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

Abstract

\u3cp\u3eIon 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\u3csub\u3eM\u3c/sub\u3e) is an important parameter affecting power consumption or power production in electrodialytic processes. In contrast to real applications, often R\u3csub\u3eM\u3c/sub\u3e is determined while using a standard 0.5M NaCl external solution. It is known that R\u3csub\u3eM\u3c/sub\u3e increases with decreasing concentration. However, the detailed effect of a salinity gradient present over an IEM on R\u3csub\u3eM\u3c/sub\u3e was not known, and is studied here using alternating and direct current. NaCl solution concentrations varied from 0.01 to 1.1M. The results show that R\u3csub\u3eM\u3c/sub\u3e is mainly determined by the lowest external concentration. R\u3csub\u3eM\u3c/sub\u3e 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\u3csub\u3eext\u3c/sub\u3eext≥0.3M, then differences of R\u3csub\u3eM\u3c/sub\u3e are small. A good approximation of experimentally determined R\u3csub\u3eM\u3c/sub\u3e can be obtained. The internal ion concentration profile is a key factor in modeling R\u3csub\u3eM\u3c/sub\u3e.\u3c/p\u3