Mercury (Hg) is a naturally occurring element in minerals, rocks, soil, water, air, plants, and animals.
The predominant forms in the atmosphere, water, and aerobic soils and sediments are elemental and mercuric mercury; while cinnabar is commonly found in mineralized ore deposits and anaerobic soils and sediments. Mercury is present throughout the atmosphere because of its relatively high vapor pressure. It vaporizes from the earth's surface and is transported in a global cycle, sometimes for hundreds of kilometers, before being deposited again with particulates, rain, or snow. The background concentrations in rocks and soils typically range between 20 and 100μg Hg/kg with a worldwide average of about 50μg Hg/kg. Natural background concentrations in the uncontaminated atmosphere are in the order of between 1 and 10 ng/m3 increasing to between 50 and 1,000,000 ng/m3 or more over mineralized areas. Mercury is transported to aquatic ecosystems via surface runoff and atmospheric deposition. Airborne concentrations associated with anthropogenic activities such as coal burning, smelting, industry, and incineration range between 100 and 100,000 ng/m3. These sources of mercury account for about 70 percent of anthropogenic mercury in the atmosphere.
The element can be divided into two major categories, organic and inorganic. Inorganic mercury includes the elemental (Hg0) silvery liquid metal (mp, 38°C; bp, 357°C) as well as mercurous ion (Hg+), mercuric ion (Hg++), and their compounds. Organic mercury includes chemical compounds which contain carbon atoms that are covalently bound to a mercury atom, such as methylmercury (CH3-Hg+).
During the latter half of the twentieth century, inorganic mercury was used extensively to produce caustic soda and chlorine (Cl) as well as to manufacture batteries, switches, street lamps, and fluorescent lamps. Gold mining, dental amalgams, pharmaceuticals, and other consumer items also consume inorganic mercury. Organic mercury applications have mostly been eliminated in agricultural fungicides, slimicides in paper pulp production, bacteriostats in water-based paints, and industrial catalysts.
The concentrations of mercury in the ocean and uncontaminated freshwater are generally believed to be less than 300 and 200 ng/l respectively. However, new ultra clean analytical techniques indicate that the actual concentrations may be three- to five-fold lower. In contaminated aquatic systems concentrations as high as 5μg Hg/l have been reported. In the water column, mercury readily adsorbs onto organic particulates, metal oxides, and clays and settles into the sediments. Historically, depending on their location, the natural background concentrations of mercury in sediments have ranged between 10 and 200g/kg. However, most aquatic systems have received some mercury contamination, and the rate has increased during the past century. Among sites that have been measured, the total concentrations have usually been from five to ten times greater than background and ranged from less than 0.5 mg Hg/kg (dry weight) in remote areas to 2,010 mg Hg/kg (dry weight) in Minamata Bay, Japan where mercury was dumped for many years into the bay by an industrial company, leading to severe public health problems.
In the aquatic ecosystem inorganic mercury is converted to methylmercury by both biotic and abiotic processes. It is then released, and aquatic organisms bioaccumulate it easily and have difficulty with metabolism and excretion. The biological half-life in fish may be as long as one to three years. Exposed organisms at each level of the food chain bioconcentrate methylmercury and pass it on to animals at the higher trophic levels.
Depending on the species of fish and the type and amount of mercury being released from the sediments, it may be magnified biologically from one thousand and 100 thousand times or more. While background levels of total mercury in freshwater and marine fishes from unpolluted waters typically range from less than 0.1 to about 0.2 mg Hg/kg, higher concentrations are found in some pelagic top predator ocean fishes such as tuna and shark, sometimes exceeding 1.5 mg/kg. Conversely, fish from contaminated waters typically contain levels between 0.5 and 5.0 mg Hg/kg and up to 35 to 50 mg Hg/kg in highly contaminated areas.
The annual worldwide production of mercury was over 1,600 tons (about 1,500 metric tons) in the early twenty-first century, with most coming from China and Kyrgyzstan. Annual output fluctuates, and the Kyrgyzstan mines have suffered problems in recent years. The U.S. Natural Resources Defense Council (NRDC) is coordinating with the Chemical Registration Center (CRC) of China's State Environmental Protection Administration to assess China's mercury supply and demand to develop a plan to stop the mining of mercury and decrease the input of mercury into the environment. The NRDC's plan focuses on eliminating export of mercury by industrialized nations and phasing out mercury mining as well as the promotion of alternatives to mercury-based production in industry. Their goal is to reduce mercury trade by 75 percent over a ten-year period.
See also Mercury poisoning .
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Frank M. D'Itri