Theoretical explanation of direct photolysis and indirect photolysis of bendazone with •OH, •SO₄⁻, and •CO₃⁻ in water: mechanism insights and ecotoxicity evaluation

Bendazone (BNTE) is an herbicide and a highly concerned pollutant in aquatic environments. Understanding the photochemical behavior of BNTE in water is crucial for evaluating its photochemical conversion process in aquatic environments. This study analyzed the direct photolysis and indirect photolysis pathways of two dissociated forms of BNTE in water through density functional theory and time-dependent density functional theory method. The results show that the reaction types of indirect photolysis of BNTE with free radicals (•OH, •SO₄⁻, and •CO₃⁻) are OH⁻ addition, SO₄⁻ addition, and CO₃⁻ addition. In the process of indirect photolysis of BNTE and free radicals, the photolysis of •OH and BNTE was the easiest, followed by •SO₄⁻. In addition, the active site of BNTE reacting with •OH is C8, and the active site of BNTE reacting with •SO₄⁻ is C10. However, the photolysis effect of •CO₃⁻ on BNTE is very small, indicating that •CO₃⁻ in water plays a secondary role in the indirect photolysis of BNTE. In the direct photolysis of BNTE, N1–C6 bond breaking is difficult to occur spontaneously in the environment due to its high endothermic property and energy barrier. The direct photolysis pathway of BNTE involves the break of the N1–S2/S2–N3/N3–C12 bond. In addition, the ecological toxicity evaluation showed that toxicity of most of the degradation products were reduced, but the toxicity level was still maintained at a harmful level. Our findings provide the photochemical fate of BNTE in aquatic environments and will help to more accurately understand their photochemical conversion mechanisms in the environment.