# Silica-Coated Magnetic Nanocomposites for Pb\({}^{2+}\) Removal from Aqueous Solution
## 3 Results and Discussion
### Structural Characterization of the Samples
#### 3.1.5 N\({}_{2}\) Adsorption-Desorption Analysis  
In Figure 5 are presented the nitrogen adsorption-desorption isotherms for the synthesized nanocomposites with the pore size distributions in the inset.  
According to IUPAC classification [32], the adsorption-desorption isotherms of the samples correspond to a type IVa isotherm with a H3 hysteresis type. Type IV isotherm is typical for mesoporous materials, showing the characteristic hysteresis loop associated with the occurrence of pore condensation. The limiting uptake over a range of high P/P\({}_{0}\) results in a plateau on the isotherm, indicating complete pore filling. The initial part of the type IV isotherm is attributed to monolayer-multilayer adsorption. H\({}_{3}\) hysteresis is associated with non-rigid aggregates of plate-like particles which generate slit-like pores.  
The specific surface area and total pore volume of nanocomposites increase with the increase of the targeted iron oxide concentration (Table 5) until a maximum of 270-275 m\({}^{2}\)/g, reached for samples MNPs35@SiO\({}_{2}\) and MNPs21@SiO\({}_{2}\). All the synthesized nanocomposites presented large and disordered pore size distributions.  
To investigate the stability of MNPs35@SiO\({}_{2}\) nanocomposite in aqueous media N\({}_{2}\) adsorption-desorption isotherms before and after adsorption process of Pb\({}^{2+}\) ions were carried out and the results are presented in Figure 6.  
Figure 5: The N\({}_{2}\) adsorption-desorption isotherms of the magnetic nanocomposites. Inset: pore size distribution curves of the samples.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Sample** & **Specific Surface Area 1 [m\({}^{2}\)/g]** & **Total Pore Volume [cc/g]** & **Pore Diameter 2 [nm]** \\ \hline
**MNPs35@SiO\({}_{2}\)** & 271 & 1.0 & 24.9 \\
**MNPs21@SiO\({}_{2}\)** & 275 & 1.3 & 3.0 \\
**MNPs15@SiO\({}_{2}\)** & 150 & 0.8 & 12.3 \\ \hline \hline \end{tabular}  
* 1 calculated by MultiBET method [26]; 2 approximated from desorption branch using BJH (Barret-Joyner–Halenda) method.  
\end{table}
Table 5: Specific surface area, total pore volume and pore diameter of the samples.  
According to IUPAC classification, both adsorption-desorption isotherms, before and after Pb\({}^{2+}\) ions adsorption, correspond to a type IVa isotherm with a H3 hysteresis. The structure of the nanocomposite has not changed after the adsorption process, indicating that the material has not degraded during the adsorption process. Specific surface area, total pore volume and pore diameter of the nanocomposite before and after the adsorption process are presented in Table 6.  
From Table 6, it can be observed that the values of specific surface areas, total pore volume and pore diameter are decreasing after adsorption process of Pb\({}^{2+}\) ions, indicating that a part of the pores are filled by the adsorbed Pb\({}^{2+}\) ions.
Figure 7 shows the appearance of the plateau of pH\({}_{\text{PZC}}\) in the range of pH between 4-8. This means that the surface of the material, at pH values below 4, is predominantly positively charged and at values greater than 8, is predominantly negatively charged. In conclusion, for good efficiency of the adsorption process of the synthesised material, it is indicated that the pH of the solution with metal ions to be in the range of 4-8.