Extraction of Gold Based on Ionic Liquid Immobilized in UiO-66: An Efficient and Reusable Way to Avoid IL Loss Caused by Ion Exchange in Solvent Extraction

Ionic liquids (ILs) have received considerable attention as a promising green solvent for extracting metal ions from aqueous solutions. However, the recycling of ILs remains difficult and challenging because of the leaching of ILs, which is caused by the ion exchange extraction mechanism and hydrolysis of ILs in acidic aqueous conditions. In this study, a series of imidazolium-based ILs were confined in a metal–organic framework (MOF) material (UiO-66) to overcome the limitations when used in solvent extraction. The effect of the various anions and cations of the ILs on the adsorption ability of AuCl(4)(−) was studied, and 1-hexyl-3-methylimidazole tetrafluoroborate ([HMIm](+)[BF(4)](−)@UiO-66) was used for the construction of a stable composite. The adsorption properties and mechanism of [HMIm](+)[BF(4)](−)@UiO-66 for Au(III) adsorption were also studied. The concentrations of tetrafluoroborate ([BF(4)](−)) in the aqueous phase after Au(III) adsorption by [HMIm](+)[BF(4)](−)@UiO-66 and liquid–liquid extraction by [HMIm](+)[BF(4)](−) IL were 0.122 mg/L and 18040 mg/L, respectively. The results reveal that Au(III) coordinated with the N-containing functional groups, while [BF(4)](−) was effectively confined in UiO-66, instead of undergoing anion exchange in liquid–liquid extraction. Electrostatic interactions and the reduction of Au(III) to Au(0) were also important factors determining the adsorption ability of Au(III). [HMIm](+)[BF(4)](−)@UiO-66 could be easily regenerated and reused for three cycles without any significant drop in the adsorption capacity.