DiBlasi, N. A., Dardenne, K., Duckworth, S., Polly, R., Rothe, J., Altmaier, M., Gaona, X.
Karlsruhe Institute of Technology, Institute for Nuclear Waste Disposal, Hermann-von-Helmholtz-Platz 1, Eggenstein-Leopoldshafen 76344 Germany
Technetium-99 (99Tc) is a low energy beta emitter with a high fission yield from both 235U (~6.1%) and 239Pu (~5.9%), making it a radionuclide of particular interest in the context of nuclear waste disposal. Tc is known to have 9 different oxidation states, with Tc(IV), Tc(V), and Tc(VII) being the most common. Notably, Tc(VII), which exists predominantly as the pertechnetate anion TcO4–, is considered relatively soluble and mobile in environmental contexts, while Tc(IV) is much less soluble in comparison. Many small organic molecules capable of strong aqueous complexation interactions with Tc may be present in different environmental and/or nuclear waste disposal scenarios where 99Tc may be disposed, and these interactions may in turn increase the solubility and mobility of Tc. For these reasons, it is essential to understand the impact of complexing organic ligands on the solubility, speciation, and redox distribution of technetium to better determine the long-term fate and transport of technetium under relevant boundary conditions. This contribution addresses these issues through a series of over- and undersaturation solubility studies under reducing, anoxic conditions coupled with advanced spectroscopic measurements. Within the current study, five carboxylate-containing organic ligands with environmental and repository relevance were investigated: ethylenediaminetetraacetic acid (EDTA), nitrilotriacetic acid (NTA), citrate, oxalate, and phthalate. Tc, added to the experiments either as a well-characterized TcIVO2(am,hyd) solid phase or as a solution of sodium pertechnetate (NaTcVIIO4), was reacted with experimental solutions with constant pH (9, 11, or 13), constant ionic strength (I = 0.7 M NaCl), and constant organic concentrations (0, 12, or 100 mM). Undersaturation experiments with TcO2(am,hyd) were all performed in the presence of SnCl2 as a redox buffer, while oversaturation experiments with TcO4– were performed in both the presence and absence of SnCl2. Solutions were analyzed using pH and Eh measurements, liquid scintillation counting for aqueous Tc concentrations, selected UV-Vis-NIR spectrophotometric measurements, and X-ray absorption spectroscopy (XAS, XANES/EXAFS). Data was interpreted in the context of oxidation state determination, speciation identification, structural elucidation, and thermochemical model generation, thus providing further necessary thermodynamic information for the determination of the fate and transport of Tc in the context of waste disposal.
Acknowledgements: This work is funded by the German Federal Ministry for Economic Affairs and Climate Action (BMWK) within the framework of the VESPAII project (02E11607C). We thank the Institute for Beam Physics and Technology (IBPT) for the operation of the storage ring, the Karlsruhe Research Accelerator (KARA).