# Facile Preparation of Graphene Oxide-MIL-101(Fe) Composite for the Efficient Capture of Uranium
## 3 Results and Discussion
### U(VI) Adsorption on the GO-MIL-101(Fe)
The larger value of \(R^{2}\) indicated that the Langmuir model fitted the adsorption  
Figure 5: The effect of different (**a**) cation ions and (**b**) anion ions on the amount of adsorbed U (VI) on MIL-101(Fe) and GO15h-MIL-101(Fe) composites (\(C_{0}\) = 10 mg L\({}^{-1}\), pH = 5.5, T = 298 K, \(m/V\) = 0.2 g L\({}^{-1}\), and I\({}_{\rm NaNO_{3}}\) = 0.01 M), (**c**) comparison of adsorption isotherms of MIL-101(Fe) and GO15-MIL-101(Fe) composites, and (**d**) adsorption isotherms of GO15-MIL-101(Fe) composite under different temperature (T = 298 K, 313 K, 328 K, pH = 5.5, \(m/V\) = 0.2 g L\({}^{-1}\), and I\({}_{\rm NaNO_{3}}\) = 0.01 M). The solid and dashed lines represent the Langmuir model and Freundlich model, respectively. isotherms of both the GO15-MIL-101(Fe) composite and MIL-101(Fe) better than the Freundlich model. This analysis confirmed the monolayer adsorption nature of the U(VI) adsorption on the samples. As summarized in Table S3, the GO15-MIL-101(Fe) composite an exhibited competitive adsorption capacity achieved from the facile one-pot synthetic process in comparison with other adsorbents. As shown in Figure 5d, the thermodynamics of U(VI) adsorption on the GO15-MIL-101(Fe) composite was studied based on the adsorption isotherms at different temperatures. The fitted parameters based on Equations (5) and (6) are listed in Table 3. Clearly, \(q_{max}\) became larger with temperature, indicating that the adsorption of uranyl ions on GO15-MIL-101(Fe) was endothermic [69]. To eValuate how temperature affects the adsorption process, the thermodynamic parameters including the Gibbs free energy change (\(\Delta G^{0}\)), the enthalpy change (\(\Delta H^{0}\)), and the entropy change (\(\Delta S^{0}\)) were achieved from the following equations:  
\[\Delta G^{0}=-RT\ln K^{0} \tag{7}\]  
\[\ln K^{0}=\frac{\Delta S^{0}}{R}-\frac{\Delta H^{0}}{RT} \tag{8}\]  
Firstly, \(K^{0}\), the equilibrium constant, was obtained from plots of \(\ln K_{d}\) as a function of \(C_{e}\) while extrapolating \(C_{e}\) to zero (\(K_{d}=\frac{C_{0}-C_{e}}{C_{e}}\times\frac{V}{m}\)) based on the experimental data. \(\Delta G^{0}\) under different temperatures can be further achieved from Equation (7). Figure S5 showed a good linear relationship between \(\ln K^{0}\) and \(1/T\) for GO15-MIL-101(Fe). The slope and intercept corresponded to \(\Delta H^{0}\) (kJ mol\({}^{-1}\)) and \(\Delta S^{0}\) (J mol\({}^{-1}\) K\({}^{-1}\)) of the adsorption, respectively. The negative \(\Delta G^{0}\) and its increased absolute value, as well as the positive \(\Delta H^{0}\), illustrated that the uptake of U(VI) by GO15-MIL-101(Fe) was spontaneous and endothermic (Table 4). This result manifested that the adsorbed heat for U(VI) dehydration was larger than the energy which was released from the process of uranyl ions binding on the surface of the adsorbents. The positive \(\Delta S^{0}\) (104.34 J mol\({}^{-1}\) K\({}^{-1}\)) and very negative \(\Delta G^{0}\) indicated the strong interaction between the GO15-MIL-101(Fe) and U(VI). The application potential of GO15-MIL-101(Fe) was investigated through several cycles of regeneration and reusing experiments. The low adsorption efficiency of GO15-MIL-101(Fe) under pH \(<\) 3 indicated that U(VI) ions may desorb from adsorbents by decreasing the solution pH. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Adsorbents**} & \multicolumn{3}{c}{**[PERSON]**} & \multicolumn{3}{c}{**[PERSON]**} \\ \cline{2-7}  & **\(q_{max}\) (mg g\({}^{-1}\))** & **b (L mg\({}^{-1}\))** & **\(R^{2}\)** & **\(K_{F}\) (mol\({}^{1-}\) L\({}^{n}\) g\({}^{-1}\))** & **n** & **\(R^{2}\)** \\ \hline
298 & 106.89 & 0.13 & 0.989 & 24.97 & 2.82 & 0.973 \\
313 & 110.15 & 0.17 & 0.988 & 28.85 & 2.96 & 0.901 \\
328 & 133.83 & 0.14 & 0.986 & 32.20 & 2.85 & 0.929 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: The thermodynamic parameters for U(VI) adsorption on and the GO15-MIL-101(Fe) composite.