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= Selenium =
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Introduction
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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 (from ) was discovered in 1817 by , 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 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. 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 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.
Physical properties
=====================
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. 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.
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
hexagonal 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 acids. With strong reducing agents, it forms
polyselenides. Selenium does not exhibit the changes in viscosity that
sulfur undergoes when gradually heated.
Isotopes
==========
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. 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. 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 | yr
|80Se |Primordial |Fission product |Stable
|82Se |Primordial |Fission product* |8.76 yr
Chemical compounds
======================================================================
Selenium compounds commonly exist in the oxidation states −2, +2, +4,
and +6. 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.
Chalcogen compounds
=====================
Selenium forms two oxides: selenium dioxide (SeO2) and selenium
trioxide (SeO3). Selenium dioxide is formed by combustion of elemental
selenium:
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:
Unlike sulfur, which forms a stable 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:
Hot, concentrated selenic acid reacts with gold to form gold(III)
selenate.
Halogen compounds
===================
Selenium reacts with fluorine to form selenium hexafluoride:
In comparison with its sulfur counterpart (sulfur hexafluoride),
selenium hexafluoride (SeF6) is more reactive and is a toxic pulmonary
irritant. Selenium tetrafluoride is a laboratory-scale fluorinating
agent.
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.
The iodides of selenium are not well known, and for a long time were
believed not to exist. There is limited spectroscopic 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 decompose under illumination.
Some selenium oxyhalides--seleninyl fluoride (SeOF2) and selenium
oxychloride (SeOCl2)--have been used as specialty solvents.
Metal selenides
=================
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:
:
Alkali metal selenides react with selenium to form polyselenides, ,
which exist as chains and rings.
Other compounds
=================
Tetraselenium tetranitride, Se4N4, is an explosive orange compound
analogous to tetrasulfur tetranitride (S4N4). It can be synthesized by
the reaction of selenium tetrachloride (SeCl4) with Metal
bis(trimethylsilyl)amides.
Selenium reacts with cyanides to yield selenocyanates:
:
Organoselenium compounds
==========================
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.
History
======================================================================
Selenium (Greek σελήνη 'selene' meaning "Moon") was discovered in 1817
by Jöns Jacob Berzelius and Johan Gottlieb Gahn. 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.
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 light meters 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 microorganisms by biochemist,
Jane Pinsent. It was discovered to be essential for mammalian life in
1957. 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).
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. 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.
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 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. 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.
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. Since its invention, solvent
extraction and electrowinning (SX/EW) production of copper produces an
increasing share of the worldwide copper supply. This changes the
availability of selenium because only a comparably small part of the
selenium in the ore is leached with the copper.
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.
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.
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.
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.
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. Like lead and sulfur, selenium improves the machinability
of steel at concentrations around 0.15%. Selenium produces the same
machinability improvement in copper alloys.
Lithium–selenium batteries
============================
The lithium-selenium (Li-Se) battery was considered for energy storage
in the family of lithium batteries in the 2010s.
Solar cells
=============
Selenium was used as the photoabsorbing 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. Only a few
years later, 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. In 2017, researchers from IBM
achieved a new record efficiency of 6.5% by redesigning the device
structure. 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. In 2024, the
first selenium-based tandem solar cell was demonstrated, showcasing a
selenium top cell monolithically integrated with a silicon bottom
cell. 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. Recent theoretical studies using
first-principles defect calculations have shown that selenium exhibits
intrinsic point defect tolerance, suggesting that interfaces and
extended defects are the primary factors limiting device performance.
As of now, the only defect-engineering strategy that has been
investigated for selenium thin-film solar cells involves crystallizing
selenium using a laser.
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:
# 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
============
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.
Other uses
============
The demand for selenium by the electronics industry is declining. Its
photovoltaic and photoconductive properties are still useful in
photocopying, 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. Cadmium selenide can be used to make
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. 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.
Pollution
======================================================================
Selenium pollution might impact some aquatic systems and may be caused
by anthropogenic factors such as farming runoff and industrial
processes. People who eat more fish are generally healthier than those
who eat less, which suggests no major human health concern from
selenium pollution, although selenium has a potential effect on
humans.
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 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.
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, 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.
Examples
==========
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
======================================================================
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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, sometimes in toxic proportions in forage (some plants may
accumulate selenium as a defense against being eaten by animals, but
other plants, such as locoweed, require selenium, and their growth
indicates the presence of selenium in soil). The selenium content in
the human body is believed to be in the range of 13-20 mg.
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 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 hydroperoxides.
The thyroid gland and every cell that uses thyroid hormone also use
selenium, 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.
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. 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 locoweeds), prince's plume
('Stanleya' sp.), woody asters ('Xylorhiza' sp.), and false goldenweed
('Oonopsis' sp.).
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).
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 a requirement for minimum and maximum levels of selenium
in infant formula.
General 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.
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. In a case of
combined iodine and selenium deficiency was shown to play a
thyroid-protecting role.
See also
======================================================================
* Abundance of elements in Earth's crust
* ACES (nutritional supplement)
* Selenium yeast
External links
======================================================================
* [
http://www.periodicvideos.com/videos/034.htm Selenium] at 'The
Periodic Table of Videos' (University of Nottingham)
* [
https://ods.od.nih.gov/factsheets/selenium-HealthProfessional/
National Institutes of Health page on Selenium]
* [
http://www.sas-centre.org/assays/trace_metals/selenium.html Assay]
*
[
https://wwwn.cdc.gov/TSP/ToxProfiles/ToxProfiles.aspx?id=153&tid=28
ATSDR - Toxicological Profile: Selenium]
* [
https://www.cdc.gov/niosh/npg/npgd0550.html CDC - NIOSH Pocket
Guide to Chemical Hazards]
* [
http://elements.vanderkrogt.net/element.php?sym=Se Peter van der
Krogt elements site]
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=========
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Original Article:
http://en.wikipedia.org/wiki/Selenium
