GO Composites as Highly Effective

May 10, 2017 - These findings indicated that GO-based composites can be used as a highly effective adsorbents for the preconcentration and immobilizat...
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Research Article pubs.acs.org/journal/ascecg

One-Pot Synthesis of LDH/GO Composites as Highly Effective Adsorbents for Decontamination of U(VI) Wensheng Linghu,*,† Hai Yang,† Yanxia Sun,† Guodong Sheng,† and Yuying Huang‡ †

College of Chemistry and Chemical Engineering, Shaoxing University, Zhejiang 312000, P. R. China Shanghai Synchrotron Radiation Facility (SSRF), Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201204, P. R. China

‡

ABSTRACT: The removal mechanism of U(VI) on Mg−Al-layered double hydroxide-supported graphene oxide (LDH/GO) composites was investigated by batch, spectroscopic, and surface complexation modeling. The batch experiments showed that the enhanced removal of U(VI) on LDH and LDH/GO composites in the presence of carbonate was observed at pH < 5.0, whereas the presence of carbonate significantly inhibited U(VI) removal at pH starting from 7.0 to 9.0. It is demonstrated that the oxygenated functional groups (i.e., −OH) were responsible for the high effective removal of U(VI) by XPS analysis. The results of XANES and EXAFS spectra indicated that adsorption of U(VI) on LDH/GO composites was inner-sphere surface complexation. According to surface complexation modeling, the removal of U(VI) on LDH/GO composites can be satisfactorily fitted by a diffuse layer model with an ion exchange (X2UO2) and two inner-sphere surface complexes (SOUO2+ and SOUO2(CO3)23− species). The maximum adsorption capacities of LDH and LDH/GO composites calculated from the Langmuir model at pH 4.5 and T = 293 K were 99.01 and 129.87 mg/g, respectively. These findings indicated that GObased composites can be used as a highly effective adsorbents for the preconcentration and immobilization of radionuclides in environmental cleanup. KEYWORDS: Uranium, LDH, Graphene oxide, Modeling, Spectroscopic techniques

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INTRODUCTION With the rapid development of nuclear-relative industries, abundant radioactive contaminants were discharged into subenvironments.1−3 Radionuclide contamination is still problematic issues at nuclear facilities.4 Uranium is the typical and common radioactive contaminant could lead to serious threats to ecological environments and human health.5−8 Therefore, the removal of uranium from aqueous solutions has been extensively investigated by various adsorbents such as Fe/Al-(hydr)oxide9−15 carbon-based materials7,16−19 and clay minerals.1,20−24 The adsorption approach is becoming an increasingly common method to remove radionuclides.25−30 In previous studies, the effect of environmental factors on the adsorption of radionuclides at the water−solid interface had been elucidated by batch techniques.31−34 Owing to the abundant oxygenated functional groups (hydroxyl, epoxy, and carboxyl groups), graphene oxide (GO) as a highly efficient adsorbent has been extensively investigated to remove radionuclides in recent years.6,35−40 However, GO was not easily separated from the liquid phase due to its excellent dispersity in aqueous solutions. Layered double hydroxide (LDH, [MII1−xMIIIx(OH)2]x+(An−)x/n·mH2O) is a typical brucite-like layered hydroxide with divalent (MII) and trivalent (MIII) metal ions in the octahedron and n-valent anion (An−).41−44 LDH can be regarded an excellent adsorbent © 2017 American Chemical Society

in environmental cleanup due to its exchangeable capacity of anions. It is determined that LDH/GO composites can be used as an excellent adsorbent, which can be separated easily from the liquid phase.45 The contribution of GO in LDH/GO composites was to significantly enhance their adsorption performance. A thorough understanding of the chemical interactions of U(VI) with LDH-based composites is essential in building a robust safety case for the long-term geological disposal of nuclear waste and for the management of radionuclide-bearing sites. Although the removal of U(VI) on LDH/GO composites was investigated, the effect of carbonate on the speciation and fate of U(VI) remains poorly understood.46 The aims of this manuscript were (1) to synthesize the LDH/GO composites and characterize them by TEM, FT-IR, and XRD techniques, (2) to investigate the effect of environmental factors (e.g., reaction time, pH, carbonate concentration, and temperature) on U(VI) removal onto LDH and LDH/GO composites by batch techniques, and (3) to determine the interaction mechanism of U(VI) and LDH and LDH/GO composites at a molecular level by XPS, XANES, EXAFS, and surface complexation modeling. This Received: April 26, 2017 Published: May 10, 2017 5608

