selenium (Se), a chemical element in the oxygen group (Group 16 [VIa] of the periodic table), closely allied in chemical and physical properties with the elements sulfur and tellurium. Selenium is rare, composing approximately 90 parts per billion of the crust of Earth. It is occasionally found uncombined, accompanying native sulfur, but is more often found in combination with heavy metals (copper, mercury, lead, or silver) in a few minerals. The principal commercial source of selenium is as a by-product of copper refining; its major uses are in the manufacture of electronic equipment, in pigments, and in making glass. Selenium is a metalloid (an element intermediate in properties between the metals and the nonmetals). The gray, metallic form of the element is the most stable under ordinary conditions; this form has the unusual property of greatly increasing in electrical conductivity when exposed to light. Selenium compounds are toxic to animals; plants grown in seleniferous soils may concentrate the element and become poisonous.
In 1817 Swedish chemist Jöns Jacob Berzelius noted a red substance resulting from sulfide ores from mines of Falun, Sweden. When this red material was investigated in the following year, it proved to be an element and was named after the Moon or the Moon goddess Selene. An ore of unusually high selenium content was discovered by Berzelius only days before he made his report to the scientific societies of the world on selenium. His sense of humour is evident in the name he gave the ore, eucairite, meaning “just in time.”
Occurrence and uses.
The proportion of selenium in Earth’s crust is about 10−5 to 10−6 percent. It has been obtained mainly from the anode slimes (deposits and residual materials from the anode) in electrolytic refining of copper and nickel. Other sources are the flue dusts in copper and lead production and the gases formed in roasting pyrites. Selenium accompanies copper in the refining of that metal: about 40 percent of the selenium present in the original ore may concentrate in copper deposited in electrolytic processes. About 1.5 kilograms of selenium can be obtained from a ton of smelted copper.
When incorporated in small amounts into glass, selenium serves as a decolourizer; in larger quantities it imparts to glass a clear red colour that is useful in signal lights. The element is also employed in making red enamels for ceramics and steel ware, as well as for the vulcanization of rubber to increase resistance to abrasion.
The allotropy of selenium is not as extensive as that of sulfur, and the allotropes have not been studied as thoroughly. Only two crystalline varieties of selenium are composed of cyclic Se8 molecules: designated α and β, both exist as red monoclinic crystals. A gray allotrope having metallic properties is formed by keeping any of the other forms at 200–220 °C and is the most stable under ordinary conditions.
An amorphous (noncrystalline), red, powdery form of selenium results when a solution of selenious acid or one of its salts is treated with sulfur dioxide. If the solutions are very dilute, extremely fine particles of this variety yield a transparent red colloidal suspension. Clear red glass results from a similar process that occurs when molten glass containing selenites is treated with carbon. A glassy, almost black variety of selenium is formed by rapid cooling of other modifications from temperatures above 200 °C. Conversion of this vitreous form to the red, crystalline allotropes takes place upon heating it above 90 °C or upon keeping it in contact with organic solvents, such as chloroform, ethanol, or benzene.
Pure selenium is obtained from the slimes and sludges formed in producing sulfuric acid. The impure red selenium is dissolved in sulfuric acid in the presence of an oxidizing agent, such as potassium nitrate or certain manganese compounds. Both selenious acid, H2SeO3, and selenic acid, H2SeO4, are formed and can be leached from residual insoluble material. Other methods utilize oxidation by air (roasting) and heating with sodium carbonate to give soluble sodium selenite, Na2SeO3·5H2O, and sodium selenate, Na2SeO4. Chlorine may also be employed: its action upon metal selenides produces volatile compounds including selenium dichloride, SeCl2; selenium tetrachloride, SeCl4; diselenium dichloride, Se2Cl2; and selenium oxychloride, SeOCl2. In one process, these selenium compounds are converted by water to selenious acid. The selenium is finally recovered by treating the selenious acid with sulfur dioxide.
