Electrochemical Oxidation of Azoic Dyes with Conductive-Diamond

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Electrochemical Oxidation of Azoic Dyes with Conductive-Diamond Anodes P. Can˜ izares,† A. Gadri,‡ J. Lobato,† B. Nasr,§ R. Paz,† M. A. Rodrigo,*,† and C. Saez† Department of Chemical Engineering, Facultad de Ciencias Quı´micas, UniVersidad de Castilla La Mancha, Campus UniVersitario s/n, 13071 Ciudad Real, Spain, Department de Chimie Industrielle, Institut Supe´ rieur des Sciences Appliquee´ s et Technologie de Gabe` s, UniVersite´ de Gabe` s, 6072 Zrig, Gabe` s, Tunisie, and Chemistry SectionsSciences Department, Sur College of Education, B.P. 484, Sur 411, Sultanate of Oman

In this work, conductive-diamond electrochemical oxidation of synthetic wastes polluted with azoic dyes is studied. Results show that this advanced oxidation process is able to complete the treatment of wastes polluted with large molecules such as the dyes studied in this work. The oxidation process starts with the breakage of the azoic group, and according to the changes in the total organic carbon and chemical oxygen demand observed during the electrolyses, it deals with the accumulation of carboxylic acids in the final stages of the treatment. Negligible amounts of refractory organic matter are found in the effluent of highly loaded waste treatment. The efficiency of the conductive-diamond electrolyses of azoic dyes does not seem to depend on the molecule of the dye oxidized but only on its concentration range. From the experimental results it seems that the primary mechanisms in the oxidation of dyes are the mediated electrooxidation by hydroxyl radicals and persulfate (highly loaded wastes) and by persulfate (diluted wastes). Introduction Conductive diamond is an emergent material with good properties for the electrochemical treatment of wastewaters polluted with organic compounds. In recent years, a great experimental effort has been made to characterize this material as anode, including laboratory-scale electrochemical-fundamentals studies, in which the properties of the conductive diamond have been clarified, and bench-scale electrolytic studies of the treatability of synthetic wastewaters polluted with different organics such as phenolic compounds, carboxylic acids, cyanides, surfactants, and herbicides, etc. From these studies,1-8 it was concluded that the use of conductive-diamond electrodes allows one to obtain high current efficiencies in the treatment of organics and also the almost complete mineralization of the organics. These facts have been related with the generation of hydroxyl radicals on the conductive-diamond surface.9 In this sense, the production of significant amounts of hydroxyl radicals allows one to consider conductive-diamond electrochemical oxidation as an advanced oxidation process (AOP). Other important properties of conductive-diamond electrodes (CDEO) are their great chemical and electrochemical stability, which enhance the average lifetime of this anodic material and allow its use in the treatment of almost any kind of wastewater. Compared to other AOP, CDEO is known to be able to mineralize almost all the organic content of a wastewater (normally nonrefractory compounds are found) with a higher efficiency. Several mechanisms4,9,10 have been proposed to justify the strong oxidative conditions found in the electrochemical treatment of organics with conductive-diamond anodes. These include direct electrooxidation on the anode surface, the previously described oxidation mediated by hydroxyl radicals, and also oxidation mediated by other oxidants electrogenerated from the electrolyte salts (0.1 peroxosulfates, peroxophosphates). * Corresponding author. Tel.: +34 926902204100 x3411. Fax: +34 926295318. E-mail: [email protected]. † Universidad de Castilla. ‡ Universite´ de Gabe`s. § Sur College of Education.

Figure 1. Structure of azoic dyes studied in this work.

