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Vertical migration of microplastics in porous media: Multiple controlling factors under wet-dry cycling
Summary
Researchers studied how microplastics move vertically through sandy soil during cycles of wetting and drying, testing four common plastic types at various particle sizes. They found that smaller, more hydrophobic particles migrated deeper, and that frequent wet-dry cycles and the presence of dissolved organic matter accelerated downward movement. The findings suggest that microplastics in agricultural soils could potentially reach groundwater, posing risks to underground water quality.
Terrestrial soils are not only a large reservoir for Microplastics (MPs), but also a possible entrance to the subsurface environment, posing potential risks to the subterranean habitats and groundwater. In this study, we examined the vertical transport of MPs of four polymers, i.e., polyethylene (PE), polyethylene terephthalate (PET), polypropylene (PP) and polyamide (PA), in porous sand media driven by wet-dry cycling. The effects of polymer properties, MP size, sand particle size, wet-dry cycles, and dissolved organic matter (DOM) on their migration behavior were investigated. Surface hydrophobicity showed a strong positive correlation with MP mobility, with PA exhibiting the greatest movement potential, followed by PE, PET, and PP. The penetration depth of MP particles increased with decreasing MP particle size (d) and increasing sand diameter (d). MP particles migrated deeper in sand media when d/d < 0.11. Furthermore, frequent wet-dry cycles and the presence of DOM promoted the vertical migration of MPs in the sand. The results revealed multiple factors influencing the vertical migration of MPs in sand, which is instructive for understanding the ecological risk of MPs in potentially contaminated soil (e.g., farmland with long-term mulching) to the subsurface environment and potential negative impact to public health.