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Selenium

Selenium

Description

Selenium is a nonmetallic element with an atomic number of 34 and an atomic weight of 78.96. Its chemical symbol is Se. Selenium is most commonly found in nature in its inorganic form, sodium selenite. An organic form of selenium, selenomethionine, is found in foods.

General use

The role of selenium in human nutrition and other therapeutic applications has provoked intense controversy over the past two decades. In contrast to such major minerals as magnesium and calcium , neither selenium's benefits nor its toxic aspects are yet fully understood. Until very recently, selenium was considered a toxic element that was not necessary to human health. In 1989, selenium was reclassified as an essential micronutrient in a balanced human diet when the National Research Council established the first recommended daily allowance (RDA) for it. It is considered a minor mineral, or a trace element, as distinct from a major mineral such as calcium or phosphorus , or an electrolyte such as sodium or chloride. There is less than 1 mg of selenium in the average human body. The selenium is concentrated in the liver, kidneys, and pancreas. In males, selenium is also found in the testes and seminal vesicles. Selenium currently has a variety of applications, ranging from standard external preparations for skin problems to experimental and theoretical applications in nutrition and internal medicine.

Alternative medicine

Naturopaths use selenium supplements to treat asthma, acne, tendinitis, infertility problems in men, and postmenopausal disorders in women. Selenium is also considered an important component in naturopathic life extension (longevity) diets , because of its role in tissue repair and maintaining the youthful elasticity of skin.

Dermatology

Selenium has been used since the 1960s in dandruff shampoos and topical medications for such skin disorders as folliculitis ("hot tub" syndrome) and tinea versicolor, a mild infection of the skin caused by the yeast-like fungus Pityrosporum orbiculare. When selenium is compounded with sulfur to form a sulfide, it has antibiotic and antifungal properties. Selenium sulfide is absorbed by the outermost layer of skin cells, the epithelium. Inside the cells, the compound splits into selenium and sulfide ions. The selenium ions counteract the enzymes that are responsible for producing new epithelial cells, thus lowering the turnover of surface skin cells. As a result, itching and flaking of the skin associated with dandruff and tinea versicolor is reduced.

Nutrition

Prior to 1989, there were no established RDA values for selenium. In 1989, the National Research Council of the National Academy of Sciences defined the RDAs for selenium as follows: Males aged 1518 years, 50 g; 1924 years, 70 g; 2550 years, 70 g; 51 years and older, 70 g. Females: aged 1518 years, 50 g; 1924 years, 55 g; 2550 years, 55 g; 51 years and older, 55 g; pregnant, 65 g; lactating, 75 g. The generally higher levels for males are related to the importance of selenium in producing vigorous sperm.

The amount of selenium in the diet is influenced by its level in the soil. Most selenium is absorbed from food products, whether plants grown in the soil or animals that have eaten the plants. Much of the selenium in foods is lost during processing. About 60% of dietary selenium is absorbed as food passes through the intestines. Selenium leaves the body in the urine and feces; males also lose some selenium through ejaculation of sperm. Selenium levels in soil vary widely, not only in different countries but also across different regions. For example, in the United States the western states have higher levels of selenium in the soil than the eastern states. South Dakota has the highest rates of soil selenium in the United States, while Ohio has the lowest.

Foods that are high in selenium contain the element in an organic form, selenomethionine. This form of selenium is considerably less toxic than inorganic sodium selenite or elemental selenium. Good sources of selenium include brewer's yeast, wheat germ , wheat bran, kelp (seaweed), shellfish, Brazil nuts, barley, and oats. Onions, garlic , mushrooms, broccoli, and Swiss chard may contain high amounts of selenium if they are grown in selenium-rich soil. Selenium is also present in drinking water in some parts of the world and can be added to drinking water as a health measure. Nursing mothers should note that human milk is much richer in selenium than cow's milk.

