Chapter 7 Donnan Dialysis

This chapter illustrates the principle of the Donnan dialysis that is incorporated with an anion exchange membrane. Strip and feed dissolving, respectively, electrolytes AX and BX are assumed to be supplied into, respectively, the extracting and feeding cells. The electrolyte concentrations at the entrance of the extracting and feeding cell are, respectively, C′′in,A, C′′in,B = 0, and C′′in,A = 0, C′′in,B. In this system, cations A (designated as driving ion) in the extracting cell transfer into the feeding cell by diffusion because of the concentration difference (C′′A > C′′in,A = 0). When the permselectivity of the cation exchange membrane is perfect, the anions X do not permeate the membrane. The process is applicable to concentrate or desalinate ions dissolving in feed. The chapter discusses the mass transport in Donnan dialysis. Flux of counter-ion in an ion exchange membrane is expressed by the following extended Nernst–Planck equation including the diffusion. The Donnan dialysis process does not use an electric current, so it is energy saving compared to electrodialysis. Compared to an ion exchange process, Donnan dialysis has advantages that it does not need regeneration and is operated continuously. The chapter also discusses various applications of Donnan dialysis.