Chemistry:Selenium

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Selenium, 34Se
SeBlackRed.jpg
Selenium
Pronunciation/sɪˈliːniəm/ ​(sil-EE-nee-əm)
Appearancegrey metallic-looking, red, and vitreous black (not pictured) allotropes
Standard atomic weight Ar, std(Se)78.971(8)[1]
Selenium in the periodic table
Hydrogen Helium
Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon
Sodium Magnesium Aluminium Silicon Phosphorus Sulfur Chlorine Argon
Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton
Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon
Caesium Barium Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury (element) Thallium Lead Bismuth Polonium Astatine Radon
Francium Radium Actinium Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium Rutherfordium Dubnium Seaborgium Bohrium Hassium Meitnerium Darmstadtium Roentgenium Copernicium Nihonium Flerovium Moscovium Livermorium Tennessine Oganesson
S
↑
Se
↓
Te
arsenic ← selenium → bromine
Atomic number (Z)34
Groupgroup 16 (chalcogens)
Periodperiod 4
Block  p-block
Element category  p-block, sometimes considered a metalloid
Electron configuration[Ar] 3d10 4s2 4p4
Electrons per shell2, 8, 18, 6
Physical properties
Phase at STPsolid
Melting point494 K ​(221 °C, ​430 °F)
Boiling point958 K ​(685 °C, ​1265 °F)
Density (near r.t.)gray: 4.81 g/cm3
alpha: 4.39 g/cm3
vitreous: 4.28 g/cm3
when liquid (at m.p.)3.99 g/cm3
Critical point1766 K, 27.2 MPa
Heat of fusiongray: 6.69 kJ/mol
Heat of vaporization95.48 kJ/mol
Molar heat capacity25.363 J/(mol·K)
Vapor pressure
P (Pa) 1 10 100 1 k 10 k 100 k
at T (K) 500 552 617 704 813 958
Atomic properties
Oxidation states−2, −1, +1,[2] +2, +3, +4, +5, +6 (a strongly acidic oxide)
ElectronegativityPauling scale: 2.55
Ionization energies
  • 1st: 941.0 kJ/mol
  • 2nd: 2045 kJ/mol
  • 3rd: 2973.7 kJ/mol
Atomic radiusempirical: 120 pm
Covalent radius120±4 pm
Van der Waals radius190 pm
Color lines in a spectral range
Spectral lines of selenium
Other properties
Natural occurrenceprimordial
Crystal structure ​hexagonal
Hexagonal crystal structure for selenium
Speed of sound thin rod3350 m/s (at 20 °C)
Thermal expansionamorphous: 37 µm/(m·K) (at 25 °C)
Thermal conductivityamorphous: 0.519 W/(m·K)
Magnetic orderingdiamagnetic[3]
Magnetic susceptibility−25.0·10−6 cm3/mol (298 K)[4]
Young's modulus10 GPa
Shear modulus3.7 GPa
Bulk modulus8.3 GPa
Poisson ratio0.33
Mohs hardness2.0
Brinell hardness736 MPa
CAS Number7782-49-2
History
Namingafter Selene, Greek goddess of the moon
Discovery and first isolationJöns Jakob Berzelius and Johann Gottlieb Gahn (1817)
Main isotopes of selenium
Iso­tope Abun­dance Physics:Half-life (t1/2) Decay mode Pro­duct
72Se syn 8.4 d ε 72As
γ –
74Se 0.86% stable
75Se syn 119.8 d ε 75As
γ –
76Se 9.23% stable
77Se 7.60% stable
78Se 23.69% stable
79Se trace 3.27×105 y β− 79Br
80Se 49.80% stable
82Se 8.82% 1.08×1020 y β−β− 82Kr
Category Category: Selenium
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Se
data m.p. cat
in calc from C diff report ref
C 221 — —
K 494 494 0
F 430 430 0
max precision 0
WD


input C: 221, K: 494, F: 430
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Se
data b.p. cat
in calc from C diff report ref
C 685 — —
K 958 958 0
F 1265 1265 0
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input C: 685, K: 958, F: 1265
comment

Selenium is a chemical element; it has the symbol Se and atomic number 34. It is a nonmetal (more rarely considered a metalloid) with properties that are intermediate between the elements above and below in the periodic table, sulfur and tellurium, and also has similarities to arsenic.[5] It seldom occurs in its elemental state or as pure ore compounds in Earth's crust. Selenium (from grc σελήνη (selḗnē) 'moon') 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 partially replaces the sulfur. Commercially, selenium is produced as a byproduct in the refining of these ores, most often during production. Minerals that are pure selenide or selenate compounds are known but rare. The chief commercial uses for selenium today are glassmaking and pigments. Selenium is a semiconductor and is used in photocells. Applications in electronics, once important, have been mostly replaced with silicon semiconductor devices. Selenium is still used in a few types of DC power surge protectors and one type of fluorescent 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.[6] 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 oxidized 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.[7]

Characteristics

Physical properties

Structure of hexagonal (gray) selenium

Selenium forms several allotropes that interconvert with temperature changes, depending somewhat on the rate of temperature change. When prepared in chemical reactions, selenium is usually an amorphous, brick-red powder. When rapidly melted, it forms the black, vitreous form, usually sold commercially as beads.[8] The structure of black selenium is irregular and complex and consists of polymeric rings with up to 1000 atoms per ring. Black selenium is a brittle, lustrous solid that is slightly soluble in CS2. Upon heating, it softens at 50 °C and converts to gray selenium at 180 °C; the transformation temperature is reduced by presence of halogens and amines.[5]

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 with different geometric arrangements, as in sulfur.[9] 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.[10][11] 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.[5]

The most stable and dense form of selenium is gray and has a chiral hexagonal crystal lattice (space group 152 or 154 depending on the chirality)[12] 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.[5] It resists oxidation by air and is not attacked by nonoxidizing acids. With strong reducing agents, it forms polyselenides. Selenium does not exhibit the changes in viscosity that sulfur undergoes when gradually heated.[8][13]

