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Highly efficient removal of uranium from aqueous solution using a magnetic adsorbent bearing phosphine oxide ligand: A combined experimental and DFT study Dingzhong Yuan, Shiao Zhang, Zhihao Xiang, Yan Liu, Yun Wang, Xinyue Zhou, Yan He, Wenjun Huang, and Qinghua Zhang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04352 • Publication Date (Web): 30 Jun 2018 Downloaded from http://pubs.acs.org on June 30, 2018

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ACS Sustainable Chemistry & Engineering

Highly efficient removal of uranium from aqueous solution using a magnetic adsorbent bearing phosphine oxide ligand: A combined experimental and DFT study Dingzhong Yuan, †, ‡, * Shiao Zhang, ‡ Zhihao Xiang, ‡ Yan Liu, ‡ Yun Wang, §, * Xinyue Zhou, ‡ Yan He, ‡, * Wenjun Huang, ‡ Qinghua Zhang, ‡, * †

Key Laboratory of Applied Chemistry of Zhejiang Province, Zhejiang University, No. 34

Tianmu Mountain Road, Hangzhou 310028, People’s Republic of China ‡

Jiangxi Province Key Laboratory of Polymer Micro/Nano Manufacturing and Devices, East

China University of Technology, No. 418 Guanglan Avenue, Nanchang 330013, People’s Republic of China §

School of Nuclear science and engineering, East China University of Technology, No. 418

Guanglan Avenue, Nanchang, 330013, People’s Republic of China

*Corresponding author: [email protected] (DZ Yuan) [email protected] (Y Wang) [email protected] (Y He) [email protected] (QH Zhang)

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ABSTRACT It is still a great challenge to develop magnetic adsorbents for the highly efficient entrapment of uranium from aqueous solution. Herein, a novel magnetic adsorbent (denoted as Fe3O4/P (AA-MMA-DVP)) bearing phosphine oxide ligand was designed and synthesized via a DPE (1,1-diphenylethylene) method based on DPE as radical controlling agent, showing an excellent adsorption capacity for uranium at pH 4.5 and outstanding selectivity in aqueous media including 14 co-existing ions. The magnetic adsorbent showed a qmax value of 413.2 mg g-1 at 298 K and pH 4.5, which was higher than that of most of other magnetic adsorbents. The outstanding selectivity (Su=95.8%) for uranium was reasonably ascribed to the strong complexation between UO22+ and P=O groups anchored on the polymer skeletion, which was evidenced by experimental results. Furthermore, the magnetic adsorbent could be isolated by magnetic force and be recycled at least five times without significant loss in adsorption capacity. This work provided a convenient synthetic route to develop a novel magnetic adsorbent with high capacity and strong selectivity for the entrapment of uranium from aqueous solution.

KEYWORDS

Magnetic adsorbent; phosphine oxide ligand; uranium; DFT calculation; Selective removal

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Introduction

Uranium is not only a main fuel for the nuclear energy, but harmful to the environment. Thus, it is very important to develop efficient methods to extract uranium from aqueous solutions.1-2 During the last two decades, magnetic nanoparticles (MNPs) have been extensively used as adsorbents for the entrapment of uranium owing to their easy separation by magnetic force, large specific surface area and high contents of surface active sites.3-6 Up to now, many works have reported on the removal of uranium by magnetic adsorbent including Fe3O4,

7

CoFe2O4,

8

modified Fe3O4 with various small

molecules, 9-13 modified with carbon materials, 14-18 modified with silicon materials, 19-23 modified Fe3O4 with polymer,

24-29

modified Fe3O4 with biochar matrix,

30-31

modified

Fe3O4 with MOF. 32 However, most of these magnetic adsorbents often suffer from many drawbacks such as low adsorption capacity and poor selectivity. Generally, the removal efficiency and selectivity for uranium mainly depends on the functional groups of the adsorbents

33-35

Amongst the various organic ligands studied for the entrapment of

uranium from aqueous media, phosphine oxide ligand has attracted extensive attention owing to its strong complexation with uranium. 36-39 Thus, the adsorbents functionalized with phosphine oxide ligand have been widely used for the removal of uranium, exhibiting excellent adsorption efficiency in uranium separation.

