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.