Experimental Study on Microplastic Transport in Heterogeneous Media

【Objective】Microplastics, as emerging pollutants, have been widely detected in soils and groundwater, yet their migration mechanisms in heterogeneous porous media remain unclear, limiting the prediction of their environmental behavior and associated risk assessment. 【Methods】This study conducted laboratory column experiments to investigate the transport characteristics of polystyrene microplastics in three typical porous media: homogeneous coarse sand (grain size 1.25 mm), homogeneous fine sand (grain size 0.25 mm), and a concentric heterogeneous structure (coarse sand core surrounded by fine sand). The effects of pH (5, 7, 9), flow rate (0.5, 1.0 ml/min), and microplastic size (200, 800 nm) on breakthrough behavior were systematically evaluated. 【Results】Heterogeneity significantly altered the migration pathways and retention patterns of microplastics, with breakthrough curves in heterogeneous media showing a typical bimodal distribution, indicating the coexistence of preferential flow (coarse sand) and retention zones (fine sand). Migration ability increased with both pH and flow rate, and microplastics of 200 nm exhibited markedly higher mobility than those of 800 nm. 【Conclusion】This study reveals the key mechanisms by which heterogeneous structures and environmental factors jointly affect microplastic transport, providing experimental evidence for modeling the behavior and assessing the risks of microplastic pollution in subsurface environments.