C Composites as an Efficient Adsorbent for the

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New Synthesis of nZVI/C Composites as an Efficient Adsorbent for the Uptake of U(VI) from Aqueous Solutions Haibo Liu, Mengxue Li, Tianhu Chen, Changlun Chen, Njud S. Alharbi, Tasawar Hayat, Dong Chen, Qiang Zhang, and Yubing Sun Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b02431 • Publication Date (Web): 25 Jul 2017 Downloaded from http://pubs.acs.org on July 25, 2017

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Environmental Science & Technology

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New Synthesis of nZVI/C Composites as an Efficient Adsorbent for

2

the Uptake of U(VI) from Aqueous Solutions

3

Haibo Liu†, Mengxue Li†, Tianhu Chen†, Changlun Chen‡, #, Njud S. Alharbi#, Tasawar Hayat∥, Dong, Chen†, Qiang Zhang†, Yubing Sun ‡, &*

4 5

†

6

Hefei, 230009, P. R. China

7

‡

Institute of Plasma Physics, Chinese Academy of Science, Hefei, 230031, P.R. China

8

#

Department of Biological Science, Faculty of Science, King Abdulaziz University,

School of Resources and Environmental Engineering, Hefei University of Technology,

Jeddah, 21589, Saudi Arabia

9 10 11 12

∥

NAAM Research Group, Kind Abdulaziz University, Jeddah, 21589, Saudi Arabia

&

School for Radiological and Interdisciplinary Sciences, Soochow University, 215123, Suzhou, P.R. China

13

ABSTRACT: New nanoscale zero-valent iron/carbon (nZVI/C) composites were

14

successfully prepared via heating natural hematite and pine sawdust at 800 °C under

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nitrogen conditions. Characterization by SEM, XRD, FTIR and XPS analyses

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indicated that the as-prepared nZVI/C composites contained a large number of

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reactive sites. The lack of influence of the ionic strength revealed inner-sphere

18

complexation dominated U(VI) uptake by the nZVI/C composites. Simultaneous

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adsorption and reduction were involved in the uptake process of U(VI) according to

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the results of XPS and XANES analyses.

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demonstrated that innersphere complexation and surface coprecipitation dominated

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the U(VI) uptake at low and high pH conditions, respectively. The uptake behaviors of

The presence of U-C/U-U shells

ACS Paragon Plus Environment


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U(VI) by the nZVI/C composites were fitted well by surface complexation modeling

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with two weak and two strong sites. The maximum uptake capacity of U(VI) by the

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nZVI/C composites was 186.92 mg/g at pH 4.0 and 328 K. Additionally, the nZVI/C

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composites presented good recyclability and recoverability for U(VI) uptake in

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regeneration experiments. These observations indicated that the nZVI/C composites

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can be considered as potential adsorbents to remove radionuclides for environmental

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remediation.

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INTRODUCTION

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The environmental contamination of radionuclides has become a more serious issue

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with the rapid development of nuclear energy.1,

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contaminant, uranium has been proven to induce carcinogenesis, teratogenesis and

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mutagenesis.3, 4 Uranium can exist in various oxidized states, e.g.

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and sparingly dissoluble U(IV) species.5-8 The mobility of U(VI) towards

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sub-environment was influenced by various environmental conditions e.g. pH and

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carbonate concentration. Therefore, the accurate prediction of U(VI) speciation at

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various water-mineral interfaces is necessary to decontaminate U(VI)-contaminated

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groundwater to values below the maximum allowable emission concentration.9-11 A

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number of previous reports have examined U(VI) decontamination using adsorbents,

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such as metal (hydr)oxides12-14 and clay materials.15-17 Sun et al. found the maximum

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uptake capacity of alumina for U(VI) was 2.76 mg/g.18 Such low uptake efficiencies

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of these natural adsorbents limit their application in practical operation.

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In recent years, nanoparticles have been extensively demonstrated as potential

2

As a toxic and radioactive


dissoluble U(VI)

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adsorbents due to their high surface area, numerous surface groups and sorption

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reactive sites; examples include carbon-based nanomaterials19-22 and nZVI.23-25 Ding

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et al. reported that the maximum uptake capacity of nano-Fe0 for U(VI) was ~215

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mg/g.26 The high, efficient uptake capacity of nZVI could be attributed to the

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simultaneous adsorption and reduction, whereas nZVI nanoparticles are prone to

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aggregation and are easily oxidized when exposed to air.27,

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researchers have attempted to explore nZVI-based composites to improve their

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dispersibility and uptake ability.29-31 Sun et al. reported that nZVI-supported graphene

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oxide presented a faster reaction rate and higher uptake capacity for U(VI) compared

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to nZVI nanoparticles.32 However, the chemical synthesis of these composites was

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rather tedious.

