Everything about Selenium totally explained
Selenium is a
chemical element with the
atomic number 34, represented by the chemical symbol
Se. It is a
nonmetal, chemically related to
sulfur and
tellurium, and rarely occurs in its elemental state in nature. It is toxic in large amounts, but trace amounts of it are necessary for cellular function in most, if not all, animals, forming the active center of the enzymes
glutathione peroxidase and
thioredoxin reductase (which indirectly reduce certain oxidized molecules in animals and some plants) and three known
deiodinase enzymes (which convert one
thyroid hormone to another). Selenium requirements in plants differ by species, with some plants apparently requiring none.
Isolated selenium occurs in several different forms, the most stable of which is a dense purplish-gray semi-metal (semiconductor) form that's structurally a trigonal polymer chain. It conducts
electricity better in the light than in the dark, and is used in
photocells (see allotropic section 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 chemical 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 all are rare.
Occurrence
Selenium occurs naturally in a number of inorganic forms, including
selenide,
selenate and
selenite. In soils, selenium most often occurs in soluble forms like selenate (analogous to sulfate), which are leached into rivers very easily by runoff.
Selenium has a biological role, and is found in organic compounds such as dimethyl selenide,
selenomethionine,
selenocysteine and
methylselenocysteine. In these compounds selenium plays an analogous role to 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.
Natural sources of selenium include certain selenium-rich soils, and selenium that has been
bioconcentrated by certain toxic plants such as
locoweed. Anthropogenic sources of selenium include coal burning and the mining and smelting of sulfide ores.
See also .
Isotopes
Selenium has six naturally occurring
isotopes, five of which are stable:
74Se,
76Se,
77Se,
78Se, and
80Se. The last three also occur as
fission products, along with
79Se which has a
halflife of 295,000 years, and
82Se which has a very long half life (~10
20 yr, decaying via double beta decay to
82Kr) and for practical purposes can be considered to be stable. 23 other unstable isotopes have been characterized.
See also Selenium-79 for more information on recent changes in the halflife of this fission product important for the dose calculations performed in the frame of the geological disposal of long-lived
radioactive waste.
History and global demand
Selenium (
Greek σελήνη
selene meaning "Moon") was discovered in 1817 by
Jöns Jakob Berzelius who found the element associated with
tellurium (named for the Earth).
Growth in selenium consumption was historically driven by steady development of new uses, including applications in rubber compounding,
steel alloying, and selenium rectifiers. By 1970, selenium in rectifiers had largely been replaced by
silicon, but its use as a photoconductor in plain paper copiers had become its leading application. During the 1980s, the photoconductor application declined (although it was still a large end-use) as more and more copiers using organic photoconductors were produced. Presently, the largest use of selenium world-wide is in glass manufacturing, followed by uses in chemicals and pigments. Electronic use, despite a number of continued applications, continues to decline.
In 1996, continuing research showed a positive correlation between selenium supplementation and
cancer prevention in humans, but widespread direct application of this important finding wouldn't add significantly to demand owing to the small doses required. In the late 1990s, the use of selenium (usually with
bismuth) as an additive to
plumbing brasses to meet no-lead environmental standards, became important. At present, total world selenium production continues to increase modestly.
Reference in popular culture
In the plot of the hit movie
Evolution staring
David Duchovny It is discovered that selenium, an ingredient in some dandruff shampoos, is as poisonous to invading alien life-forms as arsenic is to humans.
Selenium and health
Although it's 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). It is a component of the unusual
amino acids
selenocysteine and
selenomethionine. In humans, selenium is a
trace element nutrient which 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).
Glutathione peroxidase (GSH-Px) catalyzes certain reactions which remove reactive oxygen species such as peroxide:
» 2 GSH+ H
2O
2
-GSH-Px → GSSG + 2 H
2O
Selenium also plays a role in the functioning of the
thyroid gland by participating as a
cofactor for the three known
thyroid hormone deiodinases.
Dietary selenium comes from nuts, cereals, meat, fish, and eggs.
Brazil nuts are the richest ordinary dietary source (though this is soil-dependent, since the Brazil nut doesn't require high levels of the element for its own needs). High levels are found in meats such as
kidney,
crab and
lobster, in that order.
Toxicity
Although selenium is an essential
trace element, it's toxic if taken in excess. Exceeding the
Tolerable Upper Intake Level of 400 micrograms per day can lead to
selenosis. Symptoms of selenosis include a garlic odour on the breath, gastrointestinal disorders, hair loss, sloughing of nails, fatigue, irritability and neurological damage. Extreme cases of selenosis can result in
cirrhosis of the liver,
pulmonary edema and death.
Elemental selenium and most metallic
selenides have relatively low toxicities because of their low
bioavailability. By contrast,
selenate and
selenite are very toxic, and have modes of action similar to that of arsenic.
Hydrogen selenide is an extremely toxic, corrosive gas. Selenium also occurs in organic compounds such as dimethyl selenide,
selenomethionine,
selenocysteine and
methylselenocysteine, all of which have high
bioavailability and are toxic in large doses.
Nano-size selenium has equal efficacy, but much lower toxicity.
Selenium poisoning of water systems may result whenever new agricultural runoff courses through normally dry undeveloped lands. This process leaches natural soluble selenium compounds (such as selenates) into the water, which may then be concentrated in new "wetlands" as the water evaporates. High selenium levels produced in this fashion have been found to have caused certain congenital disorders in wetland birds.
