PCB Contamination by Furans

Strong heating of PCBs in the presence of a source of oxygen can result in the production of small amounts of furans. These compounds are structurally similar to dioxins; they differ only in that the molecules are missing one oxygen atom in the central ring. The furan ring contains five atoms, one of which is oxygen and the other four of which are carbon atoms that participate in double bonds:

furan furan

The dibenzofurans (DFs) have a benzene ring fused to opposite sides of the furan ring:

dibenzofuran dibenzofuran

As with dioxins, all chlorinated dibenzofuran congeners are planar; i.e., all C, O, H, and CI atoms lie in the same plane. They are formed from

PCBs by the elimination of the atoms X and Y bonded to two carbons that are ortho in position to those that link the rings and that lie on the same side of the C—C link between the rings:

PCB dibenzofuran

PCB dibenzofuran

The atoms X and Y can both be chlorine, or one can be hydrogen and the other one chlorine, so the molecule eliminated can be Cl2 or C1H (i.e., HC1), respectively. A more detailed analysis of the nature of the specific furans that result from particular PCB congeners is given in Box 11-2.

Most of the chlorine in the original PCB molecule is still present in the dibenzofuran; polychlorinated dibenzofurans are known commonly as PCDFs, The numbering scheme for substituents is the same as that for dioxins (PCDDs); note, however, that by convention the numbering starts next to a carbon that forms the single C—C bond opposite the oxygen.

9 1

9 1

While there exist 75 different chlorine-substituted dibenzo-p-dioxins, there are 135 dibenzofuran congeners, since the symmetry of the ring system is lower for furans. In particular, although the furans have the same left-right symmetry as dioxins, they do not have their up-down symmetry.

PROBLEM 11-9

Draw the structures of all the 16 unique dichlorodibenzofurans, and deduce the numbering required in their names. [Hint: Use a systematic procedure to generate all, but include only those congeners that correspond to unique molecules; i.e., be careful to eliminate duplicates. For example, start by placing one chlorine at C'l and then generate all the possible isomers corresponding to different positions for the second chlorine. Then place the first chlorine at C-2 and repeat the procedure, noting that the 1,2-dichbro isomer is generated both times. Continue the procedure with the first chlorine at C-3, etc.J

BOX 11-2

Predicting the Furans That Will Form from a Given PCB

In deducing the nature of the polychlori-nated dibenzofuran (PCDF) that would he formed from, a particular PCB, it should be remembered that free rotation occurs about the single bond joining the two rings in the original biphenyl in all PCBs at the elevated temperatures of the reaction. Thus HC1 elimination in 2,3'-dichlorobiphenyl gives both 4- and 2-chlorodibenzofuran.

Ortha Meta Migration Pcb

N^otation

4-chlorodibenzofuran

N^otation

H CI

4-chlorodibenzofuran

At the high temperatures of this reaction, some interchange of the adjacent substituents in the 2 and 3 positions (ortho and meta) of any given ring can occur as a prelude to HC1 elimination; in particular chlorine can move from an ortho to a meta position, and hydrogen from meta to ortho, preceding HCI elimination. For example, when 2,6,2',6'-tetrachlorobiphenyl (see next page) is heated in air, some of its molecules lose a pair of ortho chlorines to give a dichlorodibenzofu-ran, and some first interchange CI and H in one ring to eliminate HC1 and produce a trichlorodibenzofuran. Free rotation about the C—C bond does not occur after the interchange, as presumably the elimination occurs immediately.

H CI

2 -chlorodibenzofuran

Almost all commercial PCB samples are contaminated with some PCDFs, but this usually amounts to only a few ppm in the originally manufactured liquids. However, if the PCBs are heated to high temperatures and if some oxygen is present, conversion of PCBs to PCDFs increases the level of contamination by orders of magnitude. The furan concentration in used PCB cooling fluids is found to be greater than in the virgin materials, presumably due to the moderate heating that the fluid undergoes during its normal use.

