Selenium Prices, Occurrence, Extraction and Use

Selenium [zeleːn] is a chemical element with the element symbol Se and the ordinal number 34. In the periodic table it is in the 4. Period as well as the 6. Main group, or the 16. IUPAC group, so one of the chalcogens. It comes in several modifications, the most stable is the gray, metal-like shape.

History

Selenium (Greek σελήνη [selḗnē], "moon") Xnumx was discovered by Jöns Jakob Berzelius in the lead chamber sludge of a sulfuric acid factory; first Berzelius held the substance for tellurium (from the Latin tellus 'earth') to which selenium bears some similarities; Thus, when burning both elements, a pronounced smell of radish develops. Xnumx concluded in his experiments that Berzelius was a new element; to refer to the similarity to tellurium (earth) he called it selenium (moon).

occurrence

In inanimate nature and in industrial processes, inorganic compounds in particular play a role. In the living nature dominate the organic compounds. In yeasts and plants selenium occurs mainly as selenomethionine. As an essential trace element selenium is part of the 21. biogenic amino acid selenocysteine, as well as contained in bacteria, archaea and eukaryotes. Animals do not produce selenomethionine, but selenocysteine ​​does. Selenocysteine ​​is the specific catalytic component of selenium-dependent enzymes. In contrast, selenomethionine is nonspecifically incorporated into many proteins in place of methionine, without functioning; it is considered as a selenium storage form. The amount of selenium in food depends strongly on the selenium content of the soil. Low-selenium soils in Europe are found particularly in Germany, Scotland, Denmark, Finland, parts of the Balkan countries and in Switzerland. In some selenium poor areas selenathaltige fertilizers are fed to the soil, z. In Finland since 1984.

In small amounts, solid selenium occurs naturally. Selenium minerals such as Clausthalite and Naumannite are rare.

Selenium, usually in the form of metal selenides, is the companion of sulphurous ores of the metals copper, lead, zinc, gold and iron. When these ores are roasted, the solid selenium dioxide in the fly ash or in the downstream sulfuric acid production collects as selenious acid.

Selenium can be enriched in tragacanth, brassica or garlic as Se methylselenocysteine. The richest known source of selenium among foods is Brazil nut.

Extraction and presentation

Industrially, selenium is obtained as a by-product of electrolytic copper and nickel production from the anode sludge by roasting.

Reduction to elemental selenium is by sulfur dioxide:

On a laboratory scale, selenium can be prepared via the reaction of selenious acid with hydrogen iodide.

Organically bound selenium

For some years now, an organic source of selenium has been used in food supplements and animal nutrition (approved as 2005 in animal nutrition in the EU since May) by cultivating certain brewer's yeasts of the type Saccharomyces cerevisiae (Sel-Plex, Lalmin (TM)) on selenium-rich nutrient medium ( Molasses and sodium selenite). Yeasts synthesize high levels of selenomethionine as an amino acid and thus bind up to 2000 ppm of selenium in an organic way. The largest plant for the production of such natural Selenhehefen 2004 was built in São Pedro in the Brazilian state of Paraná.

Features

Selenium occurs like sulfur in several modifications:

Red selenium, which is soluble in carbon disulfide, consists of approximately 30% of Se8 rings and 70% of Se8 + n, which converts to the gray semiconductor metal above 80 ° C. Elemental red selenium is an insulator.
Black amorphous selenium that transforms above 60 ° C into the black, glassy selenium. Both forms convert to the gray, semi-metallic modification upon heating above 80 ° C.
Gray "metallic" selenium is the most stable modification and behaves like a semi-metal.
Above the melting point of 220 ° C it forms a black liquid. The selenium vapor produced by further increase in temperature is yellow.
When deposited from the vapor phase on a cooler surface (much lower than the melting point) it precipitates in the form of hexagonal, metallic-gray crystal needles.

The band gap of selenium is about 1,74 eV (at the boundary of visible light to infrared).

Exposure changes its electrical conductivity. In addition, it shows a photovoltaic effect. The conduction is not caused by electrons in a conduction band, but by conduction of holes (see in electrical conductivity and defect electron), so positively charged electron defects, which, among other things, the sign of the Hall effect is negative. As a mechanism for this hole line, a so-called "hopping conductivity" (the holes from one crystal defect to the next) is proposed.

When heated in air selenium burns with blue flame to selenium dioxide, SeO2. Above 400 ° C, it reacts with hydrogen to form hydrogen selenide, H2Se. With metals it usually forms selenides, for example sodium selenide, Na2Se.

