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182 A. Branzini and M.S. Zubillaga

its compounds have multifarious industrial uses. They are used in the electroplating
industry as anticorrosive and antibiofouling agents, in steel production, automobile
manufacturing, and catalytic manufacture, and in the production of chromic acid
and specialty chemicals. These anthropogenic activities produce general Cr con-
tamination in the environment and have increased its bioavailability and mobility
(Shanker et al. 2005). Among the factors that affect the Cr speciation in soil and
water and its uptake into animals and plants include organic matter content, ferrous
ion content, redox state, and pH (Kotas and Stasicka 2000). However, Cr is in
general not bioaccumulated and there is no increase in concentration of the metal in
food chains. In the natural environment, chromium occurs as two oxidation states or
valences: chromium (III) and chromium (VI). The stable forms of Cr are the
trivalent Cr(III) and the hexavalent Cr(IV) species, although there are various
other valence states which are unstable and short-lived in biological systems.
There is a great difference between Cr(III) and Cr(VI) with respect to toxicological
and environmental properties, and they must always be considered separately.
Cr(III) is less mobile, less toxic than Cr(VI), and is mainly found bound to organic
matter in soil (Becquer et al. 2003). In general, chromium (VI) is favored by higher
pH, aerobic conditions, low amounts of organic matter, and the presence of
manganese and iron oxides which oxidize chromium (III). Cr(III) is an essential
nutrient for animals and humans in amounts of 50–200 μg per day, being necessary
for the metabolism of insulin. On the other hand, Cr(VI) has been demonstrated to
have a number of adverse effects ranging from causing irritation to cancer. Effects
in humans occupationally exposed to high levels of chromium or its compounds,
primarily Cr(VI) by inhalation, may include irritating respiratory effects, possible
circulatory effects, effects on stomach and blood, liver and kidney effects, and
increased risk of death from lung cancer. Although Cr is present in all plants, it has
not been proved to be an essential element for plants. Several factors affect the
availability of Cr for the plant, including the soil pH, interactions with other
minerals or organic chelating compounds, and carbon dioxide and oxygen
concentrations. Little Cr is translocated from the site of absorption; however, the
chelated form is transported throughout the plant. Chromium in high concentrations
can be toxic for plants, and the main feature of Cr intoxication is chlorosis, which is
similar to iron deficiency.

10.1.7 Mercury

Mercury is a peculiar metal. Most conspicuous is its fluidity at room temperature,
but more important for the possible exposure of humans and the environment to
mercury are two other properties:
• Under reducing conditions in the environment, ionic mercury changes to the
uncharged elemental mercury, which is volatile and may be transported over
long distances by air.