Carbon Nanofiber Hybrids

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Plasma-Facilitated Synthesis of Amidoxime/Carbon Nanofiber Hybrids for Effective Enrichment of 238U(VI) and 241Am(III) Yubing Sun, Songhua Lu, Xiangxue Wang, Chao Xu, Jia-Xing Li, Changlun Chen, Jing Chen, Tasawar Hayat, Ahmed Alsaedi, Njud S. Alharbi, and Xiangke Wang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b02745 • Publication Date (Web): 10 Oct 2017 Downloaded from http://pubs.acs.org on October 10, 2017

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Environmental Science & Technology

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Plasma-Facilitated Synthesis of Amidoxime/Carbon Nanofiber Hybrids for

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Effective Enrichment of 238U(VI) and 241Am(III)

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Yubing Sun†,‡, Songhua Lu†, Xiangxue Wang‡, Chao Xu , Jiaxing Li†, Changlun

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Chen†, Jing Chen＆, Tasawar Hayat⊥, ∥, Ahmed Alsaedi∥,Njud S. Alharbi∥, Xiangke

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Wang‡, §*

＆

6

†

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230031, P.R. China

8

‡

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University, Beijing 102206, P.R. China

Institute of Plasma Physics, Chinese Academy of Science, P.O. Box 1126, Hefei,

College of Environmental Science and Engineering, North China Electric Power

10

＆

11

of Nuclear and New Energy Technology, Tsinghua University, Beijing, 100084, P.R.

12

China

13

§

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Institutions, School for Radiological and Interdisciplinary Sciences, Soochow

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University, Suzhou 215123, P.R. China

16

∥

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

17

⊥

Department of Mathematics, Quaid-I-Azam University, Islamabad 44000, Pakistan

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Collaborative Innovation Center of Advanced Nuclear Energy Technology, Institute

Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher Education

* Corresponding Author: E-mail: [email protected] (X. Wang); Phone:

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+86-10-61772890; fax: +86-10-61772890

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ABSTRCT. Plasma- and chemical-grafted amidoxime/carbon nanofiber hybrids

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(p-AO/CNFs and c-AO/CNFs) were utilized to remove

238

U(VI) and 241Am(III) from

ACS Paragon Plus Environment


22

aqueous solutions, seawater and groundwater. Characteristic results indicated the

23

more nitrogen-containing groups of p-AO/CNFs compared to c-AO/CNFs. The

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maximum adsorption capacities of p-AO/CNFs at pH 3.5 and T = 293 K (588.24 mg

25

238

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were significantly higher than those of c-AO/CNFs (263.18 and 22.77 mg/g for

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238

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highly effective, low-cost and environmentally friendly method. Adsorption of

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U(VI) on AO/CNFs from aqueous solutions was significantly higher than that of

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U(VI) adsorption from seawater and groundwater, moreover AO/CNFs displayed

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the highest effective selectivity for

32

Adsorption of

33

shell) by EXAFS analysis, which was supported by surface complexation modeling.

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Three inner-sphere complexes gave excellent fits to pH-edge and isothermal

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adsorption of

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utilization of plasma-grafted, AO-based composites to the preconcentration and

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immobilization of lanthanides and actinides in environmental remediation.

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INTRODUCTION

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The contamination of uranium has been as a worldwide concern due to excessive

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uranium mining and milling operations around the world.1 Over last several decades,

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many investigations regarding the removal of radionuclides with various adsorbents

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have been extensively investigated, including clay minerals,2-5 metal (hydr)oxides6-9

U(VI) and 40.79 mg

U(VI) and

241

241

Am(III) per gram from aqueous solutions, respectively)

Am(III), respectively), which indicated that plasma-grafting was a

238

238

238

U(VI) compared to the other radionuclides.

U(VI) onto AO/CNFs created inner-sphere complexes (e.g., U-C

U(VI) on the AO/CNFs. These observations are crucial for the


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and carbon-based nanoparticles.10-13 Although substantial progress has been achieved

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in several decades, the removal of U(VI) at low pH and under trace concentrations is

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still a challenge due to their limited adsorption capacities. Additionally, americium

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(241Am(III)) is a radiotoxic nuclear wastes that can be discharged towards the

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environments during the processing, storage and disposal of nuclear wastes. In our

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previous studies, the removal of

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investigated by various techniques.14-16 However, few studies regarding the removal

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of

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important to develop new materials to remove uranium and americium from aqueous

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solutions at extreme conditions with the faster adsorption rates, higher selectivities

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and larger adsorption capacities.

