Electrochemical Oxidation of Polyhydroxybenzenes on Boron

Ind. Eng. Chem. Res. 2004, 43, 6629-6637

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APPLIED CHEMISTRY Electrochemical Oxidation of Polyhydroxybenzenes on Boron-Doped Diamond Anodes P. Can ˜ izares, C. Sa´ ez, J. Lobato, and M. A. Rodrigo* Department of Chemical Engineering, Facultad de Ciencias Quı´micas, Universidad de Castilla La Mancha, Campus Universitario s/n, 13071 Ciudad Real, Spain

The electrochemical oxidation of phenol (Ph), hydroquinone (HQ), and 1,2,4-trihydroxybenzene (THB) using a diamond thin-film anode has been studied. Within the parameter ranges used (temperature 15-60 °C, initial concentration 1.1-36 mmol dm-3, current density 15-60 mA cm-2), almost complete mineralization of the organic waste is obtained. Carbon dioxide is the sole final product, and the main intermediates are carboxylic acids C4 and C2. Both direct and mediated electrochemical oxidation processes are involved in the electrochemical treatment. Sulfates contained in the waste favor the formation of electrogenerated reagents (persulfates). The instantaneous current efficiency was found to depend only on the controlling mechanism (mass transfer or electrode kinetics). The current efficiency of the process is 1 if it is kinetically controlled and decreases linearly to 0 from the chemical oxygen demand limit value if it is diffusion controlled. Taking into account the information obtained in previous works and the results of the voltammetric and galvanostatic studies of the present work, a simple mechanistic model is proposed to justify the processes involved in the electrochemical oxidation of polyhydroxybenzenes. Introduction Many industrial processes generate wastes containing small concentrations of refractory organic compounds. Under appropriate circumstances, the organic compounds contained in these wastes can be economically recovered, but usually the best method to treat these wastes is their destruction by oxidation techniques (photochemical, chemical, electrochemical, and/or wet oxidation). Within these techniques, electrochemical oxidation appears as one of the most promising technologies for the treatment of these wastes. This is a consequence of the development of new anodic materials (such as boron-doped diamond (BDD)) which allows achievement of the complete mineralization of the wastes with high current efficiencies. Phenol is frequently used as a model compound in the evaluation of oxidation techniques, since it is representative of the pollutants contained in actual wastes. As a consequence, in recent years, electrochemical treatment of phenolic wastewater has been the subject of many works.1-9 The main outcome of these works is that the electrode material has a great influence on the treatment results. The use of low-oxygen-overvoltage anodes9-13 (e.g., DSA, platinum) does not lead to the total mineralization of the wastes, and frequently polymers or carboxylic acids are obtained as final products. Likewise, these electrodes achieve low current efficiencies. By contrast, the use of high-oxygen-overvoltage anodes8,14-20 (e.g., PbO2, BDD) leads to the * To whom correspondence should be addressed. Fax: +34 926 29 53 18. E-mail: [email protected].

complete mineralization of the waste and results in higher current efficiencies. Several authors10,12,14,18,19 propose that the electrochemical oxidation of phenol on these anodes leads initially to the addition of hydroxyl groups generated in the system (by oxidation of water) to form polyhydroxybenzenes and quinonic compounds, the latter of which can suffer aromatic ring cleavage to yield carboxylic acids. Finally, these acids are oxidized to carbon dioxide. Other authors1,2,9,10,21 assume the formation of the same intermediates by direct electrochemical reactions on the anode surface, or by the combination of both direct and mediated electrochemical oxidation processes. A similar behavior is observed in the electrochemical treatment of wastes polluted with substituted phenols such as chlorophenol,13,22-24 nitrophenol,25 or aniline.26 The electrochemical oxidation of hydroxyl-substituted phenols has been less studied. Only a few papers have mentioned the electrochemical treatment of polyhydroxybenzenes, and they are focused mainly on the electrochemical oxidation of dihydroxybenzenes13,27-31 (hydroquinone, catechol). In these works the same behavior as that obtained for the electrochemical treatment of phenols is observed. Intermediates formed and current efficiencies depend strongly on the oxygen overvoltage of the anode material. The goal of the work described here was to study the electrochemical oxidation of aqueous wastes containing phenol (Ph), hydroquinone (HQ), and 1,2,4-trihydroxybenzene (THB) using boron doped diamond thin-film electrodes, and to determine the influence of the waste characteristics and the main operation parameters (temperature and current density) on the process.

