Molybdenum

Notable Characteristics Molybdenum is a transition metal. The pure metal is silvery white in color and very hard, and has one of the highest melting points of all pure elements. In small quantities, molybdenum is effective at hardening steel. Molybdenum is important in plant nutrition, and is found in certain enzymes, including xanthine oxidase. Applications Over 2/3 of all molybdenum is used in alloys. Molybdenum use soared during World War I, when demand for tungsten made tungsten scarce, and high strength steels were at a premium. Molybdenum is used to this day in high strength alloys, and high temperature steels. Special molybdenum containing alloys, such as the Hastelloys[?] ®, are notably heat resistant and corrosion resistant. Molybdenum is used in aircraft and missile parts, and in filaments. Molybdenum finds use as a catalyst in the petroleum industry, especially in catalysts for removing organic sulfurs from petroleum products. Mo-99 is used in the nuclear isotope industry. Molybdenum oranges are pigments, ranging from red-yellow to a bright red orange, used in paints, inks, plastics, and rubber compounds. Molybdenum disulphide is a good lubricant, especially at high temperatures. Molybdenum is also used in some electronic applications, as the conductive metal layers in thin-film transistors (TFTs). History Molybdenum (from the Greek molybdos meaning "lead-like") is not found free in nature, and the compounds that can be found were, until the late 18th century, confused with compounds of other elements, such as carbon or lead. In 1778 Carl Wilhelm Scheele was able to determine that molybdenum was separate from graphite and lead, and was able to isolate the oxide of the metal from molybdenite[?]. In 1782 Hjelm[?] was able to isolate an impure extract of the metal by reducing the oxide with carbon. Molybdenum was little used and remained in the laboratory until the late 19th century. Then a French company, Schneider and Co[?], tried molybdenum as an alloying agent in armor plate and noted its useful properties. Occurrence Though molybdenum is found in such minerals as wulfenite[?] (MoO₄Pb) or powellite (CaMnO₄), the main commercial source of molybdenum is molybdenite (MoS₂). Molybdenum is mined directly, and is also recovered as a by-product of copper mining. Molybdenum is present in ores from 0.01% to about 0.5%. About half of the world's molybdenum is mined in the United States. Biological Role There is a trace requirement for molybdenum in plants, and soils can be barren due to molybdenum deficiencies. Plants and animals generally have molybdenum, present in amounts of a few parts per million. In plants, molybdenum is involved in the pathways of nitrogen fixation and nitrate reduction, and in animals, molybdenum is involved in the pathways of purine degredation and formation of uric acid. In some animals, adding a small amount of dietary molybdenum enhances growth. Isotopes Molybdenum has six stable isotopes and almost two dozen radioisotopes, the vast majority of which have half lives measured in seconds. Mo-99 is used in sorpation generators[?] that are used to create Tc-99 for the nuclear isotope industry. The market for Mo-99 products is estimated to be on the order of $100 million US dollars a year. Precautions Molybdenum dusts and molybdenum compounds, such as molybdenum trioxide and water soluble molybdates, may have slight toxicities if inhaled or ingested orally. Laboratory tests suggest, compared to many heavy metals, that molybdenum is of relatively low toxicity. Acute toxicity in humans is an unlikely because the dose required would be exceptionally large. There is the potential for molybdenum exposure in mining and refining operations, as well as the chemical industry, but to date, no instance of harm from this exposure has been reported. Though water-soluble molybdenum compounds can have a slight toxicity, those that are insoluble, such as the lubricant molybdenum disulfide, are considered to be nontoxic. However, environmental chains of events can end in serious molybdenum-related health consequences. In 1996 in Sweden an increase in acid rain near Uppsala caused a depletion in the natural foods of moose in nearby rural areas; this caused the moose to venture into the fields of oat farmers who had been heavily liming their soil to compensate for the effect of the acid; the lime caused changes to the levels of cadmium and other trace metals in the soil, causing the oat crops to uptake trace molybdenum in large quantities; ingestion of the oats by hundreds of moose brought on a severe disturbance in the ratio of molybdenum to copper in their livers, which caused emaciation, hair discoloration, ulcers, diarrhea, convulsions, blindness, osteoporosis and finally heart failure.

OSHA regulation specify that maximum exposure in a 8 hour day, 40 hour week to molybdenum be 15 milligrams per cubic meter. NIOSH recommended exposure limit is 5000 mg per cubic meter.

External links

• WebElements.com - Molybdenum (http://www.webelements.com/webelements/elements/text/Mo/key)
• EnvironmentalChemistry.com - Molybdenum (http://environmentalchemistry.com/yogi/periodic/Mo)
• The International Molybdenum Association (http://www.imoa.info/)
• Database of Molybdenum in the Environment (http://www.imoa.info/imoadata/PHYSI/SUMMARY.HTM)