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Owing to its environmentally friendly, accessible raw materials and its low cost,

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carbon nanofiber (CNF) is an emerging class of carbon-based nanoparticles that have

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recently seen promising use as an adsorbent to remove a variety of organic19-24 and

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inorganic pollutants.25-28 For example, Sun et al. observed that CNF displayed high

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adsorption capacities for radionuclides such as 125 mg U(VI)/g and 91 mg Eu(III)/g

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at pH 4.5.29 Similar results have been obtained for heavy metals, where it was

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reported the high adsorption capacities of CNF for heavy metals such as 423.7 mg

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Pb(II)/g and 221.3 mg Cr(VI)/g.30 However, these CNFs displayed poor selectivities

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towards certain radionuclides of interest. Therefore, further investigation in improving

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the selectivity of CNF to uranium is of great importance. Recently, numerous studies

241

241

Am(III) with carbon nanotubes was extensively

Am(III) with other carbon-based materials have been published.17, 18 Thus, it is



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have focused on amidoxime-based composites as promising candidates for U(VI)

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sorption due to their high capacities and affinities in chelating uranyl ions.31-35 Zhang

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et al. used EXAFS and theoretical calculations to investigate the extraction

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mechanism of uranium amidoxime ligands with an η2-binding mode, but limited

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information regarding the experimental data has been reported.36 To our knowledge,

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the interaction mechanism of efficient, selective enrichment of U(VI) on amidoxime

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composites has not yet been explored under different experimental conditions.

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Poly(amidoxime) can be easily grafted onto carbon-based materials through a

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plasma-induced grafting method. This plasma method can produce chemically active

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species to change the properties of an adsorbent’s surface.37-40 The objectives of this

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study were to (1) characterize the nanostructure of plasma- and chemical-grafted

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AO/CNFs (called p-AO/CNFs and c-AO/CNFs, respectively) by SEM, FT-IR and

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XPS techniques, (2) elucidate the effect of water chemistry on 238U(VI) and 241Am(III)

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adsorption by two kinds of AO/CNFs using batch techniques, and (3) investigate the

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interaction mechanism between two kinds of AO/CNFs with

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EXAFS and surface complexation modeling. This study will provide insights into the

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preparation of AO/CNFs with high adsorption performances and high selectivities for

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radionuclides using a plasma-grafting method, which is more efficient, less expensive

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and environmentally friendlier than the chemical method.

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EXPERIMENTAL DETAILS

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Materials. Bacterial cellulose was obtained as a gift from Hainan Yeguo Food


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U(VI) using XPS,

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Company Limited. The reagents used in this study (dimethylformamide-DMF,

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acrylonitrile-AN, hydroxylamine-NH2OH.HCl, Na2CO3) were of analytical grade and

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purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai). The

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stock solutions (60 mg/L) were prepared by dissolving uranyl nitrate (> 99.9 % purity,

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Sigma-Aldrich) into a 4 % HNO3 solution under anaerobic conditions (volume ratio

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of H2/N2= 5/95, O2 concentration < 0.5 ppm). The 241Am(III) standard solutions were

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purchased from Isotope Product Laboratories (IPL, Burbank, USA).

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Synthesis of AO/CNFs. p-AO/CNFs and c-AO/CNFs were synthesized through the

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methods of plasma-discharge and chemical polymerization, respectively. Firstly, the

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CNF was obtained by the pyrolysis of bacterial cellulose at 170 °C for 3 h under N2.41

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Then, 1.0 g of CNF was added to a conical flask, which was then sealed by silicone

96

caulk. The N2 plasma method conditions were 70 W, 10 Pa, 60 Ma and 650 V.38 After

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the N2 plasma method, the conical flask was heated to 60 °C and 100 mL of a DMF

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solution containing 50 mL of AN was added under continuous stirring, which was

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continued for 8 h. Then, hybrids were repeatedly washed with DI water until

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propylene could not be detected by high-performance liquid chromatography. Finally,

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the aforementioned hybrids were reacted with NH2OH.HCl to convert the AN group

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to an amidoxime (AO) group.36,

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hybrids and 2.5 g of NH2OH.HCl were added into 150 mL of CH3OH/H2O (v/v=

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50/50) and stirred for 12 h at 60 °C. Solution pH was adjusted to 7.0 by adding

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saturated Na2CO3 solution. Then, the products were washed by DI water (60 mL) and

42

238

U(VI)

Briefly, the procedure is as follows: 2.0 g of



106

methanol (60 mL) three times each. The p-AO/CNFs were obtained by drying them in

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a vacuum oven at 60 °C overnight. The c-AO/CNFs were also used to compare their

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sorption performance. Briefly, 1.0 g of CNF and 100 mL of a DMF solution

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containing 50 mL of AN were added to a conical flask, and the suspension was

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reacted under continuous stirring at 60 °C for 8 h. Suspension pH was adjusted to 7.0,

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and 2.5 g of NH2OH.HCl was added. Then, the mixture was washed by DI water (60

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mL) and methanol (60 mL) three times each and finally dried in a vacuum oven at

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60 °C for 12 h. The contents of amidoxime groups of p- and c-AO/CNFs determined

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by HCl adsorption method were approximately 33 and 26 mmol/g, respectively,

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indicating that the plasma-grafting was the more effective, inexpensive and

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environmentally friendly method. More details on the determination of the grafting

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yield are provided in Supporting Information (SI).