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In this study, nZVI/C composites were directly synthesized by heating mixtures of

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natural hematite and pine sawdust under N2 condition. The aims of this study are to

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characterize the as-prepared nZVI/C composites by SEM, XRD, and FTIR analysis;

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to investigate the influence of the aqueous chemical conditions on U(VI) uptake by

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the nZVI/C using

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by the nZVI/C by XPS, XANES and EXAFS analyses; and to simulate U(VI) uptake

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by the nZVI/C according to surface complexation modeling. The successful

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preparation and effective utilization of the nZVI/C composites as highly effective

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adsorbents for the uptake of radionuclides for environmental cleanup was expected.

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Therefore, many

batch experiments; to demonstrate the uptake mechanism of U(VI)

65 66



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EXPERIMENTAL SECTION

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Materials. Natural hematite (< 0.75 µm) and pine sawdust (< 180 µm) were obtained

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from the Yichang iron mine (Hubei, China) and a local pine mill (Hefei, China),

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respectively. The chemical composition of natural hematite mainly contains 72.10 wt%

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of Fe2O3 and minor impurities such as 15.4 wt% of SiO2 and 4.96 wt% Al2O3. The

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pine sawdust was pulverized using a pulverizer and then sieved to less than 180 µm. A

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1.0×10−3 mol/L of U(VI) solution was prepared using UO2(NO3)2 (spectrographic

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purity, Sigma-Aldrich). Other chemicals used in this study were of analytical grade.

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Preparation and Characterization of the nZVI/C Composites. The nZVI/C

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composites were synthesized by heating natural hematite and pine sawdust at 800 °C

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under N2 conditions. Briefly, 2.0 g of pine sawdust and 1.0 g of hematite were

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homogeneously mixed under constant stirring conditions and then pre-heated at

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100 °C for 5 h. Afterwards, the mixed powders were pyrolyzed at 800 °C for 20 min

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under nitrogen protection. The as-prepared nZVI/C composites were obtained by

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cooling to room temperature under N2 conditions.

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The morphology of the nZVI/C composites was examined by SEM-EDS (JEM-2010,

83

Japan). The mineralogy of the nZVI/C composites was examined by XRD

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(Dandonghaoyuan 2700 diffractometer) at 300 mA and 45 kV using Cu-Kα radiation.

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FTIR measurements were recorded on a VERTEX-70 Fourier transform infrared

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spectrometer using KBr pellets. The potentiometric acid-base titration was conducted

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by a titrator (Mettler Toledo DL50, Switzerland). The XPS spectra were conducted

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with an electron spectrometer (Thermo Escalab 250, USA) at 150 W with Al-Kα


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radiation. The binding energies of the XPS spectra were calibrated with the C 1s peak

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at 284.6 eV. The specific surface area (SBET) of the nZVI/C was calculated by a

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TriStar II 3020 instrument. Magnetization curves and magnetic susceptibility were

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measured by a magnetometer (Lakeshore Cryotronic, USA) and a MS2WFP

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permeameter (Bartington, OX28 4GG, UK), respectively.

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Batch Uptake Experiments. Batch uptake experiments of U(VI) (C0 = 10 mg/L) by

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the nZVI/C composites

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and 3 mL of 20 mg/L U(VI) solution were pre-reacted for 12 h, and then, 2.4 mL of a

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0.5 g/L nZVI/C composite solution (total volume: 6 mL) was added to the

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aforementioned solutions. The uptake kinetics and isotherms were examined at pH 4.0

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at different times and initial U(VI) concentrations, respectively. The pH of the

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aqueous solutions was adjusted usingHNO3 and/or NaOH with a concentration of

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0.001-1.0 mol/L. The solid-liquid phases were separated by centrifugation followed

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by filtering via a 0.22-µm membrane. The blank uptake tests were simultaneously

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arranged. The amount of adsorbed U(VI) by the nZVI/C composites was computed

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from the difference between the initial and equilibrium concentration of U(VI) in the

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aqueous solutions. The concentration of U(VI) was measured by a spectrophotometry.

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All experimental data are the average of triplicate determinations.

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Preparation of XPS, XANES and EXAFS Samples. To further explore the

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mechanism of U(VI) uptake by the nZVI/C composites, the uptake samples under

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different pH conditions (pH 3 and 6) and different reaction times (1 and 7 days) were

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characterized by XPS and EXAFS analyses. The samples for these characterizations

were carried out. Briefly, 0.6 mL of 0.1 mol/L NaCl solution



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were prepared under glovebox conditions. First, 0.1 g of the nZVI/C composite and

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50 mL of a 0.1 mol/L NaCl solution were added into a 500-mL conical flask. Then, 50

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mL of 100 mg/L U(VI) solution was slowly mixed with the suspension. Then, the

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suspension pH was regularized to 3.0 or 6.0 with 0.01~1 mol/L HCl/NaOH. After

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equilibrium and centrifugation, wet wastes were collected for the analysis of the

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EXAFS spectra, whereas the samples for XPS analysis were obtained after drying.

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Uranium L3-edge EXAFS spectra were conducted at the BL14W1 beamline of the

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Shanghai Synchrotron Radiation Facility using a Si(111) double crystalline

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monochromator under a liquid nitrogen cryostat in order to avoid any redox reactions.

