Goldschmidt Conference: Mayordomo Talk

99Tc retention on Fe(II)Al(III)-Cl layered double hydroxides

1Helmholtz-Zentrum Dresden-Rossendorf Bautzner
Landstraße 400 01328 Dresden (Germany)
2The Rossendorf Beamline at ESRF, 71 Ave. des Martyrs,
38043 Grenoble (France)

To assess the safety of nuclear waste repositories,
possible incidents have to be considered like canister
corrosion and as a consequence the release of radionuclides.
Among them, the fission product 99Tc is of high concern due
to its long half-life (2.13∙105 years) and the high mobility of
the Tc(VII)O4- oxoanion that is barely adsorbed by common
mineral phases. However, Tc migration decreases under
reducing conditions due to formation of Tc(IV), whose main
species is a highly insoluble solid TcO2.Under the reducing
and corrosive conditions in the near-field of the repository,
Fe2+ will act as a reducing agent for redox sensitive
radionuclides (when present in the groundwater or sorbed on
mineral surfaces). Furthermore, secondary mineral phases like
Fe(II)-Al(III)-Cl, a layered double hydroxide (LDH), can be
formed when Fe2+ interacts with Al2O3 at circumneutralalkaline
pH [1]. LDH phases are so-called anionic clays and
they are known to retain pollutants by anion exchange,
incorporation, surface complexation and in the case of Fe(II)-
Al(III)-Cl via reduction promoted by the structural Fe2+ [2].
We have analysed the 99Tc uptake by Fe(II)-Al(III)-Cl
LDH under varying pH (4 to 11), ionic strength (0 to 0.1 M)
and Tc concentration (10-9 to 10-3 M). At pH < 6.5, the solid
to liquid distribution coefficient, (log Kd in mL/g), ranges
from (2 to 6) and increases with decreasing ionic strength and
increasing pH. At pH > 6.5, log Kd (6.5±0.3) are independent
of pH and ionic strength. Tc K-edgeX-ay absorption
spectroscopy showed in all cases a reduction to Tc(IV) and
enabled us to elucidate the surface bound speciation of Tc on
a molecular level.

We thank the German Federal Ministry of Economic
Affairs and Energy (BMWi) for the VESPA II project
funding and Dr. K. Dardenne and Dr. J. Rothe for their help
at KARA.

[1]    E. J. Elzinga,” Environ. Sci. Technol., vol. 46, no. 9,
        pp. 4894–4901, 2012.
[2]    C. Forano, U. Costantino, V. Prévot, and C. T.
        Gueho, Layered double hydroxides (LDH), vol. 5. 2013.