Cation dependent dynamics of sequential and cotransport processes of micro-polystyrene and soil colloids in porous media.

Interactions between microplastics (MPs) and soil colloids critically influence pollutant mobility in subsurface environments, yet remain insufficiently characterized under transient flow conditions. This study explored the effects of ionic strength, ion type, and cation exchange on the co-transport and release of polystyrene (PS) MPs and soil colloids in saturated quartz sand. In NaCl solutions, pre-deposited PS particles or soil colloids increased surface roughness and chemical heterogeneity of the sand, inhibiting subsequent particle transport. PS particles preferentially attached to pre-deposited soil colloids, whereas most subsequent soil colloids adhered directly onto sand surfaces. Flushing with deionized water induced significant remobilization of both particles due to reduced ionic strength, which was well captured by a transport model coupled with ion dynamics. In CaCl2 solutions, the impact of pre-deposited particles varies with ionic strength. At lower ionic strength, competition for attachment sites promoted particle transport, while at higher ionic strength, increased surface roughness and heterogeneity inhibited mobility. Unlike Na+, Ca2+ facilitated a staggered arrangement of soil colloids and pre-deposited PS particles through cation bridging, resulting in strong retention and minimal release during deionized water flushing. Subsequent cation exchange from Ca2+ to Na + weakened adhesive forces and triggered substantial release, with soil colloids showing greater sensitivity than PS. Overall, the interactions between suspended soil colloids and PS MPs are governed by cation valence and concentration, reflecting a balance between steric hindrance and surface heterogeneity. These findings provide insights into how pre-deposited and suspended PS and soil colloids influence each other's transport behavior and highlight the critical role of cation exchange in regulating mobility in subsurface environments.