Small is better: Impact of polyelectrolyte Mw on layer-by-layer nanofiltration membranes for PFAS removal

For removal of linear and negative charged Per- and polyfluoroalkyl substances (PFAS), highly negatively charged and small pore NF membranes are urgently needed. This work reports, for the first time, an attempt to regulate the pore size and surface charge of polyvinylamine/sodium polystyrene sulfonate (PVAm/PSS) layer-by-layer nanofiltration (LBL NF) membranes by varying the polyelectrolytes' molecular weights (Mw) for PFAS removal. In general, PVAm/PSS NF membranes exhibited a strong negative surface charge, with 99.9 % Na₂SO₄ rejection. Smaller pore size and greater polycation PVAm overcompensation were observed for PVAm of lower Mw. Short-chain PVAm showed small hydrodynamic radius and fast diffusion, which dominated the LBL assembly and resulted in intrinsically positively charged membranes and small pore size. The removal of PFAS during a 24-hour period demonstrated an initial decline, followed by an increase until reaching a stable value; this unique pattern originated from PFAS adsorption onto the overcompensated, positively charged PVAm in the separation layer, leading to subsequent charge reversal as well as pore size reduction. The PFAS removal rate was positively correlated with the molecular weight, polarizability, and hydrophobicity of PFAS, justifying that the membrane pore size and PFAS molecular dimensions are key removal factors. The study highlighted the importance of the assembly kinetics in regulating LBL membrane pore size and charges, as well as solute-adsorption-induced changes in membrane physicochemical characteristics and their subsequent impact on PFAS removal.