graphene oxide composite and application for uranium removal

Accepted Manuscript Facile Fabrication of Magnetic Cucurbit[6]uril/Graphene Oxide Composite and Application for Uranium Removal Lang Shao, Xiaofang Wang, Yiming Ren, Shaofei Wang, Jingrong Zhong, Mingfu Chu, Hao Tang, Lizhu Luo, Donghua Xie PII: DOI: Reference:

S1385-8947(15)01473-4 http://dx.doi.org/10.1016/j.cej.2015.10.062 CEJ 14338

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

22 August 2015 21 October 2015 25 October 2015

Please cite this article as: L. Shao, X. Wang, Y. Ren, S. Wang, J. Zhong, M. Chu, H. Tang, L. Luo, D. Xie, Facile Fabrication of Magnetic Cucurbit[6]uril/Graphene Oxide Composite and Application for Uranium Removal, Chemical Engineering Journal (2015), doi: http://dx.doi.org/10.1016/j.cej.2015.10.062

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Facile Fabrication of Magnetic Cucurbit[6]uril/Graphene Oxide Composite and Application for Uranium Removal

Lang Shaoa, Xiaofang Wang*b, Yiming Rena, Shaofei Wanga, Jingrong Zhonga, Mingfu Chua, Hao Tanga, Lizhu Luob, and Donghua Xieb

a

Institute of Materials, China Academy of Engineering Physics, PO Box 9071-11, Mianyang

621907, China. b

Science and Technology on Surface Physics and Chemistry Laboratory, PO Box 9-35,

Mianyang 621908, China.

*Corresponding author: Xiaofang Wang E-mail: [email protected]

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Abstract We report on the facile, reliable fabrication of novel magnetic cucurbit[6]uril/graphene oxide (CB[6]/GO/Fe3O4) composite by using a facial in-situ co-precipitation method, for which the formation process and possible formation mechanism were also investigated. Moreover, the composite was used as an adsorbent for the extraction of uranium(VI) from aqueous solution. Herein, GO nanosheets were chosen as a substrate to immobilize CB[6] and Fe3O4. During the co-precipitation of Fe3O4 nanoparticles on surface of GO nanosheets, CB[6] molecules was fixed through hydrogen bonding simultaneously. The as-prepared composite exhibited a high efficiency of magnetic separability, competitive adsorption performance and acceptable reusability and stability for U(VI) removal, indicating a potential application in uranium-bearing wastewater treatment.

Keywords: Removal; Uranium; Cucurbit[6]uril; Graphene oxide; Magnetite; Composite.

1. Introduction With the rapid development of nuclear industry, the problem of the treatment of uraniumbearing wastewater becomes increasingly prominent. Uranium(VI) is one of the toxic radioactivity elements due to its mutagenic and carcinogenic characteristics [1-4]. U(VI) released into the environment can be hazardous to ecosystem and human health. Therefore, it is significant to remove U(VI) from wastewater before it is discharged into the environment. Various technologies have been developed to remove and recycle U(VI) from wastewater, such as biosorption, solvent extraction, flocculation, co-precipitation, ion exchange, adsorption and so on [3], [5-10]. Because of its cost-effectiveness, high efficiency and easy operation, adsorption is

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one of the most promising methods for U(VI) removal and has attracted more and more attention. Recently, some materials such as zeolites, carbon materials, magnetic composite, polymer and metal organic frameworks etc., have been studied to remove radionuclides from wastewater [11-21]. Among them, magnetic composite materials with unique separable feature under an external magnetic field have attracted considerable interest during the past decades [13, 17, 22-25]. Although there have been many studies on the application of magnetic composite for U(VI) removal, it is still desirable to develop new magnetic composite exhibiting a rapid U(VI) removal efficiency. Cucurbit[n]urils [26] (CB[n], n = 5 – 10), a new family of macrocyles, with highly symmetrical pumpkin shaped and two identical dipolar portal ends composed of carbonyl functional groups, have attracted extensive research interests for their potential application in molecular recognition, supra-molecular assemblies, molecular catalysis, and environmental pollution treatment [27-30]. Furthermore, due to large numbers of hydrophilic portals carbonyl functional groups, CB[n] molecules are expected to have strong affinities to actinyl ions, which are stronger than other macrocyles, such as crown-ethers and cyclodextrins. As far as actinide complexation with CB[n] is concerned, Thuéry and co-workers reported a series of framework type materials using CB[5], CB[6] and CB[7] as ligands partially coordinated to uranyl [31, 32]. The interactions of uranyl species with these ligands vary from direct coordination to hydrogen bonding. Sundararajan and co-workers investigated the various coordination modes of CB[5] and CB[6] host molecules to uranyl through density functional theory [33]. The results suggest that CB[n] can be efficiently used for binding actinyl ions as compared to other macrocylic host molecules. Therefore, CB[n] would be good candidate materials for U(VI) removal. CB[6] as an important member of CB[n] with easy purifying, high yield, and low cost, comparing to other

