Removal of Dyes, Sugars, and Amino Acids from NaCl Solutions Using Multilayer Polyelectrolyte Nanofiltration Membranes

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 m3/m2·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.