The adsorption of perfluorooctanoic acid and perfluorooctane sulfonic acid by gibbsite

This study investigated the adsorption of two common PFAS compounds: perfluorooctanoic acid (PFOA) and perfluorooctane sulfonic acid (PFOS) by gibbsite (Al(OH)₃). In order to obtain a mechanistic interpretation of adsorption processes occurring at the solid-solution interface, the adsorption of PFOA and PFOS was investigated under varying conditions of pH, ionic strength, and solution composition. A combination of analytical and computational methods were used to elucidate the bonding mechanisms responsible for retention, and to identify the specific chemical reactions that occur. Microelectrophoresis techniques indicated a shift in gibbsite’s isoelectric point (IEP) from pH 9.94 to pH 7.12 following equilibration with PFOS, suggesting the formation of inner-sphere bonds. By contrast, no significant shift in gibbsite’s IEP was observed following equilibration with PFOA, suggesting outer-sphere adsorption mechanisms. Batch envelope experiments demonstrated an increase in PFOA and PFOS adsorption with decreasing pH and ionic strength. For 10 mM ionic strength conditions, almost 50% of PFOA and 80% of PFOS was adsorbed. In the presence of sulfate and phosphate, PFOS adsorption was significantly reduced and PFOA adsorption was effectively inhibited. Contrary to what other adsorption studies have reported in the literature, this study did not find any evidence to support the divalent cation bridging hypothesis, as the presence of Ca and Cu did not significantly influence PFAS adsorption. The direct bonds that form between PFAS analytes and gibbsite were also examined through ATR-FTIR spectroscopy. Both analytes were examined in their dissolved and adsorbed forms for comparison. The carboxyl and sulfonate functional groups from both analytes shifted to different wavenumbers in the presence of gibbsite, suggesting chemical interaction. Finally, the triple layer surface complexation model was employed to identify the specific chemical reactions involved in PFAS retention, and to generate equilibrium constants for the proposed reactions. The results of this study suggest PFOS and PFOA adsorbs to gibbsite through a combination of inner-sphere and outer-sphere adsorption mechanisms.