Browsing by Author "Griffith, CS"
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- ItemAqueous leachability of metakaolin-based geopolymers with molar ratios of Si/Al=1.5-4(Elsevier, 2008-08-31) Aly, Z; Vance, ER; Perera, DS; Hanna, JV; Griffith, CS; Davis, J; Durce, DThe leachability in water of metakaolin based geopolymers with molar ratios of Na/Al = 1 and Si/Al = 1.5-4.0 has been investigated in order to optimise the composition for the immobilisation of nuclear waste. Formulations with Si/Al of around 2 are the most suitable using the ASTM/PCT leach test method. The variability of the leach results is discussed with reference to the microstructure, compressive strength and the degree of polymerisation of the geopolymers as observed here by solid state nuclear magnetic resonance, XRD and infrared measurements. © 2008, Elsevier Ltd.
- ItemElectrochemical behavior of zirconium in LiCl - KCl eutectic molten salt(IOP Publishing, 2010-04-25) Fabian, CP; Caravaca, C; Griffith, CS; Luca, V; Lumpkin, GRNuclear fission energy increasingly represents a potentially important option for generating clean, CO2-free electricity and the nuclear community is considering advanced reactor and fuel cycle technologies. Pyroelectrochemical processes based on molten salt electrochemistry have become important for the potential recycle of metal and other advanced nuclear fuels. Zirconium is one of the troublesome fission products that is expected to be present in such used fuels and hence it is important to acquire an understanding of its electrochemistry in molten salts. The present investigation addresses the electrochemical reduction of Zr4+ in LiCl - KCl eutectic molten salt using transient techniques such as cyclic voltammetry (CV), constant-current chronopotentiometry (CP) and square wave voltammetry (SWV) at 425, 475 and 525 °C. The CV data clearly indicate reduction of Zr4+ proceeds through an initial adsorption process followed by two or three reaction steps dependent on the temperature. Results show three previously unobserved reaction steps at 425 and 475 °C, possibly due to improved pretreatment of the salt. The first cathodic peak is observed at -1.18 V (vs. Ag/AgCl). The second and third peaks are observed at -1.26 and -1.33 V (vs. Ag/AgCl), respectively with a separation of 60-70 mV. The CV data at 525 °C show only two cathodic peaks at about -1.1 and -1.25 V (vs. Ag/AgCl)which may indicate the effect of temperature on the mechanism of the electrochemical process. The overall reaction mechanism assessed by these electrochemical techniques will be presented. Moreover, electrodeposition of zirconium on aluminium metal will also be presented along with XRD analysis of the salt residue and zirconium-aluminium alloy. Finally, the kinetics of the electrochemical process have been studied using Electrochemical Impedance Spectroscopy and the results analyzed using the Randles-Ershler equivalent circuit model to determine the charge-transfer resistance (Rct). © 2010 The Electrochemical Society
- ItemHybrid inorganic-organic adsorbents, Part 1: synthesis and characterization of mesoporous zirconium titanate frameworks containing coordinating organic functionalities(American Chemical Society, 2010-12-22) Griffith, CS; Reyes, MDL; Scales, N; Hanna, JV; Luca, VA series of functional hybrid inorganic−organic adsorbent materials have been prepared through postsynthetic grafting of mesoporous zirconium titanate xerogel powders using a range of synthesized and commercial mono-, bis-, and tris-phosphonic acids, many of which have never before been investigated for the preparation of hybrid phases. The hybrid materials have been characterized using thermogravimetric analysis, diffuse reflectance infrared (DRIFT) and 31P MAS NMR spectroscopic techniques and their adsorption properties studied using a 153Gd radiotracer. The highest level of surface functionalization (molecules/nm2) was observed for methylphosphonic acid (3 molecules/nm2). The level of functionalization decreased with an increase in the number of potential surface coordinating groups of the phosphonic acids. Spectral decomposition of the DRIFT and 31P MAS NMR spectra showed that each of the phosphonic acid molecules coordinated strongly to the metal oxide surface but that for the 1,1-bis-phosphonic acids and tris-phosphonic acids the coordination