Photocatalytic reduction of uranium(VI) by magnetic ZnFe2O4 under visible light

Journal Pre-proof Photocatalytic Reduction of Uranium(VI) by magnetic ZnFe2 O4 under Visible Light Peng-liang Liang (Investigation) (Writing - original draft), Li-yong Yuan (Conceptualization)Writing - Review and editing) (Project administration), Hao Deng (Formal analysis), Xu-cong Wang (Validation), Lin Wang (Visualization), Zi-jie Li (Visualization), Shi-zhong Luo (Conceptualization) (Supervision), Wei-qun Shi (Conceptualization) (Resources) (Supervision) (Funding acquisition)

PII:

S0926-3373(20)30103-X

DOI:

https://doi.org/10.1016/j.apcatb.2020.118688

Reference:

APCATB 118688

To appear in:

Applied Catalysis B: Environmental

Received Date:

6 October 2019

Revised Date:

15 January 2020

Accepted Date:

24 January 2020

Please cite this article as: Liang P-liang, Yuan L-yong, Deng H, Wang X-cong, Wang L, Li Z-jie, Luo S-zhong, Shi W-qun, Photocatalytic Reduction of Uranium(VI) by magnetic ZnFe2 O4 under Visible Light, Applied Catalysis B: Environmental (2020), doi: https://doi.org/10.1016/j.apcatb.2020.118688

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Photocatalytic

Reduction

of

Uranium(VI)

by

magnetic ZnFe2O4 under Visible Light Peng-liang Lianga,b, Li-yong Yuana,*, Hao Denga, Xu-cong Wanga , Lin Wanga, Zi-jie Lia ,

a

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Shi-zhong Luob,*, Wei-qun Shia,*,

Laboratory of Nuclear Energy Chemistry, Institute of High Energy Physics, Chinese Academy of

b

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Sciences, Beijing 100049, China

Beijing Key Laboratory of Bioprocess, College of Life Science and Technology, Beijing University of

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Chemical Technology, Beijing 100029, China

AUTHOR INFORMATION Corresponding Author

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* E-mail:[email protected] (L.Y. Yuan) Tel: +86-010-88235242 * E-mail: [email protected] (S.Z. Luo) Tel: +86-010-64438015

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* E-mail: [email protected] (W. Q. Shi). Tel: +86-010-88233968, Fax: +86-010-88235294

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Graphical Abstract

100

1 st

3 rd

2 nd

80

Visible light C/C0

60

U(VI)

U(IV)

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40

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20

0 30 60 90 1200

30 60 90 120

Irradiation time (min）

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ZnFe2O4

30 60 90 1200

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0

Highlights

Visible-light-driven photoreduction of U(VI) is achieved using ZnFe2O4 as catalyst.



50 ppm of uranium(VI) was almost completely removed in 60 min.



The photocatalytic activity of ZnFe2O4 is dependent on its morphology.



ZnFe2O4 rods own good stability, recyclability and magnetic separability.

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

ABSTRACT: Visible-light-driven photocatalytic reduction of uranium(VI) is becoming an effective manner to remove uranium(VI) from waste water, whereas applicable catalysts are extremely limited. Herein, we report a first study of visible-light-driven photocatalytic reduction of uranium(VI) using visible light responsive ZnFe2O4. The ZnFe2O4 catalysts with different morphologies were successfully obtained and well characterized. The photoreduction of uranium(VI) under visible light was achieved

over these ZnFe2O4 samples with the activity order of rods > microspheres > nanoparticles. Using ZnFe2O4 rods, for example, the 50 ppm of uranium(VI) was almost completely removed in 60 min, representing one of the most effective visible-light-driven photocatalytic removal. The effects of catalyst dosage, hole scavenger (CH3OH) dosage and solution pH on the photocatalytic reactions, as well as the photoreduction mechanisms were investigated in detail. In addition, ZnFe2O4 rods own good stability, recyclability and magnetic separability. All these features make ZnFe2O4 a promising photocatalyst for radioactive environmental remediation.

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KEYWORDS: Morphology, ZnFe2O4, uranium(VI) photoreduction, visible light, magnetic separation

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1. INTRODUCTION

Uranium, as the major constituents of nuclear fuel, was usually released into the natural

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environment from nuclear industry activities, such as uranium mining and radioactive material disposal. The removal of uranium pollutants became a urgent and meaningful issue in view of the threats to ecosystem and human health induced by uranium’s chemotoxicity and radiotoxicity [1]. A variety of

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technologies such as evaporation [2], solvent extraction [3], adsorption [4-8] and chemical precipitation [9], had been developed in last decades to prevent radioactive contamination. Although some of these

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technologies have achieved encouraging results，their limitations and demerits require attentions. For example, the evaporation was hindered by a high energy costs [2], while the adsorption and the chemical precipitation may be accompanied by second pollution [10-11].

