Interaction between Eu(III) and Graphene Oxide Nanosheets

May 2, 2012 - The interaction mechanism between Eu(III) and graphene oxide nanosheets (GONS) was investigated by batch and extended X-ray absorption f...
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Interaction between Eu(III) and Graphene Oxide Nanosheets Investigated by Batch and Extended X-ray Absorption Fine Structure Spectroscopy and by Modeling Techniques Yubing Sun, Qi Wang, Changlun Chen, Xiaoli Tan, and Xiangke Wang* Key Laboratory of Novel Thin Film Solar Cells, Institute of Plasma Physics, Chinese Academy of Science, Post Office Box 1126, Hefei, 230031, Peopleʼs Republic of China S Supporting Information *

ABSTRACT: The interaction mechanism between Eu(III) and graphene oxide nanosheets (GONS) was investigated by batch and extended X-ray absorption fine structure (EXAFS) spectroscopy and by modeling techniques. The effects of pH, ionic strength, and temperature on Eu(III) adsorption on GONS were evaluated. The results indicated that ionic strength had no effect on Eu(III) adsorption on GONS. The maximum adsorption capacity of Eu(III) on GONS at pH 6.0 and T = 298 K was calculated to be 175.44 mg·g−1, much higher than any currently reported. The thermodynamic parameters calculated from temperature-dependent adsorption isotherms suggested that Eu(III) adsorption on GONS was an endothermic and spontaneous process. Results of EXAFS spectral analysis indicated that Eu(III) was bound to ∼6−7 O atoms at a bond distance of ∼2.44 Å in the first coordination shell. The value of Eu−C bond distance confirmed the formation of inner-sphere surface complexes on GONS. Surface complexation modeling gave an excellent fit with the predominant mononuclear monodentate >SOEu2+ and binuclear bidentate (>SO)2Eu2(OH)22+ complexes. This paper highlights the application of GONS as a suitable material for the preconcentration and removal of trivalent lanthanides and actinides from aqueous solutions in environmental pollution management.

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adsorption capacities of Cd(II) and Co(II) on GONS at pH ∼ 6.0 and T = 303 K were about 106.3 and 68.2 mg·g−1, respectively. Accuracy in characterizing and modeling of adsorption phenomena plays an important role in the design of adsorption treatment units. However, there is no study on the interaction between trivalent lanthanides and actinides with GONS by spectroscopy and modeling techniques. Extended X-ray absorption fine structure (EXAFS) spectroscopy, a method for the identification of local atomic structures (i.e., bond distance, coordination number of neighbors, and type of near neighbors surrounding a specific atom) at the molecular level, has been conducted extensively to determine the interaction mechanisms between heavy metals and solid phases.5,6,9,21−23 On the other hand, surface complexation modeling has already been applied extensively to simulate the adsorption behaviors at the solid−water interface.24−27 Fan et al.9 studied the adsorption of Eu(III) on attapulgite by using EXAFS technique and constant capacitance model. The authors found that the bond distance REu−O decreased from 2.415 to 2.360 Å, and the coordination number of Eu−O path decreased from ∼9.94 to ∼8.56 with pH increasing from 1.76 to 9.50. The main adsorption species (≡SwOHEu3+ and ≡X3Eu0) were

INTRODUCTION With the rapid development of nuclear energy and nuclear industry, a large amount of radioactive waste is produced. For work safety in the nuclear industry and for human health, the removal of long-lived radionuclides [i.e., half-lives (t1/2) of Np236, 1.54 × 105 years; Pu244, 8.0 × 107 years; U238, 4.468 × 109 years; etc.] from nuclear waste solutions is an important environmental concern in nuclear waste management. Europium [Eu(III)] is usually chosen as a chemical analogue of trivalent lanthanides [Ln(III)] and actinides [An(III)] in nuclear waste.1 The interactions between Eu(III) and alumina,2−4 TiO2,5,6 clay minerals7−10 and carbon-based materials11−14 have been studied extensively over the past decades. In these studies, the effects of environmental factors such as pH, ionic strength, and humic substances on Eu(III) adsorption on the solid particles were investigated. However, low adsorption capacity limited their practical application in the removal of radionuclides from large volumes of aqueous solution. Graphene has been the hotspot in multidisciplinary areas in recent years due to its excellent mechanical,15 thermal,16 and electrical properties.17 Graphene oxide nanosheets (GONS) are also regarded as suitable materials for sequestration of heavy metal ions due to their excellent adsorption performance.18−20 Yang et al.18 noted that Cu2+ adsorption on graphene oxide at pH 5.0 and T = 293 K could be aggregated with the adsorption capacity of 46.6 mg·g−1. Zhao et al.20 found that the maximum © 2012 American Chemical Society

Received: Revised: Accepted: Published: 6020

February 22, 2012 April 29, 2012 May 2, 2012 May 2, 2012 dx.doi.org/10.1021/es300720f | Environ. Sci. Technol. 2012, 46, 6020−6027

