AskDefine | Define selenium

Dictionary Definition

selenium n : a toxic nonmetallic element related to sulfur and tellurium; occurs in several allotropic forms; a stable gray metallike allotrope conducts electricity better in the light than in the dark and is used in photocells; occurs in sulfide ores (as pyrite) [syn: Se, atomic number 34]

User Contributed Dictionary

English

Etymology

A word derived by Swedish chemist Berzelius in 1818, from Σελήνη (selēnē, "moon").

Noun

  1. a nonmetallic chemical element (symbol Se) with an atomic number of 34.

Derived terms

Translations

References

External links

For etymology and more information refer to: http://elements.vanderkrogt.net/elem/se.html (A lot of the translations were taken from that site with permission from the author)

Extensive Definition

Selenium () is a chemical element with the atomic number 34, represented by the chemical symbol Se. It is a nonmetal, chemically related to sulfur and tellurium, and rarely occurs in its elemental state in nature. It is toxic in large amounts, but trace amounts of it are necessary for cellular function in most, if not all, animals, forming the active center of the enzymes glutathione peroxidase and thioredoxin reductase (which indirectly reduce certain oxidized molecules in animals and some plants) and three known deiodinase enzymes (which convert one thyroid hormone to another). Selenium requirements in plants differ by species, with some plants apparently requiring none.
Isolated selenium occurs in several different forms, the most stable of which is a dense purplish-gray semi-metal (semiconductor) form that is structurally a trigonal polymer chain. It conducts electricity better in the light than in the dark, and is used in photocells (see allotropic section below). Selenium also exists in many non-conductive forms: a black glass-like allotrope, as well as several red crystalline forms built of eight-membered ring molecules, like its lighter chemical cousin sulfur.
Selenium is found in economic quantities in sulfide ores such as pyrite, partially replacing the sulfur in the ore matrix. Minerals that are selenide or selenate compounds are also known, but all are rare.

Occurrence

Selenium occurs naturally in a number of inorganic forms, including selenide, selenate and selenite. In soils, selenium most often occurs in soluble forms like selenate (analogous to sulfate), which are leached into rivers very easily by runoff.
Selenium has a biological role, and is found in organic compounds such as dimethyl selenide, selenomethionine, selenocysteine and methylselenocysteine. In these compounds selenium plays an analogous role to sulfur.
Selenium is most commonly produced from selenide in many sulfide ores, such as those of copper, silver, or lead. It is obtained as a byproduct of the processing of these ores, from the anode mud of copper refineries and the mud from the lead chambers of sulfuric acid plants. These muds can be processed by a number of means to obtain free selenium.
Natural sources of selenium include certain selenium-rich soils, and selenium that has been bioconcentrated by certain toxic plants such as locoweed. Anthropogenic sources of selenium include coal burning and the mining and smelting of sulfide ores.

Isotopes

Selenium has six naturally occurring isotopes, five of which are stable: 74Se, 76Se, 77Se, 78Se, and 80Se. The last three also occur as fission products, along with 79Se which has a halflife of 295,000 years, and 82Se which has a very long half life (~1020 yr, decaying via double beta decay to 82Kr) and for practical purposes can be considered to be stable. 23 other unstable isotopes have been characterized.
See also Selenium-79 for more information on recent changes in the halflife of this fission product important for the dose calculations performed in the frame of the geological disposal of long-lived radioactive waste.

History and global demand

Selenium (Greek σελήνη selene meaning "Moon") was discovered in 1817 by Jöns Jakob Berzelius who found the element associated with tellurium (named for the Earth).
Growth in selenium consumption was historically driven by steady development of new uses, including applications in rubber compounding, steel alloying, and selenium rectifiers. By 1970, selenium in rectifiers had largely been replaced by silicon, but its use as a photoconductor in plain paper copiers had become its leading application. During the 1980s, the photoconductor application declined (although it was still a large end-use) as more and more copiers using organic photoconductors were produced. Presently, the largest use of selenium world-wide is in glass manufacturing, followed by uses in chemicals and pigments. Electronic use, despite a number of continued applications, continues to decline.
In 1996, continuing research showed a positive correlation between selenium supplementation and cancer prevention in humans, but widespread direct application of this important finding would not add significantly to demand owing to the small doses required. In the late 1990s, the use of selenium (usually with bismuth) as an additive to plumbing brasses to meet no-lead environmental standards, became important. At present, total world selenium production continues to increase modestly.

Reference in popular culture

In the plot of the hit movie Evolution staring David Duchovny It is discovered that selenium, an ingredient in some dandruff shampoos, is as poisonous to invading alien life-forms as arsenic is to humans.