DOI: 10.1021/acssuschemeng.7b01303 ACS Sustainable Chem. Eng. 2017, 5, 5608−5616

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Figure 1. Characterization of LDH/GO composites. (A, B) TEM images of LDH and LDH/GO composite, respectively. (C) FT-IR spectra. (D) XRD pattern. pressure. Surface charging effects were corrected with the C 1s peak at 284.6 eV as a reference. Batch Adsorption. The triple adsorption experiments of U(VI) on LDH and LDH/GO composites (m/v = 1.5 g/L) were conducted in polycarbonate centrifuge tubes at pH 4.5 and in the presence of 0.01 mol/L NaClO4 solutions (as the inert anion). Briefly, the suspension of LDH/GO composites were pre-equilibrated with NaClO4 solutions overnight, and then, the U(VI) solution was provided to reach the desired concentration. The pH value of suspensions was adjusted by adding negligible volumes of 0.01−1.0 mol/L HClO4 or NaOH solutions. After equilibrium, the solid phase was separated from the liquid phase by centrifugation at 5000 rpm for 30 min and then filtered using 0.22 μm nylon membrane filters. The recycling experiments were conducted in six successive cycles by U(VI) adsorption− desorption experiments. Briefly, after U(VI) adsorption, a U(VI)containing wet solid phase was orderly desorbed by adding 0.01 mol/L HNO3, 0.2 mol/LNa2CO3, and 0.01 mol/L EDTA solutions.23 The concentration of U(VI) in supernatant was measured by inductively coupled plasma-mass spectrometry (ICP-MS, Agilent 7500CX, USA). The adsorbed amount of U(VI) on the LDH/GO composites can be calculated by the difference of the initial and equilibrated concentration. Surface Complexation Modeling. The modeling approach allowed for a clear determination of the reactive surface sites of LDH/GO composites for U(VI) adsorption. A diffuse layer model of surface complexation modeling was done by a visual MINETQ program.48 Equilibrium constants for surface complexes were obtained by simulating adsorption edges for three surface complexation reactions. The speciation was used by mass balance constraints to optimize the equilibrium constants. The equilibrium constants were obtained from the apparent stability constants by converting the concentration of each surface species to its mole fraction. Preparation and Analysis of XANES and EXAFS Spectra. The samples for XANES and EXAFS analysis were prepared in a glovebox (purged with 95% N2 and 5% H2). Briefly, the suspension of LDH/ GO composites was pre-equilibrated with NaClO4 (0.01 mol/L) 24 h, and then, UO22+ solutions were slowly added into the aforementioned suspension under vigorous stirring conditions. The values of pH in

manuscript highlighted the application of GO-based composites for removal of radionuclides in environmental cleanup.

