PHOTOCHEMICAL TRANSFORMATION OF POLYCHLORINATED PHENOLS

To increase our understanding of the photocatalytic effects in the tra nsformation of chlorinated organic compounds, the detailed mechanism o f the direct photolysis of polychlorinated phenols has been establishe d under aerobic and anaerobic conditions. The quantum yield of photoly sis of 2,4,5-trichlorophenol (2,4,5-TCP) has been determined at differ ent wavelengths and pH values. It has been shown that direct photolysi s 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) an d dibenzofurans (PCDF). Based on the 2,4,5-TCP direct photolysis produ cts, it has been concluded that chlorophenol free radicals are formed as a result of C-Cl cleavage, which probably act as the origin of high ly chlorinated PCDD and PCDF. To utilize visible light in the photocat alytic destruction of organic pollutants, we investigated the kinetics of transformation of polychlorinated phenols in systems involving sin glet oxygen formation (dye-sensitized reactions with Rose Bengal, Meth ylene Blue and Eosin). It has been shown that superoxide radical is no t reactive towards 2,4,5-TCP, whereas singlet oxygen results in the de structive oxidation of polychlorinated phenols without formation of PC DD and PCDF among the reaction products. The detailed mechanism of the interaction of singlet oxygen with 2,4,5-TCP has been investigated. I t has been concluded that the initial trichlorophenol is dechlorinated and singlet oxygen interaction with 2,4,5-TCP proceeds via a mechanis m with the formation of hydrogen peroxide without intermediate superox ide radicals. (C) 1997 Elsevier Science S.A.