Polychlorinated biphenyls (PCBs) are a class of organic compounds with 1 to 10 chlorine atoms attached to biphenyl, which is a molecule composed of two benzene rings. The chemical formula for PCBs is C12H10-xClx, where x = 1-10. PCBs were widely used for many applications, especially as dielectric fluids in transformers and capacitors and coolants. Due to PCB's toxicity and classification as persistent organic pollutants, PCB production was banned by the United States Congress in 1976 and by the Stockholm Convention on Persistent Organic Pollutants in 2001.
PCB congeners are odorless, tasteless, clear to pale-yellow, viscous liquids (the more highly chlorinated mixtures are more viscous and deeper yellow). They are formed by electrophilic chlorination of biphenyl with chlorine gas. Theoretically 209 different PCB congeners are possible, although only about 130 are found in commercial PCB mixtures. PCBs have low water solubilities — 0.0027-0.42 ng/L for Aroclors, and low vapor pressures at room temperature, but they have high solubilities in most organic solvents, oils, and fats. They have high dielectric constants, very high thermal conductivity, high flash points (from 170 to 380 °C) and are chemically fairly inert, being extremely resistant to oxidation, reduction, addition, elimination, and electrophilic substitution. The density varies from 1.182 to 1.566 kg/L. Other physical and chemical properties vary widely across the class. As the degree of chlorination increases, melting point and lipophilicity increase, but vapour pressure and water solubility decrease.
PCBs readily penetrate skin, PVC (polyvinyl chloride), and latex (natural rubber). PCB-resistant materials include Viton, polyethylene, polyvinyl acetate (PVA), polytetrafluoroethylene (PTFE), butyl rubber, nitrile rubber, and Neoprene.
PCBs are very stable compounds and do not degrade readily. Their destruction by chemical, thermal, and biochemical processes is extremely difficult, and presents the risk of generating extremely toxic dibenzodioxins and dibenzofurans through partial oxidation. Intentional degradation as a treatment of unwanted PCBs generally requires high heat or catalysis (see Methods of destruction below).
PCBs were used as coolants and insulating fluids ('transformer oil') for transformers and capacitors especially in components of early fluorescent light fittings, the PRR GG1 locomotive's electrical transformers, plasticizers in paints and cements, stabilizing additives in flexible PVC coatings of electrical wiring and electronic components, pesticide extenders, cutting oils, reactive flame retardants, lubricating oils, hydraulic fluids, sealants (for caulking in schools and commercial buildings), adhesives, wood floor finishes (such as Fabulon and other products of Halowax in the U.S.), paints, de-dusting agents, water-proofing compounds, casting agents, vacuum pump fluids, fixatives in microscopy, surgical implants, and in carbonless copy ("NCR") paper.
PCBs, originally termed "chlorinated diphenyls," were commercially produced as complex mixtures containing multiple isomers at different degrees of chlorination. In the United States, commercial production of PCBs was taken over in 1929 by Monsanto Company from Swann Chemical Company. Manufacturing levels increased in response to the electrical industry's need for a "safer" (than flammable mineral oil) cooling and insulating fluid for industrial transformers and capacitors. PCBs were also commonly used as stabilizing additives in the manufacture of flexible PVC coatings for electrical wiring and electronic components to enhance the heat and fire resistance of the PVC.
The toxicity associated with PCBs and other chlorinated hydrocarbons, including polychlorinated naphthalenes was recognized very early due to a variety of industrial incidents. A conference about the hazards was organized at Harvard School of Public Health in 1937, and a number of publications referring to the toxicity of various chlorinated hydrocarbons were published before 1940. Robert Brown reminded chemists in 1947 that Arochlors were "objectionably toxic. Thus the maximum permissible concentration for an 8-hr. day is 1 mg/m3 of air. They also produce a serious and disfiguring dermatitis". However, PCB manufacture and use continued with few restraints until the 1970s.