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MATERIALS AND METHODS

Synthesis of LDH/GO Composites. LDH/GO composites were synthesized by a hydrothermal process.43 First, GO was obtained by the oxidation of flake graphite ( 8.0. For LDH/GO composites, the significant increase in U(VI) adsorption was observed at pH 2.0−5.0 and then kept the adsorption of U(VI) at pH 5.0−8.0, whereas a slight decrease in U(VI) adsorption was found at pH > 8.5. At pH 6.0, approximately 91% and 99.8% of U(VI) was removed by LDH and LDH/GO composites, respectively. Compared to LDH, the pH-edge adsorption of U(VI) on LDH/GO composites was shifted to the lower pH values. These results indicated that GO enhanced the adsorption of U(VI). Figure 2B shows the effect of carbonate concentration on adsorption of U(VI) onto LDH and LDH/GO composites. At pH < 5.0, the adsorption of U(VI) on LDH and LDH/GO composites in the presence of carbonate was significantly higher than that of U(VI) on LDH and LDH/GO composites in the absence of carbonate. However, carbonate inhibited the adsorption of U(VI) on LDH and LDH/GO composites at pH > 7.0. It is demonstrated that negative carbonato−uranyl complexes (i.e., UO2(CO3)22− and UO2(CO3)34− species) were obtained in the presence of carbonate conditions.14,20 As shown in Table 1, the pHPZC values (pH at point of zero charge) of LDH and LDH/GO composites were calculated to be 7.1 and 6.5, respectively. Therefore, the increased adsorption of U(VI) on LDH and LDH/GO composites in the presence of carbonate could be attributed to the electrostatic attraction between the positive charge of LDH and LDH/GO composites and negative charge of carbonato−uranyl complexes at pH < 5.0, whereas the inhibited adsorption of U(VI) on LDH and LDH/GO composites at pH > 7.0 could be ascribed the electrostatic repulsion between the negative charge of LDH and LDH/GO composites and negative charge of carbonato− uranyl complexes. The high-level adsorption of U(VI) on LDH and LDH/GO composites at pH 6.0−7.0 could be surface coprecipitation and/or a surface complexation reaction of U(VI) with LDH and LDH/GO composites. Adsorption Kinetics and Isotherms. Figure 3A and B shows the adsorption kinetics and isotherms of U(VI) on LDH and LDH/GO composites, respectively. As shown in Figure 3A, the remarkable increase in U(VI) adsorption on LDH and LDH/GO composites was observed within a reaction time of 6 h and then kept the high-level adsorption at a reaction time more than 6 h. Approximately 100% and 86% of U(VI) were removed by LDH/GO composites and LDH, respectively. The kinetic data were fitted by pseudo-first-order and pseudosecond-order kinetic models. The linear forms of pseudo-firstorder and pseudo-second-order kinetic models can be described by eqs 1 and 2, respectively

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RESULTS AND DISCUSSION Characterization. The morphology of LDH/GO composites was characterized by TEM images. The hexagonal LDH platelets with 20 nm thicknesses revealed good crystallinity due to the slow nucleation process (Figure 1A). As shown in Figure 1B, the LDH nanoplates were uniformly deposited on the surface of GO nanosheets, which was consistent with the previous studies.43,51 Figure 1C shows the FT-IR spectrum of LDH/GO composites. The significant peaks at 3450 and 1365 cm−1 could be ascribed to the stretching vibration of O−H groups associated with the interlayer water molecules and CO32− ions in the interlayer of Mg/Al LDH, respectively.43 However, the bands at 1730, 1620, and 1220 cm−1 were attributed to the stretching vibration of CO, CC, and C− O groups of GO, respectively.17,26,52 The abundant bands at 500−800 cm−1 could be due to the stretching/bending vibrations of metal−oxygen lattices such as Mg/Al−O, O− Mg/Al−O, and Mg/Al−O−Mg/Al groups.46 The result of the FT-IR spectrum showed that the LDH nanoplate was successfully synthesized on the GO surface; moreover, the oxygenated functional groups of GO provided much more reactive sites for U(VI) removal. As shown by the XRD pattern in Figure 1D, the diffraction peaks at 2θ = 11.5°, 23.6°, 35.1°, 39.5°, 47.2°, 60.8°, and 62.2° can be indexed into the (003), (006), (012), (015), (018), (110), and (113) planes of Mg/Al LDH (JCPDS No. 15-0087), respectively.53 No diffraction peaks of GO (e.g., (002) plane of GO at 9.12) and other impurities were observed. In addition, sharp and symmetric peaks revealed high crystallization. As shown in Table 1, the Table 1. Selective Properties of LDH/GO Composites pHPZC SBET (m2/g) average pore size (nm)

