Transport of nanoplastics in saturated iron oxide-coated gravel: Effects of flow velocity, ionic strength and surface property of nanoplastics

• New information on nanoplastics transport in the aggregated gravel was provided. • Increasing IS gave oposite results on negatively & positively charged nanoplastics transport. • high flow rates promote nanoparticle transport in gravel. • Surface properties of nanoplastics had distinct effects on the transport behavior. • Nanoplastics exhibited greater transport ability in iron oxide-coated gravel. This study investigates the transport and deposition behavior of nanoplastics with different surface charges in heterogeneous porous media, focusing on the effects of flow velocity, ionic strength and surface properties of nanoplastics. Controlled laboratory experiments were conducted using polystyrene (PS), carboxylate-modified polystyrene (CPS), and amine-modified polystyrene (APS) latex microspheres in gravel and iron oxide-coated gravel media under varying flow rates and ionic strengths. The results show that enhanced flow velocities significantly promote nanoplastic transport by reducing overcoming deposition potentials at solid-liquid interfaces. Elevated ionic strength induces contrasting retention behaviors: increasing deposition of negatively charged nanoplastics (PS and CPS) while reducing the retention of positively charged APS. Iron oxide coatings paradoxically increase nanoplastic recovery in effluents by altering surface roughness and blocking intra-granular porosity, despite lower energy barriers for attachment. This study highlights the importance of considering both physical and chemical heterogeneity in predicting nanoplastic behavior in complex porous media. The mechanistic insights derived from this work provide a valuable framework for environmental risk quantification and the development of targeted remediation approaches for nanoplastic-contaminated aquifers.