Mechanism for the adsorption of Per- and polyfluoroalkyl substances on kaolinite: Molecular dynamics modeling

Per- and polyfluoroalkyl substances (PFAS) have been detected in ecosystems globally and pose a serious threat to human health. Clay minerals are promising adsorbents for PFAS removal from contaminated water, thus it is crucial to understand the interactions between PFAS and clay minerals. This paper performed molecular dynamics simulations to reveal the complex behaviors and fundamental mechanisms of the adsorption of perfluoroalkyl sulfonic acids (PFSA) on the siloxane and hydroxyl external basal surfaces of kaolinite. The results demonstrate that adsorption is favorable on both the surfaces; however, their mechanisms are distinct. On the hydroxyl surface, PFSA anions exhibit a steady “point adsorption” mode, with the diffusion coefficients considerably lower than those in bulk aqueous solution. The terminal sulfonate groups form hydrogen bonds with the surface hydrogen atoms like a “three-pin plug”, with a matching molecular geometry. The molecular force and molecular electrostatic potential analyses suggest that the Coulomb electrostatic attraction force from kaolinite to the terminal sulfonate groups of PFAS anions is the predominant driving force of adsorption for the hydroxyl surface. The potential of mean force (PMF) calculations show that the PFSA anions have to go over an energy barrier, which is induced by hydration of the hydroxyl surface, to reach the stable adsorption position. On the siloxane surface, PFSA anions exhibit a mobile “surface adsorption” mode, with a relatively high freedom along the plane parallel to the surface. The diffusion coefficients of the PFSA anions adsorbed on the siloxane surface are slightly lower than those in bulk aqueous solution. The non-polar CF chains lie flat above the siloxane surface due to the hydrophobic interaction, while the terminal sulfonate groups stay relatively far away from the siloxane surface. A greater van der Waals repulsive force from the outer water molecules to the CF chains is the predominant driving force of adsorption, and pushes PFSA anions to the siloxane surface as PFSA anions approach kaolinite. The minimum values of the PMF of adsorbed PFSA anions on the siloxane surface are lower than those on the hydroxyl surface, indicating that adsorption stability is higher for the siloxane surface.