HAZARDOUS WASTE TREATMENT - Environmentally Conscious Materials Handling - page 174

where:

x = amount of material adsorbed (mg or g)

m = mass of adsorbent (mg or g)

C = concentration of material remaining in solution after

adsorption is complete (mg/ L )

a and b = constants

If adsorption follows the Langmuir isotherm, a linear relationship should result
when the inverse of x/m is plotted against the inverse of C . Values of the
constants a and b can be determined from the slope and intercept of the plot.
Freundlich (1926) developed an empirical equation to describe the adsorp-
tion process (Eckenfelder, 2000). His development was based on the assumption
that the adsorbent had a heterogeneous surface composed of different classes of
adsorption sites, with adsorption on each class of site following the Langmuir
isotherm. Freundlich offered the following equation:
x/m KC 1 /n =

where:

x = amount of solute adsorbed (mg, g)

m = mass of adsorbent (mg, g)

C = concentration of contaminant remaining in solution after

temperature

K and n = constants that must be evaluated for each solute and

temperature

The Freundlich equation can be put in a useful form by taking the log of both
sides. If the plot of log x/m versus C yields a straight line, then the adsorption
data should follow the Freundlich theory.
In a granular activated carbon process, a continuous-flow fixed-bed column
is often used as a way of contacting wastewaters with GAC. The wastewater is
applied to the top of the column and withdrawn from the bottom. The carbon
is held in place with an underdrain system at the bottom of the column. In an
attempt to limit the headloss build-up due to the removal of particulate matter,
backwashing and surface washing are usually required within the carbon column.
A GAC system must allow for the removal of spent carbon-regeneration and
addition of new carbon.
A laboratory absorption study should be conducted to evaluate the feasibility
and economics of adsorption. Column tests simulate the actual operation of a
full-scale unit. In the laboratory, 2-inch-diameter columns are filled with the
carbon to be tested and the contaminated wastewater runs through. The volume
within the carbon bed in which adsorption occurs is called the mass transfer
zone (MTZ, or adsorption zone). After a certain volume of the wastewater passes
through the carbon column, breakthrough is reached, which indicates the point