CHARACTERIZATION OF MERCURY IN SEDIMENT CORES FROM THE Y-12 NATIONAL SECURITY COMPLEX IN OAK RIDGE, TN, USA

The Y-12 National Security Complex (Y-12 NSC), a Department of Energy facility in Oak Ridge, TN, USA, used 11 million kg of liquid mercury (Hg⁰) between 1950 and 1963 for lithium isotope separation processes. Mercury released during this period still remains in soils, sediment, groundwater, and in and under buildings at the facility. Mercury speciation in sediment cores collected within Y-12 NSC was examined to understand the reactivity and mobility of the Hg and to provide information to help design of efficient remediation strategies. A combination of total Hg (HgT) analysis, gaseous Hg(0) headspace analysis and Hg sequential extractions was applied to obtain the speciation of Hg in soil cores. HgT concentrations in the soil ranged from 0.05 to 8400 mg/kg and the highest concentrations were associated with deposits of liquid Hg⁰. However, Hg was also found in the divalent state, Hg(II), likely as Hg-sulfide species and associated with organic matter in the solid phase. Groundwater appears to influence the concentration and speciation of Hg in the sediment. At depths below the groundwater table, where reducing conditions prevail, concentrations of Hg are higher than in the sediment zones above and solid phase Hg-sulfide species are the dominate Hg form in these environments. This suggests that the groundwater can be a source of Hg in the sediments. Using scanning electron microscopy coupled with energy dispersive X-ray spectroscopy we found that the Hg⁰ beads in the core samples were coated with mercury oxide, HgO. When added to water, Hg(0) beads containing these oxide coatings released 4 to 30 times more Hg relative to Hg(0) beads without the coating. The formation of HgO around the Hg⁰ beads in-situ likely enhances the mobility of mercury since the solubility of HgO in water is greater than Hg⁰. The knowledge gained with regard to how mercury speciates in the contaminated soil sediments will help guide the development of effective remediation strategies since technologies can be tailored to specific Hg species.