# Gelatin-Siloxane Hybrid Monoliths as Novel Heavy Metal Adsorbents
## 3 Results and Discussion
### Resistance of Gelatin Beads to Shaking in Water Solutions
#### 3.6.4 The Effect of Metal Ions Concentration  
The effect of metal ions concentration on adsorption is shown in Figure 10. The effect of the initial concentration of metal ions on the adsorption efficiency was tested at the adsorbent dose of 0.4 \(\pm\) 0.02 g. Due to the less effective adsorption of metal ions on the hybrid GS 1:1 further experiments and calculations were made only for the GS 2:1 hybrid monolith. The results confirmed that the increase in the initial concentration of metal ions had a positive effect on their adsorption and resulted in the increased amount of the adsorbed metal. This phenomenon can be explained by the fact that the adsorption capacity increased as there were still enough active sites on hybrid monoliths to fill. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & \multicolumn{3}{c}{**Pseudo-First Order**} & \multicolumn{3}{c}{**Pseudo-Second Order**} \\ \cline{2-7}
**Metal Ions** & \(k_{1}\) & \(q_{e}\) & \(\mathbf{R}^{2}\) & \(k_{2}\) & \(q_{e}\) & \(\mathbf{R}^{2}\) \\ \cline{2-7}  & **min\({}^{-1}\)** & **mg/g** & & **g/(mg min)** & **mg/g** & \\ \hline \multicolumn{7}{c}{GS 2:1} \\ \hline Cu & 0.00568 & 0.893 & 0.915 & 4.389 & 1.045 & 0.993 \\ \hline Pb & 0.00564 & 1.92 & 0.974 & 1.340 & 1.892 & 0.978 \\ \hline Cd & 0.00231 & 1.33 & 0.968 & 1.910 & 1.584 & 0. Langmuir and Freundlich adsorption isotherm models were used to describe the adsorption of studied metals ions on gelatin-siloxane monoliths. The Langmuir isotherm is given by the following equation [35]:  
\[q_{e}=\frac{q_{max}\cdot K_{L}\cdot C_{e}}{1+K_{L}\cdot C_{e}}\]  
where \(q_{e}\) (mg/g) is the equilibrium adsorption capacity; \(q_{max}\) (mg/g) -the maximum adsorption capacity; \(K_{L}\)-the Langmuir constant; \(C_{e}\) [mg/L]-the equilibrium concentration after the adsorption process. The Freundlich isotherm is given by equation:  
\[q_{e}=K_{F}C_{e}^{\frac{1}{6}}\]  
where \(q_{e}\) (mg/g) and \(C_{e}\) (mg/L), \(K_{F}\)- the Freundlich constant and \(1/n\)-the intensity of adsorption. The Langmuir and Freundlich isotherm constants with the correlation coefficients are shown in Table 4.  
The Langmuir model describes the maximum adsorption of metal ions on completely homogeneous surface with negligible interaction between adsorbed molecules. The values of \(q_{max}\) and \(K_{L}\) were calculated to be 2.69 and 0.0193 (Cu), and 3.5 and 0.2507 (Pb) and 3.09 and 7.9 (Cd), respectively. The adjustment of the isotherm to the experimental data is not very good as indicated by the correlation coefficient values (R\({}^{2}\)), which range from 0. 0.849 to 0.938. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Metal Ions**} & **pH** & \multicolumn{2}{c}{**Langmuir Isotherm**} & \multicolumn{3}{c}{**Freundlich Isotherm**} \\ \cline{2-7}  & & \(q_{max}\) & \(K_{L}\) & **R\({}^{2}\)** & \(K_{F}\) & \(n\) & **R\({}^{2}\)** \\  & & **mg/g** & **L/mg** & & **mg/g** & **(L/mg)\({}^{(\)**L/In**\()}\)** & \\ \hline Cu & 2.688 & 0.0193 & 0.938 & 0.4720 & 5.5889 & 0.971 \\ \hline Pb & 3.500 & 0.2507 & 0.849 & 0.7111 & 2.0151 & 0.909 \\ \hline Cd & 3.088 & 7.9000 & 0.877 & 0.1816 & 6.5908 & 0.817 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Isotherm model constants and correlation coefficients for metal ions adsorption. The Freundlich \(K_{F}\) and \(n\) constants were estimated from 0.1816 to 0.7111 and from 2.015 to 6.591. Based on the obtained results, data are assumed to be more in line with the Freundlich model. It is particularly important to compare adsorption capacity of obtained hybrid monoliths with the values denoted for other adsorbents, also based on the biopolymers. Experimental maximum adsorption capacity for hybrid monoliths was 3.75 mg/g for Pb(II), 1.76 mg/g for Cu(II) and 1.5 mg/g for Cd(II).