Remediation of PAH-Contaminated Soils and Sediments: A ...

Solubility
Solubility of PAH compounds in water is dependent upon temperature, pH, ionic
strength (concentration of soluble salts), and other organic chemicals (i.e.
dissolved organic carbon) (Pierzynski et al., 2000).
Solubility is estimated by:
� chemical structure
� octonol-water partition coefficients
Chemical structure: In general, as the number of benzene rings in a PAH
compound increases, solubility decreases (Wilson and Jones, 1993). There are,
of course, exceptions to the rule. Symmetry, planarity, and the presence of
substituents affect PAH solubility in organic solvents. Solubility has been found to
increase in linearly-fused PAH as the number of rings increase because the
bonds become weaker (olifinic) in character, but has not observed in angularlyfused
PAH (Harvey, 1997). Planar PAHs are less reactive (i.e. less soluble) and
biologically less toxic (Dabestani and Ivanov, 1999). Substituted PAHs are those
in which a functional group in the compound has been replaced with another
functional group. For example, in a methyl-substituted PAH, one of the functional
groups has been replaced by a univalent compound with the general formula
CH3 - .
Alternant PAH compounds that are planar and symmetrical require a relatively
high energy of solubilization because of their ability to fit closely in a lattice. Thus,
they tend to be less soluble (Harvey, 1997). As the compounds deviate from
planarity or symmetry they tend to be more soluble in organic solvents. Methyl
and polar substitution may also increase the solubility of PAHs in certain solvents
(Harvey, 1997). Most byproducts of PAH biological and chemical degradation
tend to be more polar and have higher solubility in the environment than the
parent compounds.
Octanol-water partition coefficients: There are substantial amounts of data
on the relationship between aqueous solubility and octonol-water partition
coefficients (Kow) for the partitioning of PAH between water and organic matter in
soils (Mackay and Callcott, 1998). There is an inverse relationship between Kow
and solubility which is determined with the following equation:
Kow = amount of organic chemical in octanol (mg L -1 )
amount of organic chemical in water (mg L -1 )
The octanol-water coefficient is often expressed as the log Kow. Naphthalene has
a log Kow of 3.37, while Indeno[1,2,3-cd]pyrene Kow is 6.50. In this case,
naphthalene is more soluble than Indeno[1,2,3-cd]pyrene. This is also in
agreement with the influence of chemical structure on solubility. Solubility and
Kow of select PAH are provided in Table 1.
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