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:''SE redirects here. For other uses of the abbreviation, see SE . For the ancient Chinese string instrument, see Se (instrument) ''. Selenium is a Chemical Element in the Periodic Table that has the symbol '''Se''' and Atomic Number 34. It is a toxic Nonmetal that is chemically related to Sulfur and Tellurium . It occurs in several different forms but one of these is a stable gray Metal like form that conducts Electricity better in the light than in the dark and is used in Photocell s. This element is found in Sulfide ores such as Pyrite . PRODUCTION Selenium is a common byproduct of copper refining, or the production of sulfuric acid {Link without Title} {Link without Title} {Link without Title} . Isolation of selenium is often complicated by the presence of other compounds and elements. Commonly though 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 selenium. APPLICATIONS
HISTORY Selenium ( Greek σελήνη ''selene'' meaning "Moon") was discovered in 1817 by Jöns Jakob Berzelius who found the element associated with Tellurium . Growth in selenium consumption was driven by the 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 By 1970, selenium in rectifiers had largely been replaced by Silicon , but its use as a photoconductor in plain paper copiers had become its leading application.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. 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 Brass es to meet no-lead environmental standards became important. OCCURRENCE Selenium occurs in a number of inorganic forms, including elemental selenium, Selenide , Selenate and Selenite . Selenium also occurs in organic compounds such as dimethyl selenide, Selenomethionine and Selenocysteine . 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 {Link without Title} . ''See also .'' ISOTOPES Selenium has at least 29 Isotopes , of which 5 are stable, and 6 are Nuclear Isomer s. SELENIUM AND HEALTH Although it is toxic in large doses, selenium is an essential for adults is 55 micrograms per day. Liver and Brazil Nuts are particularly rich sources of selenium. A list of selenium rich foods can be found on The Office of Dietary Supplements Selenium Fact Sheet . Toxicity :Although selenium is an essential of the liver, Pulmonary Edema and death {Link without Title} . :Elemental selenium and most metalic and Selenocysteine , all of which have high Bioavailability and are toxic in large doses. : Selenium Poisoning as a result of agricultural runoff was also found to have caused birth defects in 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. The Recommended Dietary Allowance for adults is 55 micrograms per day. :Selenium defficiency can lead to Necrosis , leading to weakening of the heart. Kashin-Beck disease results in Atrophy , degeneration and Necrosis of Cartilage tissue. 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 defficiency has also been associated with Goitre , Cretinism and Recurrent Miscarriage in humans {Link without Title} . Controversial Health Effects Cancer :Several studies have suggested a link between cancer and selenium deficiency A study conducted on the effect of selenium suplementation on the recurrence of skin cancers did not demonstrate a reduced rate of reccurence of skin cancers, but did show a significantly reduced occurrence of total cancers[http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=%209315315&dopt=Abstract . Selenium may help prevent cancer by acting as an supplementation on incidence of Prostate Cancer . HIV/AIDS :Some research has indicated a geographical link between regions of selenium deficient soils and peak incidences of HIV / AIDS infection. For example, much of Sub-Saharan Africa is low in selenium. However, Senegal is not, and also has a significantly lower level of AIDS infection than the rest of the continent. AIDS appears to involve a slow and progressive decline in levels of selenium in the body. Whether this decline in selenium levels is a direct result of the replication of the HIV virus {Link without Title} or related more generally to the overall malabsorption of nutrients by AIDS patients remains debated. :Low selenium levels in , 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-cell s, further weakening the immune system {Link without Title} . :Regardless of the cause of depleted selenium levels in AIDS patients, studies have shown that selenium deficiency does strongly corelate with the progression of the disease and the risk of death {Link without Title} {Link without Title} {Link without Title} . Selenium supplementation may help mitigate the symptoms of AIDS and reduce the risk of mortality. It should be emphasized that the evidence to date does not suggest that selenium can reduce the risk of infection or the rate of spread of AIDS , but rather treat the symptoms of those who are already infected. IN POPULAR CULTURE See selenium's entries at Fictional Applications Of Real Materials COMPOUNDS
''See also and Organoselenium Chemistry .'' EXTERNAL LINKS
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