Selenium

2007 Schools Wikipedia Selection. Related subjects: Chemical elements

34 arsenic ← selenium → bromine S ↑ Se ↓ Te Periodic Table - Extended Periodic Table
General
Name, Symbol, Number	selenium, Se, 34
Chemical series	nonmetals
Group, Period, Block	16, 4, p
Appearance	gray-black, metallic luster
Atomic mass	78.96 (3) g/mol
Electron configuration	[Ar] 3d10 4s2 4p4
Electrons per shell	2, 8, 18, 6
Physical properties
Phase	solid
Density (near r.t.)	(gray) 4.81 g·cm−3
Density (near r.t.)	(alpha) 4.39 g·cm−3
Density (near r.t.)	(vitreous) 4.28 g·cm−3
Liquid density at m.p.	3.99 g·cm−3
Melting point	494  K (221 ° C, 430 ° F)
Boiling point	958 K (685 ° C, 1265 ° F)
Critical point	1766 K, 27.2 MPa
Heat of fusion	(gray) 6.69 kJ·mol−1
Heat of vaporization	95.48 kJ·mol−1
Heat capacity	(25 °C) 25.363 J·mol−1·K−1
Vapor pressure P/Pa 1 10 100 1 k 10 k 100 k at T/K 500 552 617 704 813 958
Atomic properties
Crystal structure	hexagonal
Oxidation states	±2, 4, 6 (strongly acidic oxide)
Electronegativity	2.55 (Pauling scale)
Ionization energies ( more)	1st: 941.0 kJ·mol−1
	2nd: 2045 kJ·mol−1
	3rd: 2973.7 kJ·mol−1
Atomic radius	115 pm
Atomic radius (calc.)	103 pm
Covalent radius	116 pm
Van der Waals radius	190 pm
Miscellaneous
Magnetic ordering	no data
Thermal conductivity	(300 K) (amorphous) 0.519 W·m−1·K−1
Thermal expansion	(25 °C) (amorphous) 37 µm·m−1·K−1
Speed of sound (thin rod)	(20 °C) 3350 m/s
Young's modulus	10 GPa
Shear modulus	3.7 GPa
Bulk modulus	8.3 GPa
Poisson ratio	0.33
Mohs hardness	2.0
Brinell hardness	736 MPa
CAS registry number	7782-49-2
Selected isotopes
Main article: Isotopes of selenium iso NA half-life DM DE ( MeV) DP 72Se syn 8.4 d ε - 72As γ 0.046 - 74Se 0.87% Se is stable with 40 neutrons 75Se syn 119.779 d ε - 75As γ 0.264, 0.136, 0.279 - 76Se 9.36% Se is stable with 42 neutrons 77Se 7.63% Se is stable with 43 neutrons 78Se 23.78% Se is stable with 44 neutrons 79Se syn 1.13×106 y β- 0.151 79Br 80Se 49.61% Se is stable with 46 neutrons 82Se 8.73% 1.08×1020 y β-β- 2.995 82Kr
References

Selenium ( IPA: /səˈliːniəm/) is a chemical element with atomic number 34, with the chemical symbol Se. Selenium is not found in the free state in nature. It is a nonmetal that is chemically related to sulfur and tellurium. It is toxic in large amounts, but trace amounts of it, forming the active centre of certain enzymes, are necessary for the function of all cells in (probably) all living organisms.

Isolated selenium occurs in several different forms, but the most stable of these is a dense gray semimetal (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 allotropic section below). Selenium also exists in many nonconductive forms: a black glass-like substance, as well as several red crystalline forms built of eight-membered ring molecules, like its lighter cousin sulfur.

Selenium is found in highest quantities in sulfide ores such as pyrite.

Occurrence

Selenium occurs naturally in a number of inorganic forms, 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 in biology also occurs in organic compounds such as dimethyl selenide, selenomethionine and selenocysteine. 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 .

Isotopes

Selenium has at least 28 isotopes, of which 5 are stable, and 6 are nuclear isomers.

History and changing global demand for selenium

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 would not 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.

Selenium and health

Although it is toxic in large doses, selenium is an essential micronutrient in all known forms of life. 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 thioredoxin reductase. It also plays a role in the functioning of the thyroid gland by participating as a cofactor for thyroid hormone deiodinases . Dietary selenium comes from cereals, meat, fish, and eggs. The recommended dietary allowance for adults is 55 micrograms per day. Liver and Brazil nuts are particularly rich sources of selenium. A list of selenium rich foods can be found on The Office of Dietary Supplements Selenium Fact Sheet.

Toxicity

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 . 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 due to 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 and selenocysteine, all of which have high bioavailability and are toxic in large doses.

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 it evaporates. High selenium levels produced in this fashion have been found to have caused certain birth defects 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. The Dietary Reference Intake for adults is 55 micrograms per 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 deficiency has also been associated with goitre, cretinism and recurrent miscarriage in humans .

Controversial Health Effects

Cancer

Several studies have suggested a link between cancer and selenium deficiency . A study conducted on the effect of selenium supplementation on the recurrence of skin cancers did not demonstrate a reduced rate of reccurence of skin cancers, but did show a significantly reduced occurrence of total cancers . 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 long-term study of selenium levels in over 60,000 participants did not show any correlation between selenium levels and risk of 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 did not 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 proved 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 of the study showed that in just 72 hours, the efficacy of treatment using chemotherapeutic drugs, such as Taxol and Adriamycin, with selenium yeast is significantly higher than the treatment using the drugs alone. The finding was shown in various cancer cells (breast, lung, small intestine, colon, liver).

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 . Selenium supplementation may help mitigate the symptoms of AIDS and reduce the risk of mortality. It should be emphasized that the evidence to date does not suggest that selenium can reduce the risk of infection or the rate of spread of AIDS, but rather treat the symptoms of those who are already infected.

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 Se₈ molecules, similar to many allotropes of sulfur.

Non biological applications

Glass and ceramic use (largest use world-wide)

• Used to give a red colour to glasses and enamels
• Used to remove colour from glass, as it will counteract the green colour that ferrous impurities impart.

Chemical use

• Selenium is a catalyst in many chemical reactions and is widely used in various industrial and laboratory syntheses.

Manufacturing and materials use

• Used with bismuth in brasses to replace more-toxic lead.
• Used to improve the abrasion resistance in vulcanized rubbers.

Electronic use

• Used in photocopying
• Used in photocells, light meters and solar cells because of its photovoltaic and photoconductive properties.
• Once widely used in rectifiers

All these uses have been replaced by silicon-based devices, or are in the process of being replaced.

Photographic uses

• Used in the toning of photographs (in printing); sold as a toner by numerous photographic manufacturers including Kodak and Fotospeed.
• Artistic use intensifies and extends the tonal range of black and white photographic images,
• Use increases the permanence of print photographic images.

All photographic uses rapidly declining in the modern era of digital photography and digital photographic printing.

Biological use

• Used widely in vitamins and food supplements, in small doses (typically 50 to 200 micrograms per day for adult humans. Some livestock feeds are fortified as well).

In popular culture

See selenium's entries at fictional applications of real materials.

Compounds

• Mercury selenide (HgSe)
• Hydrogen selenide (H₂Se)
• Selenium dioxide (SeO₂)
• Selenic acid (H₂SeO₄)
• Selenous acid (H₂SeO₃)
• Selenium sulfides: Se₄S₄, SeS₂, Se₂S₆
• Sodium selenite (Na₂SeO₃)
• Zinc selenide (ZnSe)