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Friday, March 6, 2020

Polychlorinated biphenyl

From Wikipedia, the free encyclopedia

Polychlorinated biphenyl
Polychlorinated biphenyl structure.svg
Chemical structure of PCBs. The possible positions of chlorine atoms on the benzene rings are denoted by numbers assigned to the carbon atoms.
Identifiers
UN number UN 2315
Properties
C12H10−xClx
Molar mass Variable
Appearance Light yellow or colorless, thick, oily liquids
Hazards
NFPA 704 (fire diamond)
Flammability code 2: Must be moderately heated or exposed to relatively high ambient temperature before ignition can occur. Flash point between 38 and 93 °C (100 and 200 °F). E.g. diesel fuelHealth code 1: Exposure would cause irritation but only minor residual injury. E.g. turpentineReactivity code 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g. liquid nitrogenSpecial hazards (white): no codeNFPA 704 four-colored diamond
2
1
0
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).

PCB warning label on a power transformer known to contain PCBs.

A polychlorinated biphenyl (PCB) is an organic chlorine compound with the formula C12H10−xClx. Polychlorinated biphenyls were once widely deployed as dielectric and coolant fluids in electrical apparatus, carbonless copy paper and in heat transfer fluids.

Because of their longevity, PCBs are still widely in use, even though their manufacture has declined drastically since the 1960s, when a host of problems were identified. With the discovery of PCBs' environmental toxicity, and classification as persistent organic pollutants, their production was banned by United States federal law in 1978, and by the Stockholm Convention on Persistent Organic Pollutants in 2001. The International Agency for Research on Cancer (IARC), rendered PCBs as definite carcinogens in humans. According to the U.S. Environmental Protection Agency (EPA), PCBs cause cancer in animals and are probable human carcinogens. Many rivers and buildings, including schools, parks, and other sites, are contaminated with PCBs and there has been contamination of food supplies with the substances.

Some PCBs share a structural similarity and toxic mode of action with dioxins. Other toxic effects such as endocrine disruption (notably blocking of thyroid system functioning) and neurotoxicity are known. The maximum allowable contaminant level in drinking water in the United States is set at zero, but because of the limitations of water treatment technologies, a level of 0.5 parts per billion is the de facto level.

The bromine analogues of PCBs are polybrominated biphenyls (PBBs), which have analogous applications and environmental concerns.

Physical and chemical properties

Physical properties

The compounds are pale-yellow viscous liquids. They are hydrophobic, with low water solubilities: 0.0027–0.42 ng/L for Aroclors, but they have high solubilities in most organic solvents, oils, and fats. They have low vapor pressures at room temperature. They have dielectric constants of 2.5–2.7, very high thermal conductivity, and high flash points (from 170 to 380 °C).

The density varies from 1.182 to 1.566 g/cm3. Other physical and chemical properties vary widely across the class. As the degree of chlorination increases, melting point and lipophilicity increase, and vapour pressure and water solubility decrease.

PCBs do not easily break down or degrade, which made them attractive for industries. PCB mixtures are resistant to acids, bases, oxidation, hydrolysis, and temperature change. They can generate extremely toxic dibenzodioxins and dibenzofurans through partial oxidation. Intentional degradation as a treatment of unwanted PCBs generally requires high heat or catalysis.

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.

Structure and toxicity

PCBs are derived from biphenyl, which has the formula C12H10, sometimes written (C6H5)2. In PCBs, some of the hydrogen atoms in biphenyl are replaced by chlorine atoms. There are 209 different chemical compounds in which one to ten chlorine atoms can replace hydrogen atoms. PCBs are typically used as mixtures of compounds and are given the single identifying CAS number 1336-36-3 . About 130 different individual PCBs are found in commercial PCB products.

Toxic effects vary depending on the specific PCB. In terms of their structure and toxicity, PCBs fall into two distinct categories, referred to as coplanar or non-ortho-substituted arene substitution patterns and noncoplanar or ortho-substituted congeners. 

Structures of the twelve dioxin-like PCBs
Coplanar or non-ortho
The coplanar group members have a fairly rigid structure, with their two phenyl rings in the same plane. It renders their structure similar to polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans, and allows them to act like PCDDs, as an agonist of the aryl hydrocarbon receptor (AhR) in organisms. They are considered as contributors to overall dioxin toxicity, and the term dioxins and dioxin-like compounds is often used interchangeably when the environmental and toxic impact of these compounds is considered.
Noncoplanar
Noncoplanar PCBs, with chlorine atoms at the ortho positions can cause neurotoxic and immunotoxic effects, but only at concentrations much higher than those normally associated with dioxins. They do not activate the AhR, and are not considered part of the dioxin group. Because of their lower toxicity, they are of less concern to regulatory bodies.
Di-ortho-substituted, non-coplanar PCBs interfere with intracellular signal transduction dependent on calcium which may lead to neurotoxicity. ortho-PCBs can disrupt thyroid hormone transport by binding to transthyretin.

Alternative names

Commercial PCB mixtures were marketed under the following names:

Aroclor mixtures

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 four-digit number. In general, the first two digits refer to the product series as designated by Monsanto (e.g. 1200 or 1100 series); the second two numbers indicate the percentage of chlorine by mass in the mixture. Thus, Aroclor 1260 is a 1200 series product and contains 60% chlorine by mass. It is a myth that the first two digits referred to the number of carbon atoms; the number of carbon atoms do not change in PCBs. The 1100 series was a crude PCB material which was distilled to create the 1200 series PCB product.

The exception to the naming system is Aroclor 1016 which was produced by distilling 1242 to remove the highly chlorinated congeners to make a more biodegradable product. "1016" was given to this product during Monsanto's research stage for tracking purposes but the name stuck after it was commercialized.

Different Aroclors were used at different times and for different applications. In electrical equipment manufacturing in the US, 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.

Production

One estimate (2006) suggested that 1 million tonnes of PCBs had been produced. 40% of this material was thought to remain in use. Another estimate put the total global production of PCBs on the order of 1.5 million tonnes. The United States was the single largest producer with over 600,000 tonnes produced between 1930 and 1977. The European region follows with nearly 450,000 tonnes 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 East region of Slovakia there is still 21 500 tons of PCBs stored.

Applications

The utility of PCBs is based largely on their chemical stability, including low flammability and high dielectric constant. In an electric arc, PCBs generate incombustible gases.

Use of PCBs is commonly divided into closed and open applications. Examples of closed applications include coolants and insulating fluids (transformer oil) for transformers and capacitors, such as those used in old fluorescent light ballasts, hydraulic fluids, lubricating and cutting oils, and the like. In contrast, the major open application of PCBs was in carbonless copy ("NCR") paper, which even presently results in paper contamination.

Other open applications were as plasticizers in paints and cements, stabilizing additives in flexible PVC coatings of electrical cables and electronic components, pesticide extenders, reactive flame retardants and sealants for caulking, adhesives, wood floor finishes, such as Fabulon and other products of Halowax in the U.S., de-dusting agents, waterproofing compounds, casting agents. It was also used as a plasticizer in paints and especially "coal tars" that were used widely to coat water tanks, bridges and other infrastructure pieces.

Environmental transport and transformations

PCBs have entered the environment through both use and disposal. The environmental fate of PCBs is complex and global in scale.

Water

Because of their low vapour pressure, PCBs accumulate primarily in the hydrosphere, despite their hydrophobicity, in the organic fraction of soil, and in organisms. The hydrosphere is the main reservoir. The immense volume of water in the oceans is still capable of dissolving a significant quantity of PCBs.

As the pressure of ocean water increases with depth, PCBs become heavier than water and sink to the deepest ocean trenches where they are concentrated.

Air

A small volume of PCBs has been detected throughout the earth's atmosphere. The atmosphere serves as the primary route for global transport of PCBs, particularly for those congeners with one to four chlorine atoms.

In the atmosphere, PCBs may be degraded by hydroxyl radicals, or directly by photolysis of carbon–chlorine bonds (even if this is a less important process).

