Electrochemical removal of antibiotics from wastewaters

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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].

mailto:[email protected]

http://dx.doi.org/10.1016/j.apcatb.2005.11.035

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

� �


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


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)


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,


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 allows
to 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 avoids
the 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


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.


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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