Page 63 - Partition & Adsorption of Organic Contaminants in Environmental Systems

54 CONTAMINANT PARTITION AND BIOCONCENTRATION

ceptually analogous to that of a solvent–water partition process. Data from
laboratory and field studies on contaminant bioconcentration into fish and
their correlations with solvent–water partition coefficients are presented later
in this chapter. It is helpful to begin with an appreciation of the general and
specific features of the solute partitioning in relation to solute and solvent
properties.

5.2 OCTANOL–WATER SYSTEMS

Among current studies of the partition effects of nonionic organic compounds
in various solvent–water mixtures, the partition coefficients in octanol–water
mixtures have received the utmost attention because of the observed correla-
tions between the octanol–water partition coefficients and the partition effects
with natural organic substances and biological components. Part of the reasons
for the success of n-octanol as a surrogate for natural organic matter and/or
biological components has to do with the polar-to-nonpolar balance of the
molecule through its hydrophilic OH and lipophilic alkyl chain that mimics to
some extent the overall polarity of the natural organic matter and of biologi-
cal materials. Here the term polarity is used to refer in a general sense to the
ability of molecules to engage in hydrogen bonding and/or polar interactions
as opposed to nonspecific dispersion (i.e., induced dipole–induced dipole)
interactions. From the solubility-model standpoint, the octanol–water system
is the one in which the partition behavior of most organic solutes with the
solvent (octanol) follow closely the criterion of Raoult’s law [Eqs. (3.10) and
(3.11)] because the molecular-size disparity between the solute and octanol is
generally not very significant.
To elucidate the relative effects of the factors that affect the octanol–water
partition coefficient (K ow), we recall Eq. (3.11), with changes in the subscripts
for the related parameters, for solutes at low concentrations in both octanol
and water phases, as follows:
*
logK ow =- logS w - logV o - logg * o - log( g w g * w) (5.1)

in which S w, g w, and g* w are as defined earlier, V * o is the molar volume of the
water-saturated octanol, and g* o is the solute activity coefficient (Raoult’s law)
in the water-saturated octanol. Equation (5.1) was derived by Chiou et al.
(1982b) on the assumption that the molar volume of octanol–saturated water
w ) is the same as the molar volume of pure water (V
(V * o = 0.018L/mol),because
the solubility of octanol in water at room temperature is relatively small,
-3
4.5 ¥ 10 M. The solubility of water in octanol is 2.3M, and thus V * o is com-
puted as 0.12L/mol (instead of V o = 0.157L/mol for pure octanol) on the basis
of volume additivity based on component mole fractions. The S w value for a
solid compound is that of the corresponding supercooled liquid at room tem-
perature (25°C), as defined earlier by Eq. (3.9). A list of the measured