2009. "Experimental and Computational Studies on Collective Hydrogen Dynamics in Ammonia Borane: Incoherent Inelastic Neutron Scattering." Journal of Chemical Physics 130(2):article no. 024507. doi:10.1063/1.3042270 Abstract Incoherent inelastic neutron scattering can be used as a sensitive probe of the vibrational dynamics in chemical hydrogen storage materials. Thermal neutron energy loss measurements at 10K are presented and compared to the vibrational power spectrum calculated using ab initio molecular dynamics of pure and deuterated ammonia borane (NH3BH3, NH3BD3, and ND3BH3). A harmonic vibrational analysis on NH3BH3 clusters was also explored to check for consistency with experiment and the power spectrum. The measured neutron spectra and computed ab initio power spectrum compare extremely well (50 to 500 cm-1) and some assignment of modes to simple motion is possible, however, it is found that the lowest modes (below 250 cm-1) are dominated by collective motion. This work was supported by the U.S. Department of Energy's (DOE) Office of Basic Energy Sciences, Chemical Sciences program. The Pacific Northwest National Laboratory is operated by Battelle for DOE.
2009. "Neutron Powder Diffraction and Molecular Simulation Study of the Structural Evolution of Ammonia Borane from 15 to 340 K." Journal of Physical Chemistry A 113(9):5723-5735. doi:10.1021/jp900839c Abstract The structural behavior of perdeuterated, 11B-enriched ammonia borane, ND311BD3, was investigated by neutron powder diffraction measurements collected over the temperature range from 15 to 340 K and by molecular dynamics simulation. In the low temperature orthorhombic phase, the progressive displacement of the borane group under the amine group was observed leading to the rotation of the B-N bond parallel to the c-axis. The structural phase transition at 225 K is marked by dramatic change in the dynamics of both the amine and borane group that is problematic to extract from the metrics provided by Rietveld analysis of the NPD data alone but is evident in the molecular dynamics simulation and other spectroscopic evidence. This study highlights the valued added by complimentary experimental approaches and coupled computational studies.
2008. "Materials for Hydrogen Storage: Structure and Dynamics of Borane Ammonia Complex." Dalton Transactions (33):4514-4522. doi:10.1039/b718138h Abstract The activation energies for rotations in low temperature orthorhombic ammonia borane were analyzed and characterized in terms of electronic structure theory. The perdeuterated, 11B- enriched ammonia borane 11BD3ND3 sample was synthesized and the structure was refined from neutron powder diffraction data at 175 K. This temperature has been chosen as median of the range of previously reported NMR measurements of these rotations. A representative molecular cluster model was assembled from the refined geometry and the activation energies were calculated and characterized by analysis of the environmental factors that control the rotational dynamics. The barrier for independent NH3 rotation, Ea = 12.7 kJ/mol, largely depends on the molecular conformational torsion in the solid state geometry. The barrier for independent BH3 rotation, Ea = 38.3 kJ/mol, results from the summation of the effect of molecular torsion and large repulsive intermolecular hydrogen-hydrogen interactions. However, a barrier of Ea = 31.1 kJ/mol was calculated for rotation with preserved molecular conformation. Analysis of the barrier heights and the corresponding rotational pathways shows that rotation of the BH3 group involves strongly correlated rotation of the NH3 end of the molecule. This observation suggests that the barrier from previously reported measurement of BH3 rotation, corresponds to H3B—NH3 correlated rotation. Support for this work by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Chemical Sciences Division is gratefully acknowledged. Pacific Northwest National Laboratory is operated by Battelle for the US Department of Energy.
2008. "Thermodynamic Model for Solubility of TcO2•xH2O in Aqueous Oxalate System." Journal of Solution Chemistry 37(11):1471-1487. doi:10.1007/s10953-008-9328-5 Abstract The solubility of hydrous technetium (IV) oxide (TcO2•xH2O) was studied across a broad range of pH values extending from 1.5 to 10 and in oxalate concentrations from dilute (10-6 m) to complete saturation with respect to sodium bioxalate at lower pH values and to saturation with respect to sodium oxalate at higher pH values. The solubility was measured at very long equilibriation times (i.e. as long a 1000+ days). The thermodynamic modeling results show that the dominant species in solution must have at least one more hydroxyl moiety present in the complex (e.g. TcO(OH)Ox- versus TcOOx(aq)) than proposed by previous investigators. The inclusion of the single previously unidentified species TcO(OH)Ox- in our aqueous thermodynamic model explains a wider range of observed solubility data for TcO2•xH2O in the presence of oxalate and over a broad range of pH values. Inclusion of this species is also supported by the recently proposed thermodynamic data for the TcO(OH)+ hydrolysis species which shows an enhanced stability for this species at pH values as low as one.
