Bioavalibility in Soils - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 728

physical properties of the contaminants and to understand their biogeochemical fate,
requiring the need to investigate transport properties, sorption, volatilization, and
biodegradation of organic contaminants. The more of the contaminant present in
the pore water the more bioavailable it is. A popular aphorism used for predict-
ing solubility is “ like dissolves like ” (Williamson et al. 2007 ). This indicates that
a contaminant will dissolve best in a solvent of similar polarity. This is a rather
simplistic view, since it ignores many solvent-solute interactions, but it is a use-
ful rule-of-thumb. The behaviour of organic contaminants is often characterized by
octanol-water partitioning, expressed as the octanol-water partitioning coefficient
(K ow ). The larger the K ow , the stronger the contaminants are bound to the soil solid
phase and the less bioavailable they are. In general, the hydrophobicity and hence
the K ow increase with increasing number of C-atoms.
The solubility of organic contaminants nearly always increases with temperature.
The solubility equilibrium is relatively straightforward for non-ionic and non-polar
substances such as benzene. When dissolved in water, the benzene molecules remain
intact and are generally surrounded by water molecules. This behavior differs from
that of ionic contaminants or metal salts. When an ionic contaminant such as sodium
chloride (NaCl) dissolves in water, the sodium chloride lattice dissociates into indi-
vidual ions that are solvated or surrounded by water molecules, thus increasing the
ionic strength of the solution. In turn, increases of ionic strength affect the surround-
ing by water of neutral organic contaminants. Effectively, at high ionic strength
conditions, the solubility of organic contaminants and hence their bioavailability is
reduced.
Apart from temperature and ionic strength, sorption to (dissolved) organic mat-
ter is the main factor affecting bioavailable concentrations of organic contaminants.
Sorption increases with increasing organic matter content of the soil, whereas con-
centrations of organics in the pore water increase with increasing dissolved organic
carbon (DOC) concentrations. In contrast to a large number of aquatic organisms,
for which only the truly dissolved contaminant has been shown to be bioavailable,
it can, however, not be ruled out that contaminants sorbed onto dissolved organic
carbon can interact with biota. Amongst others, this is due to differences in the rates
of release of organics from dissolved organic carbon versus release kinetics from
soil solid organic material. Karickhoff et al. ( 1979 ) were one of the first authors to
show the equilibrium concept of partitioning of organic contaminants by reporting
that the contaminant-specific organic-carbon normalized partition coefficient ( K oc )
is proportional to K ow :

a ∗ (Log) K ow + (Log) K oc ≈ b (16.2)

where a and b are contaminant specific constants.
Subsequently, the K oc may be used to predict the degree of contaminant par-
titioning of hydrophobic organics between soil organic carbon and pore water:

K oc = K d / f oc , with K d = C w / C solid phase (16.3)