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T. Rasool and W. Baba
sites might be damaged which might be the reason for the decrease in adsorption
capacity. Also according to an adsorption theory by increasing the temperature the
adsorption may decrease because the ions previously adsorbed may begin to desorb
due to elevating temperatures (Ho et al. 2005).
3.7 Equilibrium Isotherm Modeling
The experimental equilibrium data was subjected to different isotherms to get a
deeper insight of the process of adsorption onto the ABC. The equilibrium data was
tested by using Langmuir (1918), Freundlich (1906) and Harkins-Jura isotherms
(1944). In order to optimize the adsorption process, the understanding of the forces
governing the interaction between the ions and adsorbent is necessary.
Langmuir adsorption model assumes that only specific homogenous sites on the
surface of an adsorbent are responsible for adsorption and intermolecular forces
depend directly on the distance from the adsorbent surface and may show a rapid
decrease with the movement of adsorbate away from adsorbent surface. Further the
model assumes that the surface on which adsorption takes place is homogenous in
structure and the sites available for adsorption are all identical in terms of energy. The
Freundlich isotherm is an empirical isotherm used to describe the heterogeneity of
system. The Harkins-Jura isotherms isotherm is employed for multilayer adsorption.
Heterogeneous pore distribution accounts for the multilayer adsorption. The details
of adsorption and kinetic models and the parameters used is compiled in Table 1.
Langmuir adsorption Isotherm can be expressed in the form of a dimensionless
constant separation factor (R s ), expressed as:
R s =
1
1 + K b C o
where C o is the maximum initial metal ion concentration (mg/l) and K b (l/mg)
is the Langmuir constant. R s provides information about the adsorption behavior
as irreversible (R s = 0); linear (R s = 1); favorable (0 < R s < 1) and unfavorable
(R s > 1) (Rashid et al. 2016; Kilic et al. 2013). The calculated isotherm parameters
for the adsorption of Zn(II) and Ni(II) using Langmuir, Freundlinch and Harkins-Jura
models are given in Table 2.
The analysis of results from the Table 2 shows that overall Langmuir adsorption
isotherm is a best fit for the experimental data of both Zn(II) and Ni(II) ions as
compared to other isotherms. Freundlich isotherm model indicates the heterogeneity of the surface and simultaneously supports the adsorption of the metal ions on
to the adsorbent. Freundlich isotherm also suggests that chemisorption is governing the adsorption process over physiorption. This indicates both physiorption and
chemisorption are playing their role in the process of adsorption for removing the
metal ions from aqueous solutions. Besides, it is not restricted to the monolayer as in
T. Rasool and W. Baba
sites might be damaged which might be the reason for the decrease in adsorption
capacity. Also according to an adsorption theory by increasing the temperature the
adsorption may decrease because the ions previously adsorbed may begin to desorb
due to elevating temperatures (Ho et al. 2005).
3.7 Equilibrium Isotherm Modeling
The experimental equilibrium data was subjected to different isotherms to get a
deeper insight of the process of adsorption onto the ABC. The equilibrium data was
tested by using Langmuir (1918), Freundlich (1906) and Harkins-Jura isotherms
(1944). In order to optimize the adsorption process, the understanding of the forces
governing the interaction between the ions and adsorbent is necessary.
Langmuir adsorption model assumes that only specific homogenous sites on the
surface of an adsorbent are responsible for adsorption and intermolecular forces
depend directly on the distance from the adsorbent surface and may show a rapid
decrease with the movement of adsorbate away from adsorbent surface. Further the
model assumes that the surface on which adsorption takes place is homogenous in
structure and the sites available for adsorption are all identical in terms of energy. The
Freundlich isotherm is an empirical isotherm used to describe the heterogeneity of
system. The Harkins-Jura isotherms isotherm is employed for multilayer adsorption.
Heterogeneous pore distribution accounts for the multilayer adsorption. The details
of adsorption and kinetic models and the parameters used is compiled in Table 1.
Langmuir adsorption Isotherm can be expressed in the form of a dimensionless
constant separation factor (R s ), expressed as:
R s =
1
1 + K b C o
where C o is the maximum initial metal ion concentration (mg/l) and K b (l/mg)
is the Langmuir constant. R s provides information about the adsorption behavior
as irreversible (R s = 0); linear (R s = 1); favorable (0 < R s < 1) and unfavorable
(R s > 1) (Rashid et al. 2016; Kilic et al. 2013). The calculated isotherm parameters
for the adsorption of Zn(II) and Ni(II) using Langmuir, Freundlinch and Harkins-Jura
models are given in Table 2.
The analysis of results from the Table 2 shows that overall Langmuir adsorption
isotherm is a best fit for the experimental data of both Zn(II) and Ni(II) ions as
compared to other isotherms. Freundlich isotherm model indicates the heterogeneity of the surface and simultaneously supports the adsorption of the metal ions on
to the adsorbent. Freundlich isotherm also suggests that chemisorption is governing the adsorption process over physiorption. This indicates both physiorption and
chemisorption are playing their role in the process of adsorption for removing the
metal ions from aqueous solutions. Besides, it is not restricted to the monolayer as in