There is no widely recognized deficiency syndrome for selenium, unlike the syndromes associated with calcium or magnesium (hypocalcemia and hypomagnesemia, respectively). However, many researchers who

have investigated Keshan disease, a form of heart disease in children, believe that it is caused by selenium deficiency. The disease can be prevented but not cured with supplemental selenium; it responds to treatment with 50 g per day. The symptoms of Keshan disease, which is named for the region of China where it was discovered, include enlargement of the heart and congestive heart failure. The soil in the Keshan region is low in selenium. The researchers observed that the local Chinese treat Keshan disease with astragalus (Astragalus membranaceus ), a plant that absorbs selenium from the soil.

Selenium toxicity is still a matter of controversy. It is a known fact that humans can tolerate higher levels of selenium in its organic form (selenomethionine) than in its inorganic forms. Humans can show symptoms of selenium toxicity after doses as low as 1 mg of sodium selenite. On the other hand, some researchers speculate that the organic forms of selenium may accumulate in the body and interfere with the functioning of sulfur molecules in the body, or that they may cause genetic mutations. These long-term questions await further research. In addition, researchers disagree about how much selenium will produce symptoms of toxicity. It has been suggested that toxicity can result from a daily intake of 2 mg in people who already have body stores of 2.5 mg of selenium or higher. Another measurement suggests that selenium toxicity may occur wherever the food or water regularly contains more than 5 or 10 parts per million of selenium. Patients with symptoms of selenium toxicity usually have blood plasma levels of 100 g/dl or higher, which is about four times the upper limit of normal levels.

The symptoms of selenium toxicity are not always clearly defined. People living in areas of selenium-rich soil sometimes develop heart, eye, or muscular problems. Eating foods containing high amounts of selenium over a long period of time increases the risk of tooth decay. It is thought that the selenium may compete with the fluoride in teeth, thus weakening their structure. Other symptoms associated with high levels of selenium include a metallic taste in the mouth, garlic-like breath odor, dizziness, nausea , skin inflammation, fatigue , and the loss of hair or nails. The symptoms of acute selenium poisoning include fever , kidney and liver damage, and eventual death.

Internal medicine

Selenium is most widely recognized as a substance that speeds up the metabolism of fatty acids and works together with vitamin E (tocopherol) as an antioxidant. Antioxidants are organic substances that are able to counteract the damage done to human tissue by oxidation (the breakdown of fatty acids). Selenium's antioxidant properties have been studied with respect to several diseases and disorders. In addition to its antioxidant properties, selenium also appears to work as an anti-inflammatory agent in certain disorders.

CARDIOVASCULAR DISEASES. Low levels of selenium have been associated with high risk of heart attacks and strokes. It is thought that the antioxidant properties of selenium can help prevent atherosclerosis (narrowing and hardening of the arteries) by decreasing the formation of fatty deposits in the arteries. It does so by soothing the inflamed arterial walls and binding the free radicals that damage the tissues lining the arteries. Other studies indicate that selenium reduces the symptoms of angina pectoris.

CATARACTS. Cataracts in the eye contain only one-sixth as much selenium as normal lens tissue. The healthy lens requires adequate levels of three antioxidant enzymes: superoxide dismutase, catalase, and glutathione peroxidase. Glutathione peroxidase in the human eye is dependent on selenium, which suggests that a selenium deficiency speeds up the progression of cataracts.

CANCER. Low dietary levels of selenium have been associated with an increased incidence of cancer . Cancers of the respiratory system and the gastrointestinal tract seem to be especially sensitive to the level of selenium in the body. In a recent study, patients with histories of skin cancer were given 200 g of selenium per day. Results indicated that the patients had a reduced incidence of rectal, prostate, and lung cancers as well as a lower rate of mortality from all cancers. In addition, cervical dysplasias (abnormal growths of tissue) in women are associated with low levels of selenium in the patient's diet. In animal studies, as little as 14 parts per million of selenium added to the water or food supply is associated with a decreased incidence of cancer. It is not yet known, however, exactly how selenium protects against cancer. Some researchers believe that it may prevent mutations or decrease the rate of cell division, particularly on the outer surfaces of the body. A recent study of the effects of a selenium compound on mammary tissue indicates that selenium may inhibit the growth of tumors in deeper layers of tissue, not just cancers arising from the epithelium.

