Chemistry of PCBs
Polychlorinated biphenyls (PCBs) are the chlorinated derivatives of a class of aromatic organic compounds called biphenyls and are manufactured by the direct chlorination of the biphenyl ring system (Sittig, et al., 1980). Their empirical formula is the following: C12H10-xClx. PCBs are also referred to by the trade names Aroclor, Phenoclor and Kanechlor. The 209 PCB congeners are named as follows:
Chemical properties of PCBs include a high dielectric constant, high solubility in hydrocarbons and virtual insolubility in water (i.e. high lipophilicity). They are nonvolatile, chemically inert and do not undergo oxidation, reduction, addition, elimination or electrophillic substitution reactions except under extreme conditions (Sittig, et al., 1980). Most PCBs are oily liquids whose colour darkens and viscosity increases with rising chlorine content. In terms of toxicity the most toxic congeners are the ones in which there is a coplanar conformation with chlorine substituents on the meta and para positions of the phenyl rings.
Studies have been conducted on the solubility of PCBs in water and it was determined that there was an inverse correlation between PCB solubility and degree of chlorination. The solubility of Aroclor is less than 0.1m g/L in fresh water and 0.04m g/L in marine water (Sittig, et al., 1981). The octanol water partition coefficient for tri, tetra and pentachlorobiphenyls is in the range of 10,000 to 20,000. Soil or sediment characteristics that affect the mobility of the PCBs include soil density, particle size distribution, moisture content, and permeability. In general soil sorption is more likely to occur in the more heavily chlorinated PCB congeners.
Due to their persistence in the environment and the fact that they are poorly metabolized PCBs accumulate in the environment. Analytical methods used to determine PCB levels in the air and water include gas chromatography or gas chromatography plus mass spectrometry.
Polychlorinated biphenyls are widely used as electrical insulators for capacitors and transformers, an ingredient in paints, and as thermotransfer mediums because of their high chemical stability. In summary the chemical properties that make PCBs ideal for use in industry are also the reasons for their toxic affect on our environment.
Manufacture and Distribution of PCBs
Polychlorinated Biphenyls were first produced in 1929 for a wide variety of uses that ranged from being an extender in insecticide production to being used as an insulator in transformer production. PCBs have a number of useful physical properties that can be amplified by mixing different PCB congeners. It is their unique physical properties that made them attractive compounds for industries. As more uses were found for PCBs, their production increased exponentially from 1,000,000 kg in the 1930's to an estimated high of 20 x 108 kg in 1975 (Neely,1994). The first indication that PCB's may be damaging to human health occurred four decades after PCB's were first introduced into the environment. Preliminary studies suggested that PCB's may pose a serious health threat to humans, and at the same time there were indications of wide spread distribution and longevity throughout the environment. As more attention was turned towards PCB's, it became clearer that PCB's were having a negative impact on many biological systems. And as the extent of PCB contamination became more of a public issue, pressure was mounted to ban their production and importation. Major producers such as Monsanto Co. voluntarily stopped production of open system PCB's (insecticides, coatings) but continued with the production of closed system PCBs (transformers, capacitors) until government legislation eventually banned all production and importation of polychlorinated biphenyls.
While it is believed that the total amount of PCBs produced annually has declined since the 1970s, this does not necessarily imply that the toxicological threat posed by these compounds has also decreased. Few safe disposal options exist for PCBs. In Germany, for example, PCBs are being buried in the shafts of salt mines. In such disposal facilities, residues will eventually find their way to watercourses and to the sea (Phillips, 1994). This implies that ecotoxicological problems created by PCB contamination will be evident for many years to come, despite the restrictions on PCB utilization. The investigations of Tanabe and co-workers have been instrumental in defining the non-ortho coplanar PCBs as those isomers of greatest toxicological threat. Studies have also shown that the non-ortho coplanar PCBs are very persistent in aquatic environments. Significant levels of these PCBs may therefore bioconcentrate in exposed organisms. It is also probable that such compounds are transferred through the food web (Phillips, 1994). Such a transfer results in biomagnification of PCB concentration as one moves up the food chain.