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CB[n] molecules, is the most promising large-scale produced adsorbent for the adsorption of U(VI). However, CB[6] is less soluble in aqueous environment, which may restrict the accessible active sites of CB[6] towards U(VI), and hence its practical applications have been limited. Dispersion and immobilization of CB[6] in the surface of a suitable material can in principle avoid this disadvantage. To the best of our knowledge, the only way to immobilize CB[6] onto surfaces is functionalization of CB[6] in a strongly oxidative environment to generate reactive functional groups that could further react with functional groups on the surface of a substrate [27]. However, a major shortfall of this method is that the synthesis process is usually tedious, therefore the development of a facile immobilization strategy is of great interest for practical applications. Graphene oxide (GO) nanosheet, as an ideal two-dimensional material, with large specific surface area, high mechanical stability and abundant oxygen-containing functional groups, has proven to be good candidate for supporting other functional nanomaterials [34]. Recently, many efforts have been devoted to the preparation of GO-based magnetic composites for their applications in metal ion removal. For example, Li et al. [35] investigated the removal of Cu(II) and fulvic acid by GO decorated with Fe3O4; Zhao et al. [17] grafted amidoxime on magnetite/graphene oxide composites for the uptake of uranyl. Therefore, it would be attractive to introduce GO as the substrate to fabricate magnetic cucurbit[6]uril/graphene oxide (CB[6]/GO/Fe3O4) composite, which can integrate the collective advantages of CB[6], Fe3O4 and GO. Herein, we report a novel magnetic composite CB[6]/GO/Fe3O4 fabricated via a facile in-situ co-precipitation method. GO nanosheets were covered with hydrogen-bonded CB[6] molecules and a large number of well dispersed Fe3O4 nanoparticles (Fe3O4 NPs). The resultant magnetic

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composites exhibit competitive adsorption performance and acceptable reusability and stability for U(VI) removal in aqueous solution, suggesting their very promising application prospect. Furthermore, the composites can be easily recycled by magnetic separation avoiding secondary pollution and loss. 2. Material and methods 2.1 Materials GO was synthesized from graphite flake (Alfa Aesar) by the Hummers method [36] by using concentrated H2SO4, KMnO4 and H2O2 as the oxidants. CB[6] (C36H36N24O12·5H2O) was purchased from Taiyuan Aisiweida Chemical Technology Co., Ltd. (China). The structure of CB[6] is shown in Figure S1 in the Supplementary data. FeCl3·6H2O and FeSO4·7H2O were purchased from Tianjin Kemiou Chemical Reagent Co., Ltd. (China). All chemicals used were of analytical grade or better and used without further purification. 2.2 Syntheses CB[6]/GO/Fe3O4: 0.25 g GO was dispersed in 150 mL of deionized water and sonicated for 1 h in order to effectively exfoliation. Subsequently, 0.25 g of CB[6] was added and the mixed solution was stirred at 50 °C for 0.5 h. Then, 1.488 g FeCl3·6H2O and 0.768 g FeSO4·7H2O according to the molar ratio of Fe2+: Fe3+ = 1: 2 were added and the reaction system was continued to be stirred under the protection of nitrogen condition for 0.5 h at 80 °C. Next, 15 mL of the ammonia solution (28%) was gradually dropped into the mixture system and the reaction was maintain for 0.5 h. The obtained product was collected under an external magnetic field and washed with distilled water and ethanol. Finally, the obtained product was dried in vacuum oven at 60 °C for 24 h.