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The analysis and fitting of the EXAFS data were conducted using IFEFFIT 7.0

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software.33

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Surface Complexation Modeling. The uptake behaviors of U(VI) by the nZVI/C

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composites were simulated using the surface equilibrium program Visual MINEQL

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model.34 MINEQL contains sub-routines for computing the surface complexation with

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a non-electrostatic model.35 Due to the inhomogeneity, weak (SwOH) and strong

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(SsOH) sites were considered in the surface complexation reactions. The relative

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concentrations of SwOH and SsOH derived from the fitting of potentiometric data

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were calculated to be 2×10-3 and 5×10-5 mol/g, respectively.

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RESULTS AND DISCUSSION

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Characterizations. The morphology of the prepared nZVI/C composites was

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characterized by SEM. As presented in Figure 1A, granular iron was uniformly

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dispersed on the surface of fibrous carbon containing massive porous structures,


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which is consistent with a previous study.36 As seen in the energy dispersive X-ray

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(EDX) spectrum in the inset of Figure 1A, the main constituents of the as-prepared

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nZVI/C composites were C (54.56 wt %), O (16.22 wt %) and Fe (23.96 wt %).

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According to the elemental mapping analysis (Figure S1 in the SI), Fe was uniformly

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dispersed on the surface of the composites, indicating that the nZVI/C composites

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were successfully synthesized by this method. As shown in the XRD patterns in

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Figure 1B, the characteristic peaks at 2θ = 44.8 and 65.1° were indexed as the (110)

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and (200) planes of poorly crystalline metallic iron, respectively.37 The results of

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XRD analysis indicated that Fe(III) can be reduced to metallic iron by pine sawdust at

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800 °C under N2 conditions. The formation of nZVI was further demonstrated by

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magnetization curve measurements. As displayed in Figure S2 of the SI, the high

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saturation magnetization (σs = 57.4 emu/g, magnetic field ± 45 kOe, at room

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temperature) and magnetic susceptibility (2.96×10-4 m3/kg) of the nZVI/C composites

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revealed the as-prepared nZVI/C composites to have high magnetism.38 The inset

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graph in Figure S2B indicated that the nZVI/C composites were well dispersed and

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susceptible to magnetic separation. Therefore, the nZVI/C composites could be

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facilely separated from aqueous solution by a permanent magnet. Figure 1C displays

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the FT-IR spectra of the nZVI/C composites. The peaks at 3629 and 1701 cm-1

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revealed the stretching and bending vibrations of the –OH groups, respectively.10 The

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characteristic C-H adsorption band appeared at 2894 cm-1.39 The peaks at 2349, 1565

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and 1429 cm-1 corresponded to stretching vibrations of the C=O bonds, and the peak

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at 1048 cm-1 was assigned to C-O groups.20 The results of FT-IR analysis suggested



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various oxygen-containing functional groups were introduced during the heating

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process, which provided more uptake sites for U(VI). Figure 1D shows the XPS

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survey scans of U(VI) uptake on the nZVI/C composites. For the original nZVI/C

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composites, three main peaks involving C, O and Fe were observed, whereas the

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occurrence of U 4f peaks after U(VI) uptake revealed that U(VI) was adsorbed by the

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nZVI/C composites. The relative intensities of O 1s were gradually weakened after

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U(VI) uptake, and the O 1s peaks shifted to high wavenumbers. In addition, the

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relative intensity of U 4f at pH 3 was somewhat weaker than that at pH 6.0, revealing

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that an increase in solution pH enhanced the uptake of U(VI) by the nZVI/C.40 Based

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on the zeta potential in Table 1, the pHpzc of the nZVI/C composites was determined

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to be 7.27, indicating that the nZVI/C composites were positively and negatively

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charged at pH < 7 and pH > 7.5, respectively. BET surface area and average pore size

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of the nZVI/C composites was 46.3 m2/g and 13 nm, respectively (Table 1). The

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characteristic results indicated that the nZVI/C composites were successfully

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synthesized and presented large numbers of oxygen-containing functional groups,

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providing more sites for U(VI) uptake.

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Uptake Kinetics. Figure 2A presents the uptake kinetics of U(VI) by the nZVI/C

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composites (m/v = 1.0, 0.2 g/L and pH 4.0). The uptake rate and amount of U(VI)

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rapidly increased in first 2 h and then achieved uptake equilibrium at reaction times

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over 9 h. Approximately 9.4 and 34.8 mg/g of adsorbed amount was obtained over the

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nZVI/C composites at m/v = 1 and 0.2 g/L, respectively. Two kinetic models were

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employed to simulate the kinetic data. More details on the kinetic models are


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presented in the SI. As displayed in Table S1 in the SI, the uptake kinetics of U(VI)

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by the nZVI/C composites was fitted well by the pseudo-second-order kinetic model

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(R2 > 0.999), which implies ion exchange or chemical adsorption dominated the

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instantaneous adsorption process.

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Effect of pH and Ionic Strength. Figure 2B displays the influence of solution pH on

182

U(VI) uptake by the nZVI/C composites as a function of ionic strength, respectively.

