Molecular Insights into Adsorption and Mobility of Per- and Polyfluoroalkyl Substances at Hydrated Silica Surfaces

The widespread use of poly- and perfluoroalkyl substances (PFASs) has raised significant environmental and health concerns. Understanding their adsorption mechanisms on silica surfaces is crucial to predicting their fate in subsurface environments. In this study, we employed molecular dynamics simulations to investigate the adsorption behaviors of three representative PFAS compounds (PFHxA, PFPeS, and PFOA) on structurally distinct silica surfaces (Q2, Q3, and Q4). Our results indicate that PFAS adsorption configurations and interfacial dynamics are influenced by both surface hydroxylation and molecular structure. On Q2 and Q3 surfaces, PFAS headgroups preferentially formed hydrogen bonds with surface hydroxyls, whereas hydrophobic tails penetrated the interfacial water layer. In contrast, Q4 surfaces favored reversed configurations, a phenomenon primarily driven by hydrophobic interactions. PFAS mobility appeared more restricted on Q2 and Q3 surfaces compared with hydrophobic Q4 surfaces. Potential of mean force analyses suggested that Q3 surfaces with an intermediate hydroxyl density exhibited the strongest binding affinities. Additionally, short-chain PFAS showed higher affinities for Q2 and Q3 surfaces compared with Q4. These findings provide molecular-level insights into factors that may influence PFAS retention and transport in silica-rich environments and could inform the design of materials for PFAS remediation.