Electrooxidation of Brown-Colored Molasses Wastewater. Effect of the

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Ind. Eng. Chem. Res. 2009, 48, 1298–1301

Electrooxidation of Brown-Colored Molasses Wastewater. Effect of the Electrolyte Salt on the Process Efficiency P. Can˜izares,† M. Herna´ndez,‡ M. A. Rodrigo,*,† C. Saez,† C. E. Barrera,‡ and G. Roa‡ Department of Chemical Engineering, Facultad de Ciencias Quı´micas, UniVersidad de Castilla La Mancha, Campus UniVersitario s/n, 13071 Ciudad Real, Spain, and Facultad de Quı´mica, UniVersidad Auto´noma del Estado de Me´xico, Paseo Colo´n intersection Paseo Tollocan S/N, C.P. 50120 Toluca, Estado de Me´xico, Mexico

In this work, synthetic melanoidin wastes have been treated with conductive-diamond electrochemical oxidation in a bench-scale plant. The results show a significant influence of the current density and also of the electrolyte salts on the efficiency of the treatment. The electrochemical oxidation of the melanoidins in the presence of carbonate or perchlorate does not lead to the complete mineralization of the organics, but to the accumulation of refractory compounds. Conversely, the oxidation of these synthetic wastes in the presence of chlorine, sulfates, or phosphates yields the complete removal of the chemical oxygen demand and total organic carbon of the wastes. In addition, the efficiency (amount of melanoidin removed per Ah) also depends on the electrolyte salt. Thus, the presence of chloride and phosphate anions allows better results to be obtained. These observations have been explained taking into account the important role of the mediated oxidation carried out by the oxidizing electrogenerated agents (peroxodisulfate, peroxodiphosphate, hypochlorite,...) from the oxidation of the electrolyte salts. The results obtained as a function of the current density also confirm the important role of electrogenerated oxidants on the treatment efficiency. Introduction Melanoidins, natural condensation products of sugars and amino acids, are nitrogenous polymers and copolymers produced by nonenzymatic browning reactions (Maillard reactions).1 They are discharged in huge amounts by various agro-based industries, especially cane-molasses-based distilleries and several types of fermentation factories.2 The complexity of these wastewaters makes difficult the development of effective technologies for removing their color and organic content. Nevertheless, during recent years several methods have been studied including coagulation-flocculation,3 adsorption,4,5 and biological treatments using fungi.6,7 Due to different problems (i.e., lack of nutrients in the case of biological treatments, high content of soluble compounds in the case of coagulation, etc.), these processes usually show low efficiencies, and they also used to be quite expensive. As a consequence, presently, the treatment of these wastes is a nonsolved problem, and other treatment technologies such as ozonation,8 Fenton oxidation,9 and electroFenton oxidation10 can be proposed as suitable alternatives. In this context, electrochemical oxidation is a promising technique for wastewater treatment.11-13 In recent years, many studies have been carried out on the electrochemical treatment of organic compounds, and several anode materials have been tested.14-17 However, several of them have shown rapid loss of efficiency due to surface fouling, while others can only oxidize selectively the pollutants (Ti/IrO2, Pt), resulting in the accumulation of large amounts of oxidation-refractory organics. Complete mineralization of organics to CO2 has only been obtained using high oxygen overvoltage anodes, such as SnO2,18 PbO2,19 and boron-doped diamond.20,21 In this context, conductive-diamond thin-film electrodes are emerging as excellent materials for several applications, such as electrosynthesis, * To whom correspondence should be addressed. Tel.: +34 926902204100, ext 3411. Fax: +34 926295318. E-mail: manuel.rodrigo@ uclm.es. † Universidad de Castilla La Mancha. ‡ Universidad Auto´noma del Estado de Me´xico.

electroanalysis, waste treatment,22 etc. These electrodes present some useful properties including high resistance to corrosion, high thermal stability, hardness, and good electrical conductivity.23 These electrodes are also known to produce hydroxyl radicals from the water discharge on their surfaces.24,25 This radical is a very powerful oxidant (E0 ) 2.80 V versus SHE) which usually leads to a very effective oxidation process. In addition, the global oxidation process with conductive-diamond anodes is known to be complemented by direct electrooxidation on the surface and also by mediated oxidation with other oxidants electrogenerated on the electrode surface from the electrolyte salts.26 This work is focused on the study of the conductive-diamond electrochemical oxidation (CDEO) of synthetic wastes containing melanoidins as a possible answer to an actual industrial problem. In it, we also try to increase knowledge about the oxidation mechanisms involved in CDEO of melanoidins (model of a complex pollutant). To do this, the influence of the electrolyte media and current density on the process efficiency has been analyzed. Experimental Section Preparation of Synthetic Melanoidin. Synthetic melanoidin was prepared by mixing glucose and glycine in equimolar concentration (1 M) in the presence of sodium bicarbonate (0.5 M). The mixture was heated for 7 h at 95 °C, and then the pH value of the sample was adjusted to 7. Milli-Q water was used for dilutions. Its organic load is around 2500 mg dm-3 of chemical oxygen demand (COD) and 1000 mg dm-3 of total organic carbon (TOC). Analytical Procedure. The carbon concentration was monitored using a Shimadzu TOC-5050 analyzer. COD was determined using a HACH DR2000 analyzer. UV-vis spectra were obtained using a Shimadzu 1603 spectrophotometer and quartz cells. Conductive Diamond Electrochemical Oxidation. CDEO assays were carried out in two cell types: a single-compartment27 and a double-compartment electrochemical flow cell working under

