Metal–organic frameworks (MOFs) have recently been receiving increasing attention in various scientific fields, including nuclear industry, due to their unique properties. In this work, the acid-resistant chromium-based MOF, MIL-101, and its amino derivatives were prepared to explore their potential usage in separation, removal and/or recovery of radionuclides from aqueous solutions. The synthesized MIL-101-NH₂, MIL-101-ED (ED = Ethanediamine), and MIL-101-DETA (DETA = Diethylenetriamine) were characterized by X-ray diffraction spectrometry (XRD), infrared spectrometry (IR), N₂ adsorption–desorption measurements, scanning electron microscopy (SEM) and thermogravimetric analysis (TGA), which confirm the successful modification of amino groups and the preservation of porous structures. The sorption performances of these materials toward U(VI) from an aqueous solution were investigated in detail. It was found that all the amine-grafted MOFs were highly efficient in capturing U(VI) compared to raw MIL-101. The sorption capacity of these MOFs for U(VI) sorption follows the order of MIL-101-DETA > MIL-101-ED > MIL-101-NH₂ > MIL-101, in which MIL-101-DETA possesses the highest sorption capacity of 350 mg g⁻¹ at pH ∼5.5. Moreover, the sorbed U(VI) can be easily desorbed by lowering the pH (pH ≤ 3.0), and the prepared materials also display a desirable selectivity toward U(VI) in a solution containing a range of competing ions. Based on the FTIR and EXAFS characterizations, the sorption mode of U(VI) onto MOFs is fully discussed. This work promises to provide a facile approach for developing acid-resistant MOFs toward a highly efficient and selective extraction of radionuclides from aqueous solutions.

Although many heterocyclic N-donor ligands have shown excellent competence for separating actinides from lanthanides, an explanation for why some ligands work whereas others fail is very fundamental but greatly needs to be addressed for designing novel and efficient extractants. In this work, we systematically investigated four phenanthroline-derived ligands, DHDIPhen, BQPhen, Ph2-BTPhen and CyMe₄-BTPhen, and their coordination geometrical properties and formation reactions with Am(III) and Eu(III) ions by quasi-relativistic density functional theory. The calculated hardness of ligands, which may help to determine their selectivity toward actinides and lanthanides, yielded an order, from the softest to the hardest, as follows: Ph2-BTPhen < CyMe₄-BTPhen < BQPhen < DHDIPhen. It shows that the intramolecular hydrogen bonds and size of a ligand cavity are two dominant factors for metal-ion complexation. Natural population analysis (NPA) reveals that the 5d/6d orbitals of Eu/Am accept significantly more electrons than other orbitals, but partial density of states and molecular orbital analysis prove that the d orbitals with more accepted electrons have little contribution to the metal–ligand bonds. The thermodynamic results suggest that ligand protonation does have a great influence on the complexation of ligands with metal ions but does not change the selectivity of ligands toward metal ions. This work can help in-depth understanding the differences of selectivity of various structurally similar ligands and provide more theoretical insights for designing more innovative ligands for Ln/An separation.

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