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-NH2, 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 La3+, Ce3+, Nd3+, Sm3+, and Gd3+ 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-NH2 < MIL-101-ED < MIL-101-DETA < MIL-101-PMIDA. MIL-101-PMIDA showed superior REE adsorption capacities compared to other MOFs, with Gd3+ being the element most efficiently adsorbed by the material. The adsorption of Gd3+ 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 Gd3+ on MIL-101-PMIDA was proposed by considering the surface complexation and electrostatic interaction between the functional groups and Gd3+ 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.