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 (SLi,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