Interaction of Layer-by-Layer Nanofiltration Membranes and PFAS: Filtration after Adsorption

Removal of perfluoroalkyl and polyfluoroalkyl substances (PFAS) by nanofiltration (NF) membranes has been well-documented, but the impact of PFAS interaction with the NF separation layer on the permselectivity of layer-by-layer (LBL) NF membranes has yet to be explored. The work reported two different LBL membranes prepared with pairs of long-chain polyelectrolytes (PVAm/PSS, L/L) and short-chain polyelectrolytes (S/S) for analysis of the impact of PFAS adsorption on removal. Both L/L and S/S membranes showed high Na2SO4 rejection (99.9%) and PFAS removal rates in the range of 73.6∼98.8%. A strong positive correlation between the PFAS removal rate and molecular dimensions suggested size exclusion as the main separation mechanism. After filtration, the pore size of both membranes decreased and the surface zeta potential became more negative. The adsorption of PFAS in the S/S membranes was significantly greater than that of the L/L membranes; electrostatic interaction of PFAS with positively charged overcompensated PVAm was the key contribution. The adsorption of PFAS in the S/S membranes caused increased MgCl2 rejection and reduced Na2SO4 rejection. Contrary PFAS adsorption behavior in PEC and LBL membrane suggested PVAm overcompensation varied with the assembly process. This work highlights solute/membrane interactions as a new key factor for designing NF membranes for organic removal.