Cadmium is a very soft, silvery-white metallic element. Its atomic number is 48, with the atomic symbol Cd. It is so soft that it can be cut with a knife. Cadmium has many chemical similarities to zinc, but is less reactive with acids than is zinc. Cadmium is clearly toxic to many animal species, and during the past few decades has become familiar to the public mainly due to its undesirable presence in fertilizers and elsewhere, rather than for its positive industrial applications. Metallic cadmium is rarely used industrially in pure form.
Cadmium was discovered in 1817 by the German chemist Friedrich Strohmeyer. He noticed that some samples of zinc carbonate (calamine) changed color when heated. Pure calamine, however, did not. He surmised there must be an impurity present and eventually isolated it by heating and reducing the zinc carbonate. What he isolated was cadmium metal. Strohmeyer coined the name cadmium, derived from the Latin word cadmia which means calamine.
Previous Element: Silver
Next Element: Indium
|Phase at Room Temp.||solid|
|Melting Point (K)||594.1|
|Boiling Point (K)||1038|
|Heat of Fusion (kJ/mol)||6.109|
|Heat of Vaporization (kJ/mol)||107|
|Heat of Atomization (kJ/mol)||112|
|Thermal Conductivity (J/m sec K)||96.9|
|Electrical Conductivity (1/mohm cm)||146.413|
|Number of Isotopes||44 (8 natural)|
|Electron Affinity (kJ/mol)||---|
|First Ionization Energy (kJ/mol)||867.7|
|Second Ionization Energy (kJ/mol)||1631.4|
|Third Ionization Energy (kJ/mol)||3616.2|
|Atomic Volume (cm3/mol)||13|
|Ionic Radius2- (pm)||---|
|Ionic Radius1- (pm)||---|
|Atomic Radius (pm)||151|
|Ionic Radius1+ (pm)||---|
|Ionic Radius2+ (pm)||109|
|Ionic Radius3+ (pm)||---|
|Common Oxidation Numbers||+2|
|Other Oxid. Numbers||---|
|In Earth's Crust (mg/kg)||1.50x10-1|
|In Earth's Ocean (mg/L)||1.1x10-4|
|In Human Body (%)||0.00007%|
|Regulatory / Health|
|OSHA Permissible Exposure Limit (PEL)||TWA: 0.005 mg/m3|
|OSHA PEL Vacated 1989||TWA: 0.005 mg/m3|
|NIOSH Recommended Exposure Limit (REL)||IDLH: 9 mg/m3|
Mineral Information Institute
Jefferson Accelerator Laboratory
Because cadmium is located just below zinc and above mercury in the Periodic Table, its physical and chemical properties are rather similar to those of zinc, and to a lesser degree, mercury. Most cadmium in nature occurs as an atomic substitution for zinc in zinc minerals, usually making up less than one percent of the mineral. Only a few relatively pure cadmium minerals are known. The best known of these is the mineral greenockite (cadmium sulfide, CdS), but even this mineral forms rare and rather small crystals.
In addition, cadmium can occur as an impurity in phosphate minerals. Some natural phosphate ores contain several hundred parts per million (ppm) of cadmium, and are thus undesirable to use as fertilizers.
Most cadmium used in industry is recovered from sphalerite (zinc sulfide), the principle ore of zinc where cadmium atoms replace some of zinc atoms in the sphalerite. On a worldwide basis, zinc ores around the world average about 1/400th as much cadmium as zinc. Although some zinc deposits have a higher cadmium/zinc ratio than others, those countries producing zinc from zinc ores also have the potential to produce significant quantities of cadmium. The cadmium is removed when zinc metal is purified in a refinery.
Cadmium is therefore produced in countries where zinc is refined, not necessarily in the countries where zinc ore is mined. China, Japan, and Korea are the world’s largest producers, with Mexico, the United States, the Netherlands, India, the United Kingdom, Peru, and Germany next. About 15 other countries produce smaller amounts.
Some cadmium is recovered from the recycling of nickel-cadmium batteries, which is required by law in some countries so that the cadmium is not discarded into the environment.
Movement in the Environment
Approximately 30,000 tons of cadmium are released to the environment annually, about half from the weathering of rocks into surface water bodies. Forest fires and volcanoes also release cadmium to the atmosphere. Release of cadmium from human activities is estimated in the range of 4000 to 13,000 tons per year, with major contributions from mining activities, burning of fossil fuels (e.g., coal fired electrical plants) and from the burning of household waste. Because of regulations, only small amounts currently enter water from the disposal of wastewater in Western countries; however, cadmium releases to water are significant in such countries as China, India, Russia and Brazil. Fertilizers often contain some cadmium that can enter the soil when fertilizers are applied to crops. Cadmium can also enter the soil or water from spills or leaks at hazardous waste sites, if large amounts of dissolved cadmium are present at the site. The form of cadmium at these sites is important since many forms do not easily dissolve in water.
