The surface complexes of the oxoanions of Se(VI) and Tc(VII) at mineral-water interfaces

⁷⁹Se and ⁹⁹Tc are fission products of the nuclear fuel cycle and, thus, are of concern during the safety assessment of a nuclear waste disposal site. For a comprehensive description of their mobility in the near and far field of a deep geological repository, the interactions of respective dissolved species with mineral surfaces are of special interest. In particular, the oxoanions Se(VI)O₄ ²⁻ and Tc(VII)O₄ ⁻ are considered as mobile species in aquifer due to their high solubility and negative charge.
In this study, the surface reactions of these anions were studied by in situ vibrational spectroscopy which is a dedicated technique for the real time monitoring of the surface processes at the water-mineral interface [1, 2]. For the selenate anion, the formation of two different types of outer-sphere complexes was observed depending on the mineral’s surface. Although both types of sorption complexes behave like typical outer-sphere complexes in terms of macroscopic properties, the different spectral signatures clearly reflect two different molecule symmetries. From the spectra, a slightly distorted tetrahedral geometry, which is close to the aqueous species, and a bidendately coordinated species showing a C_2v symmetry were derived. These surface species are denoted as “extended” and “classical” outer sphere complexes, respectively [3, 4].
In homology, spectroscopic sorption experiments with Tc(VII) were performed. The aqueous species should share the same symmetry as for Se(VI), that is a tetrahedral TcO₄ ⁻ ion. From preliminary results of the vibrational spectroscopic sorption experiments, mainly outer-sphere complexes were found which are obviously less specific as it was found for Se(VI).

[1] Foerstendorf, H. et al. (2012) J. Colloid Interface Sci. 377, 299–306. [2] Müller, K. et al. (2015) Environ. Sci. Technol. 49, 2560–2567. [3] Jordan, N. et al. (2011) Geochim. Cosmochim. Acta 75, 1519–1530. [4] Jordan, N. et al. (2013) Geochim. Cosmochim. Acta 103, 63–75.