PCBs are widespread contaminants in the environment primarily in soil and freshwater systems (Erikson, 1986). Their persistence in the environment has been a growing concern due to their low degradability, toxicity, mutagenicity and because of their tendency to bioaccumulate (Bedard & Habel, 1990). These compounds have been shown to undergo biodegradation under a variety of conditions in the laboratory and in the field. Polychlorinated biphenyls (PCBs) are a group of industrial chemicals that share a common structure.
PCBs were commercially produced as complex mixtures beginning in 1929 and are not known to occur naturally in the environment. Monsanto Corporation was a major producer of PCBs and marketed them under the trade name Aroclor from 1930-1977. Principle uses of PCBs such as Aroclor include uses in transformers, capacitors, printing inks, paints, dedusting agents, pesticides, plasticisers, lubricant inks, paint additives etc., and they were marketed for these uses (Erickson, 1986). It is because of PCBs chemical and physical stability as well as their electrical insulating properties that make them so commercially attractive.
PCBs occur as liquids or solids, and they are clear to light yellow in colour. They have no odour or taste. Because of their thermal stability they do not easily burn, hence their past popularly as coolants, insulating materials and their wide use for electrical applications. PCBs are difficult to oxidize and reduce. They have very low water solubility and therefore are highly lipophilic (log p > 6 for some). PCBs also have high electrical restivity or low electrical conductance. Commercial PCB mixtures were synthesized by chlorination of biphenyl with chlorine gas. The average degree of chlorination was controlled by the reaction conditions in order to yield the desired chemical and physical properties (Erickson, 1986). Today, the production of PCBs has been ceased in North America with the exception of small quantities manufactured strictly for research purposes. PCBs are an environmental hazard due to their inability to degrade in the environment. They are highly lipophilic leading to their profound persistence and ability to bioaccumulate. These `foreign' compounds are not readily metabolized by most living organisms and despite the cessation of their production remain a widely distributed health, ecological and environmental concern.
Transport of PCBs within the Environment
PCB contamination in the environment has come exclusively from human activities. Thus high PCB concentration areas tend to be around industrialized areas, like the Great Lakes. PCBs enter the general environment mainly by leakage of supposedly closed systems, from landfill sites, incineration of waste, agricultural lands, industrial discharges and sewage effluents. PCBs are also widely dispersed in the atmosphere where they are transported by winds and fall to the surface in precipitation. Approximately 98% of the PCBs entering the ocean are currently deposited form the atmosphere. Factors such as air temperature, wind speed, storm frequency, rainfall rates and the volatility of individual PCB isomers influence the pattern and rates of PCB movement in the atmosphere. The principal transport route for PCBs through aquatic systems is from waste streams into receiving waters, with further downstream movement occurring by solution and readsorption onto particles as well as by the movement of sediments. This leaves the marine environment as one of the final sinks for PCBs.
Aquatic Systems - PCBs are found in higher concentrations in the sediments of aquatic systems due to their chemical and physical properties which cause high sorption reactions. Sorption increases with chlorine content, surface area and with the organic content of the sorbent. Therefore, PCBs sorb onto falling sediments that eventually end up as bottom sediments. PCBs are associated particularly with suspended sediments of a diameter less than 0.15mm (US EPA, 1980). The release of PCBs from sediments to overlying waters can occur by slow desorption, especially when PCB concentrations are high or when sudden hydrographic activity like flooding or dredging causes sediments to be resuspended and redistributed. Translocation can also occur through biological activity (Halter and Johnson, 1977). Desorption of PCBs from particulate is more likely to occur from lower chlorinated, more water-soluble PCB congeners (Wood et al, 1987).
Air Systems (Great Lakes) - Airborne contamination has been recognized as a significant source of PCB contamination in the Great Lakes Basin since the mid-1980’s. This is especially true for Lake Superior which is still largely isolated from industrial and municipal sources (Hileman, 1988). Strachan and Eisenreich (1988) estimate that 90% of the PCBs in Lake Superior come from that source.
