# Silica-Coated Magnetic Nanocomposites for Pb\({}^{2+}\) Removal from Aqueous Solution
## 3 Results and Discussion
### Adsorption Thermodynamics  
Usually, adsorption processes are significantly influenced by temperature. Therefore, to investigate the feasibility and the physiochemical characteristics, the effect of temperature (in the range of 298-318 K) on the adsorption of Pb\({}^{2+}\) ions on the magnetic nanocomposite adsorbent MNPs35@SiO\({}_{2}\) was followed. The thermodynamic parameters were calculated from the slope and the intercept, by using linear plot of lnK\({}_{\rm d}\) function of 1/T, presented in Figure 12. Thermodynamic parameters: Gibbs free energy (\(\Delta\)G\({}^{\circ}\)), free enthalpy (\(\Delta\)H\({}^{\circ}\)), free entropy (\(\Delta\)S\({}^{\circ}\)) and regression coefficient R\({}^{2}\) values are listed in Table 7.  
The positive enthalpy value \(\Delta\)H\({}^{\circ}\), 36.1 kJ-mol\({}^{-1}\) showed that the adsorption process is endothermic in nature. When the adsorbent material is dispersed in solution, the intermolecular hydrogen from the water molecules interacts with oxygen-containing groups from the adsorbent surface, partially  
Figure 11: Adsorption isotherm of Pb\({}^{2+}\) onto magnetic nanocomposite MNPs35@SiO\({}_{2}\). Figure 12: Adsorption isotherm of Pb\({}^{2+}\) onto magnetic nanocomposite MNPs35@SiO\({}_{2}\). converting them into hydrogen bonds. This tends to occupy the active sites of the adsorbent and consequently decrease the adsorption of Pb\({}^{2+}\) ions [40]. The adsorption process in the solid-liquid system is a combination of two processes: (i) the desorption of the solvent (water) molecules adsorbed on the surface of the adsorbent, and (ii) the adsorption of the adsorbate species. In consequence, the energy required for the adsorption process is the energy consumed to displace the water molecules from the surface by the Pb\({}^{2+}\) ions [41]. At the same time, the adsorption of Pb\({}^{2+}\) ions involve electrostatic interactions and also possible complexation, that is consistent with the adsorption abilities at high temperature. The value of the Gibbs free energy, \(\Delta\)G\({}^{\circ}\), \(-\)0.7 kJ\(\cdot\)mol\({}^{-1}\), at 298 K, calculated from the experimental data, is negative, indicating that the adsorption of metal ions on the magnetic nanocomposite is a natural spontaneous process, and becomes more negative with the increase in temperature, \(-\)0.7, \(-\)1.9 and \(-\)3.1 kJ\(\cdot\)mol\({}^{-1}\) for 298 K, 308 K and 318 K, respectively, suggesting that the adsorption of Pb\({}^{2+}\) ions increases with the temperature. This can be attributed to effective surface growth contact between the adsorbent material and the metal ions. Also, this could be possible because the mobility of the Pb\({}^{2+}\) ions in the solution increases with temperature and that the affinity of the metal ions on the magnetic nanocomposite is higher at high temperatures. The positive value of \(\Delta\)S\({}^{\circ}\), 123.4 J\(\cdot\)(mol\(\cdot\)K)\({}^{-1}\) reflects an increase in randomness during the process of Pb\({}^{2+}\) ions adsorption on magnetic nanocomposite, and the existence of the affinity between the adsorbent and Pb\({}^{2+}\) ions [41; 42]. At the same time, the adsorption of Pb\({}^{2+}\) ions involve electrostatic interactions and also possible complexation, which is consistent with the adsorption abilities at high temperature. The value of the Gibbs free energy, \(\Delta\)G\({}^{\circ}\), calculated from the experimental data, is negative, indicating that the adsorption of metal ions on the magnetic nanocomposite is a natural spontaneous process, and becomes more negative with the increase in temperature, suggesting that the adsorption of Pb\({}^{2+}\) ions increases with the temperature. This can be attributed to effective surface growth contact between the adsorbent material and the metal ions. Also, this could be possible because the mobility of the Pb\({}^{2+}\) ions in the solution increases with temperature and that the affinity of the metal ions on the magnetic nanocomposite is higher at high temperatures.