.D
(Orbeco-Hellige water analysis system model 975-MP) and
voltammetric analyses (AMEL1 5000 and VoltaLab1) were
also performed.
2.4. Modeling
The flow pattern within the cell can be approximated by
means of a plug flow reactor (PFR). Conversely, the
recirculation tanks can be assumed as continuously stirred
(CSTR). It is possible to build a theoretical model of the system
depicted in Fig. 3 that permits to predict the concentration
evolution of initial molecule (C/C0) versus the electrolysis time
during the electrochemical oxidation.
Based on the mass balances listed in Table 1, by setting
C2 = C one can easily derive:
C
C0
¼ e�tðKMA=VMaÞ (1)
3. Results and discussion
3.1. Effect of high anodic potential and low electrolyte
concentration
The use of high-applied electrode potentials allows indeed
significant anodic surface activation with both OH� radicals
Table 1
Mass balances over the anode cell compartment and the mixing tank flushed by
the anodic solution
Anodic compartment PFR Reservoir CSTR
QðC1 � C2Þ ¼I
zF¼ KMAC1
C1 ¼C2
aa� 1� KMA
Q
VMdC2dt
� �¼ QðC2 � C1Þ ¼ QC2 1� 1
a
� �
C. Carlesi Jara et al. / Applied Catalysis B: Environmental 70 (2007) 479–487482
Table 2
Ofloxacin oxidation in the electrochemical (divided) reactor: calculated trans-
port coefficients and time required for specific percent abatements using
different anode materials
Initial concentration
(mg/l)
Anode
type
KMA
(m3 s�1)
Required time (min)
for abatement of
50% 90%
25 Ti/Pt 4.1 � 10�7 112.7 374.3
50 Ti/Pt 5.0 � 10�7 92.4 306.9
50 DSA 1.0 � 10�7 462.0 1535.0
50 Graphite 1.9 � 10�7 239.0 793.9
50 3D GAC 3.8 � 10�5 30.8 102.3
Applied current: 4A; electrolyte: Na2SO4 0.02N; treated volume: 4 l; average
linearity factor for Eq. (1) higher then 0.9.
Fig. 4. Results of iodometric measurement of ‘‘bulk’’ oxidants in terms of: (a)
mg l�1 of NaOCl when using NaCl electrolyte ((*) 0.02N; (~) 0.2N); (b)
relative concentration C/Co of H2O2 when using Na2SO4 electrolyte.
and/or superior oxides, and minimizes fouling phenomena
caused by in situ generation of adsorbed polymeric material.
When using the Na2SO4 electrolyte, the organic concentra-
tion abatement showed a pseudo-first order kinetic behaviour in
agreement with the theoretical model. The transport coefficient
in the electrochemical reactor is independent of the electrode
material and is a function of the mean linear velocity of the
electrolyte. High flow rates should thus enhance the mass
transfer coefficient towards the anode.
The organic molecules mass flow that reaches the anodic
surface per Ah is proportional to the mass and the concentration
of the molecule (see the first two lines in Table 2) and decreases
with the electrolyte conductivity. An increase of the electrolyte
concentration (Na2SO4) does not improve the organic abate-
ment kinetics. Conversely, a small improvement is obtained
when diminishing this parameter, at the price of a considerable
increase in energy consumption.
A further effect of poor electrolyte conductivity for 3D
electrodes is that the electric current will tend to favour the
electronic conduction pathway provided by the electrode
material rather than the ionic path through the solution.
3.2. Electro-generated oxidants
The indirect oxidation through electro-generated oxidants is
mainly attributable to hydrogen peroxide when working with
Na2SO4 as electrolyte [10] and to sodium hypochlorite when
using NaCl [12] (Fig. 4a). The concentration of ‘‘bulk’’
oxidants was measured by means of iodometric titration.
The sodium hypochlorite concentration produced in the cell,
increases linearly with the electrolysis time and is function of
the initial salt content (NaCl). No high Faradic efficiency can be
achieved mainly due to gaseous chlorine generation and loss.
However, the anodic solution containing sodium hypochlorite
shows a very high activity for organics oxidation. This explains
the evident increase of abatement achieved when NaCl is used
instead of Na2SO4 electrolyte (see Fig. 5 for the Ofloxacin
case). After an initial increase, the concentration of chlorine/
hypochlorite during electrolysis can be assumed to be a
constant and if a pseudo-first order is assumed for the related
bulk reaction, the model described in Section 2.4 can still
be adapted with a new, increased kinetic constant. This
contribution of the indirect oxidation is though limited by the
possibility of producing a high quantity of noxious chloro-
organics compounds [12].
