Electrochemical Oxidation of Aqueous Carboxylic Acid Wastes Using

in Conductive-Diamond Electrolyses. P. Cañizares , C. Sáez , J. Lobato , R. Paz , M. A. Rodrigo. Journal of The Electrochemical Society 2007 154...
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Electrochemical Oxidation of Aqueous Carboxylic Acid Wastes Using Diamond Thin-Film Electrodes P. Can ˜ izares, J. Garcı´a-Go´ mez, J. Lobato, and M. A. Rodrigo* Departamento de Ingenierı´a Quı´mica, Facultad de Ciencias Quı´micas, Universidad de Castilla La Mancha, Campus Universitario s/n, 13071 Ciudad Real, Spain

The electrochemical oxidation of three carboxylic acids (formic, oxalic, and maleic) using diamond thin-film electrodes was studied using voltametric and galvanostatic electrolysis techniques. The voltametric study shows an anodic current peak that indicates the existence of a direct oxidation reaction at the electrode surface. Galvanostatic electrolysis studies confirm the existence of mediated oxidation reactions and indicate almost total mineralization of the waste with a virtually direct transformation of carboxylic acids into carbon dioxide (no intermediates were detected) regardless of the current intensity, initial organic acid concentration, temperature, and supporting media. Experimental results obtained in the galvanostatic electrolysis study can be fitted satisfactorily (CV < 10%) using a simple model that considers both kinetic and mass-transfer processes. Introduction The use of electrochemical oxidation for the destruction of the organic materials contained in industrial wastewaters has undergone rapid development in recent years in terms of the treatment of aqueous waste. Indeed, at present there are several commercial processes in use. The main problem associated with such electrochemical treatments is their high investment and operating costs. In an attempt to solve this problem, more research aimed at gaining a better understanding of the process and, therefore, obtaining a less expensive procedure has been undertaken in recent years.1-6 Electrochemical oxidation studies are mainly applied to the destruction of aromatic and halogenated compounds contained in industrial wastewaters. Thus, the oxidation processes of phenol7-13 and chlorophenols14-18 have been widely reported in the literature. In the oxidation process, these compounds are ultimately transformed into carbon dioxide through several steps that involve the formation of intermediate products such as quinones and carboxylic acids. The study of the electro-oxidation processes of wastes containing carboxylic acids does not seem to be a particularly important aspect in this area because the high degree of biodegradability of these materials implies biological treatment as the method of choice in these cases. Nevertheless, if a complete study of the oxidation of aromatic compounds were to be undertaken, the mechanism of destruction of these intermediate compounds would be an important factor in understanding the process in order to make it more efficient. Indeed, several works19-21 have been published with this specific aim in mind. The low oxidizability of these compounds and the importance of mediated oxidation reactions in the global oxidation process represent the main conclusions of these papers. In this context, the goal of the work described here was to study the electrochemical oxidation of several carboxylic acids using diamond thin-film electrodes and * To whom all correspondence should be addressed. Email: [email protected].

to determine the influence of the temperature, the characteristics of the waste, and the current density on the evolution of the process. Experimental Details Analytical Procedures. Carbon concentrations were monitored using a Shimadzu TOC-5050 analyzer. Chemical oxygen demand (COD) was determined using a HACH DR200 analyzer (method 8000). Organic compounds were identified and quantified by high-performance liquid chromatography (HPLC). The separation and analysis of carboxylic acids was performed on a Supelcogel H column, with a mobile phase of a 0.15% phosphoric acid solution at a flow rate of 0.15 cm3/min. The UV detector was set at 210 nm. Determination of the Instantaneous Current Efficiency (ICE). The COD method was used for the determination of the current efficiency for the oxidation of phenol.7 In this method, the COD was measured during electrolysis and the ICE was calculated using the relation

ICE )

