Environmental Engineering Reference
In-Depth Information
Both
RCF
-regressions (Eqs.
9.5
and
9.6
), the experimental data of Briggs et al.
(
1982
) and the concentration ratio between bulk soil and soil pore water,
K
SW
,fora
typical soil (
OC
0.2 kg kg
−
1
) are plotted in Fig.
9.3
.For
low
K
OW
values
RCF
is higher than
K
SW
, due to the higher water content of roots.
For higher
K
OW
values Briggs'
RCF
regression and the
K
SW
equation (Eq.
9.8
) yield
similar results. This suggests that the sorption capacity of roots equals that of soil,
because the content of organic carbon in soil (in this case 2.5%) is similar to the
lipid content of roots (about 2
0.025 kg kg
−
1
and
W
=
=
3%, including waxes and lignin), and the slope of
the log
K
OW
in the regressions is similar (0.81 for
K
OC
in Eq.
9.7
and 0.77 for
RCF
in Eq.
9.5
).
−
9.3.4 Partition Coefficients for Stem and Leaves
Briggs et al. (
1983
) measured the sorption to macerated barley stems and pre-
dicted
K
stem/xylem sap
(L kg
−
1
), which is the concentration of contaminants in stem
tissue divided by the concentration in xylem sap, related to the log
K
OW
of the
contaminants:
8,
r
2
log(
K
stem/xylemsap
−
0.82)
=
0.95 log
K
OW
−
2.05 (
n
=
=
0.96).
(9.9)
Trapp et al. (
1994
) interpreted the regressions derived for sorption to roots and
stems as equilibrium partition coefficients between plant tissue and water,
K
PW
(L kg
−
1
), and introduced the general equation:
LaK
OW
K
PW
=
W
+
(9.10)
where
W
(L kg
−
1
) and
L
(kg kg
−
1
) are water and lipid content of the plant,
b
is
a correction factor for differences between solubility in octanol and sorption to
plant lipids (in the regressions of Briggs et al. (
1982
,
1983
)
b
was 0.77 for roots
and 0.95 for leaves), and
a
is a factor correcting density differences between water
and n-octanol (1/
1.22 L kg
−
1
, where
ρ
Octanol
is the density of octanol).
When parameterized accordingly, this equilibrium approach gives the same results
for roots as the Briggs
RCF
-regression.
Stems and leaves are in contact with air. The sorption equilibrium of contami-
nants between leaves and air can be described as follows:
ρ
Octanol
=
K
LA
=
C
L
/
C
A
=
K
LW
/
K
AW
(9.11)
where
K
LA
is the partition coefficient between leaves and air (L kg
−
1
),
K
LW
(L kg
−
1
)
is the partition coefficient between leaves and water (Eq.
9.10
) and
K
AW
(L L
−
1
)is
the partition coefficient between air and water (also known as the dimensionless
Henry's Law constant). Instead of estimating
K
LA
from
K
OW
and
K
AW
,
K
LA
was
often directly fitted to
K
OA
, i.e. the partition coefficient between octanol and air
(e.g., Kömp and McLachlan
1997
).
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