2009. "Uranium Extraction From Laboratory Synthesized, Uranium-Doped Hydrous Ferric Oxides ." Environmental Science & Technology 43:2341-2347. Abstract The extractability of uranium (U) from synthetic hydrous ferric oxides has been shown to decrease as a function of mineral ripening, consistent with the hypothesis that the ripening process decrease contaminant lability. To evaluate this process, three hydrous ferric oxide (HFO) suspensions were co-precipitated with uranyl (UO22+) and maintained at pH 7.0 ± 0.1. Uranyl was added to the HFO post-precipitation in a fourth suspension. Two suspensions also contained either co-precipitated silicate (Si-U-HFO) or phosphate (P-U-HFO). After precipitation of the HFOs, at time intervals of one week, one month, six months, one year, and 2 years, aliquots of the suspensions were contacted with five solutions for a range of time. The extracts were analyzed for U and iron (Fe). The results are consistent with the hypothesis that U and Fe extractability will decrease as the mineral phase ripens. All extracting solutions exhibited some degree of selectivity for U, as the proportional extraction of U exceeded that for congruent dissolution. Micro X-ray diffraction analysis indicates the transformation from an amorphous phase to a material containing substantial proportions of crystalline goethite and hematite, except the P-U-HFO which remained primarily amorphous. Further analysis of the co-precipitates by the Mössbauer technique and scanning electron microscopy (SEM) provides further evidence of mineralogic ripening
2009. "Physical control on CCl4 and CHCl3 desorption from artificially contaminated and aged sediments with supercritical carbon dioxide ." Chemosphere 74(4):494-500. Abstract The long-term interactions of carbon tetrachloride (CCl4) and chloroform (CHCl3) with sediments that are low in organic matter (OM) are not well studied and documented in the literature. In this study, CCl4 and CHCl3 were mixed with supercritical carbon dioxide (CO2) and loaded onto columns packed with two sediments with low OM content and different textures, to establish contamination and achieve expedited artificial aging. The columns were subsequently leached with a simulated groundwater under hydraulically saturated conditions. Scanning electron microscopy was used to inspect the morphology of sediment single particles, determine the degree of particle association in aggregates and qualitatively estimate porosity and the length of possible diffusional pathways that might affect the overall contaminant desorption rates in sediments with low sorption capacity. Results demonstrated that most of contaminant inventories were rapidly released in the first pore volume of effluent, although a small portion of contaminants’ total mass exhibited time-dependent desorption. The calculated Kd values of CCl4 or CHCl3 partition were negligibly small. The transport behavior of both contaminants was similar and it was simulated well with a distributed (multiple)-rate (DR) statistical model, which accounted for the apparent contaminant mass transfer through diffusional pathways of different lengths, towards the advective pores. The distribution of contaminant mass between equilibrium and kinetic fractions, the distribution of the individual rate constants, and the average rate constants calculated with the parameters of the γ-distribution function (β and η) of the DR model, were sediments (texture) dependent, indicating that contaminant desorption during the late stage of leaching was driven by concentration gradients (i.e., diffusion) within sediment matrix porosity.
2009. "Mineralogical transformations controlling acid mine drainage chemistry." Chemical Geology 262(3-4):169-178. Abstract The role of Fe(III) minerals in controlling acid mine drainage (AMD) chemistry was studied using samples from two AMD sites [Gum Boot (GB) and Fridays-2 (FR)] located in northern Pennsylvania. Chemical extractions, X-ray diffraction (XRD), scanning electron microscopy (SEM), and Fourier transform infrared spectroscopy (FTIR) were used to identify and characterize Fe(III) phases. The mineralogical analysis revealed that schwertmannite and goethite were the principal Fe(III) phases in the sediments. Schwertmannite transformation occurred at the GB site where poorly-crystallized goethite rich in surface-bound sulfate was initially formed. In contrast, no schwertmannite transformation occurred at the FR site. The goethite in GB sediments had spherical morphology due to preservation of schwertmannite structure by adsorbed sulfate. Results of chemical extractions showed that poorly-crystallized goethite was subject to further crystallization accompanied by sulfate desorption. Changes in sulfate speciation preceded its desorption, with a conversion of bidentate- to monodentate-bound sulfate surface complexes. Laboratory sediment incubation experiments were conducted to evaluate the effect of mineral transformation on water chemistry. Incubation experiments were carried out with schwertmannite-containing sediments and AMD waters with different pH and chemical composition. The pH decreased to 1.9-2.2 in all suspensions and the concentrations of dissolved Fe and S increased significantly. Regardless of differences in the initial water composition, pH, Fe and S were similar in suspensions of the same sediment. XRD measurements revealed that schwertmannite transformed into goethite in GB and FR sediments during laboratory incubation. The incubation experiment demonstrated that schwertmannite transformation controlled AMD water chemistry during “closed system” laboratory contact.