[CODt+∆t - CODt]FV 8I∆t

(1)

where CODt and CODt+∆t are the CODs (g of O2 dm-3) at times t and t + ∆t (s), respectively, I is the current intensity (A), F is the Faraday constant (96 485 C equiv-1), V is the volume of the electrolyte (dm3), and 8 is a dimensional factor for unit consistence [32 g of O2 (mol O2)-1/4 mol e-1‚(mol O2)-1]. Electrochemical Cell. The oxidation of carboxylic acids was carried out in a single-compartment electrochemical flow cell (Figure 1). Diamond-based materials were used as the anode, and stainless steel (AISI 304) was taken as the cathode. Both electrodes were circular (100 mm diameter) with a geometric area of 78 cm2 each and with an electrode gap of 9 mm. The electrolyte was stored in a 500 cm3 glass tank and circulated through the electrolytic cell by a centrifugal pump. A heat exchanger was used to maintain the temperature at the desired set point.

10.1021/ie020594+ CCC: $25.00 © 2003 American Chemical Society Published on Web 02/07/2003

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Figure 1. Experimental setup: (a) layout of the pilot plant; (b) detail of the electrochemical cell section. Table 1. Assays Carried Out for Each Compound Tested (Formic, Oxalic, and Maleic Acids)

assay: brief description 1: 2: 3: 4: 5:

reference concentration current density temperature supporting medium

concn of carboxylic acid (mmol dm-3)

supporting medium

pH

T (°C)

current density (mA cm-2)

10 20 10 10 10

H2SO4/Na2SO4 H2SO4/Na2SO4 H2SO4/Na2SO4 H2SO4/Na2SO4 H3PO4/Na3PO4

2 2 2 2 2

20 20 20 60 20

30 30 60 30 30

Voltammetric Study. Electrochemical measurements were obtained using a conventional threeelectrode cell in conjunction with a computer-controlled potentiostat/galvanostat (model PGP 201; Voltalab 21, Radiometer-Copenhagen). Diamond was used as the working electrode, Hg/Hg2Cl2‚KCl(sat) as a reference, and stainless steel (AISI 304) as a counter electrode. Voltammetry experiments were performed in motionless solutions. Preparation of the Diamond Electrode. Borondoped diamond films (BDD) were provided by CSEM (Switzerland). These electrodes were produced by the hot-filament chemical vapor deposition (HF CVD) technique on single-crystal p-type Si 〈100〉 wafers (0.1 Ω cm, Siltronix). The temperature range of the filament was 2440-2560 °C, and that of the substrate was maintained at 830 °C. The reactive gas used was methane in an excess of dihydrogen (1% CH4 in H2). The dopant gas was trimethylboron with a concentration of 3 ppm. The gas mixture was supplied to the reaction chamber at a flow rate of 5 dm3 min-1, giving a growth rate of 0.24 µm h-1 for the diamond layer. The diamond film thickness obtained was about 1 µm. This HF CVD process produces a columnar, random texture, polycrystalline film. Prior to use in galvanostatic electrolysis assays, the electrode was polarized for 30 min with a 1 M H2SO4 solution at 50 mA cm-2 to remove any kind of impurity from its surface. Experimental Procedures. Galvanostatic electrolyses were carried out to determine the influence of the main parameters in the process. The average composition of the wastewater used in the experiments was 10 mmol dm-3 of carboxylic acid (formic, oxalic, or maleic),

