The effect of specific ions on the permselectivity of ion exchange membranes

Open Access
Author:
Cassady, Harrison Jasper
Graduate Program:
Chemical Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
April 19, 2016
Committee Members:
  • Mike Hickner, Thesis Advisor
Keywords:
  • Ion-exchange membrane
  • permselectivity
  • membrane potential
  • counter-ion
  • co-ion
  • reverse electrodialysis
  • AEM
  • CEM
  • Permselectivity
  • Membrane potential
  • Counter-ion
  • Co-ion
  • Reverse electrodialysis
Abstract:
Ion exchange membranes are a class of selective barriers that prevent the passage of some aqueous ions (co-ions), while allowing other ions (counter-ions) to pass unimpeded. Ion exchange membranes form a critical component in many clean energy and ionic separation applications, and can be used to generate electricity, store and covert energy, desalinate sea water and treat highly toxic mine waste. In the effort to slow global warming, new clean energy technologies are needed to replace traditional fossil fuel sources. The improvement of ion exchange membranes will allow technologies based on these barriers to become more economical, and will provide new tools to fight climate change. Water uptake and permselectivity were measured for five sulfonated poly(ether sulfone) cation exchange membranes with varying degrees of functionalization from 20% to 60%. Tests were conducted in aqueous salt solutions of LiCl, NaCl, KCl, Li2SO4, Na2SO4 and K2SO4, to isolate the effect of counter-ions and co-ions on membrane permselectivity. Water uptake ranged from 0.13 g water / g polymer to 0.76 g water / g polymer depending on the degree of functionalization and salt used, but was not found to describe the permselectivity differences between salts as the counter-ion and co-ion were varied. This lack of correlation between water uptake and permselectivity is counter to some previous reports. Counter-ion binding affinity, charge density and dilute solution mobility were identified as factors influencing membrane permselectivity. Specifically, counter-ions with higher binding affinities to the fixed charge group of the polymer showed lower permselectivities due to counter-ion condensation. Co-ion polarizability was identified as the primary factor for co-ion effects on permselectivity, with more polarizable co-ions resulting in lower membrane permselectivities.