Activated carbon

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Activated carbon, also called activated charcoal or activated coal, is a general term which covers carbon material mostly derived from charcoal. For all three variations of the name, "activated" is sometimes substituted with "active". By any name, it is a material with an exceptionally high surface area. Just one gram of activated carbon has the surface area of approximately two tennis courts, typically determined by nitrogen gas adsorption, and includes a large amount of microporosity. Sufficient activation for useful applications may come solely from the high surface area, though often further chemical treatment is used to enhance the absorbing properties of the material.

1 Production
2 Properties
3 Applications
4 Footnotes
5 References
6 See also
7 External links

[edit] Production

It can generally be produced in two different processes:

Physical activation: a process in which the precursor is developed into activated carbons using gases. This is generally done by using one or combining the following processes-(i) Carbonization: a process where material with carbon content is pyrolysed at temperatures in the range 600-900ºC, in absence of air (usually in inert atmosphere with gases like nitrogen, argon) (ii) Activation/Oxidation: a process in which the raw material or carbonised material is exposed to oxidizing atmospheres (carbon dioxide, oxygen or steam) at temperatures above 250 ºC usually in the temperature range 600-1200ºC.
Chemical activation: is the other method used for the preparation of activated carbons, which involves impregnation with chemicals such as acids like phosphoric acid or bases like potassium hydroxide, sodium hydroxide or salts like zinc chloride, followed by carbonization process at temperatures in the range 450-900ºC. It is believed that carbonization and activation step proceeds simultaneously in chemical activation. This technique can be problematic in some cases because, for example, zinc trace residues may remain in the endproduct. However, chemical activation is preferred over physical activation owing to the lower temperature and shorter time needed for activating material.

The source material can be several carbonaceous materials, e.g. nutshells, wood, coal.

Saturated activated carbon can be regenerated by heating.

[edit] Properties

A gram of activated carbon may have a surface area in excess of 400 m², with 1500 m² being readily achievable. For comparison, a tennis court is about 260 m². Carbon aerogels, while more expensive, have even higher surface areas, and are used in special applications.

Under an electron microscope, the structure of activated carbon looks a little like ribbons of paper which have been crumpled together, intermingled with wood chips. There are a great number of nooks and crannies, and 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 and a pressure of 1/10,000 of an atmosphere.

Physically, activated carbon binds materials by Van der Waals force, specifically London dispersion force.

Activated carbon does not bind well to certain chemicals, including lithium, alcohols, glycols, ammonia, strong acids and bases, metals and most inorganic minerals, such as sodium, iron, lead, arsenic, fluorine, and boric acid. Activated carbon does adsorb iodine very well and in fact the iodine number, mg/g, (ASTM D28 Standard Method test) is used as an indication of total surface area.

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 sulphide (H2S), ammonia (NH3), formaldehyde (HCOH), radio-isotopes (Iodine-131) and mercury (Hg). This property is known as CHEMIsorption.

[edit] Applications

Activated carbon is used in metal extraction, water purification, medicine, sewage treatment, air filters in gas masks and filter masks, filters in compressed air and gas purification, and many other applications.

[edit] Environmental applications

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

Spill cleanup
Groundwater remediation
Drinking water filtration
Volatile organic compound capture from painting, dry cleaning and other processes

[edit] Medical applications

Activated carbon is used to treat poisonings and overdoses following oral ingestion. It prevents absorption of the poison by the gastrointestinal tract. In cases of suspected poisoning, medical personnel either administer activated carbon on the scene or at a hospital's emergency department. Activated carbon has become the treatment of choice for many poisonings, and other decontamination methods such as ipecac induced emesis or stomach pumps are now used rarely. The recommended dose in adults is 25 to 100 grams. Pediatric dosages are 10 to 50 g or 0.5 to 1.0 g/kg.<ref name="ClinToxicol2005-Chyka">Chyka P, Seger D, Krenzelok E, Vale J (2005). "Position paper: Single-dose activated charcoal.". Clin Toxicol (Phila) 43 (2): 61-87. PMID 15822758.</ref> Incorrect application (e.g. into to the lungs) results in pulmonary aspiration which can sometimes be fatal if immediate medical treatment is not initiated.<ref name="Chest1989-Elliott">Elliott C, Colby T, Kelly T, Hicks H (1989). "Charcoal lung. Bronchiolitis obliterans after aspiration of activated charcoal". Chest 96 (3): 672-4. PMID 2766830.</ref> The use of activated charcoal is contraindicated when the ingested substance is an acid, an alkali, or a petroleum product.

For pre-hospital use, it comes in plastic tubes or bottles, commonly 12.5 or 25 grams, pre-mixed with water. The trade names include InstaChar, SuperChar, Actidose, and Liqui-Char, but it is commonly called simply Activated Charcoal.

[edit] Gas purification

Filters with activated carbon are usually used in compressed air and gas purification to remove oil vapour, odor, and other hydrocarbons from compressed air and gas. The most common designs use a 1 stage or 2 stage filtration principle where activated carbon is embedded inside the filter media. Activated charcoal is also used in spacesuit Primary Life Support Systems.

[edit] Vodka purification

Activated carbon filters can be used to filter vodka of organic impurities. Since the activated carbon does not bind well to alcohols, the percentage of ethanol is not significantly affected, but the carbon will bind to and remove many organic impurities which can affect color, taste, and odor. Passing an organically impure vodka through an activated carbon filter 6-12 times (or through the same number of filters in one pass) will result in vodka with an identical alcohol content and significantly increased organic purity, as judged by odor and taste. <ref>Practical Applications of the Philosopher's Stone, Oh My God It Burns!</ref>