Applications of Chemical Kinetics in Environmental Systems - Elements of Environmental Engineering: Thermodynamics and Kinetics - page 313

where k r is a pseudo-first-order rate constant. Zepp (1992) has summarized how each
of the terms comprising k r can be obtained experimentally.

6.3 THE AIR ENVIRONMENT
In this section, the use of concepts from chemical kinetics and mass transfer theory
for modeling the F&T of chemicals in the air environment and in the design of air
pollution control devices is addressed.

6.3.1 F&T M ODELS

6.3.1.1 Box Models

It is possible to explore environmental systems through the use of box models . These
“models” simulate the complex behavior of a natural system by applying useful sim-
plifications. For example, box models are derived from the concept of CSTRs. Each
phase is considered to be a well-mixed compartment and pollutants exchange between
the different compartments.
CSTRboxmodelsareusedinunderstandingthetransportofchemicalsfrommobile
and stationary sources (automobiles, chemical, and coal-powered power plants) to
the atmosphere, transport and fate of pollutants between air and water, sediment and
water, and between soil and air. The sediment-water system is pictorially represented
in Figure 6.25a. Chemicals enter the water stream either with the inflow, or by wet
and dry deposition from the atmosphere.A third pathway is the resuspension of solids

We t
deposition

(a) (b)

Solar flux

Evaporation
volatilization

Exchange with
upper atmosphere

Dry
deposition

Water
level
change

Air

Lower
atmosphere

Current Wind

current

Water

Euent

Reactions
(chemical/
photo)

Influent

Euent

Influent

Reactions

Sedimentation
Dissolution

Surface
sources

Deposition

Soil/sediment Soil

FIGURE 6.25 Schematic of typical “box” models for (a) a natural aquatic stream, and (b)
atmosphere. The various transport and fate processes in each case are shown.