Synergistic integration of crown ether and Girard's reagent T in nanofiltration membranes for high-efficiency lithium extraction from salt lake brine

Polyethylenimine (PEI)-based nanofiltration (NF) membranes shows considerable for Li⁺ extraction from salt lake brines originating from their strong positive charge, yet their dense selective layer often leads to low permeability, hindering practical applications. To address this challenge, we developed a dual-modification strategy by incorporating hydrophilic Girard's reagent T (GRT) into the aqueous phase of PEI, followed by secondary interfacial polymerization (IP) with a Li⁺-selective benzo-crown ether (DAB14C4). The GRT's quaternary ammonium groups synergized with DAB14C4's precise ion-sieving capability, simultaneously boosting membrane charge density and Li⁺/Mg²⁺ selectivity. The resulting DAB14C4@PEI/GRT@PSF membrane exhibited exceptional performance: a pure water flux of 13.7 ± 0.8 L m^-2·h^-1·bar^-1, enhanced surface charge (20.38 mV vs. 7.74 mV for the pristine membrane at pH 7), and high MgCl₂ rejection. Most notably, it achieved unprecedented Li⁺/Mg²⁺ separation (S_Li,Mg = 63.63) for a mixed solution (with Mg²⁺/Li⁺ mass ratio = 20:1), far surpassing conventional positively charged NF membranes. Molecular dynamics (MD) simulations revealed that DAB14C4 preferentially binds Li ⁺ over Mg²⁺, owing to Li⁺’s lower dehydration energy and higher diffusion rate. This work provides a feasible dual-functional modification strategy for developing high-flux, selective NF membrane for Li⁺/Mg²⁺ separation.