5000 mg of Na2SO4 dm-3, and amounts of H2SO4 suitable enough to give a pH of 2. The pH was kept constant at these values during electrolysis by the continuous introduction of sulfuric acid (or sodium hydroxide) to the electrolyte reservoir. In some experiments to determine the influence of the initial organic matter concentration, current density, and temperature, these values were increased to 20 mmol dm-3, 60 mA cm-2, and 60 °C, respectively. Likewise, to determine the influence of supporting media, an experiment using Na3PO4/H3PO4 media was carried out. Table 1 presents the conditions applied in each experimental run for every compound tested. The cell potential was constant during each electrolysis, indicating that neither appreciable deterioration of the electrode nor passivation phenomena occurred. The electrolyte flow rate through the cell was 1250 cm3 min-1 (the linear velocity of the fluid was 2.31 cm s-1). Results and Discussion Voltammetric Study. Figures 2-4 show cyclic voltammograms obtained with a scan rate of 50 mV s-1 for different carboxylic acid (formic, oxalic, and maleic) concentrations in solutions containing 5000 mg of Na2SO4 dm-3 at pH 2 (sulfuric acid). In every case, an anodic current peak can be observed (≈2.3 V vs saturated calomel electrode). The peak potential is close to the water decomposition region, and it is often overlapped with oxygen evolution. Formic acid voltammograms show no differences in the oxygen evolution region, regardless of the presence and concentration of the organic acid. Conversely, for oxalic and maleic acids,

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Figure 2. Voltammetric study of BDD anodes on sulfuric acid/ sodium sulfate (5000 mg of Na2SO4 dm-3; pH 2) solutions containing different concentrations of formic acid (one scan): 1, 0 mg dm-3; 2, 50 mg dm-3; 3, 100 mg dm-3; 4, 200 mg dm-3; 5, 500 mg dm-3.

Figure 5. Voltammetric study of BDD anodes on sulfuric acid/ sodium sulfate (5000 mg of Na2SO4 dm-3; pH 2) solutions containing 100 mg dm-3 of maleic acid: 1, 25 mV s-1, first scan; 2, 25 mV s-1, second scan; 3, 50 mV s-1, first scan; 4, 50 mV s-1, second scan).

Figure 3. Voltammetric study of BDD anodes on sulfuric acid/ sodium sulfate (5000 mg of Na2SO4 dm-3; pH 2) solutions containing different concentrations of oxalic acid (one scan): 1, 0 mg dm-3; 2, 50 mg dm-3; 3, 100 mg dm-3; 4, 200 mg dm-3; 5, 500 mg dm-3.

Figure 6. Evolution of carbon fractions in the galvanostatic oxidation of formic, oxalic, and maleic acids. Formic acid oxidation: 9, organic carbon; 0, inorganic carbon. Oxalic acid oxidation: 2, organic carbon; 4, inorganic carbon. Maleic acid oxidation: [, organic carbon; ], inorganic carbon. Experimental conditions: n0, 10 mmol dm-3; T, 20 °C; j, 30 mA cm-2; supporting medium, H2SO4/Na2SO4.

Figure 4. Voltammetric study of BDD anodes on sulfuric acid/ sodium sulfate (5000 mg of Na2SO4 dm-3; pH 2) solutions containing different concentrations of maleic acid (one scan): 1, 0 mg dm-3; 2, 50 mg dm-3; 3, 100 mg dm-3; 4, 200 mg dm-3; 5, 500 mg dm-3).

an increase in the concentration leads to an advance of oxygen evolution. This fact is evidenced by the overlapping effect of oxygen evolution and the anodic peak and suggests the existence of a direct electrochemical reaction involving oxidation of the carboxylic acids. In the case of formic acid, the intensity of the anodic oxidation peak is small, and so such effects are not observed in the oxygen evolution region. A reverse peak is not observed in any of the cases studied here, indicating that the anodic peak corresponds to an irreversible reaction. The observed advance in the oxygen evolution region can also be justified20 assuming that the forma-

tion of hydroxyl radicals in the potential region of water decomposition can cause an important change in the electrochemical activity of the BDD electrode. Figure 5 shows cyclic voltammograms for maleic acid (two cycles) obtained with two different scan rates (25 and 50 mV s-1) in solutions containing 5000 mg of Na2SO4 dm-3 at pH 2 (sulfuric acid). It can be seen that there is no difference between the profiles, indicating that stable intermediates or polymers are not formed at the electrode surface. Similar behavior was also observed experimentally for oxalic and formic acids. Galvanostatic Electrolysis. Figure 6 shows the evolution of the total organic and inorganic carbon concentration in the electrochemical oxidation of the three carboxylic acids tested for a specific set of experimental conditions (n0, 10 mmol dm-3; T, 20 °C; j, 30 mA cm-2; supporting medium, H2SO4/Na2SO4). As can be seen, the electrochemical process can successfully treat the three carboxylic acids under investigation, transforming these compounds into carbon dioxide. The formation of intermediates could not be observed by HPLC, suggesting that the oxidation of the carboxylic acids can be considered as an electrochemical combustion reaction. Figure 7 shows the variation of the concentrations of the different carboxylic acids with the specific current charge passed as a function of the three parameters studied (supporting medium, temperature, and current density). It can be observed that the nature of the supporting medium has a marked influence on the

