Environmental Engineering Reference
In-Depth Information
mass transfer at the soil surface. The solution for the flux from the surface is (Valsaraj
et al., 1999)
]
0
exp
k
a
t
D
g
R
F
erfc
k
a
t
D
g
R
F
F
A
=
k
a
[
A
(6.216)
[A]
0
D
g
R
F
/
t
, which is
the solution to Fick's diffusion equation. Note that
F
A
is a function of both
D
g
and
R
F
, both of which depend on the soil porewater content. With increasing water in the
pore space,
D
g
and
R
F
will decrease. The decrease in
R
F
will be far more significant
since
K
SA
has been observed to be a sensitive function of porewater content (Guilhem,
1999). Experimental data have verified this prediction (Figure 6.59). Thus flux will
be higher from a wet soil or sediment and lower from a dry sediment. In other words,
pesticides and organic compounds will be far more volatile from wet than dry soils.
Note that at
t
=
0,
F
A
=
k
a
[A]
0
and as
t
→∞
,
F
A
=
π
60
250
DBF
Model (wet)
Model (dry)
PHE
Model (wet)
Model (dry)
50
200
40
150
dry air
30
humid
air
humid air
100
20
50
10
0
0
0
200
400
600
t
,
h
800
1000
1200
0
200
400
600
800
1000
1200
t
,
h
10
0.010
PYR
Model (wet)
Model (dry)
8
0.008
Water
6
0.006
4
0.004
2
0.002
0
0.000
0
200
400
600
800
1000
1200
0
200
400
600
800
1000 1200
t
,
h
t
,
h
FIGURE 6.59
Effects of change in air relative humidity on dibenzofuran, phenanthrene,
pyrene, and water flux from a 6.5% moisture University Lake sediment. Model curves for both
humid air over “wet” sediment and dry air over “dry” sediment are shown. (Data fromValsaraj
et al. 1999.
Environmental Science and Technology
33, 142-148.)
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