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. 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.

The focus of pK_a calculations has primarily been on stable molecules, with limited studies comparing radical cations and stable cations. In this study, we comprehensively investigate models with implicit solvent and explicit water molecules, direct and indirect calculation approaches, as well as methods for calculating free energy, solvation energy, and quasi-harmonic oscillator approximation for para-substituted aniline radical cations (R-PhNH₂^•+) and anilinium cations (R-PhNH₃⁺) in the aqueous phase. Properly including and positioning explicit H₂O molecules in the models is important for reliable pK_a predictions. For R-PhNH₂^•+, precise pK_a values were obtained using models with one or two explicit H₂O molecules, resulting in a root mean square error (RMSE) of 0.563 and 0.384, respectively, for both the CBS-QB3 and M062X(D3)/ma-def2QZVP methods. Further improvement was achieved by adding H₂O near oxygen-containing substituents, leading to the lowest RMSE of 0.310. Predicting pK_a values for R-PhNH₃⁺ was more challenging. CBS-QB3 provided an RMSE of 0.349 and the M062X(D3)/ma-def2QZVP method failed to calculate pK_a accurately (RMSE > 1). However, by adopting the double-hybrid functional method and adding H₂O near the R substituent group, the calculations were significantly improved with an average absolute difference (ΔpK_a) of 0.357 between the calculated and experimental pK_a values. Our study offers efficient and reliable methods for pK_a calculations of R-PhNH₂^•+ (especially) and R-PhNH₃⁺ based on currently mature quantum chemistry software.