New Insights into the Aquatic Photochemistry of Sulfonamide Antibiotics : Apparent Photolysis, Toxicity Evolution and Mechanisms

Ge, L. and Wang, J. and Sweetman, A. and Zheng, Z. and Zhang, P. and Cui, N. (2026) New Insights into the Aquatic Photochemistry of Sulfonamide Antibiotics : Apparent Photolysis, Toxicity Evolution and Mechanisms. Water, Air, and Soil Pollution, 237 (14): 820. ISSN 0049-6979

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Abstract

As emerging contaminants, sulfonamide antibiotics (SAs) widely exist in aqueous systems and photo-degrade significantly in surface waters, requiring new insights into their aqueous apparent photolytic mechanisms and photo-modified toxicity. In this study, reaction kinetics, self-sensitized photo-oxidation, transformation pathways, antibacterial activity and photo-induced toxicity were investigated during the apparent photolysis of nine SAs including sulfamethoxazole (SMX), sulfisoxazole, sulfathiazole, sulfamethizole, sulfadimethoxine (SDM), sulfadiazine, sulfachloropyridazine, sulfamerazine and sulfamethazine. Under simulated sunlight irradiation (λ > 290 nm), most five-membered heterocyclic SAs underwent photodegradation at a faster rate than all six-membered heterocyclic SAs, which was ascribed to their chemical structures including different heterocyclic groups and associated substituents (− CH3). The scavenging experiments indicated that SMX underwent self-sensitized photo-degradation via O2•−, 1O2 and •OH, and direct photo-degradation via 3P*. The contribution of O2•− (40.71% − 59.01%) and 1O2 (29.13%) mediated self-sensitized photo-oxidation was found to be the highest under acidic and alkaline conditions, respectively. Photo-product identification was carried out with HPLC-MS2. The transformation pathways of five-membered heterocyclic SMX involved photo-cleavage and photo-polymerization, while six-membered heterocyclic SDM underwent S − N bond cleavage, hydroxylation, desulfonation, methylation and addition reaction at sulfonyl-N. Based on multiple bioassays (Escherichia coli and Vibrio fischeri), the antibacterial activity changes and photo-modified toxicity evolution depended on dominant photo-degradation pathways and primary intermediates. Furthermore, the Ecological Structure–Activity Relationship (ECOSAR) model confirmed the comparable or higher toxicities of the intermediates compared to the parent compounds. These findings provide new photochemical mechanistic insights into the large class of SAs, which allows a better assessment of their photochemical persistence, fate and risks in aquatic environments.