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Characterization. The nanostructure and morphology of the AO/CNFs were

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illustrated by SEM (S-4800, Hitachi, Japan). The SEM samples were prepared as

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follows: 10.0 mg of the samples was dispersed into 5.0 mL of ethanol in the ultrasonic

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conditions inside the golvebox, and then suspensions were added on a copper grid,

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and then transported to the microscope after dried on a glass slide. FT-IR and XPS

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spectra were obtained on a PerkinElmer IR-843 spectrometer and an Axis-Ultra

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(Kratos, UK) with an Al Kα line at 10-8 Torr, respectively. Samples for XPS analysis

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were calibrated by the C 1s sp2 spectra at 284.6 eV. The elemental contents of the

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samples were obtained from an elemental analyzer (Carlo-Erba 1106, Italy). The ζ


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potential of the AO/CNFs was determined by a Malvern Zetasizer (632.8 nm, He-Ne

128

laser).

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Batch Sorption Protocols. All sorption experiments were conducted in triplicate in

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an anaerobic chamber with a background electrolyte of 0.01 mol/L NaCl at 293 K.

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Briefly, an uniform suspension of AO/CNFs (1.2 g/L) was firstly obtained by mixing

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0.6 g AO/CNFs solid and 500 mL DI water under ultrasonic conditions, and then 3

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mL AO/CNFs suspension and 3 mL of

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solution (background electrolyte) was added into 10 mL polycarbonate tubes to obtain

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the desired solid to liquid ratio (m/v) of 0.6 g/L. The adsorption isotherms of

136

241

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compare the sorption performance of AO/CNFs. The compositions of seawater and

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groundwater were summarized in Table S1. Suspension pH of each tube was adjusted

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by a negligible volume of NaOH or HCl solution (0.1-1.0 mol/L). The regeneration of

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the sorbent was conducted in five consecutive cycles to evaluate its stability and

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reusability. In each cycle, the sorbent was eluted by a 0.1 mol/L HCl solution and then

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washed with DI water three times for reuse. The suspensions were stirred under

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reciprocal shaker 24 h and the each tube was centrifuged at 7400 g for 15 min. The

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concentrations of

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inductively coupled plasma-mass spectrometry (ICP-MS, 7700x Agilent Technologies)

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and liquid-scintillation-counting (Liquid Scintillation Analyzer, Packard 3100 TR/AB,

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PerkinElmer) with the ULTIMA GOLD AB scintillation cocktail, respectively. The

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U(VI)/241Am(III) solution with the NaCl

Am(III)/238U(VI) from spiked seawater and groundwater were also conducted to

238

U(VI) and

241

Am(III) in the supernatants were measured by



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sorption amounts were calculated by the difference in the concentrations before and

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after the sorption experiments.

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EXAFS Analysis. The U LIII-edge EXAFS spectra were taken on the XAS beamline,

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with a double-crystal Si(111) monochromator, at the Shanghai Synchrotron Radiation

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Facility (SSRF). EXAFS spectra were obtained in the fluorescence model with a

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64-element Ge detector. The ATHENA software package was used to normalize the

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merged spectra,43 and ARTEMIS was employed to fit the EXAFS data based on the

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theoretical phase and amplitude function from the crystallographic data of uraninite.44

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However, the samples of

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by EXAFS analysis due to fluorescence quenching effects.14, 16

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RESULTS AND DISCUSSION

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Characterization. The morphologies of the two kinds of AO/CNFs were investigated

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by SEM. As shown in Figure 1A and 1B, the two kinds of AO/CNFs had the rough

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and tidy nanofibers. No significant difference between the morphologies of the two

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kinds of AO/CNFs was observed, indicating that plasma technique is a reliable and

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effective approach for CNF functionalization. As shown by the FT-IR spectra in

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Figure 1C, the characteristic bands at ~3400, 1655, 1500, 1290 and 935 cm-1 were

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attributed to the stretching vibrations of O-H/N-H, C=C, C=N, C-N, and N-O groups,

166

respectively.42 Additionally, the relative intensity of the N-O groups in the

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p-AO/CNFs was remarkably higher than c-AO/CNFs, indicating that more

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amidoxime groups were grafted onto the surface of the CNFs by the plasma method.

