Cation separations in electrodialysis through membranes coated with polyelectrolyte multilayers

This paper examines K+/Mg2+ selectivities and cation fluxes in diffusion dialysis and electrodialysis through membranes coated with poly(sodium 4-styrenesulfonate) (PSS)/protonated poly(allylamine) (PAH) multilayer films. In both dialysis techniques, K+/Mg2+ selectivities reach values >100, and with (PSS/PAH)5-coated nanofiltration membranes the K+ flux in electrodialysis is 45-times the flux in diffusion dialysis. Thus, the applied electric current can increase flux without decreasing selectivity. However, the K+ transference number is at most ∼0.35 because protons and anions also carry current. Ion fluxes and K+/Mg2+ selectivities depend on the anion of the K+/Mg2+ salts. Sulfate decreases the surface charge on (PSS/PAH)5-coated membranes and reduces K+/Mg2+ selectivities to ∼40 in both diffusion dialysis and electrodialysis through films on porous alumina. Chlorine generated during electrodialysis with chloride salts damages (PSS/PAH)5-coated membranes, and selectivities decline dramatically after 60 min. Future work should examine selectivities among more valuable ions and methods for increasing stability and the transference numbers for the ions of interest.