Ci

Thus the ground states of the vinyl and aryl halides are more stable than the corresponding ground states of aliphatic halides.

This results in a greater energy of activation (Ea) for carbocation formation and a slower rate of reaction. This is shown qualitatively in Figure 8-1.

The reactivity of aryl halides is enhanced by electron-withdrawing groups. For example, a nitro substituent greatly accelerates the rate of displacement of aryl-bound halogens:

This is due to the formation of an intermediate that is stabilized by the electron-withdrawing substituents. Halide ion displacement from an aromatic ring containing electron-withdrawing groups might thus occur under environmental conditions.

Carbocation intermediate

Carbocation intermediate

FIGURE 8-1 Reaction coordinate for the conversion of halogenated organics to products by S^1 or E1 processes.

8.4 MICROBIAL DEGRADATION OF ORGANICS

Biodegradation is the key to the destruction of organic compounds in the environment (Section 8.2). Proteins and nucleic acids of dead plants and animals are degraded rapidly because they are metabolized by microorganisms prevalent in the environment. Xenobiotics—compounds that occur rarely in the environment, such as crude oil, nylon or polychlorinated biphenyls—either are not degraded or are degraded slowly because the population of microorganisms available to metabolize them is small. The presence of xenobiotics in the environment may promote the growth of strains of microorganisms that do metabolize these compounds. Microorganisms mutate rapidly, so in some cases there is a mutant that can live by metabolizing the xenobiotic. This is why microbiologists who are looking for a population of microorganisms that degrade a particular xenobiotic search the soil samples in the vicinity of a spill of that compound.

The degradation of xenobiotics is usually carried out by a cooperating group of microorganisms called a consortium. Different steps in the degradation are carried out by different microorganisms. For example, the degradation of 3-chlorobenzoate is performed by three different groups of bacteria, as follows:

COO"

COO"

The hydrogen generated in step 2 is utilized by the group of microorganisms doing the reductive dehalogenation in step 1 in the process outlined in reaction

The degradation of organic materials to inorganic compounds of C, H, N, P, the process of mineralization, is the ultimate goal of the degradation of xenobiotics. Mineralization is usually measured by the conversion of carbon to C02. But biodegradation may result in the intermediate conversion of the carbon to new microorganisms, not achieving mineralization until these intermediate products die. This is not a concern, since the realistic goal of biodegradation is the conversion of a xenobiotic compound to a biotic one that is readily metabolized by a large population of microorganisms, hence will not accumulate in the environment. These biological compounds will eventually be degraded to C02. For example, the acetate formed in the degradation of 3-chlorobenzoate is readily metabolized by both anaerobic and aerobic microorganisms. The crucial step in the degradation of 3-chlorobenzoate is the reductive elimination of chloride ion, a reaction that is carried out by relatively few microorganisms.

Both anaerobes and aerobes degrade xenobiotics. Anaerobes often use reductive processes to degrade organics, and aerobes use oxidative processes. Both carry out other processes in which there is no change in the oxidation state of the xenobiotic.

The anaerobic dehalogenation of a pesticide was discovered during an investigation of the loss of potency of 7-hexachlorocyclohexane [Lindane: see Section 8.9, reaction (8-40) ] in a cattle tick bath. The cattle were made to swim through the bath containing hexachlorocyclohexane to kill the ticks. Since the chemical is expensive, the same bath was used for several years. The potency of the bath decreased slowly with time, but when fresh pesticide was added to it, the rate of loss of the hexachlorocyclohexane dropped more rapidly. The rapid loss in activity was traced to microorganisms, which evolved in the bath over several years. These microorganisms metabolized the hexa-chlorocyclohexane by removal of its chlorines as chloride.

8.4.1 Reductive Degradation

Bacterially mediated reductive dehalogenation is an important pathway for the destruction of unreactive haloorganics. The reaction usually proceeds in anaerobic environments such as marine sediments and landfills with the replacement of halogens with hydrogens. The principal pathway for the reductive dechlorination of 2,3-dichlorobenzoate is shown in reaction (8-22). The benzoate formed is readily degraded by other microorganisms, as shown in Section 6.4.

COO COO COO"

Cl Cl

Reductive dechlorination is an important first step in the degradation of heavily chlorinated organics like pentachlorophenol (PCP). PCP is rapidly converted to tetrachlorophenols, but the subsequent loss of chloride in the process of forming tri-, di-, and monochlorophenols proceeds much more slowly than the initial loss of chloride ion. The faster initial rate reflects the higher oxidation potential of PCP, which makes it easier to reduce than its more lightly chlorinated metabolites.