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The uranium species in natural environment mainly consist of highly mobile hexavalent uranium(VI) and relatively immobile tetravalent uranium(IV), thus reduction of soluble uranium(VI) into sparingly soluble uranium(IV) is believed to be a plausible approach to fight against the uranium pollutants [12]. Toward this end, chemical reduction [13-15], photocatalytic reduction [16] and

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microbial reduction [17-18] have been explored, among which, photocatalytic reduction as a simple, green and efficient method has been highly emphasized in last decades. Up to now, various kinds of materials, such as titanium dioxide and its composites [10, 19-22], Ti3C2/SrTiO3 heterostructure [23] and K2Ti6O13 hybridized graphene oxide [24], have been evaluated as photocatalyst for uranium(VI) removal. These catalysts exhibited excellent photocatalytic activity towards uranium(VI), but holding clear demerits in activation by ultraviolet light. Visible-light-driven photocatalysts including of Fe2O3graphene oxide composites [2], titanate/niobite nanocomposite [25], Sn-doped In2S3 [26], and nonmetallic element (B, S, P) doped graphitic carbon nitride [27-30] have also been reported for

photoreduction of uranium(VI). The application of these catalysts, however, is always limited by either low

photocatalytic

efficiency (e.g.

Fe2O3-graphene

oxide

composites

[2],

titanate/niobite

nanocomposite [25]) or high cost (e.g. Sn-doped In2S3 [26]) of the catalysts. In addition, all these powder photocatalysts are difficult to separate and recover during the practical application. The magnetic materials which are separated easily in the presence of external magnetic field are attractive candidates to overcome the problem [31-32]. In a word, finding versatile photocatalysts with strong visible light response, low cost and ready availability becomes an imperative task. Zinc ferrite (ZnFe2O4) as one of the transition-metal spinal ferrites has attract widespread attention

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because of its low cost, visible light absorbance (Eg≈1.9 eV), extraordinary photochemical stability and magnetic recyclability. It shows excellent performance in many fields, such as magnetic materials[33],

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photocatalysts [34], gas sensors [35], supercapacitors [36], lithium ion batteries [37] and photoelectrodes [38]. Among these applications, much attention has been paid to its photocatalytic

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performance which includes degradation of organic pollutants [34, 39-40], NO removal [41-42], H2 production [43-44] ,Cr(VI) reduction [45] and so on. With a conduction band position around at -0.72

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V vs. normal hydrogen electrode (NHE), ZnFe2O4 is obviously thermodynamically feasible to photocatalytic reduction of uranium(VI) since the reduction potentials of UO22+/U4+ (0.267 V) and

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UO22+/ UO2 (0.411 V) are more positive [10, 39], although no data on this topic have been reported. Up to now, a wealth of methods including hydrothermal method [43], solvothermal method [46], thermal decomposition [47], urea combustion [37], sol-gel [48], coprecipitation [41] have been reported to

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prepare ZnFe2O4, and different preparation methods often produce different morphologies of ZnFe2O4 [49]. For example, Zhou [46] et al. prepared ZnFe2O4 hollow flower-like microspheres by a solvothermal strategy, while Jia [47] et al. obtained porous ZnFe2O4 nanorods by the decomposition of ZnFe2(C2O4)3 nanorods precursor. A variety of morphologies of ZnFe2O4, such as nanoparticles [50], rods [38, 47], nanocubes [49], yolk-shell and double shell structures [51], nano-flakes [36] and

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microspheres [46, 52] have been synthesized by various methods. It is well known that the shape of materials has a great influence on its properties [53-56], these different morphologies always result in different physical and chemical properties of ZnFe2O4 [34], thus affecting its photocatalytic performance. Manisha et al. for example, investigated the effect of the morphologies of ZnFe2O4 on the photocatalytic degradation of dyes. It was concluded that ZnFe2O4 nanorods exhibited better photocatalytic activity compared to other three morphologies of ZnFe2O4 [57]. Whether the different

forms of ZnFe2O4 affect the photocatalytic reduction of uranium(VI)? A careful study is required for the answer. In this work, we conducted a first study of visible-light-driven photocatalytic reduction of uranium(VI) using ZnFe2O4 as catalysts. The aims of this work are to (1) develop a simple and applicable system for photocatalytic reduction of uranium(VI) under visible light and (2) characterize the correlation between the morphologies of ZnFe2O4 and the performance on the photoreduction of uranium(VI). To this end, the visible light responsive and magnetically separable ZnFe2O4 with

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different forms of nanoparticles, rods and microspheres were obtained by tailoring the reaction conditions. Then these catalysts were used to photocatalytic reduction of uranium(VI) into uranium(IV) under visible light. With ZnFe2O4 rods as the photocatalytic model, the effects of catalyst doses, hole

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scavenger (CH3OH) doses and solution pH on the photocatalytic reduction of uranium(VI) were investigated in detail. The plausible photocatalytic mechanism was also proposed based on XPS, Mott-

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Schottky test and photocurrent test. Considering the practical application, the stability, recyclability and magnetic separability of the catalyst were also assessed. To the best of our knowledge, this work

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represents the first study on the visible-light-driven photocatalytic reduction of uranium(VI) by magnetic ZnFe2O4.

2.1. Materials synthesis

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2. EXPERIMENTAL SECTION

All the chemicals and reagents were of analytical grade and used as such without any further

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purification. Zinc sulfate heptahydrate (ZnSO4·7H2O) and ferrous ammonia sulfate hexahydrate ((NH4)Fe(SO4)·6H2O) were purchased from Beijing Chemical Works, China. Ethylene glycol (C2H6O2), oxalic acid dihydrate (C2H2O4·2H2O), zinc acetate dihydrate (Zn(CH3COO)2·2H2O) and iron nitrate nonahydrate (Fe(NO3)3·9H2O) were purchased from Sinopharm Chemical Reagent Co., Ltd, China. Glycerol (C3H8O3) and isopropanol (C3H8O) were purchased from Shanghai Aladdin Bio-Chem

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Technology Co., Ltd, China.

