Zn Layered Double Oxide@Carbon

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Fabrication of Magnetic Fe/Zn Layered Double Oxide@Carbon Nanotube Composites and Their Application for U(VI) and 241Am(III) Removal Haijun Chen, Zhibin Zhang, Xiangxue Wang, Jing Chen, Chao Xu, Yunhai Liu, Zhimin Yu, and Xiangke Wang ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00528 • Publication Date (Web): 17 Apr 2018 Downloaded from http://pubs.acs.org on April 18, 2018

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ACS Applied Nano Materials

Fabrication of Magnetic Fe/Zn Layered Double Oxide@Carbon Nanotube Composites and Their Application for U(VI) and 241

Am(III) Removal

Haijun Chen†,‡,1, Zhibin Zhang†,‡,1, Xiangxue Wang†, Jing Chenǁ, Chao Xuǁ, Yunhai Liu‡, Zhimin Yu&, Xiangke Wang*†,‡,§

†

College of Environmental Science and Engineering, North China Electric Power

University, Beijing 102206, P.R. China ‡

School of Chemistry, Biological and Materials Sciences, East China University of

Technology, Nanchang, 330013, P. R. China &

Department of Biology and Environmental Engineering, Hefei University, Hefei

230000, P.R. China ǁ

Collaborative Innovation Center of Advanced Nuclear Energy Technology, Institute

of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084, PR China *: Corresponding Author, E-mail: [email protected] (X.K. Wang). 1

These authors contributed equally to this work.

1

ACS Paragon Plus Environment

ACS Applied Nano Materials 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

ABSTRACT: Magnetic Fe/Zn layered double oxide decorated carbon nanotubes (M-Fe/Zn-LDO@CNTs) composites were synthesized via a hydrothermal method with further calcination treatment. The fabricated M-Fe/Zn-LDO@CNTs was characterized by XPS, FTIR, XRD, UV-Vis, SEM, TEM, and then applied for U(VI) elimination under diversiform experimental conditions, such as pH, temperature, ionic strength, contact time and coexisting ions. The experimental results demonstrated that the U(VI) adsorption performance of M-Fe/Zn-LDO@CNTs was much higher than bare CNTs. The thermodynamic parameters revealed that U(VI) adsorption on M-Fe/Zn-LDO@CNTs was endothermic and spontaneous. More importantly, the adsorption performance of U(VI) on M-Fe/Zn-LDO@CNTs was still higher than ~89% after five cycles. Furthermore, ~95.9 % of

241

Am(III) could be removed by

M-Fe/Zn-LDO@CNTs from solution at pH = 8.0, which confirmed that M-Fe/Zn-LDO@CNTs was an excellent material for efficient removal of long-lived lanthanides and actinides from environmental aqueous solutions in radionuclides’ pollution remediation. KEYWORDS: M-Fe/Zn-LDO@CNT nanocomposites; Adsorption; Radionuclide; U(VI); 241Am(III)

INTRODUCTION With the increasing of energy demands, nuclear power becomes one of the major energy sources, while nuclear waste treatment is a challenging environment concern which needs to be solved 1. Especially, the long-lived actinides are significant

2


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hazardous even at trace level due to their long-term radiological and chemical toxicities. Uranium is the major material for nuclear energy used in nuclear reactors and also a dominant component in the nuclear waste

2, 3

. The major species of

uranium in natural environmental are U(IV) and U(VI). U(VI) is a long half-life radiation element with high solubility. The uranium-containing wastewater are released into natural environment from nuclear fuel legacy of uranium-contaminated site, ore mining, manufacturing and processing, which can cause high toxicity to human beings 4. Uranium is also naturally present in soils, minerals and sea water where its concentration is low (approximately 3-9 µg·L-1 or 2.6 mg·kg-1 in soil)

5, 6

and this enlarges the difficulty of enrichment and utilization. Thus, in order to reduce the environmental pollution and improve the utilization efficiency of uranium sources, it is necessary to remove uranium from contaminated wastewater. Various efficiency separation methods including chemical extraction chemical precipitation 13-16

9, 10

, biochemical reductive precipitation

11, 12

7, 8

,

and adsorption

have been employed for uranium removal from uranium polluted wastewater.

Among these methods, adsorption method is a widely used technique for efficient elimination of uranium from aqueous environment. Based on its rapid process and high selectivity performance, adsorption has been considered as one of the most significant and potential technology in environmental radionuclides’ pollution management. Recently, various materials such as clay minerals graphene oxides

21-23

17, 18

, chitosan composites19, 20,

and metal-organic frameworks24 have been employed for the

3



Page 4 of 38

elimination of heavy metal pollutants. Carbon nanotubes (CNTs) are novel and interesting materials, and have attracted tremendous attention due to their structural stability and physiochemical properties. Very recently, researchers focused on magnetic composites in the field of nuclear waste management. Magnetic separation has been developed to facilitate the collection of U(VI) and adsorbents from wastewater, and magnetic materials also have many advantages such as excellent adsorption capacity, quick separation and ease of operation, making them compatible with environmental purification and other techniques. Some studies were carried out to understand the interactions between magnetic CNT-based composites and pollutions (including organic pollutants and heavy metal ions). Chen et al.

