Mercury
Mercury 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 100 μg Hg/kg with a worldwide average of about 50 μg 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.
The annual worldwide production from cinnabar was about 11,500 metric tons in 1990. 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 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.
Over the centuries the symptoms of inorganic mercury poisoning were well documented by the exposure of miners and industrial workers as mercury accumulated in their brains, kidneys, and livers. Loose teeth, tremors, and psychopathological symptoms were common at low exposure, but removal from the source would often enable the victims to recover. However, the effects of organic alkyl mercurials, such as methylmercury, were more severe. With a half-life in the human body of about seventy days, continued exposure elevates the levels. It also crosses the blood/brain and placental barriers, attacking the central nervous system and inducing teratogenic changes in the fetus. The neurological symptoms include: loss of coordination in walking; slurred speech; constriction of the field of vision; loss of sensation, especially in the fingers, toes, and lips; and loss of hearing. Severe poisoning can cause coma, blindness, and death.
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 5 μg Hg/l have been reported. In the water column, mercury readily adsorbs onto organic particulates, metal oxides, and clays. Then they settle into the sediments. Historically, depending on their location, the natural background concentrations of mercury in sediments have ranged between 10 and 200 μg/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 2010 mg Hg/kg (dry weight) in Minamata Bay, Japan.
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 metabolize and excrete it very poorly. 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 1,000 and 100,000 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.
Several standards have been developed to protect the public's health from the threat of mercury poisoning. The maximum permissible concentration allowed by the United States Environmental Protection Agency (EPA) under its drinking water standards is 2 μg Hg/l. The United States Food and Drug Administration guideline for mercury in seafood is 1 mg Hg/kg freshweight; however, some states, such as Michigan, adhere to a more restrictive guideline of 0.5 mg Hg/kg freshweight. The Food and Agriculture Organization of the United Nations (FAO), on the other hand, recommends a provisional tolerable intake (PTI) of 0.3 mg mercury per week for a person weighing 154 lb (70 kg), of which no more than 0.2 should be in the methylated form.
Biological Methylation; Birth Defects; Food Chain/Web; Minamata Disease; Teratogen; Water Pollution; Xenobiotic
Resources
Books
D'Itri, F. M., et al. An Assessment of Mercury in the Environment. Washington, DC: National Academy of Sciences, 1978.
Periodicals
D'Itri, F. M. "Mercury Contamination: What We Have Learned Since Minamata." Environmental Monitoring and Assessment 19 (1991): 165–82.
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