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observed in 0.01 mol·L−1 NaClO4 solution, whereas ≡X3Eu0, ≡SwOHEu3+, and ≡SsOEu(OH)20 were the dominant species in 0.1 mol·L−1 NaClO4 solution. Bouby et al.6 studied the adsorption of Eu(III) on titanium dioxide by using EXAFS technique and a modified acid−base MUSIC model, and the EXAFS results revealed rough conservation of the coordination with 8−9 O atoms in the first coordination shell and surface hydroxyl groups upon adsorption. It was conceivable that mono- and multidentate species contributed to Eu(III) adsorption to adsorbents. The objectives of this paper were to (1) synthesize GONS and characterize the microscopic and macroscopic surface properties of GONS by using scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transformed infrared spectroscopy (FT-IR), potentiometric acid−base titration, and X-ray photoelectron spectroscopy (XPS); (2) investigate the effects of pH, ionic strength, initial Eu(III) concentrations, and temperature on Eu(III) adsorption on GONS by batch technique; (3) simulate the adsorption behavior of Eu(III) on GONS as a function of pH by using surface complexation modeling to determine the nature and significance of Eu(III) complexes; and (4) discuss the interaction mechanism between Eu(III) and GONS by using Eu LIII-edge EXAFS spectroscopy. This study highlighted the extensive applicability of this novel material in the removal of trivalent lanthanides and actinides from large volumes of aqueous solutions.

pH 11.0 with 0.05 mol·L−1 NaOH titrant at a variable increment (0.008 up to 0.15 mL). 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 using 150 W Al Kα radiation. Adsorption Edge Experiments. Adsorption experiments were conducted with 0.2 g·L−1 GONS and 3.35 × 10−4 mol·L−1 Eu(III) under N2 conditions at T = 25 ± 1 °C in the presence of 0.01 mol·L−1 NaClO4. The pH of suspension was adjusted to be in the range 2.0−11.0 by adding negligible volume of 0.1 or 0.05 mol·L−1 HClO4 or NaOH solution. To eliminate the effect of Eu(III) adsorption on tube walls, the adsorption of Eu(III) without GONS was carried out under the same experimental conditions. The bulk suspensions of GONS and NaClO4 were pre-equilibrated for 24 h, and then Eu(III) stock solution and radiotracer 152+154Eu(III) were spiked into the bulk suspension gradually in order to avoid forming Eu(OH)3(s) precipitate at pH > 7.0. The suspensions were shaken for 48 h to ensure that the adsorption reaction could achieve adsorption equilibrium. The preliminary experiments found that 48 h was adequate for the suspension to obtain equilibrium. Subsequently an aliquot of the suspension (2.0 mL) was removed and the total activity of 152+154Eu(III) (Atot) was determined. The solid and liquid phases were separated by centrifugation at 9000 rpm for 30 min, and then the supernatant was poured into a syringe and filtered through a 0.22-μm membrane. The concentration of Eu(III) in supernatant was analyzed by liquid scintillation counting on a Packard 3100 TR/AB liquid scintillation analyzer (Perkin-Elmer) with the scintillation cocktail (Ultima Gold AB, Packard). Adsorption percentage (R) was calculated as R (%) = 100(1 − AL/Atot), where Atot and AL were the total activity of 152+154 Eu(III) in the system and that of 152+154Eu(III) in supernatant after centrifugation and ultrafiltration, respectively. Surface Complexation Modeling. The interaction of Eu(III) with GONS as a function of pH at 0.01 mol·L−1 NaClO4 was simulated by using the diffuse double layer model (DDLM) with the aid of FITEQL v 4.0 code.29 The FITEQL code has been extensively used for the calculation of chemical equilibrium constants and the fitting of adsorption behaviors at solid−water interfaces.30−33 The chemical equilibrium constants of Eu(III) species and the corresponding distribution of Eu(III) species are shown respectively in Table S1 and Figure S1 in Supporting Information. The equilibrium adsorption constants (log K) assumed in modeling of Eu(III) adsorption on GONS were obtained by best fitting of the experimental Eu(III) adsorption data (Table 1), and surface acidity constants (pK1 and pK2) were calculated by linear extrapolation of pK to

