## 4 Nanostructured Materials for Uranium Removal
### Application of a Novel Graphene Oxide-Activated Carbon Felt Composite for U(VI) Removal  
[PERSON] et al. (2013) [67], prepared a composite material, Graphene oxide (GO)-activated carbon felt(ACF)(GO-ACF) and tested for the removal of U(VI) from aqueous solution, comparing the simple ACF and the composite GO-ACF materials. The adsorption of U(VI) on ACF is remarkably improved by the presence of GO, covalently bonding with ACF. The maximum sorption capacity of GO-ACF for U(VI) was evaluated to be 298 mg g\({}^{-1}\) at pH 5.5, i.e., much higher than that of ACF (173 mg g\({}^{-1}\)), suggesting that the carboxyl functional groups of GO-ACF can play an important role in the sorption. Application of a Three-Dimensional Layered Double Hydroxide-Graphene Hybrid Material for U(VI) Removal  
Graphene has recently attracted attention for its U(VI) sorption and recent studies have reported a substantially high maximum sorption capacity of 299 mg g\({}^{-1}\) at the pH value 4 [79], mainly due to the formation of inner-sphere surface complexes of U(VI) on GO. In the past few years, the layered double hydroxides (LDHs) containing transition metals, have also been employed as effective adsorption materials. Their large interlayer space and the high concentration of active sites have allowed the preparation of several multi-functional LDH materials to be used as anion exchangers, adsorbents, or magnetic materials. [PERSON] et al. (2015) [80] synthesized a 3-D hierarchical composite with graphene sandwiched between two layers of NiAl-LDH nanosheets, by using a simple and cost effective in situ growth procedure; during the in situ crystallization process NiAl-LDH nanosheets grew on the surface of GO@AlOH sheets, obtained by mixing boehmite AlOOH primer sol with GO solution, under specific conditions [80]. When examined as an adsorbent for U(VI) removal, several advantages of this NiAl-LDH composite material make it specifically attractive, because: (1) it is manufactured using an easy, non-toxic synthesis procedure; (2) the larger specific surface area of 257 m\({}^{2}\) g\({}^{-1}\) can provide more adsorptive sites; (3) it can be used for the effective adsorption/removal of uranium(VI) ions. The adsorption of U(VI) onto rGO/LDH was carried out by varying the pH values (2-12); it was found that the adsorption capacity was highly dependent on pH value. At pH less than 4, U(VI) is present in solution predominantly in the form of UO\({}_{2}\)\({}^{2+}\) with a lower sorption capacity, mainly due to the competition of H\({}^{+}\) ions for the binding sites of adsorbent. At the pH range 4-8, the hydrolysis of uranyl ions occurs, producing several uranium species that include UO\({}_{2}\)(OH)\({}^{+}\), (UO\({}_{2}\))(OH)\({}_{2}\), (UO\({}_{2}\))\({}_{3}\)(OH)\({}_{5}\)\({}^{+}\) and (UO\({}_{2}\))\({}_{2}\)(OH)\({}_{2}\)\({}^{2+}\), which are available for adsorption onto rGO/LDH, and the sorption reaches a maximum value, especially in the absence of carbonates. In pH values \(>\) 8, the fraction of (UO\({}_{2}\))\({}_{3}\)(OH)\({}_{7}\)\({}^{-}\) anions (known for their generally low sorption affinity) increases, leading to a decrease of uranium(VI) uptake. As a consequence, the pH value 4 is considered optimum for the adsorption of U(VI) onto rGO/LDH with maximum sorption capacity of the rGO/LDH composite for uranium (VI) 278 mg g\({}^{-1}\). Therefore, rGO/LDH is a novel adsorbent, exhibiting a bright future for practical application, regarding the removal of U(VI) from aqueous solutions [80]. Application of Graphene Oxide and Its Amine-Functionalized Composite (GO-NH\({}_{2}\)) for U(VI) Removal  
A new amine-functionalized graphene oxide (GO-NH\({}_{2}\)) nanosheet was prepared via covalently grafting reaction by [PERSON] et al. (2016) [81]. The adsorption capacities of GO and of GO-NH\({}_{2}\) were found to be 97 and 215 mg g\({}^{-1}\) at 298 K, respectively. These results showed that the adsorption capacity of GO was significantly improved by amine functionalization.