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
SeO32- 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 SeO42- 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.
Full
access: http://suo.im/5o9N28
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