Biogeochemical Cycles - Coastal Lagoons: Ecosystem Processes and Modeling for Sustainable Use and Development - page 158

The concentrations of pollutants in other environmental compartments can be
estimated by applying appropriate partition coefficients to the water concentra-
tions. However, reliable adsorption isotherms and equilibrium constants for ion
exchange are available only for a very limited number of chemicals and sorbents
of importance for environmental modeling. Therefore, scientifically defensible
estimation methods are needed. To a certain extent, it is possible to predict the
adsorption behavior of a particular sorbate-sorbent combination. The solubility of
the dissolved chemical is by far the most significant factor in determining the
intensity of the driving forces. The greater the affinity of the chemical compound
for the solvent the less likely it is to move toward an interface to be adsorbed. For
an aqueous solution this means that the more hydrophilic the chemical is, the less
likely it is to be adsorbed. Conversely, hydrophobic substances will easily be
adsorbed from aqueous solutions. Many organic compounds, e.g., sulfonated ali-
cyclic benzenes, have a molecular structure consisting of both hydrophilic and
hydrophobic groups. The hydrophobic parts will be adsorbed at the surface and
the hydrophilic parts will tend to stay in the water phase. Hydrophobic organic
chemicals, such as DDE and PCB, strongly sorb to solids and tend to concentrate
in the bottom sediment. 144
The value of the partition coefficient is dependent on numerous factors in
addition to the fraction of organic carbon of the sorbing particles. Of these, perhaps
the most potentially significant and controversial is the effect of particle (suspended
solids) concentration. Based on empirical evidence, the partition coefficient appears
to be inversely related to the (suspended) solids concentration. The partition coeffi-
cient can be calculated by

K p = f oc ⋅ K oc (4.38)

where
K p

= sediment/water partition coefficient suitable for natural waters

f oc = the decimal fraction of organic carbon present in the particulate matter

[mass/mass]

K oc = organic carbon-normalized partition coefficient [(mg chemical/kg organic

carbon)/(mg chemical/l of H 2 O)]

Once an estimate of K oc is obtained, the calculation of K p is straightforward.
For a wide variety of organic chemicals, the octanol/water partition coefficient
K ow is a good estimator of the organic carbon-normalized partition coefficient K oc .
Octanol was chosen as a reference because it is a model solvent with properties that
make it similar to organic matter and lipids in nature.
The octanol/water partition coefficient K ow is related to the solubility of a chem-
ical in water. 137 The less soluble a chemical is in water, the higher is its octanol/water
partition coefficient (log K ow ), and the more likely it is to sorb to the surfaces of
sediment or suspended particles. 119 K ow values for a few organic compounds are
reported by Schnoor. 131 If the octanol/water partition coefficient cannot be reliably
measured, or is not available in databases, it can be estimated from solubility and