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Figure 7(a) and 7(b) display the pseudo-first-order and the pseudo-second-order plots versus time sequentially. In Figure 7(a), ln (qeqt) versus t graph is represented. The plot shows linearity with regression coefficient 0.9615 and 0.9388 for cadmium and nickel ions adsorption, respectively. While the t/qt against t-plot describing the pseudo-second-order law (Figure 7(b)) represents a better fitting for the data (r2 > 0.99). On the other hand, the calculated qm values are in agreement with those obtained experimentally (Table 3). The obtained data clearly show the adsorption kinetics of both metal ions and suggests that kinetics in Cd(II) and Ni(II) adsorption follows the pseudo-second-order kinetic law. These findings agree well with previously reported data (Chen et al. 2008; Montazer-Rahmati et al. 2011).
Table 3

Rate constants for Cd(II) and Ni(II) ions adsorption on the adsorbent

Metal ionst1/2 (s)D× 1015 (cm2s−1)qm(exp)a (mg·g−1)First-order
 Second-order
 
k1 × 103 (min−1)qm(cal)b (mg·g−1)r2k2 × 103 (g.(mg·min)−1)qm(cal)b (mg·g−1)r2
Cd (II) 245.4 3.7 82.45 4.6 33.78 0.9615 3.01 81.3 0.9990 
Ni(II) 398.4 2.3 38.03 5.7 19.14 0.9388 3.99 37.74 0.9952 
Metal ionst1/2 (s)D× 1015 (cm2s−1)qm(exp)a (mg·g−1)First-order
 Second-order
 
k1 × 103 (min−1)qm(cal)b (mg·g−1)r2k2 × 103 (g.(mg·min)−1)qm(cal)b (mg·g−1)r2
Cd (II) 245.4 3.7 82.45 4.6 33.78 0.9615 3.01 81.3 0.9990 
Ni(II) 398.4 2.3 38.03 5.7 19.14 0.9388 3.99 37.74 0.9952 
Figure 7

Kinetics of Cd(II) and Ni(II) adsorption onto the (x)ZnO(1 − x)Fe2O3: (a) pseudo-first-order plot and (b) pseudo-second-order plot.

Figure 7

Kinetics of Cd(II) and Ni(II) adsorption onto the (x)ZnO(1 − x)Fe2O3: (a) pseudo-first-order plot and (b) pseudo-second-order plot.

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