Combined application of XAFS spectroscopy and sequential leaching for determination of arsenic speciation in coal

Both XAFS spectroscopy and sequential leaching have been used in previous studies to determine the modes of occurrence of arsenic and other elements in coal. However, a direct comparison of the two methods, carried out on identical fractions, has never been made. In this study, 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 by arsenic XAFS spectra. This procedure enables a rigorous evaluation to be made of the arsenic species leached at each stage of the sequential leaching method. We observed an approximate linear correlation (r(2) > 0.98 for an Ohio bituminous coal, r(2) > 0.90 for a North Dakota lignite) between the height of the edge step in the XAFS analysis and the concentration of arsenic in the leached residues measured by INAA. Data from the leaching protocol indicate that two major arsenic 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 substitutes for sulfur in pyrite (arsenic/pyrite), whereas the arsenic leached by HNO3 from the North Dakota lignite is present in two forms: an As3+-O species, believed to be organically bound, in addition to arsenic/pyrite. Minor arsenate (5-15% of the original As) persists in both coals after the final leaching with HNO3. The arsenate species extracted in HCl are most likely oxidation products derived 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 studies be carried out on identical suites of samples.