Photodegradation mechanism of sulfonamides with excited triplet state dissolved organic matter: A case of sulfadiazine with 4-carboxybenzophenone as a proxy

Highlights: • Excited triplet state of dissolved organic matter (³DOM^*) is largely responsible for the enhanced photodegradation of sulfadiazine. • Electron followed by proton transfer is a major mechanism for the reactions of sulfadiazine with ³DOM^* proxies. • Two reaction sites (amino- or sulfonyl-N) and sulfadiazine radicals were identified in the reactions of sulfadiazine with ³DOM^* proxies. - Abstract: Excited triplet states of dissolved organic matter (³DOM*) are important players for photodegradation sulfonamide antibiotics (SAs) in sunlit natural waters. However, the triplet-mediated reaction mechanism was poorly understood. In this study, we investigated the reaction adopting sulfadiazine as a representative SA and 4-carboxybenzophenone (CBBP)as a proxy of DOM. Results showed that the excited triplet state of CBBP (³CBBP*) is responsible for the photodegradation of sulfadiazine. The reaction of ³CBBP* with substructure model compounds verified there are two reaction sites (amino-or sulfonyl-N atoms) of sulfadiazine. Density functional theory calculations were performed, which unveiled that electrons transfer from the N reaction sites to the carbonyl oxygen atom of ³CBBP* moiety, followed by proton transfers, leading to the formation of sulfadiazine radicals. Laser flash photolysis experiments were performed to confirm the mechanism. Thus, this study identified that the photodegradation mechanism of SAs initiated by ³DOM*, which is important for understanding the photochemical fate, predicting the photoproducts, and assessing the ecological risks of SAs in the aquatic environment