Page 147 - Soil and water contamination, 2nd edition

134 Soil and Water Contamination

of kilometres from the source. The largest source of cadmium in the human food chain is,
however, the application of phosphate fertilisers , in which cadmium is present as impurity.
In addition, the spreading of contaminated sewage sludge on agricultural land contributes to
the human cadmium intake.

7.6 NICKEL

Nickel is a ferromagnetic transition metal sharing many chemical properties with iron . It
occurs naturally in the Earth’s crust in various forms such as nickel sulphides and oxides.
Nickel may occur in the oxidation state s 1-, 0, 2+, 3+, and 4+. However, the aqueous
chemistry of nickel is primarily concerned with the 2+ oxidation state. Other oxidation states
occur in special complexes and oxides. In alkaline solutions, Ni(II) hydroxide can be oxidised
to a hydrated Ni(IV) oxide. Like zinc, nickel is specifically adsorbed in soils and sediments
and the binding strength increases with ageing of nickel (Hem, 1989).
Nickel is an essential nutrient that is only required in very small amounts. In plants,
symptoms of nickel toxicity are generally induced by iron deficiency: chlorosis and
foliar necrosis. Excess nickel affects nutrient absorption by roots, root development, and
metabolism, and it inhibits photosynthesis and transpiration. Nickel can replace other metals
located at active sites in metallo-enzymes and disrupt their functioning. Its absorption into
the human body is affected by factors of consumption, the acidity of the gut, and various
binding or competing substances, including other metals such as iron, magnesium, zinc,
and calcium. An excess of nickel in tissues can cause damage to chromosomes and other
cell structures, alter hormone and enzyme activities, and affect membrane permeability and
immune function. As a result, changes in glucose tolerance, blood pressure, response to
stress, growth rate, bone development, and resistance to infection are possible. Nickel has
also been associated with possible carcinogenic effects (ATSDR, 2013).
Since the body’s requirement for nickel is so low, excessive exposure from the
environment, leading to toxicity is very common, e.g. from tobacco, dental implants,
stainless steel kitchen utensils, coins, and inexpensive jewellery. The most common
trouble is allergy of the skin to nickel due to direct contact. On the other hand, toxicity by
consumption of food products grown on contaminated soil is very rare and would require
1000 times the amount normally consumed in food.
The nickel used by industries comes from mined ores or from recycled scrap metal
and has a wide range of industrial uses. When alloyed with other elements, nickel imparts
toughness, strength, resistance to corrosion and various other electrical, magnetic, and heat-
resisting properties. It is primarily used in making various steels and alloys (e.g. stainless steel)
and in electroplating. Minor applications include the use in ceramics, permanent magnet
materials, and nickel–cadmium batteries.

7.7 CHROMIUM

Chromium is found in the environment in three major states: chromium (0), chromium(III),
and chromium(VI). The predominant oxidation state in the environment is trivalent
chromium. Ultramafic igneous rocks contain the most chromium. Chromite (FeCr O ),
2 4
which is highly resistant to weathering , may be concentrated in the lateric soils overlying
such rocks. In general, chromium (III) is largely present in soil as relatively unavailable,
insoluble chromium or chromium–iron oxyhydroxides . It can also exist as a substitute
for Al(III) in the octahedral groups of aluminosilicates. The solubility of Cr(III) in soil is
dependent on pH and decreases dramatically at pH values greater than 4.5. In aqueous

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