| References | Selenium is a chemical element with atomic number 34, with the chemical symbol Se. It is a nonmetal, chemically related to sulfur and tellurium, and rarely occurring 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 certain enzymes. Selenium is an essential component of the enzyme glutathione peroxidase (GSH-Px) which catalyzes the reaction:
2 GSH+ H2O2---------GSH-Px > GSSG + 2 H2O Selenium requirements in plants differ by species, with some plants apparently requiring none.[1] Isolated selenium occurs in several different forms, the most stable of which is a dense purplish-gray semimetal (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 nonconductive 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. |
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| 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 and selenocysteine. 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. |
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| 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. | ||||||
| History and changing global demand for selenium | Selenium (Greek 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.[3] 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. |
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| Production and allotropic forms | Selenium is a common byproduct of copper refining, or the production of sulfuric acid.[35][36][37] 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. |
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| Non-biological applications | Chemistry
Selenium is a catalyst in many chemical reactions and is widely used in various industrial and laboratory syntheses. It is also widely used in structure determination of proteins and nucleic acids by X-ray crystallography (incorporation of one or more Se atoms helps with MAD phasing.) Manufacturing and materials use The largest use of selenium world-wide is in glass and ceramic manufacturing, where it is used to give a red color to glasses, enamels and glazes as well as to remove color from glass by counteracting the green tint imparted by ferrous impurities. Selenium is used with bismuth in brasses to replace more toxic lead. It is also used to improve abrasion resistance in vulcanized rubbers. Electronics Because of its photovoltaic and photoconductive properties, selenium is used in photocopying, photocells, light meters and solar cells. It was once widely used in rectifiers. These uses have mostly been replaced by silicon-based devices, or are in the process of being replaced. The most notable exception is in power DC surge protection, where the superior energy capabilities of selenium suppressors make them more desirable than metal oxide varistors. Sheets of amorphous selenium convert x-ray images to patterns of charge in xeroradiography and in solid-state flat panel x-ray cameras. Photography Selenium is used in the toning of photographic prints, and it is sold as a toner by numerous photographic manufacturers including Kodak and Fotospeed. Its use intensifies and extends the tonal range of black and white photographic images as well as improving the permanence of prints. |
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