EXAFS of Frozen Elemental Mercury and its Implications for Abandoned Mercury Mine Wastes

Mercury in the environment is a large concern from both ecosystem and human health perspectives. Mine wastes at inoperable mercury mines throughout California can be highly elevated in mercury concentration, with some materials having mercury concentrations above 2% wt. It is of prime importance to know the speciation of Hg within these sediments to gauge potential bioavailability of Hg from these impacted areas of California. In order to assess Hg speciation within impacted sediments, Hg EXAFS studies have previously been conducted at room temperature. However, elemental Hg at room temperature is a liquid, which results in very little structure in the EXAFS region, thus making it difficult to determine relative proportions of elemental Hg in sediments. Hg LIII EXAFS of Hg mine wastes previously analyzed at room temperature were re-analyzed at 77K to determine what proportion of the Hg speciation consists of elemental Hg. By using least squares fitting it is shown that some Hg impacted mine wastes contain up to 25% elemental Hg. The same sediment samples used in the frozen EXAFS studies were also used in measuring the flux of Hg leaving the sediments during both light and dark exposures. Ratios between light and dark exposures for Hg fluxes, when normalized to concentration, showed some samples having nearly 60 times more Hg released when exposed to light versus the dark. The light/dark ratios of the mine sediments did not relate to overall Hg concentrations and could not be explained by Hg speciation determined from room temperature EXAFS. By doing EXAFS at 77K we see that there is a correlation between Hg light:dark fluxes and percentages of elemental Hg, with higher ratios correlating with higher Hgo percentages. Sediment samples with light:dark <3 show no evidence of elemental Hg from EXAFS, while samples that show ratios at nearly 60 contained ~25% elemental Hg. This research illustrates that a significant species of Hg has been neglected from the overall Hg speciation in environmental samples. The correlation between elemental Hg and light:dark fluxes should allow researchers to determine which mine wastes have the highest possibility of gaseous Hg emissions into the surrounding environment. In order to accurately determine speciation of Hg within sediments by EXAFS cryostat temperatures are needed and a rethinking of previous work on Hg speciation in the environment needs to be done.