LDH

LDH/GO

0 mV at pH 7.1 45.64 3.8

0 mV at pH 6.5 28.73 3.2

N2−BET specific surface area and average pore size of asprepared LDH/GO composites were calculated to be 28.73 m2/g and 3.2 nm, respectively, which were lower than those of LDH (45.64 m2/g and 3.8 nm for specific surface area and average pore size, respectively). The characteristic results indicated the occurrence of abundant oxygen-containing functional groups of LDH/GO composites. Effect of pH and Carbonate Concentration. Figure 2A shows the adsorption of U(VI) on LDH and LDH/GO composites at different pH conditions. One can see that the slight increase in U(VI) adsorption on LDH was observed at pH < 4.0, whereas the adsorption of U(VI) on LDH was significantly increased at pH 4.0−6.5 and then kept high-level

ln(qe − qt) = ln qe − k f × t

(1)

t /qt = 1/(ks × qe 2) + t /qe

(2)

where qe and qt (mg/g) are the amount of U(VI) adsorbed at equilibrium and at time t, respectively. Here, kf and ks refer to the rate constants of pseudo-first-order and pseudo-secondorder kinetic models, respectively. The optimized parameters of pseudo-first-order and pseudo-second-order kinetic models are shown in Table 2. As shown in Table 2, the removal kinetics of of U(VI) on LDH and LDH/GO composites can be satisfactorily simulated by the pseudo-second-order kinetic model with high correlation coefficients (R2 = 1) compared to the pseudo-first-order kinetic model (R2 < 0.6), which were consistent with previous studies.54,55 5610

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Figure 2. Effect of pH (A) and carbonate concentration (B) on U(VI) removal on LDH and LDH/GO composites: C0 = 10 mg/L, m/v = 1.5 g/L, I = 0.01 mol/L NaClO4, T = 293 K.

Figure 3. Adsorption kinetics (A) and isotherms (B) of U(VI) removal by LDH and LDH/GO and LDH/GO2 composites: pH 4.5, m/v = 1.5 g/L, I = 0.01 mol/L NaClO4, T = 293 K.

log Q e = log KF + 1/n log Ce

Table 2. Parameters of Pseudo-First-Order and PseudoSecond-Order Kinetic Models for Adsorption of U(VI) on LDH and LDH/GO Composites pseudo-first-order model composites

qe (mg/g)

LDH LDH/GO

3.53 5.12

0.1164 0.0232

where Ce (mg/L) and Qm (mg/g) are the equilibrated concentration and maximum removal capacity of LDH or LDH/GO composites at complete monolayer coverage. Here, KL (L/mg) and KF are Langmuir and Freundlich constants, respectively, and 1/n is the heterogeneity of the adsorption sites. All parameters of Langmuir and Freundlich models are shown in Table 3. As shown in Table 3, the adsorption of U(VI) on LDH and LDH/GO composites can be satisfactorily fitted by the Langmuir model. The maximum adsorption capacities of LDH and LDH/GO composites calculated from the Langmuir model at pH 4.5 and 293 K were 99.01 and 129.87 mg/g, respectively, which indicated that the introduction of GO in LDH/GO composites can significantly increase the maximum adsorption capacity of LDH. As compared in Table 4, the maximum adsorption capacity of LDH/GO composites was significantly higher than that of metal (hydr)oxides (e.g., 11.6, 5.59, and 50.62 mg/g for nanoporous alumina,9 hematite,56 and ferrihydrite,57 respectively) and carbon-based nanoparticles (e.g., 26.18 and 97.5 mg/g for

pseudo-second-order model

kf (h−1)

