Photochemical transformation of pyrisoxazole: mechanisms, kinetics and eco-toxicity evaluation

Pyrisoxazole (SYP) is widely used in the control of fungal diseases in crops, but its photochemical transformation mechanisms in the environment and the ecological risks of its degradation products remain unclear. In this study, density functional theory (DFT) and time-dependent density functional theory (TDDFT) were employed to systematically investigate the photochemical transformation mechanism of SYP. The study compares direct photolysis with the indirect photolysis pathway mediated by hydroxyl radicals (·OH), and found that indirect photodegradation is the main mechanism. The dominant reaction is the abstraction reaction, followed by a series of transformations such as hydroxylation, demethylation, and ring cleavage to complete the transformation. The simulation half-life, calculated using rate constants, was 18.34 h, which is in good agreement with experimental data. Toxicity assessment shows that SYP is very toxic or toxic to aquatic organisms, accompanied by developmental toxicity and mutagenicity. After photodegradation, the acute toxicity of most products decreased to a not harmful level, the chronic toxicity of all products was reduced, and the mutagenicity turned negative, with only some retaining developmental toxicity. This suggests that the photodegradation process of SYP can effectively reduce its long-term ecological hazards. The results provide a theoretical basis for scientifically assessing the environmental behavior and ecological risks of SYP.