When using sodium sulphate as the electrolyte, the increase
in the bulk concentration of H2O2 was less evident. Tests
developed using different electrolyte concentrations allowed to
establish that the oxidation of antibiotics is in this case
practically independent of this parameter. As a consequence,
the oxidation process with Na2SO4 as electrolyte should be
mainly occurs at the anode surface.
The results of further tests developed with an initial content
of hydrogen peroxide added on purpose, pointed out that this
species is actually consumed during the runs (Fig. 4b). It is
likely that H2O2 decomposition with formation of OH� radicals,
occurs over the metallic oxide layer formed on the anode at high
electrode potential. H2O2 conversion is very fast and occurs
within a time scale (minutes) much lower than that typical of
the organics abatement process, so it can hardly affect the
oxidation process.
3.3. Ofloxacin oxidation
The Ofloxacin molecule (Fig. 1a) could actually be oxidized
over all the tested anodes, in the electrode potential range
starting from 0.6 to 0.7 V (versus Hg/Hg2SO4, K2SO4sat). A
typical voltammetric cycle on a graphite anode is shown in
Fig. 6. The overall electrochemical process corresponds to the
C. Carlesi Jara et al. / Applied Catalysis B: Environmental 70 (2007) 479–487 483
Fig. 5. Ofloxacin U.V. spectra for periodically withdrawn anolyte samples during the runs in the divided cell apparatus hosting a titanium platinised anode with different
electrolyte types and current densities: (A) 0.02N Na2SO4; 200 A m�2; (B) 0.02N Na2SO4; 400 A m�2; (C) 0.02N NaCl; 200 A m�2; (D) 0.02N NaCl, 400 A m�2.
transfer of one or two electrons followed by an irreversible
chemical reaction with oxygen species, as detailed below.
The earlier introduced Fig. 5 shows U.V. spectra for samples
periodically withdrawn from the anolyte, for two different
electrolyte types and two current densities. The degradation
path is analogous for the different electrolyte types and applied
current densities. All the absorbance peaks become reduced
with electrolysis time except those at wavelength of about
230 nm, which can be assigned to the benzene ring [13].
Besides, it is important to notice that the absorbance of the
treated solutions is continuously decreasing which is a clear
sign that either the original molecule concentration or that of
Fig. 6. Single voltammetric cycle for Ofloxacin (200 mg l�1). Working elec-
trode: graphite; electrolyte H2SO4 0.5 M; scan rate: 50 mV s�1; initial potential:
0 V; no stirring.
any eventual intermediate compound with similar absorbance
spectrum is progressively decreasing.
The reaction medium has a strong influence on the
electrochemistry of Ofloxacin. Its degradation is typically
characterised N-demethylation, mainly involving coupling of
radical cations with superoxide radical anions [14,15]. The by-
products are further abated only after long electrolysis times.
For instance, the aromatic nuclei of benzene rings get oxidized
with a mono electronic transfer to give a cationic radical which
then undergoes a very rapid depronotation.
The described phenomenon is more evident with increasing
the current (Fig. 5B) and using NaCl as electrolyte (Fig. 5C and
D) due to the effect of bulk sodium hypochlorite mentioned in
Section 3.2.
An increase of the imposed current density brings about a non
linear increase of the abatement rate, since the process is under a
mixed control of charge and mass transfer. Under these con-
ditions, the reactive oxidant species (i.e. superior oxides) reach
saturation over the available electrode active sites. Such superior
oxides are produced according to the following reaction [16]:
MOX þH2O ! MOXþ1þ 2Hþ þ 2e�; (2)
whose kinetics can be expressed with Eq. (3):
r ¼ I
zF� I2R
qe
(3)
C. Carlesi Jara et al. / Applied Catalysis B: Environmental 70 (2007) 479–487484
Fig. 7. Experimental relationship between the applied current density and the
Ofloxacin oxidation kinetics. Experimental conditions: Na2SO4 0.02N; divided
cell; titanium platinised anode.
Eq. (3) takes into account the current consumption due to
heat generation (Joule effect), which becomes important for
high current values (heat effects due to electrochemistry
and coupled chemical degradation reactions are considered
negligible).
