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 (e_aq^-), 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 e_aq^--mediated reduction, whereas Cl^- and SO₄^2- 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.