Facet-dependent adsorption of PFOS and 6:2 Cl-PFESA onto hematite surfaces

Assessing the transport and fate of the sulfonate-containing per- and polyfluoroalkyl substances (PFAS) in groundwater requires a molecular-scale understanding of their interactions with minerals, particularly environmentally abundant iron oxides such as hematite. Here, we systematically investigated the adsorption of perfluorooctane sulfonate (PFOS) and its chlorinated ether alternative, 6:2 chlorinated polyfluoroalkyl ether sulfonate (6:2 Cl-PFESA) on hematite surfaces using batch experiments, in situ spectroscopy, surface complexation modeling, and density functional theory (DFT) calculations. The adsorption was regulated by Fe-coordinated hydroxyl (Fe-OH) sites, PFAS molecular structure, and aqueous chemistry, transitioning from monolayer to multilayer with increasing concentration. Elevated pH suppressed adsorption through enhanced electrostatic repulsion, whereas Ca²⁺ promoted retention via Fe-OH-Ca²⁺-sulfonate ternary complexation, attenuating the pH dependence of adsorption. Spectroscopic evidence supported outer-sphere complexation at Fe-coordinated surface hydroxyl sites, as well as hydrophobic interactions from fluorinated chains. For 6:2 Cl-PFESA, the ether bond engaged in the initial interfacial response before sulfonate anchoring. CD-MUSIC modeling identified the (012) facet as the highest-affinity hematite surface. DFT calculations supported facet-dependent intrinsic single-molecule binding tendencies in the order (012) > (214) > (110) > (001) under idealized surface conditions. These molecular-scale insights establish a facet-specific mechanistic framework for predicting PFOS and 6:2 Cl-PFESA adsorption at hematite-water interfaces, thereby improving assessments of their retention and mobility in iron oxide-rich subsurface environments under variable groundwater chemistry.