The reductive dehalogenation of aliphatic compounds has also been observed. For example, 1,1,1-trichloroethane (methylchloroform) can be reduced to the corresponding dichloro derivative:

(Cl)3CCH3 + H+ + 2e" -- (Cl)2CHCH3 + CP (8-23)

8.4.2 Oxidative Degradation

The energy derived from the oxidation of xenobiotics with atmospheric oxygen drives the process of oxidative degradation. These oxidations occur mainly with lightly chlorinated chloroorganics because they require the attack of an electrophilic oxygen on an electron-rich center such as an aromatic ring or a double bond. The delocalization of these electrons by electronegative chlorine atoms decreases the electron density on the aromatic ring; hence these compounds are less readily oxidized by molecular oxygen.

The overall oxidative degradation of chlorobenzene is shown in reaction (8-24).

COOH COOH

COOH COOH

COOH COOH

In step a, a microbial dioxygenase enzyme catalyzes the formation of a cis-dihydrodiol, which is then oxidized (step b) to a halogenated catechol. Enzyme-catalyzed oxidative ring cleavage takes place either between the two phenol groups (step c) or adjacent to one of the phenol groups (step d) to give dicarboxylic acids, which are readily degraded to simpler compounds and eventually to CO2, H2O, and CP.

8.4.3 Hydrolytic Degradation

Hydrolytic cleavage of halogens, esters, amides, and other groupings are catalyzed by both aerobes and anaerobes. Dichloromethane (methylene chloride), a compound that decomposes very slowly in the presence of water, is hydrolyzed by microorganisms to formaldehyde. The enzyme-catalyzed reaction usually proceeds by the sn2 displacement of chloride ion by the thiol peptide glutathione (G-SH). The resulting intermediate is hydrolyzed to glutathione and formaldehyde:

G-SH + CH2Cl2->G-SCH2G->+G-S = CH2 -^G-SH + CH2O + H+

HCl CP

The microorganisms that catalyze this reaction utilize formaldehyde as an energy source.

8.5 TOXICITY

Acute or immediate toxicity to humans and animals is not usually a problem for most commercial haloorganic compounds. The toxicity associated with some haloorganics is of the chronic type, where the deleterious effect appears 2-30 years after the initial exposure. For example, vinyl chloride, the monomer polymerized to poly(vinyl chloride) (Figure 7-3), evidently caused cancer in factory workers making the polymer twenty years after they started working with it. Since these workers all contracted an unusual type of liver tumor that was not observed in the general population, it was concluded that vinyl chloride was the causative agent.

Most commercial halocarbons are nonpolar compounds that are removed from the bloodstream by the liver. There, their presence induces the synthesis of the enzyme cytochrome P-450, which catalyzes the oxidative degradation of the nonpolar organics in the liver. However, if the compounds are heavily chlorinated, they may have high oxidation potentials and are oxidized slowly, or not at all, by cytochrome P-450 (Section 7.2.4). The high levels of this enzyme that are induced in the liver catalyze the oxidation of other nonpolar organics such as steroids. Since other steroid hormones are produced by cytochrome P-450 oxidation, the presence of excess cytochrome P-450 can change the normal hormonal balance.

Some of these haloorganics have many deleterious health effects other than causing cancer. Low levels can cause endocrine, immune, and neurological effects. Haloorganics vary dramatically in chronic and acute toxicity according to structure, and examples of specific problems are presented when selected types of compound are discussed in the sections that follow.

8.5.1 Environmental Hormones

The possible role of industrial chemicals in human health was debated extensively in the 1990s. Some chronic effects on humans have been anticipated since the observation of the decline in the number of eagles and hawks in the 1960s and 1970s, that correlated with the buildup of chloroorganics in the environment. There was some reassurance that humans might not be affected by these compounds because of the resurgence of the birds in the 1980s following the ban of the use of DDT in 1972. However, there were still reports of the decrease in populations of some birds and the apparent feminiza-tion of males in which enhanced levels of chlorinated organics were found.

The observation of abnormal sexual development of a number of forms of wildlife and the detection of higher levels of chloroorganics in the same animals led to the proposal that some industrial chemicals have hormonal activity that mimics or inhibits the activities of the sex hormones. Estrogen, a mixture of three steroid hormones, has essential roles in both males and females. A specific ratio of estrogen to androgen (male hormones) is required during prenatal and postnatal development for sex differentiation and for the development of reproductive organs. In females estrogen also prepares the uterus to accept the fertilized egg, helps with lactation, and lowers the risk of heart attacks, but stimulates the development of uterine and breast cancer. The presence of too much estrogen in males inhibits sperm production and the development of the testes.

