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 adsorbing properties of the material.
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, automobile emissions control and many other applications.
It can generally be produced in two different processes:
- Chemical activation: Mostly acids are mixed with the source material in order to cauterise the fine pores. This technique can be problematic because, for example, zinc trace residues may remain in the endproduct.
- 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.
Activated carbon may have a surface area in excess of 500 m²/g, with 1000 m²/gram being readily achievable. A tennis court is about 260 m².
Under an electron microscope, the structure of activated carbon looks something like ribbons of paper which have been crumpled together, with a few wood chips thrown in for good measure. There are lots and lots of nooks and crannies, and many areas where flat surfaces of graphite-like material run parallel to each other, separated by 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.
Saturated active carbon can be regenerated by heating.