241

Am(III) on AO/CNF composites cannot be investigated


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As shown by the high-resolution N 1s XPS spectra in Figure 1D, the binding energies

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of N 1s in p-AO/CNFs were significantly shifted higher compared to those of

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c-AO/CNFs. The fitted peaks of the XPS spectra for the two kinds of AO/CNFs can

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be deconvoluted into pyridinic (398.5 eV), pyrrolic (399.9 eV), graphitic (401.3 eV)

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and N-oxide (402.9 eV) moieties by the N-O, C=N and C-N groups,42 which further

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indicated that amidoxime groups had been satisfactorilly grafted onto the CNFs

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surface. Additionally, the relative peak area of the N-O in p-AO/CNFs was

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significantly higher than that in c-AO/CNFs, which agrees with the FT-IR results. As

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shown by the thermal gravimetric analysis (TGA) in Figure S1A, the degradation

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temperatures for the two kinds of AO/CNFs occurred at ~ 500 °C, whereas the

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maximum weight loss was observed at ~ 680 °C, indicating that AO/CNFs had a

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strong thermal stability. The significant decrease in the weight at ~ 550-650 °C could

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be due to the pyrolysis of functional groups (i.e., carboxyl, hydroxyl and amidoxime

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groups) in AO/CNFs.45 The components of the two AO/CNFs, as obtained by XPS

183

and elemental analysis, are shown in Table S2. The relative amount of N (6.57 % as

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determined by XPS analysis) for p-AO/CNFs was significantly higher than that of

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c-AO/CNFs (4.26 %), as summarized in Table S2. These results confirmed that the

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AO/CNFs had much more oxygen- and nitrogen-bearing functional groups and had

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good thermal stabilities.46 Additionally, more nitrogen- containing functional groups

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were grafted onto the surface of the p-AO/CNFs compared to the c-AO/CNFs.

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Effect of Reaction Time. Figure 2A shows the adsorption of 238U(VI) and 241Am(III)



238

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U(VI) and

241

190

onto the two kinds of AO/CNFs at different reaction time.

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adsorption onto both of AO/CNFs dramatically enhanced with increasing reaction

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time from 0 to 1 h, and remained the high level adsorption at reaction times of more

193

than 3 h. As shown in the inset of Figure 2A, the adsorption rates of

194

241

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kinetics data were simulated to pseudo first order and pseudo second order kinetic

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models. More details concerning this fitting are provided in the SI. As shown in

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Figure S2 and Table S3 in the SI, the adsorption kinetics of

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on the two kinds of AO/CNFs can be well simulated by a pseudo second order kinetic

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model (correlation coefficients (R2) >0.999) compared to pseudo first order kinetic

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mode (R2 < 0.91), which was in accordance with previous studies. 47-49

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Effect of pH and Ionic Strength. Figure 2B shows the effect of pH on 238U(VI) and

202

241

203

AO/CNFs observably increased at pH 1.0- 5.0, and then kept the high adsorption at

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pH 5.0-6.5, while the adsorption of

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enhanced adsorption of 241Am(III) onto the two kinds of AO/CNFs was also observed

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at pH 3.0-5.0.

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markedly higher than c-AO/CNFs. As shown by the zeta potentials in Figure S1B,

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two kinds of AO/CNFs had a negative charge at pH > 2.5. The distribution and

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corresponding equilibrated parameters of

210

aqueous solutions is shown in Figure S3 and Table S4, respectively. The main U(VI)

238

Am(III)

U(VI) and

Am(III) on p-AO/CNFs were significantly higher than those of c-AO/CNFs. The

Am(III) adsorption onto the two kinds of AO/CNFs.

238

U(VI) and

241

238

238

238

U(VI) and

241

Am(III)

U(VI) adsorption on

U(VI) was slightly decreased at pH > 6.5. The

Am(III) adsorption on p-AO/CNFs at pH 2.0-4.0 was

238

U(VI) and

241


Am(III) species in the

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species was UO22+ and various positive U(VI) speciation (e.g., UO2OH+,

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(UO2)4(OH)7+ and (UO2)3(OH)5+) were observed at pH < 5.0 and pH 5.0-8.0,

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respectively. However, the negative carbonato-uranyl complexes were observed at

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pH > 8.0. The surface co-precipitate (i.e., schoepite) was observed at pH 6.0 -8.0

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(Figure S3A). As shown in Figure S3B, the main

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AmOH2+, Am(OH)2+ and Am(OH)3(aq) at pH < 5.5, 5.5 -7.5, 7.5 -11.5 and > 11.5,

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respectively. Thus, the increased adsorption of 238U(VI) and 241Am(III) at pH 2.0 -5.0

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was due to the electrostatic-attraction of positively charged UO22+/Am3+ species and

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negative charge of AO/CNFs. The high adsorption of

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attributed to the inner-sphere surface complexation/surface precipitate, while the

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adsorption of 238U(VI) was decreased at pH > 7.5 due to the electrostatic-repulsion of

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the negative charge AO/CNFs and the negative carbonato uranyl species such as

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UO2(CO3)34- and UO2(CO3)22-.

