Photochemical transformation of polychlorinated phenols

To increase our understanding of the photocatalytic effects in the transformation of chlorinated organic compounds, the detailed mechanism of the direct photolysis of polychlorinated phenols has been established under aerobic and anaerobic conditions. The quantum yield of photolysis of 2,4,5-trichlorophenol (2,4,5-TCP) has been determined at different wavelengths and pH values. It has been shown that direct photolysis of polychlorinated phenols is accompanied by chloride ion detachment from different positions of the benzene ring with the formation of 2,5-dichlorohydroquinone and dichlorocyclopentadiene derivatives in the case of 2,4,5-TCP, as well as polychlorinated dibenzodioxins (PCDD) and dibenzofurans (PCDF). Based on the 2,4,5-TCP direct photolysis products, it has been concluded that chlorophenol free radicals are formed as a result of C-Cl cleavage, which probably act as the origin of highly chlorinated PCDD and PCDF. To utilize visible light in the photocatalytic destruction of organic pollutants, we investigated the kinetics of transformation of polychlorinated phenols in systems involving singlet oxygen formation (dye-sensitized reactions with Rose Bengal, Methylene Blue and Eosin). It has been shown that superoxide radical is not reactive towards 2,4,5-TCP, whereas singlet oxygen results in the destructive oxidation of polychlorinated phenols without formation of PCDD and PCDF among the reaction products. The detailed mechanism of the interaction of singlet oxygen with 2,4,5-TCP has been investigated. It has been concluded that the initial trichlorophenol is dechlorinated and singlet oxygen interaction with 2,4,5-TCP proceeds via a mechanism with the formation of hydrogen peroxide without intermediate superoxide radicals. (C) 1997 Elsevier Science S.A