Tetravalent Actinide Ions Research Articles

How colloid nature drives the interactions between actinide and carboxylic surfactant in sol: Towards a mesostructured nanoporous actinide oxide material

HypothesisThe key to prepare a mesostructured porous material by a soft-template route coupled to a colloidal sol–gel process is to control the surfactant-colloid interface. In the case of tetravalent actinide ions, their high reactivity in aqueous media always leads to uncontrolled and irreversible condensation. The addition of a complexing agent to the sol may moderate these reactions and enhances the interaction between the colloids and the surfactant to in fine prepare a mesostructured nanoporous actinide oxide material. ExperimentsSeveral colloidal sols were prepared without and with formic acid as complexing agent by varying the molar ratios between thorium, carboxylic surfactant and pH. Small and Wide Angle X-ray Scattering were used to characterize the nature of the colloids, their interaction with the surfactant and the final ThO2 materials. FindingsDepending on the colloid nature, hexagonal or worm-like hybrid mesophase is formed. The thermal treatment of the worm-like mesophase with a sufficient amount of Th-formic acid hexameric species coated at the surface of surfactant micelles generates micrometric ThO2 nanofibers. This material having an accessible porosity opens new perspectives to be impregnated with minor actinide solutions offering a promising safety method for the fabrication of mixed oxide nuclear fuel and the minor actinide transmutation.

Liquid–Liquid Extraction and Supported Liquid Membrane Transport of Neptunium(IV) Across a Flat-Sheet Supported Liquid Membrane Containing a TREN-DGA Derivative

ABSTRACT Liquid–liquid extraction and liquid membrane transport behavior of tetravalent actinide ions viz. Th(IV), Np(IV), and Pu(IV) were investigated for the first time using a diglycolamide (DGA) based dendrimer with a tris(2-aminoethyl)amine (TREN) scaffold as the organic extractant. The generation of 1 dendrimer with six DGA pendent moieties (termed as TREN-G1-DenDGA) extracted Np(IV) more effectively than the other two ions, the trend being Np(IV) > Pu(IV) > Th(IV). The extraction studies of Np(IV) from 3 M HNO3 indicated a 1:1 (metal:ligand) species and the extraction efficiency increased with increasing nitric acid concentration (1–6 M). The transport efficiency of Np(IV) increased with the nitric acid concentration (1–6 M) as well as with the ligand concentration. A very low concentration of 5.75 × 10−4 M ligand, when used as the carrier, resulted in the transport of ca. 25% metal ion transport in 5 h, which increased to >85% with 4.4 × 10−3 M ligand. The transport efficiency of the metal ion across the SLM followed the trend Np(IV) > Th(IV) > Pu(IV). The membrane stability was not satisfactory as seen over a period of 5 days suggesting long-term use may require regular replenishment of the carrier solvent. The effective diffusion coefficient (D eff) of Np(IV)-TREN-G1-DenDGA were determined by the lag-time method and was found to be 5.1 × 10−8 cm2/s.

Highly efficient uptake of tetravalent actinide ions from nitric acid feeds using an extraction chromatography material containing tetra-n-butyl diglycolamide and a room temperature ionic liquid

An extraction chromatography (XC) material containing N,N,N’,N’-tetra-n-butyl diglycolamide (TBDGA) and 1-butyl-3-methylimidazolium bis(trifluoromethanesulphonyl) imide (C4mim∙NTf2), a room temperature ionic liquid, was used for the uptake of the tetravalent actinide ions Th(IV), Np(IV), and Pu(IV) from nitric acid feed solutions. The uptake of the metal ions followed the trend: Th(IV) < Np(IV) < Pu(IV), which is the same as that of their ionic potential values. While a decrease in the Np(IV) and Pu(IV) uptake was seen with increasing HNO3 concentration at lower acidities, an opposite trend was observed at higher acidities. Th(IV) uptake was not affected with the acid concentration. In view of the very high uptake and its importance in the nuclear fuel cycle, the major part of the studies was carried out with Pu(IV) ion. The loaded Pu(IV) was back extracted from the XC material using a mixture of 0.5 M oxalic acid and 0.5 M nitric acid. The Pu(IV) uptake by the XC material was fitted into different kinetic and isotherm models. The results conformed to the pseudo-second order kinetic model and the Langmuir monolayer sorption model. Column studies were carried out using a feed having 1.6 mg/L Pu solution in 3 M HNO3. While the breakthrough was obtained after passing ca. nine bed volumes, a sharp elution peak was obtained with >99% recovery in about seven bed volumes.

