Mechanistic insights into acetochlor degradation by the UV/sulphite process: a transition state calculation approach

To mitigate the environmental risks posed by the herbicide acetochlor in aquatic ecosystems, this study systematically evaluated its degradation across three advanced UV-based reductive processes: UV/Na2SO3, UV/Na2S, and UV/Na2S2O4. Under 254 nm irradiation, the UV/sulphite (UV/Na2SO3) system demonstrated superior degradation efficiency, achieving optimal removal under neutral-to-weakly alkaline conditions (pH 7.0–10.0) with rates scaling proportionally to the initial sulphite concentration. Mechanistic investigations revealed that hydrated electrons (eaq−), sulphite radicals (SO3•−), and hydrogen atoms (H•) collectively drove the degradation, contributing 58.72%, 37.95%, and 3.33%, respectively. Matrix effect evaluations indicated that NO3−, NO2−, and HCO₃− significantly inhibited the eaq−-mediated reduction, whereas Cl− and SO42− exhibited negligible interference. Crucially, density functional theory (DFT) and transition state calculations were employed to elucidate the microscopic degradation pathways. These theoretical models confirmed that the initial electron-induced dechlorination at the C–Cl bond is a barrierless process. Furthermore, transition state analysis uncovered a water-assisted, stepwise hydrogen transfer and hydrolysis mechanism, wherein water clusters function as essential proton shuttles to facilitate C–N bond activation. By seamlessly integrating experimental kinetics with advanced computational modelling, this study delineates the precise degradation mechanisms of acetochlor, providing a robust theoretical foundation and technical framework for the application of UV/sulphite processes in water remediation.