PCBs are persistent organic pollutants and have entered the environment through both use and disposal. The environmental transport of PCBs is complex and nearly global in scale. The public, legal, and scientific concerns about PCBs arose from research indicating they were likely carcinogens having the potential to adversely impact the environment and therefore undesirable as commercial products. Despite active research spanning five decades, extensive regulatory actions, and an effective ban on their production since the 1970s, PCBs still persist in the environment and remain a focus of attention.
The only North American producer, Monsanto Company, marketed PCBs under the trade name Aroclor from 1930 to 1977. These were sold under trade names followed by a 4 digit number. The first two digits generally refer to the number of carbon atoms in the biphenyl skeleton (for PCBs this is 12), the second two numbers indicate the percentage of chlorine by mass in the mixture. Thus, Aroclor 1260 has 12 carbon atoms and contains 60% chlorine by mass. An exception is Aroclor 1016, which also has 12 carbon atoms, but has 42% chlorine by mass. Different Aroclors were used at different times and for different applications. In electrical equipment manufacturing in the USA, Aroclor 1260 and Aroclor 1254 were the main mixtures used before 1950, Aroclor 1242 was the main mixture used in the 1950s and 1960s until it was phased out in 1971 and replaced by Aroclor 1016.
Manufacture peaked in the 1960s, by which time the electrical industry had lobbied the U.S. Congress to make them mandatory safety equipment. In 1966, they were determined by Swedish chemist Dr. Soren Jensen to be an environmental contaminant, and it was Dr. Jensen, according to a 1994 article in Sierra, who named them PCBs. Previously, they had simply been called "phenols" or referred to by various trade names, such as Aroclor, Kennechlor, Pyrenol, Chlorinol and others.
Their commercial utility was based largely on their chemical stability, including low flammability, and desirable physical properties, including electrical insulating properties. Their chemical and physical stability has also been responsible for their continuing persistence in the environment, and the lingering interest decades after regulations were imposed to control environmental contamination.
In 1973 the use of PCBs was banned in "open" or "dissipative" sources, such as:
However, they continued to be allowed in "totally enclosed uses" such as transformers and capacitors, which, in certain failure modes or out-of-specification conditions, can leak, catch fire, or explode. It was Ward B. Stone of the New York State Department of Environmental Conservation (NYSDEC) who first published his findings in the early 1970s that PCBs were leaking from transformers and had contaminated the soil at the bottom of utility poles. Concern over the toxicity and persistence (chemical stability) of PCBs in the environment led the United States Congress to ban their domestic production in 1979, although some use continues in closed systems such as capacitors and transformers.
"Enclosed uses" of PCBs include:
In the UK, closed uses of PCBs in new equipment were banned in 1981, when nearly all UK PCB synthesis ceased, but closed uses in existing equipment containing in excess of 5 litres of PCBs were not stopped until December 2000.
In Japan, PCBs were first produced by Kanegafuchi Chemical Co. Ltd. (Kaneka) in 1954 and production continued until 1972 when the Japanese government banned the production, use, and import of PCBs.
Estimates have put the total global production of PCBs on the order of 1.5 million tons. The United States was the single largest producer with over 600,000 tons produced between 1930 and 1977. The European region follows with nearly 450,000 tons through 1984. It is unlikely that a full inventory of global PCB production will ever be accurately tallied, as there were factories in Poland, East Germany, and Austria that produced unknown amounts of PCBs.
Between approximately 1947 and 1977 General Electric Company (GE) released up to 1,300,000 pounds (590,000 kg) of PCBs into the Hudson River. The PCBs came from the company's two capacitor manufacturing plants at Hudson Falls and Fort Edward in New York State.
In 1976, because of concern over continuing high levels of PCBs in local fish and other aquatic organisms, and the unacceptable risk to the health of consumers of such fish, the NYSDEC banned all fishing in the Upper Hudson River, as well as commercial fishing of striped bass and several other species in the Lower Hudson River, and also issued advisories restricting the consumption of fish caught within a 20-mile (30 km) long segment of the Hudson River from Hudson Falls to Troy.