CI ci CI CI

Cl Ci H

2,6,2 ' ,6 ' -tetrachlorobiphenyl heat

1,9-dichlorodibenzofuran

Cl H Cl

Cl H Cl

1,4,9-trichlorodibenzofuran

PROBLEM 1

For each PCB shown below, deduce which furans would be expected to be produced by Cl2 or HQ elimination when the PCB is heated in air. Write the correct name for each PCDF.

CI CI

PROBLEM 2

Recently it has been discovered that upon strong heating in air PCBs can also react by elimination of two ortho hydrogen atoms (one on each ring) as H2. Decide which, if any, additional PCDFs will be produced if the PCBs in Problem 1 can eliminate H2.

CI CI CI

Furan production also occurs if one attempts to burn PCBs with anything but an unusually hot flame.

Other Sources of Dioxins and Furans

In addition to the sources discussed above, polychlorinated dibenzofurans and dibenzo-f>-dioxins are also produced as by-products in a myriad of processes, including the bleaching of pulp, the incineration of garbage and hospital waste, the recycling of metals and sintering of iron ore, and the production of common solvents such as tri- and perchbroethene.

Pulp-and-Paper Mills

Pulp-and-paper mills that use elemental chlorine, Cl2, to bleach pulp are dioxin and furan sources. These contaminants, among many other chlorinated compounds, result from the reaction of the chlorine with some of the organic molecules released by the pulp. The tan color of the pulp that has undergone the initial stages of processing is due to the light-absorbing properties of the lignin component of the original wood fibers. A generalized structure for lignin is shown in Figure 11-4- In order to make white paper, the residual 10%) component of the lignin still present after initial processing must be removed, usually by bleaching the pulp with oxidizing agents. If you examine the generalized structure of lignin in Figure 11-4, you can observe several sites of monosubstituted phenols and phenolic ethers as well as ortho-suhstituted phenyl diethers. From these structural components, it is not difficult to imagine how lignin can serve as a precursor to furans and dioxins when it reacts with chlorinating agents such as Cl2-

Worksheets Connect The Dots
FIGURE 11-4 Generalized structure of lignin. [Source: M. C. Cann and M. E. Connelly, Real-World Cases in Green Chemistry (Washington D.C.: American Chemical Society, 2000).]

More furans than dioxins are formed in the bleaching of pulp by elemental chlorine. The furan congeners of highest concentrations in the pulp are 1,2,7,8-TCDF and the more toxic 2,3,7,8-TCDF. Unfortunately, the most abundant dioxin produced by the pulp-and-paper bleaching process is the highly toxic 2,3,7,8-TCDD congener. The paper and effluent contain dioxins at parts-per-trillion levels, which resulted in total releases in the past, in North America, of several hundred grams of 2,3,7,8-TCDD annually.

Because of the problems of producing dioxins and furans, the use of elemental chlorine as a bleaching agent for paper was banned in the United States as of April 2001. Most pulp-and-paper mills there and in other developed countries switched their bleaching agent from elemental chlorine to chlorine dioxide, C102, from which the furan and dioxin output is much smaller, even undetectable in many cases. The difference is due to the mechanism by which the compounds attack the pulp's residual lignin. Elemental chlorine reacts to insert chlorine as a substituent on the aromatic rings in lignin, yielding products that are soluble in alkali and that can then be washed away. Experiments suggest that during the oxidation of the lignin, two of the component benzene rings can couple together to form a dibenzofuran or dibenzo-p-dioxin system that subsequently is chlorinated and in the process becomes detached from the lignin system. In contrast, chlorine dioxide destroys the aromaticity of the benzene rings by free-radical processes and therefore produces fewer chlorinated products that contain six-membered rings.

Some mills now produce paper pulp without any use of chlorine compounds. Ozone, hydrogen peroxide, and even high-pressure oxygen are the alternative bleaching agents used in these totally chlorine free (TCF) pulp mills. Mills that still use chlorine to bleach now remove contaminants from wastewater by treatments such as reverse osmosis (see Chapter 14).

