Anodic mineralization of organic substrates in chloride-containing aqueous media

The possibility of eliminating organic pollutants from industrial wastes, by anodic mineralization or “incineration” has been discussed in recent years [1-4]. The electrochemical wastewater treatment may be of particular interest when the effluent contains biorefractory organics and/or high amounts of organic carbon and requires some pre-treatment to allow further biological purification. The mineralization process takes place as an extreme case of anodic oxidation, together with the oxygen evolution reaction. The formation of adsorbed hydroxyl radicals is a necessary condition for the oxidative attack of the organic substrate to take place and also for the oxygen evolution [4]. At high oxygen overvoltage anodes, like PbO2, Sb- or F-doped SnO2 , typically the anodic mineralization of organic substrates takes place with better faradaic yields. In the case of lead dioxide, however, problems of service life and of release of lead ions in the treated effluent, may represent serious drawbacks in a practical application. For tin dioxide-based electrodes, the short service life is again an inconvenience. While the improvement of these anodes is under investigation in several research groups, it may be of interest to investigate the possibility to use stable anodes, like galvanic platinum, iridium dioxide-based DSA’s®, improving the faradaic yield of the electrochemical mineralization. This aim may be achieved using inorganic mediators of the oxidation of the organic substrate. In this respect active chlorine may be of particular interest, and has been discussed for the case of electrochemical mineralization of phenol [5,6]. In the presence of chlorides the electrochemical treatment can be carried out at much lower potentials, compared with those required for the non-mediated (direct) anodic oxidation. Galvanic Pt and other DSA’s materials based on iridium and ruthenium oxides, can be used, the optimal choice depending on pH and other compositional features of the effluent. An important drawback of electrolytic treatments in chloride solutions may be the formation of chloroderivatives of the organic substrates and of their oxidation products. In such cases an electrochemical treatment would result in an increase in toxicity of the wastewater and, possibly, also in stability of the residual chlorinated substrates. Accordingly, it is important to study the mechanism of oxidative degradation of different organic molecules, in different chloride-containing media, and at different anodes, to find optimal conditions for the electrochemical treatment, ensuring good faradaic yields for mineralization process, avoiding the formation of chlorocompounds. In the present work two model substrates have been studied: glucose and phenol. The first may be of interest considering that saccharides are important components in effluents from food industry (e.g.: olive mill wastewaters). The second has been studied in different researches on electrochemical abatement of organic pollutants and many results are available for comparison, including some aspects of its reactivity in chloride media. Experiments on electro-oxidation of glucose have been carried out at Ti/Pt and Ti/PbO2 electrodes, in presence of NaCl concentrations between 0.5 and 5 g dm-3. The substrate, in COD units, was 10.000 mg O2 dm-3 have been used, in consideration of the high COD values nor-mally met in effluents like olive-mill ones. The initial solution pH was 12. Electrolyses were carried out at different temperatures between 15 and 50°C. At Ti/Pt electrodes complete mineralization of glucose could be achieved at current densities > of 500 A m-2, under different electrolysis conditions.The mineralization rate was larger the lower the electrolysis temperature. The mineralization rate of glucose was found to increase with the NaCl concentration for values between 0.5 and 3 g dm-3. Only a small influence could be observed above this value. At Ti-PbO2 electrodes a substantially similar situation was found. At Ti/Pt electrodes, in the absence of NaCl, the abatement of COD was very small even after long electrolysis times. At PbO2, on the contrary, complete mineralization was achieved, even without the active chlorine mediator. Interestingly, a treatment of glucose solutions with sodium hypochlorite allowed a decreas of COD to only one half of its initial value. An important part of the overall reaction is therefore a surface stage. No organic chlorinated compounds were detected during the electrolysis. At the end of the electrolysis it was found that part of the original chloride was converted to chlorate and hypochlorite. In the first stage of the electrolysis the main intermediate seems to be gluconic acid. Under analogous conditions also the mineralization of phenol (1000 ppm) in alkaline media (pH 13) could be achieved. Two unidentified complex quinonic species were present during the first stages of the electrolysis. As expected from the literature, maleic, fumaric and oxalic acid are then formed. No chlorophenols or organochloro compounds were detected. References 1 S. Stucki, R. Koetz, B. Carcer, W. Suter, J. Appl. Electrochem. 21, 99 (1991) 2 Ch.Comninellis and E. Plattner, Chimia 42, 250 (1988) 3 Ch.Comninellis and C. Pulgarin, J. Appl. Electrochem. 23, 108 (1993) 4 Ch. Comninellis, Electrochim. Acta 39, 1863 (1994) 5 A. Boscolo, F. Gottardi, M. Tavan, R. Amadelli, A. De Battisti, A. Barbieri, G. Battaglin, J. Appl. Electrochem.. 24, 1052 (1994) 6 Ch. Comninellis and A. Nerini, J. Appl. Electrochem. 25, 23 (1995)