R2 0.5836 0.4077

qe (mg/g) 17.67 20.04

ks (g/(mg × h)) −5

3.11 × 10 1.12 × 10−5

R2 1 1

Figure 3B shows removal isotherms of U(VI) on LDH and LDH/GO composites. It is observed that removal of U(VI) on LDH was significantly lower than that of LDH/GO composites. It should be noted that the U(VI) on LDH/GO composites was lower than that LDH/GO2 composites, indicating that the U(VI) removal increased with increasing GO concentration. The data of removal isotherms were simulated by Langmuir and Freundlich models. The equations of Langmuir and Freundlich models can be described by eqs 3 and 4, respectively Ce/Q e = 1/(KL × Q m) + Ce/Q m

(4)

(3)

Table 3. Parameters of Langmuir and Freundlich Models for Adsorption of U(VI) on LDH and LDH/GO composites Langmuir model

Freundlich model 2

composites

KL (L/mg)

Qm (mg/g)

R

LDH LDH/GO

0.0628 0.1851

99.01 129.87

0.9978 0.9988 5611

log KF (mg/g)/(mg/g) 1.9109 3.1775

n

1/n

R2

0.7587 0.5675

0.9968 0.9829

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composites was attributed to the abundant oxygen-containing functional groups of GO. Surface Complexation Modeling. Figure 6A and B shows the surface complexation modeling of U(VI) on LDH and LDH/GO composites using a double-layer model, respectively. The optimized parameters of surface complexation modeling are summarized in Table 5. It is observed that the adsorption of U(VI) on LDH and LDH/GO composites can be satisfactorily fitted by the double-layer model with cation exchange sites (X2UO2) and two inner-sphere complexation sites (SOUO2+ and SOUO2(CO3)23− species) at pH < 7.0, whereas the fitted results under-evaluated the experimental data at pH > 8.0. As shown in Figure 6A and B, the X2UO2 species was observed at pH < 4.0, whereas SOUO2+ and SOUO2(CO3)23− species dominated the adsorption of U(VI) on LDH at pH 4.0−6.0 and pH > 6.0, respectively. However, the main species of U(VI) adsorption on LDH/GO composites were SOUO2+ and SOUO2(CO3)23− species at pH < 4.0 and pH > 5.0, respectively. The results of surface complexation modeling indicated that the adsorption of U(VI) on LDH and LDH/GO composites at pH < 4.0 was cation exchange. However, the adsorption of U(VI) on LDH and LDH/GO composites at pH > 5.0 was inner-sphere surface complexation. XANES and EXAFS Analysis. Figure 7A and B shows uranium L3-edge XANES and EXAFS spectra of standards (U(IV)O2(s), U(VI)O22+) and uranium-containing LDH/GO at different pH conditions, respectively. As shown in Figure 7A, the positions of the adsorption edge for U(IV)O2(s) and U(VI)O22+ were ∼17174 and 17176 eV, respectively. The position and type of uranium-containing LDH/GO were consistent with the U(VI)O22+ standard, indicating that uranium redox states in LDH/GO composites were U(VI) species. The results of XANES spectra revealed that the removal of U(VI) on LDH/GO composites was mainly adsorption without reduction. The uranium L3-edge EXAFS spectra for uraniumcontaining LDH/GO composites at different pH are shown in Figure 7B. Table 6 summarizes the optimized parameters (e.g., subshell, coordination number, Debye−Waller factor) of fitting results. An attempt to distinguish a U-Oeq shell into two subshelsl (e.g., U-Oeq1 and U-Oeq2) resulted in the convergence of two subshells at a similar bond distance; therefore, only the U-Oeq shells were obtained in this study. As shown in Figure 7B, the FT features of uranium-containing samples and reference standard at ca. 1.45 and 1.95 Å can be satisfactorily fitted by two axial oxygens (U-Oax) at ca. 1.8 Å

