Anion-Exchange Membranes Based on Cardo Polybenzimidazole and Zinc Ions for Selective Extraction of Monovalent Ions by Electrodialysis

Abstract

Anion-exchange membranes based on cardo polybenzimidazole and zinc(II) ions with various molar ratios of metal ions (0, 0.05, 0.25, 0.50, and 1.00) were prepared by casting solutions of metal–polymer complexes. The introduction of zinc(II) ions into the structure of cardo polybenzimidazole is accompanied by the formation of a cross-linked polymer matrix due to the coordination of metal ions to the pyridine nitrogen atoms of the monomer unit (–N=) and the appearance of anion-conducting properties. Increasing the metal content leads to an increase in the ionic conductivity of the membranes in the nitrate form from 1.8 × 10^–8 to 8.3 × 10^–6 S/cm. It is shown that the obtained materials demonstrate preferential transfer of monovalent anions (chloride, fluoride, nitrate) compared to divalent anions (sulfate) during electrodialysis. The selectivity coefficients reach 9.7 and 14 for chloride–sulfate and nitrate–sulfate pairs, and 5.7 and 9.1 for chloride–fluoride and nitrate–fluoride pairs, respectively. The obtained selectivity coefficient values exceed the corresponding values for the commercial Neosepta AMX anion-exchange membrane. At the same time, long-term cycling of the obtained membranes in galvanostatic mode leads to their degradation with leaching of zinc ions.