Geology Reference
In-Depth Information
with measuring the freely dissolved chemical are likely to provide large
uncertainties in the BCFs. Furthermore, bioconcentration is unlikely to
be a significant exposure pathway for these 'superhydrophobic' chem-
icals relative to dietary intake.
K
ow
is also used to describe partitioning to general organic phases
within the environment and K
oc
(introduced above) can be directly
estimated from K
ow
according to the empirically derived relationship
derived initially by Karickhoff
18
and refined by Seth et al.
19
where
K
oc
¼
0
:
35 K
ow
ð
6
:
5
Þ
this relationship is useful for calculating the sorption of chemicals to
soils and sediments from water; specifically to the organic carbon
fraction. It is worth noting that K
oc
represents a solid/liquid partition
coefficient and as such has units of L kg
1
, whereby K
oc
can be
represented as
K
oc
¼
C
oc
ð
moles
=
kg
Þ
C
w
ð
moles
=
L
Þ
ð
6
:
6
Þ
where C
oc
is the chemical concentration sorbed to the natural organic
carbon fraction in soil (
B
0.02 or 2%) and C
w
is the dissolved water
concentration. As K
ow
is dimensionless, the constant in Equation 6.5 has
units of L kg
1
. Experimentally, K
oc
is determined by introducing (or
'spiking') a chemical into a water/soil mixture (slurry) that is stirred at a
constant temperature, followed by filtration and analysis of the water at
different time intervals. The resulting K
d
value is then divided by the
organic carbon fraction (f
oc
) of the test soil, whereby
K
oc
¼
K
d
f
oc
ð
6
:
7
Þ
Multiplying K
d
(which also has units of L kg
1
) by soil density (
B
2.4 kg
L
1
) allows the dimensionless form of K
oc
to be used.
6.4.1.2 Henry's Law Constant and Air-Water Partitioning (K
aw
). Air-
water partitioning is described by the Henry's Law constant (H), which
is defined as the ratio of the partial pressure (p) of a chemical in the air to
its mole fraction (or molar concentration) dissolved in water (C
w
)at
equilibrium, according to
p
ð
Pa
Þ
C
w
ð
mol
=
m
3
Þ
H
¼
ð
6
:
8
Þ