There have been many programs of remediation work to reduce the PCB pollution. In 1984, approximately 200 miles (320 km) of the Hudson River was designated a Superfund site, and attempts to cleanup the Upper Hudson River began, including the removal in 1977-8 of 180,000 cubic yards (140,000 m3) of contaminated river sediments near Fort Edward. In 1991, further PCB pollution was found at Bakers Falls near the former GE Hudson Falls factory, and a program of remediation was started. In August 1995, a 40-mile (64 km) reach of the Upper Hudson was re-opened to fishing but only on a catch-and-release basis. Removal of contaminated soil from Rogers Island was completed in December 1999. In 2002, the EPA announced a further 2,650,000 cubic yards (2,030,000 m3) of contaminated sediments in the Upper Hudson River would be removed.
From the late 1950s through 1977, Westinghouse Electric used PCBs in the manufacture of capacitors in its Bloomington, Indiana plant. Reject capacitors were hauled and dumped in area salvage yards and landfills, including Bennett's Dump, Neal's Landfill and Lemon Lane Landfill. Workers also dumped PCB oil down factory drains which contaminated the city sewage treatment plant. The City of Bloomington gave away the sludge to area farmers and gardeners, creating anywhere from 200 to 2000 sites which remain unaddressed. Over 2 million pounds of PCBs were estimated to have been dumped in Monroe and Owen Counties. Although federal and state authorities have been working on the sites' environmental remediation, many areas remain contaminated. Concerns have been raised regarding the removal of PCBs from the karst limestone topography, and regarding the possible disposal options. To date, the Westinghouse Bloomington PCB Superfund site case does not have a RI/FS (Remedial Investigation/Feasibility Study) and ROD (Record of Decision), although Westinghouse signed a US Department of Justice Consent Decree in 1985.[19 ]. The 1985 Consent Decree required Westinghouse to construct an incinerator that would incinerate PCB-contaminated materials. However, due to public opposition to the incinerator, the State of Indiana passed a number of laws that delayed and blocked the construction of the incinerator. Consent Decree parties began to explore alternative remedies in 1994 for six of the main PCB contaminated sites.[19 ]
Much of the Great Lakes area is still heavily polluted with PCBs, despite extensive remediation work. Locally caught fresh water fish and shellfish are contaminated with PCBs and their consumption is restricted.
From 1959 to 1971, Waukegan Harbor in Illinois on Lake Michigan was contaminated with PCB's discharged by the Outboard Marine Corp.
PCBs (manufactured through most of the 20th century) originating from Monsanto Chemical Company in Anniston, Alabama leaked into Snow creek, then Choccolocco Creek, then Logan Martin Lake. In the early 2000s, class action lawsuits (led, in at least one case, by the late Johnnie Cochran) were settled by local land owners, including those on on Logan Martin Lake, and Lay Reservoir (downstream on the Coosa River), for the PCB pollution.
Today, the highest pollution levels remain concentrated in Snow and Choccolocco Creeks. Concentrations in fish have and continue to decline over time, however, sediment disturbance can resuspend the PCBs from the sediment back into the water column and food web.
In 1999, the Dioxine affair caused serious trouble for the Belgian government when PCBs were found in chicken and eggs.
The chemical plant Chemko in Strážske (east Slovakia) was an important producer of polychlorinated biphenyls for the former communist block (Comecon) until 1984. Chemko contaminated a large part of east Slovakia, especially the sediments of the Laborec river and reservoir Zemplínska šírava.
In December 2008 a number of Irish news sources reported that testing had revealed "extremely high" levels of PCBs in pork products, ranging from 80 to 200 times the EU's upper safe limit of 1.5 pg/μg i.e. 0.12 to 0.3 parts per billion.[27 ] The PCB levels involved are small in comparison to other contamination incidents, indeed in 1986 the breast milk of healthy nursing mothers in the US contained between 1020 to 1770 ppb of PCBs and the PCB contaminated rice-bran oil that caused mass poisoning (Yu-Cheng) in Taiwan in 1979 contained between 53,000 to 99,000 ppb of PCBs.
