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Tuesday, April 2, 2024

Creosote

From Wikipedia, the free encyclopedia
Wood railroad ties before (right) and after (left) infusion with creosote, being transported by railcar at a facility of the Santa Fe Railroad, in Albuquerque, New Mexico, in March 1943. This U.S. wartime governmental photo reports that "The steaming black ties in the [left of photo]... have just come from the retort where they have been infused with creosote for eight hours." Ties are "made of pine and fir... seasoned for eight months" [as seen in the untreated railcar load at right].

Creosote is a category of carbonaceous chemicals formed by the distillation of various tars and pyrolysis of plant-derived material, such as wood, or fossil fuel. They are typically used as preservatives or antiseptics.

Some creosote types were used historically as a treatment for components of seagoing and outdoor wood structures to prevent rot (e.g., bridgework and railroad ties, see image). Samples may be found commonly inside chimney flues, where the coal or wood burns under variable conditions, producing soot and tarry smoke. Creosotes are the principal chemicals responsible for the stability, scent, and flavor characteristic of smoked meat; the name is derived from Greek κρέας (kreas) 'meat', and σωτήρ (sōtēr) 'preserver'.

The two main kinds recognized in industry are coal-tar creosote and wood-tar creosote. The coal-tar variety, having stronger and more toxic properties, has chiefly been used as a preservative for wood; coal-tar creosote was also formerly used as an escharotic, to burn malignant skin tissue, and in dentistry, to prevent necrosis, before its carcinogenic properties became known. The wood-tar variety has been used for meat preservation, ship treatment, and such medical purposes as an anaesthetic, antiseptic, astringent, expectorant, and laxative, though these have mostly been replaced by modern formulations.

Varieties of creosote have also been made from both oil shale and petroleum, and are known as oil-tar creosote when derived from oil tar, and as water-gas-tar creosote when derived from the tar of water gas. Creosote also has been made from pre-coal formations such as lignite, yielding lignite-tar creosote, and peat, yielding peat-tar creosote.

Creosote oils

The term creosote has a broad range of definitions depending on the origin of the coal tar oil and end-use of the material.

With respect to wood preservatives, the United States Environmental Protection Agency (EPA) considers the term creosote to mean a pesticide for use as a wood preservative meeting the American Wood Protection Association (AWPA) Standards P1/P13 and P2. The AWPA Standards require that creosote "shall be a pure coal tar product derived entirely from tar produced by the carbonization of bituminous coal."

Currently, all creosote-treated wood products—foundation and marine pilings, lumber, posts, railroad ties, timbers, and utility poles—are manufactured using this type of wood preservative. The manufacturing process can only be a pressure process under the supervision of a licensed applicator certified by the State Departments of Agriculture. No brush-on, spray, or non-pressure uses of creosote are allowed, as specified by the EPA-approved label for the use of creosote.

The use of creosote according to the AWPA Standards does not allow for mixing with other types of "creosote type" materials—such as lignite-tar creosote, oil-tar creosote, peat-tar creosote, water-gas-tar creosote, or wood-tar creosote. The AWPA Standard P3 does however, allow blending of a high-boiling petroleum oil meeting the AWPA Standard P4.

The information that follows describing the other various types of creosote materials and its uses should be considered as primarily being of only historical value. This history is important, because it traces the origin of these different materials used during the 19th and early 20th centuries. Furthermore, it must be considered that these other types of creosotes – lignite-tar, wood-tar, water-gas-tar, etc. – are not currently being manufactured and have either been replaced with more-economical materials, or replaced by products that are more efficacious or safer.

For some part of their history, coal-tar creosote and wood-tar creosote were thought to have been equivalent substances—albeit of distinct origins—accounting for their common name; the two were determined only later to be chemically different. All types of creosote are composed of phenol derivatives and share some quantity of monosubstituted phenols, but these are not the only active element of creosote. For their useful effects, coal-tar creosote relies on the presence of naphthalenes and anthracenes, while wood-tar creosote relies on the presence of methyl ethers of phenol. Otherwise, either type of tar would dissolve in water.

Creosote was first discovered in its wood-tar form in 1832, by Carl Reichenbach, when he found it both in the tar and in pyroligneous acids obtained by a dry distillation of beechwood. Because pyroligneous acid was known as an antiseptic and meat preservative, Reichenbach conducted experiments by dipping meat in a dilute solution of distilled creosote. He found that the meat was dried without undergoing putrefaction and had attained a smoky flavor. This led him to reason that creosote was the antiseptic component contained in smoke, and he further argued that the creosote he had found in wood tar was also in coal tar, as well as amber tar and animal tar, in the same abundance as in wood tar.

Soon afterward, in 1834, Friedrich Ferdinand Runge discovered carbolic acid (phenol) in coal-tar, and Auguste Laurent obtained it from "phenylhydrate", which was soon determined to be the same compound. There was no clear view on the relationship between carbolic acid and creosote; Runge described it as having similar caustic and antiseptic properties, but noted that it was different, in that it was an acid and formed salts. Nonetheless, Reichenbach argued that creosote was also the active element, as it was in pyroligneous acid. Despite evidence to the contrary, his view held sway with most chemists, and it became commonly accepted wisdom that creosote, carbolic acid, and phenylhydrate were identical substances, with different degrees of purity.

Carbolic acid was soon commonly sold under the name "creosote", and the scarcity of wood-tar creosote in some places led chemists to believe that it was the same substance as that described by Reichenbach. In the 1840s, Eugen Freiherr von Gorup-Besanez, after realizing that two samples of substances labelled as creosote were different, started a series of investigations to determine the chemical nature of carbolic acid, leading to a conclusion that it more resembled chlorinated quinones and must have been a different, entirely unrelated substance.

Independently, there were investigations into the chemical nature of creosote. A study by F.K. Völkel revealed that the smell of purified creosote resembled that of guaiacol, and later studies by Heinrich Hlasiwetz identified a substance common to guaiacum and creosote that he called creosol, and he determined that creosote contained a mixture of creosol and guaiacol. Later investigations by Gorup-Besanez, A.E. Hoffmann, and Siegfried Marasse showed that wood-tar creosote also contained phenols, giving it a feature in common with coal-tar creosote.

Historically, coal-tar creosote has been distinguished from what was thought of as creosote proper—the original substance of Reichenbach's discovery—and it has been referred to specifically as "creosote oil". But, because creosote from coal-tar and wood-tar are obtained from a similar process and have some common uses, they have also been placed in the same class of substances, with the terms "creosote" or "creosote oil" referring to either product.

Wood-tar creosote

Constituency of distillations of creosote from different woods at different temperatures

Beech Oak Pine

200–220 °C 200–210 °C 200–210 °C 200–210 °C
Monophenols 39.0 % 39.0 % 55.0 % 40.0%
Guaiacol 19.7 % 26.5 % 14.0 % 20.3%
Creosol and homologs 40.0% 32.1% 31.0% 37.5%
Loss 1.3% 2.4% . . . 2.2%

Wood-tar creosote is a colourless to yellowish greasy liquid with a smoky odor, produces a sooty flame when burned, and has a burned taste. It is non-buoyant in water, with a specific gravity of 1.037 to 1.087, retains fluidity at a very low temperature, and boils at 205-225 °C. In its purest form, it is transparent. Dissolution in water requires up to 200 times the amount of water as the base creosote. This creosote is a combination of natural phenols: primarily guaiacol and creosol (4-methylguaiacol), which typically constitutes 50% of the oil; second in prevalence are cresol and xylenol; the rest being a combination of monophenols and polyphenols.

Composition of a typical beech-tar creosote



Phenol C6H5OH 5.2%
o-Cresol (CH3)C6H4(OH) 10.4%
m-Cresol and p-cresol (CH3)C6H4(OH) 11.6%
2-Ethylphenol C6H4(C2H5)OH 3.6%
Guaiacol C6H4(OH)(OCH3) 25.0%
3,4-Xylenol C6H3(CH3)2OH 2.0%
3,5-Xylenol C6H3(CH3)2OH 1.0%
Various phenols C6H5OH 6.2%
Creosol and homologs C6H3(CH3)(OH)(OCH3) 35.0%

The simple phenols are not the only active element in wood-tar creosote. In solution, they coagulate albumin, which is a water-soluble protein found in meat, so they serve as a preserving agent, but also cause denaturation. Most of the phenols in the creosote are methoxy derivatives: they contain the methoxy group (−O−CH3) linked to the benzene nucleus. The high level of methyl derivates created from the action of heat on wood (also apparent in the methyl alcohol produced through distillation) make wood-tar creosote substantially different from coal-tar creosote. Guaiacol is a methyl ether of pyrocatechin, while creosol is a methyl ether of methyl-pyrocatechin, the next homolog of pyrocatechin. Methyl ethers differ from simple phenols in being less hydrophilic, caustic, and poisonous. This allows meat to be successfully preserved without tissue denaturation, and allows creosote to be used as a medical ointment.

