Np(V) sorption onto zirconia: a combined spectroscopy, batch and modeling study

When assessing the long-term safety of a nuclear waste repository, the interactions of dissolved long-lived radionuclides, such as the actinide neptunium, with corroded phases in the near-field of the repository have to be considered. Zirconia (ZrO₂) is the main corrosion product of the zircaloy cladding material of nuclear fuel rods and can constitute a first barrier against the release of mobilized radionuclides into the environment.
To gain a detailed understanding of the Np(V) sorption processes at the zirconia‒water interface, a comprehensive multimethod approach was pursued. Molecular level information about the Np(V) surface species were derived by in situ Attenuated Total Reflection Fourier Transform Infrared Spectroscopy (ATR FT-IR) and Extended X-ray Absorption Fine Structure Spectroscopy (EXAFS). The Np L₃-absorption edge (17,610 eV) is commonly used for EXAFS investigations of neptunium. However, the Zr K absorption edge (17,998 eV) is close in energy to the Np L₃-edge, reducing the k-range that can be evaluated. Since attempts to use the Np L₂-edge (21,600 eV) with an energy above the Zr K-edge were not successful, the Np L₃-edge EXAFS spectra had to be used to gain information about the molecular environment of the sorbed Np(V) surface species. The short Np-Zr distance of approximately 3.6 Å derived from EXAFS spectra revealed the predominant formation of bidentate inner-sphere Np(V) surface complexes. ATR FT-IR experiments were conducted at different pH values and a shift of the asymmetric stretching vibration of Np(V) (𝜈₃(NpO₂⁺) towards lower energies was observed at acidic pH, revealing the interactions between Np(V) and ZrO₂. Furthermore, the sorption process was only slightly reversible, also indicating the formation of Np(V) inner-sphere complexes. However, with increasing pH, vibrational surface modes of the ZrO₂ matrix appeared, which were overlapping with Np(V) stretching frequency and impeding the investigation of the pH-dependent surface speciation of Np(V).
Batch sorption experiments (varying ionic strength, Np(V) concentration, and solid-to-liquid ratio (m/V)) as well as a sorption isotherm experiment at pH 6 were conducted to study the sorption processes of the Np(V)‒ZrO₂ system on the macroscopic scale. The sorption of Np(V) was independent of ionic strength, also indicating the formation of Np(V) inner-sphere surface complexes. This was supported by zeta potential measurements in the presence of neptunium, where a shift to higher pH values of the isoelectric point of the neat ZrO₂ was observed. With increasing m/V the Np(V) sorption edge was shifted towards lower pH values, indicating the presence of different kinds of sorption sites, which was also deduced from the shape of the sorption isotherm.
Reliable information about the number and denticity of surface species obtained by spectroscopic and macroscopic investigations enable modeling approaches such as surface complexation modeling (SCM) to be robust. The results derived by SCM will in turn contribute to a more reliable prediction of the environmental fate of neptunium.