Activated carbon, also called activated charcoal, activated coal, or carbo activatus, is a form of carbon processed to have small, low-volume pores that increase thesurface area available for adsorption or chemical reactions. Activated is sometimes substituted with active.
Due to its high degree of microporosity, just one gram of activated carbon has a surface area in excess of 500 m2, as determined by gas adsorption. An activation level sufficient for useful application may be attained solely from high surface area; however, further chemical treatment often enhances adsorption properties.
Physical adsorption is the primary means by which activated carbon works to remove contaminants from water. Carbon’s highly porous nature provides a large surface area for contaminants (adsorbates) to collect. In simple terms, physical adsorption occurs because all molecules exert attractive forces, especially molecules at the surface of a solid (pore walls of carbon), and these surface molecules seek other molecules to adhere to.
Activated carbon attracts organic chemicals from vapor and liquid streams cleaning them of unwanted chemicals. It does not have a great capacity for these chemicals, but is very cost effective for treating large volumes of air or water to remove dilute concentrations of contamination. For a better perspective, when individuals ingest chemicals or are experiencing food poisoning, they are instructed to drink a small amount of activated carbon to soak up and remove the poisons.
The large internal surface area of carbon has many attractive forces that work to attract other molecules. Thus, contaminants in water are adsorbed (or held) to the surface of carbon by surface attractive forces similar to gravitational forces. Adsorption from solution occurs as a result of differences in adsorbate concentration in the solution and in the carbon pores.
The adsorbate migrates from the solution through the pore channels to reach the area where the strongest attractive forces are. With this understanding of how the adsorption process works, we must then understand why it works, or why water contaminants become adsorbates. Water contaminants adsorb because the attraction of the carbon surface for them is stronger than the attractive forces that keep them dissolved in solution.
Those compounds that are more adsorbable onto activated carbon generally have a lower water solubility, are organic (made up of carbon atoms), have a higher molecular weight and a neutral or non-polar chemical nature. It should be pointed out that for water adsorbates to become physically adsorbed onto activated carbon, they must be both dissolved in water and smaller than the size of the carbon pore openings so that they can pass into the carbon pores and accumulate.
