First insight into the environmental fate of N-acetylated sulfonamides from wastewater disinfection to solar-irradiated receiving waters.

The worldwide detection of emerging transformation products of organic micropollutants has raised accumulating concerns owing to their unknown environmental fate and undesired toxicity. This work first explored the reaction kinetics and mechanisms of the prevalent N-acetylated sulfonamides (N⁴-AcSAs, the typical sulfonamide metabolites) from wastewater disinfection to solar-irradiated receiving waters. The transformation scenarios included chlorination/bromination, photodegradation, and solar/chlorine treatment. The halogenations of two N⁴-AcSAs (N⁴-acetylated sulfadiazine, N⁴-AcSDZ; N⁴-acetylated sulfamethoxazole, N⁴-AcSMX) were pH-dependent at pH 5.0-8.0, and the reactions between the neutral forms of oxidants and anionic N⁴-AcSAs dominated the process. Furthermore, solar-based photolysis significantly eliminated N⁴-AcSAs in small water bodies with low dissolved organic carbon levels, while the indirect photolysis mediated by hydroxyl radicals and carbonate radicals contributed the most. The presence of chlorine residues in solar-irradiated wastewater effluents promoted the decay of N⁴-AcSAs, in which the generated hydroxyl radicals and ozone played a major role. Product analysis suggested the main transformation patterns of N⁴-AcSAs during the above scenarios included electrophilic attack, bond cleavage, SO₂ extrusion, hydroxylation, and rearrangement. Multiple secondary products maintained higher persistence, mobility, and toxicity to aquatic organisms than N⁴-AcSAs. Overall, the natural and engineered transformations of such micropollutants underlined the necessity of including their degradation products in future chemical management and risk assessment.