Pore-scale visualization and microscale barrier mechanisms of microplastics transport in bio-based hydrogel modified soils

Soil stabilization techniques, as a potential barrier method for microplastics (MPs) migration, remain underexplored in terms of their impact on MPs transport behaviors. To elucidate the influence of soil stabilization on MPs transport behaviors, a pore-scale dynamic visualization system was employed to unravel the intricate transport and retention mechanisms of MPs in bio-based hydrogel (BH)-modified porous media. Breakthrough curves (BTCs) and effluent concentrations collected from flow cells were used to quantitatively characterize MPs transport. Videos of MPs flow process captured from different scenarios revealed MPs can move along bulk flow in porous media via sliding, rolling, and saltating patterns. Scanning electron microscopy and nuclear magnetic resonance analyses unveiled microscopic retention mechanisms due to BH modification. Notably, BH-modified soil introduced a novel recirculation mode, attributed to stagnation zones created by BH. Compared with natural soil, where MPs typically deposit via sedimentation and physical straining, BH-modified soil exhibited a distinct deposition mechanism primarily driven by network entrapment. The MPs mobility showed pronounced sensitivity to environmental parameters with significant barrier effects observed under increased BH contents, higher sand density, and vertical flow conditions compared to horizontal flow. The maximum reduction of MPs mass recovery was 28.5 %. Our investigation visually deciphers the complex transport and retention patterns of microplastics in modified porous media, substantially expanding the current understanding of MPs' environmental fate and transport mechanisms.