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The uptake of the nZVI/C composites was sharply increased at pH 2 - 4 and then kept

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its high uptake at pH 4 - 6, whereas a significant decrease in U(VI) uptake was found

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as pH was more than 6.0. As exhibited in Table 1, negatively and positively charged

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nZVI/C composites were found at pH < 7 and pH > 7.5, respectively. As shown in

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Figure S4 in the SI, the main speciation of U(VI) was UO22+ at solution pH < 4,

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positively charged U(VI) species were found at pH 4 - 8, and negatively charged

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U(VI) species were found at pH > 8. Hence, the increase in U(VI) uptake by the

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nZVI/C composites at pH 2 - 6 cannot be explained by electrostatic interactions,

191

which may attribute to the reduction/surface complexation of U(VI) on the nZVI/C

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composites. However, a decrease in U(VI) uptake by the nZVI/C at pH > 6.0 was

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attributed to the electrostatic repulsion between the nZVI/C composites and the U(VI)

194

species.41, 42

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The ionic strength generally affect the surface potential and the thickness of

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electrically diffused double layer, thus further influencing the binding of the adsorbed

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U(VI) ions.43 The influence of ionic strength on the uptake of U(VI) by the nZVI/C

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composites is illustrated in Figure 2B. The results indicate that the uptake of U(VI)



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was irrelevant to the ionic strength of different concentrations of NaCl solutions,

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implying inner-sphere complexation predominated the uptake of U(VI) by the nZVI/C

201

composites, according to previous reports.44-46

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Uptake Isotherms. To explore the capacity of the nZVI/C composites to U(VI), the

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uptake isotherms of U(VI) by the nZVI/C composites at different temperatures were

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measured. As illustrated in Figure 3A, the uptake of U(VI) by the nZVI/C composites

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considerably increased with an increase in temperature. In this study, Langmuir,

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Freundlich and D-R isotherm models were employed to analyze the uptake isotherms.

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More details of the description of the three models are displayed in the SI. As

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illustrated in Figure S5 and Table S3, the Langmuir model exhibited a better fit for the

209

uptake process (R2 > 0.995), implying the binding energy was uniform over the

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surface of the nZVI/C composites.32,

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(1.80-2.49 mol2/J2) calculated from the D-R model implied that the U(VI) uptake

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process on the nZVI/C composites was favorable. As summarized in Table 2, the

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maximum uptake capacity (186.9 mg/g) of U(VI) on the nZVI/C composite was

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larger

215

graphene/iron oxide composite (69.5 mg/g),50 fungus/attapulgite composite (125.0

216

mg/g)51 and graphene oxide (151.5 mg/g)52 but lower than that of nano-Fe0 (215.2

217

mg/g),53

218

oxide-supported polyaniline (245.1 mg/g),40 which indicates that the nZVI/C

219

composites are a potential material for the treatment of contaminated water.

220

Regeneration of the nZVI/C composites was conducted at 298 K by five successive

than

47, 48

montmorillonite@carbon

fungus-Fe3O4

As shown in Table S3, the β values

composites

bio-nanocomposites

(20.8

(223.9


mg/g),49

mg/g)54

and

magnetic

graphene

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adsorption−desorption cycles (Figure 3B). After five adsorption−desorption cycles,

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the maximum uptake capacity of U(VI) by the nZVI/C composites was reduced from

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103.11 to 79.03 mg/g. The slight decrease in the maximum uptake capacity may be

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due to mass loss during the adsorption−desorption process. The results of the

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regeneration experiments illustrated that the nZVI/C composites presented good

226

recyclability and recoverability for the uptake of radionuclides in environmental

227

cleanup.

228

As shown in Figure 3A, the uptake isotherm was the highest at 328 K and lowest at

229

298 K, which suggested higher temperatures can promote U(VI) uptake on the

230

nZVI/C composites. Several aspects can be used to explain this phenomenon: (i) a

231

high temperature may correspondingly contribute to the activity and proportion of

232

U(VI) ions, the potential charge of the nZVI/C composites and the relationship of the

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nZVI/C composites with U(VI) ions; (ii) a high temperature changed the pore size of

234

the nZVI/C composites and increased the number of uptake sites due to the breakage

235

of internal bonds; and (iii) a high temperature may also favored the diffusion of U(VI)

236

into the nZVI/C composites.55, 56 The thermodynamic parameters of U(VI) uptake are

237

shown in Table S4. The positive value of ∆H° (7.58 kJ/mol) suggests the uptake

238

process can be considered to be an endothermic physisorption. The negative value of

239

∆G° (-5.40 kJ/mol at 298 K) demonstrates the U(VI) uptake on the nZVI/C

240

composites is a spontaneous process. The positive ∆S° value (43.59 J/mol/K)

241

indicates that the molecular arrangement becomes more disorganized. These results

242

indicates the uptake of U(VI) by the nZVI/C composites is an endothermic and



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spontaneous process.