Selenium is a common component of ores valued for their content of silver or copper; it becomes concentrated in the slimes deposited during electrolytic purification of the metals. Methods have been developed to separate selenium from these slimes, which also contain some silver and copper. Melting the slime forms silver selenide, Ag2Se, and copper(I) selenide, Cu2Se. Treatment of these selenides with hypochlorous acid, HOCl, gives soluble selenites and selenates, which can be reduced with sulfur dioxide. Final purification of selenium is accomplished by repeated distillation.
The most outstanding physical property of crystalline selenium is its photoconductivity: on illumination, the electrical conductivity increases more than 1,000-fold. This phenomenon results from the promotion or excitation of relatively loosely held electrons by light to higher energy states (called the conduction levels), permitting electron migration and, thus, electrical conductivity. In contrast the electrons of typical metals are already in conduction levels or bands, able to flow under the influence of an electromotive force.
The electrical resistivity of selenium varies over a tremendous range, depending upon such variables as the nature of the allotrope, impurities, the method of refining, temperature, and pressure. Most metals are insoluble in selenium, and nonmetallic impurities increase the resistivity.
Illumination of crystalline selenium for 0.001 second increases its conductivity by a factor of 10 to 15 times. Red light is more effective than light of shorter wavelength.
Advantage is taken of these photoelectric and photosensitivity properties of selenium in the construction of a variety of devices that can translate variations in light intensity into electric current and thence to visual, magnetic, or mechanical effects. Alarm devices, mechanical opening and closing devices, safety systems, television, sound films, and xerography depend upon the semiconducting property and photosensitivity of selenium. Rectification of alternating electrical current (conversion into direct current) has for years been accomplished by selenium-controlled devices. Many photocell applications using selenium have been replaced by other devices using materials more sensitive, more readily available, and more easily fabricated than selenium.
Selenium is found in many different foods. Seafood and organ meats are the richest sources, according to the National Institutes of Health (NIH). Other foods that have selenium include muscle meats, grains and dairy products. It can also be found in drinking water in some places.
The amount of selenium found in the soil affects the amount of selenium found in the food that is grown in it, though soil PH and other things factor into the selenium level, as well. Soil in the Central Northern states are particularly rich in selenium, according to the National Geochemical Survey.
Some plants, such as garlic and Brazil nuts, tend to accumulate selenium if they are grown in selenium-rich soil. One ounce of Brazil nuts — about six kernels — can provide as much as 543 micrograms of selenium. People only need trace amounts, 55 micrograms or millionths of a gram, each day. The American Cancer Society recommends that the maximum dose in a supplement not exceed 200 micrograms of selenium per day.
Interesting Selenium Facts.
Here are some interesting facts about selenium:
▪️Brazil nuts are high in selenium, even if they are grown in soil that is not rich in the element. A single nut provides enough selenium to meet the daily requirement for a human adult.
▪️English electrical engineer Willoughby Smith (1828–1891) discovered that selenium reacts to light (photoelectric effect), leading to its use as a light sensor in the 1870s. Scottish-born American inventor Alexander Graham Bell (1847–1922) made a selenium-based photophone in 1879.
▪️The primary use of selenium is to decolorize glass, to color glass red, and to make the pigment China Red. Other uses are in photocells, in laser printers and photocopiers, in steels, and in semiconductors.
▪️There are six natural isotopes of selenium. One is radioactive, while the other five are stable. However, the half-life of the unstable isotope is so long that it is essentially stable. Another 23 unstable isotopes have been produced.
▪️Some plants require high levels of selenium to survive, so the presence of those plants means the soil is rich in the element.
▪️Liquid selenium exhibits extremely high surface tension.
▪️Selenium is important to several enzymes, including antioxidant enzymes glutathione peroxidase and thioredoxin reductase and the deiodinase enzymes that convert thyroid hormones into other forms.
▪️Approximately 2,000 tons of selenium are extracted annually worldwide.
▪️Selenium is most commonly produced as a byproduct of copper refining.
Robert C. Brasted, “Selenium”، www.britannica.com
Alina Bradford (28-11-2016), “Facts About Selenium”، www.livescience.com
Anne Marie Helmenstine (30-11-2018), “19 Interesting Selenium Facts”، www.thoughtco.com