Mercury

Background

Mercury is one of the basic chemical elements. It is a heavy, silvery metal that is liquid at normal temperatures. Mercury readily forms alloys with other metals, and this makes it useful in processing gold and silver. Much of the impetus to develop mercury ore deposits in the United States came after the discovery of gold and silver in California and other western states in the 1800s. Unfortunately, mercury is also a highly toxic material, and as a result, its use has severely declined over the past 20 years. Its principal applications are in the production of chlorine and caustic soda, and as a component of many electrical devices, including fluorescent and mercury-vapor lamps.

Mercury has been found in Egyptian tombs dating to about 1500 b.c., and it was probably used for cosmetic and medicinal purposes even earlier. In about 350 b.c., the Greek philosopher and scientist Aristotle described how cinnabar ore was heated to extract mercury for religious ceremonies. The Romans used mercury for a variety of purposes and gave it the name hydrargyrum, meaning liquid silver, from which the chemical symbol for mercury, Hg, is derived.

Demand for mercury greatly increased in 1557 with the development of a process that used mercury to extract silver from its ore. The mercury barometer was invented by Torricelli in 1643, followed by the invention of the mercury thermometer by Fahrenheit in 1714. The first use of a mercury alloy, or amalgam, as a tooth filling in dentistry was in 1828, although concerns over the toxic nature of mercury prevented the widespread use of this new technique. It wasn't until 1895 that experimental work by G.V. Black showed that amalgam fillings were safe, although 100 years later scientists were still debating that point.

Mercury found its way into many products and industrial applications after 1900. It was commonly used in batteries, paints, explosives, light bulbs, light switches, pharmaceuticals, fungicides, and pesticides. Mercury was also used as part of the processes to produce paper, felt, glass, and many plastics.

In the 1980s, increasing understanding and awareness of the harmful health and environmental effects of mercury started to greatly outweigh its benefits, and usage began to drop sharply. By 1992, its use in batteries had dropped to less than 5% of its level in 1988, and overall use in electrical devices and light bulbs had dropped 50% in the same period. The use of mercury in paints, fungicides, and pesticides has been banned in the United States, and its use in the paper, felt, and glass-manufacturing processes has been voluntarily discontinued.

Worldwide, production of mercury is limited to only a few countries with relaxed environmental laws. Mercury mining has ceased altogether in Spain, which until 1989 was the world's largest producer. In the United States, mercury mining has also stopped, although small quantities of mercury are recovered as part of the gold refining process to avoid environmental contamination. China, Russia (formerly the USSR), Mexico, and Algeria were the largest producers of mercury in 1992.

Raw Materials

Mercury is rarely found by itself in nature. Most mercury is chemically bound to other materials in the form of ores. The most common ore is red mercury sulfide (HgS), also known as cinnabar. Other mercury ores include corderoite (Hg₃S₂Cl₂), livingstonite (HgSb₄S₈), montroydite (HgO), and calomel (HgCl). There are several others. Mercury ores are formed underground when warm mineral solutions rise towards the earth's surface under the influence of volcanic action. They are usually found in faulted and fractured rocks at relatively shallow depths of 3-3000 ft (1-1000 m).

Other sources of mercury include the dumps and tailing piles of earlier, less-efficient mining and processing operations.

The Manufacturing
Process

The process for extracting mercury from its ores has not changed much since Aristotle first described it over 2,300 years ago. Cinnabar ore is crushed and heated to release the mercury as a vapor. The mercury vapor is then cooled, condensed, and collected. Almost 95% of the mercury content of cinnabar ore can be recovered using this process.

Here is a typical sequence of operations used for the modern extraction and refining of mercury.

Mining

Cinnabar ore occurs in concentrated deposits located at or near the surface. About 90% of these deposits are deep enough to require underground mining with tunnels. The remaining 10% can be excavated from open pits.

• 1 Cinnabar is dislodged from the surrounding rocks by drilling and blasting with explosives or by the use of power equipment. The ore is brought out of the mine on conveyor belts or in trucks or trains.

Roasting

Because cinnabar ore is relatively concentrated, it can be processed directly without any intermediate steps to remove waste material.

