hzdr.dePh.D. projects
Metabolites Interaction with Technetium and Minerals
Nuclear waste must be safely disposed on a long-term to isolate it from the biosphere. Technetium-99 (99Tc) is one of the concerning radionuclides present in the nuclear waste. 99Tc is produced in nuclear reactors due to the fission of 235U and 239Pu in a high yield (6%). It is a beta emitter with a half-life of 0.21 million years (1) and it highly contributes to the radiation emitted in the nuclear waste. It could be a potential hazardous material if released into the environment.
Significant emissions of Tc into the biosphere can cause damages to living organism after long-term exposure, in particular via accumulation in the food chain (2). Hence, it is essential to gain understanding on the basic reaction of Tc to avoid contamination and improve the safety assessments for nuclear waste repositories and to establish viable Tc remediation strategies.
The available literature shows that Tc mobilization depends on its speciation in aqueous solution, which is strongly influenced by the redox conditions (3). Under oxidizing conditions and absence of stabilizing ligands, Tc main species is pertechnetate (Tc(VII)O4−), a highly soluble anion in water that hardy sorbs on the minerals (4). Under reducing conditions, Tc(IV) is the main species of Tc. In comparison to Tc(VII), Tc(IV) migration in water is reduced either because it forms a poorly soluble solid TcO2, incorporates on mineral structures, or forms complexes on the mineral surface. The reduction of Tc(VII) to Tc(IV) can be triggered by minerals that are present in the repository – either because they are originally present in the geotechnical and geologic barriers or they are formed by secondary reaction, e.g. corrosion of canister steel or by bio-mineralization. Especially good Tc reducing agents are Fe(II) minerals such as pyrite, green rust or magnetite (3). Concerning microbial activity, microorganisms may excrete metabolites that in turn can influence both, the chemistry of Tc and the structural properties of the minerals and their Tc immobilization capacities (5).
Therefore, it is important to understand the impact of the metabolites on the Fe(II) minerals dissolution as well as on the chemical behavior of Tc and its retention by Fe(II) minerals. This knowledge is crucial to assess Tc mobility in the near and far field of the nuclear waste repository. However, a systematic work has to be performed, as only few works could be identified on this topic.
This PhD study aims to develop the basic understanding of metabolites interaction with Fe(II) minerals and Tc in a more realistic scenario. Acetic and succinic acid have been selected as metabolites to investigate 1) their influence on pyrite dissolution and 2) their complexation with Tc 3) finally study in a ternary system their impact on Tc retention. Various analytical techniques such as, inductively coupled plasma mass-spectroscopy (ICP-MS), high performance liquid chromatography (HPLC), liquid scintillation counting (LSC), scanning electron microscopy (SEM), Raman microscopy, X-ray diffraction (XRD), IR-UV-visible spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, electron paramagnetic resonance (EPR) spectroscopy and synchrotron-based techniques, such as X-ray absorption spectroscopy at the Rossendorf beamline at the ESRF (6) will be used for characterization.
In a current study, pyrite dissolution experiments with acetate at 3.5-11.5 pH range were carried out, which show increasing pyrite dissolution and negligible sorption of acetate on pyrite. The scanning electron microscope image of pyrite dissolution experiment (without metabolite) shows the dissolution features and precipitates (Figure 1).
Figure 1. Scanning electron microscope (SEM) image of pyrite without metabolite experiment (A) grains and (B) enlarged view of dissolution features and precipitates.
This Ph.D. research is developed in the frame of the NukSiFutur Young Investigators group TecRad (02NUK072), funded by the German Federal Ministry of Education and Research (BMBF).
References
- Johannsen B, Spies H. Chemistry of technetium (V) as relevant to nuclear medicine. Topics Curr. Chem. 1996;176:77-121.
- Grambow B. Mobile fission and activation products in nuclear waste disposal. J Contam Hydrol. 2008 Dec;102(3–4):180–6.
- Meena AH, Arai Y. Environmental geochemistry of technetium. Environ Chem Lett. 2017 Jun;15(2):241–63.
- Rodríguez DM, Mayordomo N, Scheinost AC, Schild D, Brendler V, Müller K, et al. New Insights into 99 Tc(VII) Removal by Pyrite: A Spectroscopic Approach. Environ Sci Technol. 2020 Mar 3;54(5):2678–87.
- Brookshaw DR, Pattrick RAD, Lloyd JR, Vaughan DJ. Microbial effects on mineral–radionuclide interactions and radionuclide solid-phase capture processes. Mineral Mag. 2012 Jun;76(3):777–806.
- Scheinost AC, Claussner J, Exner J, Feig M, Findeisen S, Hennig C, et al. ROBL-II at ESRF: a synchrotron toolbox for actinide research. J Synchrotron Radiat. 2021 Jan 1;28(1):333–49.