In a number of instances industrial chemicals or their breakdown products have been shown to have estrogen-like activity that interferes with the sex hormones in wildlife. For example, the feminization of seagulls was attributed to the higher levels of chloroorganics in these birds. PCBs appear to exert the same effect in terns living near a toxic waste site containing these chemicals. PCBs also demonstrate estrogen-like activity in turtles. Laboratory studies have established that o,p-DDT, p, p'-methoxychlor [structure given in reaction (8-35)], kepone, and some PCBs and the phenols derived from them are estrogen mimics. Nonchlorinated organics have also been shown to be estrogen mimics. Examples include bisphenol A, a degradation product of polycar bonate polymers, and alkylated phenols such as p-nonylphenol, a degradation product of polyethoxylate detergents (Section 7.2.2.3).

The interpretation of the effect of a mixture of environmental compounds found in humans on their health is complicated by the varied biological responses triggered by the presence of the compounds. Some industrial compounds have been shown to have antiestrogenic effects. These include 2,3,7,8-tetrachlorodibenzo-p-dioxin [TCDD: see Section 8.10.3, reaction (8-58)] and polychlorinated benzofurans [Section 8.7.3, reaction (8-30)]. In principle, these compounds could negate the effects of the environmental estrogens.

Another problem with TCDD is that it may cause endometriosis, a disorder in women in which tissue migrates from the uterus to the abdomen, ovaries, bowels, and bladder. This can result in internal bleeding and infertility. The

Kepone

Kepone

Bisphenol A

Bisphenol A

possible role of TCDD in endometriosis was suggested by a long-term study of the reproductive effects of TCDD in monkeys. TCDD was included in the diet of 24 female monkeys for four years and then no more was administered. In the 6-to 10-year period after the initial administration of the TCDD, 3 of the monkeys died of endometriosis. Investigation of the remaining monkeys revealed that 75% of those given a high dose of TCDD had moderate to severe cases of endometriosis, while only 42% of those given a weak dose had the ailment. None of the control monkeys had endometriosis.

More recently it has been discovered that DDE, a breakdown product of DDT, has antiandrogen activity. Antiandrogens compete with the male sex hormones for binding at androgen receptor sites and thus inhibit the activity of the androgens. The net result of this inhibition is comparable to the presence of compounds with estrogen activity. It appears likely that the feminization of newborn alligators living in a lake containing high levels of DDE should really be called demasculinization resulting from the antiandrogenic effect of the DDE.

There is general agreement on the assignments of estrogenic, antiestrogenic, or antiandrogenic properties to the compounds listed in Table 8-2. Disagreements arise when these data are used to explain recent health problems in humans. For example, the increases in breast cancer observed in older women over the past 50 years have been tentatively attributed to long-term exposure to environmental hormones. The assumption is that since estrogen is a cause of breast cancer, environmental hormones will also cause cancer. Correlations have been reported between the PCB levels in tissue and DDE present in serum in women who have breast cancer, but there is no evidence of a direct link between breast cancer and these compounds. It has also been proposed that the 40% decline in sperm counts in men over the past 50 years is due to exposure to estrogen-like and antiandrogen compounds in the environment. In addition,

TABLE 8-2

Proposed Hormonal Activity of Environmental Compounds

Estrogen-like Antiestrogens Antiandrogen

PCBs 2,3,7,8-Tetrachlorodioxin DDE

o,p-DDT Polychlorinated benzofurans

Methoxychlor Benzo(a)pyrene

Kepone

Toxaphene

Dieldrin

Endosulfan

Atrazine

Bisphenol A

Nonylphenol it has been proposed that there is a correlation between the increase in testicular cancer and the presence of estrogen-like compounds in the tissue of males. As in the case of breast cancer, there is no unambiguous link between the medical observations and environmental compounds.

An important argument against the important role of environmental chemicals as estrogens in humans is the observation that the cruciferous vegetables (brussel sprouts, cauliflower, kale, etc.) contain bioflavonoids that exhibit estrogenic activity.2 It is calculated that the estrogenic potential of the compounds in these vegetables is 107 times higher than that of the environmental chemicals because they constitute such a high proportion of our diet. Scientists on both sides of the debate agree that more data are required before conclusive statements can be made concerning the importance of environmental estrogens on humans.