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CB[6]/GO/Fe3O4-38.9%: CB[6]/GO/Fe3O4-38.9% was prepared following a similar procedure to CB[6]/GO/Fe3O4, but the adding of FeCl3·6H2O and FeSO4·7H2O is half to that of CB[6]/GO/Fe3O4. GO/Fe3O4-50.7%: GO/Fe3O4-50.7% was prepared following a similar procedure to CB[6]/GO/Fe3O4, but absence of CB[6]. 2.3 Characterizations Scanning electron microscopy (SEM) images were taken on a Sirion 200 electron microscope. Transmission electron microscopy (TEM) images were taken on a Tecnai F20 microscope. Powder X-ray diffraction data were collected on a Rigaku X-ray diffractometer using Cu Kα radiation (λ = 1.5418 Å). Thermogravimetric analysis was performed on a Perkin-Elmer TGA7 unit in air at a heating rate of 10 K min−1. Elemental analysis was carried out on a Vario Micro elemental analyzer. Magnetic measurement was conducted using commercial PPMS-9 (USA Quantum Design Cop.). Fourier transformed infrared spectroscopy (FT-IR) spectroscopy measurements were recorded with a Bruker Optics VERTEX 70 spectrometer in the range of 400–4000 cm–1 at a resolution of 4 cm−1 using the KBr pressed pellet technique. X-ray photoelectron spectroscopy (XPS) measurements were studied using an ESCALAB 250 X-ray photoelectron microprobe. Inductively coupled plasma-atomic emission spectroscopy (ICP-AES) analyses were performed on a Perkin-Elmer Optima 3300 DV ICP instrument. 2.4 Adsorption tests Batch experiments of U(VI) adsorption on CB[6]/GO/Fe3O4 were carried out in plastic vials for which adsorption onto the vial walls was negligible under the experimental conditions. In the adsorption experiments, CB[6]/GO/Fe3O4 was firstly dispersed in a certain amount of deionized water under ultrasonication, and then, solutions of uranyl nitrate were added into

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CB[6]/GO/Fe3O4 suspension. The pH value was measured using a combined glass pH electrode and adjusted by adding a small amount of 0.05 mol L−1 HNO3 or NaOH solutions. After equilibration, the solid phase was conveniently separated by an external magnet. The concentration of uranyl ions in the residual solution was determined by UV-vis spectrometry (UV-1800, Shimadzu Co.) at the wavelength of 651.5 nm using arsenazo-III as the chromogenic agent. 2.5 Regeneration and recyclability measurement After adsorption experiments, U(VI) loaded CB[6]/GO/Fe3O4 was washed with 0.01 mol/L HCl solution and Milli-Q water thoroughly until U(VI) ions were not detected in the rinsing solution. And then, the composites were dried in a vacuum oven at room temperature. Thus the regenerated composites were obtained and reused in adsorption experiments. 2.6 Stability tests The stability of CB[6]/GO/Fe3O4 was measured by examining the quantity of iron ion eluted in 0.01 mol/L HCl solution by means of ICP-AES. 10 mg CB[6]/GO/Fe3O4 was dispersed in 20 mL of HCl solution (0.01 mol/L) and sonicated for 1 h. The solid was collected under an external magnetic field and applied to next cycle of stability test, and the supernatant was used for ICPAES analysis. 3. Results and discussion 3.1 Characterizations of CB[6]/GO/Fe3O4 The microstructures of GO, and the as-prepared CB[6]/GO/Fe3O4 composite were characterized by SEM and TEM measurements. The TEM image for GO is shown in Figure S2. It can be observed that the as-synthesized GO shows the sheet-like structure with a smooth surface and tiny wrinkles. After combination with CB[6] and Fe3O4 to form CB[6]/GO/Fe3O4

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composite, Fe3O4 NPs are disorderly decorated on the surface of GO nanosheets, as shown in Figure 1a. TEM image (Figure 1b) clearly shows that GO nanosheets are the matrix of the Fe3O4 NPs, and the average size of the Fe3O4 NPs is about 13 nm (the inset in Figure 1b). Meanwhile, some wrinkles are observed on the surface of the CB[6]/GO/Fe3O4 composite. However, CB[6] cannot be observed, which is presumably ascribed to the amorphous nature of CB[6] that existed in the molecular state. The crystal phase and structural information of CB[6]/GO/Fe3O4 composite was obtained by XRD measurement (Figure 1c). The diffraction peaks are consistent with the standard XRD data for the cubic phase Fe3O4 (JCPDS 76-1849) with a face-centered cubic (fcc) structure. Because the strong signals of the Fe3O4 NPs tend to overwhelm the weak carbon peaks, no peaks from carbon are observed. Elemental analyses (see Table S1) show the loading of CB[6] moieties was ca. 13.1% according to the mass percent of N atoms. The thermogravimetric curve of CB[6]/GO/Fe3O4 was measured in the temperature range of 25 to 800 °C (Figure S3). The weight losses between 25 and 100 °C correspond to the removal of the physically adsorbed water molecules (4.13%), and the weight losses of 34.81% between 100 and 800 °C correspond to the gradual decomposition and release of GO and CB[6] in the composite. Combine thermogravimetric analysis and Elemental analysis, the percentage composition of CB[6], GO, and Fe3O4 are 13.1%, 21.7% and 61.1%, respectively. The room temperature magnetization saturation value was measured to be 44.20 emu/g for CB[6]/GO/Fe3O4 (Figure 1d). The nonlinear, reversible magnetization curve with no hysteresis exhibits characteristic super-paramagnetic behavior. Thus, CB[6]/GO/Fe3O4 composite can be easily manipulated by an external magnetic field, which is important for fast separation application (the inset of Figure 1d).

