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. ⁹³ Zr and ⁹⁹ Tc 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^IV ₄ (OH)₈ (H₂ O)₁₆ ]⁸⁺ (and related polymers) is dissociated by perrhenate/pertechnetate to yield isostructural dimers, M₂ (OH)₂ (XO₄ ^- )₆ (H₂ O)₆  ⋅ 3H₂ 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 ⁹³ Zr-⁹⁹ Tc 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^IV ₂ (OH)₂ (Re^VII O₄ ^- )₆ (H₂ O)₆ ⋅dimer.