Page 62 - Materials Chemistry, Second Edition

Site Assessment and Remedial Investigation 45

= (22)(0.535) = 11.78 mm-Hg
= 0.0155 atm = 15,500 ppmV

The partial pressure of ethyl benzene in the pore space can also be esti-
mated from Equation (2.18)
= (7)(1 − 0.535) = 3.25 mm-Hg
= 0.0043 atm = 4,300 ppmV
Discussion:
The vapor concentrations are those in equilibrium with the solvent.
The equilibrium can occur in a confined space or a stagnant phase.
If the system is not totally confined, the vapor tends to move away
from the source and creates a concentration gradient (the vapor con-
centration decreases with the distance from the solvent). However,
in the vicinity of the solvent, the vapor concentration would be at or
near the equilibrium value.

2.4.2 Liquid–Vapor Equilibrium
The compound in the void space of the vadose zone may enter the soil mois-
ture via dissolution or absorption. Equilibrium conditions exist when the
rate of the compound entering the soil moisture equals the rate of compound
volatilizing from the soil moisture.
Henry’s Coefficient. Henry’s law is used to describe the equilibrium rela-
tionship between the liquid concentration and the vapor concentration. At
equilibrium, the partial pressure of a gas above a liquid is proportional to the
concentration of the chemical in the liquid. Henry’s law can be expressed as

A
A P = H C A (2.19)
where
P = partial pressure of compound A in the vapor phase
A
H = Henry’s constant of compound A
A
C = concentration of compound A in the liquid phase
A
This equation shows a linear relationship between the liquid and vapor
concentrations. The higher the liquid concentration is, the higher the vapor
concentration will be. It should be noted that in some air pollution books or
references, Henry’s law is written as C = H P . This Henry’s constant is the
A
A A
inverse of the one used in this book and most of the site-remediation articles.
Henry’s law can also be expressed in the following form:
G = HC (2.20)

where C is the COC concentration in the liquid phase and G is the corre-
sponding concentration in the gas phase.