Influence mechanism of attapulgite on the migration of carboxylated polystyrene nanoplastics and the role of environmental factors

Nanoplastics have raised significant concerns due to their potential ecological risks and contamination of soil-groundwater environments. Previous studies have investigated the effects of clay minerals on nanoplastics migration, but focused on single minerals and pure quartz sand media, without involving the unique layer-chain structure of attapulgite (ATP) and the differential impacts of humic acid (HA) and oxalic acid (OA) on migration behavior. This study investigates the migration mechanisms of carboxyl-modified polystyrene nanoplastics (CPSNPs) in saturated porous media containing a representative natural mineral- ATP. A combination of characterization techniques, Derjaguin-Landau-Verwey-Overbeek (DLVO) interaction theory, one-dimensional column transport experiments and numerical transport model was employed. The effects of environmental factors (ATP content, flow velocity, pH, electrolytes, and organic acid) on CPSNPs migration were systematically investigated. Results suggest that ATP, with its rough surface, abundant pores, and high specific surface area, significantly enhances CPSNPs retention through adsorption and "blocking effect". As ATP content increased from 1 % to 9 %, the mass recovery rate of CPSNPs decreased from 85.39 % to 55.97 %. Fourier-transform infrared spectroscopy (FTIR) indicated that CPSNPs adsorbed onto ATP without significant chemical structural changes, primarily through physical adsorption. Additionally, HA and OA were found to influence CPSNPs migration differently, with HA enhancing and OA inhibiting CPSNPs movement. Divalent cations (Ca) strongly hinder CPSNPs migration compared to monovalent cations (Na), due to stronger charge neutralization and aggregation effects. The findings can provide essential insights into CPSNPs transport and retention in natural porous media.