The use of chlorine to disinfect drinking water and the chlorinated byproducts that are formed in the process are discussed in Chapter 14-

Green Chemistry: H202, an Environmentally Benign Bleaching Agent for the Production of Paper

TCF bleaching agents for paper such as hydrogen peroxide (H202), ozone, and diatomic oxygen have been developed. While TCF agents eliminate the formation of dioxins and furans, these methods in general are problematic because these oxidizing agents are not as strong as elemental chlorine or chlorine dioxide. Thus they generally require longer reaction times and higher temperatures (more energy input), and they lead to significant breakdown of the cellulose fibers, which weakens the paper, requiring more wood to produce the same amount of paper.

Terry Collins of Carnegie Mellon University earned a Presidential Green Chemistry Challenge Award in 1999 for his development of compounds

FIGURE 11-5 Tetraamido-macrocyclic ligands (TAML): activators for hydrogen peroxide, [Source: M. C. Cann and M. E, Connelly, Real-World Cases in Creen Chemistry (Washington D.C.: American Chemical Society, 2000).]

known as tetraamido-macroeyclie ligands (TAMLs, Figure 11-5), which enhance the oxidizing strength of hydrogen peroxide. Hydrogen peroxide is a particularly enticing oxidizing reagent since its by-products are water and oxygen, which are environmentally benign. The use of TAML in conjunction with hydrogen peroxide reduces the temperature and reaction times normally required for bleaching paper with hydrogen peroxide, thus making hydrogen peroxide a viable alternative for this process. See also the feature article "Little Green Molecules" immediately following the Introduction to this book»

The TAMLs can be modified by varying the alkyl groups (R) on the right side of the structure in Figure 11-5. Changing these groups influences the lifetime of these catalysts. In uses such as the bleaching of paper it is important for the TAML catalysts to decompose in a relatively short period of time so they do not become a burden to the environment. However, they must last long enough to fulfill their role as catalysts for hydrogen peroxide. TAML catalysts not only offer significant promise for the bleaching of paper, they are also being considered for use in laundry applications, the disinfection of water, and the decontamination of biological warfare agents such as anthrax.

Fires and Incineration as Sources of Dioxins and Furans

Fires of many kinds, including forest fires and those in incinerators, release various congeners of the dioxin and furan families into the environment; these chemicals are produced as minor by-products from the chlorine and organic matter in the fuel. Dioxin and furan production seem unavoidable whenever combustion of organic matter occurs in the presence of chlorine, unless steps are taken to ensure complete combustion by using very high flame temperatures. Some environmentalists worry particularly about the dioxin emissions when the chlorine-containing plastic PVC is incinerated or involved in other fires. Indeed, research on the combustion of newspapers indicates that chlorinated dioxin and furan production rises as the amount of salt or PVC present also increases. In many environmental samples of combustion products, several dozen different dioxin congeners are found, all in comparable amounts. Congeners with relatively high numbers of chlorine substituents usually are the most prevalent.

Incinerators now are the largest anthropogenic source of dioxins in the environment. Dioxins and furans are formed in the postcombustion zone of incinerators, where the temperature is much lower (250-500°C) than in the flame itself (see Chapter 16). They are formed during the oxidative degradation of the graphite-like structures in the soot particles that were produced during the incomplete combustion of the waste. Trace metal ions in the original waste probably catalyze the process. The small amounts of chlorine in the waste provide more than enough of this element required to partially chlorinate the furans and dioxins. The dibenzofuran and dibenzo-p-dioxin ring systems are formed at high temperatures (> 650°C); chlorination progressively occurs when the temperature cools below 650°C and gradually is reduced to 200°C.

Characteristically, incineration produces a greater mass of furans than dioxins. The yields of specific dioxin congeners increase with the degree of chlorination through to OCDD, whereas the peak production of furans occurs with four to six chlorines. In contrast to waste incineration, industrial coal combustion generates little dioxin because it burns much more completely, generating little soot to decompose later into dioxins and furans.