Brendan Smith, the Minister for Agriculture, Fisheries & Food, stated that pork contamination was caused by PCB contaminated feed that was used on 9 out of the 400 of Ireland's pig farms and only one feed supplier was involved. Smith added that 38 beef farms also used the same contaminated feed, but those farms were quickly isolated and no contaminated beef entered the food chain. While the contamination was limited to just 9 pig farms, the Irish government requested the immediate withdrawal and disposal of all pork-containing products produced in Ireland and purchased since September 1, 2008.
This request for withdrawal of pork products was confirmed in a press release by the Food Safety Authority of Ireland on December 6.
Due to their low vapour pressure, in the environment PCBs goes mainly in the hydrosphere (despite their hydrophobicity, the great amount of water in the oceans can dissolve a fair great amount of PCBs), in the organic fraction of soil, and in organisms.
However, a small amount of PCBs have been detected globally in the atmosphere, from the most urbanized areas that are the centers for PCB pollution, to regions north of the Arctic Circle. Whereas the hydrosphere is the main reservoir, the atmosphere serves as the primary route for global transport of PCBs, particularly for those congeners with 1 to 4 chlorine atoms.
Atmospheric concentrations of PCBs tend to be lowest in rural areas, where they are typically in the picogram per cubic meter range, higher in suburban and urban areas, and highest in city centres, where they can reach 1 ng/m³ or more. In Milwaukee, an atmospheric concentration of 1.9 ng/m³ has been measured, and this source alone was estimated to account for 120 kg/year of PCBs entering Lake Michigan. Concentrations as high as 35 ng/m³, 10 times higher than the EPA guideline limit of 3.4 ng/m³, have been found inside some houses in the U.S.
Volatilization of PCBs in soil was thought to be the primary source of PCBs in the atmosphere, but recent research suggests that ventilation of PCB-contaminated indoor air from buildings is the primary source of PCB contamination in the atmosphere.
In biosphere, PCBs can be degradated by either bacteria or eukariotes, but the speed of the reaction depends on both the number and the disposition of chlorine atoms in the molecule: less substituted, meta- or para- substituted PCBs undergoes biodegradation faster than more substituted congeners.
In eukariotes, PCBs may be oxidated by mixed function oxidase enzyme.
The toxicity of PCBs to animals was first noticed in the 1970s when emaciated seabird corpses with very high PCB body burdens washed up on beaches. Since seabirds may die far out at sea and still wash ashore, the true sources of the PCBs were unknown. Where they were found is no reliable indicator of where they had died.
The toxicity of PCBs varies considerably among congeners. The coplanar PCBs, known as non-ortho PCBs because they are not substituted at the ring positions ortho to (next to) the other ring, (i.e. PCBs 77, 126, 169, etc), tend to have dioxin-like properties, and generally are among the most toxic congeners. Because PCBs are almost invariably found in complex mixtures, the concept of toxic equivalency factors (TEFs) has been developed to facilitate risk assessment and regulatory control, where more toxic PCB congeners are assigned higher TEF values. One of the most toxic compounds known, 2,3,7,8-tetrachlorodibenzo[p]dioxin, is assigned a TEF of 1.
A few studies of workers indicate that PCBs were associated with specific kinds of cancer in humans, such as cancer of the liver and biliary tract. Polychlorinated biphenyls (PCBs) have been shown to mimic the action of oestrogen in breast cancer cells and can enhance breast carcinogenesis. Rats that ate food containing high levels of PCBs for two years developed liver cancer. The Department of Health and Human Services (DHHS) has concluded that PCBs may reasonably be anticipated to be carcinogens. The EPA and the International Agency for Research on Cancer (IARC) have determined that PCBs are probably carcinogenic to humans. PCBs are also classified as probable human carcinogens by the National Cancer Institute, World Health Organization, and the Agency for Toxic Substances and Disease Registry. Recent research by the National Toxicology Program has confirmed that PCB126 (Technical Report 520) and a binary mixture of PCB126 and PCB153 (Technical Report 531) are carcinogens.