2008. "Spectroscopic Studies of the Phase Transition in Ammonia Borane: Raman spectroscopy of single crystal NH3BH3 as a function of temperature from 88 to 330 K." Journal of Chemical Physics 128(3):Art. No. 034508. doi:10.1063/1.2820768 Abstract Raman spectra of single crystal ammonia borane, NH3BH3, were recorded as a function of temperature from 77 to 300 K using Raman microscopy and a variable temperature stage. The orthorhombic to orientationally disordered tetragonal phase transition at 225 K was clearly evident from the decrease in the number of vibrational modes. However some of the modes in the orthorhombic phase appeared to merge 10 to 12 K below the phase transition perhaps suggesting the presence of an intermediate phase. Factor group analysis of vibrational spectra for both orthorhombic and tetragonal phase is provided. To our knowledge this is first reported vibrational spectra in the BH and NH stretching region of single crystal NH3BH3 in the orthorhombic phase.
2008. "Quasielastic neutron scattering of -NH3 and -BH3 rotational dynamics in orthorhombic ammonia borane." Chemical Physics Letters 459(1-6):85-88. doi:10.1016/j.cplett.2008.04.130 Abstract Neutrons scattering techniques are ideally suited to directly probe H in materials due to the large incoherent scattering cross-section of hydrogen atom, and have been invaluable in providing direct insight into the local fluctuations and large amplitude motions in AB. Dihydrogen bonding may have a significant affect on materials to be used to store hydrogen for fuel-cell powered applications. We have noticed a trend of low temperature release of H2 in materials composed of hydridic and protonic hydrogen. This phenomenon has caught our attention and motivated our interest to gain more insight into dihydrogen bonding interactions in AB. We present results from a thorough Quasielastic Neutron Scattering (QENS) investigation of diffusive hydrogen motion in NH311BH3 and ND311BH3 to obtain (1) a direct measure of the rotational energy barriers the protonated species and (2) a confirmation of the 3-site jump model for rotational motion. The amplitude of the energy barrier of rotation of BH3 and NH3 determined by QENS are compared to those determined for BD3 and ND3 determined by 2H NMR studies.
2008. "The Effects of Chemical Additives on the Induction Phase in Solid-State Thermal Decomposition of Ammonia Borane." Chemistry of Materials 20(16):5332-5336. doi:10.1021/cm801253u Abstract The solid-state decomposition of ammonia borane (AB) alone and in the presence of chemical additives was investigated by a series of experimental methods to develop an approach for reducing the induction period for hydrogen release. Gas chromatography techniques were used to measure the yield of hydrogen as a function of time under isothermal conditions between 70 and 90 °C, and the polyaminoborane (PAB) products produced from hydrogen loss from AB show significant cross linking by 11B NMR spectroscopy. Raman microscopy was used to follow the transformation of crystalline AB to amorphous AB with the subsequent formation of the diammoniate of diborane (DADB). A gas burette was used to monitor the time-dependent release of hydrogen from AB in the presence of chemical additives. The combination of these approaches provides insight into the mechanism of hydrogen release from solid AB. The release of molecular hydrogen is described by a process involving sequential induction (disruption of dihydrogen bonds), nucleation (formation of DADB), and growth (hydrogen release through dehydrocoupling). Addition of DADB or ammonium chloride to neat AB significantly reduces the induction time for hydrogen release. The authors wish to acknowledge support from the U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy. This work was performed as part of the Center of Excellence in Chemical Hydrogen Storage and in collaboration with the International Partnership for the Hydrogen Economy. Pacific Northwest National Laboratory is operated for the U.S. Department of Energy by Battelle.
2008. "Molecular Structure and Dynamics in the Low Temperature (Orthorhombic) Phase of NH3BH3." Journal of Physical Chemistry A 112(18):4277-4283. doi:10.1021/jp711696 Abstract Variable temperature 2H NMR experiments on the orthorhombic phase of selectively deuterated NH3BH3 spanning the static to fast exchange limits of the borane and amine motions are reported. New values of the electric field gradient (EFG) tensor parameters have been obtained from the static 2H spectra of Vzz = 5.509(±0.275)×1014 statvolt/cm2 and ! = 0.00±0.05 for the borane hydrogens and Vzz = 9.615(±0.481)×1014 statvolt/cm2 and ! = 0.00±0.05 for the amine hydrogens. The molecular symmetry inferred from the observation of equal EFG tensors for both the boron and amine hydrogens is in sharp contrast with the Cs symmetry derived from diffraction studies. The origin of the apparent discrepancy has been investigated using molecular dynamics methods in combination with electronic structure calculations of NMR parameters, bond lengths, and bond angles. The computation of parameters from a statistical ensemble rather than from a single set of atomic Cartesian coordinates gives values that are in close quantitative agreement with the 2H NMR electric field gradient tensor measurements and are more consistent with the molecular symmetry revealed by the NMR spectra. This work was supported by the U.S. Department of Energy's (DOE) Office of Basic Energy Sciences, Chemical Sciences program. The Pacific Northwest National Laboratory is operated by Battelle for DOE.
