Chloride green rust as scavenger of technetium: immobilization and spectroscopic studies

Techntium-99 (⁹⁹Tc) is one of the most concerning fission products due to its long half-life (2.13∙10⁵ years) and the high mobility of the anion pertechnetate (TcO₄⁻) [1]. Tc migration decreases significantly when Tc(VII) is reduced to Tc(IV). This scavenging step can be induced by Fe(II) minerals, which have been widely studied due to their versatility, low cost, and ubiquity [2]. In addition, Fe(II) minerals will play an important role in the near-field of the nuclear waste repository, in case that corrosion of the waste canisters will occur and radioactive material be leaked in the environment.
Green rust is formed when Fe²⁺ interacts with Fe(III) minerals [3]. Thus, its presence is expected in both the near- and far-field of a repository. It is a Fe(II)-Fe(III) hydroxide that can immobilize radionuclides by adsorption, anion exchange, and reduction mechanisms. A previous work reports the interaction of green rust with Tc, but the results are limited to very narrow experimental conditions [4]. Thus, further studies are needed to both identify the optimal Tc scavenging conditions by green rust and the mechanism responsible of Tc retention.
Our studies consisted of a combination of batch contact studies, microscopic and spectroscopic analysis. Batch contact studies were performed under a wide range of conditions, i.e. pH (3.5 - 11.0), Tc concentration (nM - mM), and ionic strength (0.0 - 0.1 M). X-ray powder diffraction, Raman microscopy, X-ray photoelectron spectroscopy (XPS), and X-ray absorption spectroscopy provided information on Tc oxidation state and speciation as well as on secondary redox products related to the Tc interaction with chloride green rust (GR(Cl)). In addition, re-oxidation experiments have been performed for one year to analyze the Tc retention reversibility.
The results show that GR(Cl) removes Tc from solution with efficiencies between 80% (Kd = 8.0∙10³ mL/g) and ≈ 100% (Kd = 9.9∙10⁵ mL/g) for pH > 6.0 (Figure 1). In contrast, Tc removal for pH < 6.0 drops with decreasing pH, and ranges from 80% to 50% (Kd = 2.0∙10³ mL/g), reaching a minimum at pH 3.5.

XPS analysis reveals the predominance of Tc(IV) at all evaluated pH values (3.5 to 11.5), supporting that Tc reductive immobilization is the main retention mechanism. Re-oxidation experiments show that Tc is slowly solubilized when time increases.
The analysis of the extended X-ray absorption fine structure indicates a change of the Tc(IV) atomic environment depending on pH and Tc loading. The most probable structural rearrangements are represented by Tc(IV) sorption on Fe(III) minerals formed as secondary phases with Tc polynuclear species contribution.

The authors acknowledge the German Federal Ministry of Education and Research (BMBF) for the financial support of the NukSiFutur TecRad young investigator group (02NUK072) [5].

References
[1] Meena, A. H. and Arai, Y (2017). Environmental geochemistry of technetium. Env. Chem. Lett. 15: 241-263.
[2] Pearce, C. I. et al. (2020). Technetium immobilization by materials through sorption and redox-driven processes: A literature review. Sci. Total Environ. 716: 132849.
[3] Usman, M. et al (2018). Magnetite and green rust: Synthesis, Properties, and Environmental Applications of Mixed-Valent Iron Minerals. Chem. Rev. 118: 3251-3304.
[4] Pepper, S. et al. (2003). Treatment of radioactive wastes: An X-ray absorption spectroscopy study of the reaction of technetium with green rust. J. Colloid Interface Sci. 268: 408-412.
[5] TecRad webpage: https://www.hzdr.de/db/Cms?pNid=1375, vis. Feb 9th 2023.