Electrodialysis through nafion membranes coated with polyelectrolyte multilayers yields >99% pure monovalent ions at high recoveries

Separation of monovalent and multivalent ions is important in processes ranging from water softening to recycling of Li. Previous studies show that cation-exchange membranes coated with multilayer polyelectrolyte films exhibit monovalent/divalent ion selectivities as high as 1000 in electrodialysis (ED) under ideal conditions with low ion recoveries. This work shows that even at 60–80% monovalent-ion recoveries, ED through Nafion membranes coated with protonated poly(allylamine)/poly(4-styrenesulfonate) multilayers can separate equimolar K+ and Mg2+ or Li+ and Mg2+ to give 99.5% pure monovalent ion. Moreover, the use of a low-volume receiving phase allows ED to concentrate the monovalent ion. Typical current efficiencies are around 70% for these separations. Selectivities appear to increase when the membrane-coating procedure uses a higher polyelectrolyte concentration in the deposition solution. Because the limiting current declines with ED time due to depletion of the monovalent ion in the source phase, use of a gradually decreasing applied current enhances current efficiency compared to ED at a constant high current. The membranes also show Li+/Mg2+ selectivities around 100 when Mg2+ is in a 20-fold excess in the source phase at an ionic strength of 0.6 M, but selectivity decreases at even higher ionic strengths.