As of 2002, selenium is being studied as a possible chemopreventive for prostate cancer . The researchers hope to learn more about the mechanisms by which selenium slows the progress of an established cancer as well as discover a preventive strategy that makes use of selenium.

PERIODONTAL DISEASE. Selenium appears to speed up the healing of fragile gum tissue as well as opposing the actions of free radicals, which are extremely damaging to gum tissue.

RHEUMATOID ARTHRITIS. Selenium may be useful for treating several autoimmune diseases, especially lupus and rheumatoid arthritis (RA). It has been discovered that patients suffering from RA have low selenium levels. Selenium is necessary for production of the enzyme glutathione peroxidase, which reduces the production of inflammatory substances in the body (prostaglandins and leukotrienes) as well as opposing free radicals. Although supplemental selenium by itself has not been shown to cause improvement in RA, selenium taken together with vitamin E appears to have measurable positive results.

OSTEOARTHRITIS. Recent research in Germany indicates that selenium is beneficial in the prevention and treatment of osteoarthritis (OA), particularly OA resulting from physical wear and tear or structural problems in the patient's joints. Selenium supplements are even more effective when given together with vitamins in treating OA.

Preparations

Selenium is available in topical preparations and as a dietary supplement.

External preparations

Selenium sulfide for the treatment of dandruff is available as over-the-counter (OTC) scalp preparations or shampoo containing 1% or 2.5% solutions of the drug. A topical 2.5% solution of selenium sulfide is available for the treatment of tinea versicolor. Common trade names include Exsel, Selsun, and Selsun Blue.

Dietary supplements

Selenium is widely available in vitamin/mineral dietary supplements and in nutritional antioxidant formulas. Although the average diet supplies enough selenium, some naturopaths recommend daily supplements of 100200 g for adults and 30150 g for children. Sexually active males are advised to take higher doses. Some naturopaths recommend taking selenium together with vitamin E on the grounds that their combined effect is greater than the sum of their individual effects. There are at present no definitive studies on the positive effects on health of selenium taken as a dietary supplement.

Precautions

Topical preparations

Persons using selenium compounds to control dandruff or tinea versicolor should be careful to avoid applying the product to damaged or broken skin. In addition to irritating skin, selenium can enter the body through broken skin. This process is known as percutaneous absorption and can cause selenium toxicity if the preparation is used for a long period of time. Patients should wash their hands carefully after applying the selenium product to affected areas. Doing so will minimize absorption through small breaks in the skin of the hands.

Nutritional supplements

It is difficult to assess the effectiveness of dietary supplements containing selenium because there is little agreement on standards for interpreting selenium levels in human blood. Depending on their intake, healthy adults may have blood plasma levels of selenium in the range of 825 g/dl. In addition, most of the selenium in the body is not carried in the blood but is stored in tissue. Analysis of hair has not been useful in measuring selenium. In the absence of a useful test, people who wish to take supplemental selenium should first find out whether they live in an area that already has high levels of selenium in the drinking water and soil. Most people will probably not need more selenium than is in standard vitamin/mineral supplements. In addition, the body seems to utilize selenium more efficiently when it is taken together with vitamin E.

Side effects

The side effects of contact with compounds containing selenium sulfide include stinging of the skin; irritation of the lining of the eyes; hair discoloration or loss; and oily scalp. Both topical products and megadoses of selenium taken by mouth can cause selenium toxicity. The symptoms of selenium toxicity include nausea, vomiting , tiredness, abdominal pain , a garlicky breath odor, and the loss of hair and fingernails. These symptoms usually last 1012 days after the selenium preparation is discontinued.

