How Active Carbon Filtration Works

The right combination of particles in different sizes will allow adsorption of a large range of polluters.

Enlarging the carbon's surface area bigger (creating cracks and branched channels) is done by exposing the carbon to high temperatures and steam in anaerobic conditions. Enlarging the carbons surface area provides the carbon high adsorption ability and allows the negative ions of the polluters to adsorb to the carbon's positive charged surface.

The adsorption and absorption capabilities of the carbon is affected of many factors, but basically affected of:

The raw material's source
The division of the pores capacity inside the carbon

There are two types of adsorption:

Physical Adsorption:

In this process, the adsorbed materials are located on the pores' surface area and are drawn with low electrostatic forces (Van der Vals). This adsorption produces a small amount of heat. An example of physical adsorption: is removal of organic materials.

Chemical Adsorption:

In this type of adsorption there's a chemical interaction between the adsorbed material and the active carbon. This adsorption produces a large amount of heat. An example of chemical adsorption is the removal of inorganic compounds and formaldehyde. Organic apolaric materials are absorbed quickly on the active carbon. The more soluble the material is, the less its capability to adsorb, and that's why variables such as temperature and pH which affect its solubility, indirectly affect its adsorption.

Adsorption Capabilities:

Since pores in different sizes adsorb different sized molecules, loading tests are done:

A. IODIN NUMBER: The results of this test give indication to the effectiveness of the adsorption of small molecules on the general surface area of the carbon.

B. MOLLASSES NUMBER: Gives information of large molecules adsorption capabilities.

C. TANIN NUMBER: This value helps in the assessment of carbon's capabilities in removing odours and flavours from water.

Active Carbon's features:

Density Bulk
Content Ash Total - To determine the amount of the mineral materials in the carbon
Hardness/Wear
Volume Pore Total (ml/g) A large amount of pores shows a high effectiveness of adsorption.
Radius Pore: The average radius is measured in Microns and changes by the carbon's type.

The pore volume segmentation:

Each carbon type has a different segmentation;
MICROPORES, MESOPORES and MACROPORES. Carbon that adsorbs different types of gas molecules is characterized with MICROPORES. Carbon that is recommended for dichloride is characterized with MESOPORES.

Sources:

1. Learn the factors that affect carbon's adsorption capabilities - Bob Thomas Water Technology February 2000

2. WQA - Glossary of Terms 1993 Water Quality Association