Transport Mechanisms of Microplastics in Clean Sand under Cyclic Hydraulic Gradients

Understanding the transport mechanisms of microplastics (MPs) in porous media is critically important for pollution control and mitigation. MP transport in soils and sediments under fluctuating hydraulic gradients induced by extreme weather events has become increasingly recognized due to intensifying climate change. This study aims to unveil the MP transport behaviors under cyclic hydraulic gradient coupled with pore network changes of sand filters by conducting experiments using a modified permeameter. The transport of MPs in sand can be significantly affected by the potential clogging, indicated by the variation of permeability and MP mass flux. These two important indexes characterize the extent of decline of filtering capability in porous media and the risk of MP leakage into the environment. The clogging of 20-μm MPs caused by constant hydraulic gradients of 10.5 and 21 can be unclogged by cyclic hydraulic amplitudes of 5.25 and 21, respectively, with the effect enhanced by increasing the hydraulic frequency from 0.01 to 0.02 Hz. Once the unclogging happens, the restoration of permeability and mass flux can be observed compared to the steady flow conditions. Overall, the clogging and unclogging effects are simultaneously dependent on the average hydraulic gradient, amplitude of oscillation, and frequency of the cyclic hydraulic condition. This study highlights the importance of considering the potential effect of future climate changes when predicting the MP leakage in soil filters.