Technetium speciation in carbonate media and further developments on spectroelectrochemical methods

Technetium (₄₃Tc) is an inherent radioactive element with isotopes ranging from ⁸⁵Tc to ¹²⁰Tc [1]. Among them,⁹⁹Tc is the most environmentally relevant since it is a high-yield fission product of ²³⁵U and ²³⁹Pu, and the ground-state nuclear isomer of metastable ⁹⁹Tc (^99mTc)– the isotope worldwide most commonly used for radiodiagnosis. ⁹⁹Tc is a long-lived beta minus emitter (τ_½ = 2.13×10⁵ years) and its aqueous speciation and migration behavior is influenced by the (physico-)chemical conditions (e.g, pH, E_h, presence of ligands, etc.). It is known that Tc^VIIO₄⁻ barely interacts with minerals and, thus, its mobility in water is high. On the contrary, the migration of Tc^IV is limited since it forms low-soluble species (e.g., TcO₂ or Tc-sulfides), surface complexes on minerals, and/or it is incorporated into the mineral structures [2]. Studying Tc mobility is a matter of concern for the safety assessment of the nuclear waste repository and radioecology. Thus, several works focused on aqueous Tc speciation based on redox changes and the chemical composition of the solution [3–5].

In this work, we have studied the Tc speciation when KTc^VIIO₄ is electrochemically reduced in carbonate solutions at varying pH (8.2–10.0), Tc concentration (0.5–9.5 mM), carbonate concentration (5–1000 mM), and the applied potential. Tc^VII reduction was monitored in situ by UV-vis, by using a spectro-electrochemical cell. At −0.85 V a pink solution (λ_max = 512 nm) was obtained, corresponding to a Tc^IV carbonate species [3], whereas reduction at −0.95 V yields a bluish green solution (λ_max = 630 nm), associated with a Tc^III carbonate complex [3]. Additionally, the obtained solutions were then investigated by ⁹⁹Tc NMR. The −0.85 V specimen gives rise to a resonance at ~1600 ppm, characteristic for Tc^V [6]. The solution yielded at −0.95 V, besides the aforementioned Tc^V signal, reveals one additional signal at ~152 ppm, corresponding to the chemical shift range of Tc^III [6].

These unprecedented NMR data on aqueous Tc carbonate species, complemented by UV-Vis spectroscopical analysis, advance the mechanistic understanding of Tc redox behavior, and help to improve safety and risk analyses for nuclear waste management.

In addition, this work will show the further developments on spectroelectrochemical methods to study the structural behavior of Tc and other redox-active elements in solution (by in situ NMR) and in solid phase (by in situ IR) as a function of the redox potential. These advanced techniques will help to determine the mobility of redox-active elements under varying redox conditions, which in turn will be useful for the safety assessment of the nuclear waste repository.

 

Acknowledgements:

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] E.V. Johnstone, J. Chem. Educ. (2017) 320–326. [2] A.H. Meena, Env. Chem Lett. (2017) 241–263. [3]           J. Paquette, Can. J. Chem. (1985) 2369–2373 [4] M. Chotkowski, J. Electroanal. Chem. (2018) 83–90. [5] D.M. Rodríguez, Inorg. Chem. (2022) 10159–10166. [6] V.A. Mikhalev, Radiochemistry (2005) 319–333.