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

©2001 CRC Press LLC
3.3 KEY CHEMICAL PROPERTIES AND CLASSES
3.3.1 Key Properties
In Chapter 5, we discuss physicochemical properties in more detail and, in
Chapter 6, we examine reactivities. It is useful at this stage to introduce some of
these properties and identify how they apply to different classes of chemicals.
It transpires that we can learn a great deal about how a chemical partitions in
the environment from its behavior in an air-water-octanol (strictly 1-octanol) system
as shown later in Figure 3.2. There are three partition coefficients, KAW,
KOW,
and
KOA,
only two of which are independent, since KOA
must equal KOW/KAW.
These can
be measured directly or estimated from vapor pressure, solubility in water, and
solubility in octanol, but not all chemicals have measurable solubilities because of
miscibility. Octanol is an excellent surrogate for natural organic matter in soils and
sediments, lipids, or fats, and even plant waxes. It has approximately the same C:H:O
ratio as lipids. Correlations are thus developed between soil-water and octanol-water
partition coefficients, as discussed in more detail later.
An important attribute of organic chemicals is the degree to which they are
hydrophobic.
This implies that the chemical is sparingly soluble in, or “hates,” water
and prefers to partition into lipid, organic, or fat phases. A convenient descriptor of
this hydrophobic tendency is KOW.
A high value of perhaps one million, as applies to
DDT, implies that the chemical will achieve a concentration in an organic medium
approximately a million times that of water with which it is in contact. In reality,
most organic chemicals are approximately equally soluble in lipid or fat phases, but
they vary greatly in their solubility in water. Thus, differences in hydrophobicity are
largely due to differences of behavior in, or affinity for, the water phase, not differences
in solubility in lipids. The word lipophilic is thus unfortunate and is best avoided.
The chemical’s tendency to evaporate or partition into the atmosphere is primarily
controlled by its vapor pressure, which is essentially the maximum pressure that a
pure chemical can exert in the gas phase or atmosphere. It can be viewed as the
solubility of the chemical in the gas phase. Indeed, if the vapor pressure in units of
Pa is divided by the gas constant, temperature group RT, where R is the gas constant
(8.314 Pa m3/mol
K), and T is absolute temperature (K), then vapor pressure can
be converted into a solubility with units of mol/m3.
Organic chemicals vary enormously
in their vapor pressure and correspondingly in their boiling point. Some
(e.g., the lower alkanes) that are present in gasoline are very volatile, whereas others
(e.g., DDT) have exceedingly low vapor pressures.
Partitioning from a pure chemical phase to the atmosphere is controlled by vapor
pressure. Partitioning from aqueous solution to the atmosphere is controlled by KAW,
a joint function of vapor pressure and solubility in water. A substance may have a
high KAW,
because its solubility in water is low. Partitioning from soils and other
organic media to the atmosphere is controlled by KAO
(air/octanol), which is conventionally
reported as its reciprocal, KOA.
Partitioning from water to organic media,
including fish, is controlled by KOW.
Substances that display a significant tendency
to partition into the air phase over other phases are termed volatile organic chemicals
or VOCs. They have high vapor pressures.