Environmental Engineering Reference
In-Depth Information
found in indoor air have different chemical properties and, as a consequence,
cannot be effectively removed by generic removal processes. The most com-
monly used technique for removing gas/vapor-phase substances from
indoor air and ventilation airstreams is adsorption. Other control approaches
that have been attempted include absorption, catalytic oxidation/reduction,
botanical air cleaning, and ozonation.
A. Adsorption
Adsorption is a process by which gas, vapor, or liquid-phase substances are
physically removed from fluids (including air) by adherence to, and retention
on, solid sorbents. This adherence is due to Van der Waal's forces acting on
the surface of solids to hold molecules to their surface. The sorbing material
is the adsorbent (or sorbent); the adsorbed molecules, the adsorbate (or
sorbate). Sorbate condenses (capillary condensation) in the submicroscopic
pores of the sorbent.
Though adsorption is a chemical/physical phenomenon, chemical reac-
tions generally do not take place. However, when molecules are adsorbed
on a surface, heat is released (heat of adsorption) which is approximately
equal to the amount of heat produced when a gas/vapor condenses. As a
consequence, sorbate is present on the sorbent as a liquid. Desorption pro-
cesses require sufficient energy to convert the sorbate to a gas or vapor.
A variety of materials have good sorbent properties. They include acti-
vated carbons, activated alumina, silica gel, zeolites, porous clay minerals,
and molecular sieves. These materials are widely used in industrial and
commercial applications where certain sorbent properties are required. They
are used in air cleaning, water softening, and as “kitty litter.” Sorbents have
high surface-to-volume ratios. Their structure consists of large numbers of
submicroscopic pores and channels. Most sorption occurs in pores that have
cross-sectional diameters of 10 to 30 angstrom units (Å).
Sorbents may be polar or nonpolar. Metal oxide-, silicaceous-, and active
earth-type sorbents are polar. Since polar compounds attract each other, and
water is strongly polar, sorbents such as silica gel sorb and retain water
preferentially. As such, they cannot effectively remove gases (other than
water vapor) in the humid atmospheres common to most air cleaning appli-
cations. Activated carbons are nonpolar and have limited affinity for water
vapor. They preferentially sorb and retain organic vapors.
B. Activated carbons
Activated carbons are commonly used for solvent recovery and air cleaning
in industrial applications, and air cleaning in indoor applications. They are
produced in a two-step process in which carbonaceous materials such as
wood (primarily hardwoods), coal, coconut and other shells, fruit pits, etc.,
are heated in a neutral atmosphere and then oxidized at high temperature.
Substances that cannot be easily carbonized are volatilized, and numerous
