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Geological and Environmental Applications of Synchrotron Radiation Workshop
Speakers Lee A. Groat B.Sc. (Hons.), Queen's University, 1982 Brett Moldovan Brett is currently a Ph.D. candidate at the University of Saskatchewan. His graduate studies are in the area of geochemistry; more specifically, he is studying the mobility and long-term stability of arsenic in uranium mine tailings. Brett is a full-time employee of Cameco Corporation, works as a research scientist, and is also a member of the Users' Advisory Committee for the Canadian Light Source. Brett will provide an update of his research, focusing on his synchrotron work where he investigated the mineralogical characterization of arsenic in uranium mine tailings. Ronald Cavell Professor, Department of Chemistry, University of Alberta Abstracts Lee A. Groat Department of Earth and Ocean Sciences, University
of British Columbia Applications of Synchrotron Radiation to Earth Materials The properties of X-radiation from synchrotron sources that distinguish it from more conventional sources are intensity, collimation, and tunability. Earth scientists are only now beginning to realize how these properties can enhance the study of natural materials. The properties of synchrotron radiation have greatly reduced data collection times and improved the signal-to-background ratio for the familiar techniques of single-crystal and powder diffraction. The advantages of the former with synchrotron radiation are as follows: (1) data may be collected from very small samples; (2) information may be obtained from weak or "forbidden" reflections; (3) data may be collected from poor-quality samples; and (4) short-wavelength radiation may be used to reduce both absorption and extinction effects. Applications of single-crystal diffraction include crystal structure determination and unambiguous identification. Many natural materials are not available as single crystals but as microcrystalline powders. X-ray diffraction patterns collected using synchrotron radiation exhibit very high resolution and low instrumental background. Moreover, the peak shapes are much simpler. Recent developments in image plate X-ray detector systems are particularly exciting. Energy-dispersed powder X-ray diffraction is optimized for in situ experiments on samples in extreme environments. The entire diffraction pattern is collected at the same time. The scattering angle is fixed and the diffracted intensities are measured as a function of X-ray energy. λ is replaced by ch/E and the Bragg equation becomes E = ch / (2d sin θ). Applications of powder XRD include: (1) identification, particularly of low-abundance phases in a mixture, or of crystalline components in an amorphous matrix; (2) quantitative analysis; and (3) complete structural analysis of minerals from powder diffraction. Less familiar are the X-ray techniques that exploit the tunability of synchrotron radiation. Anomalous scattering can provide information on the site distribution of close neighbours in the periodic table, as well as site selectivity of different oxidation sites of the same element. Both XRD and XAFS techniques become surface sensitive when X-rays are incident at glancing angles. By altering the angle of incidence, X-rays will probe into the material to a greater or lesser extent. As there is no requirement for a vacuum environment when hard X -rays are employed, the realistic "weathering" conditions of air and water contact can be readily simulated. X-ray absorption spectroscopy, particularly X-ray Absorption Fine Structure (XAFS), can be used with either crystalline or non-crystalline samples. The latter includes amorphous solids, liquids, and glasses. XAFS in particular can provide information concerning local order-disorder and the type of atoms coordinating the absorber. The Synchrotron X-Ray Fluorescence Microprobe (SXRFM) has been found to be an efficient instrument for the accurate and non-destructive measurement of a large suite of trace elements in natural and synthetic minerals and glasses. The lower limit of detection (LLD) is 1 to 10 ppm for a spot size of 10 µm, compared to 100 ppm for Electron MicroProbe Analysis (EMPA). Brett Moldovan1,2, M. Jim Hendry1, and De-Tong Jiang3 1. Department of Geological Sciences, University of
Saskatchewan, Saskatoon, Saskatchewan, Canada Geochemical and Mineralogical Controls on Arsenic Release from a Uranium Mine Tailings Facility, Northern Saskatchewan, Canada. Arsenic contained within uranium mine tailings is one of the primary contaminants of concern in terms of its potential to affect downstream receptors. Effective removal and safe deposition of this unwanted impurity is a key issue facing several mining industries. Arsenic-rich uranium mine tailings from the Rabbit Lake in-pit tailings management facility (RLITMF) in northern Saskatchewan, Canada, were investigated to determine the mineralogy and long-term stability of secondary arsenic precipitates formed from iron-rich hydrometallurgical solutions. Tailings have been emplaced in the mined-out Rabbit Lake pit for 17 years, making it a useful location to study the long-term stability of secondary arsenic precipitates contained within the tailings. Solids and surrounding pore waters containing arsenic within tailings impoundments present a useful location to study the geochemistry of arsenic in contaminated sites. The long-term stability of arsenic within mine tailings is controlled by such factors as pH, redox conditions, and temperature, as well as the arsenic mineralogy present in the tailings. Arsenic concentrations in the tailings pore fluids ranged from 0.24 to 140 mg/l. In all cases, the arsenic concentrations in the tailings pore water exceeded the guidelines set forth for drinking water standards, currently set at 0.010 mg/l. The toxicity and mobility of arsenic is dependent upon the valence state of arsenic present in aqueous environments, with As3+ being up to 60 times more toxic than As5+. It was found that greater than 90% of the arsenic in the tailings pore fluids existed in the preferred As5+ state. Total arsenic and iron concentrations in six iron-rich samples of the mine tailings ranged from 56 to 6,000 µg/g and from 12,600 to 30,200 µg/g, respectively (Fe/As molar ratios of 5.3 to 303). Based on stability field diagrams generated from pH, Eh, and temperature measurements on tailings samples (mean values of 9.79, +162 mV, and 2.8°C, respectively), it was concluded that arsenic and iron in the tailings were stable as As5+ and Fe3+. Synchrotron-based X-ray absorption spectroscopic studies of tailings samples, fresh mill precipitates, and reference compounds showed that the arsenic in iron-rich areas of the tailings existed as the stable As5+ and was adsorbed to 2-line ferrihydrite through inner sphere bidentate linkages. Furthermore, under the conditions in the RLITMF, the 2-line ferrihydrite did not undergo any measurable conversion to more crystalline goethite or hematite, even in tailings discharged to the RLITMF 10 years prior to sampling. Dr. Ronald Cavell Department of Chemistry, University of Alberta, Edmonton, Alberta Research at and Access to Synchrotron Radiation Sources No abstract available. |
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