Phillip D. Palmer

Synthesis of Imido Analogs of the Uranyl Ion

Here we describe the synthesis of two imido analogs of the uranyl ion, UO2+2, in which the oxygens are replaced by divalent alkyl or aryl nitrogen groups: U(NtBu)2I2(THF)2 (1) and U(NPh)2I2(THF)3 (2) (where tBu is tert-butyl and THF is tetrahydrofuran). Both compounds have been fully characterized by standard analytical techniques, including x-ray crystallography, and the chemical bonding between the metal center and the nitrogen ligands was quantified by using hybrid density functional theory calculations. As expected for a uranyl analog, these complexes exhibit linear N-U-N linkages and very short U-N bonds. In addition, the theoretical calculations show strong involvement of the 5f and 6d electrons in the U-N bonding.

Solubilization of plutonium hydrous oxide by iron-reducing bacteria

The removal of plutonium from soils id challenging because of its strong sorption to soils and limited solubility, Microbial reduction of metals is known to affect the speciation and solubility of sparingly soluble metals in the environment, notably iron and manganese. The similarity in reduction potential for [alpha]-FeOOH(s) and hydrous PuO[sub 2](s) suggests that iron-reducing bacteria may also reduce and solubilize plutonium. Bacillus strains were used to demonstrate that iron-reducing bacteria mediate the solubilization of hydrous PuO[sub 2](s) under anaerobic conditions. Up to [approximately]90% of the PuO[sub 2] was biosolubilized in the presence of nitrilotriacetic acid (NTA) within 6-7 days. Biosolubilization occurred to a lesser extent ([approximately] 40%) in the absence of NTA. Little PuO[sub 2] solubilization occurred in sterile culture media or in the presence of a non-iron-reducing Escherichia coli. These observations suggest a potentially attractive, environmentally benign strategy for the remediation of Pu-contaminated soils. 26 refs., 5 figs., 2 tabs.

Neptunium and Plutonium Solubilities in a Yucca Mountain Groundwater

Solubilities of neptunium and plutonium were studied in J-13 groundwater (ionic strength of about 3.7 mmol; total dissolved carbonate of 2.8 mmol) from the proposed Yucca Mountain Nuclear Waste Repository site, Nevada, at three different temperatures (25, 60, and 90 °C) and pH values (6.0, 7.0, and 8.5). Experiments were performed from both over- and undersaturation at defined CO2 partial pressures. The solubility of 237Np from oversaturation ranged from a high of (9.40 ± 1.22) × 10-4 M at pH 6.0 and 60 °C to a low of (5.50 ± 1.97) × 10-6 M at pH 8.5 and 90 °C. The analytical results of solubility experiments from undersaturation (temperatures of 25 and 90 °C and pH values 6, 7, and 8.5) converged on these values. The 239/240Pu solubilities ranged from (4.70 ± 1.13) × 10-8 M at pH 6.0 and 25 °C to (3.62 ± 1.14) × 10-9 M at pH 8.5 and 90 °C. In general, both neptunium and plutonium solubilities decreased with increasing pH and temperature. Greenish-brown crystalline Np2O5·xH2O was identified as the solubil...

OXIDATION STATE DETERMINATION OF PLUTONIUM AQUO IONS USING X-RAY ABSORPTION SPECTROSCOPY

Abstract Four oxidation states (III, IV, V, and VI) of Pu may coexist under environmentally relevant conditions. An efficient method to determine the states actually present in various matrices would enhance the ability to model the fate and transport of plutonium in process streams and in the environment. This communication establishes that the L3 X-ray absorption near-edge structure (XANES) spectra of Pu are primarily determined by the valence state and the presence or absence of the trans dioxo moiety, consistent with previous U and Np XANES studies. The edge energies were observed to shift progressively to higher energy with increasing valence, with an average 1.68 eV increase per formal oxidation state increase. In addition, the general spectral shape of the (III) and (IV) species is clearly different from the dioxo-containing (V) and (VI) species, with the first maximum much larger and sharper for the (III) and (IV) spectra than for the (V) and (VI) spectra.

The Formation of Pu(IV)-Colloid by the Alpha-reduction of Pu(V) or Pu(VI) in Aqueous Solutions

The effects of alpha-particle self-irradiation have been investigated in solutions of {sup 239}Pu in the pH range from 1.3 to > 2.2. The initial solutions contained Pu(VI), Pu(V), colloidal Pu(IV) or various mixtures of these. Composition versus time was determined for periods up to 618 days. Colloidal Pu(IV) formed slowly in mixtures with initial average oxidation states greater than about 4.5. Colloidal Pu(IV) reacts slowly to give mixtures of Pu(V) and Pu(VI). The two most important reactions in the system appear to be: the disproportionation of Pu(V) to give Pu(VI) and colloidal Pu(IV), and the alpha-particle induced reduction of Pu(VI) to give Pu(V). Steady-states were reached very slowly and required at least 400 days for most of the mixtures. No evidence was found for equilibrium in the disproportionation reaction. 20 refs., 4 figs., 3 tabs.

