Atsushi Ikeda-Ohno

Speciation and Structural Study of U(IV) and -(VI) in Perchloric and Nitric Acid Solutions

In order to elucidate the uranium solution chemistry at the high HNO(3) concentrations typically employed for the reprocessing of spent nuclear fuels, speciation and complex structures of U(IV) and U(VI) are studied in aqueous HNO(3) solutions, as well as in HClO(4) solutions, by means of UV-visible-near-infrared and X-ray absorption spectroscopies and density functional theory calculations. In 1.0 M HClO(4), U(IV) exists as a spherical cation of U(4+), which is surrounded by 9-10 water molecules in the primary coordination sphere, while it forms a colloidal hydrous oxide, U(IV)O(2) x nH(2)O, at a lower acidic concentration of 0.1 M HClO(4). U(VI) exists as a transdioxo uranyl cation, UO(2)(2+), and forms a 5-fold pure hydrate complex of [U(VI)O(2)(H(2)O)(5)](2+) in 1.0 M HClO(4). With increasing HNO(3) concentration, the water molecules of the U(IV) and U(VI) hydrate complexes are successively replaced by planar bidentate coordinating nitrate ions (NO(3)(-)), forming dominant species of [U(IV)(H(2)O)(x)(NO(3))(5)](-) in 9.0 M HNO(3) and [U(VI)O(2)(NO(3))(3)](-) in 14.5 M HNO(3), respectively. The present multitechnique approach also suggests the formation of two intermediate U(VI) species, a 5-fold mononitrato complex ([U(VI)O(2)(H(2)O)(3)(eta(2)-NO(3))](+)) and a 6-fold dinitrato complex ([U(VI)O(2)(H(2)O)(2)(eta(2)-NO(3))(2)](0)), involving an increase in the total coordination number on the uranyl(VI) equatorial plane from 5 to 6 with increasing HNO(3) concentration. The presence of unidentate coordinate nitrato complexes or tetranitrato U(VI) complexes is less probable in the present HNO(3) system.

Electrochemical and Complexation Behavior of Neptunium in Aqueous Perchlorate and Nitrate Solutions

Electrochemical and complexation properties of neptunium (Np) are investigated in aqueous perchlorate and nitrate solutions by means of cyclic voltammetry, bulk electrolysis, UV-visible absorption, and Np L(III)-edge X-ray absorption spectroscopies. The redox reactions of Np(III)/Np(IV) and Np(V)/Np(VI) couples are reversible or quasi-reversible, while the electrochemical reaction between Np(III/IV) and Np(V/VI) is irreversible because they undergo structural rearrangement from spherical coordinating ions (Np(3+) and Np(4+)) to transdioxoneptunyl ions (NpO2(n+), n = 1 for Np(V) and 2 for Np(VI)). The redox reaction of the Np(V)/Np(VI) couple involves no structural rearrangement on their equatorial planes in acidic perchlorate and nitrate solutions. A detailed analysis on extended X-ray absorption fine structure (EXAFS) spectra suggests that Np(IV) forms a decaaquo complex of [Np(H2O)10](4+) in 1.0 M HClO4, while Np(V) and Np(VI) exist dominantly as pentaaquoneptunyl complexes, [NpO2(H2O)5](n+) (n = 1 for Np(V) and 2 for Np(VI)). A systematic change is observed on the Fourier transforms of the EXAFS spectra for all of the Np oxidation states as the nitrate concentration is increased in the sample, revealing that the hydrate water molecules are replaced by bidentate-coordinating nitrate ions on the primary coordination sphere of Np.

