Journal of Membrane Science 346 (2010) 163–171 Contents lists available at ScienceDirect Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci Transport limitations in ion exchange membranes at low salt concentrations Piotr Długoł ˛ ecki a,b , Benoît Anet b , Sybrand J. Metz b , Kitty Nijmeijer a,∗ , Matthias Wessling a a Membrane Technology Group, University of Twente, Institute of Mechanics, Processes and Control Twente (IMPACT), P.O. Box 217, 7500 AE Enschede, The Netherlands b Wetsus, Centre of Excellence for Sustainable Water Technology, P.O. Box 1113, 8900 CC Leeuwarden, The Netherlands article info Article history: Received 17 March 2009 Received in revised form 12 September 2009 Accepted 16 September 2009 Available online 22 September 2009 Keywords: Membrane resistance Limiting current density Chronopotentiometry Ion transport number Ion exchange membrane abstract In this work we show that the electrical resistance of ion exchange membranes strongly depends on the solution concentration: especially at low solution concentrations (<0.1 M NaCl) we observe a very strong increase in electrical resistance of the membrane with decreasing concentration. To understand and clarify this behavior we systematically investigate the influence of the solution concentration on ion transport phenomena in two anion exchange membranes (Neosepta AMX and Fumasep FAD) and two cation exchange membranes (Neosepta CMX and Fumasep FKD) in the concentration range from 0.017 M to 0.5 M NaCl and for different hydrodynamic conditions. The results are highly valuable for processes that operate in the low concentration range (<0.5 M) such as reverse electrodialysis, electrodialysis, microbial fuel cells and capacitive deionization, where the standard membrane characterization values as usually determined in 0.5 M NaCl solutions do not represent the practical application. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Ion exchange membranes are widely used in processes such as electrodialysis, diffusion dialysis, capacitive deionization and Donnan dialysis. Recently also the application of ion exchange membranes for energy generation from, e.g. the mixing of fresh and salt water in a process called reverse electrodialysis or from wastewater in microbial fuel cells gained attention [1–8]. Reverse electrodialysis (RED) is a non-polluting, sustainable method for energy generation by controlled mixing of fresh and salt water. RED converts the free energy of mixing of a concentrated and a diluted salt solution into electrical power through an alternating series of anion and cation exchange membranes. RED can be applied wher- ever two solutions of different salinity are mixed, e.g. where river water flows into the sea [1–4,8]. Membrane properties and especially the electrical resistance of the membrane, or membrane resistance in short, can have signif- icant impact on the overall RED stack performance [9,10]. In most literature, it is assumed that the membrane resistance is indepen- dent of the concentration and corresponds to the value determined in a standard characterization solution (0.5 M NaCl) [2,8–12]. Veer- man et al. performed a study on the shortcut current losses in the RED process, assuming that the membrane resistance is indepen- dent of the salt concentration [10]. The measured cell resistances were about a factor two higher than the calculated values, which ∗ Corresponding author. Tel.: +31 53 489 4185; fax: +31 53 489 4611. E-mail address: d.c.nijmeijer@utwente.nl (K. Nijmeijer). made the authors to hypothesize that especially at low concen- trations, membrane resistance cannot be considered independent of the concentration. The effect of strongly increasing resistance at lower concentrations was also observed in biological fuel cells, where ion exchange membranes are applied as separators, between the anode and cathode compartment [13]. The concentration dependence of the membrane resistance, especially at lower salt concentrations (<0.5 M NaCl) is hardly stud- ied in literature and only few papers exist that address this topic [14–16], although it can significantly affect the performance of ion exchange membrane processes that use solutions in the low salt concentration range [9,15]. This work investigates the influence of the solution concentra- tion on ion transport phenomena in ion exchange membranes. The relationship between the membrane resistance, the limiting cur- rent density and the counter ion transport number of commercially available membranes in the concentration range of 0.017–0.5 M NaCl and at various hydrodynamic conditions is investigated. The results are highly valuable for electromembrane processes operating at low salt concentrations, such as reverse electrodialysis, microbiological fuel cells, electrodialysis and capacitive deioniza- tion. 2. Theoretical background Ion exchange membranes are membranes with fixed anionic or cationic exchange groups that are able to transport either cations or anions. The distribution and concentration of these charged groups determines specific membrane properties such as mem- 0376-7388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2009.09.033