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

©2001 CRC Press LLC
7.1 INTRODUCTION
CHAPTER 7
Intermedia Transport
The Level II calculations described in Chapter 6 contain the major weakness
that they assume environmental media to be in equilibrium. This is rarely the case
in the real environment; therefore, the use of a common fugacity (or concentrations
related by equilibrium partition coefficients) is usually, but not always, invalid.
Reasons for this are best illustrated by an example.
Suppose we have air and water media as illustrated in Figure 7.1, with emissions
of 100 mol/h of benzene into the water. There is only slow reaction in the water
(say, 20 mol/h), but there is rapid reaction (say, 80 mol/h) in the air. This implies
that benzene is evaporating from water to air at a rate of 80 mol/h. The question
arises: is benzene capable of evaporating at 80 mol/h, or will there be a resistance
to transfer that prevents evaporation at this rate? If only 40 mol/h could evaporate,
the evaporated benzene may react in the air phase at 40 mol/h, but it will tend to
build up in the water phase to a higher concentration and fugacity until the rate of
reaction in the water increases to 60 mol/h. The benzene fugacity in the air will thus
be lower than the fugacity in water, and a nonequilibrium situation will have developed.
The ability to calculate how fast chemicals can migrate from one phase to
another is the challenging task of this chapter. The topic is one in which there still
remain considerable uncertainty and scope for scientific investigation and innovation.
We begin it by listing and categorizing all the transport processes that are likely to
occur.
7.2 DIFFUSIVE AND NONDIFFUSIVE PROCESSES
7.2.1 Nondiffusive Processes
The first group of processes consists of nondiffusive, or piggyback, or advective
processes. A chemical may move from one phase to another by piggybacking on