was highly variable resulting in a proportion of free or loosely coordinated phosphonic acid groups. Functionalization of a porous mixed metal oxide framework with the tris-methylenephosphonic acid (ATMP-ZrTi-0.33) resulted in a hybrid with the highest affinity for 153Gd3+ in nitric acid solutions across a wide range of acid concentrations. The ATMP-ZrTi-0.33 hybrid material extracted 153Gd3+ with a Kd value of 1 × 104 in 0.01 M HNO3 far exceeding that of the other hybrid phases. The unfunctionalized mesoporous mixed metal oxide had negligible affinity for Gd3+ (Kd < 100) under identical experimental conditions. It has been shown that the presence of free or loosely coordinated phosphonic acid groups does not necessarily translate to affinity for 153Gd3+. The theoretical cation exchange capacity of the ATMP-ZrTi-0.33 hybrid phase for Gd3+ has been determined to be about 0.005 mmol/g in 0.01 M HNO3. This behavior and that of the other hybrid phases suggests that the surface-bound ATMP ligand functions as a chelating ligand toward 153Gd3+ under these acidic conditions. © 2010, American Chemical Society
- ItemImmobilisation of cations and anions in geopolymers(Materials Science and Technology (MS&T), 2007-09-16) Vance, ER; Perera, DS; Aly, Z; Walls, P; Zhang, YJ; Cassidy, DJ; Griffith, CS
- ItemImpact of chloride on uranium(VI) speciation in acidic sulfate ion exchange systems: towards seawater-tolerant mineral processing circuits(Elsevier, 2017-07-25) Moon, EM; Ogden, MD; Griffith, CS; Wilson, A; Mata, JPUsing X-ray absorption spectroscopy, we have identified the mechanism by which chloride impacts the extraction of U(VI) by a weak base anion exchange resin from acidic sulfate solutions. The amount of chloride present affects U(VI) speciation both in the feed solutions and adsorbed by the resin, and we find three distinct U(VI) species taken up by the resin across the salinity gradient, directly corresponding to changes in levels of U(VI) extraction. These findings are integral to the effective design of mineral processing circuits incorporating seawater—a cheaper and more sustainable water source than fresh water. © 2017 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V.
- ItemMesoporous zirconium titanium oxides. Part 3. Synthesis and adsorption properties of unfunctionalized and phosphonate-functionalized hierarchical polyacrylonitrile-f-127-templated beads(American Chemical Society, 2009-10-06) Sizgek, GD; Griffith, CS; Sizgek, E; Luca, VA method is presented for the preparation of zirconium titanate mixed oxides in bead form having hierarchical pore structure. This method entailed the use of both preformed polyacrylonitrile (PAN) polymer beads and surfactants as templates. The templates were removed by calcination at temperatures below about 500°C, resulting in mixed oxide beads with trimodal pore size distributions and interconnected pores. The pore size distributions as determined using nitrogen adsorption−desorption showed clear maxima at 4.5 and 45 nm length scales and also clear evidence of microporosity. The macroporous framework morphology was a replica of the PAN beads with radial structure. The mesoporous framework possessed wormhole-like pores with pore size of about 6 nm that was consistent with the F-127 triblock copolymer template used. The mixed oxide beads exhibited surface areas of 215 and 185 m2/g after calcination at 500 and 600°C. Thermal stability up to 650°C is unprecedented for bulk systems. The adsorption properties were characterized using uranyl as the target cation and the mass transport in the beads with the present hierarchical architectures has been shown to be exceptional. The beads were functionalized with 4-amino,1-hydroxy,1,1-bis-phosphonic acid (HABDP) and amino-tris-methylene phosphonic acid (ATMP) and the adsorption properties for the extraction of uranyl sulfate complexes from acidic solution examined. Of the two molecules investigated, ATMP functionalization resulted in the best extraction efficiency with equilibrium uptake of about 90% of uranium available in solution between pH 1 and 2. The beads could potentially be utilized as catalysts, catalyst supports, adsorbents, and separation materials. © 2009, American Chemical Society
