Immobilization of technetium by iron corrosion phases: Lessons learnt and future perspectives

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. Depending on the chemical properties of environmental relevant systems Tc is expected to mainly occur as Tc(VII) and as Tc(IV) under oxidizing and reducing conditions, respectively. The anion pertechnetate (Tc(VII)O₄⁻) is known to barely interact with mineral surfaces; this, in turn, enhances its migration in groundwater and favours its entry into 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 favourable. 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, (Pearce et al., 2019) or metabolically induced by microbial cascades (Newsome et al., 2014).
We have studied the immobilization of Tc by various Fe(II)-containing phases: Fe²⁺ pre-sorbed on alumina nanoparticles (Mayordomo et al., 2020), Fe(II)-Al(III) layered double hydroxide (Mayordomo et al., 2021), and Fe(II) sulfides (Rodriguez et al., 2020; Rodríguez et al., 2021). We have combined sorption experiments, with microscopic and spectroscopic techniques (scanning electron microscopy, Raman microscopy, X-ray photoelectron spectroscopy, infrared spectroscopy, and X-ray absorption spectroscopy) to elucidate the mechanisms responsible of Tc(VII) reductive immobilization.
Those works 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 (HZDR, 2022), 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.
We aim at generating valuable thermodynamic data (complex formation constants, solubility constants of minerals, redox potentials and Tc distribution coefficients) that will be uses 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) and the German Federal Ministry of Economic Affairs and Climate Action (BMWK, former BMWi) for the financial support of Vespa II (02E11607B).

References

HZDR 2022, accessed March 29th. https://www.hzdr.de/db/Cms?pNid=1375.

Mayordomo, N., Rodríguez, D.M., Rossberg, A., Foerstendorf, H., Heim, K., Brendler, V., Müller, K., 2021. Analysis of technetium immobilization and its molecular retention mechanisms by Fe(II)-Al(III)-Cl layered double hydroxide. Chem. Eng. J. 408, 127265. doi:10.1016/j.cej.2020.127265

Mayordomo, N., Rodríguez, D.M., Schild, D., Molodtsov, K., Johnstone, E. V., Hübner, R., Shams Aldin Azzam, S., Brendler, V., Müller, K., 2020. Technetium retention by gamma alumina nanoparticles and the effect of sorbed Fe2+. J. Hazard. Mater. 388, 122066. doi:10.1016/j.jhazmat.2020.122066

Newsome, L., Morris, K., Lloyd, J.R., 2014. The biogeochemistry and bioremediation of uranium and other priority radionuclides. Chem. Geol. 363, 164–184. doi:10.1016/j.chemgeo.2013.10.034

Pearce, C.I., Moore, R.C., Morad, J.W., Asmussen, R.M., Chatterjee, S., Lawter, A.R., Levitskaia, T.G., Neeway, J.J., Qafoku, N.P., Rigali, M.J., Saslow, S.A., Szecsody, J.E., Thallapally, P.K., Wang, G., Freedman, V.L., 2019. Technetium immobilization by materials through sorption and redox-driven processes: A literature review. Sci. Total Environ. 132849. doi:10.1016/j.scitotenv.2019.06.195

Rodriguez, D.M., Mayordomo, N., Scheinost, A.C., Brendler, V., Müller, K., Stumpf, T., 2020. New insights into 99Tc(VII) removal by pyrite : A spectroscopic approach. Env. Sci. Technol. 54, 2678–2687. doi:10.1021/acs.est.9b05341

Rodríguez, D.M., Mayordomo, N., Schild, D., Shams Aldin Azzam, S., Brendler, V., Müller, K., Stumpf, T., 2021. Reductive immobilization of 99Tc(VII) by FeS2: the effect of marcasite. Chemosphere 281, 130904. doi:10.1016/j.chemosphere.2021.130904