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EXPERIMENTAL SECTION Materials. The synthesis of GONS was described in our previous literature.20 Briefly, flake graphite was strongly oxidized by using KMnO4 and concentrated H2SO4 under ultrasonication conditions, and then H2O2 was added to eliminate the excess MnO4− anions. GONS were obtained by centrifugation at 18 000 rpm for 30 min. The detailed processes are depicted in Supporting Information. The N2-BET specific surface area (SSA) of the prepared GONS was measured to be 120 m2·g−1, which was significantly lower than the theoretical value (∼2620 m2·g−1). It was assumed that the powder of GONS could be easily aggregated together, which can result in the partial overlapping and coalescing of nanosheets.19,28 Eu(III) stock solution at 0.1 mol·L−1 was prepared from Eu2O3 (purity 99.99%) after dissolution, evaporation, and dilution with 0.01 mol·L−1 HClO4 solution. All chemicals used in this experiment were analytical-grade, and all the solutions were prepared with Milli-Q water. Characterization of GONS. GONS were characterized by SEM, TEM, FT-IR, potentiometric acid−base titration, and XPS. The SEM image was obtained on a field emission scanning electron microscope (FEI-JSM 6320F). The TEM image was employed with a JEOL transmission electron microscope (JEM-2010, Japan). FT-IR spectroscopy measurements were mounted by using a Perkin-Elmer 100 spectrometer in KBr pellet at room temperature. The potentiometric acid−base titration was performed to determine the chemical properties of GONS by use of a computercontrolled titration system (DL50 automatic titrator, Mettler Toledo). Briefly, 0.05 g of GONS was spiked into 0.01 mol·L−1 NaClO4 background electrolyte at T = 25.0 ± 0.5 °C, and the sample was purged with argon gas for 2 h to exclude atmospheric CO2(g). The initial pH of suspension was adjusted to pH 3.0 by adding 0.01 mol·L−1 of HClO4 with vigorous stirring for 1 h, and then the suspension was slowly titrated to

Table 1. Parameters Assumed in Modeling Eu(III) Adsorption on GONS specific surface area (SBET) surface site densitya surface acidity constants (pK1, pK2) pHPZC (point of zero charge)b equilibrium adsorption constants (log K)c WSOS/DFd

120 m2·g−1 18.1 sites·nm−2 3.10, 4.70 3.9 4.52 (eq 1), −7.88 (eq 2) 8.48

a

Surface site density = [SOH]NA/SBET. bpHPZC = (pK1 + pK2)/ 2. Log K values are equilibrium adsorption constants of eqs 1 and 2 in this study. dWSOS/DF = weighted sum of squares divided by degree of freedom; 0.1 < WSOS/DF < 20.

c

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Figure 1. Characterization of GONS: (A) SEM image, (B) TEM image, (C) FT-IR spectrum, and (D) acid−base titration curve.

Figure 2. Adsorption of Eu(III) on GONS. (A) Effect of pH. CEu(III) = 3.35 × 10−4 mol·L−1, m/V = 0.2 g·L−1, I = 0.01 mol·L−1 NaClO4, and T = 298 K. () DDLM; (---) Eu(III) surface complexes. (B) Effect of ionic strength. CEu(III) = 3.35 × 10−4 mol·L−1, m/V = 0.2 g·L−1, and T = 298 K.

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RESULTS AND DISCUSSION Characterization of GONS. The characterization results of TEM, SEM, FT-IR, and potentiometric acid−base titration of the prepared GONS are given in Figure 1. The SEM image (Figure 1A) shows that GONS are closely agglomerated together by thin and randomly aggregated nanosheets, and the average thickness of GONS presents ∼5 nm, at the resolution limit of the instrument used in terms of TEM image (Figure 1B). According to the FT-IR spectrum (Figure 1C), different oxygen-containing groups (i.e., C−O group at 1220 and 1100 cm−1, CO group at 1730 cm−1, and CC group at 1620 cm−1) are found,37 which indicates that large amounts of

zero surface charge. The calculation of pK1 and pK2 are described in detail in Figure S2 (Supporting Information). EXAFS Analysis. The Eu LIII-edge X-ray absorption spectra were recorded at Shanghai Synchrotron Radiation Facility (SSRF, Shanghai, China). The EXAFS spectra were collected in transmission mode [crystalline Eu2O3 and crystalline Eu(OH)3] or in fluorescence mode [aqueous Eu(III) ions and adsorption samples] by using a multielement solid-state Ge detector. Analysis of EXAFS data was performed by using Athena and Artemis interfaces to IFFEFIT 7.0 software.34−36 Collection and analysis of EXAFS data are described in detail in Supporting Information. 6022

