Environmental Chemistry

Selenium

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Black, vitreous/glassy amorphous (with thin layer of grey selenium) allotrope of selenium. (Black, vitreous/glassy amorphous (with thin layer of grey selenium) allotrope of selenium.)

Background

Selenium is a gray, metallic element. Its atomic number is 34 and its symbol is Se. The Swedish scientist Jons Jacob Berzelius discovered selenium in 1817. In studying the sulfuric acid produced in a particular Swedish factory, he discovered an impurity which he eventually identified as selenium. Selenium occurs in three distinct forms: as a non-crystalline, gray metal; it can form as a deep red to black powder; and it can form as red crystals. It is stable in air and in water. Selenium is actually an important trace element to mammals and some plants. Too much selenium in a mammal’s diet is poisonous and has been shown to cause deformities. When there is not enough selenium, a mammal can also have health problems. For example, sheep that graze in areas with too little selenium in the soil eventually have a problem known as “white muscle disease.” Lack of selenium has also been connected to strokes in humans. The percentage amount of selenium in a healthy human is 0.00002 %.

Name

Previous Element: Arsenic

Next Element: Bromine
34

Se

78.96
Physical Properties
Color black
Phase at Room Temp. solid
Density (g/cm3) 4.285
Hardness (Mohs) 2
Melting Point (K) 490.2
Boiling Point (K) 958
Heat of Fusion (kJ/mol) 5.1
Heat of Vaporization (kJ/mol) ---
Heat of Atomization (kJ/mol) 227
Thermal Conductivity (J/m sec K) 0.52
Electrical Conductivity (1/mohm cm) 0
Source Cu smelting by-product
Atomic Properties
Electron Configuration [Ar]3d104s24p4
Number of Isotopes 6
Electron Affinity (kJ/mol) 194.97
First Ionization Energy (kJ/mol) 940.9
Second Ionization Energy (kJ/mol) 2044.5
Third Ionization Energy (kJ/mol) 2973.7
Electronegativity 2.55
Polarizability (Å3) 3.8
Atomic Weight 78.96
Atomic Volume (cm3/mol) 18.4
Ionic Radius2- (pm) 184
Ionic Radius1- (pm) ---
Atomic Radius (pm) 119
Ionic Radius1+ (pm) ---
Ionic Radius2+ (pm) ---
Ionic Radius3+ (pm) ---
Common Oxidation Numbers -2,+2
Other Oxid. Numbers +4, +6
Abundance
In Earth's Crust (mg/kg) 5×10-2
In Earth's Ocean (mg/L) 2×10-4
In Human Body (%) 0.00002%
Regulatory / Health
CAS Number 7782-49-2 powder
OSHA Permissible Exposure Limit (PEL) TWA: 0.2 mg/m3
OSHA PEL Vacated 1989 TWA: 0.2 mg/m3
NIOSH Recommended Exposure Limit (REL) TWA: 0.2 mg/m3
IDLH: 1 mg/m3
Sources:
University of Wisconsin General Chemistry
Mineral Information Institute
Jefferson Accelerator Laboratory
EnvironmentalChemistry.com
 

Selenium was named after the Greek word selene, meaning moon. This is a reference to the silvery-gray color of metallic, non-crystalline selenium. There is a mineral called selenite which is also named after the word selene; however, selenite does not contain selenium.

Sources

Minerals containing selenium are very uncommon. Rarely, ores that contain high concentrations of selenium have been discovered. Most selenium is recovered as a by-product of processing copper ores. This appears to be the only affordable source of selenium. It is estimated that the copper deposits that are yet to be discovered will produce 2.5 times the amount of selenium in the presently known copper ores. Continued search and research will therefore lead to the discovery not only of future copper ores, but also of the selenium found within them.

Currently, less than one-fifth of the refined selenium production comes from recycling. Almost all of this recycling is of selenium-containing photo-receptors used in photo copiers.

The nations producing selenium include the United States, Belgium, Canada, Chile, Germany, Japan, Sweden, Philippines, Finland, Peru, Zambia, and other countries. The United States imports selenium, primarily from Canada, Philippines, Belgium, Japan, and other nations.

Uses

Selenium is known as a photovoltaic substance. This means that it converts light energy directly into electricity. It also displays what is called a photoconductive action, in which electrical conductivity increases as more and more light shines on the selenium. These unique features make selenium useful for photocells used to power everything from hand held calculators to large-scale photocells used to convert sunlight into electrical energy which is then stored in batteries.

Selenium has other interesting electrical properties. It can be used in devices to convert alternating current (AC) electricity to direct current (DC) electricity. Therefore, selenium is used in special electrical converters where an AC power supply must be changed into a DC current. These special converters are called rectifiers. Ultimately, less than one-fifth of the selenium consumed annually is used in these various electrical applications.

Even more selenium is used in the production of glass. It is used to remove the color from the glass used to make bottles. It is used in specialized sheet glass for windows where it reduces the amount of heat that enters a building from sunlight. The glass industry consumes more than one-third of the selenium used each year.

It is also used to make a variety of chemicals and pigments. This accounts for about one-fifth of the annual selenium consumption.

The remainder is used in a variety of applications. At one time, selenium was important in the manufacture of the drums in copying machines that transfer the image to the paper (newer copiers no longer use selenium on the image drum). It is also used in anti-dandruff shampoos, steel alloys, human dietary supplements, and rubber production.

Substitutes and Alternative Sources

Newer technologies are replacing some of the applications of selenium. For instance, high purity silicon is now being used in the production of rectifiers (see Uses above). Other elements are being used in the photoelectric applications. Cerium oxide is being used in glass production in place of selenium. Coal deposits contain 1.5 parts per million selenium. This is 80 times the amount of selenium found in copper deposits! Unfortunately, a method of removing this selenium from coal has not been developed. This could prove to be a significant source of selenium should technology advance.

Further Reading

 

 

Disclaimer: This article is taken wholly from, or contains information that was originally published by, the Mineral Information Institute. Topic editors and authors for the Encyclopedia of Earth may have edited its content or added new information. The use of information from the Mineral Information Institute should not be construed as support for or endorsement by that organization for any new information added by EoE personnel, or for any editing of the original content.

 

Glossary

Citation

Institute, M. (2014). Selenium. Retrieved from http://www.eoearth.org/view/article/51cbeedd7896bb431f69ac86

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