Positively charged thin-film composite hollow fiber nanofiltration membrane via interfacial polymerization and branch polyethyleneimine modification for Mg2+/Li+ separation

It is a vital technical challenge to extract lithium from salt-lake brine which has very high Mg²⁺ / Li⁺ mass ratio via green and low-cost methods, as these two cations have quite similar ionic hydration radius. Positively charged nanofiltration membranes can separate Li⁺ and Mg²⁺ through Donnan exclusion. In this work, a kind of hollow fiber nanofiltration membrane with positively charged skin layer was successfully fabricated via interfacial polymerization followed by a surface modification with branched polyethyleneimine (BPEI). The resultant membrane has a large number of amine groups, and thus shows positively charged surface with high rejection for divalent cation ions via Donnan exclusion. This gives it very high selectivity for the separation of Li⁺ ions from salt-lake brine. Under optimized conditions, it achieves a water permeance of up to 126.2 L m^-2 h^-1 MPa⁻¹ at a transmembrane pressure difference of 4 bar, and a MgCl₂ rejection of 94.6% with 2000 mg L^-1 aqueous MgCl₂ solution as feed. Meanwhile, it achieves a Mg²⁺ / Li⁺ selectivity of nearly 24 for MgCl₂ and LiCl salt mixture solution with an overall concentration of 2000 mg L^-1 and a Mg²⁺ / Li⁺ mass ratio of 150 : 1 as feed, which is high as compared with most of the literature, demonstrating its potential in the practical application of Mg²⁺ and Li⁺ separation.