Pertechnetate/perrhenate-capped Zr/Hf-Dihydroxide Dimers: Elucidating Zr-TcO₄ Co-Mobility in the Nuclear Fuel Cycle

Spent nuclear fuel contains heavy element fission products that must be separated for effective reprocessing for a safe and sustainable nuclear fuel cycle. Zr-93 and Tc-99 are high-yield fission products that co-transport in liquid-liquid extraction processes. Here we seek atomic-level information of this co-extraction process, as well as fundamental knowledge about Zr-IV (and Hf-IV) aqueous speciation in the presence of topology-directing ligands such as pertechnetate (TcO4-) and non-radioactive surrogate perrhenate (ReO4-). In this context, we show that the flat tetrameric oxyhydroxyl-cluster [M-4(IV)(OH)(8)(H2O)(16)](8+) (and related polymers) is dissociated by perrhenate/pertechnetate to yield isostructural dimers, M-2(OH)(2)(XO4-)(6)(H2O)(6) & sdot; 3H(2)O (M=Zr/Hf-IV; X=Re/Tc-VII), elucidated by single-crystal X-ray diffraction. We used these model compounds to understand the pervasive Zr-93-Tc-99 coextraction with further speciation studies in water, nitric acid, and tetrabutylphosphate (TBP) -kerosene; where the latter two media are relevant to nuclear fuel reprocessing. SAXS (small angle X-ray scattering), compositional evaluation, and where experimentally feasible, ESI-MS (electrospray ionization mass spectrometry) showed that perrhenate/pertechnetate influence Zr/Hf-IV-speciation in water. In Zr-XO4 solvent extraction studies to simulate fuel reprocessing, we provide evidence that TcO4- enhances extraction of Zr-IV, and compositional analysis of the extracted metal-complexes (Zr-ReO4 study) is consistent with the crystallized Zr-2(IV)(OH)(2)((ReO4-)-O-VII)(6)(H2O)(6)& sdot;dimer.