Overview

The essence of the Mercury Cycle is the evaporation of inorganic mercury from both natural and man-made sources into the atmosphere where it is then oxidized in the upper atmosphere and returned back to earth, most commonly in precipitation, in its inorganic mercury form. It is dispersed evenly throughout the environment and the inorganic mercury is biomethylized by bacteria into the more toxic formation, methyl mercury. Once converted, the methyl mercury then enters the food chain and biomagnifies up the food chain (#Clarkson, 2002).

There are 6 universally recognized steps to the Mercury Cycle (#UWEC and #Purdue):
1. Degassing of mercury from rock, soils, and surface waters, or emissions from volcanoes and from human activities.
2. Movement in gaseous form through the atmosphere.
3. Deposition of mercury on land and surface waters.
4. Conversion of the element into insoluble mercury sulfide.
5. Precipitation or bioconversion into more volatile or soluble forms such as methyl mercury.
6. Reentry into the atmosphere or biomagnify up the food chain.

Sources of Mercury

Though many sources of mercury are naturally existing, the current levels of mercury level is estimated to be 2 to 5 times greater than its preindustrial level due to high levels of mining and coal combustion (#Princeton, 2004). Sources include:

• Burning of Fossil Fuels, especially coal
  • Coal fired power plants are the largest source of inorganic mercury release in the US and account for 33% of all man-made inorganic mercury released into the environment worldwide (#Princeton, 2004).

    
    Structure retrieved from Science Berkeley

• Liquid mercury used in mining
  • Large quantities of liquid mercury are used to extract gold after the mercury is heated and evaporates (#Clarkson, 2002).

• • Release also occurs when virgin mercury is also mined from mercury ore (#EPA, 2006).

• Industrial Uses
  • Fluorescent lamps, dental fillings, thermometers, manometers, electrical and electronic switches

• Waste Disposals
  • Combustion of waste and medical waste products release both inorganic and organic mercury into the atmosphere. mercury also leeches into the soil and groundwater surrounding landfills (#Princeton, 2004).

• Natural Sources including Volcanic Activity, Forest Fires

Mercury Methylation

The exact mechanism by which bacteria biomethylizes inorganic mercury into the organic methylmercury is not known precisely (#USGS).
From #Environment Canada:
"In the environment, mercury is transformed into methylmercury when the oxidized, or mercuric species (Hg2+), gains a methyl group (CH3). The methylation of Hg2+ is primarily a natural, biological process resulting in the production of highly toxic and bioaccumulative methylmercury compounds (MeHg+) that build up in living tissue and increase in concentration up the food chain, from microorganisms like plankton, to small fish, then to fish eating species like otters and loons, and humans."

"A variety of microorganisms, particularly methanogenic (methane producing) and sulfate-dependant bacteria are thought to be involved in the conversion of Hg2+ to MeHg under anaerobic (oxygen poor) conditions found, for example, in wetlands and river sediments, as well as in certain soils. Methylation occurs primarily in aquatic, low pH (acidic) environments with high concentrations of organic matter."

External Links

• Sierra Club on the Mercury Cycle

• Oceana on the Mercury Cycle

• City of Vancouver, BC on the Mercury Cycle

References

Environment Canada. "Mercury and the Environment: Environment and Health Biogeochemistry". Last updated: 2004-02-04. Accessed 9-19-07.

EPA. Great Lakes Pollution Prevention and Toxics Reduction. "Background Information on Mercury Sources and Regulations". Last Updated on March 8th, 2006. Accessed 9-19-07.

Princeton's Site about the Mercury Cycle

United States Geological Service (USGS). "Mercury Contamination of Aquatic Ecosystems". 1997.

University of Wisconsin - Eau Claire. "Mercury in the Environment and Water Supply". Accessed 9-19-07.

Purdue University College of Engineering. "Mercury Cycle". Accessed 9-19-07.