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. In 2008, concentrations as high as 35 ng/m3, 10 times higher than the EPA guideline limit of 3.4 ng/m3, have been documented 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 research suggests ventilation of PCB-contaminated indoor air from buildings is the primary source of PCB contamination in the atmosphere.

Biosphere

In the biosphere, PCBs can be degraded by the sun, bacteria or eukaryotes, 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 undergo biodegradation faster than more substituted congeners.

In bacteria, PCBs may be dechlorinated through reductive dechlorination, or oxidized by dioxygenase enzyme. In eukaryotes, PCBs may be oxidized by the cytochrome P450 enzyme.

Biomagnification is the increasing concentration of a substance, such as a toxic chemical, in the tissues of tolerant organisms at successively higher levels in a food chain.
 
Like many lipophilic toxins, PCBs undergo biomagnification and bioaccumulation primarily due to the fact that they are easily retained within organisms. Plastic pollution, specifically Microplastics, are a major contributor of PCB's into the biosphere and especially into marine environments. PCB's concentrate in marine environments because freshwater systems, like rivers, act as a bridge for plastic pollution to be transported from terrestrial environments into marine environments. It has been estimated that 88-95% of marine plastic is exported into the ocean by just 10 major rivers. An organism can accumulate PCBs by consuming other organisms that have previously ingested PCB's from terrestrial, freshwater, or marine environments. The concentration of PCB's within an organism will increase over their lifetime; this process is called bioaccumulation. PCB concentrations within an organism also change depending upon which trophic level they occupy. When an organism occupies a high trophic level, like orcas or humans, they will accumulate more PCB's than an organism that occupies a low trophic level, like phytoplankton. If enough organisms with a trophic level are killed due to the accumulation of toxins, like PCB, a trophic cascade can occur. PCB's can cause harm to human health or even death when eaten. PCBs can be transported by birds from aquatic sources onto land via feces and carcasses.

Biochemical metabolism

Overview

PCBs undergo xenobiotic biotransformation, a mechanism used to make lipophilic toxins more polar and more easily excreted from the body. The biotransformation is dependent on the number of chlorine atoms present, along with their position on the rings. Phase I reactions occur by adding an oxygen to either of the benzene rings by Cytochrome P450. The type of P450 present also determines where the oxygen will be added; phenobarbital (PB)-induced P450s catalyze oxygenation to the meta-para positions of PCBs while 3-methylcholanthrene (3MC)-induced P450s add oxygens to the orthometa positions. PCBs containing orthometa and metapara protons can be metabolized by either enzyme, making them the most likely to leave the organism. However, some metabolites of PCBs containing orthometa protons have increased steric hindrance from the oxygen, causing increased stability and an increased chance of accumulation.

Species dependent

Metabolism is also dependent on the species of organism; different organisms have slightly different P450 enzymes that metabolize certain PCBs better than others. Looking at the PCB metabolism in the liver of four sea turtle species (green, olive ridley, loggerhead and hawksbill), green and hawksbill sea turtles have noticeably higher hydroxylation rates of PCB 52 than olive ridley or loggerhead sea turtles. This is because the green and hawksbill sea turtles have higher P450 2-like protein expression. This protein adds three hydroxyl groups to PCB 52, making it more polar and water-soluble. P450 3-like protein expression that is thought to be linked to PCB 77 metabolism, something that was not measured in this study.

Temperature dependent

Temperature plays a key role in the ecology, physiology and metabolism of aquatic species. The rate of PCB metabolism was temperature dependent in yellow perch (Perca flavescens). In fall and winter, only 11 out of 72 introduced PCB congeners were excreted and had halflives of more than 1,000 days. During spring and summer when the average daily water temperature was above 20 °C, persistent PCBs had halflives of 67 days. The main excretion processes were fecal egestion, growth dilution and loss across respiratory surfaces. The excretion rate of PCBs matched with the perch's natural bioenergetics, where most of their consumption, respiration and growth rates occur during the late spring and summer. Since the perch is performing more functions in the warmer months, it naturally has a faster metabolism and has less PCB accumulation. However, multiple cold-water periods mixed with toxic PCBs with coplanar chlorine molecules can be detrimental to perch health.

Sex dependent

Enantiomers of chiral compounds have similar chemical and physical properties, but can be metabolized by the body differently. This was looked at in bowhead whales (Balaena mysticetus) for two main reasons: they are large animals with slow metabolisms (meaning PCBs will accumulate in fatty tissue) and few studies have measured chiral PCBs in cetaceans. They found that the average PCB concentrations in the blubber were approximately four times higher than the liver; however, this result is most likely age- and sex-dependent. As reproductively active females transferred PCBs and other poisonous substances to the fetus, the PCB concentrations in the blubber were significantly lower than males of the same body length (less than 13 meters).

Health effects

Labelling transformers containing PCBs

The toxicity of PCBs varies considerably among congeners. The coplanar PCBs, known as nonortho PCBs because they are not substituted at the ring positions ortho to (next to) the other ring, (such as PCBs 77, 126 and 169), 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 regulation, where more toxic PCB congeners are assigned higher TEF values on a scale from 0 to 1. One of the most toxic compounds known, 2,3,7,8-tetrachlorodibenzo[p]dioxin, a PCDD, is assigned a TEF of 1.

Exposure and excretion

In general, people are exposed to PCBs overwhelmingly through food, much less so by breathing contaminated air, and least by skin contact. Once exposed, some PCBs may change to other chemicals inside the body. These chemicals or unchanged PCBs can be excreted in feces or may remain in a person's body for years, with half lives estimated at 10–15 years. PCBs collect in body fat and milk fat. PCBs biomagnify up the food web and are present in fish and waterfowl of contaminated aquifers. Human infants are exposed to PCBs through breast milk or by intrauterine exposure through transplacental transfer of PCBs and are at the top of the food chain.

Signs and symptoms

Humans

The most commonly observed health effects in people exposed to extremely high levels of PCBs are skin conditions, such as chloracne and rashes, but these were known to be symptoms of acute systemic poisoning dating back to 1922. Studies in workers exposed to PCBs have shown changes in blood and urine that may indicate liver damage. In Japan in 1968, 280 kg of PCB-contaminated rice bran oil was used as chicken feed, resulting in a mass poisoning, known as Yushō disease, in over 1800 people. Common symptoms included dermal and ocular lesions, irregular menstrual cycles and lowered immune responses. Other symptoms included fatigue, headaches, coughs, and unusual skin sores. Additionally, in children, there were reports of poor cognitive development. Women exposed to PCBs before or during pregnancy can give birth to children with lowered cognitive ability, immune compromise, and motor control problems.

There is evidence that crash dieters that have been exposed to PCBs have an elevated risk of health complications. Stored PCBs in the adipose tissue become mobilized into the blood when individuals begin to crash diet. PCBs have shown toxic and mutagenic effects by interfering with hormones in the body. PCBs, depending on the specific congener, have been shown to both inhibit and imitate estradiol, the main sex hormone in females. Imitation of the estrogen compound can feed estrogen-dependent breast cancer cells, and possibly cause other cancers, such as uterine or cervical. Inhibition of estradiol can lead to serious developmental problems for both males and females, including sexual, skeletal, and mental development issues. In a cross-sectional study, PCBs were found to be negatively associated with testosterone levels in adolescent boys.

High PCB levels in adults have been shown to result in reduced levels of the thyroid hormone triiodothyronine, which affects almost every physiological process in the body, including growth and development, metabolism, body temperature, and heart rate. It also resulted in reduced immunity and increased thyroid disorders.

Animals

Animals that eat PCB-contaminated food even for short periods of time suffer liver damage and may die. In 1968 in Japan, 400,000 birds died after eating poultry feed that was contaminated with PCBs. Animals that ingest smaller amounts of PCBs in food over several weeks or months develop various health effects, including anemia; acne-like skin conditions (chloracne); liver, stomach, and thyroid gland injuries (including hepatocarcinoma), and thymocyte apoptosis. Other effects of PCBs in animals include changes in the immune system, behavioral alterations, and impaired reproduction. PCBs that have dioxin-like activity are known to cause a variety of teratogenic effects in animals. Exposure to PCBs causes hearing loss and symptoms similar to hypothyroidism in rats.