244

XPS Analysis. To investigate the uptake mechanism of U(VI)

245

the total survey and high-resolution XPS spectra (U 4f and Fe 2p) of the nZVI/C

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composites before and after U(VI) uptake are shown in Figure 4. After U(VI) uptake,

247

the bands at 382.6 and 393.4 eV appeared and are attributed to the characteristic

248

doublets of U 4f5/2 and U 4f7/2, respectively, indicating that a significant number of

249

U(VI) was adsorbed by the nZVI/C composites (Figure 4A). Additionally, the U 4f7/2

250

and U 4f

251

located at 380.5 and 391.9 eV and U(VI) peaks located at 382.1 and 392.9 eV, which

252

implies the adsorbed U(VI) was partially reduced to U(IV) by metallic Fe at the

253

surface of the nanoparticles.57 As shown in Figure 4B, the peaks at 710.73 and 727.16

254

eV correspond to the Fe 2p3/2 and Fe 2p1/2 peaks of the Fe(II)/Fe(III)-bearing oxides,

255

respectively.58 After U(VI) uptake, the presence of the Fe(0) peak (at 718.7 eV)

256

revealed that the nanoparticles can still act as electron donors under favorable

257

conditions. Approximately 51.4 and 62.8 % of Fe(II) was detected in the U(VI) uptake

258

samples at pH 3 and 6, respectively. In addition, the relative abundance of the Fe–O

259

peak (at 578 cm-1) at pH 6 was significantly higher than that at pH 3, which suggests

260

that the zero-valent iron was oxidized to iron (hydr)oxides at high pH.32 The XPS

261

analysis results illustrated that the high uptake of U(VI)

262

simultaneous adsorption and reduction processes.

263

XANES and EXAFS Analysis. Figure 5A and 5B show the XANES and EXAFS

264

spectra of the U(VI)-containing nZVI/C composites under different environmental

5/2

at the molecular level,

peaks were clearly deconvoluted into non-stoichiometric U(IV) peaks


was ascribed to

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conditions, respectively. As shown in Figure 5A, the energies of the adsorption edge

266

of U(VI)O22+ and U(IV)O2 were located at 17176 and 17173 eV, respectively.32 The

267

energy of the adsorption edge of the U(VI)-containing nZVI/C composites at pH 3.0

268

after 1 day was close to that of U(VI)O22+ (~17175 eV), whereas the adsorption edge

269

was shifted to lower energy with increasing reaction time (pH 6.0 for 7 days). At high

270

pH and long reaction time, the adsorption edge was similar to that of U(IV)O2

271

(~17173.8 eV). The XANES spectra exhibited the adsorbed U(VI) was gradually

272

reduced to U(IV) with an increase of time; moreover, the extent of U(VI) reduction

273

under high pH conditions is significantly higher than that under low pH conditions.

274

Figure 5B shows the uranium L3-edge EXAFS spectra of the standard nZVI/C

275

composites under different environmental conditions. The corresponding structural

276

parameters are displayed in Table 3. As shown in Figure 5B, all of the FT features of

277

the uranium-containing samples at ~1.4 Å can be satisfactorily fitted by axial oxygen

278

(U-Oax with CN = 2.0 at ~1. 78 Å),39, 54, 59 whereas the second FT feature at 1.9 Å can

279

be satisfactorily fitted by two sub-shells of equatorial oxygen (U-Oeq1 with CN = 3 at

280

~2.3 Å and U-Oeq2 shell with CN = 2 at ~2.5 Å).60, 61 For uraninite (U(IV)O2), data for

281

the two shells (i.e., U-O shell with CN = 8.0 at ~2.35 Å and U-U shell with CN = 12.0

282

at ~3.86 Å) were derived from a previous study.62 After reaction at pH 3.0 for 1 day,

283

the second FT feature can be fitted by ~0.6 carbon (U-C shell at 2.91 Å),63-65 whereas

284

the U-U shell with CN = 2.0 at ~4.36 Å was detected for the samples reacted at pH

285

3.0 for 7 days.66,

286

predominated the U(VI) uptake at pH 3 under short-term conditions, whereas the

67

These findings revealed that inner-sphere complexation



287

adsorbed U(VI) was gradually co-precipitated and/or was reduced with increasing

288

reaction times. It is noted that the U-U shell of the sample reacted at pH 6.0 for 7 days

289

showed significantly reduced bond distances (at 4.04 Å) compared to the sample

290

reacted at pH 3.0 for 7 days (at 4.36 Å), whereas the CN of the samples reacted at pH

291

6.0 for 7 days (CN = 7.5) were remarkably larger than that of samples reacted at pH

292

3.0 for 7 days (CN = 2.0). The fitting results of the samples reacted at pH 6.0 for 7.0

293

days were very similar to those of the U-U shell of uraninite (U(IV)O2), indicating the

294

adsorbed U(VI) was gradually reduced to U(IV) with an increase of reaction time and

295

pH. The EXAFS analysis indicated inner-sphere complexation dominated the U(VI)

296

uptake by the nZVI/C composites at low pH, whereas the adsorbed U(VI) was

297

gradually reduced to U(IV) with an increase of pH and reaction time, which is

298

consistent with the batch sorption experiments.

