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

• 3DOM∗ is largely responsible for the enhancing photolysis of sulfapyridine.
• Aniline-N group of sulfapyridine was identified as the reaction site with 3DOM∗.
• Anionic sulfapyridine exhibits higher 3DOM∗ reactivity than that of neutral form.
• Reaction of sulfapyridine with 3DOM∗ is via electron followed by proton transfer.

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 (3DOM∗) is largely responsible for the photodegradation of sulfapyridine. The reaction of 3DOM∗ 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 3DOM∗ 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 3DOM∗ moiety, followed by proton transfer, and leading to the formation of sulfapyridine radicals. The photolytic mechansim of sulfapyridine initiated by 3DOM∗ is helpful in understanding the photochemical fate and assessing the ecological risks of SAs in the aquatic environment.