Cancer

In 2013, the International Agency for Research on Cancer (IARC) classified dioxin-like PCBs as human carcinogens. According to the U.S. EPA, PCBs have been shown to cause cancer in animals and evidence supports a cancer-causing effect in humans. Per the EPA, studies have found increases in malignant melanoma and rare liver cancers in PCB workers.

In 2013, the International Association for Research on Cancer (IARC) determined that the evidence for PCBs causing non-Hodgkin lymphoma is "limited" and "not consistent". In contrast an association between elevated blood levels of PCBs and non-Hodgkin lymphoma had been previously accepted. PCBs may play a role in the development of cancers of the immune system because some tests of laboratory animals subjected to very high doses of PCBs have shown effects on the animals' immune system, and some studies of human populations have reported an association between environmental levels of PCBs and immune response.

History

Old power transformers are a major source of PCBs. Even units not originally filled with PCB may be contaminated, since PCB and oil mix freely and any given transformer may have been refilled from hoses or tanks also used with PCBs.

In 1865 the first "PCB-like" chemical was discovered, and was found to be a byproduct of coal tar. Years later in 1881, German chemists synthesized the first PCB in a laboratory. Between then and 1914, large amounts of PCBs were released into the environment, to the extent that there are still measurable amounts of PCBs in feathers of birds currently held in museums.

In 1935, Monsanto Chemical Company (now Solutia Inc) took over commercial production of PCBs from Swann Chemical Company which had begun in 1929. PCBs, originally termed "chlorinated diphenyls", were commercially produced as mixtures of isomers at different degrees of chlorination. The electric industry used PCBs as a non-flammable replacement for mineral oil to cool and insulate industrial transformers and capacitors. PCBs were also commonly used as heat stabilizer in cables and electronic components to enhance the heat and fire resistance of PVC.

In the 1930s, the toxicity associated with PCBs and other chlorinated hydrocarbons, including polychlorinated naphthalenes, was recognized because of a variety of industrial incidents. Between 1936 and 1937, there were several medical cases and papers released on the possible link between PCBs and its detrimental health effects. In 1936 a U.S. Public health Service official described the wife and child of a worker from the Monsanto Industrial Chemical Company who exhibited blackheads and pustules on their skin. The official attributed these symptoms to contact with the worker's clothing after he returned from work. In 1937, a conference about the hazards was organized at Harvard School of Public Health, and a number of publications referring to the toxicity of various chlorinated hydrocarbons were published before 1940.

In 1947 Robert Brown reminded chemists 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".

In 1954 Japan, Kanegafuchi Chemical Co. Ltd. (Kaneka Corporation) first produced PCBs, and continued until 1972.

Through the 1960s Monsanto Chemical Company knew increasingly more about PCBs' harmful effects on humans and the environment, per internal leaked documents released in 2002, yet PCB manufacture and use continued with few restraints until the 1970s.

In 1966, PCBs were determined by Swedish chemist Sören Jensen to be an environmental contaminant. Jensen, according to a 1994 article in Sierra, named chemicals PCBs, which previously, had simply been called "phenols" or referred to by various trade names, such as Aroclor, Kanechlor, Pyrenol, Chlorinol and others. In 1972, PCB production plants existed in Austria, West Germany, France, the UK, Italy, Japan, Spain, the USSR and the US.

In the early 1970s, Ward B. Stone of the New York State Department of Environmental Conservation (NYSDEC) first published his findings that PCBs were leaking from transformers and had contaminated the soil at the bottom of utility poles.

There have been allegations that Industrial Bio-Test Laboratories engaged in data falsification in testing relating to PCBs. In 2003, Monsanto and Solutia Inc., a Monsanto corporate spinoff, reached a US$700 million settlement with the residents of West Anniston, Alabama who had been affected by the manufacturing and dumping of PCBs. In a trial lasting six weeks, the jury found that "Monsanto had engaged in outrageous behavior, and held the corporations and its corporate successors liable on all six counts it considered – including negligence, nuisance, wantonness and suppression of the truth."

Existing products containing PCBs which are "totally enclosed uses" such as insulating fluids in transformers and capacitors, vacuum pump fluids, and hydraulic fluid, are allowed to remain in use. The public, legal, and scientific concerns about PCBs arose from research indicating they are 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.

Pollution due to PCBs

Belgium

In 1999, the Dioxin Affair occurred when 50 kg of PCB transformer oils were added to a stock of recycled fat used for the production of 500 tonnes of animal feed, eventually affecting around 2,500 farms in several countries. The name Dioxin Affair was coined from early misdiagnosis of dioxins as the primary contaminants, when in fact they turned out to be a relatively small part of the contamination caused by thermal reactions of PCBs. The PCB congener pattern suggested the contamination was from a mixture of Aroclor 1260 & 1254. Over 9 million chickens, and 60,000 pigs were destroyed because of the contamination. The extent of human health effects has been debated, in part because of the use of differing risk assessment methods. One group predicted increased cancer rates, and increased rates of neurological problems in those exposed as neonates. A second study suggested carcinogenic effects were unlikely and that the primary risk would be associated with developmental effects due to exposure in pregnancy and neonates. Two businessmen who knowingly sold the contaminated feed ingredient received two-year suspended sentences for their role in the crisis.

Italy

The Italian company Caffaro, located in Brescia, specialized in producing PCBs from 1938 to 1984, following the acquisition of the exclusive rights to use the patent in Italy from Monsanto. The pollution resulting from this factory and the case of Anniston, in the US, are the largest known cases in the world of PCB contamination in water and soil, in terms of the amount of toxic substance dispersed, size of the area contaminated, number of people involved and duration of production.

The values reported by the local health authority (ASL) of Brescia since 1999 are 5,000 times above the limits set by Ministerial Decree 471/1999 (levels for residential areas, 0.001 mg/kg). As a result of this and other investigations, in June 2001, a complaint of an environmental disaster was presented to the Public Prosecutor's Office of Brescia. Research on the adult population of Brescia showed that residents of some urban areas, former workers of the plant, and consumers of contaminated food, have PCB levels in their bodies that are in many cases 10-20 times higher than reference values in comparable general populations. PCBs entered the human food supply by animals grazing on contaminated pastures near the factory, especially in local veal mostly eaten by farmers' families. The exposed population showed an elevated risk of Non-Hodgkin lymphoma, but not for other specific cancers.

Japan

In 1968, a mixture of dioxins and PCBs got into rice bran oil produced in northern Kyushu. Contaminated cooking oil sickened more than 1860 people. The symptoms were called Yushō disease.

In Okinawa, high levels of PCB contamination in soil on Kadena Air Base were reported in 1987 at thousands of parts per million, some of the highest levels found in any pollution site in the world.

Republic of Ireland

In December 2008, a number of Irish news sources reported testing had revealed "extremely high" levels of dioxins, by toxic equivalent, in pork products, ranging from 80 to 200 times the EU's upper safe limit of 1.5 pg WHO-TEQDFP/μg i.e. 0.12 to 0.3 parts per billion.

Brendan Smith, the Minister for Agriculture, Fisheries and Food, stated the pork contamination was caused by PCB-contaminated feed that was used on 9 of Ireland's 400 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 1 September 2008. This request for withdrawal of pork products was confirmed in a press release by the Food Safety Authority of Ireland on December 6.

It is thought that the incident resulted from the contamination of fuel oil used in a drying burner at a single feed processor, with PCBs. The resulting combustion produced a highly toxic mixture of PCBs, dioxins and furans, which was included in the feed produced and subsequently fed to a large number of pigs.