2007. "Chromium(III) Hydroxide Solubility in the Aqueous K+-H+-OH--CO2- HCO3-- CO32--H2O System: A Thermodynamic Model." Journal of Solution Chemistry 36(10):1261-1285. doi:10.1007/s10953-007-9179-5 Abstract Chromium(III)-carbonate reactions are expected to be important in managing high-level radioactive wastes. Extensive studies on the solubility of amorphous Cr(III) hydroxide solid in a wide range of pH (3-13), at two different fixed partial pressures of CO2(gas) (0.003 or 0.03 atm.), and as functions of K2CO3 concentrations (0.01 to 5.8 m) in the presence of 0.01 M KOH and KHCO3 concentrations (0.001 to 0.826 m) at room temperature (22 ± 2°C) were carried out to obtain reliable thermodynamic data for important Cr(III)-carbonate reactions. A combination of techniques (XRD, XANES, EXAFS, UV-Vis-NIR spectroscopy, thermodynamic analyses of solubility data, and quantum mechanical calculations) was used to characterize solid and aqueous species. The Pitzer ion-interaction approach was used to interpret the solubility data. Only two aqueous species [Cr(OH) (CO3)22-) and Cr(OH)4CO33-] are required to explain Cr(III)-carbonate reactions in a wide range of pH, CO2(gas) partial pressures, and bicarbonate and carbonate concentrations. Calculations based on density functional theory support the existence of these species. The log K0 values of reactions involving these species [{Cr(OH)3(am) + 2CO2(gas) = Cr(OH)(CO3)22- + 2H+}and {Cr(OH)3(am) + OH- + CO32- = Cr(OH)4(CO3)3-}] were found to be –(19.07 0.41), -(4.19 0.19), respectively. No other data on any Cr(III)-carbonato complexes are available for comparisons.
2006. "Stability Constants of Technetium (IV) Oxalate Complexes as a Function of Ionic Strength." Radiochimica Acta 94(3):137-141. doi:10.1524/ract.2006.94.3.137 Abstract Solvent extraction methods were used to determine the stability constants of Tc(IV) with oxalate anions in NaCl solutions ranging in concentration from 0.5 M to 2.0 M. All experiments were conducted in an atmosphere-controlled chamber under Ar atmosphere (< 1.0ppm O2). A reducing agent (hydrazine) was used during extractions to maintain technetium in the tetravalent oxidation state. Independent tests confirmed that the oxidation state of technetium did not change during extractions. The distribution ratio of Tc(IV) between the organic and aqueous phases was found to decrease as the concentration of oxalic acid increased. At the oxalic acid concentrations used in these experiments, the complexes TcO(Ox) and TcO(Ox)22- were found to be the dominant aqueous species. Based on these data, the thermodynamic stability constants of Tc(IV) with oxalate complexes were calculated by the Specific Ion Interaction Theory (SIT).
2006. "The Aqueous Complexation of Thorium with Citrate under Neutral to Basic Conditions." Radiochimica Acta 94(4):205-212. doi:DOI 10.1524/ract.2006.94.4.205 Abstract The aqueous complexation of thorium with citrate was investigated under neutral to basic conditions and over a broad range of ionic strengths. The solubility data for ThO2(am) as a function of citrate concentration indicate the presence of stable species with citrate-to-metal ratios of between two to three. The dependence of the ThO2(am) solubilities on hydrogen ion concentration can also be readily explained by the classical assumption of hydrolysis of the central Th(IV) ion to form mixed thorium-hydroxide-citrate complexes. 13C NMR spectra of the species in solution confirm that the citrate-to-metal ratio of the species in solution is between two and three and show that the citrate attaches to the metal in a bidentate fashion through oxygens on the -carboxylate and -alkoxyl groups, rather than through the carboxylate groups. The 13C NMR spectra, as well as a density functional theory (DFT) electronic structure study of the presumptive complexes, suggests that the associated α-hydroxyl proton can be displaced during complex formation. These findings indicate an alternative explanation for the observed changes in solubility as a function of hydrogen ion concentration, the displacement of protons from the citrate alkoxyl groups via metal binding. Removal of protons from the alkoxyl groups or hydrolysis of the central Th(IV) cannot be distinguished by thermodynamic measurements, however the species with the α-hydroxyl proton removed (i.e., ThOH(Cit)25- and Th(Cit)38-) would appear to better represent the microscopic binding. Apparent equilibrium constants for the solution phase reactions of these species and the hydrous thorium oxide have been calculated as a function of ionic strength.
2006. "Chemical Speciation of Heterogeneously Reduced Pu in Synthetic Brines ." Radiochimica Acta 94(5):249-259. Abstract 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 5M 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 •zH2O, 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.
