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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
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