Selenium (pronounced /sɨˈlɛniəm/ si-LEN-ee-əm
or /sɨˈliːniəm/ si-LEE-nee-əm)
is a chemical element with the atomic
number 34, represented by the chemical
symbol Se, an atomic
mass of 78.96. It is a nonmetal,
chemically related to sulfur and tellurium, and rarely occurs in its
elemental state in nature.
Detailed description
Isolated
selenium occurs in several different forms, the most stable of which is a dense
purplish-gray semi-metal (semiconductor)
form that is structurally a trigonal polymer chain. It conducts electricity
better in the light than in the dark, and is used in photocells
(see section Allotropes
below). Selenium also exists in many non-conductive forms: a black glass-like
allotrope, as well as several red crystalline forms built of eight-membered ring
molecules, like its lighter cousin sulfur.
Selenium is found in economic quantities in sulfide ores such as pyrite, partially replacing the sulfur in the ore matrix. Minerals that are selenide or selenate compounds are also known, but are rare. The chief commercial uses for selenium today are in glassmaking and in chemicals and pigments. Uses in electronics, once important, have been supplanted by silicon semiconductor devices.
History and global demand
Selenium
(Greek σελήνη selene meaning "Moon") was
discovered in 1817 by Jöns Jakob Berzelius,[4]
who found the element associated with tellurium
(named for the Earth). It was discovered as a byproduct of sulfuric
acid production.
It came to medical notice later because of its toxicity to humans working in industry. It was also recognized as an important veterinary toxin. In 1954, the first hints of specific biological functions of selenium were discovered in microorganisms. Its essentiality for mammalian life was discovered 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, it was shown that selenocysteine is encoded by the codon TGA. The recoding mechanism was worked out first in bacteria and then in mammals (see SECIS element).
Occurrence
Selenium
occurs naturally in a number of inorganic forms, including selenide,
selenate,
and selenite. In soils, selenium most often occurs in soluble forms such as
selenate (analogous to sulfate), which are leached into rivers very easily by
runoff.
Selenium has a biological role, and it is found in organic compounds such as dimethyl selenide, selenomethionine, selenocysteine, and methylselenocysteine. In these compounds selenium plays a role analogous to that of sulfur.
Selenium is most commonly produced from selenide in many sulfide ores, such as those of copper, silver, or lead. It is obtained as a byproduct of the processing of these ores, from the anode mud of copper refineries and the mud from the lead chambers of sulfuric acid plants. These muds can be processed by a number of means to obtain free selenium.
Production and allotropic forms
Native
selenium is a rare mineral, which does not usually form good crystals, but,
when it does, they are steep rhombohedrons or tiny acicular (hair-like)
crystals.[7]
Isolation of selenium is often complicated by the presence of other compounds
and elements.
Most elemental selenium comes as a byproduct of refining copper or producing sulfuric acid.[8][9]
Industrial production of selenium often involves the extraction of selenium dioxide from residues obtained during the purification of copper. Commonly, production begins by oxidation with sodium carbonate to produce selenium dioxide. The selenium dioxide is then mixed with water and the solution is acidified to form selenous acid (oxidation step). Selenous acid is bubbled with sulfur dioxide (reduction step) to give elemental selenium.
Indicator plants
Certain
species of plants are considered indicators of high selenium content of the
soil, since they require high levels of selenium in order 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.)[19]
Deficiency
Selenium
deficiency is relatively rare in healthy, well-nourished individuals. It can
occur in patients with severely compromised intestinal
function, those undergoing total parenteral nutrition,
and also[31]
on advanced-aged people (over 90). Also, people dependent on food grown from
selenium-deficient soil are also at risk. However, although New
Zealand has low levels of selenium in its soil, adverse health effects
have not been detected.[32]
Selenium deficiency may only occur when a low selenium status is linked with an additional stress such as chemical exposure or increased oxidant stress due to vitamin E deficiency.[33]
Controversial health effects
Non-biologic applications
Selenium is used with bismuth in brasses to replace more toxic lead. It is also used to improve abrasion resistance in vulcanized rubbers.
Sheets of amorphous selenium convert x-ray images to patterns of charge in xeroradiography and in solid-state, flat-panel x-ray cameras.
Early photographic light meters used selenium but this application is now obsolete.