• 2 The ore is first crushed in one or more cone crushers. A cone crusher consists of an interior grinding cone that rotates on an eccentric vertical axis inside a fixed outer cone. As the ore is fed into the top of the crusher, it is squeezed between the two cones and broken into smaller pieces.
• 3 The crushed ore is then ground even smaller by a series of mills. Each mill consists of a large cylindrical container laying on its side and rotating on its horizontal axis. The mill may be filled with short lengths of steel rods or with steel balls to provide the grinding action.
• 4 The finely powdered ore is fed into a furnace or kiln to be heated. Some operations use a multiple-hearth furnace, in which the ore is mechanically moved down a vertical shaft from one ledge, or hearth, to the next by slowly rotating rakes. Other operations use a rotary kiln, in which the ore is tumbled down the length of a long, rotating cylinder that is inclined a few degrees off horizontal. In either case, heat is provided by combusting natural gas or some other fuel in the lower portion of the furnace or kiln. The heated cinnabar (HgS) reacts with the oxygen (02) in the air to produce sulfur dioxide (SO₂), allowing the mercury to rise as a vapor. This process is called roasting.

Condensing

• 5 The mercury vapor rises up and out of the furnace or kiln along with the sulfur dioxide, water vapor, and other products of combustion. A considerable amount of fine dust from the powdered ore is also carried along and must be separated and captured.
• 6 The hot furnace exhaust passes through a water-cooled condenser. As the exhaust cools, the mercury, which has a boiling point of 675° F (357° C), is the first to condense into a liquid, leaving the other gases and vapors to be vented or to be processed further to reduce air pollution.
• 7 The liquid mercury is collected. Because mercury has a very high specific gravity, any impurities tend to rise to the surface and form a dark film or scum. These impurities are removed by filtration, leaving a liquid mercury that is about 99.9% pure. The impurities are treated with lime to separate and capture any mercury, which may have formed compounds.

Refining

Most commercial-grade mercury is 99.9% pure and can be used directly from the roasting and condensing process. Higher purity mercury is needed for some limited applications and must be refined further. This ultrapure mercury commands a premium price.

• 8 Higher purity can be obtained through several refining methods. The mercury may be mechanically filtered again, and certain impurities may be removed through oxidation with chemicals or air. In some cases the mercury is refined through an electrolytic process, in which an electric current is passed through a tank of liquid mercury to remove the impurities. The most common refining method is triple distillation, in which the temperature of the liquid mercury is carefully raised until the impurities either evaporate or the mercury itself evaporates, leaving the impurities behind. This distillation process is performed three times, with the purity increasing each time.

Shipping

• 9 Commercial-grade mercury is poured into wrought-iron or steel flasks and sealed. Each flask contains 76 lb (34.5 kg) of mercury. Higher purity mercury is usually sealed in smaller glass or plastic containers for shipment.

Quality Control

Commercial-grade mercury with 99.9% purity is called prime virgin-grade mercury. Ultrapure mercury is usually produced by the triple-distillation method and is called triple-distilled mercury.

Quality control inspections of the roasting and condensing process consist of spot checking the condensed liquid mercury for the presence of foreign metals, since those are the most common contaminants. The presence of gold, silver, and base metals is detected using various chemical-testing methods.

Triple-distilled mercury is tested by evaporation or spectrographic analysis. In the evaporation method, a sample of mercury is evaporated, and the residue is weighed. In the spectrographic analysis method, a sample of mercury is evaporated, and the residue is mixed with graphite. Light coming from the resulting mixture is viewed with a spectrometer, which separates the light into different color bands depending on the chemical elements present.

Health and Environmental Effects

Mercury is highly toxic to humans. Exposure may come from inhalation, ingestion, or absorption through the skin. Of the three, inhalation of mercury vapor is the most dangerous. Short-term exposure to mercury vapor can produce weakness, chills, nausea, vomiting, diarrhea, and other symptoms within a few hours. Recovery is usually complete once the victim is removed from the source. Long-term exposure to mercury vapor produces shaking, irritability, insomnia, confusion, excessive salivation, and other debilitating effects.

In normal situations, most exposure to mercury comes from the ingestion of certain foods, such as fish, in which the mercury has accumulated at high levels. Although mercury is not absorbed in great quantities when passing through the human digestive system, ingestion over a long period of time has been shown to have cumulative effects.

In industrial situations, mercury exposure is a far more serious hazard. Mining and processing mercury ore can expose workers to mercury vapor as well as to direct contact with the skin. The production of chlorine and caustic soda can also cause significant mercury exposure hazards. Dentists and dental assistants can be exposed to mercury while preparing and placing mercury amalgam fillings.