10.1021/ie049807g CCC: $27.50 © 2004 American Chemical Society Published on Web 09/15/2004

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Figure 1. Arrangement of the pilot plant. Detail of the electrochemical cell section.

Experimental Section Analytical Procedure. The carbon concentration was monitored using a Shimadzu TOC-5050 analyzer. Chemical oxygen demand (COD) was determined using a HACH DR200 analyzer. Carboxylic acids were monitored by HPLC using a Supelcogel H column (mobile phase, 0.15% phosphoric acid solution; flow rate, 0.15 mL min-1). The UV detector was set at 210 nm. Aromatics were also monitored by HPLC using a Nucleosil C18 column (mobile phase, 60% water-40% methanol; flow rate, 0.50 mL min-1). In this case the UV detector was set to 270 nm. Determination of the Instantaneous Current Efficiency (ICE). The chemical oxygen demand method was used for the determination of the current. In this method, the COD was measured during electrolysis and the instantaneous current efficiency was calculated using the relation

ICE )

[CODt+∆t - CODt]FV 8I∆t

(I)

where CODt and CODt+∆t are the chemical oxygen demand (in g of O2 dm-3) at times t and t + ∆t (in

seconds), respectively, I is the current intensity (A), F is the Faraday constant (96 487 C mol-1), V is the volume of the electrolyte (dm3), and 8 is a dimensional factor for unit consistency [(32 g of O2‚mol-1 of O2)/(4 mol of e-‚mol-1 of O2)]. Electrochemical Cell. The oxidation of polyhydroxybenzenes was carried out in a single-compartment electrochemical flow cell (Figure 1). Diamond-based material was used as anode and stainless steel (AISI 304) as the cathode. Both electrodes were circular (100 mm diameter) with a geometric area of 78 cm2 each and an electrode gap of 9 mm. The electrolyte was stored in a glass tank (500 mL) and circulated through the electrolytic cell by means of a centrifugal pump. A heat exchanger was used to maintain the temperature at the desired set point. The experimental setup also contained a cyclone for gas-liquid separation and a gas absorber to collect the carbon dioxide contained in the gases evolved from the reactor into sodium hydroxide. Preparation of the Diamond Electrode. Borondoped diamond (BDD) films were provided by CSEM (Switzerland) and synthesized by the hot filament chemical vapor deposition technique (HF CVD) on single-crystal p-type Si 〈100〉 wafers (0.1 Ω cm, Siltronix). The temperature range of the filament was 2440-2560 °C, and the temperature of the substrate was 830 °C. The reactive gas was methane in excess dihydrogen (1% CH4 in H2). The dopant gas was trimethylboron with a concentration of 3 mg dm-3. 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 resulting diamond film thickness was about 1 µm. This HF CVD process produces a columnar, random texture, and polycrystalline films with an average resistivity of 0.01 Ω cm. Prior to use in galvanostatic electrolysis assays, the electrode was polarized during 30 min with a 1 M H2SO4 solution at 50 mA cm-2 to remove any kind of impurity from its surface. Voltammetry Experiments. Electrochemical measurements were obtained using a conventional threeelectrode cell in conjunction with a computer-controlled

