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One-pot synthesis of LDH/GO composites as high effective adsorbent for the decontamination of U(VI) Wensheng Linghu, Hai Yang, Yanxia Sun, Guodong Sheng, and Yuying Huang ACS Sustainable Chem. Eng., Just Accepted Manuscript • Publication Date (Web): 10 May 2017 Downloaded from http://pubs.acs.org on May 17, 2017

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ACS Sustainable Chemistry & Engineering

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One-pot synthesis of LDH/GO composites as high effective adsorbent

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for the decontamination of U(VI)

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Wensheng Linghu†*, Hai Yang†, Yanxia Sun†, Guodong Sheng†, Yuying Huang‡

4

†

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312000, P.R. China

6

‡

7

Physics, Chinese Academy of Sciences, Shanghai 201204, P.R. China

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*Corresponding authors: Email: [email protected] (Wensheng Linghu, No. 508

9

Huancheng Rd. Shaoxing)

College of Chemistry and Chemical Engineering, Shaoxing University, Zhejiang

Shanghai Synchrotron Radiation Facility (SSRF), Shanghai Institute of Applied

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ABSTRACT: The removal mechanism of U(VI) on Mg-Al-layered double

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hydroxides–supported graphene oxide (LDH/GO) composites was investigated by

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batch, spectroscopic and surface complexation modeling. The batch experiments

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showed that the enhanced removal of U(VI) on LDH and LDH/GO composites in the

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presence of carbonate was observed at pH < 5.0, whereas the presence of carbonate

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significantly inhibited U(VI) removal at pH starting from 7.0 to 9.0. It is demonstrated

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that the oxygenated functional groups (i.e., -OH) were responsible for the high

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effective removal of U(VI) by XPS analysis. The results of XANES and EXAFS

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spectra indicated that adsorption of U(VI) on LDH/GO composites was inner-sphere

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surface complexation. According to surface complexation modeling, the removal of

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U(VI) on LDH/GO composites can be satisfactorily fitted by diffuse layer model with

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an ion exchange (X2UO2), two inner-sphere surface complexes (SOUO2+ and

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SOUO2(CO3)23- species). The maximum adsorption capacities of LDH and LDH/GO

ACS Paragon Plus Environment


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composite calculated from Langmuir model at pH 4.5 and T = 293 K were 99.01 and

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129.87 mg/g, respectively. These findings indicated that GO-based composites can be

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used as a high effective adsorbent for the preconcentration and immobilization of

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radionuclides in environmental cleanup.

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KEYWORDS: Uranium, LDH, Graphene oxide, Modeling, Spectroscopic techniques

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INTRODUCTION

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With the rapid development of nuclear-relative industries, abundant radioactive

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contaminants were discharged into sub-environments.1-3 Radionuclide contamination

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is still problematic issues at nuclear facilities.4 Uranium is the typical and common

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radioactive contaminant could lead to serious threats to ecological environments and

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human health.5-8 Therefore, the removal of uranium from aqueous solutions has been

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extensively investigated by various adsorbents such as Fe/Al-(hydr)oxide9-15

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carbon-based materials7,

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becoming increasingly common method to remove radionuclides.25-30 In hitherto

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studies, the effect of environmental factors on the adsorption of radionuclides at the

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water-solid interface had been elucidated by batch techniques.31-34

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Owing to the abundant oxygenated functional groups (hydroxyl, epoxy and carboxyl

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groups), graphene oxide (GO) as highly efficient adsorbent has been extensively

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investigated to remove radionuclides in recent years.6, 35-40 However, the GO was not

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easily separated from liquid-phase due to its excellent dispersity in aqueous solutions.

