The fate, impacts and potential risks of photoaging process of the microplastics in the aqueous environment

Microplastics are ubiquitous in various environments with a wide distribution. When exposed to the natural environment, microplastics undergo photoaging under ultraviolet irradiation, accompanied by a series of physicochemical property changes such as bond breakage and oxygen addition. It is of great significance to understand the physical and chemical properties of photoaged microplastics and their potential environmental risks, but information is limited and lacking in summary to date. This paper summarizes the properties of photoaged microplastics, the factors influencing their aging, and the increased environmental risks following microplastics exposure to ultraviolet radiation in aquatic environments. Firstly, the apparent changes in photoaging behavior on microplastics' physical and chemical properties were outlined, including morphological characteristics, chemical structure, crystallinity, hydrophobicity, Zeta potential and leaching behavior. Then, the factors affecting the photoaging process were discussed comprehensively, including microplastic properties, environmental media, dissolved organic matter (DOM), clay minerals and inorganic anions. In addition, the environmental risks of photoaged microplastics as pollutant transport vectors and disruption of elemental cycles were analyzed. Notably, photoaged microplastics may enhance the induction of biological toxicity and human exposure risks. Finally, knowing the summarized above, some issues that deserve more attention for future research are forecasted. Since the UV-induced weathering of microplastics is a widespread phenomenon with implications for humans, society and the environment, this review could offer valuable insights for future research directions and environmental risk assessments of microplastics.