Because mercury poses a serious health hazard, its use and release to the environment has come under increasingly tight restrictions. In 1988, it was estimated that 24 million lb/yr (11 million kglyr) of mercury were released into the air, land, and water worldwide as the result of human activities. This included mercury released by mercury mining and refining, various manufacturing operations, the combustion of coal, the discarding of municipal refuse and sewage sludge, and other sources.

In the United States, the Environmental Protection Agency (EPA) has banned the use of mercury for many applications. The EPA has set a goal of reducing the level of mercury found in municipal refuse from 1.4 million Ib/yr (0.64 million kg/yr) in 1989 to 0.35 million lb/yr (0.16 million kg/yr) by 2000. This is to be accomplished by decreasing the use of mercury in products and increasing the diversion of mercury from municipal refuse through recycling.

The Future

Mercury is still an important component in many products and processes, although its use is expected to continue to decline. Improved handling and recycling of mercury are expected to significantly reduce its release to the environment and thereby reduce its health hazard.

Where to Learn More

Books

Brady, George S., Henry R. Clauser, and John A. Vaccari. Materials Handbook, 14th Edition. McGraw-Hill, 1997.

Heiserman, David L. Exploring Chemical Elements and Their Compounds. TAB Books, 1992.

Kroschwitz, Jacqueline I., executive editor, and Mary Howe-Grant, editor. Encyclopedia of Chemical Technology, 4th edition. John Wiley and Sons, Inc., 1993.

Stwertka, Albert. A Guide to the Elements. Oxford University Press, 1996.

Periodicals

Raloff, J. "Mercurial Airs: Tallying Who's to Blame." Science News (February 19, 1994): 119.

Spencer, Peter, and G. Murdoch. "Mercury in Paint." Consumers' Research Magazine (January 1991): 2.

Stone, R. "Mercurial Debate." Science (March 13, 1992): 1356-1357.

Other

http://www.intercorr.com/periodic/80.htm [This website contains a summary of the history, sources, properties, and uses of mercury.]

—Chris Cavette

Mercury

Mercury is the innermost and second smallest planet (4,878 kilometers [3,024 miles] in diameter) in the solar system (Pluto is the smallest). It has no known moons. As of the beginning of the twenty-first century, Mariner 10 had been the only spacecraft to explore the planet. It flew past Mercury on March 29 and September 21, 1974, and on March 16, 1975. Mariner 10 imaged only about 45 percent of the surface and only in moderate detail. As a consequence, there are still many questions concerning the history and evolution of Mercury. Two new missions to Mercury will be launched this decade. An American mission called MESSENGER will be launched in March 2004. It will make two flybys of Venus and two of Mercury before going into Mercury orbit in April 2009. A European mission called Bepi Colombo, after a famous Italian celestial dynamicist, is scheduled for launch in 2009.

Motion and Temperature

Mercury has the most elliptical and inclined (7 degrees) orbit of any planet except Pluto. Its average distance from the Sun is only 0.38 astronomical unit (AU). Because of its elliptical orbit, however, the distance varies from 0.3 AU when it is closest to the Sun to 0.46 AU when it is farthest away. Mercury's orbital velocity is the greatest in the solar system and averages 47.6 kilometers per second (29.5 miles per second). When it is closest to the Sun, however, it travels 56.6 kilometers per second (35.1 miles per second), and when it is farthest away it travels 38.7 kilometers per second (24 miles per second).

Mercury's rotational period is 58.6 Earth days and its orbital period is 87.9 Earth days. It has a unique relationship between its rotational and orbital periods: It rotates exactly three times on its axis for every two orbits around the Sun. Because of this relationship, a solar day (sunrise to sunrise) lasts two Mercurian years, or 176 Earth days.

Because Mercury is so close to the Sun, has no insulating atmosphere, and has such a long solar day, it experiences the greatest daily range in surface temperatures (633°C [1,171°F]) of any planet or moon in the solar system. Mercury's maximum surface temperature is about 450°C (842°F) at the equator when it is closest to the Sun, but drops to about -183°C (-297°F) at night.

