Actinide Ions Research Articles

Extraction of actinide ions using three CMPO-functionalized pillar[5]arenes in a room temperature ionic liquid

Abstract Extraction of actinide ions from nitric acid feeds was investigated using solutions of three CMPO (carbamoyl methyl phosphine oxide) functionalized pillar[5]arenes with varying spacer lengths in a room temperature ionic liquid, [C8mim][NTf2]. The extraction of Am3+ was found to decrease with increasing nitric acid concentration suggesting a mechanism analogous to a cation-exchange mechanism. The nature of the extracted species was determined by the conventional slope analysis method to be ML type which was subsequently confirmed by luminescence spectroscopy. Studies carried out to determine the thermodynamic parameters suggested highly exothermic reactions with negative entropy values. The reusability and recycling studies involved stripping and radiolytic stability of the solvent systems which suggested reasonably encouraging results. While a DTPA solution in guanidine carbonate was found to back extract the extracted metal ion from the ionic liquid phase, the radiolytic stability of the CMPO-pillararenes were reasonably good.

On the isotropy of induced multipole moments in heavy ion complexes.

Polarization effects have been proven to play an important role in the theoretical description of chemical processes. With this respect, we report ab initio calculations describing multiconfigurational and relativistic effects as well as electron correlation to determine dipole- and quadrupole polarizabilities for all trivalent actinide and lanthanide ions. The results are used in an iterative point-multipole model to compute polarization interaction for different metal compounds. Significant differences between the usage of isotropic polarizabilities for the coordinating ligands compared to the full tensor representation is found. Quadrupole contributions are found to be negligible for symmetric geometries but can contribute up to 30 kJ/mol to the polarization energy in specific cases. The employed method allows a parameter free study of multipole interactions for a wide variety of systems. © 2017 Wiley Periodicals, Inc.

Cation‐Directed Isomerization of the U28 Uranyl‐Peroxide Cluster

The conformational dynamics of nanometer‐sized actinide ions are exceptionally sensitive to the choice of counter‐cations. A new means of following these dynamics in solution is presented that follows 1H NMR signals. The direct bond between hydrogen and phosphorus atoms in the bridging phosphonic groups of the [(UO2)28(O2)20(PHO3)24(H2O)12]32– (U28) cluster allows unparalleled recording of the orientations of these bridges in situ. The µ3‐PHO3 bridges are organized into two supersets of conformers (facing inward vs. outward from the cluster), but each of these supersets additionally have four subsets that can be identified based upon orientations of the lone pairs of electrons in the associated oxygen atoms. The ensemble of these subsets changes over days or weeks depending upon bulk solution chemistry, temperature, and pressure. They also reflect cations trapped within the molecule. The 1H NMR spectra at room temperature indicate that the molecular orientation of this enormous ion tunes itself in response to solution composition, which suggests a strategy for selecting host–guest combinations.

Impact of uranium carbide organics treated by prolonged boiling and electrochemical oxidation upon uranium and plutonium solvent extraction

Abstract The dissolution of uranium or uranium-plutonium carbide fuel in nitric acid leads to ~50% carbon evolved as carbon dioxide, the remainder remains in the solution as soluble organics. These dissolved organic molecules interfere with the solvent extraction of uranium and plutonium by complexing to the actinide ions and decreasing the efficiency of their extraction. Experiments reported here describe two series of experiments assessing the uranium carbide dissolution liquor treatment by prolonged boiling and electrochemical oxidation. Plutonium losses to aqueous and solvent raffinates are observed for untreated liquors, highlighting that mineralisation of dissolved organics is necessary to reduce the complexing effects of organic acids to an extent that permit efficient operation of a solvent extraction process both in the first solvent use (considered here) and for maintaining solvent quality during industrial solvent reuse in the highly active cycle. Solution carbon analysis and 30% TBP solvent extraction batch tests of uranium and plutonium originating from dissolved uranium carbide liquors untreated and after treatment are compared. These experiments demonstrate the reprocessing of uranium carbides by direct dissolution coupled to a mineralisation process, can achieve near quantitative uranium and high plutonium recoveries (99.9%).

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.