In order to calculate the optimal current density to be
applied, it would be useful to establish the relationship between
the applied current and the demolition kinetics that depends
on all cited operating parameters as well as on the particular
oxidation pathways followed. This relationship is represented
for the Ofloxacin in Fig. 7a, whereas the relationship KM/i
versus i is shown in Fig. 7b in logarithmic scale.
Based on the current efficiency definition in Eq. (4):
h ¼ ilimi¼�
KM
i
�zFC1A (4)
two operational regions can be identified: in the first region
there is a positive slope increase until the theoretical maximal
efficiency is reached, while in the second one a negative sharp
decrease indicates the occurrence of simultaneous parasite
electrochemical reactions (i.e. mainly O2 evolution).
The experimental D value can be calculated from Eq. (5):
Fig. 8. Ofloxacin concentration C/Co (*) and related C.O.D. values (~) vs.
time in the divided cell apparatus equipped with the titanium platinised anode.
Experimental conditions: Na2SO4 0.02N; applied current density: 200 A m�2.
@ logðKM=iÞ@ logðiÞ ¼ D (5)
As a consequence, one can write:
KM ¼ KM;oið1þDÞ (6)
where KM,o is a function of operating conditions (Re and Sc
numbers) and D depends on different factors such as the
heating effects, changes on the turbulent regime inside the
reactor provoked by gas generation, etc. In order to predict the
current flowing at any particular time during an electrolysis run,
a quantitative model for diffusion, convection and migration of
molecules is needed to complement the model for the electron
transfer step(s). However, due to ion solvation effects and
diffuse layer interactions in solution, migration is notoriously
difficult to predict accurately for real systems. For such a
reason, semi-empirical equations like Eq. (6) are often pre-
ferred for quantitative estimations.
The C.O.D. variation during electrolysis is shown in Fig. 8. A
fair correspondence between the concentration reduction of the
original molecule and the C.O.D abatement trend can be
observed. After a further addition (at a run time equal to 250 min)
of an Ofloxacin amount (so as to restore the initial 50 mg l�1) the
cell performance on Ofloxacin oxidation (for Ti/Pt and DSA
types electrodes) remains unchanged. This indicates that no
electrode fouling by generated polymeric material or absorbed
ions is present at the high operating potentials employed. This
plays in favour of long term durability of the system.
3.4. Lincomycin oxidation
Two possible sites for oxidative attack are present in the
Lincomycin molecule (Fig. 1b): the thiomethyl group and the
pyrrolidine nitrogen. Unlike Ofloxacin and other organic
substances (dyes, phenolic contains groups, etc.), Lincomycin
has proven to be very difficult to oxidize even at high electrode
potential as well as in the presence of NaCl as electrolyte; no
clear oxidation peak on anodic materials tested in acid solutions
can be detected by means of the voltammetric method. The
main reason for this behaviour lies in the effect of substituting
nitrogen inside the aromatic framework, which renders the
Lincomycin molecule considerably more difficult to oxidize.
Besides, it is known that most tertiary amines, like Lincomycin,
C. Carlesi Jara et al. / Applied Catalysis B: Environmental 70 (2007) 479–487 485
Fig. 9. Cell potential versus electrolysis time for different applied current
densities for divided and undivided cell. Electrolyte: 0.02N Na2SO4.
are oxidized slowly by hydrogen peroxide or other oxidising
agents at low pH value [17].
However, after long-time electro-oxidation runs, a certain
reduction on the initial C.O.D. concentration was achieved
(about 30%; initial concentration 150 mg l�1; volume treated
0.5 l; charge passed: 2 Ah). In this case, the U.V. spectrum of
the treated solution cannot be shown because the Lincomycin
spectrum is rather poor. It is indeed characterized by just a weak
absorbance at about 187 nm.
