SCIENCE CHINA Chemistry. Ozonated graphene oxides as high efficient sorbents for Sr(II) and U(VI) removal from aqueous solutions

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1 SCIENCE CHINA Chemistry ARTICLES July 2016 Vol.59 No.7: doi: /s z Ozonated graphene oxides as high efficient sorbents for Sr(II) and U(VI) removal from aqueous solutions Xia Liu 1,2, Xiangxue Wang 1,2, Jiaxing Li 1,2,3,4,5* & Xiangke Wang 1,3,4,5* 1 School of Chemistry and Environment, North China Electric Power University, Beijing , China 2 Key Laboratory of Novel Thin Film Solar Cells; Institute of Plasma Physics, Chinese Academy of Sciences, Hefei , China 3 Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher Education Institutions, Suzhou , China 4 School for Radiological and Interdisciplinary Sciences (RAD-X), Soochow University, Suzhou , China 5 NAAM Research Group, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia Received October 25, 2015; accepted November 10, 2015; published online April 28, 2016 Ozone was used to oxidize graphene oxides (GO) to generate ozonated graphene oxides (OGO) with higher oxygen-containing functional groups. The as-prepared OGO was characterized by Fourier transformed infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Based on the results of potentiometric acid-base titrations, the total carboxylic acid concentration on OGO surface was calculated to be 3.92 mmol/g, which was much higher than that on GO surface. The results of adsorption experiments indicated that the adsorption capacities of OGO for Sr(II) and U(VI) removal were improved significantly after ozonization. ozone, graphene oxide, adsorption, Sr(II), U(VI) Citation: Liu X, Wang XX, Li JX, Wang XK. Ozonated graphene oxides as high efficient sorbents for Sr(II) and U(VI) removal from aqueous solutions. Sci China Chem, 2016, 59: , doi: /s z 1 Introduction Due to the random disposal of wastewater, heavy metal pollution has become a worldwide environment concern [1]. Uranium is one of the most hazardous heavy metal ions because of its high toxicity to human and its radioactivity [2]. Considering public health and ecosystem stability, it is necessary to remove U(VI) ions from polluted wastewater before they are released into the natural environment [3]. Radioactive strontium (Sr) was produced not only as a waste fission product from nuclear power plants, but also in the reprocessing of nuclear fuels. With a half life of 28 years, 90 Sr, once consolidated into bone, continues to -irradiate localized tissues with the eventual development *Corresponding authors ( lijx@ipp.ac.cn; xkwang@ncepu.edu.cn) of bone sarcomas and leukemia [4]. Thus the removal of radionuclides from wastewater is of a great importance to ecosystems and mankind. Up to now, many methods including adsorption, precipitation, electrodialysis, chemical coagulation-flocculation, membrane filtration, and ion exchange have been applied to remove Sr(II) and U(VI) from wastewater [5]. Among all these methods, sorption technology is one of the most effective choices because of its simplicity of design, convenience, low cost, high sorption efficiency, and wide adaptability. However, low sorption capacities or efficiencies limit their practical applications [6]. As a result, searching for new adsorbents to solve these problems is of paramount importance. Graphene oxide (GO) has a wide range of functional groups, such as epoxy, hydroxyl, and carboxylate groups [7]. Due to these functional groups, GO is hydrophilic and readily disperses in water to form stable colloidal suspen- Science China Press and Springer-Verlag Berlin Heidelberg 2016 chem.scichina.com link.springer.com

2 870 Liu et al. Sci China Chem July (2016) Vol.59 No.7 sions. Few layered GO has been demonstrated to have a higher adsorption capacity than any of today s nanomaterials in the removal of Sr(II) and U(VI) from aqueous solutions [8 10]. For the full utilization of graphene building blocks in environmental remediation, some functional nanoparticles are combined with graphene to generate composites with additional properties to graphene. By adding these functional groups, the sorption capacity of GO was elevated. For example, Zhang et al. [11] developed a novel method for the synthesis of hierarchical polyaniline/go nanocomposites with a high Cr(VI) adsorption capacity. Hu et al. [12] studied graphene oxide/polypyrrole as an effective sorbent for U(VI) removal from aqueous solutions. Chen et al. [13] found that PAO-g-rGO (PAO represents polyamidoxime) composites exhibited high adsorption capacity towards Sr(II), Co(II) and Eu(III) ions in aqueous solution. Chen et al. [14] used magnetite/graphene oxide to remove Sr(II) with better