Biologic applications
Detection in biological fluids
Selenium
may be measured in blood, plasma, serum or urine to monitor excessive
environmental or occupational exposure, confirm a diagnosis of poisoning in
hospitalized victims or to assist in a forensic investigation in a case of
fatal overdosage. 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 being eliminated in the urine. Cancer patients receiving
daily oral doses of selenothionine may achieve very high plasma and urine
selenium concentrations.[61]
Evolution in biology
Over three billion years ago, blue-green algae were the most primitive oxygenic photosynthetic organisms and are ancestors of multicellular eukaryotic algae.[62] Algae that contain the highest amount of antioxidant selenium, iodide, and peroxidase enzymes were the first living cells to produce poisonous oxygen in the atmosphere. It has been suggested that algal cells required a protective antioxidant action, in which selenium and iodides, through peroxidase enzymes, have had this specific role.[62][63] Selenium, which acts synergistically with iodine,[64] is a primitive mineral antioxidant, greatly present in the sea and prokaryotic cells, where it is an essential component of the family of glutathione peroxidase (GSH-Px) antioxidant enzymes; seaweeds accumulate high quantity of selenium and iodine.[62] In 2008, a study showed that iodide also scavenges reactive oxygen species (ROS) in algae, and that its biological role is that of an inorganic antioxidant, the first to be described in a living system, active also in an in vitro assay with the blood cells of today’s humans.
Detailed description
Selenium is found in economic quantities in sulfide ores such as pyrite, partially replacing the sulfur in the ore matrix. Minerals that are selenide or selenate compounds are also known, but are rare. The chief commercial uses for selenium today are in glassmaking and in chemicals and pigments. Uses in electronics, once important, have been supplanted by silicon semiconductor devices.
History and global demand
Selenium
(Greek σελήνη selene meaning "Moon") was
discovered in 1817 by Jöns Jakob Berzelius,[4]
who found the element associated with tellurium
(named for the Earth). It was discovered as a byproduct of sulfuric
acid production.It came to medical notice later because of its toxicity to humans working in industry. It was also recognized as an important veterinary toxin. In 1954, the first hints of specific biological functions of selenium were discovered in microorganisms. Its essentiality for mammalian life was discovered 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, it was shown that selenocysteine is encoded by the codon TGA. The recoding mechanism was worked out first in bacteria and then in mammals (see SECIS element).
Occurrence
Selenium
occurs naturally in a number of inorganic forms, including selenide,
selenate,
and selenite. In soils, selenium most often occurs in soluble forms such as
selenate (analogous to sulfate), which are leached into rivers very easily by
runoff.Selenium has a biological role, and it is found in organic compounds such as dimethyl selenide, selenomethionine, selenocysteine, and methylselenocysteine. In these compounds selenium plays a role analogous to that of sulfur.
Selenium is most commonly produced from selenide in many sulfide ores, such as those of copper, silver, or lead. It is obtained as a byproduct of the processing of these ores, from the anode mud of copper refineries and the mud from the lead chambers of sulfuric acid plants. These muds can be processed by a number of means to obtain free selenium.
Production and allotropic forms
Native
selenium is a rare mineral, which does not usually form good crystals, but,
when it does, they are steep rhombohedrons or tiny acicular (hair-like)
crystals.[7]
Isolation of selenium is often complicated by the presence of other compounds
and elements.Most elemental selenium comes as a byproduct of refining copper or producing sulfuric acid.[8][9]
Industrial production of selenium often involves the extraction of selenium dioxide from residues obtained during the purification of copper. Commonly, production begins by oxidation with sodium carbonate to produce selenium dioxide. The selenium dioxide is then mixed with water and the solution is acidified to form selenous acid (oxidation step). Selenous acid is bubbled with sulfur dioxide (reduction step) to give elemental selenium.
Indicator plants
Certain
species of plants are considered indicators of high selenium content of the
soil, since they require high levels of selenium in order 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.)[19]
Deficiency
Selenium
deficiency is relatively rare in healthy, well-nourished individuals. It can
occur in patients with severely compromised intestinal
function, those undergoing total parenteral nutrition,
and also[31]
on advanced-aged people (over 90). Also, people dependent on food grown from
selenium-deficient soil are also at risk. However, although New
Zealand has low levels of selenium in its soil, adverse health effects
have not been detected.[32]Selenium deficiency may only occur when a low selenium status is linked with an additional stress such as chemical exposure or increased oxidant stress due to vitamin E deficiency.[33]
Controversial health effects
Cancer
Several
studies have suggested a possible link between cancer and selenium deficiency.[35][36][37][38]
One study, known as the NPC, was conducted to test the effect of selenium
supplementation on the recurrence of skin cancers on selenium-deficient men. It
did not demonstrate a reduced rate of recurrence of skin cancers, but did show
a reduced occurrence of total cancers, although without a statistically
significant change in overall mortality.[39]
The preventative effect observed in the NPC was greatest in those with the
lowest baseline selenium levels.