Interactions

Topical preparations containing selenium may interact with the metals in costume jewelry. Patients should remove all their jewelry before applying the shampoo or lotion.

With regard to dietary supplements, there is some evidence that vitamin C inactivates selenium within the digestive tract. Persons who are concerned about their selenium intake may prefer to take supplemental selenium in the absence of vitamin C.

Resources

BOOKS

Baron, Robert B., M.D. "Nutrition." In Current Medical Diagnosis and Treatment, edited by Lawrence M. Tierney, M.D., et al. 39th ed. New York: McGraw-Hill, 2000.

Beers, Mark H., M.D., and Robert Berkow, M.D., eds. The Merck Manual of Diagnosis and Therapy. Whitehouse Station, NJ: Merck Research Laboratories, 2002.

Berger, Timothy G., M.D. "Skin, Hair, and Nails." In Current Medical Diagnosis and Treatment, edited by Lawrence M. Tierney, M.D., et al. 39th ed. New York: McGraw-Hill, 2000.

Burton Goldberg Group, comp. Alternative Medicine: The Definitive Guide. Fife, WA: Future Medicine Publishing, 1995.

Murray, Michael, N.D., and Joseph Pizzorno, N.D. Encyclopedia of Natural Medicine. Rocklin, CA: Prima Publishing, 1991.

PERIODICALS

Dong, Y., C. Ip, and H. Ganther. "Evidence of a Field Effect Associated with Mammary Cancer Chemoprevention by Methylseleninic Acid." Anticancer Research 22 (1A)(January-February 2002): 27-32.

Kurz, B., B. Jost, and M. Schunke. "Dietary Vitamins and Selenium Diminish the Development of Mechanically Induced Osteoarthritis and Increase the Expression of Antioxidative Enzymes in the Knee Joint of STR/1N Mice." Osteoarthritis and Cartilage 10 (February 2002): 119-126.

Nelson, M. A., M. Reid, A. J. Duffield-Lillico, and J. R. Marshall. "Prostate Cancer and Selenium." Urology Clinics of North America 29 (February 2002): 67-70.

Tan, J., W. Zhu, W. Wang, et al. "Selenium in Soil and Endemic Diseases in China." The Science of the Total Environment 4 (February 2002): 227-235.

Ujiie, S., and H. Kikuchi. "The Relation Between Serum Selenium Value and Cancer in Miyagi, Japan: 5-Year Follow-Up Study." Tohoku Journal of Experimental Medicine 196 (March 2002): 99-109.

Rebecca J. Frey, PhD

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Selenium (revised)

SELENIUM (REVISED)

Note: This article, originally published in 1998, was updated in 2006 for the eBook edition.

Overview

Selenium is a member of the chalcogen family. The chalcogens are elements in Group 16 (VIA) of the periodic table. The periodic table is a chart that shows how chemical elements are related to one another. Other chalcogens are oxygen, sulfur, tellurium, and polonium. The name chalcogen comes from the Greek word chatkos, meaning "ore." The first two members of the family, oxygen and sulfur, are found in most ores.

Selenium is a metalloid. A metalloid is an element that has some characteristics of a metal and some of a non-metal.

Selenium and tellurium are often associated with each other. They tend to occur together in the Earth and have somewhat similar properties. They have many uses in common. In recent years, some important new uses have been found for selenium. It is now used in the manufacture of plain paper photocopiers and laser printers, in photovoltaic cells that convert sunlight into electricity, and in X-ray systems for medical applications.

SYMBOL
Se

ATOMIC NUMBER
34

ATOMIC MASS
78.96

FAMILY
Group 16 (VIA)
Chalcogen

PRONUNCIATION
suh-LEE-nee-um

Discovery and naming

Selenium was discovered in 1818 by Swedish chemists Jons Jakob Berzelius (1779-1848) and J. G. Gahn (1745-1818). The men were studying the chemicals used in making sulfuric acid at a plant where they had just become part-owners. Among these chemicals they found a material that they thought was the element tellurium. Tellurium had been discovered some 30 years earlier, mixed with some gold deposits in Hungary.

