Direct electrochemical degradation of organic wastes in aqueous media

The goals of research reported here include the discovery of optimal electrocatalytic materials and electrolysis conditions for rapid degradation of organic compounds to CO2(g). Concepts of the designated procedure are summarized with emphasis on the carboxylic acids generated as intermediate products during anodic degradation of 4-chlorophenol and benzoquinone. Data are summarized for various electrode materials including oxide-covered Pt, as well as PbO2 and SnO2 films that are heavily doped with altervalent metallic cations. The anodic degradation reactions have in common the fact that they involve transfer of O-atoms from H2O in the solvent phase to the oxidation products. Therefore, variations in reactivity of electrode materials are ascribed to differences in the extent of electrocatalytic participation of these electrode surfaces within the anodic O-transfer mechanisms. Results are explained on the basis of the following mechanistic speculations: (1) a prerequisite of the required anodic O-transfer reactions is the discharge of H2O to generate adsorbed hydroxyl radicals; (2) a co-requisite is preadsorption of the reactant species; (3) the O-transfer step occurs from (OH)(ads) to (R)(ads); and (4) an inevitable but undesirable concomitant reaction is the anodic evolution of O-2(g). The expected benefit of elevated anode temperature is verified for degradation of acetic acid at a Pt anode. Also, cyanuric acid generated as a stable product of photocatalytic degradation processes is demonstrated to be degraded by anodic reaction at a Pt electrode. Recovery of nitrate is approximately 70% during degradation of cyanuric acid and it is speculated that some volatile N-species is lost during the procedure. (C) 2000 Elsevier Science Ltd. All rights reserved.