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Figure 1. (a) SEM and (b) TEM images (Inset is column chart of Fe3O4 size distribution) , (c) XRD pattern, and (d) magnetization curve of CB[6]/GO/Fe 3O4. The FT-IR spectra of GO, CB[6], and CB[6]/GO/Fe3O4 are shown in Figure 2. In the FT-IR spectrum of pure CB[6] (Figure 2a), the characteristic peak at 3456 cm−1 is due to the crystal water. Peaks at 3001 and 2931 cm−1 are ascribed to C–H bonds stretching vibrations for methylene. The peak at 1743 cm−1 is attributed to the allophanyl C=O stretching vibration. The peak at 1477 cm−1 is ascribed to C–H bonds bending vibrations for methylene. In the FT-IR spectrum of GO (Figure 2b), the peaks at 1726 cm−1, 1614 cm−1, 1354 cm−1 and 1052 cm−1 correspond to carbonyl and carboxyl C=O, aromatic C=C, carboxyl O=C–O, and alkoxy C–O–C stretching vibrational modes, respectively. The stretching vibrations of O–H bonds for hydroxyl and carboxyl O–H can be observed approximately in the region of 3400 – 3000 cm−1. After

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combination with CB[6] and Fe3O4 (Figure 2c), it can be seen clearly that, the strong peak at 588 cm−1 is assigned to the Fe–O vibration of Fe3O4 NPs. As the peak from Fe–O at 588 cm−1 is too strong, peaks of GO are hided, most of them can hardly be observed except the peak at 1614 cm−1. Other characteristic absorption bands in the spectrum are related to CB[6]. It should be noted that, the C=O peak (1743 cm−1) for pure CB[6] shifts to a lower position at 1735 cm−1 after forming CB[6]/GO/Fe3O4 composite, indicating the existence of hydrogen bonding between CB[6] and GO. A similar result has been observed for nanohybrid materials formed by doxorubicin hydrochloride and GO via hydrogen bonding [37]. In addition, in contrast with those broad absorption bands of pure GO and CB[6], the peak at 3424 cm−1 in the spectrum of CB[6]/GO/Fe3O4 narrows, and absorption intensity increases, which further confirmed the presence of hydrogen-bonded carbonyl groups between allophanyl C=O of CB[6] and the hydroxyl O–H and carboxyl O–H groups of GO. These results suggest that CB[6] molecules were immobilized on the surface of GO through hydrogen bonding interaction and CB[6]/GO/Fe3O4 was successfully fabricated.

Figure 2. The FT-IR spectra of (a) CB[6], (b) GO, and (c) CB[6]/GO/Fe 3O4.

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Figure 3. The C1s spectra of (a) GO and (b) CB[6]/GO/Fe3O4. The change of functional groups on the GO and CB[6]/GO/Fe3O4 composite was further investigated by XPS. The wide scan XPS spectrum (Figure S4) of the CB[6]/GO/Fe3O4 composite shows photoelectron lines at a binding energy of about 285, 400, 530, and 711 eV attributed to C 1s, N 1s, O 1s, and Fe 2p, respectively. In the spectrum of Fe 2p (Figure S5), the peaks of Fe 2p3/2 and Fe 2p1/2 are located at 711.11 and 724.88 eV, indicating the formation of the Fe3O4 phase in the composite [38]. The High-resolution C 1s XPS spectra of the CB[6]/GO/Fe3O4 composite and the start material GO are presented in Figure 3. They could be fitted to several components: the non-oxygenated ring C (ca. 285.0 eV), the C atom in C–N bond (ca. 285.9 eV), the C atom in C–O bond (ca. 287.6 eV), and the carbonyl C (ca. 289.2 eV). The

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relative contents of the different functional groups calculated from the fitted curves are also shown in Table S2. It can be seen that the C=O groups, are rising from 3.7% to 13.8% after coprecipitation, implying that CB[6] molecules with large amount of allophanyl C=O groups were loaded onto the surface of GO successfully. It is beneficial for the adsorption of U(VI) because of the possibility to form strong complexes between C=O groups and uranyl ions. 3.2 Formation mechanism of CB[6]/GO/Fe3O4

