Distinction of strontium isotope ratios between water-soluble and bulk coal fly ash from the United States

Coal ash is one of the largest industrial waste streams in the United States and contains elevated concentrations of hazardous elements that could pose environmental and human health risks. Due to the relatively high strontium concentrations in coal ash and the distinctive Sr-87/Sr-86 ratios, strontium isotopes have been utilized to trace coal ash contaminants in impacted water resources. While previous studies have characterized the Sr isotope ratios for the water-soluble Sr derived from coal ash, the strontium isotopic composition of the bulk coal ash has not been systematically evaluated. In the current study, we report the Sr-87/Sr-86 ratios of bulk and water-soluble Sr in fly ash derived from coals of the three major coal-producing basins in the U.S., the Appalachian Basin, Illinois Basin, and Powder River Basin. The data show a wide range of Sr-87/Sr-86 ratios in the bulk fly ash samples (0.710788-0.722311), with the most radiogenic Sr-87/Sr-86 ratios in fly ash originating from Illinois Basin coals (0.718029 +/- 0.003365; n = 16), followed by those from Appalachian Basin coals (0.712641 +/- 0.000860; n = 16), and those from Powder River Basin coals (0.711663 +/- 0.000489; n = 7). The Sr-87/Sr-86 ratios in the water leachates are significantly lower than the Sr-87/Sr-86 ratios of the bulk fly ash for Illinois Basin and Appalachian Basin (0.711660 +/- 0.001172 and 0.710694 +/- 0.000317, respectively, p-value < 0.01), whereas there is no significant difference for the Powder River Basin fly ash (0.711666 +/- 0.000456, p-value = 0.13). These isotopic differences most likely reflect the different mineralogical compositions of the studied fly ash and the differential solubility of carbonate phases with low Sr-87/Sr-86 ratios relative to more radiogenic silicate phases. Overall, the distinction between the strontium isotope ratios of bulk fly ash versus water-soluble fly ash is important, as it refines the application of strontium isotopes as a forensic tracer for tracking coal ash spills and contamination in the environment.