Interior and Magnetic Field

Mercury's internal structure is unique in the solar system. Mercury's small size and relatively large mass (3.3 × 10²³ kilograms [7.3 × 10²³ pounds]) means that it has a very large density of 5.44 grams per cubic centimeters (340 pounds per cubic foot), which is only slightly less than Earth's (5.52 grams per cubic centimeter [345 pounds per cubic foot]) and larger than Venus's (5.25 grams per cubic centimeter [328 pounds per cubic foot]). Because of Earth's large internal pressures, however, its uncompressed density is only 4.4 grams per cubic centimeter (275 pounds per cubic foot), compared to Mercury's uncompressed density of 5.3 grams per cubic centimeter (331 pounds per cubic foot). This means that Mercury contains a much larger fraction of iron than any other planet or moon in the solar system. The iron core must be about 75 percent of the planet diameter, or some 42 percent of its volume. Thus, its rocky outer region is only about 600 kilometers (370 miles) thick.

Mercury is the only terrestrial planet , aside from Earth, with a significant magnetic field. The maintenance of terrestrial planet magnetic fields is thought to require an electrically conducting fluid outer core surrounding a solid inner core. Therefore, Mercury's magnetic field suggests that Mercury currently has a fluid outer core of unknown thickness.

Exosphere

Mercury has an extremely tenuous atmosphere with a surface pressure a trillion times less than Earth's. This type of tenuous atmosphere is called an exosphere because atoms in it rarely collide. Mariner 10 identified the presence of hydrogen, helium, and oxygen in the atmosphere and set upper limits on the abundance of argon. These elements are probably derived from the solar wind . Later Earth-based telescopic observations detected sodium and potassium in quantities greater than the elements previously known. Sodium and potassium could be released from surface rocks by their interaction with solar radiation or by impact vaporization of micrometeoroid material. Both sodium and potassium show day-to-day changes in their global distribution.

Polar Deposits

High-resolution radar observations show highly reflective material concentrated in permanently shadowed portions of craters at the polar regions. These deposits have the same radar characteristics as water ice. Mercury's rotation axis is almost perpendicular to its orbit, and therefore Mercury does not experience seasons. Thus, temperatures in permanently shaded polar areas should be less than -161°C (-258°F). At this temperature, water ice is stable, that is, it is not subject to evaporation for billions of years. If the deposits are water ice, they could originate from comet or water-rich asteroid impacts that released the water, which was then cold-trapped in the permanently shadowed craters. Sulfur has also been suggested as a possible material for these deposits.

Geology and Composition

In general, the surface of Mercury can be divided into four major terrains: heavily cratered regions, intercrater plains , smooth plains , and hilly and lineated terrain. The heavily cratered uplands record the period of heavy meteoroid bombardment that ended about 3.8 billion years ago.

The largest relatively fresh impact feature seen by Mariner 10 is the Caloris basin, which has a diameter of 1,300 kilometers (806 miles). The floor structure consists of closely spaced ridges and troughs.

Directly opposite the Caloris basin (the antipodal point) is the unusual hilly and lineated terrain that disrupts preexisting landforms, particularly crater rims (see top image on following page). The hilly and lineated terrain is thought to be the result of seismic waves generated by the Caloris impact and focused at the antipodal region.

Mercury's two plains units have been interpreted to be old lava flows. The older intercrater plains are the most extensive terrain on Mercury (see bottom image on this page). The intercrater plains were created during the period of late heavy meteoroid bombardment. They are thought to be volcanic plains erupted through a fractured crust. They are probably about 4 to 4.2 billion years old.

The younger smooth plains are primarily associated with large impact basins. The largest occurrence of smooth plains fill and surround the Caloris basin, and occupy a large circular area in the north polar region that is probably an old impact basin about 1,500 kilometers (930 miles) in diameter. They are similar to the lunar maria and therefore are believed to be lava flows that erupted relatively late in Mercurian history. They may have an average age of about 3.8 billion years. If so, they are, in general, older than the lunar maria.

Three large radar-bright anomalies have been identified on the unimaged side of Mercury. High-resolution radar observations indicate that two of these are similar to the radar signature of a fresh impact crater, and another has a radar signature unlike any other in the solar system. One or both of these craters could account for the polar deposits if they were the result of comets or water-rich asteroid impacts.

Mercury displays a system of compressive faults (or thrust faults ) called lobate scarps . They are more-or-less uniformly distributed over the part of Mercury viewed by Mariner 10. Presumably they occur on a global scale. This suggests that Mercury has shrunk. Stratigraphic evidence indicates that the faults formed after the intercrater plains relatively late in Mercurian history. The faults were probably caused by a decrease in Mercury's size due to cooling of the planet. The amount of radius decrease is estimated to have been about 2 kilometers (1.2 miles).