224

Figure S4A and S4B show

225

different ionic strengths, respectively. Outer-sphere surface complexation was

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influenced by ionic strength, while no effect of ionic strength on inner-sphere surface

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complexation was demonstrated.50-52 Ionic strength had no effect on

228

adsorption, suggesting that the

229

was an inner sphere surface-complexation. It is noted that the adsorption of

230

and

231

to the occurrence of more available reactive sites such as nitrogen- and

241

238

241

Am(III) species were Am3+,

238

U(VI) at pH 5.0 -7.0 was

U(VI) adsorption onto the c- and p-AO/CNFs at

238

238

U(VI)

U(VI) adsorption onto the two kinds of AO/CNFs 238

U(VI)

Am(III) on p-AO/CNFs was significantly higher than that of c-AO/CNFs due



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oxygen-containing function groups.

233

Adsorption Isotherms. Figure 3A shows the isothermal adsorption of 238U(VI) on c-

234

and p-AO/CNFs. Uranium loading concentration on p-AO/CNFs (e.g., 450 mg

235

uranium/g at equilibrated concentration of 2.5 mg/L) was remarkably higher than that

236

on c-AO/CNFs (170 mg uranium/g). As shown in Figure 3A and Figure S5A, the

237

sorption of U(VI) from seawater and groundwater was notably lower than that of

238

U(VI) sorption from aqueous solution due to strong competition between U(VI) and

239

other ions (such as Na+, Mg2+, Cl- and SO42- and HCO3-) for adsorption active sites.

240

The adsorption isotherms were simulated using the Langmuir and Freundlich models.

241

More details concerning the fitting of the Langmuir and Freundlich models are

242

described in the SI. As shown in Table S5, Langmuir model gave better fits for the

243

adsorption of

244

capacity of p-AO/CNFs at pH 3.5 and 293 K was 588.2 mg/g for

245

aqueous solutions, whereas the maximum adsorption capacities of p-AO/CNFs for

246

238

247

were 248.14 and 398.41 mg/g (Table S5), respectively. As shown in Table 2, the

248

adsorption capacity of p-AO/CNFs was significantly higher than those of other

249

AO-based composites such as AO/macroporous beads (3.5 mg/g),53 AO/mesoporous

250

carbon (4.6 mg/g),54 AO/mesoporous imprinted polymer (19.1 mg/g),55 AO/poly

251

(propylene) fibers (44 mg/g),56 AO/multiwalled carbon nanotubes (145 mg/g)57 and

252

AO/ polypropylene membranes (380.0 mg/g).58 These observations indicated that

238

U(VI) onto AO/CNFs (R2 > 0.995). The maximum adsorption 238

U(VI) from

U(VI) from seawater (at pH 8.1 and 293 K) and groundwater (at pH 7.8 and 293 K)


Page 13 of 34


253

AO/CNF composites displayed a high adsorption performance for 238U(VI).

254

Figure 3B shows the adsorption performance of U(VI) on p-AO/CNFs with the other

255

radionuclides and heavy metals. The maximum adsorption capacities of p-AO/CNFs

256

at pH 3.5 were 167.34, 135.87, 40.79, 57.57, 69.97, 67.64 and 42.61 mg/g for Th(IV),

257

Eu(III), Am(III), Sr(II), Ni(II), Co(II) and Cs(I), respectively. This demonstrated that

258

p-AO/CNFs showed a highly selective adsorption for U(VI) compared to the other

259

radionuclides and heavy metals, due to η2-binding mode.58 Figure S5B shows the

260

regeneration

261

adsorption-desorption cycles. The maximum adsorption capacities of the p- and

262

c-AO/CNFs decreased from 588.23 to 530.68 mg/g and from 263.15 to 233.69 mg/g,

263

respectively. This slight decrease in the maximum adsorption capacity of the two

264

kinds of AO/CNFs could be attributed to mass loss during the adsorption/desorption

265

process. These findings showed that AO/CNFs had good recoverability and

266

recyclability for the removal of radionuclides in environmental remediation.

