Chemical Fact Sheet
Selenium
| Chemical Abstract Number (CAS #) | 7782-49-2 |
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| Synonyms | CI-77805; ELEMENTAL-SELENIUM; SELEN-(POLISH); SELENIUM-(COLLOIDAL); SELENIUM-ALLOY; SELENIUM-BASE; SELENIUM-DUST; SELENIUM-ELEMENTAL; SELENIUM-HOMOPOLYMER; CASWELL-NO.-732; EPA-Pesticide-Chemical-Code-072001; Gray-selenium |
| Analytical Methods | 200.7 - 200.8 - 6010 - 6020 - 270.2 |
| Molecular Formula | Se |
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Synopsis
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Selenium - (Gr. Selene, moon), Se; at. wt. 78.96(3); at. no. 34; m.p. (gray) 221 deg C; b.p. (gray) 685 deg C; sp. gr. (gray) 4.79, (vitreous) 4.28; valence -2, +4, or +6. Discovered by Berzelius in 1817, who found it associated with tellurium, named for the earth. Selenium is found in a few rare minerals, such as crooksite and clausthalite. In years past it has been obtained from flue dusts remaining from processing copper sulfide ores, but the anode muds from electrolytic copper refineries now provide the source of most of the world's selenium. Selenium is recovered by roasting the muds with soda or sulfuric acid, or by smelting them with soda and niter. Selenium exists in several allotropic forms. Three are generally recognized, but as many as six have been claimed. Selenium can be prepared with either an amorphous or crystalline structure. The color of amorphous selenium is either red, in powder form, or black, in vitreous form. Crystalline monoclinic selenium is a deep red; crystalline hexagonal selenium, the most stable variety, is a metallic gray. Natural selenium contains six stable isotopes. Twenty four other isotopes and isomers have been characterized. The element is a member of the sulfur family and resembles sulfur both in its various forms and in its compounds. Selenium exhibits both photovoltaic action, where light is converted directly into electricity, and photoconductive action, where the electrical resistance decreases with increased illumination. These properties make selenium useful in the production of photocells and exposure meters for photographic use, as well as solar cells. Selenium is also able to convert a.c. electricity to d.c., and is extensively used in rectifiers. Below its melting point selenium is a p-type semiconductor and is finding many uses in electronic and solid-state applications. It is used in Xerography for reproducing and copying documents, letters, etc. It is used by the glass industry to decolorize glass and to make ruby-cblored glasses and enamels. It is also used as a photographic toncr, and as an additive to stainless steel. Elemental selenium has been said to be practically nontoxic and is considered to be an essential trace element; however, hydrogen selenide and other selenium compounds are extremely toxic, and resemble arsenic in their physiological reactions. Hydrogen selenide in a concentration of 1.5 ppm is intolerable to man. Selenium occurs in some soils in amounts sufficient to produce serious effects on animals feeding on plants, such as locoweed, grown in such soils. Selenium is priced at about $150/kg. It is also available in high-purity form at a cost of about $250/kg (99.999%). |
| Use | In the glass industry as a decolorizing agent. AS INGREDIENT OF TONING BATHS IN PHOTOGRAPHY; AS PIGMENT IN MFR RUBY-, PINK-, ORANGE-, OR RED-COLORED GLASS; AS METALLIC BASE IN MAKING ELECTRODES FOR ARC LIGHTS, ELECTRICAL INSTRUMENTS & APPARATUS; AS RECTIFIER IN RADIO & TELEVISION SETS; IN SELENIUM PHOTOCELLS, IN SEMICONDUCTOR FUSION MIXTURES, SELENIUM CELLS. IN TELEPHOTOGRAPHIC APPARATUS; AS VULCANIZING AGENT IN PROCESSING OF RUBBER; AS CATALYST IN DETERMINATION OF NITROGEN BY KJELDAHL METHOD; FOR DEHYDROGENATION OF ORGANIC COMPOUNDS. IN STEEL & COPPER (DEGASIFIER & MACHINEABILITY IMPROVER). Selenium is used extensively in the manufacture and production of glass, pigments, rubber, metal alloys, textiles, petroleum, medical therapeutic agents, and photographic emulsions. |
| Consumption Patterns | Electronic and photocopier components, 35%; Glass manufacturing, 30%; Chemical and pigments, 25%; and Other, 10% (1985). |
| Apparent Color | LIQUID IS A BROWNISH RED; SELENIUM EXISTS IN SEVERAL ALLOTROPIC FORMS. 3 ARE GENERALLY RECOGNIZED ... SELENIUM CAN BE PREPD WITH EITHER AMORPHOUS OR CRYSTALLINE STRUCTURE. AMORPHOUS IS EITHER RED, IN POWDER FORM OR BLACK, IN VITREOUS FORM. CRYSTALLINE MONOCLINIC PRISM IS DEEP RED; CRYSTALLINE HEXAGONAL FORM, THE MOST STABLE VARIETY, IS A METALLIC GRAY. |