Kenya

In Kenya, a number of cases have been reported in the 2010s of thieves selling transformer oil, stolen from electric transformers, to the operators of roadside food stalls for use in deep frying. When used for frying, it is reported that transformer oil lasts much longer than regular cooking oil. The downside of this misuse of the transformer oil is the threat to the health of the consumers, due to the presence of PCBs.

Slovakia

The chemical plant Chemko in Strážske (east Slovakia) was an important producer of polychlorinated biphenyls for the former communist bloc (Comecon) until 1984. Chemko contaminated a large part of east Slovakia, especially the sediments of the Laborec river and reservoir Zemplínska šírava.

Slovenia

Between 1962 and 1983, the Iskra Kondenzatorji company in Semič (White Carniola, Southeast Slovenia) manufactured capacitors using PCBs. Due to the wastewater and improperly disposed waste products, the area (including the Krupa and Lahinja rivers) became highly contaminated with PCBs. The pollution was discovered in 1983, when the Krupa river was meant to become a water supply source. The area was sanitized then, but the soil and water are still highly polluted. Traces of PCBs were found in food (eggs, cow milk, walnuts) and Krupa is still the most PCB-polluted river in the world.

Spain

Several cetacean species have very high mean blubber PCB concentrations likely to cause population declines and suppress population recovery. Striped dolphins, bottlenose dolphins and killer whales were found to have mean levels that markedly exceeded all known marine mammal PCB toxicity thresholds. The western Mediterranean Sea and the south-west Iberian Peninsula were identified as “hotspots”.

United Kingdom

Monsanto manufactured PCBs at its chemical plant in Newport, South Wales, until the mid- to late-1970s. During this period, waste matter, including PCBs, from the Newport site was dumped at a disused quarry near Groes-faen, west of Cardiff, and Penhros landfill site from where it continues to be released in waste water discharges.

United States

Alabama

PCBs (manufactured through most of the 20th century until the early 2000s) originating from Monsanto Chemical Company in Anniston, Alabama were dumped into Snow Creek, which then spread to Choccolocco Creek, then Logan Martin Lake. In the early 2000s, class action lawsuits were settled by local land owners, including those on Logan Martin Lake, and Lay Reservoir (downstream on the Coosa River), for the PCB pollution. Donald Stewart, former Senator from Alabama, first learned of the concerns of hundreds of west Anniston residents after representing a church which had been approached about selling its property by Monsanto. Stewart went on to be the pioneer and lead attorney in the first and majority of cases against Monsanto and focused on residents in the immediate area known to be most polluted. Other attorneys later joined in to file suits for those outside the main immediate area around the plant; one of these was the late Johnnie Cochran.

In 2007, the highest pollution levels remained concentrated in Snow and Choccolocco Creeks. Concentrations in fish have declined and continue to decline over time; sediment disturbance, however, can resuspend the PCBs from the sediment back into the water column and food web.

Great Lakes

In 1976 environmentalists found PCBs in the sludge at Waukegan Harbor, the southwest end of Lake Michigan. They were able to trace the source of the PCBs back to the Outboard Marine Corporation that was producing boat motors next to the harbor. By 1982, the Outboard Marine Corporation was court-ordered to release quantitative data referring to their PCB waste released. The data stated that from 1954 they released 100,000 tons of PCB into the environment, and that the sludge contained PCBs in concentrations as high as 50%.

In 1989, during construction near the Zilwaukee bridge, workers uncovered an uncharted landfill containing PCB-contaminated waste which required $100,000 to clean up.

Much of the Great Lakes area were still heavily polluted with PCBs in 1988, despite extensive remediation work.

Indiana

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 2,000 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 Remedial Investigation/Feasibility Study (RI/FS) and Record of Decision (ROD), although Westinghouse signed a US Department of Justice Consent Decree in 1985. The 1985 consent decree required Westinghouse to construct an incinerator that would incinerate PCB-contaminated materials. Because of public opposition to the incinerator, however, the State of Indiana passed a number of laws that delayed and blocked its construction. The parties to the consent decree began to explore alternative remedies in 1994 for six of the main PCB contaminated sites in the consent decree. Hundreds of sites remain unaddressed as of 2014. Monroe County will never be PCB-free, as noted in a 2014 Indiana University program about the local contamination.

On 15 February 2008, Monroe County approved a plan to clean up the three remaining contaminated sites in the City of Bloomington, at a cost of $9.6 million to CBS Corp., the successor of Westinghouse. In 1999, Viacom bought CBS, so they are current responsible party for the PCB sites.

Massachusetts

Pittsfield, in western Massachusetts, was home to the General Electric (GE) transformer, capacitor, and electrical generating equipment divisions. The electrical generating division built and repaired equipment that was used to power the electrical utility grid throughout the nation. PCB-contaminated oil routinely migrated from GE's 254-acre (1.03 km2) industrial plant located in the very center of the city to the surrounding groundwater, nearby Silver Lake, and to the Housatonic River, which flows through Massachusetts, Connecticut, and down to Long Island Sound. PCB-containing solid material was widely used as fill, including oxbows of the Housatonic River. Fish and waterfowl who live in and around the river contain significant levels of PCBs and are not safe to eat.

New Bedford Harbor, which is a listed Superfund site, contains some of the highest sediment concentrations in the marine environment.

Investigations into historic waste dumping in the Bliss Corner neighborhood have revealed the existence of PCBs, among other hazardous materials, buried into soil and waste material.

Missouri

In 1982 Martha C. Rose Chemical Inc. began processing and disposing of materials contaminated with PCB's in Holden, Missouri, a small rural community about 40 miles east of Kansas City. From 1982 until 1986, nearly 750 companies, including General Motors Corp., Commonwealth Edison, Illinois Power Co. and West Texas Utilities, sent millions of pounds of PCB contaminated materials to Holden for disposal. Instead, according to prosecutors, the company began storing the contaminated materials while falsifying its reports to the EPA to show they had been removed. After investigators learned of the deception, Rose Chemical was closed and filed for bankruptcy. The site had become the nation's largest waste site for the chemical PCB. In the four years the company was operational, the EPA inspected it four times and assessed $206,000 in fines but managed to collect only $50,000.

After the plant closed the state environmental agency found PCB contamination in streams near the plant and in the city's sewage treatment sludge. A 100,000 square-foot warehouse and unknown amounts of contaminated soil and water around the site had to be cleaned up. Most of the surface debris, including close to 13 million pounds of contaminated equipment, carcasses and tanks of contaminated oil, had to be removed. Walter C. Carolan, owner of Rose Chemical, and five others pleaded guilty in 1989 to committing fraud or falsifying documents. Carolan and two other executives served sentences of less than 18 months; the others received fines and were placed on probation. Cleanup costs at the site are estimated at $35 million.

New York

Pollution of the Hudson River is largely due to dumping of PCBs by General Electric from 1947 to 1977. GE dumped an estimated 1.3 million pounds of PCBs into the Hudson River during these years. This pollution caused a range of harmful effects to wildlife and people who eat fish from the river or drink the water.

Love Canal is a neighborhood in Niagara Falls, New York that was heavily contaminated with toxic waste including PCBs. Eighteen Mile Creek in Lockport, New York is an EPA Superfund site for PCBs contamination.

PCB pollution at the State Office Building in Binghamton was responsible for what is now considered to be the first indoor environmental disaster in the United States. In 1981, a transformer explosion in the basement spewed PCBs throughout the entire 18-story building. The contamination was so severe that cleanup efforts kept the building closed for 13 years.

North Carolina

One of the largest deliberate PCB spills in American history occurred in the summer of 1978 when 31,000 gallons (117 m^3) of PCB-contaminated oil were illegally sprayed by the Ward PCB Transformer Company in 3-foot (0.91 m) swaths along the roadsides of some 240 miles (390 km) of North Carolina highway shoulders in 14 counties and at the Fort Bragg Army Base. The crime, known as "the midnight dumpings", occurred over nearly 2 weeks, as drivers of a black-painted tanker truck drove down one side of rural Piedmont highways spraying PCB-laden waste and then up the other side the following night.

