Effect of polystyrene microplastics on the degradation of sulfamethazine: The role of persistent free radicals

Microplastic (MPs) pollution is increasingly becoming a global environmental problem. MPs entering the environment are subjected to various aging processes, among which photoaging is the most important process leading to MPs oxidation. Persistent free radicals (EPFRs) are formed on the surface of MPs during photoaging, but it is not clear whether EPFRs on the surface of MPs can produce reactive oxygen species (ROS) and thus degrade organic pollutants. In this study, with polystyrene (PS) as the representative plastic and sulfamethazine (SMT) degradation as the target pollutant, the effect and mechanism of light-induced PS on SMT degradation were investigated by experiment and theoretical calculation. It was found that PS can stimulate the production of ROS under sunlight, which can significantly improve the degradation rate of SMT. Through quenching experiment and free radical trapping experiment, it was found that the mechanism of PS promoting the degradation of SMT was mainly due to the production of hydroxyl radical (·OH) in the system, and ·OH was the main ROS species affecting the oxidative degradation of SMT. The characterization results show that the high reactive oxygen generation ability of PS under solar irradiation was due to the abundant photoactive oxidation functional groups on its surface. In addition, the key reaction sites of SMT were predicted by density functional theory (DFT) calculation. The results of different calculations consistently showed that the sulfonamide group of SMT, the pyrimidine heterocycle and the amino group of aniline are the reaction sites of ·OH priority attack. The main intermediates were determined by UHPLC-HRMS/MS. Combined with theoretical calculation, it was proposed that the oxidative degradation pathway of SMT mainly includes SN bond cleavage, SMILES rearrangement and SO group removal. This study clarified the effect of PS on the degradation of organic pollutants under light, and provided theoretical guidance for the degradation mechanism.