419
extensively used in industrial applications and for water and wastewater treatment.
In the past few years, natural and synthetic adsorbents such as activated carbon,
zeolites, polymers, clays, graphene, carbon nanotubes, and synthetic materials have
been used to remove a wide range of dyes from aquatic environments (Salama 2017,
2019; Salama and Hesemann 2018).
16.6 Adsorption
The adsorption is a popular process especially in removal of pollutants from wastewater owning to its simplicity, facile operation design using a variety of adsorbents,
high efficacy of purification process, no sedimentation, and smaller investment
(lower operating costs), in particular when the adsorbent is cheap and available
(Siahkamari et al. 2017; Yang et al. 2017b; Li et al. 2017b). The adsorption involves
an interfacial mass transfer process. Different modes of such mass transfer can
occur between the fluid and solid phases such as liquid–liquid, gas–liquid, gas–
solid, or liquid–solid. The properties of the adsorptive and adsorbent depend on
their composition. The nature of the adsorption can be physical or chemical. The
former is known as physisorption. In this case, the forces of attraction are the weak
forces of van der Waals, and the results of the process are reversible. In addition,
adsorption occurs at or below the critical temperature of the adsorbed material. The
latter is known as chemisorption, during which the forces of attraction between the
molecules of the adsorbed material and the solid surface are due to chemical bonding. Completely different from the physisorption, the chemisorption takes place
only in a monolayer, and in addition, the chemically adsorbed material on the solid
surface is difficult to remove due to the stronger forces. In appropriate conditions,
both physisorption and chemisorption can happen simultaneously or intermittently.
Physisorption is associated with reduced free energy and entropy of the adsorption
system, and as a result, the process is exothermic (Saadi et al. 2013).
The adsorption process occurs in three stages (Tan and Hameed 2017):
Step (1) External diffusion, which involves the diffusion of adsorptive molecules
within the fluid bulk to the outer surface of the adsorbent
Step (2) Intraparticle diffusion or intra-pore diffusion, in which the adsorptive molecules are transferred from the adsorbent surface into the pores
Step (3) The surface reaction in which the adsorptive molecules attach to the inner
surface of the adsorbent pores
The first two steps of this process are the “transition steps” as shown in Fig. 16.5.
Each step imposes resistance to the uptake of the adsorptive molecules. The
overall adsorption rate is calculated by the ultimate resistance obtained from the
sum of the resistances of all three steps. Reducing the resistance of each step leads
to the higher rate of adsorption. Usually the third step occurs faster than the first two
ones because it creates less resistance. Each of the first two steps having the highest
16 Hybrid Adsorbents for Dye Removal from Wastewater
extensively used in industrial applications and for water and wastewater treatment.
In the past few years, natural and synthetic adsorbents such as activated carbon,
zeolites, polymers, clays, graphene, carbon nanotubes, and synthetic materials have
been used to remove a wide range of dyes from aquatic environments (Salama 2017,
2019; Salama and Hesemann 2018).
16.6 Adsorption
The adsorption is a popular process especially in removal of pollutants from wastewater owning to its simplicity, facile operation design using a variety of adsorbents,
high efficacy of purification process, no sedimentation, and smaller investment
(lower operating costs), in particular when the adsorbent is cheap and available
(Siahkamari et al. 2017; Yang et al. 2017b; Li et al. 2017b). The adsorption involves
an interfacial mass transfer process. Different modes of such mass transfer can
occur between the fluid and solid phases such as liquid–liquid, gas–liquid, gas–
solid, or liquid–solid. The properties of the adsorptive and adsorbent depend on
their composition. The nature of the adsorption can be physical or chemical. The
former is known as physisorption. In this case, the forces of attraction are the weak
forces of van der Waals, and the results of the process are reversible. In addition,
adsorption occurs at or below the critical temperature of the adsorbed material. The
latter is known as chemisorption, during which the forces of attraction between the
molecules of the adsorbed material and the solid surface are due to chemical bonding. Completely different from the physisorption, the chemisorption takes place
only in a monolayer, and in addition, the chemically adsorbed material on the solid
surface is difficult to remove due to the stronger forces. In appropriate conditions,
both physisorption and chemisorption can happen simultaneously or intermittently.
Physisorption is associated with reduced free energy and entropy of the adsorption
system, and as a result, the process is exothermic (Saadi et al. 2013).
The adsorption process occurs in three stages (Tan and Hameed 2017):
Step (1) External diffusion, which involves the diffusion of adsorptive molecules
within the fluid bulk to the outer surface of the adsorbent
Step (2) Intraparticle diffusion or intra-pore diffusion, in which the adsorptive molecules are transferred from the adsorbent surface into the pores
Step (3) The surface reaction in which the adsorptive molecules attach to the inner
surface of the adsorbent pores
The first two steps of this process are the “transition steps” as shown in Fig. 16.5.
Each step imposes resistance to the uptake of the adsorptive molecules. The
overall adsorption rate is calculated by the ultimate resistance obtained from the
sum of the resistances of all three steps. Reducing the resistance of each step leads
to the higher rate of adsorption. Usually the third step occurs faster than the first two
ones because it creates less resistance. Each of the first two steps having the highest
16 Hybrid Adsorbents for Dye Removal from Wastewater