Cadmium that is attached to fine particulate matter can enter the air and travel a considerable distance before settling to the Earth surface as dust, rain or snow. The cadmium metal itself, of course, does not decompose in the environment, but it can alter its chemical form. Most forms of cadmium remain for a long residence time in the same place where they first entered the environment. Some forms of cadmium that enter natural waters can bind to soil, and some portion will remain in the water. Some forms of cadmium in soil can enter water or be taken up by plants. Fish, plants, and animals can take some forms of cadmium into their bodies from air, water, or food. Cadmium can change molecular form in the human or other faunal body, but it also remains in the body for a very long time (on the order of years).
Exposure to Cadmium
Tobacco smoke and certain food products are the chief sources of cadmium exposure for the general population. Average cadmium levels in foods range from two to 40 parts of cadmium per billion (ppb). Lowest levels are in fruits and beverages, and highest levels are in leafy vegetables and potatoes. Air pollutant levels of cadmium in USA cities are low, ranging from less than one to 40 nanograms per cubic meter. Concentrations greater than 40 ng/m³ may occur in urban areas with high levels of air pollution from the burning of fossil fuels. The level of cadmium in most drinking water supplies is less than one ppb, well below the USA drinking water standard of 50 ppb. Levels in drinking water, however, may vary greatly depending on local conditions. The average level of cadmium in relatively unpolluted soil is about 250 ppb. At hazardous waste sites, cadmium levels have been measured in soil at about 4 parts cadmium per million. In the USA, the average person ingests food with about 30 micrograms (µg) of cadmium in it each day, but only about one to three µg per day of that cadmium from food is absorbed and enters the body. Cadmium exposure from tobacco is also a serious health concern. Smokers may double their daily intake of cadmium compared with nonsmokers. Each cigarette may contain from 1 to 2 µg of cadmium, and 40–60% of the cadmium in the inhaled smoke can pass through the lungs into the body. This means that smokers may take in an additional one to three µg of cadmium into their body per day from each pack of cigarettes smoked.
The single most important use of cadmium is in the production of nickel-cadmium ("Ni-Cad") batteries. About three quarters of the cadmium consumed annually is used to make batteries. Nickel-cadmium batteries are rechargeable and have found wide use in cellular phones, hand-held cordless power tools, cameras, portable computers, and a wide variety of household products. These applications account for the majority of the Ni-Cad batteries produced. The remaining represents batteries used for emergency power supplies in hospital rooms, for emergency lights, telephone exchanges, etc.
Cadmium is useful in a small number of other applications. Cadmium sulfide (also called cadmium yellow) is used as a paint pigment. Cadmium is used to make low-temperature melting alloys, such as solder and Wood’s Metal for indoor sprinkler systems. The latter is an alloy of 50% Bi, 25% Pb, 12.5% Sn, and 12.5% Cd which melts at about 160 degrees Fahrenheit, the temperature of a very hot shower. Cadmium compounds are used both in black and white and color television tubes. It is used as a stabilizing compound in plastics.
Cadmium also has the physical property of being able to absorb neutrons. As a result, it is used in nuclear reactor control rods to dampen the nuclear reaction and keep the fission reactions under control.
China is the world’s largest consumer of cadmium, primarily for manufacturing batteries. Worldwide consumption of cadmium is stable or slightly declining, as its use is becoming more restricted due to environmental rules.
Soluble compounds of cadmium are poisonous, although the metallic and the sulfide forms are not soluble and therefore not highly toxic.
Substitutes and Alternative Uses
Due to the poisonous nature of cadmium, small Ni-Cad batteries are being replaced by lithium-ion batteries and nickel-metal hydride batteries. This will obviously reduce cadmium consumption as this replacement increases. Presently, lithium-ion batteries are more expensive than Ni-Cad, which will affect the pace at which this change occurs. Cerium sulfide can be used in place of cadmium sulfide as a paint pigment.
World reserves of cadmium are more than adequate for the foreseeable future, especially since the amount of cadmium produced depends on zinc smelter output, not the market for cadmium.
References and Further Reading
- Kirk-Othmer Encyclopedia of Chemical Technology. Cadmium. (4-th edition ed.). New York: John Wiley & Sons. 1994.
- Arnold F.Holleman, Egon Wiberg and Nils Wiberg, Nils; (1985). Cadmium (in German). Lehrbuch der Anorganischen Chemie (91–100 ed.). Walter de Gruyter. pp. 1056–1057. ISBN 3-11-007511-3.
- Agency for Toxic Substances and Disease Registry. Case Studies in Environmental Medicine (CSEM) Cadmium.
- F.Albert Cotton (1999). Survey of Transition-Metal Chemistry. Advanced Inorganic Chemistry (6 ed.). John Wiley and Sons. pp. 633. ISBN 0471199575.
- Common Minerals and Their Uses, Mineral Information Institute.
- More than 170 Mineral Photographs, Mineral Information Institute.