42
2 Thermodynamics, Kinetics and Desorption Studies …
However, data generated using Langmuir and Freundlich isotherm were presented
in Table 2.4. As can be seen from the table, in the Langmuir model, the maximum
capacity of adsorption for the adsorbent, Q m (mg/g) for metal ions is high and
was observed to increase as temperature increases from 25 to 45 °C. This is an
indication that adsorption process is endothermic, and that molecular interaction is
non-covalent. Pb(II) has the highest maximum adsorption capacity of 147.3 mg/g,
followed by Cu(II); hence, adsorption process was found to follow the sequence
Pb(II) > Cu(II) > Ni(II) > Zn(II). This observation showed that Pb(II) ions were
the most effective ion on the surface of the absorbent having the strongest affinity
towards multifunctional group. The K L values of Langmuir model were found to
follow the same trend as Q m values; hence, R L values calculated using 2.11 give
0.28, 0.2, 0.16 and 0.22 for Cu(II), Pb(II), Ni(II) and Zn(II), respectively. These
values indicate favourable Langmuir adsorption processes. Also, obtained R
2 values
of Langmuir model are greater than 0.98, which is very close to unit, indicating
stronger mathematic data fit. On the other hand, the data generated from Freundlich
adsorption model gives high values of K F which indicates high adsorption intensity
[36]. Obtained n values were greater than 1, and this is an indication for convenient
and favourable adsorption for all considered heavy metal ions. With R
2 values greater
or equal to 0.763, which is far from unity is an indication that Freundlich model was
not efficient in describing the experimental data on the adsorption of metal ions onto
the modified chitosan surface. Similar result was obtained by Doˇ gan et al. [37] who
depicted that Langmuir adsorption isotherm was reasonably correlated with experimental data in metal ions adsorption. Therefore, in this study, the Langmuir model
best fits the obtained experimental data, suggesting that metal ions’ adsorption onto
the modified chitosan bead surface is dominated by homogenous distribution onto
the adsorption active sites.
2.9 Conclusion
In this work, an environmentally friendly, low-cost and high removal efficient adsorbent was synthesized and applied in adsorption studies. The synthesized modified
grafted cross-linked chitosan (G/CR-CS) adsorbent was characterized by FTIR,
XRD and SEM. The characterization revealed the presence of carbonyl (C=O),
hydroxyl (O–H) and amine (N–H) functional groups that enable good binding performances towards metal ion micropollutants. The metal ion removal from aqueous
solution was found to increase with increase in contact time, temperature and
pH until an adsorption equilibrium is reached. The metal ion adsorption kinetics
onto the modified G/CR-CS surface were best fitted with the pseudo-second-order
kinetic study showing that chemical adsorption is the rate limiting step. The values
for the thermodynamics parameters of G
o , H
o and S
o indicate spontaneous
and endothermic adsorption process. Modified G/CR-CS was efficiently fitted by
the Langmuir adsorption isotherm which indicates surface monolayer adsorption
coverage.
2 Thermodynamics, Kinetics and Desorption Studies …
However, data generated using Langmuir and Freundlich isotherm were presented
in Table 2.4. As can be seen from the table, in the Langmuir model, the maximum
capacity of adsorption for the adsorbent, Q m (mg/g) for metal ions is high and
was observed to increase as temperature increases from 25 to 45 °C. This is an
indication that adsorption process is endothermic, and that molecular interaction is
non-covalent. Pb(II) has the highest maximum adsorption capacity of 147.3 mg/g,
followed by Cu(II); hence, adsorption process was found to follow the sequence
Pb(II) > Cu(II) > Ni(II) > Zn(II). This observation showed that Pb(II) ions were
the most effective ion on the surface of the absorbent having the strongest affinity
towards multifunctional group. The K L values of Langmuir model were found to
follow the same trend as Q m values; hence, R L values calculated using 2.11 give
0.28, 0.2, 0.16 and 0.22 for Cu(II), Pb(II), Ni(II) and Zn(II), respectively. These
values indicate favourable Langmuir adsorption processes. Also, obtained R
2 values
of Langmuir model are greater than 0.98, which is very close to unit, indicating
stronger mathematic data fit. On the other hand, the data generated from Freundlich
adsorption model gives high values of K F which indicates high adsorption intensity
[36]. Obtained n values were greater than 1, and this is an indication for convenient
and favourable adsorption for all considered heavy metal ions. With R
2 values greater
or equal to 0.763, which is far from unity is an indication that Freundlich model was
not efficient in describing the experimental data on the adsorption of metal ions onto
the modified chitosan surface. Similar result was obtained by Doˇ gan et al. [37] who
depicted that Langmuir adsorption isotherm was reasonably correlated with experimental data in metal ions adsorption. Therefore, in this study, the Langmuir model
best fits the obtained experimental data, suggesting that metal ions’ adsorption onto
the modified chitosan bead surface is dominated by homogenous distribution onto
the adsorption active sites.
2.9 Conclusion
In this work, an environmentally friendly, low-cost and high removal efficient adsorbent was synthesized and applied in adsorption studies. The synthesized modified
grafted cross-linked chitosan (G/CR-CS) adsorbent was characterized by FTIR,
XRD and SEM. The characterization revealed the presence of carbonyl (C=O),
hydroxyl (O–H) and amine (N–H) functional groups that enable good binding performances towards metal ion micropollutants. The metal ion removal from aqueous
solution was found to increase with increase in contact time, temperature and
pH until an adsorption equilibrium is reached. The metal ion adsorption kinetics
onto the modified G/CR-CS surface were best fitted with the pseudo-second-order
kinetic study showing that chemical adsorption is the rate limiting step. The values
for the thermodynamics parameters of G
o , H
o and S
o indicate spontaneous
and endothermic adsorption process. Modified G/CR-CS was efficiently fitted by
the Langmuir adsorption isotherm which indicates surface monolayer adsorption
coverage.