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Issue 10, 2018
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Differences in bulk and microscale yttrium speciation in coal combustion fly ash

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Abstract

Coal combustion ash is a promising alternative source of rare earth elements (REE; herein defined as the 14 stable lanthanides, yttrium, and scandium). Efforts to extract REE from coal ash will depend heavily on the location and speciation of these elements in the ash. This study sought to identify the major chemical forms of yttrium (Y), as a representative REE in coal fly ash samples selected from major coal sources in the United States. Y speciation was evaluated using both bulk scale analyses (sequential extractions, Y K-edge X-ray absorption near-edge spectroscopy – XANES) and complementary analyses at the micron scale (micro-focus X-ray fluorescence and micro-XANES). Sequential selective extractions revealed that the REE were primarily in the residual (unextracted fraction) of coal fly ash samples. Extraction patterns for yttrium resembled those of the lanthanides, indicating that these elements were collectively dispersed throughout the aluminosilicate glass in fly ash. Bulk XANES analysis indicated that Y coordination states resembled a combination of Y-oxides, Y-carbonate, and Y-doped glass, regardless of ash origin. However, in the microprobe analysis, we observed “hotspots” of Y (∼10–50 μm) in some samples that included different Y forms (e.g., Y-phosphate) not observed in bulk measurements. Overall, this study demonstrated that yttrium (and potentially other REEs) are entrained in the glass phase of fly ash and that microscale investigations of individual high-REE regions in fly ash samples do not necessarily capture the dominant speciation.

Graphical abstract: Differences in bulk and microscale yttrium speciation in coal combustion fly ash

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Supplementary files

Article information


Submitted
13 Jun 2018
Accepted
10 Sep 2018
First published
28 Sep 2018

This article is Open Access

Environ. Sci.: Processes Impacts, 2018,20, 1390-1403
Article type
Paper

Differences in bulk and microscale yttrium speciation in coal combustion fly ash

R. K. Taggart, N. A. Rivera, C. Levard, J. Ambrosi, D. Borschneck, J. C. Hower and H. Hsu-Kim, Environ. Sci.: Processes Impacts, 2018, 20, 1390
DOI: 10.1039/C8EM00264A

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