Interaction of Tc with iron(II) phosphate

Technetium (Tc) is an element originating mostly from the fission of ²³⁵U and ²³⁹Pu with a yield of 6%.¹ Therefore, ⁹⁹Tc is mainly found in high-level radioactive waste, e.g. from nuclear power or reprocessing plants.² The waste disposal is the subject of numerous studies due to the long half-life of many radionuclides (e.g. ⁹⁹Tc: 2.1 · 10⁵ years)¹ and their high radiotoxicity. One of the most accepted concepts is the deep geological underground repository. A multiple barrier system is planned to reduce the risk of a worst-case scenario, when water ingress could induce the corrosion of the canister containing the waste and, thus, radionuclide release. For the long-term safety, including the construction of effective barriers, the interaction of the radionuclides with different minerals present in the repository needs to be studied at a fundamental level. Tc shows a complex redox chemistry and is considered very mobile compared to cationic radionuclides, due to the presence of the negatively charged TcO₄⁻ under oxidising conditions. However, Tc migration decreases when Tc(VII) is reduced to Tc(IV) since it forms precipitates or is immobilized by mineral surfaces, e.g. with Fe(II) minerals (Fig. 1).³ Vivianite (Fe₃(PO₄)₂ · 8 H₂O) is a naturally occurring Fe(II) mineral under reducing conditions⁵ and can be formed by microorganisms.⁶ Phosphate phases are already being considered as an immobilisation matrix for other radionuclides relevant in deep geological repositories (e.g. ²³⁵U, ²³⁷Np, ²³⁹Pu, ²⁴³Am).⁷ ⁸ This study investigates the retention of Tc by synthetic vivianite particles as a function of pH, Tc concentration and ionic strength on a macroscopic and molecular scale. In addition, Tc(IV) reoxidation experiments were performed.⁴ The synthesis of vivianite was carried out by precipitation from a solution mixture of an iron(II) sulphate and ammonium hydrogen phosphate, as described by Roldán et al..⁹ The product was characterised by Raman microscopy, Mössbauer spec