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

©2001 CRC Press LLC
5.1 INTRODUCTION
5.1.1 The Nature of Partitioning Phenomena
CHAPTER 5
Phase Equilibrium
There are two distinct tasks that must be addressed when predicting equilibrium
partitioning of chemicals in the environment. First, we must fully understand how
chemicals behave under ideal, laboratory conditions of controlled temperature and
well defined, pure phases. This is the task of physical chemistry. Second is the
translation of these partitioning data into the more complex and less defined conditions
of the environment where phases vary in composition and properties.
In both cases, we are concerned with the equilibrium distribution of a chemical
between phases as illustrated in the simple two-compartment system of Figure 5.1.
A small volume of nonaqueous phase (e.g., a particle of organic or mineral matter,
a fish, or an air bubble) is introduced into water that contains a dissolved chemical
such as benzene. There is a tendency for some of the benzene to migrate into this
new phase and establish a concentration that is some multiple of that in the water.
In the case of organic particles, the multiple may be 100 or, if the phase is air, the
multiple may be only 0.2. Equilibrium becomes established in hours or days between
the benzene dissolved in the water and the benzene in, or on, the nonaqueous phase.
Analytical measurements may give the total or average concentration that includes
the nonaqueous phase and may differ considerably from the actual dissolved water
concentration. The phase may subsequently settle to the lake bottom or rise to the
surface, conveying benzene with it. Clearly, it is essential to establish the capability
of calculating these concentrations and thus the fractions of the total amount of
benzene that remain in the water, and enter the second phase. In some cases, 95%
of the benzene may migrate into the phase, and in others only 5%. These systems
will behave quite differently.
The aim is to answer the question, “Given a concentration in one phase, what
will be the concentration in another phase that has been in contact with it long
enough to achieve equilibrium?” This task is part of the science of thermodynamics