NeptuniumV Retention by Siderite under Anoxic Conditions: Precipitation of NpO2-Like Nanoparticles and of NpIV Pentacarbonate

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.

X-ray absorption spectroscopy and actinide electrochemistry: a setup dedicated to radioactive samples applied to neptunium chemistry

A spectroelectrochemical setup has been developed to investigate radioactive elements in small volumes (0.7 to 2 ml) under oxidation-reduction (redox) controlled conditions by X-ray absorption spectroscopy (XAS). The cell design is presented together with in situ XAS measurements performed during neptunium redox reactions. Cycling experiments on the NpO22+/NpO2+ redox couple were applied to qualify the cell electrodynamics using XANES measurements and its ability to probe modifications in the neptunyl hydration shell in a 1 mol l-1 HNO3 solution. The XAS results are in agreement with previous structural studies and the NpO22+/NpO2+ standard potential, determined using Nernst methods, is consistent with measurements based on other techniques. Subsequently, the NpO2+, NpO22+ and Np4+ ion structures in solution were stabilized and measured using EXAFS. The resulting fit parameters are again compared with other results from the literature and with theoretical models in order to evaluate how this spectroelectrochemistry experiment succeeds or fails to stabilize the oxidation states of actinides. The experiment succeeded in: (i) implementing a robust and safe XAS device to investigate unstable radioactive species, (ii) evaluate in a reproducible manner the NpO22+/NpO2+ standard potential under dilute conditions and (iii) clarify mechanistic aspects of the actinyl hydration sphere in solution. In contrast, a detailed comparison of EXAFS fit parameters shows that this method is less appropriate than the majority of the previously reported chemical methods for the stabilization of the Np4+ ion.

High Sorption and Selective Extraction of Actinides from Aqueous Solutions

The selective elimination of long-lived radioactive actinides from complicated solutions is crucial for pollution management of the environment. Knowledge about the species, structures and interaction mechanism of actinides at solid–water interfaces is helpful to understand and to evaluate physicochemical behavior in the natural environment. In this review, we summarize recent works about the sorption and interaction mechanism of actinides (using U, Np, Pu, Cm and Am as representative actinides) on natural clay minerals and man-made nanomaterials. The species and microstructures of actinides on solid particles were investigated by advanced spectroscopy techniques and computational theoretical calculations. The reduction and solidification of actinides on solid particles is the most effective way to immobilize actinides in the natural environment. The contents of this review may be helpful in evaluating the migration of actinides in near-field nuclear waste repositories and the mobilization properties of radionuclides in the environment.

Technetium immobilization by chukanovite and its oxidative transformation products: Neural network analysis of EXAFS spectra

The uptake of the fission product technetium (Tc) by chukanovite, an Fe^II hydroxy carbonate mineral formed as a carbon steel corrosion product in anoxic and carbonate-rich environments, was studied under anoxic, alkaline to hyperalkaline conditions representative for nuclear waste repositories in deep geological formations with cement-based inner linings. The retention potential of chukanovite towards Tc^VII is high in the pH range 7.8 to 12.6, evidenced by high solid-water distribution coefficients, log R_d ~ 6, and independent of ionic strength (0.1 or 1 M NaCl). Using Tc K-edge X-ray absorption spectroscopy (XAS) two series of samples were investigated, Tc chukanovite sorption samples and coprecipitates, prepared with varying Tc loadings, pH values and contact times. From the resulting 37 XAS spectra, spectral endmembers and their dependence on chemical parameters were derived by self-organizing (Kohonen) maps (SOM), a neural network-based approach of machine learning. X-ray absorption near-edge structure (XANES) data confirmed the complete reduction of Tc^VII to Tc^IV by chukanovite under all experimental conditions. Consistent with mineralogical phases identified by X-ray diffraction (XRD), SOM analysis of the extended X-ray absorption fine-structure (EXAFS) spectra revealed the presence of three species in the sorption samples, the speciation predominately controlled by pH: Between pH 7.8 and 11.8, TcO₂-dimers form inner-sphere sorption complexes at the surface of the initial chukanovite as well as on the surface of secondary magnetite formed due to redox reaction. At pH ≥ 11.9, Tc^IV is incorporated in a mixed, chukanovite-like, Fe/Tc hydroxy carbonate precipitate. The same species formed when using the coprecipitation approach. Reoxidation of sorption samples resulted in a small remobilization of Tc, demonstrating that both the original chukanovite mineral and its oxidative transformation products, magnetite and goethite, contribute to the immobilization of Tc in the long term, thus strongly attenuating its environmental transport.