Very little is known about the surface composition of Mercury. A new color study of Mariner 10 images has been used to derive some compositional information of the surface over some of the regions viewed by Mariner 10. The smooth plains have an iron content of less than 6 percent by weight, which is similar to the rest of the regions imaged by Mariner 10. The surface of Mercury, therefore, may have a more homogeneous distribution of elements that affect color than does the Moon. At the least, the smooth plains may be low-iron basalts . The MESSENGER mission is designed to accurately determine the composition of the surface.

Geologic History

Knowledge about Mercury's earliest history is very uncertain. The earliest known events are the formation of the intercrater plains (more than 4 billion years ago) during the period of heavy meteoroid bombardment. These plains may have been erupted through fractures caused by large impacts in a thin crust. Near the end of heavy bombardment the Caloris basin was formed by a large impact that caused the hilly and lineated terrain from seismic waves focused at the antipodal region. Eruption of lava within and surrounding the large basins formed the smooth plains about 3.8 billion years ago. The system of lobate scarps formed after the intercrater plains, and resulted in a planetary radius decrease of about 2 kilometers (1.2 miles). Scientists will have to await the results of the MESSENGER and Colombo missions to fully evaluate the geologic history of Mercury.

Origins

How Mercury acquired such a large fraction of iron compared to the other terrestrial planets is not well determined. Three hypotheses have been put forward to explain the enormous iron core. One involves an enrichment of iron due to dynamical processes in the innermost part of the solar system. Another proposes that intense bombardment by solar radiation in the earliest phases of the Sun's evolution vaporized and drove off much of the rocky fraction of Mercury, leaving the core intact. A third proposes that a planetsized object impacted Mercury and blasted away much of the planet's rocky mantle, again leaving the iron core largely intact. Discriminating among these hypotheses may be possible from the chemical makeup of the surface because each one predicts a different composition. MESSENGER is designed to measure the composition of Mercury's surface, so it may be possible to answer this vital question in the near future.

see also Exploration Programs (volume 2); Planetary Exploration, Future of (volume 2); Robotic Exploration of Space (volume 2).

Robert G. Strom

Bibliography

"The Planet Mercury: Mariner 10 Mission." (various papers and authors) Journal of Geophysical Research 80, no. 17 (1975): 2342-2514.

Strom, Robert G. Mercury: The Elusive Planet. Washington, DC: Smithsonian Institution Press, 1987.

——. "Mercury: An Overview." Advances in Space Research 19, no. 10 (1997):1,471-1,485.

——. "Mercury." In Encyclopedia of the Solar System, eds. Weissman, P. R., L. Mc-Fadden, and T. V. Johnson. San Diego: Academic Press, 1999.

Villas, Faith, Clark R. Chapman, and Mildred S. Matthews, eds. Mercury. Tucson:University of Arizona Press, 1988.

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.

See also Biological methylation; Birth defects; Food chain/web; Minamata disease; Teratogen; Water pollution; Xenobiotic

[Frank M. D'Itri ]

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.

MERCURY

Mercury (Hg) is a naturally occurring silvery metal that has been associated with adverse health effects throughout history. Elemental mercury is a liquid at room temperature, and, because of this, Aristotle named mercury "quicksilver." There are three forms of mercury: elemental mercury (Hg⁰), organic mercury (e.g., methylmercury), and inorganic mercury (e.g., Hg⁺, Hg²⁺). Many different organic and inorganic mercury compounds are found in nature because of mercury's ability to form covalent or ionic bonds with other chemicals. Mercury has numerous commercial uses—including its use in the extraction of gold from ores—and is an ingredient in alkaline batteries (approximately 0.025% of battery content), mercury vapor lamps, thermostats, and mercury amalgam fillings (in the United States, 50% of a dental filling is made of mercury). Humans can be exposed to mercury compounds via the oral, inhalation, and dermal routes. The primary source of exposure to mercury compounds is attributed to the ingestion of fish and other seafood (marine mammals, crustaceans) that have bioaccumulated mercury compounds. Dental amalgams, which leach mercury, are another source.