40-44

However,

traditional modification method to introduce phosphine oxide ligand onto the adsorbents was often by wet-chemistry method,

45-46

which has many drawbacks including

complexity, substrate dependency and a high speed of waste manufacture. Meanwhile, the amount of phosphine oxide groups introduced onto the adsorbents through 3



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wet-chemistry method is also limited, due to a difficult and time consuming surface activation is often required during the process of direct bonding of functional groups to the adsorbents matrix. All these drawbacks would prevent traditional wet-chemistry method from wide application of modifying adsorbents with phosphine oxide ligand. Thus, it is still a great challenge to develop a convenient synthetic route to get magnetic adsorbents with phosphine oxide ligand to extract uranium efficiently from aqueous media. Recently, it has been reported a novel polymerization method named DPE (1,1-diphenylethylene) method based on DPE as radical controlling agent for the preparation of MNPs, showing a great prospect of application in catalysis, separations,

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and controlled drug release.

48

47

nuclear

Generally, the DPE method contains two

steps. 49 Firstly, methyl methacrylate and acrylic acid was copolymerized in the presence of DPE to obtain a DPE-containing amphiphilic precursor, which could react with Fe3O4 nanoparticles easily via bonding of -OH of Fe3O4 with -COOH of the precursor. Secondly, the third functional monomer and residual monomer would be initiated by the activated precursor anchored on Fe3O4 surface to form functional MNPs, after the third functional monomer was thrown to the reaction system. Thus, compared to traditional method, DPE method for preparation of MNPS for the removal of the target species has obvious advantages in the following: (1) many functional groups could be introduced onto magnetic adsorbents directly without traditional wet chemical modification, 33 since functional monomers could be added during the polymerization process; (2) owing to functional monomers could be added during the third polymerization process, functional

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groups could also be distributed controllably on the surface of MNPS, which not only results in the high adsorption capacity of templates, but offers excellent accessibility to the target substance and low mass-transfer resistance; (3) since the subsequent polymerization would occur controllably on the Fe3O4 surface, the magnetic content of obtained MNPs is usually high,

49

making MNPS separate from the aqueous solutions

quickly by an external magnet. Taking into accounts of these merits, the DPE method could be regarded as an alternative to prepare MNPs functionalized with phosphine oxide ligand for the highly efficient entrapment of U (VI) from aqueous media. Herein, we reported a new magnetic adsorbent modified with phosphine oxide ligand (denoted as Fe3O4/P(AA-MMA-DVP)) synthesized by DPE method. The prepared magnetic adsorbent was characterized comprehensively and the sorption behavior of uranyl ions was also evaluated in detail using batch experiments under different sorption conditions. Insights of nature of interaction between UO22+ with the magnetic adsorbent was also revealed by performing DFT calculation and XPS analysis. Very interestingly, Fe3O4/P(AA-MMA-DVP) not only shows highly selectivity for uranium, but gives a maximum adsorption capacity value of 413.2 mg g-1. This work might provide a facile synthetic route to develop magnetic adsorbents to extract U (VI) efficiently from aqueous media in the future.

Experimental

Synthesis of Fe3O4/P(AA-MMA-DVP) functionalized phosphine oxide ligand. Fe3O4 nanoparticles were synthesized via co-precipitation.

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The DPE method was


chosen to synthesize the magnetic adsorbent. 49 The detail synthetic process was described in the Supporting information. The polymer shell P (AA-MMA-DVP) of the magnetic adsorbent was cleaved according to the reference.

