Diaminomaleonitrile functionalized double-shelled hollow MIL-101 (Cr) for selective removal of uranium from simulated seawater
Introduction
Nowadays, nuclear energy has attracted much attention because of the increased demand for clean and higher density energy. The key for the long-term development of nuclear energy is utilize the resource of Uranium, the main source of which at present are terrestrial ores that could run out in less than a hundred years at the current rate of consumption [1]. However, the uranium in oceans is about 4.5 billion tons, which is 1000 times the reserves of uranium on land. But the low concentration (about 3.3 ppb) and amounts of coexisting metal ions severely restrain the exploitation of this resource [2]. Many kinds of adsorbent materials have been applied in the extraction of uranium from the ocean [3], however, a significant challenge remains to adopt a material which effectively selects and recovers uranium from the ocean.
Metal-organic frameworks (MOFs) are a class of hybrid crystalline materials, which utilize metal ions and organic ligands to construct a 3D network structure via coordinated bonds [4]. Owing to their ultrahigh surface area, tunable structure and controlled porosity, high crystallinity and stability [5], they has been widely investigated and used in many areas [6], [7], [8], [9]. Also, several types of MOFs have shown promise for efficient recovery of toxic and radioactive ions including uranium; most of these MOFs have been used as adsorbents after functionalization by organic groups [6], [10], [11], [12], [13], [14], [15]. MIL-101(Cr) is a well-known MOF with two types of mesoporous zeotypic cages and microporous windows, having high stability in aqueous solutions [16]. Furthermore, as a substrate material, MIL-101 possesses unsaturated Cr (III) sites [17], which chelate with electron-rich organic groups. From previous reports, several N-containing groups have been introduced on to MIL-101 by means of coordinatively unsaturated sites (CUS) [10], [18], [19], [20]. In particular, N atoms have good selectivity toward uranium [21], [22]. Nevertheless, even taking into account these beneficial properties, the adsorption amount of functionalized MIL-101 series materials has never exceeded 500 mg g−1. Arising from the above, we endeavor to enhance the adsorption capacity of functionalization MIL-101 which use the CUS, enabling availability of numerous accessible metal sites and contact of uranium with functional groups.
Hollow nanostructure materials, with their unique core-shell structure, have been widely applied in many areas, such as drug delivery, catalysis, adsorption of toxic ions [23], [24], [25], [26], [27]. Recently, Huo et al. have reported on the construction of hollow MIL-101 [28] and found that the increased catalytic activity is most likely due to the exposure of the metal sites in each layer especially in the case of the decrease in hollow MIL-101’s surface area. Also, the hierarchical structure provides more accessible sites than those reported in other articles [29], [30]. Therefore, amine groups can easily graft onto the multi-shelled MIL-101 with numerous accessible unsaturated metal sites; its hierarchical structure enhances the contact of uranium with nitrogen atoms.
In our work, we prepared a double-shelled hollow MIL-101(DSHM) by means of a two-step crystal growth and subsequent acetic acid etching progress. To improve the uranium selectivity and adsorption capacity of DSHM, we used a post-synthetic method mentioned above, modifying it with the addition of diaminomaleonitrile (DAMN) which contains two nucleophilic –NH2 and –CN groups that both coordinate with metal sites and uranium. The obtained product was characterized by XRD, nitrogen sorption, FT-IR spectrometry and transmission electron microscope (TEM). We studied in detail the adsorption of uranium on varying pH, contact times and different concentrations of uranium solution. We discuss the mechanism of uranium adsorption. The adsorption performance of our adsorbent was stable under five cycles of adsorption-desorption. Finally, we studied the selectivity of uranium adsorption under the condition of competing ions for the separation of uranium from simulated seawater.
Section snippets
Experimental section
Chemical Materials: Cr(NO3)3·9H2O and terephthalic acid (H2BDC) were purchased from Aladdin Ltd. Diaminomaleonitrile and acetic acid were purchased from Alfa Aesar. Dimethylforamide (DMF) and rthanol were purchased from Alfa Aesar. All of the chemical materials were used without further purification, and we used deionized water for all the solutions during the experiments.
Double-shelled MIL-101 was synthesized in a step-by-step growth and subsequent etching progress. Briefly, 0.8 g of Cr(NO3)3
Results and discussion
Characterization of Materials in Fig. 1 shows the results of double-shelled MIL-101(DSHM) and its derivatives DSHM-DAMN. From the FT-IR spectrum in Fig. 1 (1), a new stretching vibration of adsorption appears at 2235 cm−1, which confirms the presence of CN groups [31]. The three peaks appearing in the range of 3100–3500 cm−1, are mainly caused by the N–H stretching vibration from two NH2 groups. From the X-ray diffraction (XRD) patterns in Fig. 1 (2), we have successfully synthesized
Conclusion
In this article, double-shelled hollow MIL-101 MOF materials were successfully fabricated by a step-by-step synthesis and subsequent acid etching. Modification of diaminomaleonitrile onto the hollow MOF material was achieved by using a facile CUS method which successfully combines the characteristics of the hollow material with a large number of exposed and accessible unsaturated metal sites. By studying uranium adsorption kinetics, we find that the kinetic data fitted the pseudo-second-order
Acknowledgments
This work was supported by National National Science Foundation of China (NSFC 51872057), the Application Technology Research and Development Plan of Heilongjiang Province (GX16A009), Fundamental Research Funds of the Central University and Natural Science Foundation of Heilongjiang Province (QC2018010).
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