HAZARDOUS WASTE TREATMENT - Environmentally Conscious Materials Handling - page 173

DDT; and phenolic compounds, soluble organic dyes, and chlorinated solvents
such as carbon tetrachloride; and fuels such as gasoline, kerosene and oil.
Activated carbon can be used in the form of granular activated carbon (GAC)
or powdered activated carbon (PAC). GAC has been used more commonly for the
treatment of hazardous wastes because it is more effective in removing pollutants
that have poor adsorption characteristics. It can also be regenerated by thermal
methods and is therefore more economical. Powdered activated carbon (PAC) is
more commonly used in biological treatment systems.
In theory, adsorption takes place at the surface of the activated carbon when the
pollutant moves from the aqueous dilute wastewater into the carbon. The forces
mainly responsible for adsorption onto activated carbon are the weak Van der
Waals forces, electrical attraction, and the hydrophobic character of the organic
material (LaGrega et al. 2001). The majority of organic molecules are adsorbed
onto the large surface area within the pores of a carbon particle. The transfer of
contaminants (adsorbate) from solution to the carbon surface (adsorbent) contin-
ues until equilibrium is reached with the concentration of contaminant remaining
in solution.
The main factors influencing adsorption include magnitude and surface area
of the adsorbent; pore size distribution of the adsorbent; nature and the concen-
tration of the solute; temperature and pH of the solution; and the design and
mode of operation of the activated carbon system. The equilibrium distribution
of the contaminant between the liquid and solid phases helps define the capacity
of a particular system. Therefore, adsorption isotherms are used to relate the con-
centrations of the adsorbed compound in each of the two phases. Isotherms are
equilibrium relationships that would determine the degree to which adsorption
will occur and the distribution of the contaminant between the liquid phase and
the adsorbed phase at a specific temperature. For example, in the adsorption of a
hazardous compound such as phenol, the isotherms determine what solid-phase
phenol concentration in mg/gram corresponds to any given solution-phase con-
centration in mg/L.
To assess the feasibility of adsorption for a particular application, it is nec-
essary to perform an adsorption isotherm study on the waste stream of interest.
Data derived from an isotherm study will describe the performance of the acti-
vated carbon and will give important information. Some of the more familiar
isotherm models are Langmuir and Freundlich.
Langmuir's isotherm is based on the assumption that all the adsorption sites
have equal affinities for molecules of the adsorbate and that the presence of
adsorbed molecules at one site will not affect the adsorption of molecules at an
adjacent site. When a single contaminant is involved, Langmuir isotherms may
provide closer approximations.
The Langmuir equation (Langmuir 1918) is commonly written as follows
(Eckenfelder 2000):

x/m = abC /( 1 + aC )