Table 4. Comparison of Adsorption Performance of Various Adsorbents for U(VI) adsorbents nanoporous alumina hematite ferrihydrite carbon nanotubes GO LDH LDH/GO composite

exp. conditions pH pH pH pH pH pH pH

4.5, 5.5, 5.5, 5.0, 5.0, 4.5, 4.5,

T T T T T T T

= = = = = = =

298 298 298 298 293 293 293

K K K K K K K

Qm (mg/g)

ref

11.6 5.59 50.62 26.18 97.5 99.01 129.87

9 56 57 58 59 this study this study

multiwalled carbon nanotubes58 and GO,59 respectively). Therefore, LDH/GO composites can be regarded as promising adsorbents for the removal and immobilization of U(VI) from aqueous solutions at low pH conditions. Figure 4A and B shows the recycling experiments of U(VI) removal on LDH and LDH/GO composites, respectively. After six recycling experiments, the decreased extent of maximum adsorption capacity for LDH/GO composites (from 129.87 to 108.45 mg/g) was slightly lower than that of GO composites (from 99.01 to 72.69 mg/g), indicating that LDH/GO composites can be long-term use in the wastewater treatment. The results of recycling experiments revealed that LDH/GO composites can be used as an environmentally renewable and low-cost adsorbent due to the presence of the excellent capacity of regeneration and efficient performance. XPS Analysis. Figure 5A and B shows the total scan and high resolution U 4f XPS analysis of LDH/GO composites before and after U(VI) removal, respectively. As shown in the total survey spectra in Figure 5A, there are four predominant elements of LDH/GO composites (i.e., C 1s, O 1s, Al 2p, and Mg 2p) before U(VI) adsorption, whereas the weak U 4f peaks and the decreased relative intensity of O 1s were observed after U(VI) adsorption. These evidences indicated that U(VI) was attached on the surface of LDH/GO composites. After U(VI) desorption, the relative intensity of O 1s was significantly higher than that of O 1s after U(VI) adsorption, indicating that oxygen-containing functional groups were responsible for the highly effective adsorption of U(VI) on LDH/GO composites. As shown in Figure 5B, the U 4f peaks can be deconvoluted into two subpeaks at ∼381 and 392 eV, which corresponded to U 4f7/2 and U 4f5/2, respectively.14 The results of XPS analysis indicated that the adsorption of U(VI) on LDH/GO

Figure 4. Recycling of LDH (A) and LDH/GO composites (B) for U(VI) removal: pH 4.5, m/v = 1.5 g/L, I = 0.01 mol/L NaClO4, T = 293 K. 5612

DOI: 10.1021/acssuschemeng.7b01303 ACS Sustainable Chem. Eng. 2017, 5, 5608−5616

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Figure 5. XPS analysis of LDH/GO after U(VI) adsorption (LDH/GO-U-A) and desorption (LDH/GO-U-D): (A) total scans and (B) high resolution of U 4f: C0 = 10 mg/L, pH 4.5, m/v = 1.5 g/L, I = 0.01 mol/L NaClO4, T = 293 K.

Figure 6. Surface complexation modeling of U(VI) on LDH (A) and LDH/GO (B) under different pH conditions: C0 = 10 mg/L, m/v = 1.5 g/L, I = 0.01 mol/L NaClO4, T = 293 K.

and about five equatorial oxygens (U-Oeq) at ∼2.35 Å (Table 6), respectively.60 The EXAFS spectra of uranium-containing LDH/GO composites at pH 3.0 and 6.0 were significantly different from EXAFS spectra of UO22+ species. For LDH/GO composites at pH 3.0 and 6.0, the third FT feature can be simulated by ∼1.3 alumina/magnesium (U−Al/Mg shell) at ∼3.18 Å (Table 6).20 Moreover, the coordination number of

Table 5. Optimized Parameters of U(VI) Adsorption on LDH/GO Composites by Surface Complexation Modeling reactions

log K

2> XNa + UO22+ = (>X)2UO2 + 2Na+ >SOH + UO22+ = >SOUO2+ + H+ >SOH + 2H2CO3 + UO22+ = >SOUO2 (CO3)23− + 5H+

3.2 −2.7 −18.9

Figure 7. Uranium L3-edge XANES spectra (A) and Fourier transform (FT) of EXAFS spectra (B) for U(IV)O2(s), U(VI)O22+, and U(VI)-containing LDH/GO composites: pH 3.0 and 6.0, m/v = 1.5 g/L, T = 293 K, I = 0.01 mol/L NaClO4. 5613

