Linear Î²-Cyclodextrin Polymer Functionalized Multiwalled Carbon

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Materials and Interfaces

Linear #-CD polymer functionalized MWCNTs as novel nanoadsorbent for high effective removal of U(VI) from aqueous solution based on inner-sphere surface complexation Jin-Hua Xue, Hui Zhang, DX Ding, Nan Hu, Yongdong Wang, and Yong-Sheng Wang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b05453 • Publication Date (Web): 20 Feb 2019 Downloaded from http://pubs.acs.org on February 22, 2019

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Industrial & Engineering Chemistry Research

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Linear β-CD polymer functionalized MWCNTs as novel nanoadsorbent for high effective

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removal of U(VI) from aqueous solution based on inner-sphere surface complexation

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Jin-Hua Xue a,b, Hui Zhang a, DX Ding a,*, Nan Hu a, Yong-Dong Wang a,

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Yong-Sheng Wang b,* aKey

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Discipline Laboratory for National Defense for Biotechnology in Uranium Mining and Hydrometallurgy, University of South China, Hengyang, PR China 421001

bCollege

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of Public Health, University of South China, Hengyang, PR China 421001

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10 11 12 13 14 15 16 17 18 19 20 _______________________________________________

Corresponding author. Tel: +86734 8282534; fax: +86734 8281771.



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E-mail address: [email protected] (D. X. Ding); [email protected] (Y.S. Wang). ACS Paragon Plus Environment

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Abstract Linear β-cyclodextrin polymer functionalized multiwalled carbon nanotubes (MWCNTs- CDP)

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were synthesized and employed as a novel nanoadsorbent to remove uranyl ions in wastewater

24

solutions. The characterization of MWCNTs-CDP, with FT-IR, XRD, SEM and BET, suggested

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that the CDP was successfully modified on the MWCNTs surfaces. The removing efficiency of

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uranyl ions in aqueous solution by MWCNTs-CDP was investigated via varying various

27

experimental conditions. We found that the removal of uranyl ions correlated with both pH and

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temperature, and the optimum pH and temperature were 6.0 and 323.15 K, respectively. The

29

adsorption of uranyl ions was very fleet at the initial 1 h, and then reached the adsorption equilibrium

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after 3 h. The data from adsorption dynamic experiment could be commendably fitted by the pseudo-

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second-order model (R2 > 0.982), suggesting that chemisorption might be the rate-controlling

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mechanism. The Langmuir model (R2>0.995) manifested that the maximum adsorption capacity of

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uranyl ions on MWCNTs-CDP boosted from 66.16 to 89.54 mg g-1 upon the change of temperature

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from 293.15 to 323.15 K. This result was better than that of some reported adsorbents. The

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thermodynamic analysis proved that the adsorption of uranyl ions on MWCNTs-CDP was a

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spontaneous process and endothermic one. The adsorption mechanism of uranyl ions on MWCNTs-

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CDP was verified to be dominated by inner-sphere surface complexation. Our results indicated that

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MWCNTs-CDP can be utilized as a novel nanoadsorbent for high effective removal of uranyl ions

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in actual uranyl-bearing effluents.

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Keywords: Linear β-cyclodextrin polymer; Multiwalled carbon nanotubes; Nanoadsorbent;

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Uranium; Dynamics; Thermodynamics

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1. Introduction

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In recent years, the rapid development of nuclear energy has resulted in the growing usage of

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uranium (U) in nuclear power stations, which could cause the radioactive nuclear waste being

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discharged into aquatic systems. The possible risk of the aquatic system contaminated by uranium

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is a salient environmental concern because of its severe lesion towards human health.1, 2 Accordingly,

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the removal of uranium from aqueous solutions is of great importance towards environmental

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control and the public health. Adsorption technique has been developed because of its conspicuous

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advantages of having lower cost, convenient operation, and extensive applicability in the disposal

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of various pollutants,3-5 and employed for removing uranyl ions in effluent water.6-8 In the past

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decades, the rapid progress in nanotechnology provides intriguing new opportunities for developing

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nanomaterials.

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materials for the removal of uranyl ions in uranium-containing wastewaterer, such as carbon

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nanotubes (CNTs)/ multiwalled carbon nanotubes (MWCNTs),23-32 mesoporous carbon,33-35 metal-

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organic frameworks,

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46

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Among the various adsorbents, CNTs/MWCNTs have attracted considerable interests because of

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their good stability towards acid–base, excellent mechanical properties, large surface area and

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higher thermal/radiation resistance.23, 24 However, the inherent hydrophobicity and easy aggregation

61

of CNTs/MWCNTs in aqueous solution might impede their adsorption behaviors, and decrease the

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removing efficiency of pollutants, consequently limiting their wide application in real work.27 To

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overcome this limitation, the modification of CNTs/MWCNTs by grafting various functional groups

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has been developed to improve their dispersibility and removal capacity.29-32 β-Cyclodextrin (β-CD)

9-22

Especially, prodigious efforts have been made to discover novel adsorption

36-39

titanate nanotubes,

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natural/synthetic polymers, graphitic carbon nitride,

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40-44

leather waste,

biomass,

49

45

tripolyphosphate LDH,

and Fe3O4@PDA@TiO2.

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is cyclic oligosaccharides consisting of seven glucopyranose units linked by α-1, 4-bonds, and

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possesses an unique structure with a hydrophilic periphery and a hydrophobic interior.51 The

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hydroxyl groups in the hydrophilic periphery of β-CD are reported to be able to bind metal ions

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through hydrogen bonding.

69

CD-based composites have been used to clean up various pollutants.51, 52 Nevertheless, no reports

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on uranium removal from aqueous solution were discovered by linear β-CD polymer functionalized

71

MWCNTs as an adsorbent to the best of our knowledge.

29

Due to these favorable physicochemical properties, a variety of β-

72

In this work, MWCNTs-CDP was firstly synthesized by grafting linear β-CD polymer (CDP)

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onto the surface of MWNCTs, which was proved by the characterization of MWCNTs-CDP. The

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adsorption behavior of uranyl ions on MWCNTs-CDP was investigated in detail. We found that not

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only the physico-chemical stability of MWCNTs-CDP could be ameliorated, but also the

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adsorption capacity for uranyl ions has been obviously raised upon modifying CDP onto MWCNTs.

