ASSESSMENT OF MERCURY SPECIATION IN COAL ASH BY SEQUENTIAL EXTRACTIONS

Coal combustion products (CCPs) such as fly ash represent one of the largest anthropogenic waste streams and are typically enriched with toxic elements including mercury. The presence of mercury in CCPs would be especially hazardous if the metal was converted to a bioaccumulative form (e.g., methylmercury) after disposal or unintended release to the environment. While work to determine total mercury content in fly ash has previously been conducted, little is known about the speciation and reactivity of mercury in fly ash. Speciation of inorganic mercury is a major factor controlling mobility, reactivity, biological availability, and subsequent conversion to methylmercury by sediment bacteria. Here, we used a five-step sequential extraction procedure to operationally define mercury species present in fly ash from three different power plants and in sediment-ash mixtures obtained from the Emory River that was the site of the Tennessee Valley Authority (TVA) Kingston Fossil Plant coal ash spill in 2008. The extractants in this method consisted of deionized water (F1), 0.01N HCl + acetic acid (F2), 1 N KOH (F3), 12 N nitric acid (F4), and aqua regia (F5). Results showed that fly ash contained 0.17 mg/kg of Hg, a concentration that was larger than native Emory River sediments upstream of the spill (0.03 mg/kg) and less than historically contaminated sediments located downstream of the site (0.83 mg/kg). The sequential extraction procedure showed that the largest percentages of mercury in fly ash were released in the F3 KOH step (33% ± 3%) and the F4 nitric acid step (60% ± 4%). The F3 fraction typically corresponds to organic-bound Hg in sediments while the F4 fraction corresponds to “strongly complexed” forms of Hg. Mercuric sulfide (HgS) particles, which are known to persist in the rivers from historical sources, typically fall in the F5 fraction. Therefore, these results suggest that Hg in fly ash does not have the same reactivity as HgS particles originating from other sources. Our study highlights the need to consider reactivity and bioavailability when assessing the risks of mercury in coal ash. Future research will include comparisons of speciation with historically contaminated sediments near the TVA spill site and laboratory experiments that assess the methylation potential of these forms of mercury in river sediments.