adsorption capacity and easier separation. However, none of these methods are completely satisfactory due to the difficult preparation of the composites in large scale and the generation of secondary waste products. Thus, the development of new, cost-effective and more environmental friendly methods is still needed. Ozone can be acquired conveniently by air ozone generator, which is economic and non-polluted. Herein, we present a chemical modification of GO by ozone to generate ozonated graphene oxide (OGO) with higher oxygencontaining functional groups [15]. The as-prepared OGO was characterized by Fourier transformed infrared spectroscopy (FTIR), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and acid-base titrations. The higher adsorption capacity was evaluated and measured by batch sorption experiments in the preconcentration and solidification of Sr(II) and U(VI) ions. 2 Experimental 2.1 Synthesis and purification of GO and OGO Preparation of graphene oxide Graphene oxide nanosheets were prepared by using the modified Hummers method [16,17] from the natural flake graphite (average particle diameter of 20 m, 99.95% purity, Qingdao Tianhe Graphite Co. Ltd., China) using concentrated H 2 SO 4 and KMnO 4 to oxidize the graphite layer. Briefly, 1.0 g of graphite, 1.0 g of NaNO 3, and 40 ml of H 2 SO 4 were mixed and stirred in a three-neck flask, and then 6.0 g of KMnO 4 was slowly added. Once added, the solution was transferred to a 35±1 C water bath and stirred for about 1 h; and then 80 ml Milli-Q water was added; and the solution was stirred for 30 min at the temperature of 90±1 C. Afterwards, 150 ml Milli-Q water was added and 6 ml of H 2 O 2 (30%) was added slowly, turning the color of the solution from dark, brown to yellow. The desired products were obtained by rinsing with plenty of deionized water Preparation of ozonated graphene oxide GO/H 2 O dispersion was ozonized by bubbling O 3 gas through the dispersion, which was placed in a water bath and subjected to mild sonication for 2 h (SGF-YW laboratory ozone generator from Saige Ltd. of Zhenjiang (China), the ozone production rate is 5 g/h) [15,18]. A noticeable color change of the solution to light brown was observed after ozonation. The concentration of the as-prepared OGO dispersion was determined by evaporating water to weigh the residual solid. Most of the OGO were dissolved in specific concentration (0.6 g/l) as stock solution. And some OGO dried in a vacuum tank was characterized and used in the following experiments. 2.2 Characterization of GO and OGO OGO and GO were characterized by FTIR, SEM, XPS and potentiometric acid-base titrations. FTIR spectroscopy measurements were performed using a Perkin-Elmer 100 spectrometer (USA) in KBr pellet at room temperature. The potentiometric acid-base titrations were performed to determine the surface chemical properties of OGO and GO by use of a computer controlled titration system (DL50 automatic titrator, Mettler Toledo, Switzerland). Briefly, 0.3 g/l OGO (or GO) was purged with argon gas for 2 h to exclude atmospheric CO 2 (g). The initial suspension ph was adjusted to ph 3.0 by adding 0.01 mol/l HNO 3 with vigorous stirring for 1.0 h, and then the suspension was slowly titrated to ph 11.0 with 0.5 mol/l NaOH for GO and 0.05 mol/l for OGO titrant at a variable increment (0.008 up to 0.15 ml) [18]. The data sets of ph versus net consumption of H + or OH were used to obtain intrinsic acidity constants. The XPS measurements were conducted with a Thermo Escalab 250 electron spectrometer (USA) using 150 W Al K radiations. 2.3 Batch sorption experiments Adsorption experiments of Sr(II) and U(VI) on OGO were carried out by batch technique in 10 ml polyethylene centrifuge tubes. The radionuclide stock solution, NaNO 3 solution, adsorbent (OGO) stock solution and Milli-Q water were added into the 10 ml polyethylene test tubes to achieve the desired concentrations of different components. Negligible amount of concentrated HNO 3 or NaOH was added into the suspensions to adjust the ph to desired values. The tubes were placed in an oscillator and kept shaken for 24 h to achieve adsorption equilibration. The solid and liquid phases were separated by centrifugation at 9000 r/min for 15 min, and then the supernatant was poured into a syringe and filtered through a 0.22-μm membrane. The concentrations of Sr(II) in the filtrate were measured by atomic absorption spectroscopy. The concentration of U(VI) was

