insecticides that polluted water and sediments. In rural areas the greater problem is from
herbicides which, because they must be applied directly to cropland, have an inherent
tendency to get into runoff and into water sources. Fortunately, pesticides have become
generally safer and less persistent in the environment. Although pesticide use has leveled
off, it is still enormous, around 350 million kilograms per year for U.S. agriculture as
well as large amounts for nonagricultural uses including forestry, landscaping, gardening,
food distribution, and home pest control.
Insecticides
Figure 7.7 shows some insecticides that either are potential water pollutants or
have been water pollutants in the past. The insecticides that caused the greatest water
pollution problems, and that still do so in developing countries where their use is still
allowed, are the organochlorine insecticides. There were many of these, probably best
exemplified by DDT and including others, such as chlordane. DDT was widely sprayed
in the environment and was instrumental in killing mosquitos that carry malaria. Highly
persistent chlordane was the most effective insecticide against termites and was buried
around buildings to prevent termite infestation.
The organochlorine insecticides were dominant from the 1940s until the 1960s. In
general, they are not particularly toxic to humans and other animals. However, DDT
and related compounds have an even more detrimental characteristic because of their
tendency to undergo bioaccumulation in fish and other organisms, concentrating in fat
tissue. Furthermore, as organisms that have accumulated these compounds are eaten,
and these organisms, in turn, are eaten by larger animals, the organochlorine compounds
become progressively more concentrated in fat tissue, a process called biomagnification.
It was this phenomenon that caused the birds of prey — eagles, falcons, hawks — to
become so contaminated with DDT that they produced soft egg shells that broke before
young could hatch. This threatened a number of species of birds, including the Bald Eagle
that is the U.S. symbol, leading to the banning of DDT and most other organochlorine
insecticides.
For a time, as DDT and other organochlorine insecticides lost favor, organophosphate
insecticides, which are organic derivatives of phosphoric acid, H 3 PO 4 , came into common
use. These insecticides had a big advantage in being biodegradable with no tendency to
undergo bioaccumulation. Two examples are methyl parathion and malathion. Parathion,
once the most widely used organophosphate insecticide, is a very effective insecticide
and, because of its rather rapid biodegradability, it was not usually a significant water
pollutant. It acts by inhibiting the action of acetylcholinesterase, an enzyme essential
to nerve function. This is the same mode of action of military poison “nerve gases,”
such as sarin, and a significant number of fatal poisonings occurred due to parathion
exposure. Although parathion is now banned, malathion remains on the market and is
only about 1/100 as toxic to mammals as is parathion. This is because, as shown in its
structure above, malathion can be cleaved with addition of water by enzymes possessed
by humans and other mammals, but not by insects.
180 Green Chemistry, 2nd ed