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Three isotherm models, namely Freundlich, Langmuir and D–R, were used to describe the equilibrium data of As(III) on nZVI-D. The Freundlich isotherm, Langmuir isotherm and D–R parameters are displayed in Table 2. It can be seen that the experimental data of As(III) adsorption by nZVI-D can be well fitted with Langmuir adsorption isotherms due to high correlation coefficients (R2 = 0.99), more than those of the Freundlich (R2 = 0.95) and D–R (R2 = 0.65) adsorption isotherms. Therefore, the removal of As(III) by nZVI-D implied that a monolayer (chemisorption) of As(III) covered the surface of the adsorbent and that the surface contained a finite number of particular adsorption sites (Karagöz et al. 2008). The maximum adsorption capacity (qm) was 2,734 μg/g and RL of the Langmuir isotherm was 0.025–0.044, which was obtained from the intercept of the plots of Ce/qe versus Ce, at a different initial concentration (Table 2). The RL value of the Langmuir isotherm in a range of 0–1 indicates the nature of the adsorption process (Monash & Pugazhenthi 2009), therefore, RL in the study was in the range of 0–1, which indicated a favorable adsorption process for As(III) on the nZVI-D surface. It can also be concluded that the effect of the initial concentration affected the absorption rate, but that this is not a linear relationship, that is, when the initial concentration is doubled, adsorption capacity to absorb will not be doubled. The correlation coefficient of the D–R isotherm is lower than that of the Freundlich and Langmuir isotherms, suggesting that the As(III) adsorption onto nZVI-D is not a physical process.

Table 2

The values of different adsorption isotherms for As(III) adsorption of nZVI-D at different As(III) initial concentrations ranging between 1,000 and 2,000 μg/L (temperature: 30 °C, 0.75 g/L of nZVI-D, initial pH 6)

Langmuir constants Freundlich constants D–R constants 
qm (μg/g) 2,734.49 n 7.05 qD (mg/g) 1,830.12 
KL (L/mg) 0.0201 KF ((μg/g)•(L/μg)1/n706.15 β 8.12 × 10–5 
RL 0.025–0.044 R2 0.948 E 7.85 
R2 0.994   R2 0.647 
Langmuir constants Freundlich constants D–R constants 
qm (μg/g) 2,734.49 n 7.05 qD (mg/g) 1,830.12 
KL (L/mg) 0.0201 KF ((μg/g)•(L/μg)1/n706.15 β 8.12 × 10–5 
RL 0.025–0.044 R2 0.948 E 7.85 
R2 0.994   R2 0.647 

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