10.1021/ie801038t CCC: $40.75  2009 American Chemical Society Published on Web 12/31/2008

Ind. Eng. Chem. Res., Vol. 48, No. 3, 2009 1299 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)

+ 5.6 2.74 108 1300

a batch operation mode. A cationic exchange membrane (STEREOM L-105) was used to separate the compartments in the double-compartment cell.28 In both cases, diamond-based material was used as the anode and stainless steel (AISI 304) as the cathode. Both electrodes were circular (100 mm diameter) with a geometric area of 78 cm2 and an electrode gap of 9 mm. The samples of wastewater were stored in a glass tank (0.6 dm3) and circulated through the electrolytic cell by means of a centrifugal pump (flow rate 2.5 dm3 min-1). A heat exchanger coupled with a controlled thermostatic bath (Digiterm 100, JP Selecta, Barcelona, Spain) 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. Boron-doped diamond 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 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. Bulk Electrolyses. Bench-scale electrolyses under galvanostatic conditions (the current density ranges from 15 to 60 mA cm-2) were carried out. The synthetic melanoidin wastewaters used in the experiments contained a 35 mM concentration of different supporting electrolytes: NaCl, Na2SO4, KH2PO4, HClO4, and HNO3. Results and Discussion Figure 1 shows the changes in the COD, TOC, pH, and cell potential during the CDEO of a synthetic waste containing melanoidins in single- and double-compartment electrochemical flow cells. As can be observed, in spite of the complex structure of the melanoidins, the complete removal of the COD and TOC is obtained in both systems. The use of a double-compartment cell does not lead to an improvement in the removal efficiency, indicating that, in spite of the complex structure of the melanoidins, reversible electrochemical processes are not favored. Thus, the single cell seems to be enough for an efficient process. In addition, the use of an ion exchange membrane in the double-compartment flow cell leads to an increase in the ohmic losses and, thus, in the cell potential. This increase is directly related to the operation cost, and hence, it advices against the use of the double-compartment cell for this particular treatment. Likewise, it is important to note that the cell potential is maintained constant during the electrolysis in the single-compartment cell, indicating that there is no formation of nonconductive layers on the surface of the electrodes or electrode corrosion during the treatment. The strange behavior of the cell potential in the double-compartment cell can be explained by the increase in the ionic conductivity of the anolyte and catholyte during the initial stages of the oxidation caused by important changes in the pH in both zones (initial decrease in the cell potential) and by the later fouling of the membrane (final increase in the cell potential). In this context, it is important to note that the changes in the pH in

Figure 1. Electrolyses of synthetic melanoidins in a single-compartment (SC) and a double-compartment (DC) electrochemical flow cell: (a) changes in COD/COD0 (9, SC; 2, DC) and in the TOC removal (0 SC; 4 DC) removal, (b) variation of pH (9, SC; 2 anolyte DC, 4 catholyte DC), (c) variation of the cell potential (9, SC; 2, DC). Experimental conditions: j ) 30 mA cm-2, T ) 25 °C, NaCl, natural pH.

both systems are very different. The significant changes in the pH of the anolyte and catholyte (double-compartment cell) can be explained by the anodic water oxidation and cathodic water reduction, respectively. In the single-compartment cell, the production of hydroxyl anions on the cathode (from water reduction) can be compensated by the protons generated from water oxidation in the anode (both processes develop in the same compartment). However, it is important to notice that the latter process coexists with the anodic oxidation of organics (and/or inorganic compounds), and consequently, the proton generation rate on the anode is lower than that of hydroxyl anions. Thus, the anions generated in the cathode are only partially compensated, and therefore, the pH increases. In addition to the described processes, the buffer effect of the carbonate/bicarbonate couple and some possible reactions involving functional groups of melanoidins can also be responsible for changes in the pH, and their influence should not be neglected in a detailed study of the changes of the pH. Figure 2 shows, for the single-compartment cell, the changes in the COD/TOC ratio and in the UV-vis spectra during the electrolysis of the synthetic melanoidin waste. It can be observed that the COD/TOC ratio remains almost constant during the first stage of the oxidation (up to 20 Ah dm-3) and then starts to increase. This means that during this first stage (which corresponds to the oxidation of 80% of the initial COD) the main oxidation state of carbon in the organic species existing during the treatment does not change, despite the important decrease in the COD. This can only be explained assuming that all the organic intermediates produced during this first stage of

