Kinetics of polystyrene nanoplastic deposition on SiO₂ and Al₂O₃ surfaces: Ionic strength effects

Nanoplastic pollution is an emerging environmental threat to the critical zone. The transport of nanoplastic particles in subsurface environments can be determined mainly by soil minerals because they provide surfaces that interact with nanoplastic particles. However, the interactions between mineral surfaces and nanoplastics are poorly understood. In this study, the deposition kinetics of polystyrene-nanoplastic particles onto representative oxide surfaces SiO₂ and Al₂O₃ at circumneutral pH were investigated using a quartz crystal microbalance, with variations in the ionic strength (0.1-100 mM) of the well-dispersed nanoplastic particles suspension. While polystyrene-nanoplastic particles deposited minimally on the SiO₂ surface at an ionic strength of < 100 mM (∼10 ng/cm²), substantial deposition occurred at 100 mM (3.7 ± 0.4 μg/cm²). On the Al₂O₃ surface, a significant amount of polystyrene-nanoplastic particle was deposited from the lowest ionic strength (4.5 ± 0.8 μg/cm²⁾. The deposition mass at 100 mM NaCl was two times higher (7.2 ± 0.2 μg/cm²) than on the SiO₂ surface, while the deposition rates were similar between the two surfaces (10-15 Hz/min). Our results indicate that alumina most likely exerts a stronger influence than quartz on the transport of nanoplastic particles in soils and groundwater aquifers. The deposition kinetics strongly depends on the mineral surface and solution ionic strength, and these quantitative results can serve as validation data in developing transport modeling of nanoplastic in subsurface environments.