2009. "Promotion of Hydrogen Release from Ammonia Borane with Mechanically Activated Hexagonal Boron Nitride." Journal of Physical Chemistry C 113(3):1098-1103. doi:10.1021/jp8087385 Abstract We present the activation of hydrogen release from AB by the presence of mechanically activated h–BN. Hydrogen is released at lower temperatures and the dehydrogenation is less exothermic than neat AB. This approach provides all the benefits of a light-weight scaffold without additional foreign contamination that might complicate AB recycling. Several beneficial effects that pertain to the hydrogen desorption properties of the mixtures of AB:nano-BN are notable, such as the decrease of hydrogen desorption temperature, the decrease in NH3 formation as well as the decrease of the exothermicity of hydrogen desorption with increasing the nano-BN concentration. The lower exothermicity of H2 release from AB in the mesoporous silica and the nano-BN may be due physical and chemical interactions between the PAB oligomers and the interface of the silica scaffold or BN support. Pacific Northwest National Laboratory is operated by Battelle for the US Department of Energy.
2009. "Physicochemical Characterization of Capstone Depleted Uranium Aerosols III: Morphologic and Chemical Oxide Analyses." Health Physics 93(3):276-291. doi:10.1097/01.HP.0000298229.60229.10 Abstract The impact of depleted uranium (DU) penetrators against an armored target causes erosion and fragmentation of the penetrators, the extent of which is dependent on the thickness and material composition of the target. Vigorous oxidation of the DU particles and fragments creates an aerosol of DU oxide particles and DU particle agglomerations combined with target materials. Aerosols from the Capstone DU aerosol study, in which vehicles were perforated by DU penetrators, were evaluated for their oxidation states using X-ray diffraction (XRD) and particle morphologies using scanning electron microscopy/energy dispersive spectrometry (SEM/EDS). The oxidation state of a DU aerosol is important as it offers a clue to its solubility in lung fluids. The XRD analysis showed that the aerosols evaluated were a combination primarily of U3O8 (insoluble) and UO3 (relatively more soluble) phases, though intermediate phases resembling U4O9 and other oxides were prominent in some samples. Analysis of particle residues in the micrometer-size range by SEM/EDS provided microstructural information such as phase composition and distribution, fracture morphology, size distribution, and material homogeneity. Observations from SEM analysis show a wide variability in the shapes of the DU particles. Some of the larger particles appear to have been fractured (perhaps as a result of abrasion and comminution); others were spherical, occasionally with dendritic or lobed surface structures. Amorphous conglomerates containing metals other than uranium were also common, especially with the smallest particle sizes. A few samples seemed to contain small chunks of nearly pure uranium metal, which were verified by EDS to have a higher uranium content exceeding that expected for uranium oxides. Results of the XRD and SEM/EDS analyses were used in other studies described in this issue of The Journal of Health Physics to interpret the results of lung solubility studies and in selecting input parameters for dose assessments.
2008. "Hydrothermal Syntheses of Colloidal Carbon Spheres from Cyclodextrins." Journal of Physical Chemistry C 112(37):14236-14240. doi:10.1021/jp801343y Abstract Colloidal carbon spheres have been prepared from aqueous alpha-, beta-, and gamma-cyclodextrin (CD) solutions in closed systems under hydrothermal conditions at 160 oC. Both liquid and solid-state 13C NMR spectra taken for samples at different reaction times have been used to monitor the dehydration and carbonization pathways. CD slowly hydrolyzes to glucose and forms 5-hydroxymethyl furfural (HMF) followed by carbonization into colloidal carbon spheres. The isolated carbon spheres are 70-150 nm in diameter, exhibit a core-shell structure, and are comprised of a condensed core (C=C) peppered with resident chemical functionalities including carboxylate and hydroxyl groups. Evidence from 13C solid-state NMR and FT-IR spectra reveal that the evolving carbon spheres show a gradual increase in the amount of aromatic carbon as a function of reaction time and that the carbon spheres generated from gamma-CD contain significantly higher aromatic carbon than those derived from alpha- and beta-CD.
