Overlimiting mechanisms of heterogeneous cation- and anion-exchange membranes: A side-by-side comparison

Abstract

Overlimiting ionic transfer attainable at ion-exchange membranes is a potential way to intensify electromembrane separation processes, such as electrodialysis. The primary mechanisms responsible for overlimiting current are electroconvection, natural convection, and water splitting. For the first time, the present study experimentally investigates all three mechanisms on heterogeneous anion- and cation-exchange membranes, using direct membrane observation and indirect measurement of pH changes in 0.01 M KCl solution. Although the anion- and cation-exchange systems are designed to exhibit the same behavior with different counterions, our results indicate significant differences in the overlimiting ionic transfer. The strong polarization of cation-exchange membranes produced an intensive electroconvective vortex with characteristic velocities in hundreds of μm/s superimposed by an upward electrolyte motion caused by natural convection. No pH changes associated with the water-splitting reaction were observed. The same conditions applied to the anion-exchange membrane resulted in profound pH changes, weaker electroconvection, and natural convection, reaching similar characteristic velocities to the other studied system. Generally, anion-exchange systems were characterized by more significant fluctuations in measured characteristics and were susceptible to fouling by one of the negatively charged markers. The obtained data provide a unified description of phenomena that may occur in the electrodialysis of low-concentrated ionic solutions.