Selenium and health

Although it is toxic in large doses, selenium is an essential micronutrient for animals. In plants, it occurs as a bystander mineral, sometimes in toxic proportions in forage (some plants may accumulate selenium as a defense against being eaten by animals, but other plants such as locoweed require selenium, and their growth indicates the presence of selenium in soil). It is a component of the unusual amino acids selenocysteine and selenomethionine. In humans, selenium is a trace element nutrient which functions as cofactor for reduction of antioxidant enzymes such as glutathione peroxidases and certain forms of thioredoxin reductase found in animals and some plants (this enzyme occurs in all living organisms, but not all forms of it in plants require selenium).
Glutathione peroxidase (GSH-Px) catalyzes certain reactions which remove reactive oxygen species such as peroxide:
2 GSH+ H2O2---------GSH-Px → GSSG + 2 H2O
Selenium also plays a role in the functioning of the thyroid gland by participating as a cofactor for the three known thyroid hormone deiodinases.
Dietary selenium comes from nuts, cereals, meat, fish, and eggs. Brazil nuts are the richest ordinary dietary source (though this is soil-dependent, since the Brazil nut does not require high levels of the element for its own needs). High levels are found in meats such as kidney, crab and lobster, in that order.

Toxicity

Although selenium is an essential trace element, it is toxic if taken in excess. Exceeding the Tolerable Upper Intake Level of 400 micrograms per day can lead to selenosis. Symptoms of selenosis include a garlic odour on the breath, gastrointestinal disorders, hair loss, sloughing of nails, fatigue, irritability and neurological damage. Extreme cases of selenosis can result in cirrhosis of the liver, pulmonary edema and death.
Elemental selenium and most metallic selenides have relatively low toxicities because of their low bioavailability. By contrast, selenate and selenite are very toxic, and have modes of action similar to that of arsenic. Hydrogen selenide is an extremely toxic, corrosive gas. Selenium also occurs in organic compounds such as dimethyl selenide, selenomethionine, selenocysteine and methylselenocysteine, all of which have high bioavailability and are toxic in large doses. Nano-size selenium has equal efficacy, but much lower toxicity.
Selenium poisoning of water systems may result whenever new agricultural runoff courses through normally dry undeveloped lands. This process leaches natural soluble selenium compounds (such as selenates) into the water, which may then be concentrated in new "wetlands" as the water evaporates. High selenium levels produced in this fashion have been found to have caused certain congenital disorders in wetland birds.

Deficiency

Selenium deficiency is relatively rare in healthy well-nourished individuals. It can occur in patients with severely compromised intestinal function, or those undergoing total parenteral nutrition. Alternatively, people dependent on food grown from selenium-deficient soil are also at risk. In the USA, the Dietary Reference Intake for adults is 55 µg/day. In the UK it is 75 µg/day for adult males and 60 µg/day for adult females. 55 µg/day recommendation is based on full expression of plasma glutathione peroxidase. Selenoprotein P is a better indicator of selenium nutritional status, and full expression of it would require more than 66 µg/day.
Selenium deficiency can lead to Keshan disease, which is potentially fatal. Selenium deficiency also contributes (along with iodine deficiency) to Kashin-Beck disease. The primary symptom of Keshan disease is myocardial necrosis, leading to weakening of the heart. Kashin-Beck disease results in atrophy, degeneration and necrosis of cartilage tissue. Keshan disease also makes the body more susceptible to illness caused by other nutritional, biochemical, or infectious diseases. These diseases are most common in certain parts of China where the soil is extremely deficient in selenium. Studies in Jiangsu Province of China have indicated a reduction in the prevalence of these diseases by taking selenium supplements.
Selenium is also necessary for the conversion of the thyroid hormone thyroxine (T4) into its more active counterpart, triiodothyronine, and as such a deficiency can cause symptoms of hypothyroidism, including extreme fatigue, mental slowing, goitre, cretinism and recurrent miscarriage. A study conducted on the effect of selenium supplementation on the recurrence of skin cancers did not demonstrate a reduced rate of recurrence of skin cancers, but did show a significantly reduced occurrence of total cancers. Dietary selenium prevents chemically induced carcinogenesis in many rodent studies. In these studies, organic seleno-compounds are more potent and less toxic than selenium salts (e.g., selenocyanates, selenomethionine, selenium-rich Brazil nuts, or selenium-enriched garlic or broccoli). Selenium may help prevent cancer by acting as an antioxidant or by enhancing immune activity. Not all studies agree on the cancer-fighting effects of selenium. One study of naturally occurring levels of selenium in over 60,000 participants did not show a significant correlation between those levels and cancer. The SU.VI.MAX study concluded that low-dose supplementation (with 120 mg of ascorbic acid, 30 mg of vitamin E, 6 mg of beta carotene, 100 µg of selenium, and 20 mg of zinc) resulted in a 31% reduction in the incidence of cancer and a 37% reduction in all cause mortality in males, but did not get a significant result for females. The SELECT study is currently investigating the effect of selenium and vitamin E supplementation on incidence of prostate cancer. However, selenium has been proven to help chemotherapy treatment by enhancing the efficacy of the treatment, reducing the toxicity of chemotherapeutic drugs, and preventing the body's resistance to the drugs. One study of cancer cells in vitro showed that chemotherapeutic drugs, such as Taxol and Adriamycin, were more toxic to strains of cancer cells grown in culture when selenium was added.
Low selenium levels in AIDS patients have been directly correlated with decreased immune cell count and increased disease progression and risk of death. Selenium normally acts as an antioxidant, so low levels of it may increase oxidative stress on the immune system leading to more rapid decline of the immune system. Others have argued that HIV encodes for the human selenoenzyme glutathione peroxidase, which depletes the victim's selenium levels. Depleted selenium levels in turn lead to a decline in CD4 helper T-cells, further weakening the immune system.
Regardless of the cause of depleted selenium levels in AIDS patients, studies have shown that selenium deficiency does strongly correlate with the progression of the disease and the risk of death.

