Distinction of strontium isotope ratios between water-soluble and bulk coal fly ash from the United States
Introduction
Coal ash represents one of the largest industrial solid waste streams in the United States. Approximately 102 million tons of coal ash were produced in the U.S. in 2018, over 40% of which was disposed of in surface impoundments and landfills (American Coal Ash Association, 2019). Coal ash is generally composed of fly ash, bottom ash, boiler slag, and flue gas desulfurization (FGD) products. Of these, fly ash has raised the greatest environmental concerns due to its relatively greater abundance, finer particles, and elevated concentrations of potentially toxic trace elements (e.g., As, Se, Cd, Cr, and Pb), which, upon release to the environment, may pose risks to human and ecological health (Carlson and Adriano, 1993; Córdoba et al., 2012; Izquierdo and Querol, 2012; Kosson et al., 2002; Nelson et al., 2010; Swaine, 1994; Thorneloe et al., 2010; Twardowska et al., 2003). The massive volume of coal ash that has been generated in the U.S. and its potential toxicity pose major challenges for coal ash management and disposal. There have been several incidents of accidental coal ash spills caused by infrastructure failures, including events at the Tennessee Valley Authority (TVA) Kingston Fossil Plant (Harriman, Tennessee) in 2008; Dan River Steam Station (Eden, North Carolina) in 2014; and Sutton Steam Plant (Wilmington, North Carolina) in 2018 (Lemly, 2015; Ruhl et al., 2012, Ruhl et al., 2010, Ruhl et al., 2009; Vengosh et al., 2019). Contaminants may also enter the environment due to ash pond leaking and/or direct discharge of coal ash effluents (Harkness et al., 2016; Ruhl et al., 2012, Ruhl et al., 2014). In order to delineate the environmental impacts of released coal ash, a variety of geochemical and isotopic tracers (e.g., boron, strontium, radium, mercury, and lead isotopes) have been used to identify the presence of coal ash and elucidate the pathways of coal ash contamination in the environment (Bartov et al., 2013; Deonarine et al., 2013; Hurst et al., 1993, Hurst et al., 1991; Hurst and Davis, 1981; Lauer et al., 2015; Ruhl et al., 2014; Wang et al., 2019).
Strontium is a trace element with an average concentration of ~ 110 mg/kg in raw coals (Ketris and Yudovich, 2009). Following coal combustion, strontium is further enriched in coal ash (global mean value ~ 740 mg/kg; Ketris and Yudovich, 2009), and in some cases may reach up to ~ 3000 mg/kg in fly ash (Hurst and Davis, 1981; Straughan et al., 1981). Strontium is also readily mobilized in the aquatic environments (Hurst et al., 1993, Hurst et al., 1991; Ruhl et al., 2014), and its mobilization is independent of redox conditions (Ruhl et al., 2014), rendering Sr isotopes an ideal tracer for coal ash contamination in water.
There are four naturally occurring stable isotopes of Sr, of which 88Sr (natural abundance = ~ 82.6%), 86Sr (natural abundance = ~ 7.0%), and 84Sr (natural abundance = ~ 0.6%) are non-radiogenic, while 87Sr (natural abundance = ~ 9.9%), the decay product of 87Rb (half-life t1/2 = 48.8 billion years), is radiogenic (Åberg, 1995). Due to the differences in age, lithology (e.g., mafic versus felsic rocks), and chemical composition (e.g., Rb/Sr ratio), different geologic materials have large variations in the 87Sr/86Sr ratio, rendering it a useful tool for a variety of geological studies (e.g., evolution of seawater composition) and environmental studies (e.g., tracing anthropogenic contamination) (Åberg, 1995; Burke et al., 1982; Dasch, 1969; Hodell et al., 1991; Sherman et al., 2015; Spiro et al., 2019; Veizer and Compston, 1974). Because fly ash has a relatively high Sr concentration and distinctive 87Sr/86Sr signatures, it has been used to detect coal ash contamination in the environment in previous studies. Hurst and Davis (1981) and Hurst et al. (1991) were the first to demonstrate the potential of using 87Sr/86Sr ratio as an indicator for airborne fly ash as well as effluents derived from coal-fired power plants in the southwestern United States. Brubaker et al. (2013) performed both column and sequential batch leaching experiments on four site-specific fly ash samples and found a temporal increase in the 87Sr/86Sr ratios of leachates, resulting in a range from 0.7107 to 0.7138, which was associated with the isotopic composition of coal ash pond effluents in West Virginia (0.7124) as