Research at HZDR focuses on the chemistry of technetium (99Tc), a fission product with a half-time of 2.13×105 years. Boundary conditions are defined by the near and far field of a nuclear waste repository in Germany.
Although Tc has different oxidation states, Tc(VII) and Tc(IV) are the most stable ones under environmental conditions. Tc(VII) main species is the anion pertechnetate (TcO4-). As it is highly soluble and hardly interacts with minerals it is considered to behave like a conservative tracer, i.e. long—term safety assessment does generally neglect any retardation. In contrast, Tc(IV) is mainly found as a solid with a low solubility product (TcO2) and its formation limits the Tc migration in groundwater. Thus, the reduction of Tc(VIII) to Tc(IV) is a relevant step to be considered for its environmental remediation. However, the mechanism involved in this three-electron reduction is not fully understood yet. In consequence, our work is divided into two different fields and aims to fill the gaps encountered in Tc thermodynamics. Consequently, this shall also provide a better understanding of Tc retention on e.g. Fe(II) mineral phases.
I) We study the interaction of Tc(VII) and Tc(IV) with different minerals relevant in the repository and the environment: iron corrosion phases (like green rust), environmental ubiquitous minerals (such as pyrite) or layered double hydroxides (LDH). We explore the conditions under which Tc retention is maximum and elucidate the molecular mechanisms responsible for Tc immobilization on the minerals (surface complexation, precipitation, incorporation and/or anion exchange). Our methodological toolbox contains b-spectrometry, vibrational (infrared and raman) spectroscopy, UV-vis, XPS, EXAFS and XANES.
II) We explore the chemistry of the Tc reduction by means of spectroelectrochemical measurements under variable potentials in presence of different electrolytes relevant in the environment (Cl-, ClO4-, NO3- or HCO3-). Eventually, thermodynamic data of the reduction processes and the complex formations shall be derived, together with analyzing the conditions required for the occurrence of rare Tc oxidation states (other than IV and VII) in solution.
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