Cadmium (Cd)
Cadmium (Cd) is a rare metal, classified as element 48 in the periodic table, and it shares similarities with zinc in both chemical and physical properties. This lustrous, silver-white metal is primarily obtained as a by-product from the smelting of zinc, lead, and copper ores, particularly from sulfide deposits. Major producers include China, South Korea, Canada, Kazakhstan, and Japan. Cadmium is utilized mainly in batteries, especially nickel-cadmium types, as well as in alloys, coatings, pigments, and plastic stabilizers.
However, cadmium is highly toxic, posing significant health risks, including kidney disease, bone softening, and potential fetal damage. Its accumulation in the body and environmental contamination have raised concerns, leading to stringent regulations on its presence in drinking water and aquatic environments. Historically, cadmium was first isolated in 1817 and has been linked to health issues such as itai-itai disease in Japan due to industrial pollution. Efforts to recycle cadmium and seek alternative materials are ongoing to mitigate its environmental impact while maintaining its industrial applications, particularly in aerospace and automotive sectors.
Cadmium (Cd)
Where Found
Cadmium, a rare element, does not occur in nature in its elemental form. Its ore deposits are of insufficient concentration to permit direct mining. Cadmium is found in sulfides of zinc, lead, and copper and is typically obtained as a by-product of zinc. Main producers are China, South Korea, Canada, Kazakhstan, and Japan.
Primary Uses
Cadmium is used principally in batteries and in alloys. It is also used in coatings and plating, pigments, and stabilizers for plastics and similar synthetic products.
Technical Definition
Cadmium (abbreviated Cd), atomic number 48, belongs to Group IIB of the periodic table of the elements and resembles zinc in its chemical and physical properties. It has eight stable isotopes and an average molecular weight of 112.40. Pure cadmium is a lustrous, silver-white, malleable metal. Its specific gravity is 8.65, its melting point is 321° Celsius, and its boiling point is 765° Celsius.
Description, Distribution, and Forms
Cadmium is a rare element that is chemically and physically similar to zinc. Its concentration in the lithosphere is 0.1 to 0.2 gram per metric ton, making it the sixty-seventh most abundant element. The few known cadmium minerals include greenockite, hawleyite, cadmoselite, monteponite, otavite, and saukovite, none of which occurs in commercial deposits. Cadmium is concentrated principally in sulfide deposits. It frequently substitutes for zinc in zinc minerals, where it occurs as an impurity or a surface coating; it is found to a much lesser extent with lead and copper.
Cadmium is softer than zinc, is capable of taking a high polish, and alloys readily with other metals. Its characteristics make it particularly useful to the alloy, plating, and coating industries, some of its chief consumers. The United States produced approximately 750 metric tons of cadmium in 2008, and total world refinery production was about 21,000 metric tons. Cadmium is a toxic element; its toxicity has led to a search for alternative industrial materials and has heightened efforts to recycle cadmium-containing products.
Cadmium-containing zinc deposits occur in many diverse geological settings. Commercially, strata-bound deposits are the most important source of cadmium. Internationally, chief producers of cadmium are China, South Korea, Canada, Kazakhstan, and Mexico.
The most common cadmium minerals are the sulfides hawleyite and greenockite. Typically these occur only as impurities or surface incrustations in zinc ores. Zinc sulfide ores such as sphalerite and wurtzite are the main commercial source of cadmium.
History
Cadmium was isolated and identified in 1817 by Friedrich Stromeyer. One of the earliest uses of cadmium sulfides was as a paint pigment. Commercial production of cadmium as a by-product of zinc smelting began in the nineteenth century. Cadmium was first produced in the United States on a pilot-plant scale; during World War I, production increased rapidly, and it continued to rise in the following decades.
Cadmium is toxic to almost all human body systems and plays no known part as a trace element in human metabolism. Cadmium elimination proceeds slowly enough that the element can accumulate in the body over time, with storage primarily in the kidneys and liver. Chronic exposure can lead to irreversible kidney disease and fluid in the lungs as well as to osteomalacia, an extremely painful softening of the bones. Cadmium can also induce hypertension and can cross the placenta to cause fetal damage. Victims of acute exposure may exhibit symptoms similar to those of food poisoning.
