Geoscience Reference
In-Depth Information
Fig. 7.1 Isothermal
evaporation (25 C) of
natural water leading to
CaCO
3
precipitation 2[Ca
2+
]
[ [Alk] (with pCO
2
= 10
-3.5
atm). The concentration
factor is the degree by which
the water has been reduced by
volume compared to the
initial solution (Stumm and
Morgan
1996
)
initial composition
Alk (HCO
3
)
y
Ca
2+
9
0
0.2
0.4
0.6
pH
precip.
CaCO
3
meq liter
-1
Ca
2+
10
-2
8.5
10
-3
8
Alk
10
-4
pH
7.5
10
-5
CO
2-
3
12
5 0 0
CONCENTRATION FACTOR
50
100
phase, which may be considered a kind of compound ''solubility'' in the atmo-
sphere. Partitioning between the liquid and the gas phases is described by Henry's
law and is expressed as
½A
ð
aq
Þ
P
A
¼ K
H
ð
7
:
5
Þ
where [A
(aq)
] is the concentration of contaminant A in the aqueous solution, p
A
is
the vapor pressure of A (atm), and K
H
(sometimes also denoted H) is the Henry's
constant. The physicochemical significance of Henry's law is reflected by the fact
that there is a linear relationship between the activity of a volatile species in the
liquid phase and its activity in the gas phase. Table
7.1
shows the vapor pressure
and Henry's constant for selected organic compounds at 25 C.
The rate of mass transfer of a substance across a water-gas boundary is con-
trolled by the diffusion film model as well. Gas transfer from a water source is
faster than from a solid source, and the chemical does not undergo a chemical
reaction during the transfer process. Under these conditions, the interface con-
centration may be interpreted in terms of the Henry's constant (K
H
) which
indicates whether the controlling resistance is in the liquid or the gas film. When
K
H
\5, a water film is the controlling factor, while a gas film controls the behavior
when K
H
[ 500.
7.3 Vapor Pressure-Volatilization Relationship
If a chemical is placed in an empty vessel that is greater in volume than the chemical
itself, a portion of the chemical volatilizes to fill the remaining free space of the
vessel with vapors. The pressure in the vessel at equilibrium is affected only by the
