Page 270 - Soil and water contamination, 2nd edition

14

Gas exchange

14.1 HENRY’S LAW

So far, we have considered the mass exchange within or between the liquid or dissolved

phase and the solid phase . However, gas solution, exsolution, and volatilisation can

cause significant mass exchange of atmospheric gases (O , N ), gases produced by the
2 2
decomposition of organic matter (CO , CH , NH , H S), and organic chemicals between
2 4 3 2
the liquid phase and the atmosphere. The concentration of dissolved gas in solution is
commonly modelled using Henry’s law , which relates the concentration of dissolved gas in
solution to the partial pressure of that gas in an atmosphere in contact with the solution and
is an example of a chemical equilibrium constant (see Chapter 2):
P
K (14.1)
H
C
aq
-1
where K = the Henry’s law constant usually expressed in units like atm l mol or
H
3
-1
atm m mol (note that some textbooks define the Henry’s law constant as the reciprocal
of the Henry’s law constant given here; compare Equation 2.9), P = partial pressure of the

chemical in the gas phase (atm), and C = the equilibrium molar concentration of the
aq
-1
3
chemical in solution (mol l or mol m ). The Henry’s law constant can also be expressed in
dimensionless form, by dividing the gas concentration in air by the aqueous concentration in
3
-1
-1
-3
the same units (mol l , mol m , mg l , or mg m ):
C
K' H air (14.2)
C aq
where K ' = the dimensionless form of the Henry’s law constant (-) and C = the molar
H air
-1
3

concentration of the gas in air (mol l or mol m ). The relation between partial pressure P
and the concentration in air is given by the ideal gas law :
n P
C
air
V R T
K (14.3)
K' H H
R T
where n = the number of moles of the chemical in air (mol), V = the air volume (l or
3
m ), P = the partial vapour pressure of the gas (atm), R = the gas constant (= 0.082058
-1
-1
(l atm mol K )), and T = the absolute temperature (K). The Henry’s law constant is
tabulated in many handbooks. It can also be estimated by dividing the vapour pressure of a
chemical at a given temperature by its aqueous solubility at that temperature. Table 14.1 lists
the dimensionless Henry’s law constant and vapour pressure for some selected chemicals.
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