The chemical behavior is similar to sulfur, but selenium is more difficult to oxidize. The reaction with nitric acid forms "only" selenious acid, a selenium (IV) compound.

isotope

The selenium has a variety of isotopes. Of the six naturally occurring isotopes, five are stable. The shares are distributed as follows: 74Se (0,9%), 76Se (9,0%), 77Se (7,6%), 78Se (23,6%), 80Se (49,7%) and 82Se (9,2%).

82Se is the only naturally occurring radioactive isotope with about 1020 years, one of the longest currently known half-lives ever. In addition, 22 radioactive isotopes are known, among which 75Se with a half-life of 120 days and 79Se with a half-life of 327.000 years have special significance. 75Se is used to construct special gamma-ray sources for nondestructive testing of e.g. B. welds application. 75Se is used in nuclear medicine in conjunction with methionine as a tracer for the assessment of pancreatic function and with homotaurocholic acid (SeHCAT) to assess the absorption of bile acids. 79Se is a component of spent nuclear fuel, where it is produced by the splitting of uranium at a frequency of 0,04%.

The rarest of the stable isotopes 74Se has gained some importance as a speculative object. It is offered again and again at very high prices in the market. However, apart from a few very specialized research applications where it is for marking purposes, no specific technical use is known for this material.

Usage

Selenium is essential for all life forms. Selenium compounds are therefore offered as a dietary supplement and processed into feed and fertilizer additives. In the glass industry, it is used for decolorizing green glasses and for producing red-colored glasses. Other applications:

• Exposure drums for photocopiers and laser printers
• Semiconductor manufacturing
• Latex additive to increase the abrasion resistance
• Toner for black and white photographs to increase the contrast (light tones remain unchanged, you can achieve darker blacks, the dark parts are more plastic overall), durability increase (not clearly demonstrated) and easy
• Coloring of the dark parts of the picture to aubergine-colored (also to plasticity increase)
  for the production of red color pigments based on cadmium selenide (because of the cadmium content today rather rare)
• Alloy additive to improve machinability for free-cutting steels and copper alloys
• Use in the selenium rectifier and the selenium cell, but today largely replaced by silicon (semiconductors).
  for the blackening of aluminum, brass o. Ä. (Selenium dioxide)
• with copper and indium part of the photoactive layer of CIGS solar cells
• in analog exposure meters for photography
• Anti-dandruff hair shampoos and prevention / therapy of pityriasis versicolor, one caused by a yeast fungus
• skin disease
• supportive in HIV therapy (favorable effect on HIV viral load controversial)
• Reaction with Grignard compounds, R-Mg-Hal, leads to organoselenium compounds, R-Se-Mg-Hal, from which selenols, R-Se-H, can be prepared by hydrolysis
• As zinc selenide it is used to produce optically highly reflective surfaces, but in the infrared it is transparent and is used here for the production of windows and focus lenses for z. B. CO2 laser used
• Larger amounts of selenium dioxide are consumed in the electrolysis of manganese. The addition of selenium dioxide reduces the
• Energy consumption during electrolysis. Per ton of manganese are consumed up to 2 kg of selenium dioxide.

 

Biological significance

Selenium is an essential trace element for humans, animals and many bacteria. In dairy cattle feeding, selenium is added as the natural selenium content of the feed is often insufficient to supply livestock. German feed legislation mentions supplementing the selenium supply only the two inorganic selenium sources sodium selenite and selenate as feed additives. These two compounds are economically very cheap, but have the disadvantage of low bioavailability. Selenium, however, is highly toxic at higher concentrations, with very narrow margins between levels of deficiency and toxic levels. In addition, the toxicity of selenium is dependent on the chemical binding form.

Selenium is contained in selenocysteine, an amino acid in the active site of the enzyme glutathione peroxidase and many other proteins. Because of its high reactivity with oxygen, selenium plays an important role in the protection of cell membranes from oxidative destruction (radical scavengers) in animals and humans. The selenium-containing enzyme glutathione peroxidase, which is found in all animal cells, is crucially involved in the breakdown of membrane-damaging oxidants and radical-derived products. Reduced glutathione peroxidase activity explains a number of selenium deficiency syndromes. Such a connection is discussed for cardiovascular diseases. Experimental hypertension in rats can also be substantially reduced by prophylactic selenium administration. Interesting in this context is the protective selenium effect in the cryopreservation of heart muscle fragments.

Selenocysteine ​​also participates in the catalytic mechanism of other enzymes and is present in many proteins whose importance has not yet been clarified.

