Concentration polarization in ion-exchange membrane electrodialysis: The events arising in an unforced flowing solution in a desalting cell

Abstract

Using the NaCl concentration distribution observed by measuring the refraction index in a boundary layer in an unforced flowing state in a desalting cell, ionic fluxes, current density, solution velocity and potential gradient in the boundary layer were evaluated. Solution velocity was divided into the terms of electro-osmosis, concentration-osmosis and natural convection. The greater part of the solution velocity was due to natural convection, which is a horizontal component of ascending flow produced by the decrease in the solution density near the membrane surface in the boundary layer. Ionic fluxes and current density were divided into the terms of diffusion, migration and convection. Na + ion transport in the boundary layer on the desalting surface of a cation-exchange membrane was suppressed due to the lower mobility of Na + ions. Furthermore, water dissociation was strongly restricted on the surface of the cation-exchange membrane. Accordingly, Na + ion transport on the surface of the cation-exchange membrane placed in a diluted NaCl solution was promoted at the limiting current density by the increase of boundary layer thickness, the solution velocity in the boundary layer and the intensity of NaCl concentration oscillation on the membrane surface. In this circumstance, ionic transport due to the convection fluxes, which do not carry an electrical current, converted to migration fluxes and carried an electrical current. In this instance, diffusion fluxes did not support an electrical current because of lower mobility of Na + ions comparing that of Cl − ions. On the other hand, Cl − ion transport in the boundary layer on the desalting surface of the anion-exchange membrane was not suppressed because of the greater mobility of Cl − ions. Furthermore, water dissociation was not strongly restricted on the anion-exchange membrane. Accordingly, phenomena like the increase of the boundary layer thickness and the solution velocity observed in the boundary layer on the cation-exchange membrane did not occur on the anion-exchange membrane. On the anion-exchange membrane, convection fluxes were converted to diffusion fluxes and carry an electrical current. In this instance, migration fluxes did not support an electrical current because the contribution of diffusion fluxes is sufficiently effective. The potential gradient was divided into the terms of ohmic potential and diffusion potential. The ohmic potential gradient on the cation-exchange membrane was positive, and that on the anion-exchange membrane was negative. The diffusion potential gradient was positive on both the cation-exchange membrane and the anion-exchange membrane. By integrating the potential gradient distribution in a boundary layer, a current density versus voltage drop curve was obtained.