MECHANISMS OF AQUEOUS FILM FORMING FOAM-DERIVED PER- AND POLYFLUOROALKYL SUBSTANCE ADSORPTION ON AQUIFER MINERALS
Per- and polyfluoroalkyl substances (PFASs) are persistent environmental contaminants of increasing concern, frequently detected in groundwater at sites with a history of aqueous film forming foam (AFFF) use, particularly at civilian and military airports. These areas, known as PFAS “source zones,” often contain high concentrations of PFASs in underlying aquifers due to decades of repeated AFFF application during firefighting training exercises. Once in the subsurface, PFASs can persist for extended periods, slowly leaching from the vadose zone into groundwater. Their transport is governed in part by adsorption processes, which retard migration through interactions with aquifer solids. While solid-water distribution coefficients (Kd values) have been reported for some commonly measured PFASs of concern, such as perfluorooctanoic acid (PFOA) and perfluoroctane sulfonic acid (PFOS), the adsorption behavior of many PFASs present in AFFF formulations remains poorly characterized. This data gap hinders accurate modeling and prediction of PFAS fate and transport in contaminated environments. The objective of this study was to measure Kd values for up to 40 PFASs known to be constituents of AFFF on four model aquifer minerals. Two iron oxides (ferrihydrite and goethite) and two silica clays (kaolinite and montmorillonite) were selected to represent environmentally relevant mineral phases with distinct surface properties. These minerals were chosen for their ubiquity in natural subsurface environments and their contrasting physicochemical characteristics, which offer complementary insights into the mechanisms governing PFAS adsorption. Batch isotherm experiments were conducted at multiple pH levels to assess the influence of pH on adsorption behavior. Zeta potential measurements and estimated pKa values were used to interpret trends in the adsorption data. Results showed that electrostatic interactions and hydrogen bonding play significant roles in PFAS-mineral adsorption. While perfluoroalkyl PFASs with sulfonic acid head groups and longer perfluoroalkyl chains generally exhibited higher Kd values compared to their carboxylic acid and shorter-chain counterparts, polyfluoroalkyl PFASs exhibit deviations from these trends. Despite these interactions, all PFASs studied showed evidence of complete desorption under the tested conditions, highlighting their potential for long-term mobility in the subsurface. The findings from this work provide essential adsorption data for a broad range of PFASs and mineral surfaces relevant to natural aquifer systems. These data can support more accurate simulations of PFAS transport from source zones, contributing to improved risk assessment and remediation strategies at contaminated sites.