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SO52- + H2O f H2O2 + SO42-

Figure 7. Influence of the current density, temperature, and supporting medium on the evolution of the concentration of the different carboxylic acids: (a) formic acid; (b) oxalic acid; (c) maleic acid. Experimental conditions: [, 1, reference; 9, 3, current density; 2, 4, temperature; O, 5, supporting medium.

reaction of formic acid, with a smaller (but still significant) influence in the degradation of oxalic and maleic acids, and that high temperature contributes to decreases in the oxidation rates of all three carboxylic acids. On the other hand, it can also be seen that increasing the current density leads to slight increases in the oxidation rates for oxalic and formic acids but results in lower oxidation rates for maleic acid. The effect of the supporting medium and temperature on the oxidation can be explained in terms of the presence of oxidizing reagents in the electrolyte solution. It has been reported22 that electrolysis with diamond anodes in media containing sulfate ions generates peroxodisulfate (eq 2), a powerful oxidizing agent that can oxidize organic materials and thus increase their degradation rate in comparison to a supporting medium that cannot generate this reagent. Peroxodisulfate is chemically decomposed at high temperature and is transformed into oxygen (eq 3). Thus, the effect of this compound at high temperatures is less significant.

2SO42- f S2O82- + 2e-

(2)

S2O82- + H2O f 2SO42- + 2H+ + 1/2O2

(3)

Peroxodisulfate can also decompose to hydrogen peroxide according to reactions (4) and (5). Hydrogen peroxide is also capable of oxidizing organic matter, although it is a less powerful oxidant than peroxodisulfate.

S2O82- + H2O f SO52- + SO42- + 2H+

(4)

(5)

To confirm the presence of electrogenerated oxidants in the electrolyte, I-/I2 assays were performed on every sample obtained from the electrochemical treatment. This technique can detect and quantify (by titration with thiosulfate) all of the oxidant species capable of oxidizing I- to I2, including peroxodisulfate and hydrogen peroxide. In all cases the presence of an oxidant was not detected. To explain the observed influence of these oxidants, which is represented in Figure 7, it must be assumed that the amounts of these compounds formed are small and that they are rapidly consumed by reaction with the carboxylic acids present in the waste. Both of these possibilities lead us to believe that these reactions take place in a zone close to the anode surface. The influence of the current density can also be explained in terms of the presence of mediated electrochemical reactions. The increase in the oxidation rate (with respect to the specific current charge passed) observed for formic and oxalic acids can be explained by taking into account the higher quantities of electrogenerated reagents formed with increasing current density. Conversely, the decrease observed for maleic acid is characteristic of an oxidation performed mainly at the electrode surface (directly or by physisorbed hydroxyl radicals) because in these cases an increase in the current density leads to an increase in the concentration limit of organics and thus to a decrease in the efficiency of the process. In all cases only slight differences are observed, indicating that both mechanisms could be involved in the oxidation of the three acids tested. Figure 8 shows the ICE vs concentration (mmol dm-3) graph for a given set of experimental conditions, together with the theoretical evolution obtained according to a previously proposed model,23-25 in which the efficiency of the electrochemical oxidation of organic materials is assumed to be diffusion-rate-controlled. It can be observed that, for oxalic and formic acids, the current efficiency increases in a linear manner with the concentration of organics even though their values are smaller than expected, especially in the case of formic acid. For maleic acid, on the other hand, higher ICE values (with respect to the other acids) and an unusual shape can be observed. These graphs indicate that more than one oxidation mechanism is involved in the process. Model of the Process. To describe the electrochemical oxidation of phenolic aqueous wastes, our group has recently proposed a cell model25 which relies upon the existence of two zones in the electrochemical reactor (Figure 9): a first zone close to the anode surface with a thickness equivalent to the Nernst diffusion layer (reaction zone), where electrochemical and some strong mediated oxidation processes develop, and the remaining reactor volume (bulk zone), where some mediated oxidation reactions can develop. In both zones the concentration of every compound is considered to be constant with position and only time-dependent. Mass transport processes between both zones were quantified by assuming that the local rate of exchange is proportional to the concentration difference in the two zones. This description allows one to simplify significantly the mathematical complexity of the reaction system and can yield, together with an appropriate kinetic model, good agreements between experimental and simulation results.

