# 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)
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{\(\mathbf{C_{0}\ (mg\ L^{-1})}\)} & \multicolumn{3}{c}{**Pseudo-First-Order**} & \multicolumn{3}{c}{**Pseudo-Second-Order**} \\ \cline{2-7}  & \(\mathbf{k_{1}\ (min^{-1})}\) & \(q_{e}\ (\mathbf{mg\ g^{-1})}\) & \(R^{2}\) & \(\mathbf{k_{2}\ (g\ \mathbf{mg^{-1}\ \min^{-1})}}\) & \(q_{e}\ (\mathbf{mg\ g^{-1})}\) & \(R^{2}\) \\ \hline MIL-101(Fe) & 0.361 & 18.40 & 0.577 & 0.0485 & 18.88 & 0.999 \\ GO15-MIL-101(Fe) & 0.179 & 36.93 & 0.961 & 0.0117 & 38.46 & 0.999 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The kinetic parameters for U(VI) adsorption on MIL-101(Fe) and GO15-MIL-101(Fe). Figure 4: (**a**) The adsorption efficiency of the MIL-101(Fe) and GO-MIL-101(Fe) composite towards U(VI) as a function of pH (\(C_{0}\) = 10 mg L\({}^{-1}\), T=298 K, \(m/V\) = 0.2 g L\({}^{-1}\), and \(\mathrm{I_{NaNO_{3}}}\) = 0.01 M), (**b**) the effect of pH on the zeta potential of the GO10 (15, 20)-MIL-101(Fe) composite and MIL-101(Fe), the effect of (**c**) adsorbent dose and (**d**) ionic strength on the adsorption efficiency of MIL-101(Fe) and GO15-MIL-101(Fe) composites. (**e**) The amount of uranyl ions adsorbed on MIL-101(Fe) and GO15-MIL-101(Fe) composites under different contact times. The experimental condition is \(C_{0}\) = 10 mg L\({}^{-1}\), pH = 5.5, T = 298 K, \(m/V\) = 0.2 g L\({}^{-1}\), and \(\mathrm{I_{NaNO_{3}}}\) = 0.01 M for (**b–e**). (**f**) pseudo-second-order simulation result for the adsorption of the U(VI) on MIL-101(Fe) and GO15-MIL-101(Fe) composites. in the presence of Ca\({}^{2+}\) and CO\({}_{3}\)\({}^{2-}\), while that for MIL-101(Fe) was largely reduced. Several studies have eVidenced that in the presence of Ca\({}^{2+}\) and dissolved CO\({}_{3}\)\({}^{2-}\), the formation of Ca\({}_{2}\)UO\({}_{2}\)(CO\({}_{3}\))\({}_{3}\)(aq) can decrease the adsorption of U(VI) on adsorbents [67]. eVen in aqueous solutions without Ca\({}^{2+}\), the existence of CO\({}_{3}\)\({}^{2-}\) with the concentration of 10 mM can greatly influence the speciation of U(VI) [68]. (UO\({}_{2}\)CO\({}_{3}\)(aq)) become dominant U(VI) species at pH = 5.5, leading to the decreased adsorption capacity. The slightly affected performance of GO15-MIL-101(Fe) indicated that the synergistic effect between GO and MIL-101(Fe) could greatly improve the capability of the adsorbents in resisting the interference of coexisted ions. The high retention of adsorption efficiency in simulated groundwater and surface water (Figure S4) indicated the potential of GO15-MIL-101(Fe) in uranium elimination from various wastewaters. The maximum adsorption capacity of MIL-101(Fe) and composite samples were determined by plotting the adsorption isotherms (\(q_{e}\) vs. \(C_{e}\)) under pH = 5.5 and T = 298 K. Figure 5c indicated that with the increasing of equilibrium U(VI) concentration, the equilibrium adsorption capacity increased rapidly at first and then reached a saturated value. Obviously, the GO15-MIL-101(Fe) composite possessed much higher adsorption capacity than pure MIL-101(Fe). The adsorption isotherms were simulated through the Langmuir model (Equation (5)) and Freundlich model (Equation (6)). The Langmuir model describes monolayer adsorption onto the homogeneous surface with equivalent active sites. In contrast, the Freundlich model refers to multilayer adsorption on a surface with a heterogeneous surface. \[q_{e}=\frac{bq_{max}C_{e}}{1+bC_{e}} \tag{5}\]  
\[q_{e}=K_{F}C_{e}^{1/n_{F}} \tag{6}\]  
where \(q_{max}\) refers to the maximum adsorption capacity, \(b\) is the Langmuir constant (L mg\({}^{-1}\)), \(K_{F}\) and \(n_{F}\) represent the Freundlich constants corresponding to the capacity and intensity of adsorption, respectively. As listed in Table 2, the calculated \(q_{max}\) was 106.89 mg g\({}^{-1}\) for the GO15-MIL-101(Fe) composite, which was much larger than that of MIL-101(Fe) (68.93 mg g\({}^{-1}\)), respectively.