Intramolecular deactivation processes of electronically excited Lanthanide(III) complexes with organic acids of low molecular weight

The luminescence of Lanthanide(III) complexes with different model ligands was studied under direct as well as sensitized excitation conditions. The research was performed in the context of studies dealing with deep-underground storages for high-level nuclear waste. Here, Lanthanide(III) ions served as natural analogues for Actinide(III) ions and the low-molecular weight organic ligands are present in clay minerals and furthermore, they were employed as proxies for building blocks of humic substances, which are important complexing molecules in the natural environment, e.g., in the far field of a repository site. Time-resolved luminescence spectroscopy was applied for a detailed characterization of Eu(III), Tb(III), Sm(III) and Dy(III) complexes in aqueous solutions. Based on the observed luminescence the ligands were tentatively divided into two groups (A, B). The luminescence of Lanthanide(III) complexes of group A was mainly influenced by an energy transfer to OH-vibrations. Lanthanide(III) complexes of group B showed ligand-related luminescence quenching, which was further investigated. To gain more information on the underlying quenching processes of group A and B ligands, measurements at different temperatures (77K≤T≤353K) were performed and activation energies were determined based on an Arrhenius analysis. Moreover, the influence of the ionic strength between 0M≤I≤4M on the Lanthanide(III) luminescence was monitored for different complexes, in order to evaluate the influence of specific conditions encountered in host rocks foreseen as potential repository sites.

Solvation Properties of the Actinide Ion Th(IV) in DMSO and DMSO:Water Mixtures through Polarizable Molecular Dynamics.

We have studied the solvation of Th4+ in water, in dimethyl sulfoxide (DMSO), and in their equimolar mixture by using molecular dynamics based on an Amoeba-derived polarizable force field. We have performed an extended structural analysis in order to provide a complete picture of the chemical-physical features of the interaction of Th4+ with the two solvents in their pure and mixed states. Through our simulations we found that, very likely, the first solvation shell in DMSO is not unlike the one found in pure water and contains 9 solvent molecules. The residence time of first shell of DMSO molecules is however much longer than the residence time of water. For the 1:1 mixture we present computational evidence that both water and DMSO participate in the solvation of Th4+ with a slight preference for the latter.

C[sbnd]F bond breaking by bare actinide monocations in the gas phase: a relativistic DFT study

Investigations of the CF bond activation by actinide monocation An+ (An=Ac, Th, Pa, U, Np, Pu and Am) are carried out using relativistic density functional theory (DFT) computations. Originally, the aim of the study is to compare the ability of different actinide ions to break strong bonds particularly in the context of accidental radioactive dissemination. The An+ reaction with the fluorinated hydrocarbon CH3F was selected as a representative system in this context. Unexpectedly, the considered An+ were found to react differently. Via linear transit (LT) and intrinsic reaction coordinate (IRC) calculations, three reaction mechanisms for the CF bond activation, leading to the An–F+ formation, were revealed; the first one, i.e. ‘harpoon’ mechanism which was observed in the case of Pu+, Am+, while the second called ‘insertion-elimination’ mechanism concerned the case of Th+, Pa+, U+ and Np+. DFT computations highlight the particular case of the Ac+ system which presents two different mechanisms according to its spin state: a mechanism qualified as ‘harpoon-like’ for the triplet state and an ‘insertion-elimination’ mechanism for the singlet state. The activation barrier for the fluorine elimination from CH3F is weak for all the studied systems, from 0.9 kcal/mol for Th+ to 8.2 kcal/mol for Am+. Th+ is found as the most effective ion to activate the CF bond and a considerable exergonic character (−81.5 kcal/mol) for this reaction is expected. The performed orbital, population and charge analyses permitted to reveal the role of the actinide 7s, 6d and 5f orbitals and of electron transfers during the reaction.

Potential application of graphene oxide membranes for removal of Cs(I) and Sr(II) from high level-liquid waste

Abstract As a new carbon-based membrane material used for ionic and molecular sieving, the graphene oxide (GO) membrane also has potential application in radiochemical separation. In this work, the permeation performances of some typical metallic elements in high level-liquid waste (HLLW) through the ionic sieving 5-μm-thick GO membranes were investigated. It was found that the permeation rates of ions through GO membranes markedly decreased with the increase of hydrated ionic radii. Cs(I) and Sr(II) permeated through GO membranes quickly, whereas lanthanide ions and actinide ions infiltrated much more slowly, thus providing the feasibility of separation according to the difference of hydrated ionic radii. Moreover, the concentrations of initial metal ions and acidity in feed solutions also had influences on the permeation of ions through GO membranes, and higher initial metal ions and feed solution acidity were favorable to the removal of Cs(I) and Sr(II). The barrier separation test indicated that GO membranes had promising ability for the separation of Cs(I) and Sr(II) from lanthanide and actinide ions due to the difference of hydrated ion radius, and the separation factor of Cs(I)/U(VI) and Sr(II)/U(VI) would reach about 10 and 3 in single penetration. The presented results demonstrated that the membrane separation based on GO membrane would be a promising candidate for the removal of Cs(I) and Sr(II) from HLLW.