299

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

300

modeling of U(VI) uptake as functions of different pH values and initial U(VI)

301

concentrations, respectively. The batch uptake experiments showed that U(VI) uptake

302

on the nZVI/C composites was independent of ionic strength, and therefore, cation

303

exchange was not considered in this study. As shown in Figure 6A, the double layer

304

model gave better fits for U(VI) uptake on the nZVI/C composites at pH < 4.0,

305

whereas slight underestimations and overestimations of the fitted results were

306

observed at pH 5.0-8.0 and pH > 8.0, respectively. The main uptake species were

307

SsUO2+ and SsUO2(OH)2- at pH < 6.5 and pH > 6.5, respectively. A small amount of

308

SwUO2+ and SwUO2(OH)2- was observed at pH 2.0-7.0, which was further


Page 14 of 31

Page 15 of 31


309

demonstrated by the uptake isotherms at pH 4.0. As shown in Figure 6B,

310

approximately 80 and 20 % of U(VI) was bound at the strong (SsUO2+) and weak

311

(SwUO2+) sites of the nZVI/C composites at pH 4.0, respectively. The results

312

suggested that the uptake of U(VI) was dominated by the numerous strong sites,

313

which should be due to the strong redox of U(VI) to U(IV) by nZVI and high

314

effective surface complexation of U(VI) by various functional groups in the nZVI/C

315

composites. This study gives insight into the further development of nZVI/C

316

composites obtained by a simple method for use as highly effective adsorbents in

317

environmental cleanup.

318

ASSOCIATED CONTENT

319

Supporting Information

320

Additional characterization such as thermogravimetric analysis and TEM images,

321

calculation of the uptake kinetics, the solubility product, radionuclides in the aqueous

322

solutions, the Langmuir and Freundlich models, and XPS and EXAFS analyses. This

323

material is available free of charge via the Internet at http: //pubs.acs.org.

324

AUTHOR INFORMATION

325

Corresponding Authors: *Phone: 86-551-65593308. Fax: 86-551-65591310. E-mail:

326

[email protected] (Y. Sun).

327

Notes

328

The authors declare no competing financial interest.

329

ACKNOWLEDGMENTS

330

Financial support from the Natural Science Foundation of China (No. 41402030,



331

41572029), the Fundamental Research Funds for the Central Universities

332

(JZ2017HGTB0196), the Jiangsu Provincial Key Laboratory of Radiation Medicine

333

and Protection and the Priority Academic Program Development of the Jiangsu

334

Higher Education Institutions is acknowledged.

335

REFERENCES

336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369

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structural model for ferrihydrite II: Adsorption of uranyl and carbonate. Geochim. Cosmochim. Acta 2009, 73, 4437-4451. (13) Sun, Y. B.; Yang, S. T.; Sheng, G. D.; Wang, Q.; Guo, Z. Q.; Wang, X. K. Removal of U(VI) from aqueous solutions by the nano-iron oxyhydroxides. Radiochim. Acta 2012, 100, 779-784. (14) Yu, S.; Wang, J.; Song, S.; Sun, K.; Li, J.; Wang, X.; Chen, Z.; Wang, X. One-pot synthesis of graphene oxide and Ni-Al layered double hydroxides nanocomposites for the efficient removal of U(VI) from wastewater. Sci. Chin. Chem. 2017, 60, 415-422. (15)Troyer, L. D.; Maillot, F.; Wang, Z. M.; Mehta, V. S.; Giammar, D. E.; Catalano, J. G. Effect of phosphate on U(VI) sorption to montmorillonite: Ternary complexation and precipitation barriers. Geochim. Cosmochim. Acta 2016, 175, 86-99. (16) Kowal-Fouchard, A.; Drot, R.; Simoni, E.; Ehrhardt, J. J. Use of spectroscopic techniques for uranium (VI)/montmorillonite interaction modeling. Environ. Sci. Technol. 2004, 38, 1399-1407. (17) Brown, G. E., Jr; Gelabert, A.; Wang, Y.; Cismasu, C.; Ha, J.; Ona-Nguema, G.; Benzerara, K.; Morin, G.; Yang, Y.; Juillot, F.; Guyot, F.; Calas, G.; Yoon, T. H.; Bargar, J. R.; Eng, P. J.; Templeton, A. S.; Trainor, T. P.; Spormann, A. M. Synchrotron X-ray studies of bacteria-mineral-metal ion interactions. Geochim. Cosmochim. Acta 2008, 72, A116-A116. (18) Sun, Y.; Yang, S.; Sheng, G.; Guo, Z.; Tan, X.; Xu, J.; Wang, X. Comparison of U(VI) removal from contaminated groundwater by nanoporous alumina and non-nanoporous alumina. Sep. Purif. Technol. 2011, 83, 196-203. (19) Sun, Y.; Wang, X.; Song, W.; Lu, S.; Chen, C.; Wang, X. Mechanistic insights into the decontamination of Th(iv) on graphene oxide-based composites by EXAFS and modeling techniques. Environ. Sci.: Nano 2017, 4, 222-232. (20) Sun, Y.; Wang, Q.; Chen, C.; Tan, X.; Wang, X. Interaction between Eu(III) and graphene oxide nanosheets investigated by batch and extended X-ray absorption fine structure spectroscopy and by modeling techniques. Environ. Sci. Technol. 2012, 46, 6020-6027. (21) Ding, C.; Cheng, W.; Wang, X.; Wu, Z. Y.; Sun, Y.; Chen, C.; Wang, X.; Yu, S. H. Competitive sorption of Pb(II), Cu(II) and Ni(II) on carbonaceous nanofibers: A spectroscopic and modeling approach. J. Hazard. Mater. 2016, 313, 253-261. (22) Wang, X.; Yang, S.; Shi, W.; Li, J.; Hayat, T.; Wang, X. Different interaction mechanisms of Eu(III) and Am-243(III) with carbon nanotubes studied by batch, spectroscopy technique and theoretical calculation. Environ. Sci. Technol. 2015, 49, 11721-11728. (23) Zhang, Z.; Liu, J.; Cao, X.; Luo, X.; Hua, R.; Liu, Y.; Yu, X.; He, L.; Liu, Y. Comparison of U(VI) adsorption onto nanoscale zero-valent iron and red soil in the presence of U(VI)-CO3/Ca-U(VI)-CO3 complexes. J. Hazard. Mater. 2015, 300, 633-642. (24) Tsarev, S.; Collins, R. N.; Fahy, A.; Waite, T. D. Reduced uranium phases produced from anaerobic reaction with nanoscale zerovalent iron. Environ. Sci. Technol. 2016, 50, 2595-2601. (25) Sheng, G.; Yang, P.; Tang, Y.; Hu, Q.; Li, H.; Ren, X.; Hu, B.; Wang, X.; Huang,



