Selenium (Se)
Selenium (Se) is a chemical element with the atomic number 34, belonging to Group VI of the periodic table. It has properties similar to sulfur and exists in several forms, including gray, crystalline, and red varieties. Dispersed throughout the Earth's crust, selenium primarily occurs in volcanic regions and is often found in mineral sulfides. While it is an essential trace element required for the health of mammals, including humans, selenium can be toxic in high concentrations. It plays a significant role as an antioxidant in the enzyme glutathione peroxidase and contributes to the activity of vitamin E.
Selenium's most common industrial use is in the glass industry to prevent discoloration, but it is also utilized in various applications such as pigments, rubber production, and dietary supplements for livestock and humans. Seleniferous soils, which have elevated selenium levels, are found in regions across North America and other parts of the world, and these areas can sometimes experience selenium toxicity in wildlife and livestock. Despite its importance, higher concentrations of selenium in the environment can lead to adverse health effects, as seen in historical cases of animal mortality linked to selenium overload. Overall, selenium exemplifies the balance required in nutrient intake, as both deficiency and excess can have significant health implications.
Selenium (Se)
Where Found
Selenium is widely distributed in the Earth’s crust but does not occur in ore deposits of sufficient concentration to permit direct mining. In nature, selenium is principally found with metal sulfides. Seleniferous soils—soils with high selenium concentrations—are found in Canada, the United States, Mexico, Colombia, and Ireland.
Primary Uses
Selenium’s most common industrial application is in the glass industry. It is also used as a nutritional supplement in domesticated animals such as poultry, cattle, and swine.
Technical Definition
Selenium (abbreviated Se), atomic number 34, belongs to Group VI of the periodic table of the elements and resembles sulfur in its chemical and physical properties. It has six naturally occurring isotopes and an average molecular weight of 78.96. Pure selenium has gray, crystalline, and red forms. Its density is 4.79 grams per cubic centimeter; it has a melting point of 217° Celsius and a boiling point of 685.4° Celsius.
Description, Distribution, and Forms
Selenium is a widely distributed element of volcanic origin that has chemical properties resembling sulfur. It occurs as inorganic oxides such as selenate and selenite, as elemental selenium, and as selenide, depending on the alkalinity and aeration of the environment. Selenium also has a variety of soluble and volatile organic forms, such as dimethyl selenide and selenomethionine, an amino acid analog. Most selenium is combined with metals, as in ferroselite and challomenite, or appears as a trace contaminant in metal sulfides such as galena and pyrite, where it replaces sulfur because of their similarity in size. Coal and oil deposits also have appreciable selenium contents.
Soluble selenium forms are used as nutritional supplements for mammals, since selenium is an essential trace element. Selenium also plays a crucial role as an antioxidant in the enzyme glutathione peroxidase, and it contributes to the activity of vitamin E. High concentrations of selenium are toxic, however, and some notable instances of widespread animal death have occurred when human activity made selenium more available for plants and animals to absorb. On a commercial scale, about 1,600 metric tons of selenium are produced annually (1,560 in 2007, 1,590 in 2008), but this amount is dwarfed by the selenium released into soil, air, and water that occurs as a result of general industrial activity.
The highest average selenium concentrations occur in organic deposits such as coal (3.4 milligrams of selenium per kilogram) and oil shale (2.3 milligrams of selenium per kilogram). However, selenium is found in virtually all materials on Earth at low concentrations. Average concentrations of selenium in crustal material range from 0.05 to 0.14 part per million (milligrams per kilogram). Areas where selenium concentrations are low (for example, the Pacific Northwest or the northeastern United States) occur where the underlying sedimentaryrock predates the Cretaceous period.
Certain soils are called “seleniferous” because they formed in material with elevated selenium levels. These soils typically developed from shale that was formed in the Cretaceous period. In the United States, these soils were deposited primarily in South Dakota, Montana, Wyoming, Nebraska, Kansas, Utah, Colorado, and New Mexico. Seleniferous soils tend to have selenium concentrations ranging from 1 to more than 80 milligrams of selenium per kilogram of soil, and it is in areas with these soils that selenium toxicity has historically occurred. Seleniferous soils also occur in Canada (in the provinces of Alberta, Manitoba, and Saskatchewan), Mexico, Hawaii, Colombia, China, and Ireland. Wells in seleniferous soils can contain as much as 1 milligram of selenium per liter.
