Selenium
In 1817, Jöns Jacob Berzelius and Johan Gottlieb Gahn stood before a red solid precipitate in the lead chambers of their chemistry plant near Gripsholm, Sweden. They had been processing pyrite samples from the Falun Mine to produce sulfuric acid when this unexpected substance appeared. The material gave off an odor like horseradish when burned, which did not match typical arsenic compounds they expected. Berzelius initially wrote to Alexander Marcet claiming it was a tellurium compound because the smell resembled known tellurium substances. However, further analysis revealed no tellurium existed in the Falun Mine minerals. In 1818, Berzelius sent a second letter describing a newly found element similar to both sulfur and tellurium. He named the new element selenium after Selene, the Greek goddess of the Moon, to recall its relationship with tellurium, which was named for the Earth.
When prepared through chemical reactions, selenium usually appears as an amorphous brick-red powder. Rapidly melted selenium forms a black vitreous solid sold commercially as beads. This black form consists of polymeric rings containing up to 1000 atoms per ring. Upon heating, the black material softens at 50 degrees Celsius and converts to gray selenium at 180 degrees Celsius. Gray selenium possesses a chiral hexagonal crystal lattice consisting of helical polymeric chains where the Se-Se distance measures 237.3 picometers. The most stable and dense form of selenium is gray, showing appreciable photoconductivity unlike other allotropes that act as insulators. Red alpha, beta, and gamma forms contain nearly identical puckered cyclooctaselenium rings with eight atoms each. These red forms have relatively low monoclinic crystal symmetry and vary in packing density, with the alpha form being the most dense.
Selenium dioxide forms by combustion of elemental selenium and dissolves in water to create selenous acid. Selenium trioxide decomposes to the dioxide above 185 degrees Celsius and exists only as thermodynamically unstable laboratory products. Hydrogen sulfide reacts with aqueous selenous acid to produce selenium disulfide, which consists of 8-membered rings varying in composition. Selenium tetrafluoride serves as a laboratory-scale fluorinating agent while selenium hexafluoride acts as a toxic pulmonary irritant. Metal selenides like mercury selenide, lead selenide, and zinc selenide function as semiconductors used in various electronic applications. Organic derivatives include selenomethionine, diphenyldiselenide, and benzeneselenol, structurally analogous to corresponding organosulfur compounds. Tetraselenium tetranitride stands out as an explosive orange compound synthesized from selenium tetrachloride and ammonia.
About 2000 tonnes of selenium were produced worldwide in 2011, mostly in Germany, Japan, Belgium, and Russia. Electrolytic metal refining produces selenium as a byproduct obtained from the anode mud of copper refineries. The largest commercial use accounts for about 50% of consumption and involves glass production where selenium compounds confer red color. This red color cancels out green or yellow tints arising from iron impurities typical for most glass. China remains the dominant consumer at 1500 to 2000 tonnes per year with metallurgy and glass manufacturing each taking 30% of usage. Selenium rectifiers first appeared in 1933 but have mostly been replaced by silicon-based devices except in power DC surge protection. Amorphous selenium thin films serve as photoconductors in flat-panel X-ray detectors capturing incident photons directly into electric charge.
The selenium content in the human body measures between 13 and 20 milligrams according to common references. In humans, selenium functions as a cofactor for reduction of antioxidant enzymes such as glutathione peroxidases found in animals and some plants. Increased dietary selenium reduces mercury toxicity effects though effective only at low to modest doses of mercury. The US Recommended Dietary Allowance for teenagers and adults is 55 micrograms daily while selenium-specific supplements typically contain either 100 or 200 micrograms per serving. Both elemental selenium and especially selenium salts are toxic even in small doses causing selenosis symptoms including diarrhea, fatigue, hair loss, joint pain, nail brittleness, nausea, headache, tingling, vomiting, and fever. Selenium deficiency investigation continues regarding its role in inducing autoimmune reactions in Hashimoto's disease thyroid gland conditions.
In Belews Lake North Carolina, 19 species of fish were eliminated due to wastewater containing 150 to 200 micrograms of selenium per liter discharged from 1974 to 1986. Organoselenium compounds can concentrate over 200,000 times by zooplankton when water concentrations range between 0.5 and 0.8 micrograms per liter. Phytoplankton can concentrate inorganic selenium by a factor of 3000 further concentrating along the food chain as predators consume selenium-rich prey. Reproduction of mallard ducks becomes impaired at dietary concentrations of 7 micrograms per liter while many benthic invertebrates tolerate up to 300 micrograms per liter in their diet. Fish affected by selenium may experience swelling of gill lamellae impeding oxygen diffusion across gills and blood flow within them. Selenium poisoning passes from parents to offspring through eggs and may persist for many generations causing malformed fish fetuses with problems feeding or respiring.
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Common questions
Who discovered selenium and when was it found?
Jöns Jacob Berzelius and Johan Gottlieb Gahn discovered selenium in 1817 while processing pyrite samples from the Falun Mine near Gripsholm, Sweden. They identified the element as a red solid precipitate that emitted an odor resembling horseradish during their chemical experiments.
What is the origin of the name selenium and how does it relate to other elements?
Berzelius named the new element selenium after Selene, the Greek goddess of the Moon, to reflect its relationship with tellurium which was named for the Earth. This naming convention highlights the periodic table connection between these two similar elements discovered in the early 19th century.
How many tonnes of selenium were produced worldwide in 2011 and where was it mostly made?
About 2000 tonnes of selenium were produced worldwide in 2011 with major production occurring in Germany, Japan, Belgium, and Russia. China remains the dominant consumer annually consuming between 1500 to 2000 tonnes primarily for metallurgy and glass manufacturing applications.
What are the health effects of selenium deficiency and toxicity in humans?
Selenium functions as a cofactor for reduction of antioxidant enzymes such as glutathione peroxidases found in animals and some plants while both elemental forms and salts can cause selenosis symptoms including diarrhea fatigue hair loss joint pain nail brittleness nausea headache tingling vomiting and fever. The US Recommended Dietary Allowance for teenagers and adults is 55 micrograms daily yet increased dietary intake reduces mercury toxicity effects only at low to modest doses.
Why did fish populations disappear from Belews Lake North Carolina during the 1974 to 1986 period?
Seventeen species of fish were eliminated due to wastewater containing 150 to 200 micrograms of selenium per liter discharged into Belews Lake North Carolina from 1974 to 1986. Selenium poisoning passed from parents to offspring through eggs causing malformed fish fetuses with problems feeding or respiring while organoselenium compounds concentrated over 200,000 times by zooplankton.