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(6) is proposed to model the oxidation process, where kox (the maximum oxidation rate) is multiplied by ICE to take into account the oxidants that attack organic matter under total diffusion control and by the adjustable parameter σ to determine the effectiveness of the oxidation. Parameter kox is assumed to include the

r ) kox(ICE)σ Figure 8. Evolution of ICE with the organics concentration: 9, formic acid; 2, oxalic acid; b, maleic acid; O, phenol.25 Experimental conditions: n0, 10 mmol dm-3; T, 20 °C; j, 30 mA cm-2; supporting medium, H2SO4/Na2SO4.

(6)

oxidation carried out directly by the electrode surface and those performed by indirect electroreagents. Its value can be estimated from the current intensity I (C s-1) and the Faraday constant F (96 485 C mol-1) using expression (7). The ICE can be calculated from eq 8

kox ) I/F

(7)

assuming23-25 that this parameter is proportional to the concentration of organics (carboxylic acids) up to unity, before becoming constant at 1. Parameter σ depends on

if I < Ilim ICE ) 1 ICE ) [S1]/[S1]lim if I > Ilim

Figure 9. Sketch of the model for process characterization: [Si], concentration of compound Si; M/M•, redox couple of an inorganic reagent; ×, average concentration of each organic compound in the two zones.

In this paper, to describe the electrochemical oxidation of wastewaters polluted with carboxylic acids, the same cell model has been considered. The mass-transfer coefficient was calculated from a standard Fe(CN)63+/ Fe(CN)62+ current limit test, obtaining a value of 2.83 × 10-5 m s-1. This parameter was supposed to be constant (not depending on the composition) because the organics concentration in the wastewater was low and the fluid-flow conditions of the experiments did not change. The diffusion coefficient was evaluated using the Wilke and Chang equation,26 obtaining a value of 1.41 × 10-9 m2 s-1. The diffusion layer thickness was estimated from the k and D values as 4.99 × 10-5 m (average value) using the Nernst model. Taking into account the electrode surface value, the volume of the reaction zone is taken to be 0.39 cm3. Experimentally, inorganic oxidants were not found in the system although their presence was confirmed by the increase in the organic oxidation rate observed in sulfuric acid media. This fact indicates that these compounds must react rapidly with carboxylic acids to form carbon dioxide or oxygen (side reaction), with both reactions developing mainly in the reaction zone, because the concentration of oxidants is higher in this region. In this zone, the nature of the oxidation process cannot be identified because there is no possibility of discerning whether the oxidation of a carboxylic acid is performed directly on the electrode surface or by an inorganic oxidant. For this reason, kinetic expression

}

(8)

the waste composition and on the operation conditions. In the literature a similar parameter can be found,27 but its value is only related to the electrode properties. Oxidation reactions in the bulk zone can only occur for large excesses of electrogenerated inorganic oxidants in comparison to the carboxylic acid concentration. The rate of this reaction must be low because such oxidizing species are present in very low concentrations (not detected by I-/I2 assays). A first-order kinetic model with respect to carboxylic acid is assumed (eq 9) in order to quantify their effect.

