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 ca ses acetate electrolyte. The potential range studied was from the onset of PCP oxidation to the onset of oxygen evolution. For most of the potential r ange the reaction was a one-electron oxidation resulting in 2,3,4,5,6-penta chloro-4-pentachlorophenoxy-2,5-cyclohexadienone. The product is essentiall y insoluble and precipitated on the electrode surface. Various thicknesses, crystallinities, and patterns of deposit were observed to form, giving ris e to different current-time or potential-time responses depending on factor s including electrode material, current density, electrolyte, PCP concentra tion, and reaction time. At very low over-potentials the reaction product b ecame complex and the reaction took place with less than one electron per m olecule and with the appearance of some chloride. Under these conditions th e reaction is felt to be an electrochemically initiated, chain reaction pol ymerization. This involves the electrochemical formation of a radical which attacks the pentachlorophenoxalate substrate, forming a radical anion inte rmediate. This intermediate decomposes, yielding a chloride ion and reformi ng a neutral radical (though now of higher molecular weight), thus continui ng the reaction. The application of such electrochemically initiated conden sation 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.