The anodic electrochemistry of pentachlorophenol

The aqueous-phase electrochemical oxidation of pentachlorophenol (PCP) was studied. The oxidation was carried out at pyrolytic carbon, glassy carbon, gold, and platinum anodes in pH 6-7 solutions with phosphate and in some cases acetate electrolyte. The potential range studied was from the onset of PCP oxidation to the onset of oxygen evolution. For most of the potential range the reaction was a one-electron oxidation resulting in 2,3,4,5,6-pentachloro-4-pentachlorophenoxy-2,5-cyclohexadienone. The product is essentially insoluble and precipitated on the electrode surface. Various thicknesses, crystallinities, and patterns of deposit were observed to form, giving rise to different current-time or potential-time responses depending on factors including electrode material, current density, electrolyte, PCP concentration, and reaction time. At very low over-potentials the reaction product became complex and the reaction took place with less than one electron per molecule and with the appearance of some chloride. Under these conditions the reaction is felt to be an electrochemically initiated, chain reaction polymerization. This involves the electrochemical formation of a radical which attacks the pentachlorophenoxalate substrate, forming a radical anion intermediate. This intermediate decomposes, yielding a chloride ion and reforming a neutral radical (though now of higher molecular weight), thus continuing the reaction. The application of such electrochemically initiated condensation reactions for removal of PCP and some other phenols from wastewater is discussed. (C) 1999 The Electrochemical Society. S0013-4651(98)12-006-2. All rights reserved.