Photoassisted titanium dioxide mediated degradation of organophosphorus pesticides by hydrogen peroxide

The photooxidation of organophosphorus pesticides (OPP) such as methamidophos, phorate, malathion, diazion and EPN, was investigated in UV-TiO2, UV-H2O2 and UV-TiO2-H2O2 systems. Apparent first-order rate constants were used to describe the photodecomposition behaviors of OPP. Batch results demonstrate that the degradation efficiencies of OPP increased in the order of phorate > methamidophos > malathion approximate to diazion > EPN. The photooxidation of methamidophos and phorate is primarily attributed to direct photolysis. Whereas the presence of TiO2 promotes the degradation efficiencies of diazion, malathion and EPN. Hydrogen peroxide was found to be an intermediate in UV-TiO2 system during the photocatalytical reaction process. Longer irradiation decreased H2O2 concentration due to the high electron-hole recombination rate on the surface of TiO2. Adding H2O2 is more effective on the photocatalytic oxidation of OPP than TiO2. Threefold to tenfold of the quantum efficiency of the UV-H2O2 system was obtained for the UV-TiO2 system. Moreover, the quantum efficiencies for degradation of OPP decreased when light intensity is increased from 100 to 450 W, although increasing light intensity can increase the apparent first-order rate constants of OPP. (C) 1997 Elsevier Science S.A.