Optimization of flux and selectivity in Cl−/SO42− separations with multilayer polyelectrolyte membranes

Alternating layer-by-layer deposition of polycations and polyanions on porous substrates is a convenient and versatile method for forming high-flux nanofiltration (NF) membranes. This study compares the NF performance of four different types of multilayer polyelectrolyte membranes in Cl−/SO42− separations. Membranes were prepared using two polycations, poly(allylamine hydrochloride) (PAH) and poly(diallyldimethylammonium chloride) (PDADMAC), and two polyanions, poly(acrylic acid) (PAA) and poly(styrene sulfonate) (PSS). NF solution fluxes decreased in the order PSS/PDADMAC, PSS/PAH > PAA/PDADMAC ≫ PAA/PAH, which is consistent with permeability being controlled by the charge density (ionic cross-linking) in the system, as suggested previously. Interestingly, SO42− rejections were also highest for the PSS/PDADMAC system, making these membranes the most promising for Cl−/SO42− separations. In the best case (PSS/PDADMAC)3PSS deposited on a porous alumina support showed a 96% rejection of SO42−, a chloride/sulfate selectivity of 26, and a solution flux of 2.7 m3/m2 day at 4.8 × 105 Pa. In a direct comparison with a commercial NF270 membrane, the (PSS/PDADMAC)3PSS membranes allowed a 2.7-fold higher flux along with greater passage of Cl−, but the NF270 system exhibited higher (99.5%) SO42− rejections.