Migration and retention of polystyrene nanoplastics in porous media regulated by walnut shell biochar

The extensive accumulation of plastic waste in soil-groundwater poses a significant threat to subsurface environments. This study systematically elucidates the regulatory mechanisms governing the transport of polystyrene nanoplastics (PSNPs) in porous media amended with walnut shell biochar (WBC) through a combination of column experiments, Derjaguin-Landau-Verwey-Overbeek (DLVO) analysis, and two-site kinetic modeling. The results indicate that the incorporation of WBC markedly retards the migration of PSNPs. Owing to the pronounced microscopic surface roughness and well-developed lamellar pore structures of the biochar, the repulsive potential barrier between PSNPs and WBC is consistently lower than that between PSNPs and quartz sand, thereby providing abundant irreversible deposition sites. The modeling confirms that the synergistic effects of roughness-induced physical entrapment and short-range hydrophobic interactions drive the enhanced immobilization of the contaminants. Furthermore, the transport process of PSNPs is highly sensitive to hydrodynamic and hydrochemical variations. Increasing the WBC mass fraction, ionic strength, or acidity significantly compresses or neutralizes the electrical double layer, minimizing electrostatic repulsion and intensifying retention; here, divalent calcium ions exhibit a superior charge-neutralization and bridging capability compared to monovalent sodium ions. Conversely, elevated flow velocities, alkaline conditions, and the presence of humic acid enhance PSNPs mobility due to enhanced hydrodynamic shear, strengthened electrostatic repulsion, or competitive site occupation. These findings clarify the environmental behavior and interfacial mechanisms of nanoplastics under soil amendment practices, providing a scientific baseline for risk mitigation within soil-groundwater regimes.