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Layered double hydroxide (LDH, [MII1-xMIIIx(OH)2]x+(An-)x/n·mH2O) is typical

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brucite-like layered hydroxides with divalent (MII) and trivalent (MIII) metal ions in

16-19

and clay minerals.1,

20-24


Adsorption approach is

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the octahedron and n-valent anion (An-).41-44 LDH can be regarded the excellent

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adsorbent in environmental cleanup due to its exchangeable capacity of anions. It is

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determined that LDH/GO composites can be used as an excellent adsorbent, which

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can be separated easily from liquid-phase.45 The contribution of GO in the LDH/GO

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composites was to significantly enhance their adsorption performance. A thorough

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understanding of the chemical interactions of U(VI) with LDH-based composites is

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essential in building a robust safety case for the long-term geological disposal of

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nuclear waste and for the management of radionuclide-bearing sites. Although the

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removal of U(VI) on LDH/GO composites was investigated, the effect of carbonate

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on the speciation and fate of U(VI) remains poorly understood.46

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The aims of this manuscript were (1) to synthesize the LDH/GO composites and

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characterize them by TEM, FT-IR and XRD techniques; (2) to investigate the effect of

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environmental factors (e.g., reaction time, pH, carbonate concentration and

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temperature) on U(VI) removal onto LDH and LDH/GO composites by batch

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techniques; (3) to determine interaction mechanism of U(VI) and LDH and LDH/GO

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composites at a molecular level by XPS, XANES, EXAFS and surface complexation

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modeling. This manuscript highlighted the application of GO-based composites for

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removal of radionuclides in environmental cleanup.

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MATERIALS AND METHODS

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Synthesis of LDH/GO Composites. The LDH/GO composites were synthesized by a

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hydrothermal process.43 Firstly, the GO was obtained by the oxidation of flake

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graphite (< 30 micros, Qingdao, China) under the concentrated H2SO4 conditions.47



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Briefly, 1.0 g graphite and 0.5 g NaNO3 were added into 100 mL fuming sulphric acid

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under ice-bath conditions. Then 4.5 g KMnO4 was slowly added the aforementioned

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suspension, and MnO4- was eliminated by adding 6 mL of H2O2 solution. Then, 7.8

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mmol MgCl2·6H2O, 2.6 mmol AlCl3·6H2O and 9.2 mmol hexamethylene tetramine

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were added into 20 mL 2.0 g/L GO solutions under vigorous stirring conditions for 1

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h. The aforementioned suspensions were transferred into 50 mL Teflon-lined

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stainless- steel autoclave and then were heated at 140 °C for 12 h. After cooled to

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room temperature, the black slurries were washed with Milli-Q water and

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centrifugation it several times. The LDH/GO composites were obtained by drying it at

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vacuum oven at 60 °C overnight. To identify the contribution of GO, the LDH/GO2

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composited was also synthesized by adding the same amount of Mg2+ and Al3+ into

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the twice concentration of GO solution (20 mL 4.0 g/L) under the same experimental

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conditions.

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UO2(NO3)2.6H2O (analytical purity, Sigma-Aldrich) into 0.01 mol/L HNO3 solution

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under glovebox conditions.

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Characterization. The morphology of as-prepared LDH/GO composites was

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characterized by TEM (JEOL-2010 microscope). The surface groups of LDH/GO

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composites were determined by FT-IR (Nicolet Magana-IR 750 spectrometer) from

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400 to 4000 cm-1. The mineralogy of as-prepared composites were recorded by XRD

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pattern (Philips X’Pert Pro Super X-ray Diffractometer) with Cu-Kα radiation

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(λ=1.5416 Å). The change in surface electron state of LDH/GO composites was

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performed by XPS (VG Scientific ESCALAB Mark II spectrometer) equipped with

The

stock

solutions

of

U(VI)

were

prepared


by

dissolving

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an Mg Kα radiation source (1253.6 eV) at 10 kV and 5 mA under 10-8 Pa residual

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pressure. Surface charging effects were corrected with the C 1s peak at 284.6 eV as a

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reference.

