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Photodegradation behavior of a new fungicide isopyrazam in water under simulated sunlight and ultraviolet (UV) irradiation was studied. The photolysis half-life (t 1/2 ) of isopyrazam in purified water was 195 h under simulated sunlight, which was decreased to 46−88 h, 28−51 h, and 13−18 h in the presence of NO 3 − , Fe 3+ , and riboflavin, respectively. UV irradiation accelerated the photolysis of isopyrazam with a half-life of 30 min and revealed varied degradation rates in acidic (0.011 min −1 ), neutral (0.024 min −1 ), and alkaline solutions (0.022 min −1 ). Nine transformation products under simulated sunlight and UV were found, and photolytic pathways were proposed to include cleavage of C−N bonds, hydroxylation, nitration, demethylation, dehydrofluorination, and photoisomerization. The acute toxicity of defluorinated isopyrazam (TP 4) and the chronic toxicity of isomerized isopyrazam (TP 9) to aquatic organisms were approximately twice that of isopyrazam. These findings provide insights into environmental risk assessment and management of water pollution.

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