Page 184 - Soil and water contamination, 2nd edition

Organic pollutants 171

and some became comatose. At moderate levels, a decreased ability to reproduce, resistance
disorders, and effects on the liver and kidneys have been observed. Liver cancer has been
observed in laboratory rodents that ate HCH for a long period of time and HCH may
reasonably be anticipated to be a carcinogen to humans (ATSDR, 2013).

9.5.3 Hexachlorobenzene (HCB)
Hexachlorobenzene (HCB: C Cl ) consists of a benzene ring with 6 chlorine groups
6 6
attached to it and occurs as a white crystalline solid. HCB is man-made and does not
occur naturally in the environment. It is formed as a byproduct during the manufacture of
other chemicals. Small amounts can also be formed during the incineration of municipal
waste. HCB used to be widely used as a fungicide to protect the seeds of onions and
sorghum, wheat, and other grains. It was also used to make fireworks, ammunition, and
synthetic rubber. It has not been used commercially in the developed countries since the
1970s.
HCB is very persistent and can remain in the environment for a long time. Its half-life
in soil and surface water is 3–6 years. HCB is barely soluble in water and binds strongly to
organic matter. For this reason, plants take up HCB in only very small amounts. Studies in
animals show that eating HCB for a long time can damage the liver, thyroid gland, nervous
system, bones, kidneys, blood, and immune system (ATSDR, 2013). HCB is classified as a
probable human carcinogen.

9.5.4 Dichlorodiphenyl trichloroethane (DDT )

Dichlorodiphenyl trichloroethane (DDT : see Figure 9.1) is a white, crystalline solid with
no odour, which does not occur naturally in the environment. It is an organochlorine
pesticide developed during World War Two to control insects for agriculture and to assist
in the elimination of insects known to spread diseases such as malaria. It was banned in
the developed countries in 1972 but it is still used in Third World countries. Although
not used for a long time, it is still present in the environment of developed countries
due to its persistence. Illegal use of old stock and long-range atmospheric transport
from countries in which DDT is still being used also contributes to the ubiquity of this
chemical in the environment. Commercial DDT preparations are contaminated by
chemicals similar to DDT, namely DDD (dichlorodiphenyldichloroethane) and DDE
(dichlorodiphenyldichloroethylene). DDD was also used to kill pests, but this use has also
been banned. DDE has no commercial use.
In air, DDT is broken down relatively rapidly under the influence of UV light. The
half-life of DDT is about 2 days. In soils and sediments, DDT binds strongly to the
organic fraction and does not dissolve easily in water. The biodegradation of DDT by
microorganisms in soil proceeds slowly and the half-life of DDT in soil ranges from 5 to
8 years, depending on soil type (WHO, 1989). The microorganisms break down DDT
to DDE and DDD. DDT accumulates in plant tissues and fatty parts of fish, birds, and
mammals and has a high potential to biomagnify in the food chain. DDT is a hormone
disruptor which acts in a similar way to oestrogen and binds to the cell’s oestrogen receptors.
This results in the impaired reproductive success in many of the higher organisms living
in the aquatic environment. It also affects the nervous system. In acute exposure, DDT is
-1
highly toxic to aquatic invertebrates at concentrations as low as 0.3 μg l . DDT is also highly
-1
toxic to fish: the 96 hour LC50s reported range from 1.5 to 56 μg l (WHO, 1989). The
sensitivity of birds to DDT varies greatly. Predatory birds are especially sensitive, as DDT
and its metabolites are responsible for the thinning of eggshells and the consequent increased
egg breakage (Alloway and Ayres, 1997).

10/1/2013 6:44:40 PM
Soil and Water.indd 183 10/1/2013 6:44:40 PM
Soil and Water.indd 183