The goal of the work described here is to increase the understanding of the oxidation of organics with conductivediamond electrochemical oxidation. To do this, the electrochemical oxidation of three azoic dyes (eriochrome black T, methyl orange, and Congo red) has been studied. The main characteristics of these dyes are shown in Figure 1. These dyes are colored substances with a complex chemical structure (many functional groups) and a high molecular weight. Consequently, they can also be used as model compounds of large-molecule pollutants.11-13 These compounds are also highly soluble in water and persistent, once discharged into a natural environment. Thus, their removal from industrial effluents is also a subject of major importance from the environmental point of view.14 Experimental Section Analytical Procedure. The carbon concentration was monitored using a Shimadzu TOC-5050 analyzer. Chemical oxygen demand (COD) was determined using a HACH DR2000

10.1021/ie051427n CCC: $33.50 © 2006 American Chemical Society Published on Web 04/12/2006

Ind. Eng. Chem. Res., Vol. 45, No. 10, 2006 3469 Table 1. Characteristic of the BDD Electrode Used in This Work parameter

value

Scotch test adhesion electrical resistance (Ω) BDD-film thickness (µm) BDD-film Raman sp3/sp2 BDD-film boron concentration (ppm) CALT (kA‚h‚cm-3)

+ 5.6 2.74 108 1300 50

analyzer. UV-visible spectra were obtained using a Shimadzu 1603 spectrophotometer and quartz cells. Determination of the Average Current Efficiency and Instantaneous Current Efficiency. The average current efficiency (ACE) and the instantaneous current efficiency (ICE) were calculated15 using eqs 1 and 2, respectively:

[COD0 - CODt]FV 8It

(1)

[CODt - CODt+∆t]FV 8I∆t

(2)

ACE ) ICE )

where COD0, CODt, and CODt+∆t are the initial chemical oxygen demand (in g of O2 dm-3) and the chemical oxygen demand 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), t is the time (in seconds), V is the volume of the electrolyte (dm3), and 8 is a dimensional factor for unit consistence (32 g ‚mol-1 O2/(4 mol‚mol-1 O2)). Conductive Diamond Electrochemical Oxidation. The oxidation of the different dyes was carried out in a singlecompartment electrochemical flow cell, previously described in other works of our group.8 A diamond-based material was used as the anode, and stainless steel (AISI304) was used as the cathode. Both electrodes were circular (100 mm in 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 cm3) and circulated through the electrolytic cell by means of a centrifugal pump. A heat exchanger was used to maintain the temperature at 25 °C. The experimental setup also contained a cyclone for a gas-liquid separation, as well as a gas absorber to collect the carbon dioxide contained in the gases evolved from the reaction into sodium hydroxide. Boron-doped diamond films were provided by CSEM (Neuchaˆtel, 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 Siltronise). The main characteristics of the BDD used in this work are summarized in Table 1. Electrolyses were carried out in galvanostatic mode. During the electrolyses no control of pH was carried out. The cell potential was constant during each electrolysis reaction, indicating that appreciable deterioration of the electrode or passivation phenomena did not take place. The electrolyte flow rate through the cell was 1250 cm3‚min-1. The linear velocity of the fluid was 2.31 cm‚s-1, and the space velocity was 2.08 min-1. Voltammetry Experiments. Electrochemical measurements were obtained using a conventional three-electrode cell in conjunction with a computer-controlled potensiostat/galvanostat (Auto lab model PGCTAT 30, Ecochemie B.V, Utrecht, The Netherlands). Diamond was used as the working electrode, Hg/ Hg2Cl2‚KCl (saturated) as a reference electrode and stainless steel (AISI304) as counter electrode. Voltammetric experiments were performed in unstirred solutions (200 mL). The anode was anodically polarized for 10 min with a 1 M H2SO4 solution at 0.1 A prior to each experiment.

Figure 2. Changes in the COD (9) and in the TOC (0) with the current charge passed during electrolyses of EBT-polluted synthetic wastes. Operation conditions: current density, 300 A‚m-2; T, 25 °C. Waste composition: sodium sulfate Na2SO4, 5000 mg‚dm-3; natural pH. COD0: (a) 100 and (b) 1813 mg‚dm-3 O2.

Figure 3. Exponential changes in the COD during the conductive-diamond electrochemical oxidation experiments of EBT. Operation conditions: current density, 300 A‚m-2; T, 25 °C. Waste composition: Na2SO4, 5000 mg‚dm-3; natural pH. COD0: (2) 100, (0) 1184, and (9) 1813 mg‚dm-3O2.