Chlorine Content of Dioxin and Furan Emissions

The profile of estimated annual global PCDF and PCDD emissions for the congeners with four to eight chlorines, i.e., those believed to be toxic, is shown in Figure 11-6a. As discussed previously, furans outnumber dioxins, and furans peak with congeners having four chlorines, whereas the dioxin

a 200

a 200

F4 F5 F6 F7 F8

F4 F5 F6 F7 F8

D4 D5 D6 D7 D8

F4 F5 F6 F7 F8

F4 F5 F6 F7 F8

D4 D5 D6 D7 D8

FIGURE 11-6 Annual

PCDD and PCDF (a) emissions and (b) deposition rates after reactions with the hydroxyl radical OH, The letters F and D represent furans and dioxins; the numbers indicate the number of chlorine atoms per molecule. [Source:). i. Baker and R. A. Hites, "Is Combustion a Major Source of Polychlorinated Dibenzo-p-Dioxins arid Dibenzofurans to the Environment?" Environmental Science and Technology 34 (2000): 2879,]

peak is less pronounced and occurs with about six chlorines. Furans with these intermediate amounts of chlorine have toxicities similar to that of 2,3,7,8-TCDD, whereas fully chlorinated dioxin molecules have low toxicities. Consequently, the threat to human health from furans in the environment may exceed that from dioxins.

The mass of the dioxin and fiiran compounds that are eventually deposited from air onto soil and sediments, the primary mechanism by which dioxins eventually enter the food chain, is shown in Figure 11-6b. The loss in mass between emission and deposition is greater the fewer the number of chlorines present; hence the significant difference between amounts emitted and deposited for the tetrachloro and pentachloro congeners, but not those more heavily chlorinated, in Figure 11-6. This differentiation occurs because the principal loss mechanism is attacked at an unsubstituted carbon by the hydroxy! free radical, OH (followed by atmospheric oxidation of the resulting radical, as expected from the principles discussed in Chapters 3 and 5), and the rate of this initial reaction is greater the fewer the chlorines present. The amount of OCDD, the octachloro congener of dioxin, that is found to be deposited greatly exceeds the estimate from this figure, which is based mainly on combustion sources. Some scientists believe that the discrepancy arises because much additional OCDD is created in water droplets in air by the sunlight-initiated photochemical decomposition of PCP, pentachlorophenol, which eventually results in coupling of two PCPs to produce OCDD.

Very small concentrations of dioxins—particularly highly chlorinated ones—were present in the environment in the preindustrial era, presumably as a result of forest fires, volcanoes, etc. Indeed, forest fires are still probably Canada's largest source of dioxins. According to the analysis of soils and the sediments in lakes, the g reatest anthropogenic input of dioxins and furans to the environment in developed countries began in the 1930s and 1940s, and peaked in the 1960s and 1970s. The principal sources were combustion/ incineration, the smelting and processing of metals, the chemical industry, and existing environmental reservoirs. Inadvertent production of dioxins continues today, but at a slower rate—about half of the maximum, according to some sediment samples. The decrease in emissions resulted from deliberate steps taken by industrialized nations to reduce the production and dispersal of these toxic by-products. In particular, dioxin emissions from large sources in the United States declined by 75% from 1987 to 1995 alone, primarily due to reductions in air emissions from municipal and medical waste incinerators. New regulations should increase the reduction to 95%. However, uncontrolled combustion of nonpoint sources such as rural backyard trash burning in barrels—especially when plastics such as PVC are included in the mix—has not yet been brought under control.

Once created, dioxins and furans are transported from place to place mainly via the atmosphere (Chapter 12). Eventually they are deposited and can enter the food chain, becoming bioaccumulated in plants and animals. As previously mentioned, our exposure to them arises almost entirely through the foods that we eat. In the next section, we try to answer the question of what effects, if any, this exposure has on our health.

Continue reading here: The Health Effects of Dioxins Furans and PCBs

Was this article helpful?

0 0