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.
|Number of pages||14|
|Journal||Environmental Sciences: Processes and Impacts|
|State||Published - Oct 2018|
Bibliographical noteFunding Information:
This work was supported by the U.S. National Science Foundation (#CBET-1510965 and CBET-1510861), the NSF Partnership for International Research and Education (#OISE-1243433), and the U.S. Department of Energy (DE-FE0026952). RKT was partly supported by a Graduate Scholarship in Solid Waste Research from the Environmental Research and Education Foundation.
© The Royal Society of Chemistry.
ASJC Scopus subject areas
- Environmental Chemistry
- Public Health, Environmental and Occupational Health
- Management, Monitoring, Policy and Law