Electrochemical oxidation of phenols

Abstract

Electrochemical oxidation of chlorinated phenols was studied. DOI: 10.1134/S1070427207080198 Chlorinated phenols are superecotoxicants exerting mutagenic, cancerogenic, and bioaccumulating effect on humans [1]. Their cancerogenic properties become stronger with an increase in the number of chlorine atoms in the molecules. Along with exhibiting intrinsic high toxicity, chlorophenols are direct precursors of substances that are still more hazardous for human health: polychlorinated dibenzodioxins, which are formed by condensation of two chlorophenol molecules of any structure. The main sources of chlorophenols in the environment are pulp-and-paper plants where these compounds are formed in the step of cellulose bleaching and breakdown of lignin; also, chlorinated phenols are intermediates in the production of wood preservatives, dyes, pesticides, herbicides, plastics, etc. There are two ways to reduce the environmental pollution by chlorophenols: implementation of new processes that do not involve formation of these superecotoxicants, which is not always acceptable because of the possible loss of the product quality, and improvement of wastewater treatment processes. The most promising processes for solving this problem are oxidation processes, such as ozonation, catalytic decomposition, and photolysis. Today, growing attention is paid to the development of new combined oxidation methods (AOP, Advanced Oxidation Processes) for treating wastewater containing difficultly oxidizable organic compounds. These methods are based on liquid-phase chain oxidation of impurities by the generated highly reactive activated oxygen species, primarily OH . radicals. The combined methods are potentially cleaner environmentally, because they allow, in principle, complete mineralization of difficultly oxidizable pollutants [2 6]. Previously, we have demonstrated the efficiency of the electrocoagulation method based on the reaction between two or several substances having different electrochemical potentials on passing water and air through their mixture. When a system contains hydrogen peroxide, Fenton’s reagent can be formed, which promotes deep degradation of phenols [7]. It has been found that the extent and rate of phenol oxidation depend on many factors, including the presence and nature of substituents [8]. In this study, we examined the electrochemical oxidation of difficultly oxidizable organic compounds in aqueous solutions (with chlorinated phenols as an example) and considered the possibility of photochemical initiation of radical-chain reactions with simultaneous electrochemical oxidation of iron.