Gas-phase structure, bonding, and fragmentation chemistry of the An (IV)-TMPDCAM complexes studied using mass spectrometry and theoretical calculation (An = Th and U).

Pyridine-2,6-dicarboxamides (PDCAMs) exhibit a certain extraction ability for tetravalent actinide ions, but quite limited information regarding the structures and reactivities of the corresponding An4+ -PDCAMs complexes is available. Neutral diamides can form multiply charged complexes with lanthanide and actinide cations, which are well suited for gas-phase investigations using electrospray ionization (ESI) mass spectrometry in conjunction with theoretical calculation. Binary Th (TMPDCAM)3 4+ /U (TMPDCAM)3 4+ (TMPDCAM =N,N,N',N'-tetramethylpyridine-2,6-dicarboxamide) complexes were generated in the gas phase via ES) of Th (ClO4 )4 /U (ClO4 )4 and TMPDCAM mixtures in acetonitrile; collision-induced dissociation (CID) was employed to reveal their fragmentation behaviors; the structure and bonding were investigated by density functional theory (DFT) calculation. An (TMPDCAM)3 4+ (An = Th and U) tetracations dominated the ESI mass spectra of An (ClO4 )4 and TMPDCAM mixtures in acetonitrile. DFT calculations indicate that the two An (TMPDCAM)3 4+ complexes have C3 geometry, and the bonding analyses demonstrate that the thorium or uranium center interacts with both Ocarbonyl and Npyridine , but the latter is weaker. CID of Th (TMPDCAM)3 4+ generated a series of multiply charged thorium-containing products via bond cleavages of the TMPDCAM ligand, whereas U (TMPDCAM)3 4+ yielded only oxygen extraction product UO (TMPDCAM)2+ and hydrolysis product UO (OH)+ . An4+ (An = Th and U) can form stable tetrapositive complexes in the gas phase on coordination of three neutral TMPDCAM ligands. The structure and bonding analyses indicate that the two An (TMPDCAM)3 4+ complexes possess twisted tricapped trigonal prismatic geometry and the An4+ centers are coordinated by six Ocarbonyl and three Npyridine atoms while the interactions between An4+ and Ocarbonyl are stronger. The fragmentation chemistry of Th (TMPDCAM)3 4+ and U (TMPDCAM)3 4+ is quite different from each other, which reveals that the gas-phase chemistry of quadruply charged actinide-diamide complexes is affected by the metal centers with distinct properties.

Carrier mediated transport of actinides using hexa–n-hexylnitrilotriacetamide (HHNTA)

Solutions of N,N,N’,N’,N”,N”-hexa-n-hexylnitrilotriacetamide (HHNTA) in 20 % isodecanol-80 % n-dodecane (v/v) were employed for the supported liquid membrane (SLM) transport of U(VI), Np(IV), and Pu(IV) from nitric acid feed solutions using PTFE (polytetrafluoroethylene) flat sheet membranes. Preliminary transport studies revealed poor transport of U(VI) when compared with that of the tetravalent ions Np(IV) and Pu(IV) which may find application for their separation from acidic radioactive feeds. In view of this, all subsequent studies were carried out using Np(IV) and Pu(IV). The SLM studies performed with 0.08 M HHNTA as the carrier extractant indicated increased transport of the tetravalent actinide ions with increasing nitric acid and carrier extractant concentrations, while using 0.5 M HNO3 + 0.5 M oxalic acid as the strip (or receiver) phase composition. However, while > 80 % transport of Np(IV) and Pu(IV) was obtained for a feed phase of 3 M HNO3, a sharp decrease in the transport rate was seen at a feed acidity of 6 M HNO3 probably due to anionic hexanitrato complex formation of the metal ions and also because of lower availability of the free ligand in the membrane phase at higher nitric acid concentrations. The membrane stability was poor suggesting the need for continuous replenishment for long term use.