Figure 4. TEM images and optical digital photos for the samples collected stepwise. (a) and (a’): GO suspensions; (b) and (b’): after mixed with bulk CB[6]; (c) and (c’): after the addition of Fe2+/Fe3+ ions (denoted as ɑ-Fe(OOH)/CB[6]·Fen+/GO); (d) and (d’): after the addition of ammonia solution (denoted as CB[6]/GO/Fe 3O4). In order to explore the formation mechanism of CB[6]/GO/Fe3O4 composite, the structural features of the intermediate solids at different reaction stages were investigated. Figure 4 shows TEM images and optical digital photos for the samples collected stepwise after adding different reaction raw materials. Evidently, coagulation happened (Figure 4b’) after mixing GO suspensions (Figure 4a’) with bulk CB[6], probably due to the existence of interactions between bulk CB[6] and GO. The coagulation consists of GO nanosheets and the bulk CB[6] particles, as shown in Figure 4b, which indicates that the solubility of CB[6] is very low in GO suspensions.

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However, after the addition of Fe2+/Fe3+ ions, the white bulk CB[6] particles disappeared (Figure 4c’), suggesting that the solubility of CB[6] is increased with the presence Fe2+/Fe3+ ions. This is presumably due to coordination of Fe2+/Fe3+ ions to the portal carbonyl oxygen of CB[6] [39]. TEM image of the obtained solids clearly shows amounts of needle-like ɑ-Fe(OOH) particles, with length up to 100 nm are deposited on the surfaces of the GO nanosheets (Figure 4c), which can be demonstrated by XRD pattern and FT-IR spectrum (Figure S6 and S7). When the ammonia solution was poured into the mixture system (Figure 4d’), the co-precipitation process led to the disappearance of the tiny nanoneedles, and Fe3O4 NPs with a size of ca. 13 nm are disorderly decorated on the surface of GO nanosheets (Figure 4d). Based on the above results, it is suggested that the introduction of Fe2+ and Fe3+ ions acts as not only the starting source to generate Fe3O4 NPs but also the complexing agent to improve the dissolution of the bulk CB[6]. To confirm the specific function of GO nanosheets for the facile synthesis of CB[6]/GO/Fe3O4, we attempted to prepare CB[6]/Fe3O4 composite following a similar procedure to CB[6]/GO/Fe3O4 without adding GO nanosheets. As illustrated in Figure S8, the undissolved bulk CB[6] particles were present throughout the entire synthesis process and dramatic agglomeration of Fe3O4 NPs is observed, which means that GO, supplying hydrogen atoms for the formation of hydrogen bonding between CB[6] and GO, is also a key factor to facilitate the dissolution of the bulk CB[6]. Based on the above analysis, a possible formation mechanism for the as-prepared CB[6]/GO/Fe3O4 is proposed as illustrated in Figure 5. Firstly, GO nanosheet is physically mixed with bulk CB[6] particles. Then Fen+ (n = 2, 3) ions may react with CB[6] to form complex CB[6]·Fen+ as shown in Eq. (1), leading to the enhanced dissolution of bulk CB[6]. Synchronously, the formed CB[6]·Fen+ complex molecules are immobilized on the surface of

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GO nanosheets through hydrogen bonding interaction as expressed in Eq. (2), which can promote the Eq. (1) reaction equilibrium move forward, and hence, the bulk CB[6] was dissolved completely. During the preparation, the hydrolysis of Fe3+ result in the formation of α-Fe(OOH) nano-needles simultaneously as shown in Eq. (3). At last, the as-prepared CB[6]/GO/Fe3O4 composite was obtained after the addition of ammonia solution as shown in Eq. (4).

Figure 5. Schematic diagram of a possible formation mechanism of CB[6]/GO/Fe 3O4. 3.3. Adsorption of U(VI) on CB[6]/GO/Fe3O4 3.3.1 Effect of contact time The effect of contact time for CB[6]/GO/Fe3O4 on the adsorption capacity for U(VI) at pH = 5.0 is plotted in Figure 6. It can be seen that the adsorption of U(VI) on CB[6]/GO/Fe3O4 composite is very fast especially in the initial several minutes, almost 80% of the U(VI) adsorption was achieved in only 10 min, which is extremely short compared to that of 2 h in Fe3O4/GO [25] and 1 h in amidoximated Fe3O4/GO [17], and then the adsorption process attains equilibrium after 150 min. The fast adsorption rate at the initial stage may be assigned to strong

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chemisorption or complexation between U(VI) and functional groups of CB[6]/GO/Fe3O4 composite, and the highly accessible 2D structure of GO substrate. The fast adsorption performance is advantageous for the practical application of CB[6]/GO/Fe3O4 composites to adsorb U(VI) from large volumes of aqueous solution.

