Activated Carbon > The Adsorption Process
In the adsorption process, molecules are concentrated
on the surface of the activated carbon.
What makes molecules
adsorb on activated carbon?
Adsorption is caused by a type of Van der Waals
Force which exists between molecules. The force
acts in a similar way to gravitational forces
between planets. These sources are extremely short
ranged and therefore sensitive to the distance
between the carbon surface and the adsorbate molecule.
They are also additive, meaning the adsorption
force is the sum of all interactions between all
the atoms. The short range and additive nature
of these forces results in activated carbon having
the strongest physical adsorption forces of any
known material.
Gas Phase Adsorption
This is a condensation process where the adsorption
forces condense the molecules from the bulk phase
within the pores of the activated carbon. The
driving force for adsorption is the ratio of the
partial pressure and the vapor pressure of the
compound. The adsorption is the ratio of the partial
pressure and the vapor pressure of the compound.
The adsorption capasity for non-polar organics
increases with the boiling point, molecular weight
and concentration of the air contaminant. Low
molecular weight (less than 50) and/or highly
polar compounds such as formaldehyde, methane,
ethanol, etc., will not be readily adsorbed at
low concentrations.
Liquid Phase Adsorption
The molecules go from the bulk phase to being
adsorbed in the pores in a semi-liquid state.
The driving force for adsorption is the ratio
of the concentration to the solubility of the
compound.
What compounds are adsorbed?
All compounds are adsorbable to some extent. In
practice, activated carbon is used for the adsorption
of mainly organic compounds along with some larger
molecular weight inorganic compounds such as iodine
and mercury.
In general, the adsorbability of a compound increases with:
- Increasing molecular weight
- A higher number of functional groups such as double bonds or halogen compounds
- Increaseing polarisability of the molecule