Many of the sources of airborne PCBs to the Great Lakes Basin, especially those with unusually high concentrations, tend to originate from the southern U.S., Mexico, and Latin America (Atmospheric Environment Service, 1992).
Soil Systems - Sorption reactions also affect transport in soils. PCBs that are sorbed by soils, especially highly chlorinated ones, remain significantly immobile against leaching. They are also unlikely to be taken up by plants and therefore are not readily mobile in soil systems. However, because PCBs have a moderate vapour pressure, vapour phase transport may allow for redistribution or migration through the saturated soil pores
Fate of Compound
Vast quantities of PCB’s have been dumped, spilled or have leaked into the environment. These toxins have accumulated in the air, water, and land, and pose health hazards to many animals, and perhaps some plants.
The dominant mechanism by which PCB’s enter the Great Lakes and most other bodies of water is by atmospheric deposition. PCB’s have a tendency not to stay dissolved in water and therefor volatize back into the atmosphere. Because of this, large quantities of PCB’s volatize out of lakes as well as being deposited into them from the large reservoir of synthesized organic compounds moving in regional and global air masses.
PCB’s in water directly and indirectly is the main source of the toxic effects for animal life. PCB’s in water or sediments may be filtered through fish, crustaceans, or mollusks via their gills and because they are lipophillic they make their way to the fatty tissues of these animals. These animals are in turn eaten by birds; or mammals like humans or bears. In these animals again the PCB’s accumulate in fatty tissues, and their concentration again becomes greater due to biomagnification. Birds’ eggs often have the highest concentrations of PCB’s that is easily testable because they are at the end of the food chain and their yolk is rich in fatty materials. In the Great Lakes gull eggs are often collected for chemical analysis because toxic chemicals will be detectable in them long before their levels in the open water are measurable. Analysis of eggs at different sites allows us to compare the extent of contamination between different areas.
The State of the Lakes is an illustration showing areas of concern for levels is PCB’s in the environment of the Great Lakes, developed through testing of bird eggs and fish.
PCB’s accumulate in humans too, especially if their diet contains fish or animals that eat fish from polluted areas. A study was done in Norway on breast milk and it was found that the milk contained approximately 372 ng/g milk fat of PCB’s with PCB-126 being the main contributor. (Johansen, et.al., 1994) This study concluded that breast-feeding did not significantly increase risk to infants. Simply being born to a mother that has high levels of PCB’s in her body may contribute to a decrease in birth weight, head circumference, short-term memory, and some cognitive skills. (The report of the Atmospheric Deposition Monitoring Taskforce, 1987)
In recent years their production, importation and use have been banned or tightly controlled in many countries. They were considered suitable for use in capacitors and transformers in fire sensitive locations. However, fires involving equipment containing PCB's can produce toxic by-products such as polychlorinated dibenzo-para-dioxins (commonly known as dioxins) and polychlorinated dibenzofurans. Consequently, PCB-containing equipment is now being phased out for all uses. Tredi's PCBs disposal plant is located at Saint-Vulbas, 40 km from Lyon in southern France and has been operated by Tredi since 1981. Tredi Saint-Vulbas specializes in: incineration of highly halogenated organic waste including chlorides, bromides, fluorines and iodines, incineration of PCBs, decontamination of PCB insulated electrical equipment
The plant is equipped with 2 high temperature incinerators and 9 autoclaves:
Electrical transformers are drained and then decontaminated in the autoclave. Metal components are recycled, and the PCBs and wood and cellulose components of the transformer are incinerated.
Electrical capacitors are drained, shredded and then incinerated. The plant also handles other contaminated solid waste such as soils, concrete and sludge.
All activities at Tredi Saint-Vulbas are supported and monitored by the plant laboratory which is fully equipped for analysis of clients waste and the site operations. Operating licenses for the plant are issued by French government authorities and monitored by a local industrial zone surveillance committee made up of local residents, local government authorities and Tredi personnel. Tredi provides a worldwide service to respond to all problems involving PCBs.