Figure 6. The effect of contact time on the U(VI) adsorption on CB[6]/GO/Fe 3O4. C(U)initial = 36 mg/L, Cadsorbent = 0.2 g/L, pH = 5.0 and T = 298 K. 3.3.2 Influence of pH and ionic strength The adsorption capacity for U(VI) is plotted in Figure 7 as a function of pH in 0, 0.01 and 0.1 mol/L NaClO4 solutions, respectively. It clearly see that, the pH value had great influence on the U(VI) adsorption of CB[6]/GO/Fe3O4. The adsorption increases gradually at pH ranging from 2.0 to 6.5, reaches a plateau at pH 6.5 – 7.5, followed by a steep decrease at pH value higher than 7.5. Such a pH-dependent adsorption may be attributed to the surface charges of functional groups of CB[6]/GO/Fe3O4 as well as the species of U(VI) in different pH value. U(VI) species distribution in solution calculated by Visual MINTEQ version 3.1 is shown in Figure S9. At lower pH, the dominant species is UO22+, and the surface of CB[6]/GO/Fe3O4 is positively charged due to the protonation reaction. Therefore, the low adsorption capacity is owing to the electrostatic repulsion effect between UO22+ and positively charged binding groups of

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CB[6]/GO/Fe3O4. With the increasing of pH, U(VI) species transforms gradually from UO22+ to multi-nuclear hydroxide complexes such as (UO2)2(OH)22+ and (UO2)3(OH)5+, and the repulsion between multi-nuclear hydroxide complexes and CB[6]/GO/Fe3O4 has weakened through deprotonation of the functional groups. It should be noted that solid species of schoepite can be observed in the pH range of 5.5–8.5. Similar trends were reported by other researchers [40]. Thus, the removal process of U(VI) is achieved through simultaneous precipitation of schoepite as well as adsorption of (UO2)3(OH)5+. When pH values are greater than 8.0, the prominent U(VI) species are carbonate complexes, i.e., (UO2)(CO3)34- and (UO2)(CO3)22-, leading to the decreased adsorption of U(VI). This was demonstrated previously for mineral sorbents [41, 42]. Since in a pH range above 5.5, precipitation occurred in the uranium solutions, the experiments were conducted at a pH of 5.

Figure 7. The effect of pH and ionic strength on the U(VI) adsorption on CB[6]/GO/Fe3O4. C(U)initial = 36 mg/L, Cadsorbent = 0.2 g/L, T = 298 K and contact time = 24 h. The adsorption of U(VI) is independent of ionic strength at all pH values, suggesting that the adsorption of U(VI) is dominated by inner-sphere surface complexation rather than by outersphere surface complexation or ion exchange [17, 43]. Large amount of allophanyl C=O and

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other oxygen-containing functional groups on the surfaces of CB[6]/GO/Fe3O4 can form strong complexes with U(VI), which was also observed in other adsorbents for U(VI) removal. 3.3.3 Adsorption isotherms

Figure 8. Adsorption isotherms of U(VI) on CB[6]/GO/Fe 3O4, CB[6]/GO/Fe3O4-38.9% and GO/Fe3O4-50.7%. The solid line stands for Langmuir model and the dash line stands for Freundlich model, pH = 5.0, C(U)initial= 4 ~ 75 mg/L, Cadsorbent = 0.2 g/L, T = 298 K and contact time = 24 h. The adsorption isotherm is the most important information, which indicates how the adsorbate distribute between the liquid and the solid phase once the equilibrium is attained. In the research, the adsorption data were correlated to the widely used Langmuir and Freundlich isotherm models to understand the adsorption mechanism (Figure 8). The Langmuir and Freundlich isotherm models are expressed in Eq. (5) and Eq. (6), respectively.

where Ce is the concentration of the adsorbate in solution at equilibrium (mg/L), Qe is the amount of adsorbed adsorbate at equilibrium (mg/g), Qmax represents the maximum adsorption

17

capacity of the adsorbent (mg/g), and KL is the Langmuir adsorption constant related to the energy of adsorption (L/mg). KF is the Freundlich constant related to the adsorption capacity of adsorbent (mg/g); n is the Freundlich exponent related to adsorption intensity. Table 1. Adsorption constants for Langmuir and Freundlich isotherm models. Langmuir

Freundlich

Qmax (mg/g)

KL (L/mg)

R2

KF [mg/g (L/mg)1/n]