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The studies on adsorption dynamic, thermodynamic and adsorption isotherm demonstrated that the

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adsorptive behaviors of the adsorbent were able to proceed fast and spontaneously, and obeyed

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pseudo-second-order model and Langmuir isotherm. The application of MWCNTs-CDP for

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removing uranyl ions from real wastewater solution further verified the reliability of this strategy.

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This work might be not only helpful to develop effective nanoadsorbent, but also irradiative to

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discover new applications of CNTs/MWCNTs in the environmental pollution cleanup.

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2. Materials and methods

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2.1. Chemicals

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All chemicals are of reagent grade. The deionized water was utilized throughout this work. β-

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CD was obtained from China National Medicines Corporation Ltd. (Beijing, China). MWCNTs and Page 4 of 39 ACS Paragon Plus Environment

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3-Glycidyloxypropyltrimethoxysilane (GPTMS) were purchased from Sigma-Aldrich (St. Louis,

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MO, USA). The other chemicals were acquired by Shanghai Aladdin Bio-Chem Technology Co.,

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Ltd. (Shanghai, China). All reagents were prepared as previously reported.

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dissolved in concentrated sulfuric acid and H2O2 first, and then diluted in deionized water to get

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1000 mg L-1 U(VI) stock solution. All the working solutions were freshly prepared from the stock

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solutions as indicated in the context.

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2.2. Preparation of MWCNTs-CDP

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Briefly, U3O8 was

94

The synthetic route of MWCNTs-CDP is illustrated in Fig. S1 (see Supporting Information).

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Synthesis of linear β-CD polymer (CDP). The linear β-CD polymer was synthesized according

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to the protocol by Renard et al. with a minor modification. 53 Briefly, 2.5 g β-CD was dissolved in

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a 5 mL of 1 M NaOH solution with stirring overnight at room temperature, followed by adding 3.5

98

mL of Epichlorohydrin to the above solution. After 3 h incubation, the acetone was added to stop

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the reaction, which was removed afterwards by decantation. After filtration, the resultant sample

100

was washed with HCl solution recurringly until the filtrate pH was approx 12, dried at 50 °C for 12

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h. Acetone was added to the sample after cooling, grinded for purification. Then the white product

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was obtained by filtrating and drying at 80 °C for 12 h under vacuum.

103

Synthesis of MWCNTs-CDP. In a 250 mL round bottomed flask, 0.5 g raw MWCNTs were

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immersed in a 100 mL solution of HNO3 and H2SO4 (v/v=1:3), dispersing well by ultrasonic for 10

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min. Then the mixture was refluxed for 24 h at 140 °C, following by cooling to room temperature,

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the oxidized MWCNTs (MWCNTs-COOH) were washed by deionized water several times up to

107

neutral pH, and dried for 24 h at 70 °C under vacuum. Thereafter, 0.3 g MWCNTs-COOH was

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dispersed in 50.0 mL of sulfoxide chloride and 1.5 mL of coupling agent (DMF), refluxing at 70 °C Page 5 of 39 ACS Paragon Plus Environment


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for 24 h. Then the residual sulfoxide chloride was removed by distillation. Subsequently, 45.0 mL

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Ethylenediamine (EDA) was added to the flask and reacted for 24 h at 120 °C. Thereafter, the

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obtained product was diluted with deionized water and ﬁltered using a 0.45 μm PTFE membrane,

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then washed by excess methanol several times. The as-prepared MWCNTs-NH2 was dried for 24 h

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at 70 °C in a vacuum oven, following by the addition of 0.2 g MWCNTs-NH2 into a 50 mL of

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CH2Cl2 with stirring overnight. Then 2.5 mL of 3-Glycidyloxypropyltrimethoxysilane (GPTMS)

115

was added into the above-mentioned solution, reacting for 3 h with magnetic stirring. Subsequently,

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the pH was adjusted to around 4 with the HCl. When the solution became milky white, it was

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incubated with 0.3 g CDP for 3 h. Finally, the resultant product was ﬁltered using a 0.45 μm PTFE

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membrane, then put it in a vacuum oven for drying 24 h at 50 °C.

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2.3. Characterization

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A Rigaku X-ray power diffractometer was utilized to record the X-ray diffraction (XRD)

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patterns of MWCNTs-CDP using Cu-Kα radiation (40 kv, 150 mA).54 A Carl Zeiss Ultra 55 field

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emission scanning electron microscope (SEM) was employed to observe the morphologies of

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MWCNTs-CDP. Fourier transform infrared (FT-IR) spectra were measured on a Perkin-Elmer 100

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Fourier Transform Infrared spectrometer. The specific surface area, the pore size distribution and

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pore volume was analyzed by N2-physisorption using an Aytosorb-1C instrument (Quantachrome,

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USA). 36, 55

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2.4. Experimental procedure

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Adsorption experiments were performed based on a batch technique.

50

Briefly, 0.02 g

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MWCNTs-CDP and an appropriate amount of U(VI) solution were added in turn in a glass vial.

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Thereafter, the adjustment of pH was made by 0.1 M NaOH or 0.1 M HCl solutions. The mixture Page 6 of 39 ACS Paragon Plus Environment

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was put in a shaker bath, and shaken at a speed of 200 rpm for 5 h to accomplish adsorption

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equilibrium. 50 The centrifugation was carried out to separate the solid phase from liquid phase. 36

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The collected effluent was filtered with a 0.22 μm pore size filter paper.

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concentration of uranyl ions in the supernatant was determined using a inductively coupled plasma

135

mass spectrometry (ICP-MS, see Supporting Information). Standard U(VI) solutions with various

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concentrations were prepared by 2% HNO3.

