Transport of nanoplastics in porous media with ferrihydrite–humic acid coprecipitation: Effect of solution chemistry, nanoplastic size and aging degree

Nanoplastics (NPs) pose escalating environmental risks owing to their high mobility in porous media, yet the influence of abundant iron oxide–natural organic matter (NOM) complexes on the transport of pristine and UV-aged NPs remains poorly understood. This study investigated how ferrihydrite–humic acid (Fh–HA) coprecipitates affect the migration of different-sized and aged polystyrene NPs (PS NPs) using quartz sand (QS) column experiments under varying solution chemistry conditions. At 1 mM NaCl and pH 5.5, Fh–HA coprecipitation enhanced NP transport compared to a single Fh coating, improving transport efficiency by 25.5%. Raising pH to 9.5 restored NP transport efficiency in QS–Fh (83.4%) and QS–Fh–HA (86.1%). Under 10 mM Ca 2+ , NP transport efficiency remained relatively unaffected in QS–Fh–HA (7.678%) compared with in QS (6.790%) or QS-Fh (6.942%) due to steric and charge effects. Larger NPs (800 nm) were more susceptible to mechanical straining, whereas 24-h aged NPs exceeded 80% transport efficiency across all media, with the highest value in QS–Fh–HA (90.1%), driven by increased surface charge and hydrophilicity. COMSOL simulations and XGBoost analysis further proved that Fh–HA coatings exerted a weaker antagonistic retention effect. These findings reveal that Fh–HA weakens Fh-mediated NP retention by introducing steric hindrance and additional negative surface charge, while UV-aging further enhances NP mobility by diminishing hydrophobic interactions and strengthening electrostatic repulsion. This study advances the mechanistic understanding and predictive modeling of NP transport in porous media enriched with iron oxide–NOM complexes.