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544


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Page 21 of 31


545

Figure Captions

546

Figure 1. Characterization of nZVI/C composites. A: SEM image; B: XRD patterns;

547

C: FT-IR spectra; D: XPS spectra.

548

Figure 2. A: Uptake kinetics of U(VI) on nZVI/C composites, CU(VI) = 10 mg/L, I =

549

0.01 mol/L NaCl, T = 298 K, pH = 4.0. B: Effect of ionic strength on U(VI) uptake on

550

nZVI/C composite, CU(VI) = 10 mg/L, m/V = 0.2 g/L, T = 298 K, pH = 4.0.

551

Figure 3. Uptake isotherms (A) and recycling uptake (B) of U(VI) on nZVI/C

552

composites, CU(VI) = 10 mg/L, m/V = 0.2 g/L, I = 0.01 mol/L NaCl, pH = 4.0.

553

Figure 4. XPS spectra of nZVI/C composites before and after uptake of U(VI) at

554

different pH (3.0 and 6.0), A: U 4f; B: Fe 2p, CU(VI) = 10 mg/L, m/V = 0.2 g/L, I =

555

0.01mol/L NaCl, reaction time = 7 days.

556

Figure 5. Uranium L3-edge XANES (A) and EXAFS (B) spectra of uranium-

557

containing nZVI/C composites, CU(VI) = 10 mg/L, m/V = 0.2 g/L, I = 0.01 mol/L

558

NaCl.

559

Figure 6. Surface complexation modeling of pH-edge (A) and uptake isotherms (B)

560

for U(VI) uptake on nZVI/C composites, m/V = 0.2 g/L, I = 0.01 mol/L NaCl, dot:

561

experimental data; Lines: fitted data.



Page 22 of 31

Table 1.The selective parameters of nZVI/C composites

562

SBET (m2/g)

Pore diameter (nm)

Pore volume (cc/g)

pHpzc

46.3

13

0.15

7.27

σs (emu/g)

χ (10 m3/kg)

57.4

296.35

-6

563 564

Table 2. Comparison of uptake capacity of nZVI/C composites with other

565

absorbents.

Adsorbent sample

Solution conditions

qmax(mg/g)

Refs.

montmorillonite@carbon

T = 298K, pH =3.95

20.8

[48]

Magnetic graphene/iron oxides

T = 293 K, pH = 5.5

69.5

[49]

Fabrication of fungus/attapulgite

T = 303K, pH = 4.0

125.0

[50]

Graphene oxides

T = 293 K, pH = 4.0

151.5

[51]

T = 298 K, pH = 5.0

215.2

[52]

Fungus-Fe3O4 bio-nanocomposites

T = 303 K, pH = 5.0

223.9

[53]

