Mineral surface-specific nanoplastic adsorption: Insights from quartz crystal microbalance experiment and molecular modeling simulations

Nanoplastic (NP) transport in soil and natural water is primarily controlled by adsorption onto mineral surfaces, with long-range electrostatic interactions traditionally considered the main force. This study focuses on the role of hydrophobic and hydrophilic interactions in the nanoplastic adsorption. We performed quartz crystal microbalance (QCM) deposition experiments and molecular dynamics (MD)-based potential of mean force (PMF) calculations for the adsorption of carboxylated polystyrene (CPS) NPs on SiO₂ and Al₂O₃ surfaces under environmentally relevant ionic strength conditions. QCM measurements showed that increasing ionic strength enhanced NP deposition on SiO₂ but reduced it on Al₂O₃. Atomistic PMF calculations corroborated these results, revealing more negative free energy of CPS-NP adsorption on SiO₂ and more positive on Al₂O₃ with increasing ionic strength. Contrasting with traditional DLVO theory, our MD simulations predicted a constant Stern-layer thickness independent of ionic strengths and demonstrated CPS-NP adsorption to SiO₂ via hydrophobic benzene groups and to Al₂O₃ via hydrophilic carboxyl groups. Higher electrolyte concentrations strengthened hydrophobic interactions on SiO₂ by disrupting interfacial water structure, while accumulated ions hindered NP deposition on Al₂O₃. These findings highlight the critical role of hydrophobic and hydrophilic interactions in NP-mineral systems, which is often neglected in predicting the environmental transport of NPs.