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also added to the standard uranium samples. The amount of uranyl ions adsorbed on MWCNTs-

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CDP q (mg g−1), the adsorption percentage and the adsorption distribution coefficient Kd (mL g-1)

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were defined as follows: 56, 57

140 141

36

36

Subsequently, the

A 0.5 ppm of bismuth as an internal standard was

（C - C ） q= 0 e V m C  Ce Adsorption %  0  100% C0 Kd 

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(1) (2)

(C0  Ce ) V  Ce m

(3)

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Where C0 and Ce are the initial and equilibrium concentrations of the uranyl ions (mg L-1), m and V

144

are the mass of adsorbent (g) and the volume of the uranium solution (L), respectively. 50

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Desorption experiment was carried out according to reported literature.

50

After adsorption

146

equilibrium, the uranyl ions-loading MWCNTs-CDP was washed for three times by deionized water,

147

and then immersed in a 50 mL of 0.5 M NaHCO3 solution for 3 h at room temperature. The

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MWCNTs-CDP was collected from the mixture solution via centrifugation, which was used for the

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next adsorption-desorption experiment. The experiment was repeated three times.

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3. Results and discussion

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3.1. Characterization of the as-prepared MWCNTs–CDP

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The structure and morphology characterizations of MWCNTs–CDP were carried out by XRD, Page 7 of 39 ACS Paragon Plus Environment


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FT-IR and SEM (see Supporting Information). 58-60 The XRD patterns of β-CD, CDP, MWCNTs

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and MWCNTs–CDP are illustrated in Figs. 1A and 1B. As shown in Fig. 1A, β-CD exhibits main

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diffraction peaks at 2θ values of 10.5°, 12.3°, 16.9° and 22.6°, suggesting that β-CD mainly exists

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in a crystalline form. However, CDP is largely in an amorphous form with trace amount of

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crystalline one (Fig. 1A), confirming the successful crosslinking of β-CD to form β-cyclodextrin

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polymers. Fig. 1B displays a sharp peak at 2θ = 26.1° and a broad weak peak at 2θ = 44.3°, which

159

are related to the characteristics of MWCNTs. The XRD patterns of MWCNTs-CDP were similar

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to that of MWCNTs, suggesting that the CDP was successfully modified on the surface of MWCNTs

161

and the structure of MWCNTs was undamaged in the CDP modified process.

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The data from FT-IR also supported above-mentioned conclusion. The FT-IR spectrum of β-CD

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displays a wide peak of the –OH stretching vibration at 3300 – 3500 cm-1 (Fig. 1C).

52, 61, 62

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peak of 2920 cm−1 is attributed to asymmetric C–H stretching vibration, 1160 cm−1 for C–C/C–O

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stretching one, and 1030 cm−1 for antisymmetric glycosidic C–O–C vibration. 63

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spectrum of CDP was very similar to that of β-CD except for the peak intensity of CDP becoming

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obviously weakening, which could be attributed to the effect of the intermolecular crosslinking in

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β-CDs by epichlorohydrin. Fig. 1D illustrates that the FT-IR spectrum of MWCNTs-CDP shows

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the typical absorption features of CDP, such as the stretching vibrations at 1024 cm−1 for C–O–C,

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2924 cm−1 for –CH2–, 3424 cm−1 for –OH, and 1456 cm-1 for C–H bending vibration,

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respectively. These results provide direct evidence for the successful grafting of CDP on the surface

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of MWCNTs.

The

The FT-IR

64, 65

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The microstructures of CDP, MWCNTs and MWCNTs-CDP were observed by SEM. Figs. 2A

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and 2B indicate that the CDP has a more compact stacking morphology in comparison with β-CD. Page 8 of 39 ACS Paragon Plus Environment

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The surface of MWCNTs is smooth and tidy (Fig. 2C), and MWCNTs-CDP becomes rough and

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disorder along with some wrinkles on its surface (Fig. 2D). Furthermore, the average diameter of

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MWCNTs-CDP is larger than that of MWCNTs, suggesting that CDP is successfully grafted on

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MWCNTs surfaces. This was consistent with the results from both XRD and FT-IR spectra.

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Table S1 (see Supporting Information) indicates that the BET surface area

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decreases from

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7.08 to 4.33 m2 g-1 when the β-CD is cross-linked by epichlorohydrin. Meanwhile, the pore volume

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and pore diameter are slightly lessened. It is noteworthy that the surface modification of MWCNTs

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by CDP also results in the decrease of the BET surface area from 186.5 to 131.8 m2 g-1 along with

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the slight decrement of the pore volume and pore diameter. These phenomenons prompt the

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MWCNTs-CDP having the intermediate porous property between MWCNTs and CDP, suggesting

185

that the CDP was successfully modified onto the MWCNTs surface.

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3.2. Influence of pH and ionic strength

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The influence of pH on the adsorption of UO22+ on MWCNTs-CDP was investigated in the ionic

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strengths of 0.001–0.1 M NaNO3 solutions. Fig. 3A shows that the adsorption of UO22+ on

189

MWCNTs-CDP significantly increases with the increase of pH from 2.0 to 6.0 with a maximum of

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pH 6.0, and then gradually declines upon increasing pH from 6.5 to 10.0. This observation suggests

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that the adsorption of UO22+ onto MWCNTs-CDP would be dominated by surface complexation.31

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As mentioned above, the multiple –OH groups of CDP are grafted on the surface of MWCNTs,

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which can display different states along with the variation of pH. At lower pH, the hydroxyl groups

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of MWCNTs-CDP could be protonated, leading to the decrease of the nucleophilicity of oxygen

195

atoms in the –OH groups of MWCNTs-CDP towards U(VI). This is unfavourable for forming the

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complexes with U(VI) on the surface of adsorbent. While the increasing of pH could cause the Page 9 of 39 ACS Paragon Plus Environment


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deprotonation of the –OH of adsorbent. The empty orbits of UO22+ could more readily be occupied

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by the oxygen atoms with lone-pair electron, which favors the formation of a complex of U(VI).

