Transport of Ligand Coordinated Iron and Chromium through Cation-Exchange Membranes

Abstract

Fluxes of negatively charged ligand-coordinated iron, Fe(CN) 6 3/4- , and chromium, CrPDTA 1/2− , through two promising commercial cation-exchange membranes, Aquivion E87–05S and Fumasep E-620(K), were measured as functions of current density. The magnitude of the partial current density reached a maximum of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mfenced close="∣" open="∣" separators=""> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>43</mml:mn> </mml:mrow> </mml:mfenced> </mml:math> μ A cm −2 at the maximum applied current density magnitude of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mfenced close="∣" open="∣" separators=""> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>43</mml:mn> </mml:mrow> </mml:mfenced> </mml:math> mA cm −2 for Fe(CN) 6 3/4− transport through Aquivion, or 0.1% of the total current density. Fumasep E-620(K) blocks practically all crossover of both compounds. Both membranes sorb more Fe(CN) 6 3/4− and CrPDTA 1/2− than predicted by Donnan equilibrium, and low crossover rates can be attributed mainly to slow diffusion, not charge-based rejection of co-ions. The magnitude of the diffusion coefficient appears to correlate with hydraulic permeability. Although Aquivion E87–05S and Fumasep E-620(K) have significant and observable differences in membrane crossover rates, cells built with the DI-soaked membranes offer similarly high coulombic efficiency, indicating the relatively small contribution that crossover makes to inefficiency over a single cycle.