well as the composition of the Appalachian Basin coals (Vengosh et al., 2013). Ruhl et al. (2014) conducted a more systematic analysis of the Sr isotope ratios in water leachates from coal ash of different feed coal sources across the United States and showed distinguishable 87Sr/86Sr ratios in water-soluble Sr. The Sr isotopic fingerprints of coal ash water leachates, combined with other geochemical tools (e.g., boron isotopes), have subsequently been used as evidence for the mobilization of contaminants from a coal ash spill in Kingston, TN (Ruhl et al., 2014) and for the leaking of coal ash ponds into adjacent surface water and shallow groundwater (Harkness et al., 2016). Strontium isotopes have also helped to distinguish between naturally occurring and coal ash derived contaminants in groundwater near coal ash ponds in North Carolina (Vengosh et al., 2016) and used as a biogenic tracer of coal ash contamination and bioaccumulation in lake ecosystems (Brandt et al., 2018). While these studies have characterized and utilized the isotope ratio of the soluble fraction of Sr, limited data exist on the Sr isotopic composition of bulk coal ash. Spivak-Birndorf et al. (2012) compared the Sr isotopic ratios from both full digestion and sequential leaching of 11 U.S. fly ash samples, comprising both Class-F and Class-C fly ash, and showed that the water leachable fraction of Sr in fly ash has a lower 87Sr/86Sr ratios relative to the bulk fly ash. This suggests that that Sr with variable 87Sr/86Sr exists in different phases, both soluble and insoluble, within the heterogeneous fly ash matrix (Spivak-Birndorf et al., 2012).
This study aims to provide a more comprehensive dataset of the concentrations and isotopic compositions of Sr in coal fly ash from coals of the three major coal-producing basins in the United States: the Appalachian Basin, Illinois Basin, and Powder River Basin (East, 2013). These basins accounted for 21.4%, 14.1%, and 40.2% of the total U.S. coal production in 2018, respectively (U.S. Energy Information and Administration, 2019). We compare the Sr concentrations and 87Sr/86Sr ratios of both the bulk and water-soluble fractions of fly ash. Furthermore, we address the environmental applications and limitations of using 87Sr/86Sr ratios in both soluble and bulk coal ash for tracing contamination associated with the migration of both coal ash effluents and solids in the environment.
Section snippets
Sample collection
A total of 45 fly ash samples, collected from 14 U.S. coal-fired power plants between 2004 and 2015, were analyzed in this study. Specific sample collection information is presented in Table 1. Since coal-fired power plants sometimes burn feed coal by blending coals from multiple coal basins, careful attention was paid to collect fly ash samples specifically from power plants burning feed coal from only one known coal basin during the time of sampling (Table 1). It should be noted that all the
Concentrations of strontium in bulk and water-soluble fractions of fly ash
The concentration of Sr in the bulk fly ash extracted by full digestion shows large variations, ranging from 137 to 3323 mg/kg (Table 2). The fly ash samples from the Illinois Basin have the lowest bulk Sr concentration (mean = 269 ± 114 mg/kg, n = 22), ranging from 137 to 477 mg/kg, and the Appalachian Basin samples have a higher mean concentration (924 ± 307 mg/kg, n = 16) and a broader range, between 480 and 1527 mg/kg (Fig. 1A). The Powder River Basin samples have the highest bulk Sr
Conclusion
This study provides new data on the concentrations and isotopic compositions of Sr in coal fly ash originated from coals of the three major coal-producing basins in the United States, the Appalachian Basin, Illinois Basin, and Powder River Basin. The data show large variations in the Sr concentrations of both bulk (total digestion) and water-soluble Sr in fly ash, with fly ash samples derived from Illinois Basin coals having the highest mean Sr concentration, followed by fly ash samples from
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
Zhen Wang was supported by Duke University Nicholas School of the Environment. Heileen Hsu-Kim was partially supported by the U.S. National Science Foundation (CBET-1510965 and CBET-1510861). We thank three anonymous reviewers for their critical reviews that helped to improve the quality of this paper.
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