The adsorption of perfluorooctanoic acid and perfluorooctane sulfonic acid by gibbsite

Abstract

Perfluoroalkyl substances (PFAS) represent a global source of environmental contamination and have been linked to a wide variety of health issues. Despite their abundance, little is known about the mechanistic interactions between PFAS and common soil minerals. This study investigated the adsorption of perfluorooctanoic acid (PFOA) and perfluorooctane sulfonic acid (PFOS) by gibbsite (Al(OH) 3 ). To elucidate their bonding mechanisms, microelectrophoresis and attenuated total reflection Fourier‐transform infrared (ATR‐FTIR) spectroscopic analysis were performed. The electro‐kinetic experiments identified a shift in the isoelectric point of gibbsite from pH 9.94 to 7.12 following equilibration with PFOS. However, no such shift was observed with PFOA adsorption. The ATR‐FTIR analysis examined both analytes in their dissolved and adsorbed states and showed the ionizable functional groups shifted to different wavenumbers in the presence of gibbsite, suggesting inner sphere complexation. Adsorption envelope studies were performed under varying conditions of pH, ionic strength, and solution composition. Results showed an increase in PFAS adsorption with decreasing pH and ionic strength. For 10 mM and low pH conditions, almost 50% of PFOA and 80% of PFOS were adsorbed. In the presence of sulfate and phosphate, PFOS adsorption was reduced and PFOA adsorption was inhibited. In the presence of calcium and copper, adsorption of both analytes was unaffected. Finally, surface complexation modeling was used to predict PFAS adsorption under varying pH and ionic strength conditions. The results indicate the PFAS examined in this study adsorb to gibbsite through a combination of inner sphere and outer sphere mechanisms.