Ultraefficiency Mg²⁺/Li⁺ Separation by Loose Nanofiltration Membrane: The Host–Guest Recognition of Crown Ether

Nanofiltration (NF) stands as a pivotal method for lithium resource recovery. Nonetheless, traditional NF membranes predominantly rely on size exclusion and the Donnan effect, necessitating elevated operating pressures and encountering substantial hurdles in terms of ion selectivity for lithium ions. Inspired by the host-guest recognition of crown ethers toward alkali metal ions, we prepared a crown ether-functionalized loose NF membrane (CE-LNF) via interfacial polymerization of diamino-dibenzo-14-crown-4 (DAB14C4) and trimesoyl chloride on a chloromethylated polysulfone substrate. The preparation conditions of the CE-LNF membranes were systematically optimized. Results revealed that the polyamide (PA) layer structure could be precisely tuned by changing the solvent and concentration of DAB14C4. The diffusion dialysis process with low energy consumption was applied in the Mg²⁺/Li⁺ separation experiment. Interestingly, the CE-LNF membrane achieved a unique "Li⁺ repulsion-Mg²⁺ permeation" result owing to the efficient host-guest recognition of Li⁺ by DAB14C4 on the PA layer. During the diffusion process, the transmembrane flux of Mg²⁺ (0.5 mol·m^-2·h^-1) surpassed that of Li⁺ (0.01 mol·m^-2·h^-1) by 50-fold, leading to a significant reduction in the Mg²⁺/Li⁺ ratio in the feed (from 19.5:1 to 5.2:1) and generating a Mg²⁺-enriched permeate. This work proposes a novel strategy for Mg²⁺/Li⁺ separation, where CE-LNF-based continuous diffusion dialysis serves as an energy-efficient, nonpressure-driven pretreatment for high Mg²⁺/Li⁺ ratio brines. The process not only produces high-purity Mg²⁺ solutions but also facilitates subsequent Li⁺ recovery via secondary CE-LNF diffusion or conventional extraction methods.