Application of graphene oxides and graphene oxide-based nanomaterials in radionuclide removal from aqueous solutions

Sci. Bull. DOI 10.1007/s11434-016-1168-x

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Feature Article

Chemistry

Application of graphene oxides and graphene oxide-based nanomaterials in radionuclide removal from aqueous solutions Xiangxue Wang • Shujun Yu • Jie Jin • Hongqing Wang • Njud S. Alharbi • Ahmed Alsaedi • Tasawar Hayat • Xiangke Wang

Received: 29 June 2016 / Revised: 12 August 2016 / Accepted: 16 August 2016  Science China Press and Springer-Verlag Berlin Heidelberg 2016

Abstract With the fast development of nanoscience and nanotechnology, the nanomaterials have attracted multidisciplinary interests. The high specific surface area and large numbers of oxygen-containing functional groups of graphene oxides (GOs) make them suitable in the preconcentration and solidification of radionuclides from wastewater. In this paper, mainly based on the recent work carried out in our laboratory, the efficient elimination of radionuclides using GOs and GO-based nanomaterials as adsorbents are summarized and the interaction mechanisms are discussed from the results of batch techniques, surface complexation modeling, spectroscopic analysis and theoretical calculations. This review is helpful for the understanding of the interactions of radionuclides with GOs and GO-based nanomaterials, which is also crucial for the application of GOs and GO-based nanomaterials in Xiangxue Wang and Shujun Yu contributed equally to this work. X. Wang  S. Yu  J. Jin (&)  X. Wang (&) School of Environment and Chemical Engineering, North China Electric Power University, Beijing 102206, China e-mail: [email protected] X. Wang e-mail: [email protected] H. Wang  X. Wang School of Chemistry and Chemical Engineering, University of South China, Hengyang 421001, China N. S. Alharbi Biotechnology Research Group, Department of Biological Sciences, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia A. Alsaedi  T. Hayat  X. Wang NAAM Research Group, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia

environmental radionuclide pollution management and also helpful in nuclear waste management. Keywords Graphene oxides  Radionuclides  Sorption  Interaction mechanism  Theoretical calculation  Spectroscopic analysis

1 Introduction With the quick development of nuclear science and energy, more and more nuclear power plants have been built and large amounts of nuclear wastes have been generated. In the nuclear fuel cycle options (i.e., from the extraction of uranium for the fabrication of nuclear fuel, the application of nuclear fuel in nuclear power plants, and at last in the spent fuel process to the geological disposal, etc.), it is inevitable to produce large volumes of wastewater containing different kinds of long-lived radionuclides, especially some important fission products, lanthanides and actinides, which should be eliminated from the radioactive wastewater before they are discharged into the natural environment [1, 2]. Different kinds of techniques such as sorption, (co)precipitation, ion exchange, solidification, and membrane separation, have been extensively applied to eliminate radionuclides from wastewater [3, 4]. Among these methods, sorption technique has widely been applied in large scale and in real environmental pollution management because of its simple operation, low cost and applications in large scale [5–8]. The application of sorption technique in the removal of radionuclides from large volumes of aqueous solutions has been studied extensively by using batch techniques, and the interaction mechanisms were discussed from the results of surface complexation

