Geology Reference
In-Depth Information
distribution ratio (K
d
), describe partitioning between a heterogeneous
solid phase (soil) and soil-water (containing a variety of dissolved solids
and organic material). Furthermore, a distribution ratio accounts for all
the speciated forms of a particular chemical, as well as the various
different sorption mechanisms and correctly describes the distribution of
a chemical rather than a particular partitioning process. An example of a
useful partitioning coefficient applied to soils and sediments is the
organic-carbon to water partitioning coefficient (K
oc
), which specifically
describes chemical partitioning between the organic-carbon fraction of a
soil or sediment and water and is frequently used to describe pesticide
fate in agricultural soils.
6.4.1 Important Partitioning Coefficients
There are several important partitioning coefficients, besides the funda-
mental properties of vapour pressure and aqueous solubility, which are
essential for understanding chemical transfer in the environment and
these are illustrated in Figure 2 and include the air/water (K
aw
), n-octanol/
water (K
ow
)andn-octanol/air (K
oa
) partitioning coefficients. Indeed,
chemical vapour pressure (the 'solubility' of a chemical in air) and
aqueous solubility explain the 'partitioning' of a chemical from the
pure-liquid or solid-state into air and water, respectively. Transfer of
either vapour or dissolved chemical to other environmental compart-
ments, media or phases can be explained by specific partition coefficients.
Chemical
P
S
AIR
WATER
Kaw
Koa
Kow
ORGANIC PHASE
'Octanol like'
e.g. fats/lipids/waxes
organic carbon in soil
Figure 2 Key physical-chemical properties that control the environmental partitioning of
an organic chemical. The fundamental properties of vapour pressure (P) and
aqueous solubility (S) determine the partitioning from the pure liquid or pure
solid-state of the chemical into air and water, respectively
