Page 140 - Soil and water contamination, 2nd edition

7

Heavy metals

7.1 INTRODUCTION

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
-3
sodium , whereas others define it as a metal with a density above 3.5–6 g cm . 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 non-
metals, 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).
At their natural concentrations, many metals play an essential role in biochemical
processes and are thus required in small amounts by most organisms for normally healthy
growth (e.g. Zn , Cu , Se, Cr ). Other metals, however, are not essential and do not cause
deficiency disorders if absent (e.g. Cd , Pb , Hg , Sn, and the semi-metal As). Figure 7.1
shows typical dose–response curves for these two types of trace metals . If ingested in
excessive quantities, virtually all heavy metals are toxic – especially to animals and humans –
although organisms are also able to adapt themselves, at least partly, to increased levels of
metals. Most heavy metals accumulate in organism tissues (bioaccumulation ) and as they
are transferred through the food chain (biomagnification ). Metals generally produce their
toxicity by forming complexes with organic compounds (ligands ). The modified molecules
lose their ability to function properly, causing the affected cells to malfunction or die. Metals
commonly bind to biological compounds containing oxygen, sulphur, and nitrogen , which
may inactivate certain enzyme systems. This is especially true for enzymes that are directly or
indirectly involved in ATP production. In acute poisoning, large excesses of metal ions can
disrupt membrane and mitochondrial function and the generation of free radicals . In most
cases this leads to general weakness and malaise.
In the environment, most heavy metals are present as cations , though some semi-metals
3-
may occur as oxyanions (e.g. arsenate AsO ). Heavy metals occur naturally in the Earth’s
4
crust as impurities isomorphously substituted for various macroelement constituents in the
lattices of many primary and secondary minerals. The heavy metal content varies greatly both
within and between different types of rocks. Typical ranges of heavy metal concentrations
in major igneous and sedimentary rock types, as well as in fresh water, are given in Table
7.1. The maximum concentrations of trace elements are commonly found in areas near ore

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