Geoscience Reference
In-Depth Information
Table 3.2
Characteristics of major radionuclides that occur in soil
Isotope
Half-life (yr)
Principal radiation
Main occurrence
14
5.7 9 10 3
C
b
Natural and nuclear reactor
40 K
1.3 9 10 9
b
Natural
90 Sr
28
b
Nuclear reactor
134 Cs
2
b, c
Nuclear reactor
137 Cs
30
b, c
Nuclear reactor
239 Pu
2.4 9 10 4
a, X-rays X-rays, nuclear reactor
Reprinted from Zhu and Shaw ( 2000 ). Copyright 2002 with permission of Elsevier
3.5 Heavy Metals and Metalloids
The term heavy metal refers to any metallic chemical element that has a relatively
high density (usually specific density of more than 5 g/mL) and is toxic or poi-
sonous at low concentrations. Examples of heavy metals include arsenic (As),
cadmium (Cd), chromium (Cr), mercury (Hg), lead (Pb), and thallium (Tl). The
sources, uses, and environmental effects of several exemplary specific metals are
discussed briefly here.
Heavy metals are natural components of the earth's crust. They cannot be
degraded or destroyed. To a small extent, they generally enter human and animal
bodies via food, drinking water, and air. However, exposure to increasingly higher
amounts of pollutants from this group usually is due to technological progress that,
in many cases, is linked to the ability to extract and process metals. Therefore,
already several thousands of years ago, polluted zones were identified, and some
effects of metal poisoning were known during the period of development of methods
for use of metals (Nariagu 1996 ; Jarup 2003 ; Maskall and Thornton 1998 ).
In trace amounts, some heavy metals (e.g., copper, nickel, selenium, zinc) are
essential to all organisms, to accomplish specific catalytic functions. However, at
levels exceeding these requirements, all metals can disturb the metabolism by
binding nonspecifically to biomolecules and inflicting oxidative damage, due to
their ability to catalyze redox reactions. This can result in the deactivation of
essential enzymatic reactions, damage to cellular structures (especially mem-
branes), and DNA modification (mutagenesis). In humans, exposure to high levels
of metals can cause acute toxicity symptoms, while long-term exposure to lower
levels can trigger allergies and even cancers.
3.5.1 Arsenic
Arsenic is historically the poison of choice for many murders, both in fiction and
reality (e.g., Christie 1924 ; CNN 1998 ). The element is considered a metalloid
(having both metallic and nonmetallic properties) and is widely distributed in the
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