Graphene oxide supported

T = 298 K, pH = 3.0

245.1

[40]

nZVI/C composite

T = 328K, pH = 4.0

186.9

This work

Nano-Fe

0

566


Page 23 of 31


567

Table 3 Structural parameters of uranium L3-edge EXAFS spectra for standards

568

and U(VI)-containing samples Samples

σ2(Å 2)c

1.78 2.35 2.54

2.0 3.1 2.2

0.0030 0.0064 0.0089

U(IV)O2(s)d

U-Oax U-U U-Oax U-Oeq U-C U-Odis

2.35 3.86 1.77 2.46 2.94 2.49 4.32 1.78 2.32 2.54 2.98 1.78 2.32 2.54

8.0 12.0 2.0 6.0 2.0 8.0 2.0 2.0 3.1 2.5 0.61 2.0 3.2 2.2

0.0046 0.0029 0.0024 0.0105 0.0024 0.0097 0.0061 0.0033 0.0061 0.0078 0.0105 0.0033 0.0087 0.0114

U-C U-U U-Oax U-Oeq1 U-Oeq2 U-C

2.96 4.36 1.78 2.31

1.9 2.0 2.0 3.2

0.0264 0.0357 0.0033 0.0065

2.55 2.95

2.3 1.6

0.0068 0.0123

U-U

4.05

7.5

0.0257

nZVI/C pH 3.0 7 days

nZVI/C pH 6.0 7 days

570

CNb

U-Oax U-Oeq1 U-Oeq2

nZVI/C pH 3.0 1 day

a

R(Å)a

U(VI)O22+

UO2CO3e

569

Shells

U-U1 U-Oax U-Oeq1 U-Oeq2 U-C U-Oax U-Oeq1 U-Oeq2

R: bond distance; b CN: coordination number; cσ2: Debye-Waller factor; d data from Schofield et al. (2008)74;

e

data from Catalano et al. (2005)77.



Page 24 of 31

571

Table 4. The optimized parameters of surface complexation modeling of U(VI)

572

uptake on nZVI/C composites Reaction types

Reaction equations

log K

Protonation

SOH + H+ =SOH2+

5.9

Deprotonation

SOH = SO- + H+

-6.3

Surface

SwOH + UO22+ = SwUO2+ + H+

1.1

complexation

SwOH + UO22+ +2H2O = SwUO2(OH)2- + 3H+

-11.8

SsOH + UO22+ = SsUO2+ + H+

4.3

SsOH + UO22+ +2H2O = SsUO2(OH)2- + 3H+

-9.7

573


Page 25 of 31


574 M

(B)

(A)

M

I M

pH6 7day

pH3 7day

pH3 1day

nZVI/C

20

575

(C)

30 40 50 2-theta (degree)

60 pH6 7day

(D) C 1s O 1s

pH6 7day

I

U 4f

Fe 2p

pH3 7day

pH3 7day

1701 1565 1429

nZVI/C

nZVI/C

1048

2349

2894 576 577 578 579

578

3629

pH3 1day

pH3 1day

3600 3000 2400 1800-1 1200 600 Wavenumber (cm )

300 400 500 600 700 800 900 Binding Energy (eV)

Figure 1



Page 26 of 31

580 100 B

A

35

Qe(mg/g)

0.2g/L 1 g/L

25 20 15

581 582

60 40 0.01 M NaCl 0.1 M NaCl 1 M NaCl

20

10 5

U sorption(%)

80

30

0 0

10

20 30 Time(h)

40

2

4

Figure 2


6

pH

8

10

12

Page 27 of 31


583

160

298K 313K 328K

140 120

A

Qe(mg/g)

Qe(mg/g)

B

80

100 80 60 40

60 40 20

20 0 0 584 585

100

10

20

30 40 Ce(mg/L)

50

60

0

1

2

Figure 3


3 4 Cycle times

5


Page 28 of 31

586 (B)

(A)

Fe(III)

Relative Intensity (c/s)

NZVI/C U(VI)

Fe 2p

Fe(0) nZVI/C

pH3,7day Fe(II)

U(IV) Fe(0) pH3 7days

pH6,7day

pH6 7days

396

587 588

393

390 387 384 381 Binding Energy (eV)

378

708

712 716 720 724 728 Binding Energy (eV)

Figure 4

589 590 591 592


732

Page 29 of 31


(B)

q Oe U-

UOa x

(A)

U-U

(IV)

U

O2(s)

(VI)

U FT(χ(k2)

U-C

pH3.0 1day pH3.0 7days

U(IV) U(VI) pH3.0 1day pH3.0 7days pH6.0 7days 17170

593 594

17175

17180 17185 Energy (eV)

17190

17195

2+

O2

pH6.0 7days 0

1

o

2

R(A)

Figure 5


3

4

5


100 (B)

(A)

90

Page 30 of 31

80 +

+

60

SsOUO2

60

50

Qe (mg/g)

Amount of U(VI) (%)

80

SsOUO2

70

-

SsOUO2(OH)2

40

40

30

+

-

SwOUO2(OH)2

+

20

SwOUO2

SwOUO2

20

10

-

0 2

595 596

SwOUO2(OH)2

0 3

4

5

6

pH

7

8

9

10

11

0

10

20

Figure 6


-

SsOUO2(OH)2

30 40 Ce (mg/g)

50

60

Page 31 of 31


TOC


C Composites as an Efficient Adsorbent for the

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