Optimized ion selectivity in nanofiltration membranes via poly(allylamine hydrochloride) (PAH) and poly (styrene sulfonate) (PSS) layer-by-layer self-assembly

Layer-by-layer (LBL) assembly is a simple and effective technique for modifying nanofiltration (NF) membranes, enabling tailored surface charge and enhanced anti-fouling properties. These features make LBL-modified membranes suitable for applications such as resource recovery, seawater desalination, and water treatment. Among polyelectrolyte combinations, poly (styrene sulfonate) (PSS) as the anionic and poly(allylamine hydrochloride) (PAH) as the cationic components have garnered attention for their stability, particularly in water softening and the battery industry. However, limited research exists on PAH/PSS multilayers for modifying polymeric flat-sheet NF membranes. This study investigates the modification of a commercial NF membrane containing a semiaromatic active layer (Fortilife-XN) using LBL-assembled PAH/PSS multilayers to improve cation selectivity and divalent cation rejection. Various bilayer configurations were tested to assess their impact on membrane performance. The modified membranes were characterized using deionized water and tested with a synthetic water solution mimicking the composition of surface water from a Mediterranean river affected by industrial discharges and hydric stress. The membrane modified with 6.5 bilayers exhibited the highest selectivity between single charge and double charge cations. At 11 bar transmembrane pressure, Mg(II) rejection reached 95 %, with K(I)/Mg(II) and Na(I)/Mg(II) selectivity increasing by 151 % and 113 %, respectively. Additionally, K(I)/Ca(II) and Na(I)/Ca(II) selectivity improved by 183 % and 146 %, with Ca(II) rejection at 92 %. These results demonstrated the potential of PAH/PSS multilayer coatings for developing high-selectivity NF membranes suited for hardness removal and resource recovery applications. • A polymeric nanofiltration membrane was successfully modified using PAH/PSS polyelectrolyte multilayers. • The surface properties of membrane were enhanced via a layer-by-layer (LbL) assembly technique. • The (PAH/PSS)₆.₅ configuration demonstrated optimal ion selectivity performance. • Surface morphology and chemical composition were thoroughly characterized using multiple analytical techniques.