As discussed, PCBs exhibit a wide range of toxic effects. These effects may vary depending on the specific PCB. Similar to dioxin, toxicity of coplanar PCBs and mono-ortho-PCBs are thought to be primarily mediated via binding to aryl hydrocarbon receptor (AhR). Because AhR is a transcription factor, abnormal activation may disrupt cell function by altering the transcription of genes. The concept of toxic equivalency factors (TEF) is based on the ability of a PCB to activate AhR.
However, not all effects may be mediated by the AhR receptor, and PCBs doesn't alter estrogen concentrations to the same degree as other ligands of the AhR receptor such as PCDD and PCDF, possibly reflecting the reduced potency of PCBs to induce CYP1A1 and CYP1B1. Examples of other actions of PCBs include di-ortho-substituted non-coplanar PCBs interfering with intracellular signal transduction dependent on calcium; this may lead to neurotoxicity. Ortho-PCBs may disrupt thyroid hormone transport by binding to transthyretin.
Landfill – Large quantities of PCBs have been placed in landfill sites, mainly in the form of transformers and capacitors. Many municipal sites are not designed to contain these pollutants and PCBs are able to escape into the atmosphere or ground water. No emissions above background are seen if the landfill is designed correctly.
These can be separated into three distinct categories: physical, microbial, and chemical destruction.
Incineration – Although PCBs do not ignite themselves, they can be combusted under extreme and carefully controlled conditions. The current regulations require that PCBs are burnt at a temperature of 1200 °C for at least two seconds, in the presence of fuel oil and excess oxygen. A lack of oxygen can result in the formation of PCDDs, PCDFs and dioxins, or the incomplete destruction of the PCBs. Such specific conditions mean that it is extremely expensive to destroy PCBs on a tonnage scale, and it can only be used on PCB-containing equipment and contaminated liquid. This method is not suitable for the decontamination of affected soils.
Ultrasound – In a similar process to combustion, high power ultrasonic waves are applied to water, generating cavitation bubbles. These then implode or fragment, creating microregions of extreme pressures and temperatures where the PCBs are destroyed. Water is thought to undergo thermolysis, oxidising the PCBs to CO, CO2 and hydrocarbons such as biphenyl, and releasing chlorine. The scope of this method is limited to those congeners which are the most water soluble; those isomers with the least chlorine substitution.
Irradiation – If a deoxygenated mixture of PCBs in isopropanol or mineral oil is subjected to irradiation with gamma rays then the PCBs will be dechlorinated to form biphenyl and inorganic chloride. The reaction works best in isopropanol if potassium hydroxide (caustic potash) is added. Solvated electrons are thought to be responsible for the reaction. If oxygen, nitrous oxide, sulfur hexafluoride or nitrobenzene is present in the mixture then the reaction rate is reduced. This work has been done recently in the US often with used nuclear fuel as the radiation source.
Pyrolysis – Destruction of PCBs with pyrolysis using plasma arc processes, like incineration uses heat, however unlike incineration, there is no combustion. The long chain molecules are broken with extreme temperature provided by an electric arc in an inert environment. Adequate post pyrolisis post treatment of the resultant products is required in order to prevent the risk of back reactions.
Much recent work has centered on the study of micro-organisms that are able to decompose PCBs. Generally, these organisms work in one of two ways: either they use the PCB as a carbon source, or destruction takes place through reductive dechlorination, with the replacement of chlorine with hydrogen on the biphenyl skeleton. However, there are significant problems with this approach. Firstly, these microbes tend to be highly selective in their dechlorination, with lower chlorinated biphenyls being readily transformed, and with preference to dechlorination in the para and meta positions. Secondly, microbial dechlorination tends to be rather slow acting on PCB as a soil contaminant in comparison to other methods. Finally, while microbes work well in laboratory conditions, there is often a problem in transferring a successful laboratory strain to a natural system. This is because the microbes can access other sources of carbon, which they decompose in preference to PCBs.
