Heavy metals are generally defined as metals with relatively high densities,
atomic weights, or atomic numbers. These metals include transition metals
such as lanthanides and actinides as well as semi-metals (metalloids) such as
Arsenic and Antimony. The term heavy metal can also be referred to elements of
the periodic table of elements with atomic number 21 or higher (eg. Zinc). This
term heavy metal mainly came about
with dialogues about pollutants that were released to the environment in the
form of air, water or soil contaminants. Some heavy metals are chiefly known
for their toxicity to man and other organisms. On the other hand, several of them
(eg. Zinc, Iron, Copper, Chromium, Cobalt) are vital for metabolic function in
many organisms in their right quantities (Ariel
et al, 1973).

 

Despite
their significance (some of the heavy metals) as micronutrients for animals,
plants and many microbes, all heavy metals prove toxic to living organisms
through metabolic interference and genetic mutations. This usually is based on
the route, and dose of these heavy metals that enters the organism. This toxic
impacts span from fitness reduction, through to reproductive interference,
carcinoma and in extreme exposure, being lethal.

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Various
ways for classifying heavy metals have been adopted and usually been based on
atomic number, toxicity levels among other factors; meanwhile, the use of this
term has centered around the treatment of certain metallic elements that have
effect on animals when released in high concentrations into the environment.
Based on the extent of relevance, the leading heavy metals of consideration in
environmental science are Silver, Nickel, Zinc, Copper, Manganese, Mercury,
Cadmium, Chromium. In some cases, lanthanides and actinides are excluded from
heavy metals while mostly transition metals (elements with incomplete d-shell)
are considered (Baird and Cann, 2012).

 

Heavy metals mostly show good electrical and heat
conductivities in their natural pure state. Some literature refers to
“heavy” metal to a metal on the periodic table with specific gravity ranging
from 3.5-6 g/cm3, rather than the atomic number as shown above. The
transition metals which are mostly considered as heavy metals show varying
chemical species or oxidation states and can form variety of complexes with
some chemical species or ligands. All of the transition metals have two or more
oxidation states (Brady and Holum, 1995).

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