Concepts from Chemical Reaction Kinetics - Elements of Environmental Engineering: Thermodynamics and Kinetics - page 189

5 Concepts from Chemical

Reaction Kinetics

Environmental systems are dynamic in nature. Changes with time are important in
understanding the F&T of chemicals and also in process design for waste treatment.
The time scales of change in natural systems (weathering of rocks, atmospheric
reactions, biological, or thermal-induced reactions) range from a few femtoseconds
(10 − 15 s) to as large as billions of years. Thermodynamics does not deal with
the questions regarding time-varying properties in environmental systems. Chem-
ical kinetics plays the key role in determining the time-dependent behavior of
environmental systems.
Chemical kinetics is the study of changes in chemical properties with time due to
reaction in a system . For example, principles from chemical kinetics can be used to
answer the following questions:

1. At what rate does a pollutant disappear from or transform in an environmental
compartment or a waste treatment system?
2. What is the concentration of the pollutant in a given compartment at a given
time?
3. At what rate will a chemical move between different environmental compart-
ments, and how fast will it be transferred between phases in a waste treatment
system?

To answer questions (1) and (2), rates and mechanisms of reactions are needed. How-
ever, question (3) requires knowledge of kinetic data as well as momentum and mass
transfer information between different phases.
In this chapter, we will discuss concepts from chemical kinetics that form the basis
for the discussion of rates and mechanisms in environmental engineering. Chemical
kinetics covers a broad range of topics.At its simplest level, it involves empirical stud-
ies of the effects of variables (temperature, concentration, and pressure) on various
reactions in the environment. At a slightly advanced level, it involves elucidation of
reaction mechanisms.At its most advanced level, it involves the use of powerful tools
from statistical and quantum mechanics to understand the molecular rearrangements
accompanying a chemical reaction. We shall not deal with the last issue since it falls
beyond the scope of this topic.Ingeneral,theapplicationsofchemicalkineticsinenvi-
ronmental engineering are limited to the following: (i) experimentally establishing the
relationship between concentration, temperature, and pressure in chemical reactions;
(ii) using the empirical laws to arrive at the reaction mechanism; and (iii) using the
rate data in models for predicting the F&T of pollutants in the natural environment,
and in process models for designing waste treatment systems.

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