Further recent developments have focused on testing enzymes and vitamins extracted from microbes which show PCB activity. Especially promising seems to be the use of vitamin B12, in which a cobalt ion is in oxidation state (III) under normal redox conditions. Using titanium (III) citrate as a strong reductant converts the cobalt from Co(III) to Co(I), giving a new vitamin known as B12s, which is a powerful nucleophile and reducing catalyst. This can then be used on PCBs, which it dechlorinates in a rapid and selective manner.
Many chemical methods are available to destroy or reduce the toxicity of PCBs.
Nucleophilic aromatic substitution is a method of destroying low concentration PCB mixtures in oils, such as transformer oil. Substitution of chlorine by polyethylene glycols) occurs in under two hours under a blanket of nitrogen, to prevent oxidation of the oil, to produce aryl polyglycols, which are insoluble in the oil and precipitate out.
Between 700 and 925 °C, H2 cleaves the carbon-chlorine bond, and cleaves the biphenyl nucleus into benzene yielding HCl without a catalyst. This can be performed at lower temperatures with a copper catalyst, and to yield biphenyl. However, since both of these routes require an atmosphere of hydrogen gas and relatively high temperatures, they are prohibitively expensive.
Reaction with highly electropositive metals, or strong reducing agents such as sodium naphthalide, in aprotic solvents results in a transfer of electrons to the PCB, the expulsion of a chloride ion, and a coupling of the PCBs. This is analogous to the Wurtz reaction for coupling halogenoalkanes. The effect is to polymerise many molecules, therefore reducing the volatility, solubility and toxicity of the mixture. This methodology is most successful on low strength PCB mixtures and can also be performed electrochemically in a partly aqueous bicontinuous microemulsion.
The solution photochemistry of PCBs is based on the transfer of an electron to a photochemically excited PCB from a species such as an amine, to give a radical anion. This either expels a chloride ion and the resulting aryl radical extracts a hydrogen atom from the solvent, or immediately becomes protonated, leading to the loss of a chlorine atom. It is useful only for water soluble PCBs.
The major pathway for atmospheric destruction of PCBs is via attack by OH radicals. Direct photolysis can occur in the upper atmosphere, but the ultraviolet wavelengths necessary to excite PCBs are shielded from the troposphere by the ozone layer. It has, however, been shown that higher wavelengths of light (> 300 nm) can degrade PCBs in the presence of a photosensitizer, such as acetone.
The Schwartz reaction is the subject of much study, and has significant benefits over other routes. It is advantageous since it proceeds via a reductive process, and thus yields no dioxins through oxidation. The proposed reaction scheme involves the electron transfer from a titanium (III) organometallic species to form a radical anion on the PCB molecule which expels chlorine to eventually form the relatively non-toxic biphenyl.
For a complete list of PCB congeners, see PCB Congener List. Note that biphenyl, while not technically a PCB congener due to its lack of chlorine substituents, is still typically included in the literature.
|PCB Homolog||CASRN||Cl Substituents||Number of Congeners|
[[File:|thumb|Labeling transformers containing PCBs]]
Polychlorinated biphenyls (often shortened to PCBs) are organic chemical compounds that contain chlorine atoms. They were used in industrial chemistry, and to make certain transformers, capacitors, and cooling fluids. The Stockholm Convention on Persistent Organic Pollutants declared them to be among the worst chemical polluants. Their use has been banned worldwide, since 2001. The mode of action of these substances is similar to that of dioxin.
Polychlorinated biphenyls were widely used, and they can be found in the atmosphere, the soil, and the bodies of water anywhere on the earth. In the short term, these substances are not very toxic, but they accumulate in the body. They are teratogens, and may cause cancer.