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modeling, spectroscopic analysis and theoretical calculations. From the batch sorption results, the interaction mechanism can be postulated from the results, such as outer-sphere surface complexation or ion exchange is mainly ionic strength-dependent and pH-independent whereas inner-sphere surface complexation is mainly pHdependent and ionic strength-independent. From the surface complexation modeling simulation, the formation of the surface complexes of radionuclides on solid surfaces can be provided and the interaction mechanism may be postulated from the assumption of the species, such as inner-sphere surface complexes, outer-sphere surface complexes, multi-layered surface complexes, surface (co)precipitation, etc. [9, 10]. From the spectroscopy measurements, the species and the microstructures of radionuclides on solid particles at molecular level and the functional groups which form surface complexes with radionuclides can be achieved directly from the spectrum analysis, which are crucial and useful to evaluate the interaction mechanism of radionuclides at solid–liquid interfaces [11, 12]. For example, the X-ray absorption fine structure (XAFS) spectroscopy, which includes X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy, is helpful to achieve the microstructures and species at molecular level. The XANES spectra can provide the evidence for the identification of oxidation–reduction state of radionuclides at solid particles directly, whereas the EXAFS spectra can provide the information of specific bonding type, coordination number and the corresponding microstructures at molecular level [13, 14]. The analysis of XAFS spectra is crucial to get the information of the microstructures and the species of radionuclides on solid particles. The time resolved laser fluorescence spectroscopy (TRLFS) can provide the information of the number of water molecules in the first coordination sphere from the analysis of the fluorescence lifetime, which is crucial to identify the formation of outer-sphere or innersphere surface complexes [15–19]. From the loss of water molecular number in the sorption process, the interaction mechanism and the binding state of radionuclides at solid/ water interfaces can be evaluated. The Raman spectroscopy can identify the molecular structure of radionuclides from the peak position and fundamental vibrations to determine the binding state of the sorption species [20]. The X-ray photoelectron spectroscopy (XPS) can be used to obtain the chemical composition of radionuclides with functional groups, the oxidation states, and bonding relationships of radionuclides with surrounding spheres. The theoretical calculation such as density functional theory (DFT) has shown to be a very useful and powerful tool to describe and evaluate the physical and chemical properties of radionuclide interaction with solid particles [21–27]. From the

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binding energies of the radionuclides interaction with different functional groups, one can understand the interaction mechanism of radionuclides and to identify the sorption properties such as physical sorption or chemical sorption, which are helpful to interpret the experimental results. Generally, from the results of batch experiments, advanced spectroscopic analysis and theoretical calculation, one can understand the interaction mechanism of radionuclides at solid-water interfaces clearly. In recent years, graphene oxides (GOs) and GO-based nanomaterials have attracted multidisciplinary interests because of their unique physicochemical properties such as high surface area, high chemical stability, large pore volume structure and abundant oxygen-containing functional groups [28–35]. These special properties make GOs suitable for efficient elimination of inorganic and organic chemical pollutants from solutions. Zhao et al. [36] firstly synthesized sulfonated GOs and applied the sulfonated GOs as adsorbents to remove organic pollutants in environmental pollution cleanup, and found that the prepared GOs had the highest adsorption capacity in the preconcentration of persistent aromatic pollutants among today’s nanomaterials, which was also evidenced from the DFT calculations. They also applied GOs to remove Co(II) and Cd(II) ions from aqueous solutions using batch technique and found that the adsorption capacities were much higher than any related materials currently reported. Most importantly, they found that the presence of humic acid reduced Co(II) and Cd(II) sorption at the applied pH values (i.e., pH \ 8), which was quite different to the effect of humic acid on the sorption of heavy metal ions on clay minerals and oxides, suggesting that the GOs provided much more accessible unoccupied sites and functional groups for the binding of metal ions rather than humic acid. The surface adsorbed humic acid occupied parts of sorption sites on GO surfaces, thereby reduced the available sites and functional groups and decreased the sorption capacity [37]. The interactions of radionuclides with GOs and GObased materials were further studied under different experimental conditions, and the results showed that the GOs and GO-based materials had high sorption capacities in the removal of radionuclides, which was attributed to the large number of oxygen-containing functional groups on the surface or edge sites [38, 39]. From the above mentioned literatures, one can see that GOs and GO-based nanomaterials have high surface areas and large number of functional groups, which are favorable for the preconcentration of radionuclides from aqueous solutions. Although the applications of GOs and GO-based nanomaterials in the elimination of radionuclides from wastewater have been studied extensively, the summary on the interaction mechanisms of radionuclides with GOs is still scarce [34]. In this review paper, mainly based on the

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recent work carried out in our laboratory, we discussed the interactions of radionuclides with GOs from the viewpoint of surface complexation modeling, the spectroscopy analysis and computational theoretical calculations to understand the interaction mechanism.