3 Liu et al. Sci China Chem July (2016) Vol.59 No analyzed by the Dichlorophosphonoazo III Spectrophotometer Method at the wavelength of 669 nm. The amount of Sr(II) or U(VI) adsorbed on the adsorbent composites was calculated from the difference between the initial concentration (C 0 ) and the equilibrium one (C e ). Adsorption performance was expressed in terms of adsorption percentage (%) and distribution coefficient (K d ), which were calculated from the following equations: C0 Ce Sorption (%) 100% (1) C0 C0 Ce V Kd (2) Ce m where C 0 (mg/l) is the initial concentration, C e (mg/l) is the equilibrium concentration, m (g) is the mass of the absorbent composite, and V (ml) is the volume of the suspension. All the experimental data were the averages of duplicate or triplicate determinations. The relative errors of the data were within 5%. 3 Results and discussion 3.1 Characterization of GO and OGO The FTIR spectra of GO and OGO are presented in Figure 1. The most remarkable vibrations are associated with different types of oxygen functional groups on GO, such as the overlapping bands centered around 1100 cm 1 ( C O ), 1403 cm 1 ( C OH ), 1730 cm 1 ( C=O ), 1620 cm 1 ( C=C ) and 1220 cm 1 ( C O C ) [15]. These vibrational intensities increased after ozonization, which were consistent with the observed changes in the XPS data, indicating an increment of oxygen-containing functional groups on GO surface. The SEM images give the morphologies and sizes of OGO and GO. From Figure 2(a), OGO exhibited a smooth layer structure with an average size of about several micrometers whereas Figure 2(b) depicted the morphology of Figure 1 FTIR spectra of GO and OGO powder samples (color online). Figure 2 SEM images of GO (a) and OGO (b) samples, and TEM images of GO (c) and OGO (d). GO film, which showed a heavily-stacked layered structure [15]. From the picture we can imagine that OGO was better oxidized because of the formation of oxygenated functional groups in graphite oxide, which makes them easier to exfoliate. From TEM images (Figure 2(c, d)), the thinner lamella can be seen as the oxidation degree increased. XPS survey of GO and OGO are shown in Figure 3(a) with typical peaks of C1s and O1s. Two overlapping peaks can be observed in high-resolution C1s XPS spectra for GO and OGO, as depicted in Figure 3(b). The peaks at a low binding energy of ev and at a high binding energy of ev can be assigned to graphitic and oxidized carbon, respectively [8]. Compared with GO, OGO exhibited a decreased intensity in the amount of graphitic carbon and a significant increase in the oxidized carbon, which indicates that the ozone treatment results in further surface oxidation of carbon atoms, and therefore, introduced additional oxygen-containing functional groups onto the surface. The content of carbon in GO is 64.75% while that of carbon in OGO has decreased to 61.31% and the ratio of C/O in GO is 1.89 and that of C/O in OGO is These data also confirm that the oxygen-containing functional groups increase by ozone treatment. The oxygen-containing functional groups on the surfaces of GO and OGO were also investigated by XPS spectroscopy analysis (Figure 3(c, d)). The O1s XPS spectra of GO and OGO can be deconvoluted into three peaks as 531.4±0.2, 532.7±0.2 and 533.7±0.2 ev, which corresponded to COOH, >C=O and C OH bonds, respectively [19]. As seen in Figure 3(c), after the GO was ozonized by bubbling ozone gas, the peak fractions of COOH and >C=O reduced and the peak fraction of C OH increased. Quantitative analyses of the O1s spectra of GO and OGO are shown in Table 1. The suspensions of GO and OGO, as well as structural illustrations, are depicted in Figure 4. It can be seen that

4 872 Liu et al. Sci China Chem July (2016) Vol.59 No.7 Figure 3 (a) XPS spectra of GO and OGO samples; XPS high resolution C1s spectra of GO and OGO films (b), and O1s spectrum of GO (c) and OGO (d) (color online). Table 1 OGO GO XPS O1s spectra analysis Peak BE (ev) FWHM (ev) a) Fraction (%) COOH C=O C OH COOH C=O C OH a) Full width at half maximum. the color of OGO suspension was much lighter than that of GO suspension, indicating a less conjugated structure of OGO as compared with GO by forming epoxide functional groups on OGO surface after ozone treatment [15]. 