Civil Engineering Reference
In-Depth Information
teristics. Because the carbon is quite dense, it generally has a high adsorption capacity.
Most waterworks use coal-derived carbons.
Wood-derived carbons are generally softer than their coal counterparts, and thus
more friable. They are less resistant to breakage and may generate dust problems
during storage or transfer.
Carbon has two dominant forms in nature: graphic, which is flat, and diamond,
which is tetrahedral. Activated carbon consists mainly of randomly organized graphite
platelets as shown in Figure 17-1(a). Dr. M. Greenbank
10
defined activated carbon as
a ''crude form of graphite with random or amorphous structure, which is highly porous
over a broad range of pore sizes, from visible cracks and crevices to cracks and
crevices of molecular size.'' London dispersion forces (a form of Van der Waals force)
is very strong over short distances and provides for the attraction between a molecule
and the flat graphite surface or platelet. The strength of the adsorption force is deter-
mined by the distance between the molecule and graphite platelets In areas with a
high density of graphite platelets, the adsorption forces will be very high as shown in
Figure 17-1(b). The type of raw materials and production process will determine the
activated carbon adsorption characteristics.
Adsorption is a dynamic process where molecules are continuously attached to and
released from the surface similar to the ion-exchange process. Therefore, molecules
compete for available surface area. The compounds with the highest surface attraction
force and highest concentration will eventually dominate the surface coverage. This
phenomenon of competitive adsorption also means that it is possible to displace a
previously adsorbed compound with another compound that has a greater surface at-
traction. To prevent the undesirable, accidental bleed through a previously adsorbed
compound, it is good practice to closely monitor the effluent quality of the contactor.
In certain critical applications it may be necessary to design GAC systems with two
beds in series, operated in lead and polish mode (as discussed in more detail later in
this chapter).
ADSORPTIVE CHARACTERIZATION OF ACTIVATED CARBONS
The most important characteristic of an adsorbent is the quantity of adsorbate it can
accumulate. Simple capacity tests—such as the iodine number, molasses decolorizing
index, threshold odor test, phenol value, tannin value, and others—may be used as an
indirect measure of adsorptive capacity.
11-14
These capacity measures provide a general
guide to the ability of an activated carbon to remove organics.
The iodine number is the number of milligrams of iodine adsorbed per gram of
carbon when the carbon is in equilibrium, under specified conditions, with a solution
of 0.02
N
iodine concentration. It is an approximate measure of the adsorptive capacity
of a carbon for small molecules such as iodine. The molasses decolorizing index is a
measure of the adsorptive capacity of the carbon for color bodies in a specified mo-
lasses solution, compared to a standard carbon. Therefore, it is a measure of the ad-
sorptive capacity for large molecules such as complex carbohydrates.
The adsorptive capacity of an activated carbon for a specific compound is best
determined experimentally. Because competitive adsorption with other compounds in
the water and the specific environmental conditions (temperature, pH, total dissolved
solids [TDS], etc.) can significantly affect the adsorptive capacity, the experimental
tests should be conducted under conditions closely matching the ultimate operating
conditions.
