Mechanistic insights into the pore-scale transport and retention of microplastics: bridging physical and geochemical controls.

Porous media with constricted throats and heterogeneous surfaces dictate microplastic transport and retention within the soil-groundwater continuum; however, the multifaceted mechanisms governing their subsurface accumulation remain poorly understood. This study quantifies microplastic migration pathways, adsorption hotspots, and breakthrough dynamics in aquifers using a high-resolution pore-scale Lagrangian framework that couples hydrodynamic drag, Brownian motion, dielectrophoretic (DEP) forces, and surface adsorption. Our results demonstrate that while Brownian perturbations fundamentally drive particle-wall collisions, DEP forces and minerals with high surface charge magnitudes serve as deterministic geochemical controls on particle immobilization. We show that geometric singularities, such as narrow throats and sharp bends, act as permanent retention hotspots regardless of bulk porosity. A non-monotonic size effect was identified, where particles of an intermediate size (3 µm) exhibited the highest maximum velocity. Environmental forcing significantly modulates these processes: warming groundwater (0 °C to 25 °C) enhances Brownian-driven filtration, more than doubling the retention capacity from 1.2% to 2.65%. Conversely, higher inlet velocities intensify hydrodynamic drag and confine trajectories to preferential flow paths, thereby suppressing dispersive adsorption. Furthermore, pore-network architecture (diagonal vs. transverse flow) can shift the aquifer's role from a transport conduit to a long-term pollutant sink. By bridging the gap between microscopic physical forces and geochemical controls, this study provides a mechanistic tool to assess the vulnerability of groundwater resources and informs long-term subsurface risk management.