268Some of these elements are actually necessary for humans in minute amounts (cobalt,
copper, chromium, manganese, nickel) while others are carcinogenic or toxic, affecting,
among others, the central nervous system (manganese, mercury, lead, arsenic), the
kidneys or liver (mercury, lead, cadmium, copper) or skin, bones, or teeth (nickel,
cadmium, copper, chromium).
Heavy metal pollution can arise from many sources but most commonly arises from the
purification of metals, e.g., the smelting of copper 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. Currently, plants or
microrganisms are tentatively used to remove some heavy metals such as mercury.
Plants which exhibit hyper accumulation can be used to remove heavy metals from soils
by concentrating them in their bio matter. Some treatment of mining tailings has occurred
where the vegetation is then incinerated to recover the heavy metals. One of the largest
problems associated with the persistence of heavy metals is the potential for
bioaccumulation and biomagnification causing heavier exposure for some organisms than
is present in the environment alone. Coastal fish (such as the smooth toadfish) and
seabirds (such as the Atlantic Puffin) are often monitored for the presence of such
contaminants.
Living organisms require varying amounts of "heavy metals". Iron, cobalt, copper,
manganese, molybdenum, and zinc are required by humans. Excessive levels can be
damaging 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.
Toxic Metals
Toxic metals are metals that form poisonous soluble compounds and have no biological
role, i.e. are not essential minerals, or are in the wrong form. Often heavy metals are
thought as synonymous, but lighter metals also have toxicity, such as beryllium, and not
all heavy metals are particularly toxic, and some are essential, such as iron. The definition
may also include trace elements when considered in abnormally high, toxic doses. A
difference is that there is no beneficial dose for a toxic metal with no biological role.
Toxic metals sometimes imitate the action of an essential element in the body, interfering
with the metabolic process to cause illness.
Many metals, particularly heavy metals are toxic, but some heavy metals are essential,
and some, such as bismuth, have a low toxicity. Most often the definition includes at least
cadmium, lead, mercury and the radioactive metals. Metalloids (arsenic, polonium) may
be included in the definition.