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oxygen-containing functional groups (hydroxyl, carboxyl, carbonyl, and epoxy groups) are present on the surface of GONS. The pHPZC (point of zero charge) is measured to be 3.9 in terms of eq S6 (Supporting Information), which is similar to that from potentiometric acid−base titration (pHPZC = 4.3, Figure 1D). The surface site density of GONS was calculated as 18.1 sites·nm−2 by fitting potentiometric acid−base titration data with the aid of FITEQL v4.0 code (Figure S3, Supporting Information), which is approximately 3 times higher than that of oxidized multiwalled carbon nanotubes (MWCNTs) (6.2 sites·nm−2).13 Figure S4 (Supporting Information) shows the highresolution O 1s, C 1s, and Eu 3d XPS spectra of Eu(III) adsorbed on GONS and activated carbon (AC) at pH 4.5 in 0.01 mol·L−1 NaClO4 solution. The various kinds of oxygencontaining functional groups can be readily identified by the high-resolution O 1s and C 1s XPS spectra. The Eu(III)adsorbed GONS present three major O 1s peaks positioned at 531.65 eV (bridging OH), 532.66 eV (COO), and 533.41 eV (adsorbed H2O).3,4,19,20,38 The O 1s XPS spectrum indicates a considerable degree of oxidation with different functional groups, which is expected to form strong surface complexes with Eu(III) species easily. Compared to the C 1s spectrum of AC, enhanced intensity of the OC group (at 287.8 eV) is observed on the surface of GONS, indicating that flake graphite is strongly oxidized by using KMnO4 and concentrated H2SO4 under ultrasonication conditions.19 The adsorbed Eu(III) is also recorded by the Eu 3d XPS spectrum, indicating the adsorbed Eu(III) is chemically presented within the nearsurface region of GONS. Effect of pH. The adsorption curves of Eu(III) on GONS as a function of pH in 0.01 mol·L−1 NaClO4 are shown in Figure 2A. Approximately 65% of Eu(III) is adsorbed on GONS at pH ∼ 2.0; Eu(III) adsorption increases slightly with increasing pH ranging from 2.0 to 7.0 and then maintains the high level at pH > 7.0. This adsorption behavior is different from the results of Eu(III) adsorption on TiO2,5,6 on attapulgite,9 and on MWCNTs.12,13 In these studies, the amounts of Eu(III) adsorbed increased from approximately 0 to 100% at pH 2.0− 6.0, which showed that the adsorption edge of Eu(III) on these adsorbents shifted to lower pH as compared to that of Eu(III) on GONS [∼100% of Eu(III) was adsorbed at pH 7.0]. The shift of pH adsorption edge could be attributed to the different Eu(III) surface loading. It is found that the adsorption capacity of Eu(III) on GONS (∼1.1 × 10−3 mol·g−1 at pH 2.0 in 0.01 mol·L−1 NaClO4) is more than 2 orders of magnitude higher than that of Eu(III) on attapulgite (∼1.2 × 10−6 mol·g−1 at pH 2.0 in 0.01 mol·L−1 NaClO4).9 Surface Complexation Modeling. The adsorption curves of Eu(III) on GONS in Figure 2A were simulated by use of a diffuse double-layer model (DDLM) with the aid of FITEQL v4.0 code.29 From the distribution of Eu(III) species in aqueous solution (Figure S1, Supporting Information), the main species of Eu(III) are Eu3+ at pH < 5.5, Eu2(OH)24+ at pH 5.5−9.0, and Eu(OH)3(aq) at pH 9.0−11.0. Therefore, we optimized the adsorption data of Eu(III) on GONS by using Eu3+ and Eu2(OH)24+ species throughout a wide pH range to simplify their adsorption processes. The main adsorption reactions can be described by >SOH + Eu 3 + = >SOEu 2 + + H+

2>SOH + 2Eu 3 + + 2H 2O = ( >SO)2 Eu 2(OH)2 2 + + 4H+

(2)

In eqs 1 and 2, >SOH represents a structurally undefined, average functional group (assumed to be an amphoteric hydroxyl group) on the surface of GONS. The log K values of eqs 1 and 2 are obtained by best fitting of Eu(III) adsorption data on GONS (Table 1). On the basis of the estimated experimental uncertainties (0.1 < VWSOS/DF < 20.0), the calculated curves in Figure 2A show the relatively subtle differences with experimental data over a wide pH range, which indicates that the DDLM can fit the experimental adsorption data very well. Clearly, the two reactions are sufficient to depict Eu(III) adsorption on GONS in the pH range from 2.0 to 11.0. Fan et al.9 also modeled the adsorption of Eu(III) on attapulgite by using DDLM. It was found that log K value (−1.95) for >SOHEu2+ in 0.01 mol·L−1 NaClO4 was more than 6 orders of magnitude lower than the results in this study, whereas the similar intrinsic surface acidity constants for attapulgite and GONS were observed (i.e., log K2int = 4.79 for attapulgite and log K2int = 4.70 for GONS in 0.01 mol·L−1 NaClO4). The higher log K values substantiate the higher adsorption capacity of Eu(III) on GONS. As shown in Figure S5 (Supporting Information), one can see that the main >SOH2+ sites and >SO− sites are observed at pH < 3.0 and pH > 7.0, respectively. The SOH sites are approximately 60% at pH 4.0. The calculated pHPZC (∼3.9) of GONS also shows that the surface charge of GONS is positive at pH < pHPZC, and the surface of GONS presents massive negative surface charge at pH > pHPZC. Therefore, the positively charged Eu(III) ions [Eu2(OH)24+ species] can be easily adsorbed on the negatively charged GONS because of the strong electrostatic interaction at pH > pHPZC. However, approximately 65% of Eu(III) cations are adsorbed on GONS at pH ∼ 2.0, whereas the surface of GONS is positively charged at pH < pHPZC. This phenomenon is inconsistent with electrostatic interaction, which is presumably due to the restoration of a graphitic network of sp2 bonds.17 There are enough functional groups on GONS surfaces that can form surface complexes with Eu(III), which thereby results in high Eu(III) sorption at low pH values. According to the distribution of Eu(III) surface complexes (Figure 2A), the predominant adsorbed Eu(III) species on the GONS are the mononuclear monodentate >SOEu2+ complex at pH < 5.0 and the binuclear bidentate (>SO)2Eu2(OH)22+ complex at pH > 8.0. Combined with the results of EXAFS (discussed later), the interaction mechanism between Eu(III) and GONS is mainly dominated by inner-sphere surface complexation from mononuclear monodentate complexes at pH 6.3 to binuclear bidentate surface complexes at pH 9.0. Effect of Ionic Strength. The effect of ionic strength on the adsorption of Eu(III) on GONS in 0.01 and 0.1 mol·L−1 NaClO4 are shown in Figure 2B. The increasing ionic strength has no effect on the adsorption of Eu(III) on GONS throughout a wide pH range, which shows that inner-sphere surface complexation dominates Eu(III) adsorption on GONS. According to the calculation of two-site Langmuir model (Figure S6, Supporting Information), ∼ 95.5% of inner-sphere complexes are observed on GONS surface. Zhao et al.19,20 found that the adsorptions of Pb(II), Co(II), and Cd(II) on GONS were also independent of ionic strength, which were consistent with the results in this paper. The occurrence of

