Unraveling the Np(V) sorption on ZrO₂: A batch, spectroscopic and modeling combined approach

The interactions of the long-lived actinide neptunium with the corrosion product zirconia (ZrO₂) have to be considered in the safety assessment of a repository for radioactive waste. The sorption of Np(V) on ZrO₂ was investigated in the absence of carbonate at the macroscopic and molecular scale. At the macroscopic level, the Np(V) uptake was independent of ionic strength and the isoelectric point of the pristine zirconia was increased, both suggesting the presence of inner-sphere Np(V) surface complexes. The Np(V) sorption isotherms indicated the presence of strong and weak sorption sites. Molecular level information were derived from in situ attenuated total reflection Fourier-transform infrared spectroscopy and extended X-ray absorption fine structure spectroscopy (EXAFS), which confirmed the presence of Np(V) inner-sphere complexes. EXAFS experiments revealed the formation of a bidentate inner-sphere surface complex in the weak sorption site regime. The derived information at the macroscopic and molecular levels were used to parametrize a charge distribution multi-site complexation (CD-MUSIC) model. The derived thermodynamic constants can help to better predict the environmental fate of Np(V) in the context of nuclear waste repository assessments and can also support the appraisal of safety-relevant scenarios for the extended interim storage of spent nuclear fuel.