Table 1. Experimental Conditions Studied in This Work exptl run

compound

C0/mmol dm-3

supporting media

pH

T/°C

j/mA cm-2

Ph1 Ph2 Ph3 Ph4 Ph5 Ph6 Ph7 Ph8 HQ1 HQ2 HQ3 HQ4 HQ5 HQ6 HQ7 HQ8 HQ9 THB1 THB2 THB3 THB4 THB5 THB6 THB7 THB8 THB9

phenol phenol phenol phenol phenol phenol phenol phenol hydroquinone hydroquinone hydroquinone hydroquinone hydroquinone hydroquinone hydroquinone hydroquinone hydroquinone 1,2,4-trihydroxybenzene 1,2,4-trihydroxybenzene 1,2,4-trihydroxybenzene 1,2,4-trihydroxybenzene 1,2,4-trihydroxybenzene 1,2,4-trihydroxybenzene 1,2,4-trihydroxybenzene 1,2,4-trihydroxybenzene 1,2,4-trihydroxybenzene

5.3 10 10 10 10 10 10 21 1.1 1.1 1.1 10 10 10 10 10 36 1.1 1.1 1.1 10 10 10 10 10 36

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

2 2 12 2 2 2 2 2 2 12 2 2 12 2 2 2 2 2 12 2 2 12 2 2 2 2

25 25 25 25 25 60 15 25 25 25 25 25 25 25 25 60 25 25 25 25 25 25 25 25 60 25

30 30 30 15 60 30 30 30 30 30 30 30 30 15 60 30 30 30 30 30 30 30 15 60 30 30

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Figure 2. Linear sweep voltammograms on BDD anodes of phenol, hydroquinone, and 1,2,4-trihydroxybenzene solutions (2.5 mmol dm-3) on sodium sulfate media (5000 mg dm-3) at pH 2 (a) and 12 (b). (1) No organic matter; (2) HQ; (3) THB; (4) Ph. Auxiliary electrode, stainless steel AISI 304; reference electrode, SCE; scan rate ) 100 mV s-1.

potentiostat/galvanostat (Autolab Model PGSTAT 30, Eco Chemie B.V., Utrecht, The Netherlands). Diamond was used as the working electrode, Hg/Hg2Cl2‚KCl (saturated) as a reference electrode, and stainless steel (AISI 304) as a counter electrode. All electrodes were circular (10 mm diameter) with a geometric area of 7.8 cm2 each. Voltammetry experiments were performed in unstirred solutions (200 mL). The anode was anodically polarized during 5 min with a 1 M H2SO4 solution at 0.1 A prior to each experiment. Bulk Electrolysis. Galvanostatic electrolyses were carried out to determine the main intermediates formed in the process. The synthetic wastewaters used in the experiments contained different concentrations of phenol (Ph), hydroquinone (HQ), or 1,2,4-trihydroxybenzene (THB), 5000 mg dm-3 Na2SO4, and H2SO4 in suitable amounts to give a pH of 2 (or NaOH to reach a pH of 12). The pH was kept constant by the continuous introduction of sulfuric acid (or sodium hydroxide) to the electrolyte reservoir. Table 1 presents the conditions applied in each experimental run. The cell potential was constant during each electrolysis, indicating that appreciable deterioration of the electrode or passivation phenomena did not take place. The electrolyte flow rate through the cell was 2500 cm3 min-1. The linear velocity of the fluid was 4.62 cm s-1, and the space velocity was 4.16 min-1. Results and Discussion Voltammetric Study. Linear sweep voltammograms with BDD electrodes of solutions containing 2.5 mM Ph, HQ, or THB, and 5000 mg dm-3 Na2SO4 at pH 2 (a) and pH 12 (b) are shown in Figure 2. The curve obtained under the same experimental conditions but without

Figure 3. Cyclic voltammograms on BDD anodes of phenol (a), hydroquinone (b), and 1,2,4-trihydroxybenzene (c) solutions (2.5 mmol dm-3) on sodium sulfate media (5000 mg dm-3) at pH 2. (1) First cycle; (2) second cycle; (3) third scan. Auxiliary electrode, stainless steel AISI 304; reference electrode, SCE; scan rate ) 100 mV s-1.

