Activated charcoal, more commonly known as activated carbon, is a form of carbon that has been treated to change its properties. The treatment can be done through a physical or a chemical process but the end result is an increase in the surface area and the creation of millions of micro pores on the carbon. The carbon precursor or source of carbon material may come from a variety of different sources such as coconut shells, coal, wood, sawdust, petroleum pitch, lignite, and peat. While several different forms of activated carbon can be created, the final product will always work as a result of the adsorption of substances onto and into the pores of the activated carbon.
How it’s formed
The formation of activated carbon is a result of a heating process that ‘activates’ the carbon. There are two slightly different treatment processes, physical and chemical. The physical activation begins with the carbonization of the carbon material in a process known a pyrolysis.
Pyrolysis involves heating the materials at a high temperature without the presence of oxygen. Usually the temperature is around 600 to 900 degrees Celsius and the material is commonly not in a vacuum but in a sealed container containing an inert atmosphere that will not react with the carbon. Once this process is completed, the carbon is then activated by further heating at higher temperatures, around 600 to 1200 degrees Celsius, in the presence of oxygen.
The chemical activation is very similar to the physical activation. Before pyrolysis begins, chemicals are added or impregnated into the carbon material. The exact chemicals can range depending on the desired activated carbon but they are typically a strong acid, salt, or a strong base. The pyrolysis step is then completed but the temperature required for this step is usually lower than with the physical activation, at around 450 to 900 degrees. This makes the chemical activation preferred to physical as it requires lower temperatures and often the surface area produced in this activation is larger.
The activation processes result in expansion and the formation or pores and micro pores on the surface and inside of the carbon. The pores are small but incredibly numerous. This greatly increases the surface area of the resultant carbon. Typically one gram of activated carbon has more than 100 square meters of surface area and the surface area can even reach as high as over 1000 square meters. To put this in perspective, a sample of activated carbon that could easily be placed in the open palm of a person’s hand has more surface area than a regulation sized tennis court.
How activated carbon works
The process by which the activated carbon works is called adsorption. This term should not be confused with the very similarly worded ‘absorption.’ The change in prefix from ‘ab’ to ‘ad’ makes a large difference. Absoprtion is the uptake of a substance by a material through physical contact or by passing through it, similar to how a sponge or paper towel absorbs water. Adsorption onto the activated carbon is rather different and involves the chemical attraction of a substance to a material.
What makes activated carbon so useful is that many different substances that people want removed from air or water are chemically attracted to activated carbon through van der Waals forces. When chemicals travel near the activated carbon, they are attracted to it. The chemicals will be removed from the medium (air or water) and become trapped on or in the activated carbon. This can happen through three difference means. Chemicals could be attracted and adsorb to the exterior of the carbon, the chemicals could be trapped in one of the pores, or the chemicals could adsorb to the interior walls of the activated carbon. Whichever case occurs, the substance is removed from the medium.
Over time, the activated carbon’s usefulness will diminish because the available sites which adsorption can occur will continue to be used up. As the sites diminish, the efficiency of the activated carbon will also diminish. Once this happens, there are two choices. The activated carbon, should it be in a tank or filter, can be backwashed which involves the pushing of water through the activated carbon which will disrupt the adsorption and remove the substances adsorbed onto the carbon. The other option is reactivation, which involves putting the carbon through a similar process through which it was formed. Instead of altering the carbon, the goal is to burn off the substances adsorbed onto the carbon. Using this method, around 70% of the activated carbon can be reused after the process is complete.
Applications
As a result of the properties of activated carbon, it has many different industrial, commercial, or even residential applications. One large industrial use of activated carbon is for filtering metal plating solution that industries use in order to remove impurities. Probably the most significant application that affects/benefits people is filtration and purification of water. Activated carbon is found in many water and wastewater plants as well as many household water filtration units. The substance is also widely used for environmental cleanup and remediation. In addition to water, air purification is also something that activated carbon can be used for. Activated carbon can remove many volatile organic compounds and other harmful pollutants from the air. It can also remove odor from the air and is sometimes applied to wastewater for the sole purpose or reducing smell.
