Microplastic retention in soils: Pore-scale mechanisms revealed by high-resolution micro-computed tomography and digital rock physics.

Microplastic (MP) pollution is a growing concern for soil health, water quality, and biogeochemical processes. Evaluating the long-term environmental impacts of MPs requires understanding the processes controlling their mobility in soils. However, conventional approaches mainly rely on column outlet measurements or destructive sampling, leaving pore-scale MP behaviour poorly resolved. Here, we developed a systematically optimised, non-destructive workflow that combines small-scale column experiments, high-resolution X-ray micro-computed tomography (µCT), and digital rock physics. This approach enables three-dimensional pore-scale visualisation and quantification of retained small-sized MPs (down to 2 µm) within saturated soil-relevant porous media under controlled hydraulic conditions. Our results show that MP retention behaviour does not decrease monotonically with increasing flow velocity. Instead, high-flow conditions cause more localised MP accumulation, resulting in measurable decreases in soil permeability. At low flow velocities, MP retention was primarily associated with diffusion-enhanced delivery toward grain surfaces. With increasing flow velocity, advective transport became dominant, resulting in lower but more evenly distributed MP retention. Under high-flow conditions, however, hydrodynamic multi-particle bridging blocked pore throats, resulting in a permeability reduction of up to 4.6%. By linking pore-scale retention mechanisms with changes in hydraulic properties, this study provides new mechanistic insight into MP transport in saturated porous media. The proposed workflow provides a basis for future studies aimed at improving the prediction of MP transport in subsurface porous media.