Environmental Engineering Reference
In-Depth Information
CO
2
mass arising from the initial concentration of NaHCO
3
should equal [CO
2
]
T,w
in
the aqueous phase at equilibrium plus the [CO
2
]
g
in the gas phase. Thus,
V
w
(
1
×
10
−
3
)
=
V
w
[
CO
2
]
T,w
+
V
G
[
CO
2
]
g
,
(4.34)
CO
2
·
H
2
O
w
7.3
×
10
−
3
100
·
1.29
×
CO
2
·
H
2
w
.
1
1
×
10
−
3
=
+
(4.35)
Therefore [CO
2
·
H
2
O]
w
=
7.3
×
10
−
6
M and [CO
2
]
g
=
9.4
×
10
−
6
M.
E
XAMPLE
4.9 B
ENZENE
C
ONCENTRATION
N
EAR A
W
ASTEWATER
L
AGOON
The atmosphere above a wastewater lagoon contained 0.01
μ
g/P of benzene. The
average surface water concentration in the lagoon was measured as 10
μ
g/P. If the
temperature of air and surface water was 20
◦
C, in what direction is the flux of benzene?
C
i
=
0.01
μ
g/L,
C
i
=
10
μ
g/L.
K
aw
at 298 K is 0.23 (fromAppendix 1). Using the
temperature variation for benzene (Table 4.2), we obtain at 293 K,
K
aw
=
0.23. The
driving force for flux is therefore
(
0.01
−
0.01
×
10
−
9
/
0.23
)
=
0.01 g/cm
3
. Since the
flux is positive, we conclude that the impoundment acts as a source of benzene to the air.
4.2 AIR-WATER EQUILIBRIUM IN ATMOSPHERIC CHEMISTRY
Atmospheric moisture is present in the form of hydrometeors (
fog
,
rain
,
cloud
droplets
,
aerosols
, and
hydrosols
).An
aerosol
is a stable suspension of solid or liquid
particles or both in air, whereas a
hydrosol
is a stable suspension of particles in water.
Aerosols provide the medium for a variety of atmospheric reactions. In the atmo-
sphere, solar radiation is absorbed and scattered by both particles and gas molecules
that comprise aerosols and hydrosols. Atmospheric aerosol mass is rarely lower than
1
g/m
3
in urban
areas. The concentration of most trace gases in the atmosphere is in the vicinity of
10
3
-10
4
g/m
3
close to the earth's surface and occasionally increases to 100
μ
μ
g/m
3
. Thus, except for some rare cases (such as rural areas), the trace gases
in the atmosphere far exceed the aerosol concentrations. Aerosols, however, play a
larger role in the hydrologic cycle by providing condensation nuclei for both fog and
cloud formation. Sea spray from the ocean surface provides a rich source of liquid
droplets that upon evaporation of water form sea-salt crystals. Some of the liquid
droplets arise from condensation of organic vapors when the vapor pressure exceeds
the saturation point. An example of this type of atmospheric droplets is the aerosol
generated from incomplete combustion of wood or agricultural residues.
The primary focus of interest for us in these cases is the equilibrium between
the dispersed phase and the continuous phase that contain trace gases (organic and
inorganic). We wish to analyze the extent of removal of molecules from the gas phase
by various processes.
μ
Search WWH ::
Custom Search