ZnFe2O4 nanoparticles (named as ZFO-1) was of analytical grade and was purchased from

Shanghai Aladdin Bio-Chem Technology Co., Ltd, China. ZnFe2O4 rods (named as ZFO-2) was synthesized according to previously reported work [47]. Solution A: 2 mmol ZnSO4·7H2O and 4 mmol (NH4)Fe(SO4)·6H2O were dissolved in the mixture of 10 mL deionized water and 30 mL ethylene glycol, followed by 30 min stirring. Solution B: 6 mmol C2H2O4·2H2O was dissolved in the identical mixture. Solution B was dripped into Solution A and kept

stirring for 60 min. Then the mixed suspension was transferred to 100 mL teflon-lined autoclave and maintained at 120 oC for 24 h. The obtained yellow solid was washed with water and ethanol and dried at 80 oC for 12 h. Finally, the sample was annealed in a muffle furnace at 400 oC for 2 h with a heating rate of 1 oC/min. ZnFe2O4 microspheres (named as ZFO-3) was obtained according to previously reported work [46]. 1 mmol Zn(CH3COO)2·2H2O and 2 mmol Fe(NO3)3·9H2O were dissolve in the mixture of 8 mL glycerol and 30 mL isopropanol. Then the homogeneous solution kept in a 50 mL teflon-lined autoclave was maintained at 180 oC for 12 h in an oven. A green solid was obtained by centrifuged and

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washed with water and ethanol. Finally, the solid after dried at 80 oC for 12 h was transferred to a muffle furnace and annealed at 400 oC for 2 h with a heating rate of 2 oC/min.

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2.2. Material characterizations

The crystallographic properties of ZnFe2O4 were acquired from X-ray diffraction (XRD) with a

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Bruker D8 Advance diffractometer using Cu Kα radiation at a scan rate of 5 o/min. Fourier transforminfrared (FI-IR) spectra of the ZnFe2O4 samples was recorded on a Bruker Tensor 27 spectrometer. KBr

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pellets in the scan range of 4000-400 cm-1 were used for analysis. Raman spectra was collected using a HORIBA Scientific spectrometer with a 473 nm laser as the excitation source (Ciel model, Laser

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Quantum, Ltd.).The porous feature of ZnFe2O4 was investigated by N2 adsorption-desorption isotherms using a Micromeritics ASAP 2020 apparatus with prior degassing of the product under vacuum at 120 °C overnight. SEM images of these samples were performed on a S-4800 Field Emission Scanning

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Electron Microscopy (FESEM: Hitachi S-4800) at an accelerating voltage of 10 kV. TEM and HRTEM analyses were carried out with a Tecnai G2 F20 transmission electron microscope (Fei-Tecnai G2-F20) at an accelerating voltage of 200 kV. UV-Vis absorption reflectance spectra was obtained using a Hitachi U-3900 UV-Vis spectrophotometer. BaSO4 was used as a reflectance standard in a UV-Vis diffuse reflectance experiment. X-ray photoelectron spectroscopy (XPS) of samples were taken on a

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Kratos Axis ultra (DLD) equipped with Al Kα source. All the binding energies were referenced to the C 1s peak at 248.8 eV of the surface adventitious carbon. Magnetism of samples was measured at room temperature using vibrating sample magnetometers (VSM, LakeShore 7404) in the field range of 18000 to +18000 Oe. 2.3. Photocatalytic reduction of uranium(VI) Photocatalytic tests were carried out in a 200 mL glass photo-reactor which cooled by circulation water (25 ± 0.2 oC). A 300 W xenon lamp with a 420 nm cut off filter was used as the light source.

Typically, 10 mg of ZnFe2O4 was added into 37.5 mL deionized water and stirred for 1h. Then 24 mmol methanol (hole scavenger) and 12.5 mL of 200 mg/L uranium(VI) solution was added into the mixture, followed by the pH adjustment with negligible amounts of 0.1 mol /L NaOH or HCl solutions. Before light irradiation, the suspension was bubbled with N2 for 2 h under magnetic stirring to ensure the anaerobic condition and achieve the adsorption-desorption equilibrium. After that, the suspension was irradiated during which 0.4 mL suspension was taken out and filtered through 0.22 um Nylon syringe filters at certain time intervals. The uranium(VI) concentration in the filtrate was measured by

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inductively coupled plasma optical emission spectrometer (ICP-OES, Horiba JY2000-2, Japan)，and the reduction amount of uranium(VI) was obtained by determining Ct/C0, where C0 and Ct are the

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uranium(VI) concentration at adsorption-desorption equilibrium without irradiation and after t min irradiation, respectively. The effects of catalyst doses (0.1-0.6 g/L), hole scavenger doses (methanol, 048 mM) and pH (3-6) on the photocatalytic reduction of uranium(VI) were investigated using ZFO-2 as

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the catalyst.

2.4. Electrochemical and photoelectrochemical measurements

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The photocurrent and Mott-Schottky plots were measured at an electrochemical analyser (CHI 660D electrochemical workstation, Chenhua Instrument, Shanghai, China). A 300 W Xenon lamp was

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used as the light source and 0.1 mol /L NaOH aqueous solution was used as the electrolyte. A three electrode system was used in which a Pt foil as the counter electrode and a silver-silver chloride as the reference electrode were used. The working electrode was prepared by dip-coating ZnFe2O4 onto a 1.0

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cm×1.0 cm FTO glass.

