Indirect photodegradation of sulfadimidine and sulfapyridine: Influence of CDOM components and main seawater factors.

This study investigated the indirect photodegradation of sulfadimidine (SM) and sulfapyridine (SP) in the presence of chromophoric dissolved organic matter (CDOM), and studied the influences of main marine factors (salinity, pH, NO and HCO). Reactive intermediate (RI) trapping experiments demonstrated that triplet CDOM (CDOM*) played a major role in the photodegradation of SM with a 58% photolysis contribution, and the contributions to the photolysis of SP were 32%, 34% and 34% for CDOM*, hydroxyl radical (HO·) and singlet oxygen (O), respectively. Among the four CDOMs, JKHA, with the highest fluorescence efficiency, exhibited the fastest rate of SM and SP photolysis. The CDOMs were composed of one autochthonous humus (C1) and two allochthonous humus (C2 and C3). C3, with the strongest fluorescence intensity, had the strongest capacity to generate RIs and accounted for approximately 22%, 11%, 9% and 38% of the total fluorescence intensity of SRHA, SRFA, SRNOM and JKHA, respectively, indicating the predominance of CDOM fluorescent components in the indirect photodegradation of SM and SP. These results demonstrated the photolysis mechanism: The photosensitization of CDOM occurred after its fluorescence intensity decreased, and a large number of RIs (CDOM*, HO· and O, etc.) were generated by energy and electron transfer, then these RIs reacted with SM and SP to cause photolysis. The increase in salinity stimulated the photolysis of SM and SP consecutively. The photodegradation rate of SM first increased and then decreased with increasing pH, whereas the photolysis of SP was remarkably promoted by high pH but remained stable at low pH. NO and HCO had little effect on the indirect photodegradation of SM and SP. This research may contribute to a better understanding of the fate of SM and SP in the ocean and provide new insights into the transformation of other sulfonamides (SAs) in marine ecological environments.