Aquatic photolysis of strobilurin fungicide kresoxim-methyl: kinetics, pathways, and effects of adjuvants

Kresoxim-methyl (KM), an extensively used strobilurin fungicide, displays broad-spectrum fungicidal activity. Despite its prevalent application in agriculture and frequent detection in the environment, specific studies on the photolytic fate of KM in natural surface waters are quite limited. To address this, the present study delves into the photochemical transformation of KM under light irradiation using a high-pressure mercury lamp. Photodegradation of KM followed pseudo-first-order kinetics, with a degradation rate constant of 0.067 ± 0.005 h−1, corresponding to a half-life of approximately 10.3 ± 0.7 h, and the quantum yield was estimated to be 6.95 × 10−3. Transient species identification experiments demonstrated the involvement of singlet oxygen (1O2) and singlet (1KM*) and triplet (3KM*) states. Four different photodegradation pathways, i.e., photoisomerization, hydrolysis, hydroxylation, and ether cleavage, were proposed. Toxicity assessments demonstrated that photodegradation effectively reduced the ecotoxicity of KM. Two typical pesticide adjuvants, namely, Tween 20 and sodium dodecylbenzenesulfonate (SDBS), were selected to evaluate their impacts on KM photolysis. While Tween 20 exhibited a remarkable increase in the photodegradation of KM, SDBS showed a concentration dependent behavior, with an inhibitory effect at low concentrations (<200 μM) and promotional effects at higher concentration levels. The present study elucidated the photolytic KM degradation mechanism, as well as its possible inhibition and enhancement, which is of significant importance for evaluating the environmental fate and ecotoxicological risks of KM in sunlit surface waters.