Selenium
Selenium | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Pronunciation | /sɪˈliːniəm/ | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Appearance | grey metallic-looking, red, and vitreous black (not pictured) allotropes | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Standard atomic weight Ar°(Se) | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Selenium in the periodic table | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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kJ/mol | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Heat of vaporization | 95.48 kJ/mol | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Molar heat capacity | 25.363 J/(mol·K) | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Vapor pressure
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Atomic properties | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Johann Gottlieb Gahn (1817) | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Isotopes of selenium | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Selenium is a
Selenium is found in
Although
Characteristics
Physical properties
Selenium forms several
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,
The most stable and dense form of selenium is gray and has a
Isotopes
Selenium has seven naturally occurring
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[19][20] |
80Se | Primordial | Fission product | Stable |
82Se | Primordial | Fission product* | 8.76×1019 yr[8][a] |
Chemical compounds
Selenium compounds commonly exist in the oxidation states −2, +2, +4, and +6.
Chalcogen compounds
Selenium forms two oxides: selenium dioxide (SeO2) and selenium trioxide (SeO3). Selenium dioxide is formed by combustion of elemental selenium:[12]
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:[21]
Unlike sulfur, which forms a stable trioxide, selenium trioxide is thermodynamically unstable and decomposes to the dioxide above 185 °C:[12][21]
Selenium trioxide is produced in the laboratory by the reaction of anhydrous potassium selenate (K2SeO4) and sulfur trioxide (SO3).[22]
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.[21]
Selenium trioxide may be synthesized by dehydrating selenic acid, H2SeO4, which is itself produced by the oxidation of selenium dioxide with hydrogen peroxide:[23]
Hot, concentrated selenic acid reacts with gold to form gold(III) selenate.[24]
Halogen compounds
Selenium reacts with fluorine to form selenium hexafluoride:
In comparison with its sulfur counterpart (
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.[26]
The iodides of selenium are not well known, and for a long time were believed not to exist.[27] There is limited spectroscopic evidence that the lower iodides may form in bi-elemental solutions with nonpolar solvents, such as carbon disulfide[28] and carbon tetrachloride;[27] but even these appear to decompose under illumination.[29]
Some selenium oxyhalides—
Metal selenides
Analogous to the behavior of other chalcogens, selenium forms
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).[12][31][32] It can be synthesized by the reaction of selenium tetrachloride (SeCl4) with [((CH
3)
3Si)
2N]
2Se.[33]
Selenium reacts with cyanides to yield selenocyanates:[12]
Organoselenium compounds
Selenium, especially in the II oxidation state, forms a variety of organic derivatives. They are structurally analogous to the corresponding
History
Selenium (
In 1873,
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.[46][47] It was discovered to be essential for mammalian life in 1957.[48][49] 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).[50]
Occurrence
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.[51] 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.[52][53]
In living systems, selenium is found in the amino acids
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.[52][53] Ocean water contains significant amounts of selenium.[56][57]
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.[58]
Anthropogenic sources of selenium include coal burning, and the mining and smelting of sulfide ores.[59]
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.[60][61] Since its invention, solvent extraction and electrowinning (SX/EW) production of copper produces an increasing share of the worldwide copper supply.[62] This changes the availability of selenium because only a comparably small part of the selenium in the ore is leached with the copper.[63]
Industrial production of selenium usually involves the extraction of
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.[66]
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.[66][67]
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.[68]
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.[69] Like lead and sulfur, selenium improves the machinability of steel at concentrations around 0.15%.[70][71] Selenium produces the same machinability improvement in copper alloys.[72]
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.[73] 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.[74] 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 1950s, 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.[75] In 2017, researchers from IBM achieved a new record efficiency of 6.5% by redesigning the device structure.[76] 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.[77] In 2024, the first selenium-based tandem solar cell was demonstrated, showcasing a selenium top cell monolithically integrated with a silicon bottom cell.[78] However, a significant deficit in the open-circuit voltage is currently the main limiting factor to further improve the efficiency, necessitating defect-engineering strategies for selenium thin-films to enhance the carrier lifetime.[79] As of now, the only defect-engineering strategy that has been investigated for selenium thin-film solar cells involves crystallizing selenium using a laser.[80]
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:[81][82]
- 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.
