Concentration polarization in ion-exchange membrane electrodialysis—the events arising in a flowing solution in a desalting cell

Abstract

Limiting current density and the solution velocity in the boundary layer formed on the desalting surface of an ion-exchange membrane were measured in a flowing NaCl solution in a desalting cell. Applying the measured limiting current density and the solution velocity to the extended Nernst–Planck equation, the thickness of the boundary layer and NaCl concentration distribution in the boundary layer were calculated. The thickness of the boundary layer on an anion-exchange membrane was larger than that on a cation-exchange membrane. The reason of this phenomenon is attributed to that the transport rate of counter-ions in a solution on the anion-exchange membrane is larger than that on the cation-exchange membrane. That is, the mobility of Cl − ions is larger than that of Na + ions. Ionic fluxes and current density in the boundary layer were divided into the terms of diffusion, migration and convection. They were evaluated using the NaCl concentration distribution and the solution velocity in the boundary layer, and the limiting current density. The ionic fluxes are carried steadily by the diffusion and migration and the contribution of the convection is negligible, because the boundary layer thickness and the solution velocity in the boundary layer are low in a flowing solution in a desalting cell. Counter-ion fluxes for the ion-exchange membrane which are transported across the boundary layer were confirmed to be greater than co-ion fluxes. This is because the direction of diffusion and migration are the same each other for counter-ions, but are opposite for co-ions. The potential gradient was divided into the terms of ohmic potential and diffusion potential, and were evaluated from the NaCl concentration change in the boundary layer. The potential gradient across the boundary layer formed on the cation-exchange membrane was greater than that on the anion-exchange membrane, because the direction of ohmic potential gradient and diffusion potential gradient across the boundary layer are the same each other for the cation-exchange membrane, but are opposite for the anion-exchange membrane. The convection fluxes decreased at the membrane surface at the limiting current density. This event arose because the convection fluxes, which do not carry an electrical current, convert to diffusion fluxes or migration fluxes that carry an electrical current.