organic matter is also shown for the sake of comparison. As can be observed, for every compound an anodic oxidation peak close to the oxygen evolution potential region (2.2-2.4 V vs SCE) appears. These peaks are overlapped with the oxygen evolution process, and due to this, it seems that the oxygen evolution begins at lower potentials. This trend is specially marked for THB. In addition to this peak, other peaks appear in the anodic oxidation of each of the tested compounds. Thus, the phenol voltammogram shows two overlapped peaks in the 1.3-1.8 V vs SCE potential region, and the hydroquinone voltammogram shows a peak in the 0.8-1.0 V vs SCE potential region. The voltammogram of THB shows two anodic oxidation peaks which seem to be related to those that appear in the Ph and HQ voltammograms. The first peak is slightly shifted toward the left, compared with the peak measured for HQ. The apparent potential of the second matches the second peak of the phenol voltammogram. In alkaline conditions, the peaks are softer and overlapped. However, the shift of oxygen evolution process is also observed.

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Figure 5. (a) Variation of COD with the specific electrical charge passed in the electrochemical oxidation of wastes containing 10 mM phenol, hydroquinone, and 1,2,4-trihydroxybenzene (pH 2; T ) 25 °C; j ) 30 mA cm-2). 9, Ph; 0, HQ; 2, THB. (b) Variation of COD with the specific electrical charge passed in the electrochemical oxidation of wastes containing hydroquinone (pH 2; T ) 25 °C; j ) 30 mA cm-2). 9, 1.1 mM; 0, 10 mM; 2, 36 mM.

anodic oxidation peaks. This behavior has also been observed in the anodic oxidation of other phenols on BDD electrodes, and it has been previously reported for chlorophenol12,22,24,32 and nitrophenol.25,33,34 According to these remarks, it seems reasonable that these peaks correspond to the oxidation of the phenolic compound to the phenoxy radical and the subsequent oxidation of this phenoxy radical to the corresponding phenoxy cation (eq 1). Figure 4. Cyclic voltammograms on BDD anodes of phenol (a), hydroquinone (b), and 1,2,4-trihydroxybenzene (c) solutions (2.5 mmol dm-3) on sodium sulfate media (5000 mg dm-3) at pH 2. (1) First cycle; (2) second cycle. Auxiliary electrode, stainless steel AISI 304; reference electrode, SCE; scan rate ) 100 mV s-1; A, anodic start of the CV.

Figure 3 shows consecutive cyclic voltammograms of acid aqueous solutions containing 2.5 mM Ph, HQ, and THB. It can be observed that every peak decreases in size at the second scan. In this range of potential, reverse peaks were not observed. This indicates that these peaks may correspond to irreversible processes, suggesting electrochemical-chemical oxidation mechanisms. Figure 4 shows voltammograms obtained in a wider range of potentials (from hydrogen to oxygen evolution). The appearance of a reduction peak (p4), which was identified as the reduction of quinonic compounds, can be observed.24,25 According to these results, it seems that the anodic oxidation of these compounds shares the same initial reaction stages, as indicated by the presence of related

Ar-OH f Ar-O• f Ar-O+

(1)

It has been reported that the two electrochemically formed compounds are very reactive and can couple to form polymers3 or undergo the addition of hydroxyl groups (from water or hydroxyl radicals generated in the potential region of water decomposition) to form polyhydroxybenzenes.8,35-37 The formation of this polymer can lead to the development of a passivating film on the surface of the electrode, causing the decrease in size of the peaks with the number of scans. On the other hand, the formation of polyhydroxybenzenes is known to be the first step in the aromatic ring cleavage in oxidation mechanisms. Below in this work (in the Bulk Electrolysis section) are described the presence of traces of several polyhydroxybenzenes and the conversion of the phenolic compounds into carboxylic acids C2 and C4 during the treatment of wastes polluted with Ph, HQ, and THB in a bench-scale plant. Bulk Electrolysis. Figure 5 shows the variation of the chemical oxygen demand (COD) in the electrolyte with the specific electrical charge passed (A h dm-3) as a function of the organic pollutant (Ph, HQ, THB) and

