19
1.8 Adsorption Isotherms, Kinetics, and Thermodynamic
Studies of Activated Carbon
1.8.1 Adsorption Isotherms
The study of isotherm data is fundamentally vital to ascertain the strength of the
adsorbent. The relationship of the adsorbate (heavy metal ion) with an adsorbent
(activated carbon) could be well explained by adsorption isotherms, which thus
helps in maximizing the use of adsorbent. It is a usual tradition to relate the quantity
of heavy metal ions uptake by an adsorbent and its concentration in solution by different isotherm models (Hadi et al. 2015).
In 1906, Freundlich introduced an empirical equation which was used to illustrate a heterogeneous system of non-identical adsorption sites. In the model, it was
assumed that each adsorbate molecule could be adsorbed only on one adsorption
site. However, besides the good representation it gives in some scenarios as depicted
by the high correlation values, a well-accepted meaning of the fraction 1/n claimed
to be relating adsorption intensity was unclear in various examined systems
(Gokhale et al. 2008; Zhao et al. 2010). The inability of Freundlich’s model to
respond as Henry’s law at low dilution systems has also been faulted. In addition,
even with an evident increase in concentration, no definite adsorption uptake was
observed; thereby no peak was noticed as opposed to a real system. Thus, the validity
of this empirical isotherm only holds for narrow range of concentration (Foo and
Hameed 2010).
Another model was developed on the assumption that adsorption occurs at the
homogeneous sites with uniform level of energy (Langmuir 1916). This proposed
model by Langmuir was initially expressed to explain gas–solid phase adsorption
onto activated carbon but has since been employed to describe the response of various biosorbents. The isotherm hinge on the following theories, viz., (1) adsorption
is restricted to monolayer coverage, (2) all surface sites are identical with the capacity to only take one adsorbed molecule, and (3) the potential of a molecule being
adsorbed on a specific site is independent of its neighboring sites residency.
Noticeable in the Langmuir isotherm is the saturation capacity (q max ) which is
expected to go along with saturation of a finite quantity of identical sites, and fundamentally, it should be independent of temperature. But, this is contrary to the real
situation as narrow to moderate increase (Aydın et al. 2008; Dundar et al. 2008;
Malkoc and Nuhoglu 2005) and decrease (Djeribi and Hamdaoui 2008; Padmavathy
2008) in q max with temperature is mostly noticed. Furthermore, the value of K L Ce is
much lower than 1 at utmost dilution which means it can be neglected; therefore, the
Langmuir correlation can follow Henry’s law. Thus, the isotherm equation makes
the prediction of a uniform monolayer adsorption ability at elevated concentration
(Hadi et al. 2015).
A need for another model was brought up by Sips in 1948 which took the combined expression of Langmuir and Freundlich isotherms. The isotherm assumed that
one molecule may occupy more than one site (Sips 1948b). Despite the finite value
1 Synthesis of Activated Carbons for Heavy Metals Removal
1.8 Adsorption Isotherms, Kinetics, and Thermodynamic
Studies of Activated Carbon
1.8.1 Adsorption Isotherms
The study of isotherm data is fundamentally vital to ascertain the strength of the
adsorbent. The relationship of the adsorbate (heavy metal ion) with an adsorbent
(activated carbon) could be well explained by adsorption isotherms, which thus
helps in maximizing the use of adsorbent. It is a usual tradition to relate the quantity
of heavy metal ions uptake by an adsorbent and its concentration in solution by different isotherm models (Hadi et al. 2015).
In 1906, Freundlich introduced an empirical equation which was used to illustrate a heterogeneous system of non-identical adsorption sites. In the model, it was
assumed that each adsorbate molecule could be adsorbed only on one adsorption
site. However, besides the good representation it gives in some scenarios as depicted
by the high correlation values, a well-accepted meaning of the fraction 1/n claimed
to be relating adsorption intensity was unclear in various examined systems
(Gokhale et al. 2008; Zhao et al. 2010). The inability of Freundlich’s model to
respond as Henry’s law at low dilution systems has also been faulted. In addition,
even with an evident increase in concentration, no definite adsorption uptake was
observed; thereby no peak was noticed as opposed to a real system. Thus, the validity
of this empirical isotherm only holds for narrow range of concentration (Foo and
Hameed 2010).
Another model was developed on the assumption that adsorption occurs at the
homogeneous sites with uniform level of energy (Langmuir 1916). This proposed
model by Langmuir was initially expressed to explain gas–solid phase adsorption
onto activated carbon but has since been employed to describe the response of various biosorbents. The isotherm hinge on the following theories, viz., (1) adsorption
is restricted to monolayer coverage, (2) all surface sites are identical with the capacity to only take one adsorbed molecule, and (3) the potential of a molecule being
adsorbed on a specific site is independent of its neighboring sites residency.
Noticeable in the Langmuir isotherm is the saturation capacity (q max ) which is
expected to go along with saturation of a finite quantity of identical sites, and fundamentally, it should be independent of temperature. But, this is contrary to the real
situation as narrow to moderate increase (Aydın et al. 2008; Dundar et al. 2008;
Malkoc and Nuhoglu 2005) and decrease (Djeribi and Hamdaoui 2008; Padmavathy
2008) in q max with temperature is mostly noticed. Furthermore, the value of K L Ce is
much lower than 1 at utmost dilution which means it can be neglected; therefore, the
Langmuir correlation can follow Henry’s law. Thus, the isotherm equation makes
the prediction of a uniform monolayer adsorption ability at elevated concentration
(Hadi et al. 2015).
A need for another model was brought up by Sips in 1948 which took the combined expression of Langmuir and Freundlich isotherms. The isotherm assumed that
one molecule may occupy more than one site (Sips 1948b). Despite the finite value
1 Synthesis of Activated Carbons for Heavy Metals Removal