Results and Discussion Figure 2 shows the changes in the COD and total organic carbon (TOC) with the specific electrical charge passed during the electrolyses of synthetic wastes polluted with different initial concentrations of eriochrome black T (EBT). It can be observed that both parameters are satisfactorily reduced during the treatments (almost complete mineralization of the pollutants was obtained) and that the specific electrical charges required to complete the treatment are not proportional to the initial organic load of the waste. The process seems to be more efficient for high initial concentrations of pollutant. During the initial stages, changes in the COD are more abrupt than those in the TOC, as it can be expected taking into account the complex structure of the EBT and the existence of many functional groups. The large molecular weight of the EBT molecule should favor the formation of a great variety of intermediates (changes in the COD) without carbon dioxide formation (changes in the TOC). The COD changes with time in accordance with an exponential law (Figure 3), which can be explained either in terms of masstransfer limitations (for a direct electrooxidation) or in terms of first-order chemical kinetics (for a mediated electrooxidation). Figure 4 shows the changes in the UV-visible spectra of the different samples obtained during one of the electrolyses. This analytical technique can give qualitative information about

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Figure 4. Changes in the UV-vis spectra with the time passed during electrolyses of a EBT-polluted synthetic waste. Operation conditions: current density, 300 A‚m-2; T, 25 °C. Waste composition: Na2SO4, 5000 mg‚dm-3; natural pH. COD0: 100 mg‚dm-3 O2.

the main intermediates and consequently about the oxidation pathways. As it can be observed, the UV-visible spectra present four main bands at 225, 285, 340, and 540 nm. The intensity of the first band increases in the initial stages and later decreases. This observation can be related to the presence of several organic intermediates that should appear at the initial stages of the treatment. The intensities of the three later bands decrease approximately in the same ratios during the treatment, indicating that the EBT is the main species present in the treated waste during an important stage in the batch electrolytic process. The total discoloration of the solution (98%) is obtained after 10 A‚h‚dm-3 (breakage of the azoic group). For this current charge passed, the removals of TOC and COD are of 60 and 87%, respectively. Figure 5 shows the influence of the waste COD and that of the current density in the instantaneous current efficiencies obtained during batch electrolyses of EBT. The obtained results are compared with that obtained using a model that assumes direct electrochemical reaction in the anode surface with efficiencies only limited by mass transport.4,8,9,16 The model trends summarized the results obtained in the treatment of many compounds such as phenol, chlorophenols, nitrophenols, and naphthol, etc., which have been used to validate it. As it can be seen, the efficiencies obtained in the electrolyses of the dye are very low, especially if compared with that obtained for this technology in the treatment of other wastes.8,17-23 This supports the importance of the nature of the pollutant, especially in cases such as in the one studied here which the complexity of the pollutant is high. This observation is especially important for low concentrations of COD in which the differences between the experimental data obtained in this work and the model proposed in the literature are very significant. One important observation is that although the direct electrooxidation model predicts that for a given COD the efficiency that can be obtained decreases linearly with the current density, the experimental data obtained in this work show an important increase. This suggests that in this case the oxidation of organics is carried out by mediated electrooxidation and not by direct electrooxidation in the anode surface. Figure 5b shows the energy consumption required to remove the COD as a function of the COD removal percentage. As expected (taking into account the lower efficien-

Figure 5. Influence of the current density in the efficiency of the process (Na2SO4, 5000 mg‚dm-3; natural pH; T, 25 °C). (a) Changes in the instantaneous current efficiency (ICE) with the residual COD during electrolyses: (4) 600 and (9) 300 A‚m-2 of current density. (b) Energy consumption during electrolyses assays versus the COD removal: (9) 1184 and (0) 1813 mg‚dm-3 O2.

cies obtained), the values obtained are higher that other values proposed in the literature21 for slighter pollutants (range of 35-50 kW‚h‚kg-1 COD removed). Likewise and despite the greater cell voltage, the use of high current densities does not affect significantly the energy consumption required to remove 1 kg of COD, due to the higher efficiency of the process. To try to obtain more information about the mechanisms (direct or mediated oxidation) of the oxidation process, several voltammetric measurements were carried out. Thus, Figure 6a shows a cyclic voltammogram of an aqueous solution containing EBT (100 mg‚dm-3) and NaSO4 (5000 mg‚dm-3). As it can be observed, the presence of ETB leads to the appearance of an anodic oxidation shoulder at values of potential close to those

