Salt and water transfer in concentrating hypersaline electrolyte solutions by electrodialysis

Abstract

Many chemical engineers have sought to concentrate hypersaline electrolyte solutions, but the transfer processes are much complex. During electrodialysis, various mass transfer phenomena that develop in an ion exchange membrane (IEM) determine the concentration limit/efficiency; this limit typically decreases as feed concentrations increase. Here, salt and water transfer through an IEM was experimentally examined to assess electro-migration, diffusion, electro-osmosis, and osmosis individually via single-stage batch-mode electrodialysis. For precise quantification, LiCl, NaCl, and KCl solutions were used to establish solute-specific, concentration-dependent coefficients. Currentless experiments provided diffusion permeabilities and osmotic coefficients, while current-driven experiments enabled derivation of current-associated (electro-osmotic) coefficients and water transport numbers. Most concentration loss was attributed to water transfer, particularly electro-osmosis driven by hydrated counter-ions, followed by osmosis, whereas salt transfer contributed minimally in this regard. A substantial increase in the water transport number, associated with a concentration decrease in the diluate channel, significantly influenced concentration loss (i.e., lower concentration limit). The order of concentration loss increase was NaCl < KCl < LiCl; importantly, LiCl exhibited the highest water transport number due to the high hydration free energy of the Li + ion. These findings indicate potential modifications to salt and water transfer through IEMs to enhance concentration efficiency, supporting future efforts to concentrate hypersaline electrolyte solutions.