| Odor | Odorless |
| Boiling Point | 690 deg C |
| Melting Point | 170-217 deg C |
| Molecular Weight | 78.96 +/- 3 |
| Density | 4.26-4.81 |
| Sensitivity Data | Elemental selenium dusts produce respiratory tract irritation manifested by nasal discharge, loss of smell, epistaxis, and cough. |
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Chemical and Physical Properties |
COMBINES DIRECTLY WITH HYDROGEN, WITH THE HALOGENS (EXCLUDING IODINE); OXIDIZED TO SELENIOUS ACID BY NITRIC ACID, TO SELENIC ACID BY SULFURIC ACID; REDUCES HOT AQUEOUS SOLN OF SILVER AND GOLD SALTS WITH FORMATION OF SILVER SELENIDE AND METALLIC GOLD, RESPECTIVELY DARK RED-BROWN TO BLUISH-BLACK SOLID; SOL IN CARBON DISULFIDE, METHYLENE IODIDE, BENZENE OR QUINOLINE; DENSITY: 4.28; SOFTENS @ 50-60 DEG C & BECOMES ELASTIC @ 70 DEG C; FORMED WHEN MOLTEN SELENIUM IS COOLED RAPIDLY; WHEN FRESHLY PRECIPITATED, REACTS WITH WATER @ 50 DEG C FORMING SELENIOUS ACID & HYDROGEN /AMORPHOUS SELENIUM/ DARK RED, TRANSPARENT CRYSTALS; TWO MONOCLINIC FORMS: BOTH FORMS ARE METASTABLE & CHANGE INTO GRAY FORM ON HEATING; MELTING POINT: BELOW 200 DEG C; DENSITY (ALPHA-FORM) 4.46 /RED SELENIUM/ SOLUBLE IN CONCENTRATED NITRIC ACID; CRYSTALLINE SELENIUM IS A P-TYPE SEMICONDUCTOR /RED SELENIUM/ Lustrous gray to black hexagonal crystals; insol in alcohol; very slightly sol in carbon disulfide (2 mg/100 ml); sol in ether; conducts electricity & rectifies alternating current; conductivity increases up to 1000 times on exposure to light; density 4.81 @ 20 deg C/4 deg C; melting point 217 deg C; Mohs' hardness: 2.0; Latent heat of fusion 16.4 cal/g; Latent heat of vaporization 20.6 kcal/mol; Linear coefficient of thermal expansion per degree C= 37X10-6; Specific heat (28 deg C): 0.084 cal/g/deg C; Surface tension (217 deg C): 92.5 dynes/cm; Thermal conductivity (25 deg C): 0.0007-0.00183 cal/(cm)(deg C)/sec. /Gray selenium/ MELTING POINT: TRANSITION POINT TO HEXAGONAL: 60-80 DEG C /AMORPHOUS SELENIUM/ EXHIBITS BOTH PHOTOVOLTAIC ACTION, WHERE LIGHT IS CONVERTED DIRECTLY TO ELECTRICITY; EXHIBITS PHOTOCONDUCTIVE ACTION, WHERE ELECTRICAL RESISTANCE DECREASES WITH INCREASED ILLUMINATION SOL IN CHLOROFORM /GRAY SELENIUM/ SOL IN CARBON DISULFIDE: 0.1 G/100 CC @ 46.6 DEG C /RED SELENIUM/ BP: 684.9 +/- 1 deg C /Gray selenium/. |
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Environmental Impact |
SELENIUM DIOXIDE IS THE PRIMARY PROBLEM INVOLVED WITH MOST INDUSTRIAL EXPOSURE TO THE ELEMENT SINCE THE OXIDE IS FORMED WHEN SELENIUM IS HEATED. THE DIOXIDE ITSELF FORMS SELENIOUS ACID WITH WATER OR SWEAT, & THE ACID IS AN IRRITANT. Exposure to selenium may occur in the following occupations: arc light electrode makers, copper smelter workers, electric rectifier makers, glass makers, organic chemical synthesizers, pesticide makers, photographic chemical makers, pigment makers, plastic workers, pyrite roaster workers, rubber makers, semiconductor makers, sulfuric acid makers, and textile workers. SELENIUM CMPD RELEASED DURING FOSSIL FUEL COMBUSTION MAY POSE A POTENTIAL INHALATION HAZARD TO PEOPLE. MOST PROBABLE FORMS OF SELENIUM RELEASED DURING COMBUSTION ARE SELENIOUS ACID AND ELEMENTAL SELENIUM. ACUTE POISONING THROUGH INHALATION MAY OCCUR IN INDUSTRY (SELENIUM DUST & FUMES, SELENIUM DIOXIDE & HYDROGEN SELENIDE). |
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Environmental Fate |
There are no true deposits of selenium anywhere and it cannot economically be recovered from the earth directly. CONSTITUTES ABOUT 0.09 PPM OF THE EARTH'S CRUST. OCCURS IN NATURE USUALLY IN THE SULFIDE ORES OF THE HEAVY METALS; FOUND IN SMALL QUANTITIES IN PYRITE; IN THE MINERALS CLAUSTHALITE (PBSE), NAUMANNITE ((AG,PB)SE), TIEMANNITE (HGSE); IN SELENOSULFUR. Present in the major oceans and in inland waters resulting in the presence of selenium in drinking water. Selenium accompanies sulfur in volcanic effluents. Soils in the neighborhood of volcanos tend to have enriched amounts of selenium. Selenium concentration in geological materials: Igneous rocks 0.05 ppm; shales 0 - 0.6 ppm; sandstones 0.0 - 0.05 ppm; limestones 0.08 ppm; soils 0.2 ppm. The earth's crust is said to have an average selenium concentration of 0.03 of 0.08 ppm. Selenium is the most strongly enriched element in coal, being present as an