Adsorption of Per- and Polyfluoroalkyl Substances on Gibbsite: Insights from First-Principles Molecular Dynamics Simulations

The complexation onto Al oxide is critical in governing the retention and environmental fate of per- and polyfluoroalkyl substances (PFAS), yet molecular-scale mechanisms remain poorly understood. This study employs first-principles molecular dynamics (FPMD) simulations to elucidate the dynamic interfacial behavior and complexation processes of perfluorooctanoic acid (PFOA) and perfluorooctanesulfonate (PFOS) interacting with the different crystal surfaces of gibbsite in aqueous solution. On the basal surface, PFOA and PFOS form outer-sphere complexes mainly via hydrogen bonding between their oxygen-containing functional groups and surface hydroxyl moieties. In contrast, the edge surface facilitates inner-sphere complexation, where both compounds adopt a monodentate coordination mode through ligand exchange with the water ligand of the edge Al sites. Free-energy calculations demonstrate that both inner-sphere complexes are thermodynamically favorable and spontaneous, highlighting the critical role of edge sites in driving strong PFAS retention. Under highly alkaline pH conditions, the formation of edge surface-Ca²⁺-PFOA/PFOS complexes becomes the dominant inner-sphere species, and this cation-mediated mechanism stabilizes PFAS at this interface. This study quantitatively reveals the crystal surface-dependent retention and control mechanisms of PFAS on gibbsite by providing atomic- and electronic-level insights into how mineral surface topology governs the distribution and fixation of PFAS.