Isotopes

Main page: Physics:Isotopes of selenium

Selenium has seven naturally occurring isotopes. Five of these, 74Se, 76Se, 77Se, 78Se, 80Se, are stable, with 80Se being the most abundant (49.6% natural abundance). Also naturally occurring is the long-lived primordial radionuclide 82Se, with a half-life of 8.76×1019 years.[14] The non-primordial radioisotope 79Se also occurs in minute quantities in uranium ores as a product of nuclear fission. Selenium also has numerous unstable synthetic isotopes ranging from 64Se to 95Se; the most stable are 75Se with a half-life of 119.78 days and 72Se with a half-life of 8.4 days.[15] Isotopes lighter than the stable isotopes primarily undergo beta plus decay to isotopes of arsenic, and isotopes heavier than the stable isotopes undergo beta minus decay to isotopes of bromine, with some minor neutron emission branches in the heaviest known isotopes.

Selenium isotopes of greatest stability
Isotope Nature Origin Half-life
74Se Primordial Stable
76Se Primordial Stable
77Se Primordial Fission product Stable
78Se Primordial Fission product Stable
79Se Trace Fission product 327000 yr[16][17]
80Se Primordial Fission product Stable
82Se Primordial Fission product* 8.76×1019 yr[14][lower-alpha 1]

Chemical compounds

Selenium compounds commonly exist in the oxidation states −2, +2, +4, and +6.

Chalcogen compounds

Structure of the polymer SeO2: The (pyramidal) selenium atoms are yellow.

Selenium forms two oxides: selenium dioxide (SeO2) and selenium trioxide (SeO3). Selenium dioxide is formed by combustion of elemental selenium:[8]

Se + O
2 → SeO
2

It is a polymeric 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:[18]

3 Se + 4 HNO
3 + H
2O → 3 H
2SeO
3 + 4 NO

Unlike sulfur, which forms a stable trioxide, selenium trioxide is thermodynamically unstable and decomposes to the dioxide above 185 °C:[8][18]

2 SeO
3 → 2 SeO
2 + O
2 (ΔH = −54 kJ/mol)

Selenium trioxide is produced in the laboratory by the reaction of anhydrous potassium selenate (K2SeO4) and sulfur trioxide (SO3).[19]

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:

H
2SeO
3 + 2 H
2S → SeS
2 + 3 H
2O

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.[18]

Selenium trioxide may be synthesized by dehydrating selenic acid, H2SeO4, which is itself produced by the oxidation of selenium dioxide with hydrogen peroxide:[20]

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Hot, concentrated selenic acid reacts with gold to form gold(III) selenate.[21]

Halogen compounds

Selenium reacts with fluorine to form selenium hexafluoride:

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In comparison with its sulfur counterpart (sulfur hexafluoride), selenium hexafluoride (SeF6) is more reactive and is a toxic pulmonary irritant.[22] With different stoichiometry, the elements form selenium tetrafluoride, a laboratory-scale fluorinating agent, instead.

The only stable chlorides 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 bromides are all known, and recapitulate the same stability and structure as the chlorides.[23]

The iodides of selenium are not well known, and for a long time were believed not to exist.[24] There is limited spectroscopic evidence that the lower iodides may form in bi-elemental solutions with nonpolar solvents, such as carbon disulfide[25] and carbon tetrachloride;[24] but even these appear to decompose under illumination.[26]

Some selenium oxyhalides—seleninyl fluoride (SeOF2) and selenium oxychloride (SeOCl2)—have been used as specialty solvents.[8]

Metal selenides

Structures of two polyselenide anions[27]

Analogous to the behavior of other chalcogens, selenium forms hydrogen selenide, H2Se. It is a strongly odiferous, toxic, and colorless gas. It is more acidic than H2S. In solution it ionizes to HSe−. The selenide dianion Se2− forms a variety of compounds, including the minerals from which selenium is obtained commercially. Illustrative selenides include mercury selenide (HgSe), lead selenide (PbSe), zinc selenide (ZnSe), and copper indium gallium diselenide (Cu(Ga,In)Se2). These materials are semiconductors. With highly electropositive metals, such as aluminium, these selenides are prone to hydrolysis, which may be described by this idealized equation:[8]

[math]\ce{ Al2Se3 + 6 H2O -> 2 Al(OH)3 + 3 H2Se }[/math]

Alkali metal selenides react with selenium to form polyselenides, Se2−n, which exist as chains and rings.

Other compounds

Tetraselenium tetranitride, Se4N4, is an explosive orange compound analogous to tetrasulfur tetranitride (S4N4).[8][28][29] It can be synthesized by the reaction of selenium tetrachloride (SeCl4) with [((CH3)3Si)2N]2Se.[30]

Selenium reacts with cyanides to yield selenocyanates:[8]

[math]\ce{ 8 KCN + Se8 -> 8 KSeCN }[/math]

Organoselenium compounds

Main page: Chemistry:Organoselenium chemistry

Selenium, especially in the II oxidation state, forms a variety of organic derivatives. They are structurally analogous to the corresponding organosulfur compounds. Especially common are selenides (R2Se, analogues of thioethers), diselenides (R2Se2, analogues of disulfides), and selenols (RSeH, analogues of thiols). Representatives of selenides, diselenides, and selenols include respectively selenomethionine, diphenyldiselenide, and benzeneselenol. The sulfoxide in sulfur chemistry is represented in selenium chemistry by the selenoxides (formula RSe(O)R), which are intermediates in organic synthesis, as illustrated by the selenoxide elimination reaction. Consistent with trends indicated by the double bond rule, selenoketones, R(C=Se)R, and selenaldehydes, R(C=Se)H, are rarely observed.[31]