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in the enhanced sequestration of UO22+ byzerovalent iron nanoparticles: An advanced approach utilizing XPS and EXAFS. Appl. Catal., B 2016, 193, 189−197. (3) Yang, S. T.; Zhao, D. L.; Sheng, G. D.; Guo, Z. Q.; Sun, Y. B. Investigation of solution chemistry effects on sorption behavior of radionuclide Cu-64(II) on Illite. J. Radioanal. Nucl. Chem. 2011, 289, 467−477. (4) Wang, X. X.; Fan, Q. H.; Yu, S. J.; Chen, Z. S.; Ai, Y. J.; Sun, Y. B.; Hobiny, A.; Alsaedi, A.; Wang, X. K. High sorption of U(VI) on graphene oxides studied by batch experimental and theoretical calculations. Chem. Eng. J. 2016, 287, 448−455. (5) Wang, Q.; Chen, L.; Sun, Y. B. Removal of radiocobalt from aqueous solution by oxidized MWCNT. J. Radioanal. Nucl. Chem. 2012, 291, 787−795. (6) Sun, Y. B.; Yang, S. T.; Sheng, G. D.; Guo, Z. Q.; Wang, X. K. The removal of U(VI) from aqueous solution by oxidized multiwalled carbon nanotubes. J. Environ. Radioact. 2012, 105, 40−47. (7) Sun, Y. B.; Wu, Z.-Y.; Wang, X. X.; Ding, C. C.; Cheng, W. C.; Yu, S.-H.; Wang, X. K. Macroscopic and microscopic investigation of U(VI) and Eu(III) adsorption on carbonaceous nanofibers. Environ. Sci. Technol. 2016, 50, 4459−4467. (8) Sheng, G.; Hu, J.; Alsaedi, A.; Shammakh, W.; Monaquel, S.; Ye, F.; Li, H.; Huang, Y.; Alshomrani, A. S.; Hayat, T.; Ahmad, B. Interaction of uranium (VI) with titanate nanotubes by macroscopic and spectroscopic investigation. J. Mol. Liq. 2015, 212, 563−568. (9) Sun, Y. B.; Yang, S. T.; Sheng, G. D.; Guo, Z. Q.; Tan, X. L.; Xu, J. Z.; Wang, X. K. Comparison of U(VI) removal from contaminated groundwater by nanoporous alumina and non-nanoporous alumina. Sep. Purif. Technol. 2011, 83, 196−203. (10) Sun, Y. B.; Yang, S. B.; Wang, Q.; Alsaedi, A.; Wang, X. K. Sequestration of uranium on fabricated aluminum co-precipitated with goethite (Al-FeOOH). Radiochim. Acta 2014, 102, 797−804. (11) Sun, Y. B.; Wang, Q.; Yang, S. T.; Sheng, G. D.; Guo, Z. Q. Characterization of nano-iron oxyhydroxides and their application in UO22+ removal from aqueous solutions. J. Radioanal. Nucl. Chem. 2011, 290, 643−648. (12) Sun, Y. B.; Chen, C. L.; Tan, X. L.; Shao, D. D.; Li, J. X.; Zhao, G. X.; Yang, S. B.; Wang, Q.; Wang, X. K. Enhanced adsorption of Eu(III) on mesoporous Al2O3/expanded graphite composites investigated by macroscopic and microscopic techniques. Dalton Trans. 2012, 41, 13388−13394. (13) Ding, C. C.; Cheng, W. C.; Sun, Y. B.; Wang, X. K. Novel fungus-Fe3O4 bio-nanocomposites as high performance adsorbents for the removal of radionuclides. J. Hazard. Mater. 2015, 295, 127− 137. (14) Ding, C. C.; Cheng, W. C.; Sun, Y. B.; Wang, X. K. Effects of Bacillus subtilis on the reduction of U(VI) by nano-Fe0. Geochim. Cosmochim. Acta 2015, 165, 86−107. (15) Ding, C. C.; Cheng, W. C.; Jin, Z. X.; Sun, Y. B. Plasma synthesis of beta-cyclodextrin/Al(OH)3 composites as adsorbents for removal of UO22+ from aqueous solutions. J. Mol. Liq. 2015, 207, 224− 230. (16) Sun, Y. B.; Yang, S. B.; Ding, C. C.; Jin, Z. X.; Cheng, W. C. Tuning the chemistry of graphene oxides by a sonochemical approach: application of adsorption properties. RSC Adv. 2015, 5, 24886−24892. (17) Sun, Y. B.; Yang, S. B.; Chen, Y.; Ding, C. C.; Cheng, W. C.; Wang, X. K. Adsorption and desorption of U(VI) on functionalized graphene oxides: A combined experimental and theoretical study. Environ. Sci. Technol. 2015, 49, 4255−4262. (18) Ding, C. C.; Cheng, W. C.; Sun, Y. B.; Wang, X. K. Determination of chemical affinity of graphene oxide nanosheets with radionuclides investigated by macroscopic, spectroscopic and modeling techniques. Dalton Trans. 2014, 43, 3888−3896. (19) Cheng, W. C.; Wang, M. L.; Yang, Z. G.; Sun, Y. B.; Ding, C. C. The efficient enrichment of U(VI) by graphene oxide-supported chitosan. RSC Adv. 2014, 4, 61919−61926. (20) Sun, Y. B.; Zhang, R.; Ding, C. C.; Wang, X. X.; Cheng, W. C.; Chen, C. L.; Wang, X. K. Adsorption of U(VI) on sericite in the

