PFAS removal from reverse osmosis and nanofiltration brine by granular activated carbon: Thermodynamic insights into...

We explored an underexplored area in water treatment by examining the removal of per- and polyfluoroalkyl substances (PFAS) from reverse osmosis/nanofiltration (RO/NF) brine. We first compared multiple RO/NF membranes, revealing that DK and NF270 showed sub-optimal removal (<90 %) of C4-C8 PFAS, SW30 had low flux (<15 L/m²/h at 8 bar), and NFX exhibited significant adsorption of perfluorosulfonic acids (e.g., 8 µmol/m²). To address the PFAS-enriched brine generated from membrane treatment, we further evaluated activated carbon (GAC) and anion-exchange (AIX) resin, both of which efficiently removed moderate- and long-chain PFAS from brine. Although AIX outperformed GAC, the ion exchange contribution was small for short-chain PFAS like perfluorobutanoic acid (PFBA, C4) but increased with chain length, driven by the hydrophobic effect facilitating the migration to near-surface regions of resins. Equilibrium batch experiments and thermodynamic modeling revealed a disproportionate salinity impact on PFAS adsorption by GAC, with short-chain PFAS (e.g., PFBA) experiencing more pronounced adsorption reduction than longer-chained homologs as NaCl concentrations increased. This reduction was driven by a significant change in a free energy component unrelated to the hydrophobic or electrostatic interactions, likely due to the competitive adsorption of Cl^- ions and short-chain PFAS anions or the formation of hydration shells around Na⁺ and Cl^- ions, obstructing the pathways for weakly hydrophobic PFAS (e.g., PFBA) within the GAC pore network. The salting-out effect was found to be unimportant. This study provides new insights into salinity-dependent sorptive removal of PFAS from high-ionic-strength water such as RO/NF brine.