Product Code Database
Selenium is a chemical element; it has symbol Se and atomic number 34. It has various physical appearances, including a brick-red powder, a vitreous black solid, and a grey metallic-looking form. It seldom occurs in this elemental state or as pure ore compounds in Earth's crust. Selenium (σελήνη ) was discovered in 1817 by Jöns Jacob Berzelius, who noted the similarity of the new element to the previously discovered tellurium (named for the Earth).
Selenium is found in metal sulfide ores, where it substitutes for sulfur. Commercially, selenium is produced as a byproduct in the refining of these ores. Minerals that are pure selenide or selenate compounds are rare. The chief commercial uses for selenium today are glassmaking and . Selenium is a semiconductor and is used in . Applications in electronics, once important, have been mostly replaced with silicon semiconductor devices. Selenium is still used in a few types of Direct current and one type of Fluorescence quantum dot.
Although trace amounts of selenium are necessary for cellular function in many animals, including humans, both elemental selenium and (especially) selenium salts are toxic in even small doses, causing selenosis. Symptoms include (in decreasing order of frequency): diarrhea, fatigue, hair loss, joint pain, nail brittleness or discoloration, nausea, headache, tingling, vomiting, and fever.
Selenium is listed as an ingredient in many multivitamins and other dietary supplements, as well as in infant formula, and is a component of the antioxidant enzymes glutathione peroxidase and thioredoxin reductase (which indirectly reduce certain Redox molecules in animals and some plants) as well as in three deiodinase enzymes. Selenium requirements in plants differ by species, with some plants requiring relatively large amounts and others apparently not requiring any.
The red α, β, and γ forms are produced from solutions of black selenium by varying the evaporation rate of the solvent (usually CS2). They all have a relatively low, monoclinic crystal symmetry (space group 14) and contain nearly identical puckered cyclooctaselenium (Se8) rings as in sulfur. The eight atoms of a ring are not equivalent (i.e. they are not mapped one onto another by any symmetry operation), and in fact in the γ-monoclinic form, half the rings are in one configuration (and its mirror image) and half in another. The packing is most dense in the α form. In the Se8 rings, the Se–Se distance varies depending on where the pair of atoms is in the ring, but the average is 233.5 pm, and the Se–Se–Se angle is on average 105.7°. Other selenium allotropes may contain Se6 or Se7 rings.
The most stable and dense form of selenium is gray and has a chiral crystal lattice (space group 152 or 154 depending on the chirality) consisting of helical polymeric chains, where the Se–Se distance is 237.3 pm and Se–Se–Se angle is 103.1°. The minimum distance between chains is 343.6 pm. Gray selenium is formed by mild heating of other allotropes, by slow cooling of molten selenium, or by condensing selenium vapor just below the melting point. Whereas other selenium forms are insulators, gray selenium is a semiconductor showing appreciable photoconductivity. Unlike the other allotropes, it is insoluble in CS2. It resists oxidation by air and is not attacked by nonoxidizing . With strong reducing agents, it forms polyselenides. Selenium does not exhibit the changes in viscosity that sulfur undergoes when gradually heated.
It is a solid that forms monomeric SeO2 molecules in the gas phase. It dissolves in water to form selenous acid, H2SeO3. Selenous acid can also be made directly by oxidizing elemental selenium with nitric acid: (2025). 9780123526519, Academic Press. ISBN 9780123526519
Unlike sulfur, which forms a stable sulfur trioxide, selenium trioxide is thermodynamically unstable and decomposes to the dioxide above 185 °C:
Selenium trioxide is produced in the laboratory by the reaction of anhydrous potassium selenate (K2SeO4) and sulfur trioxide (SO3).
Salts of selenous acid are called selenites. These include silver selenite (Ag2SeO3) and sodium selenite (Na2SeO3).
Hydrogen sulfide reacts with aqueous selenous acid to produce selenium disulfide:
Selenium disulfide consists of 8-membered rings. It has an approximate composition of SeS2, with individual rings varying in composition, such as Se4S4 and Se2S6. Selenium disulfide has been used in shampoo as an antidandruff agent, an inhibitor in polymer chemistry, a glass dye, and a reducing agent in fireworks.