History

Selenium (Greek σελήνη selene meaning "Moon") was discovered in 1817 by Jöns Jacob Berzelius and Johan Gottlieb Gahn.[32] Both chemists owned a chemistry plant near Gripsholm, Sweden, producing sulfuric acid by the lead chamber process. Pyrite samples from the Falun Mine produced a red solid precipitate in the lead chambers, which was presumed to be an arsenic compound, so the use of pyrite to make acid was discontinued. Berzelius and Gahn, who wanted to use the pyrite, observed that the red precipitate gave off an odor like horseradish when burned. This smell was not typical of arsenic, but a similar odor was known from tellurium compounds. Hence, Berzelius's first letter to Alexander Marcet stated that this was a tellurium compound. However, the lack of tellurium compounds in the Falun Mine minerals eventually led Berzelius to reanalyze the red precipitate, and in 1818 he wrote a second letter to Marcet describing a newly found element similar to sulfur and tellurium. Because of its similarity to tellurium, named for the Earth, Berzelius named the new element after the Moon.[33][34]

In 1873, Willoughby Smith found that the electrical conductivity of grey selenium was affected by light.[35][36] 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 light meters and similar devices. Selenium's semiconductor properties found numerous other applications in electronics.[37][38][39] The development of selenium rectifiers began during the early 1930s, and these replaced copper oxide rectifiers because they were more efficient.[40][41][42] 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 microorganisms by biochemist, Jane Pinsent.[43][44] It was discovered to be essential for mammalian life in 1957.[45][46] In the 1970s, it was shown to be present in two independent sets of enzymes. This was followed by the discovery of selenocysteine in proteins. During the 1980s, selenocysteine was shown to be encoded by the codon UGA. The recoding mechanism was worked out first in bacteria and then in mammals (see SECIS element).[47]

Occurrence

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Native selenium in sandstone, from a uranium mine near Grants, New Mexico

Native (i.e., elemental) selenium is a rare mineral, which does not usually form good crystals, but, when it does, they are steep rhombohedra or tiny acicular (hair-like) crystals.[48] Isolation of selenium is often complicated by the presence of other compounds and elements.

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.[49][50]

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.[51][52]

Certain soils are selenium-rich, and selenium can be bioconcentrated 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.[49][50] Ocean water contains significant amounts of selenium.[53][54]

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.[55]

Anthropogenic sources of selenium include coal burning, and the mining and smelting of sulfide ores.[56]

Production

Selenium is most commonly produced from selenide in many sulfide ores, such as those of copper, nickel, or lead. Electrolytic metal refining is particularly productive of selenium as a byproduct, obtained from the anode mud of copper refineries. Another source was the mud from the lead chambers of sulfuric acid plants, a process that is no longer used. Selenium can be refined from these muds by a number of methods. However, most elemental selenium comes as a byproduct of refining copper or producing sulfuric acid.[57][58] Since its invention, solvent extraction and electrowinning (SX/EW) production of copper produces an increasing share of the worldwide copper supply.[59] This changes the availability of selenium because only a comparably small part of the selenium in the ore is leached with the copper.[60]

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 acidified to form selenous acid (oxidation step). Selenous acid is bubbled with sulfur dioxide (reduction step) to give elemental selenium.[61][62]

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.[63]

Applications

Manganese electrolysis

During the electrowinning of manganese, the addition of selenium dioxide decreases the power necessary to operate the electrolysis cells. China is the largest consumer of selenium dioxide for this purpose. For every tonne of manganese, an average 2 kg selenium oxide is used.[63][64]

Glass production

The largest commercial use of selenium, accounting for about 50% of consumption, is for the production of glass. Selenium compounds confer a red color to glass. This color cancels out the green or yellow tints that arise from iron impurities typical for most glass. For this purpose, various selenite and selenate salts are added. For other applications, a red color may be desired, produced by mixtures of CdSe and CdS.[65]

Alloys

Selenium is used with bismuth in brasses to replace more toxic lead. The regulation of lead in drinking water applications such as in the US with the Safe Drinking Water Act of 1974, made a reduction of lead in brass necessary. The new brass is marketed under the name EnviroBrass.[66] Like lead and sulfur, selenium improves the machinability of steel at concentrations around 0.15%.[67][68] Selenium produces the same machinability improvement in copper alloys.[69]

Lithium–selenium batteries

The lithium–selenium (Li–Se) battery is one of the most promising systems for energy storage in the family of lithium batteries.[70] The Li–Se battery is an alternative to the lithium–sulfur battery, with an advantage of high electrical conductivity.

Solar cells

Selenium was used as the photoabsoring layer in the first solid-state solar cell, which was demonstrated by the English physicist William Grylls Adams and his student Richard Evans Day in 1876.[71] Only a few years layer, Charles Fritts fabricated the first thin-film solar cell, also using selenium as the photoabsorber. However, with the emergence of silicon solar cells in the 1950's, research on selenium thin-film solar cells declined. As a result, the record efficiency of 5.0% demonstrated by Tokio Nakada and Akio Kunioka in 1985 remained unchanged for more than 30 years.[72] In 2017, researchers from IBM achieved a new record efficiency of 6.5% by redesigning the device structure.[73] Following this achievement, selenium has gained renewed interest as a wide bandgap photoabsorber with the potential of being integrated in tandem with lower bandgap photoabsorbers.[74] However, a large deficit in the open-circuit voltage is currently the main limiting factor to further improve the efficiency, which calls for defect-engineering of selenium thin-films to enhance the carrier lifetime.[75] To date, the only defect-engineering strategy that has been investigated for selenium thin-film solar cells involves crystallizing selenium using a laser.[76]

Photoconductors

Amorphous selenium (α-Se) thin films have found application as photoconductors in flat-panel X-ray detectors. These detectors use amorphous selenium to capture and convert incident X-ray photons directly into electric charge. Selenium has been chosen for this application among other semiconductors owing to a combination of its favorable technological and physical properties:[77][78]

  1. Amorphous selenium has a low melting point, high vapor pressure, and uniform structure. These three properties allow quick and easy deposition of large-area uniform films with a thickness up to 1 mm at a rate of 1–5 µm/min. Their uniformity and lack of grain boundaries, which are intrinsic to polycrystalline materials, improve the X-ray image quality. Meanwhile the large area is essential for scanning the human body or luggage items.
  2. Selenium is less toxic than many compound semiconductors that contain arsenic or heavy metals such as mercury or lead.
  3. The mobility in applied electric field is sufficiently high both for electrons and holes, so that in a typical 0.2 mm thick device, c. 98% of electrons and holes produced by X-rays are collected at the electrodes without being trapped by various defects. Consequently, device sensitivity is high, and its behavior is easy to describe by simple transport equations.

