Chronopotentiometric study on the simultaneous transport of EDTA ionic species and hydroxyl ions through an anion-exchange membrane for electrodialysis applications

Abstract

The use of EDTA as a complexing agent in electrodialysis has gained increasing attention in several industrial applications. Variations in pH at the membrane surface are known to alter the chemical equilibrium of EDTA species during electrodialysis. In this work, we investigate the simultaneous transport of EDTA species and hydroxyl ions through an anion-exchange membrane under alkaline conditions, and the influence of their relative concentrations on relevant membrane transport properties. Chronopotentiometric and current-voltage curves were obtained using solutions at different pH and electrolyte concentrations. Intense oscillations in membrane potential drop and a reduction in membrane resistance over time were verified for the solution at pH 11, where EDTA4− and OH− ions presented similar molar concentrations. While the intense oscillations can be associated with the onset of electroconvection, the reduction in membrane potential drop can be explained based on the alteration of the chemical equilibrium in the system. At an advanced stage of concentration polarization, the increased exclusion of protons in the membrane phase induces an equilibrium shift and the dissociation of HEDTA3− into EDTA4− and H+ ions. The formed EDTA4− ions migrate to the enriched solution, while protons are released back to the diluting diffusion boundary layer, increasing the conductivity in this region. The equilibrium shift in the membrane/electrolyte system and intense electroconvection at the membrane surface were also noted with the solution at pH 12. When the concentrations of OH− and EDTA species are similar, the equilibrium shift and the occurrence of overlimiting phenomena become more prominent.