Extraction Characteristics of Selenium as Affected by Coal Fly Ash Type, Water Extractant, and Extraction Time

During the combustion of coal, inorganic constituents that are naturally enriched in the coal are further concentrated in the coal combustion residuals (CCRs): bottom ash and fly ash. And of the inorganic constituents concentrated in the CCRs, especially fly ash, selenium (Se) is of particular concern due to the transference of Se from the coal to the ash through a physical, volatilization-condensation adsorption process. And the Se-laden leachate from coal ash landfills can be transported with the groundwater and become a human and ecological threat.

In this paper, the effects of ash type (i.e., fresh and weathered), water-extractant type (i.e., deionized water, rainwater, and groundwater), and extraction time (i.e., 2 and 6 hours) on Se, arsenic (As), and chromium (Cr) concentrations were investigated from Class C, subbituminous coal fly ash produced at the Flint Creek Power Plant (Benton County, AR).

The results showed that water-extractable Se concentrations differed (p = 0.03) between ash types across water-extractants, but were unaffected (p > 0.05) by extraction times. Unexpectedly, fresh ash water-extractable Se concentrations were below minimum detection limits (i.e., 2.0 μg·L^-1) for all treatments. In contrast, averaged over extraction times, the water-extractable Se concentration from weathered ash was greatest (p < 0.05) with groundwater and rainwater, which did not differ and averaged 60.0 μg·L^-1, compared to extraction with deionized water (57.6 μg·L^-1). Selenite SeO₃^2- was greater (p < 0.001) in the fresh (3.85 mg·L^-1) than in the weathered ash (0.70 mg·kg^-1), while selenate SeO₄^2- concentration was greater (p < 0.001) in the weathered (0.67 mg·kg^-1) than in fresh ash (0.48 mg·kg^-1).

In conclusion, fresh and weathered sub-bituminous Powder River Basin (PRB) fly ash had significantly different water-extractable Se, As, and Cr characteristics, with fresh ash water-soluble concentrations at or near the minimum detection limits (MDLs) across all treatments. Differences between fresh and weathered ash were likely due to differences in the redox status of elements in the two ashes. Selenite was shown to be the dominate form present in the fresh ash, likely preventing the release of Se during the water-extraction study due to its low solubility.  

Implications of this research include a better understanding of the past, present, and future environmental and health risk potential associated with the release of watersoluble Se, As, and Cr to aid in the development of sustainable fly ash management strategies.

Article by Mark A. Cantrell, et al, from USA.   

           

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