13C NMR characterization of actinyl(VI) carbonate complexes in aqueous solution

Abstract The ligand exchange reaction between free and coordinated carbonate on 242 Pu VI O 2 ( 13 CO 3 ) 3 4− and 243 Am VI O 2 ( 13 CO 3 ) 3 4− systems was examined by variable temperature 13C pulsed nuclear magnetic resonance (NMR) techniques. A modified Carr-Purcell-Meiboom-Gill NMR pulse sequence was used for the experimental determination of ligand exchange parameters for paramagnetic actinide complexes. Eyring analysis provided activation parameters of ΔG 295 ‡ = 53 kJ M −1 , ΔH ‡ = 35 kJ M −1 and ΔS 295 ‡ = −61 J M −1 K −1 for the plutonyl triscarbonate system, suggesting an associative transition state for the plutonyl(VI) carbonate self-exchange reaction.

Chemical Speciation of Neptunium(VI) under Strongly Alkaline Conditions. Structure, Composition, and Oxo Ligand Exchange

Hexavalent neptunium can be solubilized in 0.5-3.5 M aqueous MOH (M = Li(+), Na(+), NMe4(+) = TMA(+)) solutions. Single crystals were obtained from cooling of a dilute solution of Co(NH3)6Cl3 and NpO2(2+) in 3.5 M [N(Me)4]OH to 5 °C. A single-crystal X-ray diffraction study revealed the molecular formula of [Co(NH3)6]2[NpO2(OH)4]3·H2O, isostructural with the uranium analogue. The asymmetric unit contains three distinct NpO2(OH)4(2-) ions, each with pseudooctahedral coordination geometry with trans-oxo ligands. The average Np═O and Np-OH distances were determined to be 1.80(1) and 2.24(1) Å, respectively. EXAFS data and fits at the Np L(III)-edge on solid [Co(NH3)6]2[NpO2(OH)4]3·H2O and aqueous solutions of NpO2(2+) in 2.5 and 3.5 M (TMA)OH revealed bond lengths nearly identical with those determined by X-ray diffraction but with an increase in the number of equatorial ligands with increasing (TMA)OH concentration. Raman spectra of single crystals of [Co(NH3)6]2[NpO2(OH)4]3·H2O reveal a ν1(O═Np═O) symmetric stretch at 741 cm(-1). Raman spectra of NpO2(2+) recorded in a 0.6-2.2 M LiOH solution reveal a single ν1 frequency of 769 cm(-1). Facile exchange of the neptunyl oxo ligands with the water solvent was also observed with Raman spectroscopy performed with (16)O- and (18)O-enriched water solvent. The combination of EXAFS and Raman data suggests that NpO2(OH)4(2-) is the dominant solution species under the conditions of study and that a small amount of a second species, NpO2(OH)5(3-), may also be present at higher alkalinity. Crystal data for [Co(NH3)6]2[NpO2(OH)4]3·H2O: monoclinic, space group C2/c, a = 17.344(4) Å, b = 12.177(3) Å, c = 15.273 Å, β = 120.17(2)°, Z = 4, R1 = 0.0359, wR2 = 0.0729.

Synthese und Struktur des ersten Transuran‐Kronenether‐Einschlußkomplexes: [NpO2([18]krone‐6)][ClO4]

Eindeutig in der ersten Koordinationssphare des Dioxoactinoid-Ions befindet sich der Kronenetherligand im Komplex-Ion [NpO2([18]krone-6)]+ 1. Dieses entsteht relativ leicht durch Zugabe von [18]Krone-6 zu einer wasrigen Losung von NpO2+ oder NpO22+ in verdunnter HClO4 oder CF3SO3H. Rontgenstrukturanalyse und spektroskopische Daten von 1[ClO4] belegen die Komplexstruktur zweifelsfrei.

Chemical speciation of heterogeneously reduced Pu in synthetic brines

Summary X-ray absorption fine structure (XAFS) spectroscopy has been used to determine the speciation of Pu precipitates prepared by the heterogeneous reduction of Pu(VI) with Al and Fe in 5 M NaCl and an ERDA-6 brine, a simulant from the Waste Isolation Pilot Plant in Carlsbad, New Mexico. NaOCl was added to some of these solutions to determine its effect on Pu speciation. Analysis of the Pu LIII spectra showed that all solids consisted of PuO2+x-y(OH)2y·z H2O, compounds with characteristics identical to those prepared by hydrolysis and with Pu-O and Pu-Pu distances identical to those treated at elevated temperature. Additionally, reduction with Al gave compounds with different site distributions than reduction with Fe, and reduction with Al or the addition of NaOCl appeared to suppress the formation of oxo groups and their associated Pu(V) sites.