Membrane resistance: The effect of salinity gradients over a cation exchange membrane

• IEM resistance largely depends on the lowest external solution ion concentration.
• An IEM can be regarded as existing from two conductive phases placed in series.
• These two phases stem from structural inhomogeneity of the membrane pore.
• The ion concentration profile in the membrane defines largely the IEM conductance.
• The profile is influenced by the size of the (electro-) osmotic water flux.

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 (RM) is an important parameter affecting power consumption or power production in electrodialytic processes. In contrast to real applications, often RM is determined while using a standard 0.5 M NaCl external solution. It is known that RM increases with decreasing concentration. However, the detailed effect of a salinity gradient present over an IEM on RM 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 RM is mainly determined by the lowest external concentration. RM 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, cext<0.3 M. The membrane conductivity is limited by the conductivity of the gel phase when cext≥0.3 M, then differences of RM are small. A good approximation of experimentally determined RM can be obtained. The internal ion concentration profile is a key factor in modeling RM.

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