What is activated carbon?
Activated carbon or active carbon is a carbonaceous material with many micropores, which give it the structure of a crystalline solid, with a highly developed internal porosity and a significant absorbency capacity.
How is activated carbon produced?
To manufacture activated carbon, carbon is produced from carbonaceous materials such as wood or fruit peels like walnut or coconut; however, it is also made from petroleum, pitch, and polymers. The material with which it is made affects the size of the pores, as well as their regenerative characteristics.
The activation process can be carried out physically or chemically:
- Physical activation: carried out by means of a thermal process executed in furnaces with temperatures close to one thousand degrees Celsius in two successive stages:
- Carbonization: gives rise to the porous structure via the removal of hydrogen and oxygen.
- Gasification: the char is exposed to an oxidizing atmosphere to remove carbon atoms and volatile products. This increases the pore volume and the specific surface area.
- Chemical activation: the raw material is impregnated with a chemical agent capable of reducing the formation of volatile matter and tar and is then heated in a furnace that reaches 500 to 700 degrees Celsius. The chemicals may be phosphoric acid, zinc chloride, or potassium hydroxide. The chemical process can leave residues even after washing the activated carbon.
How are the pores of activated carbon classified?
According to the International Union of Pure and Applied Chemistry (IUPAC), the pores of activated carbon are classified according to the size of their radius:
- Micropores: when less than 2 nanometers.
- Mesopores: when ranging from 2 to 50 nanometers.
- Macropores: when exceeding 50 nanometers.
Nanometers are an infinitesimal unit of measurement that is equivalent to one-millionth of a meter. This value is represented by the formula (1 nm = 10−9 m); however, it is easier to understand when put as 1 millimeter is equivalent to one million nanometers.
What are the applications of activated carbon?
- Wastewater treatment: activated carbon is used to remove toxic and polluting organic compounds and chemicals from waters prior to discharge.
In the domestic sphere, activated carbon is used to filter drinking water in order to eliminate pollutants, heavy metals, and unwanted organic compounds.
- Medicine: used to treat cases of intoxication or poisoning. It is administered orally to absorb and eliminate toxins or dangerous substances from the digestive system.
- Pharmaceutical: used to improve the stability of products in manufacturing medicines, given its ability to remove impurities.
- Nutrition: used to purify sugar, discolor oils and fats, remove impurities and unwanted odors, and filter alcoholic beverages.
- Air purification: used to absorb volatile organic compounds (VOCs), chemical vapors, and gases that are toxic or create a bad odor.
It is also used in gas masks and respirators to filter inhaled air.
- Cosmetics: helps remove impurities from the skin and absorb excess oil, which is why it is often found in face masks and cleansers.
- Dentistry: used in toothpaste and whitening products to remove stains from teeth and improve oral hygiene.
What happens to activated carbon when it reaches its maximum absorption capacity?
If activated carbon is used in products (as with cosmetics), it is single-use. However, most industrial applications reuse activated carbon. When activated carbon reaches its maximum absorption capacity, it is said to be saturated and loses its properties; however, it is possible to subject it to a reactivation process.
How is carbon reactivated?
It is possible to reactivate carbon by subjecting it to a thermal process at high temperatures, which:
- Dries the material.
- Evaporates the volatile compounds it has absorbed.
- Decomposes the non-volatile compounds it has absorbed. This phase transforms the carbon into amorphous carbon.
- Gasifies the amorphous carbon to constitute the micropores. This last process is possible thanks to modern technologies. Gasification enables the reactivation of a large amount of carbon that, due to its applications, could not be reactivated before.
The reactivation of activated carbon reduces CO2 emissions into the atmosphere by some 80%.