Influence of the type of anion membrane on solvent flux and back diffusion in electrodialysis of concentrated NaCl solutions T. Rottiers a , K. Ghyselbrecht b , B. Meesschaert b , B. Van der Bruggen c , L. Pinoy a,n a Department of Chemical Engineering, Cluster Sustainable Chemical Process Technology, KU Leuven @ KAHO St.-Lieven, Gebroeders Desmetstraat 1, B-9000 Gent, Belgium b Department of Microbial and Molecular Systems, Cluster for Bioengineering Technology, KU Leuven @ KULAB, Zeedijk 101, B-8400 Oostende, Belgium c Department of Chemical Engineering, KU Leuven, W. de Croylaan 46, B-3001 Leuven, Belgium HIGHLIGHTS  Back diffusion of ions is hardly influenced by the used type of anion membrane.  Solvent flux by electro-osmosis depends only on the applied current density.  Solvent flux by electro-osmosis is multiple times higher than by osmosis. article info Article history: Received 28 December 2013 Received in revised form 4 April 2014 Accepted 5 April 2014 Available online 13 April 2014 Keywords: Electrodialysis Solvent flux Back diffusion Brine abstract The potential of using electrodialysis as a technology for separations in highly concentrated solutions is limited by back diffusion of ions and solvent flux from the diluate to the concentrate. Solvent flux dilutes the concentrate stream while simultaneously back diffusion forms a limitation to reach high ion concentrations. In this study the diffusion flux of sodium and chloride in two different combinations of ion-exchange membranes, namely PC-SK/PC-SA with standard anion exchange membranes and PC-SK/ PC-MVA with monovalent selective anion membranes, are studied for feed solutions with very high concentrations. It was found that diffusion fluxes of both ions are hardly influenced by the type of anion- selective membrane and are solely concentration dependant. To assure an efficient separation concentration gradients must be small to minimize back diffusion. Furthermore, the quantitative influence of operating parameters on the solvent flux caused by osmosis and electro-osmosis was investigated. Electro-osmosis was found to be independent from the concentration gradient and proportional with the current density. Osmosis only depends on the concentration gradient. When the concentration gradient is below 1 mol L 1 the flux by osmosis was at least eight times smaller than by electro-osmosis. Membranes with high steric hindrance are therefore necessary to minimize the electro-osmotic flux. & 2014 Elsevier Ltd. All rights reserved. 1. Introduction Concentrated salt effluents from various industries are proble- matic due to the high cost of treatment (Romero Barranco et al., 2001) and environmental impact (Palomar et al., 2012). Van der Bruggen et al. (2003), Kim (2011) and Perez-Gonzalez et al. (2012) reviewed different possibilities to treat or to discharge concen- trated salt streams. These possibilities include recuperation of the dissolved salts by vacuum evaporation. Prior to an evaporative crystallizer, electrodialysis can be used as a concentration method (Jiang et al., 2014; Tanaka et al., 2003; Turek et al., 2005). Electrodialysis is generally considered an economically feasible desalination technology when the salinity of the salt stream is less than 5 g L 1 (Strathmann, 2010). However, electrodialysis has the technological potential to concentrate more concentrated solutions (Schoeman et al., 2005). Korngold et al. (2009) for example, used electrodialysis to treat a reverse osmosis brine stream, saturated with CaSO 4 , to increase the concentration from 1.5 to 10% and to concentrate brine solutions of various concentra- tions to approximately 20% (Korngold et al., 2005). Electrodialysis was also used by Turek et al. (2005) to concentrate a coal-mine brine up containing 290 g L 1 NaCl and by Reig et al. (2014) to evaluate the efficiency of concentrating a seawater reverse osmosis brine from approximately 70 to 245 g L 1 NaCl. The desalination of concen- trated NaCl brines was simulated using the Nernst–Planck approach by Fidaleo and Moresi (2011). Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/ces Chemical Engineering Science http://dx.doi.org/10.1016/j.ces.2014.04.008 0009-2509/& 2014 Elsevier Ltd. All rights reserved. n Corresponding author. Tel.: þ32 9 265 86 37. E-mail address: Luc.Pinoy@kuleuven.be (L. Pinoy). Chemical Engineering Science 113 (2014) 95–100