A pore-scale investigation of microplastics migration and deposition during unsaturated flow in porous media

Microplastics are ubiquitous in the natural environment and have the potential to endanger the natural environment, ecology and even human health. A series of microfluidic experiments by using soft lithography technology were carried out to investigate the effect of flow rate, particle volume fraction, particle size and pore/throat ratio on microplastics migration and deposition at the pore scale. We discovered a range of deposition patterns of the spherical microplastics from no particle deposition, to discontinuous particle layer, and to continuous particle layers in the retained liquid in the pores, depending on the particle size and volume fraction. Several metrics, including air saturation, probability of particle detainment, expansion ratio and thickness of residual liquid, were quantified to examine the role of various parameters on particle migration and retention of microplastics. At low flow rate (Q = 0.05 μL/min), microplastics migration and deposition were sensitive to changes in particle volume fraction, particle size and pore/throat ratio. In contrast, at high flow rates (Q > 5 μL/min), the migration and retention of particles were mainly controlled by strongly channelized air invasion pattern, while the particle volume fraction, particle size and pore/throat size ratio have only secondary influence. At intermediate range of flow rates, microplastics migration and deposition were dramatically impacted by flow rate, particle volume fraction, particle size and pore/throat ratio. This work improves the understanding of the mechanisms of particle migration and retention in porous media and can provide a reference for more accurate assessment of the exposure levels and times of microplastics in soil and groundwater systems.