2006. "Dynamics of Ammonia Borane Using Neutron Scattering." Physica B Condensed Matter 385(Pt.1 ):266-268. Abstract We have used both the backscattering (HFBS) and time-of-flight (DCS) neutron spectrometers to investigate the proton dynamics in ammonia borane, a compound of intense interest as a model for 'chemical hydrogen storage' materials. Results indicate that the deposition of ammonia borane on a mesoporous silicate results in longer proton residence times and lower energy barriers for proton motion compared to bulk ammonia borane. The reduced activation energy for proton motions may partly explain the improved thermolysis and lowering the activation barrier for the loss of the first equivalent of H2. In addition, the phonon density of states for neat ammonia borane compares well with other spectroscopic results, with the intense peak at 22 meV assigned to the librational NH3 and BH3 modes, whereas ammonia borane on MCM-41 displays a broad, featureless spectrum indicating a poorly crystalline material.
2005. "Bond-valence Sums for Tc—O Systems from EXAFS Data." Inorganica Chimica Acta 358(4):865-874. Abstract Literature data for structures containing exclusively Tc—O bonds were used to calculate unit-valence parameters R0 for Tc(VII), Tc(VI), Tc(V) (six- and five-coordinate), Tc(IV), and Tc(III). A second method of estimating R0 was developed to validate the calculated values for these oxidation states because crystallographic data are limited. The method was first tested and shown to be valid using literature data for Cr, Mn, Fe, and Co complexes. The validated R0 values for Tc were used to calculate bond-valence sums (BVS) for Tc solids and aqueous solutions using EXAFS data for the bond distances and coordination numbers. The calculated BVS showed good agreement with the expected values for the assumed Tc oxidation states.
2005. "Thorium Reactions in Borosilicate-Glass/Water Systems." Radiochimica Acta 93(8):443-455. doi:10.1524/ract.2005.93.8.443 Abstract SUMMARY-Studies were conducted on the dissolution of Th-doped borosilicate glass, a complex assemblage of 24 different elements in various proportions, in a wide range of pH values (0.5 to 12) and carbonate (as high as 6.2 m) and bicarbonate (as high as 1.0 m) concentrations, and as a function of time to determine whether the dissolution exhibits an equilibrium phenomenon and to ascertain whether existing thermodynamic data can be used to interpret these results. Measurable Th concentrations were observed in either the very acidic (pH 0.5 to 4) solutions or alkaline solutions containing relatively high carbonate/bicarbonate concentrations. Steady state Th concentrations were reached over time from both the oversaturation and undersaturation directions, indicating that Th concentrations are controlled by an equilibrium solubility phenomenon...
2005. "Solubility of (UO2)3(PO4)2.4H2O in H+-Na+-OH--H2PO-4-HPO2-4-PO3-4-H2O and its Comparison to the Analogous PuO2+2 System." Journal of Solution Chemistry 34(4):469-498. Abstract ABSTRACT-The objectives of this study were to address uncertainties in the solubility product of (UO2)3(PO4)24H2O(c) and in the phosphate complexes of U(VI), and more importantly to develop needed thermodynamic data for the Pu(VI)-phosphate system in order to ascertain the extent to which U(VI) and Pu(VI) behave in an analogous fashion. Thus studies were conducted on (UO2)3(PO4)24H2O(c) and (PuO2)3(PO4)24H2O(lc) solubilities for long equilibration periods (up to 870 days) in a wide range of pH values (2.5 to 10.5) at fixed phosphate concentrations of 0.001 and 0.01 M, and in a range of phosphate concentrations (0.0001 to 1.0 M) at fixed pH values of about 3.5. A combination of techniques (XRD, DTA/TG, XAS, and thermodynamic analyses) was used to characterize the reaction products...
2005. "Charge Distribution and Local Structure and Speciation in the UO2+x and PuO2+x Binary Oxides for x <= 0.25." Journal of Solid State Chemistry 178(2):521-535. Abstract The local structure and chemical speciation of the mixed valence, fluorite-based oxides UO2+x (0.00pxp0.20) and PuO2+x/PuO2+x*y(OH)2y * zH2O have been determined by U/Pu LIII XAFS spectroscopy. The U spectra indicate (1) that the O atoms are incorporated as oxo groups at short (1.75A ˚ ) U–O distances consistent with U(VI) concomitant with a large range of U displacements that reduce the apparent number of U neighbors and (2) that the UO2 fraction remains intact implying that these O defects interact to form clusters and give the heterogeneous structure consistent with the diffraction patterns. The PuO2+x system, which does not show a separate phase at its x ¼ 0:25 endpoint, also displays (1) oxo groups at longer 1.9A ˚ distances consistent with Pu(V+d), (2) a multisite Pu–O distribution even when x is close to zero indicative of the formation of stable species with H2O and its hydrolysis products with O2*, and (3) a highly disordered, spectroscopically invisible Pu–Pu component. The structure and bonding in AnO2+x are therefore more complicated than have previously been assumed and show both similarities but also distinct differences among the different elements.