(1) 6023

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Figure 3. Adsorption isotherms of Eu(III) adsorption on GONS. (A) Adsorption isotherms at different pH. m/V = 0.2 g·L−1, I = 0.01 mol·L−1 NaClO4, and T = 298 K. (B) Adsorption isotherms at different temperatures. m/V = 0.2 g·L−1, pH = 4.5, and I = 0.01 mol·L−1 NaClO4. Solid lines represent Langmuir model.

plenty of surface oxygen functional groups on GONS surface are favorable to form strong complexes with Eu(III) ions on the surfaces of GONS.19 Adsorption Isotherms. The adsorption isotherms of Eu(III) on GONS at pH 4.5 and 6.0 and T = 298 K were conducted by batch technique. Activated carbon (AC) is also employed to compare the differences in adsorption performance. As shown in Figure 3A and Figure S7 (Supporting Information), one can see that the sorption behaviors of Eu(III) on GONS and AC can be fitted by Langmuir model very well. The maximum adsorption capacities (qmax) of Eu(III) on GONS calculated from Langmuir model are 161.29 mg·g−1 at pH 4.5 and 175.44 mg·g−1 at pH 6.0, which are at least 7 times higher than the value calculated for Eu(III) on AC (qmax ∼ 20 mg·g−1 at pH 4.5) in this study (Table S2, Supporting Information). As shown in Table S2, the Ka value of AC (0.3976 L·mg−1 at pH 4.5) is higher than that of GONS (0.1845 L·mg−1 at pH 4.5) (see Supporting Information), indicating that a higher proportion of inner-sphere complexes occurs at AC. It is worth pointing out that the amount of Eu(III) desorbed from Eu(III)-loaded GONS is ∼1.5 times higher than that from Eu(III)-loaded AC (Figure S8, Supporting Information), which is in good agreement with the Ka values calculated from Langmuir model. Comparing to qmax values of Eu(III) adsorption on other adsorbents (Table 2) such as TiO2,5,39 ZSM-5 zeolite,40 clay minerals,8 MWCNTs,41 and AC,11 one can see that GONS present the highest adsorption capacity of today’s materials. It is demonstrated that the delocalized π electron systems of graphene layer can be assumed as a Lewis base to form electron donor−acceptor complexes with metal ions,42,43 which can significantly enhance the surface complexation between Eu(III) ions and GONS.44 Thermodynamic Data. The effect of temperature on Eu(III) adsorption onto GONS at pH 4.5 is given in Figure 3B. Adsorption capacity is highest at T = 338 K and lowest at T = 298 K, which shows that Eu(III) adsorption on GONS is promoted at higher temperature. The standard free energy change (ΔG°) for Eu(III) adsorption on GONS can be calculated by eq S21 (Supporting Information). The standard enthalpy change (ΔH°) and the standard entropy change (ΔS°) are calculated from the linear plot of ln K° versus 1/T for Eu(III) adsorption on GONS. Detailed processes of calculation of the thermodynamic parameters (i.e., ln K°, ΔG°, ΔH°, and ΔS°) are described in Figures S9 and S10

Table 2. Comparison of Maximum Adsorption Capacity of Eu(III) Adsorption on Various Adsorbents exptl conditions adsorbents commercial titanium dioxide bare TiO2 ZSM-5 sodium montmorillonite synthetic saponite synthetic fluorotetrasilicic mica MWCNTs activated carbon activated carbon graphene oxide nanosheet graphene oxide nanosheet

pH

T (K)

asdsorption capacity (mg·g−1)

