Hydrophobic Ion Pairing as a Pretreatment to Enhance the Removal of Short-Chain Per- and Polyfluoroalkyl Substances by Granular Activated Carbon

Hydrophobic ion pairing (HIP) was used as a novel pretreatment approach to enhance the removal of short-chain per- and polyfluoroalkyl substances (PFAS) by granular activated carbon (GAC). The formation of ion pairs between PFAS and cationic surfactants was tested through controlled batch experiments with varying types and dose of HIP (e.g., cetyltrimethylammonium chloride (CTAC)) and surface tension measurements. Interactions between PFAS and CTAC were observed to be disturbed in high ionic strength solution, likely due to competitive electrostatic interactions from the presence of inorganic ions. Addition of CTAC (2 μM) prior to GAC treatment in batch adsorption experiments greatly improved the sorption of short-chain PFAS (<C₇) by 54-111% but suppressed the sorption of long-chain PFAS (>C₇) by 23-96%. The changes in sorption capacities were hypothesized to result from the altered physicochemical properties of the formed ion pair complex, thereby affecting its uptake on GAC. Both the linear free energy relationship (LFER) calculation and molecular dynamic (MD) simulation suggested that short-chain PFAS sorption was improved via enhanced hydrophobicity of formed ion pairs with a low self-assembly tendency, while long-chain PFAS sorption was inhibited due to the formation of large complexes with a high self-assembly tendency. Rapid small-scale column tests were performed for tap water spiked with an equal mass of perfluorobutanoate (PFBA), perfluorobutanesulfonate (PFBS), and perfluorooctanesulfonate (PFOS). The bed volume for 50% breakthrough was improved by 350% and 230% for PFBA and PFBS, respectively, in the presence of CTAC and PFOS.