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Figure 7. Percentage of COD removal vs the ratio of specific electrical charge passed/stoichiometrical requirements of the current. Operation conditions: current density, 300 A‚m-2; T, 25 °C. Waste composition: Na2SO4, 5000 mg‚dm-3; natural pH. EBT, COD0: ([) 100, (×) 1184, and (2) 1813 mg‚dm-3 O2. CR, COD0: (O) 225, (+) 729, and (4) 1292, MO, COD0: (b) 183, (0) 706, (9) 1268 mg‚dm-3 O2.

Figure 6. Cyclic voltammograms on BDD anodes of EBT (100 mg‚dm-3) and NaSO4 (5000 mg‚dm-3) solutions: (a) five consecutive cycles.; (b) comparison of the first cycle (1) with the cycle (2) after anodic polarization (5 min at a potential of 2.8 V vs SCE). Scan rate: 100 mV‚s-1. Counter electrode: Pt. Reference electrode: SCE.

of the water decomposition. This shoulder can be explained in terms of a peak very close to the oxygen evolution potential region and partially overlapped with this process. This peak indicates the existence of a direct electrochemical reaction in the potential region of stability of the electrolyte. The peak decreases in size in the second scan, and the voltammograms are superposed in later cycles, suggesting the formation of polymers by this reaction. As it can be observed in Figure 6b, the polarization of the electrode during 5 min at a potential of 2.8 V vs SCE (over water oxidation potential) allows the former shape to be obtained again. This suggests that hydroxyl radicals formed during the oxidation of water (or other more-stable mediated electroreagents such as peroxosulfates formed directly on the surface24 or by hydroxyl-radical oxidation) can oxidize the polymer formed on the BDD surface. In this context, it has been recently demonstrated the role of the hydroxyl radicals in the CDEO.9 Likewise, it also indicates that the mechanisms involved in the direct and in the mediated electrooxidation with conductive diamond are different, being more severe than those occurring in the supporting electrolyte unstability region (mediated oxidation processes). The peak obtained behaves similarly to others obtained in voltammograms of aromatic compounds,25-27 and in these cases it was related to the formation of aromatic radicals which later could couple to yield polymeric materials. Thus, this should not be interpreted in terms of the oxidation of a given functional group but on the oxidation of the aromatic rings of the molecules of azoic colorants. Results similar to those described for the electrolyses of EBT are also observed for the electrolyses of Congo red (CR) and methyl orange (MO) (the other azoic dyes studied in this work), and consequently the behavior seems to be characteristic of the oxidation of the azoic dyes. The almost complete mineralization of the pollutants confirms conductive-diamond electrochemical oxidation as a very promising technology for the treatment of almost any kind of pollutant contained in wastewaters. Figure 7 shows the percentage of COD removal vs the ratio of thencurrent charge passed/stoichiometric requirements of the current. No differences can be observed between the electrolyses of the different azoic dyes tested in this work when they are