2008. "Facile stabilization of gold-silver alloy nanoparticles on cellulose nanocrystal." Journal of Physical Chemistry C 112(13):4844-4848. doi:10.1021/jp710767w Abstract Uniform Au-Ag alloy nanoparticles have been synthesized on cellulose nanocrystal (CNXL) by co-reduction method of corresponding metal ions. CNXL that plays a dual role of a matrix and of a stabilizer has been used to obtain stable dispersions of alloy nanoparticles. The composition of alloy nanoparticles indicates quantitative deposit of metal ions on CNXL surface followed by reduction. The sizes of alloy nanoparticles were controlled in the range of 3-7 nm by capping agent, sodium citrate, and their average diameter was increased with an increase of Ag content. Aqueous suspensions of Au-Ag alloy nanoparticles and their dried films are characterized by UV-Vis spectroscopy, Field Emission-Scanning Electron Microscope (FE-SEM), Transmission Electron Microscope (TEM), and X-ray Diffraction (XRD).
2007. "Studying Cellulose Fiber Structure by SEM, XRD, NMR and Acid Hydrolysis." Carbohydrate Polymers 68(2):235-241. doi:10.1016/j.carbpol.2006.12.013 Abstract Cotton linters were partially hydrolyzed in dilute acid and the morphology of remaining macrofibrils studied with Scanning Electron Microscopy (SEM) under various magnifications. The crystal region (microfibril bundles) in the macrofibrils was not altered by hydrolysis, and only amorphous cellulose was hydrolyzed and leached out from the macrofibrils. The diameter of microfibril bundles was 20-30 nm after the amorphous cellulose was removed by hydrolysis. XRD experiments confirm the unaltered diameter of the microfibrils after hydrolysis. The strong stability of these microfibril bundles in hydrolysis limits both the total sugar monomer yield and the size of nano particles or rods produced in hydrolysis. The large surface potential on the remaining microfibril bundles drives the agglomeration of macrofibrils.
2007. "Hydrothermal Dehydration of Aqueous Fructose Solutions in a Closed System ." Journal of Physical Chemistry C 111(42):15141-15145. doi:10.1021/jp074188l Abstract The synthesis of materials with targeted size and shape has attracted much attention. Specifically, colloidal spheres with targeted and uniform sizes have opened the door for a variety of applications associated with drug delivery, and manipulation of light (photonic band-gap crystals). Surface modification is a key to realizing many of these applications owing to the inherent inert surface.The remarkable transformation of carbohydrate molecules including sugars to homogeneous carbon spheres is found to readily occur by a dehydration mechanism and subsequent sequestering in aqueous solutions that are heated at 160-180oC in a pressurized vessel. Under such conditions, these molecules actually dehydrate even though they are dissolved in water. Size-tunable metal and metal oxides with uniform shells have also been prepared by using carbon spheres as templates.
2007. "Experimental Limitations Regarding the Formation and Characterization of Uranium-Mineral Phases in Concrete Waste Forms." Cement and Concrete Research 37(2):151-160. Abstract Predicting the long-term fate of low-level radioactive waste forms requires understanding how the radionuclides interact with the waste form. Concrete encasement is one method being considered for containment of low-level radioactive wastes. The necessary data to conduct an accurate performance assessment of such a waste form requires understanding the behavior and interactions of the radionuclides with the concrete matrix. The formation of uranium mineral phases has been investigated in simulated concrete pore fluids and Ordinary Portland Cement/Pulverized Fuel Ash (fly ash) concrete waste forms. X-Ray diffraction analyses of uranium precipitates from concrete pore fluids suggest diuranate salts, uranium-oxyhydroxides, and –silicates as solubility limiting phases. Scanning electron microscopy – energy dispersive spectroscopic analyses of uranium-spiked concrete suggests that under conditions both under-saturated and over-saturated with respect to the formation of uranium mineral phases, uranyl-oxyhydroxide phases precipitate within the initial two weeks. Subsequently, uranyl-silicate phases form after approximately one month and uranyl-phosphate phases provide a significant contribution to the long-term control over uranium in concrete waste forms after two months. This investigation demonstrates the importance of investigating the solubility of complex contaminants such as uranium in the complete matrix (i.e. concrete matrix versus pore fluids) and suggests the importance of secondary uranium mineral phases in the long-term retention within concrete waste forms.
