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Mineral surface-specific nanoplastic adsorption: Insights from quartz crystal microbalance experiment and molecular modeling simulations
Summary
This study investigated how nanoplastics stick to mineral surfaces commonly found in soil and water — specifically quartz (SiO2) and alumina (Al2O3) — using both lab experiments and molecular computer simulations. The two minerals behaved oppositely: higher salt concentrations increased nanoplastic deposition on quartz but reduced it on alumina, explained by differences in hydrophobic versus hydrophilic surface interactions. Understanding these mineral-specific adsorption behaviors is important for predicting how nanoplastics move through soils and aquifers and whether they could reach drinking water sources.
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.