John M. Berg

Characterization of the nitrate complexes of Pu(IV) using absorption spectroscopy, 15N NMR, and EXAFS

Abstract The nitrate complexes of Pu(IV) are studied in solutions containing nitrate at concentrations up to 13 molar (M) using absorption spectroscopy, 15N nuclear magnetic resonance (NMR), and extended X-ray absorption fine structure (EXAFS). Four major nitrato complexes are observed and identified as Pu(NO3)3+, Pu(NO3)22+, Pu(NO3)4, and Pu(NO3)62−. The Pu(NO3)31+ and Pu(NO3)51− complexes are not observed here and an upper limit of 0.10 can be set on the fraction for each of these species.

Analysis and spectral assignments of mixed actinide oxide samples using laser-induced breakdown spectroscopy (LIBS).

In this paper, we report for the first time the identification and assignments of complex atomic emission spectra of mixed actinide oxides using laser-induced plasma spectroscopy or laser-induced breakdown spectroscopy (LIBS). Preliminary results of LIBS measurements on samples of uranium dioxide (UO2)/plutonium dioxide (PuO2) and UO2/PuO2/americium dioxide (AmO2)/neptunium dioxide (NpO2) simulated fuel pellets (or mixed actinide oxide samples) are reported and discussed. We have identified and assigned >800 atomic emission lines for a UO2/PuO2/AmO2/NpO2 fuel pellet thus far. The identification and assignments of spectral emission lines for U, Pu, and Am are consistent with wavelength data from the literature. However, only a few emission lines have been assigned with a high degree of confidence for Np compared with atomic emission data from the literature. We also indicate where atomic emission lines for Cm would most likely appear in the spectral regions shown. Finally, we demonstrate that a LIBS system with a resolving power of approximately 20 000 is adequate for analyzing complex mixtures of actinide elements within the same sample.

Spectroscopic and computational investigation of actinium coordination chemistry

Actinium-225 is a promising isotope for targeted-α therapy. Unfortunately, progress in developing chelators for medicinal applications has been hindered by a limited understanding of actinium chemistry. This knowledge gap is primarily associated with handling actinium, as it is highly radioactive and in short supply. Hence, AcIII reactivity is often inferred from the lanthanides and minor actinides (that is, Am, Cm), with limited success. Here we overcome these challenges and characterize actinium in HCl solutions using X-ray absorption spectroscopy and molecular dynamics density functional theory. The Ac–Cl and Ac–OH2O distances are measured to be 2.95(3) and 2.59(3) Å, respectively. The X-ray absorption spectroscopy comparisons between AcIII and AmIII in HCl solutions indicate AcIII coordinates more inner-sphere Cl1– ligands (3.2±1.1) than AmIII (0.8±0.3). These results imply diverse reactivity for the +3 actinides and highlight the unexpected and unique AcIII chemical behaviour.

Tetrahydrofuran adducts of uranium tetrachloride

Abstract Pale green crystals of UCl 4 (THF) 3 ( 1 ) were isolated from saturated THF solutions after standing undisturbed for 12 h. The molecular structure of the compound was determined by single crystal X-ray diffraction. The molecule adopts a unique structure based on a distorted pentagonal bipyramid with two chlorides occupying the axial positions and the other two chlorides occupying adjacent positions in the pentagonal plane. Crystal data for 1 (UCl 4 O 4 C 12 H24 ) at 203 K: monoclinic space group P 2 1 a =7.835(1), b =14.428(1), c =8.456(1) A, β=100.97(1)°, Z =2, D calc =2.110 g cm −3.

Speciation Model Selection by Monte Carlo Analysis of Optical Absorption Spectra: Plutonium(IV) Nitrate Complexes

Standard modeling approaches can produce the most likely values of the formation constants of metal–ligand complexes if a particular set of species containing the metal ion is known or assumed to exist in solution equilibrium with complexing ligands. Identifying the most likely set of species when more than one set is plausible is a more difficult problem to address quantitatively. A Monte Carlo method of data analysis is described that measures the relative abilities of different speciation models to fit optical spectra of open-shell actinide ions. The best model(s) can be identified from among a larger group of models initially judged to be plausible. The method is demonstrated by analyzing the absorption spectra of aqueous Pu(IV) titrated with nitrate ion at constant 2 molal ionic strength in aqueous perchloric acid. The best speciation model supported by the data is shown to include three Pu(IV) species with nitrate coordination numbers 0, 1, and 2. Formation constants are β1 = 3.2 ± 0.5 and β2 = 11.2 ± 1.2, where the uncertainties are 95% confidence limits estimated by propagating raw data uncertainties using Monte Carlo methods. Principal component analysis independently indicates three Pu(IV) complexes in equilibrium.

