Role of Polyelectrolyte Concentration in Layer-by-Layer-Modified Sulfonated Polyether Sulfone Nanofiltration Membranes: Implications for Ion Selectivity and pH Resistance

Layer-by-layer (LbL) modification of nanofiltration membranes using polyelectrolytes is widely studied, though the influence of PE concentration on flat-sheet polymeric membranes has been insufficiently explored. In this study, acidic-resistant sulfonated polyether sulfone (HYDRACoRe 70pHT) membranes were modified using poly(diallyldimethylammonium chloride) (PDADMAC) and poly(styrenesulfonate) (PSS) polyelectrolytes at concentrations of 0.01, 0.1, and 1.0 wt·%, applying 5.5 bilayers ending with PDADMAC. Membrane performance was evaluated by a nanofiltration setup based on hydraulic permeability, ion rejection, and selectivity. The membrane modified with a PE concentration of 0.01 wt·% demonstrated the best performance. At a transmembrane pressure of 12 bar, Mg(II) rejection increased from 73 ± 2% to 88 ± 1% (a 21% increase with respect to the virgin membrane) and Ca(II) rejection from 74 ± 2% to 87 ± 1% (a 18% increase). Selectivity improvements were also notable, with Mg(II) selectivity over Na(I) and K(I) increasing by 94% and 173%, respectively, and Ca(II) selectivity over Na(I) and K(I) increasing by 74% and 133%, respectively. Beyond membrane modification, the stability of the resulting membranes was thoroughly investigated. All membranes were soaked in 0.1 M HNO3 and NaOH solutions for 2 months after testing. Characterization via FTIR, contact angle, FE-SEM, XPS, and zeta potential confirmed successful multilayer formation and evaluated changes after exposure to an extreme pH environment. The optimized 0.01 wt·% membrane exhibited the highest stability under extreme pH conditions, despite partial degradation affecting all modified membranes.