25

studied

Sr(II) and Ni(II) adsorption to MWCNTs/iron oxide composites and Ma et al.

5

studied U(VI) adsorption to polysulfide/layered double hydroxide. However, to the best of our knowledge, the CNT-supported magnetic LDO composites for the removal of U(VI), especially 241Am(III) from aqueous solution is still unavailable. In our previous paper26, we synthesized Ca/Al-LDH@CNT composites via co-precipitation and the hydrothermal aged treatment, and applied the composites for the elimination of

241

Am(III) and U(VI) ions from wastewater solutions. The surface

complexation between Ca/Al and U(VI) contributed to U(VI) uptake. However, the separation of Ca/Al-LDH@CNT is difficult because of its small particles. Herein, we developed a novel method to synthesize magnetic Fe/Zn-LDO@CNTs composites, which were easily to be separated from solution by magnetic separation. The synthesis of M-Fe/Zn-LDO@CNTs was based on the fabrication of magnetic

4


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nanocomposites via a hydrothermal method with further calcination treatment, but the Ca/Al-LDH@CNTs was synthesized via co-precipitation and the hydrothermal aged treatment. The influence of experimental conditions (such as coexisting ions, temperature, pH and contact time) on adsorption process was studied via various techniques. Furthermore,

241

Am(III) adsorption on M-Fe/Zn-LDO@CNTs was also

investigated to explore the practical application for the efficient removal of actinides at extra-low concentrations. This study provides new insight to the excellent practical applications of M-Fe/Zn-LDO@CNTs in radionuclides’ environmental pollution remediation.

MATERIALS AND METHODS Materials. Carbon nanotubes (CNTs) used in this research with the average outer diameter of 20-30 nm, inner diameter of 5-10 nm and length of 10-30 µm were purchased

from

Aladdin

Industrial

Corporation

(Shanghai,

China).

The

Fe(NO3)3·9H2O, Zn(NO3)2·6H2O and other chemicals were purchased from Sinopharm Chemical Reagent Co. (China) in analytic purity grade.

Synthesis of M-Fe/Zn-LDO@CNTs Composite. The synthetic pathway of M-Fe/Zn-LDO@CNTs was shown in Scheme 1, and the M-Fe/Zn-LDO@CNTs was synthesized by the hydrothermal method and further calcination (Supporting Information, SI). Generally, the synthesis of M-Fe/Zn-LDO@CNTs involved a three-step methodology. (1) Starting by mixture process, the CNTs, Na2CO3, Fe(NO3)3·9H2O

and

Zn(NO3)2·6H2O

were

dissolved

5


into

Milli-Q

water.


Page 6 of 38

Simultaneously, the pH was controlled close to 8.5 via adding alkali. (2) In order to obtain uniform mixture, the suspension was stirred for 2 h. Then the suspension was heated at 120 oC for 12 hours in Teflon-lined stainless-steel autoclave. (3) After the nanocomposites

were

annealed

under

N2

condition

at

650

o

C,

the

M-Fe/Zn-LDO@CNTs composites were successfully obtained. More detailed synthesis processes of M-Fe/Zn-LDO@CNTs were described in Supporting Information.

Characterization of Materials. CNTs and as-prepared M-Fe/Zn-LDO@CNTs samples were characterized via diversiform spectral analysis techniques (e.g., transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FT-IR) in detail). The FTIR measurement was mounted in KBr pellet at room temperature (IRTracer-100, Shimadzu, Japan). The XRD characterization was performed by using X-ray diffraction (Philips X’ Pert Pro Super X-ray diffractometer) with Cu Kα radiation at room temperature. The SEM images were performed using a field emission scanning electron microscope (Hitachi S4800) with the energy of 20 kV, and the TEM images were obtained using Tecnai F-20 microscopy. The XPS measurements were obtained on a TE-250 electron spectrometer. The surface charging effect was corrected by C 1s peak at 284.4 eV as the reference.

Adsorption Experiments. The whole adsorption experiments were performed in polyethylene centrifuge tubes, and the adsorbent, adsorbate, background ions (NaNO3)

6


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were mixed in polyethylene centrifuge tubes. More corresponding discussions were shown in SI. The desired pH was adjusted using HNO3 or NaOH. And then, the suspensions (0.1 g·L-1 M-Fe/Zn-LDO@CNTs or bare CNTs, 30 mg·L-1 U(VI) and 0.001 ~ 0.1 M NaNO3) were oscillated for 24 hours to achieve the equilibration, and the solid phase was magnetically separated from solutions. The U(VI) concentration was determined by UV-vis spectrophotometer (UV-2550, Shimadzu) at the wavelength of 650 nm with the Arsenazo-III method. The adsorption efficiency was described as [adsorption (%) = (C0－Ce)/C0×100%] (where C0 (mg·L-1) was the initial concentration and Ce ( mg·L-1) was equilibrium concentration), and the distribution coefficient Kd = (C0－Ce)/Ce×V/m (where V and m were the volume of the suspension and the mass of adsorbent, respectively). To reveal the removal efficiency of M-Fe/Zn-LDO@CNTs toward other actinides, the sorption of 241Am(III) on M-Fe/Zn-LDO@CNTs was also investigated under different pH values. The whole experiments were carried out in duplicate; the experimental data was the average values within relative error of ~5%.

