A dual-pathway modeling framework for rainfall-driven transport of microplastics in soil-water systems

The contamination and environmental fate of microplastics (MPs) in soil-water systems have attracted growing attention. This study systematically investigates the adsorption mechanisms and transport behavior of polyethylene (PE), polypropylene (PP), and polyethylene terephthalate (PET) MPs in sandy loam soil, utilizing DLVO theory, adsorption kinetics, isotherm experiments, and simulated rainfall tests. A novel dual-pathway quantitative model is developed to characterize rainfall-driven horizontal and vertical transport of MPs. The results demonstrate that the attachment of MPs in soil follows pseudo-second-order kinetics, with the Freundlich isotherm providing a better fit for the adsorption behavior. Furthermore, both DLVO theory and adsorption experiments confirm that PET MPs exhibit the strongest soil adsorption capacity. When the total rainfall amount is constant, as rainfall becomes more intense and shorter in duration, it results in a significant decrease in cumulative infiltration and key microplastic transport metrics, including horizontal flux, total vertical migration, and maximum penetration depth，Which confirming that MPs migration is strongly governed by rainfall patterns. PP MPs exhibit the highest horizontal mobility owing to its buoyancy, whereas PE MPs displays the deepest vertical penetration. PET MPs are largely retained in shallow layers with its strong affinity for soil particles. And the modified exponential decay models are developed to quantify horizontal transport against runoff timing ( [Formula: see text] , R²= 0.81-0.99) and vertical transport with migration depth(M(d)=M₀×e^-kd+C, R²= 0.94-0.99). These findings provide a robust quantitative framework for predicting MPs migration pathways in terrestrial environments and informing targeted pollution control strategies.