Production and allotropic forms

Selenium is a common byproduct of copper refining, or the production of sulfuric acid. Isolation of selenium is often complicated by the presence of other compounds and elements. Commonly, production begins by oxidation with sodium carbonate to produce sodium selenite. The sodium selenite is then acidified with sulfuric acid producing selenous acid. The selenous acid is finally bubbled with sulfur dioxide producing elemental red amorphous selenium.
Selenium produced in chemical reactions invariably appears as the amorphous red form-- an insoluble brick red powder. When this form is rapidly melted, it forms the black, vitreous form which is usually sold industrially as beads. The most thermodynamically stable and dense form of selenium is the electrically conductive gray (trigonal) form, which is composed of long helical chains of selenium atoms. The conductivity of this form is notably light sensitive. Selenium also exists in three different deep red crystalline monoclinic forms, which are composed of Se8 molecules, similar to many allotropes of sulfur.

Non-biological applications

: Selenium is used with bismuth in brasses to replace more toxic lead. It is also used to improve abrasion resistance in vulcanized rubbers.
Sheets of amorphous selenium convert x-ray images to patterns of charge in xeroradiography and in solid-state flat panel x-ray cameras.

Biological applications

Compounds

Selenium also occurs in the III oxidation state, but only in the Se412+ cation; Se(III) compounds are not otherwise known.

References

External links

selenium in Afrikaans: Selenium
selenium in Arabic: سيلينيوم
selenium in Bengali: সেলেনিয়াম
selenium in Belarusian: Селен
selenium in Bosnian: Selen
selenium in Bulgarian: Селен
selenium in Catalan: Seleni
selenium in Czech: Selen
selenium in Corsican: Seleniu
selenium in Welsh: Seleniwm
selenium in Danish: Selen
selenium in German: Selen
selenium in Estonian: Seleen
selenium in Modern Greek (1453-): Σελήνιο
selenium in Spanish: Selenio
selenium in Esperanto: Seleno
selenium in Basque: Selenio
selenium in Persian: سلنیوم
selenium in French: Sélénium
selenium in Friulian: Seleni
selenium in Irish: Seiléiniam
selenium in Manx: Shellainium
selenium in Galician: Selenio
selenium in Korean: 셀레늄
selenium in Armenian: Սելեն
selenium in Hindi: सेलेनियम
selenium in Croatian: Selenij
selenium in Ido: Selenio
selenium in Indonesian: Selenium
selenium in Icelandic: Selen
selenium in Italian: Selenio
selenium in Hebrew: סלניום
selenium in Javanese: Selenium
selenium in Kannada: ಸೆಲೆನಿಯಮ್
selenium in Swahili (macrolanguage): Seleni
selenium in Haitian: Selenyòm
selenium in Latin: Selenium
selenium in Latvian: Selēns
selenium in Luxembourgish: Selen
selenium in Lithuanian: Selenas
selenium in Lojban: lurcmu
selenium in Hungarian: Szelén
selenium in Macedonian: Селен
selenium in Marathi: सेलेनियम
selenium in Dutch: Seleen
selenium in Japanese: セレン
selenium in Norwegian: Selen
selenium in Norwegian Nynorsk: Selen
selenium in Occitan (post 1500): Selèni
selenium in Uzbek: Selen
selenium in Low German: Selen
selenium in Polish: Selen
selenium in Portuguese: Selênio
selenium in Romanian: Seleniu
selenium in Quechua: Silinyu
selenium in Russian: Селен
selenium in Sicilian: Sileniu
selenium in Simple English: Selenium
selenium in Slovak: Selén
selenium in Slovenian: Selen
selenium in Serbian: Селен
selenium in Serbo-Croatian: Selen
selenium in Finnish: Seleeni
selenium in Swedish: Selen
selenium in Tamil: செலீனியம்
selenium in Thai: ซีลีเนียม
selenium in Vietnamese: Selen
selenium in Turkish: Selenyum
selenium in Ukrainian: Селен
selenium in Chinese: 硒
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