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T. Rasool and W. Baba
the behavior of adsorption seems to be independent of initial concentration of metal
ions. It can be seen that higher percentage of both the metal ions were removed at
lower concentrations. In previous studies, similar adsorption capacity behavior at
lower metal ions concentration have been observed and reported (Gong et al. 2005;
Manzoor et al. 2013; Ullah et al. 2013). But the amount of Zn(II) and Ni(II) metal
ions adsorbed per unit mass of adsorbent improved with increase in metal ion initial
concentrations in both the cases. For Zn(II), the maximum adsorption capacity came
out to be 100.78 mg/g and for Ni(II), it was found to be 108.67 mg/g. At higher
concentrations the available sites might be occupied while as at lower concentrations
more binding sites were available for the metal ions to be adsorbed. At low concentrations the binding sites of the algae bentonite composite remained unsaturated.
The findings reiterate that initial metal ion concentration plays a vital role in the process of adsorption and therefore the optimization of initial metal ion concentration
is necessary for an efficient adsorption process. The maximum removal efficiencies
for Zn(II) and Ni(II), however, were found to be 77% and 82%, respectively.
3.5 Effect of Contact Time
The rate of adsorption being an characteristic parameter in the design of batch reactors, the effect of contact time was evaluated for the exchange of adsorbate from the
solution on the adsorbent. The effect of contact time for the adsorption of Zn(II) and
Ni(II) metal ions on to the ABC was analyzed for 129 min with initial ion concentration of 100 mg/l for both Zn(II) and Ni(II) ions at 303.15 K. The adsorbent dosage
was taken as 1 g/l for both Zn(II) and Ni(II) metal ions. A 100 ml metal ion solution
was used, so 100 mg of adsorbent was used for adsorption of Zn(II) and Ni(II) ions,
respectively. The pH of the solution was fixed at an optimum values of 5 for Zn(II)
and 6 for Ni(II), respectively. The uptake of both the Zn(II) and Ni(II) species was
fast at an initial stage of contact time but became slower towards the equilibrium as
indicated from the curves presented in Fig. 7. This is due to the fact that during the
initial stage of adsorption, vacant sorption active sites available for attachment were
large in number. The equilibrium was reached in 50 min for Zn(II) and that for Ni(II),
it was attained in about 60 min. The adsorption capacities of ABC at equilibrium
was found to be 64.87 mg/g for Zinc and that for nickel, it was on the higher side
than that of Zn(II), at 87.2 mg/g. Thus it seems that that the process of adsorption
had followed a two step process, faster initial regime and a slower second regime.
As contact time was further increased the adsorption capacities didn’t increase any
further. This behavior of faster first phase and slower second phase can be explained
by the fact that initially active sites were readily available on the adsorbent surface
while as during the second phase, the exhaustion of vacant sites leads to the slower
adsorption rate. The behavior is in complete agreement with some earlier studies on
various adsorbent materials (Marandi 2011; Safa and Bhatti 2011).
T. Rasool and W. Baba
the behavior of adsorption seems to be independent of initial concentration of metal
ions. It can be seen that higher percentage of both the metal ions were removed at
lower concentrations. In previous studies, similar adsorption capacity behavior at
lower metal ions concentration have been observed and reported (Gong et al. 2005;
Manzoor et al. 2013; Ullah et al. 2013). But the amount of Zn(II) and Ni(II) metal
ions adsorbed per unit mass of adsorbent improved with increase in metal ion initial
concentrations in both the cases. For Zn(II), the maximum adsorption capacity came
out to be 100.78 mg/g and for Ni(II), it was found to be 108.67 mg/g. At higher
concentrations the available sites might be occupied while as at lower concentrations
more binding sites were available for the metal ions to be adsorbed. At low concentrations the binding sites of the algae bentonite composite remained unsaturated.
The findings reiterate that initial metal ion concentration plays a vital role in the process of adsorption and therefore the optimization of initial metal ion concentration
is necessary for an efficient adsorption process. The maximum removal efficiencies
for Zn(II) and Ni(II), however, were found to be 77% and 82%, respectively.
3.5 Effect of Contact Time
The rate of adsorption being an characteristic parameter in the design of batch reactors, the effect of contact time was evaluated for the exchange of adsorbate from the
solution on the adsorbent. The effect of contact time for the adsorption of Zn(II) and
Ni(II) metal ions on to the ABC was analyzed for 129 min with initial ion concentration of 100 mg/l for both Zn(II) and Ni(II) ions at 303.15 K. The adsorbent dosage
was taken as 1 g/l for both Zn(II) and Ni(II) metal ions. A 100 ml metal ion solution
was used, so 100 mg of adsorbent was used for adsorption of Zn(II) and Ni(II) ions,
respectively. The pH of the solution was fixed at an optimum values of 5 for Zn(II)
and 6 for Ni(II), respectively. The uptake of both the Zn(II) and Ni(II) species was
fast at an initial stage of contact time but became slower towards the equilibrium as
indicated from the curves presented in Fig. 7. This is due to the fact that during the
initial stage of adsorption, vacant sorption active sites available for attachment were
large in number. The equilibrium was reached in 50 min for Zn(II) and that for Ni(II),
it was attained in about 60 min. The adsorption capacities of ABC at equilibrium
was found to be 64.87 mg/g for Zinc and that for nickel, it was on the higher side
than that of Zn(II), at 87.2 mg/g. Thus it seems that that the process of adsorption
had followed a two step process, faster initial regime and a slower second regime.
As contact time was further increased the adsorption capacities didn’t increase any
further. This behavior of faster first phase and slower second phase can be explained
by the fact that initially active sites were readily available on the adsorbent surface
while as during the second phase, the exhaustion of vacant sites leads to the slower
adsorption rate. The behavior is in complete agreement with some earlier studies on
various adsorbent materials (Marandi 2011; Safa and Bhatti 2011).