Coating of Anion-Exchange Membranes with Polyelectrolyte Multilayers to Increase Chloride/Sulfate Selectivity in Electrodialysis

This research focuses on modification of anion-exchange membranes to increase the monovalent/divalent anion selectivity in electrodialysis. Alternating adsorption of poly(4-styrenesulfonate) and protonated poly(allylamine) on a Fujifilm anion-exchange membrane increases the transient Cl^-/SO₄^2- transport selectivities remarkably from ∼1.3 to more than 100, and the current efficiency for such separations reaches ∼90%. Similarly high NO₃^-/SO₄^2- selectivities also occur. However, selectivity declines overs several hours of electrodialysis, possibly because transport does not achieve a steady state even after 2 h. Monovalent anion transport reaches steady state in minutes, and this work is currently investigating the source of possible long induction times for SO₄^2- transport. Increasing the Cl^- and SO₄^2- source-phase concentrations from 0.01 to 0.1 M decreases the selectivity (over the first 2 h of electrodialysis) from 180 to 50, perhaps because the membrane gets closer to steady state. When the current density increases from 0.63 to 2.23 mA cm^-2, the selectivity decreases from 180 to 10 and the current efficiency decreases from ∼90% to ∼50%, which likely results from an overlimiting current. Interestingly, when the polyelectrolyte multilayer film faces the source phase, the membrane shows increased selectivity in electrodialysis. In contrast, in Donnan dialysis the membrane is more selective when the polyelectrolyte multilayer faces the receiving phase. This difference may stem from different concentration polarization in Donnan dialysis and electrodialysis. The combination of highly selective anion-exchange membranes and cation-exchange membranes in electrodialysis may allow purification of salt mixtures containing divalent and/or multivalent ions.