Photochemical transformation of sunscreen agent benzophenone-3 and its metabolite in surface freshwater and seawater

• Direct photolysis of benzophenone-3 and its metabolite exhibits pH dependance.
• Photolytic rates of both compounds are lower in seawater than those in freshwater.
• Triplet-excited DOM is largely responsible for indirect photolysis in seawater.

The occurrence of sunscreen agents and their metabolites in surface waters gives rise to public concerns. However, little is known about the environmental fate of these pollutants at present, especially for their metabolites. In this study, we investigated the photochemical of sunscreen agents and their metabolites in natural waters, adopting benzophenone-3 (BP-3) and its human metabolite 4-hydroxybenzophenone (4-OH-BP3) as examples. Results show that only anionic forms of both BP-3 and 4-OH-BP3 can undergo direct photodegradation. The photolytic rates of both compounds in natural waters are faster as compared to those in pure water. Radical scavenging experiments revealed that triplet-excited dissolved organic matter (3DOM∗) was responsible for the indirect photodegradation of BP-3 and 4-OH-BP3 in seawater, whereas in freshwater, the indirect photodegradation of these two compounds was attributed to 3DOM∗ and ·OH. 1O2 plays a negligible role in their photodegradation because of the weak 1O2 reactivity. Furthermore, we probed the contribution of ·OH and 3DOM∗ to the photodegradation of both compounds in freshwater, and the results revealed that ·OH accounted for 56% and 59% of the observed photodegradation for BP-3 and 4-OH-BP3, respectively, whereas 3DOM∗ accounted for 43% and 12% of the observed photodegradation for BP-3 and 4-OH-BP3, respectively. These results are helpful in assessing the ecological risk of BP-3 and its metabolite in the aquatic environment.

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