3. RESULTS AND DISCUSSION

3.1. Characterizations of the prepared ZnFe2O4 samples The crystal structure and phase purity of the prepared samples were determined using powder Xray diffraction (PXRD). As shown in Fig. 1(a), all the samples exhibit diffraction peaks at 2θ values of

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18.3o, 30.1o, 35.3o, 36.9o, 43.0o, 53.3o, 56.8o, 62.3o, 70.7o, and 73.8o, which are indexed to the (111), (220), (311), (222), (400), (422), (511), (440), (620) and (533) planes of cubic spinel ZnFe2O4 (JCPDS:01-077-0011), respectively [58]. No other discernable peaks were observed, indicating that single phase ZnFe2O4 samples with high purity were obtained. The average crystallite sizes for ZFO-1, ZFO-2 and ZFO-3 are 18.2, 16.9 and 18.7 nm, respectively, which was estimated from the values of full-width at half-maximum of the most intense (311) reflection using the Scherrer equation. Fig. S1 and Fig. S2 display the FT-IR and Raman spectra of the prepared ZnFe2O4 samples, respectively. The

characteristic absorption peaks of ZnFe2O4 assigned to the stretching vibrations of Zn-O modes and Fe-O modes are observed at 550 and 415 cm-1 [50], respectively, while the characteristic Raman bands of ZnFe2O4 appear in the range of 200-900 cm-1 [59]. Given the fact that the surface area and porous characteristics of a catalyst directly affect its photocatalytic activity, the porous structure of the prepared ZnFe2O4 samples were determined by N2 adsorption-desorption isotherms. As can been seen from Fig. 1(b), all samples show similar isotherms and are hithermost to the shape of type IV cures, revealing a typical mesoporous structure [45]. The

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BET specific surface area (SBET) for ZFO-1, ZFO-2 and ZFO-3 are 24, 82 and 47 m2/g, respectively. And the average pore sizes of the three samples are 30.0, 2.9 and 10.2 nm, respectively (Table1), which further confirms the mesoporous structure of the prepared ZnFe2O4. Among these catalysts, ZFO-2

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shows the maximum specific surface area and maximum pore volume, which could provide more reaction sites for photocatalytic reduction of uranium(VI), thus being expected to have the best

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ZFO-3

JCPDS:01-077-0011

30

40

50

60

2-Theta(degree)

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20

70

80

Pore volume(cm3/g·nm)

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100

ZFO-2

10

150

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（ 422） （ 511） （ 440）

（ 400）

(a)

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ZFO-1

（ 220） （ 331） （ 222）

（ 111）

Intensity(a.u.)

photocatalytic performance [40].

0.001

0.000 0

15

30

45

60

Pore width(nm)

50

0 0.0

(b)

ZFO-1 ZFO-2 ZFO-3

0.002

0.2

0.4

0.6

0.8

Relative Pressure(P/P0)

1.0

(c)

(d)

(e)

(f)

(g)

(h)

Fig. 1. Characterizations of the prepared ZnFe2O4 samples. (a) XRD patterns; (b) N2 adsorption-desorption isotherms and BJH pore size distribution; SEM images of ZFO-1 (c), ZFO-2 (d), and ZFO-3 (e); TEM and

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HRTEM images of ZFO-1 (f), ZFO-2 (g), and ZFO-3 (h).

FESEM of the prepared ZnFe2O4 samples was recorded to characterize their morphologies. As

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shown in Fig. 1(c), ZFO-1 displays a disordered arrangement of roughly spherical nanoparticles, and the size of these aggregated nanoparticles is at the range of 20-40 nm (Fig. S3(a)). Unlike ZFO-1, 1D

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rod shaped structure with the diameter of 300-800 nm and the length of several micrometers is observed for ZFO-2 (Fig. 1(d)). By careful discrimination (Fig. S3(b)), it is found that the 1D rod

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structure is composed of numerous nanoparticles. ZFO-3 is found to be well-dispersed microspheres structure with the size at the range of 200-500 nm (Fig. 1(e)), and these microspheres are composed of

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numerous nanosheets as evidenced by the enlarged FESEM image (Fig. S3(c)). Subsequently, TEM as well as HRTEM images of the three samples was also recorded to understand their microstructure in detail. Fig. 1(f) displays the result of ZFO-1, which illustrates that nanoparticles with the size in the

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range of 20-40 nm were formed without any specific morphology. Fig. 1(g) shows that ZFO-2 exhibits rod shaped structure with a diameter above 200 nm, and the rod shaped structure consists of numerous nanoparticles and nanopores, which is in good consistency with the FESEM results. The nanopores not only provide more active sites for photocatalytic reactions, but also facilitate the diffusion of the metal ions, both of which benefit photocatalytic reduction of uranium(VI). ZFO-3 is made up of numerous

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well-dispersed microspheres with the diameter of 500 nm on the basis of TEM images in Fig. 1(h). Fig. 1(f)-Fig. 1(h) also show (220) and (111) planes of cubic spinel ZnFe2O4 (JCPDS:01-077-0011), which further prove the successful preparation of ZnFe2O4. All the above results clearly suggest that the prepared ZnFe2O4 samples show very different forms, and these samples can thus be used to investigate the morphological dependence on the photoreduction of uranium(VI) by ZnFe2O4. Table 1. Surface area, pore volume and average pore size of the prepared ZnFe2O4 samples

Surface area

Sample

Pore

2

volume Average pore size

3

(m /g)

