McKay, Donald. "Front matter" Multimedia Environmental Models ...

Another important classification of organic chemicals is according to their dissociating
tendencies in water solution. Some organic acids, notably the phenols, will
form ionic species (phenolates) at high pH. The tendency to ionize is characterized
by the acid dissociation constant KA,
often expressed as pKA,
its negative base ten
logarithm.
In concert with partitioning characteristics, the other set of properties that determine
environmental behavior is reactivity or persistence, usually expressed as a halflife.
It is misleading to assign a single number to a half-life, because it depends on
the intrinsic properties of the chemical and on the nature of the environment. Factors
such as sunlight intensity, hydroxyl radical concentration, the nature of the microbial
community, as well as temperature vary considerably from place to place and time
to time. Here, we use a semiquantitative classification of half-lives into classes,
assuming that average environmental conditions apply. Different classes are defined
for air, water, soils, and sediments. The classification is that used in a series of
“Illustrated Handbooks” by Mackay, Shiu, and Ma is shown below in Table 3.4.
Table 3.4 Classes of Chemical Half-Life or Persistence, Adapted from
the Handbooks of Mackay et al., 2000
The half-lives are on a logarithmic scale with a factor of approximately 3 between
adjacent classes. It is probably misleading to divide the classes into finer groupings;
indeed, a single chemical may experience half-lives ranging over three classes,
depending on environmental conditions such as season.
We examine, in the following sections, a number of classes of compounds that
are of concern environmentally. In doing so, we note their partitioning and persistence
properties. The structures of many of these chemicals are given in Figure 3.1.
Table 3.5 gives suggested values of these properties for selected chemicals.
Figure 3.2 is a plot of log KAW
versus log KOW
for the chemicals in Table 3.5
on which lines of constant KOA
lie on the 45° diagonal. This graph shows the wide
variation in properties. Volatile compounds tend to lie to the upper left, water-soluble
compounds to the lower left, and hydrophobic compounds to the lower right. Assuming
reasonable relative volumes of air (650,000), water (1300), and octanol (1), the
percentages in each phase at equilibrium can be calculated. The lines of constant
percentages are also shown. Lee and Mackay (1995) have used equilateral triangular
diagrams to display the variation in partitioning properties in a format similar to
that of Figure 3.2.
©2001 CRC Press LLC
Class Mean half–life (hours) Range (hours)
1 5 30,000