Selenium
For other uses, see Selenium (disambiguation)
SE redirects here. For other uses of the abbreviation, see
SE. For the ancient Chinese string instrument, see
Se (instrument)''.
Selenium is a
chemical element in the
periodic table that has the symbol
Se and
atomic number 34. 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 center of certain enzymes, are necessary for the function of all cells in (probably) all living organisms.
Isolated selenium occurs in several different forms but one of these is a stable gray
metallike form that conducts
electricity better in the light than in the dark and is used in
photocells. This element is found in
sulfide ores such as
pyrite.
Selenium occurs in a number of inorganic forms, including elemental selenium,
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 also occurs in organic compounds such as dimethyl selenide,
selenomethionine and
selenocysteine.
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[
1].
See also Selenide minerals.Selenium has at least 29
isotopes, of which 5 are stable, and 6 are
nuclear isomers.
Selenium (
Greek σελήνη
selene meaning "Moon") was discovered in
1817 by
Jöns Jakob Berzelius who found the element associated with
tellurium.
Growth in selenium consumption was driven by the 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 By 1970, selenium in rectifiers had largely been replaced by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. 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.
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 [
2]. 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[
3]. 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[
4].
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[
5]. 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 as a result of new agricultural runoff through normally dry lands, with associated sudden leaching of natural soluble selenium which was then concentrated in new "wetlands," has been found to have caused birth defects in wetland birds. [
6]
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, and contributes (along with iodine deficiency) to Kashin-Beck disease[
7]. 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 [
8]. 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[
9].
Controversial Health Effects
Cancer
:Several studies have suggested a link between cancer and selenium deficiency[
10][
11][
12][
13][
14][
15][
16]. A study conducted on the effect of selenium suplementation 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[
17]. 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[
18]. The SU.VI.MAX study [
19] 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 [
20]. The
SELECT study is currently investigating the effect of selenium and
vitamin E supplementation on incidence of
prostate cancer.
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[
21] or related more generally to the overall malabsorption of nutrients by AIDS patients remains debated.
This thesis is further weakened by the fact that AIDS is a phenomenon that is only 30 years old, whereas the communities in sub-Saharan Africa have been in existence for millions of years. If selenium levels were that important a factor in the AIDS epidemic, one would logically argue that AIDS would have long been a presence there. Furthermore, AIDS has been particularly prominent in a few sub-Saharan countries with a vast copper mining infrastructure, notably the Democratic Republic of Congo and Zambia which further invalidates the thesis that low selenium levels have anything to do with the spread of AIDS. Lastly, there has been no quantifiable definition, or threshold of what "low selenium levels" represents, which makes the idea of making a comparison impossible to verify.
Low selenium levels in
AIDS patients have been directly correlated with decreased immune cell count and increased disease progression and risk of death[
22]. 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[
23].
Regardless of the cause of depleted selenium levels in AIDS patients, studies have shown that selenium deficiency does strongly corelate with the progression of the disease and the risk of death[
24][
25][
26]. 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.
Selenium is a common byproduct of copper refining, or the production of sulfuric acid[
27][
28][
29]. 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 selenium.
*Used in
photocells,
light meters and
solar cells because of its photovoltaic and photoconductive properties.
*Used in
rectifiers (this is a classic use which has since been largely replaced by silicon diodes).
*Used to remove color from
glass, as it will counteract the green color that ferrous impurities impart.
*Used to give a red color to glasses and
enamels.
*Used to improve the abrasion resistance in
vulcanized rubbers.
*Used in
photocopying.
*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.
See selenium's entries at fictional applications of real materials.
*
Mercury selenide (HgSe)
*
Hydrogen selenide (H
2Se)
*
Selenium dioxide (SeO
2)
*
Selenic acid (H
2SeO
4)
*
Selenous acid (H
2SeO
3)
*
Selenium sulfides: Se
4S
4, SeS
2, Se
2S
6*
Sodium selenite (Na
2SeO
3)
*
Zinc selenide (ZnSe)
See also Selenium compounds and organoselenium chemistry.*
Los Alamos National Labs Chemistry Division - Selenium*
WebElements.com - Selenium*
National Institutes of Health page on Selenium*
ATSDR - Toxicological Profile: Selenium*
Peter van der Krogt elements site
