Unveiling the role of Pseudodesulfovibrio aespoeensis in the immobilization of technetium-99

Technetium (Tc) is a radionuclide with 45 identified isotopes being ⁹⁹Tc the most abundant in the environment. This isotope is a β⁻ emitter presenting a half-life (t_1/2) of 2.13 × 10⁵ years [1]. The abundance of ⁹⁹Tc in the environment is mainly due to anthropogenic activities such as the fission of ²³⁵U and ²³⁹Pu in nuclear reactors (electricity production) and nuclear weapon detonation tests, as well as due the decay product of the worldwide-used isotope ^99mTc for radiodiagnosis [2]. Due to its long half-life, ⁹⁹Tc will be one of the critical radionuclides present in the high radioactive waste that will be finally stored in deep geological repositories (DGRs). This disposal consist of a multi-barrier system where bacteria will be present and can potentially influence the migration of Tc. After the DGR final sealing, the activity of some bacteria (e.g., sulfate-reducing bacteria; SRB) could compromise the integrity of the repository barriers, which could lead to the release of radionuclides from the waste. However, some SRB could be beneficial and mediate the reductive immobilization of mobile Tc^VII to less mobile Tc^IV [3]. Therefore, unveiling the role of repository-relevant bacteria in the interaction with ⁹⁹Tc under well-defined conditions is crucial for the safety assessment of a DGR.

This study investigates the role of the SRB Pseudodesulfovibrio aespoeensis ASPO-2, isolated from the groundwater of a DGR-related underground laboratory [4], in the direct reductive immobilization of Tc^VII. The efficiency of Tc immobilization at circumneutral pH was assessed using liquid scintillation counting (LSC). Additionally, the localization of Tc on or in bacterial cells and its retention mechanisms were analyzed through scanning electron microscopy (SEM) and several spectroscopic techniques (XPS and XAS). Our results suggest that the presence of P. aespoeensis in the DGR environment could affect the mobilization of Tc limiting the migration of this radionuclide into the bio- and geosphere

The authors acknowledge funding from the German Federal Ministry of Education and Research for NukSiFutur young investigator group TecRad (02NUK072) and MFM-M for the funding obtained from the Ramón Areces Foundation.

[1] Johnstone et al., 2017, J. Chem Educ, 94, 320−326

[2] Meena and Arai, 2017, Environ Chem Lett, 15:241–263

[3] Lloyd et al., 1998, Geomicrobiol J, 15:1, 45-58

[4] Motamedi and Pedersen, 1998, Int J Syst Evol Microbiol, 48:1, 311–315