ZIF-8 Used for the Selective Recovery of Heavy Rare Earth Elements from Mining Wastewater

In this study, a sample of 2-methylimidazole zinc salt (ZIF-8) demonstrated high selectivity for the recovery of heavy rare earth elements (REEs) from real rare earth mining wastewater. Results show that the distribution coefficient values of Y³⁺ (4.02 × 10⁴ mL·g^-1), Gd³⁺ (7.8 × 10⁴ mL·g^-1), and Dy³⁺ (6.8 × 10⁴ mL·g^-1) are orders of magnitude higher than those of K⁺ (359.51 mL·g^-1), Mn²⁺ (266.67 mL·g^-1), Ca²⁺ (396.42 mL·g^-1), and Mg²⁺ (239.48 mL·g^-1). Moreover, the desorption efficiency of heavy REEs exceeded 40%. Advanced characterizations and density functional theory (DFT) calculations were utilized to elucidate that the heavy REEs were more likely to bind to the nitrogen atoms of imidazole groups on ZIF-8 compared to non-REEs. Furthermore, the adsorption and desorption of heavy REEs primarily depend on the chemical interaction confirmed by adsorption kinetics, isotherm model, and thermodynamic analysis, which involves the dissociation of water and the formation of REE-O bonds. Finally, the ZIF-8 exhibits a remarkable recovery efficiency of over 40% for heavy REEs in column tests conducted over 7h. The findings reported here provide new insights into the selective recovery of heavy REEs from real mining wastewater.