Table 6. Optimized Parameters of Uranium L3-edge EXAFS Spectra for Reference and Uranium-Containing Samples shell

R (Å)a

CNb

σ2 (Å2)c

U-Oax U-Oeq

1.78 2.35

2.0 4.9

0.0033 0.0054

LDH/GOpH3

U-Oax U-Oeq U-Mg/Al

1.78 2.33 3.18

2.0 4.8 1.22

0.0026 0.0039 0.0087

LDH/GOpH6

U-Oax U-Oeq U-Mg/Al

1.78 2.31 3.17

2.0 4.6 1.20

0.0041 0.0038 0.0072

samples U

a

(VI)

O22+

Bond distance. bCoordination number. cDebye−Waller factor.

the U−Mg shell decreased with increasing pH from 3.0 to 6.0 (CN = 1.22 and 1.20 for pH 3.0 and 6.0, respectively). The results of EXAFS spectra indicated that inner-sphere surface complexation dominated the removal of U(VI) on LDH/GO composites, which was consistent with the batch adsorption experiments.

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CONCLUSIONS The LDH/GO composites were successfully synthesized by a hydrothermal method. The adsorption kinetics and isotherms of U(VI) on LDH and LDH/GO composites can be satisfactorily fitted pseudo-second-order kinetic and Langmuir model, respectively. The adsorption of U(VI) on LDH and LDH/GO composites at pH < 5.0 significantly increased with increasing carbonate concentration, whereas the carbonate accumulated the desorption of U(VI) from LDH and LDH/ GO composites at pH 7.0−9.0. According to XPS analysis, oxygen-containing functional groups were responsible for the adsorption of U(VI) on LDH/GO composites. The results of surface complexation modeling indicated that the adsorption of U(VI) on LDH and LDH/GO composites at pH < 4.0 and pH > 5.0 was cation exchange and inner-sphere surface complexation, respectively. These observations indicated that LDH/GO composites can be a promising adsorbent for the highly efficient adsorption of radionuclides from aqueous solutions in environmental cleanup.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Shaoxing University of Research Startup Project (20155029), Natural Science Foundation of Zhejiang Province (LY15B070001) and National Natural Science Foundation of China (21577093, 21207092).

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