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This could facilitate the adsorption of UO22+ onto the MWCNTs-CDP surface.31 This conclusion is

200

also demonstrated by the distribution of uranium species calculated by Visual MINTEQ 3.0. As

201

shown in Fig. 3B, UO22+ ions predominate in acidic solutions of pH < 4. The competition between

202

H+ ions and UO22+ for –OH groups on the MWCNTs-CDP surface causes a decrease of the

203

adsorption capacity. With raising solution pH, the hydrolysis of UO22+ is enhanced to form various

204

hydroxyl complexes of U(VI), mainly including (UO2)3(OH)5+ and UO2OH+ at pH 6. These species

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could more easily combine with the oxygen atoms in the –OH groups on MWCNTs-CDP owing to

206

the deprotonation of the –OH groups to enhance their nucleophilicity towards U(VI). Hence, the

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adsorption of uranyl ions onto MWCNTs-CDP acquires a maximum. At pH 8.0–10.0, UO2(CO3)34-

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and UO2(CO3)22- are the predominant species, which are unfavourable for adsorbing UO22+ on the

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surface of MWCNTs- CDP due to the empty orbits of UO22+ being occupied by CO32–. Consequently,

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the adsorption of uranyl ions onto MWCNTs-CDP declined in this scope of pH. 24, 29

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Fig. 3A illustrates that the adsorption of UO22+ is mightily affected by ionic strength in the pH

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2.0–4.0, implying that the dominant mechanism may be outer-sphere complexation. Moreover, the

213

adsorption percentage of U(VI) decreases with the increase of ionic strength, probably attributing

214

to the decrease of the activity coefficient of UO22+ ions to hinder the migration of UO22+ ions from

215

bulk solution to the surface of adsorbent. However, upon increasing pH, the change of ionic strength

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has no obvious effect on the adsorption percentage, which suggests that inner-sphere surface

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complexation mechanism might be predominated in this pH range. 46, 52 This result further verifies

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the above-mentioned supposition. Page 10 of 39 ACS Paragon Plus Environment

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3.3. Effect of contact time, initial U(VI) concentration and temperature

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To estimate the influence of contact time on the U(VI) adsorption, an experiment was carried

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out in the present of 0.02 g of MWCNTs-CDP at pH 6.0. Fig. 3C displays a rapid increase of the

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adsorption amount of uranyl ions with the increase of contact time in the initial 1 h. Thereafter it

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slows down until the adsorption equilibrium is realized. At the initial U(VI) concentrations of 0.25,

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0.5, 1.0 mg L−1, the adsorption equilibriums were achieved in approximate 100, 120, 180 min,

225

respectively. The quick adsorption in the initial adsorptive stage is attributed to the availability of

226

higher number of adsorption sites (such as –OH group) and the strong binding of uranyl ions with

227

–OH groups on MWCNTs-CDP. It is also related to the quick diffusion of uranyl ions from the

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solution onto the MWCNTs-CDP surface through the driving force of concentration gradient. 46 At

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later stage, the concentration gradient gradually decreases owing to the accumulation of U(VI) on

230

the adsorbent, causing the reduction of adsorption rate. In short, the adsorption of U(VI) on the

231

MWCNTs-CDP is very fast. The contact time of 3 h can realize adsorption equilibrium, suggesting

232

the potential application of MWCNTs-CDP in continuous effluent disposal.

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The influences of the initial concentration of UO22+ ions and temperature were also investigated.

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Fig. 3D shows the increase of the adsorption capacity of MWCNTs-CDP from 0.24 mg g–1 to 2.4

235

mg g–1 with the raise of the initial concentrations of UO22+ ions ranging from 0.1 mg L–1 to 1 mg L–1

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at 323.15 K. This may be because of a higher initial U(VI) concentration being able to provide

237

greater concentration gradient and stronger driving force. Fig. 3D indicates that the Kd values keep

238

constant upon raising the initial UO22+ concentration. However, the Kd value increases with the

239

change of temperature from 293.15 to 323.15 K. This result is in agreement with the

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physicochemical property of Kd, namely, the Kd value is a function of temperature. Page 11 of 39 ACS Paragon Plus Environment


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3.4. Adsorption dynamics, adsorption isotherm and thermodynamics

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The appropriate dynamic models could provide valuable information to understand the potential

243

mechanism in the adsorption process. Thereby, the experimental dynamic data were simulated with

244

pseudo-first-order model (ln (qe – qt) = ln qt – k1t), and pseudo-second-order model

245

( t 

246

equilibrium time and any time t (min), k1 (min-1) and k2 (g mg-1 min-1) are the rate constants of

247

pseudo-first-order and pseudo-second-order equations,

248

including k1, k2 , qe, cal were calculated from the intercepts and slopes of the linear plots of ln (qe –

249

qt) versus t, and t/qe versus t, 46 and listed in Table S2 (see Supporting Information). It can be seen

250

that the correlation coefficient of R2 > 0.982 for pseudo-second-order model was larger than those

251

for pseudo-first- order one (R2 ≤ 0.980). This result suggested that the adsorption kinetic data of

252

UO22+ ions on MWCNTs-CDP can be commendably fitted by the pseudo-second-order model in

253

comparison with pseudo-first-order model. 46,

254

qe,cal was more close to the experimental adsorption capacity qe,exp, suggesting that the rate

255

controlling mechanism might be chemisorptions rather than physical sorption.31, 46 The adsorption

256

behavior might involve the coordinating interaction of U(VI) ions with –OH groups on MWCNTs-

257

CDP. 31 This result was in good agreement with above–mentioned conclusion.

qt

1 t ),  2 k 2 qe qe

46, 67, 68

where, qe and qt (mg g-1) are the amount of UO22+ ions adsorbed at

67, 68

46

respectively. The kinetic parameters

In addition, the calculated adsorption capacity

258

Langmuir and Freundlich models were also adopted to understand the UO22+ adsorption

259

mechanism. 6, 7 The Langmuir isotherm model is verified to be effective for monolayer adsorption

260

to a surface containing a finite number of identical sites, describing by the following equation: 6, 7

261 262

qe 

bqmax Ce 1  bCe

(4)

Eq. (4) is able to be expressed by a linear form: 6, 7 Page 12 of 39 ACS Paragon Plus Environment

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Ce C 1   e qe bqmax qmax

(5)

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herein, qe (mg g-1) is the amount of the adsorbed U(VI) per unit mass of adsorbent at equilibrium,

265

Ce (mg L-1), qmax (mg g-1) and b are the U(VI) concentration at equilibrium, maximum adsorption

266

capacity, and Langmuir constant, respectively. 6, 7 While Freundlich isotherm model describes the

267

adsorption onto heterogeneity surface, usually expressing as: qe  K F Ce n , and its linear form is

268

expressed as: ln qe = ln KF + 1/n ln Ce,69 herein KF and n represent the adsorption capacity and the

269

dependence degree of adsorption with equilibrium concentration, respectively. The adsorption

270

isotherms of UO22+ on MWCNTs-CDP at various temperatures were illustrated in Fig. 4A and 4B.