1/n

R2

CB[6]/GO/Fe3O4

66.81

0.521

0.982

31.084

0.214

0.897

CB[6]/GO/Fe3O4-38.9%

122.48

0.838

0.936

63.39

0.192

0.934

GO/Fe3O4-50.7%

49.19

0.140

0.985

11.560

0.369

0.930

Sample

Langmuir and Freundlich isotherm parameters calculated from fitting processes are listed in Table 1. It can be seen that the Langmuir equation fits the experimental data better than the Freundlich model with a higher correlation coefficient (R2) of 0.982, implying that the adsorption process results in the formation of a monolayer coverage of U(VI) on CB[6]/GO/Fe3O4, and the complexation between U(VI) and functional groups of CB[6]/GO/Fe3O4 is the predominant uptake mechanism. The maximum adsorption capacity of CB[6]/GO/Fe3O4 was evaluated as 66.81 mg/g. To investigate the contribution of each component to the U(VI) removal, CB[6]/GO/Fe3O4 with 38.9% Fe3O4 content (denoted as CB[6]/GO/Fe3O4-38.9%) and GO/Fe3O4 without adding CB[6] with 50.7% Fe3O4 content (denoted as GO/Fe3O4-50.7%) were prepared and used for U(VI) adsorption tests. The results are illustrated in Figure 8 and Table 1. Under the same experimental conditions, the maximum adsorption capacity of CB[6]/GO/Fe3O4-38.9% is 122.48 mg/g, which is higher than that of CB[6]/GO/Fe3O4 (Fe3O4 content is 61.1%). The results demonstrate that Fe3O4 content is unfavorable for the adsorption capacity of magnetic composites, which is probably due to that Fe3O4 NPs decrease

18

the surface adsorption area of CB[6]/GO/Fe3O4. As for GO/Fe3O4-50.7%, the maximum adsorption capacity is 49.19 mg/g, which is smaller than that of CB[6]/GO/Fe3O4, indicating that the introduction of CB[6] actually enhanced the adsorption capacity of CB[6]/GO/Fe3O4. Table 2 compares the maximum adsorption capacity of present adsorbents with that reported in the literatures. Generally, a direct comparison with other adsorbents is very difficult owing to the different experimental conditions adopted. The summary simply provides an overview of the potential of the present adsorbents against other materials. Table 2 confirms that magnetic cucurbit[6]uril/graphene oxide composites have a competitive adsorption performance among the U(VI) adsorption materials, especially magnetic composites. Table 2. Comparison of the U(VI) adsorption capacity of CB[6]/GO/Fe3O4 with others. Adsorbents

Experimental conditions

Qmax (mg/g) Ref

Carboxymethylated polyethyleneimine Ambient temperature, pH = 151.5 functionalized nanoporous carbons 4.0

[44]

Poly(2-hydroxyethyl methacrylate– T = 293 K, pH = 6.0 methacryloylamidoglutamic acid)

[45]

204.8

Sargassum biomass

Ambient temperature, pH = 560 4.0

[46]

Graphene oxide nanosheets

T = 293 K, pH = 5.0

97.5

[43]

Fe3O4/graphene oxide

T = 293 K, pH = 5.5

69.5

[25]

284.9

[17]

Amidoximated oxide

magnetite/graphene T = 298 K, pH = 5.0

Colloidal magnetite

Ambient temperature, pH = 1.4 7.0

[47]

Quercetin modified Fe3O4 nanoparticles

T = 298 K, pH = 5.0

12.3

[48]

Magnetic Fe3O4@SiO2

T = 298 K, pH = 6.0

52

[49]

Polyacrylamide coated -Fe3O4

T = 293 K, pH = 5.0

220.9

[24]

Manganese oxide coated zeolite

T = 293 K, pH = 6.0

17.6

[50]

CB[6]/GO/Fe3O4

T = 298 K, pH = 5.0

66.8

This work

19

CB[6]/GO/Fe3O4-38.9%

T = 298 K, pH = 5.0

122.5

This work

GO/Fe3O4-50.7%

T = 298 K, pH = 5.0

49.2

This work

3.3.4 Effect of temperature and adsorption thermodynamics To evaluate the influence of temperature on the adsorption process of U(VI) onto CB[6]/GO/Fe3O4, thermodynamic parameters were calculated at various temperatures. The adsorption capacities as a function of temperature are plotted in Figure 9a, which show that the uptake of U(VI) increases with increasing temperature, indicating the endothermic nature of the process.