Industrial pollution has introduced cadmium into surface water, groundwater, and the air. Zinc mine tailings can be a source of environmental cadmium in regions where transporting waters are acidic. In Western Europe, where landfilling is less common than in the United States, incineration of plastics is a potential source of cadmium release to the atmosphere.
In the late 1960s, itai-itai (literally “ouch-ouch”) disease was diagnosed in several localities in Japan. The disease, characterized by osteomalacia, multiple bone fractures, and kidney dysfunction, was linked to elevated levels of cadmium in body tissues and bone. High concentrations of cadmium derived from mine tailings, metal smelters, and other industrial sources were found in soil and drinking water.
In the United States, the 1974 Safe Drinking Water Act set the maximum allowable concentration for cadmium in drinking water at 10 micrograms per liter. Regulatory criteria have also been established for aquatic environments: 5.0 micrograms per liter for salt water, and between 0.4 to 12.0 micrograms per liter for fresh water, depending on water’s hardness and the sensitivity of resident fish species.
Environmental regulations based on cadmium’s toxicity have led to a search for alternative materials. Cadmium plating is still necessary where surface characteristics of the coating are critical, notably in aircraft parts. Recycling cadmium-containing products and recovering the element for reuse can diminish its release into the environment through landfilling or incineration.
Obtaining Cadmium
Cadmium is entirely a by-product metal. It is obtained principally from the smelting and refining of sulfide ores of zinc, lead, and copper. Cadmium can be volatilized from the ores, recovered from the dust and fumes produced during ore roasting and sintering, or precipitated from electrolytic refining slimes.
Uses of Cadmium
Cadmium is a key component of alkaline nickel-cadmium batteries, accounting for 83 percent of world consumption. Although its use in consumer electronics has been declining as a result of the more popular lithiumion technologies, nickel-cadmium batteries are seeing new industrial applications, including electric vehicles and photovoltaic (solar-energy) systems.
Cadmium is also an important element in alloys. When alloyed with nickel or with silver and copper, it forms a high-pressure antifriction metal for automobile bearings. It hardens copper and makes silver resistant to tarnish. Cadmium plating forms a thin, rustless surface alloy, especially on iron and steel; it is electroplated onto vehicle and aircraft parts such as bolts, nuts, and locks to make them corrosion-resistant. It also provides an adhering bond between iron and other plating metals. Cadmium compounds are used in chemicals, photographic materials, television picture tubes, rubber, soaps, textile printing, and fireworks. Cadmium serves as a stabilizer for plastics and similar synthetic products. Cadmium sulfide forms a durable yellow pigment used in paints, glass, and enamels.
Bibliography
Adriano, Domy C. “Cadmium.” In Trace Elements in Terrestrial Environments: Biogeochemistry, Bioavailability, and Risks of Metals. 2d ed. New York: Springer, 2001.
Bhattacharyya, M. H., et al. “Biochemical Pathways in Cadmium Toxicity.” In Molecular Biology and Toxicology of Metals, edited by Rudolfs K. Zalups and James Koropatnick. New York: Taylor & Francis, 2000.
Dobson, S. Cadmium: Environmental Aspects. Geneva, Switzerland: World Health Organization, 1992.
Greenwood, N. N., and A. Earnshaw. “Zinc, Cadmium, and Mercury.” In Chemistry of the Elements. 2d ed. Boston: Butterworth-Heinemann, 1997.
Khan, Nafees A., and Samiullah, eds. Cadmium Toxicity and Tolerance in Plants. Oxford, England: Alpha Science, 2006.
Massey, A. G. “Group 12: Zinc, Cadmium, and Mercury.” In Main Group Chemistry. 2d ed. New York: Wiley, 2000.
Nordberg, G. F., R. F. M. Herber, and L. Alessio, eds. Cadmium in the Human Environment: Toxicity and Carcinogenicity. New York: Oxford University Press, 1992.
Scoullos, Michael J., ed. Mercury, Cadmium, Lead: Handbook for Sustainable Heavy Metals Policy and Regulation. Boston: Kluwer Academic, 2001.
Natural Resources Canada.
U.S. Geological Survey.