organoselenium compound, a chelated species, or as an adsorbed element. Various industries discharge small amounts of selenium into the air, water, and soil of the immediate vicinity. The forms of selenium in soil depend on soil pH and redox. At equilibrium, most soil selenium should be elemental selenium. In areas of acid or neutral soils, the amount of biologically available selenium should steadily decline. The decline may be accelerated by active agricultural or industrial practices. In dry areas with alkaline soils and oxidizing conditions, elemental selenium and selenides in rocks and volcanic soils may oxidize sufficiently to maintain the availability of biologically active selenium. The contents of copper, molybdenum, sulphur, zinc, selenium, iron, manganese, and the copper/molybdenum ratio were determined in different native plant species from a mountain area of central southern Norway. The overall mean values and ranges (mg/kg DM) were copper: 6.0, 0.9-27.2; molybdenum: 0.25, 0.01-3.57; zinc: 77, 8-320; selenium: 0.05, less than 0.01-0.32; iron: 208, 15-2245; manganese: 338, 31-3784; sulfur: (g/100 g DM) 0.20, 0.03-0.56; copper/molybdenum: 79, 1-7955. Levels of the individual elements showed considerable variability, both between and within plant groups. Mineral contents were compared with the established requirements for sheep and cattle, the following conclusion being drawn. The levels of zinc, sulphur, iron, and manganese were found to be adequate for ruminants. The mean value for selenium in human milk has been reported to be 0.018 ppm with a range of 0.007 to 0.033 ppm. Inhalation, contact with eyes or skin, swallowed. Selenium, sodium selenite, sodium selenate, or selenium dioxide can affect the body if they are inhaled, if they come in contact with the eyes or skin, or if they are swallowed. The mean value for selenium in human milk has been reported to be 0.018 ppm with a range of 0.007 to 0.033 ppm. |
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Disposal |
At the time of review, criteria for land treatment or burial (sanitary landfill) disposal practices are subject to significant revision. Prior to implementing land disposal of waste residue (including waste sludge), consult with environmental regulatory agencies for guidance on acceptable disposal practices. Depending upon the industrial source, selenium occurs in either the particulate or the soluble form. For the former, physical treatment processes such as sedimentation or filtration are quite effective. Lime precipitation of soluble selenium has been shown to be ineffective. Alum coagulation/coprecipitation is somewhat effective, with removal efficiencies approaching 50% achievable. Iron coagulation/coprecipitation is capable of above 80% selenium removal, with efficiency increasing with decreasing pH. Activated carbon adsorption is not effective, except in one reported instance involving high pH pretreatment. The most efficient process for soluble selenium appears to be anion exchange. Preoxidation of the selenite to selenate is reported to enhance exchange capacity. Chemical Treatability of Selenium; Concentration Process: Chemical Precipitation; Chemical Classification: Metals; Scale of Study: Full Scale/Continous flow; Type of Wastewater Used: Domestic; Influent concentration: <2.5 ppb/ 6.5 ppb. Results of Study: 0% reduction with lime; (Lime dose of 350-400 ppm as calcium oxide at pH= 11.3). Chemical Treatability of Selenium; Concentration Process: Chemical Precipitation; Chemical Classification: Metal; Scale of Study: Pilot Scale; Type of Wastewater Used: Synthetic; Influent concentration: 100 ppb Results of Study: 75% reduction with ferric chloride; (3 coagulants used: 220 ppm of alum at pH= 6.4; 40 ppm of ferric chloride at pH= 6.2; 415 ppm of lime at pH= 11.5; chemical coagulation was followed by dual media filtration). |
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Atmosphere |
The atmospheric level of selenium in most urban regions ranges from 0.1 to 10 ng/cu m. A greater part of the atmospheric selenium is bound to fly ash and to suspended particles that contain from 1.4 to 11 ug/g and from 1 to 110 ug/g, respectively. Up to 90% of the selenium content in ambient air is emitted during the burning of fossil fuels. Selenium dioxide is formed during combustion of elemental selenium present in fossil fuels. |
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