r ) Kb[S1]

(9)

In this kinetic model, it is clear that the reaction rate in the bulk zone is nil (Kb ) 0) if the efficiency factor in the reaction zone is below unity (σ < 1) because for such conditions all inorganic oxidants react in the reaction zone and therefore cannot reach the bulk zone. System Resolution. By using the mass transport and kinetics expressions proposed and taking into account the stoichiometric coefficients that relate the carboxylic acid tested with the electrons involved in the oxidation (ne: 2 for formic acid, 2 for oxalic acid, and 12 for maleic acid) and with the carbon dioxide generated (ν12), the mass balances of carboxylic acids (S1) and carbon dioxide (S2) for the experimental system used in this work are expressed in eqs 10-13. To determine

Vr

Vb

d[S1] ) k([S1]* - [S1]) - Kb[S1]Vb dt

(10)

Vb

d[S2] ) k([S2]* - [S2]) + Kb[S1]Vb dt

(11)

d[S1]* 1 ) -k([S1]* - [S1]) - Kox(ICE)σ (12) dt ne

the influence of the different conditions on the adjustable parameters, the concentrations of the different organic species were calculated, for every experiment, using total carbon (TC), total organic carbon (TOC), and HPLC data. The differential equation system was solved

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Vr

ν12 d[S2]* K (ICE)σ ) -k([S2]* - [S2]) + dt ne ox

(13)

using the first-order Euler discretization method. Model parameters (σ and Kb) were calculated by a nonlinear least-squares regression method. The results obtained (simulation versus experimental data) for three different experimental runs (one for each component studied) are shown in Figure 10. It can clearly be seen that good agreement is obtained in all cases. These agreements are parametrized in Table 2 as a coefficient of variation. It can be seen that the values of this parameter are always below 10%, with an average value at 3.4%. Likewise, Table 2 shows the values of σ and Kb obtained for each compound and the experimental conditions employed. From these values, it must be concluded that the oxidizability of maleic acid is greater than those of the formic and oxalic acids because, for all experimental conditions tested, only diffusion limitations exist in the reaction zone (σ ) 1) and even some mediated reactions develop in the bulk zone (Kb > 0). For oxalic and formic acids, values of 1 for parameter σ are also obtained when the ratio oxidants concentration/organics concentration is high (excess of oxidants). Thus, it can be observed that when the values of the current intensity are high (hence, the inorganic oxidant concentration must be higher than that in the reference conditions), the value of σ increases up to 1 (assay 3) and the oxidation reactions begin to develop in the bulk zone (Kb > 0). Likewise, it can be observed that when the temperature is increased (assay 4) or there are insufficient sulfates in the media (assay 5), the value of σ decreases (the concentration of inorganic oxidants is lower than that in the reference experiments). The same behavior is observed when the initial carboxylic acid concentration is increased (assay 2) because in these conditions there are not enough inorganic oxidants to oxidize the carboxylic acid and hence the process efficiency σ decreases. In the last cases, because σ < 1, the reaction does not develop in the bulk zone (Kb ) 0). Conclusions From this work the following conclusions can be drawn: (1) Electrochemical oxidation using diamond thin-film electrodes can be successfully carried out for treating aqueous carboxylic acid wastes. Almost total mineralization of the waste is obtained regardless of the current intensity, initial organic acid concentration (within the range studied), and temperature. (2) The oxidation of carboxylic acids both is carried out directly at the electrode surface (according to the voltamometric study) and is mediated by inorganic electroreagents (according to the galvanostatic electrolysis study). (3) The inorganic electrogenerated reagent must react quickly in a zone close to the anode surface because I-/I2 assays were performed on every sample obtained in the galvanostatic electrolysis experiments and these compounds were not detected in any case. (4) The final product of the electrochemical oxidation of formic, oxalic, and maleic acids with diamond thinfilm electrodes is carbon dioxide. Intermediates were not found regardless of the conditions maintained in the galvanostatic electrolysis experiments.

