Membrane resistance: The effect of salinity gradients over a cation exchange membrane A.H. Galama a,b , D.A. Vermaas b,c , J. Veerman d , M. Saakes b , H.H.M. Rijnaarts a , J.W. Post a,b , K. Nijmeijer c,n a Wageningen University, Sub-Department of Environmental Technology, P.O. Box 8129, 6700 EV Wageningen, The Netherlands b Wetsus, Centre of Excellence for Sustainable Water Technology, P.O. Box 1113, 8900 CC Leeuwarden, The Netherlands c Membrane Science & Technology, University of Twente, MESA þ Institute for Nanotechnology, P.O. Box 217, 7500 AE Enschede, The Netherlands d REDstack B.V., P.O. Box 199, 8600 AD Sneek, The Netherlands article info Article history: Received 12 March 2014 Received in revised form 21 May 2014 Accepted 23 May 2014 Available online 2 June 2014 Keywords: Ion exchange membrane Membrane resistance Salinity gradient Electro-osmosis Osmosis abstract Ion exchange membranes (IEMs) are used for selective transport of ions between two solutions. These solutions are often different in concentration or composition. The membrane resistance (R M ) is an important parameter affecting power consumption or power production in electrodialytic processes. In contrast to real applications, often R M is determined while using a standard 0.5 M NaCl external solution. It is known that R M increases with decreasing concentration. However, the detailed effect of a salinity gradient present over an IEM on R M was not known, and is studied here using alternating and direct current. NaCl solution concentrations varied from 0.01 to 1.1 M. The results show that R M is mainly determined by the lowest external concentration. R M can be considered as two resistors in series i.e. a gel phase (concentration independent) and an ionic solution phase (concentration dependent). The membrane conductivity is limited by the conductivity of the ionic solution when the external concentration, c ext o0.3 M. The membrane conductivity is limited by the conductivity of the gel phase when c ext Z0.3 M, then differences of R M are small. A good approximation of experimentally determined R M can be obtained. The internal ion concentration profile is a key factor in modeling R M . & 2014 Elsevier B.V. All rights reserved. 1. Introduction Ion exchange membranes are widely used for concentrating and/or selective transport of dissolved charged particles, for example in electrodialysis (ED) for desalination purposes [1–3]. Although ED is in practice most used for brackish water desalination, it recently gained interest as a seawater (pre) desalination technology [4,5]. In addition, an electrodialysis stack can be used for the production of salinity gradient energy in the opposite process i.e. reversed electro- dialysis (RED), by mixing river water and seawater [6–9]. The salt concentrations of the solutions in a RED stack are comparable to those in a seawater ED stack. In the ED process low energy consumption is desired and in RED high power production is targeted. In both situations low stack resistances are a prerequisite. Generally, membrane resistances are determined at an external salt concentration of 0.5 M NaCl and a temperature of 25 1C. Literature shows that membrane resistance depends on the concentration of the external solution [10–16]. In practical applications of ED or RED, the concentration at either side of the ion exchange membrane differs. It is, however, unknown how this determines the membrane resistance. Recent research indicated that the membrane resistance is signifi- cantly higher compared to high salinities at both sides of the membrane when a solution low in salinity is present at one side of the membrane and liquid with a high salinity at the other side [16]. This previous research indicated that the actual membrane resistance in practical applications may be an order of magnitude higher than specified in standard resistance characterization measurements with 0.5 M solutions at both sides of the membranes. Although the membrane resistance between external solutions of unequal concentration is particularly interesting for many practical applications, no systematic quantitative experimental research has been performed on this topic. Veerman et al. attempted to model the membrane resistance in cases with different salinity at both sides quantitatively [8]. To validate this model, and gain fundamental knowledge on membrane resistance in practical applications, this paper presents experimental results on the resistance of ion exchange membranes, having different salinities at both sides (0.01–1.1 M NaCl). This experimental data provides a solid fundament for a model, presented in this research, to estimate the (cat-) ion exchange membrane resistance, even Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/memsci Journal of Membrane Science http://dx.doi.org/10.1016/j.memsci.2014.05.046 0376-7388/& 2014 Elsevier B.V. All rights reserved. n Corresponding author. Tel.: þ31 53 489 4185. E-mail address: d.c.nijmeijer@utwente.nl (K. Nijmeijer). Journal of Membrane Science 467 (2014) 279–291