John G. Darab

The structure of Na2O–Al2O3–SiO2 glass: impact on sodium ion exchange in H2O and D2O

Abstract The kinetics of matrix dissolution and alkali-exchange for a series of sodium aluminosilicate glass compositions was determined at constant temperature and solution pH(D) under conditions of silica-saturation. Steady state release rate for sodium was 10–50 times faster than the rate of matrix dissolution, demonstrating that alkali exchange is an important long-term reaction mechanism that must be considered when modeling systems near saturation with respect to dissolved glass components. Sodium release rates were 30% slower in D 2 O compared to rates in H 2 O; but matrix dissolution rates were unaffected. These results are consistent with rupture of the OH bond as the rate-limiting reaction in Na + –H + exchange whereas matrix dissolution is controlled by OH − or H 2 O catalyzed hydrolysis of SiOSi and SiOAl bonds. Changes in Na exchange rate with increasing Al 2 O 3 content could not be reconciled with changes in the number of non-bridging oxygen (NBO) sites in the glass alone. A simple model was used to estimate a structural energy barrier for alkali ion exchange using NaO bond length and co-ordination as measured by Na K-edge X-ray absorption spectroscopy, and binding energy shifts for SiONa sites measured by X-ray photoelectron spectroscopy (XPS). The energy barrier was calculated to increase from 34 kJ mol −1 for Na 2 O·2SiO 2 glass to 49 kJ mol −1 for a glass containing 15 mol% Al 2 O 3 , consistent with stronger bonding of Na on NBO sites and increasing mechanical stiffness of the glass network with increasing Al content. The calculated ion-exchange enthalpies were then used to calculate Na ion-exchange rates as a function of glass composition. Agreement between the calculated and measured Na ion exchange rates was excellent.

An X-ray absorption fine structure study of copper(I) chloride coordination structure in water up to 325°C

Abstract X-ray absorption fine structure (XAFS) spectroscopy was used to measure the Cl − and H 2 O coordination structure about Cu 1+ in water at temperatures up to 325°C including the coordination numbers, symmetry, distances and the amount of bond disorder. The linear dichloro Cu 1+ species, [CuCl 2 ] − , is especially stable and it is predominant from 100°C to 325°C in the presence of excess Cl − . Even for solutions with 2.0 m NaCl, only the dichloro Cu 1+ species is observed with no evidence of higher Cl − coordination. There is no evidence of hydration waters in the first-solvation shell of this dichloro-species.

New experimental developments for in situ XAFS studies of chemical reactions under hydrothermal conditions

Abstract New experimental developments for in situ X-ray absorption fine structure spectroscopy (XAFS) studies of hydrothermal systems are described. Improvements in materials of construction and the design of spectroscopic cells have allowed application of XAFS as a powerful method to derive the molecular structures of reacting species. Two different spectroscopic cells are described that are suitable for in situ XAFS studies of aqueous solutions under hydrothermal conditions. One cell consists of a titanium alloy body with a corrosion-resistant platinum–iridium insert and diamond windows for X-ray transmission. Using this cell, XAFS spectra were acquired on aqueous tungstate solutions up to 400°C. The results demonstrate how XAFS can be used to study the speciation of isopolytungstates and their equilibria at high temperatures. Results from model calculations using the FEFF ab initio code are used to explain the observed spectral changes with changes in pH and temperature. The second XAFS cell consists of a simple fused-silica capillary having a 180 μm inner diameter. High-quality spectra of a 0.2 molal nickel bromide solution were collected in these small capillaries demonstrating the feasibility of the approach to the study of a wide range of geochemical systems. Further, the XAFS spectra show that the nickel bromide solution irreversibly reacts with the fused-silica capillary at 425°C. Based upon these observations, a potential use of the fused-silica capillary is suggested for in situ studies of silicate formation, or, in general, for studies of chemical interactions of hydrothermal brine solutions with quartz.

Behavior of simulated Hanford slurries during conversion to glass

Mixtures of simulated Hanford low-activity aqueous waste and glass precursor additives were heat treated at various temperatures up to 1,000 C and characterized using X-ray diffraction, X-ray fluorescence spectroscopy, scanning electron microscopy, gas chromatography-mass spectroscopy, and inductively coupled plasma-mass spectroscopy. It was found after heating the simulant mixture up to 700 C, the majority of the sodium nitrite present in the original waste reacted with the silica and perhaps boron species in the additives to form C- and N-containing gases, oxygen gas, and lower melting phases which contributed to the observed batch volume expansion beginning at these temperatures. Elemental analyses of minor components in the simulant mixture showed that I, Cl, and S exhibited significant losses during heating, presumably due to volatilization or entrainment. For the case of I, nearly 90% on an elemental basis was lost from the simulant mixture after processing to 1,000 C, whereas Re exhibited less than 40% loss.