- ItemMicrocrystalline hexagonal tungsten bronze. 1. Basis of ion exchange selectivity for cesium and strontium(American Chemical Society, 2009-07-06) Griffith, CS; Luca, V; Hanna, JV; Pike, KJ; Smith, ME; Thorogood, GJThe structural basis of selectivity for cesium and strontium of microcrystalline hexagonal tungsten bronze (HTB) phase NaxWO3+x/2·zH2O has been studied using X-ray and neutron diffraction techniques, 1D and 2D 23Na magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy, and radiochemical ion exchange investigations. For the HTB system, this study has shown that scattering techniques alone provide an incomplete description of the disorder and rapid exchange of water (with tunnel cations) occurring in this system. However, 1D and 2D 23Na MAS NMR has identified three sodium species within the HTB tunnels—species A, which is located at the center of the hexagonal window and is devoid of coordinated water, and species B and C, which are the di- and monohydrated variants, respectively, of species A. Although species B accords with the traditional crystallographic model of the HTB phase, this work is the first to propose and identify the anhydrous species A and monohydrate species C. The population (total) of species B and C decreases in comparison to that of species A with increasing exchange of either cesium or strontium; that is, species B and C appear more exchangeable than species A. Moreover, a significant proportion of tunnel water is redistributed by these cations. Multiple ion exchange investigations with radiotracers 137Cs and 85Sr have shown that for strontium there is a definite advantage in ensuring that any easily exchanged sodium is removed from the HTB tunnels prior to exchange. The decrease in selectivity (wrt cesium) is most probably due to the slightly smaller effective size of Sr2+; namely, it is less of a good fit for the hexagonal window, ion exchange site. The selectivity of the HTB framework for cesium has been shown unequivocally to be defined by the structure of the hexagonal window, ion exchange site. Compromising the geometry of this window even in the slightest way by either (1) varying the cell volume through changes to hydration or sodium content or (2) introducing disorder in the a−b plane through isomorphous substitution of molybdenum is sufficient to reduce the selectivity. Indeed, it is our hypothesis that this applies for all cations which are strongly bound by the HTB framework. © 2009, American Chemical Society
- ItemMicrocrystalline hexagonal tungsten bronze. 2. Dehydration dynamics(American Chemical Society, 2009-07-06) Luca, V; Griffith, CS; Hanna, JVLow-temperature (25−600°C) thermal transformations have been studied for hydrothermally prepared, microcrystalline hexagonal tungsten bronze (HTB) phases AxWO3+x/2·zH2O as a function of temperature, where A is an exchangeable cation (in this case Na+ or Cs+) located in hexagonal structural tunnels. Thermal treatment of the as-prepared sodium- and cesium-exchanged phases in air were monitored using a conventional laboratory-based X-ray diffractometer, while thermal transformations in vacuum were studied using synchrotron X-ray and neutron diffraction. Concurrent thermogravimetric, diffuse reflectance infrared (DRIFT), and 23Na and 133Cs magic angle spinning (MAS) NMR spectroscopic studies have also been undertaken. For the cesium variant, cell volume contraction occurred from room temperature to about 350°C, the regime in which water was “squeezed” out of tunnel sites. This was followed by a lattice expansion in the 350−600°C temperature range. Over the entire temperature range, a net thermal contraction was observed, and this was the result of an anisotropic change in the cell dimensions which included a shortening of the A−O2 bond length. These changes explain why Cs+ ions are locked into tunnel positions at temperatures as low as 400°C, subsequently inducing a significant reduction in Cs+ extractability under low pH (nitric acid) conditions. The changing Cs+ speciation as detected by 133Cs MAS NMR showed a condensation from multiple Cs sites, presumably associated with differing modes of Cs+ hydration in the tunnels, to a single Cs+ environment upon thermal transformation and water removal. While similar lattice contraction was observed for the as-prepared sodium variant, the smaller radius of Na+ caused it to be relatively easily removed with acid in comparison to the Cs+ variant. From 23Na MAS NMR studies of the parent material, complex Na+ speciation was observed with dehydrated and various hydrated Na+ species being identified, and a subsequent dynamic interchange within this speciation was observed upon thermal treatment. © 2009, American Chemical Society