2 Batch experiments GOs contain large amounts of hydrophilic oxygen-containing functional groups such as carboxyl (COOH) group, epoxy (C–O–C) group, and hydroxyl (OH) group, on the both basal planes and edges, which can form strong surface complexes with radionuclides. The high surface area and rich functional groups offer GOs excellent performance for removing radionuclides such as Cs(I) [40], Sr(II) [41], Eu(III) [42], and U(VI) [43] from aqueous solutions. Besides the high specific surface area and large number of oxygen-containing functional groups, the c-axis spacing of GOs (0.87 nm) is enough for most of metal ions/radionuclides (e.g., r * 0.48 nm for Pb(II), 0.43 nm for Cd(II), 0.42 nm for Co(II), 0.41 nm for Sr(II), and 0.33 nm for Cs(I)) to enter the interlayer space of GOs, thereby can form strong complexes which are very difficult to be desorbed from GOs. The delocalized p electron systems of graphene layer as Lewis base can form electron donor– acceptor complexes with radionuclides as Lewis acid through Lewis acid–base interaction, which also contributes to the high sorption capacity of radionuclide on GOs [37]. GOs have high dispersion properties in aqueous solutions due to their hydrophilic nature, thereby it is difficult to separate GOs from aqueous solution by using the traditional separation techniques after the GOs are applied as adsorbents in the sorption process, and this may also increase the real application cost [44]. The magnetic Fe3O4 nanoparticles in GO composites could serve as a good stabilizer against the aggregation of GO nanosheets because of the strong van der Waals interactions between the layers of graphene nanosheets [45]. The magnetic composites can be easily separated from solution using magnetic separation, and thereby can be used in large scale in possible real applications. Meanwhile, the good magnetic recycle ability and the high stability of the materials can prevent the release of nanoscaled composites into aqueous solutions effectively, which avoids the GO-based magnetic nanocomposites to generate unknown damage to environment [46]. Liu et al. [47] synthesized the magnetite/graphene oxide (M/GO) composites with the magnetite particle sizes of 10–15 nm by a chemical reaction (Fig. 1). The sorption kinetic of radioactive Co(II) ions on the as-prepared M/GO composites was well simulated by the pseudo-second-order model. The pH-dependent and

ionic strength-independent sorption curves indicated that Co(II) sorption on M/GO was mainly dominated by innersphere surface complexation at low pH values, whereas the sorption of Co(II) was accomplished by simultaneous surface (co)precipitation and inner-sphere surface complexation at high pH values. From the temperature-dependent sorption isotherms, the thermodynamic parameters were calculated, which indicated that Co(II) sorption process on the M/GO composites was spontaneous and endothermic. The Co(II)-bounded M/GO composites were separated and recovered from the solution quickly by using easy magnetic separation method, which was important for the real application in large scale. It is also interesting to notice that the M/GO composites have high stability and can be used for many times with little decrease of sorption capacity. The high stability of GOs under some harsh conditions such as very low pH values and very high pH values can also assure the application of GOs in real nuclear waste management. Nanoscale zero-valent iron (NZVI) can be deposited on GOs to prepare NZVI/GO composites, which can not only increase the stability and dispersion of bare NZVI, but also improve the electron transfer and increase the preconcentration of metal ions by the coupling of the advantages of NZVI reduction and GO high adsorption capacity. Sun et al. [48] reported a chemical reduction technology for the synthesis of reduced GO nanosheets supported NZVI (NZVI/rGO). The presence of rGO on NZVI nanoparticles can increase the reaction rate and the removal capacity of U(VI) from aqueous solutions significantly, which is mainly attributed to the chemisorbed –OH groups of rGO and the massive enrichment of Fe2? ions on rGO surfaces, which is also evidenced by the XPS spectrum analysis [48]. The surface adsorbed U(VI) can be reduced to U(IV), which can reduce the mobility of U(VI) in the environment. The sorption and reduction mechanism of U(VI) to U(IV) is shown in Fig. 2. Li et al. [49] synthesized the NZVI/rGOs composites by using a H2/Ar plasma technique from GOs-bound Fe ions and applied the as-prepared NZVI/rGOs composites in the removal and reductive immobilization of perrhenate (ReO4-). The redox reactions of Re(VII) on the NZVI/ rGOs composites can be expressed by the following equations 3Fe 0 ? 2ReO 4- ? 8H ??3Fe 2? ? 2ReO2 ? 4H2O and 3Fe2? ? ReO4- ? 4H??3Fe3? ? ReO2 ? 2H2O. The Fe0 nanoparticles can be oxidized by ReO4easily, and thus formed Fe2? ions can also reduce ReO4- to ReO2 through the above mentioned equations. It is well known that the chemistry characteristic of Tc(VII) is similar to that of Re(VII). Thus the NZVI/rGOs composites have the potentials in the preconcentration and reductive immobilization of TcO4-, which is a kind of very important fission product in spent nuclear fuels. Recently, we [50] reviewed