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Figure 4. Changes in the removal COD during electrolyses in a single compartment of synthetic melanoidins with different supporting electrolytes and current densities: (O) Na2SO4, (0) NaCl, (4) KH2PO4. Experimental conditions: specific electrical charge passed 10 Ah dm-3, T ) 25 °C, natural pH.

Figure 2. Changes in the (COD/COD0)/(TOC/TOC0) ratio (a) and absorbance (b) with the specific electrical charge passed during electrolyses in a single compartment of synthetic melanoidin wastewaters. Onset of part b: absorbance of melanoidin at 285 nm of wavelength. Experimental conditions: j ) 30 mA cm-2, T ) 25 °C, NaCl; natural pH.

Figure 3. Changes in COD/COD0 during electrolyses in a single compartment of synthetic melanoidins with different supporting media: (*) carbonates, (O) carbonates + Na2SO4, (0) carbonates + NaCl, (4) carbonates + KH2PO4, (b) carbonates + HClO4, (2) carbonates + HNO3. Experimental conditions: j ) 30 mA cm-2, T ) 25 °C, natural pH.

the CDEO are rapidly oxidized to carbon dioxide. This observation is confirmed by the UV-vis spectra (part b of the figure) in which the appearance of additional peaks during the treatment is not observed. Likewise, the absorbances for any wavelength fall proportionally to the COD. Hence, it can be assumed that once the CDEO starts the oxidation of a melanoidin molecule remains up to the formation of carbon dioxide. This has to be explained taking into account that oxidation is developed mainly on the nearness of the electrode surface where very large amounts of oxidants are formed. The selectivity of these oxidants should be very small, and they attack indiscriminately all the intermediates formed and restrain the transfer of intermediates from this reaction zone to the bulk. The efficiency of CDEO is known to be strongly dependent on the type of supporting electrolyte. It is well-known that different oxidizing agents can be formed after the oxidation of the anions or cations existing in the waste, and these new oxidizing species can have an important role in the results of the electrolytic treatment. Figure 3 shows the influence on the treatment results of the use of different electrolytes. The first observation that can be noticed is the significant influence of the type of electrolyte on the

formation of oxidation-refractory organics during the treatment. The CDEO of wastes containing carbonates or perchlorates as electrolytes does not lead to the complete removal of the COD but to the accumulation of organics (around 20% of the initial COD). However, when Na2SO4, NaCl, or KH2PO4 is added as a supporting electrolyte, the complete removal of the organic load is obtained. The differences observed not only are important in the extension of the treatment, but also are significant in the oxidation rates and efficiencies. Thus, it can be observed that the electrochemical treatment of melanoidins in NaCl or KH2PO4 medium is more effective than in other electrolytic media. In the first stages of the oxidation process, Na2SO4 medium also seems to be effective, although from a given electrical charge passed the global oxidation rate decreases significantly and the removal efficiency becomes lower. These observations are important because they show that the hydroxyl radicals produced by CDEO are not enough to reduce completely the COD of the waste and that the contribution of other oxidizing agents produced during the CDEO from the electrolyte salts is not negligible. In fact, in the literature it is proposed that the CDEO of aqueous solutions containing chloride, sulfate, phosphate, and carbonate anions promotes the production of hypochlorite,29 peroxodisulfates,30 peroxodiphosphates,28,31,32 and peroxocarbonates.33 These chemical species are very powerful oxidants with high standard reduction potentials. However, their particular effect on a given organic does notdependontheoxidationpotentialbutonkineticconsiderations,34-37 and it can be evaluated from the experimental behavior of the CDEO. Thus, the obtained results seem to indicate the low oxidizability of melanoidins by peroxocarbonates and by peroxodisulfates, perchlorate, or nitrate. Hence, in these cases the oxidation of melanoidins should mainly be caused by direct oxidation or hydroxyl radical mediated oxidation.25,26 Conversely, the oxidation of these nitrogenous polymers by hypochlorite and peroxodiphosphate seems to be very favored. In these cases, the direct and the hydroxyl radical mediated oxidations are efficiently complemented by the oxidation with these powerful oxidants. One of the most important operation parameters in the electrolysis of wastes is the current density. In Figure 4, the effect of this parameter on the results for three electrolytic media which allowed the complete oxidation of melanoidins to be obtained can be observed. To compare all results in the same graph, the COD removal for an arbitrarily selected charge passed of 10 Ah dm-3 is shown. This current charge was selected taking into account that at this particular charge the decay in the COD with the charge was still linear and, hence, the rate was not mass-transfer-controlled. As can be observed, two very different behaviors can be discerned: in the electrolysis of the sulfatecontaining waste the results do not depend on the current density,

