CROWLEY, SHARON S.; PALMER, CURTIS A.; KOLKER, ALLAN; FINKELMAN, ROBERT B.; KOLB, KATHLEEN C. and WILLETT, JASON C., U.S. Geological Survey, Reston, VA

The modes of occurrence of As, Hg, Se, and Cr were examined in three samples of bituminous coals from some of the major coal beds used for power generation in the United States: (1) a sample from the Illinois No. 6 coal bed, (2) a washed fraction from the Pittsburgh coal bed, and (3) a blended, low-sulfur "compliance" coal sample from the Elkhorn and Hazard coal beds. Sequential leaching of each coal sample (using ammonium acetate, hydrochloric acid, hydrofluoric acid, and nitric acid) was used as an indirect method for determining the modes of occurrence of the trace elements. Microprobe analysis and scanning electron microscopy were used as direct methods for determining the mode of occurrence of the trace elements.
Our data suggest that the bulk of the As in the Pittsburgh and Illinois No. 6 coals is in pyrite. High percentages (mean = 70 %) of As were leached by nitric acid; and microprobe analyses confirm the presence of As in some of the pyrite grains analyzed (0.01-0.08 wt. %). In the Elkhorn/Hazard coal, however, 30 percent of the As is leached by hydrochloric acid and 25 percent by nitric acid. Leaching of As by hydrochloric acid may be interpreted as the leaching of arsenates which were formed from the oxidation of arsenic-bearing pyrite.
In general, pyrite grains observed in the Elkhorn/Hazard sample have As concentrations that are slightly higher (range: 0.01-0.27 wt. %) than samples of the Pittsburgh or Illinois No. 6 coals. Leaching data for Hg, Se, and Cr in all three samples are inconclusive, but indicate that some Cr may be associated with silicates (illite) and some Se may be associated with the organic constituents.

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