Spectroscopic investigations of the U(VI) sorption onto the zircaloy corrosion product ZrO2

For a safety assessment of a repository for nuclear waste, the interactions of actinides with corroded phases in the near-field must be taken into account. Most commercial fission reactors use uranium-based fuels and the spent nuclear fuel still contains approximately 95 % of uranium, making it the largest fraction of the spent nuclear fuel by mass. Zirconia (ZrO₂) is the main corrosion product of the zircaloy cladding material of nuclear fuel rods and can act as a first barrier against the release of mobilized radionuclides from the spent nuclear fuel into the environment. Furthermore, the complexation of dissolved radionuclides with common inorganic ligands, such as carbonate, in the groundwater can have a significant influence on the formation and structure of actinide surface species and thus their mobility in the environment.
In situ Attenuated Total Reflection Fourier Transform Infrared Spectroscopy (ATR FT-IR) and Extended X-ray Absorption Fine Structure Spectroscopy (EXAFS) were applied to investigate the U(VI) speciation at the ZrO₂-water interface. A pH-dependent speciation of U(VI) at the zirconia surface could be observed under inert gas conditions with EXAFS and the preliminary results indicated the presence of two inner-sphere U(VI) surface species with different structural environments. The EXAFS results can be compared to the literature results of Lomenech et al., where also two U(VI) surface species on the ZrO₂ surface were observed (a tridentate U(VI) surface species at pH 3 and a bidentate surface species at higher pH) [1,2].
The sorption of U(VI) onto ZrO₂ under inert gas conditions was also studied with ATR FT-IR at pH 3.5 and 5.5 and a pH-dependent U(VI) speciation was observed, supporting the findings from the EXAFS investigations. The observed red shift of the asymmetric stretching vibration of the free uranyl aqua ion (𝜈₃(UO₂²⁺)) in the presence of ZrO₂ at pH 5.5 was due to the U(VI)‒ZrO₂ interactions. At a lower pH of 3.5 a second U(VI) surface species with a less pronounced red shift of the ν₃ vibration was identified.
A pH-dependence of the sorption of atmospheric carbonate on the zirconia surface was observed and a spectral splitting (Δ𝜈) of approximately 200 cm⁻¹ of the symmetric and asymmetric stretching vibration modes indicated the presence of bidentate bound carbonate species on the surface. The U(VI) sorption onto zirconia pre-equilibrated with atmospheric carbonate was also studied at pH 5.5 and 3.5. Compared to the experiments conducted under inert gas conditions, the red shift of the ν₃ mode of U(VI) at pH 5.5 was more pronounced in the presence of carbonate, indicating an influence of carbonate on the formed U(VI) surface species. In addition, changes in the frequency of the asymmetric and symmetric stretching vibrations of carbonate sorbed to the zirconia surface were observed in the presence of U(VI), also hinting towards structural changes in the surface species.
EXAFS and ATR FT-IR investigations provided valuable structural information about the formed U(VI) sorption species on the ZrO₂ surface in the presence and absence of carbonate. The improved molecular level understanding of such sorption processes will enable more reliable predictions of the environmental fate of U(VI). Such results will be complemented with batch sorption experiments as well as thermodynamic surface complexation modeling.

[1] Lomenech, C. et al. (2003) Radiochim. Acta 91(8), 453-461.
[2] Lomenech, C. et al. (2003) J. Colloid Interface Sci. 261(2), 221-232.