Industry uses selenium to manufacture photocells, photographic exposure meters, solar cells, and rectifiers for home entertainment equipment. Further uses are in metal alloys, xerography, red or black glass, anti-dandruff shampoos, pigments in plastics, paints, dyes, enamels, inks, textiles, rubber (accelerator and vulcanising agent), photographic emulsions, and petroleum. It is also used in veterinary medicine, in medical therapeutic agents, as a nutritional feed additive for poultry and livestock, and as a fungicide and insecticide. Uses in electronics and photography account for the majority of selenium use, followed by the glass industry, then pigments. Some applications for specific selenium compounds follow. Selenium dioxide is the most widely used selenium compound in industry. It is used as an oxidising agent in drug and other chemical manufacture, a catalyst in organic syntheses, and an anti-oxidant in lubricating oils. Selenium sulfide is used in anti-dandruff shampoo. Selenous acid can be found in gun blueing solution which is used to clean the metal parts of a gun.
Substance name: Selenium & compounds
CASR number: 7782-49-2
Molecular formula: Se
Synonyms: No synonyms.
Selenium compounds include selenium (IV) oxide or selenium dioxide (CAS# 7446-08-4), selenium (VI) fluoride or selenium hexafluoride (CAS# 7783-79-1), selenium (II) hydride or hydrogen selenide (CAS# 7783-07-5), selenium (IV) oxychloride (CAS# 7791-23-3), selenium (IV) sulfide or selenium disulfide (CAS# 7488-56-4), selenic acid (CAS# 7783-08-6), and selenous acid (CAS# 7783-00-8).
Selenium is an odourless metalloid (an element which has both metallic and non-metallic properties). It can be a grey (the 'metallic' and most stable form), red or black solid.
Atomic Number: 34
Atomic Mass: 79.0
Melting Point: 221°C (grey selenium)
Boiling Point: 685°C
Specific Gravity: 4.4-4.8
Properties vary widely depending on the particular compound. Some physical properties for selected selenium compounds follow.
- Selenium disulfide is a light orange to red powder. Its melting point is less than 100°C and it decomposes when heated.
- Selenium hydride is an extremely poisonous gas with revolting odour. Its melting point is -66°C, its boiling point is -41°C, and the compound is thermodynamically stable to 280°C.
- Selenium dioxide is hygroscopic and melts at 315°C. It has a specific gravity of 3.95.
- Selenous acid forms hygroscopic, colourless crystals that melt at 70°C. Its boiling point is 315°C, with a specific gravity 3.0.
- Selenic acid forms white crystals that melt at 58°C. Its boiling point is 260°C.
- Selenium hexafluoride is a very toxic colourless gas (melting point -39°C, boiling point -35°C).
- Selenium oxychloride has a vapour density 5.7 and a melting point of 9°C.
Selenium can exist in four different oxidation states, (II-), (0), (IV) and (VI), forming various salts. In aquatic environments, the (IV) and (VI) oxidation states are predominant. Selenium (IV) is known as selenite and selenium (VI) is known as selenate. Various chemical properties for selenium and selected selenium compounds follow.
- Selenium burns in air when heated to give selenium dioxide. It reacts when heated with halogens, most metals and non-metals. Non-oxidising acids do not affect selenium.
- Selenium dioxide can be obtained by treating selenium with hot nitric acid. It dissolves in water to give solutions of selenous acid.
- Selenic acid evolves oxygen when heated above 200°C. It is a strong oxidising agent. Selenous acid and selenic acid are both water soluble.
- Selenium dioxide is slightly water soluble.
- Selenium disulfide and selenium hexafluoride are both insoluble in water.
- Selenium hydride has a water solubility of 0.70 g/100 mL.
The National Pollutant Inventory (NPI) holds data for all sources of selenium and compounds in Australia.
The severity of health effects will depend on how much selenium a person has been exposed to, for how long, the nature of the selenium compound(s), and current state of health. Trace amounts of selenium are essential for humans and most diets usually provide enough selenium to meet the daily requirement. Diets lacking selenium have resulted in heart problems and muscle pain. Diets with too much selenium can be harmful at levels 5 to 10 times higher than the daily requirement. Accidentally swallowing a large quantity of selenium supplement pills could be life-threatening without immediate medical treatment. Eating too much selenium over long periods of time can result in excessive tooth decay, discolouration of the skin and teeth, brittle hair, deformed nails, fatigue, irritability, depression and pallor. Feeling and control in the arms and legs may also be lost. People exposed to very high levels of selenium in the workplace have reported headaches, dizziness, fatigue, irritation (eye, nose, throat and bronchial tubes), collection of fluid in the lungs, and severe bronchitis. The exact levels at which these effects occur are not known. Upon contact with skin, selenium compounds have caused rashes, swelling, and pain. It may also protect against the toxicity of heavy metals such as cadmium, mercury and silver.
More details about two selenium compounds follow.
