Effects and mechanisms of aged 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 effects 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 MPs inhibited the photodegradation of SMX, and the photodegradation rate (kobs) of SMX was negatively correlated with the aging degree of PS MPs (R2 = 0.998). The aged 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 (3SMX*). According to sample characterization, aging PS MPs formed more unsaturated chromophores and produced organic intermediates that enhanced photon absorption. Additionally, aged 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 aged MPs in sunlit surface waters.