Coupled transport phenomena in overlimiting current electrodialysis

The long-term goals of this work is to develop a better understanding of the coupled transport phenomena role in enhancement of salt ions transfer at electrodialysis of moderate-dilute and dilute electrolyte solutions in membrane modules of various constructions. Dependences of partial salt ions and H+(OH-) fluxes through an anion-exchange membrane as a function of voltage applied to a membrane or a membrane channel were measured. On the base of the data obtained, reasons of the growth of salt ions 'overlimiting' mass transfer in electrodialysis desalination channels with intermembrane distance and different NaCl solution concentration were analysed. It is shown that the role of gravitational convection may be important in desalination channels with a quite great intermembrane distance and electrolyte solution concentration. In this case a salt ions concentration at a membrane-solution interface does not decrease to values sufficient for promoting the H+ and OH- generation and electroconvection. In desalination channels with small intermembrane distances, when treating dilute solutions, the salt concentration at the membrane surface reaches very small values. It promotes the H+ and OH- generation and a space charge region arising. An additional salt ions transfer occurs due to the exaltation effect as well as to the electroconvection. In this case the gravitational convection contribution is negligible. (C) 1998 Elsevier Science B.V. All rights reserved.