HIV/AIDS
Some
research has indicated a geographical link between regions of
selenium-deficient soils and peak incidences of HIV/AIDS
infection. For example, much of sub-Saharan Africa is low in selenium. However, Senegal
is not, and also has a significantly lower level of AIDS infection than the
rest of the continent. AIDS appears to involve a slow and progressive decline
in levels of selenium in the body. Whether this decline in selenium levels is a
direct result of the replication of HIV[50]
or related more generally to the overall malabsorption of nutrients by AIDS
patients remains debated.
Tuberculosis
Some
research has suggested that selenium supplementation, along with other
nutrients, can help prevent the recurrence of tuberculosis.[56]
Diabetes
A
well-controlled study showed that selenium intake is positively correlated with
the risk of developing type 2 diabetes.
Because high serum selenium levels are positively associated with the
prevalence of diabetes, and because selenium deficiency is rare,
supplementation is not recommended in well-nourished populations such as the
U.S.[57]
Mercury
Experimental
findings have demonstrated a protective effect of selenium on methylmercury
toxicity, but epidemiological studies have been inconclusive in linking
selenium to protection against the adverse effects of methylmercury.[58]
Non-biologic applications
Chemistry
Selenium
is a catalyst in many chemical reactions and is widely used in various
industrial and laboratory syntheses, especially organoselenium chemistry.
It is also widely used in structure determination of proteins and nucleic acids
by X-ray crystallography (incorporation of one or more Se atoms helps with MAD
and SAD phasing.)
Manufacturing and materials use
The
largest use of selenium worldwide is in glass
and ceramic
manufacturing, where it is used to give a red color to glasses, enamels
and glazes as well as to remove color from glass by counteracting the green
tint imparted by ferrous impurities.Selenium is used with bismuth in brasses to replace more toxic lead. It is also used to improve abrasion resistance in vulcanized rubbers.
Electronics
Because
of its photovoltaic and photoconductive
properties, selenium is used in photocopying,
photocells,
light
meters and solar cells. It was once widely used in rectifiers.
These uses have mostly been replaced by silicon-based devices, or are in the
process of being replaced. The most notable exception is in power DC surge
protection, where the superior energy capabilities of selenium suppressors
make them more desirable than metal oxide varistors.Sheets of amorphous selenium convert x-ray images to patterns of charge in xeroradiography and in solid-state, flat-panel x-ray cameras.
Photography
Selenium
is used in the toning of photographic prints,
and it is sold as a toner by numerous photographic manufacturers including Kodak
and Fotospeed.
Its use intensifies and extends the tonal range of black and white photographic
images as well as improving the permanence of prints.Early photographic light meters used selenium but this application is now obsolete.
Biologic applications
Medical use
The
substance loosely called selenium
sulfide (approximate formula SeS2) is the active ingredient in
some anti-dandruff shampoos.[59]
The selenium compound kills the scalp fungus Malassezia,
which causes shedding of dry skin fragments. The ingredient is also used in
body lotions to treat Tinea
versicolor due to infection by a different species of Malassezia
fungus.[60]
Nutrition
Selenium
is used widely in vitamin preparations and other dietary supplements, in small doses (typically 50 to
200 micrograms per day for adult humans). Some livestock feeds
are fortified with selenium as well.
Detection in biological fluids
Selenium
may be measured in blood, plasma, serum or urine to monitor excessive
environmental or occupational exposure, confirm a diagnosis of poisoning in
hospitalized victims or to assist in a forensic investigation in a case of
fatal overdosage. 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 being eliminated in the urine. Cancer patients receiving
daily oral doses of selenothionine may achieve very high plasma and urine
selenium concentrations.[61]
Evolution in biology
Over three billion years ago, blue-green algae were the most primitive oxygenic photosynthetic organisms and are ancestors of multicellular eukaryotic algae.[62] Algae that contain the highest amount of antioxidant selenium, iodide, and peroxidase enzymes were the first living cells to produce poisonous oxygen in the atmosphere. It has been suggested that algal cells required a protective antioxidant action, in which selenium and iodides, through peroxidase enzymes, have had this specific role.[62][63] Selenium, which acts synergistically with iodine,[64] is a primitive mineral antioxidant, greatly present in the sea and prokaryotic cells, where it is an essential component of the family of glutathione peroxidase (GSH-Px) antioxidant enzymes; seaweeds accumulate high quantity of selenium and iodine.[62] In 2008, a study showed that iodide also scavenges reactive oxygen species (ROS) in algae, and that its biological role is that of an inorganic antioxidant, the first to be described in a living system, active also in an in vitro assay with the blood cells of today’s humans.
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