BIODEGRADATION OF PCBs
As a result of their very stable properties, PCBs are synthetic compounds that are not readily degraded. The degradation of these compounds entails difficult mechanisms of chemical, biochemical or thermal destruction (Erickson, 1986).
Biodegradation, that is, the degradation of compounds by bacteria or other microorganisms, is a slow yet possible method for destroying PCBs in both aerobic and anaerobic environments. It is the only process known to degrade PCBs in soil systems or aquatic environments. The specific processes involved are aerobic oxidative dechlorination or hydrolytic dehalogenation and anaerobic reductive dechlorination. Theoretically, the biological degradation of PCBs should result to give CO2, chlorine and water. This process involves the removal of chlorine from the biphenyl ring followed by cleavage and oxidation of the resulting compound (Boyle et al., 1992). Persistence of PCBs in the environment increases with the degree of chlorination of the congener. Those compounds with a high degree of chlorination such as 1248, 1254 and 1260 are resistant to biodegradation and degrade slowly in the environment.
Aerobic Oxidative Dehalogenation involves the oxidation of PCBs by aerobic microbes, especially by bacteria of the genus Pseudomonas. This involves the addition of oxygen to the biphenyl ring (Boyle et al., 1992). Further research by Bevinakatti and Ninnekar (1992) proposed the degradation of biphenyls also by the Micrococcus sp. The metabolic pathway used by this family of bacteria resembles that described for the Pseudomonas sp. which is illustrated below.
Figure 1: A possible pathway for the aerobic oxidative dehalogenation of PCBs. (Bevinakatti and Ninnebar, 1992)
By way of 1,2- dioxygenative ring cleavage, benzoate results as a common by-product of biphenyl degradation. Although different bacterial species seem to produce benzoate through PCB metabolism, further breakdown of benzoate seems to differ among the different microbes. Nevertheless, the by-products produced are less toxic compounds to people and the environment (Bevinakatti and Ninnebar, 1992).
Since PCBs are more persistent with increasing chlorination of the congener, aerobic biodegradation involving biphenyl ring cleavage, is restricted to the lightly chlorinated congeners (U.S. DHHS, 1992).
Anaerobic Reductive Dechlorination involves the replacement of chlorine with a hydrogen atom on the biphenyl ring. This type of degradation transforms the more highly chlorinated congeners to less chlorinated ones. Specifically, the monochlorobiphenyls and ortho-substituted dichlorobipenyls are degraded in this manner. Byproducts of this process are less toxic and can usually be degraded by the aerobic microbes. (Ye et al., 1992).
Figure 2 Potential pathway for anaerobic degradation of a highly chlorinated congener to a less chlorinated one (Fish and Principe, 1994).
The different pathways of dechlorination observed may be explained by the different microbial populations that exist in the environment (Alder et al., 1993). However, a similarity between degradation patterns exists. The para- and meta-substituted congeners are more commonly degraded than ortho-substituted congeners. Only a few ortho-substituted congeners have been reported to degrade (Fish and Principe, 1994). Moreover, anaerobic degradation has most commonly been observed under methanogenic conditions.
This may lead one to conclude that anaerobic reductive dechlorination occurs under methanogenic conditions, if not inhibited by sulfate-reducing conditions. Sulfates have a higher affinity for electrons than the chloroaromatics (Alder et al., 1993).
Indeed, many environmental factors can affect the degradation of biphenyls, both aerobically and anaerobically. Rates are quite variable depending on the conditions present in the environment. These factors may include;
>type of microbial population
>available nutrients
>pH
>temperature
As previously stated, more highly chlorinated congeners are less readily degraded than the less chlorinated congeners. The position of chlorine atoms on the rings also affects the rate of biodegradation. Not only are PCBs with para- and meta-substituted rings more easily degraded than the ortho- substituted compounds, but PCBs containing all chlorines on one ring are biodegraded faster than those which contain chlorines throughout both rings. It has been suggested that both aerobic and anaerobic conditions are affected with the addition of certain nutrients.
