Effects of interlayer cations and hydration on PFAS adsorption and mobility in montmorillonite: A molecular dynamics...

Per- and polyfluoroalkyl substances (PFAS) are persistent contaminants of growing global concern because of their high environmental stability, mobility, and remediation difficulty. However, molecular-scale understanding of their behavior in clay minerals remains limited, particularly for volatile PFAS precursors (e.g., Fluorotelomer alcohol, FTOH) and their comparison with ionic species (perfluorooctanoic acid, PFOA and perfluorooctanesulfonic acid, PFOS) in cation-exchanged systems under variable hydration conditions. In this study, molecular dynamics simulations were conducted to investigate the adsorption and transport behaviors of FTOH, PFOA, and PFOS in homoionic montmorillonite (Na-, K-, and Cs-MMT) at different moisture contents. The results show that PFAS adsorption on MMT is primarily governed by the interactions between the oxygen-containing hydrophilic head groups and the H/O sites on the clay surface, while the fluorinated tails contribute negligibly to direct surface affinity. Under low-moisture conditions, PFAS adopt a typical "head-anchored, tail-escaping" configuration. Among the three cation systems, Na-MMT exhibits the strongest adsorption, shortest interaction distance, and smallest mobility, followed by K-MMT and Cs-MMT. Increasing moisture content weakens adsorption but generally promotes PFAS mobility in nanopores, with PFOA showing the greatest increase in diffusion. These findings provide new molecular-level insights into the distinct behaviors of vapor and ionic PFAS in clay nanopores, with implications for predicting their environmental fate and improving clay-based containment strategies.