Tellurium is a rare element. Berzelius decided to study the sample more carefully. He took it back to his laboratory in Stockholm. There, he found that he and Gahn had been mistaken. The substance was similar to tellurium, but it also had different properties. They realized they had found a new element. Berzelius suggested naming the element selenium, from the Greek word selene, for "moon." The name seemed a good choice because the element tellurium is named after the Latin word tellus for "Earth." Just as the Earth and the Moon go together, so do tellurium and selenium.

Physical properties

Selenium exists in a number of allotropic forms. Allotropes are forms of an element with different physical and chemical properties. One allotrope of selenium is an amorphous red powder. Amorphous means "without crystalline shape." A lump of clay is an example of an amorphous material. A second allotrope of selenium has a bluish, metallic appearance. A number of other allotropes have properties somewhere between these two forms.

The amorphous forms of selenium do not have specific melting points. Instead, they gradually become softer as they are heated. They may also change from one color and texture to another.

The crystalline (metallic) form of selenium has a melting point of217°C (423°F ) and a boiling point of 685°C (1,260°F). Its density is 4.5 grams per cubic centimeter.

Selenium from the Greek word for moon, selene.

Some of the most important physical characteristics of selenium are its electrical properties. For example, selenium is a semiconductor. A semiconductor is a substance that conducts an electric current better than non-conductors, but not as well as conductors. Semiconductors have many very important applications today in the electronics industry. Selenium is often used in the manufacture of transistors for computers, cellular phones, and hand-held electronic games.

Selenium is also a photoconductor, a material that changes light energy into electrical energy. Furthermore, it becomes better at making this conversion as the light intensity or brightness increases.

Chemical properties

Selenium is a fairly reactive element. It combines easily with hydrogen, fluorine, chlorine, and bromine. It reacts with nitric and sulfuric acids. It also combines with a number of metals to form compounds called selenides. An example is magnesium selenide (MgSe). One of its interesting reactions is with oxygen. It burns in oxygen with a bright blue flame to form selenium dioxide (SeO2). Selenium dioxide has a characteristic odor of rotten horseradish.

Selenium and tellurium are often associated with each other. They tend to occur together in the Earth and have somewhat similar properties.

Occurrence in nature

Selenium is a very rare element. Scientists estimate its abundance at about 0.05 to 0.09 parts per million. It ranks among the 25 least common elements in the Earth's crust. It is widely distributed throughout the crust. There is no ore from which it can be mined with profit. Instead, it is obtained as a by-product of mining other metals. It is now produced primarily from copper, iron , and lead ores. The major producers of selenium in the world are Japan, Canada, Belgium, the United States, and Germany.

Isotopes

There are six naturally occurring isotopes of selenium, selenium-74, selenium-76, selenium-77, selenium-78, selenium-80, and selenium-82. Isotopes are two or more forms of an element. Isotopes differ from each other according to their mass number. The number written to the right of the element's name is the mass number. The mass number represents the number of protons plus neutrons in the nucleus of an atom of the element. The number of protons determines the element, but the number of neutrons in the atom of any one element can vary. Each variation is an isotope.

About a dozen radioactive isotopes of selenium are known also. A radioactive isotope is one that breaks apart and gives off some form of radiation. Radioactive isotopes are produced when very small particles are fired at atoms. These particles stick in the atoms and make them radioactive.

Only one radioactive isotope of selenium is used commercially, selenium-75. This isotope is used to study the function of two organs in the body, the pancreas and the parathyroid gland. (The pancreas helps with digestion and the parathyroid gland releases hormones.) The radioactive selenium is injected into the blood stream. It then goes primarily to one or both of these two organs. The isotope gives off radiation when it reaches these organs. A technician can tell whether the organs are functioning properly by the amount and location of radiation given off.