5.5

298

5.0

6

4.5 5.0 5.0

298 298 298

1.5 3.28 1.02

38 39 8

5.0 5.0

298 298

0.71 1.0

8 8

4.5 5.0 4.5

298 298 298

1.4 46.5 20.12

4.5

298

161.29

6.0

298

175.44

ref

40 11 this study this study this study

(Supporting Information). Negative ΔG° values (−24.9 kJ·mol−1 at 298 K, −27.2 kJ·mol−1 at 318 K, and −29.2 kJ·mol−1 at 338 K) indicate that the adsorption of Eu(III) on GONS is a spontaneous process, and the decrease of ΔG° with increasing temperature indicates that the adsorption of Eu(III) on GONS is more favorable at higher temperature. The positive ΔH° value (7.14 kJ·mol−1) suggests that Eu(III) adsorption on GONS is an endothermic process. It is determined that the dehydration of Eu(III) from aqueous Eu(III) complex ion is an endothermic process, but attachment of Eu(III) to the surface of GONS is an exothermic process. It is plausible to assume that the energy of dehydration exceeds the exothermicity of the Eu(III) ions attached to the surface of GONS.44 The value of ΔS° was calculated to be 107.6 J·mol−1·K−1. Zhao et al.20 calculated ΔH° and ΔS° for Cd(II) adsorption on GONS to be 7.39 kJ·mol−1 and 100.1 J·mol−1·K−1, respectively, which are consistent with values of Eu(III) adsorption on GONS. EXAFS Spectra Analysis of Reference Samples and Selected Adsorption Samples. The k2-weighted Eu EXAFS spectra and corresponding Fourier transform data of reference 6024

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silicate hydrates.22 Fourier transforms display a slight difference between aqueous Eu(III) ions and Eu(III) adsorbed on GONS at pH 6.3 and pH 9.0, but the oscillations at k ∼ 6.5 Å−1 for Eu(III) adsorbed on GONS at pH 6.3 and 9.0 are wider than those of aqueous Eu(III) ions. It is found that a Fourier transform obtained for Eu(III) adsorbed on GONS at pH 9.0 looks similar to tht of crystalline Eu(OH)3, but some difference is observed at R ∼ 4.2 Å. The fit to the EXAFS data is simultaneously done for reference samples and Eu(III) adsorbed on GONS at pH 6.3 and 9.0 (---, Figure 4A). The structural parameters [i.e., coordination number of neighbors, bond distance, and EXAFS Debye−Waller factor (σ2)] derived from EXAFS analyses are summarized in Table 3. The REu−O value of ∼2.45 Å for Eu(III)

samples and selected adsorption samples at pH 6.3 and 9.0 are shown in Figure 4. The position of the absorption line at

Table 3. EXAFS Results of Reference Samples and Eu(III) Adsorbed on GONS at Eu LIII-Edgea Figure 4. (A) k2-weighted Eu LIII-edge EXAFS spectra and (B) corresponding Fourier transforms of the reference samples and selected adsorption samples at pH 6.3 and 9.0. () EXAFS results; (---, panel A) fitting results; (---, panel B) related imaginary parts. m/V = 0.2 g·L−1, CEu(III) = 3.35 × 10−4 mol·L−1, I = 0.01 mol·L−1 NaClO4, and T = 298 K.

shell

Rb (Å)

Eu−O Eu−Eu

2.403(8) 3.647(9)

Eu−O Eu−Eu

2.327(4) 3.665(3)

Eu(OH)

Eu2O

∼6983 eV shows that Eu is trivalent in all samples in terms of X-ray absorption near-edge structure spectra (Figure S11, Supporting Information).23 Figure 4A shows the normalized, background-subtracted and k2-weighted EXAFS spectra of reference samples [i.e., aqueous Eu(III) ions, crystalline Eu(OH)3, and crystalline Eu2O3] and Eu(III) adsorbed on GONS at pH 6.3 and 9.0. For aqueous Eu(III) ions, a single-wave frequency of monotonically decreasing amplitude for k > 3 Å−1 is observed, which is consistent with the presence of a single ordered coordination shell.4 The EXAFS spectra for both crystalline Eu2O3 and crystalline Eu(OH)3 display evident frequencies compared to aqueous Eu(III) ions. The difference not only is attributed to multiple backscattering paths in the first coordination shells but also can be related to the presence of higher atomic shells. 22 Compared to reference samples, the broaden oscillation at k ∼ 6.5 Å−1 for Eu(III) adsorbed on GONS at pH 6.3 and 9.0 are due to the formation of inner-sphere surface complexes. Stumpf et al.23 found that the obtained emission decay of Eu(III) was relatively broad at 581.6 nm, based on the results of timeresolved laser fluorescence spectroscopy. According to the lifetime (193 ± 6 μs), ∼5 water molecules were observed in the first Eu(III) coordination shell, indicating the formation of actinide and lanthanide inner-sphere surface complexes. Figure 4B shows the radial structure functions (RSFs) and related imaginary parts (---) of reference samples and Eu(III) adsorbed on GONS at pH 6.3 and 9.0, which are uncorrected for phase shift. Fourier transforms all display a first peak near 2.0 Å, which is related to the contributions from oxygen atoms of the nearest coordination shell. High-amplitude contributions at R ∼3.6 Å for crystalline Eu2O3 and crystalline Eu(OH)3 originate mainly from the next-nearest Eu backscattering shells.22 Only a weak contribution at ∼3.1 Å for crystalline Eu(OH)3 is seen. Aqueous Eu(III) ions may originate either from multiple backscattering paths within the first coordination shell or from single backscattering paths from second and more distant hydration spheres. The results are in quite good agreement with the results of Eu(III) interaction with calcium