Derivation of wood-tar creosote from resinous woods

Because wood-tar creosote is used for its guaiacol and creosol content, it is generally derived from beechwood rather than other woods, since it distills with a higher proportion of those chemicals to other phenolics. The creosote can be obtained by distilling the wood tar and treating the fraction heavier than water with a sodium hydroxide solution. The alkaline solution is then separated from the insoluble oily layer, boiled in contact with air to reduce impurities, and decomposed by diluted sulfuric acid. This produces a crude creosote, which is purified by re-solution in alkali, re-precipitation with acid, then redistilled with the fraction passing over between 200° and 225° constituting the purified creosote.[21]

When ferric chloride is added to a dilute solution, it will turn green: a characteristic of ortho-oxy derivatives of benzene. It dissolves in sulfuric acid to a red liquid, which slowly changes to purple-violet. Shaken with hydrochloric acid in the absence of air, it becomes red, the color changing in the presence of air to dark brown or black.

In preparation of food by smoking, guaiacol contributes mainly to the smoky taste, while the dimethyl ether of pyrogallol, syringol, is the main chemical responsible for the smoky aroma.

Historical uses

Industrial

Soon after it was discovered and recognized as the principle of meat smoking, wood-tar creosote became used as a replacement for the process. Several methods were used to apply the creosote. One was to dip the meat in pyroligneous acid or a water of diluted creosote, as Reichenbach did, or brush it over with them, and within one hour the meat would have the same quality of that of traditionally smoked preparations. Sometimes the creosote was diluted in vinegar rather than water, as vinegar was also used as a preservative. Another was to place the meat in a closed box, and place with it a few drops of creosote in a small bottle. Because of the volatility of the creosote, the atmosphere was filled with a vapour containing it, and it would cover the flesh.

The application of wood tar to seagoing vessels was practiced through the 18th century and early 19th century, before the creosote was isolated as a compound. Wood-tar creosote was found not to be as effective in wood treatments, because it was harder to infuse the creosote into the wood cells, but still experiments were done, including by many governments, because it proved to be less expensive on the market.

Medical

Even before creosote as a chemical compound was discovered, it was the chief active component of medicinal remedies in different cultures around the world.

In antiquity, pitches and resins were used commonly as medicines. Pliny mentions a variety of tar-like substances being used as medicine, including cedria and pissinum. Cedria was the pitch and resin of the cedar tree, being equivalent to the oil of tar and pyroligneous acid which are used in the first stage of distilling creosote. He recommends cedria to ease the pain in a toothache, as an injection in the ear in case of hardness of hearing, to kill parasitic worms, as a preventive for infusion, as a treatment for phthiriasis and porrigo, as an antidote for the poison of the sea hare, as a liniment for elephantiasis, and as an ointment to treat ulcers both on the skin and in the lungs. He further speaks of cedria being used as the embalming agent for preparing mummies. Pissinum was a tar water that was made by boiling cedria, spreading wool fleeces over the vessels to catch the steam, and then wringing them out.

Portrait of Bishop Berkeley by John Smybert, 1727

The Pharmacopée de Lyon, published in 1778, says that cedar tree oil is believed to cure vomiting and help medicate tumors and ulcers. Physicians contemporary to the discovery of creosote recommended ointments and pills made from tar or pitch to treat skin diseases. Tar water had been used as a folk remedy since the Middle Ages to treat affections like dyspepsia. Bishop Berkeley wrote several works on the medical virtues of tar water, including a philosophical work in 1744 titled Siris: a chain of philosophical reflexions and inquiries concerning the virtues of tar water, and divers other subjects connected together and arising one from another, and a poem where he praised its virtues. Pyroligneous acid was also used at the time in a medicinal water called Aqua Binelli (Binelli's water), a compound which its inventor, the Italian Fedele Binelli, claimed to have hemostatic properties in his research published in 1797. These claims have since been disproven.

Given this history, and the antiseptic properties known to creosote, it became popular among physicians in the 19th century. A dilution of creosote in water was sold in pharmacies as Aqua creosoti, as suggested by the previous use of pyroligneous acid. It was prescribed to quell the irritability of the stomach and bowels and detoxify, treat ulcers and abscesses, neutralize bad odors, and stimulate the mucous tissues of the mouth and throat. Creosote in general was listed as an irritant, styptic, antiseptic, narcotic, and diuretic, and in small doses when taken internally as a sedative and anaesthetic. It was used to treat ulcers, and as a way to sterilize the tooth and deaden the pain in case of a tooth-ache.

Creosote was suggested as a treatment for tuberculosis by Reichenbach as early as 1833. Following Reichenbach, it was argued for by John Elliotson and Sir John Rose Cormack. Elliotson, inspired by the use of creosote to arrest vomiting during an outbreak of cholera, suggested its use for tuberculosis through inhalation. He also suggested it for epilepsy, neuralgia, diabetes, and chronic glanders. The idea of using it for tuberculosis failed to be accepted. Use for this purpose was dropped, until the idea was revived in 1876 by British doctor G. Anderson Imlay, who suggested it be applied locally by spray to the bronchial mucous membrane. This was followed up in 1877 when it was argued for in a clinical paper by Charles Bouchard and Henri Gimbert. Germ theory had been established by Pasteur in 1860, and Bouchard, arguing that a bacillus was responsible for the disease, sought to rehabilitate creosote for its use as an antiseptic to treat it. He began a series of trials with Gimbert to convince the scientific community, and claimed a promising cure rate. A number of publications in Germany confirmed his results in the following years.

Later, a period of experimentation with different techniques and chemicals using creosote in treating tuberculosis lasted until about 1910, when radiation therapy seemed more promising. Guaiacol, instead of a full creosote solution, was suggested by Hermann Sahli in 1887. He argued it had the active chemical of creosote and had the advantage of being of definite composition and having a less unpleasant taste and odor. A number of solutions of both creosote and guaiacol appeared on the market, such as phosphotal and guaicophosphal, phosphites of creosote and guaiacol; eosot and geosot, valerinates of creosote and guaicol; phosot and taphosot, phosphate and tannophospate of creosote; and creosotal and tanosal, tannates of creosote. Creosote and eucalyptus oil were also a remedy used together, administered through a vaporizor and inhaler. Since then, more effective and safer treatments for tuberculosis have been developed.

In the 1940s, Canadian-based Eldon Boyd experimented with guaiacol and a recent synthetic modification—glycerol guaiacolate (guaifenesin)—on animals. His data showed that both drugs were effective in increasing secretions into the airways in laboratory animals, when high-enough doses were given.

Current uses

Industrial

Wood-tar creosote is to some extent used for wood preservation, but it is generally mixed with coal-tar creosote, since the former is not as effective. Commercially available preparations of "liquid smoke", marketed to add a smoked flavour to meat and aid as a preservative, consist primarily of creosote and other constituents of smoke. Creosote is the ingredient that gives liquid smoke its function; guaicol lends to the taste and the creosote oils help act as the preservative. Creosote can be destroyed by treatment with chlorine, either sodium hypochlorite, or calcium hypochlorite solutions. The phenol ring is essentially opened, and the molecule is then subject to normal digestion and normal respiration.

Medical

The guaifenesin developed by Eldon Boyd is still commonly used today as an expectorant, sold over the counter, and usually taken by mouth to assist the bringing up of phlegm from the airways in acute respiratory tract infections. Guaifenesin is a component of Mucinex, Robitussin DAC, Cheratussin DAC, Robitussin AC, Cheratussin AC, Benylin, DayQuil Mucous Control, Meltus, and Bidex 400.

Seirogan is a popular Kampo medicine in Japan, used as an anti-diarrheal, and has 133 mg wood creosote from beech, pine, maple or oak wood per adult dose as its primary ingredient. Seirogan was first used as a gastrointestinal medication by the Imperial Japanese Army in Russia during the Russo-Japanese War of 1904 to 1905.

Creomulsion is a cough medicine in the United States, introduced in 1925, that is still sold and contains beechwood creosote. Beechwood creosote is also found under the name kreosotum or kreosote.

