XAFS Spectroscopy Analysis of Selected Elements in Fine Particulate Matter Derived from Coal Combustion

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Abstract

X-ray absorption fine structure (XAFS) spectroscopy has been used to investigate the valence states and molecular structures of sulfur (S), chromium (Cr), arsenic (As), and zinc (Zn) in fine particulate matter (PM) separated from coal fly ash produced in a laboratory combustion system. A cyclone separator was used to separate the PM into fractions 2.5 μm in diameter (PM2.5+). Sulfate was the dominant form of S present. However, thiophene was present in many samples, and sulfite was significant in two of the PM2.5+ samples. For Cr and As, the valence states were of primary concern. Analysis of the X-ray absorption near-edge structure (XANES) regions of the spectra revealed that the toxic Cr6+ valence state accounted for approximately 10 to 30% of the Cr in PM derived from the fly ash of several coals from the western United States. However, only Cr3+ was observed in a number of PM samples from eastern U.S. bituminous coal fly ash. For As, only As5+ as arsenate was detected in all samples examined. However, subtle spectral differences were observed between the arsenate forms observed in western and eastern coal fly ash PM. Zn exhibited a rich chemistry, with significant variation in the major phases observed for eastern coal fly ash PM (principally ZnFe2O4) and western coal fly ash PM (mixed phases). These phase differences undoubtedly reflect the differences in the suites of inorganic constituents contained in the parent coals.

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