Applications of Equilibrium Thermodynamics - Elements of Environmental Engineering: Thermodynamics and Kinetics - page 119

4 Applications

of Equilibrium
Thermodynamics

Chemical thermodynamics has numerous applications in environmental engineering.
We will discuss specific examples that are relevant in understanding (a) the fate, trans-
port, and transformations of chemicals in air, water, sediment, and soil environments,
and (b) the design of waste treatment and control operations. The discussion will
primarily involve the equilibrium partitioning of chemicals between different phases.
In general, the distribution of species i between any two phases A and B at a given
temperature is given by

C i A
C i B ,

=

K AB (T) (4.1)

where C i B and C i A are concentrations of component i in phases B andA, respectively,
at equilibrium.

4.1 AIR-WATER PHASE EQUILIBRIUM
In a number of situations in the environment, the two phases air and water co-exist.
The largest area of contact is between these two phases in the natural environment.
Exchange of mass and heat across this interface is extensive. In some cases one
phase will be dispersed in the other. A large number of waste treatment operations
involve comminuting either of the phases to maximize the surface area and mass
transfer. Firstly, we shall discuss the area of fate and transport modeling (chemo-
dynamics). Secondly, we will discuss an area within the realm of separation processes
for waste treatment.
Consider the exchange of a compound at the air-water interface (Figure 4.1).
Compounds transfer between air and water through volatilization and absorption.
Gas bubbles transport materials from the ocean floor to the atmosphere. Ejections
of particulates attached to the bubbles occur upon bubble bursting at the interface.
Dissolved compounds and particulates in air can be deposited on sea or land via
attachment or dissolution in fog, mist, and rain. As discussed in Chapter 2, Henry's
law describes the equilibrium at this interface. The air-water equilibrium constant
K aw at any temperature is given by

C i a
C i w .

K aw (T) = (4.2)

119