Environmental Engineering Reference
In-Depth Information
Figure 9.3
Effect of ionic strength on adsorption capacity of bentonite and CTAB-MBn
for Congo red [37].
a large number of vacant surface sites are available for adsorption and then
the remaining vacant surface sites are difficult to occupy due to repulsive
forces between the dye molecules on the bentonite surface [27].
In the literature, the pseudo-first- and pseudo-second-order kinetic mod-
els are generally used to describe adsorption rates and capacities [17]. The
pseudo-second-order model is most prominent [72,97]. Toor and Jin [25],
Vimonses
et al.
[39] and Baskaralingam
et al.
[83] have mentioned that this
model is more likely to fit adsorption behavior over the whole time range.
This model is also in agreement with the rate-controlling step being chemical
adsorption, where valence electrons are shared between dye and adsorbent.
The formula of a pseudo-second-order model [114] obtained by Toor
and Jin [25] is given below (Figure 9.4).
kq t
(+kqt)
22
(9.1)
q=
e
Pseudo-second-order:
t
1
2e
where
q
e
and
q
t
are the amounts (mg/g) of solute bound at the interface
at the equilibrium and after time
t
(min), respectively, and
k
2
is the rate
constant of the pseudo-second-order adsorption (g/mg min).
9.3.5 Adsorption Isotherms
The adsorption equilibrium isotherm is important for describing how
the adsorbate molecules partition between the liquid and the solid
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