Electrochemical removal of antibiotics from wastewaters

Electrochemical removal of antibiotics from wastewaters

Applied Catalysis B: Environmental 70 (2007) 479–487 www.elsevier.com/locate/apcatb Electrochemical removal of antibiotics from wastewaters C. Carles...

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Applied Catalysis B: Environmental 70 (2007) 479–487 www.elsevier.com/locate/apcatb

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 mgcm2 A1 h1 for the bidimensional 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 adsorption, 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].

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Nomenclature List of Symbols A Anodic surface area (m2) C Organic pollutant concentration (mg l1) C1 Anodic compartment inlet concentration of organic pollutant (mg l1) C2 Anodic compartment outlet concentration of organic pollutant (mg l1) E Electrode potential (V) F Faraday constant (C mol1) I Electrical current (A) i Current density (A m2) ilim Diffusive limit current density (A m2) KM Transport coefficient in the electrochemical reactor (m s1) MOX Metal oxide Q Recycled flow between anodic compartment and mixing tank (m3 s1) qe Electro-generated heat (removal from mixing tank by heat exchange) (J) R Cell electrical resistance (V) r Electrochemical reaction rate (s1) 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

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 electrooxidation 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

Fig. 1. The Ofloxacin (a) and Lincomycin (b) molecules.

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 l1 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:    

platinised titanium (Tokuyama Soda, Japan); rigid graphite (purity 99.5% Good Fellow, England); Ti/IrO2/Ta2O5 (DSA1 type anodes, DeNora, Italy) and 3D GAC: three-dimensional anode consisting of a fixed bed (60 g dry) of activated carbon pellets (Camel EnvirotechMultisorb 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

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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  102 m1; the electrodes surface was 2  102 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 m2, 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.

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 ¼ etðK M A=V M aÞ C0

(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

Fig. 3. Scheme of the closed recirculating loop of electrolyte solution containing the organic pollutants throughout the anodic compartment and its mixing tank.

I ¼ K M AC1 zF KMA a1  Q

QðC 1  C 2 Þ ¼ C1 ¼

C2 a

Reservoir CSTR    V M dCdt2 ¼ QðC 2  C 1 Þ ¼ QC 2 1  a1

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Table 2 Ofloxacin oxidation in the electrochemical (divided) reactor: calculated transport coefficients and time required for specific percent abatements using different anode materials Initial concentration (mg/l)

25 50 50 50 50

Anode type

Ti/Pt Ti/Pt DSA Graphite 3D GAC

KMA (m3 s1)

Required time (min) for abatement of

7

4.1  10 5.0  107 1.0  107 1.9  107 3.8  105

50%

90%

112.7 92.4 462.0 239.0 30.8

374.3 306.9 1535.0 793.9 102.3

Applied current: 4A; electrolyte: Na2SO4 0.02N; treated volume: 4 l; average linearity factor for Eq. (1) higher then 0.9.

and/or superior oxides, and minimizes fouling phenomena caused by in situ generation of adsorbed polymeric material. When using the Na2SO4 electrolyte, the organic concentration 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 abatement 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

Fig. 4. Results of iodometric measurement of ‘‘bulk’’ oxidants in terms of: (a) mg l1 of NaOCl when using NaCl electrolyte ((*) 0.02N; (~) 0.2N); (b) relative concentration C/Co of H2O2 when using Na2SO4 electrolyte.

contribution of the indirect oxidation is though limited by the possibility of producing a high quantity of noxious chloroorganics 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

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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 m2; (B) 0.02N Na2SO4; 400 A m2; (C) 0.02N NaCl; 200 A m2; (D) 0.02N NaCl, 400 A m2.

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

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 byproducts 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 conditions, 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 þ H2 O ! MOXþ1 þ 2Hþ þ 2e ;

(2)

whose kinetics can be expressed with Eq. (3): Fig. 6. Single voltammetric cycle for Ofloxacin (200 mg l1). Working electrode: graphite; electrolyte H2SO4 0.5 M; scan rate: 50 mV s1; initial potential: 0 V; no stirring.



I I2R  zF qe

(3)

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As a consequence, one can write: K M ¼ K M;o ið1þDÞ

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):   ilim KM ¼ h¼ (4) zFC 1 A i i

(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 preferred 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 l1) 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,

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): @ logðK M =iÞ ¼D @ logðiÞ

(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 m2.

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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 l1; 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) Organic 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) Reduction 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,

485

Fig. 9. Cell potential versus electrolysis time for different applied current densities for divided and undivided cell. Electrolyte: 0.02N Na2SO4.

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 electrocatalytic 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

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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 performances, 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

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 g1. 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

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 m2; electrolyte: 0.02N Na2SO4.

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 electrochemical reaction pathways.

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