Coal-tar creosote

Composition of a typical coal-tar creosote


Aromatic hydrocarbons

Polycyclic aromatic hydrocarbons (PAHs), alkylated PAHs, benzenes, toluenes, ethylbenzenes, and xylenes (BTEX)

75.0–90.0%
Tar acids / phenolics

Phenols, cresols, xylenols, and naphthols

5.0–17.0%
Tar bases / nitrogen-containing heterocycles

Pyridines, quinolines, benzoquinolines, acridines, indolines, and carbazoles

3.0–8.0%
Sulfur-containing heterocycles

Benzothiophenes

1.0–3.0%
Oxygen-containing heterocycles

Dibenzofurans

1.0–3.0%
Aromatic amines

Aniline, aminonaphthalenes, diphenylamines, aminofluorenes, and aminophenanthrenes, cyano-PAHs, benz acridines

0.1–1.0%

Coal-tar creosote is greenish-brown liquid, with different degrees of darkness, viscosity, and fluorescence depending on how it is made. When freshly made, the creosote is a yellow oil with a greenish cast and highly fluorescent, and the fluorescence is increased by exposure to air and light. After settling, the oil is dark green by reflected light and dark red by transmitted light. To the naked eye, it generally appears brown. The creosote (often called "creosote oil") consists almost wholly of aromatic hydrocarbons, with some amount of bases and acids and other neutral oils. The flash point is 70–75 °C and burning point is 90–100 °C, and when burned it releases a greenish smoke. The smell largely depends on the naphtha content in the creosote. If there is a high amount, it will have a naphtha-like smell, otherwise it will smell more of tar.

In the process of coal-tar distillation, the distillate is collected into four fractions; the "light oil", which remains lighter than water, the "middle oil" which passes over when the light oil is removed; the "heavy oil", which sinks; and the "anthracene oil", which when cold is mostly solid and greasy, of a buttery consistence. Creosote refers to the portion of coal tar which distills as "heavy oil", typically between 230 and 270 °C, also called "dead oil"; it sinks into water but still is fairly liquid. Carbolic acid is produced in the second fraction of distillation and is often distilled into what is referred to as "carbolic oil".

Derivation and general composition of coal-tar creosote

Commercial creosote contains substances from six groups. The two groups occur in the greatest amounts and are the products of the distillation process—the "tar acids", which distill below 205 °C and consist mainly of phenols, cresols, and xylenols, including carbolic acid—and aromatic hydrocarbons, which divide into naphthalenes, which distill approximately between 205 and 255 °C, and constituents of an anthracene nature, which distill above 255 °C. The quantity of each varies based on the quality of tar and temperatures used, but generally, the tar acids won't exceed 5%, the naphthalenes make up 15 to 50%, and the anthracenes make up 45% to 70%. The hydrocarbons are mainly aromatic; derivatives of benzene and related cyclic compounds such as naphthalene, anthracene, phenanthrene, acenaphthene, and fluorene. Creosotes from vertical-retort and low temperature tars contain, in addition, some paraffinic and olefinic hydrocarbons. The tar-acid content also depends on the source of the tar—it may be less than 3% in creosote from coke-oven tar and as high as 32% in creosote from vertical retort tar. All of these have antiseptic properties. The tar acids are the strongest antiseptics but have the highest degree of solubility in water and are the most volatile; so, like with wood-tar creosote, phenols are not the most valued component, as by themselves they would lend to being poor preservatives. In addition, creosote contains several products naturally occurring in coal—nitrogen-containing heterocycles, such as acridines, carbazoles, and quinolines, referred to as the "tar bases" and generally make up about 3% of the creosote—sulfur-containing heterocycles, generally benzothiophenes—and oxygen-containing heterocycles, dibenzofurans. Lastly, creosote contains a small number of aromatic amines produced by the other substances during the distillation process and likely resulting from a combination of thermolysis and hydrogenation. The tar bases are often extracted by washing the creosote with aqueous mineral acid, although they're also suggested to have antiseptic ability similar to the tar acids.

Commercially used creosote is often treated to extract the carbolic acid, naphthalene, or anthracene content. The carbolic acid or naphthalene is generally extracted to be used in other commercial products. American produced creosote oils typically have low amounts of anthracene and high amounts of naphthalene, because when forcing the distillate at a temperature that produces anthracene the soft pitch will be ruined and only the hard pitch will remain; this ruins it for use in roofing purposes, and only leaves a product which isn't commercially useful.

Historical uses

Industrial

The use of coal-tar creosote on a commercial scale began in 1838, when a patent covering the use of creosote oil to treat timber was taken out by inventor John Bethell. The "Bethell process"—or as it later became known, the full-cell process—involves placing wood to be treated in a sealed chamber and applying a vacuum to remove air and moisture from wood "cells". The wood is then pressure-treated to imbue it with creosote or other preservative chemicals, after which vacuum is reapplied to separate the excess treatment chemicals from the timber. Alongside the zinc chloride-based "Burnett process", use of creosoted wood prepared by the Bethell process became a principal way of preserving railway timbers (most notably railway sleepers) to increase the lifespan of the timbers, and avoiding having to regularly replace them.

Besides treating wood, it was also used for lighting and fuel. In the beginning, it was only used for lighting needed in harbour and outdoor work, where the smoke that was produced from burning it was of little inconvenience. By 1879, lamps had been created that ensured a more complete combustion by using compressed air, removing the drawback of the smoke. Creosote was also processed into gas and used for lighting that way. As a fuel, it was used to power ships at sea and blast furnaces for different industrial needs, once it was discovered to be more efficient than unrefined coal or wood. It was also used industrially for the softening of hard pitch, and burned to produce lamp black. By 1890, the production of creosote in the United Kingdom totaled approximately 29,900,000 gallons per year.

In 1854, Alexander McDougall and Robert Angus Smith developed and patented a product called McDougall's Powder as a sewer deodorant; it mainly consisted of carbolic acid derived from creosote. McDougall, in 1864, experimented with his solution to remove entozoa parasites from cattle pasturing on a sewage farm. This later led to widespread use of creosote as a cattle wash and sheep dip. External parasites would be killed in a creosote diluted dip, and drenching tubes would be used to administer doses to the animals' stomachs to kill internal parasites.

Wooden street pavers in Chicago

Creosoted wood blocks were a common road-paving material in the late 19th and early 20th centuries, but ultimately fell out of favor because they did not generally hold up well enough over time.

Two later methods for creosoting wood were introduced after the turn of the century, referred to as empty-cell processes, because they involve compressing the air inside the wood so that the preservative can only coat the inner cell walls rather than saturating the interior cell voids. This is a less effective, though usually satisfactory, method of treating the wood, but is used because it requires less of the creosoting material. The first method, the "Rüping process" was patented in 1902, and the second, the "Lowry process" was patented in 1906. Later in 1906, the "Allardyce process" and "Card process" were patented to treat wood with a combination of both creosote and zinc chloride. In 1912, it was estimated that a total of 150,000,000 gallons were produced in the US per year.

Medical

Coal-tar creosote, despite its toxicity, was used as a stimulant and escharotic, as a caustic agent used to treat ulcers and malignancies, cauterize wounds, and prevent infection and decay. It was particularly used in dentistry to destroy tissues and arrest necrosis.

Current uses

Industrial

Coal-tar creosote is the most widely used wood treatment today; both industrially, processed into wood using pressure methods such as "full-cell process" or "empty-cell process", and more commonly applied to wood through brushing. In addition to toxicity to fungi, insects, and marine borers, it serves as a natural water repellent. It is commonly used to preserve and waterproof railroad ties, pilings, telephone poles, power line poles, marine pilings, and fence posts. Although suitable for use in preserving the structural timbers of buildings, it is not generally used that way because it is difficult to apply. There are also concerns about the environmental impact of the leaching of creosote into aquatic ecosystems.

Due to its carcinogenic character, the European Union has regulated the quality of creosote for the EU market and requires that the sale of creosote be limited to professional users. The United States Environmental Protection Agency regulates the use of coal-tar creosote as a wood preservative under the provisions of the Federal Insecticide, Fungicide, and Rodenticide Act. Creosote is considered a restricted-use pesticide and is only available to licensed pesticide applicators.

Oil-tar creosote

Derivation and general composition of water-gas-tar creosote

Oil-tar creosote is derived from the tar that forms when using petroleum or shale oil in the manufacturing of gas. The distillation of the tar from the oil occurs at very high temperatures; around 980 °C. The tar forms at the same time as the gas, and once processed for creosotes contains a high percentage of cyclic hydrocarbons, a very low amount of tar acids and tar bases, and no true anthracenes have been identified. Historically, this has mainly been produced in the United States on the Pacific coast, where petroleum has been more abundant than coal. Limited quantities have been used industrially, either alone, mixed with coal-tar creosote, or fortified with pentachlorophenol.

