Abstract:

The transport and retention of micro/nano-plastics (M/NPs) in porous media is a critical concern in soil and groundwater contamination. However, limitations in visualization techniques have hindered a deeper understanding of the microscopic mechanisms governing M/NP transport under two-phase flow conditions. Using a three-dimensional visualization apparatus, we directly observed and quantified the distribution characteristics of M/NPs within pore spaces, with a specific focus on their retention at grain-grain contacts, solid surfaces, and fluid-fluid interfaces during imbibition. The effects of M/NP size and imbibition rate on their spatial distribution were further investigated. Our results reveal that M/NP size predominantly governs the selection of retention sites by altering the M/NP-solid and M/NP-fluid interface interaction energies. Conversely, the magnitude of interaction energy between M/NPs determines their potential to form aggregated clusters. The flow rate primarily influences the overall retention by altering the solid-wetting phase interfacial area. Moreover, distinct mechanisms govern retention at different locations: straining dominates at grain-grain contacts, while interfacial adsorption prevails at solid surfaces and fluid-fluid interfaces. Under high flow rates, M/NPs retained on solid surfaces can be readily detached due to enhanced shear forces. In contrast, the size of aggregated M/NP clusters at grain-grain contacts increases with flow velocity. This study elucidates the synergistic mechanisms by which M/NP size and flow velocity govern M/NP migration and retention, providing a theoretical basis for predicting the transport and fate of contaminants in multiphase subsurface environments.

Key words: micro/nano plastics, porous medium, two-phase flow, retention mechanism, distribution characteristics