Ab Initio Ligand-Field Theory Analysis and Covalency Trends in Actinide and Lanthanide Free Ions and Octahedral Complexes.

Actinide chemistry is gaining increased focus in modern research, particularly in the fields of energy research and molecular magnetism. However, the structure-function and structure-property relationships of actinides have still not been studied as intensely as those for transition metals. In this work, we report a detailed ab initio study of the spectroscopic, magnetic, and bonding properties of the trivalent actinide free ions and their associated hexachloride complexes in octahedral symmetry. The electronic structures of these systems are examined using complete active-space self-consistent-field calculations followed by second-order N-electron valence perturbation theory, including both scalar relativistic and spin-orbit-coupling effects. The computed energies and wave functions are further analyzed by means of ab initio ligand-field theory (AILFT) and finally chemically interpreted by means of the angular overlap model (AOM). The derived Slater-Condon and spin-orbit parameters have allowed us to systematically rationalize the spectroscopic and magnetic properties of the investigated free ions and complexes along the entire actinide series. Overall, the AILFT- and AOM-derived parameters accurately reproduce the multireference electronic structure calculations. The small observed discrepancies with respect to experimentally derived ligand-field parameters are essentially due to an underestimation of the electronic correlation, which arises from both the constrained size of the active space (restricted to the f orbitals) and the limit of the perturbation approach to account for dynamical correlation. Our analysis also provides insight into the metal-ligand covalency trends along the series. Consistent with natural population analysis, the nephelauxetic (Slater-Condon parameters) and relativistic nephelauxetic (spin-orbit-coupling) reductions determined for these complexes indicate a decrease in the covalency along the series. These trends also hold, to varying extents, for the corresponding tetravalent derivatives, as well as the lanthanide analogues.

Synthesis, structural and extraction studies of new bifunctional ligand with uranium

A new bifunctional ligand system (N,N-dialkyl carbamoyl methyl) (2-pyridyl-N-Oxide) sulfide C5H4NOSCH2CONR2, (where R=isopropyl (L1), isobutyl (L2) and butyl (L3) and octyl (L4)) have been synthesized. The coordination chemistry of this ligand system with UO2(NO3)2·6H2O has been carried out. All the ligands and complexes were characterized by CHN, IR and NMR techniques. Single crystal X-ray study of one of the complexes (2) shows that the uranium atom is surrounded by eight oxygen atoms in a hexagonal-bi-pyramidal geometry. Ligand is bonded to metal through carbamoyl and N-oxide groups in a chelating fashion. Thermogravimetric studies of the complexes shows that the ligands are completely incinerable. Extraction studies of the actinides with ligand L4 show that this new bifunctional ligand has very good distribution ratio (D) values for tetravalent and hexavalent actinides ions from nitric acid medium.

On the development of polarizable and Lennard-Jones force fields to study hydration structure and dynamics of actinide(III) ions based on effective ionic radii.

In this contribution, we show how it is possible to develop polarizable and non-polarizable force fields to study hydration properties of a whole chemical series based on atomic properties such as ionic radii. In particular, we have addressed the actinide(III) ion series, from U3+ to Cf3+, for which X-ray absorption data and effective ionic radii are available. A polarizable force field has been re-parameterized improving the original one [M. Duvail et al., J. Chem. Phys. 135, 044503 (2011)] which was based on solid state ionic radii. The new force field does not depend on solid state properties but directly on the liquid phase ones, and it can be used to study these ions in liquid water without any ambiguity. Furthermore, we have shown that it is possible to parameterize also a non-polarizable potential using standard Lennard-Jones and Coulombic forces, which can be transferred to other systems in condensed phase. The structural and dynamical properties of these two force fields are compared to each other and with data available in the literature, providing a good agreement. Moreover, we show the comparison with experimental X-ray absorption data that are very well reproduced by both force fields.

