Page 178 - Soil Degradation, Conservation and Remediation

6.2 Sources of Soil Pollutants 167

Fig. 6.11 Two organotin
fungicides

Fig. 6.12 Structure

of phthalimide

Fig. 6.13 Structure
of benzimidazole

• Benzimidazoles. Benzimidazole is a systemic fungicide. The pentagonal ring in
histamine is known as an imidazole ring. Its fusion with a benzene nucleus gives
the benzimidazole (Fig. 6.13 ).
6.2.6.4 Persistence of Pesticides in Soil

Persistence may be defined as the tendency of a given compound, a pesticide in
this case, to conserve its molecular integrity and chemical, physical and functional
characteristics in a medium through which it is transported and distributed after
being released into the environment. Many organic compounds persist for long
periods in soils, subsoils, aquifers, surface waters, and aquatic sediment. The low-
and high- molecular weight compounds that resist biodegradation are known as
recalcitrant molecules. Many pesticides, mainly herbicides, have this characteristic
(Navarro et al. 2007 ).
Organochlorine pesticides are considered persistent because they are stable
in the environment and can resist degradation. This ability of organochlorine
pesticides makes them highly effective and therefore widely used but at the same
time helps them to build up their residues in soil and water. DDT was used in
large scales to control typhus and malaria. The properties that made these chemicals
such effective insecticides also made them environmental hazards; they accumulate
in human fat tissue. Some of the organochlorine pesticides have been banned for use
in US and European countries, but others are the active ingredients of some home
and garden products and some agricultural and environmental pest-control products.
There are four broad groups of organochlorine pesticides: