Activated Carbon: A Common Adsorptive Air Filter

Adsorptive air filters work mainly through physical adsorption and chemical adsorption. Activated carbon filters are currently the most commonly used adsorptive filters in the air-purification industry. Other adsorbents include activated alumina, silica gel, zeolites and photocatalysts made from nanoscale TiO2.

Physical Adsorption

Physical adsorption can capture almost any gas, although its effectiveness varies with the type of gas.

In general, gases with higher molecular weights or higher boiling points are more readily adsorbed. Nonpolar molecules are more readily adsorbed than polar molecules, while organic gases, total volatile organic compounds (TVOCs), solvent vapors and odors are also readily captured.

Chemical Adsorption

In the air-filtration industry, activated carbon filter media is commonly used as a base material because it already has adsorptive properties. To increase its capacity for a particular gas, such as formaldehyde or VOCs, or for several gases, the activated carbon can be chemically impregnated. The resulting process is called chemical adsorption, and the material is known as impregnated activated carbon.

Activated carbon that has not undergone chemical impregnation is called unimpregnated activated carbon. In chemical adsorption, the adsorbate is held by chemical bonds and cannot be desorbed for reuse. Chemical adsorption with impregnated carbon can capture acidic and corrosive gases such as SO2, NOx and H2S, alkaline gases such as NH3, and formaldehyde, also known as formalin.

Adsorption that does not involve an obvious chemical reaction is called physical adsorption and depends mainly on van der Waals forces. When free airborne molecules with high boiling points at or above room temperature contact activated carbon, some condense into liquid inside the micropores and remain there through capillary action. Others fill micropores similar to their molecular size and effectively become integrated with the material.

Gases such as nitrogen, oxygen, carbon dioxide, hydrogen and chlorine have very low boiling points and are not readily adsorbed by activated carbon. Ordinary activated carbon is hydrophobic, so its capacity to adsorb water vapor is also limited. Activated carbon can also capture certain airborne microorganisms and kill them.