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The adsorption capacities of the as-obtained ZVINP/MCM-41 to NDMA were measured individually at pH 7.6 with 20 mg ZVINP/MCM-41 and varied NDMA concentration (1.0–6.0 μg/L). The Henry, Langmuir, and Freundlich models expressed in Equations (7)–(9) were used for modeling these adsorption isotherm data. All the isotherms showed a similar shape and were nonlinear over a wide range of aqueous equilibrium concentrations, shown in Figure 4.
formula
7
formula
8
formula
9
The fitted constants along with regression coefficients (R2) are summarized in Table 2; qm is the maximum adsorption capacity (μg/g). The data for NDMA adsorbed at equilibrium (qe, μg/g) and the equilibrium metal concentration (ce, μg/L) were fitted to the Langmuir adsorption model. The data fit well to the model with correlation coefficients (R2) of 0.996. The Langmuir model is based on three assumptions: (1) the adsorption of molecules is a monolayer adsorption; (2) the adsorption on the adsorbent surface is uniform; (3) there is no interaction among adsorbed molecules. Therefore, the removal of NDMA onto ZVINP/MCM-41 is monolayer uniform adsorption. From Table 2, the calculated maximum adsorption capacity is 23.5 μg/g, which is in agreement with the above-mentioned conclusion obtained from reaction kinetics.
Figure 4

(a) Langmuir, (b) Freundlich, and (c) Henry adsorption isotherm fits of NDMA removal on ZVINP/MCM-41.

Figure 4

(a) Langmuir, (b) Freundlich, and (c) Henry adsorption isotherm fits of NDMA removal on ZVINP/MCM-41.

Table 2

Constants and correlation coefficients of adsorption isotherms

ModelLangmuirFreundlichHenry
Parameter qm B R2 KF n R2 K R2 
Results 23.5 μg/g 0.053 L/μg 0.996 2.62 13.5 0.842 0.00173 L/g 0.356 
ModelLangmuirFreundlichHenry
Parameter qm B R2 KF n R2 K R2 
Results 23.5 μg/g 0.053 L/μg 0.996 2.62 13.5 0.842 0.00173 L/g 0.356 
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