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

©2001 CRC Press LLC
air Z = P S /RT, water Z = C S , sorbed Z = C S K SW
A saturated solution thus has an activity of 1.0. A zero or near-zero vapor pressure
can be used to calculate Z for air as zero or near zero.
In some cases, we may have to go farther, because we are uncertain about the
solubility C S . The simple expedient is then to multiply through by C S to give a new
equilibrium criterion A as
or, for air,
fC S /P S = A = C ARTC S /P S = C W = C S/K SW
Z = P S /RTC S , water Z = 1.0, sorbed phase Z = K SW.
We are now using the water concentration or the equivalent equilibrium water
concentration as the criterion of equilibrium. This has been termed the aquivalent
concentration (Mackay and Diamond, 1989) and can be used for metals in ionic
form when the solubility is meaningless.
The essential procedure is that, for most organic substances, Z is defined in air
as 1/RT, then all other Z values are deduced from it. In the “aquivalence” approach,
Z is arbitrarily set to 1.0 in water, and all other Z values are deduced from this basis
using partition coefficients. This approach is used in the EQC model for involatile
substances (Mackay et al., 1996).
5.6.7 Some Environmental Implications
Viewing the behavior of a solute in the environment in terms of Z introduces
new and valuable insights. A solute tends to migrate into (or stay in) the phase of
largest Z. Thus, SO 2 and phenol tend to migrate into water, freons into air, and DDT
into sediment or biota. The phenomenon of bioconcentration is merely a manifestation
of Z in biota, which is much higher (by the bioconcentration factor) than Z
in the water. Occasionally, a solute such as inorganic mercury changes its chemical
form becoming organometallic (e.g., methylmercury). Its Z values change, and the
mercury now sets out on a new environmental journey with a destination of the new
phase in which Z is now large. In the case of mercury, the ionic form will sorb to
sediments or dissolve in water but will not appreciably bioconcentrate. The organic
form experiences a large Z in biota and will bioconcentrate. The metallic form tends
to evaporate.
Some solutes, such as DDT or PCBs, have very low Z values in water because
of their highly hydrophobic nature; i.e., they exert a high fugacity even at low
concentration, reflecting a large “escaping tendency.” They will therefore migrate
readily into any neighboring phase such as sediment, biota, or the atmosphere.
Atmospheric transport should thus be no surprise, and the contamination of biota
in areas remote from sites of use is expected. With this hindsight, it is not surprising
that these substances are found in the tissues of Arctic bears and Antarctic penguins!
From the environmental monitoring and analysis viewpoint, it is preferable to
sample and analyze phases in which Z is large, because it is in these phases that