Polybenzimidazole-Modified\nCation-Exchange Membrane\nwith High Monovalent Ion Selectivity for Electrodialysis Separation\nof Alkaline/Alkaline Earth Metals

One of the keys to sustainable mining and recycling of\nalkaline/alkaline\nearth metals is using membrane technology for mono/divalent ion separation.\nDeveloping membrane materials with high selectivity could significantly\nenhance minerals’ extraction and recycling. In this work, \npolybenzimidazole (PBI) was incorporated into the matrix of the commercial\ncation-exchange Nafion-117 membrane via in situ polymerization of\n1,2,4,5-benzenetetraamin (BTA) and teraphthalaldehyde, forming a hybrid\ncation-exchange membrane with high selectivity for monovalent ions.\nThe optimum membrane was obtained within 6 min of contact time with\nBTA (denoted as PBI-6m). Fourier transform infrared and Raman spectroscopy\nand small-angle X-ray scattering confirmed the successful penetration\nof PBI into the membrane structure. PBI-6m was tested in electrodialysis\nseparation of mixed electrolyte solutions containing Li, Na, K, Mg,\nCa, and Sr chlorides for a current range of 0.4–3.0 mA/cm<sup>2</sup> and concentration range of 3–9 mM of each electrolyte.\nThe modified membrane exhibited a high selectivity to less hydrated\nand monovalent ions, providing a high separation for Na/Mg, Li/Mg,\nand Na/Ca of up to 27, 11, and 3.8, respectively, with a current efficiency\nof more than 80%. The membrane behavior was scrutinized through current–voltage\npolarization experiments and impedance spectroscopy in individual\nand mixed ionic environments. The results demonstrated that the separation\nmechanism is based on the low mobility of the divalent ions, which\nis explained by the ionic blockage emanating from the reduction of\npores and channels’ diameter upon the integration of PBI.