Page 179 - Materials Chemistry, Second Edition

162 Practical Design Calculations for Groundwater and Soil Remediation

(d) Convert the vapor concentration into a volume concentration by
using Equation (2.1):

1 ppmV benzene = (78.1)/24.5 = 3.2 mg/m @25°C
3
47,500 mg/m = 47,500 ÷ 3.2 = 14,800 ppmV
3
Discussion:
The actual concentration of the extracted vapor would be smaller than
14,800 ppmV, due to the fact that not all the air flows through the
impacted zone. In addition, limitations of mass transfer were not
considered in these calculations.

Example 5.6: Estimate the Extracted Vapor Concentration
(in the Absence of the Free Product)
For the site discussed in Example 5.5, the average benzene concentration of
the soil samples, taken from the impacted zone, dropped to 250 mg/kg after
three months of soil venting. Estimate the extracted soil vapor concentration
from soil venting.

Solution:
Since the equilibrium constants (i.e., K and H) will stay the same,
p
and assuming the volumetric water content also stays the same,
the extracted vapor concentration (G) will drop to half of the initial
value because the soil concentration dropped to half of the initial
value.

  (0.35)(0.45) + (1.6)(2.6) + (0.35)(1 45%)−  
 

250 =   0.23 0.23  × G
 1.8  


So, G = 47.5 × (250/500) = 23.75 mg/L = 23,750 mg/m = 7.400 ppmV
3

Discussion:
1. The equilibrium relationships used in this book are linear (i.e., G =
HC and S = K C), so that the relationship between X and G is also
p
linear.
2. Mass-transfer limitations may play a more important role when the
soil concentration becomes lower, which may make the extracted
vapor concentration smaller than the calculated value.