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The kinetics of the adsorption process were evaluated by fitting the experimental data with the pseudo-first-order and pseudo-second-order models. The values obtained from the models are shown in Table 3. It was found that the experimental kinetic data fitted the pseudo-second-order (R2 = 0.998) better than the pseudo-first-order (R2 = 0.952). Thus, the pseudo-second-order kinetic model is more likely to describe the overall rate of the adsorption process of As(III) on nZVI-D. Based on pseudo-second-order kinetic model ascription, chemical adsorption involves the valence electron forces shared between the adsorbent and heavy metal (Erdem et al. 2016). In addition, the values of k2 (Figure 5(a)) decreased with the increase of initial As(III) concentration (1,000–1,500 μg/L) and became constant at 1,750–2,000 μg/L. It is shown that when the initial concentrations increased, the adsorption constant rate (k2) between 1,750 and 2,000 μg/L was not significant. A high driving force of high concentration was thought to affect the adsorption rate. However, the absorption rate obtained from this experiment was not consistent with the absorption theory because the active sites of the absorbent were limited. Therefore, Figure 5(b) was plotted in order to explain the adsorption capacity of the active sites. It can be seen that the available active sites of absorbent decreased with the increase of initial concentration. Furthermore, more than 95% of the active site was used to absorb As(III) at an initial concentration of 1,750μg/L. The low value of k2 (between 1,750 and 2,000 μg/L) displayed in Figure 5(a) may indicate that the surface of the absorbent at high concentration was covered by As(III) particles, which is demonstrated by a small change of qt/qm in Figure 5(b).
Table 3

Adsorption kinetic model rate constants for As(III) adsorption on nZVI-D at different initial concentrations (temperature: 30 °C; 0.75 g/L of nZVI-D, initial pH 6)

C0 (μg/L) 1,000 1,250 1,500 1,750 2,000 
qe,exp (μg/g) 1,827.60 2,068.16 2,216.90 2,484.22 2,574.35 
Pseudo-first-order 
 qe,cal (μg/g) 657.66 590.28 661.69 903.52 1,013.94 
 k1 (L/min) 0.175 0.120 0.121 0.124 0.108 
 R2 0.952 0.877 0.849 0.920 0.941 
Pseudo-second-order 
 qe,cal (μg/g) 1,378.43 1,521.45 1,658.85 1,865.77 1,857.52 
 k2 × 10–3 (g/μg·min) 1.05 1.30 0.93 0.61 0.60 
 R2 0.995 0.998 0.997 0.995 0.993 
Intra-particle diffusion 
 ki1 (μg/g·min1/2133.06 198.15 278.08 302.99 246.70 
 ki2 (μg/g·min1/26.69 15.86 13.18 11.97 23.41 
C0 (μg/L) 1,000 1,250 1,500 1,750 2,000 
qe,exp (μg/g) 1,827.60 2,068.16 2,216.90 2,484.22 2,574.35 
Pseudo-first-order 
 qe,cal (μg/g) 657.66 590.28 661.69 903.52 1,013.94 
 k1 (L/min) 0.175 0.120 0.121 0.124 0.108 
 R2 0.952 0.877 0.849 0.920 0.941 
Pseudo-second-order 
 qe,cal (μg/g) 1,378.43 1,521.45 1,658.85 1,865.77 1,857.52 
 k2 × 10–3 (g/μg·min) 1.05 1.30 0.93 0.61 0.60 
 R2 0.995 0.998 0.997 0.995 0.993 
Intra-particle diffusion 
 ki1 (μg/g·min1/2133.06 198.15 278.08 302.99 246.70 
 ki2 (μg/g·min1/26.69 15.86 13.18 11.97 23.41 
Figure 5

(a) Rate constant (k2) of pseudo-second-order adsorption (g/μg·min) and (b) experimental adsorption capacity per maximum adsorption capacity (qt/qm) of nZVI-D at various As(III) initial concentrations (C0 between 1,000 and 2,000 μg/L).

Figure 5

(a) Rate constant (k2) of pseudo-second-order adsorption (g/μg·min) and (b) experimental adsorption capacity per maximum adsorption capacity (qt/qm) of nZVI-D at various As(III) initial concentrations (C0 between 1,000 and 2,000 μg/L).

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