Impact of manganese and iron oxides on the dissolution of elemental mercury

Localized deposits of elemental Hg (Hg(0)) are present in soils worldwide as a result of releases from industrial facilities, such as chloro-alkli facilities, and artisanal Hg gold mining activities. In Oak Ridge, TN, USA, the DOE Y-12 National Security Complex (Y-12 NSC), used 11 million kg of Hg(0) between 1950 and 1963 for lithium isotope separation. Mercury released during this period still remains in sediment, groundwater, and in and under buildings at the facility, as shown in recently recovered sediment cores. Scanning electron microscopy (SEM) was used to examine Hg-ladden cores. SEM images show distinct intact or fragmented spherical solids which appeared to be associated with Hg(0) beads. Further characterization of Hg beads demonstrated that crystalline mercury oxide (HgO) was present on the beads surface. The HgO coating present on the Hg(0) beads were found to facilitate dissolution of Hg(I,II) into water. Laboratory experiments were conducted by adding coated and uncoated Hg(0) beads to water and measuring the release of oxidized Hg (Hg(I,II)). Compared with uncoated Hg(0) bead standards, coated beads of Hg(0) collected from Y-12 soil cores rapidly (e.g., 10 minutes) released 15-45 times more Hg(I,II) into solution. Studies were conducted to evaluate factors affecting the development and dissolution of the HgO coating. Beads of Hg(0) exposed to ambient air for 8-10 days did not release significantly higher amounts of Hg(I,II) than beads exposed to N2; indicating that oxygen itself does not have the ability to increase the oxidation rate of the Hg(0). When pure Hg(0) was added to a soil from Y-12 for 10 days the released Hg(I,II) was 14 times greater than a Hg(0) bead without soil treatment. Mixing and incubating Hg(0) beads with sand, kaolinite or Fe¬2¬O¬3, respectively, increased the release of Hg(I, II) into solution by a factor of 2-3 compared to the clean untreated Hg(0) standard bead; which was less than Hg(0) mixed with soil. Hg(0) beads mixed and incubated in Mn solids resulted in the greatest production of HgO coatings and release of Hg(I,II). Incubations with MnO2 resulted in the same order of magnitude increase as the soil incubations, while Hg(0) incubated in Mn2O3 for 10 days produced Hg(I,II) concentrations that were 290 times greater than the clean beads. Energy dispersive X-ray spectroscopy mapping of cores collected from Y-12 NSC show an association between Mn oxide minerals and elevated Hg concentrations providing additional evidence that Mn is interacting with Hg in soils. The oxidizing potential of Mn minerals in soils and sediments is well documented with other metals but has not been demonstrated with mercury. These findings have important implications for understanding the mobility of Hg in contaminated environments since the solubility and mobility of Hg(I,II) is greater than that of Hg(0).