The major available forms of inorganic selenium found in well-aerated alkaline soils are selenate, selenite, and elemental selenium.Selenate andselenite are water soluble and can leach out of soil. Elemental selenium is relatively insoluble. In poorly drained environments, selenium will be found as selenide, usually combined with some type of metal such as lead, copper, or iron. These are relatively immobile forms of selenium. When selenides are exposed to air, however, the selenium reoxidizes to form selenates and selenites, which are much more easily taken up by plants.
Some plants accumulate selenium. Examples are milk vetch (Astragalus), goldenweed (Haplopappus), prince’s plume (Stanleya), and woody aster (Xylorhiza). The milk vetch can accumulate up to 20 grams of selenium per kilogram of tissue. The presence of these plants in an area is sometimes an indication that the soil may contain high selenium concentrations. In selenium-accumulating plants, the selenium is found as a water-soluble compound such as selenium methylselenosysteine. In plants that do not accumulate selenium, the form is usually selenomethionine. Selenium can also take the place of sulfur in sulfur-containing organic compounds.
Selenium would not have an impact on other natural resources, except in localized areas, were it not for human activity, such as disposing of fly ash from coal-burning power plants and irrigating arid alkaline soils. The amount of selenium released each year by industrial activity is approximately fifteen times more than that which is released naturally.
Selenium is usually a minor constituent of drinking water, appearing in concentrations ranging from less than 0.1 to 100 micrograms per liter. The upper limit for the allowable selenium content of drinking water in the United States was set at 10 micrograms per liter by the 1974 Safe Drinking Water Act. Wells from seleniferous soils in Colorado and Montana, however, can contain as much as 1 milligram of selenium per liter.
Wildlife is a vital natural resource that can be adversely affected by selenium as a direct result of human activity redistributing selenium to excess in the environment. A notable example of this occurred in the Kesterson reservoir in the San Joaquin Valley of California. The Kesterson Reservoir was a series of twelve shallow ponds that were designed to be an evaporation basin for the drainage waters of the western San Joaquin Valley. Originally this was surface water, but by 1981 almost all the water entering the reservoir was subsurface agricultural drainage water from irrigated agricultural fields. Because of interest in saving some of Northern California’s disappearing wetlands, water that entered the reservoir was diverted and used to preserve wetlands in the adjacent Kesterson National Wildlife Refuge in Merced County, California.
By 1983 the incidence of embryo deformity and mortality among aquatic birdsnesting in the Kesterson Reservoir was alarmingly high. No one immediately suspected, when the drainage water was used in the wetlands, that it contained almost 4.2 milligrams of selenium per liter—a selenium concentration one thousand times greater than in naturally occurring drainage in the region. As a consequence, phytoplankton in the reservoir accumulated selenium to levels 100 to 2,600 times greater than normal. Since these plankton formed the base of the food chain in the reservoir, the levels of selenium in the fish, frogs, snakes, birds, and mammals also increased to levels 12 to 120 times greater than normal (20 to 170 milligrams of selenium per kilogram). Migratory birds that fed on plants, invertebrates, and fish in the reservoir contained up to 24 times the normal level of selenium in their tissue. Between 1983 and 1985 an estimated one thousand migratory birds died as a consequence of selenium toxicity. To protect the migratory birds from future selenium exposure, the reservoir was drained in 1988 and filled with dirt, effectively burying and isolating the excess selenium.
