Optimizing Nanofiltration Membrane Layer-by-Layer Modification: Chemometric and Morphological Insights

Nanofiltration (NF) membranes are essential in wastewater treatment, battery industries, and brine management for selectively removing multivalent ions. However, fouling reduces their lifespan and necessitates harsh cleaning. The layer-by-layer (LBL) technique addresses this by modifying surface properties, enhancing rejection of divalent cations, such as magnesium and calcium, and minimizing fouling. This study evaluated a semiaromatic-based polyamide NF membrane (Fortilife-XN) modified using a LBL technique. Surface properties such as contact angle (CA), roughness, morphology, uniformity, and thickness were analyzed before and after modification. FTIR characterization revealed the membrane's structure, comprising a polyethylene terephthalate (PET) support, a polysulfone (PS) substrate, and a polyamide (PA) active surface. Poly-(sodium 4-styrenesulfonate) (PSS) was used as the polyanion, while poly-(diallyldimethylammonium chloride) (PDADMAC) and poly-(allylamine hydrochloride) (PAH) served as strong and weak polycations, respectively. Modifications in varying bilayers (1.5-6.5 BLs) with a positive terminal half-layer introduced peaks at 1034 and 923 cm^-1, corresponding to sulfonate and C-N bonds. CA and roughness analysis showed that (PDADMAC/PSS)-4.5 (CA 20°, roughness 77 nm) and (PAH/PSS)-1.5 (CA 61°, roughness 79 nm) achieved superior wettability and roughness, confirmed by initial NF testing with a permeability of around 10 L/(m² h bar). Ellipsometry, using Cauchy and Sellmeier models, measured multilayer thickness, estimating bilayers at ∼2 nm. Raman imaging visualized cross-sectional modifications, distinguishing raw and modified layers. Surface imaging revealed more uniform PAH deposition, while PDADMAC showed higher swelling with increased layers due to stronger affinity. These combined analytical techniques provided insights into the impact of LBL modification on membrane morphology and properties, aiding performance optimization. All membranes were preliminarily tested with a mixed salt solution (NaCl, CaSO₄, and MgSO₄). The average selectivity of Na/Ca for the bare membrane was 2.6 ± 0.4, increasing by 108% for (PDADMAC/PSS)-5.5 and 134% for PAH 6.5. For Na/Mg, the selectivity was 4.5 ± 0.3, rising by 49% for (PDADMAC/PSS)-5.5 and 131% for PAH 6.5.