Water-gas-tar creosote

Water-gas-tar creosote is also derived from petroleum oil or shale oil, but by a different process; it is distilled during the production of water gas. The tar is a by-product resulting from enrichment of water gas with gases produced by thermal decomposition of petroleum. Of the creosotes derived from oil, it is practically the only one used for wood preservation. It has the same degree of solubility as coal-tar creosote and is easy to infuse into wood. Like standard oil-tar creosote, it has a low amount of tar acids and tar bases, and has less antiseptic qualities. Petri dish tests have shown that water-gas-tar creosote is one-sixth as anti-septically effective as that of coal-tar.

Lignite-tar creosote

Lignite-tar creosote is produced from lignite rather than bituminous coal, and varies considerably from coal-tar creosote. Also called "lignite oil", it has a very high content of tar acids, and has been used to increase the tar acids in normal creosote when necessary. When it has been produced, it has generally been applied in mixtures with coal-tar creosote or petroleum. Its effectiveness when used alone has not been established. In an experiment with southern yellow pine fence posts in Mississippi, straight lignite-tar creosote was giving good results after about 27 years exposure, although not as good as the standard coal-tar creosote used in the same situation.

Peat-tar creosote

There have also been attempts to distill creosote from peat-tar, although mostly unsuccessful due to the problems with winning and drying peat on an industrial scale. Peat tar by itself has in the past been used as a wood preservative.

Health effects

According to the Agency for Toxic Substances and Disease Registry (ATSDR), eating food or drinking water contaminated with high levels of coal-tar creosote may cause a burning in the mouth and throat, and stomach pains. ATSDR also states that brief direct contact with large amounts of coal-tar creosote may result in a rash or severe irritation of the skin, chemical burns of the surfaces of the eyes, convulsions and mental confusion, kidney or liver problems, unconsciousness, and even death. Longer direct skin contact with low levels of creosote mixtures or their vapours can result in increased light sensitivity, damage to the cornea, and skin damage. Longer exposure to creosote vapours can cause irritation of the respiratory tract.

The International Agency for Research on Cancer (IARC) has determined that coal-tar creosote is probably carcinogenic to humans, based on adequate animal evidence and limited human evidence. The animal testing relied upon by IARC involved the continuous application of creosote to the shaved skin of rodents. After weeks of creosote application, the animals developed cancerous skin lesions and in one test, lesions of the lung. The United States Environmental Protection Agency has stated that coal-tar creosote is a probable human carcinogen based on both human and animal studies. As a result, the Federal Occupational Safety and Health Administration (OSHA) has set a permissible exposure limit of 0.2 milligrams of coal-tar creosote per cubic meter of air (0.2 mg/m3) in the workplace during an 8-hour day, and the Environmental Protection Agency (EPA) requires that spills or accidental releases into the environment of one pound (0.454 kg) or more of creosote be reported to them.

There is no unique exposure pathway of children to creosote. Children exposed to creosote probably experience the same health effects seen in adults exposed to creosote. It is unknown whether children differ from adults in their susceptibility to health effects from creosote.

A 2005 mortality study of creosote workers found no evidence supporting an increased risk of cancer death, as a result of exposure to creosote. Based on the findings of the largest mortality study to date of workers employed in creosote wood treating plants, there is no evidence that employment at creosote wood-treating plants or exposure to creosote-based preservatives was associated with any significant mortality increase from either site-specific cancers or non-malignant diseases. The study consisted of 2,179 employees at eleven plants in the United States where wood was treated with creosote preservatives. Some workers began work in the 1940s to 1950s. The observation period of the study covered 1979–2001. The average length of employment was 12.5 years. One third of the study subjects were employed for over 15 years.

The largest health effect of creosote is deaths caused by residential chimney fires due to chimney tar (creosote) build-up. This is entirely unconnected with its industrial production or use.

Build-up in chimneys

Burning wood and fossil fuels in the absence of adequate airflow (such as in an enclosed furnace or stove), causes incomplete combustion of the oils in the wood, which are off-gassed as volatiles in the smoke. As the smoke rises through the chimney it cools, causing water, carbon, and volatiles to condense on the interior surfaces of the chimney flue. The black oily residue that builds up is referred to as creosote, which is similar in composition to the commercial products by the same name, but with a higher content of carbon black.

Over the course of a season creosote deposits can become several inches thick. This creates a compounding problem, because the creosote deposits reduce the draft (airflow through the chimney) which increases the probability that the wood fire is not getting enough air for complete combustion. Since creosote is highly combustible, a thick accumulation creates a fire hazard. If a hot fire is built in the stove or fireplace, and the air control left wide open, this may allow hot oxygen into the chimney where it comes in contact with the creosote which then ignites—causing a chimney fire. Chimney fires often spread to the main building because the chimney gets so hot that it ignites any combustible material in direct contact with it, such as wood. The fire can also spread to the main building from sparks emitting from the chimney and landing on combustible roof surfaces. In order to properly maintain chimneys and heaters that burn wood or carbon-based fuels, the creosote buildup must be removed. Chimney sweeps perform this service for a fee.

Release into environment

Broken creosote piling exposed by weathering

Even though creosote is pressurized into the wood, the release of the chemical – and resulting marine pollution – occurs due to many different events: During the lifetime of the marine piling, weathering occurs from tides and water flow which slowly opens the oily outer coating and exposes the smaller internal pores to more water flow. Frequent weathering occurs daily, but more severe weather, such as hurricanes, can cause damage or loosening of the wooden pilings. Many pilings are either broken into pieces from debris, or are completely washed away during these storms. When the pilings are washed away, they come to settle on the bottom of the body of water where they reside, and then they leach chemicals into the water slowly over a long period of time. This long-term secretion is not normally noticed because the piling is submerged beneath the surface, hidden from sight.

The creosote is mostly insoluble in water, but the lower-molecular-weight compounds will become soluble the longer the broken wood is exposed to the water. In this case, some of the chemicals become water-soluble and further leach into the aquatic sediment while the rest of the insoluble chemicals remain together in a tar-like substance. Another source of damage comes from wood-boring fauna, such as shipworms and Limnoria. Though creosote is used as a pesticide preservative, studies have shown that Limnoria is resistant to wood preservative pesticides and can cause small holes in the wood, through which creosote can then be released.

Chemical reactions with sediment and organisms

Once the soluble compounds from the creosote preservative leach into the water, the compounds begin reacting with the external environment or are consumed by organisms. The reactions vary depending on the concentration of each compound that is released from the creosote, but major reactions are outlined below:

Alkylation

Alkylation occurs when a molecule replaces a hydrogen atom with an alkyl group that generally comes from an organic molecule. Alkyl groups that are found naturally occurring in the environment are organometallic compounds. Organometallic compounds generally contain a methyl, ethyl, or butyl derivative which is the alkyl group that replaces the hydrogen. Other organic compounds, such as methanol, can provide alkyl groups for alkylation. Methanol is found naturally in the environment in small concentrations, and has been linked to the release from biological decomposition of waste and even a byproduct of vegetation. The following reactions are alkylations of soluble compounds found in creosote preservatives with methanol.

m-Cresol

The diagram above depicts a reaction between m-cresol and methanol where a c-alkylation product is produced. The c-alkylation reaction means that instead of replacing the hydrogen atom on the -OH group, the methyl group (from the methanol) replaces the hydrogen on a carbon in the benzene ring. The products of this c-alkylation can be in either a para- or ortho- orientation on the molecule, as seen in the diagram, and water, which is not shown. Isomers of the dimethylphenol (DMP) compound are the products of the para- and ortho-c-alkylation. Dimethylphenol (DMP) compound is listed as an aquatic hazard by characteristic, and is toxic with long lasting effects.

Phenol

This diagram shows an o-alkylation between phenol and methanol. Unlike the c-alkylation, the o-alkylation replaces the hydrogen atom on the -OH group with the methyl group (from the methanol). The product of the o-alkylation is methoxybenzene, better-known as anisole, and water, which is not shown in the diagram. Anisole is listed as an acute hazard to aquatic life with long-term effects.

