Phototransformations of solid pentachlorophenol

The phototransformations of thin layers of pentachlorophenol (PCP), an insectide and herbicide, have been investigated in the laboratory in open air under irradiation at wavelengths >290 nm, i.e. corresponding to the sunlight UV region. The PCP disappearance rate was much higher when a 50 v/v% methanol / water mixture instead of pure methanol was used as the PCP deposition medium; changes in the product distribution also resulted although the products were the same. Analyses by GC/MS showed that tetrachlorophenols, hexachlorobenzene and pentachlorobenzene, in this order, were the monocyclic products that reached the highest amounts. Other C-6-membered products were trichlorophenol isomers and tetrachlorobenzene isomers. Hexachlorocyclopentadiene and a tetrachlorocyclo pentadiene isomer as well as five other C-5-membered cyclic products containing carbonyl groups, among which dichloromaleic anhydride was the most abundant, were also identified. Moreover, polychlorinated-p-dioxins and biphenylethers were formed, in particular octachlorodibenzo-p-dioxin reached a significant amount. With respect to PCP photodegradation in water the essential difference was the absence of products corresponding to the substitution of Cl by OH with possible subsequent oxidation to quinonic derivatives. Reactions based on the removal of one of the PCP chlorine atoms as the primary photochemical process are tentatively suggested to account for the formation of the main primary products. Consequences for the environment are also very briefly discussed. (C) 1998 Elsevier Science S.A. All rights reserved.