# Silica-Coated Magnetic Nanocomposites for Pb\({}^{2+}\) Removal from Aqueous Solution
## 3 Results and Discussion
### Structural Characterization of the Samples
#### 3.2.3 Effect of Contact Time and Temperature on the Adsorption of Pb\({}^{2+}\) Ions  
The plot of Pb\({}^{2+}\) ions adsorption versus contact time with the nanocomposite MNPs35@SiO\({}_{2}\) at three temperatures (298, 308 and 318 K) is presented in Figure 9.  
The experimental data shows that the adsorption capacity of the adsorbent material increases with contact time, reaching a maximum after 90 min. This high initial adsorption rate is due to the availability of a large number of adsorption sites present at the surface of the adsorbent at the beginning of the process. The selected contact time for further studies is set to 90 min. Increasing the temperature from 298 K to 318 K a small increase in the adsorption capacity, namely from 2.14 mg g\({}^{-1}\) to 2.33 mg g\({}^{-1}\) is obtained. This increase in adsorption capacity is economically insignificant, consequently the studies were continued at 298 K.  
2.4 Adsorption Process Mechanism for Uptake the Pb\({}^{2+}\) Ions onto the Magnetic Nanocomposite Adsorption Kinetics  
The kinetics of the Pb\({}^{2+}\) adsorption process from aqueous solution using the nanocomposite MNPs35@SiO\({}_{2}\) are studied to show the efficiency of the adsorption process. The pseudo-first-order hand the pseudo-second-order kinetic equations were applied to explore the mechanisms of the adsorption processes. The fitting curves of ln(q\({}_{\text{e}}\)-q\({}_{\text{t}}\)) and t/q\({}_{\text{t}}\) versus time are shown in Figure 10a,b, respectively. The relevant kinetic model parameters for Pb\({}^{2+}\) adsorption are presented in Table 7.  
Figure 9: The effect of contact time and temperature for the Pb\({}^{2+}\) adsorption on the magnetic nanocomposite MNPs35@SiO\({}_{2}\). Kinetic models can identify the type of adsorption mechanism of the system and the potential stages to control the rate, including mass transport processes and chemical reactions [38]. The pseudo-first-order kinetic models, or the Lagergren model and pseudo-second-order, or the [PERSON] and McKay model [39], were applied to describe the kinetics of the adsorption of Pb\({}^{2+}\). The kinetic model that describes best the adsorption process can be established based on the values of the resulting constants and the obtained regression coefficient (R\({}^{2}\)). Thus, from the data presented in Table 7, it can be observed that the kinetic model of pseudo-second-order is the model that describe the adsorption process of Pb\({}^{2+}\) ions on MNPs35@SiO\({}_{2}\) nanocomposite, since R\({}^{2}\) is in the range \(\sim\)0.9919-0.9986, depending on temperature. This correlation is in line with the literature data showing that the adsorption process of Pb\({}^{2+}\) ions is influenced by pH and temperature and that the chemical reactions between the functional groups present on the sorbent and the metal ions are the rate-controlling step throughout the sorption process [40; 41; 42]  
According to the correlation coefficient R\({}^{2}\) the Langmuir isotherm describes best the adsorption process of Pb\({}^{2+}\) on the MNPs35@SiO\({}_{2}\) nanocomposite. Moreover, q\({}_{\rm L}\) = 17.1 mg g\({}^{-1}\), is much closer to the experimental value, i.e., q\({}_{\rm m}\), exp = 14.9 mg g\({}^{-1}\). According to the literature, the fact that the adsorption process is better modelled according to the Langmuir model shows that the adsorption process is positively influenced by the increase in the concentration of metal ions [14] and that the adsorption process of Pb\({}^{2+}\) ions on the adsorbent occurs only in a monolayer, all surface sites are energetically identical housing a single Pb\({}^{2+}\) ion, and the ability of a Pb\({}^{2+}\) ion to adsorb on the surface is independent of occupying adjacent sites [43]. At the same time, the adsorption mechanism is controlled by chemosorption processes as a result of strong chelation between Pb\({}^{2+}\) ions and OH\({}^{-}\) groups and/or free electron pairs present on the surface of the adsorbent material [44].