Enhanced retention of microplastics in biopolymer-modified porous media: Insights from pore-scale visualization

Microplastics (MPs), due to their small size and high mobility, have been widely detected in soils and groundwater, posing long-term ecological risks. Developing sustainable and effective strategies to limit their transport in porous media remains a critical environmental challenge. In this study, a plant-derived biopolymer (BDH) was introduced to modify porous media and enhance the retention of polymethyl methacrylate microplastics (PMPs). A pore-scale visualization platform was employed to systematically investigate PMP transport and retention under varying BDH contents, initial PMP concentrations, flow rates, and saturation conditions. Nuclear magnetic resonance (NMR) was used to track pore structure evolution during transport. Results showed that in saturated media, PMPs were primarily retained via surface deposition and pore straining, whereas in unsaturated conditions, film straining and air-water interface capture dominated. With BDH modification, network capture by BDH became the primary retention mechanism. Increasing BDH contents significantly inhibited PMP transport, with 1.5 % BDH achieving a maximum interception rate of 66.5 %. Mechanistic analysis revealed that BDH formed a three-dimensional hydrogel network through hydrogen bonding. This hydrogel coated, encapsulated, and bridged sand particles via hydrogen bonding, reducing pore spaces and obstructing PMP transport pathways. Furthermore, partial coverage of surface charges on sand and PMPs by BDH reduced electrostatic repulsion, while hydrogen bonding between BDH and PMPs further enhanced their retention. The findings advance a deeper understanding of MP transport and retention in modified porous media, providing a scientific basis for developing green and efficient strategies to control microplastic pollution.