Bioaccumulation

Bioaccumulation is the process by which an organism takes in chemicals through ingestion, exposure, and inhalation. Bioaccumulation is broken down into bioconcentration (uptake of chemicals from the environment) and biomagnification (increasing concentration of chemicals as they move up the food chain). Certain species of aquatic organisms are affected differently from the chemicals released from creosote preservatives. One of the more studied organisms is a mollusk. Mollusks attach to the wooden, marine pilings and are in direct contact with the creosote preservatives. Many studies have been conducted using polycyclic aromatic hydrocarbons (PAH), which are low molecular hydrocarbons found in some creosote-based preservatives. In a study conducted from Pensacola, Florida, a group of native mollusks were kept in a controlled environment, and a different group of native mollusks were kept in an environment contaminated with creosote preservatives. The mollusks in the contaminated environment were shown to have a bioaccumulation of up to ten times the concentration of PAH than the control species. The intake of organisms is dependent on whether the compound is in an ionized or an un-ionized form. To determine whether the compound is ionized or un-ionized, the pH of the surrounding environment must be compared to the pKa or acidity constant of the compound. If the pH of the environment is lower than the pKa, then the compound is un-ionized which means that the compound will behave as if it is non-polar. Bioaccumulation for un-ionized compounds comes from partitioning equilibrium between the aqueous phase and the lipids in the organism. If the pH is higher than the pKa, then the compound is considered to be in the ionized form. The un-ionized form is favored because the bioaccumulation is easier for the organism to intake through partitioning equilibrium. The table below shows a list of pKas from compounds found in creosote preservatives and compares them to the average pH of seawater (reported to be 8.1).

Compound pKa pH of Seawater Form (Ionized or Un-Ionized)
m-cresol 10.09 8.1 Un-ionized
o-cresol 10.29 Un-ionized
p-cresol 10.30 Un-ionized
2-ethylphenol 10.20 Un-ionized
guaiacol 9.98 Un-ionized
phenol 9.99 Un-ionized

Each of the compounds in the table above is found in creosote preservatives; all are in the favored un-ionized form. In another study, various species of small fish were tested to see how the exposure time to PAH chemicals affected the fish. This study showed that an exposure time of 24–96 hours on various shrimp and fish species affected the growth, reproduction, and survival functions of the organisms for most of the compounds tested.

Biodegradation

It can be seen in some studies that biodegradation accounts for the absence of creosote preservatives on the initial surface of the sediment. In a study from Pensacola, Florida, PAHs were not detected on the surface on the aquatic sediment, but the highest concentrations were detected at a depth of 8-13 centimeters. A form an anaerobic biodegradation of m-cresol was seen in a study using sulfate-reducing and nitrate-reducing enriched environments. The reduction of m-cresol in this study was seen in under 144 hours, while additional chemical intermediates were being formed. The chemical intermediates were formed in the presence of bicarbonate. The products included 4-hydroxy-2-methylbenzoic acid and acetate compounds. Although the conditions were enriched with the reducing anaerobic compounds, sulfate and nitrate reducing bacteria are commonly found in the environment. For further information, see sulfate-reducing bacteria. The type of anaerobic bacteria ultimately determines the reduction of the creosote preservative compounds, while each individual compound may only go through reduction under certain conditions. BTEX is a mixture of benzene, toluene, ethylbenzene, and xylene, that was studied in the presence of four different anaerobic-enriched sediments. Though the compound, BTEX, is not found in creosote preservatives, the products of creosote preservatives' oxidation-reduction reactions include some of these compounds. For oxidation-reduction reactions, see the following section. In this study, it was seen that certain compounds such as benzene were only reduced under sulfate-enriched environments, while toluene was reduced under a variety of bacteria-enriched environments, not just sulfate. The biodegradation of a creosote preservative in an anaerobic enrichment depends not only on the type of bacteria enriching the environment, but also the compound that has been released from the preservative. In aerobic environments, preservative compounds are limited in the biodegradation process by the presence of free oxygen. In an aerobic environment, free oxygen comes from oxygen saturated sediments, sources of precipitation, and plume e The free oxygen allows for the compounds to be oxidized and decomposed into new intermediate compounds. Studies have shown that when BTEX and PAH compounds were placed in aerobic environments, the oxidation of the ring structures caused cleavage in the aromatic ring and allowed for other functional groups to attach. When an aromatic hydrocarbon was introduced to the molecular oxygen in experimental conditions, a dihydrodiol intermediate was formed, and then oxidation occurred transforming the aromatic into a catechol compound. Catechol allows for cleavage of the aromatic ring to occur, where functional groups can then add in an ortho- or meta- position.

Oxidation-reduction

Even though many studies conduct testing under experimental or enriched conditions, oxidation-reduction reactions occur naturally and allow for chemicals to go through processes such as biodegradation, outlined above. Oxidation is defined as the loss of an electron to another species, while reduction is the gaining of an electron from another species. As compounds go through oxidation and reduction in sediments, the preservative compounds are altered to form new chemicals, leading to decomposition. An example of the oxidation of p-cresol and phenol can be seen in the figures below:

p-Cresol

This reaction shows the oxidation of p-cresol in a sulfate-enriched environment. P-cresol was seen to be the easiest to degrade through the sulfate-enriched environment, while m-cresol and o-cresol where inhibited. In the chart above, p-cresol was oxidized under an anaerobic sulfate reducing condition and formed four different intermediates. After the formation of the intermediates, the study reported further degradation of the intermediates leading to the production of carbon dioxide and methane. The p-hydroxylbenzyl alcohol, p-hydroxylbenzaldehye, p-hyrdoxylbenzoate, and benzoate intermediates all are produced from this oxidation and released into the sediments. Similar results were also produced by different studies using other forms of oxidation such as: iron-reducing organisms, Copper/Manganese Oxide catalyst, and nitrate- reducing conditions.

Phenol

This reaction shows the oxidation of phenol by iron and peroxide. This combination of iron, which comes from iron oxide in the sediment, and the peroxide, commonly released by animals and plants into the environment, is known as the Fenton Reagent. This reagent is used to oxidize phenol groups by the use of a radical hydroxide group produced from the peroxide in the p-benzoquinone. This product of phenol's oxidation is now leached into the environment while other products include iron(II) and water. P-benzoquinone is listed as being a very toxic, acute environmental hazard.

Environmental hazards

Sediment

In aquatic sediments, several reactions can transform the chemicals released by the creosote preservatives into more dangerous chemicals. Most creosote preservative compounds have hazards associated with them before they are transformed. Cresol (m-, p-, and o-), phenol, guaiacol, and xylenol (1,3,4- and 1,3,5-) all are acute aquatic hazards prior to going through chemical reactions with the sediments. Alkylation reactions allows for the compounds to transition into more toxic compounds with the addition of R-groups to the major compounds found in creosote preservatives. Compounds formed through alkylation include: 3,4-dimethylphenol, 2,3-dimethylphenol, and 2,5-dimethylphenol, which are all listed as acute environmental hazards. Biodegradation controls the rate at which the sediment holds the chemicals, and the number of reactions that are able to take place. The biodegradation process can take place under many different conditions, and vary depending on the compounds that are released. Oxidation-reduction reactions allow for the compounds to be broken down into new forms of more toxic molecules. Studies have shown oxidation-reduction reactions of creosote preservative compounds included compounds that are listed as environmental hazards, such as p-benzoquinone in the oxidation of phenol. Not only are the initial compounds in creosote hazardous to the environment, but the byproducts of the chemical reactions are environmental hazardous as well.

Other

From the contamination of the sediment, more of the ecosystem is affected. Organisms in the sediment are now exposed to the new chemicals. Organisms are then ingested by fish and other aquatic animals. These animals now contain concentrations of hazardous chemicals which were secreted from the creosote. Other issues with ecosystems include bioaccumulation. Bioaccumulation occurs when high levels of chemicals are passed to aquatic life near the creosote pilings. Mollusks and other smaller crustaceans are at higher risk because they are directly attached to the surface of wood pilings that are filled with creosote preservative. Studies show that mollusks in these environments take on high concentrations of chemical compounds which will then be transferred through the ecosystem's food chain. Bioaccumulation contributes to the higher concentrations of chemicals within the organisms in the aquatic ecosystems.

Activated carbon

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Activated_carbon
Activated carbon

Activated carbon, also called activated charcoal, is a form of carbon commonly used to filter contaminants from water and air, among many other uses. It is processed (activated) to have small, low-volume pores that greatly increase the surface area available for adsorption or chemical reactions that can be thought of as a microscopic "sponge" structure. (Adsorption, not to be confused with absorption, is a process where atoms or molecules adhere to a surface). Activation is analogous to making popcorn from dried corn kernels: popcorn is light, fluffy, and its kernels have a high surface-area-to-volume ratio. Activated is sometimes replaced by active.