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Batch Adsorption. The triple adsorption experiments of U(VI) on LDH and LDH/GO

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composites (m/v = 1.5 g/L) were conducted in polycarbonate centrifuge tubes at pH

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4.5 and in presence of 0.01 mol/L NaClO4 solutions (as an inert anion). Briefly, the

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suspension of LDH/GO composites were pre-equilibrated with NaClO4 solutions

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overnight, and then U(VI) solution was provided to reach the desired concentration.

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The pH value of suspensions was adjusted by adding negligible volumes of 0.01-1.0

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mol/L HClO4 or NaOH solution. After equilibrium, the solid phase was separated

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from liquid phase by centrifugation at 5000 rpm for 30 min and then filtered using

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0.22 µm nylon membrane filters. The recycling experiments were conducted 6

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successive cycles by U(VI) adsorption- desorption experiments. Briefly, after U(VI)

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adsorption, UVI)-containing wet solid phase was orderly desorbed by adding 0.01

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mol/L HNO3, 0.2 mol/LNa2CO3 and 0.01 mol/L EDTA solutions.23 The concentration

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of U(VI) in supernatant was measured by inductively coupled plasma-mass

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spectrometry (ICP-MS, Agilent 7500CX, USA). The adsorbed amount of U(VI) on

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LDH/GO composites can be calculated by the difference of the initial and equilibrated

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concentration.

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Surface Complexation Modeling. The modeling approach allowed for a clear

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determination of the reactive surface sites of LDH/GO composites for U(VI)

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adsorption. Diffuse layer model of surface complexation modeling was done by visual



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MINETQ program.48 Equilibrium constants for surface complexes were obtained by

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simulating adsorption edges for three surface complexation reactions. The speciation

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was used by mass balance constraints to optimize the equilibrium constants. The

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equilibrium constants were obtained from the apparent stability constants by

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converting the concentration of each surface species to its mole fraction.

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Preparation and Analysis of XANES and EXAFS spectra. The samples for

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XANES and EXAFS analysis were prepared in the golvebox (purged with 95 % N2

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and 5 % H2). Briefly, the suspension of LDH/GO composites was pre-equilibrated

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with NaClO4 (0.01 mol/L) 24 h, then UO22+ solutions were slowly added into

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aforementioned suspension under vigorous stirring conditions. The values of pH in

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aqueous solutions were adjusted to 3.0 and 6.0 by adding 0.01-1.0 mol/L HCl and/or

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NaOH solutions. After reaction 2 days, the wet solid phase was obtained by

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centrifuging the suspension at 9800 rpm for 20 min. Then wet pastes were mounted in

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Teflon sample holders sealed with Kapton tape. The uranium L3-edge XANES and

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EXAFS

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uranium-containing LDH/GO at different pH conditions were performed with Si(111)

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double-crystal monochromator at the BL14W1 beamline of Shanghai Synchrotron

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Radiation Facility (SSRF, Shanghai, China). All spectra were conducted in

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fluorescenc mode expect for crystalline U(IV)O2(s), which was recorded in

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transmission mode. The fitting of EXAFS data were carried out by Athena and

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Artemis interfaces of IFFEFIT 7.0 mode.49, 50

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RESULTS AND DISCUSSION

spectra

for

reference

standards

(e.g.,

U(IV)O2(s),


U(VI)O22+)

and

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Characterization. The morphology of LDH/GO composites was characterized by

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TEM image. The hexagonal LDH platelets with 20 nm thicknesses revealed the good

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crystallinity due to slow nucleation process (Figure 1A). As shown in Figure 1B, the

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LDH nanoplates were uniformly deposited on the surface of GO nanosheets, which

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was consistent with the previous studies.43, 51 Figure 1C shows the FT-IR spectrum of

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LDH/GO composites. The significant peaks at 3450 and 1365 cm-1 could be ascribed

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to the stretching vibration of O-H groups associated with the interlayer water

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molecules and CO32- ions in the interlayer of Mg/Al LDH, respectively.43 However,

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the bands at 1730, 1620 and 1220 cm-1 were attributed to the stretching vibration of

