Simultaneous selective separation and enrichment of bromine and cesium ions by electrochemically switched amphoteric ion membrane

A BiOBr film electrode was prepared by electrodeposition method for verifying an electrochemically switched amphoteric ion exchange (ESAIX) mechanism. Physical and electrochemical characterization results reveal that the BiOBr can switch the valence of Bi between Bi3+ and Bi(3-x)+ to selectively insert Cs+ with simultaneously release Br− at a reduction potential, while it can selectively insert Br− with simultaneously release Cs+ at an oxidation potential. The separation factors of BiOBr film electrode for Cs+/K+, Cs+/Na+, Cs+/Li+, and Cs+/Rb+ cations were 3.71, 3.23, 4.94, and 7.46, respectively, and for Br−/F−, Br−/Cl− and Br−/NO3− anions were 2.11, 1.55 and 3.83, respectively, which showed relatively excellent selectivity for Br− and Cs+. Density functional theory (DFT) calculations reveal that the selectivity of BiOBr towards Br− and Cs+ comes from the lower ion migration energy barrier in the BiOBr lattice. Based on the ESAIX mechanism, a novel electrochemically switched amphoteric ion membrane coupling permselective (ESAIM-CP) system was further developed for the simultaneous separation and enrichment of Br− and Cs+, which successfully achieved the enriched CsBr with 2 folds of the original concentration from a mixed solution of Cs2SO4 and KBr.