Environmental Engineering Reference
In-Depth Information
Table 6.1
Mobility classes,
octanol-water-distribution
coefficients
K
ow
according
to Fetter (
1994
) and the
range of solubility for
organic pollutants
Mobility class
K
ow
[mL/g]
Solubility
S
[ppm]
10
3
-1.4
10
5
Very high
1-50
4.4
High
50-150
850-3,570
Moderate
150-500
110-1,100
Low
500-2,000
30-156
10
4
Slight
2,000-2
0.275-10
10
4
Immobile
>
2
<
0.252
A generalized formulation for fast sorption is the
Freundlich
2
isotherm
c
s
¼ a
F
1
c
a
F
2
(6.6)
with coefficients
a
F
2
usually is smaller than 1, which
corresponds to the observation that for low concentrations of
c
the gradient of the
isotherm is higher than for higher concentrations. For
a
F
1
and
a
F
2
. The exponent
1, the Freundlich
isotherm also describes a linear relationship between the concentrations in both
phases. The Freundlich isotherm is favoured mostly by experimental scientists, who
fit their experimental data using a power law relationship.
The third important formulation is the
Langmuir
3
isotherm that is written as:
a
F
2
¼
a
L
1
c
a
L
2
þ c
c
s
¼
(6.7)
with coefficients
a
L
2
. The Langmuir isotherm also has the property that
for low concentrations the gradient of the isotherm is higher than for high
concentrations. In contrast to the Freundlich isotherm, the Langmuir isotherm
approaches a finite asymptote for
c !1
a
L
1
and
a
L
1
. The argu-
ment for the relevance of the Langmuir isotherm is that for high concentrations the
limited number of sorption sites at the surface of the pore space is occupied, so that
no further increase of
c
s
is possible.
Figure
6.2
depicts examples of the three major isotherm types. Formulae of
further isotherms are listed in Table
6.2
.
In the soil compartment, sorption and cation exchange are closely connected; see
the paper of Johnson et al. (
1998
), which is concerned with forest eco-systems for
an example study. Cations as Ca
2+
,Na
+
,NH
4
+
,Sr
+2
,Al
+3
exchange sorption places
if the equilibrium is disturbed when water of different composition enters. This is
a typical situation for the soil column with water from precipitation or irrigation
entering.
, given by the parameter
2
Herbert Freundlich(1880-1941), German chemist.
3
Irving Langmuir (1881-1957), US-American chemist and physicist.
