2.5 Application of Modified Chitosan for Metal Ions Adsorption
37
chitosan G/CR-CS) and a temperature range of 25–55 °C. It is observed that an
increase in temperature leads to a rise in the removal efficiency on all the considered
metal ions as shown in Fig. 2.7c. This is because the rise in temperature increases
the kinetic energy {KE} of the metal ions making them easier to contact adsorption
sites, thus improving their adsorption efficiency [1].
In addition, higher temperature may have caused bond rupture of the absorbent,
leading to an increase in adsorption active sites [32]. After optimum, the removal
efficiency decreased with increasing temperature. As the temperature exceeds 45 °C,
deterioration of the adsorbent begins. Since chitosan material cannot withstand higher
temperatures, their adsorption efficiency therefore decreases.
2.6 Thermodynamic Parameters of Adsorption
Thermodynamic parameters which existed as a result of the effect of temperature
changes on the adsorption capacity of metal ions onto modified chitosan surface
were entropy, S, enthalpy, H and the Gibbs free energy, G. The value of these
parameters determines if an adsorption process is spontaneous or not. However, the
Gibbs free energy change, G
o , is the fundamental criterion of spontaneity, and
reactions occur spontaneous at a given temperature if the G
o is a negative quantity
[32]. Relevant equations for the calculation of thermodynamic parameters are thus:
Gibbs free energy 2.3, Van ’t Hoff 2.4, equilibrium constant 2.5 and Van ’t Hoff plot
of ln K C versus 1/T as expressed in 2.6.
G
o
= H
o
−T S
o
(2.3)
G
o
= −RT ln K
(2.4)
K C = q e /C e
(2.5)
ln K C = −H
o
/RT + S
o
/R
(2.6)
where G
o , H
o and S
o are the standard free energy, enthalpy and entropy of
adsorption, respectively, R is the universal gas constant (∼8.314 J K
−1 mol
−1 ), T is
the temperature and K C is the equilibrium constant, while q e and C e are the adsorption
capacity at equilibrium and equilibrium concentration, respectively. The values of
G° and S° were determined from the slope and intercept of the plot of ln K C
against 1/T, and thermodynamic parameter values were recorded and presented in
Table 2.2.
As observed, the negative values of G
o rise as the temperature increases and this
is an indication that the adsorption reaction of heavy metal ions onto the adsorbent
is a spontaneous process. The positive value of H
o confirms that the adsorption
37
chitosan G/CR-CS) and a temperature range of 25–55 °C. It is observed that an
increase in temperature leads to a rise in the removal efficiency on all the considered
metal ions as shown in Fig. 2.7c. This is because the rise in temperature increases
the kinetic energy {KE} of the metal ions making them easier to contact adsorption
sites, thus improving their adsorption efficiency [1].
In addition, higher temperature may have caused bond rupture of the absorbent,
leading to an increase in adsorption active sites [32]. After optimum, the removal
efficiency decreased with increasing temperature. As the temperature exceeds 45 °C,
deterioration of the adsorbent begins. Since chitosan material cannot withstand higher
temperatures, their adsorption efficiency therefore decreases.
2.6 Thermodynamic Parameters of Adsorption
Thermodynamic parameters which existed as a result of the effect of temperature
changes on the adsorption capacity of metal ions onto modified chitosan surface
were entropy, S, enthalpy, H and the Gibbs free energy, G. The value of these
parameters determines if an adsorption process is spontaneous or not. However, the
Gibbs free energy change, G
o , is the fundamental criterion of spontaneity, and
reactions occur spontaneous at a given temperature if the G
o is a negative quantity
[32]. Relevant equations for the calculation of thermodynamic parameters are thus:
Gibbs free energy 2.3, Van ’t Hoff 2.4, equilibrium constant 2.5 and Van ’t Hoff plot
of ln K C versus 1/T as expressed in 2.6.
G
o
= H
o
−T S
o
(2.3)
G
o
= −RT ln K
(2.4)
K C = q e /C e
(2.5)
ln K C = −H
o
/RT + S
o
/R
(2.6)
where G
o , H
o and S
o are the standard free energy, enthalpy and entropy of
adsorption, respectively, R is the universal gas constant (∼8.314 J K
−1 mol
−1 ), T is
the temperature and K C is the equilibrium constant, while q e and C e are the adsorption
capacity at equilibrium and equilibrium concentration, respectively. The values of
G° and S° were determined from the slope and intercept of the plot of ln K C
against 1/T, and thermodynamic parameter values were recorded and presented in
Table 2.2.
As observed, the negative values of G
o rise as the temperature increases and this
is an indication that the adsorption reaction of heavy metal ions onto the adsorbent
is a spontaneous process. The positive value of H
o confirms that the adsorption