The slow overall abatement can be explained by an
intrinsically slow primary electron transfer and by the fact
that chemical reaction coupling has to take place. For the
electro-organics reactions one or two deprotonation reactions in
the bulk solution must therefore precede the electron transfer
step with the electrode material, and Lincomycin possesses a
very stable protonated group, the tertiary amine of the
pyrrolidine ring when the pH of solution is smaller than the
pKa of the molecule. Lincomycin belongs at the Lincosamides
groups that are in fact basic compounds with pKa values of
about 7.6 and the pH of the anodic solution evolves during the
electrolysis to an acid environment from a starting neutral
condition. As a consequence, the condition pH� pKa is always
valid. It is thus possible to deduce that deprotonation is actually
the rate controlling reaction step. After deprotonation has
occurred, C–S bonds in the Lincomycin molecule are the most
susceptible to breakage with simultaneous oxygen addition
[17]. This strongly suggests that the major products of
Lincomycin oxidation should be sulfoxide (S O) and sulfine
(O S O) derivates [6]. One can also suppose an additional
fragmentation pattern occurring at the aliphatic substituent of
the pyrrolidine ring, and introduction of oxygen at the
pyrrolidine nitrogen location. Studies are in progress to better
elucidate these points.
3.5. The role of the membrane
The use of a membrane entails, in general, a more complex
and expensive reactor. On the other hand, the ohmic drops
caused by the membrane (5% overall cell potential increase) are
though not as remarkable as anolyte and catholyte ohmic drops.
Conversely, the membrane offers some positive aspects in
terms of:
(1) O
rganic molecules demolition, since the membrane allowsto have a acid environment in the anodic compartment
which enhances the kinetics of electron transfer and
chemical reaction; acid dissociation (hydrolysis) represents
indeed a precursor reaction for the electrochemical
oxidation pathway;
(2) R
eduction of parasite currents, since the membrane avoidsthe formations of redox couples involving species that
after oxidization at the anode could be reduced at the
cathode.
In accordance to the Nernst law, the mentioned pH
change entails an overall cell potential reduction with
electrolysis time. This last occurrence is illustrated in Fig. 9,
where the direct relationship between the maximum cell
potential with the applied current density is clearly
represented. In particular, an increase in the current density
helps to reach more quickly the maximum potential, which
then diminishes to eventually settle at its equilibrium
value. This behaviour can be also explained on the basis of
a trans-passivation phenomenon occurring at high anodic
potential, characterized by the following stages:
� Oxidation of electrode surface (called passivation
phenomenon): slightly soluble MOX species are formed.
� Further oxidation of the original oxide to a soluble and
reactive form MOX+Y favoured by the acid environment.
By modifying the initial pH value of the anodic compartment
to basic conditions (pH 12) the phenomenon was found to be
severely delayed due to an excess of OH� anions that make the
initial oxides coverage slower (Fig. 9, triangles curve).
3.6. Effect of the anodic material nature
The Ti/Pt electrode showed the highest specific electro-
catalytic activity towards organic oxidation (see Table 2). This
is undoubtedly favoured by the strong tendency of organic
species (especially aromatic hydrocarbons) to adsorb on the
platinum electrode surface, as well as by its easy generation of
active oxygen species.
Both Ti/Pt and DSA1 electrodes indeed promote the
oxidation via formation of superficial high oxides, which allows
to address them as ‘‘active’’ electrodes [18]. Over these anodes
a layer of oxides should be the true oxidizing catalyst, which
generally promotes a selective oxidation to partially oxidized
sub products.
No decrease on the oxidation activity of both Ti/Pt (as
illustrated in Section 3.2 for the Ofloxacin) and DSA1 anodes
(already known as very stable industrially produced electrodes)
was noticed after 4 months operation. However, the C.O.D.
removal achievable, about 50% over Ti/Pt after 2 h of batch
runs, was not really satisfactory for industrial application. It is
actually well known that these electrodes do not lead to
C. Carlesi Jara et al. / Applied Catalysis B: Environmental 70 (2007) 479–487486
complete ‘‘combustion’’ of the organic pollutants to CO2 and
water [18], but tend to generate electro-inactive oxidation
intermediates.
The removal kinetics and consequently the current
efficiency obtained with the different electrodes can be
compared through the data listed in Table 2 where the values
of the transport coefficient in the electrochemical reactor for
Ofloxacin abatement at a fixed current density are reported. The
DSA1 type electrodes have rather limited oxidative perfor-
mances, mainly since they favour both a direct oxygen
production from water electrolysis (their main industrial
application is indeed oxygen evolution in sulphuric acid
media) and worse adsorption characteristics of the organic
molecules with respect to the Ti/Pt electrode.
The generated O2 probably can also take part in the bulk
oxidation of organics, through formation of organic radical by
hydrogen abstraction mechanism, followed by reaction of
organic radicals with O2 formed at the anode and further
abstraction of a hydrogen atom with the formation of organic
radicals such as hydroperoxide (relatively unstable). These
latter radicals tend to decompose and often lead to formation of
lower carbon number molecules [18]. However, this possible
oxidation pathway should be rather slow and have a low
influence on the general demolition kinetics.
