Positively Charged Polyamine Nanofiltration Membrane for Precise Ion–Ion Separation

Positively charged nanofiltration (NF) membranes offer enormous potential for lithium-magnesium separation, hard water softening, and heavy metal removal. However, fundamental performance limitations for these applications exist in conventional polyamide-based NF membranes due to the negatively charged surface and low ion-ion selectivity. We hereby innovatively develop an advanced positively charged polyamine-based NF membrane built by the nucleophilic substitution of bromine and amine groups for precise ion-ion separation. Specifically, polyethylenimine (PEI) and 1,3,5-tris(bromomethyl)benzene (TBB) are interfacially polymerized to generate an amine-linked PEI-TBB selective layer with an ultrathin thickness of ∼95 nm, an effective pore size of 6.5 Å, and a strong positively charged surface with a zeta potential of +20.9 mV at pH 7. The PEI-TBB composite membrane achieves a water permeance of 4.2 L·m^-2·h^-1·bar^-1, various divalent salt rejections above 90%, and separation factors above 15 for NaCl/MgCl₂ and LiCl/MgCl₂ mixed solutions. A three-stage NF process is implemented to achieve a Mg²⁺/Li⁺ mass ratio sharply decreasing from 50 to 0.11 with a total separation factor (S_Li,Mg) of 455. Furthermore, the polyamine-based NF membrane exhibits excellent operational stability under continuous filtration and high operational pressure, demonstrating great application potential for precise ion-ion separation.