Figure 10. Results obtained [simulation (line) versus experimental (points) data] for three experimental runs: [, formic acid; 2, oxalic acid; 9, maleic acid. Experimental conditions: n0, 10 mmol dm-3; T, 20 °C; j, 30 mA cm-2; supporting medium, H2SO4/Na2SO4. Table 2. Values of the Two Adjustable Parameters of the Model (σ, Fraction; Kb, s-1; CV, %) formic acid exptl conditions (Table 1) 1: 2: 3: 4: 5:

reference concentration current density temperature supporting medium

σ 0.5 0.5 1.0 0.6 0.3

Kb × 105 CV 0 0 10.0 0 0

2.0 3.3 4.3 2.6 2.7

oxalic acid σ 1.0 0.5 1.0 0.9 0.5

Kb × 105 CV 0 0 12.5 0 0

2.2 2.0 3.5 3.0 1.1

maleic acid σ 1.0 1.0 1.0 1.0 1.0

Kb × 105 CV 11.5 2.5 15.0 4.5 8.5

4.8 3.2 8.3 3.7 4.9

(5) Experimental results obtained in the galvanostatic electrolysis study can be fitted satisfactorily (CV < 10%) using a simple conceptual model that considers both kinetic and mass-transfer processes. The model supports some of the suggestions made in the Results and Discussion section. Acknowledgment This work was supported by the MCT (Ministerio de Ciencia y Tecnologı´a, Spain) and by the EU (European Union) through Project REN2001-0560. Literature Cited (1) Ko¨tz, R.; Stucki, S.; Carcer, B. Electrochemical Waste Water Treatment using High Overvoltage Anodes. Part I: Physical and electrochemical properties of SnO2 anodes. J. Appl. Electrochem. 1991, 21, 14. (2) Stucki, S.; Ko¨tz, R.; Carcer, B.; Suter, W. Electrochemical Waste Water Treatment using High Overvoltage Anodes. Part II: Anode Performance and Applications. J. Appl. Electrochem. 1991, 21, 99. (3) Comninellis, Ch. Electrocatalysis in the Electrochemical Conversion/Combustion of Organic Pollutants for Waste Water Treatment. Electrochim. Acta 1994, 39, 1857. (4) Savall, A. Electrochemical Treatment of Industrial Organic Effluents. Chimia 1995, 49, 23. (5) Leffrang, U.; Ebert, K.; Flory, K.; Galla, U.; Schmieder, H. Organic Waste Destruction by Indirect Electrooxidation. Sep. Sci. Technol. 1995, 30, 1883. (6) Awad, Y. M.; Abuzaid, N. S. Electrochemical Treatment of Phenolic Wastewaters.: Efficiency, Design Considerations and Economic Evaluation. J. Environ. Sci. Health 1997, A32 (5), 1393. (7) Comninellis, Ch.; Pulgarin, C. Anodic Oxidation of Phenol for Waste Water Treatment. J. Appl. Electrochem. 1991, 21, 703. (8) Comninellis, Ch.; Pulgarin, C. Electrochemical Oxidation of Phenol for Wastewater Treatment using SnO2 Anodes. J. Appl. Electrochem. 1993, 23, 108. (9) Comninellis, Ch.; Nerini, A. Anodic Oxidation of Phenol in the Presence of NaCl for Waste Water Treatment J. Appl. Electrochem. 1995, 25, 23. (10) Boudenne, J.-L.; Cerclier, O.; Gale´a, J.; Van der Vlist, E. Electrochemical Oxidation of Aqueous Phenol at a Carbon Black Slurry Electrode. Appl. Catal. 1996, 143, 185.