It is also interesting to note that biodegradation rates decrease with high levels of organic carbon present. (U.S. DHHS, 1992).
Alternatively, some other methods of PCB destruction are being used, developed and investigated. More popular is incineration and photolysis. Incineration involves exposing the PCB congeners to extremely high temperatures (1200° C), a mixing /agitation process as well as a long residence time (>2 sec) (Erickson, 1986). Photolysis on the other hand is the most popular means of chemical degradation of PCBs. This process uses the free radicals produced from sunlight to remove the chlorine atoms from the biphenyl ring. This method however, is limited to PCBs found in the air and the water (U.S. DHHS, 1992).
Other destruction methods, both chemical and biological are:
CHEMICAL TECHNIQUES BIOLOGICAL PROCESSES
| adsorption chlorinolysis catalytic dehydrochlorination microwave plasma ozonation wet air oxidation reaction with sodium Naphthalene reaction with molten sodium reaction with sodium salt in amine solvent |
activated sludge trickling filters special bacterial methods |
References:
Ackerman, D.G., Scinto, L.L., Bakshi, P.S., Delumyea, R.G., Johnson, R.J., Richard, G., Takata, A.M., and Sworzyn, E.M. 1983. Destruction and Disposal of PCBs by Thermal and Non-Thermal Methods. Noyes Data Corporation, New Jersey, USA. pages 16-19 and 370-380.
Alder, A.C., M.M. Haggblom, S.R. Oppenheimer, L.Y. Young, 1993, Reductive Dechlorination of Polychlorinated Biphenyls in Anaerobic Sediments, Environ. Sci. Technol., Vol. 27, pp. 530-538
Authority of the Minister of the Environment, Probing Our Changing Atmosphere: Air Pollution Research, Downsview, Ontario: Minister of Supply and Services Canada, 1992.
Benvinakatti, B.G., and H.Z. Ninnekar, 1992, Degradation of Biphenyl by a Micrococcus Species, Applied Microbiology and Biotechnology, Vol. 38, pp. 273-275.
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Fish, K.M., and J.M. Principe, 1994, Biotransformations of Arochlor 1242 in Hudson River Test Tube Microcosms, Applied and Environmental Microbiology, Vol. 60, No.12, pp.4289-4296.
Hileman, Bette, "The Great Lakes Cleanup Effort." Chemistry and Engineering, February 8, 1988, pp. 22-39.
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Johansen, R; Becher, G; Polder, A.; et.al; Congener-Specific Determination of Polychlorinated Biphenyls And Organochlorine Pesticides In Human Milk From Norwegian Mothers Living is Oslo; Journal of Toxicology and Environmental Health 42:157-171; Taylor and Francis (1994).
Klasson, K.T.; Barton, J.W.; Evans, B.S., et.al.; Reductive Microbial Dechlorination of Indigenous Polychlorinated Biphenyls in Soil Using a Sediment-Free Inoculum; Biotechnology Progress 12, 310-315; (1996)
Mullin, M.D.; Pochini, C.M.; McCrindle, S.; et.al. High Resolution PCB Analysis: Synthesis and Chromatographic Properties of all 209 PCB Congeners; Environ. Sci. Technol. 1984, 18, 468-476.
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Sittig, M., et al. (1981) Handbook of Toxic and Hazardous Chemicals, Noyes Publications, New Jersey.
U.S. Department of Health & Human Services, 1992, Toxicological Profile for Selected PCBs (Arochlor-1260, -1254, -1248, -1242, -1232, -1221, and 1016).
Ye, D., J.F. Quensen III, J.M. Tiedje, and S.A. Boyd, 1992, Anaerobic Dechlorination of Polychlorinated Biphenyls (Arochlor 1242) by Pasteurized and Ethanol-Treated Microorganisms from Sediments, Applied and Environmental Microbiology, Vol. 58, No.4, pp. 1110-1114.
Jill Howarth
Anita Mirabelli
Leanne Peck