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C=O, C=C, C-O groups of GO, respectively.17, 26, 52 The abundant bands at 500-800

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cm-1 could be due to the stretching/bending vibrations of metal-oxygen lattices such

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as Mg/Al-O, O-Mg/Al-O, and Mg/Al-O-Mg/Al groups.46 The result of FT-IR

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spectrum showed that LDH nanoplate was successfully synthesized on the GO surface,

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moreover the oxygenated functional groups of GO provided the much more reactive

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sites for U(VI) removal. As shown by XRD pattern in Figure 1D, the diffraction peaks

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at 2θ= 11.5, 23.6, 35.1, 39.5, 47.2, 60.8 and 62.2° can be indexed into the (003), (006),

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(012), (015), (018), (110) and (113) planes of Mg/Al LDH (JCPDS No. 15-0087),

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respectively.53 No diffraction peaks of GO (e.g., (002) plane of GO at 9.12) and other

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impurities were observed. In additions, sharp and symmetric peaks revealed the high

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crystallization. As shown in Table 1, the N2-BET specific surface area and average

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pore size of as-prepared LDH/GO composites were calculated to be 28.73 m2/g and

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3.2 nm, respectively, which were lower than that of LDH (45.64 m2/g and 3.8 nm for



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specific surface area and average pore size, respectively). The characteristic results

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indicated the occurrence of abundant oxygen-containing functional groups of

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LDH/GO composites.

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Effect of pH and Carbonate Concentration. Figure 2A shows the adsorption of

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U(VI) on LDH and LDH/GO composites at different pH conditions. One can see that

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the slight increase in U(VI) adsorption on LDH was observed at pH < 4.0, whereas

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the adsorption of U(VI) on LDH was significantly increased at pH 4.0 – 6.5, and then

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kept the high-level adsorption at pH 6.5-7.5. The decreased adsorption of U(VI) on

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LDH was discerned at pH > 8.0. For LDH/GO composites, the significant increase of

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U(VI) adsorption was observed at pH 2.0-5.0, and then remained the adsorption of

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U(VI) at pH 5.0-8.0, whereas the slight decrease of U(VI) adsorption was found at

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pH > 8.5. At pH 6.0, approximately 91 and 99.8 % of U(VI) was removed by LDH

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and LDH/GO composites, respectively. Compared to LDH, the pH-edge adsorption of

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U(VI) on LDH/GO composites was shifted to the lower pH values. These results

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indicated that the GO enhanced the adsorption of U(VI). Figure 2B shows the effect

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of carbonate concentration on adsorption of U(VI) onto LDH and LDH/GO

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composites. At pH < 5.0, the adsorption of U(VI) on LDH and LDH/GO composites

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in the presence of carbonate was significantly higher than that of U(VI) on LDH and

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LDH/GO composites in the absence of carbonate. However, the carbonate inhibited

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the adsorption of U(VI) on LDH and LDH/GO composites at pH > 7.0. It is

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demonstrated the negative carbonato- uranyl complexes (i.e., UO2(CO3)22- and

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UO2(CO3)34- species) was obtained in the presence of carbonate conditions.14, 20 As


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shown in Table 1, the pHPZC (pH at point of zero charge) of LDH and LDH/GO

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composites were calculated to be 7.1 and 6.5, respectively. Therefore, the increased

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adsorption of U(VI) on LDH and LDH/GO composites in the presence of carbonate

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could be attributed to the electrostatic attraction between positive charged of LDH and

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LDH/GO composites and negative charged of carbonato-uranyl complexes at pH
7.0 could be ascribed the electrostatic repulsion between negative charged of

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LDH and LDH/GO composites and negative charged of carbonato-uranyl complexes.

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The high-level adsorption of U(VI) on LDH and LDH/GO composites at pH 6.0- 7.0

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could be surface co-precipitation and/or surface complexation reaction of U(VI) with

187

LDH and LDH/GO composites.

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Adsorption Kinetics and Isotherms. Figure 3A and 3B shows the adsorption

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kinetics and isotherms of U(VI) on LDH and LDH/GO composites, respectively. As

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shown in Figure 3A, the remarkable increase of U(VI) adsorption on LDH and

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LDH/GO composites was observed with in reaction time of 6 h, and then kept the

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high-level adsorption at reaction time more than 6 h. Approximately 100 % and 86 %

193

of U(VI) were removed by LDH/GO composites and LDH, respectively. The kinetic

194

data were fitted by pseudo-first and pseudo-second order kinetic models. The linear

195

forms of pseudo-first and pseudo-second order kinetic models can be described by

196

Eqns. (1) and (2): respectively:

197

ln(qe - qt) =lnqe - kf×t

(1)

198

t/qt = 1/(ks×qe2) + t/qe

(2)



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where qe and qt (mg/g) were the amount of U(VI) adsorbed at equilibrium and at time

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t, respectively. kf and ks referred to the rate constants of pseudo-first and

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pseudo-second order kinetic model, respectively. The optimized parameters of

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pseudo-first and pseudo-second order kinetic model were shown in Table 2. As shown

203

in Table 2, the removal kinetics of of U(VI) on LDH and LDH/GO composites can be

204

satisfactorily simulated by pseudo-second-order kinetic model with high correlation

205

coefficients (R2 =1) compared to pseudo-first-order kinetic model (R2 < 0.6), which

206

were consistent with previous studies.54, 55

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Figure 3B shows removal isotherms of U(VI) on LDH and LDH/GO composites. It is

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observed that removal of U(VI) on LDH was significantly lower than that of

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LDH/GO composites. It should be noted that the U(VI) on LDH/GO composites was

210

lower than that LDH/GO2 composites, indicating that the U(VI) removal increased

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with increasing GO concentration. The data of removal isotherms were simulated by

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Langmuir and Freundlich models. The equations of Langmuir and Freundlich models

213

can be described by Eqns. (3) and (4), respectively:

214

Ce / Qe = 1/ (KL×Qm) + Ce / Qm

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log Qe = log KF + 1/n log Ce

216

where Ce (mg/L) and Qm (mg/g) are equilibrated concentration and the maximum

217

removal capacity of LDH or LDH/GO composites at complete monolayer coverage.

218

KL (L/mg) and KF are a Langmuir and Freundlich constant, respectively. 1/n is the

219

heterogeneity of the adsorption sites. All parameters of Langmuir and Freundlich

220

model were showed in Table 3. As shown in Table 3, the adsorption of U(VI) on LDH


(3) (4)

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and LDH/GO composites can be satisfactorily fitted by Langmuir model. The

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maximum adsorption capacities of LDH and LDH/GO composites calculated from

223

Langmuir model at pH 4.5 and 293 K were 99.01 and 129.87 mg/g, respectively,

224

which indicated that the introduction of GO in LDH/GO composites can significantly

225

increase the maximum adsorption capacity of LDH. As compared in Table 4, the

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maximum adsorption capacity of LDH/GO composites was significantly higher than

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that of metal (hydr)oxides (e.g., 11.6, 5.59 and 50.62 mg/g for nanoporous alumina9,

228

hematite56 and ferrihydrite57, respectively) and carbon-based nanoparticles (e.g., 26.18

229

and 97.5 mg/g for multiwalled carbon nanotubes58 and GO59, respectively). Therefore,

230

LDH/GO composites can be regarded as a promising adsorbents for the removal and

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immobilization of U(VI) from aqueous solutions at low pH condition.

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Figure 4A and 4B show the recycling experiments of U(VI) removal on LDH and

233

LDH/GO composites, respectively. After 6 recycling experiments, the decreased

234

extent of maximum adsorption capacity for LDH/GO composites (from 129.87 to

235

108.45 mg/g) was slightly lower than that of GO composites (from 99.01 to 72.69

236

mg/g), indicating that LDH/GO composites can be long-term use in the wastewater

237

treatment. The results of recycling experiments revealed that LDH/GO composites

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can be used as environmentally renewable and low cost adsorbent due to the presence

239

of the excellent capacity of regeneration and the efficient performance.

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XPS Analysis. Figure 5A and 5B show the total scan and high resolution U 4f XPS

241

analysis of LDH/GO composites before and after U(VI) removal, respectively. As

242

shown the total survey spectra in Figure 5A, there are four predominant elements of



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LDH/GO composites (i.e., C 1s, O 1s, Al 2p and Mg 2p) before U(VI) adsorption,

244

whereas the weak U 4f peaks and the decreased relative intensity of O 1s were

245

observed after U(VI) adsorption. These evidence indicated that U(VI) was attached on

246

the surface of LDH/GO composites. After U(VI) desorption, the relative intensity of

247

O 1s was significantly higher than that of O 1s after U(VI) adsorption, indicating that

248

oxygen-containing functional groups were responsible for the high effective

249

adsorption of U(VI) on LDH/GO composites. As shown in Figure 5B, the U 4f peaks

250

can be deconvoluted into two sub-peaks at ~ 381 and 392 eV, which were

251

corresponded to U 4f7/2 and U 4f5/2, respectively.14 The results of XPS analysis

252

indicated that the adsorption of U(VI) on LDH/GO composites was attributed to the

253

abundant oxygen-containing functional groups of GO.

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Surface Complexation Modeling. Figure 6A and 6B show the surface complexation

255

modeling of U(VI) on LDH and LDH/GO composites using double layer model,

256

respectively. The optimized parameters of surface complexation modeling were

257

summarized in Table 5. It is observed that the adsorption of U(VI) on LDH and

258

LDH/GO composites can be satisfactorily fitted by double layer model with a cation

259

exchange sites (X2UO2) and two inner-sphere complexation sites (SOUO2+ and

260

SOUO2(CO3)23- species) at pH < 7.0, whereas the fitted results under-evaluated the

261

experimental data at pH > 8.0 . As shown in Figure 6A and 6B, the X2UO2 species

262

was observed at pH < 4.0, whereas SOUO2+ and SOUO2(CO3)23- species dominated

263

the adsorption of U(VI) on LDH at pH 4.0-6.0 and pH > 6.0, respectively. However,

264

the main species of U(VI) adsorption on LDH/GO composites were SOUO2+ and


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SOUO2(CO3)23- species at pH < 4.0 and pH > 5.0, respectively. The results of surface

266

complexation modeling indicated that the adsorption of U(VI) on LDH and LDH/GO

267

composites at pH < 4.0 was cation exchange. However, the adsorption of U(VI) on

268

LDH and LDH/GO composites at pH > 5.0 was inner-sphere surface complexation.

269

XANES and EXAFS Analysis. Figure 7A and 7B show the uranium L3-edge

270

XANES and EXAFS spectra of standards (U(IV)O2(s), U(VI)O22+) and uranium-

271

containing LDH/GO at different pH conditions, respectively. As shown in Figure 7A,

272

the positions of adsorption edge for U(IV)O2(s) and U(VI)O22+ were ~17174 and 17176

273

eV, respectively. The position and type of uranium-containing LDH/GO was

274

consistent with U(VI)O22+ standard, indicating that uranium redox states in LDH/GO

275

composites was U(VI) species. The results of XANES spectra revealed that the

276

removal of U(VI) on LDH/GO composites was mainly adsorption without reduction.