Plutonium(IV) mononitrate and dinitrate complex formation in acid solutions as a function of ionic strength

Titrations of Pu(IV) with HNO3 in a series of aqueous HClO4 solutions ranging in ionic strength from 2 to 19 molal were followed using visible and near-infrared absorption spectrophotometry. The Pu 5f-5f spectra in the visible and near IR range change with complex formation. At each ionic strength, a series of spectra were obtained by varying nitrate concentration. Each series was deconvoluted into spectra of Pu4+ (aq), Pu(NO3)3− and Pu(NO3)22+ complexes, and simultaneously their formation constants were determined. When corrected for the incomplete dissociation of nitric acid, the ionic strength dependence of each formation constant can be described by two parameters, β0 and Δε using the formulae of specific ion interaction theory.

Performance of Fiber-Optic Raman Probes for Analysis of Gas Mixtures in Enclosures

The feasibility of using fiber-optic Raman probes to identify and quantify gases in enclosures is investigated by measuring and comparing detection thresholds using several probe and enclosure designs. Unfiltered, non-imaging, fiber-optic probes are shown to achieve lower detection thresholds than a filtered, imaging, fiberoptic probe, provided that light scattering within the sample enclosure is minimized and provided that a window is not used between the probe and the analyte gas. Achievable thresholds for hydrogen, oxygen, nitrogen, carbon monoxide, and methane in gas mixtures are demonstrated to be below 1 kPa with ten seconds signal acquisition and 0.1 kPa with twenty minutes signal acquisition with the use of 0.4 W of 532-nm excitation. Ambient carbon dioxide in air (.03 kPa) is shown to be detectable in a twenty minute acquisition, and ambient water vapor is well above the detection threshold. Background signals generated within the optical fibers remain the principal factors limiting detection thresholds. Factors affecting the magnitudes of these signals reaching the detector are investigated and discussed. A flat piece of light-absorbing colored glass tilted to direct reflected light away from the fiber-optic probe performs well as a beam stop to reduce background signal in a simple, cylindrical sample enclosure.

Near-Infrared Photoluminescence from a Plutonyl Ion

We report the first example of photoluminescence from electronically excited states of the plutonyl ion. Discrete emission transitions were measured between 6000 and 10,200 cm(-1) from crystalline Cs2U(Pu)O2Cl4 cooled to 75 K following pulsed laser excitation at 628 nm. An excitation spectrum in the region of 15,000-16,500 cm(-1) is compared with 4.2 K plane-polarized absorption spectra reported by Gorshkov and Mashirov. Analysis of excited-state lifetime data suggests multiple relaxation pathways in the electronic structure of PuO2Cl4(2-).

Raman Spectroscopic Study of the Aging and Nitration of Actinide Processing Anion-Exchange Resins in Concentrated Nitric Acid

Degradation of two types of anion exchange resins, Dowex 11 and Reillex HPQ, from the action of concentrated nitric acid (4 to 12 M) and radiolysis [from depleted uranium as UO22+ nitrate species and 239Pu as Pu(IV) nitrate species] was followed as a function of time with Raman vibrational spectroscopy. Elevated temperatures (∼ 50 °C) were used in the absence of actinide metal loading to simulate longer exposures of the resin to a HNO3 process stream and waste storage conditions. In the absence of actinide loading, only minor changes in the Dowex resin at acid concentrations ⩽10 M were observed, while at 12 M acid concentration, the emergence of a Raman peak at 1345 cm−1 indicates the addition of nitro functional groups to the resin. Similar studies with the Reillex resin show it to be more resistant to nitric acid attack at all acid concentrations. Incorporation of weakly radioactive depleted uranium as the UO22+ nitrate species to the ion-exchange sites of Dowex 11 under differing nitric acid concentrations (6 to 12 M) at room temperature showed no Raman evidence of resin degradation or nitration, even after several hundred days of contact. In contrast, Raman spectra for Dowex 11 in the presence of 239Pu as Pu(IV) nitrate species reveal numerous changes indicating resin alterations, including a new mode at 1345 cm−1 consistent with a Pu(IV)-nitrate catalyzed addition of nitro groups to the resin backbone.

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.