A material defined by its holes
Most carbon materials are valued for the solid framework itself: the hardness of diamond, the slipperiness of graphite, the strength of a fiber. Activated carbon is different. What matters is the space inside it. A grain smaller than a pencil eraser can contain a network of pores so fine and so numerous that its internal surface area is often described as hundreds to well over a thousand square meters per gram. That surface is where the useful work happens.
Adsorption, not absorption
Activated carbon works mainly by adsorption, meaning that molecules stick to a surface. Absorption is different: a sponge soaks liquid into its bulk. In adsorption, dissolved or airborne molecules are held on the pore walls by weak attractive forces. The more surface a given mass of carbon exposes, the more molecules it can hold, which is why pore development is the entire point of activation.
The carbon surface tends to attract nonpolar, organic molecules more strongly than small polar ones or dissolved salts. That preference explains both what activated carbon does well and what it does not.
How it is made
Activated carbon starts from a carbon-rich raw material. Common sources are coconut shell, wood, peat and various coals, and the choice influences the pore structure and hardness of the finished product. Production generally has two stages:
- Carbonization. The raw material is heated with little or no oxygen so that most non-carbon elements are driven off, leaving a char.
- Activation. The char is opened up to create pores. In physical activation this means exposure to steam or carbon dioxide at high temperature. In chemical activation the raw material is treated with an agent such as an acid or a base before heating.
Temperatures and times vary by process, and manufacturers tune them to reach the pore sizes they want.
Micropores, mesopores and macropores
Pores are classified by width. Micropores are narrower than about 2 nanometers, mesopores run from about 2 to 50 nanometers, and macropores are wider still. Each does something different.
- Micropores supply most of the surface area and are best at capturing small molecules, such as many gases and small organic compounds.
- Mesopores act as passages and also capture somewhat larger molecules.
- Macropores work like the entrances of a road network, letting molecules reach the smaller pores deeper inside.
A carbon intended for gas-phase work usually leans toward micropores, while one intended to remove large colored molecules from a liquid may need more mesopores. A molecule that is too large to fit a pore simply cannot use that surface.
Common forms
Activated carbon comes as powder, granules and shaped pellets or blocks. Powder offers fast contact but is harder to separate from a liquid. Granular carbon is packed into beds that water or air passes through. Carbon can also be formed into cloth or felt, or impregnated into other media. The format is chosen to balance pressure drop, contact time and how easily the carbon can be replaced.
Typical uses
- Drinking water and wastewater treatment, where carbon is often used to reduce certain organic compounds, tastes and odors.
- Air and gas purification, including solvent vapor capture and odor control.
- Food and beverage processing, such as decolorizing and removing unwanted flavors.
- Protective equipment and filters, where carbon helps remove vapors from the air a person breathes.
- Industrial process streams, for purifying chemicals and recovering valuable vapors.
Whether it is suitable for any particular contaminant depends on the contaminant. Carbon does not remove everything, and it is not a substitute for a full treatment design in a safety-critical application.
Saturation and reactivation
Adsorption has a limit. Once the surface is covered, the carbon is “spent” and contaminants can begin to pass through. Spent carbon may be discarded, or it may be reactivated by heating it in a furnace to drive off and destroy what it collected, then restoring part of its pore structure. Some material is lost on each cycle, and reactivation is practical mainly at larger scale.
Other porous carbons
Activated carbon is the best known, but not the only porous carbon.
- Carbon molecular sieves have very uniform pores, tuned so that smaller molecules diffuse in faster than larger ones. They are used in some gas-separation equipment.
- Carbon aerogels and foams are lightweight, open structures studied for insulation and electrodes.
- Templated and ordered porous carbons are made with controlled pore patterns for research in catalysis and energy devices.
- Porous electrode carbons are used in capacitors that store charge on a large surface; this ties into the broader topic of energy storage.
Relation to the rest of the carbon family
Activated carbon is largely disordered, which places it with the amorphous members of the family described in the overview of carbon allotropes. Its pore walls contain small fragments of graphite-like sheets, but they are not neatly stacked into crystalline graphite. That disorder is useful, because the irregular arrangement leaves gaps that become pores.
Practical cautions
Activated carbon can host bacterial growth if a water filter is left wet and unused for long periods, and fine carbon dust is a nuisance to handle. In addition, carbon beds can heat up when they adsorb some reactive vapors, so specialized applications call for guidance from the equipment supplier and applicable standards. Choosing a grade for a specific job is best done by testing it against the actual stream rather than relying on a general rule.