Extraction

Selenium is obtained as a by-product from other industrial processes. For example, when copper is refined, small amounts of selenium are produced as by-products. This selenium can be removed from the copper-refining process and purified. Selenium is also obtained as a secondary product during the manufacture of sulfuric acid.

The miracle of copying

W e sometimes forget what an amazing step forward the invention of the copy machine was. Hundreds of years ago, making a copy of a document was a long, difficult process. Some people spent their whole lives making copies of important documents. Each copy was written out by hand. The process was not only dull and monotonous, but it also resulted in many errors.

Even thirty years ago, copying was slow and difficult. For example, carbon paper allowed a person to make one or more copies while writing or typing. But every error had to be corrected on every copy. The copies were often messy and difficult to read.

Mimeograph machines made it possible to reproduce dozens of copies in a few minutes, but required handwritten or typed originals. The final product was printed in purple.

Then came the photocopy machine. Copies could be made by simply placing the original on a glass cover and pushing a button. What goes on inside a copy machine to make this happen?

An essential part of a photocopier is a drum-shaped unit or a wide moving belt. Fine selenium powder is spread on the surface of the drum or the belt. An electric charge is then applied to the selenium.

Another part of the photocopy machine consists of a set of mirrors. When the machine's "Copy" button is pushed, a bright light shines on the page being copied. The light reflects off the white parts of the page. But it is not reflected off the dark parts, such as text or images. The light reflects off the mirrors to the drum or belt.

Selenium is important because when light strikes the charged selenium, the charge disappears. The sections on the drum or belt struck by light have no charge. The sections not struck by light continue to have a charge.

Next, a toner is spread out over the surface of the drum or belt. A toner is usually finely-divided carbon. It sticks to the areas that still carry an electric charge. But it does not stick to the selection without a charge.

Finally, a piece of paper is pressed against the drum or belt. The toner sticks to the paper. A blast of heat causes the carbon to melt and stick tightly to the paper. A copy of the original document is produced by the machine.

Uses

The two most important uses of selenium are in glass-making and in electronics. Each accounts for about 30 to 35 percent of all the selenium produced each year. The addition of selenium to glass can have one of two opposite effects. First, it will cancel out the green color that iron compounds usually add to glass. If a colorless glass is desired, a little selenium is added to neutralize the effects of iron. Second, selenium will add its own colora beautiful ruby redif that is wanted in a glass product.

Selenium is also added to glass used in architecture. The selenium reduces the amount of sunlight that gets through the glass.

A growing use of selenium is in electronic products. One of the most important uses is in plain-paper photocopiers and laser printers. The element is also used to make photovoltaic ("solar") cells. When light strikes selenium, it is changed into electricity. A solar cell is a device for capturing the energy of sunlight on tiny pieces of selenium. The sunlight is then changed into electrical energy.

Currently, that process is not very efficient. Too much sunlight is lost without being converted into electricity. More efficient solar cells will be able to make use of all the free sunlight that strikes the planet every day.

About a third of all selenium produced is used as pigments (coloring agents) for paints, plastics, ceramics, and glazes. Depending on the form of selenium used, the color ranges from deep red to light orange.

Selenium is also used to make alloys. An alloy is made by melting and mixing two or more metals. The mixture has properties different from those of the individual metals. The addition of selenium to a metal makes it more machinable. Machinability means working with a metal: bending, cutting, shaping, turning, and finishing the metal, for example.

About 5 percent of all selenium produced is used in agriculture. It is added to soil or animal feed to provide the low levels of selenium needed by plants and animals.

Compounds

Very few compounds of selenium have any important practical applications. One exception is selenium sulfide (SeS2). This compound is used to treat seborrhea, or "oily skin." It is sometimes added to shampoos for people with especially oily hair. Another compound, selenium diethyldithiocarbonate (Se[SC(S)N(C2H5)2]4), is used as a vulcanizing ("toughening") agent for rubber products.

