Highly selective capture of gallium from aqueous solutions using tetradentate amidoxime functionalized MIL-53(Al) nanofiber membranes

Gallium, with an average content of about 15 mu g g-1 in the Earth's crust, ranks at the forefront of strategic metals. However, the separation of gallium remains challenging due to the similar physical and chemical properties of gallium and other impurity ions. Herein, the tetradentate amidoxime-functionalized MIL-53(Al) nanofiber membranes (PAN/MIL-53-AO) are prepared to separate gallium, aluminum, vanadium ions and so on. The resultant membranes display 231.08 mg g-1 for Ga(III) with high selectivity in static and dynamic adsorption. Specially, PAN/MIL-53-AO demonstrates excellent gallium regeneration rate of 84.25% after five cycles. According to DFT calculations, due to the tetradentate structure of MIL-53(Al)-AO, the amidoxime group binds to gallium ion with a lower binding energy (-29.46 eV). In conclusion, the experimental dates and theoretical calculations indicate that the excellent adsorption capacity is derived from the high binding ability of unique rhombic tetragonal coordination structure for gallium ion and the specific MIL-53(Al) cavity size for passage of gallium. Such performances outperform most state-of-the-art adsorption membranes. The special structure of MOF pores provides a novel approach for rapid synthesis of adsorption sites with high selectivity, with potential utilization in the selective adsorption of gallium ions.