267

XPS Analysis. Figure 4A shows the scans of the XPS spectra for the p-AO/CNFs

268

after U(VI) adsorption and desorption. Compared to the original AO/CNFs, the

269

relative intensities of O 1s and N 1s significantly decreased after U(VI) adsorption,

270

indicating that the oxygen- and nitrogen-containing functional groups were

271

responsible for the highly effective adsorption of U(VI). It should be noted that the

272

change in the relative intensity of N 1s after U(VI) desorption was significantly less

273

than that of O 1s. This demonstrated that the nitrogen-bearing groups displayed a

of

the

two

kinds

of

AO/CNFs


through

five-successive


274

stronger chemical affinity for U(VI) than the oxygen-bearing groups did. Figure 4B

275

and C shows the XPS spectra of O 1s and C 1s, respectively. XPS spectra of O 1s

276

were shifted to the lower energies after U(VI) adsorption (Figure 4B), whereas the

277

spectra were shifted to higher energies after U(VI) desorption, indicating that these

278

oxygen-bearing groups were responsible for the highly effective adsorption of U(VI).

279

The O 1s spectra of p-AO/CNFs could be deconvoluted into four sub-bands at a

280

binding energies of 530.6, 532.1, 533.1 and 534.2 eV, which were assigned to –OH,

281

C=O, C-O and adsorbed water, respectively. As shown in Figure 4C, the XPS spectra

282

of C 1s could be fitted with four peaks at 288.8, 286.4, 285.4 and 284.5eV, which

283

could be ascribed to the C=O, C-N/C-O, C=N and C-C species, respectively.54, 59, 60

284

Figure 4D shows the XPS detailed scan of p-AO/CNFs in the U 4f region after U(VI)

285

adsorption and desorption. The slight decrease in the relative intensities of U 4f after

286

desorption revealed that AO/CNFs displayed a strong chemical affinity for U(VI) at

287

low pH conditions.

288

EXAFS Analysis. EXAFS was applied to investigate the chemical species and

289

microstructure of U(VI) at the water-solid interface. Figure 5A and 5B shows the U

290

LIII-edge EXAFS spectra and the corresponding Fourier transform (FT), respectively,

291

of the two kinds of AO/CNFs. Table 3 summarized the various fitted constants such

292

as Debye-Waller factor (σ2), coordination number (CN) and bond length (R). As

293

shown in Figure 5A, no differences in the EXAFS spectra were observed between the

294

kinds of AO/CNFs. As shown in Figure 5B, the FT features at ~ 1.4 and 1.9 Å can be


Page 14 of 34

Page 15 of 34


295

fitted by a two axial oxygen shell (U-Oax at 1.8 Å) and a five or six equatorial oxygen

296

shell (U-Oeq at 1.8 Å), respectively.61 The bond lengths of U-Oeq shell on the two

297

kinds of AO/CNFs were obviously longer than

298

contribution of a longer U–N shell.36 For both AO/CNFs, it should be noted that the

299

weak peaks at 2.3 Å can be fitted by a 1.5 carbon shell (U-C) at ~ 2.9 Å, indicating

300

the inner-sphere surface complexation of U(VI) on AO/CNFs at pH 3.5. Thus,

301

AO/CNFs showed promise as adsorbents for the highly effective and selective

302

adsorption of U(VI) in nuclear waste management applications.

303

Surface Complexation Modeling. Figure 6A-D shows the surface complexation

304

modeling of

305

concentrations. The main U(VI) speciation in aqueous solutions at pH < 5, pH = 8-9

306

and pH > 9 were UO22+, (UO2)3(OH)7- and UO2(CO3)23- species, respectively (Figure

307

S3A), therefore three U(VI) species were employed to simulate the adsorption

308

behaviors. As shown in Figure 6A and 6C, the pH-edge adsorption indicated that the

309

adsorption of 238U(VI) onto the two kinds of AO/CNFs can be fitted by a double layer

310

model, within error. The main species were SOUO2+, SO(UO2)3(OH)72- and

311

SOUO2(CO3)23- species at pH < 4, pH 4 -7 and pH > 7.0, respectively. The different

312

equilibrium parameters (log K values, Table 4) were observed for the two kinds of

313

AO/CNFs due to their different surface properties arising from the different

314

pretreatment methods. These optimized parameters derived from the pH-edge

315

adsorption, were used to fit the isothermal adsorption of U(VI) on c- and p-AO/CNFs

238

those in UO22+ due to the

U(VI) adsorption onto AO/CNFs at different pH and initial


238

U(VI)


316

at pH 3.5. As shown in Figure 6B and 6D, the diffuse layer model (DLM) model gave

317

better fits to the isothermal adsorption of U(VI) on both kinds AO/CNFs. Sun et al.

318

also found that Eu(III) adsorption on graphene oxide can be well simulated with a

319

DLM using SOEu2+ and (SO)2Eu2(OH)22+ species.62 This surface complexation

320

modeling showed that U(VI) adsorption on c- and p-AO/CNFs was dominated by

321

inner-sphere surface complexation in a wide range of pH. These observations

322

indicated p-AO/CNFs as a potential adsorbent to selectively enrich uranium from

323

aqueous solutions, seawater and groundwater in the cleanup of environmental

324

pollution.

