Application of Sustainable and Low-Cost Sludge-Based Adsorbents …
63
increased with the increase in carbon dosage at constant initial dye concentration
and hence increased the overall removal efficiency of the dyes.
For an increase in dye concentration, there would be increased competition for
the active adsorption sites. The strong chemical binding of the adsorbate slows down
the adsorption process. A similar trend was reported in the adsorption of a dye Reactive Red 241 onto a commercial activated carbon [92]. The variation in percentage
removal of dye with contact time at different initial concentrations ranging from 10
to 60 mg/L is presented in Figs. 20 and 21. From the illustrations (Figs. 14, 15, 16,
17, 18, 19, 20 and 21), it was observed that the maximum amount of dye adsorption
took place within the contact time of 30 min and becomes gradual thereafter. This
indicated a quicker adsorption rate during the initial contact period.
5.2.4 Effect of Agitation Speed on Adsorption Kinetics
Agitation is an important parameter in adsorption phenomena because it influences
the distribution of the solute in the bulk solution and the formation of the external
boundary film. Figure 23a, b, respectively, shows the adsorption capacity of the
reactive dyes, Reactive Red 31 and Reactive Red 2, using three activated carbons
(CAC, SC600 and SC300) at different agitation speeds (100, 120 and 140 rpm). It
is clear from Fig. 23a, b that the uptake of dyes increased from 100 rpm to 120 rpm
and decreased slightly at the agitation speed of 140 rpm.
The slight decrease in adsorption capacity at 140 rpm was due to desorption at
the equilibrium time. Difference in agitation speed caused changes in kinetics of the
adsorption as well as the equilibrium adsorption capacity. The increase in dye uptake
at the optimum speed of 120 rpm reduced the film boundary layer surrounding
the particles, thus increasing the external film transfer coefficient and thereby the
percentage dye removal. Mckay [73] also found that the degree of agitation reduced
the boundary layer resistance and increased the mobility of the system and the
external mass transfer coefficient resulting in quicker adsorption of dye molecules
with agitation speed.
5.2.5 Effect of pH on Adsorption Kinetics
The initial pH of the dye solution is an important parameter which controls the
adsorption process in general and the adsorption capacity in particular. pH of the
solution influences the following factors:
1. The surface charge of the adsorbent
2. The degree of ionization of the adsorbate molecule and
3. The extent of dissociation of functional groups on the active sites of the
adsorbent.
Experimental results showed that the removal of dye increased with the decrease
in pH as shown in Fig. 24. The variation of adsorption capacity with pH, shown in
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increased with the increase in carbon dosage at constant initial dye concentration
and hence increased the overall removal efficiency of the dyes.
For an increase in dye concentration, there would be increased competition for
the active adsorption sites. The strong chemical binding of the adsorbate slows down
the adsorption process. A similar trend was reported in the adsorption of a dye Reactive Red 241 onto a commercial activated carbon [92]. The variation in percentage
removal of dye with contact time at different initial concentrations ranging from 10
to 60 mg/L is presented in Figs. 20 and 21. From the illustrations (Figs. 14, 15, 16,
17, 18, 19, 20 and 21), it was observed that the maximum amount of dye adsorption
took place within the contact time of 30 min and becomes gradual thereafter. This
indicated a quicker adsorption rate during the initial contact period.
5.2.4 Effect of Agitation Speed on Adsorption Kinetics
Agitation is an important parameter in adsorption phenomena because it influences
the distribution of the solute in the bulk solution and the formation of the external
boundary film. Figure 23a, b, respectively, shows the adsorption capacity of the
reactive dyes, Reactive Red 31 and Reactive Red 2, using three activated carbons
(CAC, SC600 and SC300) at different agitation speeds (100, 120 and 140 rpm). It
is clear from Fig. 23a, b that the uptake of dyes increased from 100 rpm to 120 rpm
and decreased slightly at the agitation speed of 140 rpm.
The slight decrease in adsorption capacity at 140 rpm was due to desorption at
the equilibrium time. Difference in agitation speed caused changes in kinetics of the
adsorption as well as the equilibrium adsorption capacity. The increase in dye uptake
at the optimum speed of 120 rpm reduced the film boundary layer surrounding
the particles, thus increasing the external film transfer coefficient and thereby the
percentage dye removal. Mckay [73] also found that the degree of agitation reduced
the boundary layer resistance and increased the mobility of the system and the
external mass transfer coefficient resulting in quicker adsorption of dye molecules
with agitation speed.
5.2.5 Effect of pH on Adsorption Kinetics
The initial pH of the dye solution is an important parameter which controls the
adsorption process in general and the adsorption capacity in particular. pH of the
solution influences the following factors:
1. The surface charge of the adsorbent
2. The degree of ionization of the adsorbate molecule and
3. The extent of dissociation of functional groups on the active sites of the
adsorbent.
Experimental results showed that the removal of dye increased with the decrease
in pH as shown in Fig. 24. The variation of adsorption capacity with pH, shown in