Heavy metal
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Description
A heavy metal is an arbitrary term for some heavier metallic elements. The term is often loosely used to describe metals whose compounds are in some way harmful to the environment.
Detailed description
Wikipedia[1] has a good general article on the subject, without really defining it. The Oxford English Dictionary defines it simply as a metal with a high density. Encyclopaedia Britannica does not define it. Anecdotally, some refer the term to all metals with a higher atomic weight than iron (AW >56), but this definition would include zinc, which is less dense than iron. It is therefore perhaps prudent not to try to define it without including the context.
A few examples can show the extent of different types of heavy metals.
Gold
By any measure, gold is a heavy metal, but it is generally harmless to the environment because it does not form compounds readily, being a noble metal. That having been said, gold/potassium cyanide, used in electroplating, is highly toxic to most living organisms.
Copper
Copper is used extensively in electrical and electronic equipment. Copper salts are strong fungicides, algicides and bactericides. As such, their presence in waste water can reduce or stop the anaerobic decomposition of organic waste in sewage treatment plants or septic tanks. They are also toxic to aquatic life by destroying the bottom of the food chain. Copper salts form readily by exposure of the metal to air or water. This is one of the reasons behind the EU WEEE Directive which mandates recycling of end-of-life electrical and electronic equipment. Copper, as a trace element, is considered as essential to life.
Lead
This metal is one which often springs to mind when considering pollution. Tetraethyl lead is a highly toxic organometallic compound formerly widely used as an anti-knocking additive to volatile motor fuels (it still is in some developing countries). As motor traffic grew, so the quantity of lead, as an air pollutant, increased. Lead in water pipes, crystal glass and in ceramic glazes used in tableware also allowed lead to enter the human food chain. The largest use of lead today is in lead-acid accumulators, such as car batteries, and their end-of-life recycling is mandatory to protect the environment. On the other hand, the EU RoHS Directive banned the use of lead in most solder used for electrical and electronic assemblies; however, it is unlikely that this use is harmful to the environment or to human life and is therefore likely to be a needless regulation that has had severe economic effects on industry. Lead in the human body is a cumulative toxin for the central nervous system and retards mental development in children, hence the banning of paints containing lead in toys.
Mercury
Mercury is an insidious pollutant. It is the only common metal which is liquid at room temperature and with a significant vapour pressure, meaning that exposed mercury can slowly evaporate. It forms compounds easily and enters into the food chain at all levels. The most important single source of incidental mercury pollution is through the combustion of coal, notably in power stations[2] and coking plants for iron and steel production. Efforts are being made to reduce these emissions, but they cannot be eliminated. The Minamata catastrophe[3] was due to mercury pollution of industrial waste water entering the food chain, via fish, to humans. The metal is also banned under the EU RoHS Directive. Mercury is a cumulative poison that is destructive to the central and peripheral nervous systems and retards development. Note that fluorescent tubes and compact fluorescent lamps (CFLs, economy lamps) contain minute amounts of mercury and should be disposed of, without breaking them, in an adequate recycling centre.
