Actinide redox reactions at the solid-water interface probed by XAFS

Fe(II)-bearing phases are naturally occuring in most anoxic aquifers, and form also at the surface of corroding steel containers under typical nuclear waste repository conditions. Due to their ability to reduce metal and metalloid contaminants, they are expected to play a key role for the migration behaviour of a wide range of radionuclides, including actinides and fission products. Using X-ray absorption spectroscopy as main tool, we have studied reaction end products, mechanisms and kinetics of redox processes at a range of water/mineral interfaces, including magnetite, green rust, mackinawite, chukanovite, and Fe2+-sorbed clays, but also in more redox-inert systems like cement.
Our results show that the electron transport within mineral structures and at the surface is controlling the extent and the kinetics of multi-electron redox reactions. Examples that I will show include: (1) The reduction of Pu(V) to Pu(III), which then forms a highly ordered inner-sphere sorption complex at the 111 face of magnetite, instead of the expected precipitation of PuO2 clusters. (2) The reduction of Pu(V) to PuO2 by chukanovite, and by chukanovite to a mixture of Pu(III) and PuO2. (3) The reduction of U(VI) to mixed U(IV/VI) surface complexes on Fe(II)-sorbed montmorillonite. (4) The conservation of the tetravalent oxidation state of Np in fresh and degrading cement phases and its local structure in the cemet phases. The results highlight the need for direct spectroscopic investigation of such processes, which are difficult to predict by thermodynamic methods, in order to provide reliable risk assessments.

(1) Kirsch, R.; Fellhauer, D.; Altmaier, M.; Neck, V.; Rossberg, A.; Fanghänel, T.; Charlet, L.; Scheinost, A. C. Oxidation state and local structure of plutonium reacted with magnetite, mackinawite and chukanovite. Environ. Sci. Technol. 2011, 45, 7267–7274.

(2) Chakraborty, S.; Boivin, F. F.; Banerjee, D.; Scheinost, A. C.; Mullet, M.; Ehrhardt, J.-J.; Brendle, J.; Vidal, L.; Charlet, L. U(VI) Sorption and Reduction by Fe(II) Sorbed on Montmorillonite. Environ. Sci. Technol. 2010, 44, 3779–3785.

(3) Gaona, X.; Daehn, R.; Tits, J.; Scheinost, A. C.; Wieland, E. Uptake of Np(IV) by C-S-H phases and cement paste: an EXAFS study. 2011, accepted fro publication.