A lithium ion selective membrane synthesized from a double layered Zrbased metalorganic framework (MOF-on-MOF) thin film

The ever-growing global demands for lithium (Li+) require energy-efficient techniques to separate lithium from natural resources and commercial wastewaters. We propose a facile seed-assisted in-situ growth method to prepare double-layered UiO-66-based (–(COOH)2 and –NH2) membranes for Mg2+ and Li+ separation in brine. These membranes, with sub-nanometer-sized windows, nanometer-sized cavities, and functional groups can selectively transport K+, Na+, Li+ over Mg2+ and Ca2+. A carboxyl-functionalized UiO-66-based membrane showed higher Li+ and Mg2+ selectivity (up to 90.8) than a UiO-66-NH2 membrane (65.0) by the current-voltage method using single salt solutions. Carboxylic groups greatly enhance membrane selectivity for Li+ over Mg2+. The effects and mechanisms of (i) different feed concentrations in a mono-ion system, and (ii) concentration gradients and electrical potential as driving forces for Li+ extraction in a synthetic multi-ion brine, are discussed. The UiO-66-(COOH)2/UiO-66-NH2 anodized aluminum oxide membrane exhibited excellent Mg2+/Li+ separation efficiency in synthetic Qinghai Taijiner salt lake brine (abbreviated as Taijiner brine) under concentration gradients. This research will promote metal-organic framework-based membrane designs with tailored pore morphologies and provide an innovative and eco-friendly solution for Li+ enrichment from salt lake brines.