2007. "Synthesis and stabilization of selenium nanoparticles on cellulose nanocrystal ." Materials Letters 61(21):4297-4300. doi:10.1016/j.matlet.2007.01.091 Abstract Selenium nanoparticles of 10-20 nm in diameter have been prepared using cellulose nanocrystal (CNXL) as a reducing and structure-directing agent under hydrothermal conditions. Na2SeO3 was reduced to form elemental selenium nanoparticles under hydrothermal conditions. During the hydrothermal process (120-160 oC), CNXL rods were mainly maintained and selenium nanoparticles were interfacially bound to CNXL surface. The reaction temperature affects the sizes of interfacially bound selenium nanoparticles. X-ray diffraction (XRD), field emission scanning electron microscope (FESEM), and transmission electron microscope (TEM) were employed to characterize interfacially bound selenium nanoparticles on CNXL surface.
2007. "Synthesis and Characterization of Phosphate-coated Mesoporous Titania and Cd-doping of Same via Ion-Exchange." Inorganic Chemistry Communications 10(6):642-645. doi:10.1016/j.inoche.2007.02.016 Abstract Phosphate-based mesoporous TiO2 materials were prepared by surfactant-directed method using an alkylphosphate surfactant, which produced a mesoporous titania with high surface area (~200 m2/g) and a phosphate monolayer interface. Calcination of the as-synthesized greenbody in an ozone atmosphere generated materials with higher surface area, and higher purity, than did calcination in air. These interfacial phosphate groups are convenient functionality for chemically modifying the surface via ion-exchange processes. High doping ratios of P/Ti (0.47-0.69) and Cd/P (0.37-0.40) were observed. Materials were characterized by XRD, FE-SEM, TEM, and XPS.
2007. "Simple Preparation and Stabilization of Nickel Nanocrystals on Cellulose Nanocrystal." Materials Letters 64(14-15):3215-3217. doi:10.1016/j.matlet.2006.11.036 Abstract Nickel nanocrystals were simply prepared on the carbon through a thermal reduction process at 400-500oC under N2 after Ni(II) ions were deposited and stabilized on cellulose nanocrystal (CNXL) surface. Hydroxyl groups on the CNXL anchor and stabilize Ni(II) ions. Well-dispersed Ni nanocrystals on the carbonized CNXL were about 5-12 nm in size. XRD, FESEM, and TEM were employed to characterize the products.
2007. "Accelerated Weathering of High-Level and Plutonium-bearing Lanthanide Borosilicate Waste Glasses under Hydraulically Unsaturated Conditions." Applied Geochemistry 22(9):1841-1859. doi:10.1016/j.apgeochem.2007.03.056 Abstract A can-in-canister waste package design has been proposed for disposal of excess weapons plutonium at the proposed mined geologic repository at Yucca Mountain, NV. by the U. S. Department of Energy Office of Fissile Material Disposition. This configuration consists of a high-level waste (HLW) canister fitted with a rack that holds mini-canisters containing a Pu-bearing lanthanide borosilicate (LaBS) waste glass and/or ceramic (~15% of the total canister volume). The larger canister is then filled with HLW glass, SRL-202, (~85% of the total canister volume). A 6-year pressurized unsaturated flow (PUF) test was conducted to investigate waste form/waste form interactions that may occur when water penetrates the canisters and contacts the waste forms. Volumetric water content was observed to increase steadily during PUF testing from accumulation of water mass as waters of hydration associated with alteration phases formed on the glass surface. Periodic excursions in effluent pH, electrical conductivity, and solution chemistry were monitored and correlated with the formation of a clay phase(s) during the test. Thermodynamic modeling of select effluent solution samples suggests the dominant secondary reaction product for the SRL-202 glass is a smectite di-octahedral clay phase(s), possibly nontronite [Na0.33Fe2(AlSi)4O10(OH)2•n(H2O)] or beidellite [Na0.33Al2.33Si3.67O10(OH)2]. This phase was identified in SEM images as discrete spherical particles found growing out of a gel-layer on reacted SRL-202 glass. Plutonium analyses of filtered and unfiltered solutions indicate that >80% of the Pu exiting the PUF system is as filterable particulates. In this advection-dominated system, Pu is migrating principally as colloids after being released from the LaBS glass. Analysis of reacted LaBS glass using SEM-EDS illustrates that Pu has segregated into a discrete disk-like phase, possibly PuO2. Alteration products that contain the neutron absorber Gd have not been positively identified. Separation of the Pu and its neutron absorbers during glass dissolution and transport is a potential criticality concern in the proposed repository. However, the translation and interpretation of these long-term PUF test results to actual disposed waste packages requires further analysis.
