Light induced transformations of selected organophosphorus pesticides on titanium dioxide: Pathways and by-products evaluation using LC–MS technique

Abstract

Three organophosphorus pesticides (azinphos methyl, phosphamidon and methidathion) were degraded in aqueous solution using titanium dioxide as photocatalyst. The investigation had involved the identification of the intermediate compounds and the assessment of the mineralization. HPLC/MS was used to follow the disapperance of the initial pesticide and the formation of intermediate products and allowed the identification of some important photodegradation by-products. A tentative transformation mechanism has been proposed for the selected pesticides. As common pathways, in all cases hydroxylation, bihydroxylation and detachment of the organophosphorus moiety occurs. In addition, azinphos methyl and methidation had shown to give oxidation of P S to P O. Some additional pathways were however recognized. Phosphamidon is subjected to dechlorination, while methidathion is subjected to the ring opening of the triazolidine moiety. Azinphos methyl gives the N–C bond cleavage and its transformation proceeds through six concomitant pathways with the formation of 17 intermediate compounds. Complete mineralization was achieved in all cases. While for azinphos methyl the mineralization process occurs within 3 h of irradiation, for phosphamidon up to 16 h are required. Measurement of phosphate, sulphate, chloride, nitrate and ammonium ions gave valuable information about how this process was achieved.