ABSTRACT
The term heavy metals (or trace metals ) is applied to the group of metals and semimetals ( metalloids) that have been associated with contamination and potential toxicity or ecotoxicity; it usually refers to common metals such as copper , lead , or zinc . However, the term is only loosely defined and there is no single authoritative definition (see Duffus, 2002). Some define a heavy metal as a metal with an atomic mass greater than that of sodium , whereas others define it as a metal with a density above 3.5-6 g cm-3. As mentioned above, the term is also applied to semimetals (elements that have the physical appearance and properties of a metal but behave chemically like a non-metal), such as arsenic , or nonmetals, such as selenium, presumably because of the hidden assumption that ‘heaviness’ and ‘toxicity’ are in some way identical. Despite the fact that the term heavy metals has no sound terminological or scientific basis, it is used here in the way it has been used in much of the scientific environmental literature, namely to refer to metals or semi-metals which meet the definitions given above. Common heavy metals include zinc (Zn), copper (Cu), lead (Pb), cadmium (Cd), mercury (Hg), chromium (Cr), nickel (Ni), tin (Sn), arsenic (As), and silver (Ag).