2007. "Reactive Ballistic Deposition of Porous TiO2 Films: Growth and Characterization." Journal of Physical Chemistry C 111(12):4765-4773. doi:10.1021/jp067641m Abstract Nanoporous, high-surface area films of TiO2 are synthesized by reactive ballistic deposition of titanium metal in an oxygen ambient. Auger electron spectroscopy (AES) is used to investigate the stoichiometric dependence of the films on growth conditions (surface temperature and partial pressure of oxygen). Scanning and transmission electron microscopy show that the films consist of arrays of separated filaments. The surface area and the distribution of binding site energies of the films are measured as functions of growth temperature, deposition angle, and annealing conditions using temperature programmed desorption (TPD) of N2. TiO2 films deposited at 50 K at 70º from substrate normal display the greatest specific surface area of ~100 m2/g. In addition, the films retain greater than 70% of their original surface area after annealing to 600 K. The combination of high surface area and thermal stability suggest that these films could serve as supports for applications in heterogeneous catalysis.
2006. "Electronic Stopping Forces of Heavy Ions in Metal Oxides." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 249(1-2):18-21. doi:10.1016/j.nimb.2006.03.013 Abstract Electronic energy loss of charged particles in materials is a fundamental process accountable for the unique response of materials in applications of advanced nuclear power, radiation detectors, and advanced processing of electronic devices. For over a century, the stopping of energetic ions in matter has been a subject of great experimental and theoretical interest. In spite of a long history of studies, the electronic stopping force is not adequately described over all ranges of ions, energies and targets, particularly in the case of heavy ions in compound targets. In this study, stopping powers for ions in ceramic oxides of SiO2, ZrO2, Ta2O5 and Nb2O5 have been determined using a time-of-flight energy elastic recoil detection analysis (ToF-E ERDA) set-up. In transmission geometry, the energy loss of heavy ions in the thin foils was measured over a continuous range of energies from a few 10 keV/nucleon to over a thousand keV/nucleon using the ToF data that was tagged by a Si detector with and without the stopping foils. Comparisons are made with the SRIM-2003 (The Stopping and Range of Ions in Matter) predictions, and deviations are discussed.
2006. "Residual Waste from Hanford Tanks 241-C-203 and 241-C-204. I. Solids Characterization." Environmental Science and Technology 40(12):3749-3754, and sup. info. file. Abstract Bulk X-ray diffraction (XRD), synchrotron X-ray microdiffraction (microXRD), and scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS) were used to identify the individual phases, phase associations, morphologies, particle sizes, and compositions of solids in residual sludge from single-shell underground waste tanks C-203 and C-204 at the U.S. Department of Energy’s Hanford Site in southeastern Washington state. Cejkaite [Na4(UO2)(CO3)3] was determined to be the dominant crystalline phase in the C-203 and C-204 sludges. This is only the second documented occurrence of cejkaite reported in the literature, and the first documented occurrence of this phase in radioactive wastes from DOE sites. XRD and SEM/EDS analyses of cejkaite found in the sludge solids are consistent with analyses of a natural mineral specimen of cejkaite. Characterization of residual solids from water leach and selective extraction tests indicates that cejkaite has a high solubility and a rapid rate of dissolution in water at ambient temperature, and that these sludges may also contain poorly crystalline Na2U2O7 [or clarkeite Na[(UO2)O(OH)](H2O)0-1], as well as nitratine (soda niter, NaNO3), goethite [FeO(OH)], and maghemite (Fe2O3). SEM/EDS analysis also shows that the C-204 sludge contains a solid composed of Na, Al, P, O, and possibly C that is likely amorphous. Other identified solids include Fe oxides that often also contain Cr and Ni and occur as individual particles, coatings on particles and botryoidal aggregates; a porous-looking material (or an aggregate of sub-micrometer particles) that typically contained Al, Cr, Fe, Na, Ni, Si, U, P, O, and C; Si oxide (probably quartz); and Na-Al silicate(s). The latter two solids probably represent minerals from Hanford sediment that were introduced into the tank during prior sampling campaigns or other tank-related activities. Preferential dissolution cavities were found on the surfaces of some of the Fe oxide particles present in residual solids from the water leach and selective extraction tests. If these solids contain contaminants, then their release into infiltrating water would be limited by dissolution of the low solubility Fe oxides. This process may account for at least some of the slow release of recalcitrant Tc-99 found in these sludges as discussed in the companion Part II paper by Cantrell et al.