- ItemStructural studies of the rhombohedral and orthorhombic monouranates: CaUO4, α-SrUO4, β-SrUO4 and BaUO4(Elsevier B.V., 2016-05-01) Murphy, GL; Kennedy, BJ; Johannessen, B; Kimpton, JA; Avdeev, M; Griffith, CS; Thorogood, GJ; Zhang, ZThe structures of some AUO4 (A=Ca, Sr, or Ba) oxides have been determined using a combination of neutron and synchrotron X-ray diffraction, supported by X-ray absorption spectroscopic measurements at the U L3-edge. The smaller Ca cation favours a rhombohedral AUO4 structure with 8-coordinate UO8 moieties whilst an orthorhombic structure based on UO6 groups is found for BaUO4. Both the rhombohedral and orthorhombic structures can be stabilised for SrUO4. The structural studies suggest that the bonding requirements of the A site cation play a significant role in determining which structure is favoured. In the rhombohedral structure, Bond Valence Sums demonstrate the A site is invariably overbonded, which, in the case of rhombohedral α-SrUO4, is compensated for by the formation of vacancies in the oxygen sub-lattice. The uranium cation, with its flexible oxidation state, is able to accommodate this by inducing vacancies along its equatorial coordination site as demonstrated by neutron powder diffraction. © 2016 Elsevier Inc.
- ItemStructure and phase transformations in the titanosilicate, sitinakite. The importance of water(American Chemical Society, 2010-07-27) Thorogood, GJ; Kennedy, BJ; Griffith, CS; Elcombe, MM; Avdeev, M; Hanna, JV; Thorogood, SK; Luca, VSynchrotron X-ray diffraction and neutron diffraction have been used to investigate the phase changes that the titanosilicate mineral sitinakite undergoes when dehydrated. Refinements of the powder diffraction data of the material before and after heating to 573K indicate a phase change from space group P42/mcm to P42/mbc. Upon exposure to normal atmospheric conditions for an extended period the material transforms back to P42/mcm with a reduced lattice parameter. If the material is heated in a sealed capillary, it is possible to get the two phases coexisting. The coexistence of these two phases in sealed system suggests that it is only the loss of H2O that is driving the reversible phase transformation. © 2010, American Chemical Society
- ItemSynthesis and characterization of functionalized silica-based nanohybrid materials for oxyanions adsorption(American Chemical Society, 2010-06-01) Karatchevtseva, I; Astoux, M; Cassidy, DJ; Yee, P; Bartlett, JR; Griffith, CSThis study investigates the structural evolution of a series of nanohybrid powders and coatings synthesized by direct co-condensation of amino-functionalized alkyltrialkoxysilanes and tetraalkoxysilanes with an aromatic carboxylic acid (trimesic acid, TMA) as a structure directing agent. Fourier transform infrared spectroscopy (FTIR) and 13C CP-MAS NMR results have suggested the formation of secondary (−CO−NH−) amide linkages upon interaction of TMA with the amino functionalized silane thus creating a “scaffold” around which the silica network is formed and also assisting in more homogeneous distribution of nitrogen sites within the nanohybrid structure. Functionalized silica powders were investigated for their potential to remove toxic oxyanions from mildly acidic or basic solutions. The uptake of Mo(VI), Se(VI), and Cr(VI) oxyanions was investigated as a function of the nanohybrid composition, oxyanion concentration, and solution pH using laser diffraction particle sizing, gas adsorption, and various spectroscopic techniques. The adsorption data obtained for Mo and Se could be adequately described by Langmuir adsorption isotherms, while the Freundlich isotherm is employed to fit the adsorption data for Cr. An easily accessible processing window (of pH, aging time, etc.) has been identified allowing production of continuous and uniform thin nanohybrid coatings on silicon and glass substrates. These coatings were tested as chemical barriers against Mo leaching from specially prepared Mo-doped glass. Leaching studies were conducted over 200 days in water at 90°C and the Mo leaching from coated and uncoated samples compared. © 2010, American Chemical Society