2004. "Technetium Reduction in Sediments of a Shallow Aquifer Exhibiting Dissimilatory Iron Reduction Potential ." FEMS Microbiology Ecology 49(1):151-162. Abstract Pertechnetate ion [Tc(VII)O4-] reduction rate was determined in core samples from a shallow sandy aquifer located on the U.S. Atlantic Coastal Plain. The aquifer is generally low in dissolved O2 (<1 mg L-1) and composed of weakly indurated late Pleistocene sediments differing markedly in physicochemical properties. Thermodynamic calculations, x-ray absorption spectroscopy and statistical analyses were used to establish the dominant reduction mechanisms, constraints on Tc solubility, and the oxidation state, and speciation of sediment reduction products. The extent of Tc(VII) reduction differed markedly between sediments (ranging from 0 % to 100% after 10 days of equilibration), with low solubility Tc(IV) hydrous oxide the major solid phase reduction product. The dominant electron donor in the sediments proved to be (0.5M HCl extractable) Fe(II). Sediment Fe(II)/Tc(VII) concentrations >4.3 were generally sufficient for complete reduction of Tc(VII) added [1-2.5 mmol (dry wt. sediment) g-1]. At these Fe(II) concentrations, the Tc (VII) reduction rate exceeded that observed previously for Fe(II)-mediated reduction on isolated solids of geologic or biogenic origin, suggesting that sediment Fe(II) was either more reactive and/or that electron shuttles played a role in sediment Tc(VII) reduction processes. In buried peats, Fe(II) in excess did not result in complete Tc(VII) reduction, perhaps because organic complexation of Tc(IV) limited formation of the Tc(IV) hydrous oxide. In some sands exhibiting Fe(II)/Tc(VII) concentrations <1.1, there was presumptive evidence for direct enzymatic reduction of Tc(VII). Addition of organic electron donors (acetate, lactate) resulted in microbial reduction of (up to 35%) Fe(III) and corresponding increases in extractable Fe(II) in sands that exhibited lowest initial Tc(VII) reduction and highest hydraulic conductivities, suggesting that accelerated microbial reduction of Fe(III) could offer a viable means of attenuating mobile Tc(VII) in this type of sediment system.
2004. "Thermodynamics and solubility of (UxNp1-x) O2(am) solid solution in the carbonate system." Radiochimica Acta 92(9-11):527-535. doi:10.1524/ract.92.9.527.54995 Abstract SUMMARY-The formation of a solid solution can significantly affect the solubility of a minor component. The objectives of this study were to determine the nature of U(IV) and Np(IV) solid solutions and their thermodynamic properties. For this purpose...
2004. "Chromium (III) Hydroxide Solubility in the Aqueous Na+-OH-- H2PO4-- HPO42--PO43--H2O System: A Thermodynamic Model." Journal of Solution Chemistry 33(10):1213-1242. Abstract ABSTRACT-Chromium(III)-phosphate reactions are expected to be important in managing high-level radioactive wastes stored in tanks at many DOE sites. Extensive studies on the solubility of amorphous Cr(III) solids in a wide range of pH (2.8 to 14) and phosphate concentrations (10-4 to 1.0 m) were carried out to obtain reliable thermodynamic data for important Cr(III)-phosphate reactions. A combination of techniques (XRD, XANES, EXAFS, Raman spectroscopy, total chemical composition, and thermodynamic analyses of solubility data) was used to characterize solid and aqueous species. Contrary to the data recently reported in the literature(1), only a limited number of aqueous species [Cr(OH)3H2PO4-, Cr(OH)3 (H2PO4)22-), and Cr(OH)3HPO42-] with up to about four orders of magnitude lower values for the formation constants of these species are required to explain Cr(III)-phosphate reactions in a wide range of pH and phosphate concentrations. The log K0 values of reactions involving these species [Cr(OH)3(aq) + H2PO4- Cr(OH)3H2PO4-; Cr(OH)3(aq) + 2H2PO4- Cr(OH)3(H2PO4)22-; Cr(OH)3(aq) + HPO42- Cr(OH)3HPO42-] were found to be 2.78 0.3, 3.48 0.3, and 1.97 0.3, respectively.
2004. "Thermodynamic Model for the Solubility of TcO2•xH2O(am) in the Aqueous Tc(IV) - Na+ - Cl- - H+ - OH- - H2O System." Journal of Solution Chemistry 33(2):199-226. Abstract Solubility studies of TcO2•xH2O(am) have been conducted as a function of H+ concentration from 1 x 10-5 to 6 M HCl and as function of chloride concentration from 1 x 10-3 to 5 M NaCl. These experiments were conducted under carefully controlled reducing conditions such that the preponderance of Tc is present in solution is in the reduced oxidation state and was determined to be Tc(IV) by XANES analysis. The aqueous species and solid phases were characterized using a combination of techniques including thermodynamic analyses of solubility data, XRD, and XANES, EXAFS, and UV-Vis spectroscopies. Chloride was found to significantly affect Tc(IV) concentrations through 1) the formation of Tc(IV) chloro complexes [i.e., TcCl4(aq) and TcCl62-] and a stable compound [data suggests this compound to be TcCl4(am)] in highly acidic and relatively concentrated chloride solutions, and 2) its interactions with the positively charged hydrolyzed Tc(IV) species in solutions of relatively low acidity and high chloride concentrations. A thermodynamic model was developed, which included hitherto unavailable chemical potentials of Tc(IV)-chloro species and Pitzer ion-interaction parameters for Tc(IV) hydrolyzed species with bulk electrolyte ions used in this study. The thermodynamic model presented in this paper is consistent with the extensive data reported in this study and with the reliable literature data, and is applicable to a wide range in H+ and Cl- concentrations and ionic strengths.
