Incorporation of crown ether into PEI-polyamide nanofiltration membrane for efficient Mg2+/Li+ separation

Positive charged nanofiltration (NF) membrane have been triggered for Li⁺ extraction from brine driven by the Donnan effect. However, achieving selective Li⁺ transport channels remains a significant challenge, particularly in brines with a high Mg²⁺/Li⁺ mass ratio. In this work, a rigid crown ether was incorporated into the polyethyleneimine (PEI) based polyamide (PA) membrane for enhancing the robust selectivity of the typical positive charged membrane. Di(aminobenzo)-18-crown-6 (DAB18C6) was chosen as an additional aqueous phase monomer alongside PEI, with both reacting with trimesoyl chloride (TMC) to fabricate membranes via interfacial polymerization (IP). Due to the rigidity and the unique selectivity for alkali metal ions, DAB18C6 molecules not only expand the interval of the tightly stacking PEI-TMC chain in the PA layer, which resulting in high permeability, but also provide selective channels for Li⁺ transport. Therefore, the obtained DAB18C6@PEI@PSF membrane achieving the simultaneous enhancement in permeability (8.2 L·m⁻²·h⁻¹·bar⁻¹) and ion separation factor (S_Li,Mg = 23). Molecular dynamics demonstrated that the DAB18C6 exerts a stronger force on Li⁺ compared to Mg²⁺, which facilitates the transmembrane transport of the monovalent ion and achieves a high separation factor. After three-stage NF treatment process for a simulated salt lake brine with initial Mg²⁺/Li⁺ ratio of 61.5, a permeability liquid with low Mg²⁺/Li⁺ ratio of only 0.16 was obtained, indicating that the DAB18C6@PEI@PSF membrane has promising potential application in the field of high mass ratio Mg²⁺/Li⁺ separation.