Selenium sulfide is very different from the selenium compounds found in foods and in the environment. Selenium sulfide has not caused cancer in animals when it is placed on the skin, and the use of anti-dandruff shampoos containing selenium sulfide is considered safe. When ingested it is reasonably anticipated to be a carcinogen.
Selenium hydride is the most acutely toxic selenium compound, resembling arsenic in its physiological reactions. Acute (short term) exposure to selenium hydride by inhalation results primarily in respiratory effects, such as irritation of the mucous membranes, collection of fluid in the lungs, severe bronchitis, and bronchial pneumonia.
Entering the body
Selenium can be inhaled or ingested.
Food is the primary source of exposure to selenium. Humans are usually breathing very low levels of selenium in air. Drinking water usually contains selenium at very low levels. Higher levels of selenium may be found in drinking water, usually in areas where high levels of selenium in soil contribute to the selenium content of the water. Occupational exposure to selenium in the air may occur in the metal industries, selenium recovery processes, painting, and special trades. Higher selenium levels may be encountered when living in the vicinity of these industries.
Workplace exposure standards
Safe Work Australia sets the workplace exposure standards for selenium & compounds through the workplace exposure standards for airborne contaminants. These standards are only appropriate for use in workplaces and are not limited to any specific industry or operation. Make sure you understand how to interpret the standards before you use them.
- Maximum eight hour time weighted average (TWA): 0.05 parts per million (0.16 mg/m3)
- Maximum eight hour time weighted average (TWA): 0.05 parts per million (0.16 mg/m3)
Other selenium compounds
- Maximum eight hour time weighted average (TWA): 0.1 mg/m3
Drinking water guidelines
The Australian Drinking Water Guidelines include the following guidelines for acceptable water quality:
- Maximum of 0.01 milligrams per litre of water for health purposes
Trace amounts of selenium are essential for animals, but selenium is generally not essential for plants. Many plants are selenium tolerant. The toxicity of selenium depends on whether it is in the biologically active oxidised form. This form can be found in alkaline soils, causing plant uptake of the metal to be increased. Various plants are known to accumulate selenium. The biological availability appears to be reduced in acidic or neutral soils. Some plants and micro-organisms can volatilise selenium from soils by converting selenium to volatile alkylated selenium compounds. Plants containing high levels of selenium and selenium compounds can be toxic to grazing livestock. "Blind staggers" disease is a disease in livestock that results from acute consumption of plants high in selenium. It is characterised by impaired vision, aimless wandering behaviour, reduced consumption of food and water, and paralysis. "Alkali disease" is a disease in livestock resulting from chronic consumption of high levels of selenium. It is characterised by hair loss, deformation and sloughing of the hooves, erosion of the joints of the bones, anaemia, and effects on the heart, kidney, and liver. In waters containing elevated levels of selenium fish can bioaccumulate selenium, building up high levels. Selenium and its compounds have high acute toxicity to aquatic life and mammals and moderate acute toxicity to birds. Insufficient data are available to evaluate or predict the short term effects of selenium on plants.
Entering the environment
Selenium can be transported as particles released into the atmosphere or as dissolved compounds in natural waters.
Where it ends up
Small selenium particles in the air settle to the ground or are taken out of the air by rain. Soluble selenium compounds can be found in natural waters, either as selenites or selenates.
Australian Water Quality Guidelines for Fresh and Marine Waters: (ANZECC, 1992):
Maximum of 5 microgram/L (i.e. 0.000005 g/L) and 70 microgram/L (i.e. 0.00007 g/L) in marine waters respectively.
Selenium compounds are released to the air and water in flue gas and fly ash during the combustion of coal and petroleum fuels (e.g. in coal-fired power stations), during the smelting and refining of metals such as copper, lead and zinc, from glass and ceramics manufacturing, and from refuse incinerators. It also accumulates in the residues from sulphuric acid manufacture.
Diffuse sources, and industry sources included in diffuse emissions data
Selenium can enter waterways through use of some anti-dandruff shampoos. Application as fungicides and insecticides may contribute to elevated selenium levels in the environment. Electronic and photographic waste, and photocopying accessories may be other contributors if improperly disposed of. Cigarette smoke may be another source of selenium in air.
Selenium is a naturally occurring element, widely but unevenly distributed in the earth's crust and commonly found in sedimentary rock formations. However, there are no true deposits of selenium from which selenium can be directly and economically recovered. Selenium usually occurs combined with other compounds in the environment, such as in sulfide ores of other metals (silver, copper, lead, and nickel). Soils in the neighbourhood of volcanos tend to have enriched amounts of selenium. It occurs in water as a result of natural weathering of soils and rocks. Selenium is enriched in coal.
Emissions may result from burning petrol in cars.
Anti-dandruff shampoos, home entertainment equipment, photographic equipment.
Sources used in preparing this information
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- National Health and Medical Research Council (NHMRC), Australian Drinking Water Guidelines (2011) - Updated October 2017, accessed May 2018.