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(11) Brillas, E.; Sauleda, R.; Casado, J. Degradation of 4-Chlorophenol by Anodic Oxidation, Electro-Fenton, Photoelectro-Fenton, and Peroxi-coagulation Processes. J. Electrochem. Soc. 1998, 145, 759. (12) Can˜izares, P.; Domı´nguez, J. A.; Rodrigo, M. A.; Villasen˜or, J.; Rodrı´guez, J. Effect of the Current Intensity in the Electrochemical Oxidation of Aqueous Phenol Wastes at an Activated Carbon and Steel Anode. Ind. Eng. Chem. Res. 1999, 38, 3779. (13) Awad, Y. M.; Abuzaid, N. A. The Influence of Residence Time on the Anodic Oxidation of Phenol. Sep. Pur. Technol. 2000, 18, 227. (14) Bunce, N. J.; Merica, S. G.; Lipkowski, J. Prospects for the use of Electrochemical Methods for the Destruction of Aromatic Organochlorine Wastes. Chemosphere 1997, 35, 2719. (15) Polcaro, A. M.; Palmas, S. Electrochemical Oxidation of Chlorophenols. Ind. Eng. Chem. Res. 1997, 36, 1791. (16) Boudenne, J.-L.; Cerclier, O. Performance of Carbon BlackSlurry Electrodes for 4-Chlorophenol Oxidation. Water Res. 1999, 33, 494. (17) Rodgers, J. D.; Jedral, W.; Bunce, N. J. Electrochemical Oxidation of Chlorinated Phenols. Environ. Sci. Technol. 1999, 33, 1453. (18) Azzam, M. O.; Al-Tarazi, M.; Tahboub, Y. Anodic Destruction of 4-Chlorophenol Solution. J. Hazard. Mater. 2000, B75, 99. (19) Bock, C.; MacDougall, B. The Anodic Oxidation of pBenzoquinone and Maleic Acid. J. Electrochem. Soc. 1999, 146, 2925. (20) Gandini, D.; Mahe´, E.; Michaud, P. A.; Haenni, W.; Perret, A.; Comninellis, Ch. Oxidation of Carboxylic Acid at Boron-doped Diamond Electrodes. J. Appl. Electrochem. 2000, 30, 1345.

(21) Bock, C.; Smith, A.; MacDougall, B. The Anodic Oxidation of Oxalic and Formic Acid Using WOx Based Anodes: Systematic Studies. Energy and Electrochemical Processes for a Cleaner Environment; Proceedings of the International Symposium; Electrochemical Society: Pennington, NJ, 2001. (22) Michaud, P.-A.; Mahe´, E.; Haenni, W.; Perret, A.; Comninellis, Ch. Preparation of Peroxodisulfuric Acid using Boron-doped Diamond Thin-film Electrodes. Electrochem. Solid State Lett. 2000, 3, 77. (23) Panizza, M.; Michaud, P.-A.; Cerisola, G.; Comninellis, Ch. Anodic Oxidation of 2-Naphthol at Boron-doped Diamond Electrodes. J. Electroanal. Chem. 2001, 507, 206. (24) Rodrigo, M. A.; Michaud, P.-A.; Duo, I.; Panizza, M.; Cerisola, G.; Comninellis, Ch. Oxidation of 4-Chlorophenol at Boron-doped Diamond Electrodes for Wastewater Treatment. J. Electrochem. Soc. 2001, 148, D60. (25) Can˜izares, P.; Dı´az, M.; Domı´nguez, J. A.; Garcı´a-Go´mez, J.; Rodrigo, M. A. Electrochemical Oxidation of Aqueous Phenol Wastes on Synthetic Diamond Thim-film Electrodes. Ind. Eng. Chem. Res. 2002, 41, 4187. (26) Wilke, C. R.; Chang, P. AIChE J. 1955, 1, 264. (27) Gherardini, L.; Michaud, P. A.; Panizza, M.; Comninellis, Ch.; Vatistas, N. Electrochemical Oxidation of 4-Chlorophenol for Wastewater Treatment. J. Electrochem. Soc. 2001, 148, D78.

Received for review August 2, 2002 Revised manuscript received December 18, 2002 Accepted December 21, 2002 IE020594+