Many of the persistent organic pollutants (POPs) that are believed to be environmental hormones are still being produced and used in the world. These compounds are carried from where they are manufactured or used around the globe by weather patterns. The United Nations sponsored and adopted a treaty in 2000 intended to end the production of these and other persistent chlorinated compounds throughout the world and to destroy their stocks. This is the first attempt to control these compounds globally. The "dirty dozen" compounds include the insecticides aldrin, chlordane, DDT, dieldrin, endrin, hep-tachlor, hexachlorobenzene, mirex, and toxaphene plus the industrially used polychlorinated biphenyls and the industrial by-products, dioxins and benzofurans. The treaty took effect upon ratification by 122 nations in December, 2000. The use of DDT for the control of malaria is the one exception to the ban, since the insecticide is still used for this purpose in Africa, Asia, and South America. The structures and discussions of most of these compounds are given in subsequent sections of this chapter.

8.6 CHLOROFLUOROCARBONS, HYDROFLUOROCARBONS, AND PERHALOGENATED ORGANICS

Chlorofluorocarbons (CFCs) and perhalogenated organics were used for a long time as aerosol propellants, refrigerants, solvents, and foaming agents, but this use has been curtailed. So-called halons, which contain bromine as well as fluorines, are still used to extinguish fires in critical situations such as on aircraft. Most of these compounds are nontoxic, stable, colorless, and nonflammable: ideal substances for use in a variety of industrial applications. The high stability of these compounds, which made them so attractive to industry, also explains

2S. E. Safe, Environmental and dietary estrogens in human health: Is there a problem? Environ.

Heath Perspect., 103, 346-351 (1995).

why they are implicated in two major environmental problems, global warming (Section 3.3.3) and the destruction of the ozone layer (Section 5.2.3.2).

8.6.1 Structural Types

The chlorofluorocarbons that were banned from production in the industrialized nations in 1996 (Section 5.2.3.5) are highly halogenated derivatives that do not contain a C—H bond (e.g., CCl3F, C2F2Cl4, and C3F2Cl6. The per-chlorinated compound carbon tetrachloride (CCl4), which had been used extensively as a degreasing and cleaning solvent, is as deleterious to the environment as the CFCs. Methylchloroform (1,1,1-trichloroethane, CCl3CH3), which was also used for degreasing, although it has one-tenth the ozone-destructive power of CFCs, was nevertheless a major contributor to the destruction of the ozone layer because it was used in large amounts in industry.

Bromofluorocarbons (halons), such as CBrF3, used to extinguish fires in military and commercial aircraft, are believed to act in much the same way that tetraethyllead prevents preignition in internal combustion engines (Section 6.7.4), that is, by combining with free radicals and stopping the oxidative chain reaction central to the combustion process. The production of halons that destroy the ozone layer (Section 5.2.3.2) was banned in 1994 in the industrialized countries but will continue in developing nations. There are still large supplies available that, when put to use to fight fires, will eventually end up in the ozone layer. The level of halons in the atmosphere was still increasing in 1998.

It is ironic that the same person, Thomas Midgely Jr., invented both chloro-fluorocarbons and tetraethyllead. These compounds, which were highly acclaimed when discovered in the 1920s, made possible the widespread use of refrigerators and automobiles, staples of the American culture and economy. The manufacture and use of leaded fuels in the United States continued until 1995, while the manufacture of chlorofluorocarbons was halted in 1996. The phaseout in the use of these compounds started some 10-15 years earlier than the final cutoff dates.

8.6.2 Atmospheric Lifetimes3

8.6.2.1 Chlorofluorocarbons and Perhalogenated Organics

Carbon tetrafluoride (CF4), the most stable of the perhalogenated organics, is estimated to have a lifetime in the environment that exceeds 50,000 years—

3The lifetimes referred to in this section are the reciprocal of the first-order rate constant for their degradation. See A. R. Ravishankara and E. R. Lovejoy, Atmospheric lifetime, its application and determination: CFC-substitutes as a case study, J. Chem. Soc. Faraday Trans., 90(15), 21592169 (1994).

about 10 times longer than the history of human civilization. It is formed as a by-product of the production of aluminum by the electrolysis of aluminum oxide in a melt of cryolite (Na3AlF6) (Section 12.2.5). The CF4 is formed by the reaction of fluorine with the carbon electrodes at the high temperatures (1000°C) and voltages used in the electrolytic process. About 30,000 tons of CF4 is released globally into the atmosphere each year by this process.

