Insights into photolytic mechanism of sulfapyridine induced by triplet-excited dissolved organic matter

The ubiquity of sulfonamide antibiotics (SAs) in natural waters urges insights into their fate for ecological risk assessment in the aqueous euphotic zone. In this study, we investigated the effect of dissolved organic matter (DOM) on the photolysis of SAs with sulfapyridine as a reprentative. Results show that excited triplet state DOM ((3)DOM(∗)) is largely responsible for the photodegradation of sulfapyridine. The reaction of (3)DOM(∗) with a substructure model compound of SAs confirmed that sulfapyridine has one reaction site (aniline-N). Density functional theory (DFT) calculation was performed, which indicates that the anionic sulfapyridine has higher (3)DOM(∗) reactivity than that of the neutral form, which was also confirmed by steady state photolytic experiments. In the reaction, electrons of the aniline-N transfer to the carbonyl oxygen atom of (3)DOM(∗) moiety, followed by proton transfer, and leading to the formation of sulfapyridine radicals. The photolytic mechansim of sulfapyridine initiated by (3)DOM(∗) is helpful in understanding the photochemical fate and assessing the ecological risks of SAs in the aquatic environment.