Ion separations are essential in applications such as water softening, salt purification and waste-water treatment. Membrane-based processes are attractive for such applications because of their low energy and capital costs, but success in these processes requires selective, ultrathin membrane skins. The minimal skin thickness affords high flux along with selectivity.Alternating adsorption of polycations and polyanions is a promising approach to form highly charged skins on porous membranes. Remarkably, as few as four bilayers of adsorbed poly(styrenesulfonate)/poly(allylamine hydrochloride) (PSS/PAH) on alumina membranes provide K+/Mg2+ selectivities >350 in diffusion dialysis. The same modified membranes show K+/Mg2+ selectivities of only 16 in nanofiltration (NF), however, suggesting that coupled transport of water and ions in small membrane defects reduces ion-transport selectivities. Transmembrane potential measurements show that PSS/PAH films are much more permeable to Cl- than Mg2+, and this leads to -200% rejection of trace K+ during NF of MgCl2 solutions in NF (the K+ concentration in the membrane permeate is three times that in the feed). The high diffusion dialysis selectivities of (PSS/PAH)5-coated membranes translate to electrodialysis with an accompanying increase in ion fluxes due to electromigration. Specifically, (PSS/PAH)5-coated commercial NF membranes show K+/Mg2+ selectivities around 100, and the K+ flux in electrodialysis is 45 times the flux in diffusion dialysis. However, the K+ transference number is at most ~0.35, because protons and anions carry most of the current. Electrodialysis with chloride salts damages membranes, presumably because of electrically generated chlorine. Controlling the membrane surface charge by application of an electric potential via a conductive membrane skin layer may greatly increase ion rejections and ion-transport selectivities. Dilute polymerization of polyaniline leads to a film of conducting polyaniline nanofibers on the membrane surface, and the resistance across the surface of such coated membranes is in the kΩ range. Unfortunately, initial experiments did not show a significant change in ion fluxes with an applied potential and conductive membranes, but this area requires further work.Overall, generating a highly charged and dense polymer film on a porous membrane provides remarkably selective ion transport in electrodialysis and diffusion dialysis. Whether electrical potentials applied to conductive membranes can enhance this selectivity is an open question.
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