Deficiency
Selenium deficiency is relatively rare in healthy well-nourished individuals. It can occur in patients with severely compromised
intestinal function, or those undergoing
total parenteral nutrition. Alternatively, people dependent on food grown from selenium-deficient soil are also at risk. In the USA, the
Dietary Reference Intake for adults is 55 µg/day. In the UK it's 75 µg/day for adult males and 60 µg/day for adult females. 55 µg/day recommendation is based on full expression of plasma
glutathione peroxidase.
Selenoprotein P is a better indicator of selenium nutritional status, and full expression of it would require more than 66 µg/day.
Selenium deficiency can lead to
Keshan disease, which is potentially fatal. Selenium deficiency also contributes (along with iodine deficiency) to Kashin-Beck disease. The primary symptom of Keshan disease is
myocardial necrosis, leading to weakening of the heart. Kashin-Beck disease results in
atrophy, degeneration and
necrosis of
cartilage tissue. Keshan disease also makes the body more susceptible to illness caused by other nutritional, biochemical, or infectious diseases. These diseases are most common in certain parts of China where the soil is extremely deficient in selenium. Studies in
Jiangsu Province of China have indicated a reduction in the prevalence of these diseases by taking selenium supplements.
Selenium is also necessary for the conversion of the thyroid hormone
thyroxine (T4) into its more active counterpart,
triiodothyronine, and as such a deficiency can cause symptoms of
hypothyroidism, including extreme fatigue, mental slowing,
goitre,
cretinism and
recurrent miscarriage. A study conducted on the effect of selenium supplementation on the recurrence of skin cancers didn't demonstrate a reduced rate of recurrence of skin cancers, but did show a significantly reduced occurrence of total cancers. Dietary selenium prevents chemically induced carcinogenesis in many rodent studies. In these studies, organic seleno-compounds are more potent and less toxic than selenium salts (for example, selenocyanates, selenomethionine, selenium-rich Brazil nuts, or selenium-enriched garlic or broccoli). Selenium may help prevent cancer by acting as an
antioxidant or by enhancing immune activity. Not all studies agree on the cancer-fighting effects of selenium. One study of naturally occurring levels of selenium in over 60,000 participants didn't show a significant correlation between those levels and cancer. The SU.VI.MAX study concluded that low-dose supplementation (with 120 mg of ascorbic acid, 30 mg of vitamin E, 6 mg of beta carotene, 100 µg of selenium, and 20 mg of zinc) resulted in a 31% reduction in the incidence of cancer and a 37% reduction in all cause mortality in males, but didn't get a significant result for females. The SELECT study is currently investigating the effect of selenium and
vitamin E supplementation on incidence of
prostate cancer. However, selenium has been proven to help chemotherapy treatment by enhancing the efficacy of the treatment, reducing the toxicity of chemotherapeutic drugs, and preventing the body's resistance to the drugs. One study of cancer cells in vitro showed that chemotherapeutic drugs, such as Taxol and Adriamycin, were more toxic to strains of cancer cells grown in culture when selenium was added.
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 or related more generally to the overall malabsorption of nutrients by AIDS patients remains debated. » Low selenium levels in AIDS patients have been directly correlated with decreased immune cell count and increased disease progression and risk of death. Selenium normally acts as an antioxidant, so low levels of it may increase oxidative stress on the immune system leading to more rapid decline of the immune system. Others have argued that HIV encodes for the human selenoenzyme glutathione peroxidase, which depletes the victim's selenium levels. Depleted selenium levels in turn lead to a decline in CD4 helper T-cells, further weakening the immune system.
Regardless of the cause of depleted selenium levels in AIDS patients, studies have shown that selenium deficiency does strongly correlate with the progression of the disease and the risk of death.
Tuberculosis: Some research has suggested that selenium supplementation, along with other nutrients, can help prevent the recurrence of tuberculosis.
Diabetes: A well-controlled study showed that selenium intake is positively correlated with the risk of developing type II diabetes. Because high serum selenium levels are positively associated with the prevalence of diabetes, and because selenium deficiency is rare, supplementation isn't recommended in well-nourished populations such as the U.S.
Production and allotropic forms
Selenium is a common byproduct of copper refining, or the production of sulfuric acid. Isolation of selenium is often complicated by the presence of other compounds and elements. Commonly, production begins by oxidation with sodium carbonate to produce sodium selenite. The sodium selenite is then acidified with sulfuric acid producing selenous acid. The selenous acid is finally bubbled with sulfur dioxide producing elemental red amorphous selenium.
Selenium produced in chemical reactions invariably appears as the amorphous red form-- an insoluble brick red powder. When this form is rapidly melted, it forms the black, vitreous form which is usually sold industrially as beads. The most thermodynamically stable and dense form of selenium is the electrically conductive gray (trigonal) form, which is composed of long helical chains of selenium atoms. The conductivity of this form is notably light sensitive. Selenium also exists in three different deep red crystalline monoclinic forms, which are composed of Se
8 molecules, similar to many allotropes of sulfur.
Non-biological 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 phasing.)
Manufacturing and materials use: The largest use of selenium world-wide is in glass and ceramic manufacturing, where it's 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's 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.
Nondestructive testing: Selenium is a highly effective Gamma source used in Industrial radiography.
Biological applications
Medical use: The substance loosely called selenium sulfide, SeS2, actually selenium disulfide or selenium (IV) sulfide, is the active ingredient in some dandruff shampoos. The effect of the active ingredient is to kill 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.
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.
Compounds
Selenium also occurs in the III oxidation state, but only in the Se
412+ cation; Se(III) compounds are not otherwise known.
See also and organoselenium chemistry.Further Information
Get more info on 'Selenium'.
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