Health effects

Selenium has some rather interesting nutritional roles. It is essential in very small amounts for the health of both plants and animals. Animals that do not have enough selenium in their diets may develop weak muscles. But large doses of selenium are dangerous. In some parts of California, for example, selenium has been dissolved out of the soil by irrigation systems. Lakes accumulate unusually high levels of selenium and birds and fish in the area develop health problems.

A serious selenium problem occurred at the Kesterson Reservoir in Northern California. In the Late 1970s, scientists found that birds nesting in the reservoir were developing genetic deformities. They traced the problem to high levels of selenium in the water. A large artificial lake was built and the birds were moved to the artificial lake. They were no longer allowed to nest in the dangerous waters of the reservoir.

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selenium

selenium (səlē´nēəm), nonmetallic chemical element; symbol Se; at. no. 34; at. wt. 78.96; m.p. 217°C; b.p. about 685°C; sp. gr. 4.81 at 20°C; valence -2, +4, or +6. Selenium is directly below sulfur in Group 16 of the periodic table. In chemical activity and physical properties it resembles sulfur and tellurium. Selenium exhibits allotropy, appearing in a number of forms, including a red amorphous powder, a red crystalline material, and a gray crystalline metallike form called "metallic" selenium. A remarkable property (discovered by Willoughby Smith in 1873) of the gray metallic form is that its electrical conductivity is greater in light than in darkness, and it increases as the illumination increases. This property has led to use of the metallic form in the junction rectifier and as a cathode in the photoelectric cell rectifier. Selenium is extensively used in the vulcanization of rubber, in the manufacture of red glass and some enamels, as a decolorizer of glass to counteract the green of iron compounds, in electronics, and in xerography. Selenium forms the oxides SeO2 and SeO3, the selenious (H2SeO3) and selenic (H2SeO4) acids and their respective selenite and selenate salts, a nitride, carbide, hydride, two sulfides, and various halides and oxyhalides. Selenium sometimes occurs in conjunction with sulfur deposits and often occurs as the selenide (especially of copper, lead, silver, and iron) in sulfide ores. Commercially it is obtained chiefly as a byproduct in the refining of copper. In the Great Plains region and certain other areas, selenium is absorbed from the soil by vegetation in quantities sufficient to poison livestock, thus rendering the land useless for grazing. Nonetheless, selenium is one of the elements needed in trace amounts in the animal and human diet. Fish, meat, poultry, whole grains, and dairy products are good sources of this mineral nutrient in the human diet. The element was discovered by Berzelius in 1817.

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Selenium

Selenium


melting point: 217°C
boiling point: 685°C
density: 4,819 kg/m3
most common ions: SeO32, SeO42, Se2

Selenium (from the Greek word selēnē the Moon), discovered by Swedish chemist Jöns Jakob Berzelius in 1817, ranks thirty-fourth among elements in Earth's crust. It has six naturally occurring isotopes , a large number of allotropes (elemental forms), and in compounds has oxidation states 2, +4, and +6. The gray elemental form has the unique photoelectric property of exhibiting lowered electrical resistance when struck by light, and it is used in photovoltaic cells and photocells (e.g., light meters) and in xerography. It conducts electricity in a "unipolar" manner, hence it is commonly used in electrical rectifiers. It is also used to tint glass red and to decolorize green glass.

Selenium substitutes for sulfur in amino acids to form seleno-cysteine, cystine, and methionine. The selenium-containing antioxidant glutathione peroxidase is biologically important, and selenium is a necessary trace nutrient in warm-blooded animals. Grazing animals develop a form of muscular dystrophy and other disorders when grazing in areas in which the selenium has been depleted; with selenium-depleted diets, people develop

Keshan disease, a form of cardiomyopathy. When its intake is too high, selenium disrupts enzyme function, causing poor health in mammals and birth defects and reproductive failure in birds and fish. Good sources of selenium in human diets include wheat, garlic, Brazil nuts, and walnuts.

see also Chalcogens.