Superparamagnetic graphene oxide–magnetite nanoparticle composites for uptake of actinide ions from mildly acidic feeds

Super paramagnetic graphene oxide (GO) − Fe3O4 nanoparticle composites were prepared and characterized by conventional techniques such as XRD, SEM, EDX, FT-IR, Raman, XPS, DLS and zeta potential, etc. TEM studies have confirmed nanoparticle nature of the composites. The GO-magnetic nanoparticle composites can be dispersed in mildly acidic aqueous solutions and get concentrated in a small volume under application of an external magnetic field. The composites were evaluated for the uptake of actinide ions such as Am3+, UO22+, Th4+ and Pu4+ from mildly acidic aqueous solutions. Am3+ sorption sharply increased with pH as the Kd values increased from about 10 at pH 1 to 105 at pH 3 beyond which a plateau in the Kd values was seen. Eu3+ displayed nearly comparable uptake behaviour to that of Am3+ while the uptake of other metal ions followed the trend: Pu(IV)>Th(IV)>>UO22+. The adsorption behaviour of Am3+ onto the graphene oxide − Fe3O4 nanoparticle composites fitted very well to the Langmuir as well as Temkin isotherm models. The desorption rate (using 1M HNO3) was fast and reusability study results were highly encouraging. The very high uptake values suggest possible application of the magnetic nanoparticles in radioactive waste remediation in natural ground water.

Luminescent probing of the simplest chiral α-amino acid-alanine in an enantiopure and racemic state.

Luminescent spectroscopy combined with the technique of luminescent probing with rare earth ions (europium, gadolinium, terbium) and an actinide ion (uranyl) was used to differentiate enantiopure and racemic alanine, the simplest chiral proteinogenic amino acid. Using the achiral luminescent probes, small differences between pure L and DL alanine in the solid state were strongly amplified. Based on the observed electronic transitions of the probes, the position of the triplet level of the coordinated alanine was estimated. Formation of homo- and heterochiral complexes between enantiomers of alanine and the metal ions is discussed as a possible mechanism of chiral self-discrimination.

Glycolamide-functionalized ionic liquid: Synthesis and actinide ion extraction studies

ABSTRACTA glycolamide-functionalized ionic liquid (G-FIL) was synthesized for the first time and was evaluated for the extraction of actinide ions such as Am3+, Pu4+ and UO22+ and fission product element ions such as Eu3+, Sr2+ and Cs+. The extraction of the trivalent metal ions was found to be exceptionally high at low acid concentrations, which rapidly decreased with increasing acidity. In view of the high viscosity of the G-FIL, the studies were carried out using its diluted solution in a commercial ionic liquid, viz. 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C4mim][Tf2N]).

Insight into the Complexation of Actinides and Lanthanides with Diglycolamide Derivatives: Experimental and Density Functional Theoretical Studies.

Extraction of actinide (Pu4+, UO22+, Am3+) and lanthanide (Eu3+) ions was carried out using different diglycolamide (DGA) ligands with systematic increase in the alkyl chain length from n-pentyl to n-dodecyl. The results show a monotonous reduction in the metal ion extraction efficiency with increasing alkyl chain length and this reduction becomes even more prominent in case of the branched alkyl (2-ethylhexyl) substituted DGA (T2EHDGA) for all the metal ions studied. Steric hindrance provided by the alkyl groups has a strong influence in controlling the extraction behavior of the DGAs. The distribution ratio reduction factor, defined as the ratio of the distribution ratio values of different DGAs to that of T2EHDGA, in n-dodecane follows the order UO22+ > Pu4+ > Eu3+ > Am3+. Complexation of Nd3+ was carried out with the DGAs in methanol by carrying out UV-vis spectrophotometric titrations. The results indicate a significant enhancement in the complexation constants upon going from methyl to n-pentyl substituted DGAs. They decreased significantly for DGAs containing alkyl substituents beyond the n-pentyl group, which corresponds to the observed trend from the solvent extraction studies. DFT-based calculations were performed on the free and the Nd3+ complexes of the DGAs both in the gas and the solvent (methanol) phase and the results were compared the experimental observations. Luminescence spectroscopic investigations were carried out to understand the complexation of Eu3+ with the DGA ligands and to correlate the nature of the alkyl substituents on the photophysical properties of the Eu(III)-DGA complexes. The monoexponential nature of the decay profiles of the complex revealed the predominant presence of single species, while no water molecules were present in the inner coordination sphere of the Eu3+ ion.

Synthesis and Characterization of the Actinium Aquo Ion.