The presence of strong C—F and C—Cl bonds and the absence of C—H bonds are central to the environmental stability of the CFCs and perhaloge-nated organics. The trends in the energy of dissociation of the carbon-halogen bond of halomethanes show the C—F bond to be the strongest (Table 8-1). The large energy of dissociation of the C—F bond is a consequence of the greater overlap of the bonding orbitals because of the similarity in size of the carbon and fluorine atoms. In addition, because of its high electronegativity (4.0), the fluorine atom reduces electron density on carbon and deactivates the carbon atom for reaction by SN1 or E1 processes (Section 8.3.2). Because the C—Cl bond is weaker than the C—F bond, a CFC will react to form a carbocation by ionization of the chlorine atom as follows:

Cl vF Ci F

Cl Cl Cl

The postulated carbocation intermediate in reaction (8-26) has a very high energy. Consequently, the chloride atom ionizes from the carbon to form the carbocation only at high temperatures. While CCl4 is more reactive than CF4, the relatively high strength and electronegativity (3.0) of the C—Cl bond (Table 8-1) explains the environmental stability of CCl4.

CFCs and perhalogenated organic compounds are stable in the earth's atmosphere because they do not contain C—H groups and therefore are resistant to attack by hydroxyl radicals (Section 5.2.3.5). Since abstraction by a hydroxyl radical (OH) of a chlorine atom from a CFC to form ClOH is not energetically favored, CFCs only react after diffusion up to the ozone layer, where solar UV radiation initiates the dissociation of the chlorine atom (Section 5.2.3.2). The chlorine atom catalyzes the dissociation of ozone. Since CF4 does not contain chlorine or bromine, it does not cause the destruction of the ozone layer, but its global warming potential is several thousand times greater than that of a comparable amount of carbon dioxide (Section 3.1.2).

Methylchloroform, an industrial solvent, is a compound whose production was curtailed as a result of the 1987 Montreal Protocol agreement (Section 5.2.3.5). It is not as inert as the chloroflurocarbons, with a lifetime of 5.2 years in the Northern Hemisphere and 4.9 years in the Southern Hemisphere. Its atmospheric concentration leveled off in 1991 and started to decrease in 1995. The rapid decay of methyl chloroform is due to its reaction with atmospheric hydroxyl radicals to form CH2CCl3 (Section 5.2.3.5). It was possible to calculate from these lifetimes that the concentration of hydroxyl radicals in the atmosphere of the Southern Hemisphere is 15±10% higher than in the Northern Hemisphere.

The annual production of CFCs decreased by 90% in the 1986-1995 time period. Since the lifetimes of many CFCs in the atmosphere are in the range of 50-150 years, limitations on their production will not result in a decrease in their atmospheric concentrations until about 2001-2010 (Figure 8-2). However the global agreement to halt the production and use of chloro-fluorocarbons made in the Montreal Protocol in 1987, and its subsequent modifications, have resulted in halting the increase in buildup of stratospheric chlorine in 1994 at one-third the level that might have formed if no controls had been put in place until 2010. It is predicted that the ozone levels will decrease in 2050 to those observed in the time period when the Antarctic ozone hole was discovered (1980-1984: see Section 5.2.3.3).

Cumulative stratospheric chlorine equivalent, ppb

Cumulative stratospheric chlorine equivalent, ppb

Chlorine level at which Antarctic ozone hole appeared

1979 1984 1989 1994 1999 2004 2009 2014 2019 2024 2029 2034 2039 2044 2049 2054

FIGURE 8-2 Projected trend in levels of chlorine, CFCs, HCFCs, and haloorganics in the atmosphere as a consequence of The Montreal Protocol. Redrawn from, "Looming ban on production of CFCs, halons spurs switch to substitutes," Chem. Eng. News, p. 13, Nov. 15, 1993. Used by permission of E.I. du Pont de Nemours and Company. Also see color insert.

1979 1984 1989 1994 1999 2004 2009 2014 2019 2024 2029 2034 2039 2044 2049 2054

FIGURE 8-2 Projected trend in levels of chlorine, CFCs, HCFCs, and haloorganics in the atmosphere as a consequence of The Montreal Protocol. Redrawn from, "Looming ban on production of CFCs, halons spurs switch to substitutes," Chem. Eng. News, p. 13, Nov. 15, 1993. Used by permission of E.I. du Pont de Nemours and Company. Also see color insert.