277

The uranium L3-edge EXAFS spectra for uranium-containing LDH/GO composites at

278

different pH were showed in Figure 7B. Table 6 summarized the optimized

279

parameters (e.g., sub-shell, coordination number, Debye-Waller factor) of fitting

280

results. Attempt to distinguish U-Oeq shell into two sub-shell (e.g., U-Oeq1 and

281

U-Oeq2) resulted to the convergence of two sub-shell at the similar bond distance,

282

therefore only U-Oeq shell were obtained in this study. As shown in Figure 7B, the FT

283

features of uranium-containing samples and reference standard at ca. 1.45 and 1.95 Å

284

can be satisfactorily fitted by two axial oxygen (U-Oax) at ca. 1.8 Å and ~ five

285

equatorial oxygen (U-Oeq) at ~ 2.35 Å (Table 6), respectively.60 The EXAFS spectra

286

of uranium-containing LDH/GO composites at pH 3.0 and pH 6.0 were significantly



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

287

different from EXAFS spectra of UO22+ species. For LDH/GO composites at pH 3.0

288

and pH 6.0, the third FT features can be simulated by ~ 1.3 alumina/magnesium

289

(U-Al/Mg shell) at ~ 3.18 Å (Table 6).20 Moreover, the coordination number of U-Mg

290

shell decreased with increasing pH from 3.0 to 6.0 (CN = 1.22 and 1.20 for pH 3.0

291

and 6.0, respectively). The results of EXAFS spectra indicated that inner-sphere

292

surface complexation dominated the removal of U(VI) on LDH/GO composites,

293

which was consistent with the batch adsorption experiments.

294

CONCLUSIONS

295

The LDH/GO composites were successfully synthesized by hydrothermal method.

296

The adsorption kinetics and isotherms of U(VI) on LDH and LDH/GO composites

297

can be satisfactorily fitted pseudo-second-order kinetic and Langmuir model,

298

respectively. The adsorption of U(VI) on LDH and LDH/GO composites at pH < 5.0

299

significantly increased with increasing carbonate concentration, whereas the

300

carbonate accumulated the desorption of U(VI) from LDH and LDH/GO composites

301

at pH 7.0-9.0. According to XPS analysis, oxygen-containing functional groups were

302

responsible for the adsorption of U(VI) on LDH/GO composites. The results of

303

surface complexation modeling indicated that the adsorption of U(VI) on LDH and

304

LDH/GO composites at pH < 4.0 and pH > 5.0 was cation exchange and inner-sphere

305

surface complexation, respectively. These observations indicated that LDH/GO

306

composites can be a promising adsorbent for the high efficient adsorption of

307

radionuclide from aqueous solutions in environmental cleanup.

308

Acknowledgements


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309

This work was supported by the Shaoxing University of Research Startup Project

310

(20155029).

311

References

312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349

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495



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496

Figure Captions

497

Figure 1. Characterization of LDH/GO composites. A and B: TEM images of LDH

498

and LDH/GO composite, respectively; C: FT-IR spectra; D: XRD pattern.

499

Figure 2. The effect of pH (A) and carbonate concentration (B) on U(VI) removal on

500

LDH and LDH/GO composites, C0 = 10 mg/L, m/v = 1.5 g/L, I = 0.01 mol/L NaClO4,

501

T = 293 K.

502

Figure 3. Adsorption kinetics (A) and isotherms (B) of U(VI) removal by LDH and

503

LDH/GO and LDH/GO2 composites, pH 4.5, m/v = 1.5 g/L, I = 0.01 mol/L NaClO4,

504

T = 293 K.

505

Figure 4. Recycling of LDH (A) and LDH/GO composites (B) for U(VI) removal, pH

506

4.5, m/v = 1.5 g/L, I = 0.01 mol/L NaClO4, T = 293 K.

507

Figure 5. The XPS analysis of LDH/GO after U(VI) adsorption (LDH/GO-U-A) and

508

desorption (LDH/GO-U-D), A: total scans; B: high resolution of U 4f, C0 = 10 mg/L,

509

pH 4.5, m/v = 1.5 g/L, I = 0.01 mol/L NaClO4, T = 293 K.