49

HF solution must be used to ascertain polymer shell could be cleaved

successfully. Adsorption tests. To evaluate the sorption property of the prepared adsorbent for the entrapment of uranium, static batch experiments were performed. 10 mg of the magnetic adsorbent was placed into 25 mL of U (VI) solutions or multi-ion solution at a certain value of pH regulated by utilizing negligible volumes of HNO3 or NaOH solution and determined on a digital pH meter. After stirred for a given time at a certain temperature, the U(VI)-loaded adsorbent was isolated by magnetic force. A UV-vis spectrophotometer with arsenazo (III) as the complex agent was used to determine the concentrations of UO22+ in the aqueous media. The concentrations of metal ions including uranyl ions in solution were measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES), when the uranium-selectivity of the magnetic adsorbent was evaluated. Sorption capacity qe (mg g-1) and Kd (mL g-1) were obtained according to the following two equations:

qe =

( co − ce ) × V

Kd =

w

(1)

( co − ce ) ×V ce × w

(2)

Where C0 and Ce (mg L−1) are the concentrations of UO22+ in the aqueous solution before

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and after adsorption, respectively; w is the mass of sorbent (mg); V is the bulk of the suspension (L); and Kd is the distribution coefficient (mL g-1). Uranium - selectivity (Su) to reflect the degree of the selective sorption of uranium for the magnetic adsorbent was calculated by Eqs. (3). Su =

qe −U × 100% qe −tol

(3)

Where qe-tol is the concentration of all metal cations adsorbed (mmol g-1); qe-U is the concentration of uranium adsorbed (mmol g-1). All the experiments were performed at least in three duplicates.

Reusability studies. To study the recyclability of the prepared magnetic adsorbent, adsorption-desorption experiments were performed in five consecutive cycles with CU(VI) initial of 100 mg L-1 at 298 K (m/V=0.4 g L-1, pH=4.5). After adsorption process was end, the U(VI) loaded Fe3O4/P(AA-MMA-DVP) was isolated by magnet force, and then added into 10% NaCl in 0.1 mol L-1 hydrochloric acid solution (50 mL) and agitated using a shaker at 25 ℃ for 3h. After each cycle, the magnetic adsorbent was regenerated and eluted with purified water five times and then lyophilized for the next use.

Computational Methods. To discover the adsorption mechanism, DFT calculations were carried out using the Gaussian 09 suite of programs. 50 All the geometric constructions under study were optimized by dispersion-corrected DFT method, B3LYP-D3, which was developed by the group of Grimme.

51-52

In the calculations, the ECP60MWB-SEG basis set with relative pseudo

potential was adoppted to describe uranium atoms,

53-54

which consider 60 electrons as the

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core and the other 32 electrons were treated via 6-31G (d) basis set. 55 The latter basis set was also employed for the remaining atoms, C, H, O, and P. During the calculations, symmetry or geometry constraint was not imposed. Frequency calculations were carried out at the same theoretical level to confirm all the structures found were real minima on the potential energy surface. To simulate the real solution environment, effect of the solvent water (ε=78.3553) on the sorption was considered utilizing the continuum solvent method SMD, developed by Truhlar and his co-workers. 56 The binding energy was obtained using the total energy of a complex minus the energy of the individual monomers involved in the complex. All the binding energies were corrected by the standard counterpoise procedure of Boys and Bernardi,

57

to explain the basis set

superposition error (BSSE), the corrected binding energy was defined as ∆Ec.

Results and discussions Structural analyses. Scheme 1 listed the synthetic route of the phosphonate functionalized magnetic adsorbent Fe3O4/P(AA-MMA-DVP). To investigate whether the polymerization shown in Scheme 1 proceed successfully, the FT-IR, NMR and XPS analytical methods were used in this work. An adsorption band at 585 cm-1 assigned to the stretching vibration of Fe-O bond of Fe3O4 was observed in Fig. S1 A (a). However, the intensity of characteristic absorption of Fe-O bond in Fig. S1A (b) was much weaker than that in Fig. S1A (a), and many characteristic peaks ascribed to P(AA-MMA-DVP) also appeared. Fig. S1B was the 1H NMR spectrum of P (AA-MMA-DVP). There were many characteristic peaks which assigned to DVP, MMA and AA unit, respectively. 58 for example, the signals belonged to syndiotactic (rr),

8


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atactic (mr), and isotactic (mm) methyls were at 0.84, 1.02 and 1.30 ppm, respectively.

59-61

The adsorption from 1.4 to 2.4 ppm was owed to the methylene protons (aH, cH, fH) from AA, MMA and DVP, and the absorption of methine protons (bH, gH) from AA and DVP unit 62-64

was also overlapped in this region.