8.6.2.2 Hydrochlorofluorocarbons and Hydrofluorocarbons

Hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs) have been designed to take the place of CFCs, which were banned from production in industrialized nations on January 1, 1996. Many times, a chemist who has prepared a useful but unstable compound is asked to come up with a compound that has similar properties but does not break down. The assignment in the case of the CFCs, however, was to make a less stable substitute that breaks down more rapidly in the environment. The approach used was to include C— H bonds in these substitutes for the CFCs so that they would be susceptible to oxidation by hydroxyl radicals in the atmosphere (Section 5.2.3.5). Two examples of the hundreds of HCFCs and HFCs prepared are CH3CHF2 (HFC-152a) and CF3CH2F (HFC-134a). Both have no ozone destruction potential, but both are greenhouse gases. In fact, CF3CH2F has about five times the global warming potential of CH3CHF2. The global warming potentials of CFCs, like CFCl3, are about 30 times greater than that of CH3CHF2.

The atmospheric lifetimes of CH3CHF2 and CF3CH2F are estimated to be 2 and 14 years, respectively, values that are considerable shorter than the 50-to 100-year lifetimes of the CFCs. In addition, since these compounds contain no bromine or chlorine, they will not destroy the ozone layer. There was a concern that the CF3 radical formed in the atmospheric degradation of CF3CHF2, and other CFC substitutes that contain the CF3 group, would catalyze the degradation product of ozone, via the formation of CF3OO. and CF3O. by reaction with molecular oxygen and ozone, respectively. Laboratory studies and atmospheric models showed that these oxygenated trifluoromethyl radicals would react in the lower stratosphere, where they would have a negligible effect on the level of atmospheric ozone. There is also the concern that the trifluoroa-cetic acid, an important atmospheric degradation product of CFC substitutes containing the CF3 group, would accumulate at toxic levels in ecosystems where water evaporates to form concentrated solutions of organic and inorganic compounds. While it has been demonstrated that trifluoroacetic acid is not very toxic to humans, plants, and animals, and is degraded by some anaerobic bacteria, there is still concern that it may accumulate in the ecosystems and be toxic to the life there. These conclusions were reached on the basis of modeling (calculations) and have not been verified by experimental studies.

The HCFCs and HFCs are not expected to be permanent replacements for chlorofluorocarbons. Rather, their use is planned until 2020, when they are expected to be replaced by even more benign substitutes. For example, while compounds that contain the CF3 group (e.g., CF3CHF2), do not destroy the ozone layer, they are degraded to CHF3, which has a 400-year atmospheric lifetime and 8000 times the global warming potential of carbon dioxide. Policy makers expect that chemists will find readily degradable compounds that neither destroy the ozone layer, cause global warming, nor degrade to sub stances that are toxic to any forms of life on earth. These compounds should have the desirable properties of CFCs so that all humans can continue to enjoy all the benefits of modern civilization without an increase in the cost of living. It remains to be seen whether these desirable characteristics can be achieved.

8.6.2.3 Other Compounds

Progress toward the goal of producing a new generation of halons for use as fire retardants has not been as successful as the search for effective replacements for CFCs. The current plan is to carefully husband the available supplies of CF3Br and CBrClF2 by reserving their use for situations in which other retardants cannot be used while the search for effective replacements continues. There was hope that CF3I would be a direct replacement of CF3Br, but the toxicity of the iodide prevents its use in aircraft and other closed environments. The production of halons in 1995 decreased to almost zero in both industrialized countries and in all developing countries except China, where the production increased 2.5-fold between 1991 and 1995.

Another ozone-destroying chemical, methyl bromide (Figure 8-2), is used in large amounts as a soil and grain fumigant. It is an excellent agent for the removal of fungi, bacteria, viruses, insects, and rodents from soil prior to planting crops. Unlike many other pesticides, it does not persist in the soil but is decomposed in a few weeks. Even though its atmospheric lifetime is short, its use in large amounts results in the transport of some of the pesticide to the ozone layer. It was proposed in 1992 that methyl bromide is responsible for 10% of the depletion of the ozone layer. The photochemically generated bromine atoms destroy ozone about 50 times more rapidly than do chlorine atoms (5.2.3.2). California, the largest agricultural user of methyl bromide of any U.S. state, released 8 x 106kg in 1988. It was proposed in 1992 that the use of methyl bromide in developed nations be phased out in 2005. There is great concern about this proposed ban in the agricultural community because there is no obvious replacement.

