Coupled transport and retention dynamics of microplastics in porous media

Microplastics (MPs) are emerging contaminants of increasing concern in subsurface environments because of their ability to migrate through porous media and threaten groundwater quality. Although many experimental studies have investigated MP transport, only a limited number of mathematical models exist, and these are mostly restricted to attachment-detachment processes. In reality, MPs exhibit a wide range of sizes and undergo multiple transport and retention mechanisms, including attachment, detachment, straining, blocking, ripening, agglomeration, and size exclusion. To address this gap, this study develops a unified three-dimensional mathematical framework that simultaneously incorporates these key mechanisms to provide a comprehensive description of MP transport in porous media. The governing equations are solved using a semi-implicit Crank-Nicolson finite-difference scheme and validated using four experimentally measured breakthrough curves from sand column studies. The model successfully captures early breakthrough, peak concentration, and long tailing behavior of MPs. Sensitivity analyses demonstrate the strong influence of MP size, collector grain size, attachment kinetics, and straining parameters on transport dynamics. Furthermore, three-dimensional plume simulations over a 4.8-year period reveal that blocking-dominated conditions promote long-range MP migration, whereas ripening enhances retention.