2004. "Local and Nanoscale Structure and Speciation in the PuO2+ x- y(OH)2 y • zH2O System." Journal of the American Chemical Society 126(41):13443-13458. Abstract Pu L3 X-ray absorption fine structure spectra from 24 samples of PuO2+ x (and two related Pusubstituted oxides), prepared by a variety of methods, demonstrate that (1) although the Pu sublattice remains the ordered part of the Pu distribution, the nearest-neighbor O atoms even at x) 0 are found in a multisite distribution with Pu-O distances consistent with the stable incorporation of OH- (and possibly H2O and H+) into the PuO2 lattice; (2) the excess O from oxidation is found at Pu-O distances <1.9 Å, consistent with the multiply bound “oxo”-type ligands found in molecular complexes of Pu(V) and Pu(VI); (3) the Pu associated with these oxo groups is most likely Pu(V), so that the excess O probably occurs as PuO2 + moieties that are aperiodically distributed through the lattice; and (4) the collective interactions between these defect sites most likely cause them to cluster so as give nanoscale heterogeneity in the form of domains that may have unusual reactivity, observed as sequential oxidation by H2O at ambient conditions. The most accurate description of PuO2 is therefore actually PuO2+ x- y(OH)2 yâ zH2O, with pure, ordered, homogeneous PuO2 attained only when H2O is rigorously excluded and the O activity is relatively low.
2003. "Thermodynamic model for the solubility of ThO2 (am) in the aqueous Na+ - H+ -OH- -NO3- -H2O-EDTA System." Radiochimica Acta 91(12):751-760. Abstract Thermodynamic model for the solubility of ThO2 (am) in the aqueous Na+ - H+ - OH- - NO3- - H2O-EDTA System
2003. "Comprehensive Thermodynamic Model Applicable to Highly Acidic to Basic Conditions for Isosaccharinate Reactions with Ca(II) and Np(IV)." Journal of Solution Chemistry 32(8):665-689. Abstract ABSTRACT-Isosaccharinate (ISA-) is expected to be one of the important ligands in low-level nuclear wastes. Comprehensive thermodynamic data for complexation reactions of ISA- with any of the tetravalent actinides have not been available.
2003. "Acidity and Structure of Isosaccharinate in Aqueous Solution: A Nuclear Magnetic Resonance Study." Journal of Solution Chemistry 32(8):691-702. Abstract Dilute aqueous solutions of the calcium and sodium salts of -D-isosaccharinate (ISA) have been analyzed by 13C and 1H NMR spectroscopy. The positions of the six 13C ISA NMR lines show a pH dependence that can be used to infer that the most acidic proton is the one associated with the carboxylate group, with a log k0 of -3.27 - -3.36 for the reaction H(ISA) H+ K ISA-.
2002. "Thermodynamics of the U(VI)-Ca2+-Cl- -OH- -H2O System: Solubility Product of Becquerelite." Radiochimica Acta 90:495-503. Abstract Summary-the solubility of synthetic becquerelite (Ca(UO2)6O4(OH)6.8H2O) was determined in 0.02, 0.1, and 0.5 M kCaCl2 solutions and at pCH+ values ranging from about 4 to 11...
2002. "Thermodynamic Model for the Solubility of Cr(OH)(3)(am) in Concentrated NaOH and NaOH-NaNO3 Solutions." Journal of Solution Chemistry 31(5):343-367. Abstract ABSTRACT-The objectives of this study were to develop a reliable thermodynamic model for predicting Cr(III) behavior in concentrated NaOH and in mixed NaOH-NaNO3 solutions for application to effective caustic leaching strategies for high-level tank sludges. To meet these objectives, the solubility of Cr(OH)3(am) was measured in 0.003 to 10.5 m NaOH, 3.0 m es in NaOH concentration...
