Vladimir Sladkov

The electrophoretic mobility change of desferrioxamine B (DFO) was monitored by UV absorption spectrophotometry upon increasing the thorium(IV) concentration in the background electrolyte at two acidities ([HClO₄]_Tot = 0.0316 and 0.0100 M). These data enabled to assess the speciation model and to determine the equilibrium constant of [Th(DFO)H₂]³⁺ at fixed ionic strength (I = 0.1 M (H,Na)ClO₄). Affinity capillary electrophoresis (ACE) turned out to be most helpful in identifying the complexed species by ascertaining its charge and protonation state. The assignment of the correct stoichiometry relied on the reliable estimation of the electrophoretic mobility by assuming similar hydrodynamic radii for (DFO)H₄⁺ and the chelate. The value of the apparent equilibrium constant (log β₁₁₂ = 38.7 ± 0.4) obtained by ACE compares favorably well with those reported in the literature for thorium and a range of other metal ions, according to a linear free‐energy relationship. This method is useful for studying metal‐ligand binding equilibria and provides valuable information for further modelling the behavior of tetravalent actinides under environmental conditions. Structural information about the prevalent solution species in acidic conditions was gained by DFT calculations, confirming the bishydroxamato coordination mode of Th⁴⁺ by the diprotonated ligand.

A comprehensive molecular analysis of a simple aqueous complexing system—U(VI) acetate—selected to be independently investigated by various spectroscopic (vibrational, luminescence, X-ray absorption, and nuclear magnetic resonance spectroscopy) and quantum chemical methods was achieved by an international round-robin test (RRT). Twenty laboratories from six different countries with a focus on actinide or geochemical research participated and contributed to this scientific endeavor. The outcomes of this RRT were considered on two levels of complexity: first, within each technical discipline, conformities as well as discrepancies of the results and their sources were evaluated. The raw data from the different experimental approaches were found to be generally consistent. In particular, for complex setups such as accelerator-based X-ray absorption spectroscopy, the agreement between the raw data was high. By contrast, luminescence spectroscopic data turned out to be strongly related to the chosen acquisition parameters. Second, the potentials and limitations of coupling various spectroscopic and theoretical approaches for the comprehensive study of actinide molecular complexes were assessed. Previous spectroscopic data from the literature were revised and the benchmark data on the U(VI) acetate system provided an unambiguous molecular interpretation based on the correlation of spectroscopic and theoretical results. The multimethodologic approach and the conclusions drawn address not only important aspects of actinide spectroscopy but particularly general aspects of modern molecular analytical chemistry.

This paper is an attempt to find out thorium oxide dissolution mechanism in HNO3-HF mixture. In a previous paper, several parameters effects on thorium oxide dissolution have been described, with specific focus on hydrofluoric acid effect, which can lead to an increase of the dissolution rate if present in small amount, but precipitates as ThF4 at higher content. Based on this previous study, experimental data were fitted using several dissolution models in order to find out the best one. Finally, a revisited model based on literature and considering the ThF4 formation was proposed. It describes the main steps of dissolution and is able to fit the experimental data for a wide range of solution compositions. This point is crucial since it allows considering an extrapolation of the established model to not-yet-studied conditions.

Many siderophores incorporate as bidentate chelating subunits linear and more seldomly cyclic hydroxamate groups. In this work, a comparative study of the uranyl binding properties in aqueous solution of two monohydroxamic acids, the prototypical linear N-methylacetohydroxamic acid (NMAH) and the cyclic analog 1-hydroxypiperidine-2-one (PIPOH), has been carried out. The complex [UO₂(PIPO)₂(H₂O)] crystallized from slightly acidic water solutions (pH < 5), and its molecular structure was determined by X-ray diffraction. The uranyl speciation in the presence of both ligands has been thoroughly investigated in a 0.1 M KNO₃ medium at 298.2 K by the combined use of four complementary techniques, i.e., potentiometry, spectrophotometry, Raman spectroscopy, and affinity capillary electrophoresis. Preorganization of the hydroxamate ligand for chelation by incorporation into a cyclic structure, as in PIPO⁻, results in an increase of nearly one order of magnitude in the formation constants of the uranyl complexes of 1 : 1 and 1 : 2 metal/ligand stoichiometries.

The complexation of U(VI) and Pu(VI) with succinic acid in aqueous acid solutions is investigated and compared. Affinity capillary electrophoresis (ACE) has been used to study the interaction of U(VI) with succinic acid in perchloric acid aqueous solutions (p[H] 2.0–2.5, 0.1 M ionic strength). The speciation model for uranyl is deduced in the presence of succinic acid. The existence of protonated complexes UO₂HSuc⁺ and UO₂(HSuc)₂ is supposed and the stability constant values for these complex species are calculated. The interaction of Pu(VI) with succinic acid has been investigated using UV-vis spectrophotometry. The equilibria with the formation of PuO₂(HSuc)₂, PuO₂HSuc₂ ⁻ and PuO₂(Suc)₂ ²⁻ complex species are found. The stability constant values are calculated at 0.1 M and 1 M ionic strengths. The existence of UO₂(HSuc)₂, PuO₂(HSuc)₂ and PuO₂HSuc₂ ⁻ is suggested for the first time. It is noticed that the complex species of U(VI) with succinic acid are more stable than the ones of Pu(VI). The U(VI) and Pu(VI) systems with succinic acid are also investigated using density functional theory (DFT). It is observed that actinyl has a bidentate coordination with a succinate anion. Using DFT calculations the experimental finding that uranyl–succinate species are more stable than the corresponding plutonyl–succinate ones is confirmed. This trend in stability is explained by a stronger electrostatic interaction in the uranyl–succinate complexes.

Thorium oxide is poorly soluble: unlike uranium oxide, concentrated nitric acid medium is not sufficient to get quantitative dissolution. Addition of small amounts of fluoride is required to achieve thorium oxide total dissolution. The effect of several parameters on thorium oxide dissolution in order to optimize the dissolution conditions is reported in this paper. Thus the influence of solid characteristics, dissolution method, temperature and composition of dissolution medium on ThO₂dissolution rate has been studied. No complexing agents tested other than fluoride allows total dissolution. Beyond a given HF concentration a decrease of the dissolution rate is observed due to the formation of a precipitate at the solid/solution interface. It was demonstrated by XPS measurements that this precipitate is constituted of thorium fluoride (ThF₄) formed during the ThO₂dissolution. The low concentration of HF required to achieve a total dissolution and the activation energy value measured tends to show a catalytic effect of HF on the dissolution process.

Interaction of radionuclides with inorganic and organic species present in natural environment plays an important role in their eventual dispersion. The complex equilibria established in the aqueous phase cause significant changes in the migration properties of radionuclides. Affinity capillary electrophoresis (ACE) can be fruitful in studying these equilibria. This paper reviews the recent methodological advances of the use of ACE in studying the complex equilibria of radionuclides in aqueous solutions. Special attention is paid to the determination of a number of species involved in equilibrium, species constituents (number of ligands, protonated, deprotonated), the influence of ionic strength and temperature on stability constants of complex species formed. Use of ACE for the determination of the main thermodynamic parameters (the molar Gibbs energy (Δ_rG_m), the molar enthalpy (Δ_rH_m) and the molar entropy (Δ_rS_m)) of complex formation reactions is also discussed. These data are essential to predict dispersion of radionuclides in the natural environment.