In 1992 it was found that methyl chloride, methyl bromide, and more highly halogenated methane derivatives are formed in the environment. These compounds are produced by microorganisms living in salt marshes, coastal lands, decaying vegetation and in beds of kelp (seaweed) growing on the ocean floor. This is the principal source of atmospheric methyl chloride, since the compound is not manufactured in large amounts. Methyl chloride is a source of the stratospheric chlorine that destroys the ozone layer (Section 5.2.3.2).

Since these compounds have been in the environment for a long time, it is likely that there are microorganisms that metabolize them for their carbon and energy. Indeed, in a field that was fumigated with methyl bromide, microbiolo-gists found an anaerobic microbe that uses this compound as its sole energy source. Other microorganisms were discovered in the environment, both aerobes and anaerobes, which metabolize these halomethane derivatives. Aerobes oxidize them to carbon dioxide and halide ions. Anaerobes use them to methylate the sulfide ion formed by the reduction of sulfate to generate the volatile dimethyl sulfide. These metabolic pathways proceed more rapidly than the simple hydrolysis of methyl halides to methanol but more slowly than the rate of formation of these haloorganics.

These new developments have resulted in extensive study of the sources and sinks of methyl bromide in the environment. Anthropomorphic sources of methyl bromide include its release during fumigation, biomass burning, and combustion of leaded gasoline in automobile engines (Section 6.6). These sources, together with production in the environment, are estimated to put 150 x 106 kg of methyl bromide into the atmosphere each year. The use of methyl bromide for fumigation releases 41 x 106 kg a year, or about 30% of the total amount released into the atmosphere. It is possible, however, to reduce the amount emitted during soil fumigation by a factor of 10 by covering the fields with a sheet of nonpermeable polymer while the methyl bromide is injected into the soil. The polymer sheet prevents the escape of methyl bromide to the atmosphere. If the field is covered for several weeks, the methyl bromide decomposes and is not released when the sheets of plastic are removed. If this practice is shown to be applicable on a large scale, it may not be necessary to ban the use of methyl bromide as an agricultural fumigant. At present there are no plans to rescind the ban on the production of methyl bromide in industrialized countries.

8.7 POLYCHLORINATED AND POLYBROMINATED BIPHENYLS

Polychlorinated biphenyls (PCBs) were first used industrially in the 1930s, but their use was not widespread until the 1950s. Their extreme stability made them especially effective in many applications. For example, they are resistant to acids, bases, heat, and oxidation. This stability had prompted their use as heat exchange fluids and dielectrics in transformers and capacitors. They were also used in large amounts as plasticizers and solvents for plastics and printing inks and in carbon paper. It is estimated that in the 50-year period of their use, over 1.4 billion pounds was manufactured and several hundred million pounds released in the environment.

PCBs were first available commercially in the United States in 1929. The last major manufacturer of PCBs was Monsanto, under the trade name of Arochlor. These products are identified by a four-digit number; the first two digits indicate if it is pure or a mixture, and the last the second two digits designate the weight percent chlorine present. The annual sales of Arochlors increased from 40 million pounds in 1957 to 85 million pounds in 1970. The cumulative U.S. production is estimated at one billion (109) pounds. World production was 200 million pounds in 1970. Because of the increasing environmental concerns, Monsanto stopped selling Arochlors in 1970 for use where they could not be recovered and terminated manufacture of them completely in 1977 when the EPA recommended that PCBs not be used as heat transfer fluids in the production of foods, drugs, and cosmetics.

PCB production has been banned in most of the industrial nations of the world. Russia, the principal exception, claims that because all its transformers were built on the use of PCBs as the dielectric, it must continue to manufacture PCBs for these transformers until 2005. The government has pledged to destroy all the unused stock of PCBs in 2020. Thus it appears likely that Russia will be a major source of PCBs in the environment in the twenty-first century. PCB manufacture also continues in some nonindustrialized countries in the world.

8.7.1 Environmental Problems

The stability of PCBs, which makes them so useful commercially, also results in their persistence in the environment. The first report that these compounds were prevalent and were being ingested by aquatic life came in 1966, when Jensen detected PCBs in fish. Since the production of PCBs has been banned in the United States, the current sources in the environment are evaporation from landfills, incineration of household trash in backyard barrels, volatilization from lakes and other repositories, and synthesis and use elsewhere in the world. For example, since PCBs have been dumped into and washed into the Great Lakes for many years, these lakes are now one of the major sources of atmospheric PCBs. PCBs bound to dust particles have been carried over the globe by atmospheric circulation and are present in wildlife from the Antarctic to isolated Pacific atolls. Their atmospheric concentrations are highest near urban centers in the United States and Europe, where they are present in higher quantities than in rural areas.

