Nanostructured polyaniline rice husk composite as adsorption materials synthesized by different methods

In order to establish the equilibration time for maximum uptake of metal ions onto regarded composites the contact time was varied from 10 to 80 min. The results showed that both metal ions were rapidly removed by composites after 15 min, but, a slightly significant desorption of Cd²⁺ ion can be observed in continuing contact time (figure 4) in the case of chemical method.

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Figure 5 shows the effect of varying initial metal concentration on the adsorption under contacting time of 40 min. The removal percentages of both metal ions are decreased with increasing their initial concentration. However, the removal degree was better for lead(II) ion (a), but worse for cadmium(II) ion (b) if comparing composite prepared by soaking method to that by chemical one.

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We have formula

$$\begin{equation} \frac{C}{q} = \frac{1}{{K_{\rm{L}} q_{\max } }} + \frac{1}{{q_{\max } }}C, \end{equation} \tag{ 2 }$$

where C is metal ion concentrations (mg l⁻¹) and q is adsorption capacity (mg g⁻¹) at equilibrium, K_L is the Langmuir constant (l mg⁻¹) and q_max is the maximum adsorption capacity (mg g⁻¹) [10].

The data given in figure 6 and table 2 showed that the maximum adsorption capacity q_max of lead(II) ion onto PANi–RH composite obtained by soaking method (200 mg g⁻¹) was half as much again as that by chemical one (131.5789 mg g⁻¹), while it was reduced about one-third for the case of cadmium(II) ion adsorption. However, their adsorption process fitted relatively well into the Langmuir model.

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The dimensionless Langmuir parameter R_L, which represents characteristics of adsorption process, can be defined as below:

$$\begin{equation} R_{\rm{L}} = \frac{1}{{1 + K_{\rm{L}} C_0 }}, \end{equation} \tag{ 3 }$$

where K_L is Langmuir constant (l mg⁻¹) and C₀ is initial concentration (mg l⁻¹).

The calculated R_L values given in table 3 indicating that adsorption of Pb²⁺ and Cd²⁺ ions onto both PANi–RH composites is favourable because of 0 < R_L < 1 [11], however, favourable degree is cut down with increasing initial metal ion concentration due to decreasing R_L values.

We have formula

$$\begin{equation} \log q = \log K_{\rm{F}} + (1/N_{\rm{F}} )\log C, \end{equation} \tag{ 4 }$$

where C is metal ion concentrations (mg l⁻¹) and q is adsorption capacity (mg g⁻¹) at equilibrium, K_F is Freundlich constant (mg g⁻¹) and N_F is Freundlich parameter [12].

As shown in figure 7 and table 4, the obtained results explained that adsorption of lead(II) ion fitted into Freundlich isotherm model better than that of cadmium(II) one because of higher R² values, however, it was worse than that in comparison with Langmuir model (table 2). Otherwise, it was found 1 < N_F < 5 confirming that the adsorption process was also favourable [13].

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We have formula

$$\begin{equation} q = \frac{{RT}}{b}\ln K_{\rm{T}} + \frac{{RT}}{b}\ln C, \end{equation} \tag{ 5 }$$

where C is metal ion concentrations (mg l⁻¹) and q is adsorption capacity (mg g⁻¹) at equilibrium, R is universal gas constant (8.314 J mol⁻¹ K⁻¹), T is Kelvin temperature (K), K_T is the Temkin isotherm equilibrium binding constant (l g⁻¹) and b is Temkin isotherm constant (kJ mol⁻¹) [13] (figure 8).

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The smaller Temkin correlation coefficients (R²) for cadmium ion (0.6329 and 0.7063) in comparison with those for lead(II) ion (0.8338 and 0.9614) in table 5 indicate that Temkin equation cannot be used to model the adsorption of Cd²⁺ ion onto above PANi–RH composites correctly.