| Abstract
| - Both XAFS spectroscopy and sequential leaching have been used in previous studies todetermine the modes of occurrence of arsenic and other elements in coal. However, a directcomparison of the two methods, carried out on identical fractions, has never been made. In thisstudy, residues of coal extracted after each stage of the U. S. Geological Survey's leaching protocol(sequential extraction by solutions of ammonium acetate, HCl, HF, and HNO3) are examined byarsenic XAFS spectra. This procedure enables a rigorous evaluation to be made of the arsenicspecies leached at each stage of the sequential leaching method. We observed an approximatelinear correlation (r2> 0.98 for an Ohio bituminous coal, r2> 0.90 for a North Dakota lignite)between the height of the edge step in the XAFS analysis and the concentration of arsenic in theleached residues measured by INAA. Data from the leaching protocol indicate that two majorarsenic forms occur in both coals; one is removed by leaching with HCl and the other by HNO3.The XAFS spectral signatures of the arsenic leached by HCl from both coals identify it as arsenate(AsO43-). The arsenic leached by HNO3 from the Ohio coal is identified as arsenic that substitutesfor sulfur in pyrite (arsenic/pyrite), whereas the arsenic leached by HNO3 from the North Dakotalignite is present in two forms: an As3+-O species, believed to be organically bound, in additionto arsenic/pyrite. Minor arsenate (5−15% of the original As) persists in both coals after the finalleaching with HNO3. The arsenate species extracted in HCl are most likely oxidation productsderived from the original arsenic forms upon exposure of the pulverized coals to air. Time-dependent phenomena such as oxidation make it imperative that such combined speciation studiesbe carried out on identical suites of samples.
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