Adsorption - Principles of Chemical Separations with Environmental Applications - page 183

7.3 Adsorption principles

7.3.1 Physical vs chemical adsorption

Physical adsorption occurs due to van der Waals (dispersion) or electrostatic forces. The
attraction depends on the polar nature of the fluid component being adsorbed as well as
that of the adsorbent. Van der Waals forces are directly related to the polarizability. An
estimate of the relative strength of interaction is based on the sorbate size and polariz-
ability. Electrostatic forces include polarization forces, field-dipole interactions and field
gradient-quadrupole interactions. These forces arise when the surface is polar. In the case
of a polar solvent like water with non-polar organic impurities, the organic molecules will
prefer to stick to a non-polar adsorbent such as activated carbon rather than remain in the
polar solvent. Physical adsorption is reversible. Physical sorption is sensitive to temper-
ature, relatively non-specific regarding sorbates, relatively fast kinetically, and has a low
heat of adsorption (

<

H vap ). Multiple sorbate layers can form on the sorbent surface.
Chemical adsorption (chemisorption) occurs when the attraction between the adsorbent
and the adsorbate can form a covalent bond, or when a chemical reaction occurs between
the adsorbent and adsorbate. Usually chemical adsorption will only allow a single layer of
molecules (monolayer) to accumulate on the surface of the adsorbent. Chemical adsorption
is usually irreversible. Chemisorption is typically more specific, kinetically slower and
has a larger heat of adsorption (

2

> 3 H vap ).

7.3.2 Separating mechanism

The interaction of the adsorbate with the solid surface can be due to three mechanisms:
steric, kinetic, or equilibrium. Steric interactions are due to the shape of the molecule.
One example would be the difference in adsorption strength (heat of sorption) for a linear
vs a branched hydrocarbon. Another example would be the separation of a large and
small molecule using zeolites whereby the small molecule could enter the zeolite pores
and the large molecule would be excluded (molecular sieving). Kinetic interactions are
due to the relative ease of accessibility of the adsorbate to the solid surface. Diffusion
through the fluid boundary layer to the solid surface and diffusion in the pores of the
sorbent contribute to this effect (i.e., whichever component gets to the sorption site first
wins). Equilibrium interactions relate to the thermodynamic equilibrium state of the fluid
and solid phases. Equilibrium interactions differ from kinetic effects in that a molecule
may get to the sorption site first (kinetic) but will later be displaced by the more strongly
adsorbed species due to the reversibility of the process. For physical sorption, there is a
finite rate of adsorption as well as desorption to the solid surface. When these rates are
equal, the process is at equilibrium. This is analogous to a reversible chemical reaction
where the equilibrium constant represents the balance of the forward and reverse reaction
rates.