Selenium trioxide may be synthesized by dehydrating selenic acid, H2SeO4, which is itself produced by the oxidation of selenium dioxide with hydrogen peroxide: (1980). 9780471077152 ISBN 9780471077152 The report describes the synthesis of selenic acid.
Hot, concentrated selenic acid reacts with gold to form gold(III) selenate.
In comparison with its sulfur counterpart (sulfur hexafluoride), selenium hexafluoride (SeF6) is more reactive and is a toxic pulmonary irritant. (2025). 9780471268833, Wiley-IEEE. ISBN 9780471268833 Selenium tetrafluoride is a laboratory-scale fluorinating agent.
The only stable are selenium tetrachloride (SeCl4) and selenium monochloride (Se2Cl2), which might be better known as selenium(I) chloride and is structurally analogous to disulfur dichloride. Metastable solutions of selenium dichloride can be prepared from sulfuryl chloride and selenium (reaction of the elements generates the tetrachloride instead), and constitute an important reagent in the preparation of selenium compounds (e.g. Se7). The corresponding are all known, and recapitulate the same stability and structure as the chlorides.
The of selenium are not well known, and for a long time were believed not to exist. There is limited Spectroscopy evidence that the lower iodides may form in bi-elemental solutions with nonpolar solvents, such as carbon disulfide and carbon tetrachloride; but even these appear to Photosensitivity.Rao, M. R. Aswatha Narayana. "Selenium iodide". In Proceedings of the Indian Academy of Sciences-Section A, vol. 12, pp. 410-415. Springer India, 1940.
Some selenium oxyhalides—seleninyl fluoride (SeOF2) and selenium oxychloride (SeOCl2)—have been used as specialty solvents.
Alkali metal selenides react with selenium to form polyselenides, , which exist as chains and rings.
Selenium reacts with to yield selenocyanates:
In 1873, Willoughby Smith found that the electrical conductivity of grey selenium was affected by light. This led to its use as a cell for sensing light. The first commercial products using selenium were developed by Werner Siemens in the mid-1870s. The selenium cell was used in the photophone developed by Alexander Graham Bell in 1879. Selenium transmits an electric current proportional to the amount of light falling on its surface. This phenomenon was used in the design of and similar devices. Selenium's semiconductor properties found numerous other applications in electronics. The development of selenium rectifiers began during the early 1930s, and these replaced copper oxide rectifiers because they were more efficient. These lasted in commercial applications until the 1970s, following which they were replaced with less expensive and even more efficient silicon rectifiers.
Selenium came to medical notice later because of its toxicity to industrial workers. Selenium was also recognized as an important veterinary toxin, which is seen in animals that have eaten high-selenium plants. In 1954, the first hints of specific biological functions of selenium were discovered in by biochemist, Jane Gibson. (2025). 9780306463785 ISBN 9780306463785 It was discovered to be essential for mammalian life in 1957. (2025). 9780387338262 ISBN 9780387338262 In the 1970s, it was shown to be present in two independent sets of . This was followed by the discovery of selenocysteine in proteins. During the 1980s, selenocysteine was shown to be encoded by the Genetic code. The recoding mechanism was worked out first in bacteria and then in (see SECIS element).
Selenium occurs naturally in a number of inorganic forms, including selenide, selenate, and selenite, but these minerals are rare. The common mineral selenite is not a selenium mineral, and contains no selenite ion, but is rather a type of gypsum (calcium sulfate hydrate) named like selenium for the moon well before the discovery of selenium. Selenium is most commonly found as an impurity, replacing a small part of the sulfur in sulfide ores of many metals.
In living systems, selenium is found in the amino acids selenomethionine, selenocysteine, and methylselenocysteine. In these compounds, selenium plays a role analogous to that of sulfur. Another naturally occurring organoselenium compound is dimethyl selenide.
