Effects of input concentration, media particle size, and flow rate on fate of polystyrene nanoplastics in saturated porous media

Nanoplastics have gradually attracted widespread attention, but the studies in this area are still very scarce. In this research, the adsorption, transport, long-term release, and particle fracture of polystyrene nanoplastics (PS-NPs) in saturated porous media were investigated at different media particle sizes, input concentrations, and flow rates. The increased PS-NPs concentration and sand grain size promoted the adsorption of PS-NPs onto quartz sand. In transport tests, the peak breakthrough amounts of PS-NPs ranged from 0.5761 to 0.8497, demonstrating their high mobility in saturated quartz sand. Transport of PS-NPs in saturated porous media increased with decreasing input concentration and increasing media particle sizes. The effect of input concentration could be predicted by the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, in which adsorption played a dominant role. The effect of media particle size was mainly dominated by filtration rather than adsorption. As a result of higher shear force, increasing flow rate might boost transport of PS-NPs. With increasing media particle size and flow rate, more retained PS-NPs were released, which was in line with the findings of the transport tests on the mobility of PS-NPs. Notably, PS-NPs could be broken down into smaller PS-NPs during long-term release and the percentage of released PS-NPs (<100 nm) was gradually increased from 1st to 3rd PV effluent in all media particle sizes and flow rates. The fracture of released PS-NPs from medium quartz sand was the most in relation to fine and coarse and showed a decreased trend with increasing flow rate, which was likely to be governed by the force perpendicular to the contact surface with the media particle. This study showed that PS-NPs have strong mobility in porous media and are easily broken into smaller particles during long-term release. The findings of this research provided fundamental information for clarifying transport laws of nanoplastics in porous media.