Coating of Nafion Membranes with Polyelectrolyte Multilayers to Achieve High Monovalent/Divalent Cation Electrodialysis Selectivities Nicholas White, † Maria Misovich, † Andriy Yaroshchuk, ‡ and Merlin L. Bruening* ,† † Department of Chemistry, Michigan State University, East Lansing, Michigan 48824, United States ‡ ICREA and Department of Chemical Engineering, Polytechnic University of Catalonia, av. Diagonal 647, 08028 Barcelona, Spain * S Supporting Information ABSTRACT: 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-styrenesul- fonate) (PSS)/protonated poly(allylamine) (PAH) films. Remarkably, K + / Mg 2+ ED selectivities reach values >1000, and similar monovalent/divalent cation selectivities occur with feed solutions containing K + and Ca 2+ . For comparison, the corresponding K + /Mg 2+ selectivity of bare Nafion 115 is only 1.8 ± 0.1. However, with 0.01 M KNO 3 and 0.01 M Mg(NO 3 ) 2 in the source phase, as the applied current density increases from 1.27 to 2.54 mA cm −2 , the K + /Mg 2+ 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 KNO 3 and 0.1 M Mg(NO 3 ) 2 , the K + transference number approaches unity and the K + /Mg 2+ 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. KEYWORDS: electrodialysis, layer-by-layer, polyelectrolyte, ion-exchange membranes, selectivity 1. INTRODUCTION Electrodialysis (ED) is a membrane-based separation technique for applications such as preconcentrating brines, 1 recovering organic acids from waste-salt solutions, 2 organic acid production, 3 treating wastewater effluent, 4 demineralizing milk byproducts, 5 and desalting brackish water. 6,7 In commercially viable configurations, alternating anion- and cation-exchange membranes in flow-cells create parallel diluate and concentrate streams to enable high throughput. 8 Under an applied current or potential, cations leave the diluate compart- ment through the cation-exchange membrane, whereas anions leave in the opposite direction through the anion-exchange membrane. Thus, ED effectively removes ions from feed streams, but typical ion-exchange membranes exhibit low selectivities among ions. Such selectivities are important when employing ion-exchange membranes in some ED applications, e.g., removal of SO 4 2− from sea salt, or other functions such as prevention of vanadium crossover in redox flow batteries. 9−12 Although variation of current density and concentration polarization may provide some control over ion-transport selectivity, 13 the development of ED for separating ions requires ion-exchange membranes with high selectivities. Sata and co-workers showed that polycation or polyanion coatings on ion-exchange membranes enhance selectivity among cations or anions, respectively. 14−16 Deposition of protonated poly- ethylenimine (PEI) on cation-exchange membranes increases monovalent/divalent cation selectivity, and control over the hydrophobicity and cross-linking at the membrane surface leads to Na + /Ca 2+ selectivities up to 7. 14 Additionally, increasing the hydrophilicity of anion-exchange membranes with adsorbed ethylene glycols increases SO 4 2− /Cl − selectivities from <0.1 to 0.8. 17,18 Rakib and co-workers observed a Na + /Cr 3+ selectivity of 10−20 using an electrodeposited PEI film on a Nafion cation-exchange membrane. 19 Nevertheless, these selectivities are relatively modest. In this study, we examine whether adsorption of polyelectrolyte multilayers on ion-exchange membranes can yield even higher selectivities. With the development of alternating adsorption of polycations and polyanions to form ultrathin coatings, 20 several research groups began investigating whether polyelectrolyte multilayers (PEMs) can serve as ultrathin membrane skins that show high selectivity among cations or anions. 21−23 In many cases, monovalent ions move through these membranes more readily than multivalent ones, presumably because of enhanced electrostatic and size-based exclusion of highly hydrated, multiply charged ions. Although Michaels formed membranes Received: December 18, 2014 Accepted: March 4, 2015 Research Article www.acsami.org © XXXX American Chemical Society A DOI: 10.1021/am508945p ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX