Microplastics in the soil-groundwater environment: Aging, migration, and co-transport of contaminants – A critical review

Microplastic contamination in soil has received increasing attention since excessive plastic debris has been emitted directly into the terrestrial environment. Once released into the terrestrial environment, microplastics can be aged via photo- and thermally-initiated oxidative degradation, hetero-aggregation, and bioturbation. Aging affects the physiochemical properties of microplastics with the increase of surface roughness and oxygen-containing groups, which could enhance the sorption and mobility of microplastics in the soil and groundwater environment. However, the interactions among aging, sorption, and transport of microplastics in the terrestrial system have not been unveiled. This review clarifies the key processes of microplastics transport pathways in soil and groundwater ecosystems influenced by aging and sorption under various scenarios. Co-transport of microplastics and sorbed contaminants are also addressed to help understand the risks associated with heavy metals, organic contaminants, and engineered nanoparticles in the soil environment. Overall, this review elaborates the most pressing research limitations on the present literature and highlights the future perspectives to investigate the possible broad transport pathways of microplastics in soil.