(cm /g)

(nm)

Nanoparticles (ZFO-1)

24

0.13

30

Rods (ZFO-2)

82

0.16

2.9

Microspheres (ZFO-3)

47

0.14

10.2

The optical properties of the different ZnFe2O4 samples were analyzed by the UV-Vis diffuse reflectance spectra. As can be seen from Fig. 2(a), compared to TiO2 whose absorption edge is below 400 nm, all the ZnFe2O4 samples show significant light absorbance from 200 to 700 nm, which

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indicates higher light utilization efficiency for these samples. Furthermore, the values of band gap (Eg) for ZFO-1, ZFO-2 and ZFO-3 shown in Fig. 2(b) are estimated to be 2.02, 2.08 and 1.93 eV,

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respectively. It is well documented that the particle sizes affect the band gap values of semiconductor. For example, NdVO4 [60] and SnO2 [61] with different particle sizes own different band gap values,

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respectively. In our work, the ZnFe2O4 samples with different morphologies have very different particle sizes (Fig. 1(c)-Fig. 1(h)), thus show different band gap values. Whatever, the narrow band gap

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Absorbance(a.u.)

(a)

200

300

400

500

600

Wavelegth(nm)

ZFO-1 ZFO-2 ZFO-3 TiO2

700

(ahv)2(eV2nm-2)

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values guarantee the photocatalytic activity of ZnFe2O4 samples under visible light.

800

15

(b)

ZFO-1 ZFO-2 ZFO-3

10

5

0

1.6

1.8

2.0

hv(eV)

2.2

2.4

Fig. 2. UV-Vis diffuse reflectance spectra (a) and calculated band gap of prepared ZnFe2O4 samples (b)

XPS measurement was further performed to confirm the surface chemical composition and elemental valence of ZFO-2. The coexist of Zn, Fe and O elements for ZFO-2 are observed in the full XPS spectrum (Fig. 3(a)). The characteristic peaks for Zn 2p1/2 and 2p3/2 appear at binding energies of

1045.4 and 1022.5 eV, respectively [46], as can be seen from high-resolution Zn 2p spectrum (Fig. 3(b)). As for O 1s spectrum (Fig. 3(c)), the broad curve is devolved into two peaks located at 532.8 and 531.0 eV, respectively, where the peak at 531.0 eV is assigned to the surface lattice oxygen including Zn-O and Fe-O linkage, while another peak is the characteristic signal for surface oxygen species such as OH and absorbed H2O [43]. In the high-resolution Fe 2p spectrum (Fig. 3(d)), the Fe 2p3/2 is fitted two peaks with the banding energy at 711.4 (octahedral site) and 713.4 eV (tetrahedral site). Meanwhile, other peaks centred at 719.1 and 725.6 eV are attributed to the shake-up satellite structure

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and Fe 2p1/2, respectively [52]. It is noticeable that the existence of Fe3+ in ZFO-2 is determined from the above results.

Considering all of the above results, it is apparent that ZnFe2O4 catalysts with different

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morphologies of nanoparticles, rods and microspheres have been successfully prepared. Among them, ZnFe2O4 rods are expected to show the best photocatalytic activity due to the largest specific surface

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area and pore volume. Moreover, all the ZnFe2O4 samples have narrow band gap values in the range of

(b)

Fe 2p O 1s

Zn 2p 1022.5 eV

Intensity(a.u.)

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Zn 2p

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Intensity(a.u.)

(a)

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1.93-2.02 eV and the visible-light-driven photocatalytic activity is expected.

1045.4 eV

C 1s

1000

800

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1200

600

400

200

1050 1045 1040 1035 1030 1025 1020

0

Binding Energy(eV)

Binding Energy(eV)

532.8 eV

536

534

(d)

O 1s

531.0 eV

532

530

Binding Energy(eV)

528

526

Fe 2p 711.4 eV

725.6 eV 713.4 eV 719.1 eV

Intensity(a.u.)

Intensity(a.u.)

(c)

730

725

720

715

Binding Energy(eV)

710

Fig. 3. XPS spectra for survey (a), Zn 2p (b), O 1s (c), and Fe 2p (d) of ZFO-2

3.2. Photocatalytic removal of uranium(VI) Effect of catalyst morphology: The prepared ZnFe2O4 samples with different morphologies were

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used to photocatalytic remove of uranium(VI) under visible light. The control experiment shows that the remove of uranium(VI) was negligible in the absence of the ZnFe2O4 samples (Fig. 4(a)), implying

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uranium(VI) photolysis can be ignored under the reaction conditions. During 120 min visible light irradiation, ZFO-2 shows the best photocatalytic performance and the uranium(VI) was almost

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completely removed in 60 min, while the uranium(VI) removal efficiency for ZFO-1 and ZFO-3 were 72% and 91% respectively. The reaction rate constant(k) was then calculated by pseudo first order kinetics model: ln (C0/Ct) = kt, where C0 and Ct represent the uranium(VI) concentrations before and

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after irradiation for time t, respectively. As shown in Fig. S4(a), the reaction rate constant follows the order of ZFO-2 (0.0667 min-1) > ZFO-3 (0.0310 min-1) > ZFO-1 (0.0177 min-1). This order is well

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consistent with the trend of the specific surface area and pore volume of the three ZnFe2O4 samples. That is, rod shaped ZFO-2 owns the largest specific surface area and largest pore volume, thus exhibits the best activity for the photoreduction of uranium(VI). This result is understandable considering that