compared with those of the same range of initial concentration. Only in one case (Congo red initial concentration of 1295 mg‚dm-3 O2) the electrolysis is not able to reduce completely the organic load (maximum removal about 85%). However two different and strongly marked behaviors can be observed as a function of the concentration (continuous line in the figure): for a concentration higher than 500 mg‚dm-3 COD the concentrations do not seem to have a great influence in the electrolyses results, while for a lower concentration the electrolytic process becomes less efficient. This behavior can be explained in terms of the controlling mechanisms of the electrochemical process. In the electrolyses of highly concentrated wastes the process can be considered to be kinetically controlled during an important period of the treatment. The generation of oxidants in the zone close to the anode surface is not enough to oxidize all the dye molecules which arrive at this zone. In these conditions direct electrooxidation, hydroxyl radicals mediated oxidation, and oxidation carried out by other oxidants electrogenerated in the system are favored, and consequently high efficiency is obtained. On the contrary, for diluted wastes the process is mass-transfer-controlled. The amount of oxidants formed is very high in comparison with the amount of dyes which arrives at the anode surface. The short lifetime of hydroxyl radicals favors the formation of hydrogen peroxide or oxygen with the subsequent reduction in the efficiency. The direct electrochemical process is also disfavored, and water oxidation becomes in the more important process. In these conditions the only oxidizing agent that can be formed from the salts present in the electrolyte (sodium sulfate) is persulfate. This oxidant can act in both the zone close to the anode surface and in the remaining reaction volume with a efficiency similar to that in the case of highly polluted waste. Consequently the global efficiency of the process decreases. Figure 8 shows the average current efficiencies obtained in the electrolysis of wastes polluted with different concentrations of the three azoic dyes studied in this work for an arbitrarily selected specific electrical charge passed (20 A‚h‚dm-3). It can be observed that during a given electrolysis the efficiency seems to depend only on the initial concentration of pollutants in the waste and not on the nature of the dye. Finally, Figure 9 shows the energy required to reduce the COD of a waste polluted with a dye (concentrations higher that 500 mg‚dm-3 COD) as a function of the COD removal percentage. All the experimental data lay over the same curve. In this curve two zones can be distinguished. The energy requirements increase linearly up to an 80% removal, and then they start to change exponentially. This means that this technique

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can be used in an economically adequate way for a pretreatment of the waste and that although a big removal of COD is feasible to be obtained from the technical point of view, the energy cost makes this technique unsuitable for a refining process. Acknowledgment The authors want to thank the Spanish and the Tunisian Governments for financial support (AECI Joint Project 34/04/ P/E). Figure 8. Average current efficiency (ACE) obtained during the electrolyses of the different dyes studied in this work (for an arbitrarily chosen specific electrical charge of 20 A‚h‚dm-3) vs initial COD of the waste. Operation conditions: current density, 300 A‚m-2; T, 25 °C. Waste composition: Na2SO4, 5000 mg‚dm-3; natural pH. (9) EBT; (0) CR; (2) MO.

Figure 9. Specific energy consumption during electrolyses of dyes. Operation conditions: current density, 300 A‚m-2; T, 25 °C. Waste composition: Na2SO4, 5000 mg‚dm-3; natural pH. EBT, COD0: (9) 1184 and (0) 1813 mg‚dm-3 O2. CR, COD0: ([) 729 mg‚dm-3 O2. MO, COD0: (4) 706 and (2) 1268 mg‚dm-3 O2.

can be used in an economically adequate way for a pretreatment of the waste and that although a big removal of COD is feasible to be obtained from the technical point of view, the energy cost makes this technique unsuitable for a refining process. Conclusions From this work the following conclusions can be drawn: (a) Conductive-diamond electrochemical oxidation is able to complete the treatment of waste polluted with large molecules such as the dyes studied in this work. The oxidation process starts with the breakage of the azoic group, and according to the changes in the TOC and COD during the electrolyses, it deals with the accumulation of carboxylic acids in the final stages of the treatment. Negligible amounts of refractory organic matter are found in the treatment of highly loaded wastes. (b) The efficiency of the conductive-diamond electrolyses of azoic dyes does not seem to depend on the molecule of the dye oxidized but only on its concentration range. (c) The specific electrical charges required to complete the treatment are not proportional to the initial organic load of the waste. Consequently, the process seems to be more efficient for high initial concentrations of pollutant. This can be explained in terms of the oxidation mechanisms that happen inside the electrochemical cell. In this context, from the experimental results it seems that the primary mechanisms in the oxidation of dyes are the mediated electrooxidation by hydroxyl radicals and persulfate (highly loaded wastes) and by persulfate (diluted wastes). (d) During the electrolyses of the dyes, the energy requirements increase linearly up to an 80% removal and then they start to change exponentially. This means that this technique

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ReceiVed for reView December 22, 2005 ReVised manuscript receiVed March 8, 2006 Accepted March 16, 2006 IE051427N