Page 146 - Soil and water contamination, 2nd edition

Heavy metals 133

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the 20 century, many malnourished inner-city children satisfied their cravings by eating old
paint flakes from dilapidated buildings, which caused many cases of lead poisoning. For this
reason, lead was phased out of paint pigments (EPA, 2013).
The lead used by industry comes from mined ores or from recycled scrap metal. Lead
has a wide range of uses. Its main use is in the manufacture of storage batteries. Other uses
include the production of chemicals, including petrol additives, paint pigments, pigments
for glazing ceramics, various metal products (for example, sheet lead, solder, pipes, and
fisherman’s weights), and ammunition. Contamination of air, water, and soil by lead is due
to dispersal of these chemicals in the environment via similar pathways as has been described
for zinc and copper (see previous sections). However, the pathway via the air used to be more
significant, especially in areas of heavy automobile traffic. Before the introduction of lead-
free petrol, tetraethyl lead was added to the petrol to promote more efficient combustion.
The lead content of the fuel was emitted as aerosol particulates in the exhaust gases,
which are deposited within a relatively limited range from the source. The use of lead in
ammunition for clay pigeon shooting, game hunting, and warfare is another ‘special case’
causing lead pollution. Military training grounds and clay pigeon shooting club sites can
therefore contain particularly large concentrations of lead. Although lead ammunition
contains lead as native metal, the weathering of the shot pellets, bullets, and shells in the
soil results in relatively large quantities of lead being dispersed within the soil (Alloway and
Ayres, 1997).

7.5 CADMIUM

Cadmium has some chemical similarities with zinc and occurs only in the 2+ oxidation
state . However, it is much less abundant than zinc. In general, cadmium is present in zinc
ore minerals such as sphalerite, and also in some copper ores. The solubility of cadmium in
water is mainly controlled by adsorption on cation exchange sites and coprecipitation with
manganese oxides. However, cadmium tends to be less strongly adsorbed than other divalent
metals and is, therefore, more labile in soil and sediments and more bioavailable (Hem, 1989).
Cadmium is a highly toxic metal not known to have any beneficial effects for plants
and animals. High human exposures to cadmium are rare today, but long-term, low-level
exposure may cause adverse chronic health effects. The main toxic effect in humans from
chronic exposure to the metal is kidney damage and, ultimately, kidney failure. Many
cadmium compounds are also believed to be carcinogenic (ATSDR, 2013). The normal
-1
intake of cadmium by humans amounts to about 1-3 μg d , which does not appear to cause
health problems. Most cadmium enters the body via the gastrointestinal tract by eating food
products grown on contaminated soil, although smokers may receive a considerable part of
their cadmium intake by inhaling cigarette smoke. A notorious case of cadmium poisoning
occurred in the Jintsu Valley in Japan during the 1940s, when it was found that more than
200 elderly women who had had several children had developed kidney damage and skeletal
deformities. This disease was called the ‘itai-itai ’ disease, which literally means ‘ouch-ouch’
because of the pain caused by the deformed bones (Copius Peereboom, 1976).
Cadmium used in industry is a byproduct of zinc , lead , and copper refining. It is used for
metal plating, rechargeable and non-rechargeable batteries, and for pigments used in paint,
printer ink, and plastics. The principal source of cadmium release to the general environment
is the burning of fossil fuels (in power stations, furnaces, stoves, automobiles, etc.) and the
incineration of municipal waste materials. Cadmium may also be released from zinc, lead,
or copper smelters. Cadmium becomes very volatile at temperatures above 400 °C (Alloway
and Ayres, 1997). Therefore, if materials containing cadmium are heated, the cadmium is
readily released as aerosols . These cadmium aerosols are usually deposited within several tens

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