Page 73 - Partition & Adsorption of Organic Contaminants in Environmental Systems

64 CONTAMINANT PARTITION AND BIOCONCENTRATION

(5.6)
p X = logK X - logK R
where K R is the partition coefficient of the parent solute (molecule) and
K X is that of the substituted solute with a given solvent–water system. For
substituted aromatic solutes, benzene is commonly chosen as the parent
(or reference) compound.
Since p X is a derived quantity, the factors that affect it must be contained
in the expression for partition coefficient, as illustrated with K ow and K hw .In
the absence of a specific solute interaction with a solvent or the dissociation
of the solute, the p X in octanol–water and heptane–water systems is accounted
for by Chiou et al. (1982a) by substituting Eqs. (5.1) and (5.4) into (5.6) to
give, respectively,

p X (oct-water) log[(S w ) R (S w ) X ] - log[(g* o ) X (g* o ) R ] (5.7)

and

p X(hep-water) log[(S w) R (S w) X] - log[(g h) X (g h) R] (5.8)

where the log(g w/g* w) term has been neglected because its contribution is small
for most solutes in these two systems. If one designates the first term on the
right-hand sides of Eqs. (5.7) and (5.8) as

(5.9)
log[(S w) R (S w) X] = log[(g w) X (g w) R] =D X
in which D X indicates the change in solute solubility or activity coefficient in
water when substituent X is incorporated into the parent molecule, one
obtains

p X(oct-water) D X - log[(g* o) X (g* o) R] (5.10)

and

p X (hep-water) D X - log[(g h ) X (g h ) R ] (5.11)

By Eqs. (5.10) and (5.11), the substituent contribution to solute partition co-
efficient can be estimated in terms of the water solubilities of parent and sub-
stituted molecules and their compatibilities with a specific solvent.
The p X (oct-water) and p X (hep-water) values for solutes calculated from
their K ow and K hw data and the respective D X values from their liquid S w or
g w values using benzene as the parent molecule are presented in Table 5.3. A
plot of p X (oct-water) versus D X is given in Figure 5.3, and a similar plot of
p X (hep-water) versus D X is given in Figure 5.4. As noted, the p X (oct-water)
values approach the D X values for all nonpolar substituents (e.g., alkyl and