Transport and retention mechanisms of micro/nano plastics during two-phase flow

doi:10.13745/j.esf.sf.2025.10.9

Abstract

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

CLC Number:

LIU Qiming, YANG Zhibing, WU Ting. Transport and retention mechanisms of micro/nano plastics during two-phase flow[J]. Earth Science Frontiers, 2026, 33(1): 222-235.

Figures/Tables 10

Fig.1 3D visualization experimental system

Fig.2 DLVO interaction energy between M/NPs, between M/NPs and the two-phase interface, and between M/NPs and the solid surface for M/NPs (a)—d=0.5 μm;(b)—d=1 μm;(c)—d=5 μm。

Fig.3 Distribution of the nonwetting liquid and M/NPs of different diameters (d) in porous media and displacement flow rates (Q) (a)-(c)—d=0.5 μm,Q=1、50和500 μL/min;(d)-(f)—d=1 μm,Q=1、50和500 μL/min;(g)-(i)—d=5 μm,Q=1、50和500 μL/min。比例尺代表100 μm。

Fig.4 Volume distribution of M/NPs clusters with different diameters

Fig.5 Variation of residual saturation and interfacial area with flow rate

Fig.6 Schematic diagram and microscopy image showing M/NPs clusters accumulation at: (1) grain-grain contact area, (2) solid surfaces, and (3) fluid-fluid interfaces

Fig.7 Local 3D reconstruction of 5 μm M/NPs clusters at typical accumulation sites after displacement at Q=500 μL/min

Fig.8 Volume distribution with local 3D reconstructions of 0.5 μm M/NPs clusters at typical accumulation sites

Fig.9 Volume distribution with local 3D reconstructions of 1 μm M/NPs clusters at typical accumulation sites

Fig.10 Volume distribution of 5 μm M/NPs clusters at typical accumulation sites

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