Discussion about selenium

Before a research group led by Klaus Schwarz at the National Institute of Health (USA) discovered selenium as an essential dietary constituent of animals, selenium was considered a toxic substance. In the 1930 years, veterinarians in the "Great Plains" blamed the high levels of selenium-containing plants for cattle alkalinity and blind ataxia, while a research group on black in the 1950 years reported that selenium prevented necrotic liver degeneration. At about the same time, a group of researchers from Oregon State University, including OH Muth and JE Oldfield, found a selefeficiency in weak calves. Later, Hogue proved that selenium prevents lamb's muscular dystrophy. Following these reports, researchers from various institutions have begun studies on the benefits of selenium supplementation on the performance and health of dairy cattle. It has been reported that selenium is involved primarily in the catalysis of the glutathione peroxidase (GSH-Px) system. Various isoforms of the GSH-Px destroy the peroxides (reactive oxygen compounds) formed during normal fat metabolism. When peroxides remain freely in the cell, they attack and destabilize cell membranes. Hemken explained that selenium is also involved in the detoxification of dangerous drugs or toxins. Selenium also plays a role in at least two other enzyme systems in animals: iodothyronine deiodase, an enzyme that activates the thyroid hormone T4, and thioredoxin reductase, an enzyme that regulates reducing responses. Certain plasma, heart, muscle and kidney proteins contain selenium. However, the function of selenium in these proteins is still largely unclear.

There are many different selenoproteins. The selenoproteins usually contain selenocysteine, also known as 21. Amino acid is known and is incorporated during protein biosynthesis via its own tRNA. Selenoproteins occur in this function only in animal organisms, bacteria and archaea. Depending on their soil content, plants plant nonspecific amino acids in selenium, especially in methionine (Se-methionine) and, to a lesser extent, cysteine ​​(Se-cysteine) or derivatives thereof (methyl-Se-cysteine). Only the so-called "selenium collector plants" (selenium accumulator plants, eg, "Paradiesnuss"), which are found in selenious, arid areas, also store selenium as organically bound, water-soluble selenium or selenium salts.

To date, at least 25 genes for selenoproteins have been discovered in the human genome:

• Glutathione peroxidase 1 (GSHPx-1), the cellular or classical glutathione peroxidase (in the cytosol, mitochondrial matrix);
• Glutathione peroxidase 2 (GSHPx-2), the gastrointestinal glutathione peroxidase (in the intestinal mucosa);
• Glutathione peroxidase 3 (GSHPx-3), the extracellular or plasma glutathione peroxidase (in plasma);
• Glutathione Peroxidase 4 (GSHPx-4), the phospholipid hydroperoxide glutathione peroxidase (attached to lipid membranes, structural protein in the
• Tail of sperm); → antioxidant enzymes that neutralize peroxide radicals
• Thioredoxin reductase (TrxR) → reduces thioredoxin, which is important for cell growth, but also many others
• low molecular weight and high molecular weight substrates.
• Iodothyronine 5′-deiodinases (thyroid hormone iodinases) (ID-I, ID-II, ID-III) → catalyze thyroid hormones, for example removal of an iodine atom from T4 (thyroxine), which creates T3 (triiodothyronine)
• Selenoprotein P (Se-P) → very important as a transport protein of selenium from and to the cells; contains 10 selenium atoms
• Selenoprotein W → in the musculature; Role still unknown
• Selenium Phosphate Synthetase → catalyzes the synthesis of monoselenophosphate, a precursor of selenocysteine
• H. selenoprotein, M, N, O, I, K, S, V → function of these selenoproteins is still poorly understood. Mutations of the SEPN1 gene have been described in multicore myopathy.
• Selenoprotein R = methionine sulfoxide reductase
• Selenophosphatase synthetase 2 → catalyzes the production of selenophosphate

Selenium deficiency diseases

Well-known selenium deficiency diseases are:

• Keshan disease (juvenile cardiomyopathy), named after the northeastern Chinese city of Keshan in the Heilongjiang district of Manchuria
• Selenium deficiency promotes a mutation of the harmless coxsackievirus B3 (CVB3 / 0), which thereby becomes virulent
  Occurrences: Tibet, Mongolia, Siberia
• Kashin-Beck disease of humans (nutritive articular cartilage degeneration), named after the Russian physician Nikolai Ivanovich Kashin and the American Melinda A. Beck, occurrence: Siberia, Mongolia, North Korea, China; affected are about 3 million people
• Epidemic neuropathy in humans - Occurrence: Cuba, selenium deficiency causes a mutation of the influenza A / Bangkok / 1/79 virus, which becomes virulent as a result
• White muscle disease (nutritive myodegeneration (NMD), nutritive muscular dystrophy, enzootic myodystrophy, nutritive rhabdomyolysis, nutritive rhabdomyopathy, myopathic-dyspnoic syndrome, calf rheumatism, chicken meatiness, fishmeat), occurrence: in all selenium deficient areas of the earth, animal species: juveniles of especially ruminants: calves, lambs , Kidney, dromedary and lama foci, overloading myopathy of the ruminating cattle (paralytic myoglobinuria, exercitional rhabdomyolysis), occurrence: in all selenium deficient areas of the earth, animal species: above all cattle from eight months old

Selenium as a dietary supplement

A critical review of the June 2005 Pharmaceutical Information found that the studies available to date did not suggest any benefit from adding selenium in any context. Although it seems possible to positively influence various types of cancer, on the other hand it is not improbable to favor other carcinomas. The "SELECT" study ("Selenium and Vitamin E Cancer Prevention Trial") should provide information on this and 2013 should be completed. However, this was discontinued in October 2008, as it could be demonstrated during the study that there was no improved protection compared to placebo and a benefit could be excluded. In fact, an increase in prostate cancer frequency with the addition of vitamin E and increased diabetes onset of selenium administration was observed in this study, but neither was statistically significant.

As part of the re-evaluation of data from a study, Saverio Stranges from the University of Buffalo concluded that of the 600 patients taking selenium (daily 200 μg), about ten percent had type 2 diabetes after nearly eight years. In the placebo control group, it was only six percent. To date, no potential cause for the increased risk of diabetes has been found. High selenium concentrations in the blood correlate with the risk of developing diabetes. Thus, the pharmaceutical information from February 2008 comes to the conclusion: "A critical attitude to little-used concepts, behind which, of course, a big financial interest, has once again confirmed." The study situation is not clear in this regard. For example, the study by Stranges suggests methodological errors, such as the absence of a previous family history that would have precluded an increased familial prevalence of diabetes mellitus within the selenium group, as well as the fact that the subjects studied were persons with high levels of solar radiation and Exposed to chemicals, which meant that the results were poorly transferable to "average" subjects. In addition, diabetes risk was below the American average in both the placebo and selenium groups. Other studies suggest an inhibitory effect of selenium on the development of diabetes mellitus, including a recent study by Tasnime Akbaraly (University of Montpellier) on 1162 men and women.

Even a work from the year 2012 shows a positive effect of selenium only if there is a selenium deficiency, otherwise it is more likely to develop diabetes mellitus. A large meta-study from the year 2013 shows no protective benefits of selenium substitution in terms of cardiovascular disease. Although there was an increase in diabetes 2 cases in the selenium substitution group, the difference was not significant. But there was more and more alopecia and dermatitis.
Sodium selenite and thyroid hormones

Selenium plays an important role in the production of thyroid hormones, specifically in the "activation" of thyroxine (T4) to triiodothyronine (T3).

Selenium is part of an enzyme, the thyroxine 5'-deiodase, which is responsible for the removal of an iodine atom from T4. This deiodination creates T3. A selenium deficiency leads to a deficiency of thyroxine 5'-Deiodase, whereby only a part of the available T4 can be deiodiert. Since T3 is much more effective in the metabolism, T3 deficiency results in hyperthyroidism (hypothyroidism). Additional intake of selenium supplements (sodium selenite) in high doses of 200-300 μg daily z. As in Hashimoto's thyroiditis to reduce the inflammatory activity is discussed occasionally.

proof

The quantitative determination of traces (0,003%) of selenate can be carried out electrochemically by means of polarography. In 0,1-molar ammonium chloride solution, a step appears at -1,50 V (vs SCE). In the ultra-trace range, atomic spectrometry is suitable, whereby 100 μg / l selenium can be detected by flame AAS 0,5 μg / l (ppb), by graphite tube AAS 0,01 and by hydride technology.

safety instructions

Selenium and selenium compounds are poisonous. Direct contact damages the skin (blistering) and mucous membranes. Inhaled selenium can lead to protracted lung problems.