2006. "Synthesis and Characterization of Cobalt Silicide Films on Silicon." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 249(1-2):532–535. doi:10.1016/j.nimb.2006.03.046 Abstract Cobalt silicide has emerged as a leading contact material in silicon technology due to its low resistivity, high stability and small lattice mismatch. In this study, 0.2-0.4 micron thick Co films were deposited on Si(100) wafers by RF magnetron sputtering at room temperature, and annealed at temperatures from 600 °C to 900°C in vacuum. The as-deposited and annealed samples were characterized by Rutherford backscattering spectrometry (RBS), nuclear reaction analysis (NRA), x-ray diffraction (XRD) and scanning electron microscopy (SEM). Although the Si substrates were sputter cleaned before the deposition, all the samples showed a thin oxide layer at the Si/Co interfaces. Annealing up to 700 °C did not alter the composition at the interface except small amount Co diffusion into Si. Annealing at 800 °C promotes the evaporation of the oxides from the interface and, as a result, clean CoSi2 films were formed. Although the interface appeared to be sharp within the RBS resolution, the surface topography was relatively rough with varying size of the crystal grains after high temperature annealing.
2005. "Corrosion of commercial spent nuclear fuel. 1. Formation of studtite and metastudtite." Radiochimica Acta 93:159-168. Abstract The contact of commercial spent nuclear fuel (CSNF) with water over a 2-year period led to an unexpected corrosion phase and morphology. At short hydration times, crystallites of metaschoepite [(UO₂)₈O₂(OH)₁₂] (H₂O)₁₀ were observed on the hydrated CSNF particles. Over the 2-year contact period, all evidence of metaschoepite disappeared, and the fuel particles were coated by a new alteration phase. Additionally, films of the reacted fuel were observed at the sample air-water interface of each sample. The corrosion phases on fuel powders and on the suspended films were examined by scanning electron microscopy, energy-dispersive X-ray fluorescence, and X-ray diffraction and were identified as studtite [(UO₂) (O₂) (H₂O)₂] (H₂O)₂ and metastudtite (UO₄ •2H₂O), respectively. The reason for the partitioning of the latter phase to the sample air-water interface is unclear at this time but may be due to structural differences between the two phases. Scanning electron micrographs of the CSNF powders indicated surface corrosion along grain boundaries and fragmentation of the primary solid. The occurrence of studtite and metastudtite on CSNF could have implications for the potential attenuation of released radionuclides during oxidative corrosion of CSNF in a geologic repository.
2005. "Neptunium(V) Partitioning to Uranium(VI) Oxide and Peroxide Solids." Environmental Science and Technology 39(11):4117-4124. Abstract Metaschoepite, [(UO2)8O2(OH)12]∙10H2O, and metastudtite, UO4∙4H2O, are alteration phases anticipated in a spent nuclear fuel repository following the moist oxidation of UO2 on a geologic timescale. Dissolved concentrations and hence potential mobility of other radionuclides in the fuel, such as the neptunyl cation (NpO2+), will likely be determined by the extent of their partitioning into these U(VI) solids. 237Np is of particular interest due to its potential high mobility and long half-life (2.1 x 106 years.) In this study, metaschoepite has been precipitated and subsequently transformed to studtite in the presence of dissolved Np. The metaschoepite and studtite solids that formed initially contained <10 ppm and 6,500 ppm Np, respectively. Batch dissolution studies of these solids at pH 6 demonstrate release of Np that exceeds congruent dissolution of U from metastudtite; furthermore, the released Np cation remains in solution. Thus, although the Np partitions into the metastudtite solid initially, it is released to solution over time, indicating that metastudtite is not likely to serve as a host solid for Np incorporation or sorption of the neptunyl cation on long time scales.
