# Facile Preparation of Graphene Oxide-MIL-101(Fe) Composite for the Efficient Capture of Uranium
## 1 Introduction  
The severe energy and environmental crisis calls for the development of green energy sources. Nuclear power is an effective solution for this problem as a sustainable energy source with a high efficiency. However, the radioactive contamination from the production process of nuclear energy can cause potential ecological threats and biological toxicity [1; 2; 3]. In particular, uranium-contamination is included in the major nuclide pollutants that are urgently required to be eliminated from the polluted waters [4]. The widely used techniques for the removal of uranyl ions consist of solvent extraction [5], ion-exchange [6], and adsorption. In particular, adsorption takes advantages of the rapid and highly efficient treatment of uranium by developing adsorbent materials with programmable functionalization [7]. Until now, much attention has been paid to the development of nanostructured adsorbents for higher adsorption efficiency [8; 9]. One of the most promising directions is the fabrication of composite materials which combine the functionality of individual components into one [10; 11]. The key point lies in the component selection and sophisticated structure design for the optimization of synergistic effect inside the composite materials. Among the various adsorbents, functional carbon nanomaterials including one dimensional carbon nanofiber/nanotube, two-dimensional graphene oxide (GO) [12; 13], and three dimensional functionalized carbon nanostructures take the advantages of a large surface area and easy functionalized capability, which endow them the great potential as novel adsorbents for uranium-based nuclide species [14; 15; 16; 17; 18]. [PERSON] et al. reported that the adsorption of uranyl ions on pristine carbonnanotube (CNT) reached 5.0 mmol g\({}^{-1}\) at pH = 5.0 under 298 K [19]. [PERSON] et al. demonstrated the good adsorption performance of GO towards U(VI) in aqueous solution [20]. The tedious and high-cost synthetic procedures of CNT and GO promote the studies on the fabrication of CNT/GO-based composites which maintain a good performance at a lower dosage of CNT and GO by the introduction of other components and optimizing the synergistic effects [21; 22]. Compared with CNT, fabrication of GO-based composite materials benefits the adsorption properties in several ways. On the one hand, the expansive surface of GO gives rise to the easy attachment of other nanomaterials, making them excellent substrates. On the other hand, the formation of composites through surface growth method helps overcome the aggregation problem of GO in solutions, which improves the exposure of functional groups. In the past few years, various GO-based composite adsorbents with diverse functionalities have been designed and fabricated, including a magnetic GO composite [23], GO-polymer composite [24; 25], GO-layered double hydroxides (LDHs) composite [26], etc. Metal-organic frameworks (MOFs) are a series of crystalline hybrids based on the bridging network between metal ions and organic ligand molecules [27]. Owing to their high surface area, tunable porous structure, and versatile chemical functionality, MOFs have become high-profile candidates in application fields of catalysis [28], adsorption [29; 30; 31; 32], and membrane formation [33]. Very recently, MOFs have found new opportunities in the removal of uranyl ions [34; 35]. MIL-101, as a series of MOFs with different metal centers (Al, Cr, and Fe), has received much attention in the recovery of uranium polluted waters due to its chemical stability in acid solutions [36; 37; 38]. Compared with pristine materials, functionalized MOFs have shown optimized adsorption performance with enriched binding sites [39]. The post-synthetic strategy is the commonly used method to introduce functional groups on the MIL-101 host. However, the multistep post-grafting process is time-consuming and usually involves polluting acid such as HNO\({}_{3}\)[40].