Photodegradation of the novel herbicide pyraclonil in aqueous solution: Kinetics, identification of photoproducts, mechanism, and toxicity assessment

Pyraclonil is a new type of pyrazole herbicide, whose photochemical fate in aqueous solution has not been reported yet. In this study, effects on the photolysis rate such as light source, pH, NO3−, Fe3+, fulvic acid (FA) and riboflavin (RF) were investigated. Pyraclonil photodegraded in pure water under both UV and simulated sunlight with half-lives of 32.29 min and 42.52 h, respectively. Under UV, the degradation rate of pyraclonil in pH 4 solution (0.0299 ± 0.0033 min−1) was about twice higher than that in pH 9 (0.0160 ± 0.0063 min−1). Under simulated sunlight, low concentration (0.1–1 mg/L) of FA, NO3−, Fe3+ and RF noticeably promoted the photodegradation of pyraclonil. Then, with the combination of experimental UPLC-Q-TOF/MS and computational calculation of density functional theory (DFT), fourteen transformation products (TPs) of pyraclonil were identified with possible mechanism of C–N bond cleavage, photorearrangement, demethylation, hydroxylation and oxidation. Additionally, acute toxicity assessment was conducted through ECOSAR prediction and laboratory bioassays. The prediction results indicated that toxicity of TP157 to daphnid and green algae was 1.3 and 1.4 times higher than that of the parent, respectively. The bioassay results indicated that toxicities of TP157 and TP263 to C. vulgaris were about 1.6 and 5.9 times higher than that of the parent, respectively. The results provided a reference for elucidating the potential hazards of pyraclonil to non-target organisms and promoting its rational use. [Display omitted] •Photolytic behavior of pyraclonil in water was studied for the first time.•Light source, pH, NO3−, Fe3+, FA, RF affect the photodegradation of pyraclonil.•Fourteen TPs were identified and two TPs were confirmed using LC-HRMS.•Photodegradation pathways were proposed based on the identified TPs and DFT.•Toxicities of 2 TPs to C. vulgaris were both higher than that of parent compound.