Effects of clay minerals on the transport of nanoplastics through water-saturated porous media

Clay minerals are important constituents of porous media. To date, only little is known about the transport and retention behavior of nanoplastics in clay-containing soil. To investigate the effects of clay minerals on the mobility of nanoplastics in saturated porous media, polystyrene nanoplastics (PS-NPs) were pumped through columns packed with sand and clay minerals (kaolinite and illite) at different pH and ionic strengths (IS). Mobility of PS-NPs decreased with increasing clay content attributed to physical straining effects (smaller pore throats and more complex flow pathways). Variations in pH and IS altered the surface charges of both PS-NPs and porous media and thus affecting the interaction energy. An increase of IS from 10 mM to 50 mM NaCl decreased the maximum energy barrier and secondary minimum from 142 KT to 84 KT and from -0.1 KT to -0.72 KT, respectively. Thus, the maximum C/C ratio decreased from ~51% to ~0% (pH 5.9, 3% kaolinite). Among the two clay minerals, kaolinite showed a stronger inhibitory effect on PS-NPs transport compared to illite. For instance, at the same condition (3% clay content, pH 5.9, 10 mM NaCl), the (C/C) of PS-NPs in kaolinite was ~51%, while for illite, it was ~77%. The difference in transport inhibition was mainly attributed to amphoteric sites on the edges of kaolinite which served as favorable deposition sites at pH 5.9 (pH is ~2.5 for illite and ~6.5 for kaolinite). Besides, the morphology of kaolinite was more complex than illite, which may retain more PS-NPs in kaolinite. Results and conclusions from the study will provide some valuable insights to better understand the fate of NPs in the soil-aquifer system.