Photochemistry of microplastics and its associate effect on the coexisting pharmaceuticals in an aqueous environment

Microplastics (MPs) have drawn much attention and become a global concern in recent years due to their widely presence in aqueous environments and potential threat to aquatic organisms. MPs continuously undergo different aging processes in aquatic environments, leading to changes in surface physical and chemical properties. Pharmaceuticals, another global issue, coexist with MPs in aquatic environments, because of the prevalent usage and the difficulty in removing by wastewater treatments. Previous studies have mainly focused on the adsorption effect and toxicity of MPs. However, little research has been discussed on the photochemical reactions between aged MPs and the coexisting pharmaceuticals. In this study, polystyrene (PS) and UV radiation were chosen as representative MPs and the aging process. This study was conducted to investigate the photodegradation of pharmaceuticals via pristine and aged PS-MPs under sunlight irradiation, and the generation of photoinduced reactive species on PS-MPs and the associate effect on the coexisting pharmaceuticals. To clarify the above-mentioned mechanism, electron paramagnetic resonance (EPR) spectroscopy, chemical probe method, gas purging (N₂ and O₂) and scavenger experiments were utilized. The result showed that pharmaceuticals (cimetidine, codeine and morphine) susceptible to indirect photolysis are significantly degraded in the presence of aged PS-MPs under sunlight irradiation. The photodegradation of cimetidine exhibited much faster in the presence of aged PS-MPs (>93%) within 2 h, compared to that in the presence of pristine PS-MPs (<8%). In addition, cimetidine can be absorbed on aged PS-MPs, while no absorption was observed on pristine PS-MPs. The formation of environmentally persistent free radicals (EPFRs), OH radicals, ¹O₂ and ³PS^* was verified, only ¹O₂ and ³PS^* were demonstrated the dominant reactive species on the enhanced cimetidine photodegradation. This work provides another viewpoint on how aged MPs may serve as photosensitizers and alter the fate of coexisting pharmaceuticals in aquatic environments.