2002. "Spectroscopic Investigations of the Structural Phase Transition in Gd2(Ti1-yZry)(2)O-7 Pyrochlores ." Journal of Physical Chemistry B 106(18):4663-4677. Abstract The Gd2(Ti1-yZry)2O7 pyrochlore series undergoes a structural phase transition from pyrochlore (Fd?3m) to defect fluorite (Fm?3m) that can be driven compositionally by increasing the Zr content or thermally by sintering Zr-rich compositions at temperatures above 1550 ?C. Our results demonstrate that ion-beam irradiation can also drive the structural phase transition for Zr-rich compositions. In an effort to understand the effects of composition and ion-beam irradiation on this phase transition, powder X-ray diffraction, polarized Raman, reflection infrared, and X-ray absorption spectroscopy experiments were conducted on Gd2(Ti1-yZry)2O7 pyrochlores prior to and following irradiation with 2 MeV Au2+ ions to a fluence of 5 ions/nm2. Analysis of the vibrational and X-ray absorption data suggests that the structural integrity of the pyrochlore structure is based on distorted corner-shared TiO6 or ZrO6 octahedra. The vibrational spectra indicate that both anion and cation disorder precede the compositionally-driven phase transition, while cation disorder appears to dominate the irradiation-driven transition. Analyses of the extended X-ray absorption fine structure of the Ti and Zr K-edges and the Gd LIII-edges reveal a significant change in the Gd local environment upon irradiation and with increasing Zr content. The Ti and Zr local environments are less affected by irradiation or compositional change, but show evidence of increasing disorder that can be attributed to rotations about shared polyhedral edges and corners.
2001. "Thermodynamic Model for the Solubility of PuO2(am) in the Aqueous Na+-H+-OH--Cl--H2O- ehtylenediaminetetraacetate System." Radiochimica Acta 89:67-74. Abstract The solubility of PuO2(am) was determined in the aqueous Na+-H+-OH--Cl--H2O-ethylenediaminetetraacetate system as a function of pH (1 to 13.5), ethylenediaminetetraacetate (EDTA) concentrations (0.00001 to 0.093 M), and time to obtain reliable values of Pu-EDTA complexes important in the pH range that corresponds to environmental conditions. The aqueous Pu oxidation states were determined using a combination of techniques involving laser photoacoustic and x-ray absorption near edge spectroscopy and solvent extraction. The equilibrium/steady-state Pu(IV) concentrations were reached within a few days. The aqueous Pu(IV) concentrations in equilibrium with PuO2(am) at fixed concentrations of EDTA (0.0001 or 0.001 M) as a function of pH, at fixed pH (~9), and as a function of EDTA were interpreted using a the Pitzer ion-interaction model. The model was developed based on a segment of the experimental data and was validated against extensive data not used in developing the model. The log K0 values for the formation of PuOHEDTA-, Pu(OH)2EDTA-2, and Pu(OH)3EDTA-3 according to the generalized reaction Pu4+ + xOH- +EDTA4+ = Pu(OH)xEDTA-x were determined to be 36.39, 44.74, and 49.64, respectively.
2001. "Hydroxo and Chloro Complexes/Ion-Interactions of Hf4+ and the Solubility Product of HfO2(am)." Journal of Solution Chemistry 30(11):949-967. Abstract SUMMARY-The solubility of HfO2(am) was determined at different equilibration periods from the over- and undersaturation directions, in very acidic to basic solutions (0.1 m HCl to 3.2 m NaOH), and in NaCl solutions ranging in concentrations from very dilute to as high as 5.59 m and in a pCH+ range from 1 to 4 to obtain reliable thermodynamic data for the Hf4+-Cl- -Na+-H+-OH- -H2O system...
2001. "Evidence of Edge-Sharing TiO5 Polyhedra in Ti-Substituted Pollucite, CsTixAl1-xSi2O6+x/2 ." Journal of Physical Chemistry B 105(29):6805-6811. Abstract The compositional series CsTixAl1-xSi2O6+x/2, 0=x=1, was investigated using x-ray absorption and Raman spectroscopy. The data indicate the formation of TiO5 edge-sharing polyhedra at relatively low Ti concentrations (x=0.3)...
2001. "Heavy-Ion Irradiation Effects on Structures and Acid Dissolution of Pyrochlores." Journal of Nuclear Materials 288(2-3):208-216. Abstract The temperature dependence of the critical dose for amorphization, using 0.6 MeV Bi+ ions, for A₂Ti₂O₇ pyrochlores, in which A=Y, Sm, Gd and Lu, exhibits no significant effect of A-site ion mass or size. The room temperature dose for amorphization was found to be ~ 0.18 dpa in each case. After irradiation with 2 MeV Au₂⁺ ions glancing-incidence XRD revealed that each pyrochlore underwent an irradiation-induced structural transformation to fluorite in conjunction with amorphization. The effect of amorphization on the dissolution rates of fully dense pyrochlores, at 90oC and pH 2 (nitric acid) varied from a factor of 10-15 increase for Gd₂Ti₂O₇ to none for Y₂Ti₂O₇. Significant differences were observed in the A-site dissolution rates from the crystalline pyrochlores, indicating differences in the manner in which the A-site cations are incorporated into the pyrochlore structure. These indications were supported by Raman spectroscopy.
