Applications of Chemical Kinetics in Environmental Systems - Elements of Environmental Engineering: Thermodynamics and Kinetics - page 290

and kinetics in such a porous bed medium can be modeled as a catalytic reactor,
with the aim of obtaining the degree of conversion at any defined position within the
reactor. The analysis can be used to model a waste treatment unit operation such as
activated carbon, ion-exchange, or other physicochemical treatment processes. It can
also be applied to the modeling of F&T of pollutants in a porous medium such as a
soil or sediment.

6.2 THE WATER ENVIRONMENT
In this section, we will discuss applications of chemical kinetics and reactor models in
the water environment. The first part of the discussion will be illustrations of F&T in
the natural environment. The second part will be examples in water pollution control
and treatment.

6.2.1 F ATE AND T RANSPORT

6.2.1.1 Chemicals in Lakes and Oceans

A number of anthropogenic chemicals have been introduced into our lakes, rivers,
and oceans. To elucidate the impact of these chemicals on marine species, birds,
mammals, and humans, we need a clear understanding of their F&T in the water
environment.A chemical introduced into a lake, for example, is subjected to a variety
of transport, transformation, and mixing processes. There are basically two types of
processes: (i) transport processes such as advection, dispersion, and diffusion within
and between compartments (water, sediment, and air) and (ii) reaction processes
that chemically transform compounds via photolysis, microbiological reactions, and
chemical reactions. These processes do not occur independent of one another, and
in many cases are influenced by one another. Figure 6.14 is an illustration of these
processes (Schwarzenbach et al., 1998).
Atitssimplest,amodelwillconsistofaone-dimensionalverticaltransportbetween
the sediment, water, and air.The horizontal variations in concentrations are neglected.
For a deep lake this is a good approximation, whereas for shallow water bodies the
approximation fails. One can increase the spatial resolution of the model to obtain
more sophisticated models.
To obtain the time constants for the various processes involved in the model, let us
first construct a zeroth-order chemodynamic model for the lake. In a lake we have an
epilimnion and a hypolimnion. To a good first approximation, we shall consider the
epilimnion as a well-mixed CSTR (Figure 6.15). The various processes that occur can
each be assumed to be a first-order loss process.A material balance for the compound
A can be written as

input = output + reaction + accumulation.

V d C A Q 0 C A0 = Q 0 C A + r tot V +

d t ,

(6.71)

where C A is the total concentration of compound in the epilimnion and V is the
volume of the epilimnion. Note that Q 0 /V = k F , the rate constant for flushing, and