Interaction of technetium with metabolites, microorganisms and at the mineral-water interface: radioecological consideration

Technetium-99 (⁹⁹Tc) is a long-lived fission product (2.13∙10⁵ years) of uranium-235 (²³⁵U) and plutonium-239 (²³⁹Pu) and therefore, of great concern for the long-term safe management of nuclear waste. The migration of Tc in the environment is highly influenced by the redox conditions, since Tc may be present in various oxidation states. Under environmental conditions, Tc is expected to mainly occur as Tc(VII) under oxidizing conditions and as Tc(IV) under reducing conditions. The anion pertechnetate (Tc(VII)O₄⁻) is known to barely interact with mineral surfaces; this, in turn, enhances its migration in groundwater and favors its entry in the biosphere. On the contrary, the formation of Tc(IV) limits the migration of Tc since it forms a low soluble solid (TcO2) and/or species whose interaction with minerals is more favorable. In the last decades Tc migration has been focused on the reduction of Tc(VII) to Tc(IV) by various reductants, such as Fe(II), Sn(II) or S(-II) either present in solution, taking part in mineral structures, [1] or metabolically induced by microbial cascades [2].
Most of the published studies have been focused on binary systems i.e., studies of the interaction of Tc with a given reductant. However, the environment is a complex system where different components often depend on and modify each other. Thus, Tc migration is susceptible and varies upon environmental conditions and should not be studied in an isolated manner. The young investigator group TecRad [3], funded by the German Federal Ministry of Education and Research, aims at analyzing Tc chemistry from a wider perspective. Our goal is to study the biogeochemical behavior of Tc when it interacts with: i) microorganisms, ii) metabolites, iii) Fe(II) minerals, and iv) Fe(II) minerals in presence of metabolites.
An important part of this project deals with implementing new spectro-electrochemical methods to monitor in situ the behavior of Tc in solution and at interfaces as a function of the redox potential. With these tools we aspire at characterizing the molecular structures of Tc species under a variable range of redox conditions, to broaden the understanding of the chemical behavior of the pollutant.
Our goal is to generate valuable thermodynamic data (complex formation constants, solubility constants of minerals, redox potentials and Tc distribution coefficients) that we will use to implement a geochemical modeling able to explain Tc environmental fate even under different redox conditions.

The authors acknowledge the German Federal Ministry of Education and Research (BMBF) for the financial support of NukSiFutur TecRad young investigator group (02NUK072).

References
[1] Pearce, C. et al. (2020). Technetium immobilization by materials through sorption and redox-driven processes: A literature review. Sci. Total Env. 716: 132849.
[2] Newsome, L. et al. (2014). The biogeochemistry and bioremediation of uranium and other prioriy radionuclides. Chem. Geol. 363: 164-184.
[3] TecRad webpage: https://www.hzdr.de/db/Cms?pNid=1375 vis on February 9th 2023.