Dynamics of microplastic particle deposition in porous media

Abstract:

The majority of microplastic particles in the environment are located in sediments at the bottom of oceans, seas, and lakes; in groundwater aquifers; or in contaminated soils. All of these examples involve the transport of microplastic particles in a disordered, three-dimensional (3D) porous medium. In this case, not only do confinement and tortuosity imposed by the medium alter microplastic particle transport, but the particles in turn can alter the medium by depositing onto its solid matrix as they are transported, yielding coupled dynamics that pose a challenge to current understanding. Here, we elucidate these coupled dynamics by directly visualizing microplastic particle transport and deposition in transparent, 3D porous media over a broad range of length and time scales. We find that while the pore-scale distribution of deposited particles is sensitive to particle charge, their distribution throughout the entire medium is tuned by imposed pressure in unexpectedly similar ways, independent of particle charge. Specifically, at high injection pressures, hydrodynamic stresses cause particles to both deposit on and become eroded from the solid matrix continually—strikingly, forcing them to be distributed throughout the entire medium. By contrast, at low injection pressures, the relative influence of erosion is suppressed, causing particles to be localized near the inlet of the medium. Guided by these findings, we describe the evolution of the overall permeability of the medium over time, providing a quantitative description of how deposition in turn impacts fluid flow. Our results thus deepen our understanding of the multi-scale interactions between flowing fluid, microplastic particles, and a porous medium during colloidal transport, yielding guidelines for prediction and control of microplastic transport in environmental settings.