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Coating of Nafion Membranes with Polyelectrolyte Multilayers to Achieve High Monovalent/Divalent Cation Electrodialysis Selectivities

White, Nicholas, Misovich, Maria, Yaroshchuk, Andriy, Bruening, Merlin L.
ACS Applied Materials & Interfaces 2015 v.7 no.12 pp. 6620-6628
calcium, cations, coatings, electrodialysis, electrolytes, magnesium, pH, permeability, polycyclic aromatic hydrocarbons, potassium, potassium nitrate, protons
Electrodialysis (ED) membranes typically exhibit modest selectivities between monovalent and divalent ions. This paper reports a dramatic enhancement of the monovalent/divalent cation selectivities of Nafion 115 membranes through coating with multilayer poly(4-styrenesulfonate) (PSS)/protonated poly(allylamine) (PAH) films. Remarkably, K⁺/Mg²⁺ ED selectivities reach values >1000, and similar monovalent/divalent cation selectivities occur with feed solutions containing K⁺ and Ca²⁺. For comparison, the corresponding K⁺/Mg²⁺ selectivity of bare Nafion 115 is only 1.8 ± 0.1. However, with 0.01 M KNO₃ and 0.01 M Mg(NO₃)₂ in the source phase, as the applied current density increases from 1.27 to 2.54 mA cm–², the K⁺/Mg²⁺ selectivities of coated membranes decrease from >1000 to 22. Water-splitting at strongly overlimiting current densities may lead to a local pH increase close to the membrane surface and alter film permeability or allow passage of Mg(OH)ₓ species to decrease selectivity. When the source phase contains 0.1 M KNO₃ and 0.1 M Mg(NO₃)₂, the K⁺ transference number approaches unity and the K⁺/Mg²⁺ selectivity is >20 000, presumably because the applied current is below the limiting value for K⁺ and H⁺ transport is negligible at this high K⁺ concentration. The high selectivities of these membranes may enable electrodialysis applications such as purification of salts that contain divalent or trivalent ions.