RESULTS AND DISCUSSION Characterization of CNTs and M-Fe/Zn-LDO@CNTs. The precise microstructures, sizes and morphologies of M-Fe/Zn-LDO@CNTs and bare CNTs were characterized by SEM and TEM. Figure 1a shows that the diameter of CNTs is ~20 nm and the length of CNTs is several micrometers. Figure 1b shows the SEM image of M-Fe/Zn-LDO@CNTs, indicating that the nanotube morphology and

7



Page 8 of 38

nano-particles are covered on the surface of CNTs. The TEM images (Figure 1c and d) further show the nanotube structure and LDO structure in the composites. Herein, it is worth noting that M-Fe/Zn-LDO@CNTs still possesses nanotube-like structure and the morphology is similar to CNTs, indicating that the CNTs play a template role in the synthetic process, and the composite keeps the nanotube-like structure. Such morphology structure in the composites could provide rich adsorption sites for metal ion binding from aqueous solution 16, 25. In order to explore the elemental distribution of the composites, the M-Fe/Zn-LDO@CNTs was further characterized by elemental mapping and EDS. The elemental mapping shows the homogeneous distribution of all elements in composites within the hierarchical structure (Figure 1e). Figure 1f shows the EDS patterns and insets

a

pie

graph,

which

describes

the

total

element

distribution

on

M-Fe/Zn-LDO@CNTs. The Fe element content (about 55.1%) indicates that magnetic Fe oxide (Fe2O3 and Fe3O4) are the main components in the M-Fe/Zn-LDO@CNTs composites. It is the Fe oxides that are beneficial to the magnetic separation of the composites from aqueous solutions 25. The contents of Zn, O and C are calculated to be ~32.8%, 10.4% and 1.7%, respectively. The abundance of metallic oxides and carbon-oxygen functional groups on the composites (including outer and inner surfaces)

are

beneficial

to

the

binding

of

U(VI)

or

241

Am(III)

on

M-Fe/Zn-LDO@CNTs 27. The precise morphologies and structures of the M-Fe/Zn-LDO@CNTs composites were further investigated by high resolution TEM measurement (Figure

8


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2a). It can be clearly found that the M-Fe/Zn-LDO@CNTs composites have very interesting structures, i.e., clearly lattice fringes of Fe, Fe3O4, Fe2O3 and ZnO particles are presented in whole regions

7, 28

. As shown in Figure 2a, the typical

CNTs lattice fringes reveal a d-spacing of 4.63 ± 0.05 Å, indicating the presence of CNTs’ structures 29. Moreover, the Fe nanoparticles (with a d-spacing of 1.48 ± 0.05 Å), the Fe2O3 nanoparticles (with a d-spacing of 3.33 ± 0.05 Å), the Fe3O4 nanoparticles (with a d-spacing of 2.23 ± 0.05 Å) 30 and the ZnO nanoparticles (with a d-spacing of 2.83 ± 0.05 Å)

8

present lattice fringes clearly in the composites,

which revealed that the outer surface of CNTs is covered with a series of metal oxide nanoparticles. Figure 2b shows the XRD patterns of CNTs and M-Fe/Zn-LDO@CNTs. In the XRD pattern of CNTs, the sharp peaks at 25.86o and 42.96o indicate the typical crystal phases of CNTs and the sp2 hybridized diffraction structure, respectively 25, 29. In the XRD pattern of M-Fe/Zn-LDO@CNTs (Figure 2b), the typical crystal phase of CNTs was not observed due to the effect of strong peak of metal oxides (such as Fe-O and Zn-O). And the main diffraction peaks of the composites at [100], [002], [101], [102], [103], [112] are agreed with the typical hexagonal wurtzite structure of ZnO (JCPDS no.36-1451) 31, 32. An additional small diffraction peaks at [220], [311] and [110] are corresponding to the crystalline planes of Fe3O4, Fe, Fe2O3, respectively

31

diffraction

intensities

. The diffraction of Fe and Fe3O4 are suppressed because of strong of

ZnO.

The

abovementioned

M-Fe/Zn-LDO has successfully grown on the surface of CNTs.

