Technetium-99 Reduction in Carbonate Environment: An Spectroelectrochemical and ⁹⁹Tc-NMR Study

Technetium-99 (⁹⁹Tc) is a radioactive isotope with a long half-live (211,000 a). It is produced in nuclear power plants and nuclear weapon detonation since it is a fission product of U-235 and Pu-239. In addition, 99Tc forms after gamma ray emission of metastable technetium-99 (99mTc), which is the most used isotope for cancer diagnosis at hospitals [1]. The emission of ⁹⁹Tc in the environment is hazardous for living organisms and depends on its chemical speci-ation, being especially decisive the oxidation state. Thus, several works focused on the speciation (e.g., [2–4]) and immobilization of Tc (e.g., [5,6]) based on redox changes.
The nuclear properties of Tc99 make it suitable to study Tc molecular structures by NMR [7,8] and, depending on the oxidation state and thus electron configuration, also by EPR [9] spec-troscopies. Despite the power of both these methods, they have been rarely used for envi-ronmental studies.
In this work we reduced KTcO₄ electrochemically in carbonate solutions in dependence on 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 the UV-vis range in situ using a spec-tro-electrochemical cell. At -0.85 V a pink solution (λmax 512 nm) was obtained, corresponding to a Tc(IV) carbonate species [2], whereas reduction at -0.95 V yields a bluish green solution (λmax 630 nm), associated with a Tc(III) carbonate complex [2]. The obtained solutions were then investigated by ⁹⁹Tc NMR. The −0.85 V solution reveals a resonance at ~1600 ppm, indicative of a carbonate species of Tc(V) since the chemical shift range is characteristic for Tc in +V oxidation state [7]. The other specimen yielded at −0.95 V, in addition to the former Tc(V) signal at about 1600 ppm, gives rise to one additional signal at ~152 ppm, which is in the chemical shift range expected for Tc(III) [7].
These are unprecedented NMR data on aqueous Tc carbonate species, which advance the mechanistic understanding of Tc redox behavior and help to improve safety and risk analyses for nuclear waste management.
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