Study on the photo-aging process and mechanism of polystyrene microplastics under different salinities mediated by humic acid.

Microplastics are ubiquitous in the environment, and their aging is frequently influenced by environmental factors. The dissolved organic matter (DOM) and salinity influence the aging process of microplastics. Polystyrene microplastics (PS) were employed to explore the combined influence of humic acid (HA) and inorganic salt (NaCl) on the photo-aging process of PS in this study. The salinity and HA together promoted the photo-aging of PS, mainly reflected in the reduction of particle size, enhancement of hydrophilicity, and increased oxygen content. The production of ·OH, O, O· and HO was detected in the PS suspension, and the combined influence of salinity and HA promoted the transformation of HO and reactive oxygen species (ROS). Quenching experiments signified that the photo-aging of PS mainly involved direct photo-aging process to generate ROS, followed by indirect photo-aging process. Reactive oxygen species in the solution first attacked PS by addition reactions to destroy its benzene structure, followed by Norrish I reaction mechanisms to produce small molecules such as ketones, esters and aldehydes. This study revealed the aging mechanism of PS attacked by ROS under simulated sunlight irradiation. The toxicity analysis also revealed that the toxic effects of aged PS were significantly regulated by environmental factors (e.g., HA and salinity), presenting a complex toxicity profile with dynamic changes.