325

ASSOCIATED CONTENT

326

Supporting Information

327

Complementary work that is associated with this present study is provided and

328

includes: 1) images from TGA analysis and zeta potentials, 2) details on adsorption

329

kinetics models and the Langmuir and Freundlich models, and 3) details on the batch

330

adsorption and adsorption recyclability (PDF). This material is available free of

331

charge via the Internet at http://pubs.acs.org.

332

AUTHOR INFORMATION

333

Notes

334

The authors declare no competing financial interest.

335

ACKNOWLEDGEMENTS

336

Financial support from Science Challenge Project (TZ2016004), the Research Fund


Page 16 of 34

Page 17 of 34


337

Program of Guangdong Provincial Key Laboratory of Radionuclides Pollution

338

Control and Resources (GZDX2017K002), National Natural Science Foundation of

339

China (21577032, 2147713 and 21607156), National Key Research and Development

340

Program of China (2017YFA0207002)

341

of Radiation Medicine and Protection and the Priority Academic Program

342

Development of Jiangsu Higher Education Institutions are acknowledged.

343

REFERENCES

344

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Figure Captions

541

Figure 1. Characterization of two kinds of AO/CNFs, A and B: SEM images of p-

542

and c-AO/CNFs, respectively; C: FT-IR spectra; D: high-resolution N 1s XPS spectra.

543

Figure 2. Adsorption kinetics (A) and pH effect (B) of

544

adsorption onto two kinds of AO/CNFs, C0 = 10.0 mg/L, I = 0.01 mol/L NaCl, m/v=

545

0.6 g/L, pH = 3.5, T = 293 K.

546

Figure 3. A: Adsorption isotherms of U(VI) on two kinds of AO/CNFs, I = 0.01

547

mol/L NaCl, m/v= 0.6 g/L, pH = 3.5, T = 293 K; B: Comparison of adsorption of

548

U(VI) on p-AO/CNFs with Th(IV), Eu(III), Am(III), Sr(II), Co(II), Ni(II) and Cs(I), I

549

= 0.01 mol/L NaCl, m/v = 0.6 g/L, T = 293 K.

550

Figure 4. XPS analysis of adsorption and desorption of U(VI) on p-AO/CNFs, A:

551

survey scans; B-D: high resolution of O 1s, N 1s and U 4f after U(VI) adsorption and

552

desorption, respectively, C0 = 10.0 mg/L, I = 0.01 mol/L NaCl, m/v= 0.6 g/L, pH =

553

3.5, T = 293 K.

554

Figure 5. The k2-weighted U LIII-edge EXAFS spectra (A) and corresponding Fourier

555

Transmission (FT) (B) of references and U(VI) adsorption samples, C0 = 10 mg/L,

556

m/v = 0.6 g/L, I = 0.01 mol/L NaCl, T = 293 K.

557

Figure 6. Surface complexation modeling of 238U(VI) adsorption on c- (A and B) and

558

p-AO/CNFs (C and D), A and C: pH effect, B and D: adsorption isotherm, C0 = 10.0

559

mg/L, I = 0.01 mol/L NaCl, m/v= 0.6 g/L, T = 293 K.


238

U(VI) and

241

Am(III)


Page 26 of 34

(B)

(A)

(C)

C-N

N-O

p-AO/CNFs

C=N

c-AO/CNFs O-H/N-H(3400)

C-N(1290) C=N (1500) C=C(1655)

N-O(935)

(D)

p-AO/CNFs

c-AO/CNFs

600 900 1200 1500 1800 2100 2400 2700 3000 3300 3600

Wavenumber (cm-1)

398

399

400

401

402

Binding energy (eV)

Figure 1


403

Page 27 of 34


100 (B)

(A)

90

90

80

238

70

238

60

241

U(VI)+p-AO/CNFs U(VI)+c-AO/CNFs Am(III)+p-AO/CNFs

241

Am(III)+c-AO/CNFs

50 40 30 20 10

Amount of adsorbed radionuclides (%)

Amount of adsorbed radionuclides (%)

100

80 70 60 50 40 30

238

U(VI)+p-AO/CNFs

238

20

U(VI)+c-AO/CNFs

241

Am(III)+p-AO/CNFs

10

0

241

Am(III)+c-AO/CNFs

0

0

3

6

9

12

15

Reaction time (h)

18

21

24

1

2

3

4

Figure 2


5pH 6

7

8

9

10


(A)

Page 28 of 34

(B)

600

450 400

500 238

U(VI)+p-AO/CNFs(aqueous solution) U(VI)+c-AO/CNFs(aqueous solution) 238 U(VI)+p-AO/CNFs(seawater) 238 U(VI)+c-AO/CNFs(seawater)