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the large specific surface area provides more active sites for photocatalytic reactions [40, 62], and the large pore volume facilitates the diffusion of the uranium(VI) in the catalyst. To achieve a high efficiency in the uranium(VI) removal, ZFO-2 was hereafter selected for the following experiments. In addition, the performance of the previously reported catalysts for the photoreduction of uranium(VI) was listed in Table 2. Compared to Fe2O3-GO [2], C3N4-TiO2 [10], TiO2(001) [19], Ti3C2/SrTiO3 [23]

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and Nb/TiNFs [25], ZFO-2 exhibits the best photocatalytic performance for the uranium(VI) reduction under visible light in view of the encouraging result of more than 95% uranium(VI) removal in 40 min with a catalyst dosage of 0.2 g/L. The results indicate that ZnFe2O4 is a promising and competitive photocatalyst for the removal of uranium pollutants. Table 2. Comparison of various catalysts for photocatalytic removal of U(VI) Catalyst

Light source

CU(VI) (ppm)

Ccatalyst (g/L)

pH

Removal (100%)

Ref

Fe2O3-GO

Vis

5

4

4

76(120 min)

[2]

C3N4/TiO2

UV-vis

20

0.25

6.9

67(240 min)

[10]

TiO2(001)

UV

24

0.2

5

100(180 min)

[19]

Ti3C2/SrTiO3

UV-vis

50

0.33

4

77(180 min)

[23]

Nb/TiNFs

UV-vis

50

0.2

5

46.5(240 min)

[25]

ZnFe2O4

Vis

50

0.2

5

95(40 min)

This work

Effect of catalyst dosage: The effect of catalyst dosage in the range of 0.1-0.6 g/L on the photocatalytic removal of uranium(VI) was investigated and the results are shown in Fig. 4(b) and Fig.

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S4(b). With the increase of the catalyst dosage from 0.1 to 0.2 g/L, the uranium(VI) removal efficiency increased from 82% to 98%, while the reaction rate increased from 0.0360 to 0.0667 min-1. A higher

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dosage of catalyst results in an increase of the number of photons absorbed by the catalyst and surface active sites for the photocatalytic reactions [45], thus enhancing the photocatalytic performance.

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However, an adverse effect on the photocatalytic activity was observed when the catalyst dosage was further increased. The removal efficiency for the catalyst dosage at 0.4 and 0.6 g/L, for example, was

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decreased to 95% and 77% respectively. We believe that the shielding effect is responsible for the decrease in the removal efficiency. That is, a oversaturation suspended ZnFe2O4 samples reduce the penetration of the light in the solution [63], thus decreasing the light utilization. A similar result has

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been reported during photocatalytic reduction of aqueous Cr(VI) over mesoporous ZnFe2O4 samples [45]. To achieve a high efficiency in the uranium(VI) removal, the ZnFe2O4 dosage of 0.2 g/L was

100 80

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20

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Irradiation time(min) 100

CH3OH-0 mM

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CH3OH-12 mM CH3OH-24 mM CH3OH-48 mM

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ZFO-2-0.1 g/L ZFO-2-0.2 g/L ZFO-2-0.4 g/L ZFO-2-0.6 g/L

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C/C0

60

ZFO-1 ZFO-2 ZFO-3

C/C0

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selected for the following experiments.

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pH-3 pH-4 pH-5 pH-6

Fig. 4. Photocatalytic removal of uranium(VI) by the prepared ZnFe2O4. (a) effect of catalyst morphology, [uranium(VI)]initial=50 ppm, m/V=0.2 g/L, V=50 mL, [CH3OH]=24 mM, pH=5; (b) effect of catalyst dosage,

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[uranium(VI)]initial =50 ppm, V=50 mL, [CH3OH]=24 mM, pH=5; (c) effect of hole scavenger (CH3OH) dosage, [uranium(VI)]initial=50 ppm, m/V=0.2 g/L, V=50 mL, pH=5; (d) effect of pH, [uranium(VI)]initial=50 ppm,

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m/V=0.2 g/L, V=50 mL, [CH3OH]=24 mM.

Effect of hole scavenger (CH3OH) dosage: Methanol as hole scavenger has been reported to be

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beneficial for photocatalytic reduction of uranium(VI) [20, 28]. Herein, the photocatalytic removal of uranium(VI) with different methanol dosage in the range of 0-48 mM was performed to assess the

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effect of hole scavenger. As shown in Fig. 4(c) and Fig. S4(c), the ZnFe2O4 catalyst shows a poor photocatalytic activity in the absence of methanol and almost no reduction of uranium(VI) occurs,

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which might be ascribed to the lower separation efficiency of electron-hole pairs under this condition. Upon methanol is added, however, the photocatalytic activity of ZnFe2O4 is clearly enhanced. When the methanol dosage is 24 mM, for example, the uranium(VI) removal efficiency and the reaction rate

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reach to 98% and 0.0667 min-1, respectively. The enhanced uranium(VI) photoreduction is obviously due to the lower recombination of electron-hole pairs by increasing mass transfer of methanol to ZnFe2O4 surface at higher methanol concentration, thus resulting in an enhanced photogenerated holes scavenging. Tan et al. reported a similar result during photocatalytic reduction of Selenium(VI) with methanol as hole scavenger [64]. Considering that further increasing the methanol dosage does not

