⁹⁹Tc retention on Fe(II)Al(III)-Cl layered double hydroxides

To assess the safety of nuclear waste repositories, possible incidents have to be considered like canister corrosion and as a consequence the release of radionuclides.Among them,the fission product ⁹⁹Tc is of high concern due to its long half-life (2.13∙10⁵years) and the high mobility of the Tc(VII)O₄⁻ oxoanion that is barely adsorbed by common mineral phases. However, Tc migration decreases under reducing conditions due to formation of Tc(IV), whose main species is a highly insoluble solid TcO₂.Under the reducing and corrosive conditions in the near-field of the repository, Fe²⁺ will act as a reducing agent for redox sensitive radionuclides (when present in the groundwater or sorbed on mineral surfaces). Furthermore,secondary mineral phases like Fe(II)-Al(III)-Cl, a layered double hydroxide (LDH),can be formed when Fe²⁺ interacts with Al₂O₃ at circumneutral-alkaline pH [1]. LDH phases are so-called anionic clays and they are known to retain pollutants by anion exchange, incorporation, surface complexation and in the case of Fe(II)-Al(III)-Cl via reduction promoted by the structural Fe²⁺ [2]. We have analysed the ⁹⁹Tc uptake by Fe(II)-Al(III)-Cl LDH under varying pH (4 to 11), ionic strength (0 to 0.1 M) and Tc concentration(10⁻⁹to 10⁻³ M). At pH < 6.5, the solid to liquid distribution coefficient, (log Kd in mL/g),ranges from (2 to 6) and increases with decreasing ionic strength and increasing pH. At pH > 6.5, log Kd (6.5±0.3) are independent of pH and ionic strength.Tc K-edge X-ray absorption spectroscopy showed in all cases a reduction to Tc(IV) and enabled us to elucidate the surface bound speciation of Tc on a molecular level.

[1]E. J. Elzinga,”Environ. Sci. Technol., vol. 46, no. 9, pp. 4894–4901, 2012.
[2]C. Forano, U. Costantino, V. Prévot, and C. T. Gueho, Layered double hydroxides (LDH), vol. 5. 2013.