Sulfur Partitioning During Vitrification of INEEL Sodium Bearing Waste: Status Report

The sodium bearing tank waste (SBW) at Idaho National Engineering and Environmental Laboratory (INEEL) contains high concentrations of sulfur (roughly 5 mass% of SO3 on a nonvolatile oxide basis). The amount of sulfur that can be feed to the melter will ultimately determine the loading of SBW in glass produced by the baseline (low-temperature, joule-heated, liquid-fed, ceramic-lined) melter. The amount of sulfur which can be fed to the melter is determined by several major factors including: the tolerance of the melter for an immiscible salt layer accumulation, the solubility of sulfur in the glass melt, the fraction of sulfur removed to the off-gas, and the incorporation of sulfur into the glass up to it?s solubility limit. This report summarizes the current status of testing aimed at determining the impacts of key chemical and physical parameters on the partitioning of sulfur between the glass, a molten salt, and the off-gas.

A Flow-Through Hydrothermal Method for the Synthesis of Active Nanocrystalline Catalysts

Publisher Summary The rapid thermal decomposition of precursors in solution (RTDS) process is a novel powder synthesis method involving hydrothermal reactions of particle-forming precursor species in continuously flowing solutions. It uses short reaction times, on the order of seconds, to quickly form particulate species and then terminates the growth process by abruptly removing the resulting suspension from the hydrothermal environment through a pressure let-down device. This method is an attractive approach for the large-scale generation of active nanocrystalline oxide and oxyhydroxide powders for a variety of catalytic applications. Specific benefits of the process include the nanocrystalline character of the powder products, the capability to adjust product characteristics by varying the process conditions, a wide range of product materials, the ability to generate doped single phase or biphasic powders, and the potential to operate in a continuous mode for large-scale nanocrystalline powder generation.

X-ray absorption spectroscopy and imaging of heterogeneous hydrothermal mixtures using a diamond microreactor cell

Hydrothermal synthesis is an important route to novel materials. Hydrothermal chemistry is also an important aspect of geochemistry and a variety of waste remediation technologies. There is a significant lack of information about the speciation of inorganic compounds under hydrothermal conditions. For these reasons we describe a high-temperature, high-pressure cell that allows one to acquire both x-ray absorption fine structure (XAFS) spectra and x-ray transmission and absorption images of heterogeneous hydrothermal mixtures. We demonstrate the utility of the method by measuring the Cu(I) speciation in a solution containing both solid and dissolved Cu phases at temperatures up to 325 °C. X-ray imaging of the various hydrothermal phases allows micro-XAFS to be collected from different phases within the heterogeneous mixture. The complete structural characterization of a soluble bichloro-cuprous species was determined. In situ XAFS measurements were used to define the oxidation state and the first-shell coo...

Chemistry of Rare Earth Oxalate Vitrification

Mixtures of rare earth and actinide oxalates will be vitrified into boro-aluminosilicate-based glasses for intermediate term stabilization according to current plans. The reaction chemistry involved with converting these oxalate feed stocks into glass products determines the potential for foaming, redox, and other melt and off gas related phenomena associated with this process. The authors have undertaken a detailed study of this conversion process using a variety of complementary techniques. A closed quartz crucible contained in a vertical furnace equipped with a quartz window and video camera was used to study volume expansion of the feed/melt during heating while monitoring the off-gas using a gas chromatograph-mass spectrometer. Simultaneous thermogravimetric and differential thermal analyses were conducted on small samples of feed and frit mixtures. Samples containing Ce were analyzed using established wet chemical techniques to determine Ce{sup 3+}:Ce{sup 4+} ratio (redox) as a function of temperature. The authors evaluate the results and provide a description of the reaction chemistry of these oxalate feeds during vitrification.

NMR characterization of simulated Hanford low-activity waste glasses and its use in understanding waste form chemical durability

Magic Angle Spinning Nuclear Magnetic Resonance (MAS-NMR) spectroscopy has been used to characterize the structural and chemical environments of B, Al, and Si in model Hanford low-activity waste glasses. The average {sup 29}Si NMR peak position was found to systematically change with changing glass composition and structure. From an understanding of the structural roles of Al and B obtained from MAS-NMR experiments, the authors first developed a model that reliably predicts the distribution of structural units and the average {sup 29}Si chemical shift value, {delta}, based purely on glass composition. A product consistency test (PCT) was used to determine the normalized elemental release (NL) from the prepared glasses. Comparison of the NMR and PCT data obtained from sodium boro-aluminosilicate glasses indicates that a rudimentary exponential relationship exists between the {sup 29}Si chemical shift value, and the boron NL value.

Chemical and Structural Elucidation of Minor Components in Simulated Hanford Low-Level Waste Glasses

This symposium marks to the month the fiftieth anniversary of the beginning of the atomic age with the detonation of the world’s first fission weapons in August, 1945. To support this effort, in 1943, the first full-scale nuclear reactors and processing plants needed for the production and isolation of 239Pu were built at the Hanford Engineering Works along the Columbia River in southeastern Washington. Starting in December, 1944, the Hanford Site, as it would later be called, began processing irradiated uranium fuel elements and subsequently isolating 239Pu bearing solutions.2 Nearly thirty years of specialty nuclear materials production at Hanford as well as a concomitant generation of vast amounts of solid and liquid radioactive waste ensued. With the end of the cold war, the emphasis at the Hanford Site is now directed at remediation of these radioactive waste byproducts.