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.