3.2 Potentiometric acid-base titrations The surfaces of GO and OGO can be regarded as an amphoteric surface site with surface hydroxyl groups, which can not only be protonated to form a positively charged surface site but also be deprotonated to form the negatively charged surface site [20]. Potentiometric acid-base titration provides a measurement of the sequential binding of the proton by the surface functional groups of OGO materials. Herein, GO and OGO were characterized by potentiometric acid-base titration using NaOH as the titrant. Figure 5 shows the Gran plots for the titration data of the GO and OGO. The Gran function (G) is expressed as [3]: at acidic side: G ( V + V + V ) (3) ph a 0 at b at alkaline side: ph) G ( V + V + V ) (4) b 0 at b where G a and G b are the Gran functions at the acidic and alkaline sides respectively, V 0 (ml) is the initial solution volume, V at (ml) represents the total volume of HNO 3 added before titration to achieve ph ~3, V b (ml) represents the volume of NaOH added at the different titration points. The potentiometric titration data for GO and OGO are shown in Figure 6. TOTH, the total concentration of protons consumed in the titration process, can be calculated from the following equation [21]: ( Vb Veb1) TOHT Cb (mol / L) (5) V + V + V 0 at b where C b represents the concentration of NaOH, V eb1 obtained from line regression of the Gran plot (Figure 5) and can be considered as the zero titration point of samples. Figure 6 also shows that achieving the same ph value, the amounts of OH consumed decrease in the order of OGO>GO which means that the buffer capacity of OGO is the higher than that of GO. The total concentration of surface acidic groups per solid weight (H s ) calculated from the two equivalence points on the Gran plot (V eb1 and V eb2 ) is defined by the following formula [20]: ( Veb2 Veb1) Hs Cb (mmol / L) (6) m s

5 Liu et al. Sci China Chem July (2016) Vol.59 No Figure 4 The diagram for synthesis process (color online). Figure 5 Gran plots of GO (a) and OGO (b). assumptions that YOH was used to represent strong acidic groups, while XOH was employed to represent weak acidic groups [22,23]. Three reactions are used to account for the acid-base chemistry of the GO and OGO materials: YOH+ H YOH 2 YOH YO+ H XOH XO+ H Based on the CCM (constant capacitance model) fitting, Figure 6 TOTH curves of of GO (a) and OGO (b). The results calculated from the titration curves are listed in Table 2. The total concentration of surface functional groups on OGO was 3.92 mmol/g, which was higher than those on GO (2.97 mmol/g), indicating that more functional groups were reacted with NaOH on OGO surface. This observation also explained the higher buffer capacity of OGO than GO. For modeling purposes, we proposed the following Table 2 Two equivalent points (V eb1 and V eb2 ) obtained from linear regression of the Gran plots V eb1 V eb2 H s (mmol/g) OGO GO the distributions of surface sites on GO and OGO as a function of ph calculated from titration curves with the aid of the Visal MINTEQ are shown in Figure 7. It can be seen that the distribution of surface sites on GO (Figure 7(a)) and OGO (Figure 7(b)) as a function of ph showed a similar trend. However, the total concentration of surface acidic groups and the relative ratio of XOH and YOH of GO and OGO were not the same as each other, resulting in different acid-base chemistry of the GO and OGO and thus different adsorption properties of the GO and OGO. 3.3 Effect of ph and ionic strength As shown in Figure 8(a), the sorption of U(VI) increased gradually as ph increased from 2.0 to 6.0, and then decreased at higher ph values. The ph of solution can affect both the relative distribution of U(VI) species in solution and the surface properties of OGO. From Figure 8(c), it can

6 874 Liu et al. Sci China Chem July (2016) Vol.59 No.7 Figure 7 Surface site concentrations of OGO (a) and GO as a function of ph simulated by CCM with the aid of Visal MINTEQ (b) (color online). Figure 8 Effect of ph on U(VI) (a) and Sr(II) (b) sorption onto OGO in the presence of 0.001, 0.01 and 0.1 mol/l NaNO 3 solutions (C 0 =13 mg/l, m/v= 0.1 g/l, and T=303 K); the relative amounts of species of U(VI) (c) and Sr(II) (d) as a function of ph (color online). 2 be found that UO was the dominant species at low ph, 2 which gradually changed to U(VI) hydrolysis complexes and multinuclear hydroxide complexes (UO 2 (OH) +, UO 2 (OH) 2, etc.) as the ph increased. Meanwhile, at ph< ph pzc, a positive surface charge was observed on the surface of OGO due to the protonation reaction. This positive OGO surface made it difficult to adsorb positive U(VI) species because of the electrostatic repulsion. As the ph increased to be higher than ph pzc, the OGO surface was negatively charged by the deprotonation reaction [12]. In this case, the negatively charged OGO surface facilitated the adsorption of positive U(VI) species by electrostatic interactions [2]. These electrostatic interactions reached the maximum at a ph value of around 8.0. At ph>8.0, the U(VI) ions were presented as negatively charged species (i.e., UO 2 (CO 3 ) 2 2, UO 2 (CO 3 ) 4 3 ), which repelled with the negative charged OGO surface, resulting in the decreased adsorption capacities [5]. Furthermore, the ionic strength-independent sorption of U(VI) was also observed in Figure 8(a), suggesting that the sorption of U(VI) was dominated by inner-sphere surface complexation rather than outer-sphere surface complexation or ion exchange. The Sr(II) sorption by OGO as a function of ph in different NaNO 3 solutions is presented in Figure 8(b). Adsorption of Sr(II) on OGO composites kept a stable increase from about 0.1% 90% in the ph range A change of ph in solution can promote or suppress the adsorption of metal ions [24] by impacting the distribution of Sr(II) species (Figure 8(d)) in solution and the surface charges of the adsorbents by dissociating functional groups. According to the Sr(II) hydrolysis constants of log 1 =13.29 and log 2 =

