Physical
adsorption is the primary means by which activated carbon works to remove
contaminants from liquid or vapor streams. Carbon's large surface area per unit
weight allows for contaminants to adhere to the activated carbon media.
The
large internal surface area of carbon has several attractive forces that work
to attract other molecules. These forces manifest in a similar manner as
gravitational force; therefore, contaminants in water are adsorbed (or adhered)
to the surface of carbon from a solution as a result of differences in
adsorbate concentration in the solution and in the carbon pores.
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 to adhere to other molecules.
The
dissolved adsorbate migrates from the solution through the pore channels to
reach the area where the strongest attractive forces are located. 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 exhibit this preference to adsorb are able to do so when there is enough
energy on the surface of the carbon to overcome the energy needed to adsorb the
contaminant.
Contaminants
that are organic, have high molecular weights, and are neutral, or non-polar, in
their chemical nature are readily adsorbed on activated carbon. For water
adsorbates to become physically adsorbed onto activated carbon, they must both
be dissolved in water so that they are smaller than the size of the carbon pore
openings and can pass through the carbon pores and accumulate.
Besides
physical adsorption, chemical reactions can occur on a carbon surface. One such
reaction is chlorine removal from water involving the chemical reaction of
chlorine with carbon to form chloride ions.