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improve the photoreduction of uranium(VI), 24 mM methanol was used in the subsequent experiment. Effect of solution pH: The solution pH is always a key factor for photocatalytic reduction of metal

ions in light of the pH-dependent metal ions species and surface charge of catalyst. Thus the effect of solution pH on the photoreduction of uranium(VI) was investigated here by changing the pH values from 3 to 6. As can be seen from Fig. 4(d) and Fig. S4(d), when the solution pH increased from 3 to 5, the uranium(VI) removal efficiency greatly enhanced, from almost no removal to 98% removal, suggesting a clear pH-dependent photocatalytic process. We believe that the pH-dependent surface

charge of the catalyst is responsible for the pH-dependent photoreduction. Specially, at pH 3, the uranium(VI) ions mainly exists as positive UO22+ [65], while the catalyst, i.e. ZFO-2, is also positive charged since the point of zero charge (pHPZC) of ZFO-2 is 4.8 (Fig. S5). The electrostatic repulsion between uranium(VI) ions and ZFO-2 leads to a lower uranium(VI) reduction. As the pH increases, the surface charge of ZFO-2 gradually changes from positive to negative, the electrostatic repulsion weakens or even turns into electrostatic attraction, so the uranium(VI) reduction increases. With further increasing pH to 6, however, the uranium(VI) reduction has a slight decrease. This is probably because

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that the transfer of excited e- to ZnFe2O4 surface is suppressed by the negative surface charges at a higher pH. Similar observation was reported previously for photoreduction uranium(VI) using TiO2/Fe3O4 as a catalyst [22]. Besides, it is worth noting that the uranium(VI) reduction shows a clear

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hysteresis effect at pH 4.0 compared to those at other pH. Similar phenomenon occurred in Fan’s work [20] when TiO2 was used as photocatalyst. They attributed the slow photoreduction process at primary

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stage to the abundant protons that compete with UO22+ to consume photogenerated electronics. 3.3. Photocatalytic mechanism

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XPS measurement was carried out to testify the photocatalytic reduction of uranium(VI). Fig. 5(a) is the high-resolution U 4f spectrum in ZnFe2O4 rods after photocatalytic reactions with 50 ppm

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uranium(VI) at pH 5, in which 24 mM methanol was used as hole scavenger. The curves of U 4f7/2 and U 4f5/2 compose of four peaks at 380.3, 381.4, 391.1 and 392.2 eV, respectively. Among them, the peaks at 381.4 and 392.2 eV are the characteristic peaks of uranium(VI), while the peaks at 381.4 and

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392.2 eV correspond to the formation of uranium(IV) [22]. This result confirms the successful photocatalytic reduction of uranium (VI) to uranium (IV) over ZnFe2O4 rods.

(a)

4x109

U 4f

(b) ZFO-2

3x109

1/C2(F-2cm4)

Intensity(a.u.)

U(VI)

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U(IV)

2x109

1x109

0 -1.2

405 400 395 390 385 380 375 370 365

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rrent(A cm-2)

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(c)

ZFO-2 ZFO-2+CH3OH

60 Light Off 40

-0.8

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(d) -1

-0.4

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Potential/V vs (Ag/AgCl)

Binding Energy(eV) ECB= -0.7 V

U(VI) -

e e

-

U(IV)

0

2+

4+

UO2 /U 0.267 V

2.08 eV

2+

1 2

CH3OH

UO2 /UO2 0.411 V

EVB= 1.38 V

+

+

h h ZFO-2

CO2 H2O

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Fig. 5. Characterizations of the photocatalytic mechanism. (a) U 4f spectrum of ZFO-2 after uranium(VI) photoreduction; (b) Mott-Schottky plots of ZFO-2; (c) photocurrent of ZFO-2 ; (d) the plausible mechanism for

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the photoreduction of uranium(VI)

Then, Mott-Schottky plots was recorded to determine the conduction band edges (ECB) of ZnFe2O4.

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As shown in Fig. 5(b), the positive slope indicates that ZnFe2O4 is a n-type semiconductor, consistent with the previous report [39]. The flat band potential (EFB) of ZnFe2O4 equals to -0.9 V (vs. Ag/AgCl),

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corresponding to -0.7 V (vs. NHE). As a n-type semiconductor, the ECB of ZnFe2O4 is close to its EFB potential. Due to the ECB potential is negative than the redox potentials of UO22+/U4+ (0.267 V) and

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UO22+/ UO2 (0.411 V), the photogenerated electrons of ZnFe2O4 is thermodynamically feasible to photocatalytic reduction of uranium(VI).

In addition, the ECB potential of ZnFe2O4 is also negative than redox potentials of O2/·O2- (-0.28

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V), thus the existence of molecular oxygen may have an adverse effect on the photocatalytic remove of uranium(VI) by consuming photogenerated electrons [26]. The comparative photocatalytic test (Fig. S6) under air atmosphere shows that the uranium(VI) removal efficiency was as low as 18%, revealing the importance of photogenerated electrons in the photoreduction process. Meanwhile, photogenerated holes also hinders photoreduction of uranium(VI) due to the recombination of electron-hole pairs.