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Figure 6. Variation of intermediates and final products of the electrochemical oxidation of phenol (a), hydroquinone (b), and 1,2,4trihydroxybenzene (c) with the specific electrical charge passed (C0 ) 10 mmol dm-3; pH 2; T ) 25 °C; j ) 30 mA cm-2). 9, phenolic compound; 2, inorganic carbon; 4, C4 acids; 0, C2 acids.

of their concentration. As can be observed, the complete mineralization of the organic matter contained in the waste is obtained in all cases. Moreover, it can also be observed that the rate of mineralization depends not on the particular organic but on its concentration. Figure 6 shows the variation with the specific electrical charge passed of the main intermediates involved in the oxidation process of the tested compounds (10 mmol dm-3; pH 2; 5000 mg dm-3 Na2SO4; T ) 25 °C; j ) 30 mA cm-2). As can be seen, the electrochemical oxidation of polyhydroxybenzene wastes using a BDD electrode leads to the rapid sequential formation of carboxylic acids (maleic and oxalic, principally) and carbon dioxide, as the sole final product. Some quantities of polyhydroxybenzenes and quinonic compounds were also detected in the electrochemical treatment of phenol wastes, although in trace concentrations. The influence of the pH of the waste is shown in Figure 7. As can be observed, the global oxidation rate of the electrochemical treatment of phenolic compounds does not depend on the pH, at least in the experimental conditions studied. In both studied cases, complete mineralization of organic waste is obtained. Figures 8 and 9 show the influence of the current density and temperature on the bulk electrolysis of the synthetic wastes tested. As can be seen, complete

mineralization is also obtained in every case. As expected, the oxidation rate increases with the current density, although this increase is not proportional to the current density in the whole range of COD. As will be described below, the process is kinetically controlled at high COD values (the increase in the oxidation rate proportional to the increase in current density) and mass transport controlled at low values of COD. The effect of temperature seems to be more strange. Direct electrochemical oxidation processes are almost never affected by temperature. Thus, the influence of this parameter must be interpreted in terms of the effect of mediated electroreagents. Normally, an increase in temperature leads to increases in the mediated oxidation rates due to their chemical nature. However, the effect observed in Figure 9 is that the oxidation rate at 60 °C is lower than that obtained at 25 °C. This can be easily explained in terms of the particular mediated electrogenerated reagents formed in this process. In the literature, it is proposed38,39 that, in the electrochemical oxidation on BDD electrodes of wastes containing sulfates, some peroxodisulfates can be formed (eq 2). These compounds are powerful oxidizing agents that can oxidize organic matter and thus increase their degradation rate. However, peroxodisulfate is chemically decomposed at high temperature and it can be

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Figure 7. Variation of COD with the specific electrical charge passed (C0 ) 10 mmol dm-3; sulfate media; T )25 °C; j ) 30 mA cm-2). (a) Phenol; (b) hydroquinone; (c) 1,2,4-trihydroxybenzene. 9, pH 2; 0, pH 12.

transformed into oxygen (eq 3) or hydrogen peroxide (eqs 4 and 5).

2SO42- f S2O82- + 2e-

(2)

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

(3)

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

(4)

SO52- + H2O f H2O2 + SO42-

(5)

Thus, an increase in temperature favors both the oxidation rate of the organics with peroxodisulfate (positive effects) and also peroxodisulfate decomposition (negative effects). Which of the two processes is favored must depend on temperature and also on the nature of the organic pollutant. As can be seen in Figure 9a, increasing the temperature from 15 to 25 °C mainly favors the oxidation rate of organics with peroxodisulfate, while a further increase up to 60 °C favors the negative effect of peroxodisulfate decomposition. On the