Highly efficient diglycolamide-functionalized dendrimers for the sequestration of tetravalent actinides: solvent extraction and theoretical studies

Two DGA-functionalized dendrimers were used for the extraction and bonding computations of tetravalent actinide ions Np4+and Pu4+from HNO3medium.

Stabilization of Plutonium(V) Within a Crown Ether Inclusion Complex

Crystalline coordination complexes of actinides, especially in atypical oxidation states, are not only fundamentally important for expanding the notably limited knowledge on the bonding nature of a...

Periodic trends and complexation chemistry of tetravalent actinide ions with a potential actinide decorporation agent 5-LIO(Me-3,2-HOPO): A relativistic density functional theory exploration.

A relativistic density functional theory (DFT) study is reported which aims to understand the complexation chemistry of An4+ ions (An = Th, U, Np, and Pu) with a potential decorporation agent, 5-LIO(Me-3,2-HOPO). The calculations show that the periodic change of the metal binding free energy has an excellent correlation with the ionic radii and such change of ionic radii also leads to the structural modulation of actinide-ligand complexes. The calculated structural and binding parameters agree well with the available experimental data. Atomic charges derived from quantum theory of atoms in molecules (QTAIM) and natural bond order (NBO) analysis shows the major role of ligand-to-metal charge transfer in the stability of the complexes. Energy decomposition analysis, QTAIM, and electron localization function (ELF) predict that the actinide-ligand bond is dominantly ionic, but the contribution of orbital interaction is considerable and increases from Th4+ to Pu4+ . A decomposition of orbital contributions applying the extended transition state-natural orbital chemical valence method points out the significant π-donation from the oxygen donor centers to the electron-poor actinide ion. Molecular orbital analysis suggests an increasing trend of orbital mixing in the context of 5f orbital participation across the tetravalent An series (Th-Pu). However, the corresponding overlap integral is found to be smaller than in the case of 6d orbital participation. An analysis of the results from the aforementioned electronic structure methods indicates that such orbital participation possibly arises due to the energy matching of ligand and metal orbitals and carries the signature of near-degeneracy driven covalency.

Evaluation of two aza-crown ether-based multiple diglycolamide-containing ligands for complexation with the tetravalent actinide ions Np4+ and Pu4+: extraction and DFT studies.

Two multiple diglycolamide (DGA)-containing extractants where the DGA arms are tethered to the nitrogen atoms of two aza-crown ether scaffolds, a 9-membered aza-crown ether containing three ‘N’ atoms (LI) and a 12-membered aza-crown ether containing four ‘N’ atoms (LII), were evaluated for the extraction of the tetravalent actinide ions Np4+ and Pu4+. The tripodal ligand with three DGA arms (LI) was relatively inferior in its metal ion extraction properties as compared to the tetrapodal ligand with four DGA arms (LII) and Pu4+ ion was better extracted than Np4+ ion with both the ligands. A solvation extraction mechanism, where species of the type ML(NO3)4 are extracted, was found to be operative for both the ligands involving both the tetravalent actinide ions. While the extraction of the metal ions increased with the feed nitric acid concentration up to 4 M, a sharp decline in the extraction was seen after that. Quantitative extraction (>99%) of the actinide ions was observed with LII from 4 M HNO3, suggesting the possible application of the ligands for actinide partitioning of high-level waste. The structure and the composition of the complexes were optimized by DFT computations.

