Application of Sustainable and Low-Cost Sludge-Based Adsorbents …
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Resistance to mass transfer in the adsorption processes can be either resistance
caused by external mass transfer through the particle boundary layer or resistance
due to intraparticle diffusion. The external mass transfer [74] described by the linear
and nonlinear isotherms [108] is differentiated based on their intraparticle diffusion.
Solid-phase diffusion is the dominant effect in intraparticle mass transfer during the
initial stages of adsorption. These diffusion models developed by various researches
[4, 72] have focused on various process parameters including the rate of agitation,
dye concentration and phase ratio.
2.2 Types of Adsorption
The adsorption phenomena can be classified as physisorption and chemisorption
based on the nature of the bonding between the molecule and the adsorbent surface.
Physical adsorption results from molecular condensation in the capillaries of the
solid and is dominated by the weak-by-weak van der Waals-type forces. There
is no significant redistribution of electron density in either the molecule or at the
substrate surface. Chemical adsorption results in the formation of monomolecular
layer of the adsorbate on the surface through forces of residual valence of the surface
molecules. In chemisorption, a strong chemical bond such as covalent bond is formed
between the adsorbate and substrate. The bond exhibits substantial rearrangement
of electron density. The classification and characteristics between physisorption and
chemisorption are shown in Table 11.
Table 11 Typical characteristics of adsorption processes
Characteristics
Physical adsorption
Chemical adsorption
Binding force
Due to physical force of
attraction, thus this process is
also called as van der Waals
adsorption
Due to chemical forces or
bonding, thus this process is
also called as activated
adsorption
Saturation uptake
Multilayer phenomena
Single-layer phenomena
Activation energy
No activation energy involved
May be involved
Temperature range (over
which adsorption occurs)
Adsorption is appreciable at
lower temperature below boiling
point of adsorbate
Adsorption can take place even
at higher temperature
Nature of sorbate
Amount of adsorbate removed
depends more on adsorbate than
on adsorbent
Depends on both adsorbent and
adsorbate
Crystallographic specificity Virtually independent of surface
atomic geometry
Marked variation between
crystal planes
Heat of adsorption
1 kcal/mole
50–100 kcal/mole
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Resistance to mass transfer in the adsorption processes can be either resistance
caused by external mass transfer through the particle boundary layer or resistance
due to intraparticle diffusion. The external mass transfer [74] described by the linear
and nonlinear isotherms [108] is differentiated based on their intraparticle diffusion.
Solid-phase diffusion is the dominant effect in intraparticle mass transfer during the
initial stages of adsorption. These diffusion models developed by various researches
[4, 72] have focused on various process parameters including the rate of agitation,
dye concentration and phase ratio.
2.2 Types of Adsorption
The adsorption phenomena can be classified as physisorption and chemisorption
based on the nature of the bonding between the molecule and the adsorbent surface.
Physical adsorption results from molecular condensation in the capillaries of the
solid and is dominated by the weak-by-weak van der Waals-type forces. There
is no significant redistribution of electron density in either the molecule or at the
substrate surface. Chemical adsorption results in the formation of monomolecular
layer of the adsorbate on the surface through forces of residual valence of the surface
molecules. In chemisorption, a strong chemical bond such as covalent bond is formed
between the adsorbate and substrate. The bond exhibits substantial rearrangement
of electron density. The classification and characteristics between physisorption and
chemisorption are shown in Table 11.
Table 11 Typical characteristics of adsorption processes
Characteristics
Physical adsorption
Chemical adsorption
Binding force
Due to physical force of
attraction, thus this process is
also called as van der Waals
adsorption
Due to chemical forces or
bonding, thus this process is
also called as activated
adsorption
Saturation uptake
Multilayer phenomena
Single-layer phenomena
Activation energy
No activation energy involved
May be involved
Temperature range (over
which adsorption occurs)
Adsorption is appreciable at
lower temperature below boiling
point of adsorbate
Adsorption can take place even
at higher temperature
Nature of sorbate
Amount of adsorbate removed
depends more on adsorbate than
on adsorbent
Depends on both adsorbent and
adsorbate
Crystallographic specificity Virtually independent of surface
atomic geometry
Marked variation between
crystal planes
Heat of adsorption
1 kcal/mole
50–100 kcal/mole