Eu−O Eu−O Eu−C Eu−O Eu−C Eu−Eu

Nc

σ2 d(Å 2)

9.09(0) 1.92(0)

0.0112(8) 0.0060(0)

6.48(6) 5.70(6)

0.0107(0) 0.0077(7)

3

3

Eu (aq) 2.424(1) 8.48(4) Graphene Oxide, pH 6.3 2.414(8) 6.69(0) 2.30(9) 4.00(9) Graphene Oxide, pH 9.0 2.407(5) 6.02(4) 2.31(0) 4.41(0) 3.531(2) 2.36(8)

0.0077(3) 0.0060(0) 0.0060(0) 0.0030(0) 0.0030(0) 0.0030(0)

a Conditions: T = 25 ± 1 °C, I = 0.01 mol·L−1 NaClO4. bR is the bond distance. cN is the coordination numbers of neighbors. dσ2 is the Debye−Waller factor.

adsorbed on GONS at pH 6.3 and 9.0 is an average bond distance composed of Eu−O contribution, the hydration shell Eu−OH2, and the excessive carboxylate groups Eu−O (GONS) contribution on Eu(III) adsorption on GONS.4,9,22 The coordination number of first coordination shell (Eu−O path) decreases from ∼6.69 to ∼6.02 with pH increasing from 6.3 to 9.0, which indicates that Eu(III) is coordinated with a hydration shell of ∼7 O in the first coordination shell on GONS at pH 6.3 and with ∼6 O in the first coordination shell at pH 9.0. Attempts to include the Eu−C sphere contribution to the EXAFS data for Eu(III) adsorbed on GONS at pH 6.3 suggest the formation of inner-sphere surface complexes. For Eu(III) adsorbed on GONS at pH 9.0, attempts to include the Eu−C and Eu−Eu sphere contributions to the EXAFS data are observed. However, the fitted parameters (e.g., bond distance and coordination numbers of neighbors) are significantly different from those of crystalline Eu(OH)3. Generally, the coprecipitated component would be observed at high pH conditions, but approximately 70% of Eu(III) was adsorbed on GONS at pH 4.0. Therefore, no coprecipitated component is formed under our experimental conditions. Results of EXAFS analysis suggest that the multinuclear surface complexes on the surface of GONS occurred at pH 9.0.22 The interaction mechanism between Eu(III) and GONS was determined from mononuclear monodentate complexes to binuclear bidentate 6025