Rectifiers

Selenium rectifiers were first used in 1933. They have mostly been replaced by silicon-based devices. One notable exception is in power DC surge protection, where the superior energy capabilities of selenium suppressors make them more desirable than metal-oxide varistors.Lua error: Internal error: The interpreter exited with status 1.

Other uses

Small amounts of organoselenium compounds have been used to modify the catalysts used for the vulcanization for the production of rubber.[60]

The demand for selenium by the electronics industry is declining.[63] Its photovoltaic and photoconductive properties are still useful in photocopying,[79][80][81][82] photocells, light meters and solar cells. Its use as a photoconductor in plain-paper copiers once was a leading application, but in the 1980s, the photoconductor application declined (although it was still a large end-use) as more and more copiers switched to organic photoconductors.

Zinc selenide was the first material for blue LEDs, but gallium nitride dominates that market.[83] Cadmium selenide was an important component in quantum dots. Sheets of amorphous selenium convert X-ray images to patterns of charge in xeroradiography and in solid-state, flat-panel X-ray cameras.[84] Ionized selenium (Se+24, where 24 of the outer D, S and P orbitals are stripped away due to high input energiesLua error: Internal error: The interpreter exited with status 1.) is one of the active mediums used in X-ray lasers.[85]

Selenium catalyzes some chemical reactions, but it is not widely used because of issues with toxicity.[86] 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.[87]

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.[88][89][90]

75Se is used as a gamma source in industrial radiography.[91]

Selenium is used in some anti-dandruff shampoos in the form of selenium disulfide such as Selsun and Vichy Dereos[92] brands.

Pollution

Selenium pollution might impact some aquatic systems and may be caused by anthropogenic factors such as farming runoff and industrial processes.[93] People who eat more fish are generally healthier than those who eat less,[94] which suggests no major human health concern from selenium pollution, although selenium has a potential effect on humans.[95]

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, cataracts, 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 bioaccumulated 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 aquatic birds. 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 invertebrates can tolerate selenium concentrations up to 300 μg/L of selenium in their diet.[96]

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 season could have an impact on the harmful effects of selenium on fish.[97]

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.[98] High selenium levels in waterways were found to cause congenital disorders in oviparous species, including wetland birds[99] and fish.[100] Elevated dietary methylmercury levels can amplify the harm of selenium toxicity in oviparous species.[101][102]

Cases

In Belews Lake North Carolina, 19 species of fish were eliminated from the lake due to 150–200 μg Se/L wastewater discharged from 1974 to 1986 from a Duke Energy coal-fired power plant. At the Kesterson National Wildlife Refuge in California, thousands of fish and waterbirds were poisoned by selenium in agricultural irrigation drainage.

Biological role

Main page: Biology:Selenium in biology
Elemental selenium
HazardsLua error: Internal error: The interpreter exited with status 1.
NFPA 704 (fire diamond)
Flammability code 0: Will not burn. E.g. waterHealth code 2: Intense or continued but not chronic exposure could cause temporary incapacitation or possible residual injury. E.g. chloroformReactivity code 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g. liquid nitrogenSpecial hazards (white): no codeNFPA 704 four-colored diamond
0
2
0
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references
Tracking categories (test):
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Although it is toxic in large doses, selenium is an essential micronutrient for animals. In plants, it occurs as a bystander mineral,[103] sometimes in toxic proportions in forage (some plants may accumulate selenium as a defense against being eaten by animals,[104] but other plants, such as locoweed, require selenium, and their growth indicates the presence of selenium in soil).[105]

Selenium is a component of the unusual amino acids selenocysteine and selenomethionine. In humans, selenium is a trace element nutrient that functions as cofactor for reduction of antioxidant enzymes, such as glutathione peroxidases[106] 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 certain reactions that remove reactive oxygen species such as hydrogen peroxide and organic hydroperoxides:

2 GSH + H2O2----GSH-Px → GSSG + 2 H2O

The thyroid gland and every cell that uses thyroid hormone use selenium,[107] which is a cofactor for the three of the four known types of thyroid hormone deiodinases, 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. (Only the deiodinase iodotyrosine deiodinase, which works on the last breakdown products of thyroid hormone, does not use selenium.)[108]

Selenium may inhibit Hashimoto's disease, in which the body's own thyroid cells are attacked as foreign. A reduction of 21% on TPO antibodies is reported with the dietary intake of 0.2 mg of selenium.[109]

Increased dietary selenium reduces the effects of mercury toxicity,[110][111][112] although it is effective only at low to modest doses of mercury.[113] Evidence suggests that the molecular mechanisms of mercury toxicity includes the irreversible inhibition of selenoenzymes that are required to prevent and reverse oxidative damage in brain and endocrine tissues.[114][115] The selenium-containing compound selenoneine is present in the blood of bluefin tuna.[116][117]

