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 each containing six carbon atoms. The chemical formula for all PCBs is C12H10-xClx.
PCBs were used as coolants and insulating fluids for transformers and capacitors, stabilizing additives in flexible PVC coatings of electrical wiring and electronic components, pesticide extenders, cutting oils, flame retardants, hydraulic fluids, sealants (used in caulking, etc), adhesives, wood floor finishes, paints, de-dusting agents, and in carbonless copy paper.
PCBs readily penetrate skin, PVC (polyvinyl chloride), and latex (natural rubber); organic solvents such as kerosene increase the rate of skin absorption. 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. They may be destroyed by chemical, thermal, and biochemical processes, though it is extremely difficult to achieve full destruction, and there is the risk of creating extremely toxic dibenzodioxins and dibenzofurans through partial oxidation. Because of the high thermodynamic stability of PCBs, all degradation mechanisms are difficult to sustain. Intentional degradation as a treatment of unwanted PCBs generally requires high heat or catalysis. Environmental and metabolic degradation generally proceeds quite slowly relative to most other compounds.
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. per cu.m. 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, 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, knowing all the while that they were extremely toxic. 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. Previously, they had simply been called "phenols" or referred to by various trade names, such as Aroclor, Kennechlor, Pyrenol, Chlorinol and others. However, Arochlors (chlorinated biphenyls) were known toxins by 1947. 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 1972, PCB production plants existed in Austria, then Federal Republic of Germany, France, Great Britain, Italy, Japan, Spain, USSR, and USA.
From 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 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 1977, 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 from 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.
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 New York State Department of Environmental Conservation 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 long segment of the Hudson River from Hudson Falls to Troy.
There have been many programs of remediation work to reduce the PCB pollution, mostly paid for by GE. In 1984, approximately 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 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 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 United States Environmental Protection Agency announced a further of contaminated sediments in the Upper Hudson River would be removed.
From 1959 to 1971, Waukegan Harbor in Illinois on Lake Michigan was contaminated with PCB's discharged by the Outboard Marine Corp
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.
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, [[dioxin|2,3,7,8-tetrachlorodibenzo[p]dioxin]], is assigned a TEF of 1.
However, not all effects may be mediated by the AhR receptor. For example, di-ortho-substituted non-coplanar PCBs interfere with intracellular signal transduction dependent on calcium; this may lead to neurotoxicity. Ortho-PCBs may disrupt thyroid hormone transport by binding to transthyretin.
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, with chlorine present as the inorganic ion 16. 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 subject to irradiation with gamma rays then the PCBs will be dechlorinated to form inorganic chloride and biphenyl. 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
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.
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|
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