271

The related parameters for Langmuir and Freundlich isotherms were listed in Table S3 (see

272

Supporting Information). As shown in Fig. 4A, the adsorption of UO22+ ions significantly enhances

273

with raising temperature, implying the higher temperature being favored for U(VI) adsorption on

274

MWCNTs-CDP. Moreover, all experimental data can well fit to both adsorption isotherm models

275

(Fig. 4A and 4B). The Langmuir isotherm model indicated that the maximum adsorption capacity

276

for MWCNTs-CDP changed from 66.16 mg g-1 to 89.54 mg g-1 upon

277

293.15 K to 323.15 K (Table S3, see Supporting Information), demonstrating MWCNTs-CDP

278

having much higher adsorption capacity. This might attribute to the multiple hydroxyl groups of

279

CDP being grafted onto the MWCNTs-CDP surface. These –OH groups can interact with U(VI) to

280

form the complexes, which strongly enhance the adsorption capacity of MWCNTs-CDP. The same

281

effect was also observed with Freundlich model, that is, the adsorption capacity (KF) enhanced from

282

24.26 to 60.97 upon raising the temperature in the range of 293.15 – 323.15 K. Table S3 (see

283

Supporting Information) indicates that “ 1/n ” is less than 1, suggesting that U(VI) ions are

284

favorably adsorbed by MWCNTs-CDP at the studied temperatures.

temperature change from



Page 14 of 39

285

Adsorption thermodynamic experiment was also conducted, and the parameters including

286

enthalpy (  H0), entropy (  S0), and Gibb’s free energy (  G0) for U(VI) removal by MWCNTs-

287

CDP were obtained from Eqs. (6) –(7): 46

H 0 S 0  RT R

288

ln K d  

289

G 0  H 0  TS 0

(6)

(7)

290

where R, T are gas constant (8.314 J mol-1 K-1) and thermodynamic temperature (K), respectively.

291

46 The

292

of the plots of ln Kd versus 1/T (Fig. 4C), and listed in Table S4 (see Supporting Information). The

293

positive value of ΔH0 indicates that the adsorption of UO22+ ions is endothermic. However, the

294

combination of UO22+ ions with the –OH groups on MWCNTs-CDP is exothermic. The reason for

295

this positive value may be that UO22+ ions are well dissolved in water, resulting in the formation of

296

the hydration sheath of U(VI) ions. 70 This one has to be destroyed before they being adsorbed onto

297

MWCNTs-CDP. So, the higher temperature is necessary for the dehydration process. Moreover, the

298

endothermicity in the dehydration process might be much larger than the enthalpy of adsorption.

299

The positive value of ΔS0 suggests an augmentation in the randomness on the solid-solution

300

interface in the adsorption process. 36, 70 As mentioned above, the adsorption of U(VI) on MWCNTs-

301

CDP may attributed to the formation of the coordinating complex between U(VI) and –OH groups

302

on MWCNTs-CDP, which declines the degree of freedom of U(VI) ions. However, the U(VI) ions

303

in solution could form a hydration sheath, in which water molecules are highly ordered compared

304

with that in the bulk water. When U(VI) interact with the hydration sheath of adsorbent, the

305

arranging of the ordered water molecules in the above-mentioned hydration sheaths are disturbed,

306

leading to the increase of the entropy of water molecules. 36, 70 Hence, the positive entropy related

thermodynamic parameters such as ΔH0 and ΔS0 were obtained from the slope and intercept


Page 15 of 39 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


307

to the U(VI) adsorption may be owing to the entropy enlargement of water molecules exceeding the

308

entropy reducing of U(VI) to bind the functional groups on MWCNTs-CDP.70 The negative value

309

of ΔG0 implies that the adsorption process is spontaneous and thermodynamically favorable. The

310

values of ΔG0 become more negative upon raising temperature. This prompts the more efficient

311

adsorption at high temperature due to U(VI) being readily desolvated. 36, 70

312

3.5. FT-IR and SEM analysis

313

Compared with the FT-IR spectra of MWCNTs-CDP, a strong peak emerged at 920 cm-1 for

314

MWCNTs-CDP-U(VI), attributing to the antisymmetric vibration for O=U=O group (Fig. 1D). The

315

–OH stretching vibration of MWCNTs-CDP-U(VI) at 3300-3500 cm-1 also changed obviously,

316

implying the binding of U(VI) with the hydroxyl groups on adsorbent to form the surface complexes.

317

52, 61, 62

318

CDP. Fig. 2E displays the SEM images of MWCNTs-CDP-U(VI). The presence of UO22+ ions on

319

the surface of MWCNTs-CDP significantly changes the morphology of the adsorbent, and its

320

surface is more rough with block like structure, which confirms the UO22+ ions being adsorbed on

321

the surface of MWCNTs-CDP.

322

3.6. Desorption and reusability studies

This observation implies the chemical adsorption of U(VI) on the surface of MWCNTs-

323

A good adsorbent should have these characteristics such as good selective adsorptive ability

324

and desorption-reuse property, which decide the degree of practical application of the adsorbent. 36

325

To test the reuse property of the MWCNTs-CDP, the desorption experiment of uranyl-loading

326

adsorbent was performed by means of different concentration of NaHCO3 solution as eluting agent.

