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

Abstract

Selenium (Se) contamination can be a potential groundwater concern near un-lined coal ash landfills. Of all the Environmental Protection Agency’s priority and non-priority pollutants, Se has the narrowest concentration range considered beneficial and detrimental for aquatic and terrestrial organisms. 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). 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 SeO 3 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 SeO 4 2- concentration was greater ( p < 0.001) in the weathered (0.67 mg ·kg -1 ) than in fresh ash (0.48 mg ·kg -1 ). Results from this study indicate that environmental weathering of Class C, subbituminous fly ash promotes oxidation of selenite, to the less toxic, but highly mobile selenate. The formation of hydrated ettringite [Ca 6 Al 2 (SO 4 ) 3 (OH) 12· 26H 2 O] and calcium selenite (CaSeO 3 ) likely acted as a sink for weathered ash selenite. 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.