Environmental Engineering Reference
In-Depth Information
In general, the adsorbability of gases and vapors increases with increased
molecular size and weight. Small, highly volatile molecules (very volatile
organic compounds, VVOCs) have lower adsorbability on activated carbons
than larger, lower-volatility compounds (VOCs). In an organic compound
series such as paraffins, olefins, and aromatic compounds, adsorbability
increases with increased carbon numbers.
3. Adsorption capacity
Activated carbons differ in their ability to sorb and retain sorbate molecules
determined on a sorbent weight basis. Adsorption capacity, described as the
weight of the sorbate collected per weight of sorbent, depends on (1) sorbent
surface area; (2) active sorbent pore volume; (3) gas/vapor sorbate proper-
ties; and (4) environmental factors such as temperature, relative humidity,
and pressure.
Surface areas for commonly used activated carbons range from 500 to
1400 m
2
/g. An inverse exponential relationship exists between pore size and
surface area; surface area available for sorption decreases dramatically as
pore size increases.
Adsorption capacity of activated carbons is rated relative to their ability
to sorb carbon tetrachloride (CCl
4
) vapors. A standard weight of activated
carbon is exposed to a saturated dry stream of CCl
4
at 68°F (20°C) until the
sorbent no longer increases in weight. The ratio of the weight of sorbed CCl
4
to the weight of activated carbon is the maximum possible sorption of CCl
4
.
This adsorptive capacity, expressed as CTC% (or g CTC/g carbon), ranges
from a low of 20 to a high of 90% for different activated carbons.
Though adsorption capacity is determined by using CCl
4
as a standard,
other chemical substances will differ in their adsorption capacity on a given
activated carbon. As an example, reported adsorption capacities for carcino-
genic vapors such as 1,2-dibromomethane, CCl
4
, and 1,1-dimethyl hydrazine
are 1.020 g/g, 0.741 g/g, and 0.359 g/g, respectively.
As indicated, adsorption capacity can be affected by environmental con-
ditions such as temperature, pressure, and relative humidity. Under elevated
temperatures and low pressures, significant loss in adsorption capacity
occurs. Fortunately, such temperature and pressure extremes do not occur
in nonindustrial air cleaning applications.
Under ordinary ambient (outdoor) conditions, relative humidity is the
most likely factor to affect adsorption capacity. Nonpolar activated carbons
can nevertheless sorb water vapor from the atmosphere. At high relative
humidities (>50%) significant reductions in adsorption capacities have been
reported for activated carbons.
4. Retentivity
Retentivity is a property of both the sorbate and sorbent, described as the
maximum concentration of vapor retained by a sorbent when the vapor
content of an airstream is reduced to zero. It is measured by passing clean,
