Impact of membrane and electrolyte properties on the intensity of equilibrium electroconvection in electrodialysis

Abstract

Electroconvection enhances mass transfer in electrodialysis, which allows reducing the area of expensive membranes. It is commonly believed that a significant increase in mass transfer can only be achieved through nonequilibrium electroconvection developing at overlimiting currents. However, Rubinstein and Zaltzman recently predicted theoretically that equilibrium electroconvection becomes unstable under certain conditions, and this can potentially cause an increase in mass transfer. In this work, this effect is investigated experimentally using three different anion-exchange membranes and two electrolytes (KCl and LiCl). It was found that an increase in the electrolyte concentration (from 0.02 M to 0.75 M) in the concentrate compartment of an electrodialysis cell leads to an increase in the limiting current by up to 37 %, provided that the solution in the diluate compartment has a constant concentration (0.02 M). This increase significantly exceeds the value that could be expected due to the increase in coion electrodiffusion transport through the membrane caused by the concentration increase in the concentrate compartment (<6 %). It is established that the replacement of KCl with LiCl leads to earlier transition to unstable equilibrium electroconvection and more effective solution mixing. The presence of structural heterogeneities in membranes and an increase in zeta potential also contribute to enhancement of electroconvection.