Heavy metals
A heavy metal is a member of an ill-defined subset of elements that exhibit metallic properties, which would mainly include the transition metals, some metalloids, lanthanides, and actinides. Many different definitions have been proposed—some based on density, some on atomic number or atomic weight, and some on chemical properties or toxicity.[1] The term heavy metal has been called "meaningless and misleading" in a IUPAC technical report due to the contradictory definitions and its lack of a "coherent scientific basis".[1] As discussed below, depending on context, heavy metal can include elements lighter than carbon and can exclude some of the heaviest metals. One source defines "heavy metal" as "... common transition metals, such as copper, lead, and zinc. These metals are a cause of environmental pollution (heavy-metal pollution) from a number of sources, including lead in petrol, industrial effluents, and leaching of metal ions from the soil into lakes and rivers by acid rain."[2]
Relationship to living organisms
Living organisms require varying amounts of "heavy metals." Iron, cobalt, copper, manganese, molybdenum, and zinc are required by humans. Excessive levels can be detrimental to the organism. Other heavy metals such as mercury, plutonium, and lead are toxic metals that have no known vital or beneficial effect on organisms, and their accumulation over time in the bodies of animals can cause serious illness. Certain elements that are normally toxic are, for certain organisms or under certain conditions, beneficial. Examples include vanadium, tungsten, and even cadmium.[3][4]
Heavy metal pollution
Heavy metal pollution can arise from many sources but most commonly arises from the purification of metals, e.g., the smelting of ores and the preparation of nuclear fuels. Electroplating is the primary source of chromium and cadmium. Through precipitation of their compounds or by ion exchange into soils and muds, heavy metal pollutants can localize and lay dormant. Unlike organic pollutants, heavy metals do not decay and thus pose a different kind of challenge for remediation.
A well documented environmental disaster associated with heavy metals is the Minamata disease cause by mercury pollution.
In medicine
In medical usage, heavy metals are loosely defined[1] and includes all toxic metals irrespective of their atomic weight: "heavy metal poisoning" can possibly include excessive amounts of iron, manganese, aluminium, or beryllium (the fourth lightest element) or such a semimetal as arsenic as well as heavy metals. This definition excludes bismuth, the heaviest of stable elements, because of its low toxicity.
"Hazmats"
Heavy metals in a hazardous materials (or "hazmat") setting are for the most part classified in "Misc." on the UN model hazard class, but they are sometimes labeled as a poison when being transported.[citation needed]
See also
- Metallicity
- Soil contamination
- Metal
References
- a b c John H. Duffus ""Heavy metals" a meaningless term? (IUPAC Technical Report)" Pure and Applied Chemistry, 2002, Vol. 74, pp. 793-807. doi:10.1351/pac200274050793
- A Dictionary of Chemistry. Oxford University Press, 2000. Oxford Reference Online.Oxford University Press.
- Lane TW, Morel FM (2000). "A biological function for cadmium in marine diatoms". Proc. Natl. Acad. Sci. U.S.A. 97 (9): 4627–31. doi:10.1073/pnas.090091397. PMID 10781068.
- Lane TW, Saito MA, George GN, Pickering IJ, Prince RC, Morel FM (2005). "Biochemistry: a cadmium enzyme from a marine diatom". Nature 435 (7038): 42. doi:10.1038/435042a. PMID 15875011.
Source: adapted by the editor from Wikipedia, the free encyclopedia; from the article "Heavy metals". Image Credit.
