How dissolved minerals affect the transport and retention of microplastics in unsaturated porous media*

Microplastics, which are ubiquitous and acknowledged as one of the major pollutants in the subsurface environment, commonly enter unsaturated soil through intermittent saturation events, such as rain or discharge. This process is relatively less understood due to the inherent limitation of the characterization of the transport processes in the soil. By utilizing a soil-mimicking microfluidic porous media, we investigate the impact of dissolved mineral species on the transport and deposition of microplastics in unsaturated porous media. While the transport of microplastics in saturated porous media is governed primarily by pore flow, the sudden, periodic introduction of mineral-rich immobile water pockets in unsaturated porous media alters the microplastic transport at the pore scale, leading to increased retention of the microplastics in impermeable pore space that is dependent on the mineral type and concentration. Our study offers insights into the overlooked impact of dissolved minerals within unsaturated pore spaces on the fate and transport of microplastics in soil.