This study shows that alternating polyelectrolyte deposition on porous supports can yield nanofiltration membranes that allow high water flux along with selective ion transport. Membranes composed of 4.5-5 layer pairs of poly(styrene sulfonate)/poly(allylamine hydrochloride) (PSS/PAH) on porous alumina allow water fluxes of 1-2 m 3 m -2 day -1 at 4.8 bar while exhibiting MgSO4 rejections of 96%. Rejections of CaCl2 and Na2SO4 depend on polyelectrolyte deposition conditions and the composition of the outer layer of the membrane. In general, divalent-ion rejection increases when the charge of the outer layer of the membrane has the same sign as the divalent ion being rejected. Increasing the concentration of the supporting electrolyte present during deposition of the terminating PSS layer of PSS/PAH membranes results in a higher surface charge, and hence higher Na 2SO4 rejections (up to 95%). Nanofiltration with mixed solutions of NaCl and Na2SO4 yields Cl -/SO4 2selectivities of about 30 when the top layer of PSS/PAH membranes is deposited from a solution of high ionic strength. Capping PSS/PAH films with a layer of PAA increases Cl -/SO4 2selectivities to values as high as 80. Interestingly, Cl -/SO4 2selectivities in mixed solutions are higher than those determined from single-salt measurements, presumably because diffusion potentials are different in the two cases. The high selectivities, water fluxes, and ion rejections of PSS/PAH membranes make them potentially attractive for applications in water and salt purification.

Several recent studies demonstrated highly selective ion and neutral molecule transport through multilayer polyelectrolyte nanofiltration (NF) membranes. This work examines the potential of such membranes in the selective removal of dyes, sucrose, and amino acids from NaCl solutions. Remarkably, simple deposition of 4.5-bilayer poly(styrenesulfonate) (PSS)/poly(allylamine hydrochloride) (PAH) films on porous alumina supports yields membranes that exhibit NaCl/sucrose selectivities of ∼130 and NaCl/dye selectivities >2200. These high selectivities stem from rejections of 99.4% for sucrose and >99.9% for reactive dyes, along with small (∼20%) rejections of NaCl. Moreover, the solution flux (>1.7 m 3 /m 2 ‚day at 4.8 bar) through these membranes is comparable to or greater than that reported for commercial membranes. In contrast to results with sucrose and reactive dyes, the NaCl/glutamine selectivity of a [PSS/PAH] 7 membrane is only 3.7 because of the relatively small size of glutamine. With increasing NaCl concentrations (up to 0.5 M) in feed solutions, solution fluxes decrease due to an increased osmotic pressure drop across the membrane, but this flux decrease is minimized by low rejections of NaCl. Both salt passage and rejections of organic molecules are not strong functions of NaCl concentration.