Intoxication ingestion of selenium is called selenosis. A selenium intake greater than 3000 μg / d may cause cirrhosis, hair loss and heart failure. Employees in the electronics, glass and paint industries are considered at risk. According to other sources, poisoning symptoms such as nausea and vomiting, hair loss, nail changes, peripheral neuropathy and fatigue occur as early as 400 μg / d.

selenium compounds

In compounds selenium occurs most frequently in the oxidation states -II (selenide, selenide) and + IV (tetrahalides, selenium dioxide and selenates (IV), obsolete selenites). In the selenide ions selenium also occurs with non-integer negative oxidation numbers. Rare positive oxidation numbers are + I (halides Se2X2) and + VI (selenium hexafluoride, selenic acid). Selenium compounds with the oxidation number + VI are stronger oxidizing agents than the analogous sulfur and tellurium compounds. Thus, mixtures of concentrated selenic (VI) acid with hydrochloric acid dissolve metals such as gold and platinum.

Hydrogen compounds

Hydrogen sulfide, H2Se, is a colorless, highly toxic gas produced by the reaction of selenides (MxSey) with strong acids, such as hydrochloric acid HCl. From the elements (hydrogen and selenium), the compound can be represented as a strongly endothermic compound only at temperatures above 350 ° C. Hydrogen sulfide decomposes slowly at room temperature into the elements, the decay is accelerated by the influence of light. The aqueous solution (selenic acid) reacts slightly acidic; the acidity (Ks = 1,88 10-4) is of the same order of magnitude as that of HNO2.

selenides

With most metals, selenium forms binary selenides containing the selenide anion Se2-. In addition, diselenides are known as Se22 and polyselenide senmic, which can be obtained by the reaction of a metal with an excess of selenium:

The synthesis is possible by melting the elements together or in solution. The selenides are sensitive to hydrolysis and oxidation. Besides the ionic selenides, the molecular compound carbon diselenide, Se = C = Se, is known.

 

Oxygen compounds and interchalkogens

Selenium dioxide (selenium (IV) oxide) is a colorless, crystalline solid which can be obtained by burning selenium in air. In water it forms selenious acid, H2SeO3. It is a relatively strong oxidizer and is easily reduced to selenium.

Selenium dioxide (selenium (VI) oxide) can be obtained by dehydration of selenic acid, H2SeO4. It is also a crystalline solid and a strong oxidizer.

In addition, there are the solid, crystalline, mixed-valence selenium (IV, VI) oxides Se2O5 and Se3O7.

Selenium monoxide, SeO, is known only as an unstable intermediate.

Selenium sulfide SeS ≈2 (an unstoichiometric selenium-sulfur compound consisting of sulfur-like cyclic molecules of variable size and composition, also called selenium disulfide due to the approximate ratio of SeS2).

Selenates are the salts of selenic acid with the anions SeO42-. Orthoselenates such as the trigonal bipyramidal anion SeO54 and the octahedral SeO66 are rarely observed.

Selenhalogenide

Selenium hexafluoride can be represented by the reaction of selenium with elemental fluorine. Although it is more reactive than sulfur hexafluoride, it does not react with water under normal conditions.

The most important selenium halides are the tetrahalides, but a seletetraiodide could not be synthesized. The tetrahalides can be represented by the elements. They can react as Lewis bases to form: SeX3 + as well as Lewis acids (formation of SeX62-). The known with all halogens dihalides and monohalides are unstable.

Selenium organic compounds

Organic selenium compounds occur mainly with the oxidation states

• Selane (organic selenides) RSeR, z. For example, dimethylselenide
• Diselane (Diselenide) RSeSeR
• Triselane (Triselenide) RSeSeSeR
• Selenole RSeH
• Selenenyle RSeX
• Selenoxides R-Se (= O) -R
• Selenone R2SeO2
• Selone R2C = Se, the selenium analogues of ketones

Selenpolykationen

By careful oxidation of selenium, numerous selenium polycations Senx + can be prepared and crystallized with a suitable counterion. The counterion must be a weak Lewis base because the selenium polycations are relatively strong Lewis acids. Suitable oxidizing agents are often halides of the transition metals, which at temperatures of typically 200 ° C give directly the desired compound:

Frequently, crystallization is successful under the conditions of chemical transport, but sometimes anhydrous solvents such as stannic chloride or silicon tetrabromide must be used.

If the metal halide is not a suitable oxidizing agent, as is usually the case with halides of the main group elements, the corresponding tellurium tetrahalides can be used as the oxidizing agent:

By varying the counterion and the reaction medium, a wide variety of polycations could be represented; Mixed selenium-tellurium polycations are also accessible by appropriate choice of the reactants of the synthesis.

Selenium price

Selenium prices -> Prices for strategic metals