Page 125 - Industrial Wastewater Treatment, Recycling and Reuse

Advanced Physico-chemical Methods of Treatment for Industrial Wastewaters 99

highlighted further research in the area of inorganic adsorbents and nanoma-
terial adsorbents in view of limited information on applications in industry
wastewater treatment.

2.3.2 Equilibria and Kinetics of Adsorption
The fundamental principles of adsorption are well known and have been well
reported in standard textbooks (Ruthven, 1984). Therefore, only information
pertaining to wastewater treatment is discussed here. The design of the adsorp-
tion process in wastewater treatment involves understanding of sorption
equilibria and kinetics apart from other aspects for commercial viability.
The equilibrium capacity gives the maximum theoretical capacity that can
be obtained, while in real-world operations, more useful terminologies such
as operating capacity and breakthrough capacity are commonly used. The
mathematical models for equilibrium and kinetics are most useful for design-
ing wastewater systems, and it is convenient to have the maximum possible
information through appropriate models for accurate design, scale-up, and
physical understanding of the phenomenon so that it can be extended to
similar systems.

2.3.2.1 Adsorption Isotherm
Adsorption isotherm is in simple terms a mathematical expression of equi-
librium adsorbate loading on adsorbent as a function of concentration at
constant temperature. In wastewater treatment, a highly favorable isotherm
is generally not preferred because it adversely affects regeneration. Isotherm
considerations in single use adsorbent not requiring regeneration are how-
ever, totally different requiring irreversible isotherm.
The adsorption isotherms can be classified mainly from Type I to Type
V, depending on the nature of sorption curve (IUPAC recommendation).
Different equations are available to describe adsorption isotherms, and some
common forms are listed in Table 2.2.
The Langmuir model is one of the best known, theoretically understood,
and widely applied models. Developed by Langmuir in 1916 (Langmuir,

Table 2.2 Commonly used forms of adsorption isotherm
Langmuir Freundlich
C e 1 C e 1/n
¼ + q e ¼KC e
q e a L b L a L
Dubinin-Radushkevich Redlich-Peterson
h i 2 ½ ð
1
e
ln q ¼ ln q B D RT ln 1 + C e ln K R C e =q Þ 1Š ¼ ln a R blnC e
e
D