5.4 Adsorption Capacity
105
where b o and b are the parameters of affinity at temperature T 0 and T accordingly, R
is the standard gas constant (83,144 J/mol K), T is the temperature in K and H ads
is the J mol
−1 adsorption enthalpy. And the linear type of the Van’t Hoff equation
(5.7) was considered to evaluate the adsorption enthalpy:
ln(b) = ln(b o ) −
H ads
R · T
(5.7)
Consequently, referring to the regression analysis predictions as shown in Fig. 5.3,
the enthalpy of Zn(II) adsorption on the cross-lined chitosan membranes was
observed to be 20 kJ mol
−1 . The adsorption enthalpy measured indicates endothermic
adsorption, i.e. a rise in temperature supports Zn(II) adsorption, as confirmed by the
results shown in Table 5.1. Lima and Airoldi [30] and Ho and McKay [28] reported
negative values (−27.7 kJ mol
−1 in beads and −17.7 kJ mol
−1 in powder) for chitosan
adsorption enthalpy in Zn(II). Many adsorption operations are exothermic [1, 28],
although for the adsorption copper and mercury, Ho and McKay [28] reported positive adsorption enthalpies for chitosan powder. This phenomenon may also probably
be due to the effect of temperature on the chitosan’s free volume of water, as stated
with the rise in adsorption capability with temperature rise. The essence of adsorption
may also be interpreted as physical adsorption from the adsorption enthalpy value
(20 kJ mol
−1 ) of Zn(II) onto the chitosan membrane. If a solute, like metal ions, is
adsorbed into an adsorbent material like chitosan, the adsorption process may rely
on the forms of contact (reaction) that exist between the solute and the adsorbent; for
most cases, the degree of energy that exists is needed to identify the form of adsorption wherein the process of adsorption can be either chemical (i.e. covalent bond is
the relationship between the solute and adsorbent) or physical (van der Waals forces
are the relationship) in essence; the quantity of energy used in the adsorption has
been recorded for both cases. In the case of physical adsorption (those concerning
Van der Waals), the enthalpy range was 20–100 kJ mol
−1 and the enthalpy range for
chemical adsorption (those concerning covalent bonding) is 200–500 kJ mol
−1 and
is mainly an exothermic reaction [29]. One of the most important considerations for
the adsorption operation is the quality of the bond between the adsorbate and the
adsorbent [28]. A tight bond between adsorbent and adsorbent improves adsorption
efficiency which causes the adsorbent to be quite challenging to desorb.
Chitosan membrane performance is validated with other chitosan materials and
also with other adsorbents in Table 5.2 (only the optimum value presented in the
literature are given). From this relation (Table 5.2), cross-linked chitosan membranes
have a high capacity for adsorption relative to several other forms of chitosan and
other adsorbents.
105
where b o and b are the parameters of affinity at temperature T 0 and T accordingly, R
is the standard gas constant (83,144 J/mol K), T is the temperature in K and H ads
is the J mol
−1 adsorption enthalpy. And the linear type of the Van’t Hoff equation
(5.7) was considered to evaluate the adsorption enthalpy:
ln(b) = ln(b o ) −
H ads
R · T
(5.7)
Consequently, referring to the regression analysis predictions as shown in Fig. 5.3,
the enthalpy of Zn(II) adsorption on the cross-lined chitosan membranes was
observed to be 20 kJ mol
−1 . The adsorption enthalpy measured indicates endothermic
adsorption, i.e. a rise in temperature supports Zn(II) adsorption, as confirmed by the
results shown in Table 5.1. Lima and Airoldi [30] and Ho and McKay [28] reported
negative values (−27.7 kJ mol
−1 in beads and −17.7 kJ mol
−1 in powder) for chitosan
adsorption enthalpy in Zn(II). Many adsorption operations are exothermic [1, 28],
although for the adsorption copper and mercury, Ho and McKay [28] reported positive adsorption enthalpies for chitosan powder. This phenomenon may also probably
be due to the effect of temperature on the chitosan’s free volume of water, as stated
with the rise in adsorption capability with temperature rise. The essence of adsorption
may also be interpreted as physical adsorption from the adsorption enthalpy value
(20 kJ mol
−1 ) of Zn(II) onto the chitosan membrane. If a solute, like metal ions, is
adsorbed into an adsorbent material like chitosan, the adsorption process may rely
on the forms of contact (reaction) that exist between the solute and the adsorbent; for
most cases, the degree of energy that exists is needed to identify the form of adsorption wherein the process of adsorption can be either chemical (i.e. covalent bond is
the relationship between the solute and adsorbent) or physical (van der Waals forces
are the relationship) in essence; the quantity of energy used in the adsorption has
been recorded for both cases. In the case of physical adsorption (those concerning
Van der Waals), the enthalpy range was 20–100 kJ mol
−1 and the enthalpy range for
chemical adsorption (those concerning covalent bonding) is 200–500 kJ mol
−1 and
is mainly an exothermic reaction [29]. One of the most important considerations for
the adsorption operation is the quality of the bond between the adsorbate and the
adsorbent [28]. A tight bond between adsorbent and adsorbent improves adsorption
efficiency which causes the adsorbent to be quite challenging to desorb.
Chitosan membrane performance is validated with other chitosan materials and
also with other adsorbents in Table 5.2 (only the optimum value presented in the
literature are given). From this relation (Table 5.2), cross-linked chitosan membranes
have a high capacity for adsorption relative to several other forms of chitosan and
other adsorbents.