Local Coordination about La3+ in Molten LaCl3 and Its Mixtures with Alkali Chlorides

The local structure around the La(3+) ions in molten LaCl(3) and its mixtures with alkali and alkaline earth chlorides has been investigated by using extended X-ray absorption fine structure (XAFS) and molecular dynamics (MD) techniques. Such mixtures, which are of current technological interest, are known to be thermodynamically nonideal, and there has been a good deal of work to understand the structural effects factors that contribute to the nonideality. New experimental methods allow observations at the La K-absorption edge at the high temperatures of interest, and the ability of the technique to obtain reliable information even at very low La(3+) concentrations in multicomponent mixtures is demonstrated. Both the mean La(3+)-Cl(-) interionic separation and the mean La(3+) coordination number are found to decrease as the concentration of La(3+) in the mixture decreases. The rate of decrease depends on the identity of the alkali and alkaline earth cations present in the mixtures in a way that parallels the degree of nonideality of the different systems; it is greatest for those alkali cations that coordinate Cl(-) weakly. In dilute mixtures with such cations La(3+) is able to adopt a very stable octahedral coordination geometry but this is inhibited by the presence of more strongly coordinating cations like Li(+) and Mg(2+).

Crystal structure and solution species of Ce(III) and Ce(IV) formates: from mononuclear to hexanuclear complexes.

Cerium(III) and cerium(IV) both form formate complexes. However, their species in aqueous solution and the solid-state structures are surprisingly different. The species in aqueous solutions were investigated with Ce K-edge EXAFS spectroscopy. Ce(III) formate shows only mononuclear complexes, which is in agreement with the predicted mononuclear species of Ce(HCOO)(2+) and Ce(HCOO)2(+). In contrast, Ce(IV) formate forms in aqueous solution a stable hexanuclear complex of [Ce6(μ3-O)4(μ3-OH)4(HCOO)x(NO3)y](12-x-y). The structural differences reflect the different influence of hydrolysis, which is weak for Ce(III) and strong for Ce(IV). Hydrolysis of Ce(IV) ions causes initial polymerization while complexation through HCOO(-) results in 12 chelate rings stabilizing the hexanuclear Ce(IV) complex. Crystals were grown from the above-mentioned solutions. Two crystal structures of Ce(IV) formate were determined. Both form a hexanuclear complex with a [Ce6(μ3-O)4(μ3-OH)4](12+) core in aqueous HNO3/HCOOH solution. The pH titration with NaOH resulted in a structure with the composition [Ce6(μ3-O)4(μ3-OH)4(HCOO)10(NO3)2(H2O)3]·(H2O)9.5, while the pH adjustment with NH3 resulted in [Ce6(μ3-O)4(μ3-OH)4(HCOO)10(NO3)4]·(NO3)3(NH4)5(H2O)5. Furthermore, the crystal structure of Ce(III) formate, Ce(HCOO)3, was determined. The coordination polyhedron is a tricapped trigonal prism which is formed exclusively by nine HCOO(-) ligands. The hexanuclear Ce(IV) formate species from aqueous solution is widely preserved in the crystal structure, whereas the mononuclear solution species of Ce(III) formate undergoes a polymerization during the crystallization process.

The Sulfate Coordination of Np(IV), Np(V), and Np(VI) in Aqueous Solution

The coordination and redox behavior of Np(IV), Np(V), and Np(VI) sulfate in aqueous solution were investigated by Np L(3)-edge extended X-ray absorption fine structure (EXAFS) spectroscopy, cyclic voltammetry, and density functional theory (DFT) calculations. The sulfate coordination mode, that is, monodentate versus bidentate, was determined by using neptunium-sulfur distances R(Np-S) and coordination numbers N(S) obtained by EXAFS spectroscopy. Np(VI) is coordinated by sulfate in the bidentate (R(Np-S) = 3.12 +/- 0.02 A) and monodentate (R(Np-S) = 3.61 +/- 0.02 A) modes at a low sulfate concentration of [SO(4)(2-)]/[NpO(2)(2+)] = 1. At higher [SO(4)(2-)]/[NpO(2)(2+)] ratios, bidentate coordination prevails. Approximately two bidentate sulfate groups are coordinated to Np(VI) with 2.0 M SO(4)(2-) and at pH 1.1. Np(V) is coordinated by sulfate in the bidentate (R(Np-S) = 3.16 +/- 0.02 A) and monodentate (R(Np-S) = 3.67 +/- 0.02 A) modes. However, sulfate coordination is less pronounced and does not exceed one SO(4)(2-) per Np(V) with 2.0 M SO(4)(2-). The redox reaction between the Np(VI)/Np(V) couple can be basically categorized as quasi-reversible. It becomes a more irreversible character at high sulfate concentrations due to structural rearrangement of the sulfate ligands. Finally, Np(IV) also shows bidentate (R(Np-S) = 3.06 +/- 0.02 A) and monodentate (R(Np-S) = 3.78 +/- 0.02 A) coordination modes. The sulfate coordination increases with an increasing [SO(4)(2-)]/[Np(4+)] ratio. A comparison of other tetravalent actinides shows that the monodentate sulfate coordination decreases whereas the bidentate coordination increases along the series Th(IV)-U(IV)-Np(IV). This trend was studied by DFT calculations and is discussed in terms of solvation energy and increasing number of unpaired electrons.

