Aqueous photodegradation of fenthion by ultraviolet B irradiation: contribution of singlet oxygen in photodegradation and photochemical hydrolysis

The objective of this study was to evaluate the photodegradation of the organophosphorus pesticide fenthion in the environment from a human health effect viewpoint. The major photodegradation products of fenthion in an aqueous solution under UVB irradiation (280-320nm radiation) were identified as fenthion sulfoxide. 3-methyl-4-methylthiophenol (MMTP), dimethyl phosphorothioate and 3-methyl-4-methylsulfinylphenol (MMSP). MMTP, dimethyl phosphorothioate and MMSP were discovered as novel photodegradation products of fenthion. Kinetic analysis of these products showed the formation of MMTP and dimethyl phosphorothioate by the photochemical hydrolysis of fenthion, which was accelerated under alkaline conditions. The former was further oxidized to MMSP. Fenthion sulfoxide was directly produced by the oxidative reaction of fenthion. Contribution of dissolved oxygen in this photooxidation was observed by replacing the air with nitrogen gas in the reaction system. which prevented oxidative formation of fenthion sulfoxide from fenthion and MMSP from MMTP. These oxidative compounds were also formed from fenthion in the presence of singlet oxygen (O-1(2)) generated by the visible light irradiation of rose bengal solution, while 102 scavengers, L-histidine and sodium azide (NaN3) inhibited this reaction. The aqueous photolysis mechanisms of fenthion were proposed from a kinetic photolysis experiment study as follows: there were two kinds of UVB photodegradation pathways of fenthion, one being photochemical hydrolysis of the phosphorus-O-phenyl ester to form MMTP and dimethyl phosphorothioate, and the other oxygenation triggered by 102 and producing fenthion sulfoxide and MMSP. Therefore, the steady photodegradation products of fenthion in the water environment may be fenthion sulfoxide and MMSP. (C) 2002 Elsevier Science Ltd. All rights reserved.