UV-photoaging behavior of polystyrene microplastics enhanced by thermally-activated persulfate

Aging of microplastics (MPs) under natural conditions takes several months or even years. In this study, polystyrene MPs (PS-MPs) were used as the target MPs, and these were subject to artificially accelerated aging by UV irradiation (UV254) and UV/heat-activated persulfate (UV+K2S2O8). The physicochemical characteristics of aged MPs and the interaction with tetracycline hydrochloride (TH) were investigated. Simultaneously, the fluorescence characteristics, total organic carbon content (TOC), and toxicity effects of aging filtrate were also evaluated. Furthermore, the aging path of PS-MPs was derived with reactive species analysis using an electron paramagnetic resonance (EPR) test. The results showed that obvious cracks appeared on the surface of the aged PS-MPs, with the average particle size decreasing from 83.95 ± 19.93 µm to 60.41 ± 19.58 µm in the UV treatment and to 48.02 ± 18.83 µm in the UV+K2S2O8 treatment. The oxygen-containing functional groups of aged MPs were detected after the UV and UV+K2S2O8 treatment; the carbonyl index (CI) values were 0.44 and 0.62, respectively. The TH adsorption capacity of UV-treated MPs and UV+K2S2O8-treated MPs increased from 0.176 mg g−1 for pristine MPs to 0.193 mg g−1 and 0.225 mg g−1, respectively. Especially, the TOC and luminescent inhibition toxicity of the filtrate increased after aging. EPR analysis demonstrated that SO4•− and ROS played a key role in accelerating the aging of PS-MPs. This study provided a reference for UV photoaging behavior of MPs enhanced by activated K2S2O8, which is meaningful for a comprehensive understanding of MPs behavior and the risks they introduce into the environment.