7 Liu et al. Sci China Chem July (2016) Vol.59 No [25], Sr(II) presents as Sr 2+ at ph<11.0 and no Sr(OH) 2 precipitation forms at ph<13.0. At low ph conditions, the positively charged OGO surface restricted the sorption of Sr(II) species due to electrostatic repulsion. As ph increases, the OGO surface was getting negatively charged and the adsorption capacity toward Sr(II) was enhanced by electrostatic interactions. A similar Sr(II) adsorption result was observed by Chen et al. [3] on the multiwalled carbon nanotubes. It can also be observed from Figure 8(b) that the Sr(II) adsorption decreased with increasing ionic strengths. This phenomenon can be explained by the following aspects: (1) The formed electrical double layer complexes between Sr(II) ions and graphene oxide nanosheets favored metal ion sorption when the concentration of NaNO 3 was decreased. The sorption interactions between the functional groups and metal ions were mainly ionic interaction, which was in accordance with ion exchange mechanism. (2) The activity coefficient of Sr(II) ion was affected by NaNO 3 concentrations, which limited Sr(II) ion transfer from solution to solid surfaces. (3) At higher ionic strength, the electrostatic repulsion was weakened and the particle aggregation increased, reducing the available sites to bind metal ions on graphene oxide surfaces [26,27]. The ionic strengthdependent sorption of Sr(II) on OGO suggested that the sorption of Sr(II) was mainly dominated by ion exchange or outer-sphere surface complexation, rather than by innersphere surface complexation. 3.4 Sorption isotherms and thermodynamic data Sorption isotherms The effect of temperature on radionuclide adsorption on OGO and the adsorption capacities could be acquired from adsorption thermodynamic studies. Adsorption isotherms, the function image of adsorption capacity to initial radionuclide concentration at different temperatures are shown in Figure 9. The experimental data are simulated with the Langmuir and Freundlich models, respectively. The Langmuir and Freundlich isotherm models are expressed as [19]: bqmaxce qe (7) 1 bce n q K C (8) e F e where C e (mg/l) is the equilibrium concentration of radionuclides in aqueous solution, q e (mg/g) is the amount of radionuclides adsorbed on OGO, q max is the maximum amount of radionuclides adsorbed per unit weight of OGO to form complete monolayer coverage on the surface at high equilibrium radionuclides concentration, b (L/g) is the Langmuir constant. K F and n are Freundlich constants related to the adsorption capacity and adsorption intensity, respectively. The maximum adsorption capacities of OGO calculated from the Langmuir model were mg/g at ph 5.5 and T=303 K for U(VI) and 85 mg/g at ph 8.0 and T=303 K for Sr(II), both of which were higher than the corresponding sorption values by GO at the same experimental condition (q max =180 mg/g at ph 5.5 and T=303 for U(VI) and q max =60 mg/g at ph 8.0 and T=303 K for Sr(II)). This result was consistent with Sun s work [28] Thermodynamic data The effect of temperature on U(VI) and Sr(II) adsorption onto OGO at ph 5.5 for U(V) and at ph 8.0 for Sr(II) are also shown in Figure 9. Adsorption capacity is the highest at T=323 K and is the lowest at T=303 K, which indicates that U(VI) (Figure 9(a)) and Sr(II) (Figure 9(b)) adsorption on OGO can be promoted at higher temperature. The thermodynamic parameters (G 0, S 0 and H 0 ) for U(VI) (Figure 10(a)) and Sr(II) (Figure 10(b)) adsorption on OGO can be acquired from the temperature-dependent adsorption isotherms. The average standard enthalpy change (H 0 ) and standard entropy change (S 0 ) can be calculated from the slope and y-intercept plot of lnk d versus 1/T (Figure 10) using the Van t Hoff equation [29,30]: Figure 9 (a) Sorption isotherms of U(VI) at ph 5.5±0.1, m/v=0.1 g/l, C 0 =13 mg/l, I=0.01 mol/l NaNO 3 on OGO; (b) Sr(II) at ph 8.0±0.1, m/v=0.1 g/l, C 0 =13 mg/l, I=0.01 mol/l NaNO 3 on OGO. The solid line stands for Langmuir model and the dash line stands for Freundlich mode (color online).

