Transport behavior of polystyrene nanoplastics in saturated quartz sand: coupled experimental and modeling approaches

The environmental mobility of nanoplastics (NPs) is a critical concern due to their persistence and potential ecological risks. However, the interactive effects of aging degree and solution chemistry on the transport of NPs with different sizes in saturated porous media require further systematic quantification. In this study, the transport behaviors of PS NPs with different particle sizes (100 and 800 nm) and aging degrees (pristine, UV-aged for 8 h and 24 h) were investigated through sand column experiments under varying pH, ionic strength (IS), cation valence, flow rate, and initial concentration. The relative breakthrough concentration (C/C₀) of pristine PS NPs decreased from 84% to 12% as NaCl IS increased from 1 to 100 mM, and divalent cations (Ca²⁺) further suppressed transport to 7-38% compared with 61-87% for Na⁺ at 10 mM. In contrast, UV aging enhanced NP mobility, with 24 h-aged particles exhibiting 7-15% higher C/C₀ than pristine counterparts, due to reduced particle size, increased negative surface charge density, and the introduction of oxygen-containing functional groups (OFGs). The dual-site model successfully simulated the retention behaviors under most conditions, while single-site models were required under strong adsorption scenarios. Extreme gradient boosting (XGBoost) analysis identified cation valence, IS, particle size, and aging as the dominant factors controlling PS NP migration, consistent with model-derived maximum retention densities increasing from 0.4 to 0.9 mg kg^-1 with IS and up to 1.35 mg kg^-1 in the presence of Ca²⁺. These findings indicate that aged NPs may exhibit higher mobility and persistence in subsurface environments, thereby increasing their potential to contaminate groundwater systems. The strong dependence of PS NPs transport on water chemistry further highlights the sensitivity of NP fate to hydrogeochemical changes with direct implications for environmental risk assessment.