Selective Adsorption of Rare Earth Elements over Functionalized Cr-MIL-101.

Efficient rare earth elements (REEs) separation and recovery are crucial to meet the ever-increasing demand for REEs extensively used in various high technology devices. Herein, we synthesized a highly stable chromium-based metal-organic framework (MOF) structure, Cr-MIL-101, and its derivatives with different organic functional groups (MIL-101-NH, MIL-101-ED (ED: ethylenediamine), MIL-101-DETA (DETA: diethylenetriamine), and MIL-101-PMIDA (PMIDA: N-(phosphonomethyl)iminodiacetic acid)) and explored their effectiveness in the separation and recovery of La, Ce, Nd, Sm, and Gd in aqueous solutions. The prepared materials were characterized using various analytical instrumentation. These MOFs showed increasing REE adsorption capacities in the sequence MIL-101 < MIL-101-NH < MIL-101-ED < MIL-101-DETA < MIL-101-PMIDA. MIL-101-PMIDA showed superior REE adsorption capacities compared to other MOFs, with Gd being the element most efficiently adsorbed by the material. The adsorption of Gd onto MIL-101-PMIDA was examined in detail as a function of the solution pH, initial REE concentration, and contact time. The obtained adsorption equilibrium data were well represented by the Langmuir model, and the kinetics were treated with a pseudo-second-order model. A plausible mechanism for the adsorption of Gd on MIL-101-PMIDA was proposed by considering the surface complexation and electrostatic interaction between the functional groups and Gd ions under different pH conditions. Finally, recycling tests were carried out and demonstrated the higher structural stability of MIL-101-PMIDA during the five adsorption-regeneration runs.