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while in the electrolyses with phosphate and chloride salts the higher the current density the higher the removal. Conclusions The conductive-diamond electrochemical oxidation in both single- and double-compartment flow cells can be successfully used for the treatment of molasses wastewater. The high cell potential advises against the use of the double-compartment flow cell. The variations of the COD and TOC removals and of the UV-vis spectrum seem to indicate that the organic intermediates produced during this first stage of the CDEO are rapidly oxidized to carbon dioxide. The type of electrolyte shows a marked influence on the formation of oxidation-refractory organics during the treatment. The use of carbonate and perchlorate solutions as electrolytes favors the accumulation of organics (around 20% of the initial COD). Conversely, electrolysis in the presence of phosphate and chloride salts allows the complete removal of the organic load. This fact indicates the importance of the contribution of the oxidants produced from the oxidation of the electrolyte salts. The results show the high oxidizability of melanoidin by hypochlorite and peroxodiphosphate. Percarbonate, peroxodisulfate, and perchlorate seem to be less effective. Acknowledgment The financial support of the Spanish government through the project CONSOLIDER-INGENIO 2010 (Grant CSD2006-0044) is gratefully acknowledged. This work was also supported by the JCCM (Junta de Comunidades de Castilla La Mancha, Spain) through Project PCI-08-068. Literature Cited (1) Plavsˇic´, M.; C´osovic´, B.; Lee, C. Copper complexing properties of melanoidins and marine humic material. Sci. Total EnViron. 2006, 366, 310. (2) Kumar, P.; Chandra, R. Decolourisation and detoxification of synthetic molasses melanoidins by individual and mixed cultures of Bacillus spp. Bioresour. Technol. 2006, 97, 2096. (3) Migo, V. P.; Matsumura, M.; del Rosario, E. J.; Kataoka, H. Decolorization of molasses wastewater using an inorganic flocculant. J. Ferment. Bioeng. 1993, 75, 438. (4) Bernardo, E.C.; Egashira, R.; Kawasaki, J. Decolorization of molasses wastewater using activated carbon prepared from cane bagasse. Carbon 1999, 35, 1217. (5) Figaro, S.; Louisy-Louisa, S.; Lambertb, J.; Ehrhardt, J.; Ouensanga, A.; Gaspard, S. Adsorption studies of recalcitrant compounds of molasses spentwash on activated carbons. Water Res. 2006, 40, 3456. (6) Miyata, N.; Mori, T.; Iwahori, G. K.; Fujita, M. Microbial decolorization of melanoidin-containing wastewaters: combined use of activated sludge and the fungus Coriolus hirsutus. J. Biosci. Bioeng. 2000, 89, 145. (7) Kumar, P.; Chandra, R. Decolourisation and detoxification of synthetic molasses melanoidins by individual and mixed cultures of Bacillus spp. Bioresour. Technol. 2006, 97, 2096. (8) Coca, M.; Pen˜a, M.; Gonza´lez, G. Variables affecting efficiency of molasses fermentation wastewater ozonation. Chemosphere 2005, 60, 1408. (9) Pala, A.; Erden, G. Decolorization of a baker’s yeast industry effluent by Fenton oxidation. J. Hazard. Mater. 2005, B127, 141. (10) Yavuz, Y. EC and EF processes for the treatment of alcohol distillery wastewater. Sep. Purif. Technol. 2007, 53, 135. (11) 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. (12) Can˜izares, P.; Paz, R.; Lobato, J.; Sa´ez, C.; Rodrigo, M. A. Electrochemical treatment of the effluent of a fine chemical manufacturing plant. J. Hazard. Mater. 2006, B138, 173. (13) Can˜izares, P.; Lobato, J.; Paz, R.; Rodrigo, M. A.; Sa´ez, C. Advanced oxidation processes for the treatment of olive-oil mills wastewater. Chemosphere 2007, 67, 832. (14) 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.

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ReceiVed for reView July 4, 2008 ReVised manuscript receiVed October 3, 2008 Accepted October 27, 2008 IE801038T