Evolution in biology

From about three billion years ago, prokaryotic selenoprotein families drive the evolution of selenocysteine, an amino acid. Selenium is incorporated into several prokaryotic selenoprotein families in bacteria, archaea, and eukaryotes as selenocysteine,[118] where selenoprotein peroxiredoxins protect bacterial and eukaryotic cells against oxidative damage. Selenoprotein families of GSH-Px and the deiodinases of eukaryotic cells seem to have a bacterial phylogenetic origin. The selenocysteine-containing form occurs in species as diverse as green algae, diatoms, sea urchins, fish, and chickens. Selenium enzymes are involved in the small reducing molecules glutathione and thioredoxin. One family of selenium-bearing molecules (the glutathione peroxidases) destroys peroxide and repairs damaged peroxidized cell membranes, using glutathione. Another selenium-bearing enzyme in some plants and in animals (thioredoxin reductase) generates reduced thioredoxin, a dithiol that serves as an electron source for peroxidases and also the important reducing enzyme ribonucleotide reductase that makes DNA precursors from RNA precursors.[119]

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.[118] Marine organisms retained and sometimes expanded their selenoproteomes, whereas the selenoproteomes of some terrestrial organisms were reduced or completely lost. These findings suggest that, with the exception of vertebrates, aquatic life supports selenium use, whereas terrestrial habitats lead to reduced use of this trace element.[120] 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), polyphenols (including flavonoids), tocopherols, 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.Lua error: Internal error: The interpreter exited with status 1.

The deiodinase isoenzymes constitute another family of eukaryotic selenoenzymes. Deiodinases are involved in thyroid-hormone regulation, participating in the protection of thyrocytes from damage by H2O2 produced for thyroid-hormone biosynthesis.[121] About 200 million years ago, new selenoproteins were developed as mammalian GSH-Px enzymes.[122][123][124][125]

Nutritional sources of selenium

Dietary selenium comes from meat, nuts, cereals and mushrooms. Brazil nuts are the richest dietary source (though this is soil-dependent, since the Brazil nut does not require high levels of the element for its own needs).[126][127]

The US Recommended Dietary Allowance (RDA) of selenium for teenagers and adults is 55 µg/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 the requirement of minimum and maximum levels of selenium in infant formula.[128]

The selenium content in the human body is believed to be in the 13–20 mg range.[129]

Indicator plant species

Certain species of 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 locoweeds), prince's plume (Stanleya sp.), woody asters (Xylorhiza sp.), and false goldenweed (Oonopsis sp.)[130]

Detection in biological fluids

Selenium may be measured in blood, plasma, serum, or urine to monitor excessive environmental or occupational exposure, to confirm a diagnosis of poisoning in hospitalized victims, or investigate a suspected case of fatal overdose. Some analytical techniques are capable of distinguishing organic from inorganic forms of the element. Both organic and inorganic forms of selenium are largely converted to monosaccharide conjugates (selenosugars) in the body prior to elimination in the urine. Cancer patients receiving daily oral doses of selenothionine may achieve very high plasma and urine selenium concentrations.[131]

Toxicity

Selenium at nutritional levels or low concentrations is required for cell homeostasis, playing a role as an anti-oxidant through selenoproteins, thus, act chemo-preventive against cancer. In contrast, supra-nutritional levels or higher concentrations act as pro-oxidant in tumour cells, thus can be exploited as chemo-therapeutic against cancer.[132]

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.[133] This 400 µg Tolerable Upper Intake Level is based primarily on a 1986 study of five Chinese patients who exhibited overt signs of selenosis and a follow-up study on the same five people in 1992.[134] The 1992 study found the maximum safe dietary selenium intake to be approximately 800 micrograms per day (15 micrograms per kilogram body weight), but suggested 400 micrograms per day to avoid creating an imbalance of nutrients in the diet and to accord with data from other countries.[135] In China, people who ingested corn grown in extremely selenium-rich stony coal (carbonaceous shale) have suffered from selenium toxicity. This coal was shown to have selenium content as high as 9.1%, the highest concentration in coal ever recorded.[136]

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.[137] Elemental selenium and most metallic selenides have relatively low toxicities because of low bioavailability. By contrast, selenates 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.[138] Hydrogen selenide is an extremely toxic, corrosive gas.[139] 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 ponies 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 blood samples, and 15 to 20 times higher than normal in the liver samples. Selenium was later confirmed to be the toxic factor.[140]

Relationship between survival of juvenile salmon and concentration of selenium in their tissues after 90 days (Chinook salmon[141]) or 45 days (Atlantic salmon[142]) exposure to dietary selenium. The 10% lethality level (LC10=1.84 µg/g) was derived by applying the biphasic model of Brain and Cousens[143] to only the Chinook salmon data. The Chinook salmon data comprise two series of dietary treatments, combined here because the effects on survival are indistinguishable.

In fish and other wildlife, selenium is necessary for life, but toxic in high doses. For salmon, the optimal concentration of selenium is about 1 microgram selenium per gram of whole body weight. Much below that level, young salmon die from deficiency;[142] much above, they die from toxic excess.[141]

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 immediately dangerous to life and health.[144]

Deficiency

Main page: Medicine:Selenium deficiency

Selenium deficiency can occur in patients with severely compromised intestinal function, those undergoing total parenteral nutrition, and[145] in those of advanced age (over 90). Also, people dependent on food grown from selenium-deficient soil are at risk. Although New Zealand soil has low levels of selenium, adverse health effects have not been detected in the residents.[146]

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 an additional stress, such as high exposures to mercury[147] or increased oxidant stress from vitamin E deficiency.[148]

Selenium interacts with other nutrients, such as iodine and vitamin E. The effect of selenium deficiency on health remains uncertain, particularly in relation to Kashin–Beck disease.[149] 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.[150]

In the regions (e.g. various 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.[151] Ruminants are particularly susceptible. In general, absorption of dietary selenium is lower in ruminants than other animals, and is lower from forages than from grain.[152] Ruminants grazing certain forages, e.g., some white clover varieties containing cyanogenic glycosides, may have higher selenium requirements,[152] presumably because cyanide is released from the aglycone by glucosidase activity in the rumen[153] and glutathione peroxidases is deactivated by the cyanide acting on the glutathione moiety.[154] Neonate ruminants at risk of white muscle disease may be administered both selenium and vitamin E by injection; some of the WMD myopathies respond only to selenium, some only to vitamin E, and some to either.[155]

