Opportunities and challenges of applying advanced X-ray spectroscopy to actinide and lanthanide N-donor ligand systems.

N-donor ligands such as n-Pr-BTP [2,6-bis(5,6-dipropyl-1,2,4-triazin-3-yl)pyridine] preferentially bind trivalent actinides (An3+) over trivalent lanthanides (Ln3+) in liquid-liquid separation. However, the chemical and physical processes responsible for this selectivity are not yet well understood. Here, an explorative comparative X-ray spectroscopy and computational (L3-edge) study for the An/Ln L3-edge and the N K-edge of [An/Ln(n-Pr-BTP)3](NO3)3, [Ln(n-Pr-BTP)3](CF3SO3)3 and [Ln(n-Pr-BTP)3](ClO4)3 complexes is presented. High-resolution X-ray absorption near-edge structure (HR-XANES) L3-edge data reveal additional features in the pre- and post-edge range of the spectra that are investigated using the quantum chemical codes FEFF and FDMNES. X-ray Raman spectroscopy studies demonstrate the applicability of this novel technique for investigations of liquid samples of partitioning systems at the N K-edge.

A Combined Study of Tc Redox Speciation in Complex Aqueous Systems: Wet-Chemistry, Tc K-/L3-Edge X-ray Absorption Fine Structure, and Ab Initio Calculations.

The combination of wet-chemistry experiments (measurements of pH, Eh, and [Tc]) and advanced spectroscopic techniques (K- and L3-edge X-ray absorption fine structure spectroscopy) confirms the formation of a very stable Tc(V)-gluconate complex under anoxic conditions. In the presence of gluconate and an excess of Sn(II) (at pe + pH ≈ 2), technetium forms a very stable Tc(IV)-gluconate complex significantly enhancing the solubility defined by TcO2(s) in hyperalkaline gluconate-free systems. A new setup for "tender" X-ray spectroscopy (spectral range, ∼2-5 keV) in transmission or total fluorescence yield detection mode based on a He flow cell has been developed at the INE Beamline for radionuclide science (KIT light source). This setup allows handling of radioactive specimens with total activities up to one million times the exemption limit. For the first time, Tc L3-edge measurements (∼2.677 keV) of Tc species in liquid (aqueous) media are reported, clearly outperforming conventional K-edge spectroscopy as a tool to differentiate Tc oxidation states and coordination environments. The coupling of L3-edge X-ray absorption near-edge spectroscopy measurements and relativistic multireference ab initio methods opens new perspectives in the definition of chemical and thermodynamic models for systems of relevance in the context of nuclear waste disposal, environmental, and pharmaceutical applications.

Pu Coexists in Three Oxidation States in a Borosilicate Glass: Implications for Pu Solubility.

Pu(III), Pu(IV), and a higher oxidation state of Pu, likely Pu(VI), are for the first time characterized simultaneously present in a borosilicate glass using Pu M5 edge high energy resolution X-ray absorption near edge structure (HR-XANES) technique. We illustrate that the method can be very efficiently used to determine Pu oxidation states, which control the solubility limit of Pu in a glass matrix. HR-XANES results show that the addition of excess Si3N4 is not sufficient for complete reduction of Pu to Pu(III), which has a relatively high solubility limit (9-22 wt % Pu) due to its network-modifying behavior in glasses. We provide evidence that the initially added Pu(VI) might be partly preserved during vitrification at 1200/1400 °C in Ar atmosphere. Pu(VI) could be very advantageous for vitrification of Pu-rich wastes, since it might reach solubility limits of 40 wt % comparable to U(VI).

Adsorption of Selenium and Strontium on Goethite: EXAFS Study and Surface Complexation Modeling of the Ternary Systems.

Knowledge of the geochemical behavior of selenium and strontium is critical for the safe disposal of radioactive wastes. Goethite, as one of the most thermodynamically stable and commonly occurring natural iron oxy-hydroxides, promisingly retains these elements. This work comprehensively studies the adsorption of Se(IV) and Sr(II) on goethite. Starting from electrokinetic measurements, the binary and ternary adsorption systems are investigated and systematically compared via batch experiments, EXAFS analysis, and CD-MUSIC modeling. Se(IV) forms bidentate inner-sphere surface complexes, while Sr(II) is assumed to form outer-sphere complexes at low and intermediate pH and inner-sphere complexes at high pH. Instead of a direct interaction between Se(IV) and Sr(II), our results indicate an electrostatically driven mutual enhancement of adsorption. Adsorption of Sr(II) is promoted by an average factor of 5 within the typical groundwater pH range from 6 to 8 for the concentration range studied here. However, the interaction between Se(IV) and Sr(II) at the surface is two-sided, Se(IV) promotes Sr(II) outer-sphere adsorption, but competes for inner-sphere adsorption sites at high pH. The complexity of surfaces is highlighted by the inability of adsorption models to predict isoelectric points without additional constraints.

The mechanism of Fe induced bond stability of uranyl(v).

The stabilization of uranyl(v) (UO2 1 + ) by Fe(ii) in natural systems remains an open question in uranium chemistry. Stabilization of UVO2 1+ by Fe(ii) against disproportionation was also demonstrated in molecular complexes. However, the relation between the Fe(ii) induced stability and the change of the bonding properties have not been elucidated up to date. We demonstrate that U(v) - oaxial bond covalency decreases upon binding to Fe(ii) inducing redirection of electron density from the U(v) - oaxial bond towards the U(v) - equatorial bonds thereby increasing bond covalency. Our results indicate that such increased covalent interaction of U(v) with the equatorial ligands resulting from iron binding lead to higher stability of uranyl(v). For the first time a combination of U M4,5 high energy resolution X-ray absorption near edge structure (HR-XANES) and valence band resonant inelastic X-ray scattering (VB-RIXS) and ab initio multireference CASSCF and DFT based computations were applied to establish the electronic structure of iron-bound uranyl(v).

Paving the way for examination of coupled redox/solid-liquid interface reactions: 1 ppm Np adsorbed on clay studied by Np M5-edge HR-XANES spectroscopy.

The recently emerged actinide (An) M4,5-edge high-energy resolution X-ray absorption near-edge structure (HR-XANES) technique has proven to be very powerful for oxidation state studies of actinides. In this work, for the first time, Np M5-edge HR-XANES was applied to study Np sorption on illite. By improving the experimental conditions, notably by operation of the spectrometer under He atmosphere, it was possible to measure Np M5-edge HR-XANES spectra of a sample with ≈ 1 µg Np/g illite (1 ppm). This is 30-2000 times lower than Np loadings on mineral surfaces usually investigated by X-ray absorption spectroscopy. A newly designed cryogenic configuration enabled sample temperatures of 141.2 ± 1.5 K and successfully prevented beam-induced changes of the Np oxidation state. The described approach paves the way for the examination of coupled redox/solid-liquid interface reactions of actinide ions via An M4,5-edge HR-XANES spectroscopy at low metal ion concentrations, which are of specific relevance for contaminated sites and nuclear waste disposal studies.

Exploring the electronic structure and speciation of aqueous and colloidal Pu with high energy resolution XANES and computations.

Pu L3 HR-XANES and FEFF9 computations provide evidence for band-like 6d states in colloidal Pu contrasting to narrow 6d states in molecular Pu(iv). Pu L3 HR-XANES is valuable for bond length estimation in plutonyl, whereas Pu M5 HR-XANES is an advanced tool for analysing Pu redox states and 5f unoccupied density of states.