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Microplastic retention in soils: Pore-scale mechanisms revealed by high-resolution micro-computed tomography and digital rock physics
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Scientists used detailed 3D scanning to watch how tiny microplastic particles get trapped in soil as water flows through it, something that's hard to observe otherwise. They found that surprisingly, fast-flowing water doesn't always flush microplastics out better, instead, it can cause them to clump together and clog small spaces in the soil, which reduces how well water moves through it. This matters because soil clogged with microplastics could affect groundwater filtration and how pollutants (including the microplastics themselves) travel toward the water we eventually drink.
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.
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Real-time pore-scale visualization using a microscope-coupled flow cell was used to track how fragmented microplastics move and deposit in saturated and unsaturated porous media, revealing distinct transport and retention patterns depending on water saturation conditions. The findings improve mechanistic understanding of how microplastics migrate through soils toward groundwater.
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Scientists studied how tiny plastic particles (microplastics) move through soil and sand when mixed with natural clay particles. They found that the combination of different clay types and water conditions can either help microplastics travel further underground or trap them in place. This research helps us better understand how microplastics might contaminate groundwater sources that provide our drinking water.
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