2001. "Heavy-Ion Irradiation Effects in Gd2(Ti2-xZrx)O7 Pyrochlores." Journal of Nuclear Materials 289(1-2):188-193. Abstract Gd2(Ti2-xZrx)O7 samples with 0 = x = 1.5 were single-phase and pyrochlore structured after sintering at 1600oC in air. The Gd2Zr2O7 (x=2) end member was predominantly fluorite-structured. Raman spectroscopy indicated that the level of short-range fluorite-like disorder in the unirradiated Gd2(Ti2-xZrx)O7 samples increased significantly as Zr was substituted for Ti, despite the retention of a long-range pyrochlore structure for samples with 0 = x = 1.5. Glancing-incidence X-ray diffraction indicated that pyrochlores with an ionic radii ratio rA/rB = 1.52 (x=1.5) were transformed into a radiation resistant fluorite-structure after irradiation at room temperature with 2 MeV Au2+ to a fluence of 5 ions/nm2. As the ionic radii ratio of the pyrochlore increased beyond rA/rB > 1.52, the fluorite structure became increasingly unstable with respect to the amorphous state under identical irradiation conditions.
2000. "Computer Simulation of Pu3+ and Pu4+ Substitutions in Zircon." Journal of Nuclear Materials 278(2-3):207-211. Abstract Energy minimization methods were used in atomistic computer simulations to determine the energetics of Pu(3+) and Pu(4+) substitutions and interstitials in zircon, including the effect of ion size. The lowest energy was found for Pu(4+) on Zr sites. The lowest energy for Pu(3+) substitutions was found for the defect cluster 2Pu(3+) on Zr sites plus an oxygen vacancy. Mean field calculations of unit-cell volumes for 8% Pu substitutions were in agreement with XRD data.
2000. "Effect of Electron Donor and Solution Chemistry on the Products of Dissimilatory Reduction of Technetium by Shewanella putrefaciens." Applied and Environmental Microbiology 66(6):2451-2460. Abstract To help provide a fundamental basis for use of microbial dissimilatory reduction processes in separation or immobilizing Tc-99 in waste or groundwaters.
2000. "Beta Radiation Effects in 137Cs-Substituted Pollucite." Journal of Nuclear Materials 281(1):22-33. Abstract The effect of high energy beta radiation on the long-range and local structure of 137 Cs-substituted CsAlSi2O6 (pollucite) was studied with X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) experiments at the Cs K-edge...
2000. "Spectroscopic Studies of Zinc (II) and Cobalt (II) Associated Escherichia Coli Formamidopyrimidine-DNA Glycosylase: Extended X-Ray Absorption Fine Structure Evidence for a Metal-Binding Domain." Biochemistry 39:12441-12449. Abstract Formamidopyrimidine-DNA glycosylase (Fpg) is a 30.2 kDa protein that plays an important role in the base excision repair of oxidatively damaged DNA in Escherichia coli.
2000. "Human Nucleotide Excision Repair Protein XPA: Summary of EXAFS Studies On the Zn (II), Co (II), and Cd (II) Associated Minimal DNA-Binding Domain." Protein and Peptide Letters 7(1):49-56. Abstract N/A
2000. "Cadmium Mutagenicity and Human Nucleotide Excision Repair Protein XPA: CD, EXAFS, and 1H/15N NMR Spectroscopic Studies on the Zinc (II) and Cadmium (II) Associated Minimal DNA-Binding Domain." Carcinogenesis 21(5):1051-1057.
2000. "XAS and XRD Study of Annealed Pu-238- and Pu-239-Substituted Zircons (Zr0.92Pu0.08SiO4)." Journal of Nuclear Materials 278(2-3):212-224.
1999. "A Thermodynamic Model for the Solubility of PuO2(am) in the Aqueous K+-HCO3- -CO32- -OH- -H2O System." Radiochimica Acta 86:89-99. Abstract Text Processing File: 981B J Pu Carbonate SUMMARY The solubility of PuO2(am) was determined in the aqueous K+-HCO3- -CO32- -OH- -H2O system extending to high concentrations of carbonate and bicarbonate.
1999. "A Thermodynamic Model for the Solubility of NpO2(am) in the Aqueous K+-HCO3--CO32--OH--H2O System." Radiochimica Acta 84:159-169. Abstract Text Processing File: 851b-1 Np(IV) Carbonate
1999. "Extended X-ray Absorption Fine Structure Evidence for a Single Metal Binding Domain in Xenopus Laevis Nucleotide Excision Repair Protein XPA." Biochemical and Biophysical Research Communications 254(1):109-113. Abstract There is no abstract currently available for this item
1998. "Structure and Properties of Ion-Beam-Modified (6H) Silicon Carbide." Materials Science and Engineering. A. Structural Materials: Properties, Microstructure and Processing 253(1-2):62-70.
1998. "A Thermodynamic Model for the Solubility of UO2(am) in the Aqueous K+-Na+-HCO3--CO32--OH--H2O System." Radiochimica Acta 82:17-25. Abstract SUMMARY Solubility data for UO2(am) were determined in the aqueous K+-Na+-HCO3--CO23--OH--H2O system extending to high concentrations of carbonate, bicarbonate, and mixed carbonate-hydroxide.
1998. "U and Pu L(III) XAFS of Pu-Doped Glass and Ceramic Waste Forms." Journal of Alloys and Compounds 271-273:240-243.