20

Figure 9. (a) The effect of temperature on the U(VI) adsorption on CB[6]/GO/Fe 3O4 and (b) a liner plot of lnKd versus 1/T. pH = 5.0, C(U)initial = 36 mg/L, Cadsorbent = 0.2 g/L, T=298 K, 308 K, 318 K and 328 K, and contact time = 24 h. The temperature dependence of the adsorption process is associated with changes in several thermodynamic parameters such as standard free energy (∆G0), enthalpy (∆H0) and entropy (∆S0) of adsorption, which are calculated from the slope and intercept of the linear line of lnKd versus 1/T (Fig. 9b) by using the van’t Hoff equation shown in Eq. (7) and Eq. (8):

where Kd is the distribution coefficient (mL/g) of U(VI), T is absolute temperature (K), and R is the ideal gas constant (8.314 J/(mol K)). The standard free energy values were calculated based on Eq. (9):

where ∆G0 is the standard Gibbs free energy. According to Eq. (9), the data of ∆G0 at different temperatures are obtained. The data of ∆G0, ∆H0 and ∆S0 are given in Table 3. The positive ∆S0 and negative ∆G0 values suggest the spontaneity of the adsorption process. Table 3. The thermodynamic parameters of U(VI) adsorption on CB[6]/GO/Fe3O4. ∆G0 (kJ/ mol)

∆H0

∆S0

(kJ/ mol)

(J/ mol k)

298 K

308 K

318 K

328 K

15.517

117.77

-19.58 -20.76

-21.93

-23.11

3.3.5 Stability and reusability of CB[6]/GO/Fe3O4

21

Figure 10. Reusability of CB[6]/GO/Fe3O4 in the removal of U(VI). C(U)initial = 36 mg/L, Cadsorbent = 0.2 g/L, pH = 5.0 and contact time = 24 h. The stability and reusability of CB[6]/GO/Fe3O4 are of great importance for practical application. Acid elution was the common method for U(VI) desorption from adsorbents. Table S3 demonstrates the leaching percent of Fe from CB[6]/GO/Fe3O4 at 0.01 mol/L HCl solutions. The percent of Fe leaching from CB[6]/GO/Fe3O4 was less than 1% after elution under strong sonication after 5 times, implying that CB[6]/GO/Fe3O4 is very stable in 0.01 mol/L HCl solution. Therefore, the reusability of CB[6]/GO/Fe3O4 was estimated using 0.01 mol/L HCl as the desorbing agent and the results are shown in Figure 10. One can note that the adsorption capacities of U(VI) marginally decreased from 63.75 to 57.75 mg/g after five cycles. The slight decrease in adsorption amount is ascribed to the incomplete U(VI) desorption from the surfaces of CB[6]/GO/Fe3O4. Overall, the acceptable regeneration capacity proved that CB[6]/GO/Fe3O4 has a potential application prospect for the preconcentration of U(VI) from large volumes of aqueous solutions. 4. Conclusions In summary, we have demonstrated a facile strategy for fabricating novel magnetic cucurbit[6]uril/graphene oxide (CB[6]/GO/Fe3O4) composite via a simple, low cost and easy to

22

scale-up in-situ co-precipitation method. The study of the formation mechanism shows that the introduction of Fe ions and GO is crucial for the successful fabrication of CB[6]/GO/Fe3O4 composite. In U(VI) removal experiments, the CB[6]/GO/Fe3O4 composite displayed competitive adsorption performance and acceptable recyclability owing to its strong complexation ability, high efficiency of magnetic separability and good stability. Therefore, CB[6]/GO/Fe3O4 is expected to have practical applications for the removal of U(VI) in wastewater. Such materials may also find application in the adsorption of organic contaminants in polluted water. We believe that the strategy developed here is expected to make more kinds of composites and apply for a variety of applications due to its simplicity and scalability.

Acknowledgements The authors are very grateful for the financial support of the National Natural Science Foundation of China (Grant No. 21401174), the Science and Technology Development Foundation of China Academy of Engineering Physics (2014B0301050), and Discipline Development Foundation of Science and Technology on Surface Physics and Chemistry Laboratory (XK201303, XK201306).

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Highlights 

Magnetic cucurbit[6]uril/graphene oxide composite was fabricated.



The formation mechanism was investigated.



The composite displayed competitive adsorption performance and acceptable recyclability for uranium removal.

Graphical abstract Facile Fabrication of Magnetic Cucurbit[6]uril/Graphene Oxide Composite and Application for Uranium Removal

Lang Shaoa, Xiaofang Wang*b, Yiming Rena, Shaofei Wanga, Jingrong Zhonga, Mingfu Chua, Hao Tanga, Lizhu Luob, and Donghua Xieb a

Institute of Materials, China Academy of Engineering Physics, PO Box 9071-11,

Mianyang 621907, China. b

Science and Technology on Surface Physics and Chemistry Laboratory, PO Box

9-35, Mianyang 621908, China. *Corresponding author: Xiaofang Wang E-mail: [email protected]