9


results

reveal

that


Page 10 of 38

The surface functional groups were analyzed by FTIR spectroscopy. In the FTIR spectra of CNTs and M-Fe/Zn-LDO@CNTs (Figure 2c), the wide peak of CNTs and M-Fe/Zn-LDO@CNTs at ~3420 cm-1 corresponds to the O-H stretching and bending oscillation of OH- groups 33. In addition, in the FTIR spectrum of CNTs, the peaks at 1730, 1595 and 1040 cm-1 were attributed to the functional groups of –COOH, C-C and C-O, respectively

34

. As for the M-Fe/Zn-LDO@CNTs sample, the adsorption

band around 3420 cm-1 corresponds to the O-H stretching and bending oscillation of the OH- groups, indicating that the structures of CNTs are existed steadily in M-Fe/Zn-LDO@CNTs composites. The peak at 1408 cm-1 assigned the vibration bands of C-OH 27. The peaks at 442~825 cm-1 represented the lattice vibrations of the M-O and M-O-H (M: Fe or Zn) on M-Fe/Zn-LDO@CNTs surfaces

30-32

. From the

characteristic FTIR results, the M-Fe/Zn-LDO@CNTs present various oxygenated functional groups which are beneficial to the adsorption. Figure 2d shows the magnetization cure of M-Fe/Zn-LDO@CNTs composites. The saturation magnetization of M-Fe/Zn-LDO@CNTs is 75.08 emu·g-1, which could be attributed to the fact that the M-Fe/Zn-LDO nanoparticles were combined onto CNTs’ surface via hydrothermal and further calcination process (under nitrogen atmosphere)

35, 36

. An image of magnetic separation of the M-Fe/Zn-LDO@CNTs

composites is shown in the inset of Figure 2d. The M-Fe/Zn-LDO@CNTs nanoparticles were attracted to the magnet rapidly, showing high magnetic sensitivity and easy separation of the composites. To further investigate the elemental composition and chemical state of

10


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M-Fe/Zn-LDO@CNTs composites, the XPS measurements has been carried out. The survey of the XPS spectrum of M-Fe/Zn-LDO@CNTs revealed the presence of predominant peaks such as Fe 2p, Zn 2p, O 1s and C 1s

37-40

(Figure S1), indicating

that the Fe, Zn, O and C were the predominant elements in the M-Fe/Zn-LDO@CNTs composites. Deconvolution analysis of high-resolution XPS spectrum provided an effective way to distinguish the specific chemical species or functional groups on composite surfaces. Deconvolution of C 1s spectrum resulted in three components, i.e., C-C (284.6 eV), C-O (286.3 eV), and O=C-O (289.2 eV)

37, 38

(Figure 3a).

Likewise, deconvolution of O 1s spectrum gave two component signals assigned to Zn-O (530.8 eV) and Fe-O (532.4 eV) species

36, 40

(Figure 3b), suggesting that O

element played significant effect on the synthesis process of M-Fe/Zn-LDO@CNTs composites, meanwhile the Zn-O bonds and Fe-O bonds were well merged with the composites. In addition, the XPS spectrum exhibited strong Fe 2p (Figure 3c) and Zn 2p (Figure 3d) peaks

41, 42

, which further confirmed the presence of Zn- and

Fe-containing groups being in good agreement with the EDS results. Therefore, the abovementioned SEM, TEM, XRD and XPS analyses indicated that M-Fe/Zn-LDO nanocrystals were deposited on the surface of CNTs through the calcination treatment.

Effect of pH and Ionic Strength. Figure 4a and Figure 4b show the influence of

solution

pH

on

adsorption

efficiency

of

U(VI)

onto

CNTs

and

M-Fe/Zn-LDO@CNTs, respectively. The adsorption behaviors of U(VI) on bare CNTs and M-Fe/Zn-LDO@CNTs were obviously influenced by solution pH, because

11



the change of solution pH can change the surface charge of adsorbent, the protonation/deprotonation of adsorbent, and the species of U(VI) ions. The surface charge density and pHpzc (point of zero charge) of M-Fe/Zn-LDO@CNTs were shown in Figure S2. The pHpzc was 9.7, and the surface charge density changed with pH value increasing. The surface charge of M-Fe/Zn-LDO@CNTs was positive at pH < pHpzc due to the protonation reaction. From Figure S5, in the pH range of 2.5 to 6.5, the main species of uranium were UO2CO3 and UO2(CO3)22-, the surface charge of adsorbent decreased from ~30 mV to 15 mV, while the surface complexation increased and electrostatic repulsion decreased between the adsorbent and adsorbate, hence the adsorption of U(VI) increased rapidly. The adsorption of U(VI) on M-Fe/Zn-LDO@CNTs increased rapidly with pH values rising from 2.5 to 6.5, followed by a decline at pH values higher than 7.0. This interesting variation is associated with the distribution of uranium species. In the pH range of 6.5 to 10.0, the main species of uranium was UO2(CO3)34-, the surface charge of adsorbent decreased from ~15 mV to -5 mV (the surface charge of M-Fe/Zn-LDO@CNTs became negative because of the deprotonation process), the surface complexation decreased and electrostatic repulsion increased between the adsorbent and adsorbate, especially at pH ≥ 9.7, the adsorption of U(VI) decreased rapidly. Thus, one can easily to achieve the conclusion that the hydrolysis species of uranium, such as UO22+ [UO2(OH)]+, [(UO2)2(OH)2]2+, and [(UO2)3(OH)5]+, were favorably adsorbed on CNTs and M-Fe/Zn-LDO@CNTs by strong surface complexation with pH increasing. At even higher pH values, the decrease of U(VI)