Metal aquo ions occupy central roles in all equilibria that define metal complexation in natural environments. These complexes are used to establish thermodynamic metrics (i.e., stability constants) for predicting metal binding, which are essential for defining critical parameters associated with aqueous speciation, metal chelation, in vivo transport, and so on. As such, establishing the fundamental chemistry of the actinium(III) aquo ion (Ac-aquo ion, Ac(H2O)x3+) is critical for current efforts to develop 225Ac [t1/2 = 10.0(1) d] as a targeted anticancer therapeutic agent. However, given the limited amount of actinium available for study and its high radioactivity, many aspects of actinium chemistry remain poorly defined. We overcame these challenges using the longer-lived 227Ac [t1/2 = 21.772(3) y] isotope and report the first characterization of this fundamentally important Ac-aquo coordination complex. Our X-ray absorption fine structure study revealed 10.9 ± 0.5 water molecules directly coordinated to the AcIII cation with an Ac–OH2O distance of 2.63(1) Å. This experimentally determined distance was consistent with molecular dynamics density functional theory results that showed (over the course of 8 ps) that AcIII was coordinated by 9 water molecules with Ac–OH2O distances ranging from 2.61 to 2.76 Å. The data is presented in the context of other actinide(III) and lanthanide(III) aquo ions characterized by XAFS and highlights the uniqueness of the large AcIII coordination numbers and long Ac–OH2O bond distances.

Europium binding to humic substances extracted from deep underground sedimentary groundwater studied by time-resolved laser fluorescence spectroscopy

ABSTRACTHumic substances (HSs) are ubiquitous in various environments including deep underground and play an important role in the speciation and mobility of radionuclides. The binding of Eu3+, a chemical homologue of trivalent actinide ions, to HSs isolated from sedimentary groundwater at −250 m below the surface was studied by time-resolved laser fluorescence spectroscopy combined with parallel factor analysis (PARAFAC) as a function of pH and salt concentration. PARAFAC modeling reveals the presence of multiple factors that correspond to different Eu3+ species. These factors resemble those observed for Eu3+ binding to HSs from surface environments; however, detailed comparison shows that there are some particularities in Eu3+ binding to the deep groundwater HSs. The distribution coefficients (Kd) of Eu3+ binding to the HSs calculated from the PARAFAC modeling exhibits a rather strong salt effect. At 0.01 M NaClO4, the Kd values are relatively large and comparable to those to the surface HSs; they are decreaed at 0.1 M NaClO4 by more than an order of the magnitude. The Kd values are larger for the humic acid fraction of the deep underground HSs than the fulvic acid fraction over the entire range of pH and salt concentration investigated in this study.

Actinide ion extraction using room temperature ionic liquids: opportunities and challenges for nuclear fuel cycle applications.

Studies on the extraction of actinide ions from radioactive feeds have great relevance in nuclear fuel cycle activities, mainly in the back end processes focused on reprocessing and waste management. Room temperature ionic liquid (RTIL) based diluents are becoming increasingly popular due to factors such as more efficient extraction vis-à-vis molecular diluents, higher metal loading, higher radiation resistance, etc. The fascinating chemistry of the actinide ions in RTIL based solvent systems due to complex extraction mechanisms makes it a challenging area of research. By the suitable tuning of the cationic and anionic parts of the ionic liquids, their physical properties such as density, dielectric constant and viscosity can be changed which are considered key parameters in metal ion extraction. Aqueous solubility of the RTILs, which can lead to significant loss in the solvent inventory, can be avoided by appending the extractant moieties onto the ionic liquid. While the low vapour pressure and non-flammability of the ionic liquids make them appear as 'green' diluents, their aqueous solubility raises concerns of environmental hazards. The present article gives a summary of studies carried out on actinide ion extraction and presents perspectives of its applications in the nuclear fuel cycle. The article discusses various extractants used for actinide ion extraction and at many places, comparison is made vis-à-vis molecular diluents which includes the nature of the extracted species and the mechanism of extraction. Results of studies on rare earth elements are also included in view of their similarities with the trivalent minor actinides.

Separating lanthanides and actinides from nitric acid solutions by using N,N-di(2-ethylhexyl)-diglycolamic acid (HDEHDGA).

The extraction of Eu(iii), U(vi), and Pu(iv) from 0.1-3 M HNO3 solutions by using N,N-di(2-ethyl-hexyl)-diglycolamic acid (HDEHDGA) is presented. In contrast to common organic acid extractants, such as long-chain carboxylic acids, organophosphorus acids, and derivatives of beta-diketones, losing extractability, HDEHDGA exhibits a strong ability to extract lanthanide and actinide ions from high HNO3 solutions.