Certain soils are selenium-rich, and selenium can be bioaccumulation by some plants. In soils, selenium most often occurs in soluble forms such as selenate (analogous to sulfate), which are leached into rivers very easily by runoff. Ocean water contains significant amounts of selenium.
Typical background concentrations of selenium do not exceed 1 ng/m3 in the atmosphere; 1 mg/kg in soil and vegetation and 0.5 μg/L in freshwater and seawater.
Anthropogenic sources of selenium include coal burning, and the mining and smelting of sulfide ores.
Industrial production of selenium usually involves the extraction of selenium dioxide from residues obtained during the purification of copper. Common production from the residue then begins by oxidation with sodium carbonate to produce selenium dioxide, which is mixed with water and to form selenous acid (oxidation step). Selenous acid is bubbled with sulfur dioxide (Redox step) to give elemental selenium.
About 2,000 tonnes of selenium were produced in 2011 worldwide, mostly in Germany (650 t), Japan (630 t), Belgium (200 t), and Russia (140 t), and the total reserves were estimated at 93,000 tonnes. These data exclude two major producers: the United States and China. A previous sharp increase was observed in 2004 from $4–$5 to $27/lb. The price was relatively stable during 2004–2010 at about US$30 per pound (in 100 pound lots) but increased to $65/lb in 2011. The consumption in 2010 was divided as follows: metallurgy – 30%, glass manufacturing – 30%, agriculture – 10%, chemicals and pigments – 10%, and electronics – 10%. China is the dominant consumer of selenium at 1,500–2,000 tonnes/year.
Zinc selenide was the first material for blue , but gallium nitride dominates that market. Cadmium selenide can be used to make . Sheets of amorphous selenium convert X-ray images to patterns of charge in xeroradiography and in solid-state, flat-panel X-ray cameras. Ionized selenium (Se+24, where 24 of the outer D, S and P orbitals are stripped away due to high input energies) is one of the active mediums used in X-ray lasers. 75Se is used as a gamma source in industrial radiography.
Selenium catalyzes some chemical reactions, but it is not widely used because of issues with toxicity. In X-ray crystallography, incorporation of one or more selenium atoms in place of sulfur helps with multiple-wavelength anomalous dispersion and single wavelength anomalous dispersion phasing.
Selenium is used in the toning of photographic prints, and it is sold as a toner by numerous photographic manufacturers. Selenium intensifies and extends the tonal range of black-and-white photographic images and improves the permanence of prints. Small amounts of organoselenium compounds have been used to modify the catalysts used for the vulcanization for the production of rubber. Selenium is used in some anti-dandruff shampoos in the form of selenium disulfide such as Selsun and Vichy Dereos brands.
Selenium poisoning of water systems may result whenever new agricultural run-off courses through dry lands. This process leaches natural soluble selenium compounds (such as selenates) into the water, which may then be concentrated in wetlands as the water evaporates. Selenium pollution of waterways also occurs when selenium is leached from coal flue ash, mining and metal smelting, crude oil processing, and landfill. High selenium levels in waterways were found to cause congenital disorders in oviparous species, including wetland birds and fish. Elevated dietary methylmercury levels can amplify the harm of selenium toxicity in oviparous species.
Selenium is bioaccumulation in aquatic habitats, which results in higher concentrations in organisms than the surrounding water. Organoselenium compounds can be concentrated over 200,000 times by zooplankton when water concentrations are in the 0.5 to 0.8 μg Se/L range. Inorganic selenium bioaccumulates more readily in phytoplankton than zooplankton. Phytoplankton can concentrate inorganic selenium by a factor of 3000. Further concentration through bioaccumulation occurs along the food chain, as predators consume selenium-rich prey. It is recommended that a water concentration of 2 μg Se/L be considered highly hazardous to sensitive fish and Water bird. Selenium poisoning can be passed from parents to offspring through the egg, and selenium poisoning may persist for many generations. Reproduction of mallard ducks is impaired at dietary concentrations of 7 μg Se/L. Many Benthic zone invertebrates can tolerate selenium concentrations up to 300 μg/L of selenium in their diet.
