X-ray Absorption Fine Structure (XAFS) Spectroscopic Characterization of Emissions from Combustion of Fossil Fuels

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The application of XAFS spectroscopy is described for the determination of the speciation of sulfur and various hazardous or key metals in both coarse (>2.5μm) and fine (<2.5μm) fractions of particulate matter (PM) emitted in three different fossil-fuel combustion processes. The three processes consisted of (i) combustion of residual oil, (ii) combustion of various eastern and western U. S. coals, and (iii) combustion of diesel fuel formulations in an engine. Sulfur was found to exist in mainly sulfate forms in the emissions from all three processes, although minor to significant fractions of the sulfur were also present as thiophenic sulfur, elemental sulfur, and various other sulfur forms, depending on the fossil fuel, the combustion conditions, and the particle size of the collected fraction. Combustion of residual oil tended to form metal sulfates predominantly, although the coarse fractions (PM2.5+) also contained minor amounts of sulfides of Fe, Cu and Ni, while the fine fractions (PM2.5) contained oxides instead of sulfides as their minor components. Due to the presence of aluminosilicates in the coal, metal species in coal emissions were more complex. Chromium was present entirely as Cr3+ in both fine and coarse PM from combustion of eastern U. S. coals, but significant fractions of the chromium were present as more toxic and carcinogenic Cr (VI) species in fine and coarse PM from combustion of western U. S. coals. In contrast, arsenic was present entirely as arsenate (As043-) species in all coal PM samples (and residual oil PM samples), regardless of origin. No evidence was found for the more toxic As (III) oxidation state in any sample. The zinc speciation in PM samples from combustion of coal samples was also found to vary with geographic origin of the coal. Zinc in PM samples from eastern U. S. coals was present mostly as Zn ferrite (ZnFe2O4), while the Zn speciation in PM from western U. S. coals was more complex. The XANES signatures of Zn in PM from both residual oil and diesel combustion processes were quite different from those of PM from coal combustion and suggestive of anhydrous zinc sulfate.<BR>Comparison was also made of sulfur and zinc XANES spectra for PM collected on filters from the ambient atmosphere with those from the fossil fuel combustion processes. Significant differences were observed in both sulfur and zinc speciation.

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