Photophysical and photochemical insights into the photodegradation of tricyclazole and pymetrozine in water bodies of a rice field

Environmental context Photodegradation, a natural process for removing pesticides from water, is particularly important in rice fields with their large surface area and sunlight exposure. By studying two commonly used pesticides, we show that the properties of rice field water, such as dissolved organic matter, facilitate indirect photolytic processes while reducing direct photolysis. A better understanding of the natural degradation mechanisms for pesticides will inform their safer use in agriculture. Abstract Rice paddy fields (RFs) make ecologically important contributions to agricultural wetland ecosystems in south China. In recent years, the elevated use of pesticides has resulted in concerns over the water quality in RFs and their downstream aquatic environment. Photodegradation is important for the natural elimination of nonpoint source agricultural contaminants, and RF water provides an ideal environment for the photochemical removal of pesticides, owing to its large surface area and photic conditions. Therefore, in this study, the physical and photochemical properties of water collected from the irrigation area were investigated, along with the direct and indirect photodegradation of two commonly used pesticides in RFs, tricyclazole (TC) and pymetrozine (PYM). Results indicated that allochthonous-like dissolved organic matter (DOM) was abundant in the irrigation water, possibly originating from the decomposition of rice straw. Moreover, the light screen effect and the formation rate of singlet oxygen (1O2) and the triplet excited state of DOM (3DOM*) were generally increased in RF water, while the formation of HO• was marginally reduced. Evapoconcentration and photobleaching processes were found to correlate with crop leaf coverage and had a significant impact on the levels and compositions of water constituents. Finally, the photodegradation kinetics of TC and PYM further confirmed that RF water would facilitate indirect photolysis and reduce the direct photolysis of pesticides.