ENV 440 - Lecture 25

Reading Assignment: Read the information at the web sites linked below on pesticides and the degradation of pesticides in the environment. Read Chapter 6 in Manahan.

Homework: HW-8, Due Wednesday, March 12. WARNING: Exam 2 is scheduled for Friday, March 28!!!!

NOTE: The 3-D structures included in this lecture require a plug-in for your browser called Chime. It is available free from the MDL Web site MDLChime Plug-in. Although Chime is configured to work with IE and Netscape, we recommend using Netscape 4.80 with Chime 2.60 for optimum results.

Any discussion of pesticide degradation must start with DDT for several reasons. DDT was the first synthetic organic substance use in large quantities for insect control. Because of this, the undesirable side effects of chlorinated hydrocarbons were first discovered with DDT.

The history of DDT began in 1874 when a German chemist named Zeidler discovered it. It remained a laboratory curiosity until Paul Mueller discovered its ability to control insects in 1939. DDT was rapidly adopted for widespread use in agriculture and public health programs, and proved to be the most effective agent known at eradicating diseases that are transmitted by insects (i.e., malaria). These accomplishments were deemed so significant that Mueller was awarded the 1948 Nobel Prize in medicine--for discovering DDT's insecticidal properties. The first reports of ecological problems related to DDT use were published in 1950. These reports were followed by the publication of Rachel Carson's Silent Spring in 1962. Silent Spring documented the DDT's devastating impact on the ecosystem and launched the environmental movement. The use of DDT was eliminated in the United States in 1970, and the developed countries soon adopted similar restrictions on DDT's use. Unfortunately, DDT is still in wide use in underdeveloped countries.

The problems with DDT are related to its molecular structure. DDT is the common name for 1,1,1-trichloro-2, 2-bis(p-chlorophenyl)ethane. The molecular structure is shown in Figure 25.1 below. The microbial degradation of DDT is fairly well understood. The degradation pathway is given in the University of Minnesota Biodegradation Database DDT Biodegradation Pathway

DDT is a hydrocarbon, which means that it does not dissolve in water, but does dissolve in oils and fats. It is not easily degraded to other substances, and therefore remains in the soil long after it has been applied to agricultural crops. All chlorinated hydrocarbons share these properties. The persistence of insecticides in soils is shown in Table 1.

Table 25.1 Persistence of chlorinated hydrocarbon insecticides in agricultural soils.

Insecticide	Years Since Treatment	Percent Remaining
Aldrin	14	40
Chlordane	14	40
Endrin	14	41
Heptachlor	14	16
Dilan	14	23
Isodrin	14	15
Benzene hexachloride	14	10
Toxaphene	14	45
Dieldrin	15	31
DDT	17	39

The fat solubility of chlorinated hydrocarbons (CHCs) results in CHCs being concentrated throughout the food chain. The result is that organism at the top of the food chain have excessively high CHC levels in their body fat, CHC levels that are much higher than soil or water levels. The process of bioaccumulation in a food chain is shown in Figure 25.2 below:

Figure 25.2. Diagram illustrating the process of biomagnification of a chloronated hydrocarbon in an ecosystem. (Coutersy of McGraw Hill Higher Education, BioCourse.com)

Recent data, taken from Lake Kariba, a lake in Africa, shows how a DDT level of 0.39 PPM in sediments increased in the food chain to 9.5 PPM in the Cormorant (a fish eating bird) and to 34.5 PPM in the crocodile. Figure 25.3 below shows DDT concentration changes in the Lake Kariba ecosystem.

DDT residues were present on most of the food consumed in the United States during the 1940s through the 1970s, because of the large quantities of DDT that were used on agricultural crops. DDT was retained in the body fat of people who consumed this food, as shown in Table 2. It is interesting to observe how the DDT body fat levels dropped after the use of DDT was banned in the United States (1972).

Figure 25.3 Mean DDT levels (ng/g fat) for organisms in the Lake Kariba ecosystem. Source: Berg, H., Kiibus, M. and Kautsky, N. (1992) DDT and other insecticides in the Lake Kariba ecosystem, Zimbabwe, Ambio, 21, 444-450.

Table 25.2. Average levels of DDT in human body fat for individuals living in the United States, 1942-1978 (PPM, m g/g fat).

Year	DDT level	Year	DDT level
1942	0	1970	11.6
1950	5.3	1972	9.2
1954-56	11.7	1974	6.7
1961-62	12.6	1976	5.5
1962-63	10.3	1978	4.8
1968	12.5	--	--

DDT was correlated with significant population reductions in a number of species of birds. Field observations showed that the nest of the affected species contained crushed eggs (see Figure 25.4 below). By current comparing eggshell thickness with historical data it became clear that eggshell thickness decreased f with the introduction of widespread DDT use. This data is shown in Figure 25.5.

Figure 25.4. Photograph of a crushed Caspian tern egg, next to a normal egg. Source: Marco, G. J., Hollingworth, R. M., and Durham, W., Silent Spring Revisited, American Chemical Society, Washington, DC, p 100, 1987.

Figure 25.5. Changes in the thickness of eggshells of the peregrine falcon in Britain. The arrow show when DDT first came into widespread use. Source: Ehrlich, P. R., Ehrlich, A. H. and Holdren, J. P., Ecoscience, Population, Resources, Environment, W. H. Freeman and Co., San Francisco, 1977.

Twenty-five years after banning DDT in the United States, populations of the most severely affected bird species have recovered. The American Bald Eagle is one of the most dramatic examples of this recovery.

DDT has been associated with many health problems in humans, from increased suicide levels to cancer. Banning its use in developed countries removed a major health threat for much of the world's population, but underdeveloped countries in Africa and southeast Africa still use DDT.

The carcinogenic aspects of chlorinated hydrocarbons result from metabolites formed when these substances degrade. The greatest concern in recent years has been with smaller chlorinated hydrocarbon molecules, e. g., carbon tetrachloride and chloroform. Just a few years ago, chloroform was a common ingredient of cough syrup. The metabolism of carbon tetrachloride carbon tetrachloride degradation, illustrates how dfferent microorganisms, and different biochemical pathways, transform CCl₄ to either methane (CH₄), carbon monoxide (CO), formic acid (HCO₂H) or carbon dioxide (CO₂). Phosgene is an intermediate in the pathway to CO₂. Phosgene can interact with the genes in DNA, ultimately leading to cancer.