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(7) Rabung, T.; Pierret, M. C.; Bauer, A.; Geckeis, H.; Bradbury, M. H.; Baeyens, B. Sorption of Eu(III)/Cm(III) on Ca-montmorillonite and Na-Illite. Part 1: Batch sorption and time-resolved laser fluorescence spectroscopy experiments. Geochim. Cosmochim. Acta 2005, 69, 5393−5402. (8) Okada, T.; Ehara, Y.; Ogawa, M. Adsorption of Eu3+ to smectites and fluorotetrasilicic mica. Clays Clay Miner. 2007, 55, 348−353. (9) Fan, Q. H.; Tan, X. L.; Li, J. X.; Wang, X. K.; Wu, W. S.; Montavon, G. Sorption of Eu(III) on attapulgite studied by batch, XPS and EXAFS techniques. Environ. Sci. Technol. 2009, 43, 5776−5782. (10) Galunin, E.; Alba, M. D.; Santos, M. J.; Abrao, T.; Vidal, M. Lanthanide sorption on smectitic clays in presence of cement leachates. Geochim. Cosmochim. Acta 2010, 74, 862−875. (11) Gad, H. M. H.; Awwad, N. S. Factors affecting on the sorption/ desorption of Eu(III) using activated carbon. Sep. Sci. Technol. 2007, 42, 3657−3680. (12) Accorsi, G.; Armaroli, N.; Parisini, A.; Meneghetti, M.; Marega, R.; Prato, M.; Bonifazi, D. Wet adsorption of a luminescent Eu-III complex on carbon nanotubes sidewalls. Adv. Funct. Mater. 2007, 17, 2975−2982. (13) Chen, C. L.; Hu, J.; Xu, D.; Tan, X. L.; Meng, Y. D.; Wang, X. K. Surface complexation modeling of Sr(II) and Eu(III) adsorption onto oxidized multiwall carbon nanotubes. J. Colloid Interface Sci. 2008, 323, 33−41. (14) Chen, C. L.; Wang, X. K.; Nagatsu, M. Europium adsorption on multiwall carbon nanotube/iron oxide magnetic composite in the presence of polyacrylic acid. Environ. Sci. Technol. 2009, 43, 2362− 2367. (15) Atalaya, J.; Isacsson, A.; Kinaret, J. M. Continuum elastic modeling of graphene resonators. Nano Lett. 2008, 8, 4196−4200. (16) Varshney, V.; Patnaik, S. S.; Roy, A. K.; Froudakis, G.; Farmer, B. L. Modeling of thermal transport in pillared-graphene architectures. ACS Nano 2010, 4, 1153−1161. (17) Xu, Z. W.; Chen, L.; Li, J. L.; Wang, R.; Qian, X. M.; Song, X. Y.; Liu, L. S.; Chen, G. W. Oxidation and disorder in few-layered graphene induced by the electron-beam irradiation. Appl. Phys. Lett. 2011, 98, 183112. (18) Yang, S. T.; Chang, Y. L.; Wang, H. F.; Liu, G. B.; Chen, S.; Wang, Y. W.; Liu, Y. F.; Cao, A. N. Folding/aggregation of graphene oxide and its application in Cu2+ removal. J. Colloid Interface Sci. 2010, 351, 122−127. (19) Zhao, G. X.; Ren, X. M.; Gao, X.; Tan, X. L.; Li, J. X.; Chen, C. L.; Huang, Y. Y.; Wang, X. K. Removal of Pb(II) ions from aqueous solutions on few-layered graphene oxide nanosheets. Dalton Trans. 2011, 40, 10945−10952. (20) Zhao, G. X.; Li, J. X.; Ren, X. M.; Chen, C. L.; Wang, X. K. Fewlayered graphene oxide nanosheets as superior sorbents for heavy metal ion pollution management. Environ. Sci. Technol. 2011, 45, 10454−10462. (21) Scheckel, K. G.; Sparks, D. L. Kinetics of the formation and dissolution of Ni precipitates in a gibbsite/amorphous silica mixture. J. Colloid Interface Sci. 2000, 229, 222−229. (22) Schlegel, M. L.; Pointeau, I.; Coreau, N.; Reiller, P. Mechanism of europium retention by calcium silicate hydrates: An EXAFS study. Environ. Sci. Technol. 2004, 38, 4423−4431. (23) Stumpf, T.; Curtis, H.; Walther, C.; Dardenne, K.; Ufer, K.; Fanghanel, T. Incorporation of Eu(III) into hydrotalcite: A TRLFS and EXAFS study. Environ. Sci. Technol. 2007, 41, 3186−3191. (24) Davis, J. A.; Kent, D. B. Surface complexation modeling in aqueous geochemistry. Rev. Miner. 1990, 23, 177−260. (25) Rabung, T.; Geckeis, H.; Kim, J. I.; Beck, H. P. Sorption of Eu(III) on a natural hematite: Application of a surface complexation model. J. Colloid Interface Sci. 1998, 208, 153−161. (26) Landry, C. J.; Koretsky, C. M.; Lund, T. J.; Schaller, M.; Das, S. Surface complexation modeling of Co(II) adsorption on mixtures of hydrous ferric oxide, quartz and kaolinite. Geochim. Cosmochim. Acta 2009, 73, 3723−3737.

surface complexes with increasing pH by using EXAFS spectroscopy and surface complexation modeling.

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ENVIRONMENTAL IMPLICATIONS The adsorption capacity of Eu(III) on GONS (175.44 mg·g−1 at pH 6.0 and T = 298 K) is the highest among materials used to adsorb Eu(III) from aqueous solutions, such as 5.0 mg·g−1 for titanium dioxide at pH 5.5 and T = 298 K,6 3.28 mg·g−1 for ZSM-5 at pH 5.0 and T = 298 K,39 and 1.4 mg·g−1 for MWCNTs at pH 4.5 and T = 298 K.40 From the results of Eu(III) interaction with GONS, one can see that GONS is a suitable material for the removal of trivalent lanthanides and actinides in environmental pollution cleanup. Although GONS is relatively expensive compared to natural materials, GONS will be synthesized at large scale in the near future with the development of technology. The high adsorption capacity of GONS for Eu(III) ions makes application in nuclear waste management promising.

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ASSOCIATED CONTENT

S Supporting Information *

Additional text, 11 figures, and two tables describing the preparation of GONS, collection and analysis of EXAFS data, analysis of XPS spectra, distribution of Eu(III) species, adsorption isotherms, and analysis of XANES. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-551-5592788; fax: +86-551-5591310; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the assistance of Dr. Qiang Zhang (Analysis & Testing Center of Hefei University of Technology, China) for SEM analysis. Financial support from 973 projects from Ministry of Science and Technology of China (2011CB933700) and National Natural Science Foundation of China (91126020; 21071107; 20971126; 21071147) are acknowledged.

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REFERENCES

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