Microplastics induce alterations in soil structure and hydrodynamics: A critical factor controlling cadmium transport...

Karst ecosystems face the problem of combined pollution of microplastics (MPs) and Cd because of their high geochemical background and anthropogenic pollution. However, the migration and environmental effects of Cd-MPs co-contamination in karst soils remain unclear. This study focuses on carbonate-weathered soils in karst areas, employing soil column leaching experiments to simulate the migration processes of MPs and Cd co-contaminants and to investigate the migration driving mechanism. MPs generally enhanced Cd migration ability compared with the control; however, at smaller sizes (10-15 μm) and higher concentrations (3 %), MPs inhibited Cd migration. Moreover, the cumulative leaching amount of Cd, soil leaching rate, and soil porosity showed significant positive correlations. Thus, during soil leaching, MPs primarily affect Cd migration in karst soils by altering the soil pore structure and hydrodynamic conditions rather than merely the adsorption-desorption capacity of MPs for Cd as conventionally perceived. Coarse-grained MPs (850-1000 μm) do not move downward in the soil but enhance Cd migration by enlarging soil pores and accelerating Cd leaching, exacerbating Cd pollution in deep soil and groundwater. Fine-grained MPs (10-15 μm) can migrate downward through soil pores but do not act as carriers to enhance Cd mobility. Instead, they block soil pores, reduce soil water leaching rates, and inhibit Cd migration, ultimately increasing the risk of Cd stress to surface vegetation. The potential ecological risks of MPs-Cd co-contamination in karst areas cannot be ignored, and developing prevention and control strategies based on comprehensive studies of soil MP occurrence characteristics is imperative.