8 876 Liu et al. Sci China Chem July (2016) Vol.59 No.7 Figure 10 Plots of LnK d 0 vs. 1/T for U(VI) (a) and Sr(II) sorption (b). Table 3 Values of thermodynamic parameters for Sr(II) and U(VI) sorption on OGO T (K) G 0 (kj/mol) H 0 (kj/mol) S 0 (J/(mol k)) U(VI) Sr(II) U(VI) Sr(II) U(VI) Sr(II) ln K 0 d S R H RT 0 0 where K d is the adsorption equilibrium constant. The free energy change (G 0 ) is calculated by the following equation [31]: H 0 =G 0 +TS 0 (10) Table 3 lists the thermodynamic parameters calculated from the adsorption isotherms at ph 5.5 for U(VI) and at ph 8.0 for Sr(II) at three different temperatures. The positive values of H 0 indicated an endothermic process of U(VI) and Sr(II) adsorption on OGO surface [32]. The endothermic adsorption process can be interpreted the solvation process of radionuclides. In order to adsorb the radionuclides on OGO, the solvated radionuclides are required to denude of the hydration sheath by adsorbing energy, which is higher than the energy released to bind radionuclides on OGO surface, making the whole process to be endothermic [33,34]. As expected, a negative Gibbs free energy change (G 0 ) was obtained, indicating a spontaneous process. The decrease of G 0 with the increasing temperature indicated more efficient adsorption at higher temperature [35,36]. 4 Conclusions (9) In this paper, ozonated graphene oxide was synthesized from graphene oxide. The as-prepared OGO possessed higher oxygen-containing functional groups and higher dispersion properties in aqueous solutions. The sorption capacity of the as-prepared OGO was measured by the sorption of Sr(II) and U(VI) ions from aqueous solutions. The adsorption of Sr(II) ions on OGO was strongly dependent on ph and ionic strength while the adsorption of U(VI) ions on OGO was independent of ionic strength. The thermodynamic parameters calculated from the temperaturedependent adsorption isotherms indicated that the adsorption of Sr(II) and U(VI) ions was an endothermic and spontaneous process. The OGO has high adsorption capacity toward Sr(II) and U(VI) ions due to the strong surface complexation between Sr(II)/U(VI) ions and the abundant oxygen-containing functional groups on the surfaces of OGO. Acknowledgments This work was supported by the National Natural Science Foundation of China ( , , , , ), the Chinese National Fusion Project for ITER (2013GB110005), the Fundamental Research Funds for the Central Universities (JB ), the Jiangsu Provincial Key Laboratory of Radiation Medicine and Protection and the Priority Academic Program Development of Jiangsu Higher Education Institutions are acknowledged. Conflict of interest The authors declare that they have no conflict of interest. 1 Reddad Z, Gerente C, Andres Y, Le Cloirec P. Environ Sci Technol, 2002, 36: Zhao YG, Li JX, Zhao LP, Zhang SW, Huang YS, Wang XL, Wang XK. Chem Eng J, 2014, 235: Li JX, Chen SY, Sheng GD, Hu J, Tan XL, Wang XK. Chem Eng J, 2011, 166: Chen JP. Bioresour Technol, 1997, 60: Qian LJ, Hu PZ, Jiang ZJ, Geng YX, Wu WS. Sci China Chem, 2010, 53: Chen CC, Hayes KF. Geochim Cosmochim Acta, 1999, 63:

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