Electrochemical incineration of glucose

The possibility of eliminating organic pollutants from industrial wastes, by anodic mineralization or “incineration” has been discussed in recent years (1-3). 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 (3). 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. Both electrodes have however short service life. Accordingly, it may be of interest to investigate the possibility to use stable anodes, like galvanic Pt, IrO2-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 (4). An important drawback of electrolytic treatments in chloride solutions may be the formation of chlorinated species. 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 glucose has been chosen as a model substrate, of interest considering that saccharides are important components in food industry effluents, like for instance olive-mill wastewaters. Experiments on electro-oxidation of glucose have been carried out at Ti/Pt and Ti/PbO2 elec-trodes, in absence and in presence of NaCl. In the presence of NaCl, at Ti/Pt electrodes complete mineralization of glucose could be achieved at current densities > of 500 A m-2. 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. References 1) S. Stucki, R. Kötz, B. Carcer, W. Suter, J. Appl. Electrochem. 21, 99 (1991) 2) Ch.Comninellis and E. Plattner, Chimia 42, 250 (1988) 3) Ch. Comninellis, Electrochim. Acta 39, 1863 (1994) 4) Ch. Comninellis and A. Nerini, J. Appl. Electrochem. 25, 23 (1995)