Chemical and photo-initiated aging enhances transport risk of microplastics in saturated soils: Key factors, mechanisms, and modeling

Microplastics (MPs) inevitably undergo aging transformation and transport process in environmental compartments. In this study, the polystyrene MPs were aged via three different oxidation methods including persulfate oxidation (PS), UV irradiation (UV), and UV irradiated persulfate oxidation (UVPS). All three treatments induced the great transformation of MPs, with the significant increase in surface roughness and in oxygen-containing functional groups, i.e., COOH or COOC. The UVPS aging showed synergetic effect due to the strengthened photo-initiated chemical oxidation, compared to UV and PS alone. All aged MPs exhibited the enhanced transport (34.9%-89.2%) in sandy and clay loam soils than pristine MPs (30.5%), and the synergetic effect was also observed in the transport behaviors of the UVPS MPs. Higher transport of MPs and aged MPs occurred in sandy soil than that in clay loam soil since the latter one contained high Fe minerals that tend to retain MPs, which was confirmed by the model quartz sand column experiment. Modeling on the migration of MPs retained in soil under a rainstorm scenario showed that the aged MPs had the stronger remobility and greater proportion of cumulative flux than pristine ones in the soil profile. These findings provided new insights on the fate and transport of MPs in natural soil and their potential risk to groundwater contamination.