Recent progress of capillary electrophoresis in studying the speciation of actinides

Vanadium(IV) and vanadium(V) complexation by succinic acid studied by affinity capillary electrophoresis. Simultaneous injection of two analytes in equilibrium studies

The interaction of V(IV) and V(V) with succinic acid was investigated by affinity capillary electrophoresis (ACE) in aqueous acid solutions at pH values 1.5, 2.0 and 2.4, and different ligand concentrations. V(IV) and V(V) form protonated complexes with succinic acid ligand at this pH range. The logarithms of the stability constants, measured at 0.1 mol L^-1 (NaClO₄/HClO₄) ionic strength and 25 °C, are logβ₁₁₁=7.4 ± 0.2 and logβ₁₂₂=14.1 ± 0.5 for V(IV), and logβ₁₁₁=7.3 ± 0.1 for V(V), respectively. The stability constant values, extrapolated to zero ionic strength with the Davies equation, are logβ°₁₁₁=8.3 ± 0.2 and logβ°₁₂₂=15.6 ± 0.5 for V(IV) and logβ°₁₁₁=7.9 ± 0.1 for V(V). The application of ACE to the simultaneous equilibria of V(IV) and V(V) (injection of two analytes) was also attempted. When the results were compared with those obtained when introducing only one analyte in the capillary, using the traditional version of the method, similar stability constants and precision are obtained. The possibility of studying two analytes simultaneously decreases the time needed for the determination of the constants; this feature is especially valuable when working with hazardous materials or when only small quantities of ligand are available.

Assignment of complex species by affinity capillary electrophoresis: The case of Th(IV)-desferrioxamine B

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.

Investigation of the Electrophoretic Mobility of the Actinides Th, U, Np, Pu, and Am in Different Oxidation States

The electrophoretic mobilities (μ_e) of the actinides Th and U-Am in different oxidation states (prepared in 1 M HCl and 1 M HClO₄) have been determined by capillary electrophoresis (CE)-inductively coupled plasma mass spectrometry (ICPMS) using 1 M acetic acid as the background electrolyte, which has proven to provide an excellent setup for trace analysis at environmentally relevant concentrations (1 × 10^-9 M). The values are independent of the respective acid solution. The μ_e of the Pu oxidation states +III to +VI have been measured. They agree with both the available literature data and the redox-stable analogues (Eu(III), Th(IV), Np(V), U(VI)) that have also been investigated. The trend in the μ_e for the actinides U-Pu was found to be An(III) > An(VI) > An(V) > An(IV). The μ_e values of Am(III) (μ_e(Am(III)) = 3.86 × 10^-4 cm²/(Vs)), U(IV) (μ_e(U(IV)) = 0.34 × 10^-4 cm²/(Vs)), and U(VI) (μ_e(U(VI)) = 1.51 × 10^-4 cm²/(Vs)) have been measured for the first time under these experimental conditions. Furthermore, the measured μ_e values show systematic trends that can be rationalized on the basis of the calculated species distribution of the actinides in 1 M acetic acid and the corresponding average effective charges (q_eff).

Determination of kinetic parameters of redox reactions using CE-ICP-MS: A case study for the reduction of Np(V) by hydroxylamine hydrochloride

The rate constants k of the reduction of 5 × 10^-5  M Np(V) to Np(IV) by hydroxylamine hydrochloride (HAHCl) in 1 M HCl have been determined by CE-ICP-MS in the temperature range of ϑ = 30-70°C and with varying concentrations of HAHCl from 1 to 7.2 M. The reaction was found to have (pseudo)first order kinetics with respect to HAHCl. The experimental results for k ranged from 0.0029(1) min^-1 (ϑ = 40°C, c(HAHCl) = 3 M) to 0.039(7) min^-1 (ϑ = 60°C, c(HAHCl) = 7.2 M). The activation energy of the reaction was determined as E_A  = (72 ± 10) kJ/mol. These results and a comparison with literature data show that the coupling of CE to ICP-MS provides a powerful analytical tool for the investigation of the kinetic aspects of redox reactions of actinides at low concentrations. On the basis of this proof-of-principle study, the method presented here can be extended to the investigation of the kinetic parameters of other redox systems containing different actinides (or transition metals) and oxidants/reductants.

Complexation of An(vi) with succinic acid in aqueous acid solutions: uranyl vs. plutonyl

Due to stronger electrostatic interaction in a uranyl–succinate system, complex species of U(vi) with succinate are more stable than the ones of Pu(vi).

Effects of preorganization in the chelation of UO22+by hydroxamate ligands: cyclic PIPO−vs.linear NMA−

Thanks to preorganization, 1,2-PIPOH, the six-membered ring cyclic hydroxamic acid, binds uranyl six times more strongly than its linear, methyl-substituted homolog (NMAH).

Europium (III) and Uranium (VI) complexation by natural organic matter (NOM): Effect of source

For the safe long-term storage of high-level radioactive waste (HLW), detailed information about geo-chemical behavior of radioactive and toxic metal ions under environmental conditions is important. Natural organic matter (NOM) can play a crucial role in the immobilization or mobilization of these metal ions due to its complexation and colloid formation tendency. In this study, the complexation of europium (as chemical homologue of trivalent actinides such as americium) and uranium (as main component of HLW) by ten humic acids (HA) from different sources and Suwannee NOM river extract has been analyzed. Capillary electrophoresis in combination with inductively coupled plasma mass spectrometry has been used for the evaluation of complex stability constants log β. In order to determine the complex stability constants a conservative single site model was used in this study. In dependence of their source and thus of NOM structure the log β values for the analyzed humic acids are in the range of 6.1-7.0 for Eu(III) and 5.2-6.4 for U(VI) (UO₂²⁺ ), respectively. In contrast to the results for HA the used Suwannee river NOM reveals log β values in the range of nearly two orders of magnitude lower (4.6 for Eu³⁺ and 4.5 for UO₂²⁺ ) under the geochemical conditions applied in this study.