Effect of ambient carbon dioxide on salt permeability and sorption measurements in ion-exchange membranes

Abstract

Characterizing ion sorption and transport properties in charged polymers is critical for developing fundamental understanding necessary to prepare high performance membranes. The presence of dissolved CO2 from the atmosphere in aqueous solutions can interfere with measurements of salt permeability and sorption in ion-exchange membranes, frustrating characterization of ion transport properties. In water or aqueous saline solutions, CO2 speciates to form ions such as H+ and HCO3−. NaCl or other salt permeability experiments are often performed by exposing a membrane to two salt solutions of different concentrations in a conventional diffusion cell and monitoring the conductivity rise with time in the receiving chamber (i.e., the chamber containing the lower salt concentration). H+ and HCO3− ions in the external solutions on either side of the membrane undergo ion exchange with counter-ions in cation- and anion-exchange membranes, respectively. This CO2-induced ion exchange interferes with conductivity measurements designed to measure receiver salt concentration change with time due to ion permeation through the polymer from the higher salt concentration chamber (i.e., the donor chamber). This phenomenon results in non-linear changes in downstream conductivity with time, which is most pronounced at low donor cell NaCl concentrations, especially for anion-exchange membranes. Furthermore, this effect is absent when an anion-exchange membrane in the HCO3− form is tested using NaHCO3 rather than NaCl for permeability measurements. The effect of CO2 on NaCl permeability measurements can be significantly reduced when ultra-high purity N2 gas is bubbled through the donor and receiver solutions in the diffusion cell during the experiment, making it possible to obtain true NaCl permeability values. During NaCl sorption experiments, when an ion-exchange membrane was equilibrated with a NaCl solution of 0.1M or lower, the amount of desorbed mobile counter-ions was significantly greater than the amount of desorbed mobile co-ions. The desorption of unequal amounts of ions can also be attributed to ion exchange between the membrane and solution phase and has implications for determining the mobile NaCl sorption coefficient. Procedures for addressing these issues are described.