John Harwood

Bibliography

Barceloux, Donald G. (1999). "Selenium." Journal of Toxicology: Clinical Toxicology 37(2): 145172.

Frankenberger, William T., and Engberg, Richard A., eds. (1998). Environmental Chemistry of Selenium. New York: Marcel Dekker.

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selenium

selenium A dietary essential mineral, which is part of enzymes glutathione peroxidase and thyroxine deiodinase. Through its role in glutathione peroxidase it acts as an antioxidant, and to some extent can compensate for vitamin E deficiency. Similarly, vitamin E can compensate for selenium deficiency to some extent.

Requirements are of the order of 50 μg/day; in parts of New Zealand, Finland, and China soils are especially poor in selenium and deficiency occurs. In China selenium deficiency is associated with Keshan disease. Rich sources include: fish and shellfish, mung (dahl) and red kidney beans, Brazil nuts, bread, kidney, lentils, liver, pork, rabbit, veal.

Selenium is toxic in excess; mild selenium intoxication results in production of foul‐smelling hydrogen selenide, which is excreted on the breath and through the skin. Intakes above 450 μg/day are considered hazardous.

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selenium

se·le·ni·um / səˈlēnēəm/ • n. the chemical element of atomic number 34, a gray crystalline nonmetal with semiconducting properties. (Symbol: Se) DERIVATIVES: sel·e·nide / ˈseləˌnīd; -nid/ n.

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"selenium." The Oxford Pocket Dictionary of Current English. . Encyclopedia.com. 22 Aug. 2017 <http://www.encyclopedia.com>.

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selenium

selenium (si-lee-niŭm) n. a trace element that has important antioxidant properties. It is also known to be an essential component of the enzyme that catalyses the production of triiodothyronine (T3) from thyroxine (T4) in the thyroid gland. Symbol: Se.

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"selenium." A Dictionary of Nursing. . Encyclopedia.com. 22 Aug. 2017 <http://www.encyclopedia.com>.

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selenium

selenium •columbium •erbium, terbium, ytterbium •scandium • compendium •palladium, radium, stadium, vanadium •medium, tedium •cryptosporidium, cymbidium, idiom, iridium, rubidium •indium •exordium, Gordium, rutherfordium •odeum, odium, plasmodium, podium, sodium •allium, gallium, pallium, thallium, valium •berkelium, epithelium, helium, nobelium, Sealyham •beryllium, cilium, psyllium, trillium •linoleum, petroleum •thulium • cadmium •epithalamium, prothalamium •gelsemium, premium •chromium, encomium •holmium • fermium •biennium, millennium •cranium, geranium, germanium, Herculaneum, titanium, uranium •helenium, proscenium, rhenium, ruthenium, selenium •actinium, aluminium, condominium, delphinium •ammonium, euphonium, harmonium, pandemonium, pelargonium, plutonium, polonium, zirconium •neptunium •europium, opium •aquarium, armamentarium, barium, caldarium, cinerarium, columbarium, dolphinarium, frigidarium, herbarium, honorarium, planetarium, rosarium, sanitarium, solarium, sudarium, tepidarium, terrarium, vivarium •atrium •delirium, Miriam •equilibrium, Librium •yttrium •auditorium, ciborium, conservatorium, crematorium, emporium, moratorium, sanatorium, scriptorium, sudatorium, vomitorium •opprobrium •cerium, imperium, magisterium •curium, tellurium •potassium • axiom • calcium •francium • lawrencium • americium •Latium, solatium •lutetium, technetium •Byzantium • strontium • consortium •protium • promethium • lithium •alluvium, effluvium •requiem • colloquium • gymnasium •caesium (US cesium), magnesium, trapezium •Elysium • symposium

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"selenium." Oxford Dictionary of Rhymes. . Encyclopedia.com. 22 Aug. 2017 <http://www.encyclopedia.com>.

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"selenium." Oxford Dictionary of Rhymes. . Retrieved August 22, 2017 from Encyclopedia.com: http://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/selenium