Syntheses and crystal structures of thorium(IV) and uranium(IV) tripodal metalloligands

Two tetravalent actinide tripodal metalloligands, [Th(H3L)(NO3)(MeO)]·2NO3 (1) and [U(H3L)Cl(MeO)]2[(UO2)Cl4]0.5·3Cl·2CH3OH·2H2O (2) (H3L=tris{[2-{(imidazole)methylidene} amino]ethyl}amine; CH3O=methoxide), were synthesized and structurally characterized. Both 1 and 2 contain the expected tripodal shape with the heptadentate Schiff base wrapping around Th(IV) or U(IV) ion. Further coordination of nitrato/chlorine and methoxide makes a ten-fold and a nine-fold coordination environments for Th(IV) ion (1) and U(IV) ion (2), respectively. Metalloligand 2 is a mixed valent uranium complex due to partial oxidation of U(IV) to U(VI). These tripodal metalloligands will have the potential to link secondary metal ions via deprotonated imidazole nitrogen donors to form either discrete heterometallic cages or supramolecular assemblies containing tetravalent actinide ions.

Systematic analysis of spectroscopic characteristics of the lanthanide and actinide ions with the 4fN−15d and 5fN−16d electronic configurations in a free state

Systematic consideration of the spectroscopic properties of the fN−1d excited electronic configurations of the di-, tri- and tetravalent lanthanide and actinide ions in a free state is presented. Variations of the Hartree–Fock calculated Slater parameters for the fN−1d electron configurations, spin–orbit (SO) interaction constant ζ for the f and d electrons, and averaged values of the second, fourth and sixth powers of the 4f, 5f, 5d, 6d electrons’ radial coordinate across both series were considered; functional dependencies between the mentioned quantities were obtained. It has been shown that the Coulomb interaction parameters F2(ff), F4(ff), and F6(ff) for the fN−1 core increase linearly with the atomic number Z, whereas the direct and exchange Coulomb parameters F2(fd), F4(fd), G1(fd), G3(fd), G5(fd) for the fN−1d configuration decrease linearly with Z. Since the SO interaction constant ζ1/4 is also proportional to Z, it was possible to find linear relationships between the Coulomb interaction parameters and SO constants for the f and d electrons, which effectively reduce the number of independent parameters in the free ion Hamiltonian. The constraining relations between the free ion Hamiltonian’s parameters obtained in the present paper can be used for simulations of the f–d transition spectra of these ions in various crystals.

Systematic analysis of spectroscopic characteristics of the lanthanide and actinide ions with the 4fN and 5fN (N = 1…14) electronic configurations in a free state

Systematic consideration of the spectroscopic properties of the di-, tri- and tetravalent lanthanide and actinide ions in a free state is presented. Variations of the Hartree–Fock calculated Slater parameters F2, F4, F6, spin–orbit interaction constant ζ and averaged values of the 4f, 5f electrons’ radial coordinate across both series were considered; functional dependencies between the mentioned quantities were obtained. It has been shown that the F2, F4, F6 parameters grow up linearly with the atomic number Z. The spin–orbit interaction constant ζ1/4 is also proportional to Z, and, therefore, to the Slater integrals as well. Moreover, the ratios F4/F2 and F6/F2 are practically constant for the isovalent ions in the considered series. The energy barycenters of all electronic configurations and the third, fourth and fifth ionization energies were all calculated. The energy barycenter moves up with increasing number of f-electrons until the f-shell becomes half-filled (7 f electrons) and then moves down. The ionization energies increase linearly with atomic number across the considered series. The established relations between F2, F4, F6 and ζ reduce the number of independent parameters in the free ion Hamiltonian and can be further applied to the case of crystal-field fitting calculations as the empirical constraints.

