Speciation and Complex Formation of Zirconium(IV) with Tc(VII) and Re(VII) in Liquid–Liquid Extraction Systems

The formation of Tc-(VII)-Zr-(IV) species in nitric acid and their extraction in kerosene/tributyl phosphate (TBP) phases was studied. The technetium distribution coefficient (<i>D</i> _Tc) was evaluated as a function of nitric acid concentration (1-7 M) and is in the range of 25-37, which is significantly higher compared to solutions that do not contain zirconium and confirms the synergism of their coextraction. Emphasis was placed on 1 M HNO₃ solution, which is the initial concentration used in uranium extraction during spent nuclear fuel reprocessing. The behavior of pertechnetate under these conditions is complex, primarily due to the formation of Tc-(VII)-Zr-(IV) species, a phenomenon that remains under investigation. In order to describe the coextraction mechanism, it is essential to characterize the zirconium species, with focus on intermolecular interactions. A neutral complex, [(H₂O)₃(ReO₄)₃Zr-(μ-OH)₂Zr-(ReO₄)₃(H₂O)₃]·3H₂O (1), was employed as a structural analogue to the proposed Tc-Zr species. This complex was isolated from solution and analyzed via single-crystal X-ray diffraction (SCXRD), corroborating the structural data previously reported. Furthermore, the positions of the H atoms in (1) were refined, and noncovalent interactions were analyzed using the Hirshfeld surface method. The H···O/O···H (63.0%) contacts is the main contribution to the crystal packing. The hydrolyzed form of zirconium perrhenate consists of a {Zr₂(OH)₂} core surrounded by six perrhenate anions acting as monodentate ligands and six coordination water molecules. Pertechnetate and perrhenate zirconium complexes exhibit a similar physico-chemical behavior and were studied by MALTI-TOF and powder X-ray diffraction (PXRD). The MALDI-TOF analysis shows the presence of dimeric and trimeric species.