Fluorescence properties of a uranyl(V)-carbonate species [U(V)O2(CO3)3]5− at low temperature

Fluorescence properties of a uranyl(V)-carbonate species in solution are reported for the first time. The fluorescence characteristics of the stable aqueous uranyl(V)-carbonate complex [U(V)O(2)(CO(3))(3)](5-) was determined in a frozen solution (T=153K) of 0.5mM uranium and 1.5M Na(2)CO(3) at pH 11.8 by time resolved laser-induced fluorescence spectroscopy (TRLFS). Two different wavelengths of 255nm and 408nm, respectively were used to independently of each other excite the uranyl(V)-carbonate species. The resulting U(V) fluorescence emission bands were detected between 380nm and 440nm, with a maxima at 404.7nm (excitation with 255nm) and 413.3nm (excitation with 408nm), respectively. It was found that by using an excitation wavelength of 255nm the corresponding extinction coefficient was much higher and the fluorescence spectrum better structured than the ones excited at 408nm. The fluorescence lifetime of the uranyl(V)-carbonate species was determined at 153K as 120micros. TRLFS investigations at room temperature, however, showed no fluorescence signal at all.

Neptunium Carbonato Complexes in Aqueous Solution: An Electrochemical, Spectroscopic, and Quantum Chemical Study

The electrochemical behavior and complex structure of Np carbonato complexes, which are of major concern for the geological disposal of radioactive wastes, have been investigated in aqueous Na(2)CO(3) and Na(2)CO(3)/NaOH solutions at different oxidation states by using cyclic voltammetry, X-ray absorption spectroscopy, and density functional theory calculations. The end-member complexes of penta- and hexavalent Np in 1.5 M Na(2)CO(3) with pH = 11.7 have been determined as a transdioxo neptunyl tricarbonato complex, [NpO(2)(CO(3))(3)](n-) (n = 5 for Np(V), and 4 for Np(VI)). Hence, the electrochemical reaction of the Np(V/VI) redox couple merely results in the shortening/lengthening of bond distances mainly because of the change of the cationic charge of Np, without any structural rearrangement. This explains the observed reversible-like feature on their cyclic voltammograms. In contrast, the electrochemical oxidation of Np(V) in a highly basic carbonate solution of 2.0 M Na(2)CO(3)/1.0 M NaOH (pH > 13) yielded a stable heptavalent Np complex of [Np(VII)O(4)(OH)(2)](3-), indicating that the oxidation reaction from Np(V) to Np(VII) in the carbonate solution involves a drastic structural rearrangement from the transdioxo configuration to a square-planar-tetraoxo configuration, as well as exchanging the coordinating anions from carbonate ions (CO(3)(2-)) to hydroxide ions (OH(-)).