Figure 8. Variation of COD with time (C0 ) 10 mmol dm-3; pH 2; sulfate media; T ) 25 °C). (a) Phenol; (b) hydroquinone; (c) 1,2,4trihydroxybenzene. 9, 30 mA cm-2; 0, 15 mA cm-2; 2, 60 mA cm-2.

other hand, in previous papers which describe the electrochemical oxidation of wastewaters containing sulfate and organics at different temperatures,8,25,38 it can be observed that increasing temperature in the same range always leads to more efficient oxidation processes. Hence, the nature of the organic pollutant must influence the favored process. The maximum concentrations of organic intermediates in several experiments are shown in Table 2 for comparison purposes. As can be seen, the main intermediates presented in the system are carboxylic acids. The larger amounts of oxalic acid (C2) and maleic acid (C4) can be easily explained, taking into account their lower oxidation rate.21,39,40 In accordance with the information obtained from previous studies about the electrochemical oxidation of phenolic compounds,12,20,21,24,25 the results of the bulk electrolysis study, and those of the voltammetric study, a simple mechanistic model (Figure 10), which relates the intermediates formed during the treatment throughout the most important processes, can be proposed. According to this model, the electrochemical treatment of phenol aqueous wastes leads to the formation of polyhydroxybenzenes (e.g., hydroquinone, catechol),

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Figure 10. Sketch of the simple mechanistic model proposed to explain the main processes occurring in the electrochemical treatment of THB wastes using BDD anodes.

Figure 9. Variation of COD with the specific electrical charge passed (C0 ) 10 mmol dm-3; pH 2; sulfate media; j ) 30 mA cm-2). (a) Phenol; (b) hydroquinone; (c) 1,2,4-trihydroxybenzene. 9, 25 °C; 0, 60 °C; 2, 15 °C. Table 2. Maximum Concentrations Measured (Referred to as the Initial Carbon Concentration) for the Main Intermediates Detected in the Galvanostatic Electrolysis of Phenol, Hydroquinone, and 1,2,4-Trihydroxybenzene (10 mmol dm-3) experimental run compound

Ph2

Ph3

Ph4

Ph5

Ph6

hydroquinone p-benzoquinone C4 acid C2 acid

0.4 0.4 1.2 10.2

0.0 0.0 13.9 2.8

0.0 0.0 24.2 25.8

0.0 0.0 0.0 25.8

0.0 0.0 42.4 16.4

experimental run compound

HQ4

HQ5

HQ6

HQ7

HQ8

C4 acid C2 acid

0.4 9.4

1.3 6.9

1.0 0.3

0.3 6.7

1.4 0.2

experimental run compound

THB4

THB5

THB6

THB7

THB8

C4 acid C2 acid

0.5 2.4

0.2 3.3

0.7 1.8

0.5 2.3

1.2 1.4

which are oxidized first to carboxylic acids (maleic and oxalic acid mainly) and later to carbon dioxide.24,25,30 The

Figure 11. (a) Variation of the instantaneous current efficiency (ICE) with COD in the electrochemical oxidation of wastes containing phenol, hydroquinone, and 1,2,4-trihydroxybenzene (T ) 25 °C; j ) 30 mA cm-2). 9, 10 mmol dm-3 Ph, pH 2; 2, 10 mmol dm-3 Ph, pH 12; 4, 10 mmol dm-3 HQ, pH 2; b, 10 mmol dm-3 HQ, pH 12; O, 10 mmol dm-3 THB, pH 2; 0, 10 mmol dm-3 THB, pH 12; [, 36 mmol dm-3 HQ, pH 2; ], 36 mmol dm-3 THB, pH 2. (b) Variation of COD with the specific electrical charge passed (sulfate media; T ) 25 °C). 9, 10 mmol dm-3 Ph, pH 2, 30 mA cm-2; 2, 10 mmol dm-3 HQ, pH 2, 60 mA cm-2; 0, 1.1 mmol dm-3 THB, pH 12, 30 mA cm-2. Solid line, simulation with a theoretical model.8

