Page 144 - Soil and water contamination, 2nd edition

Heavy metals 131

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animals and humans. If the zinc concentration in the soil exceeds about 300 mg kg , which
is frequently the case if the soil is contaminated, zinc may be toxic to several species of soil
microorganisms. This affects the decomposition of plant litter and hence nutrient cycling in
contaminated ecosystems. Nitrogen-fixing bacteria in the root nodules of legumes such as
clover are especially susceptible to zinc toxicity (Alloway and Ayres, 1997). As a consequence,
soil pollution by zinc may cause a noticeable shift in plant species composition.
The most severe pollution by zinc is usually found in mining and smelting areas.
Fragments of sulphides in mine tailings oxidise on weathering . The oxidation reaction of
sphalerite can be written in simplified form as:

ZnS + 2O 2 Zn 2 + + SO 4 2 (7.1)
This reaction is accompanied by the oxidation of pyrite and creates acidic solutions which
tend to decrease adsorption and hence increase the environmental mobility of zinc and other
metals .
Other anthropogenic sources of zinc are galvanised steel, sewage sludge , waste disposals,
and industrial releases. Galvanised steel is used in roofs, gutters, drainpipes, and wire fences.
Exposure to acid rain slowly dissolves these materials, and the zinc ends up in soil or runoff
water. Zinc is also widely used in domestic products such as skin care products (cosmetics,
baby creams, shampoos). Together with drainage water from galvanised surfaces, these are
the main sources of zinc pollution of sewage water, effluent from wastewater treatment
plants, and sewage sludge. Hence, spreading sewage sludge on land as a fertiliser progressively
increases the zinc concentrations in agricultural soils. The other major sources of zinc
in domestic waste in addition to discarded galvanised materials and domestic products
containing zinc are batteries, pigments, and paints. Waste disposal can therefore lead to local
soil and groundwater pollution around landfills.

7.3 COPPER
Copper may occur in the Earth’s crust as the free native metal or in the 1+ or 2+ oxidation
+
2+
2+
state s. Copper may be in solution as either Cu or Cu , but the more oxidised Cu
+
predominates due to redox reactions in aerated water and the tendency of Cu ions to
+
0
2+
disproportionate (2Cu → Cu + Cu ). Copper ions are prone to complexation , especially
-
with hydroxide and carbonate ligands . In water above neutral pH , the Cu(OH) complex is
3
formed and a strong CuCO (aq) ion pair predominates in aerated natural waters containing
3
dissolved carbonate species. These complexes are slightly soluble and may keep the copper
-1
concentration in water below 10 μg l . Moreover, adsorption and coprecipitation with ferric
oxyhydroxides may cause even smaller copper concentrations (Hem, 1989). Under reduced
conditions and in bedrock minerals, copper is contained in rather stable sulphide minerals.
Some of the common copper sulphide minerals that are important as ores also contain iron,
such as chalcopyrite (CuFeS ).
2
Like zinc, copper is an essential micronutrient that is a constituent of many enzyme
systems. Copper deficiency in soils may depress crop yields, especially in cereals. Animals
-1
and humans need copper to be able to use iron properly. Herbage with less than 5 mg kg
can cause deficiency in sheep and cattle. The most commonly observed results of copper
deficiency in cattle are diarrhoea, broken bones, infertility, anaemia, poor weight gains, and
reduced immune response. Humans need between 1 mg and 3 mg each day. It takes several
days for copper to leave the body.

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