Unraveling the Np(V) Sorption on the Nuclear Fuel Cladding Corrosion Product ZrO₂: a Batch, Spectroscopic and Modeling Combined Approach

Zirconia (ZrO₂), the main corrosion product of the zircaloy cladding material of nuclear fuel rods, might potentially act as the first barrier for radionuclides. Thus, the interactions of radionuclides, such as the long-lived actinide neptunium, with zirconia have to be considered in the safety assessment process of a repository for radioactive waste. The sorption of Np(V) onto zirconia (ZrO₂) was investigated in the absence of carbonate at the macroscopic and molecular scale. For the macroscopic description, pH-dependent batch sorption experiments under varying ionic strength (0.1 and 0.01 mol∙L⁻¹ NaCl), Np(V) concentration (1∙10⁻⁶ or 6∙10⁻⁶ mol∙L⁻¹) and solid-to-liquid ratio (m/V = 0.5 or 4 g∙L⁻¹ ZrO₂) were conducted. Np(V) sorption isotherms at pH 4.5 and 6.0 were additionally obtained at 0.01 mol∙L⁻¹ NaCl. The Np(V) uptake on zirconia strongly depends on pH, with sorption starting from acidic pH and maximum sorption was reached at pH 6 and above. Increasing the m/V ratio caused a significant shift of the sorption edge towards lower pH values. This indicates the presence of different kinds of sorption sites, which was supported by the results of the Np(V) sorption isotherms, where the shape of the isotherm suggested the presence of strong and weak sorption sites. The Np(V) uptake was independent of ionic strength, suggesting the presence of inner-sphere Np(V) surface complexes on zirconia. This was also supported by zeta potential measurements where a shift of the isoelectric point of the pristine zirconia towards higher pH values in the presence of Np(V) was observed.
Molecular level investigations by means of spectroscopic techniques, namely in situ attenuated total reflection Fourier transform Infrared Spectroscopy (ATR FT-IR) and extended X-ray absorption fine structure spectroscopy (EXAFS), confirmed the predominant presence of Np(V) inner-sphere complexes on the zirconia surface. EXAFS experiments conducted in the weak sorption site regime revealed the formation of one Np(V) bidentate inner-sphere surface complex. Spectroscopic techniques could not be applied to gain information about the presence and structure of Np(V) surface species at such low Np(V) concentrations, where the strong site regimes could be investigated.
The derived information at the macroscopic and molecular levels were used to parameterize a surface complexation model. The Np(V) sorption edges and isotherms could be described with a 1-pK three plane CD-MUSIC model. The derived thermodynamic constants are expected to help to better predict the environmental fate of Np(V) in the context of nuclear waste repository assessments and will also support the appraisal of safety-relevant scenarios for the extended interim storage of spent nuclear fuel.