Synthesis of Coordination Polymers of Tetravalent Actinides (Uranium and Neptunium) with a Phthalate or Mellitate Ligand in an Aqueous Medium

Four metal-organic coordination polymers bearing uranium or neptunium have been hydrothermally synthesized from a tetravalent actinide chloride (AnCl4) and phthalic (1,2-H2bdc) or mellitic (H6mel) acid in aqueous media at 130 °C. With the phthalate ligand, two analogous assemblies ([AnO(H2O)(1,2-bdc)]2·H2O; An = U4+ (1) or Np4+ (2)) have been isolated, in which the square-antiprismatic polyhedra of AnO8 are linked to each other via μ3-oxo groups with an edge-sharing mode to materialize infinite zigzag ribbons. The phthalate molecules play a role in connecting the adjacent zigzag chains to build a two-dimensional (2D) network. Water molecules are bonded to the actinide center or found intercalated between the layers. With the mellitate ligand, two distinct structures have been identified. The uranium-based compound [U2(OH)2(H2O)2(mel)] (3) exhibits a three-dimensional (3D) structure composed of the dinuclear units of UO8 polyhedra (square antiprism), which are further linked via the μ2-hydroxo groups. The mellitate linkers use their carboxylate groups to connect the dinuclear units, eventually building a 3D framework. The compound obtained for the neptunium mellitate ([(NpO2)10(H2O)14(Hmel)2]·12H2O (4)) reveals oxidation of the initial NpIV to NpV under the applied hydrothermal synthetic conditions, yielding the neptunyl(V) (NpO2+) unit with a pentagonal-bipyramidal NpO7 environment. This further leads to the formation of a layered assembly of the square-frame NpO7 sheets via the bridging oxygen atoms from the neptunyl oxo groups, which further coordinate to the pentagonal equatorial coordination plane of the adjacent neptunium unit (i.e., cation-cation interactions). In compound 4, the mellitate molecules act as bridging linkers between the NpO7 sheets by using four of their carboxylage groups, eventually building up a 3D structure.

Speciation of the trivalent f-elements Eu(III) and Cm(III) in digestive media

In case radioactive materials are released into the environment, their incorporation into our digestive system would be a significant concern. Trivalent f-elements, i.e., trivalent actinides and lanthanides, could potentially represent a serious health risk due to their chemo- and radiotoxicity, nevertheless the biochemical behavior of these elements are mostly unknown even to date. This study, therefore, focuses on the chemical speciation of trivalent f-elements in the human gastrointestinal tract. To simulate the digestive system artificial digestive juices (saliva, gastric juice, pancreatic juice and bile fluid) were prepared. The chemical speciation of lanthanides (as Eu(III)) and actinides (as Cm(III)) was determined experimentally by time-resolved laser-induced fluorescence spectroscopy (TRLFS) and the results were compared with thermodynamic modeling. The results indicate a dominant inorganic species with phosphate/carbonate in the mouth, while the aquo ion is predominantly formed with a minor contribution of the enzyme pepsin in the stomach. In the intestinal tract the most significant species are with the protein mucin. We demonstrated the first experimental results on the chemical speciation of trivalent f-elements in the digestive media by TRLFS. The results highlight a significant gap in chemical speciation between experiments and thermodynamic modeling due to the limited availability of thermodynamic stability constants particularly for organic species. Chemical speciation strongly influences the in vivo behavior of metal ions. Therefore, the results of this speciation study will help to enhance the assessment of health risks and to improve decorporation strategies after ingestion of these (radio-)toxic heavy metal ions.

[UO2Cl2(phen)2], a Simple Uranium(VI) Compound with a Significantly Bent Uranyl Unit (phen = 1,10-phenanthroline)

A simple synthesis based on UO2 Cl2 ⋅n H2 O and 1,10-phenanthroline (phen) resulted in the formation of a new uranyl(VI) complex [UO2 Cl2 (phen)2 ] (1), revealing a unique dodecadeltahedron coordination geometry around the uranium center with significant bending of the robust linear arrangement of the uranyl (O-U-O) unit. Quantum chemical calculations on complex 1 indicated that the weak but distinct interactions between the uranyl oxygens and the adjacent hydrogens of phen molecules play an important role in forming the dodecadeltahedron geometry that fits to the crystal structure of 1, resulting in the bending the uranyl unit. The uranyl oxygens in 1 are anticipated to be activated as compared with those in other linear uranyl(VI) compounds.