Polybenzimidazole-Modified Cation-Exchange Membrane with High Monovalent Ion Selectivity for Electrodialysis Separation of Alkaline/Alkaline Earth Metals

One of the keys to sustainable mining and recycling of alkaline/alkaline earth metals is using membrane technology for mono/divalent ion separation. Developing membrane materials with high selectivity could significantly enhance minerals’ extraction and recycling. In this work, polybenzimidazole (PBI) was incorporated into the matrix of the commercial cation-exchange Nafion-117 membrane via in situ polymerization of 1,2,4,5-benzenetetraamin (BTA) and teraphthalaldehyde, forming a hybrid cation-exchange membrane with high selectivity for monovalent ions. The optimum membrane was obtained within 6 min of contact time with BTA (denoted as PBI-6m). Fourier transform infrared and Raman spectroscopy and small-angle X-ray scattering confirmed the successful penetration of PBI into the membrane structure. PBI-6m was tested in electrodialysis separation of mixed electrolyte solutions containing Li, Na, K, Mg, Ca, and Sr chlorides for a current range of 0.4–3.0 mA/cm2 and concentration range of 3–9 mM of each electrolyte. The modified membrane exhibited a high selectivity to less hydrated and monovalent ions, providing a high separation for Na/Mg, Li/Mg, and Na/Ca of up to 27, 11, and 3.8, respectively, with a current efficiency of more than 80%. The membrane behavior was scrutinized through current–voltage polarization experiments and impedance spectroscopy in individual and mixed ionic environments. The results demonstrated that the separation mechanism is based on the low mobility of the divalent ions, which is explained by the ionic blockage emanating from the reduction of pores and channels’ diameter upon the integration of PBI.