Because it is so porous on a microscopic scale, one gram of activated carbon has a surface area of over 3,000 square metres (32,000 square feet), as determined by gas adsorption. For charcoal the equivalent figure before activation is about 2–5 square metres. A useful activation level may be obtained solely from high surface area. Further chemical treatment often enhances adsorption properties.

Activated carbon is usually derived from waste products such as coconut husks; waste from paper mills has been studied as a source. These bulk sources are converted into charcoal before being activated. When derived from coal it is referred to as activated coal. Activated coke is derived from coke.

Uses

Activated carbon is used in methane and hydrogen storage, air purification, capacitive deionization, supercapacitive swing adsorption, solvent recovery, decaffeination, gold purification, metal extraction, water purification, medicine, sewage treatment, air filters in respirators, filters in compressed air, teeth whitening, production of hydrogen chloride, edible electronics, and many other applications.

Industrial

One major industrial application involves use of activated carbon in metal finishing for purification of electroplating solutions. For example, it is the main purification technique for removing organic impurities from bright nickel plating solutions. A variety of organic chemicals are added to plating solutions for improving their deposit qualities and for enhancing properties like brightness, smoothness, ductility, etc. Due to passage of direct current and electrolytic reactions of anodic oxidation and cathodic reduction, organic additives generate unwanted breakdown products in solution. Their excessive build up can adversely affect plating quality and physical properties of deposited metal. Activated carbon treatment removes such impurities and restores plating performance to the desired level.

Medical

Activated charcoal for medical use

Activated carbon is used to treat poisonings and overdoses following oral ingestion. Tablets or capsules of activated carbon are used in many countries as an over-the-counter drug to treat diarrhea, indigestion, and flatulence. However, activated charcoal shows no effect on intestinal gas and diarrhea, and is, ordinarily, medically ineffective if poisoning resulted from ingestion of corrosive agents, boric acid, or petroleum products, and is particularly ineffective against poisonings of strong acids or bases, cyanide, iron, lithium, arsenic, methanol, ethanol or ethylene glycol. Activated carbon will not prevent these chemicals from being absorbed into the human body. It is on the World Health Organization's List of Essential Medicines.

Incorrect application (e.g. into the lungs) results in pulmonary aspiration, which can sometimes be fatal if immediate medical treatment is not initiated.

Analytical chemistry

Activated carbon, in 50% w/w combination with celite, is used as stationary phase in low-pressure chromatographic separation of carbohydrates (mono-, di-, tri-saccharides) using ethanol solutions (5–50%) as mobile phase in analytical or preparative protocols.

Activated carbon is useful for extracting the direct oral anticoagulants (DOACs) such as dabigatran, apixaban, rivaroxaban and edoxaban from blood plasma samples. For this purpose it has been made into "minitablets", each containing 5 mg activated carbon for treating 1ml samples of DOAC. Since this activated carbon has no effect on blood clotting factors, heparin or most other anticoagulants  this allows a plasma sample to be analyzed for abnormalities otherwise affected by the DOACs.

Environmental

Activated carbon is usually used in water filtration systems. In this illustration, the activated carbon is in the fourth level (counted from bottom).

Carbon adsorption has numerous applications in removing pollutants from air or water streams both in the field and in industrial processes such as:

During early implementation of the 1974 Safe Drinking Water Act in the US, EPA officials developed a rule that proposed requiring drinking water treatment systems to use granular activated carbon. Because of its high cost, the so-called GAC rule encountered strong opposition across the country from the water supply industry, including the largest water utilities in California. Hence, the agency set aside the rule. Activated carbon filtration is an effective water treatment method due to its multi-functional nature. There are specific types of activated carbon filtration methods and equipment that are indicated – depending upon the contaminants involved.

Activated carbon is also used for the measurement of radon concentration in air.

Agricultural

Activated carbon (charcoal) is an allowed substance used by organic farmers in both livestock production and wine making. In livestock production it is used as a pesticide, animal feed additive, processing aid, nonagricultural ingredient and disinfectant. In organic winemaking, activated carbon is allowed for use as a processing agent to adsorb brown color pigments from white grape concentrates. It is sometimes used as biochar.

Distilled alcoholic beverage purification

Activated carbon filters (AC filters) can be used to filter vodka and whiskey of organic impurities which can affect color, taste, and odor. Passing an organically impure vodka through an activated carbon filter at the proper flow rate will result in vodka with an identical alcohol content and significantly increased organic purity, as judged by odor and taste.

Fuel storage

Research is being done testing various activated carbons' ability to store natural gas and hydrogen gas. The porous material acts like a sponge for different types of gases. The gas is attracted to the carbon material via Van der Waals forces. Some carbons have been able to achieve binding energies of 5–10 kJ per mol. The gas may then be desorbed when subjected to higher temperatures and either combusted to do work or in the case of hydrogen gas extracted for use in a hydrogen fuel cell. Gas storage in activated carbons is an appealing gas storage method because the gas can be stored in a low pressure, low mass, low volume environment that would be much more feasible than bulky on-board pressure tanks in vehicles. The United States Department of Energy has specified certain goals to be achieved in the area of research and development of nano-porous carbon materials. All of the goals are yet to be satisfied but numerous institutions, including the ALL-CRAFT program, are continuing to conduct work in this field.

Gas purification

Filters with activated carbon are usually used in compressed air and gas purification to remove oil vapors, odor, and other hydrocarbons from the air. The most common designs use a 1-stage or 2 stage filtration principle in which activated carbon is embedded inside the filter media.

Activated carbon filters are used to retain radioactive gases within the air vacuumed from a nuclear boiling water reactor turbine condenser. The large charcoal beds adsorb these gases and retain them while they rapidly decay to non-radioactive solid species. The solids are trapped in the charcoal particles, while the filtered air passes through.

Chemical purification

Activated carbon is commonly used on the laboratory scale to purify solutions of organic molecules containing unwanted colored organic impurities.

Filtration over activated carbon is used in large scale fine chemical and pharmaceutical processes for the same purpose. The carbon is either mixed with the solution then filtered off or immobilized in a filter.

Mercury scrubbing

Activated carbon, often infused with sulfur or iodine, is widely used to trap mercury emissions from coal-fired power stations, medical incinerators, and from natural gas at the wellhead. However, despite its effectiveness, activated carbon is expensive to use. 

Since it is often not recycled, the mercury-laden activated carbon presents a disposal dilemma. If the activated carbon contains less than 260 ppm mercury, United States federal regulations allow it to be stabilized (for example, trapped in concrete) for landfilling. However, waste containing greater than 260 ppm is considered to be in the high-mercury subcategory and is banned from landfilling (Land-Ban Rule). This material is now accumulating in warehouses and in deep abandoned mines at an estimated rate of 100 tons per year.

The problem of disposal of mercury-laden activated carbon is not unique to the United States. In the Netherlands, this mercury is largely recovered and the activated carbon is disposed of by complete burning, forming carbon dioxide (CO2).

Food additive

Activated, food-grade charcoal became a food trend in 2016, being used as an additive to impart a "slightly smoky" taste and a dark coloring to products including hotdogs, ice cream, pizza bases and bagels. People taking medication, including birth control pills and antidepressants, are advised to avoid novelty foods or drinks that use activated charcoal coloring, as it can render the medication ineffective.

Smoking filtration

Activated charcoal is used in smoking filters as a way to reduce the tar content and other chemicals present in smoke which is a result of combustion, wherein it has been found to reduce the toxicants from tobacco smoke, in particular the free radicals. There are numerous brands in Europe such as Purize, as well as in India such as "PEER Next" which offer these products to consumers looking for filters with activated carbon application.

Structure of activated carbon

The structure of activated carbon has long been a subject of debate. In a book published in 2006, Harry Marsh and Francisco Rodríguez-Reinoso considered more than 15 models for the structure, without coming to a definite conclusion about which was correct. Recent work using aberration-corrected transmission electron microscopy has suggested that activated carbons may have a structure related to that of the fullerenes, with pentagonal and heptagonal carbon rings.

