Page 148 - Soil and water contamination, 2nd edition

Heavy metals 135

3+
+
0
2+
solutions the reduced forms of chromium are Cr , CrOH , Cr(OH) , Cr(OH) , and
3
2
-1
-
Cr(OH) . Chromium hydroxide (Cr(OH) ) has a minimum solubility of about 5 μg l
4 3
at about pH 9.0. Chromium(III) can form stable complexes with organic ligands at pH
values as high as 8.5. These complexes form slowly, and once formed, are difficult to break.
The presence of organic complexes can significantly influence the concentration of total
dissolved Cr(III) in the soil solution. Chromium(VI) is not thermodynamically stable in
soils except in alkaline, oxidising environments, and is readily reduced to Cr(III). Hexavalent
2-
2-
chromium may be present as the anions dichromate Cr O and chromate CrO (Hem,
2 7 4
1989). Chromium(VI) is considerably more soluble than chromium(III). Concentrations of
chromium in natural waters that have not been affected by waste disposal are commonly less
-1
than 6 μg l (see Table 7.1), although under exceptional conditions they may increase over
3+
1 to 2 orders of magnitude. Dissolution and oxidation of Cr under basic conditions may
cause the groundwater to be enriched in chromium(VI) up to concentrations greater than
-1
100 μg l .
There is no evidence that chromium has any physiological function in plants.
-1
Chromium is toxic to agronomic plants at about 5 to 100 μg g of available chromium
in soil (Hossner et al., 1998). Plants absorb chromium(VI) better than chromium(III),
whereas chromium(VI) is more toxic to plants than chromium(III). In animals and
humans, chromium(III) is considered to be an essential nutrient that helps to maintain
normal metabolism of glucose, cholesterol, and fat (ATSDR, 2013). Signs of chromium
deficiency in humans include weight loss and impairment of the body’s ability to remove
glucose from the blood. The minimum human daily requirement of chromium for optimal
-1
health is not known, but a daily ingestion of 50–200 μg d has been estimated to be safe
and adequate. Only very large doses of chromium(III) are harmful. Long-term exposure
to hexavalent chromium may cause adverse effects in the kidney and liver. Chromium(VI)
and some chromium compounds are also known carcinogens. Because small amounts of
trivalent chromium occur in many foods, most of it enters the body from dietary intake. In
the case of excess intake of chromium (VI), the chromium originates mainly from the use of
contaminated groundwater as drinking water.
The metal (chromium (0)) is a steel-grey solid with a high melting point (1857.0 °C). It
is mainly used for making steel and other alloys. Chromite is used by the refractory industry
to make bricks for metallurgical furnaces. Chromium compounds produced by the chemical
industry are used for chrome plating, the manufacture of pigments, leather tanning, wood
treatment, and water treatment. Soil and groundwater pollution by chromium is primarily
caused by leaching from waste disposal sites and industrial zones.
7.8 MERCURY
Mercury is a metal that occurs naturally in the environment in several forms. Elemental
mercury has a melting point of –38.9 °C and is thus a liquid at normal environmental
temperatures near the Earth’s surface. However, it has a relatively high vapour pressure at
these temperatures, so it is also slightly volatile. This free metallic form, which has an
-1
equilibrium solubility of 25 μg l in a closed system without a gas phase over a considerable
pH–Eh range (Hem, 1989) is the stable form in most natural water systems. However, the
concentration that would be present in water in contact with the free atmosphere is likely
to be much lower, because of mercury’s tendency to vaporise. Mercury can also combine
with other chemicals, such as chlorine, carbon, or oxygen to form either inorganic or
+
organic mercury compounds. Organic complexes such as methyl mercury (HgCH ) may be
3
produced by methane -producing bacteria in contact with metallic mercury in lake or stream
sediments. In this form, mercury is in the 2+ oxidation state . Like other trace metals , the

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