Under Governor James B. Hunt, Jr., state officials then erected large, yellow warning signs along the contaminated highways that read: "CAUTION: PCB Chemical Spills Along Highway Shoulders." The illegal dumping is believed to have been motivated by the passing of the Toxic Substances Control Act (TSCA), which became effective on August 2, 1978 and increased the expense of chemical waste disposal.

Within a couple of weeks of the crime, Robert Burns and his sons, Timothy and Randall, were arrested for dumping the PCBs along the roadsides. Burns was a business partner of Robert "Buck" Ward, Jr., of the Ward PCB Transformer Company, in Raleigh. Burns and sons pleaded guilty to state and Federal criminal charges; Burns received a three to five-year prison sentence. Ward was acquitted of state charges in the dumping, but was sentenced to 18 months prison time for violation of TSCA.

Cleanup and disposal of the roadside PCBs generated controversy, as the Governor's plan to pick up the roadside PCBs and to bury them in a landfill in rural Warren County were strongly opposed in 1982 by local residents. In October 2013, at the request of the South Carolina Department of Health and Environmental Control (SCDHEC), the City of Charlotte, North Carolina decided to stop applying sewage sludge to land while authorities investigated the source of PCB contamination. In February 2014, the City of Charlotte admitted PCBs have entered their sewage treatment centers as well.

After the 2013 SCDHEC had issued emergency regulations the City of Charlotte discovered high levels of PCBs entering its sewage waste water treatment plants, where sewage is converted to sewage sludge. The city at first denied it had a problem, then admitted an "event" occurred in February 2014, and in April that the problem had occurred much earlier. The city stated that its very first test with a newly changed test method revealed very high PCB levels in its sewage sludge farm field fertilizer. Because of the widespread use of the contaminated sludge, SCDHEC subsequently issued PCB fish advisories for nearly all streams and rivers bordering farm fields that had been applied with city waste.

Ohio

The Clyde cancer cluster (also known as the Sandusky County cancer cluster) is a childhood cancer cluster that has affected many families in Clyde, Ohio and surrounding areas. PCBs were found in soil in a public park within the area of the cancer cluster.

In Akron, Ohio, soil was contaminated and noxious PCB-laden fumes had been put into the air by an electrical transformer deconstruction operation from the 1930s to the 1960s.

South Carolina

From 1955 until 1977, the Sangamo Weston plant in Pickens, SC, used PCBs to manufacture capacitors, and dumped 400,000 pounds of PCB contaminated wastewater into the Twelve Mile Creek. In 1990, the EPA declared the 228 acres (0.92 km2) site of the capacitor plant, its landfills and the polluted watershed, which stretches nearly 1,000 acres (4.0 km2) downstream to Lake Hartwell as a Superfund site. Two dams on the Twelve Mile Creek are to be removed and on Feb. 22, 2011 the first of two dams began to be dismantled. Some contaminated sediment is being removed from the site and hauled away, while other sediment is pumped into a series of settling ponds.

In 2013, the state environmental regulators issued a rare emergency order, banning all sewage sludge from being land applied or deposited on landfills, as it contained very high levels of PCBs. The problem had not been discovered until thousands of acres of farm land in the state had been contaminated by the hazardous sludge. A criminal investigation to determine the perpetrator of this crime was launched.

Washington

As of 2015, several bodies of water in the state of Washington were contaminated with PCBs, including the Columbia River, the Duwamish River, Green Lake, Lake Washington, the Okanogan River, Puget Sound, the Spokane River, the Walla Walla River, the Wenatchee River, and the Yakima River. A study by Washington State published in 2011 found that the two largest sources of PCB flow into the Spokane River were City of Spokane stormwater (44%) and municipal and industrial discharges (20%).

PCBs entered the environment through paint, hydraulic fluids, sealants, inks and have been found in river sediment and wild life. Spokane utilities will spend $300 million to prevent PCBs from entering the river in anticipation of a 2017 federal deadline to do so. In August 2015 Spokane joined other U.S cities like San Diego and San Jose, California, and Westport, Massachusetts. in seeking damages from Monsanto.

Wisconsin

From 1954 until 1971, the Fox River in Appleton, Wisconsin had PCBs deposited into it from Appleton Paper/NCR, P.H. Gladfelter, Georgia Pacific and other notable local paper manufacturing facilities. The Wisconsin DNR estimates that after wastewater treatment the PCB discharges to the Fox River due to production losses ranged from 81,000 kg to 138,000 kg. (178,572 lbs. to 304,235 lbs). The production of Carbon Copy Paper and its byproducts led to the discharge into the river. Fox River clean up is ongoing.

Pacific Ocean

Polychlorinated biphenyls have been discovered in organisms living in the Mariana trench in the Pacific Ocean. Levels were as high as 1,900 nanograms per gram of amphipod tissue in the organisms analyzed.

Regulation

In 1972 the Japanese government banned the production, use, and import of PCBs.

In 1973, the use of PCBs in "open" or "dissipative" sources, such as plasticisers in paints and cements, casting agents, fire retardant fabric treatments and heat stabilizing additives for PVC electrical insulation, adhesives, paints and waterproofing, railroad ties was banned in Sweden.

In 1976, concern over the toxicity and persistence (chemical stability) of PCBs in the environment led the United States Congress to ban their domestic production, effective January 1, 1978, via the Toxic Substances Control Act. As the agency that was charged with implementing TSCA, the EPA banned new manufacturing of PCBs, but it allowed their continued use for electrical equipment for economic reasons. In 1979 and future years, the EPA continued to regulate PCB usage and disposal.

In 1981, the UK banned closed uses of PCBs in new equipment, and nearly all UK PCB synthesis ceased; closed uses in existing equipment containing in excess of 5 litres of PCBs were not stopped until December 2000.

Modern sources include pigments, which may be used in inks for paper or plastic products.

Methods of destruction

Physical

PCBs are technically attractive because of their inertness, which includes their resistance to combustion. Nonetheless, they can be effectively destroyed by incineration at 1000 °C. When combusted at lower temperatures, they convert in part to more hazardous materials, including dibenzofurans and dibenzodioxins. When conducted properly, the combustion products are water, carbon dioxide, and hydrogen chloride. In some cases, the PCBs are combusted as a solution in kerosene. PCBs have also been destroyed by pyrolysis in the presence of alkali metal carbonates.
Thermal desorption is highly effective at removing PCBs from soil.

Chemical

PCBs are fairly chemically unreactive, this property being attractive for its application as an inert material. They resist oxidation. Many chemical compounds are available to destroy or reduce the PCBs. Commonly, PCBs are degraded by basis mixtures of glycols, which displace some or all chloride. Also effective are reductants such as sodium or sodium naphthenide. Vitamin B12 has also shown promise.

Microbial

Some micro-organisms degrade PCBs by reducing the C-Cl bonds. Microbial dechlorination tends to be rather slow-acting in comparison to other methods. Enzymes extracted from microbes can show PCB activity. In 2005, Shewanella oneidensis biodegraded a high percentage of PCBs in soil samples. A low voltage current can stimulate the microbial degradation of PCBs.

Fungal

There is research showing that some ligninolytic fungi can degrade PCBs.

Mollusc shell

From Wikipedia, the free encyclopedia
Diversity and variability of shells of molluscs on display.
Variety of Mollusc shells (gastropods, snails and seashells).
Closed and open shells of a marine bivalve, Petricola pholadiformis. A bivalve shell is composed of two hinged valves which are joined by a ligament.
Four views of a shell of the land snail Arianta arbustorum
The mollusc (or mollusk[spelling 1]) shell is typically a calcareous exoskeleton which encloses, supports and protects the soft parts of an animal in the phylum Mollusca, which includes snails, clams, tusk shells, and several other classes. Not all shelled molluscs live in the sea; many live on the land and in freshwater.

