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

Although it may appear environmentally irrelevant to introduce Z P, there are
situations in which it is used. If there is a spill of PCB or an oil into water of
sufficient quantity that the solubility is exceeded, at least locally, the environmental
partitioning calculations may involve the use of volumes and Z values for water, air,
sediment, biota, and a separate pure solute phase. Indeed, early in the spill history,
most of the solute will be present in this phase. The difference in behavior of this
and other phases is that the pure phase fugacity (and, of course, concentration)
remains constant, and as the chemical migrates out of the pure phase, the phase
volume decreases until it becomes zero at total dissolution or evaporation. In the
case of other phases, the concentration changes at approximately constant volume
as a result of migration.
It can be useful to compare a set of calculated Z values with Z P to gain an
impression of the degree of nonideality in each phase. Rarely does a Z value of a
chemical in a medium exceed Z P, but they may be equal when ideality applies and
activity coefficients are close to 1.0.
5.4.8 Partitioning to Interfaces
Chemicals tend to adsorb from air or water to the surface of solids. An extensive
literature exists on this subject as reviewed in texts in chemistry and environmental
processes. A good review with environmental applications is given by Valsaraj (1995)
in which the fundamental Gibbs equation is developed into the commonly used
adsorption isotherms. These isotherms relate concentration at the surface to concentration
in the bulk phase. Examples are the Langmuir, BET, and Freudlich isotherms.
Generally, a linear isotherm applies at low concentrations as are usually encountered
in the environment in relatively uncontaminated situations. Nonlinear behavior
occurs at high concentrations in badly contaminated systems and in process equipment
such as carbon adsorption units.
It is often not realized that partitioning also occurs at the air-water interface,
where an excess concentration may exist. This is exploited in the solvent sublation
process for removing solutes from water using fine bubbles.
If an area of the surface is known, a surface concentration in units of mol/m 2
can be calculated, but more commonly the concentration is given in mol/mass of
sorbent, which is essentially the product of the surface concentration and a specific
area expressed in m 2 surface per unit mass of sorbent. Solids such as activated carbon
have very high specific areas and are thus effective sorbents. Partitioning to the airwater
interface can become very important when the area of that interface is large
compared to the associated volume of air or water. This occurs in fog droplets and
snow where the ratio of area to water volume is very large, or in fine bubbles where
the ratio of area to air volume is large. These ratios are (6/diameter) m 2 /m 3 .
A Z value can be defined on an area basis (mol/m 2 Pa) or for the bulk phase by
including the specific area.
Schwarzenbach et al. (1993) have reviewed mechanisms of sorption and have
summarized reported data. This partitioning is important for ionizing substances
but less important for nonpolar compounds, which sorb more strongly to organic
matter.
©2001 CRC Press LLC