Health effects

The effects of selenium intake on cancer have been studied in several clinical trials and epidemiologic studies in humans. Selenium may have a chemo-preventive role in cancer risk as an anti-oxidant, and it might trigger the immune response. At low levels, it is used in the body to create anti-oxidant selenoproteins, at higher doses than normal it causes cell death.[132]

Selenium (in close interrelation with iodine) plays a role in thyroid health. Selenium is a cofactor for the three thyroid hormone deiodinases, helping activate and then deactivate various thyroid hormones and their metabolites. Isolated selenium deficiency is now being investigated for its role in induction autoimmune reactions in thyroid gland in Hashimoto's disease.[156] However, in a case of combined iodine and selenium deficiency, selenium deficiency was shown to play a thyroid-protecting role.[157]

See also

Notes

  1. ↑ For all practical purposes, 82Se is stable.

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References

  1. ↑ Meija, Juris; Coplen, Tyler B.; Berglund, Michael; Brand, Willi A.; De Bièvre, Paul; Gröning, Manfred; Holden, Norman E.; Irrgeher, Johanna et al. (2016). "Atomic weights of the elements 2013 (IUPAC Technical Report)". Pure and Applied Chemistry 88 (3): 265–91. doi:10.1515/pac-2015-0305. 
  2. ↑ Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8. 
  3. ↑ Magnetic susceptibility of the elements and inorganic compounds, in Lide, D. R., ed (2005). CRC Handbook of Chemistry and Physics (86th ed.). Boca Raton (FL): CRC Press. ISBN 0-8493-0486-5. 
  4. ↑ Weast, Robert (1984). CRC, Handbook of Chemistry and Physics. Boca Raton, Florida: Chemical Rubber Company Publishing. pp. E110. ISBN 0-8493-0464-4. 
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  6. ↑ Fernández-Bautista, Tamara; Gómez-Gómez, Beatriz; Palacín-García, Roberto; Gracia-Lor, Emma; Pérez-Corona, Teresa; Madrid, Yolanda (2022-01-15). "Analysis of Se and Hg biomolecules distribution and Se speciation in poorly studied protein fractions of muscle tissues of highly consumed fishes by SEC-UV-ICP-MS and HPLC-ESI-MS/MS". Talanta 237: 122922. doi:10.1016/j.talanta.2021.122922. ISSN 0039-9140. PMID 34736659. https://www.sciencedirect.com/science/article/pii/S0039914021008444. 
  7. ↑ Ruyle, George. "Poisonous Plants on Arizona Rangelands". The University of Arizona. https://cals.arizona.edu/arec/pubs/rmg/1%20rangelandmanagement/2%20poisonousplants93.pdf. 
  8. ↑ 8.0 8.1 8.2 8.3 8.4 8.5 8.6 8.7 House, James E. (2008). Inorganic chemistry. Academic Press. p. 524. ISBN 978-0-12-356786-4. 
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  10. ↑ "β–Se (Al) Structure: A_mP32_14_8e". https://aflowlib.org/prototype-encyclopedia/A_mP32_14_8e.html. 
  11. ↑ "Se (Ak) Structure: A_mP64_14_16e". https://aflowlib.org/prototype-encyclopedia/A_mP64_14_16e.html. 
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  15. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named NUBASE2016
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  17. ↑ Jörg, Gerhard; Bühnemann, Rolf; Hollas, Simon et al. (2010). "Preparation of radiochemically pure 79Se and highly precise determination of its half-life". Applied Radiation and Isotopes 68 (12): 2339–2351. doi:10.1016/j.apradiso.2010.05.006. PMID 20627600. 
  18. ↑ 18.0 18.1 18.2 Wiberg, Egon; Wiberg, Nils; Holleman, Arnold Frederick (2001). Inorganic chemistry. San Diego: Academic Press. p. 583. ISBN 978-0-12-352651-9. 
  19. ↑ Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. p. 780. ISBN 978-0-08-037941-8. 
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  28. ↑ Woollins, Derek; Kelly, Paul F. (1993). "The Reactivity of Se4N4 in Liquid Ammonia". Polyhedron 12 (10): 1129–1133. doi:10.1016/S0277-5387(00)88201-7. 
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  51. ↑ Wessjohann, Ludger A.; Schneider, Alex; Abbas, Muhammad; Brandt, Wolfgang (2007). "Selenium in chemistry and biochemistry in comparison to sulfur". Biological Chemistry 388 (10): 997–1006. doi:10.1515/BC.2007.138. PMID 17937613. 
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  54. ↑ Haug, Anna; Graham, Robin D.; Christophersen, Olav A.; Lyons, Graham H. (2007). "How to use the world's scarce selenium resources efficiently to increase the selenium concentration in food". Microbial Ecology in Health and Disease 19 (4): 209–228. doi:10.1080/08910600701698986. PMID 18833333. 
  55. ↑ Rieuwerts, John (2015). The Elements of Environmental Pollution. London and New York: Earthscan Routledge. pp. 262. ISBN 978-0-415-85919-6. OCLC 886492996. https://www.worldcat.org/oclc/886492996. 
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  59. ↑ Bartos, P.J. (2002). "SX-EW copper and the technology cycle". Resources Policy 28 (3–4): 85–94. doi:10.1016/S0301-4207(03)00025-4. Bibcode: 2002RePol..28...85B. 
  60. ↑ 60.0 60.1 Naumov, A. V. (2010). "Selenium and tellurium: State of the markets, the crisis, and its consequences". Metallurgist 54 (3–4): 197–200. doi:10.1007/s11015-010-9280-7. 