A diglycolamide-functionalized task specific ionic liquid (TSIL) for actinide extraction: Solvent extraction, thermodynamics and radiolytic stability studies

A diglycolamide (DGA)-functionalized task specific ionic liquid (TSIL) was used for the extraction of actinide ions such as Am3+, Pu4+, Np4+, UO22+, NpO22+, and PuO22+. Trivalent actinide ion extraction was about one order of magnitude higher than that of the tetravalent ions, which in turn was one order of magnitude higher than that of the hexavalent actinide ions. Typically, using 3.6×10−2M solution of the DGA-TSIL in C4mim·NTf2, the D values at 3M HNO3 for Am3+, Pu4+, Np4+, UO22+, NpO22+, and PuO22+ were 34, 5.9, 3.5, 0.44, 0.38, and 0.32, respectively. The extraction of the actinide ions decreased with increasing aqueous phase acidity, which supports an ion-exchange mechanism that often operates for this type of extraction systems. The number of DGA-TSIL molecules present in the extracted species was about one for all actinide ions studied, suggesting species of the type M(TSIL)ILn+. Both the extraction constants (logKex) and the thermodynamic parameters were calculated. The ΔG values were negative in case of all actinide ions suggesting spontaneity of the extraction, which was reflected in very high D values for the tri- and tetravalent actinide ions. Stripping and radiolytic degradation studies were also carried out and EPR spectroscopic measurements were performed to understand the nature of the degraded free radical species.

Static Electric Dipole Polarizabilities of Tri- and Tetravalent U, Np, and Pu Ions

High-quality static electric dipole polarizabilities have been determined for the ground states of the hard-sphere cations of U, Np, and Pu in the III and IV oxidation states. The polarizabilities have been calculated using the numerical finite field technique in a four-component relativistic framework. Methods including Fock-space coupled cluster (FSCC) and Kramers-restricted configuration interaction (KRCI) have been performed in order to account for electron correlation effects. Comparisons between polarizabilities calculated using Dirac-Hartree-Fock (DHF), FSCC, and KRCI methods have been made using both triple- and quadruple-ζ basis sets for U(4+). In addition to the ground state, this study also reports the polarizability data for the first two excited states of U(3+/4+), Np(3+/4+), and Pu(3+/4+) ions at different levels of theory. The values reported in this work are the most accurate to date calculations for the dipole polarizabilities of the hard-sphere tri- and tetravalent actinide ions and may serve as reference values, aiding in the calculation of various electronic and response properties (for example, intermolecular forces, optical properties, etc.) relevant to the nuclear fuel cycle and material science applications.

Sructural parameters of the nearest surrounding of tri- and tetravalent actinide ions in aqueous solutions of actinide salts

Published data are generalized on different research methods for structural characterization of the nearest surrounding of trivalent and tetravalent actinide ions in aqueous solutions of their salts under standard conditions. The structural parameters such as coordination numbers, interparticle distances, the parameters of the second coordination sphere, and the types of ionic association are discussed. The quantitative characteristics of hydrated complexes of poorly known trivalent actinide ions are determined on the basis of comparative analysis with lanthanide ions.

Modelling Np(IV) Solvent Extraction between 30% Tri-Butyl Phosphate and Nitric Acid in the Presence of Simple Hydroxamic Acids

Formo- and aceto-hydroxamic acids are very effective reagents for stripping tetravalent actinide ions from a tri-butyl phosphate phase into nitric acid. A model describing the partitioning of actinide (IV) ions has been derived accounting for reactions in the aqueous and solvent phases, including complex formation with nitrate and hydroxamate ions. Predicted distribution ratios for Np(IV) are compared with experimental data in the presence of aceto-hydroxamate ions. Additionally, a value of 0.473 (±0.004) kgmol−1 for the Np(IV) ion interaction coefficient with nitrate ions (εNp4+-NO3−) was determined.