12


Page 12 of 38

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adsorption might be attributed to the electrostatic repulsion between adsorbent and adsorbate 6. More detailed discussion of acid-base properties was shown in SI. Evaluation of ionic strength effect on the removal of U(VI) is helpful to verify the application potential of adsorbent. From Figure 4a and 4b, it can be observed that the removal of U(VI) using bare CNTs was deeply affected by ionic strength, suggesting that the removal of U(VI) using bare CNTs was dominated mainly by outer-sphere surface complexation, the same as the study of Chen et al 25. Compared to

CNTs,

the

influence

of

ionic

strength

on

removal

of

U(VI)

by

M-Fe/Zn-LDO@CNTs was complicated. At pH values within 2.5 to 6.0, the adsorption increased rapidly, and the influence of ionic strength was weak, which suggested that the inner-sphere surface complexation and electrostatic attraction were the main adsorption reactions. Interestingly, at pH values within 6.0 to 10.0, with the ionic strength (INaNO3) increasing from 0.001 to 0.1 M, the adsorption performance decreased greatly. The remarkable influence of ionic strength revealed that the outer-sphere surface complexation was the main interaction at pH values within 6.0 to 10.0.

6,15,16

The reason for this phenomenon was that the ionic strength could

influence the potential value of adsorbent, which affected the adsorption capacity 16.

Thermodynamics Study. To analyze the thermodynamics process, sorption isotherms of U(VI) on bare CNTs and M-Fe/Zn-LDO@CNTs (Figure 4c and d) were investigated at the temperatures of 298.15 K, 313.15 K and 328.15 K, respectively. The U(VI) adsorption on bare CNTs and M-Fe/Zn-LDO@CNTs were promoted obviously with the temperature increasing, suggesting that the increase of temperature

13



Page 14 of 38

could promote the adsorption capacity. To better reveal the adsorption mechanism, two isotherm models (i.e., Langmuir and Freundlich) were implemented to evaluate the experimental results. The maximum adsorption of U(VI) were calculated to be 140.8 mg·g-1 on CNTs and 380.8 mg·g-1 on M-Fe/Zn-LDO@CNTs at 298.15 K (Table 1). The adsorption performance of U(VI) on M-Fe/Zn-LDO@CNTs was more excellent than that of U(VI) on CNTs, which were comparable to many excellent magnetic materials, as listed in Table S1. The relative parameters calculated from the Langmuir and Freundlich models were tabulated in Table 1. The adsorption process of U(VI) on bare CNTs and M-Fe/Zn-LDO@CNTs were well conformed to Langmuir mode (R2 > 0.98), suggesting that the dominated adsorption mechanisms were monolayer coverage and chemosorption. It was consistent with the strong chemical binding between U(VI) and reaction sites (oxygen-containing functional groups or metal-oxide bonds) on the M-Fe/Zn-LDO@CNTs composites 27. In order to further reveal thermodynamic process, various traditional thermodynamic parameters (e.g., free energy change (∆G, kJ·mol-1), enthalpy change (∆H, kJ·mol-1), entropy change (∆S, J·K-1·mol-1)) were calculated to understand the interaction mechanism. More detailed description was described in SI, and the relative parameters for the adsorption process were tabulated in Table 2. As tabulated in Table 2, the ∆G (M-Fe/Zn-LDO@CNTs: -8.39 to -15.83 kJ·mol-1, CNTs: -5.78 to -8.86 kJ·mol-1) and ∆H (CNTs: 24.7 kJ·mol-1, M-Fe/Zn-LDO@CNTs: 90.0 kJ·mol-1) confirmed that the reaction processes were spontaneous and endothermic

14


43-45

, which

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explained that why the increase of temperature was beneficial to U(VI) adsorption. Moreover, the positive values of ∆S (CNTs: 102.0 J·mol-1·K-1, M-Fe/Zn-LDO@CNTs: 328.0 J·mol-1·K-1) might be caused by the release of H2O molecules in the complexation reaction between surface functional groups and uranyl ions

46

. It

uncovered that the adsorption reaction was randomness at solid-liquid interfaces during U(VI) adsorption onto adsorbents.

Adsorption Kinetics. In order to evaluate the adsorption kinetic, the influence of contact time on adsorption reaction was shown in Figure 4e and 4f. It was obvious to see that the adsorption increased sharply at the first 200 min of contact time (process I), and then rose slowly to achieve equilibrium (process II, 200 ~ 500 min). The process I (0~200 min) might be attributed to the presence of free metal-oxide bonds and abundant free active sites 6. The rich reaction sites or free binding sites on the surface of adsorbents were available for U(VI) capture from aqueous solutions. In process II (200 ~ 500 min), most of the reaction sites on the adsorbents were occupied by adsorbate molecules, the adsorption process might be attributed to the reaction in the internal surface of nanotube structure 25, 29, 33. It was the inter-layer adsorption that main present at the time of 200 to 500 min, which lead to the slow increase of U(VI) adsorption. Compared to M-Fe/Zn-LDO@CNTs, the U(VI) adsorption on bare CNTs was simple, it was dominated by surface adsorption and reached equilibrium within 5 hours. In order to further unravel the adsorption system, the kinetic models (i.e., pseudo-first-order and pseudo-second-order) were used to evaluate mass transfer