Production

Activated carbon is carbon produced from carbonaceous source materials such as bamboo, coconut husk, willow peat, wood, coir, lignite, coal, and petroleum pitch. It can be produced (activated) by one of the following processes:

  1. Physical activation: The source material is developed into activated carbon using hot gases. Air is then introduced to burn out the gasses, creating a graded, screened and de-dusted form of activated carbon. This is generally done by using one or more of the following processes:
    • Carbonization: Material with carbon content is pyrolyzed at temperatures in the range 600–900 °C, usually in an inert atmosphere with gases such as argon or nitrogen
    • Activation/oxidation: Raw material or carbonized material is exposed to oxidizing atmospheres (oxygen or steam) at temperatures above 250 °C, usually in the temperature range of 600–1200 °C. The activation is performed by heating the sample for 1 h in a muffle furnace at 450 °C in the presence of air.
  2. Chemical activation: The carbon material is impregnated with certain chemicals. The chemical is typically an acid, strong base, or a salt (phosphoric acid 25%, potassium hydroxide 5%, sodium hydroxide 5%, potassium carbonate 5%, calcium chloride 25%, and zinc chloride 25%). The carbon is then subjected to high temperatures (250–600 °C). It is believed that the temperature activates the carbon at this stage by forcing the material to open up and have more microscopic pores. Chemical activation is preferred to physical activation owing to the lower temperatures, better quality consistency, and shorter time needed for activating the material.

The Dutch company Norit NV, part of the Cabot Corporation, is the largest producer of activated carbon in the world. Haycarb, a Sri Lankan coconut shell-based company controls 16% of the global market share.

Classification

Activated carbons are complex products which are difficult to classify on the basis of their behaviour, surface characteristics and other fundamental criteria. However, some broad classification is made for general purposes based on their size, preparation methods, and industrial applications.

Powdered activated carbon (PAC)

A micrograph of activated charcoal (R 1) under bright field illumination on a light microscope. Notice the fractal-like shape of the particles hinting at their enormous surface area. Each particle in this image, despite being only around 0.1 mm across, can have a surface area of several square centimeters. The entire image covers a region of approximately 1.1 by 0.7 mm, and the full resolution version is at a scale of 6.236 pixels/μm.

Normally, activated carbons (R 1) are made in particulate form as powders or fine granules less than 1.0 mm in size with an average diameter between 0.15 and 0.25 mm. Thus they present a large surface to volume ratio with a small diffusion distance. Activated carbon (R 1) is defined as the activated carbon particles retained on a 50-mesh sieve (0.297 mm).

Powdered activated carbon (PAC) material is finer material. PAC is made up of crushed or ground carbon particles, 95–100% of which will pass through a designated mesh sieve. The ASTM classifies particles passing through an 80-mesh sieve (0.177 mm) and smaller as PAC. It is not common to use PAC in a dedicated vessel, due to the high head loss that would occur. Instead, PAC is generally added directly to other process units, such as raw water intakes, rapid mix basins, clarifiers, and gravity filters.

Granular activated carbon (GAC)

A micrograph of activated charcoal (GAC) under scanning electron microscope

Granular activated carbon (GAC) has a relatively larger particle size compared to powdered activated carbon and consequently, presents a smaller external surface. Diffusion of the adsorbate is thus an important factor. These carbons are suitable for adsorption of gases and vapors, because gaseous substances diffuse rapidly. Granulated carbons are used for air filtration and water treatment, as well as for general deodorization and separation of components in flow systems and in rapid mix basins. GAC can be obtained in either granular or extruded form. GAC is designated by sizes such as 8×20, 20×40, or 8×30 for liquid phase applications and 4×6, 4×8 or 4×10 for vapor phase applications. A 20×40 carbon is made of particles that will pass through a U.S. Standard Mesh Size No. 20 sieve (0.84 mm) (generally specified as 85% passing) but be retained on a U.S. Standard Mesh Size No. 40 sieve (0.42 mm) (generally specified as 95% retained). AWWA (1992) B604 uses the 50-mesh sieve (0.297 mm) as the minimum GAC size. The most popular aqueous-phase carbons are the 12×40 and 8×30 sizes because they have a good balance of size, surface area, and head loss characteristics.

Extruded activated carbon (EAC)

Extruded activated carbon (EAC) combines powdered activated carbon with a binder, which are fused together and extruded into a cylindrical shaped activated carbon block with diameters from 0.8 to 130 mm. These are mainly used for gas phase applications because of their low pressure drop, high mechanical strength and low dust content. Also sold as CTO filter (Chlorine, Taste, Odor).

Bead activated carbon (BAC)

Bead activated carbon (BAC) is made from petroleum pitch and supplied in diameters from approximately 0.35 to 0.80 mm. Similar to EAC, it is also noted for its low pressure drop, high mechanical strength and low dust content, but with a smaller grain size. Its spherical shape makes it preferred for fluidized bed applications such as water filtration.

Impregnated carbon

Porous carbons containing several types of inorganic impregnate such as iodine and silver. Cations such as aluminium, manganese, zinc, iron, lithium, and calcium have also been prepared for specific application in air pollution control especially in museums and galleries. Due to its antimicrobial and antiseptic properties, silver loaded activated carbon is used as an adsorbent for purification of domestic water. Drinking water can be obtained from natural water by treating the natural water with a mixture of activated carbon and aluminium hydroxide (Al(OH)3), a flocculating agent. Impregnated carbons are also used for the adsorption of hydrogen sulfide (H2S) and thiols. Adsorption rates for H2S as high as 50% by weight have been reported. Polymer coated carbon

Woven activated carbon cloth

This is a process by which a porous carbon can be coated with a biocompatible polymer to give a smooth and permeable coat without blocking the pores. The resulting carbon is useful for hemoperfusion. Hemoperfusion is a treatment technique in which large volumes of the patient's blood are passed over an adsorbent substance in order to remove toxic substances from the blood.

Woven carbon

There is a technology of processing technical rayon fiber into activated carbon cloth for carbon filtering. Adsorption capacity of activated cloth is greater than that of activated charcoal (BET theory) surface area: 500–1500 m2/g, pore volume: 0.3–0.8 cm3/g). Thanks to the different forms of activated material, it can be used in a wide range of applications (supercapacitors, [Odor Absorbers, CBRN-defense industry etc.).

Properties

A gram of activated carbon can have a surface area in excess of 500 m2 (5,400 sq ft), with 3,000 m2 (32,000 sq ft) being readily achievable. Carbon aerogels, while more expensive, have even higher surface areas, and are used in special applications.

Under an electron microscope, the high surface-area structures of activated carbon are revealed. Individual particles are intensely convoluted and display various kinds of porosity; there may be many areas where flat surfaces of graphite-like material run parallel to each other, separated by only a few nanometers or so. These micropores provide superb conditions for adsorption to occur, since adsorbing material can interact with many surfaces simultaneously. Tests of adsorption behaviour are usually done with nitrogen gas at 77 K under high vacuum, but in everyday terms activated carbon is perfectly capable of producing the equivalent, by adsorption from its environment, liquid water from steam at 100 °C (212 °F) and a pressure of 1/10,000 of an atmosphere.

James Dewar, the scientist after whom the Dewar (vacuum flask) is named, spent much time studying activated carbon and published a paper regarding its adsorption capacity with regard to gases. In this paper, he discovered that cooling the carbon to liquid nitrogen temperatures allowed it to adsorb significant quantities of numerous air gases, among others, that could then be recollected by simply allowing the carbon to warm again and that coconut based carbon was superior for the effect. He uses oxygen as an example, wherein the activated carbon would typically adsorb the atmospheric concentration (21%) under standard conditions, but release over 80% oxygen if the carbon was first cooled to low temperatures.

Physically, activated carbon binds materials by van der Waals force or London dispersion force.

Activated carbon does not bind well to certain chemicals, including alcohols, diols, strong acids and bases, metals and most inorganics, such as lithium, sodium, iron, lead, arsenic, fluorine, and boric acid.

Activated carbon adsorbs iodine very well. The iodine capacity, mg/g, (ASTM D28 Standard Method test) may be used as an indication of total surface area.

Carbon monoxide is not well adsorbed by activated carbon. This should be of particular concern to those using the material in filters for respirators, fume hoods or other gas control systems as the gas is undetectable to the human senses, toxic to metabolism and neurotoxic.

Substantial lists of the common industrial and agricultural gases adsorbed by activated carbon can be found online.

Activated carbon can be used as a substrate for the application of various chemicals to improve the adsorptive capacity for some inorganic (and problematic organic) compounds such as hydrogen sulfide (H2S), ammonia (NH3), formaldehyde (HCOH), mercury (Hg) and radioactive iodine-131(131I). This property is known as chemisorption.

Iodine number

Many carbons preferentially adsorb small molecules. Iodine number is the most fundamental parameter used to characterize activated carbon performance. It is a measure of activity level (higher number indicates higher degree of activation) often reported in mg/g (typical range 500–1200 mg/g). It is a measure of the micropore content of the activated carbon (0 to 20 Å, or up to 2 nm) by adsorption of iodine from solution. It is equivalent to surface area of carbon between 900 and 1100 m2/g. It is the standard measure for liquid-phase applications.

