Active carbon or activated carbon is a highly porous material comprising mainly pure carbon.

A common feature of any type of carbon is its ability to generate a physicochemical phenomenon called ‘adsorption’, whereby the atoms on the surface of the material attract and retain certain types of molecules trapped in liquid or gaseous compounds.

However, the adsorption capacity of activated carbon is much higher than that of other types of carbon due to the large surface area provided by its structure in the form of graphitic plates.

That is why activated carbon is so commonly used in processes in which toxic compounds, impurities or undesirable elements in liquid or gaseous substances must be eliminated.

Activated carbon is commonly used in purification processes, in order to eliminate one or several compounds from liquids or gasses.

Examples of applications include:

• Water purification:Most Drinking Water Treatment Stations use activated carbon to reduce the amount of undesirable organic compounds in the water or to improve its organoleptic properties.
• Compounds that are not acceptable in raw materials or highly pure products, for example the ascorbic acid used to manufacture vitamin C, bleaching sugar, glucose, wine or juice, eliminating chlorine, chloramine or ozone from water, etc.
• Controlling atmospheric emissions from industrial processes: Many industrial processes generate polluting gasses that must be properly treated before going back into the atmosphere. There are several ways to treat these gasses, and one of them is filtration through a bed of granular activated carbon.

Activated carbon for water purification can be used in powder form (PAC), dosed in water as a reagent, or in granular form (GAC), as a filter through which the water passes.

The advantage of using granular carbon, versus powdered, is that when the GAC has used up all its adsorption power it can be regenerated or reactivated to re-establish its initial characteristics. so it can be reused, generating less waste and cutting costs.

The main mechanism through which contaminants are retained on the surface of GAC is physical adsorption and, as such, it is reversible. The graphite structure of AC means this solid can withstand very high temperatures, as well as brusque temperature changes. Based on these properties, there are methods known as reactivation or regeneration that can be used to eliminate the compounds that have been adsorbed by the GAC and re-establish its initial properties so it can be reused. This process normally costs 60% to 70% of the price of virgin carbon.

The reactivation process involves the following steps:

• In the first stage, the carbon is heated until it reaches the boiling point of water. The volatile molecules adsorbed by the carbon are detached and the water evaporates.
• The carbon continues to heat up to a temperature of between 300 and 450 °C. During this period, other organic molecules that are less volatile than water are desorbed.
• The organic compounds that have not been desorbed begin to decompose. This decomposition is called pyrolysis, and as a result, amorphous carbon is formed that remains deposited on the graphitic surface of the GAC.
• When the temperature reaches over 500 °C, the amorphous carbon generated from the previous step begins to react with the water vapour, oxygen, monoxide and carbon dioxide in the gas stream. As a result, other water vapour and carbon dioxide and monoxide molecules are formed, which are released into the gaseous stream. The graphitic plates that make up the structure of the GAC are less reactive than the amorphous carbon atoms, so they do not react or do not react significantly. Finally, and as a result, the initial pore structure is recovered and therefore its functionality as an adsorbent is also recovered.

Several raw materials can be used to create activated carbon, such as woods of different hardnesses, mineral coals, shells or vegetable bones, etc.

The activation process consists of rearranging the carbon atoms in the form of graphitic plates. These plates usually have a flattened shape and form spaces between one another when grouped together known as ‘pores’.

These spaces or pores increase the carbon’s surface area and drastically increase its adsorbent capacity.

There are several types of activated carbon activation (thermal and chemical), which, along with the raw material used to manufacture the carbon, influence the distribution of the size of the pores or spaces between the graphitic plates of the material.

In other words, the type of activation and the raw materials used determine the size of the pores and, consequently, the type of molecules that can be adsorbed. Because of this, different types of activated carbon are used depending on the compound to be eliminated.

Activated carbon has long been used as an adsorbent in water purification. It was originally used to adsorb tastes and odours or eliminate free chlorine left over from the disinfection stage, which gives the water an unpleasant taste and can affect other processes.

AC is no longer used only for the above purposes, but to retain the organic and synthetic pollutants that are present in most drinking water sources.

Activated carbon is recognised as the best technology available for treating water and as such, it is one of the basic purification processes at Drinking Water Treatment Stations.

Activated carbon is one of the most widely used compounds in adsorption or separation processes. Its effectiveness depends largely on the characteristics of the compound to be retained or adsorbed and its affinity with the characteristics of the carbon itself. That is why it is so important to select the right carbon for each application.

Activated carbon is highly effective at retaining organic compounds, while its efficacy against inorganic compounds depends on each case.

Any organic substance can be turned into carbon. The term charcoal encompasses wood, shells and other plant compounds that have been carbonised. For charcoal to be considered activated carbon, its atoms have to be restructured to give it that highly porous structure.

Activated carbon is a completely harmless material that can even be used to treat poisoning thanks to its ability to adsorb toxins that enter the body.