Qe (mg/g)

Qe (mg/g)

350

238

300 250 200 150

400

300

200

100 100

50 0 0

1

2

3

4

5

6

0

Ce (mg/L)

U(VI) Th(IV) Eu(III) Am(III) Sr(II) Ni(II) Co(II) Cs(I)

Figure 3


Page 29 of 34


O 1s

(A)

(B)

C 1s

p-AO/CNFs_UD

U 4f N 1s

p-AO/CNFs_UD p-AO/CNFs_UA

p-AO/CNFs_UA p-AO/CNFs C-O

p-AO/CNFs

200

400

600

800

adsorbed H2O

C=O -OH

1000

530

531

532

533

534

535

(C)

(D) C-C C=N

C-N/C-O

p-AO/CNFs_UD C=O

p-AO/CNFs_UD

p-AO/CNFs_UA

p-AO/CNFs_UA 282

283

284

285

286

287

288

289

290

291

378

Binding energies (eV)

380

382

384

386

388

390

Binding energies (eV)

Figure 4


392

394

396


(B)

U -O ax

(A)

Page 30 of 34

U-C U-U

U-Oeq

UO2CO3(s)

UO2CO (s) 3

2

FT(Κ χ)

2

χ(κ)κ

p-AO/CNFs

p-AO/CNFs

c-AO/CNFs c-AO/CNFs

3

4

5

o -1

k(A)

6

7

8

0

1

2

Figure 5


30

R(A)

4

5

6


(B) 480

100 (A) 90 80 70 60 50 40 30 20 10 0 100 (C) 90 80 70 60 50 40 30 20 10 0

Qe (mg/g)

400 320

SO(UO2)3(OH)72-

240 160

SOUO2+

SOUO2+ SOUO2(CO3)23-

80

2SOUO2(CO3)23- SO(UO2)3(OH)7

0 (D) 480 400 SOUO2+

SO(UO2)3(OH)72-

320

Qe (mg/g)

Amount of adsorbed U(VI) (%)

Amount of adsorbed U(VI) (%)

Page 31 of 34

240 SOUO2+

160

SOUO2(CO3)23-

SO(UO2)3(OH)72-

80

SOUO2(CO3)232

3

4

5

6

7

8

0

1

2

pH

Figure 6


3 4 Ce (mg/L)

5

0 6


Page 32 of 34

Table 1. Selective Properties of Two Kinds of AO/CNFs Samples

SBET (m2/g)

Zeta potential

N content (at %)

c-AO/CNFs

154.5

0 eV at pH 2.01

4.26

p-AO/CNFs

139.7

0 eV at pH 2.45

6.57

Table 2. Comparison of Adsorption Capacity of U(VI) on Various AmidoximeContaining Adsorbents Sorbents

Capacity (mg/g)

Ref.

AO/ macroporous beads

3.5

[53]

AO/mesoporous carbon

4.6

[54]

AO/mesoporous imprinted polymer

19.1

[55]

44

[56]

145

[57]

AO/polypropylene membranes

380.0

[58]

p-AO/CNFs

588.2

This study

AO/poly(propylene) fibrous AO/multiwalled carbon nanotubes


Page 33 of 34


Table 3. EXAFS Results of Reference and U(VI)-Containing Samples at LIII-Edge, T = 293 K, I = 0.01 mol/L NaCl Solution Samples

Shell

R (Å)a

CN b

σ2(Å2) c

UO2CO3(s)

U-Oax

1.785

2.0

0.0031

U-Oeq

2.564

5.2

0.0068

U-C

2.894

1.6

0.0056

U-U

3.816

0.4

0.0087

U-Oax

1.789

2.0

0.0042

U-Oeq

2.517

4.7

0.0065

U-C

2.901

1.5

0.0071

U-Oax

1.804

2.0

0.0022

U-Oeq

2.526

5.1

0.0064

U-C

2.921

1.7

0.0082

p-AO/CNFs

c-AO/CNFs

a

R: bond length; b CN: coordination numbers; c σ2: Debye-Waller factor

Table 4. Optimized Parameters of Surface Complexation Modeling for U(VI) Adsorption on c-(1) and p-AO/CNFs (2) Reactions

Log K (1)

(2)

SOH + H+ = SOH2+

3.54

3.27

SOH = SO- + H+

-5.26

-6.59

SOH + UO22+ = SOUO2+ +H+

2.61

2.97

SOH + 3UO22+ +7H2O = SO(UO2)3(OH)72- + 8H+

-2.65

-3.01

SOH + UO22+ +2CO32- = SOUO2(CO3)23- + H+

-8.09

-8.67



TOC

PLASM

CN

.

NH2OH HCl

AO/CNF


Page 34 of 34

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