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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