Bioaccumulation of selenium in aquatic environments causes fish kills depending on the species in the affected area. There are, however, a few species that have been seen to survive these events and tolerate the increased selenium. It has also been suggested that the season could have an impact on the harmful effects of selenium on fish. Substantial physiological changes may occur in fish with high tissue concentrations of selenium. Fish affected by selenium may experience swelling of the gill lamellae, which impedes oxygen diffusion across the gills and blood flow within the gills. Respiratory capacity is further reduced due to selenium binding to hemoglobin. Other problems include degeneration of liver tissue, swelling around the heart, damaged egg follicles in ovaries, , and accumulation of fluid in the body cavity and head. Selenium often causes a malformed fish fetus which may have problems feeding or respiring; distortion of the fins or spine is also common. Adult fish may appear healthy despite their inability to produce viable offspring.
Selenium is a component of the unusual selenocysteine and selenomethionine. In humans, selenium is a dietary mineral nutrient that functions as cofactor for redox 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).
The glutathione peroxidase family of enzymes (GSH-Px) catalyze reactions that remove reactive oxygen species such as hydrogen peroxide and organic .
The Thyroid and every cell that uses thyroid hormone also use selenium, which is a cofactor for the three of the four known types of Deiodinase, which activate and then deactivate various thyroid hormones and their metabolites; the iodothyronine deiodinases are the subfamily of deiodinase enzymes that use selenium as the otherwise rare amino acid selenocysteine.
Increased dietary selenium reduces the effects of mercury toxicity, although it is effective only at low to modest doses of mercury. Evidence suggests that the molecular mechanisms of mercury toxicity include the irreversible inhibition of selenoenzymes that are required to prevent and reverse oxidative damage in brain and endocrine tissues. The selenium-containing compound selenoneine is present in the blood of bluefin tuna.Michiaki Yamashita, Shintaro Imamura, Md. Anwar Hossain, Ken Touhata, Takeshi Yabu, and Yumiko Yamashita, "Strong antioxidant activity of the novel selenium-containing imidazole compound 'selenoneine, The FASEB Journal, vol. 26 no. 1, supplement 969.13, April 2012 Certain plants are considered indicators of high selenium content of the soil because they require high levels of selenium to thrive. The main selenium indicator plants are Astragalus species (including some ), prince's plume ( Stanleya sp.), woody asters ( Xylorhiza sp.), and false goldenweed ( Oonopsis sp.).
Selenoprotein families of GSH-Px and the deiodinases of eukaryotic cells seem to have a bacterial Phylogenetics origin. The selenocysteine-containing form occurs in species as diverse as green algae, diatoms, sea urchins, fish, and chickens.
Trace elements involved in GSH-Px and superoxide dismutase enzymes activities, i.e., selenium, vanadium, magnesium, copper, and zinc, may have been lacking in some terrestrial mineral-deficient areas. Marine organisms retained and sometimes expanded their selenoproteomes, whereas the selenoproteomes of some terrestrial organisms were lowered or completely lost. These findings suggest that, with the exception of , aquatic life supports selenium use, whereas terrestrial habitats lead to lowered use of this trace element. Marine fishes and vertebrate thyroid glands have the highest concentration of selenium and iodine. From about 500 million years ago, freshwater and terrestrial plants slowly optimized the production of "new" endogenous antioxidants such as ascorbic acid (vitamin C), (including flavonoids), , etc. A few of these appeared in the last 50–200 million years in fruits and flowers of angiosperm plants. In fact, the angiosperms (the dominant type of plant today) and most of their antioxidant pigments evolved during the late Jurassic period.
About 200 million years ago, new selenoproteins were developed as mammalian GSH-Px enzymes.
Signs and symptoms of selenosis include a garlic odor on the breath, gastrointestinal disorders, hair loss, sloughing of nails, fatigue, irritability, and neurological damage. Extreme cases of selenosis can exhibit cirrhosis of the liver, pulmonary edema, or death. Elemental selenium and most metallic have relatively low toxicities because of low bioavailability. By contrast, and selenites have an oxidant mode of action similar to that of arsenic trioxide and are very toxic. The chronic toxic dose of selenite for humans is about 2400 to 3000 micrograms of selenium per day. 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.
