Electrodialysis is a membrane-based separation technique for applications including preconcentrating brines, recovering organic acids from waste-salt solutions, treating wastewater effluent, demineralizing milk by-products and desalting brackish water. However, typical electrodialysis membranes exhibit low selectivies among ions. This work examines monovalent/divalent cation selectivities and current efficiency in electrodialysis through Nafion 115 membranes modified with multilayer poly(sodium 4-styrenesulfonate) (PSS)/ protonated poly(allylamine) (PAH) films. K+/Mg2+ selectivity reaches values &gt;20 for these modified membranes. In a cell with only two compartments separated by the membrane, K+ transference numbers reach 0.4. Addition of a third compartment that ioslates the anode with an anion-exchange membrane increases the K+ transference to &gt;0.5, presumably by limiting the transport of protons generated at the anode. Furthermore, selectivity increases more than ten-fold and transference numbers for K+ are &gt;0.9 when increasing the source phase KNO3 and Mg(NO3)2 concentrations from 0.01 M (each) to 0.1 M. Higher KNO3 concentrations likely decrease the extent of concentration polarization near the membrane surface. Reducing current density also increases selectivity and monovalent cation transference numbers probably because of greater diffusion limitations on monovalent ion flux at higher currents. Future work should examine film stability in electric fields and electrodialysis of more valuable ions.

Highly selective cation separations in electrodialysis through cation-exchange membranes coated with polyelectrolyte multilayers

249th National Meeting (2015)

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