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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.