Geology Reference
In-Depth Information
6.4.1.1 Octanol-Water Partitioning Coefficient (K
ow
). Octanol serves
as a very useful surrogate of fats, lipids and organic phases present in the
environment. This is largely attributable to octanol having a similar
carbon to oxygen ratio (8:1) as fatty acids and lipids. As a result K
ow
is
used to represent chemical partitioning between lipids and water (where
lipids and fats can be viewed as 'octanol-like solvents') and is an
important chemical property used in the fields of biology and pharmacy,
where K
ow
is represented by the term 'P' for partitioning coefficient. In
essence, K
ow
is the ratio of the equilibrium concentration of a chemical
between octanol (mol m
3
) and water (mol m
3
) and is a measure of a
chemical's 'water hating' tendency or hydrophobicity. As Mackay
14
points out, most organic chemicals have a fairly constant solubility in
octanol (200-2000 mol m
3
) but very different solubility in water and it
may be misleading to describe K
ow
as a chemical's 'love of fats' or
lipophilicity. As the range of K
ow
for environmental contaminants is
large (over orders of magnitude), log K
ow
values are often reported.
Perhaps the most common application of K
ow
is for predicting the
bioconcentration of low-polarity hydrophobic contaminants from water
into aquatic biota. A bioconcentration factor (BCF) can be represented
as
BCF
¼
C
org
C
w
ð
6
:
3
Þ
where C
org
is the actual concentration determined in an organism (e.g.
fish), or some 'compartment' of the organism (e.g. lipids), and C
w
is the
truly dissolved concentration of the chemical in water. From empirical
studies that have examined a range of different organisms and chemi-
cals, BCF may be predicted from simple linear relationships with K
ow
.
For example, Mackay
15
described the BCF in fish, by examining hun-
dreds of organic chemicals, as
BCF
¼
0
:
048 K
ow
ð
6
:
4
Þ
The constant 0.048 representing the
5% lipid content of fish, or as
Mackay describes, about 5% octanol by volume. However, relationships
between BCF and K
ow
appear to break down for chemicals with log
K
ow
47, as these 'superhydrophobic' substances may have difficulty
crossing cellular membranes (i.e. they have a low bioavailability) or
are rapidly egested from the organism. There is, in fact, little consensus
on the reasons for the breakdown in the linear relationship between
BCF and chemicals with log K
ow
47, but both Borga˚ et al.
16
and
Boethling et al.
17
point out that the experimental artefacts associated
E