Iodine number is defined as the milligrams of iodine adsorbed by one gram of carbon when the iodine concentration in the residual filtrate is at a concentration of 0.02 normal (i.e. 0.02N). Basically, iodine number is a measure of the iodine adsorbed in the pores and, as such, is an indication of the pore volume available in the activated carbon of interest. Typically, water-treatment carbons have iodine numbers ranging from 600 to 1100. Frequently, this parameter is used to determine the degree of exhaustion of a carbon in use. However, this practice should be viewed with caution, as chemical interactions with the adsorbate may affect the iodine uptake, giving false results. Thus, the use of iodine number as a measure of the degree of exhaustion of a carbon bed can only be recommended if it has been shown to be free of chemical interactions with adsorbates and if an experimental correlation between iodine number and the degree of exhaustion has been determined for the particular application.

Molasses

Some carbons are more adept at adsorbing large molecules. Molasses number or molasses efficiency is a measure of the mesopore content of the activated carbon (greater than 20 Å, or larger than 2 nm) by adsorption of molasses from solution. A high molasses number indicates a high adsorption of big molecules (range 95–600). Caramel dp (decolorizing performance) is similar to molasses number. Molasses efficiency is reported as a percentage (range 40%–185%) and parallels molasses number (600 = 185%, 425 = 85%). The European molasses number (range 525–110) is inversely related to the North American molasses number.

Molasses Number is a measure of the degree of decolorization of a standard molasses solution that has been diluted and standardized against standardized activated carbon. Due to the size of color bodies, the molasses number represents the potential pore volume available for larger adsorbing species. As all of the pore volume may not be available for adsorption in a particular waste water application, and as some of the adsorbate may enter smaller pores, it is not a good measure of the worth of a particular activated carbon for a specific application. Frequently, this parameter is useful in evaluating a series of active carbons for their rates of adsorption. Given two active carbons with similar pore volumes for adsorption, the one having the higher molasses number will usually have larger feeder pores resulting in more efficient transfer of adsorbate into the adsorption space.

Tannin

Tannins are a mixture of large and medium size molecules. Carbons with a combination of macropores and mesopores adsorb tannins. The ability of a carbon to adsorb tannins is reported in parts per million concentration (range 200 ppm–362 ppm).

Methylene blue

Some carbons have a mesopore (20 Å to 50 Å, or 2 to 5 nm) structure which adsorbs medium size molecules, such as the dye methylene blue. Methylene blue adsorption is reported in g/100g (range 11–28 g/100g).

Dechlorination

Some carbons are evaluated based on the dechlorination half-life length, which measures the chlorine-removal efficiency of activated carbon. The dechlorination half-value length is the depth of carbon required to reduce the chlorine concentration by 50%. A lower half-value length indicates superior performance.

Apparent density

The solid or skeletal density of activated carbons will typically range between 2000 and 2100 kg/m3 (125–130 lbs./cubic foot). However, a large part of an activated carbon sample will consist of air space between particles, and the actual or apparent density will therefore be lower, typically 400 to 500 kg/m3 (25–31 lbs./cubic foot).

Higher density provides greater volume activity and normally indicates better-quality activated carbon. ASTM D 2854 -09 (2014) is used to determine the apparent density of activated carbon.

Hardness/abrasion number

It is a measure of the activated carbon's resistance to attrition. It is an important indicator of activated carbon to maintain its physical integrity and withstand frictional forces. There are large differences in the hardness of activated carbons, depending on the raw material and activity levels (porosity).

Ash content

Ash reduces the overall activity of activated carbon and reduces the efficiency of reactivation: the amount is exclusively dependent on the base raw material used to produce the activated carbon (e.g. coconut, wood, coal, etc.). The metal oxides (Fe2O3) can leach out of activated carbon resulting in discoloration. Acid/water-soluble ash content is more significant than total ash content. Soluble ash content can be very important for aquarists, as ferric oxide can promote algal growths. A carbon with a low soluble ash content should be used for marine, freshwater fish and reef tanks to avoid heavy metal poisoning and excess plant/algal growth. ASTM (D2866 Standard Method test) is used to determine the ash content of activated carbon.

Carbon tetrachloride activity

Measurement of the porosity of an activated carbon by the adsorption of saturated carbon tetrachloride vapour.

Particle size distribution

The finer the particle size of an activated carbon, the better the access to the surface area and the faster the rate of adsorption kinetics. In vapour phase systems this needs to be considered against pressure drop, which will affect energy cost. Careful consideration of particle size distribution can provide significant operating benefits. However, in the case of using activated carbon for adsorption of minerals such as gold, the particle size should be in the range of 3.35–1.4 millimetres (0.132–0.055 in). Activated carbon with particle size less than 1 mm would not be suitable for elution (the stripping of mineral from an activated carbon).

Modification of properties and reactivity

Acid-base, oxidation-reduction and specific adsorption characteristics are strongly dependent on the composition of the surface functional groups.

The surface of conventional activated carbon is reactive, capable of oxidation by atmospheric oxygen and oxygen plasma steam, and also carbon dioxide and ozone.

Oxidation in the liquid phase is caused by a wide range of reagents (HNO3, H2O2, KMnO4).

Through the formation of a large number of basic and acidic groups on the surface of oxidized carbon to sorption and other properties can differ significantly from the unmodified forms.

Activated carbon can be nitrogenated by natural products or polymers or processing of carbon with nitrogenating reagents.

Activated carbon can interact with chlorine, bromine and fluorine.

Surface of activated carbon, like other carbon materials can be fluoralkylated by treatment with (per)fluoropolyether peroxide in a liquid phase, or with wide range of fluoroorganic substances by CVD-method. Such materials combine high hydrophobicity and chemical stability with electrical and thermal conductivity and can be used as electrode material for super capacitors.

Sulfonic acid functional groups can be attached to activated carbon to give "starbons" which can be used to selectively catalyse the esterification of fatty acids. Formation of such activated carbons from halogenated precursors gives a more effective catalyst which is thought to be a result of remaining halogens improving stability. It is reported about synthesis of activated carbon with chemically grafted superacid sites –CF2SO3H.

Some of the chemical properties of activated carbon have been attributed to presence of the surface active carbon double bond.

The Polyani adsorption theory is a popular method for analyzing adsorption of various organic substances to their surface.

Examples of adsorption

Heterogeneous catalysis

The most commonly encountered form of chemisorption in industry, occurs when a solid catalyst interacts with a gaseous feedstock, the reactant/s. The adsorption of reactant/s to the catalyst surface creates a chemical bond, altering the electron density around the reactant molecule and allowing it to undergo reactions that would not normally be available to it.

Reactivation and regeneration

World's largest reactivation plant located in Feluy, Belgium.
Activated carbon reactivation center in Roeselare, Belgium.

The reactivation or the regeneration of activated carbons involves restoring the adsorptive capacity of saturated activated carbon by desorbing adsorbed contaminants on the activated carbon surface.

Thermal reactivation

The most common regeneration technique employed in industrial processes is thermal reactivation. The thermal regeneration process generally follows three steps:

  • Adsorbent drying at approximately 105 °C (221 °F)
  • High temperature desorption and decomposition (500–900 °C (932–1,652 °F)) under an inert atmosphere
  • Residual organic gasification by a non-oxidising gas (steam or carbon dioxide) at elevated temperatures (800 °C (1,470 °F))

The heat treatment stage utilises the exothermic nature of adsorption and results in desorption, partial cracking and polymerization of the adsorbed organics. The final step aims to remove charred organic residue formed in the porous structure in the previous stage and re-expose the porous carbon structure regenerating its original surface characteristics. After treatment the adsorption column can be reused. Per adsorption-thermal regeneration cycle between 5–15 wt% of the carbon bed is burnt off resulting in a loss of adsorptive capacity. Thermal regeneration is a high energy process due to the high required temperatures making it both an energetically and commercially expensive process. Plants that rely on thermal regeneration of activated carbon have to be of a certain size before it is economically viable to have regeneration facilities onsite. As a result, it is common for smaller waste treatment sites to ship their activated carbon cores to specialised facilities for regeneration.

Other regeneration techniques

Current concerns with the high energy/cost nature of thermal regeneration of activated carbon has encouraged research into alternative regeneration methods to reduce the environmental impact of such processes. Though several of the regeneration techniques cited have remained areas of purely academic research, some alternatives to thermal regeneration systems have been employed in industry. Current alternative regeneration methods are:

Equality (mathematics)

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