Mercury (Hg) is a naturally occurring element that poses considerable health risks to humans, with high exposure levels resulting in damage to the brain, heart, kidneys, lungs, and immune system. Young children and unborn babies are particularly vulnerable to mercury, which can affect their nervous systems and impair their neurological development. As a result, mercury is one of the most strictly regulated pollutants by the Environmental Protection Agency (EPA), which controls mercury emissions from coal-fired power plants and issues consumption advisory warnings for various types of fish, the primary route of mercury exposure to humans.

Environmental mercury contamination is widespread due both to point sources and the transport/distribution of mercury on regional and global scales. Since mercury compounds possess a wide range of solubilities in water, understanding the specific forms of mercury present in a contaminated sample and the factors that influence what forms are likely to be present is critical to predicting the mobility, reactivity, and potential bioavailability of mercury in the environment. Research at SSRL by Dr. Christopher Kim of Chapman University and colleagues has resulted in the development of a sensitive technique which uses EXAFS spectroscopy to identify and quantify the proportions of different mercury species present in mercury-bearing samples; as applied to mine wastes from selected mercury and gold mine regions in California and Nevada, this represents the first in situ, non-destructive method by which to identify mercury speciation in heterogeneous samples.

The results of this research, conducted at SSRL Beam Lines 4-3 and 11-2, reveal that geological environment plays an important role in which mercury species are likely to appear, with hot-spring hydrothermal systems containing larger proportions of soluble (and potentially more toxic) mercury chloride species. The roasting of mercury-bearing ore at temperatures approaching 600°C was found to have the effect of converting cinnabar (HgS, hex.) to the more soluble metacinnabar (HgS, cub.) species. Also, total mercury concentrations were found to increase dramatically with decreasing particle size in a heterogeneous mine waste, sometimes by nearly an order of magnitude. While this raises concern due to the higher transport potential for smaller particles, EXAFS analysis also determined that the mercury associated with these small particles is more likely to be present as relatively insoluble mercury sulfides rather than soluble mercury chlorides and oxides. This type of information will provide a higher degree of sophistication in assessing and prioritizing mine sites for remediation by agencies such as the EPA and Bureau of Land Management.