On 19 April 2009, 21 Polo pony died shortly before a match in the United States Polo Open. Three days later, a pharmacy released a statement explaining that the horses had received an incorrect dose of one of the ingredients used in a vitamin/mineral supplement compound that had been incorrectly prepared by a compounding pharmacy. Analysis of blood levels of inorganic compounds in the supplement indicated the selenium concentrations were 10 to 15 times higher than normal in the and 15 to 20 times higher than normal in the liver samples. Selenium was later confirmed to be the toxic factor.
In fish and other wildlife, selenium is necessary for life but toxic in high doses. For salmon, the optimal selenium concentration is about 1 microgram selenium per gram of whole body weight. Much below that level, young salmon die from deficiency; much above, they die from toxic excess.
The Occupational Safety and Health Administration (OSHA) has set the legal limit (permissible exposure limit) for selenium in the workplace at 0.2 mg/m3 over an 8-hour workday. The National Institute for Occupational Safety and Health (NIOSH) has set a Recommended exposure limit (REL) of 0.2 mg/m3 over an 8-hour workday. At levels of 1 mg/m3, selenium is IDLH.
Selenium deficiency, defined by low (<60% of normal) selenoenzyme activity levels in brain and endocrine tissues, occurs only when a low selenium level is linked with additional stress, such as high exposures to mercury or increased oxidant stress from vitamin E deficiency. (2025). 9780192627568, Oxford University Press. ISBN 9780192627568
Selenium interacts with other nutrients, such as iodine and vitamin E. The effect of selenium deficiency on health remains uncertain, particularly concerning Kashin–Beck disease. Also, selenium interacts with other minerals, such as zinc and copper. High doses of selenium supplements in pregnant animals might disturb the zinc:copper ratio and lead to zinc reduction; in such treatment cases, zinc levels should be monitored. Further studies are needed to confirm these interactions.
In the regions (e.g., regions within North America) where low selenium soil levels lead to low concentrations in the plants, some animal species may be deficient unless selenium is supplemented with diet or injection.National Research Council, Subcommittee on Sheep Nutrition (1985). Nutrient requirements of sheep. 6th ed., National Academy Press, Washington, . are particularly susceptible. In general, absorption of dietary selenium is lower in ruminants than in other animals and is lower in forages than in grain.National Research Council, Committee on Nutrient Requirements of Small Ruminants (2007). Nutrient requirements of small ruminants. National Academies Press, Washington, . Ruminants grazing certain forages, e.g., some white clover varieties containing cyanogenic glycosides, may have higher selenium requirements, presumably because cyanide is released from the aglycone by glucosidase activity in the rumen and glutathione peroxidases are deactivated by the cyanide acting on the glutathione moiety. Neonate ruminants at risk of white muscle disease may be administered both selenium and vitamin E by injection; some of the WMD Myopathy respond only to selenium, some only to vitamin E, and some to either.Kahn, C. M. (ed.) (2005). Merck Veterinary Manual. 9th ed. Merck & Co., Inc., Whitehouse Station, .
The US Recommended Dietary Allowance (RDA) of selenium for teenagers and adults is 55 Microgram/day. Selenium as a dietary supplement is available in many forms, including multi-vitamins/mineral supplements, which typically contain 55 or 70 μg/serving. Selenium-specific supplements typically contain either 100 or 200 μg/serving. In June 2015, the US Food and Drug Administration (FDA) published its final rule establishing a requirement for minimum and maximum levels of selenium in infant formula.
Selenium (in close interrelation with iodine) plays a role in thyroid health. Selenium is a cofactor for the three thyroid hormone , helping activate and then deactivate various thyroid hormones and their metabolites. Isolated selenium deficiency is now being investigated for its role in the induction of autoimmune reactions in the thyroid gland in Hashimoto's disease. In a case of combined iodine and selenium deficiency was shown to play a thyroid-protecting role.
|
|