The peaks at 177 (4C), 53 (6C) and 51 (2C) were

attributed to the carbons of AA and MMA (Fig. S1C). The peaks at 14.8 (9C) and 62.6 (10C) were attributed to the carbons from DVP. As shown in Fig. S1D, the peak at 133.0 eV 65

attributed to P2p could be found clearly,

indicating phosphonate groups had been grafted

onto the polymer chain. The high resolution of C 1s XPS spectrum was resolved into four main peaks occurring at 287.7 eV, 286.3eV, 285.2eV and 284.9 eV, respectively (Fig. S1E), which belonged to C=O, C-O, C-P and C-C. 66 All these results suggested the polymerization described in Scheme 1 had proceed successfully. COOCH 3

Ph

COOH

KPS CH2= C

+

CH

CH2=

CH2

+ CH2= C

CH3 AA

CH3 CH

C

CH2

CH2

C

CH3

1

CH2

C

COOCH 3 Ph

Ph

Ph COOCH 3 COOCH 3

O

CH2 C

CH2

C

C CH CH2

O Fe3O4 nanoparticles

C

Ph

DPE COOCH 3

OH +

COOCH 3

COOH

Ph

MMA

CH2

CH

COOCH 3

CH3

COOH

O O P O

O P O O

O O P O

80℃

C

CH2 C

CH2

Ph

O P

CH2

CH3

C Ph

O O

P O O O

2 3

COOCH 3

COOCH 3

C

C

CH3 CH2

CH

CH2

CH2

CH2

C CH2

COOH

CH3

C

CH2CH2CH

COOCH 3

COOH

Scheme 1 synthetic route of the phosphonate functionalized magnetic adsorbent Fe3O4/P (AA-MMA-DVP)

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Fig.1 TEM image of Fe3O4 (A), Fe3O4/P (AA-MMA-DVP) (B), Photograph of Fe3O4/P (AA-MMA-DVP) dispersed in water (C), Fe3O4/P (AA-MMA-DVP) separated by magnetic force within fifteen seconds (D) Fig. 1A and 1B showed the TEM morphologies of Fe3O4 and the magnetic adsorbent, respectively. Fe3O4 nanoparticles prepared by the chemical coprecipitation had an average size of 10.0 nm with a quasi-spherical structure (Fig. 1A). From Fig. 1B, a large amount of Fe3O4 nanoparticles (dark inner) had been successfully encapsulated by polymer P(AA-MMA-DVP) (light outer). Meanwhile, Fe3O4/P(AA-MMA-DVP) had a good spherical morphology with an average diameter of 800 nm (Fig. S2). All these results clearly showed the prepared magnetic adsorbent could be prepared facilely by DPE method. Besides, the adsorbent had a good dispersity in water to form a yellow solution, indicating the adsorbent had excellent hydrophilic surface, which could help to extract uranium from aqueous media (Fig. 1C). Meanwhile, both Fe3O4 and Fe3O4/P(AAMMA-DVP) were also characterized by XRD, VSM and TGA. The XRD spectra of the two samples proved Fe3O4 had been triumphantly 10


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coated by the amorphous polymer P (AA-MMA-DVP) (Fig. S3A). From Fig. S3B, the saturation magnetic moments of the magnetic adsorbent could reach ca.16.7 emu g-1, indicating it could be easily recycled from the adsorption system by magnetic force within fifteen seconds (Fig. 1D). The TGA curve of Fe3O4/P (AA-MMA-DVP) showed the mass loss was 73.1 %, which indicated the magnetic content of the adsorbent could be up to 26.9 % (Fig. S3C). Finally, the XPS characterization of Fe3O4/P (AA-MMA-DVP)-U indicated U(VI) could be adsorbed onto Fe3O4/P (AA-MMA-DVP), due to a new signal belonging to the antisymmetric vibration of the [O=UVI=O]2+ was found in Fig. S3D. 67-69

Effect of pH effect of H+ concentration on the sorption property of the magnetic adsorbent was studied at different pH values varying from 1.0 to 9.0. From Fig. 2A, the uranium adsorption capacity of the magnetic adsorbent was very low at pH

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