No abstract

In a cooperative agreement with DOE , the USGS has participated with Physical Sciences, Inc. (PSI) in a project entitled "Toxic Substances From Coal Combustion -A Comprehensive Assessment". Samples from the Pittsburgh, Elkhorn/Hazard, Illinois No. 6, and Wyodak program coals were examined to determine the mode of occurrence of selected trace elements (As, Se, Cr, Hg, and Ni) using selective leaching, scanning electron microscopy, electron microprobe analysis, and X-ray diffraction techniques. Among other findings, our results indicate that the bulk of the arsenic in the Pittsburgh and Illinois No. 6 coals is in pyrite. High percentages (60-80%) of arsenic were leached by nitric acid, and microprobe data confirm the presence of arsenic in pyrite in each of these coals (concentrations ranging from <0.01 to 0.09 wt.% of the pyrite grains). In the Elkhorn/Hazard coal, arsenic may have several modes of occurrences. About 30 percent of the arsenic in the Elkhorn/Hazard coal was leached by hydrochloric acid, possibly indicating the presence of arsenates that were formed by the oxidation of pyrite. About 25 percent of the arsenic in the Elkhorn/Hazard coal was leached by nitric acid, suggesting an association with pyrite. Only sixty percent of the total arsenic in the Elkhorn/Hazard coal was leached. The low percentage of leachable arsenic may be accounted for by unleached pyrite grains, which were detected in solid residues from the nitric acid leach. In the Wyodak coal, arsenic probably occurs in iron oxides or carbonates (35 % arsenic leached by HCl) and clays (15% arsenic leached by HF). Arsenic in the Wyodak coal may also have an organic association, as indicated by low totals for leaching (50% unleached arsenic). In the four program coals 20 to 45 percent of the chromium was leached by hydrofluoric acid, suggesting an association with silicates (probably illite). Microprobe analysis of the Pittsburgh, Elkhorn/Hazard, and Illinois No. 6 coals confirmed the presence of chromium in illite and possibly in other clays, at concentrations that are near the detection limits. Results related to the forms of occurrence of the other trace elements (Se, Hg, and Ni) are varied; further work in Phase II is planned to determine their mode of occurrence. 2.Plots of arsenic concentration vs. maximum dimensions (in polished section) for pyrite grains in the Pittsburgh (Fig. 2a), Illinois #6 (Fig. 2b), and Elkhorn/Hazard (Fig. 2c) coal samples. Data points represent individual microprobe analyses, and generally include more than one analysis per grain. Arsenic data collected prior to 11-1-96 are adjusted to conform to results obtained with improved analytical technique used thereafter (Appendix II). 3.Plots of nickel concentration vs. maximum dimensions (in polished section) for pyrite grains in the Pittsburgh (Fig. 3a), Illinois #6 (Fig. 3b), and Elkhorn/Hazard coal samples (Fig. 3c). For the Illinois #6 and Elkhorn/Hazard coals, data suggest that smaller pyrite grains, primarily framboids, are enriched in nickel. Data poi...