510

Figure 6. Surface complexation modeling of U(VI) on LDH (A) and LDH/GO (B)

511

under different pH conditions, C0 = 10 mg/L, m/v = 1.5 g/L, I = 0.01 mol/L NaClO4,

512

T = 293 K.

513

Figure 7. Uranium L3-edge XANES spectra (A) and Fourier transform (FT) of

514

EXAFS spectra (B) for U(IV)O2(s), U(VI)O22+ and U(VI)-containing LDH/GO

515

composites at pH 3.0 and 6.0, m/v = 1.5 g/L, T = 293 K, I = 0.01 mol/L NaClO4.

516


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Table 1. The selective properties of LDH/GO composites LDH

LDH/GO

pHPZC

0 mV at pH 7.1

0 mV at pH 6.5

SBET (m2/g)

45.64

28.73

Average pore size (nm)

3.8

3.2

Table 2. Parameters of pseudo-first-order and pseudo-second-order kinetic model for the adsorption of U(VI) on LDH (1) and LDH/GO composites (2)

pseudo-ﬁrst-order

pseudo-second-order

qe(mg/g)

kf(h−1)

R2

qe(mg/g)

ks(g /(mg×h))

R2

(1)

3.53

0.1164

0.5836

17.67

3.11E-5

1

(2)

5.12

0.0232

0.4077

20.04

1.12E-5

1



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Page 22 of 31

Table 3. Parameters of Langmuir and Freundlich model for the adsorption of U(VI) on LDH (1) and LDH/GO composites (2) Langmuir KL Qm (L/mg) (mg/g)

R

2

Freundlich Log KF 1/n n (mg/g)/(mg/g)

R2

(1)

0.0628

99.01

0.9978

1.9109

0.7587

0.9968

(2)

0.1851

129.87

0.9988

3.1775

0.5675

0.9829

Table 4. Comparison of adsorption performance of various adsorbents for U(VI) Adsorbents

Exp. Conditions

Qm (mg/g)

Nanoporous alumina

pH 4.5 T= 298 K

11.6

9

Hematite

pH 5.5 T= 298 K

5.59

56

Ferrihydrite

pH 5.5 T= 298 K

50.62

57

Carbon nanotubes

pH 5.0, T= 298 K

26.18

58

GO

pH 5.0 T= 293 K

97.5

59

LDH

pH 4.5 , T= 293 K

99.01

This study

LDH/GO composite

pH 4.5 , T= 293 K

129.87

This study


Ref.

Page 23 of 31

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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+

3.2

>SOH + UO22+ = >SOUO2+ + H+

-2.7

>SOH +2H2CO3 + UO22+ = >SOUO2 (CO3)23- + 5H+

-18.9

Table 6. Optimized parameters of uranium L3-edge EXAFS spectra for reference and uranium-containing samples Samples

Shell

R(Å)a

CNb

σ2(Å2)c

U(VI)O22+

U-Oax

1.78

2.0

0.0033

U-Oeq

2.35

4.9

0.0054

U-Oax

1.78

2.0

0.0026

U-Oeq

2.33

4.8

0.0039

U-Mg/Al

3.18

1.22

0.0087

U-Oax

1.78

2.0

0.0041

U-Oeq

2.31

4.6

0.0038

U-Mg/Al

3.17

1.20

0.0072

LDH/GOpH3

LDH/GOpH6

a

: bond distance; b: coordination number; c: Debye-Waller factor.



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(A)

(B)

(C)

(D)

Figure 1


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Figure 2



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 3


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Figure 4



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Figure 5


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Figure 6



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Figure 7


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TOC

Manuscript title: One-pot synthesis of LDH/GO composites as high effective adsorbent for the decontamination of U(VI) Names of all authors: Wensheng Linghu†*, Hai Yang†, Yanxia Sun†, Guodong Sheng†, Yuying Huang‡ Brief synopsis: LDH/GO presented high adsorption capacity and good recyclability and recoverability for radionuclides in environmental cleanup.


GO Composites as ... - ACS Publications

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