327

The experiment found that the desorption percentage of U(VI) on U(VI)-loading MWCNTs-CDP

328

by NaHCO3 at concentrations of 0.02, 0.05, 0.1, 0.2 and 0.5 M were 59.41%, 71.33%, 82.57%, Page 15 of 39 ACS Paragon Plus Environment


Page 16 of 39

329

87.46% and 92.58%, respectively. Therefore, 0.5 M NaHCO3 solution was selected as an optimal

330

eluant for eluting uranyl ions from U(VI)-loading MWCNTs-CDP. 50 To evaluate the reusability of

331

the MWCNTs-CDP, the adsorption-desorption experiment was carried out repeatedly three times

332

under the same condition. 50 After three cycles of adsorption–desorption experiments, the adsorptive

333

capacity of the MWCNTs-CDP decreased from 66.16 mg g-1 to 59.35 mg g-1. This result implies

334

that the synthesized MWCNTs-CDP can be potentially utilized as an efficient adsorbent for the

335

removing and preconcentration of uranyl ions in low concentration solution.

336

3.7. Uranium adsorption test in real wastewater disposal

337

MWCNTs-CDP was further used to remove U(VI) from U(VI)-containing wastewater samples,

338

which were collected from an uranium mine in Hunan province. The components of the wastewater

339

samples are including (µg L-1): K+ (27923.65), Ca2+ (566784.57), Na+ (105793.02), Mg2+ (52146.83),

340

Al3+ (4747.01), Fe3+ (1283.44), Mn2+ (24873.57), and U(VI) (386.95). The wastewater pH was 5.8.

341

Batch sorption studies were conducted using 1.0 g MWCNTs-CDP to treat 2.5 L wastewater sample

342

at 303.15 K. Fig. 4D illustrates that about 87.46% of uranyl ions can be adsorbed on the MWCNTs-

343

CDP, and the other coexistence ions such as K+, Ca2+, Na+, Mg2+, Al3+, and Fe3+ have a small effect

344

towards the adsorption of uranyl ions on MWCNTs-CDP. However, Mn2+ can be also effectively

345

adsorbed on MWCNTs-CDP. Thereby, the MWCNTs-CDP could be utilized as one of repeated cost

346

effective adsorbents for the handling of uranium loading wastewater.

347

3.8. Comparison of the adsorption capacity of uranyl ions with other adsorbents

348

To estimate the potential application of MWCNTs-CDP in wastewater disposal, the qmax (a

349

Langmuir coefficient) was employed for comparing the adsorption capacity of MWCNTs-CDP with

350

those reported adsorbents (Table 1). Clearly, the adsorptive capacity of MWCNTs-CDP was higher Page 16 of 39 ACS Paragon Plus Environment

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351

than those of some CNTs/MWCNTs adsorbents, such as plasma functionalized MWCNTs,24

352

oxidized MWCNTs,25 CNT-PRIST and CNT- MOD16h,28 chitosan modified MWCNTs,29

353

MWCNTs grafted with chitosan30, Raw MWCNTs,32 and MWCNTs, 23 but lower than that of AO-

354

g-MWCNTs,31 and MWCNT-g-CMC.

355

groups of CDP that were grafted onto the MWCNTs surface. These –OH groups on the MWCNTs-

356

CDP can bind to U(VI) to form strong complexes, which materially increases the adsorption

357

capacity of MWCNTs-CDP. Thus, this result suggests that MWCNTs-CDP could be potential for

358

U(VI) removal in practical application. Furthermore, compared with other adsorbents, the qmax value

359

of MWCNTs-CDP was larger than those of AF-PIM-FM, 41 and amidoxime polymeric adsorbents,

360

71, 72

361

activated leather waste, 45 phosphonate grafted mesoporous carbon, 35 and amidoximated adsorbents

362

(0.6 M KOH, T=333 K). 73 The further improvement for the adsorptive capacity of MWCNTs-CDP

363

is underway.

364

4. Conclusions

32

This phenomenon might be related to the multiple –OH

but less than that of CD/HNT/iron oxide,

52

functionalized mesoporous carbon,

33

alkali-

365

In this work, a novel nanoadsorbent of MWCNTs-CDP was synthesized by chemically grafting

366

linear β-CD polymer on the surface of MWCNTs. The MWCNTs-CDP was characterized by FT-

367

IR, SEM, XRD and BET, suggesting CDP being successfully modified on MWCNTs. The as-

368

prepared MWCNTs-CDP was employed for the removing or preconcentration of uranyl ions from

369

aqueous solutions. The adsorption of uranyl ions on MWCNTs-CDP correlated with both pH and

370

temperature. The isotherm data can be fitted well to Langmuir and Freundlich models. Adsorption

371

kinetic data can be successfully fitted to pseudo-second-order model, suggesting that the

372

chemisorption might be the rate-controlling mechanism. The data from thermodynamics suggested Page 17 of 39 ACS Paragon Plus Environment


Page 18 of 39

373

that the adsorption was a spontaneous process and endothermic one. Real wastewater disposal

374

experiment suggests that uranyl ions in the wastewater could be effectively removed in the presence

375

of other high concentration coexisted ions. Therefore, the MWCNTs-CDP can be potentially utilized

376

as a cost effective adsorbent for the disposal of uranium-loading effluents.

377

ASSOCIATED CONTENT

378

Supporting Information. Schematic illustration for the synthetic route of MWCNTs-CDP,

379

Procedures for measuring XRD, FT-IR, SEM, and ICP-MS, BET results for as-prepared porous

380

materials, Kinetic parameters of U(VI) sorption on MWCNTs-CDP, Isotherm parameters for

381

adsorption of U(VI) ions by MWCNTs-CDP, and Thermodynamic parameters for U(VI) ions

382

adsorption on MWCNTs-CDP.

383

AUTHOR INFORMATION

384

Corresponding Author.

385

*E-mail: [email protected] (D.X. Ding).

386

*E-mail: [email protected] (Y.S. Wang).

387

ORCID

388

DX Ding: 0000-0002-8712-5273.

389

Yong-Sheng Wang: 0000-0001-5833-9275.

390

Notes

391

The authors declare no competing financial interest.

392

Acknowledgments

393

This work was supported by the National Natural Science Foundation of China (11505093，

394

91326106, U1401321 ， 114055081 and 11705085), the Development Program for Science and Page 18 of 39 ACS Paragon Plus Environment

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395

Technology for National Defense (B3720132001) and the Research Foundation of Education

396

Bureau of Hunan Province (13A083).