Adverse health effects from elemental and inorganic mercury compounds have been observed, particularly in occupational settings. Health consequences commonly observed from exposure to compounds such as elemental mercury vapor and mercuric chloride include tremors, bleeding gums, abdominal pain, vomiting, and kidney damage.

Health effects from organic mercury compounds have also been well-documented, primarily because of the tragic mass poisonings from organic mercurials in locations such as Minamata, Japan, and in Iraq. These mass poisonings were primarily associated with central nervous system toxicity and death. Adverse health effects observed in poisoned individuals and their offspring included ataxia, dysarthria, impaired vision and hearing, and death. Methylmercury is particularly toxic because 95 percent of an ingested dose is absorbed into the bloodstream and can cross the blood-brain and placental barriers, causing adult and fetal neurotoxicity. One of the reasons that offspring are particularly susceptible is that methylmercury can accumulate at 30 percent higher levels in fetal red blood cells than in maternal red blood cells. Besides damaging the brain and peripheral nervous system, methylmercury may also adversely affect the adult and fetal cardiovascular systems.

Research continues to be performed on the potential neurodevelopment effects of ingesting low levels of mercury in seafood. Three particularly important, ongoing studies involve residents of New Zealand and the Seychelles and Faroe Islands who consume significant portions of seafood as part of their normal diets. Analyses performed to date on mother-offspring pairs from the Seychelles identified adverse neurodevelopmental impact in offspring attributable to maternal methylmercury exposure from seafood. Mild developmental effects were also reported among offspring of New Zealand and Faroe Island residents who ingested seafood containing relatively high levels of methylmercury. These studies are particularly pertinent to assessing potential health effects among Native Arctic populations who consume marine mammals (beluga whales, ringed seals) as part of their normal diet. An increased level of mercury has been noted in the Arctic environment since the 1970s, possibly due to anthropogenic sources such as fossil fuel combustion, or possibly from increased natural releases of mercury from geologic sources. It is hypothesized that the cold Arctic climate acts as a sink for mercury; a particularly troublesome prospect for Native Arctic populations who continue to consume mercury-laden mammals and seafood.

Margaret H. Whitaker

Bruce A. Fowler

(see also: Environmental Determinants of Health; Foods and Diets; Heavy Metals; Minamata Disease; Occupational Safety and Health )

Bibliography

Agency for Toxic Substances and Disease Registry (1999). Toxicological Profile for Mercury (Update). Washington, DC: U.S. Department of Health and Human Services.

Arctic Monitoring and Assessment Programme (1999). Arctic Pollution Issues: A State of the Arctic Environment Report. Available at http://www.amap.no.

National Research Council (2000). Toxicological Effects of Methyl Mercury. Washington, DC: Committee on the Toxicological Effects of Mercury. Board on Environmental Studies and Toxicology. Commission on Life Sciences.

Tenenbaum, D. J. (1998). "Northern Overexposure." Environmental Health Perspective 106(2): A64–A69.

U.S. Environmental Protection Agency (1997). Report to Congress on Mercury. Available at http://www.epa.gov/oar/mercury.html.

World Health Organization (1990). Methyl Mercury, Vol. 101. Geneva: International Programme on Chemical Safety, WHO.

—— (1990). Inorganic Mercury, Vol. 118. Geneva: International Programme on Chemical Safety, WHO.

Mercury

Mercury is a metal with chemical similarities to zinc and cadmium. The metal is liquid at room temperature, with a freezing point at –31°C, and it is one of the most volatile metals. It occurs as the element Hg0 and as the mercuric ion Hg++, which has a great affinity for reduced sulfur (sulfide, S=). Most mercury ore deposits consist of the very insoluble mineral cinnabar (HgS), with little droplets of elemental Hg. Mercury also occurs as impurities in many other ore minerals, creating mercury contamination when these minerals are mined or processed. Most common rocks have very low Hg contents, about ten to one hundred parts per billion (ppb) Hg . Elemental mercury is barely soluble in pure water, with only twenty-five ppb Hg dissolving at room temperature, but it is more soluble at higher temperatures. The mercuric ion is very soluble in most ambient waters, but very insoluble in the presence of sulfide. Natural enrichments of mercury occur in and around ore deposits and in geothermal hot spring areas and volcanoes. Bacteria in coastal waters convert inorganic Hg ions back into the elemental state, which then evaporate from the water back into the atmosphere. The physical transport of mercury from ore regions and the vapor transport from geothermal areas and the oceans provide the natural background contamination of mercury.