15



Page 16 of 38

process, which were expressed as follows in Equation 1 and Equation 2 16:

ln(qe − qt ) = ln qe − k1t

(1)

t t 1 = + 2 qt k2 qe qe

(2)

where qt (mg·g-1) represents the sorption capacity at time t (min), qe (mg·g-1) represents the sorption capacity at equilibrium. k1 (min-1) is the pseudo-first order rate, and k2 (g·mg-1·min-1) is the pseudo-second order rate. Figure 4e, 4f and Figure S4 showed the results of kinetic model simulation, and the kinetic parameters were summarized in Table 3. As graph inset in Figure 4e and 4f, the higher R2 values (R2 = 0.99 and R2 = 0.99) indicated that the sorption of U(VI) to bare CNTs and M-Fe/Zn-LDO@CNTs were better simulated by pseudo-second order model than pseudo-first order model, indicating that the chemical reaction was mainly adsorption process on both CNTs and M-Fe/Zn-LDO@CNTs.

Effect of Co-existing Ions. The co-existing ions (such as anions CO32-, Cl-, Br-, SO42-, NO3- and cations Mg2+, Ca2+, Na+, K+) in surface water environment or the natural groundwater system could affect U(VI) adsorption onto adsorbents

33

. These

anions and cations are very important for the application of Fe/Zn-LDO@CNTs in environmental pollution remediation. In Figure 5a, as the co-existing ion concentration increased from 0.001 to 0.1 M, the affect of co-existing anion ions on the removal of U(VI) from aqueous solution was weakly, except for CO32-. The negative influence of CO32- on U(VI) adsorption onto M-Fe/Zn-LDO@CNTs might be due to two main reasons: (1) The CO32- might change surface charge distribution on the surface of M-Fe/Zn-LDO@CNTs, and thereby caused the electrostatic 16


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interaction between M-Fe/Zn-LDO@CNTs and UO22+ 47; (2) In the presence of 0.1 M CO32+ at pH =6.0 M, the dominating species of U(VI) in aqueous solution were UO2CO3

and

UO2(CO3)22-

(Figure

S5).

The

adsorption

of

U(VI)

onto

M-Fe/Zn-LDO@CNTs was inhibited by the free radical intermediates and stable carboanion in the aqueous solution, and it produced steric hindrance on the removal process between U(VI) onto M-Fe/Zn-LDO@CNTs. In Figure 5b, with the increase of Al3+, Mg2+ and Ca2+ ions’ concentration, the effect of these metal cations on adsorption reaction was obviously. The competitive adsorption might be the main reason for this phenomenon, because the Mg2+, Al3+ and Ca2+ cations occupied the adsorption sites

48, 49

. And this competitive adsorption among UO22+, Mg2+, Al3+ and

Ca2+ caused the decrease of adsorption capacity.

Recycle Performance of M-Fe/Zn-LDO@CNTs. The reusability was a crucial factor to evaluate the practical applications. Therefore, the regeneration and reusability of M-Fe/Zn-LDO@CNTs were studied to evaluate its application potential in the removal and recovery of U(VI). The 0.2 M Na2CO3 was applied as eluent in desorption experiments, and the recycle adsorption experiments were repeat five times (detail experimental operation and corresponding discussion were shown in SI). As shown in Figure S6, the removal efficiency of U(VI) by M-Fe/Zn-LDO@CNTs was still > 89% after five cycles. By virtue of excellent the regeneration performance and considerable sorption capacity for U(VI) elimination, M-Fe/Zn-LDO@CNTs were anticipated to have potentially environmental clean applications for practical U(VI)-contaminated water treatment.

17



Page 18 of 38

Adsorption Mechanism and Application Prospect. To further unravel the adsorption mechanism, the M-Fe/Zn-LDO@CNTs composites were analyzed by XPS spectroscopy before and after U(VI) adsorption. As exhibited in Figure 6a, the strong peaks of C 1s, O 1s, Zn 2p and Fe 2p indicated that C, O, Fe and Zn were the dominated elements on the surface of M-Fe/Zn-LDO@CNTs composites. The new peak of U 4f in the XPS spectrum after U(VI) adsorption indicated that U(VI) was immobilized on the M-Fe/Zn-LDO@CNTs composites. The peaks of U 4f spectra were shown in Figure 6b, it could be assigned to U6+ state contribution with the binding energies at U 4f5/2 and U 4f7/2

3, 27

, indicating that the absence of redox

reaction in removal process. In the adsorption process, the interaction between M-Fe/Zn-LDO@CNTs and U(VI) may be attributed to the strong electrostatic interaction and surface complexation (more corresponding discussion were shown in SI), because of the indented functional groups on the M-Fe/Zn-LDO@CNTs composites

14,31,49

.

The

“Zn-O”

and

“Fe-O”

functional

groups

on

the

M-Fe/Zn-LDO@CNTs also contributed to the sorption of U(VI). Overall, the above results demonstrated that M-Fe/Zn-LDO@CNTs had excellent potential for the removal of actinides (such as U(VI)) from environmental wastewater.