397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413

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by magnetic core-dual shell Fe3O4@PDA@TiO2, J. Radioanal. Nucl. Chem. 2018, 317, 613-624.

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cyclodextrin: An environmentally friendly bifunctional adsorbent for simultaneous adsorption of

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metals and cationic dyes, Environ. Sci. Technol. 2015, 49, 10570-10580.

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(52) Yang, S.T.; Zong, P.F.; Hua, J.; Sheng, G.D.; Wang, Q.; Wang, X.K. Fabrication of β-

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cyclodextrin conjugated magnetic HNT/iron oxide composite for high-efficient decontamination of

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U(VI), Chem. Eng. J. 2013, 214, 376-385.

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(53) Renard, E.; Deratani, A.; Volet, G.;

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soluble high molecule weight β-cyclodextrin-epichlorohydrin polymers, Eur. Polym. J. 1997, 33,

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49-57.

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(54) Mohammadi, N.; Khani H.; Gupta, V.K.; Amereh, E.; Agarwal, S. Adsorption process of

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methyl orange dye onto mesoporous carbon material–kinetic and thermodynamic studies, J. Colloid

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Interface Sci. 2011, 362, 457-462.

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(55) Mittal, A.; Mittal, J.; Malviya, A.; Gupta, V.K. Removal and recovery of Chrysoidine Y from

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aqueous solutions by waste materials, J. Colloid Interf. Sci. 2010, 344 , 497-507.

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(56) Gupta, V.K.; Jain, R.; Nayak, A.; Agarwal, S.; Shrivastava, M. Removal of the hazardous dye-

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Tartrazine by photodegradation on titanium dioxide surface, Mater. Sci. Eng. C 2011, 31, 1062-

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(57) Devaraj, M.; Saravanan, R.; Deivasigamani, R.K.; Gupta, V.K.; Gracia, F.; Jayadevan, S.

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Fabrication of novel shape Cu and Cu/Cu2O nanoparticles modified electrode for the determination

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of dopamine and paracetamol, J. Mol. Liq. 2016, 221, 930-941.

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(58) Saravanan, R.; Gupta, V.K.; Prakash, T.; Stephen, A. Synthesis, characterization and

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photocatalytic activity of novel Hg doped ZnO nanorod sprepared by thermal decomposition method,

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J. Mol. Liq. 2013, 178, 88-93.

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(59) Saleh, T.A.; Gupta, V.K. Synthesis and characterization of alumina nano-particles polyamide

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membrane with enhanced flux rejection performance, Sep. Purif. Technol. 2012, 89, 245-251.

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(60) Gupta, V.K.; Ali, I.; Saleh, T.A.; Siddiqui, M.N.; Agarwal S. Chromium removal from water

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by activated carbon developed from waste rubber tires, Environ. Sci. Pollut. Res. 2013, 20, 1261-

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1268.

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(61) Saleh, T.A.; Gupta, V.K. Functionalization of tungsten oxide into MWCNT and its application

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for sunlight-induced degradation of rhodamine B. J. Colloid Interface Sci. 2011, 362, 337-344.

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(62) Ahmaruzzaman, M.; Gupta, V.K. Rice husk and its ash as low-cost adsorbents in water and

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wastewater treatment, Ind. Eng. Chem. Res. 2011, 50, 13589-13613.

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(63) Banerjee, S.S.; Chen, D.H. Cyclodextrin conjugated magnetic colloidal nanoparticles as a

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nanocarrier for targeted anticancer drug delivery, Nanotechnology 2008, 19, 265601-265607.

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Magnesium (II) ions using a coumarin-derived fluorescent probe, Sens. Actuators B-Chem. 2015,

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207, 216–223.

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(65) Saravanan, R.; Gupta, V.K.; Narayanan, V.; Stephen A. Comparative study on photocatalytic

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activity of ZnO prepared by different methods, J. Mol. Liq. 2013, 181, 133-141.

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(66) Saravanan, R.; Karthikeyan, N.; Gupta, V.K.; Thirumal, E.; Thangadurai, P.; Narayanan, V.;

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Stephen A. ZnO/Ag nanocomposite: An efficient catalyst for degradation studies of textile effluents

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(67) Ghaedi, M.; Hajjati, S.; Mahmudi, Z.; Tyagi, I.; Agarwal, S.; Maity A.; Gupta, V.K. Modeling

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of competitive ultrasonic assisted removal of the dyes-methylene blue and Safranin-O using Fe3O4

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nanoparticles, Chem. Eng. J., 2015, 268, 28-37.

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Tire for the adsorption of phenolics: Effect of pre-treatment conditions, J Colloids Surface Sci. 2014,

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417, 420-430.

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O by ZnS:Cu nanoparticles loaded on activated carbon: optimization of parameters using response

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surface methodology with central composite design, RSC Adv., 2015, 5, 18438 - 18450.

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(70) Song, S.; Yin, L.; Wang, X.; Lin, L.; Huang, S.; Zhang, R.; Wang, T.; Yu, S.; Hayat, T.; Wang,

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X. Interaction of U(VI) with ternary layered double hydroxides by combined batch experiments and

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spectroscopy study, Chem. Eng. J. 2018, 338, 579-590.

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(71) Kim, J.; Oyola, Y.; Tsouris, C.; Hexel, C.R.; Mayes, R.T.; Janke, C.J.; Dai, S. Characterization

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Res. 2013, 52, 9433−9440.

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Choe, K.Y.; Schneider, E.; Lindner, H. Uptake of uranium from seawater by amidoxime-based

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(73) Das, S.; Tsouris, C.; Zhang, C.; Kim, J.; Brown, S.; Oyola, Y.; Janke, C. J.; Mayes, R. T.; Kuo,

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L.-J.; Wood, J. R.; Gill, G. A.; Dai,S. Enhancing uranium uptake by amidoxime adsorbent in

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seawater: an investigation for optimum alkaline conditioning parameters, Ind. Eng. Chem. Res. 2016, Page 28 of 39 ACS Paragon Plus Environment

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55, 4294−4302.

