Factors Affecting Hydroxide Ion Concentrations in Bipolar Membranes

Document Type : Research Paper

Authors

1 Department of Chemical and Environmental Engineering, University of Arizona, Tucson, AZ 85721, USA

2 University of Arizona

10.22079/jmsr.2021.521613.1433

Abstract

The useful lifetime of bipolar ion exchange membranes is often limited by nucleophilic attack by hydroxide ions on the  ionic groups and polymer backbone in the anion exchange layers (AELs). This is especially problematic in water treatment applications for making acid and base from salt solutions. This research investigated the effect of bulk electrolyte composition, current density, membrane thickness, ion exchange capacity, and bulk solution pH value on hydroxide ion  concentrations inside the AELs of a bipolar membrane. Onedimensional Nernst-Plank equations were solved for the species Na+, Cl-, OH- and H+ within 20-100 μm thick anion and cation exchange layers with fixed charged densities  ranging from 0.5-2.0 eq/L. In 1 M NaCl solutions at neutral pH values, hydroxide concentrations in the AEL reached as  high as 2.2 M at a current density of 100 mA/cm2. In 1 M NaOH solutions, hydroxide ion concentrations reached as high  as 3.77 M. Hydroxide concentrations in the AEL were significantly affected by the ratio of Cl- to hydroxide ions in the bulk electrolyte. Where hydroxide concentrations in the bulk electrolyte were an order of magnitude lower than chloride  concentrations, membrane hydroxide concentrations were nearly proportional to the current density. Increases in ion  exchange capacity and AEL thickness resulted in increased membrane hydroxide ion concentrations. Membrane  concentrations of hydroxide ions can be minimized by operation at low current densities, with high background  electrolyte concentrations using thin membranes with low ion exchange capacities and producing base concentrations  less than 0.1 M. 

Graphical Abstract

Factors Affecting Hydroxide Ion Concentrations in Bipolar Membranes

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