The ancestral mollusc is thought to have had a shell, but this has subsequently been lost or reduced on some families, such as the squid, octopus, and some smaller groups such as the caudofoveata and solenogastres. Today, over 100,000 living species bear a shell; there is some dispute as to whether these shell-bearing molluscs form a monophyletic group (conchifera) or whether shell-less molluscs are interleaved into their family tree.

Malacology, the scientific study of molluscs as living organisms, has a branch devoted to the study of shells, and this is called conchology—although these terms used to be, and to a minor extent still are, used interchangeably, even by scientists (this is more common in Europe).

Within some species of molluscs, there is often a wide degree of variation in the exact shape, pattern, ornamentation, and color of the shell.

Formation

The giant clam (Tridacna gigas) is the largest extant species of bivalve. The mantle is visible between the open valves
A mollusc shell is formed, repaired and maintained by a part of the anatomy called the mantle. Any injuries to or abnormal conditions of the mantle are usually reflected in the shape and form and even color of the shell. When the animal encounters harsh conditions that limit its food supply, or otherwise cause it to become dormant for a while, the mantle often ceases to produce the shell substance. When conditions improve again and the mantle resumes its task, a "growth line" is produced.

The mantle edge secretes a shell which has two components. The organic constituent is mainly made up of polysaccharides and glycoproteins; its composition may vary widely: some molluscs employ a wide range of chitin-control genes to create their matrix, whereas others express just one, suggesting that the role of chitin in the shell framework is highly variable; it may even be absent in monoplacophora. This organic framework controls the formation of calcium carbonate crystals (never phosphate, with the questionable exception of Cobcrephora), and dictates when and where crystals start and stop growing, and how fast they expand; it even controls the polymorph of the crystal deposited, controlling positioning and elongation of crystals and preventing their growth where appropriate.

The shell formation requires certain biological machinery. The shell is deposited within a small compartment, the extrapallial space, which is sealed from the environment by the periostracum, a leathery outer layer around the rim of the shell, where growth occurs. This caps off the extrapallial space, which is bounded on its other surfaces by the existing shell and the mantle. The periostracum acts as a framework from which the outer layer of carbonate can be suspended, but also, in sealing the compartment, allows the accumulation of ions in concentrations sufficient for crystallization to occur. The accumulation of ions is driven by ion pumps packed within the calcifying epithelium. Calcium ions are obtained from the organism's environment through the gills, gut and epithelium, transported by the haemolymph ("blood") to the calcifying epithelium, and stored as granules within or in-between cells ready to be dissolved and pumped into the extrapallial space when they are required. The organic matrix forms the scaffold that directs crystallization, and the deposition and rate of crystals is also controlled by hormones produced by the mollusc. Because the extrapallial space is supersaturated, the matrix could be thought of as impeding, rather than encouraging, carbonate deposition; although it does act as a nucleating point for the crystals and controls their shape, orientation and polymorph, it also terminates their growth once they reach the necessary size. Nucleation is endoepithelial in Neopilina and Nautilus, but exoepithelial in the bivalves and gastropods.

The formation of the shell involves a number of genes and transcription factors. On the whole, the transcription factors and signalling genes are deeply conserved, but the proteins in the secretome are highly derived and rapidly evolving. engrailed serves to demark the edge of the shell field; dpp controls the shape of the shell, and Hox1 and Hox4 have been implicated in the onset of mineralization. In gastropod embryos, Hox1 is expressed where the shell is being accreted; however no association has been observed between Hox genes and cephalopod shell formation. Perlucin increases the rate at which calcium carbonate precipitates to form a shell when in saturated seawater; this protein is from the same group of proteins (C-type lectins) as those responsible for the formation of eggshell and pancreatic stone crystals, but the role of C-type lectins in mineralization is unclear. Perlucin operates in association with Perlustrin, a smaller relative of lustrin A, a protein responsible for the elasticity of organic layers that makes nacre so resistant to cracking. Lustrin A bears remarkable structural similarity to the proteins involved in mineralization in diatoms – even though diatoms use silica, not calcite, to form their tests!

Development

Mollusc shells in Manchester Museum

The shell-secreting area is differentiated very early in embryonic development. An area of the ectoderm thickens, then invaginates to become a "shell gland". The shape of this gland is tied to the form of the adult shell; in gastropods, it is a simple pit, whereas in bivalves, it forms a groove which will eventually become the hinge line between the two shells, where they are connected by a ligament. The gland subsequently evaginates in molluscs that produce an external shell. Whilst invaginated, a periostracum - which will form a scaffold for the developing shell - is formed around the opening of the invagination, allowing the deposition of the shell when the gland is everted. A wide range of enzymes are expressed during the formation of the shell, including carbonic anhydrase, alkaline phosphatase, and DOPA-oxidase (tyrosinase)/peroxidase.

The form of the molluscan shell is constrained by the organism's ecology. In molluscs whose ecology changes from the larval to adult form, the morphology of the shell also undergoes a pronounced modification at metamorphosis. The larval shell may have a completely different mineralogy to the adult conch, perhaps formed from amorphous calcite as opposed to an aragonite adult conch.

In those shelled molluscs that have indeterminate growth, the shell grows steadily over the lifetime of the mollusc by the addition of calcium carbonate to the leading edge or opening. Thus the shell gradually becomes longer and wider, in an increasing spiral shape, to better accommodate the growing animal inside. The shell thickens as it grows, so that it stays proportionately strong for its size.

Secondary loss

The loss of a shell in the adult form of some gastropods is achieved by the discarding of the larval shell; in other gastropods and in cephalopods, the shell is lost or demineralized by the resorption of its carbonate component by the mantle tissue.

Shell proteins

Hundreds of soluble and insoluble proteins control shell formation. They are secreted into the extrapallial space by the mantle, which also secretes the glycoproteins, proteoglycans, polysaccharides and chitin that make up the organic shell matrix. Insoluble proteins tend to be thought of as playing a more important/major role in crystallization control. The organic matrix of shells tends to consist of β-chitin and silk fibroin. Perlucin encourages carbonate deposition, and is found at the interface of the chitinous and aragonitic layer in some shells. An acidic shell matrix appears to be essential to shell formation, in the cephalopods at least; the matrix in the non-mineralized squid gladius is basic.
In oysters and potentially most molluscs, the nacreous layer has an organic framework of the protein MSI60, which has a structure a little like spider silk and forms sheets; the prismatic layer uses MSI31 to construct its framework. This too forms beta-pleated sheets. Since acidic amino acids, such as aspartic acid and glutamic acid, are important mediators of biomineralization, shell proteins tend to be rich in these amino acids. Aspartic acid, which can make up up to 50% of shell framework proteins, is most abundant in calcitic layers, and also heavily present in aragonitic layers. Proteins with high proportions of glutamic acid are usually associated with amorphous calcium carbonate.
The soluble component of the shell matrix acts to inhibit crystallization when in its soluble form, but when it attaches to an insoluble substrate, it permits the nucleation of crystals. By switching from a dissolved to an attached form and back again, the proteins can produce bursts of growth, producing the brick-wall structure of the shell.

Chemistry

The formation of a shell in molluscs appears to be related to the secretion of ammonia, which originates from urea. The presence of an ammonium ion raises the pH of the extrapallial fluid, favouring the deposition of calcium carbonate. This mechanism has been proposed not only for molluscs, but also for other unrelated mineralizing lineages.