- ItemTungstate-based glass-ceramics for the immobilization of radio cesium(Elsevier, 2009-02-15) Drabarek, E; McLeod, TI; Hanna, JV; Griffith, CS; Luca, VThe preparation of tungstate-containing glass–ceramic composites (GCC) for the potential immobilization of radio cesium has been considered. The GCC materials were prepared by blending two oxide precursor compositions in various proportions. These included a preformed Cs-containing hexagonal tungsten bronze (HTB) phase (Cs0.3Ti0.2W0.8O3, P63/mcm) and a blend of silica and other oxides. The use of the HTB phase was motivated on the assumption that a HTB-based adsorbent could be used to remove cesium directly from aqueous high level liquid waste feeds. In the absence of the HTB, glass–ceramics were relatively easily prepared from the Cs-containing glass-forming oxide blend. On melting the mixture a relative complex GCC phase assemblage formed. The principal components of this phase assemblage were determined using X-ray powder diffraction, 133Cs MAS-NMR, and cross-sectional SEM and included glass, various zeolites, scheelite (CaWO4) and a range of other oxide phases and Cs-containing aluminosilicate. Importantly, under no circumstance was cesium partitioned into the glass phase irrespective of whether or not the composition included the preformed Cs-containing HTB compound. For compositions containing the HTB, cesium was partitioned into one of four major phases including zeolite; Cs–silica–tungstate bronze, pollucite (CsAlSi2O6), and an aluminosilicate with an Al/Si ratio close to one. The leach resistance of all materials was evaluated and related to the cesium distribution within the GCC phase assemblages. In general, the GCCs prepared from the HTB had superior durability compared with materials not containing tungsten. Indeed the compositions in many cases had leach resistances comparable to the best ceramics or glass materials. © 2009, Elsevier Ltd.
- ItemUnderstanding the supramolecular self-assembly of zirconium titanate mesophases formed from the poly(ethylene oxide) surfactant brij-58(American Chemical Society, 2010-07-13) Luca, V; Drabarek, E; Griffith, CS; Hanley, TLThe evaporation-induced self-assembly of mesoporous zirconium titanium oxide thin films prepared from precursor solutions with the composition ZrxTi1−xCl4:40 EtOH:0.005 Brij 58:h H2O (where x = 0.28, h = 10, and Brij 58 = diblock copolymer C16H33PEO20) by dip coating has been studied as a function of the mole fraction of zirconium (x), the relative humidity (RH), and the drying conditions. Extremely well-ordered mesophases and compositional homogeneity on the atomic scale were consistently obtained when the films were prepared at relative humidities above 60%, under which conditions a cubic phase was identified. The influence of humidity was monitored using a combination of in situ small-angle X-ray diffraction (SAXD) and small-angle X-ray scattering (SAXS). These techniques showed the existence of nanocrystalline phases during the initial drying process for films prepared from precursor solutions with x in the range of 0.20−0.60. The nanocrystalline phases were evidenced as multiple X-ray reflections above 8° 2θ. These reflections were ascribed to relatively crystalline nano building units of the mesophase and displayed only a transient stability in humid air, disappearing after 1 h of film conditioning in any humidity. A clear dependence of the primary d-spacing observed in the SAXD on composition for films dried at 65% RH, stabilized at 200°C, and calcined at 350°C provided strong evidence of the ability to form a complete solid solution of Zr and Ti in the oxide mesophase prepared from the precursor solutions. This compositional homogeneity was confirmed by transmission electron microscopy and further corroborated using Ti K-edge X-ray absorption spectroscopy, which probed the local chemical environment of the embedded Ti atoms. The data provided here are in stark contrast to studies of zirconium titanium mixed oxide film systems using similar preparation procedures but triblock copolymer porogens such as F-127. A hypothesis is tendered to explain the dramatic difference in the products obtained using these distinct but related copolymer templates. © 2010, American Chemical Society