Article: Activated carbon

Activated carbon (also called activated charcoal) is the more general term which includes carbon material mostly derived from charcoal. It denotes a material which has an exceptionally high surface area, typically determined by nitrogen 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.

Production

It can generally be produced in two different processes:

1. Chemical activation: Mostly acids are mixed with the source material in order to cauterize the fine pores. This technique can be problematic because, for example, zinc trace residues may remain in the endproduct.
2. Steam activation: The carbonised material is mixed with vapours and/or gases at high temperature to activate it. The source material can be several carbonic materials, e.g. nutshells, wood, coal.

Saturated active carbon can be regenerated by heating.

Properties

Activated carbon may have a surface area in excess of 400 m²/g, with 1500 m²/g being readily achievable. For comparison, a tennis court is about 260 m².

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.

Carbon aerogels, while more expensive, have even higher surface, and find use similar to activated carbon in special applications.

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

Activated carbon, however, does not bind well to:

• Lithium, strong acids and bases, metals and most inorganic minerals (examples of these are 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.
• Alcohol (such as ethanol, methanol, isopropyl alcohol, glycols, and acetone)
• Ammonia

Applications

Activated carbon is used in metal extraction (e.g. gold), water purification (especially in home aquariums), medicine, wastewater treatment, filters in gas and filter masks, filters in compressed air and gas purification, and many other applications.

Environmental applications

Carbon absorption 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

Medical applications

Activated carbon is used to treat poisonings and overdoses by oral ingestion. It prevents absorption of the poison by the stomach. In other cases of suspected poisoning, medical personnel administer activated carbon on the scene, and the stomach is pumped at the hospital.

The typical adult dosage is 25-50 grams. Pediatric dosages are 12.5-25 grams. Incorrect application to children (e.g., to the lungs) will usually result in a fatality if immediately unchecked. 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.

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.

Vodka purification

Activated carbon filters can be used to filter vodka of organic impurities. Since the activated carbon does not bind well to alcohol, the percentage of alcohol is not significantly affected, but the carbon will bind to and remove many organic impurities which can affect color, taste, and odor. ^[1] 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. ^[2]