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Fig. 1 SEM (a) and TEM (b) images, XRD pattern (c), and XPS spectrum (d) of the M/GO composite (the inset illustrates the high solution spectrum of Fe 2p peak of the composite). Reprinted with permission from Ref. [47]. Copyright (2011) American Chemical Society

Fig. 2 (Color online) Simultaneous adsorption and reduction of U(VI) on NZVI/rGO. Reprinted from Ref. [48], Copyright (2014), with permission from Elsevier

the NZVI and NZVI/GO composites for the removal of heavy metal ions and radionuclides from solutions, and concluded that NZVI and NZVI/GO materials had satisfactory removal abilities for metal ions and radionuclides, and played important roles in environmental pollution management. GOs present high sorption capacity for radionuclides. However, GOs can incur irreversible aggregation and/or polydisperse in its thickness, lateral size and shape, which

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may hinder the effective sorption behaviors and thereby reduces the sorption capacity. Therefore, many researchers grafted polymers including polyaniline [51], polyacrylamide [38], cyclodextrins [52], chitosan [53], amidoxime [54], onto GOs to introduce various functional groups and to enhance their dispersibility in solutions and thereby improves the removal ability of some radionuclides. Sun et al. [51] reported that the maximum sorption capacities of Cs(I), Sr(II), Eu(III) and U(VI) on GOs were 0.684, 0.656, 0.953, and 0.362 mmol/g, respectively, at T = 298 K and low pH value (i.e., pH 3.0), while the maximum sorption capacities of Cs(I), Sr(II), Eu(III) and U(VI) on the GO-supported polyaniline (PANI@GO) nanocomposites (Fig. 3) were enhanced to 1.39, 1.68, 1.65 and 1.03 mmol/g, respectively. The PANI has a strong affinity for radionuclides and heavy metal ions due to the large number of amine and imine functional groups, these functional groups can form very strong complexes with radionuclides on the nanocomposite surfaces [55]. The chemical binding of radionuclides with the nitrogen-containing functional groups is much stronger than that of radionuclides with the oxygen-containing functional

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Fig. 3 (Color online) Comparison of the sorption capacity of PANI@GO composites with other adsorbents for the uptake of radionuclides, pH 3.0, T = 298 K, m/V = 0.25 g/L. Reprinted with permission from Ref. [51]. Copyright (2013) American Chemical Society

groups, which is also evidenced by the XPS spectrum analysis [55].

3 Surface complexation modeling Surface complexation models (SCMs) are the efficient tools to describe the macroscopic sorption of radionuclides on GOs at solid/water interfaces. SCMs are empirical modeling approaches extending the ion-association model of aqueous solution chemistry to include chemical species on solid surfaces. Generally, the following criteria should be taken into account [9]: (1) sorption occurs at specific surface coordination sites; (2) sorption reaction can be described by the mass law equation; and (3) surface charge results from the surface complex formation reaction itself. From the acid–base titration curves, the surface properties of GOs can be achieved, which are helpful to describe the sorption properties of radionuclides on GOs as a function of solution pH, solute concentrations and ionic strength, etc. The hypothetical surface properties are applied in SCMs to describe the equilibrium relationship at solid/ water interfaces. Normally, three SCMs, i.e., the constant capacitance model (CCM), the diffuse layer model (DLM) and the triple layer model (DLM), are widely used for the simulation of interactions of radionuclides with GOs. The DLM is the simplest and generally three parameters (the sorption constants, the total number of the surface sites, and the surface acidity constants) are applied in the simulation process. The CCM has four adjustable parameters (the sorption constants, the total number of surface sites, the surface acidity constants of the surface sites, and the inner-layer capacitance). The TLM is the most complex and has five parameters for the adjustable simulation (the

