Spectroscopic and batch studies of technetium uptake by siderite

⁹⁹Tc is a long-lived (t_1/2 = 2.1 x 10⁵ years) β-emitter formed during the fission of U and is of major concern for radioactive waste disposal. Its environmental mobility is primarily governed by the oxidation states VII and IV, with Tc^VII forming the highly mobile TcO₄ ⁻ aquo anion, whereas Tc^IV is rather immobile due to the low solubility of its hydrolysis products. Redox processes, which are able to convert Tc^VII into Tc^IV, are hence of paramount importance for the safety of radioactive waste repositories. Fe^II-bearing minerals, ubiquitous in nature but also forming as corrosion products of the steel canisters foreseen as a possible first enclosure of radioactive waste, play a vital role in these redox reactions due to their high redox reactivity and high sorption capacity, as has been shown not only for Tc, but also for Se, U, Np and Pu.
We studied the Tc^VII uptake by siderite (FeCO₃), a typical Fe^II mineral in carbonate-rich environments, in the relevant pH range 7 – 12.6 under anoxic conditions by means of batch sorption experiments and by X-ray absorption spectroscopy. Sorption experiments showed that Tc retention by siderite is fast and efficient (log R_d ~5) across the investigated pH range and independent of ionic strength (0.1 – 1 M NaCl). Tc K-edge X-ray absorption near-edge structure (XANES) data confirmed that the Tc immobilization is due to the surface-mediated reduction of Tc^VII to Tc^IV. The local structure of Tc^IV in Tc siderite sorption samples and Tc siderite coprecipitates probed by extended X-ray absorption fine-structure (EXAFS) spectroscopy revealed three different species: In the pH range 7.8 to 11.8, TcO₂-dimers form inner-sphere sorption complexes at the surface of the initial siderite phase as well as on secondary magnetite or green rust formed during the redox reaction. Between pH 11.9 and 12.6, a mixed Fe/Tc hydroxocarbonate precipitate (chukanovite-like) is formed.
The results showed that siderite contributes effectively to the retention of Tc under repository conditions through formation of strong sorption complexes and precipitation of hydroxocarbonate phases with low solubility.