Plutonium redox reactions with iron oxides under anoxic conditions

The environmental fate of plutonium, the major transuranium actinide in nuclear waste, is largely impacted by its sorption onto [1] and redox reactions [2] with iron oxide minerals that form as corrosion products of steel in the "near field" and occur widely in sediments. To obtain information on oxid-ation state and local structure, we reacted 242Pu as electro-lytically prepared Pu(V) or Pu(III) (1×10-5 M) under anoxic conditions in carbonate free 0.1 M NaCl with hematite, goethite, maghemite and magnetite. Pu-LIII-edge XAFS spectra were collected after 40 d and 6 months of reaction.
Results and Discussion
After reaction of either Pu(III) or Pu(V) with hematite (α-Fe2O3), Pu associated with the solid phase ( > 99.9 % of added Pu) is mainly present as Pu(IV) and up to 30 % Pu(V). Also after reaction with goethite (γ-FeOOH) both Pu(IV) (55 %) and Pu(V) (45 %) are present. For both minerals, EXAFS spectra show no strong Fe-backscattering from the substrate and also give no evidence for the formation of a solid PuO2 phase. In contrast, EXAFS spectra of Pu reacted with maghemite (γ-Fe2O3) and magnetite (Fe3O4) are charac-terized by strong iron backscattering, indicating the formation of inner-sphere surface sorption complexes. With maghemite, oxidation state mixtures of Pu(III) and Pu(IV) or Pu(IV) and Pu(V) were found while with magnetite, Pu(III) was the predominant oxidation state [3]. However, in one case and probably due to an increased Pu / magnetite surface area ratio, formation of PuO2 after reaction of Pu(V) with magnetite was observed. These results highlight the importance of plutonium surface complexation on different iron oxides in controlling environmental [Pu] concentrations. Further, for risk assessment under reducing conditions where Fe(II)-bearing oxides such as magnetite exist, it is necessary to consider trivalent in addition to tetravalent plutonium species and PuO2(am,hyd).
[1] Novikov et al. (2006) Science 314, 638-641. [2] Powell et al. (2005) Environ. Sci. Technol. 39, 2107-2114. [3] Kirsch et al. (2011) submitted to Environ. Sci. Technol.