TOWARD UNDERSTANDING THE TRANSPORT OF NANOPLASTICS IN SUBSURFACE PORES

Nanoplastic pollution is not widely recognized as microplastic pollution because nanoparticulate plastics are difficult to collect and analyze. Recent nanoplastic toxicity studies have warranted the need for understanding the transport and fate of nanoplastics in soils and groundwater. Minerals likely determine the transport of nanoplastics (NPs) in the subsurface environment mainly through adsorption of NPs onto the pore surfaces. In the current presentation, we report the kinetics results of NPs adsorption onto SiO₂ and Al₂O₃ surfaces obtained by using quartz crystal microbalance (QCM). Carboxylated polystyrene (CPS) NPs and amine-functionalized polystyrene (APS) NPs with the particle size of ~100 nm were flown through onto SiO₂- or Al₂O₃-coated QCM sensor surface in the range of 0.1 to 100 mM NaCl solution at pH 7. Zeta potential measurement indicated that CPS-NPs and SiO₂ have negative surface charges, but APS-NPs and Al₂O₃ have positive surface charges. And thus no adsorption occurred for CPS-NPs on SiO₂ surface and for APS-NPs on Al₂O₃ surface; however, significant adsorption occurred in 100 mM NaCl solution because of the enhanced screening effect of electrolytes on repulsion between NPs and oxide surface. For NPs adsorption to oxide surface with opposite zeta potentials (i.e., CPS-NPs on Al₂O₃ surface or APS-NPs on SiO₂ surface), substantial amounts of NPs adsorbed even in 0.1 mM NaCl solution. Electrolytes reduce attraction between the NPs and oxide surface, but we found enhanced adsorption of NPs with increasing ionic strengths while the initial adsorption rates decreased with increasing ionic strength. We also present the effect of humic acid on the adsorption. Our results suggest that the transport of NPs in the subsurface pores is strongly influenced by mineral surface types and fluid chemistry. The adsorption kinetics data obtained in the current QCM study may be used as key parameters required for a reliable transport modelling of NPs in soils and groundwater.