Mercury is a naturally occurring element, unique among metals for remaining liquid at room temperature. This silvery, heavy metal, also known as quicksilver, circulates widely through various environmental compartments. Understanding how mercury is released and transforms in the environment is important due to potential health and ecological concerns.
Mercury’s Natural Release
Mercury is naturally present in the Earth’s crust, primarily within minerals such as cinnabar and embedded in materials like coal and limestone. Natural processes continuously release this element into the atmosphere, water bodies, and soils. Volcanic eruptions are a significant natural source, contributing mercury to the atmosphere through gases and ash. Geothermal activity, including hot springs and undersea vents, also mobilizes mercury from geological reservoirs.
The natural weathering and erosion of mercury-rich rocks and soils further contribute to its presence. This geological breakdown makes inorganic mercury salts available, which can then be transported by water and reside in soil. These natural emissions, alongside re-emissions of previously deposited mercury, account for a substantial portion of the element’s global atmospheric circulation.
Human Activities and Mercury Circulation
Human activities have significantly increased the amount of mercury circulating in the environment beyond natural levels. The burning of fossil fuels, particularly coal, is a major anthropogenic source of mercury air emissions. Although coal contains only trace amounts of mercury, the immense volumes combusted in power plants and industrial uses vaporize and release this mercury into the atmosphere. In the United States, coal-fired power plants alone emit approximately 50 tons of elemental mercury annually.
Artisanal and small-scale gold mining (ASGM) is the largest single anthropogenic source of mercury release. Miners use elemental mercury to bind gold particles, forming an amalgam that is then heated, often over open flames, to vaporize the mercury and leave the gold behind. This practice releases substantial quantities of mercury vapor directly into the air, with estimates suggesting over 1,000 tonnes annually from ASGM worldwide.
Industrial processes also contribute to mercury emissions. Chlor-alkali plants, which produce chlorine and caustic soda using mercury-cell technology, are a significant source of both local and global mercury pollution, releasing it into the air and water. Cement manufacturing is another contributor, as mercury present in raw materials like limestone and fuels such as coal is vaporized in high-temperature kilns, leading to atmospheric emissions. This industry accounts for about 11% of global anthropogenic mercury emissions. Waste incineration also releases mercury into the atmosphere, primarily from the combustion of discarded products containing the element, such as batteries, fluorescent lamps, and thermometers.
Environmental Transformation of Mercury
Once released into the environment, mercury undergoes various transformations, altering its form and behavior. Elemental mercury, when airborne, can travel long distances before settling onto land or water. In aquatic and terrestrial environments, elemental mercury can react with other substances to form inorganic mercury compounds, such as mercury salts, found in soils and water.
A key environmental transformation is methylation, where inorganic mercury is converted into methylmercury. This process is primarily carried out by specific microorganisms, particularly anaerobic bacteria found in low-oxygen environments like aquatic sediments and wetlands.
Methylmercury is the most toxic form of mercury and is highly soluble in lipids, which increases its uptake by living organisms. It readily bioaccumulates, meaning organisms absorb and retain it at a faster rate than they can eliminate it. This leads to biomagnification, where the concentration of methylmercury increases at higher trophic levels within a food web. Consequently, large predatory fish often contain higher levels, posing risks to fish-eating wildlife and humans who consume them.