- Selenium is less toxic than many compound semiconductors that contain arsenic or heavy metals such as mercury or lead.
- 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
Other uses
Small amounts of organoselenium compounds have been used to modify the catalysts used for the vulcanization for the production of rubber.[63]
The demand for selenium by the electronics industry is declining.
Selenium catalyzes some chemical reactions, but it is not widely used because of issues with toxicity.
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.[92][93][94]
75Se is used as a gamma source in industrial radiography.[95]
Selenium is used in some anti-dandruff shampoos in the form of selenium disulfide such as Selsun and Vichy Dereos[96] brands.
Pollution
Selenium pollution might impact some aquatic systems and may be caused by anthropogenic factors such as farming runoff and industrial processes.[97] People who eat more fish are generally healthier than those who eat less,[98] which suggests no major human health concern from selenium pollution, although selenium has a potential effect on humans.[99]
Substantial physiological changes may occur in fish with high tissue concentrations of selenium. Fish affected by selenium may experience swelling of the
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.[100]
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.[101]
Selenium poisoning of water systems may result whenever new
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
Hazards | |
---|---|
NFPA 704 (fire diamond) |
Although it is toxic in large doses, selenium is an essential micronutrient for animals. In plants, it occurs as a bystander mineral,[107] sometimes in toxic proportions in forage (some plants may accumulate selenium as a defense against being eaten by animals,[108] but other plants, such as locoweed, require selenium, and their growth indicates the presence of selenium in soil).[109]
Selenium is a component of the unusual
The glutathione peroxidase family of enzymes (GSH-Px) catalyze certain reactions that remove reactive oxygen species such as hydrogen peroxide and organic hydroperoxides:
The
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.[113]
Increased dietary selenium reduces the effects of mercury toxicity,[114][115][116] although it is effective only at low to modest doses of mercury.[117] 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.[118][119] The selenium-containing compound selenoneine is present in the blood of bluefin tuna.[120][121]
Evolution in biology
From about three billion years ago,
Trace elements involved in GSH-Px and superoxide dismutase enzymes activities, i.e. selenium,
The deiodinase
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).[130][131]
The US
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.[132]
The selenium content in the human body is believed to be in the 13–20 mg range.[133]
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.)[134]
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.[135]
Toxicity
Although selenium is an essential
Signs and symptoms of selenosis include a garlic odor on the breath, gastrointestinal disorders, hair loss,
On 19 April 2009, 21
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;[146] much above, they die from toxic excess.[145]
The
Deficiency
Selenium deficiency can occur in patients with severely compromised
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[151] or increased oxidant stress from vitamin E deficiency.[152]
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.[153] 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.[154]
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.
Health effects
The effects of selenium intake on cancer have been studied in several
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 the induction of autoimmune reactions in the thyroid gland in Hashimoto's disease.[160] In a case of combined iodine and selenium deficiency selenium deficiency was shown to play a thyroid-protecting role.[161]
See also
- Abundance of elements in Earth's crust
- ACES (nutritional supplement)
- Selenium yeast
Notes
- ^ For all practical purposes, 82Se is stable.
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- ISBN 0911910506.
- PMID 33142736.
- PMID 8427203.
External links
- Selenium at The Periodic Table of Videos(University of Nottingham)
- National Institutes of Health page on Selenium
- Assay Archived 2012-02-26 at the Wayback Machine
- ATSDR – Toxicological Profile: Selenium
- CDC – NIOSH Pocket Guide to Chemical Hazards
- Peter van der Krogt elements site