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 oxidation states in X-ray absorption near-edge structure (XANES) spectra, can be used to speciate as little as 50 ppm of chromium in ash. The results indicate that the fraction of Cr(VI) oxidation state present in combustion ash samples from most commercial combustion plants is typically below or close to the detection limit (approximately 3-5% of the total chromium). There are exceptions, however, that have been found to reach as high as 20%. Such findings provide justification for a reappraisal of whether or not chromium should be considered a significant hazardous air pollutant in coal combustion. (C) 1998 Elsevier Science Ltd. All rights reserved.