total number of surface sites, the surface acidity constants for the surface sites, the inner-layer capacitance, the outerlayer capacitance, and the sorption constants) [20]. A comprehensive understanding of the transport and the fate of radionuclides at solid/water interfaces by using the SCMs is hence crucial for the risk assessment of radionuclides [56–58]. In recent years, the SCMs have been extensively used to model the adsorption of radionuclides on GOs under different environmental conditions [59–61]. Ding et al. [42] used diffuse double-layer model (DDLM) of SCMs to simulate the adsorption of U(VI) on GOs with the aid of FITEQL v 4.0 code. The fitting results of SCMs showed that the adsorption behaviors of U(VI) on GO nanosheets can be well simulated by a DDLM with the mononuclear monodentate [SOUO2? and [SOUO2OH0 complexes (Fig. 4). As shown in Fig. 5, Sun et al. [62] also demonstrated that SCMs gave an excellent fitting for the adsorption of Eu(III) on GOs with the predominated binuclear bidentate [(SO)2Eu2(OH)22? complexes and mononuclear monodentate [SOEu2? species, which were in good agreement with the ionic strength-dependent adsorption experimental results. At low pH values, the Eu(III) was adsorbed on GOs as [SOEu2? species, and then the [(SO)2Eu2(OH)22? species became the dominate species with pH increasing. From the surface complexation modeling, the relative distribution species of radionuclides on GOs can be achieved, which is helpful to understand the interaction mechanisms of radionuclides with GOs at different pH values.

4 Spectroscopic analysis Spectroscopy technique is helpful to understand the contribution of different functional groups on the uptake of radionuclides onto GOs. From the spectroscopic analysis, the binding of radionuclides with different functional

Fig. 4 (Color online) Simulation of adsorption of Eu(III) and U(VI) on GOs with diffuse double-layer model (DDLM), a Eu(III); b U(VI). m/V = 0.20 g/L, I = 0.01 mol/L NaClO4, C0 = 10.0 mg/L, T = 303 K [42]. Reproduced by permission of The Royal Society of Chemistry

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Fig. 5 (Color online) Surface complexation modeling of Eu(III) on GOs under the different pH conditions, C0 = 10 mg/L, I = 0.01 mol/ L NaClO4 and T = 293 K. Reprinted with permission from Ref. [62]. Copyright (2012) American Chemical Society

groups can be evaluated. For example, the appearance of new peak generally suggests the formation of chemical bond between radionuclides and functional groups, whereas the shift of peak position normally indicates the formation of inner-sphere surface complexes. Thereby, the spectroscopic analysis is one important tool to understand the interaction mechanisms of radionuclides on solid particles. Sun et al. [48] studied the sorption and reduction of U(VI) to U(IV) on NZVI/GO composites using XPS and XANES techniques. They found that the –OH functional group and the massive enrichment of Fe2? contributed to the enhanced removal of U(VI) through the formation of inner-sphere surface complexes and UIVO2 precipitates were formed from the reduction of U(VI) to U(IV), which was evidenced from the short bond distance of U–Fe ˚ ) in the EXAFS analysis. The GO-supported (3.23 A polyaniline (PANI/GO) nanocomposites were further synthesized through the chemical oxidation process, and the sorption abilities of Cs(I), Sr(II), U(VI) and Eu(III) on the prepared PANI/GO nanocomposites were improved significantly compared to those of GO, activated carbon (AC) and PANI. The XPS analysis showed that the oxygencontaining and nitrogen-containing functional groups on the surfaces of PANI/GO nanocomposites were mainly responsible for the high sorption capacities of radionuclides on PANI/GO nanocomposites [51]. From the XAFS analysis (Fig. 6), one can see that U(VI) and U(IV) species are both presented in nZVI/rGO-2 and nZVI/rGO-60, respectively. The XANES spectra showed that the extent of U(VI) reduction to U(IV) was significantly increased with increasing reaction time, suggesting that part of surface adsorbed U(VI) was reduced to U(IV) on nZVI/rGO composites, and the reduction of U(VI) was increased with increasing aging time. The Fourier

