Page 250 - Materials Chemistry, Second Edition

Groundwater Remediation 233

(b) Plot the capture zone of two pumping wells (same as Example
6.7). The two open circles on Figure 6.5 are the locations of the
wells, and the square symbols define the capture zone of these
two wells. As shown, this capture zone cannot encompass the
entire plume. Consequently, two pumping wells should be
employed.

6.3 Activated-Carbon Adsorption
6.3.1 Description of the Activated-Carbon Adsorption

Adsorption is the process that collects soluble substances (adsorbates) in
solution onto the surface of the solid (the adsorbent). Activated carbon is a
universal adsorbent that adsorbs almost all types of organic compounds.
Granular or powder activated carbon has a large specific surface area. In acti-
vated-carbon adsorption, the organics leave the liquid by adsorbing onto the
activated-carbon surface. As the activated-carbon unit becomes exhausted,
as indicated by the breakthrough of COCs in the effluent, the activated-car-
bon unit needs to be regenerated or replaced.
Common preliminary design of an activated-carbon adsorption system
includes sizing the adsorption unit, determining the carbon-change (or
regeneration) interval, and configuring the carbon units, if multiple adsorp-
tion units are used.

6.3.2 Adsorption Isotherm and Adsorption Capacity
In general, the extent of adsorption depends on the characteristics of the
adsorbates (i.e., COCs) and the activated carbon, concentrations of the COCs,
and the temperature. An adsorption isotherm describes the equilibrium
relationship between the adsorbed COC concentration on the surface of the
activated carbon and the dissolved COC concentration in the bulk solution
at a given temperature. The adsorption capacity of a given activated car-
bon for a specific compound is estimated from its isotherm data. The most
commonly used adsorption models in environmental applications are the
Langmuir and Freundlich isotherms, respectively:

abC
q = (6.6)
1 + bC
q = kC n (6.7)