Mercury is a toxic element that damages the human nervous system and brain. Elemental mercury is less dangerous when it is ingested than when it is inhaled. The use of mercury in felt-making led to widespread elemental mercury poisoning of hatmakers ("mad as a hatter"), which was expressed by tremor, loss of hair and teeth, depression, and occasional death. The organic forms of mercury—methylmercury compounds, CH3Hg+ and (CH3)2Hg—are very bioavailable or are easily taken up by living organisms and rapidly enter cells, and are therefore the most hazardous. Minamata disease was an episode of mercury poisoning of a small coastal community in Japan (1954) through the direct industrial release of methylmercury in the bay. Another infamous episode of mercury contamination occurred in Iraq, where people ate wheat that was treated with a mercury-containing fungicide. The continuous flux of mercury from the atmosphere results in the low level of mercury pollution nationwide. A small fraction of the Hg++ from atmospheric deposition is converted by bacteria into the very dangerous methylmercury form. The methylmercury is then taken up by the lowest life forms and makes its way up the food chain and bioaccumulates in the larger fish. As a result, large predator fish such as bass, tuna, shark, and swordfish have the highest levels of Hg in the methylmercury form. Most states in the United States have advisories for eating only limited amounts of freshwater fish. Limiting intake of mercury-contaminated fish is especially important for pregnant women and young children. The current U.S. legal limit for Hg in fish for consumption is 1 ppm. Limits for Hg in soils vary from state to state but generally range from 10 to 20 ppm, whereas the Environmental Protection Agency's limit for drinking water is 2 ppb Hg. The Occupational Safety and Health Administration limits for Hg in the air in the workplace (for an eight-hour average) are 0.01 mg organic Hg/m3 air.

Modern sources of mercury contamination from human activities are subdivided into the following groups:

1. High-temperature combustion processes such as coal-fired power plants, incineration of solid household waste, medical waste, sewage sludge, and ore smelting.
2. Industrial waste effluents, such as from chlor-alkali plants that use liquid mercury as electrodes.
3. Effluents of wastewater treatment plants.
4. Point sources of specific industries, many of them no longer active today (such as hat making, explosives, mercury lights, herbicides, and plastics).

An overview of modern anthropogenic Hg fluxes into the environment shows that more than 80 percent of mercury is injected into the atmosphere through such combustion processes as coal-fired power plants. The combustion releases mercury as elemental vapor into the atmosphere, where it has an average residence time of about one year before it is oxidized to the mercuric form. The oxidized mercury attaches itself to small dust particles and is removed by wet and dry atmospheric deposition. As a result of this massive injection of Hg into the atmosphere—more than 100 tons of Hg per year in the United States in the late 1990s—the contaminant is distributed all over the globe. Even the polar ice caps show evidence of mercury contamination over the last 150 years, from atmospheric dispersal and deposition from anthropogenic sources. There are almost no places on earth that are not contaminated by anthropogenic mercury.

Mercury contamination is a matter of ongoing concern, and an extensive study was done for the U.S. Congress to summarize the sources, pathways, and sinks of mercury in the outdoor environment. There are several initiatives to limit the anthropogenic flux of Hg from coal-fired power plants, such as switching to mercury-poor coals and scrubbing the stack gases. Limiting or banning the production of mercury-containing materials, including switches, thermometers, thermostats, and manometers, both in the household as well as in the medical profession, would also reduce the mercury recycled back into the atmosphere from garbage incineration.

see also Bioaccumulation; Health, Human; Incineration; Ishimure, Michiko; Medical Waste; Persistent Bioaccumulative Toxic Chemicals (PBTs); Superfund.

internet resource

u.s. environmental protection agency. "mercury study report to congress." available from http://www.epa.gov/oar/mercury.html.

Johan C. Varekamp

The most common exposure to mercury in the home comes when a mercury thermometer is dropped and broken. Children should be removed from the room immediately. DO NOT VACUUM SPILLED MERCURY. Vacuuming will disperse the mercury into the air; inhaling mercury poses high risk. Mercury naturally beads and if it is on a hard surface, it can be scooped up with index cards or a file folder. Seal in a ziplock bag and call the health department or a hospital to arrange safe disposal. Call the health department if mercury has spilled on a carpet or other fabric.