For the graphite anode, lower oxidation kinetics than
metallic anodes are clearly shown in Table 2. Furthermore,
corrosion effects are observed for this electrode as a
consequence of high operating potentials and long electrolysis
time. The most important difference in the oxidation
behaviour, with respect to metallic anodes, lies in the fact
that sub-products generation is limited because a non selective
path (i.e. leading to carbon dioxide and water) seems to take
place preferentially (see Fig. 10 for Ofloxacin oxidation). An
enhancement of the demolition oxidative kinetics can though
be achieved with the use of three-dimensional electrodes
(Table 2). This electrode configuration is rather attractive and
particularly appropriate to treat low concentration solutions
[19]. Most of the contributions to the enhancement of the mass
transfer coefficient and the limiting current density in the
Fig. 10. Ofloxacin U.V. spectra for periodically (15 min) withdrawn anolyte
samples during the runs in divided cell with the graphite anode. Current density:
200 A m�2; electrolyte: 0.02N Na2SO4.
three-dimensional materials come from the increase of the
specific surface area (provide a more extensive interfacial
electrode surface for the electrochemical reaction) and from
the induced mixing [20,21].
In the structure of the 3D GAC electrode, current flows in
both electrolyte and electrode phases and the respective
conductivities of these two phases determine the associated
distribution of electrode potential and the reaction rate (non
uniform current/potential).
From Table 2 it is possible to conclude that not all the
specific area is useful for the electro-oxidation because the
theoretical surface offered by the activated carbon is much
greater than 1 m2 g�1. Furthermore, from the calculated values
(supposing that the oxidation kinetics is equal to that of the
graphite electrode) the utilized area is almost one hundred times
smaller that the one formally available in the whole set of the
GAC pellets. This suggests that the process occurs over the
external activated carbon sites of the pellet surface, which are
continuously re-generated through oxidation of adsorbed
organics.
Moreover, the structural stability of activated carbon bed is
rather low (working at relative high potential), as testified by
the dark coloration of the treated solution, caused by suspended
carbon fines and by a loss of the bed cohesion also due to the
oxidation to carbon monoxide/dioxide. This last corrosion
effect was not noticed at low electrode potentials (about 0.93–
0.98 V versus SCE) after 70 h of operation [22].
In order to quantify the CO2 formation from activated carbon
pellets oxidation inside the reactor anodic compartment, the
method described by Alvarez-Gallegos and Pletcher [23] was
adopted. The system was fit with a cool and concentrated
sodium hydroxide trap on the anodic gas outlet in order to trap
the CO2 given off during the electrolysis (no flow runs).
Samples of this solution were titrated with HCl first to pH 8.3
and then to pH 4.3.
The measured CO2 amounts are proportional to the electric
charge passed and are equal to about 12 mg CO2/Ah. This
phenomenon reduces the chances of this anodic material type
for a practical application.
4. Conclusions
The electrochemical oxidation and voltammetric tests of two
antibiotic substances, Ofloxacin and Lincomycin, has been
investigated on various anodes: Ofloxacin is oxidized
efficiently on all the anodes tested, whereas Lincomycin is
hardly oxidized because, for the nature of molecule, it is quite
difficult to induce its deprotonation, a prerequisite for the
following electron transfer step.
The electrochemistry of the oxidation process strongly
depends on the anode type and adsorption phenomena are
extremely important as they affect the kinetics of charge transfer.
For the metallic electrodes tested at high positive potentials,
the superficial oxide films, formed during operation, represent
the true catalytic media on which various organics substances
get adsorbed to follow different kinds of consequent electro-
chemical reaction pathways.
C. Carlesi Jara et al. / Applied Catalysis B: Environmental 70 (2007) 479–487 487
The adoption of a membrane to separate the anodic and
cathodic compartments is highly favourable as it enhances the
anodic reaction kinetics (mostly by keeping low pH conditions
at the anode side) and improves the current efficiency (by
hampering the occurrence of parasite redox couples). The use
of carbon type electrodes enables complete oxidation path-
ways, but the severe corrosion effect observed already at high
operating electrode potentials hampers their practical applic-
ability in these operating conditions.
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