The Mercury Problem: Global Considerations

Mercury is a basic element forming a significant portion of the earth's crust. The amount present today is identical to that present at the time the earth was formed, but the distribution of mercury has been drastically changed as a result of continued offgassing from the earth and increasingly wide industrial use and subsequent release of fumes from burning coal and oil, and the disposal of mercury-containing products. When mercury ore, or mercury-containing coal, is dug out of the earth vaporization and oxidation occur, creating elemental mercury (Hg0), ionic mercury (Hg++) and particles containing inorganic mercury (Hgpart). In the course of time the mercury compounds become a component of the atmosphere, and begin global distribution. Mercury is unique in that

  1. it is the only heavy metal that is a liquid at room temperature,
  2. it is the heaviest liquid in existence,
  3. it readily vaporizes into the atmosphere,
  4. it can form stable covalent bonds with carbon,
  5. most of its compounds are toxic, depending primarily upon their solubility in water and lipids and
  6. it has no known physiological benefit to living organisms.

In earlier geologic times mercury, as a component of highly insoluble mercury ore (cinnabar), was buried along with other minerals and only released into the atmosphere during earth movements such as volcanoes and similar thermal situations. The natural background levels of mercury have been greatly enhanced in recent times by the release of thousands of tons of mercury and its compounds into the environment as a result of anthropogenic activities such as mining, combustion of contaminated fossil fuels, and disposal of mercury-containing wastes. Consequently we are facing dramatic increases in the levels of mercury spread over the surface of the planet - in the atmosphere, in the oceans, over the land and eventually in living organisms. The industrial revolution has exacerbated a massive transformation from underground (essentially inert) deposits of cinnabar, coal and crude oil to widely dispersed mercury vapor with its capacity for transformation into ionic mercury. In the presence of bacteria in soil and sediment methylation can occur transforming the ionic inorganic mercury into organic mercury potentially increasing its eventual uptake and accumulation in living organisms.

Figure 1 is a simplified version of the mercury biogeochemical Cycle. This cycle consists of three phases - terrestrial, oceanic and atmospheric. The major pathways into the atmosphere from natural offgassing by earth movements and volcanoes are depicted on the left along with the varied emissions from industrial activities and disposal of products in which mercury is a component. Inorganic mercury compounds become widely dispersed in the atmosphere and are subjected to repeated deposition and resuspension both over the ocean and over land. In the process of this cycling some of the mercury (Hg0) is oxidized in the presence of ozone, oxygen and moisture to form ionic mercury (Hg++) which is highly water soluble and readily incorporates into rain drops. This ionic mercury (Hg++) is carried by rainwater into soil and sediment containing bacteria, algae, fungi and other organisms. These organisms react with mercury, converting some of it back to its gaseous elemental state for re-release into the atmosphere. Depending upon the conditions and the organisms present in soil and sediment the inorganic mercury can be converted or methylated into organic mercury. The organic mercury (methylmercury MeHg+, and dimethylmercury DMeHg) has the capacity to penetrate through cell membranes and react with essential proteins, amino acids and nucleic acids within the cells. In this form it is highly toxic and capable of being incorporated into the aquatic organisms that are part of the food chain of higher animals. These methylmercury compounds with their capacity to bioaccumulate and biomagnify can compromise the health of higher organisms. This inadvertent exposure to elemental and organic mercury in the environment poses a serious threat to humans as well as wildlife.

Additional References:

Mercury in the Aquatic Environment: a review of factors affecting methylation
Ullrich SM, et al 2001
Critical Reviews in Environmental Science and Technology 31 (3):241-283

Mercury Contamination of Aquatic Ecosystems
By D.P. Krabbenhoft and D.A. Rickert
November 3, 1997

Mercury Contamination of Aquatic Habitats
August 2, 2000

Low-Level Collection Techniques and Species-Specific Analytical Methods for Mercury in Water, Sediment, and Biota
By Mark L. Olson and John F. DeWild
Spring 1999
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