Effects and Mechanisms of Aging Polystyrene Microplastics on the Photodegradation of Sulfamethoxazole In Water Under Simulated Sunlight

Pharmaceutical and microplastics (MPs) have been frequently detected in aquatic environment. In this study, the effect s of polystyrene MPs (PS MPs) with different aging degrees on the photolysis of sulfamethoxazole (SMX) in simulated sunlit water were investigated. The results showed that the presence of PS M Ps inhibited the photodegradation of SMX, and the photodegradation rate ( k obs ) of SMX was negatively correlated with the aging degree of PS MPs (R 2 = 0.998). The aging PS MPs would cause light-screening effect, thereby reducing the photodegradation of SMX in sunlit water. Further, the free radical quenching experiment showed that the mechanism for inhibiting the photolysis of SMX was the reduction of the triplet excited state SMX ( 3 SMX* ). According to the sample characterizations, aging PS MPs formed more unsaturated chromophores and enhanced light absorption. Additionally, aging PS MPs also decreased the types and yields of degradation products of SMX via product analysis. This study provides an insight into the environmental behaviors of SMX and the photochemical roles of aging MPs in sunlit surface waters