Tc immobilization by Fe(II)-Al(III)-Cl layered double hydroxide phase

⁹⁹Tc is a long half-life isotope (2.13×10⁵ years) that can be found in the environment due to anthropogenic sources - nuclear energy production, tests of nuclear weapons and radiopharmacy - as it is a fission product from ²³⁵U and ²³⁹Pu and the daughter of 99mTc, used for diagnosis [1].
Although Tc has several oxidation states, Tc(VII) and Tc(IV) are the more stable ones found under oxidizing and reducing conditions, respectively, but their chemical behavior differs. While the Tc(VII) main species (TcO₄⁻) is a highly mobile anion that hardly interacts with minerals, Tc(IV) is usually found as a low soluble solid (TcO₂) whose precipitation avoids the Tc migration [2]. Therefore, in order to reduce Tc mobility it is essential to understand the conditions that favor the three electrons donation step switching between these two oxidation states.
Several works report that Fe²⁺ promotes the Tc reduction, especially when found in Fe(II)-minerals or sorbed on mineral surfaces [3], [4].
If corrosion of the nuclear waste canisters occurs, Fe²⁺ will be present in the near-field of a deep geological repository. In that case, Fe²⁺ could not only act as reducing agent but also interact with different minerals, getting sorbed or forming new mineral phases.
In fact, it has been observed that when Fe²⁺ interacts with Al₂O₃, it forms Fe(II)-Al(III)-Cl, a layered double hydroxide (LDH) [5]. The LDH phases are known to retain pollutants by different mechanisms: anion exchange, incorporation, surface complexation and, in Fe(II)-Al(III)-Cl, reduction promoted by the structural Fe²⁺ [6].
Therefore, we analysed the ⁹⁹Tc retention by Fe(II)-Al(III)-Cl LDH phase under different conditions (pH, ionic strength and Tc concentration). We observed that the affinity of the Fe(II)-Al(III)-Cl LDH phase for Tc has two different trends. For pH < 6.5, Tc retention increases with increasing pH and decreasing ionic strength, being complete in water, suggesting anion exchange as the main retention mechanism. At pH > 6.5, Tc uptake is complete and independent from the ionic strength and the pH value, suggesting Tc reduction as main uptake mechanism.
This work has been performed in the frame of VESPA II project (02E11607B), supported by the German Federal Ministry for Economic Affairs and Energy (BMWi).
References:
[1] A. H. Meena and Y. Arai, Environ. Chem. Lett., vol. 15, no. 2, pp. 241–263, 2017.
[2] K. H. Lieser and C. Bauscher, Radiochim. Acta, vol. 42, pp. 205–213, 1987.
[3] T. Peretyazhko et al., Geochim. Cosmochim. Acta, vol. 72, no. 6, pp. 1521–1539, 2008.
[4] D. Cui and T. E. Eriksen, Environ. Sci. Technol., vol. 30, no. 7, pp. 2263–2269, 1996.
[5] E. J. Elzinga, Environ. Sci. Technol., vol. 46, no. 9, pp. 4894–4901, 2012.
[6] C. Forano, et al , Layered double hydroxides (LDH), vol. 5. 2013.