Competitive adsorption of per- and polyfluoroalkyl substances (PFAS) on activated carbon: Impact of activated carbon and PFAS properties

Activated carbon (AC) has been shown to be an effective adsorbent for the removal of per- and polyfluoroalkyl substances (PFAS) from impacted waters. To aid in the development and selection of effective ACs, this study investigated the adsorption of perfluorooctanoic acid (PFOA), perfluorooctanesulfonic acid (PFOS), and perfluorobutanesulfonic acid (PFBS) individually and as mixtures, and with the addition of Suwannee River natural organic matter (NOM), on six ACs with a range of source materials and properties. Single PFAS equilibrium adsorption data were fit to the Freundlich and Langmuir isotherm models, and PFAS mixture data were fit using three multi-component Langmuir-based adsorption models. All the ACs exhibited adsorption in the order of PFOS, PFOA, then PFBS. ACs with greater hydrophobicity and larger pore volumes yielded the best PFAS removal, while ACs with large pore volume but lower hydrophobicity exhibited substantially lower adsorption. Using a Spearman rank-order correlation, adsorption of each PFAS exhibited monotonic relationships with several AC properties, indicating that adsorption performance is not dependent on one AC characteristic alone. In mixtures, competition had minimal effects on PFOS and PFOA adsorption, but resulted in reduced adsorption of PFBS when the solid phase loading exceeded 14-44 mg/g. Similarly, competition from NOM impacted PFBS far more than PFOS and PFOA, with average decreases in adsorption of 15-35 % for PFBS compared to 6-7 % for PFOA and PFOS. These results indicate preferential adsorption of more hydrophobic PFAS to ACs, and the greater impact of competing species on less hydrophobic PFAS.