Speaker
Description
Technetium-99 ($^{99}$Tc) is a long-lived fission product (t½ = 2.13 × 10⁵ a) with high yield (~6%) that poses environmental concerns due to its complex redox chemistry.[1] Under oxic conditions, it exists as the highly mobile pertechnetate anion ($\mathrm{TcO_4^-}$), whereas under reducing conditions, $\mathrm{Tc^{IV}}$ is less mobile and readily sorbs onto mineral surfaces or precipitates as $\mathrm{TcO_2 \cdot nH_2O}$ or $\mathrm{TcS_2}$.[1], [2]
The deep geological repository is considered the best practice for the long−term isolation of radioactive material from the environment. It follows the multi-barrier concept where bentonite clay is used as backfill material. Pyrite ($\mathrm{Fe^{II}S_2}$) is one of the accessory minerals found in the bentonite clay,[3] where also indigenous bacteria grow, producing metabolites. In the worst-case scenario of the safety assessment for such repository, Tc might be released in the surrounding environment. It is known that $\mathrm{Tc^{VII}}$ can be retained by pyrite in absence of any metabolites due to reductive immobilization. Tc retention mechanisms involve inner-sphere complexation of $\mathrm{Tc^{IV}\text{-}Tc^{IV}}$ dimers onto hematite at pH 6, and $\mathrm{Tc^{IV}}$ incorporation into magnetite via $\mathrm{Fe^{III}}$ substitution ≥ pH 10.[4] However, this Tc immobilization by pyrite can be influenced in presence of metabolites.
In this study, the effect of metabolites (acetate, succinate and DFOB) on pyrite solubility was studied using inductively coupled plasma mass spectrometry (ICP-MS) and high-performance liquid chromatography (HPLC). The $^{99}$Tc immobilization was investigated using liquid scintillation counting (LSC), nuclear magnetic resonance (NMR), and Tc K-edge X-ray absorption spectroscopy (XAS). Results showed the increase in pyrite solubility in presence of metabolites, especially with DFOB. The kinetics of Tc immobilization by pyrite were slightly slower in the presence of acetate and succinate. In contrast, DFOB significantly decreased the quantitative removal of $^{99}$Tc by pyrite, to a maximum of 25%. The XAS investigation showed the presence of $\mathrm{Tc^{IV}}$ in all the Tc-containing solid samples and different immobilization mechanism as a function of metabolite and pH. NMR experiments showed the presence of an aqueous Tc-DFOB complex.
Work in progress focuses on studying $^{99}$Tc migration in the environment by column experiments, and the determination of the Tc complexation constants with metabolites by solvent extraction method[5] and isothermal titration calorimetry (ITC).
Acknowledgements
The authors acknowledge the German Federal Ministry of Research, Technology and Space (BMFTR) for the financial support of the NukSiFutur young investigator group TecRad (02NUK072). The authors express their gratitude to Prof. Gareth Law and Dr. Rohan Jain for the support of this work.