615 616 617 618 619 620 621 622 623 624 625 626 627 628 629 630 631 632

Figure captions

633

Fig. 1. (A and B). XRD patterns of β-CD, CDP, MWCNTs-COOH and MWCNTs-CDP; (C and

634

D). FT-IR spectra of β-CD, CDP, MWCNTs, MWNTs-CDP and MWCNTs-CDP-U(VI).

635

Fig. 2. SEM images of CD (A), CDP (B), MWCNTs (C), MWCNTs-CDP (D), and MWCNTs-



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CDP-U(VI) (E).

637

Fig. 3. (A) Effect of pH and ionic strength on U(VI) sorption, (B) predicted U(VI) speciation in

638

aqueous solution as function of pH 2–10, (C) adsorption of U(VI) ions on MWCNTs-CDP as a

639

function of contact time and initial U(VI) concentrations, and (D) effect of initial U(VI)

640

concentration on adsorption capacity and Kd by the MWCNTs-CDP. (A)T=293.15 K, m/V =0.4,

641

CU(VI)=1.0 mg L-1. (B) CU(VI) = 1 mg L-1, PCO2 =10-3.5 atm. Calculated by Visual MINTEQ 3.0. (C)

642

T = 293.15 K, pH = 6.0, m/V = 0.4 g L-1, I = 0.01 M NaNO3. (D) C0 = 0–1.0 mg L-1, m/V = 0.4 g

643

L-1, I = 0.01 M NaNO3, t = 4 h.

644

Fig. 4. (A and B) Isotherm model for adsorption of U(VI) by the MWCNTs-CDP, (C) relationship

645

curve between ln Kd and 1/T and (D) removal efficacy of MWCNTs-CDP towards U(VI)- loading

646

real wastewater. (A and B) C0 = 0–1.0 mg L-1, m/V = 0.4 g L-1, I = 0.01 M NaNO3, t = 4 h.

647 648 649 650 651 652 653 654 655


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657 658



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659 660

661 662

Fig. 1. (A and B). XRD patterns of β-CD, CDP, MWCNTs and MWCNTs-CDP; (C and D). FT-IR

663

spectra of β-CD, CDP, MWCNTs, MWNTs-CDP and MWCNTs-CDP-U(VI).

664 665 Page 32 of 39 ACS Paragon Plus Environment

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666

667 668

Fig. 2. SEM images of β-CD (A), CDP (B), MWCNTs (C), MWCNTs-CDP (D), and MWCNTs-

669

CDP-U(VI) (E).

670 671 672 673 674 675 676 677 678 679 Page 33 of 39 ACS Paragon Plus Environment


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681

682

683

684

685


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686 687

688 689

Fig. 3. (A) Effect of pH and ionic strength on U(VI) sorption, (B) predicted U(VI) speciation in

690

aqueous solution as function of pH 2–10, (C) adsorption of U(VI) ions on MWCNTs-CDP as a

691

function of contact time and initial U(VI) concentrations, and (D) effect of initial U(VI)

692

concentration on adsorption capacity and Kd by the MWCNTs-CDP. (A)T=293.15 K, m/V =0.4,

693

CU(VI)=1.0 mg L-1. (B) CU(VI) = 1 mg L-1, PCO2 =10-3.5 atm. Calculated by Visual MINTEQ 3.0. (C)

694

T = 293.15 K, pH = 6.0, m/V = 0.4 g L-1, I = 0.01 M NaNO3. (D) C0 = 0–1.0 mg L-1, m/V = 0.4 g

695

L-1, I = 0.01 M NaNO3, t = 4 h.

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705 706

707 708

709 710 711

Fig. 4. (A and B) Isotherm model for adsorption of U(VI) by the MWCNTs-CDP, (C) relationship

712

curve between ln Kd and 1/T and (D) removal efficacy of MWCNTs-CDP towards U(VI)- loading

713

real wastewater. (A and B) C0 = 0–1.0 mg L-1, m/V = 0.4 g L-1, I = 0.01 M NaNO3, t = 4 h.

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Table 1. Comparison of U(VI) sorption capacity of MWCNTs-CDP with other adsorbents

Adsorbents Multiwalled Carbon Nanotubes

Experimental conditions

qmax (mg g-1)

Refs.

T=298, pH 5

24.9 ±1.3

23

amidoxime polymeric adsorbents

T=296 K

4.0

71

amidoxime polymeric adsorbents

T=293±2 K

3.3

72

AF-PIM-FMa

pH 4

4.04

41

CNT-PRIST

pH 5.0, T=298 K

4.28

28

Raw MWCNTs

pH 5.6, T=293K

14.28

32

Plasma functionalized MWCNTs

pH 5.0, T=298 K

17.35

24

Oxidized MWCNTs

pH 5.0, T=293 K

33.32

25

MWCNTs grafted by chitosan

pH 5.0, T=293 K

34.55

30

Chitosan modified MWCNTs

pH 5.0, T=293 K

41

29

CNT-MOD16h

pH 5.0, r.t.

45.9

28

MWCNTs-CDP

pH 6.0, T=293.15 K

66.16

This study

AALWb

pH 5, T=308 K

95.51

45

functionalized mesoporous carbon

pH 4

97

33

CD/HNT/iron oxide

pH 5.5, T=298 K

107.57

52

MWCNT-g-CMC

pH 5.0, T=298 K

111.19

32

AO-g-MWCNTs

pH 4.5, T=298 K

145

31

PGMCc

pH 4,

150

35

amidoximated adsorbents

0.6 M KOH, T=333 K

190

73

717

a Amidoxime

718

b

Alkali-activated leather waste.

719

c

Phosphonate grafted mesoporous carbon.

functionalized polymers of intrinsic microporosity (PIM-1) fibrous membrane.

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Table of Contents/Abstract Graphics The linear β-CD polymer (CDP) was grafted onto the surface of MWNCTs. The MWCNTs-CDP was employed as a novel nano-adsorbent for removing U(VI) from aqueous solutions based on the formation of the inner-sphere surface complex between MWCNTs-CDP with U(VI).

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Linear Î²-Cyclodextrin Polymer Functionalized Multiwalled Carbon

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