Insights on experimental methodologies and theoretical models for microplastics transport in soils and sediments based on meta-analysis

Much work remains to understand the sources, fates, and transport processes of microplastics (MPs) in terrestrial and water environments. Specifically, the transport of MPs in soils and sediments in various locations is underexamined. The transport mechanisms of MPs in soil are complex due to possible blocking, ripening, and straining phenomena. In this study, the methodologies to experimentally simulate MPs transport in soil are thoroughly reviewed. The factors of concern are soil grain size distribution, soil porosity, fluid ionic strength, MPs sizes and type, and the size ratio between MPs and soil. Available experiments only reasonably simulate the MPs transport in shallow beach sand. However, for other transport scenarios in soils, sediments, landfilled MSW, and landfill liners, experimental results are unavailable. Theoretical models are important for understanding the transport processes, but the model selection and fitting are still highly subjective so far. Six models based on advection–dispersion equations are used to fit the 29 sets of experimental results in the available studies. Akaike weights and post hoc analysis are used to further compare the fitting performances. Overall, the dual-site model considering both time- and depth-dependent deposition effects shows the best fitting performance for the experimental breakthrough curves and retention profiles of MPs in soil. The fitted model coefficients rely on the experimental conditions, especially fluid ionic strength and flow velocity.