  61. ↑ Hoffmann, James E. (1989). "Recovering selenium and tellurium from copper refinery slimes". JOM 41 (7): 33–38. doi:10.1007/BF03220269. Bibcode: 1989JOM....41g..33H. 
  62. ↑ Hyvärinen, Olli; Lindroos, Leo; Yllö, Erkki (1989). "Recovering selenium from copper refinery slimes". JOM 41 (7): 42–43. doi:10.1007/BF03220271. Bibcode: 1989JOM....41g..42H. 
  63. ↑ 63.0 63.1 63.2 "Selenium and Tellurium: Statistics and Information". United States Geological Survey. http://minerals.usgs.gov/minerals/pubs/commodity/selenium/. 
  64. ↑ Sun, Yan; Tian, Xike; He, Binbin et al. (2011). "Studies of the reduction mechanism of selenium dioxide and its impact on the microstructure of manganese electrodeposit". Electrochimica Acta 56 (24): 8305–8310. doi:10.1016/j.electacta.2011.06.111. 
  65. ↑ Bernd E. Langner (2005), "Selenium and Selenium Compounds", Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim. doi:10.1002/14356007.a23_525.
  66. ↑ Davis, Joseph R. (2001). Copper and Copper Alloys. ASM Int.. p. 91. ISBN 978-0-87170-726-0. https://books.google.com/books?id=sxkPJzmkhnUC&pg=PA91. 
  67. ↑ Isakov, Edmund (2008-10-31). Cutting Data for Turning of Steel. Industrial Press. p. 67. ISBN 978-0-8311-3314-6. https://books.google.com/books?id=QahG1Ou1cyEC&pg=PA67. 
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  99. ↑ Ohlendorf, H. M. (2003). Ecotoxicology of selenium. Handbook of ecotoxicology. Boca Raton: Lewis Publishers. pp. 466–491. ISBN 978-1-56670-546-2. https://books.google.com/books?id=qN0I3husm50C&pg=PA477. 
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  101. ↑ Penglase, S.; Hamre, K.; Ellingsen, S. (2014). "Selenium and mercury have a synergistic negative effect on fish reproduction". Aquatic Toxicology 149: 16–24. doi:10.1016/j.aquatox.2014.01.020. PMID 24555955. 
  102. ↑ Heinz, G. H.; Hoffman, D. J. (1998). "Methylmercury chloride and selenomethionine interactions on health and reproduction in mallards". Environmental Toxicology and Chemistry 17 (2): 139–145. doi:10.1002/etc.5620170202. 
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  105. ↑ Selenium concentrations in leaf material from Astragalus Oxyphysus (diablo locoweed) and Atriplex Lentiformis (quail bush) in the interior Coast Ranges and the western San Joaquin Valley, California (Report). 1986. Water-Resources Investigations Report 86-4066. https://www.academia.edu/61786801. 
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  108. ↑ "Selenium". Linus Pauling Institute at Oregon State University. http://lpi.oregonstate.edu/infocenter/minerals/selenium/. 
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  114. ↑ Ralston, N. V. C.; Raymond, L. J. (2010). "Dietary selenium's protective effects against methylmercury toxicity". Toxicology 278 (1): 112–123. doi:10.1016/j.tox.2010.06.004. PMID 20561558. 
  115. ↑ Carvalho, C. M. L.; Chew, Hashemy S. I.; Hashemy, J. et al. (2008). "Inhibition of the human thioredoxin system: A molecular mechanism of mercury toxicity". Journal of Biological Chemistry 283 (18): 11913–11923. doi:10.1074/jbc.M710133200. PMID 18321861. 
  116. ↑ 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
  117. ↑ Yamashita, Y.; Yabu, T.; Yamashita, M. (2010). "Discovery of the strong antioxidant selenoneine in tuna and selenium redox metabolism". World Journal of Biological Chemistry 1 (5): 144–150. doi:10.4331/wjbc.v1.i5.144. PMID 21540999. 
  118. ↑ 118.0 118.1 Gladyshev, Vadim N.; Hatfield, Dolph L. (1999). "Selenocysteine-containing proteins in mammals". Journal of Biomedical Science 6 (3): 151–160. doi:10.1007/BF02255899. PMID 10343164. https://digitalcommons.unl.edu/biochemgladyshev/77. 
  119. ↑ Stadtman, T. C. (1996). "Selenocysteine". Annual Review of Biochemistry 65 (1): 83–100. doi:10.1146/annurev.bi.65.070196.000503. PMID 8811175. 
  120. ↑ Lobanov, Alexey V.; Fomenko, Dmitri E.; Zhang, Yan et al. (2007). "Evolutionary dynamics of eukaryotic selenoproteomes: large selenoproteomes may associate with aquatic life and small with terrestrial life". Genome Biology 8 (9): R198. doi:10.1186/gb-2007-8-9-r198. PMID 17880704. 
  121. ↑ Venturi, Sebastiano; Venturi, Mattia (2007). "Evolution of Dietary Antioxidant Defences". European EpiMarker 11 (3): 1–11. https://www.researchgate.net/publication/234162439. 
  122. ↑ Castellano, Sergi; Novoselov, Sergey V.; Kryukov, Gregory V. et al. (2004). "Reconsidering the evolution of eukaryotic selenoproteins: a novel nonmammalian family with scattered phylogenetic distribution". EMBO Reports 5 (1): 71–7. doi:10.1038/sj.embor.7400036. PMID 14710190. 
  123. ↑ Kryukov, Gregory V.; Gladyshev, Vadim N. (2004). "The prokaryotic selenoproteome". EMBO Reports 5 (5): 538–43. doi:10.1038/sj.embor.7400126. PMID 15105824. 
  124. ↑ Wilting, R.; Schorling, S.; Persson, B. C.; Böck, A. (1997). "Selenoprotein synthesis in archaea: identification of an mRNA element of Methanococcus jannaschii probably directing selenocysteine insertion". Journal of Molecular Biology 266 (4): 637–41. doi:10.1006/jmbi.1996.0812. PMID 9102456. 
