Evaluating the Role of Electrostatic Forces in the Adsorption of Perfluoroalkyl Substances to Granular Activated Carbon

In response to new drinking water regulations announced by the EPA in 2024, many municipalities will need to monitor concentrations and implement new treatments to address PFAS, a class of anthropogenic organic contaminants. A leading technology for this application is GAC adsorption due to its familiarity and availability. The adsorption of four PFAS species (PFBA, PFBS, PFOA and PFOS) to Calgon Filtrasorb400 (F400) was evaluated for a range of pH and ionic strength conditions to investigate the role of electrostatic interactions in adsorption capacity. These PFAS are all strong acids, so pH was used as a control for GAC surface charge, assuming the neutral PFAS would be insignificant. High ionic strengths (IS) were used to create competition and shielding for charged sites to inhibit electrostatic interactions. The pHpzc of F400 was evaluated with the pH drift method and found to be 10.44, therefore it will be positively charged in most drinking water applications, where pH is usually between 6.5 and 8.5. Batch testing was used to calculate distribution coefficients and quantify equilibrium adsorption using the aqueous loss method. Equilibration times were much longer than expected, and 23-day concentrations were used for calculating distribution coefficients. PFBA, PFBS and PFOA all showed decreasing adsorption with increasing pH, as expected for GAC adsorption of an anionic surfactant, but PFOS showed no clear trend in adsorptive capacity to the GAC with pH. For PFBA and PFBS, when pH below was pHpzc, the highest adsorption was observed in the reactors with the lowest ionic strength and increasing ionic strength was inversely correlated with adsorptive capacity. This suggests that electrostatic interactions are important for sorption of these PFAS to GAC. A clear trend in adsorption with ionic strength was not observed for PFOA and PFOS, suggesting that electrostatic interactions are not the controlling mechanism in adsorption or that other mechanisms, like the salting out effect, served to mask the expected behavior. These findings serve as evidence that when evaluating an activated carbon for PFAS removal, the criteria should not be the same for every source water. Rather, the specific PFAS present should be identified and, with their unique chemical behaviors in mind, a treatment process or GAC selection should be implemented to address the specific source water with consideration to its unique chemical makeup, such as the pH and ionic strength.