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Fig. 6 (Color online) XANES spectra (a) and Fourier transform (FT) of EXAFS spectra (b) for reference samples and U(VI)-reacted NZVI and NZVI/GO, T = (25 ± 1) C, I = 0.01 mol/L NaClO4, pH 5.0. Reprinted from Ref. [48], Copyright (2014), with permission from Elsevier

transforms (FT) of U(VI) adsorbed samples (Fig. 6b) showed the presence of multimeric surface complexes, and UIVO2 precipitates were formed on the surface of the composites, which was important for the application of nZVI/rGO composites in the elimination, immobilization and reduction of U(VI) to U(IV) in environmental pollution management. After the reduction of U(VI) to U(IV), the mobility of U(IV) is obviously decreased, which is useful for the immobilization of U(VI) and thereby reduces the toxicity of U(VI) in the environment. 5 Theoretical calculations Computational chemistry studies can obtain the electronic structures, bonding nature and thermodynamic properties of lanthanide/actinide complexes with GOs at molecular level. Based on computational chemistry, people can explore the geometric and electronic structures, thermodynamic properties and some molecular properties of radionuclide compounds that can be compared with experimental observations, especially in the absence of experimental data. All actinide complexes are radioactive and toxic, especially transuranic elements, which hinders the experimental researches involving them. Fortunately, quantum chemical studies can provide another useful approach to disclose the electronic structures and bonding natures of actinide complexes. Shi and co-workers [24–27] have investigated the coordination modes and thermodynamic behaviors between GOs and Th(IV), U(VI), Np(V) and Pu(VI, IV) using the quasi-relativistic DFT method. Bai et al. [63] have reported the interaction of Th(IV) with GOs modified by the carboxyl and hydroxyl groups on

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the side and surface using DFT method. Three Th(IV)/ GO(COOH) (A, D and E) and four Th(IV)/GO(OH) (B, C, F, and G) structures including four nitrate anions were optimized at B3LYP/ECP60MWB-SEG/6-31G(d) level of theory as shown in Fig. 7. Three Th(IV)/GO complexes A, B and C are eight-coordinated structures, whereas three of the four nitrate anions act as bidentate ligands and the fourth nitrate anion coordinates monodentately to Th(IV). Whereas four Th(IV)/GO complexes D, E, F and G are nine-coordinated structures, all the nitrate anions are coordinated as bidentate ligands. All the average Th–O bond distances for nitrate anions are similar and range ˚ for the seven complexes, and the Th–O within 2.44–2.50 A bond distance of the hydroxyl group is longer than that of the carboxyl group for all Th(IV)/GO complexes, which suggests that the carboxyl group has stronger coordination ability as compared to the hydroxyl group. It showed that the calculated coordination number and bond lengths were in good agreement with the results of EXAFS spectra analysis. The binding energies suggested that the coordination modes for all the seven structures were favorable energetically. It is well known that the chemical properties of uranyl and thorium ions are quite different. Th(IV) ions are bare and more positively charged, while uranyl cations are linear. Therefore, the adsorption property of uranyl ions may be different with that of Th(IV) ions at the edge or on the surface of GOs. Wu et al. [26] investigated the complexes of uranyl ions with GOs, which were modified by carboxyl and hydroxyl groups, by using DFT calculations. The structures of the U(VI)/GO complexes optimized at B3LYP/ECP60MWB-SEG/6-31G(d) level of theory are shown in Fig. 8 [26]. One can see that the average U–OG

