Page 252 - Materials Chemistry, Second Edition

Groundwater Remediation 235

Example 6.9: Determine the Capacity of an Activated-Carbon Adsorber
Dewatering to lower the groundwater level for belowground construction
is often necessary. At a construction site, the contractor unexpectedly found
that the extracted groundwater contained 5 mg/L toluene. The toluene con-
centration of the groundwater has to be reduced to below 100 ppb before
discharge. To avoid further delay of the tight construction schedule, off-the-
shelf 55-gallon activated-carbon units are proposed to treat the extracted
groundwater.
The activated-carbon vendor provided the adsorption isotherm informa-
tion. It follows the Langmuir model as: q(kg toluene/kg carbon) = [0.04C /
e
(1+0.002C )] where C is in mg/L. The vendor also provided the following
e
e
information regarding the adsorber:
• Diameter of carbon packing bed in each 55-gal drum = 1.5 ft
• Height of carbon packing bed in each 55-gal drum = 3 ft
• Bulk density of the activated carbon = 30 lb/ft 3

Determine (1) the adsorption capacity of the activated carbon, (2) the
amount of activated carbon in each 55-gallon unit, and (3) the amount of the
toluene that each unit can remove before becoming exhausted.

Solution:
(a) The theoretical adsorption capacity can be found by using the
given adsorption isotherm as:

0.004C (0.004)(5)
q (kg/kg) = e = = 0.02 kg/kg
+
1 + 0.002C e 1(0.002)(5)
The actual adsorption capacity can be found by using Equation
(6.8) as:
q q = (50%)(0.02) = 0.01 kg toluene/kg activatedcarbon
design = (50%) theoretical
(b) Volume of the activated carbon inside a 55-gal drum
= (πr )(h)
2
= (π)[(1.5/2) ](3) = 5.3 ft 3
2
Amount of the activated carbon inside a 55-gal drum = (V)(ρ )
b
= (5.3 ft )(30 lb/ft ) = 159 lb
3
3
(c) Amount of toluene that can be retained in a drum before the
carbon becomes exhausted
= (amount of the activated carbon)(actual adsorption capacity)
= (159 lb/drum)(0.01 lb toluene/lb activated carbon) = 1.59 lb
toluene/drum