Cantrell produced a Master's thesis #1054 from the University of Arkansas, Fayetteville in 2014 on the leaching characteristics of selenium from coal fly ash. Selenium (Se) contamination can be a potential groundwater concern near un-lined coal ash landfills. Out of all the Environmental Protection Agency (EPA) priority and non-priority pollutants, Se has the narrowest range between what is 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 located in Benton County, AR.
Water-extractable Se concentrations differed (P = 0.03) between ash types across water-extractants, but were unaffected 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 weathered ash water-extractable Se concentration 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 (Se4+) was greater (P < 0.001) in the fresh (3.85 mg kg-1) than in the weathered ash (0.70 mg kg-1), while selenate (Se6+) 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 (Se4+), to the less toxic, but highly mobile selenate (Se6+). The formation of hydrated ettringite [Ca6Al2(SO4)3(OH)12 - 26H2O] and calcium selenite (CaSeO3) likely acted as a sink for weathered ash selenite (Se4+).
The author concluded that when performing an environmental assessment of the potential environmental impact of trace elements in fly ash, the total concentration is not nearly as important as knowing the water-soluble and speciation fractions. The water-soluble fraction is the most likely fraction to be released into the environment and the speciation is what determines the mobility and toxicity once released. Fresh and weathered subbituminous PRB fly ash had significantly different water-soluble Se, As, and Cr leaching characteristics, with fresh ash water-soluble concentrations at or very near the MDL across all treatments. Differences between fresh and weathered ash were likely due to the speciation present in the ash. Selenite (Se4+) was shown to be the dominate species present in the fresh ash, likely preventing the release of Se during the water-extraction study.
Once landfilled, environmental weathering promotes the oxidation of selenite (Se4+) to the highly mobile, water-soluble selenate (Se6+). Although Cr and As speciation analyses was not performed, differences between fresh and weathered ash water-soluble Cr concentrations indicate that the less water-soluble trivalent (Cr3+) Cr was likely dominate in the fresh ash and the more water-soluble hexavalent (Cr6+) Cr was likely dominate in the weathered ash. Weathered ash water-soluble Se and Cr concentrations indicate that there is still a significant amount of Se and Cr readily available to be released into the environment if allowed to come in contact with rainwater or groundwater.
Fresh and weathered ash water-soluble As concentrations at or near the MDL were likely due to As existing and remaining in the arsenate (As5+) form. Previous research indicates that the less water-soluble arsenate (As5+) is the dominate species present in fresh subbituminous PRB ash and it is unlikely that the arsenate (As5+) is being reduced to the more water-soluble arsenite (As3+) in the landfill (Huggins et al., 2007). Therefore, results from this study indicate that speciation is likely responsible for the differences between extraction behavior of fresh and weathered ash Se, As, and Cr concentrations.
Weathered ash water-soluble Se concentrations were greatest when extraction occurred with environmentally representative rainwater and groundwater. Similarly, weathered ash water-extractable Cr concentration was greatest with groundwater. Selenium and As extraction was unaffected by extraction time; however, weathered ash water-soluble Cr concentration increased as the extraction time increased from 2- to 6-hours. This research was not able to demonstrate if 6 hours was enough time to ensure maximum Cr extraction; therefore, a 24-hour extraction is recommended for Cr (EPRI, 1994). Water-soluble As concentrations were too near the MDL to make an informed decision for both fresh and weathered ash. Therefore, a longer extraction time and lower dilution ratio may be required to obtain measurable water-extractable As concentrations from a Class C fly ash (Wang et al., 2009). To obtain maximum fresh and weathered ash suspension pH and EC, it is recommended to perform at minimum a 6-hour extraction with deionized water for both fresh and weathered ash. Extraction with environmentally representative rainwater and groundwater offered no distinct benefits when compared to deionized water as the extractant of choice for suspension pH and EC.
It should be re-iterated that results from this study were derived from the extraction of a Class C fly ash produced from the combustion of a subbituminous PRB coal and may not be comparable to extraction from Class F fly ash, whereas the goal of this study was to better understand the behavior of a site-specific fly ash and not fly ash in a holistic sense.
Implications of this research include a better understanding of the past, present, and future environmental and health risk potential associated with the release of water-soluble Se, As, and Cr to aid in the development of sustainable fly ash management strategies.
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