Fig. 7 (Color online) Structures of the Th(IV)/GO complexes optimized using the B3LYP method. Three complexes a, d and e modified by carboxyl group and four complexes b, c, f, and g modified by hydroxyl group. Reproduced from Ref. [63] with permission from The Royal Society of Chemistry

bond length is much shorter in the anionic GO complex as compared to the corresponding neutral ones, and the hydrogen bonds appear for the uranyl complexes with the ortho-functionalized or meta-functionalized anionic GOs. Due to the cooperative effect of negatively charged GOs and the hydrogen bonds, it is easier for uranyl cations to bind with the anionic GOs than with the corresponding neutral ones. Natural bonding orbital (NBO) analysis suggests that the electrostatic interactions mainly dominate the U–O bonds for uranyl/GO complexes. Based on the changes of the Gibbs free energy, the uranyl ion is favorable to chelate with the anionic GOs in aqueous solution due to the deprotonated states of GOs. The structural and electronic properties as well as thermodynamic behaviors of the GO complexes with Np(V), Pu(VI) and Pu(IV) had been explored theoretically [24, 64]. They found that Pu(VI) and Np(V) complexes with GOs were five-coordination at the equatorial plane, while Pu(IV) complexes preferred to form eight-coordination. The analysis of NBO and QTAIM (quantum theory of atoms in molecules) showed that the Pu(IV)–O bonds of

Fig. 8 (Color online) Structures of the U(VI)/GO complexes optimized at B3LYP/ECP60MWB-SEG/6-31G(d) level of theory. M, OB and MB denote mono-functional, ortho- and meta-bifunctional GO, respectively. Deprotonated states of GOs are labeled with a superscript 0 for denotation. Adapted with permission from Ref. [26]. Copyright (2014) American Chemical Society

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Pu(IV)/GO complexes possessed more covalency, while the electrostatic interactions dominated the Np(V)–OG and Pu(VI)–OG bonds. According to the binding energies in aqueous solution, the adsorption abilities of GOs toward Np(V), Pu(VI) and Pu(IV) ions followed the below orders: GO(–COOH) [ GO(–OH) [ GO(–CO) [ GO(–O–); GO (–OH) [ GO(–CO) [GO(–COOH) [ GO(–O–), GO(–O–) [ GO(–OH) [ GO(–COOH) [ GO(–CO), respectively. These results indicate that the binding ability of the same type GOs toward the actinide ions is different, which can provide qualitative trends for experimental investigations. From the results in the aforementioned papers, one can understand that the high sorption of radionuclides on GOs and GO-based materials is mainly attributed to the surface functional groups. However, the contribution of different functional groups on radionuclide uptake to GOs is still unclear because of the presence of different kinds of oxygen-containing functional groups on GO surfaces or edges. The binding energy (BE) calculated from DFT calculations can also give some important information about the binding of radionuclides with surface groups of GOs. The BE of different functional groups with radionuclides can be calculated from the DFT calculations, and then the contribution of different group on the uptake of radionuclides can be evaluated from the BE values, i.e., the higher BE is, the stronger interaction of radionuclides with functional group is. Sun et al. [60] studied the adsorption of U(VI) on different functionalized GOs and found that adsorption capacities of U(VI) were GOs [ HOOC-GOs [ rGOs from the batch sorption experiments, whereas the amounts of desorbed U(VI) were HOOC-GOs \ GOs \ rGOs from the kinetic desorption measurements. The high sorption of U(VI) on GOs and HOOC-GOs was attributed to the innersphere surface complexation, whereas outer-sphere surface complexes were formed on rGOs. The DFT calculation showed that the BE value of [GOs-COO…UO2]? (50.5 kcal/mol) was much higher than that of [rGOs…UO2]? (8.1 kcal/mol), suggesting that the desorption of U(VI) from the –COOH groups of HOOC-GOs was much more difficult than from GOs and rGOs, which was in good agreement with the kinetic desorption experiments (Fig. 9). The DFT calculations further evidenced the batch experimental results that the –COOH group formed much stronger surface complexes with U(VI) than other oxygencontaining functional groups. The DFT calculation is crucial to understand the interaction of radionuclides with different functional groups at molecule level, which is helpful for us to understand the contribution of different functional groups for the binding of radionuclides on GOs. Such information cannot be achieved from the batch experiments, surface complexation modeling and spectroscopy analysis.

