What is adsorption?

What is adsorption?

The use of solids for removing substances from either gaseous or liquid solutions has been widely used since biblical times. This process, known as adsorption, involves nothing more than the preferential partitioning of substances from the gaseous or liquid phase onto the surface of a solid substrate. From the early days of using bone char for decolorization of sugar solutions and other foods, to the later implementation of activated carbon for removing nerve gases from the battlefield, to today’s thousands of applications, the adsorption phenomenon has become a useful tool for purification and separation.

Adsorption phenomena are operative in most natural physical, biological, and chemical systems, and adsorption operations employing solids such as activated carbon and synthetic resins are used widely in industrial applications and for purification of waters and wastewaters.

The process of adsorption involves separation of a substance from one phase accompanied by its accumulation or concentration at the surface of another. The adsorbing phase is the adsorbent, and the material concentrated or adsorbed at the surface of that phase is the adsorbate. Adsorption is thus different from absorption, a process in which material transferred from one phase to another (e.g. liquid) interpenetrates the second phase to form a “solution”. The term sorption is a general expression encompassing both processes.

Physical adsorption is caused mainly by van der Waals forces and electrostatic forces between adsorbate molecules and the atoms which compose the adsorbent surface. Thus adsorbents are characterized first by surface properties such as surface area and polarity.

A large specific surface area is preferable for providing large adsorption capacity, but the creation of a large internal surface area in a limited volume inevitably gives rise to large numbers of small sized pores between adsorption surfaces. The size of the micropores determines the accessibility of adsorbate molecules to the internal adsorption surface, so the pore size distribution of micropores is another important property for characterizing adsorptivity of adsorbents. Especially materials such as zeolite and carbon molecular sieves can be specifically engineered with precise pore size distributions and hence tuned for a particular separation.

Surface polarity corresponds to affinity with polar substances such as water or alcohols. Polar adsorbents are thus called “hydrophilic” and aluminosilicates such as zeolites, porous alumina, silica gel or silica-alumina are examples of adsorbents of this type. On the other hand, nopolar adsorbents are generally “hydrophobic”. Carbonaceous adsorbents, polymer adsorbents and silicalite are typical nonpolar adsorbents. These adsorbents have more affinity with oil or hydrocarbons than water.

Text adapted from:Slejko, F.L., Adsorption Technology, Marcel Dekker, New York, 1985.
M. Suzuki, Adsorption Engineering, Elsevier,Amsterdam, 1990
More information on adsorption related subjects in the references below or through the links page.

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Adsorbents

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The adsorbent is the separating agent used to express the difference between molecules in a mixture: adsorption equilibrium or kinetics.

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Adsorbents are mostly microporous, high specific surface materials (200 – 2000 m2/g)

Most commonly used:
Alumina (drying)
Silicagel (drying)
Zeolite molecular sieves (gas & liquid separations, drying)
highly specific, single pore size
may be fine-tuned: cations + structure
A type or LTA
X and Y or FAUjasites
Mordenite, other natural zeolites
Silicalites or ZSMx (hydrophobic, carbon like)
Ordered mesoporous materials
MCM-41, MCM-48, …
Active carbon (gas & liquid separations, guard beds)
Carbon molecular sieves (narrow pore distribution)
Others:
impregnated carbons (Cu-chlorides – CO separation)
clays (natural and pillared clays)
resins, polymers (biological, ions, large molecules)
carbon nanotubes