pH-Swing membrane adsorption of perfluoroalkyl substances: Anion-exchange brushes and role of water chemistry

The pH-swing membrane adsorption, aiming for effective removal, high membrane permeability, and facile regeneration, was developed to remediate emerging perfluoroalkyl contaminants. Compared to the collapse transformation of deprotonated tertiary-amine brushes at neutral pH (pK_b: ∼5.82), the quaternary-ammonium (QA) brushes endowed consistent membrane pore structure and higher isoelectric point (pH_IEP: 11.2 versus 7.5), resulting in a suitable pH-swing adsorption/desorption range towards groundwater conditions. Such QA-grafted membranes (QA loading: 1.1 mmol/g) not only presented >90 % removal of perfluorooctanoic acid (PFOA) at a treatment capacity of 570 L per m² of membrane area, but also enabled effective membrane regeneration that >97 % desorption was achieved at pH 12.5 and 5 % methanol. The remediation is driven by synergetic electrostatic and hydrophobic interactions, as demonstrated by (1) the zeta-potential dependent removal performance and (2) the greater maximum adsorption capacity towards the more hydrophobic perfluorooctane sulfonate than PFOA (Q_m: 0.65 mmol/g and 0.44 mmol/g, respectively). Within three pH-swing adsorption/regeneration cycles, a total PFOA removal of 82.4 % was achieved with a treatment capacity of 2,930 L/m². The impacts of ionic strength, ionic types, and natural organic matters were evaluated. Overall, the pH-swing strategy is an effective method with high permeability (165.6 L m⁻²h⁻¹bar⁻¹), stability, and tunable adsorption/regeneration processes.