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Journal of Environmental Quality Abstract - Special Submissions: Advanced In Situ Spectroscopic Techniques and Their Applications In Environmental Biogeochemistry

Combined Application of QEM-SEM and Hard X-ray Microscopy to Determine Mineralogical Associations and Chemical Speciation of Trace Metals

 

This article in JEQ

  1. Vol. 40 No. 3, p. 767-783
     
    Received: May 13, 2010


    * Corresponding author(s): markus.grafe@csiro.au
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doi:10.2134/jeq2010.0214
  1. Markus Gräfe *a,
  2. Matthew Landersa,
  3. Ryan Tapperob,
  4. Peter Austina,
  5. Bee Gana,
  6. Alton Grabscha and
  7. Craig Klaubera
  1. a CSIRO Process Science and Engineering, Australian Minerals Research Centre, 7 Conlon St., Waterford, WA 6152, Australia
    b Beamline X27A, National Synchrotron Light Source, Brookhaven National Lab., Upton, NY 11973-5000. Assigned to Associate Editor Kirk Scheckel

Abstract

We describe the application of quantitative evaluation of mineralogy by scanning electron microscopy in combination with techniques commonly available at hard X-ray microprobes to define the mineralogical environment of a bauxite residue core segment with the more specific aim of determining the speciation of trace metals (e.g., Ti, V, Cr, and Mn) within the mineral matrix. Successful trace metal speciation in heterogeneous matrices, such as those encountered in soils or mineral residues, relies on a combination of techniques including spectroscopy, microscopy, diffraction, and wet chemical and physical experiments. Of substantial interest is the ability to define the mineralogy of a sample to infer redox behavior, pH buffering, and mineral–water interfaces that are likely to interact with trace metals through adsorption, coprecipitation, dissolution, or electron transfer reactions. Quantitative evaluation of mineralogy by scanning electron microscopy coupled with micro-focused X-ray diffraction, micro–X-ray fluorescence, and micro–X-ray absorption near edge structure (mXANES) spectroscopy provided detailed insights into the composition of mineral assemblages and their effect on trace metal speciation during this investigation. In the sample investigated, titanium occurs as poorly ordered ilmenite, as rutile, and is substituted in iron oxides. Manganese's spatial correlation to Ti is closely linked to ilmenite, where it appears to substitute for Fe and Ti in the ilmenite structure based on its mXANES signature. Vanadium is associated with ilmenite and goethite but always assumes the +4 oxidation state, whereas chromium is predominantly in the +3 oxidation state and solely associated with iron oxides (goethite and hematite) and appears to substitute for Fe in the goethite structure.

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