# 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)  
Figure 4a showed the change of the adsorption efficiency of MIL-101(Fe) and GO-MIL-101(Fe) towards U(VI) as a function of pH. With pH varies ranging from 2.0 to 11.0, the adsorption percentage of uranyl ions increased slowly firstly (Stage I), followed by a rapid improvement (Stage II) and then it was maintained at a high level (Stage III). GO-MIL-101(Fe) composite materials exhibited decreased adsorption efficiency after Stage III. However, the pH turning point of Stage I-II and Stage II-III is different for MOF and GO-MOF composites. Pure MIL-101(Fe) and GO10-MIL-101(Fe) ended stage I at pH = 4, while that for GO15-MIL-101(Fe) and GO20-MIL-101(Fe) is pH = 3. Furthermore, pure MIL-101(Fe) begins stage III at pH > 7.4. In comparison, GO-MIL-101(Fe) composite reached high level of adsorption efficiency at lower pH (GO10-MIL-101(Fe)/92.2% at pH = 7, GO15-MIL-101(Fe)/98.1% at pH = 6.5, GO20-MIL-101(Fe)/96.7% at pH = 7). Considering the complexity of contaminated water sources, the adsorption efficiency of GO15-MIL-101(Fe) was tested with different initial U concentrations. Under initial U(VI) concentration of 0.035 mg L\({}^{-1}\), 1.3 mg L\({}^{-1}\) and 3 mg L\({}^{-1}\), the adsorption percentage was 40%, 93.1%, and 99.6% with 21 ug L\({}^{-1}\), 89 ug L\({}^{-1}\), and 12 ug L\({}^{-1}\) U(VI) left in water (Figure S3), respectively. The lowered adsorption percentage at U(VI) concentration of 0.035 mg L\({}^{-1}\) may originate from the reduced driving force. The final U concentration upon treatment of contaminated waters with C\({}_{0}\) = 0.035 mg L\({}^{-1}\) and 3 mg L\({}^{-1}\) by GO-MIL-101(Fe) can reach the requirement of the Chinese National Emission Standard (0.05 mg L\({}^{-1}\)) for discharged water and  
Figure 3: (**a**) The survey spectra obtained from X-ray photoelectron spectroscopy (XPS) of GO15-MIL-101(Fe) before and after the adsorption of U(VI), high-resolution spectra of U4f for (**b**) GO15-MIL-101(Fe)-U, C1s for (**c**) GO15-MIL-101(Fe) and (**d**) GO15-MIL-101(Fe)-U, O1s for (**e**) GO15-MIL-101(Fe) and (**f**) GO15-MIL-101(Fe)-U.  
the US Environmental Protection Agency (EPA) (0.03 mg L\({}^{-1}\)) for potable water. This result indicated that application potential of as-prepared composite samples in the treatment of U contaminated water in a specific concentration range. Due to the highest adsorption percentage in the whole pH range, GO-15 MIL-101(Fe) was selected as the model sorbent for further adsorption experiments. Moreover, the deprotonating condition of surface functional groups can greatly affect the performance of adsorbents. Figure 4b displayed the surface charge information of the GO-MIL-101(Fe) composites and pure MIL-101(Fe). All the samples showed a decreasing trend from positive charges with pH value increased in the range of 2.0-11.0. The positive zeta potential originated from protonation of amine group in the organic linker of MIL-101(Fe), which were the dominant components of the composite samples. Compared with pure MIL-101(Fe), the composite samples had a lower zeta potential due to the introduction of GO. At each pH, GO15-MIL-101(Fe) exhibited the most negative zeta potential. The isoelectric point of GO-15 MIL-101(Fe) appeared at the lowest pH. The zeta potential result of as-prepared samples agreed well with the effect of pH on the adsorption efficiency of the samples towards U(VI). For GO15-MIL-101(Fe), the surface negative charging greatly improved with the dissociation of oxygenated groups on GO nanosheets in the pH range of 3.0-7.0, which favored the adsorption of U(VI) cations. As shown in Figure S4, negative charged UO\({}_{2}\)(OH)\({}_{4}\)\({}^{2-}\) would dominate in aqueous solution with pH > 9.0 [17]. The repulsion between the UO\({}_{2}\)(OH)\({}_{4}\)\({}^{2-}\) species and the composite gave rise to the decreased adsorption efficiency at high pH value. The change of adsorption percentage with pH indicated the importance of the electrostatic interaction in the uptake of U(VI) on the composite.