Retention of Tc by Fe(II) solid phases: Combining wet chemistry experiments and spectroscopic data

Tc-99 is a relevant fission product in the field of nuclear waste disposal, due to its high yield in nuclear reactors, long half-life (t1/2 = 2.13∙10ˆ5 years) and redox-sensitive character. In the context of the multibarrier systems considered in repositories for nuclear waste, sorption arises as a key process to minimize the release of the radionuclides to the biosphere.[1] The anoxic corrosion of Fe in the repository generates H2(g) whilst producing a number of corrosion phases, which are expected to contribute to the reduction and sorption of radionuclides like Tc.[2,3] This study investigates the reduction and uptake of Tc-99 by Fe(OH)2(cr), and Fe(II)-silicates, as such phases are expected to form as corrosion products in the presence of dissolved silicates.[3,4]
Batch sorption experiments with NaTcO4 were carried out under anoxic conditions in the hyperalkaline pH range (pH 10 and 12.5) and with total Tc concentrations varying between 10ˆ6.5 and 10ˆ-4.5 M. A fast and strong uptake of Tc was observed with final Rd values ranging between 10ˆ6 < Rd [l/kg] < 10ˆ8. Linear sorption isotherms (with slope of ⁓ 1) were observed both at short (t = 2 days) and long contact times (t = 39 days). ... mehrTogether with thermodynamic calculations, this observation disregards any impact of solubility phenomena and supports that the uptake of Tc is solely controlled by sorption processes under the investigated boundary conditions. This work provides an improved quantitative description and some insights on the process understanding for the retention of Tc-99 by Fe(II) corrosion phases.

[1] J. P. Icenhower et al. (2010) Am. J. Sci. 8, 721-752.
[2] H. Geckeis et al. (2012) ChiuZ. 46, 282–293.
[3] S. Hagemann et al. (2021) Stability of Iron Corrosion Phases Expected in a Repository in Lower Cretaceous Clay, Gesellschaft Für Anlagen- Und Reaktorsicherheit (GRS) gGmbH, Köln Garching b. München Berlin Braunschweig.
[4] J. Goethals et al. (2024) Corros. Sci. 237, 112318.