Direct speciation of chromium in coal combustion by-products by X-ray absorption fine-structure spectroscopy

Chromium X-ray absorption fine-structure (XAFS) spectroscopy has been used to determine the relative amounts of Cr(VI) and Cr(III) in coal combustion by-products, principally fly-ash and bottom ash. The method, which is based on the relative heights of the pre-edge peaks for the different Cr oxidati on states in X-ray absorption near-edge structure (XANES) spectra, can be u sed to speciate as little as 50 ppm of chromium in ash. The results indicat e that the fraction of Cr(VI) oxidation state present in combustion ash sam ples from most commercial combustion plants is typically below or close to the detection limit (approximately 3-5% of the total chromium). There are e xceptions, however, that have been found to reach as high as 20%. Such find ings provide justification for a reappraisal of whether or not chromium sho uld be considered a significant hazardous air pollutant in coal combustion. (C) 1998 Elsevier Science Ltd. All rights reserved.