What is Activated Carbon and How is it Made?

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Activated carbon (also known as active carbon or activated charcoal), is an artificial carbonaceous (carbon-rich) material. It is typically made from wood, but also can be made using bamboo, coconut husk, peat, woods, coir, lignite, coal, pitch, and other dense carbon sources. There are a variety of similar substances that fall under the general classification of activated carbon such as activated coal and activated coke, but they all share the properties of vast surface area per mass, exceptional microporosity, and a composition of almost exclusively elemental carbon.

The pore structure of activated carbon can be seen with an electron microscope and is primarily responsible for its usefulness. Microscopes show a surface honeycombed with holes and crenelated with ridges which join to a similar structure deeper within the carbon. Due to this structure, a pound of activated carbon, for instance, can have as much as 100 acres of surface area packed into it. These small, low-volume pores allow for increased adsorption capacity (the process of chemical surface bonding, not to be confused with absorption) and allow more reactions between the carbon and other media. Activated carbon is therefore highly valued in filtering, deodorization, medical, and chemical applications, as most contaminants easily bind to it and remain trapped in the carbon microstructure via many small distance-dependent atomic attractions (known as London dispersion forces).

Because carbon is produced through a relatively inexpensive and simple series of activation processes, it is widely available for applications; however, activated carbon must be constantly changed as it becomes clogged with contaminants, water, and becomes a breeding ground for dangerous microorganisms after a certain contact time. This means activated carbon is constantly produced to meet the demands of its uses and is kept in large supply to ensure no delay exists in the replacement process.

How to Make Activated Carbon

The production process of activated carbon, or the activation of carbon, exists in two forms. A carbonaceous source such as wood, coal, peat, or any organic carbonaceous material is carbonized, which means the pure carbon is extracted by a heating method known as pyrolysis. Once the material is carbonized, the material needs to be oxidized or treated with oxygen either by exposure to CO2 or steam or by an acid-base chemical treatment. The sections below will briefly detail these processes.

Carbonization

Carbonization is the process of taking a carbon-rich piece of material and converting it to pure carbon through heating. This heating process, called pyrolysis, comes from an ancient technique for making charcoal. Very dense carbonaceous material is used in the beginning, because the result needs to be extra-porous for activated carbon purposes. Carbon-rich material is placed in a small (relative to the amount of material) furnace and cooked at extreme temperatures topping 2000 degrees Celsius. What remains is usually 20-30 percent of the beginning weight and consists of mostly carbon with a small percentage of inorganic ash. This is very similar to “coking,” a method of producing coke from charcoal, a type of carbon-based fuel.

Once the porous form of carbon is produced, it needs to undergo oxidization so it can be adsorbent, or “activated”. This can occur in one of two ways: gas or chemical treatment.

Gas Treatment

The activating of carbon can be done directly through heating carbon in a chamber while gas is pumped in. This environment exposes it to oxygen for oxidization purposes. After being oxidized, the active carbon gains its good adsorption characteristics and is ready to collect contaminants in liquid/gaseous applications. For physical gas treatment, the carbonization/pyrolysis process must take place in an inert environment at 600-900 degrees Celsius. Then, an oxygenated gas is pumped into the environment and heated between 900 and 1200 degrees Celsius, causing the oxygen to bond to the carbon's surface.

Chemical Treatment

The process of chemical treatment is slightly different from the gas activation of carbon. First, carbonization and chemical activation occur simultaneously in chemical treatment. A bath of acid, base, or other chemicals is prepared, and the material is submerged in this solution. The bath is then heated to temperatures of 450-900 degrees Celsius (much less than the heat needed for gas activation) and left for a specific amount of time. The carbonaceous material is both carbonized and activated at the same time, all at a much quicker rate than with gas activation. However, some heating processes cause trace elements from the bath to adsorb to the carbon, which can result in a more impure or ineffective active carbon.

Activated Carbon Regeneration

Soiled activated carbon can be restored to its original adsorptive capacity via regeneration procedures. There are numerous methods of carbon regeneration, but the most common technique is thermal reactivation, where the soiled carbon is dried, heated in an inert atmosphere, and gas treated. About 5-15% of the original weight of the carbon is lost in thermal regeneration, and this process is very energy-intensive; therefore, smaller companies typically send their used carbon to specialized regeneration facilities instead of performing regeneration onsite. There are numerous other regeneration techniques that aim to reduce both the yield loss and energy expenditure of thermal regeneration. These methods include thermal swing adsorption, pressure swing adsorption, microwave regeneration, chemical regeneration, microbial regeneration, electrochemical regeneration, ultrasonic regeneration, wet air oxidation, and other techniques.

Post Treatment Activated Carbon

Following oxidization, activated carbon can be processed for many kinds of uses, with several classifiable different properties. For instance, granular activated carbon (GAC) is a sand-like product with bigger grains than powdered activated carbon (PAC), and each is used for different applications.  Other varieties include impregnated carbon, which adds different elements such as silver and iodine, and polymer-coated carbons. Below, the following section provides a brief overview of the different types of available activated carbon and their beneficial qualities.