6 – Concentration Polarization

In an electrodialysis process, a boundary layer is formed on the desalting surface of the membrane and the salt concentration in the layer is depleted as a result of the concentration polarization, which increases voltage drop and energy consumption. Concentration polarization is an inevitable phenomenon that exerts an influence on the performance of an electrodialyzer. The salt concentration changes in the boundary layer are observed by measuring the light refractive indexes and the mass transport in the boundary layer is discussed based on the extended Nernst–Planck equation. The convection itself does not carry an electric current but it converts to migration and diffusion and then carries an electric current. At the overlimiting current density, a charged region develops on the membrane surface and ion transport in the boundary layer is promoted by gravitational convection or electroconvection that accompanies vortex flow, voltage fluctuation, current fluctuation, and light scattering fluctuation. The mechanism of the fluctuation is understandable based on the phenomenological equation held at the space-charged region generated on the membrane surface and the changes in salt concentration, the potential difference, and the ion–solvent flux at the membrane–solution interface. Concentration polarization generated on the concentrating surface of the membrane is not the same in principle as the phenomenon generated on the desalting surface.