  125. ↑ Zhang, Yan; Fomenko, Dmitri E.; Gladyshev, Vadim N. (2005). "The microbial selenoproteome of the Sargasso Sea". Genome Biology 6 (4): R37. doi:10.1186/gb-2005-6-4-r37. PMID 15833124. 
  126. ↑ Barclay, Margaret N. I.; MacPherson, Allan; Dixon, James (1995). "Selenium content of a range of UK food". Journal of Food Composition and Analysis 8 (4): 307–318. doi:10.1006/jfca.1995.1025. 
  127. ↑ "Selenium Fact Sheet". The Office of Dietary Supplements, National Institutes of Health. http://ods.od.nih.gov/factsheets/selenium.asp#h2.  Includes a list of selenium-rich foods.
  128. ↑ "FDA Issues Final Rule to Add Selenium to List of Required Nutrients for Infant Formula". Food and Drug Administration. https://www.fda.gov/Food/NewsEvents/ConstituentUpdates/ucm451982.htm. 
  129. ↑ A common reference for this is Schroeder, H. A.; Frost, D. V.; Balassa, J. J. (1970). "Essential trace metals in man: Selenium". Journal of Chronic Diseases 23 (4): 227–243. doi:10.1016/0021-9681(70)90003-2. PMID 4926392. 
  130. ↑ Zane Davis, T. (2008-03-27). "Selenium in Plants". p. 8. http://www.ars.usda.gov/SP2UserFiles/Place/54282000/PPClassPPSlides/3-27-08DavisSelenium.pdf. 
  131. ↑ Baselt, R. (2008). Disposition of Toxic Drugs and Chemicals in Man (8th ed.). Foster City, California: Biomedical Publications. pp. 1416–1420. ISBN 978-0-9626523-5-6. 
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  133. ↑ "Dietary Supplement Fact Sheet: Selenium". National Institutes of Health; Office of Dietary Supplements. http://ods.od.nih.gov/factsheets/selenium.asp#h7. 
  134. ↑ Panel on Dietary Antioxidants and Related Compounds, Subcommittees on Upper Reference Levels of Nutrients and Interpretation and Uses of DRIs, Standing Committee on the Scientific Evaluation of Dietary Reference Intakes, Food and Nutrition Board, Institute of Medicine (August 15, 2000). Dietary Reference Intakes for Vitamin C, Vitamin E, Selenium, and Carotenoids. Institute of Medicine. pp. 314–315. doi:10.17226/9810. ISBN 978-0-309-06949-6. http://www.nap.edu/openbook.php?record_id=9810&page=315. 
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  136. ↑ Yang, Guang-Qi; Xia, Yi-Ming (1995). "Studies on Human Dietary Requirements and Safe Range of Dietary Intakes of Selenium in China and Their Application in the Prevention of Related Endemic Diseases". Biomedical and Environmental Sciences 8 (3): 187–201. PMID 8561918. 
  137. ↑ "Public Health Statement: Health Effects". Agency for Toxic Substances and Disease Registry. http://www.atsdr.cdc.gov/toxprofiles/tp92-c3.pdf. 
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  140. ↑ "Polo pony selenium levels up to 20 times higher than normal". 2009-05-06. http://www.horsetalk.co.nz/news/2009/05/033.shtml. 
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  143. ↑ Brain, P.; Cousens, R. (1989). "An equation to describe dose responses where there is stimulation of growth at low doses". Weed Research 29 (2): 93–96. doi:10.1111/j.1365-3180.1989.tb00845.x. Bibcode: 1989WeedR..29...93B. 
  144. ↑ "NIOSH Pocket Guide to Chemical Hazards – Selenium". United States: National Institute for Occupational Safety & Health. https://www.cdc.gov/niosh/npg/npgd0550.html. 
  145. ↑ Ravaglia, G.; Forti, P.; Maioli, F. et al. (2000). "Effect of micronutrient status on natural killer cell immune function in healthy free-living subjects aged ≥90 y". American Journal of Clinical Nutrition 71 (2): 590–598. doi:10.1093/ajcn/71.2.590. PMID 10648276. 
  146. ↑ MedSafe Editorial Team. "Selenium". New Zealand Medicines and Medical Devices Safety Authority. http://www.medsafe.govt.nz/Profs/PUarticles/Sel.htm. 
  147. ↑ Ralston, N. V. C.; Raymond, L. J. (2010). "Dietary selenium's protective effects against methylmercury toxicity". Toxicology 278 (1): 112–123. doi:10.1016/j.tox.2010.06.004. PMID 20561558. 
  148. ↑ Mann, Jim; Truswell, A. Stewart (2002). Essentials of Human Nutrition (2nd ed.). Oxford University Press. ISBN 978-0-19-262756-8. 
  149. ↑ Moreno-Reyes, R.; Mathieu, F.; Boelaert, M. et al. (2003). "Selenium and iodine supplementation of rural Tibetan children affected by Kashin-Beck osteoarthropathy". American Journal of Clinical Nutrition 78 (1): 137–144. doi:10.1093/ajcn/78.1.137. PMID 12816783. 
  150. ↑ Kachuee, R.; Moeini, M.; Suori, M. (2013). "The effect of dietary organic and inorganic selenium supplementation on serum Se, Cu, Fe and Zn status during the late pregnancy in Merghoz goats and their kids". Small Ruminant Research 110 (1): 20–27. doi:10.1016/j.smallrumres.2012.08.010. 
  151. ↑ National Research Council, Subcommittee on Sheep Nutrition (1985). Nutrient requirements of sheep. 6th ed., National Academy Press, Washington, ISBN:0309035961.
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  154. ↑ Kraus, R. J.; Prohaska, J. R.; Ganther, H. E. (1980). "Oxidized forms of ovine erythrocyte glutathione peroxidase. Cyanide inhibition of 4-glutathione:4-selenoenzyme". Biochimica et Biophysica Acta (BBA) - Enzymology 615 (1): 19–26. doi:10.1016/0005-2744(80)90004-2. PMID 7426660. 
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