Selenium is a good demonstration of the adage “the dose is the poison.” Trace quantities of selenium are nutritionally essential, and blood concentrations of 0.1 milligram of selenium per liter are nutritionally sound. The minimum lethal concentration of selenium in tissue, however, is only 1.5 to 3.0 milligrams of selenium per kilogram of body weight. Symptoms of toxicity may occur when dietary intake exceeds 4 milligrams per kilogram of body weight. Selenium toxicity leads to the syndromes known as alkali disease and blind stagger. On the other end of the scale, symptoms of deficiency may appear if dietary intake is less than 0.04 milligram of selenium per kilogram of body weight. Selenium deficiency leads to a syndrome known as white muscle disease. In mammals, including humans, selenium is an essential component of the enzyme glutathione peroxidase, found in red blood cells. Glutathione peroxidase is an antioxidant; it protects tissues against oxidation by destroying hydrogen peroxide or organic hydroperoxides.
History
In 1817, selenium was purified and identified by Jöns Jacob Berzelius. However, its environmental influences, particularly its toxic effects, have been known for much longer. Marco Polo, for example, described unmistakable signs of selenium toxicity in horses, cattle, sheep, and humans during his travels across China in 1295. Selenium toxicity was described in Colombia in 1560, in South Dakota in 1857, and in Wyoming in 1908. Selenium was specifically identified as the cause of the toxicity in alkaline soils in the western United States in 1929. Its essential role in animal nutrition was identified in the 1950’s. In the mid-1980’s, the toxic effects of selenium were once more advertised when it was discovered to be the cause of widespread bird mortality at the Kesterson National Wildlife Refuge in Northern California.
Obtaining Selenium
There are no known commercially usable selenium deposits, and the concentration of selenium in soil and water is too dilute to be of economic significance. Consequently, most selenium is a by-product extracted from more abundant materials in which it is a contaminant, particularly during the refining of ores containing metal sulfides such as chalcopyrite. Most of the annual selenium production comes from the waste sludge produced during the electrolytic refining of copper.
Uses of Selenium
Selenium’s industrial uses are varied. The principal use is in the glass industry, where it is used to prevent discoloration of glass by iron oxides. Ammonium selenite is also used as a pigment in making red glass. Selenium diethyldithiocarbamate is used as a fungicide, but more important, it is used as a vulcanizing agent by the rubber industry to increase wear resistance. Selenium is also incorporated into plastics and paints because it improves resistance to heat, light, weathering, and chemical action. Selenium’s antioxidant properties cause it to be included in inks, mineral and vegetable oils, and lubricants. Cadmium selenide is found in photoelectric cells and photoconductors. In addition to its use as a dietary supplement, selenium is used in pharmaceutical remedies for eczema, fungal infections, and dandruff. Selenium also plays a nutritional role and is incorporated into dietary supplements for animals, including humans, although too much selenium in the diet can have deleterious effects.
Bibliography
Adriano, Domy C. “Selenium.” In Trace Elements in Terrestrial Environments: Biogeochemistry, Bioavailability, and Risks of Metals. 2d ed. New York: Springer, 2001.
Ehrlich, Henry Lutz, and Dianne K. Newman. Geomicrobiology. 5th ed. Boca Raton, Fla.: CRC Press, 2009.
Frankenberger, William T., Jr., and Sally Benson, eds. Selenium in the Environment. New York: Marcel Dekker, 1994.
Frankenberger, William T., Jr., and Richard A. Engberg, eds. Environmental Chemistry of Selenium. New York: Marcel Dekker, 1998.
Greenwood, N. N., and A. Earnshaw. “Selenium, Tellurium, and Polonium.” In Chemistry of the Elements. 2d ed. Boston: Butterworth-Heinemann, 1997.
Jacobs, L. W., ed. Selenium in Agriculture and the Environment: Proceedings of a Symposium. Madison, Wis.: American Society of Agronomy, Soil Science Society of America, 1989.
Massey, A. G. “Group 16: The Chalcogens—Oxygen, Sulfur, Selenium, Tellurium, and Polonium.” In Main Group Chemistry. 2d ed. New York: Wiley, 2000.
Rosenfeld, Irene, and Orville A. Beath. Selenium: Geobotany, Biochemistry, Toxicity, and Nutrition. New York: Academic Press, 1964.
Surai, Peter F. Selenium in Nutrition and Health. Nottingham, England: Nottingham University Press, 2006.
Natural Resources Canada.
U.S. Geographical Survey.