UV direct photolysis of amisulbrom in buffer solutions: Kinetics, quantum yield, products identification, DFT, mechanism and predict toxicity

Abstract

Direct photolysis is one of the most important abiotic transformations for pesticides in the aquatic environment. In this study, direct photolysis of amisulbrom was studied under low and medium pressure mercury lamps at pH 4.0, 7.0 and 9.0. The result shows that the direct photolysis rates of amisulbrom in the basic solution are 2.5–2.8 times faster than in the acidic (pH 4.0) or neutral (pH 7.0) solutions under low pressure mercury lamp, while the direct photolysis rate of amisulbrom is similar in different buffer solutions under medium pressure mercury lamp. The quantum yield of amisulbrom is determined in the wavelength range of 190–400 nm. Eight main photolysis products were identified using ultra-performance liquid chromatography quadrupole time-of-flight tandem mass spectrometry (UPLC-Q-TOF-MS/MS). Based on the identified photolysis products, the density functional theory (DFT) analysis, photolysis profiles and the possible photolysis pathway of amisulbrom are proposed. The energy gap of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) for amisulbrom is 4.76 eV and it corresponds to a photon energy of 260.5 nm. The quantitative structure–activity relationship (QSAR) model is used to predict the toxicity of amisulbrom and its TPs. The Results show that TP-I (3-((6-fluoro-2-methyl-1H-indol-1-yl)sulfonyl)-1H-1,2,4-triazol-1-ium), TP-II (3-bromo- 6-fluoro-2-methyl-1H-indole), TP-III (1-((1H-1,2,4-triazol-3-yl)sulfonyl)-3-bromo-6-fluoro- 2-methyl-1H-indole) and TP-V (6-fluoro-2-methyl-1H-indole) are more toxic than their parent.