Structure

Precious Wentletrap: the spiral shell of Epitonium scalare sea snail.
The calcium carbonate layers in a shell are generally of two types: an outer, chalk-like prismatic layer and an inner pearly, lamellar or nacreous layer. The layers usually incorporate a substance called conchiolin, often in order to help bind the calcium carbonate crystals together. Conchiolin is composed largely of quinone-tanned proteins.
The periostracum and prismatic layer are secreted by a marginal band of cells, so that the shell grows at its outer edge. Conversely, the nacreous layer is derived from the main surface of the mantle.
Some shells contain pigments which are incorporated into the structure. This is what accounts for the striking colors and patterns that can be seen in some species of seashells, and the shells of some tropical land snails. These shell pigments sometimes include compounds such as pyrroles and porphyrins.
Shells are almost always composed of polymorphs of calcium carbonate - either calcite or aragonite. In many cases, such as the shells of many of the marine gastropods, different layers of the shell are composed of calcite and aragonite. In a few species which dwell near hydrothermal vents, iron sulfide is used to construct the shell. Phosphate is never utilised by molluscs, with the exception of Cobcrephora, whose molluscan affinity is uncertain.
Shells are composite materials of calcium carbonate (found either as calcite or aragonite) and organic macromolecules (mainly proteins and polysaccharides). Shells can have numerous ultrastructural motifs, the most common being crossed-lamellar (aragonite), prismatic (aragonite or calcite), homogeneous (aragonite), foliated (aragonite) and nacre (aragonite). Although not the most common, nacre is the most studied type of layer.

Size

In most shelled molluscs, the shell is large enough for all of the soft parts to be retracted inside when necessary, for protection from predation or from desiccation. However, there are many species of gastropod mollusc in which the shell is somewhat reduced or considerably reduced, such that it offers some degree of protection only to the visceral mass, but is not large enough to allow the retraction of the other soft parts. This is particularly common in the opisthobranchs and in some of the pulmonates, for example in the semi-slugs.
Some gastropods have no shell at all, or only an internal shell or internal calcareous granules, and these species are often known as slugs. Semi-slugs are pulmonate slugs with a greatly reduced external shell which is in some cases partly covered by the mantle.

Shape

The shape of the molluscan shell is controlled both by transcription factors (such as engrailed and decapentaplegic) and by developmental rate. The simplification of a shell form is thought to be relatively easily evolved, and many gastropod lineages have independently lost the complex coiled shape. However, re-gaining the coiling requires many morphological modifications and is much rarer. Despite this, it can still be accomplished; it is known from one lineage that was uncoiled for at least 20 million years, before modifying its developmental timing to restore the coiled morphology.
In bivalves at least, the shape does change through growth, but the pattern of growth is constant. At each point around the aperture of the shell, the rate of growth remains constant. This results in different areas growing at different rates, and thus a coiling of the shell and a change in its shape - its convexity, and the shape of the opening - in a predictable and consistent fashion.
The shape of the shell has an environmental as well as a genetic component; clones of gastropods can exert different shell morphologies. Indeed, intra-species variation can be many times larger than inter-species variation.
A number of terms are used to describe molluscan shell shape; in the univalved molluscs, endogastric shells coil backwards (away from the head), whereas exogastric shells coil forwards; the equivalent terms in bivalved molluscs are opisthogyrate and prosogyrate respectively.

Nacre

Nacre, commonly known as mother of pearl, forms the inner layer of the shell structure in some groups of gastropod and bivalve molluscs, mostly in the more ancient families such as top snails (Trochidae), and pearl oysters (Pteriidae). Like the other calcareous layers of the shell, the nacre is created by the epithelial cells (formed by the germ layer ectoderm) of the mantle tissue. However, nacre does not seem to represent a modification of other shell types, as it uses a distinct set of proteins.

Evolution

The fossil record shows that all molluscan classes evolved some 500 million years ago  from a shelled ancestor looking something like a modern monoplacophoran, and that modifications of the shell form ultimately led to the formation of new classes and lifestyles. However, a growing body of molecular and biological data indicate that at least certain shell features have evolved many times, independently. The nacreous layer of shells is a complex structure, but rather than being difficult to evolve, it has in fact arisen many times convergently. The genes used to control its formation vary greatly between taxa: under 10% of the (non-housekeeping) genes expressed in the shells that produce gastropod nacre are also found in the equivalent shells of bivalves: and most of these shared genes are also found in mineralizing organs in the deuterostome lineage. The independent origins of this trait are further supported by crystallographic differences between clades: the orientation of the axes of the deposited aragonite 'bricks' that make up the nacreous layer is different in each of the monoplacophora, gastropods and bivalves.
Mollusc shells (especially those formed by marine species) are very durable and outlast the otherwise soft-bodied animals that produce them by a very long time (sometimes thousands of years even without being fossilized). Most shells of marine molluscs fossilize rather easily, and fossil mollusc shells date all the way back to the Cambrian period. Large amounts of shell sometimes forms sediment, and over a geological time span can become compressed into limestone deposits.
Most of the fossil record of molluscs consists of their shells, since the shell is often the only mineralised part of a mollusc (however also see Aptychus and operculum). The shells are usually preserved as calcium carbonate – usually any aragonite is pseudomorphed with calcite. Aragonite can be protected from recrystalization if water is kept away by carbonaceous material, but this did not accumulate in sufficient quantity until the Carboniferous; consequently aragonite older than the Carboniferous is practically unknown: but the original crystal structure can sometimes be deduced in fortunate circumstances, such as if an alga closely encrusts the surface of a shell, or if a phosphatic mould quickly forms during diagenesis.
The shell-less aplacophora have a chitinous cuticle that has been likened to the shell framework; it has been suggested that tanning of this cuticle, in conjunction with the expression of additional proteins, could have set the evolutionary stage for the secretion of a calcareous shell in an aplacophoran-like ancestral mollusc.
The molluscan shell has been internalized in a number of lineages, including the coleoid cephalopods and many gastropod lineages. Detorsion of gastropods results in an internal shell, and can be triggered by relatively minor developmental modifications such as those induced by exposure to high platinum concentrations.

Pattern formation

The pattern formation processes in mollusc shells have been modeled successfully using one-dimensional reaction-diffusion systems, in particular the Gierer-Meinhardt system which leans heavily on the Turing model.

Varieties

Monoplacophora

The nacreous layer of monoplacophoran shells appears to have undergone some modification. Whilst normal nacre, and indeed part of the nacreous layer of one monoplacophoran species (Veleropilina zografi), consists of "brick-like" crystals of aragonite, in monoplacophora these bricks are more like layered sheets. The c-axis is perpendicular to the shell wall, and the a-axis parallel to the growth direction. This foliated aragonite is presumed to have evolved from the nacreous layer, with which it has historically been confused, but represents a novelty within the molluscs.

Chitons

The chiton Tonicella lineata, anterior end towards the right
Shells of chitons are made up of eight overlapping calcareous valves, surrounded by a girdle.

Gastropods

The marine gastropod Cypraea chinensis, the Chinese cowry, showing partially extended mantle
In some marine genera, during the course of normal growth the animal undergoes periodic resting stages where the shell does not increase in overall size, but a greatly thickened and strengthened lip is produced instead. When these structures are formed repeatedly with normal growth between the stages, evidence of this pattern of growth is visible on the outside of the shell, and these unusual thickened vertical areas are called varices, singular "varix". Varices are typical in some marine gastropod families, including the Bursidae, Muricidae, and Ranellidae.
Finally, gastropods with a determinate growth pattern may create a single and terminal lip structure when approaching maturity, after which growth ceases. These include the cowries (Cypraeidae) and helmet shells (Cassidae), both with in-turned lips, the true conchs (Strombidae) that develop flaring lips, and many land snails that develop tooth structures or constricted apertures upon reaching full size.

Cephalopods

Nautilus belauensis is one of only 6 extant cephalopod species which have an external shell

Nautiluses are the only extant cephalopods which have an external shell. Cuttlefish, squid, spirula, vampire squid, and cirrate octopuses have small internal shells. Females of the octopus genus Argonauta secrete a specialised paper-thin eggcase in which they partially reside, and this is popularly regarded as a "shell", although it is not attached to the body of the animal.

Bivalves

The shell of the Bivalvia is composed of two parts, two valves which are hinged together and joined by a ligament.

Scaphopods

The shell of many of the scaphopods ("tusk shells") resembles a miniature elephant's tusk in overall shape, except that it is hollow, and is open at both ends.

Damage to shells in collections

As a structure made primarily of calcium carbonate, mollusc shells are vulnerable to attack by acidic fumes. This can become a problem when shells are in storage or on display and are in the proximity of non-archival materials, see Byne's disease.

