Technetium immobilization by chukanovite and its oxidative transformation products: Neural network analysis of EXAFS spectra

Katja Schmeide; André Rossberg; Frank Bok; Salim Shams Aldin Azzam; Stephan Weiss; Andreas C Scheinost

doi:10.1016/j.scitotenv.2021.145334

Technetium immobilization by chukanovite and its oxidative transformation products: Neural network analysis of EXAFS spectra

Sci Total Environ. 2021 May 20:770:145334. doi: 10.1016/j.scitotenv.2021.145334. Epub 2021 Jan 22.

Authors

Katja Schmeide¹, André Rossberg², Frank Bok³, Salim Shams Aldin Azzam³, Stephan Weiss³, Andreas C Scheinost⁴

Affiliations

¹ Helmholtz-Zentrum Dresden - Rossendorf, Institute of Resource Ecology, Bautzner Landstraße 400, 01328 Dresden, Germany. Electronic address: k.schmeide@hzdr.de.
² Helmholtz-Zentrum Dresden - Rossendorf, Institute of Resource Ecology, Bautzner Landstraße 400, 01328 Dresden, Germany; The Rossendorf Beamline at ESRF - The European Synchrotron, CS40220, 38043 Grenoble Cedex 9, France.
³ Helmholtz-Zentrum Dresden - Rossendorf, Institute of Resource Ecology, Bautzner Landstraße 400, 01328 Dresden, Germany.
⁴ Helmholtz-Zentrum Dresden - Rossendorf, Institute of Resource Ecology, Bautzner Landstraße 400, 01328 Dresden, Germany; The Rossendorf Beamline at ESRF - The European Synchrotron, CS40220, 38043 Grenoble Cedex 9, France. Electronic address: scheinost@esrf.fr.

PMID: 33736379
DOI: 10.1016/j.scitotenv.2021.145334

Abstract

The uptake of the fission product technetium (Tc) by chukanovite, an Fe^II hydroxy carbonate mineral formed as a carbon steel corrosion product in anoxic and carbonate-rich environments, was studied under anoxic, alkaline to hyperalkaline conditions representative for nuclear waste repositories in deep geological formations with cement-based inner linings. The retention potential of chukanovite towards Tc^VII is high in the pH range 7.8 to 12.6, evidenced by high solid-water distribution coefficients, log R_d ~ 6, and independent of ionic strength (0.1 or 1 M NaCl). Using Tc K-edge X-ray absorption spectroscopy (XAS) two series of samples were investigated, Tc chukanovite sorption samples and coprecipitates, prepared with varying Tc loadings, pH values and contact times. From the resulting 37 XAS spectra, spectral endmembers and their dependence on chemical parameters were derived by self-organizing (Kohonen) maps (SOM), a neural network-based approach of machine learning. X-ray absorption near-edge structure (XANES) data confirmed the complete reduction of Tc^VII to Tc^IV by chukanovite under all experimental conditions. Consistent with mineralogical phases identified by X-ray diffraction (XRD), SOM analysis of the extended X-ray absorption fine-structure (EXAFS) spectra revealed the presence of three species in the sorption samples, the speciation predominately controlled by pH: Between pH 7.8 and 11.8, TcO₂-dimers form inner-sphere sorption complexes at the surface of the initial chukanovite as well as on the surface of secondary magnetite formed due to redox reaction. At pH ≥ 11.9, Tc^IV is incorporated in a mixed, chukanovite-like, Fe/Tc hydroxy carbonate precipitate. The same species formed when using the coprecipitation approach. Reoxidation of sorption samples resulted in a small remobilization of Tc, demonstrating that both the original chukanovite mineral and its oxidative transformation products, magnetite and goethite, contribute to the immobilization of Tc in the long term, thus strongly attenuating its environmental transport.

Keywords: Goethite; Machine learning; Magnetite; Redox; XAS; XRD.