Photoaging of four microplastics under diverse conditions: characteristics and leachate composition studies

Despite extensive research on microplastic (MP) aging, the mechanistic distinctions under varying environmental conditions remain ambiguous. This study investigated the photoaging behavior of four microplastics (polystyrene (PS), polypropylene (PP), polyethylene (PE) and polyamide (PA)) within 120 days under different aging conditions (anoxic/UV, aerobic/UV, and aqueous/UV) and the composition evolution of their leachates. The results showed that UV aging caused surface fragmentation of microplastic particles and increased the number of oxygen-containing functional groups on the polymer surface. 120 days of UV + H₂O aging significantly elevated surface oxidation (O/C ratio), with fold increases of PS (5.8) > PE (4.5) > PP (3.4) > PA (1.2). O₂ and H₂O significantly accelerated the photoaging process of microplastics. The release characteristics of dissolved organic matter (DOM) were mainly related to the type of MPs and aging conditions. The dissolved organic carbon (DOC) concentration released by PA was significantly higher than that of the other three polymers, reaching up to 886.835 mg L^-1. Humus-like substances were the main components of MP-DOM, accounting for nearly 80%. Furthermore, the fluorescent components of the samples under UV + H₂O conditions showed significant differences at different aging stages (40, 80, and 120 days), indicating that more complex photodegradation reactions had occurred in the aquatic environment. Meanwhile, GC-MS analysis identified an increased diversity of organic compounds in MP-DOM following UV aging, characterized by the leaching of various chemical additives and their transformation products. By comparing the photoaging behavior and subsequent leachate characteristics across different polymers, this research fills critical gaps and further elucidates the aging mechanisms of microplastics.