A systematic review on the aging of microplastics and the effects of typical factors in various environmental media

Microplastics (MPs) in natural environment are derived from various discharge processes and undergo a series of aging that is influenced by different factors. This paper first described the sources of MPs in the environment. Main sources include sewage treatment plants, etc. Then, the aging processes and mechanisms of MPs were discussed, including mechanical fragmentation, photoaging, thermal aging and biodegradation. Finally, the critical factors for affecting the aging of MPs were outlined, mainly including the physicochemical property of MPs (e.g., particle size, chemical) and environmental factors (e.g., oxygen, temperature). Non-biodegradable MPs (e.g., polystyrene, polyurethane) exhibited a lower photoaging rate with inferior fragmentation than biodegradable ones (e.g., polyhydroxyalkanoates, polycaprolactone). Overall, this review systematically expounds on the occurrence of MPs and the impact of environmental factors on their aging process, which provides insight into the effective treatment of MP pollution in the future.