Removal of

241

Am(III). The experimental removal of

241

Am(III) on

M-Fe/Zn-LDO@CNTs was studied at pH 3.0, 5.0 and 8.0 (the experiments were described in SI). As shown in Figure 7, the removal percentage of 241Am(III) reached to 5.5%, 94.5% and 95.9% respectively. The adsorption results of 241Am(III) indicated that the adsorption of

241

Am(III) on M-Fe/Zn-LDO@CNTs composites were

18


Page 19 of 38 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


attributed to

241

Am(III) interaction with various oxygen-containing functional groups

50

. Based on the experimental results and consideration of the tracer-level

concentration of actinides in natural environmental or other radionuclides in nuclear wastewater, it can easily conclude that the M-Fe/Zn-LDO@CNTs might be the potential material in the efficient removal of actinides (such as

241

Am(III)) from

wastewater solutions.

CONCLUSIONS This paper reported the synthesis of M-Fe/Zn-LDO@CNTs composites using hydrothermal method and a further calcination. Series of characterization results indicated that the CNTs were modified on the composites successfully. The as-prepared

M-Fe/Zn-LDO@CNTs

exhibited

an

obviously

strip

structure

(M-Fe/Zn-LDO nanoparticles combined onto CNTs surface) and abundant active sites on the surface of composites. The maximum capacity of U(VI) adsorption achieved to be 380.8 mg·g-1 on M-Fe/Zn-LDO@CNTs at the experimental condition of pH = 6.0 and T = 298.15 K, which was triple than that of CNTs (140.8 mg·g-1). The thermodynamic parameters (i.e., ∆G: -8.4 to -15.8 kJ·mol-1, ∆H: 90.0 kJ·mol-1, ∆S: 328.0

J·mol-1·K-1)

revealed

that

the

adsorption

process

of

U(VI)

onto

M-Fe/Zn-LDO@CNTs was spontaneous and endothermic. Furthermore, the M-Fe/Zn-LDO@CNTs composite presented high removal efficiency for actinides (such as U(VI) and 241Am(III)) through the strong electrostatic interaction and surface complexation. This work provided new insight to fabricate novel magnetic composite

19



Page 20 of 38

with high efficiency in radionuclides’ elimination from wastewater solution in real work in future.

ASSOCIATED CONTENT AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (X.K. Wang) ORCID Xiangke Wang: 0000-0002-3352-1617 Author contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. And the authors Haijun Chen and Zhibin Zhang contributed equally to this work. Notes The authors declare no competing financial interest Supporting Information Synthesis of composites and experimental operation of remove

241

Am(III); XPS

spectra, Zeta potential, adsorption isotherm models and kinetic parameters. The detailed

information

of

UO22+

species

distribution;

Recycle

property

of

M-Fe/Zn-LDO@CNTs for U(VI) adsorption. This information is available free of charge on the ACS Publications website at DOI: 10.1021/acsanm.****.

ACKNOWLEDGMENTS 20


Page 21 of 38 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


This work was supported by Science Challenge Project (TZ2016004), NSFC (21577032, 21561002, 41461070 and 11475044), and China Postdoctoral Science Foundation (2016M600981).

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Page 29 of 38 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


Table 1. Parameters of U(VI) adsorption on CNTs and M-Fe/Zn-LDO@CNTs at three temperatures. CNTs Models

Parameters

M-Fe/Zn-LDO@CNTs

298.15 K

313.15 K

328.15 K

298.15 K

313.15 K

328.15 K

KL (L·mg-1)

0.13

0.051

0.071

0.15

0.92

1.01

qmax (mg·g-1)

140.79

383.49

505.59

380.84

478.46

812.03

R2

0.98

0.98

0.96

0.99

0.89

0.88

KF (mg1-n·Ln·g-1)

41.93

43.37

66.95

105.15

241.40

383.95

1/n

0.27

0.48

0.47

0.30

0.19

0.27

R2

0.97

0.93

0.90

0.91

0.76

0.70

Langmuir

Freundlich

Table 2. Thermodynamic parameters of U(VI) adsorption on CNTs and M-Fe/Zn-LDO@CNTs at different temperatures. Adsorbents CNTs M-Fe/Zn-LDO@CNTs

∆G (kJ·mol-1)

∆H

∆S

(kJ·mol-1)

(J·mol-1·K-1)

298.15 K

313.15 K

328.15 K

24.7

102.1

-5.8

-6.9

-8.8

90.0

328.0

-8.4

-13.1

-15.8

29



Table

3.

