Effects of co-present mineral colloids on the transport of microplastics in porous media: The key role of hydrochemical and hydrodynamic conditions.

Co-present mineral colloids affect the transport and distribution of microplastics (MPs) in porous media, however, the transport mechanism remains unclear. In this study, the combined effects of montmorillonite and goethite colloids on the transport/retention behavior of MPs in porous media were investigated. The results show that the type of mineral colloids affects the transport and deposition behavior of MPs in porous media. The coexistence of mineral colloids at an equal ratio promotes MPs transport, while excessively high concentrations of either goethite or montmorillonite colloids inhibit MPs transport. Coexisting mineral colloids promote the transport of MPs in coarse sand media, while play an inhibitory role in medium/fine sand media. Divalent cations reduce MPs fluidity through charge shielding and heteroaggregate formation and the inhibition ability is much higher than that of monovalent cation. Co-present mineral colloids exert an inhibitory effect on MPs transport under acidic and alkaline conditions. The high flow rate promoted the penetration of MPs and induced their deep retention through the fluid drag force. The selective adsorption of coexisting mineral colloids on natural river sand may be a key interface mechanism for regulating transport. These results provide new insights into the theory of colloid-MPs cotransport in groundwater systems.