2005. "Microscale Characterization of Uranium(VI) Silicate Solids and Associated Neptunium(V)." Radiochimica Acta 93(5):265-272. Abstract The uranium(VI) silicate phases uranophane, Ca[(UO2)(SiO3OH)]2•5H2O, and sodium boltwoodite, Na[(UO2)(SiO3OH)]•1.5H2O, were synthesized in the presence of small, variable quantities (0.5 – 2.0 mol% relative to U) of pentavalent neptunium (Np(V), as NpO2+), to investigate the nature of its association with these U(VI) solid phases. Solids were characterized by X-ray powder diffraction (XRD), gamma spectrometry (GS), scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDS), and transmission electron microscopy (TEM) with electron energy-loss spectroscopy (EELS). Neptunium concentration was determined in the bulk solid phases by GS and was found to range from 780 – 15,800 μg/g. In some cases, Np distributions between the aqueous and solid phases were monitored, and 78-97% of the initial Np was associated with the isolated solid. Characterization of individual crystallites by TEM/EELS suggests the Np is associated with the U(VI) phase. No discrete Np phases, such as Np-oxides, were observed. Because the U(VI) silicates are believed to be important solubility-controlling solids on a geologic timescale, these results suggest that the partitioning of the minor actinides to these solids must be considered when assessing the performance of a waste repository for spent nuclear fuel.
2001. "Effect of feed melting, temperature history, and minor component addition on spinel crystallization in high-level waste glass." Journal of Non-crystalline Solids 289(1-3):17-29. Abstract Spinel crystallization affects the anticipated cost and risk of high-level waste (HLW) vitrification. Spinel, (Fe,Ni) (Fe,Cr)2O4, is the primary crystalline phase that precipitates from melts containing Fe and Ni in sufficient concentrations. This study was undertaken to help design and verify mathematical models for a HLW glass melter in which spinel crystals precipitate and partially settle.
2001. "Effect of Feed Melting, Temperature History and Minor Component Addition on Spinel Crystallization in High-Level Waste Glass." Journal of Non-crystalline Solids 289(1-3):17-29. Abstract This study was undertaken to help design mathematical models for high-level waste (HLW) glass melter that simulate spinel behavior in molten glass. Spinel, (Fe,Ni,Mn) (Fe,Cr)2O4, is the primary solid phase that precipitates from HLW glasses containing Fe and Ni in sufficient concentrations. Spinel crystallization affects the anticipated cost and risk of HLW vitrification. To study melting reactions, we used simulated HLW feed, prepared with co-precipitated Fe, Ni, Cr, and Mn hydroxides. Feed samples were heated up at a temperature-increase rate (4C/min) close to that which the feed experiences in the HLW glass melter. The decomposition, melting, and dissolution of feed components (such as nitrates, carbonates, and silica) and the formation of intermediate crystalline phases (spinel, sodalite [Na8(AlSiO4)6(NO2)2], and Zr-containing minerals) were characterized using evolved gas analysis, volume-expansion measurement, optical microscope, scanning electron microscope, thermogravimetric analysis, differential scanning calorimetry, and X-ray diffraction. Nitrates and quartz, the major feed components, converted to a glass-forming melt by 880C. A chromium-free spinel formed in the nitrate melt starting from 520C and Sodalite, a transient product of corundum dissolution, appeared above 600C and eventually dissolved in glass. To investigate the effects of temperature history and minor components (Ru,Ag, and Cu) on the dissolution and growth of spinel crystals, samples were heated up to temperatures above liquidus temperature (TL), then subjected to different temperature histories, and analyzed. The results show that spinel mass fraction, crystals composition, and crystal size depend on the chemical and physical makeup of the feed and temperature history.