Kinetic

parameters

of

U(VI)

Page 30 of 38

adsorption

on

CNTs

and

M-Fe/Zn-LDO@CNTs. Models

Pseudo-first-order

Pseudo-second-order

Adsorbents

q1,cal (mg·g-1)

k1 (min-1)

R2

CNTs

45.4

7.6×10-3

0.99

85.9

6.8×10-4

0.99

134.4

4.7×10-4

0.60

143.0

1.7×10-4

0.99

Ca/Al-LDH@CNTs

k2 q2,cal (mg·g-1) (g·mg-1·min-1)

30


R2

Page 31 of 38 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


CNTs Fe(NO3)3 M-Fe/Zn-LDO@CNTs Zn(NO3)2 650 oC 3 h N2

N2

Precursor solution pH = 8.5

Tube furnace calcinations

Hydrothermal reaction 120 oC 12 h Scheme

1.

Proposed

synthetic

routes

for

M-Fe/Zn-LDO@CNTs composites

31


the

synthesis

of


Figure 1. SEM images of CNTs (a) and M-Fe/Zn-LDO@CNTs (b); TEM images of CNTs (c) and M-Fe/Zn-LDO@CNTs (d); the TEM image of M-Fe/Zn-LDO@CNTs (e); and distribution of C, O, Fe and Zn in the composites and the EDS results of M-Fe/Zn-LDO@CNTs (f) (inset: elemental distribution percentage pie graph)

32


Page 32 of 38

112

220 100 002 311 101

Intensity (a.u.)

10 nm

103

CNTs M-Fe/Zn-LDO@CNTs ZnO Fe3O4 Fe

(b)

110

CNTs

(a)

PDF#36-1451 2θ (degree)

1040

1730 1595

M-Fe/Zn-LDO@CNTs 1730

442-825 1408

3420

3420

1599

Ss = 75.08 emu·g-1

Wavenumber (cm-1)

40 s

Magnet

CNTs

Magnetization (emu·g-1)

(d)

(c) Intensity (a.u.)

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


102

Page 33 of 38

H (Oe)

Figure 2. HRTEM image of M-Fe/Zn-LDO@CNTs (a) and XRD patterns of CNTs and M-Fe/Zn-LDO@CNTs (b); FT-IR spectra of CNTs and M-Fe/Zn-LDO@CNTs (c) and magnetization curve of M-Fe/Zn-LDO@CNTs sample (d).

33



(a)

(b)

C 1s

Intensity (a.u.)

Intensity (a.u.)

C-C

C-O

O 1s Zn-O Fe-O

O=C-O

Binding energy (eV) (c)

Binding energy (eV) (d)

Fe 2p Fe 2p2/3 711.5 eV

Fe 2p1/2 725.3 eV

Intensity (a.u.)

Intensity (a.u.)

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

Page 34 of 38

Zn 2p

Zn 2p2/3 1022.2 eV

Binding energy (eV)

Zn 2p1/2 1044.9 eV

Binding energy (eV)

Figure 3. High resolved XPS spectrum of M-Fe/Zn-LDO@CNTs, C 1s XPS spectrum (a); O 1s XPS spectrum (b); Fe 2p XPS spectrum (c), Zn 2p XPS spectrum (d).

34


Page 35 of 38

(a) qe (mg·g-1)

qe (mg·g-1)

(b)

pH

pH

(c) qe (mg·g-1)

qe (mg·g-1)

(d)

Ce (mg·L-1)

Ce (mg·L-1)

(e)

(f) qe (mg·g-1)

qe (mg·g-1)

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


Contact time (min)

Contact time (min)

Figure 4. Effect of pH on U(VI) adsorption to CNTs (a) M-Fe/Zn-LDO@CNTs (b) at different NaNO3 concentrations. m/V = 0.1 g·L-1, T = 298.15 K, C[U(VI)]initial] = 30 mg·L-1. Adsorption isotherms of U(VI) on CNTs (c) and M-Fe/Zn-LDO@CNTs (d), pH = 6.0± 0.1, I = 0.01 M NaNO3. Adsorption kinetics of U(VI) on CNTs (e) and M-Fe/Zn-LDO@CNTs (f), pH = 6.0± 0.1, I = 0.01M NaNO3, C[U(VI) initial] = 30 mg·L-1, T = 298.15 K.

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(a)

qe (mg·g-1)

qe (mg·g-1)

(b)

Anions species

Cations species

Figure 5. Effect of anions (a) and cations (b) on U(VI) adsorption on M-Fe/Zn-LDO@CNTs. pH = 6.0 ± 0.1, I = 0.01 M NaNO3, C[U(VI) initial] = 30 mg·L-1, T = 298.15 K.

(a)

(b)

before adsorption O 1s Fe 2p

C 1s

U 4f

U 4f 7/2 U 4f 5/2

Intensity (a.u.)

Intensity (a.u.)

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

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Zn 2p

U 4f after adsorption

Binding energy (eV)

Binding energy (eV)

Figure 6. Wide scan XPS spectra of M-Fe/Zn-LDO@CNTs before and after U(VI) adsorption (a), and U 4f XPS spectrum (b).

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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


Adsorption percentage

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pH Figure 7. The removal percentages of different pH values. The trace amount of

241

241

Am(III) on M-Fe/Zn-LDO@CNTs at

Am(III) (~10-9 mol/L), m/V = 0.01 g/L, I

= 0.01 M NaNO3, T = 298.15 K.

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Table of contents graphic

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Zn Layered Double Oxide@Carbon

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