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Fig. 9 (Color online) The DFT-optimized geometries of the rGOs_uranyl complexes and GOs_uranyl complexes. Reprinted with permission from Ref. [60]. Copyright (2015) American Chemical Society

6 Conclusion and prospectives It is well known that GOs can be fully incinerated at high temperatures, which can minimize the production of secondary wastes. After the preconcentration of radionuclides from aqueous solutions, the GOs can be incinerated at high temperatures (i.e., GOs can be completely decomposed at T [ 800 C), and the radionuclides are left in the fly ash. The volume of the waste is obviously minimized, which can decrease the cost for nuclear waste management. Herein, on the basis of our recent work, we reviewed the interactions of several important radionuclides with GOs and GO-based nanomaterials from batch technique, surface complexation modeling, spectroscopy analysis and theoretical calculations. This paper only presents representative results of some important fission product elements and actinides interaction with GOs and GO-based nanomaterials, and the interaction of radionuclides with GOs and GO-based nanomaterials is mainly dominated by outer-sphere surface complexation at low pH values, and by inner-sphere surface complexation at high pH values. The formation of precipitates and multisurface complexes are also inevitable at high pH values because of the high loading of radionuclides on GO surfaces. After the surface loading of nanoscaled zero-valent iron on GO surface, the NZVI nanoparticles can reduce the surface adsorbed radionuclides from high valence to low-valence, such as the reduction of UO22? to UIVO2, and TcO4- to TcO2, which is crucial for the immobilization of U(VI) or Tc(VII) or other kinds of high valence radionuclides in the natural environment. From the spectroscopy analysis, the microstructures of radionuclides on GOs or GO-based nanomaterials at molecular level can be calculated from the spectrum analysis. However, different spectroscopy techniques have

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advantages and disadvantages. For XAFS analysis, the samples can be prepared as powder or solution, and the species can be derived from the structure calculated from XAFS analysis, however, the XAFS can only provide the averaged signal of the prevailing species and cannot distinguish between scattering atoms with little difference in atomic number such as C, N, O or S, Cl, Mn or Fe [20]. The XANES is sensitive for the variation of valence, whereas EXAFS is for the bond distances and coordination number. The XAFS technique can also be used in many research areas, for example bioscience such as protein structure, catalytic analysis such as catalyst measurement, energy areas such as phase change material analysis and geochemical research. For TRLFS analysis, it is non-invasive, in situ method with high sensitivity and selectivity at very low concentration, however the different species cannot be detected and the measurement is dependent on temperature and is influenced by apparatus properties. For XPS analysis, it is high surface sensitivity with the detection of most surface-concentrated elements in the top 3–30 atomic layers, however the measurements should be carried out in vacuum which possibly changes the sample properties. The theoretical calculations can further evidence the interaction of different kind of radionuclides with different functional groups of GOs, which is helpful to modify the GOs with special functional groups to improve the selective removal of radionuclides from solutions to GOs and GO-based materials. Such information is crucial to understand the interaction mechanism of radionuclides with GOs, and to evaluate the behavior of radionuclides in the natural environment. With the large application of GOs in multidisciplinary areas, parts of GOs can be released into the natural environment inevitably and thereby become environmental pollutants [65]. In near future, the study of GO properties in the natural environment should be considered such as the toxicity of GOs, the coagulation and interaction of GOs with clay minerals and oxides, the migration and diffusion of GOs in the natural environment in the presence and absence of different kinds of organic materials and heavy metal ions. The GOs themselves should be considered as one kind of pollutants in the environment. Such research is crucial to understand the physicochemical behavior of GOs in the natural environment. Acknowledgments This work was supported by the National Natural Science Foundation of China (21225730, 91326202, and 21577032), the Fundamental Research Funds for the Central Universities (JB2015001), and Furong Scholarship of Hunan Province. Conflict of interest The authors declare that they have no conflict of interest.

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