Effects of the first hydration sphere and the bulk solvent on the spectra of the f2isoelectronic actinide compounds: U4+, NpO2+, and PuO22+

The electronic spectra of the 5f(2) isoelectronic actinide compounds U(4+), NpO(2)(+), and PuO(2)(2+) have been investigated theoretically both in gas phase and in solution. In the latter case the solvent was modelled by a saturated first hydration sphere, five water molecules for NpO(2)(+), and PuO(2)(2+) and eight for U(4+), and a continuum model describing the remaining solvent. The transition energies and oscillator strengths were obtained at the spin-orbit level using the relativistic wave function based multi-configuration methods CASPT2 (complete active space with second-order perturbation theory) and MRCI + DC (Davidson corrected multi-reference configuration interaction), followed by a spin-orbit CI based on a dressed effective spin-orbit Hamiltonian. This study is an attempt to contribute to an enhanced understanding of the electronic structure of tetravalent actinide ions and actinyl(v) and (vi) ions. The spin-orbit MRCI and spin-orbit CASPT2 transitions energies have been compared for the bare ions, leading us to the conclusion that the spin-orbit CASPT2 approach is reasonably accurate and can be used with confidence for the calculation of the hydrated species. The first hydration sphere and the bulk solvent lift degeneracies, but the effect on the transition energies is fairly small for the two actinyl ions, while it is larger, up to several thousands of wave numbers for U(4+). The calculations allowed us to make assignments of the experimentally observed absorption spectra for all species. The computed transition energies and intensities compared favourably with experiment.

X-ray absorption spectroscopic study of trivalent and tetravalent actinides in solution at varying pH values

Abstract We perform X-ray absorption spectroscopy (XAS) investigations to monitor the stabilization of redox sensitive trivalent and tetravalent actinide ions in solution at acidic conditions in a pH range from 0 to 3 after treatment with holding reductants, hydroxylamine hydrochloride (NH2OHHCl) and Rongalite (sodium hydroxymethanesulfinate, CH3NaO3S). X-ray absorption near edge structure (XANES) measurements clearly demonstrate the stability of the actinide species for several hours under the given experimental conditions. Hence, structural parameters can be accurately derived by extended X-ray absorption fine structure (EXAFS) investigations. The coordination structure of oxygen atoms belonging to water ligands surrounding the actinide ions does not change with increasing pH value (approximately 11 O atoms at 2.42 Å in the case of U(IV) at pH 1, 9 O atoms at 2.52 Å for Np(III) at pH 1.5, and 10 O atoms at 2.49 Å for Pu(III) up to pH 3), indicating that hydrolysis reactions are suppressed under the given chemical conditions.

In silico prediction of atomic static electric-dipole polarizabilities of the early tetravalent actinide ions:Th4+(5f0),Pa4+(5f1), andU4+(5f2)

The dipole polarizability tensor components of the tetravalent actinide ions ${\mathrm{Th}}^{4+}$, ${\mathrm{Pa}}^{4+}$, and ${\mathrm{U}}^{4+}$ are computed using the numerical finite-field technique. Four-component correlated calculations have been performed to serve as a reference for establishing the accuracy of two- and one-component relativistic methods. A good agreement within all methods is achieved provided that extended basis sets are used to reach the complete basis set limit. The four-component correlated polarizabilities represent a database of reference values of the dipole polarizability for the early tetravalent actinide ions.

Solvent Extraction Behavior of Neptunium (IV) Ions between Nitric Acid and Diluted 30% Tri‐butyl Phosphate in the Presence of Simple Hydroxamic Acids

Formo‐ and aceto‐hydroxamic acids are very effective reagents for stripping tetravalent actinide ions such as Np(IV) and Pu(IV) ions from a tri‐butyl phosphate phase into nitric acid. Distribution data for Np(IV) in the presence of these hydroxamate ions have now been accumulated and trends established. Stability constants for aceto‐hydroxamate complexes of Np(IV) and Np(V) ions have also been determined in a perchlorate medium, and these reaffirm the affinity of hydroxamate ligands for actinide (IV) ions over actinyl (V,VI) ions.