oxidation processes in this model may occur directly on the electrode surface or can be mediated by peroxodisulfate and/or by other inorganic electrogenerated reagents at the anode surface. The aromatic ring cleavage is faster than the oxidation of carboxylic acids. This allows justification of the small measured concentrations of aromatic intermediates and the higher measured concentrations of carboxylic acid intermediates. To conclude this work, the effect of the different parameters on the current efficiencies is studied. Figure 11a shows the ICE vs COD graph for several experiments. As can be observed, obtained data match previously published models8,25,38,39 in which it is assumed that ICE depends only on the controlling mechanisms (mass transfer or electrode kinetics). Thus, two zones can be distinguished as a function of the value of COD: a kinetically controlled zone for high values of COD corresponds to ICE values close to 1, and a diffusion controlled zone corresponds to ICE values decreasing

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linearly to zero. Figure 11b shows the results of a simulation using this model. As can be observed, this model satisfactorily predicts the oxidation of phenolic compounds on diamond anodes, independently of waste characteristics (organic compound, concentration, and pH) and of the operating conditions (current density). Conclusions The following conclusions can be drawn from the work described here: (i) Electrochemical oxidation using a diamond thinfilm anode can be successfully used for treating aqueous wastes polluted with polyhydroxybenzenes. The total mineralization of the waste is obtained, regardless ofthe current intensity, initial concentration, and temperature. (ii) Carbon dioxide is the sole final product in the electrochemical treatment of the tested compounds with BDD anodes. The main intermediates are carboxylic acids C4 and C2. (iii) The instantaneous current efficiency depends only on the controlling mechanism (mass transfer or electrode kinetics). The current efficiency of the process is 1 if it is kinetically controlled and decreases linearly to 0 from the COD limit value if it is diffusion controlled. (iv) Both direct and mediated electrochemical oxidation processes are involved in the electrochemical treatment of polyhydroxybenzenes with BDD anodes. Sulfates contained in the waste favor the formation of electrogenerated reagents (persulfates). (v) Taking into account the information obtained in previous works and the results of the voltammetric and galvanostatic studies, a simple mechanistic model is proposed. According to this model, the electrochemical treatment of phenol aqueous wastes leads to the formation of polyhydroxybenzenes (e.g., hydroquinone, catechol), which are oxidized first to carboxylic acids (maleic and oxalic acids mainly) and later to carbon dioxide. The aromatic ring cleavage is faster than the oxidation of carboxylic acids. This allows justification of the small measured concentrations of aromatic intermediates and the higher measured concentrations of carboxylic acid intermediates. Acknowledgment This work was supported by the MCT (Ministerio de Ciencia y Tecnologı´a, Spain) and by the EU (European Union) through Project No. REN2001-0560. Literature Cited (1) Papouchado, L.; Sandford, R. W.; Petrie, G.; Adams, R. N. Anodic Oxidation Pathways of Phenolic Compounds. J. Electroanal. Chem. 1975, 65, 275. (2) Sharifian, H.; Kirk, D. W. Electrochemical Oxidation of Phenol. J. Electrochem. Soc. 1986, 133, 921. (3) Comninellis, Ch.; Pulgarin, C. Anodic Oxidation of Phenol for Waste Water Treatment. J. Appl. Electrochem. 1991, 21, 703. (4) Leffrang, U.; Ebert, K.; Flory, K.; Galla, U.; Schmieder, H. Organic Waste Destruction by Indirect Electrooxidation. Sep. Sci. Technol. 1995, 30, 1883. (5) Boudenne, J.-L.; Cerclier, O. Performance of Carbon BlackSlurry Electrodes for 4-Chlorophenol Oxidation. Water Res. 1999, 33, 494. (6) 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.

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Received for review March 12, 2004 Revised manuscript received May 27, 2004 Accepted June 8, 2004 IE049807G