8.7.2 Toxicity

Early concerns regarding the toxicity of PCBs were fueled by the "Yusho incident" in Japan in 1968, when more than a thousand persons ate rice oil contaminated with PCBs that had leaked from a heat exchanger used to process the rice oil. Persons who ate 0.5 g or more (average consumption was 2g) developed darkened skin, eye damage, and severe acne. It is not certain whether the subsequent deaths of some of the patients were due to the PCB poisoning. Recovery was slow, with symptoms still present after three years. Several infants were born with the same symptoms, demonstrating that PCBs can readily cross the placental barrier. Similar effects were observed on Taiwan in 1979, when cooking oil contaminated with PCBs was used by many people.

While the concentration of PCBs in lakes, rivers, and oceans is low because of the compounds' low solubility in water, concentrations in animals that feed almost exclusively on fish can build up to 100 million times the concentration in water. Animals at the end of a food chain, such as those that eat fish, consume vast amounts of fish in their lifetimes, with the result that even though an individual fish may contain a low level of PCBs, the cumulative amount consumed is very large. The PCBs remain dissolved in the livers and other fatty tissues of these animals because they are so insoluble in water and because they are degraded very slowly by cytochrome P-450 in the liver. For example, herring gulls, eagles, bottlenose dolphins, seals, killer whales, and beluga whales all have been found to contain high levels (from 10 to > 200 ppm) of PCBs. The normal PCB concentration in humans is 1-2 ppm.

It is known that mink are very sensitive to PCBs; only 5 ppm causes complete reproductive failure. In contrast, there is little decrease in survival of offspring in rats containing 100 ppm. There have been several episodes of chicken poisoning by feed containing PCBs. As little as 10-20 ppm causes enlarged livers, kidney damage, decreased egg production, and decreased hatchability of the eggs produced, as well as developmental defects in the chicks that do hatch. Similar effects have been observed in the birds living on the shores of the Great Lakes. Some species of shellfish, shrimp, and fish also exhibit sensitivity to PCBs.

There is also some concern that chronic toxic effects may occur in the animals at the end of food chains that contain high levels of PCBs. It is difficult to assign such effects unambiguously to PCBs because the animals also contain other stable chloroorganics in their fatty tissues. In addition, the biological effects vary with the degree of chlorination of the biphenyls and with the polychlorinated benzofuran [see Section 8.7.3, reaction (8-30)] content of the PCBs. Nor is it clear that all the toxic effects observed in humans who ingested PCB-containing cooking oil in Japan and Taiwan were due to the PCBs, since chlorobenzofurans present might have been implicated as well. PCBs and benzofurans have environmental half-lives of about 15 years, so significant amounts will be in the environment well into the twenty-first century.

It was reported in 2000 that PCBs and their dioxin and benzofuran impurities may inhibit the development of children. The study showed that young breast-fed children appear to have a weakened immune system in comparison to those fed formulas. The mothers' milk contained PCBs, while the formulas did not. The effect on the children's immune systems was small, but the breastfed children were eight times more likely to get chicken pox and three times more likely to have at least six ear infections. This study will have to be confirmed by independent investigators before the conclusions can be generally accepted.

8.7.2.1 Polybrominated Biphenyls [PBBs]

Polybrominated biphenyls, first sold commercially in 1970, were produced in much smaller amounts (5 x 106 lb./year) than PCBs. They were used as flame-retardant and fire proofing materials in the polymers in typewriters, calculators, televisions receivers, radios, and other appliances. Their chemical properties are similar to those of PCBs in that they are very stable. They are an environmental problem in the Pine River, in St. Louis, Michigan, where about a quarter-pound was spilled daily during manufacture. Their deleterious effect to livestock and fowl was observed in 1973 when PBBs, sold under the commercial name Firemaster, were mistakenly substituted for Nutri-master, a dairy feed supplement, in animal feed. Animals fed PBBs were not fit for human consumption, and 20,000 cattle, pigs, and sheep had to be destroyed along with 1.5 million domestic fowl. The PBBs caused weight loss, decreased milk production, hair loss, abnormal hoof growth, abortions, and stillbirths. The manufacture of PBBs in the United States was banned in 1977.

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