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Methanol can quench photogenerated holes and thus accelerate uranium(VI) photoreduction shown in Fig. 4(c). The effect of methanol is also confirmed by the photocurrent test (Fig. 5(c)), in which ZnFe2O4 alone shows weak photocurrent response, whereas high photocurrent response occurs when methanol is added to the electrolyte solution. Thus, it can be conclude that methanol is beneficial to promote uranium(VI) photoreduction by enhancing charge separation ability. Base on the above results, a plausible mechanism for the photocatalytic reduction of uranium(VI) was proposed, as shown in Fig. 5(d). Under visible light agitation, ZnFe2O4 is excited and the electron-

hole pairs are generated (eqs 1). Then the reduction of uranium(VI) is achieved by the photogenerated electrons on the conduction band of ZnFe2O4 via eqs 2 (eqs 3 may be another way for the uranium(VI) reduction. Soluble U4+, however, are difficult to distinguish from uranium(VI) by ICP measurement ). Meanwhile, the photogenerated holes on valence band are quench by methanol (eqs 4). ZnFe2O4 + hν → ZnFe2O4 (e- + h+) (1) UO22+ + 2e- → UO2(s) (2) UO22+ + 4H+ + 2e- → U4+ + 2H2O (3)

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CH3OH + 6OH- +6 h+→ CO2 + 5H2O (4) 3.4. Further practical implications

The stability and facile separation are always essential traits for a catalysts in view of low cost and

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convenient applications [63]. Herein, the stability of ZnFe2O4 rods was investigated by repeating usage of the catalyst for photoreduction of uranium(VI). The regeneration of the reacted photocatalysts was

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performed by stirring them in a 0.1 M Na2CO3 solution for 12 h followed by centrifugal separation. Then, the photocatalysts were dried at 60 oC for 8 h for next recycle usage. As shown in Fig. 6(a), the

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efficiency for the photoreduction by the ZnFe2O4 rods remained more than 95% even following three runs. Meanwhile, the XRD patterns of ZnFe2O4 rods before and after uranium(VI) photoreduction (Fig.

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S7) confirm that the crystal structure of ZnFe2O4 rods kept almost unchanged during the photocatalytic reactions. These results show that ZnFe2O4 rods possess excellent stability and reusability for the photocatalytic reduction of uranium(VI).

C/C0

60

2 nd

3 rd

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1 st

5.0

Magnetization (emu/g)

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Irradiation time(min） )

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Applied Field (Oe)

Fig. 6. (a) Cycling experiments for photoreduction of uranium(VI); (b) Magnetization curve of ZFO-2; Inset: the magnetic separation tests of ZFO-2.

The magnetization characters of prepared ZnFe2O4 samples at room temperature were investigated by vibrating sample magnetometry (VSM). As can be seen from Fig. 6(b), the magnetization saturation value of ZFO-2 was calculated to be 4.8 emu/g. This value is comparable to that of ZFO-1 (4.8 emu/g) but obviously lower than that of ZFO-3 (12.0 emu/g) (Fig. S8), which supports the conclusion from previously reported work [49] that material morphology affects its magnetic properties. Whatever, as demonstrated in the inset of Fig. 6(b), when a magnet was placed near the vial, a clear and transparent solution was obtained as the suspended ZnFe2O4 rods was gathered at the wall. The result demonstrates

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an intrinsic feature of easy separation for ZnFe2O4 rods, and this is valuable and helpful for applications in the actual case.

Overall, compared to other photocatalysts for photoreduction uranium(VI), ZnFe2O4 rods not only

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exhibit better photocatalytic activity, but also are easily to be separated and recycled, which make them promising and applicable photocatalysts for radioactive environmental remediation. However, ZnFe2O4

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rods as catalyst for uranium(VI) photoreduction are not always a “generalist”, and they also have shortcomings. For example, the uranium(VI) photoreduction is a clear pH-dependent photocatalytic

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process. And ZnFe2O4 shows poor photocatalytic performance and stability under strong acidic

4.CONCLUSIONS

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conditions. The resolution of these issues requires further and more detailed works.

In summary, we reported here a simple, efficient and applicable photocatalytic system for

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photoreduction and removal of uranium(VI) from waste water under visible light. The simpleness lies in that the photocatalytic system consists of cheap and readily available ZnFe2O4 as the catalyst and commonly used methanol as the hole scavenger. The efficiency is demonstrated by the high uranium(VI) removal of more than 98% with small catalyst dosage of 0.2 g/L in a short irradiation time of 60 min. The practicality is reflected in the characteristics of visible light excitation, magnetic

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separation, and excellent stability and reusability of the catalyst. These characteristics can reduce costs during practical applications. The finding of this work not only offers new clues for radioactive environmental remediation, but also opens up new opportunities for ZnFe2O4 in photocatalytic application.

ASSOCIATED CONTENT Supporting Information

Supplementary data associated with this article can be found in the online version.

Notes The authors declare no competing financial interest.

Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could

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have appeared to influence the work reported in this paper.

CRediT author statement

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Peng-liang Liang: Investigation, Writing- Original draft preparation. Li-yong Yuan: Conceptualization, Writing- Reviewing and Editing, Project administration. Hao Deng: Formal analysis. Xu-cong Wang:

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Validation. Lin Wang: Visualization. Zi-jie Li: Visualization. Shi-zhong Luo: Conceptualization, Supervision.

ACKNOWLEDGMENTS

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Wei-qun Shi: Conceptualization, Resources,Supervision, Funding acquisition.

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This work was supported by the National Science Fund for Distinguished Young Scholars (Grants No. 21925603) and the National Natural Science Foundation of China (Grant Nos. 21777161, 21790373, 21836001 and 21790370). The Science Challenge Project (TZ2016004) and Youth Innovation

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Promotion Association of CAS (21017020) are also acknowledged.

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