Nacre (Mother of Pearl)

From Wikipedia, the free encyclopedia
The iridescent nacre inside a nautilus shell
 
Nacre (/ˈnkər/ NAY-kər also /ˈnækrə/ NAK-rə), also known as mother of pearl, is an organic-inorganic composite material produced by some molluscs as an inner shell layer; it is also the material of which pearls are composed. It is strong, resilient, and iridescent.

Nacre is found in some of the most ancient lineages of bivalves, gastropods, and cephalopods. However, the inner layer in the great majority of mollusc shells is porcellaneous, not nacreous, and this usually results in a non-iridescent shine, or more rarely in non-nacreous iridescence such as flame structure as is found in conch pearls.

The outer layer of cultured pearls and the inside layer of pearl oyster and freshwater pearl mussel shells are made of nacre. Other mollusc families that have a nacreous inner shell layer include marine gastropods such as the Haliotidae, the Trochidae and the Turbinidae.

Physical characteristics

Structure and appearance

Schematic of the microscopic structure of nacre layers
 
Electron microscopy image of a fractured surface of nacre

Nacre is composed of hexagonal platelets of aragonite (a form of calcium carbonate) 10–20 µm wide and 0.5 µm thick arranged in a continuous parallel lamina. Depending on the species, the shape of the tablets differ; in Pinna, the tablets are rectangular, with symmetric sectors more or less soluble. Whatever the shape of the tablets, the smallest units they contain are irregular rounded granules. These layers are separated by sheets of organic matrix (interfaces) composed of elastic biopolymers (such as chitin, lustrin and silk-like proteins). This mixture of brittle platelets and the thin layers of elastic biopolymers makes the material strong and resilient, with a Young's modulus of 70 GPa (when dry). Strength and resilience are also likely to be due to adhesion by the "brickwork" arrangement of the platelets, which inhibits transverse crack propagation. This structure, at multiple length sizes, greatly increases its toughness, making it almost as strong as silicon.

The statistical variation of the platelets has a negative effect on the mechanical performance (stiffness, strength, and energy absorption) because statistical variation precipitates localization of deformation. However, the negative effects of statistical variations can be offset by interfaces with large strain at failure accompanied by strain hardening. On the other hand, the fracture toughness of nacre increases with moderate statistical variations which creates tough regions where the crack gets pinned. But, higher statistical variations generates very weak regions which allows the crack to propagate without much resistance causing the fracture toughness decreases. 

Nacre appears iridescent because the thickness of the aragonite platelets is close to the wavelength of visible light. These structures interfere constructively and destructively with different wavelengths of light at different viewing angles, creating structural colours.

The crystallographic c-axis points approximately perpendicular to the shell wall, but the direction of the other axes varies between groups. Adjacent tablets have been shown to have dramatically different c-axis orientation, generally randomly oriented within ~20° of vertical. In bivalves and cephalopods, the b-axis points in the direction of shell growth, whereas in the monoplacophora it is the a-axis that is this way inclined. The interlocking of bricks of nacre has large impact on both the deformation mechanism as well as its toughness. In addition, the mineral–organic interface results in enhanced resilience and strength of the organic interlayers.

Formation

Nacre formation is not fully understood. The initial onset assembly, as observed in Pinna nobilis, is driven by the aggregation of nanoparticles (~50–80 nm) within an organic matrix that arrange in fibre-like polycrystalline configurations. The particle number increases successively and, when critical packing is reached, they merge into early-nacre platelets. Nacre growth is mediated by organics, controlling the onset, duration and form of crystal growth. Individual aragonite "bricks" are believed to quickly grow to the full height of the nacreous layer, and expand until they abut adjacent bricks. This produces the hexagonal close-packing characteristic of nacre. Bricks may nucleate on randomly dispersed elements within the organic layer, well-defined arrangements of proteins, or may grow epitaxially from mineral bridges extending from the underlying tablet. Nacre differs from fibrous aragonite – a brittle mineral of the same form – in that the growth in the c-axis (i.e., approximately perpendicular to the shell, in nacre) is slow in nacre, and fast in fibrous aragonite.

Function

Fossil nautiloid shell with original iridescent nacre in fossiliferous asphaltic limestone, Oklahoma. Dated to the late Middle Pennsylvanian, which makes it by far the oldest deposit in the world with aragonitic nacreous shelly fossils.
 
Nacre is secreted by the epithelial cells of the mantle tissue of various molluscs. The nacre is continuously deposited onto the inner surface of the shell, the iridescent nacreous layer, commonly known as mother of pearl. The layers of nacre smooth the shell surface and help defend the soft tissues against parasites and damaging debris by entombing them in successive layers of nacre, forming either a blister pearl attached to the interior of the shell, or a free pearl within the mantle tissues. The process is called encystation and it continues as long as the mollusc lives.

In different mollusc groups

The form of nacre varies from group to group. In bivalves, the nacre layer is formed of single crystals in a hexagonal close packing. In gastropods, crystals are twinned, and in cephalopods, they are pseudohexagonal monocrystals, which are often twinned.

Commercial sources

The main commercial sources of mother of pearl have been the pearl oyster, freshwater pearl mussels, and to a lesser extent the abalone, popular for their sturdiness and beauty in the latter half of the 19th century.

Widely used for pearl buttons especially during the 1900s, were the shells of the great green turban snail Turbo marmoratus and the large top snail, Tectus niloticus. The international trade in mother of pearl is governed by the Convention on International Trade in Endangered Species of Wild Fauna and Flora, an agreement signed by more than 170 countries.

Decorative uses

Architecture

White nacre mosaic tiles in the ceiling of the Criterion Restaurant in London
 
Both black and white nacre are used for architectural purposes. The natural nacre may be artificially tinted to almost any color. Nacre tesserae may be cut into shapes and laminated to a ceramic tile or marble base. The tesserae are hand-placed and closely sandwiched together, creating an irregular mosaic or pattern (such as a weave). The laminated material is typically about 2 millimetres (0.079 in) thick. The tesserae are then lacquered and polished creating a durable and glossy surface.

Instead of using a marble or tile base, the nacre tesserae can be glued to fiberglass. The result is a lightweight material that offers a seamless installation and there is no limit to the sheet size. Nacre sheets may be used on interior floors, exterior and interior walls, countertops, doors and ceilings. Insertion into architectural elements, such as columns or furniture is easily accomplished.

Fashion

Nacre bracelet

Mother of pearl buttons are used in clothing either for functional or decorative purposes. The Pearly Kings and Queens are an elaborate example of this.

Nacre is also used to decorate watches, knives, guns and jewellery.

Musical instruments

Nacre inlay is often used for music keys and other decorative motifs on musical instruments. Many accordion and concertina bodies are completely covered in nacre, and some guitars have fingerboard or headstock inlays made of nacre (as well as some guitars having plastic inlays designed to imitate the appearance of nacre). The bouzouki and baglamas (Greek plucked string instruments of the lute family) typically feature nacre decorations, as does the related Middle Eastern oud (typically around the sound holes and on the back of the instrument). Bows of stringed instruments such as the violin and cello often have mother of pearl inlay at the frog. It is traditionally used on saxophone keytouches, as well as the valve buttons of trumpets and other brass instruments. The Middle Eastern goblet drum (darbuka) is commonly decorated by mother of pearl.

Other

Mother of pearl is sometimes used to make spoon-like utensils for caviar, so as to not spoil the taste with metallic spoons.

Manufactured nacre

In 2012, researchers created calcium-based nacre in the laboratory by mimicking its natural growth process.

In 2014, researchers used lasers to create an analogue of nacre by engraving networks of wavy 3D "micro-cracks" in glass. When the slides were subjected to an impact, the micro-cracks absorbed and dispersed the energy, keeping the glass from shattering. Altogether, treated glass was reportedly 200 times tougher than untreated glass.

Cryogenics

From Wikipedia, the free encyclopedia https://en.wikipedia.org/wiki/Cryogenics...