Enhanced Photocatalytic Removal of Uranium(VI) from Aqueous

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Enhanced Photocatalytic Removal of Uranium(VI) from Aqueous Solution by Magnetic TiO2/Fe3O4 and Its Graphene Composite Zijie Li, Zhiwei Huang, Wenlu Guo, Lin Wang, Lirong Zheng, Zhifang Chai, and Weiqun Shi Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b05313 • Publication Date (Web): 14 Apr 2017 Downloaded from http://pubs.acs.org on April 15, 2017

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Environmental Science & Technology

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Enhanced Photocatalytic Removal of Uranium(VI) from Aqueous Solution by

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Magnetic TiO2/Fe3O4 and Its Graphene Composite

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Zi-Jie Li a, Zhi-Wei Huang a, Wen-Lu Guo a, Lin Wang a, Li-Rong Zheng b, Zhi-Fang

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Chai c, Wei-Qun Shi a,*

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a

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Effects of Nanomaterials and Nanosafety, Institute of High Energy Physics, Chinese

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Academy of Sciences, Beijing, 100049, China

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b

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Academy of Sciences, Beijing, 100049, China

Laboratory of Nuclear Energy Chemistry and Key Laboratory for Biomedical

Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese

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c

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Innovation Center of Radiation Medicine of Jiangsu Higher Education Institutions,

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Soochow University, Suzhou 215123, China

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* Corresponding author. Tel.: +86-10-88233968; fax: +86-10-88235294; e-mail:

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[email protected] (W.Q. Shi)

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ABSTRACT: The separation and recovery of uranium from radioactive wastewater is

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important from the standpoints of environmental protection and uranium reuse. In the

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present work, magnetically collectable TiO2/Fe3O4 and its graphene composites were

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fabricated and utilized for the photocatalytical removal of U(VI) from aqueous

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solutions. It was found that, under ultraviolet (UV) irradiation, the photoreactivity of

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TiO2/Fe3O4 for the reduction of U(VI) was 19.3 times higher than that of pure TiO2,

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which is strongly correlated with the Fe0 and additional Fe(II) generated from the

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reduction of Fe3O4 by TiO2 photoelectrons. The effects of initial uranium

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concentration, solution pH, ionic strength, the composition of wastewater, and organic

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pollutants on the U(VI) removal by TiO2/Fe3O4 were systematically investigated. The

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results demonstrated its excellent performance in the cleanup of uranium

School of Radiological and Interdisciplinary Sciences (RAD-X), and Collaborative

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ACS Paragon Plus Environment


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contamination. As graphene can efficiently attract the TiO2 photoelectrons and thus

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decrease their transfer to Fe3O4, the photodissolution of Fe3O4 in the

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TiO2/graphene/Fe3O4 composite can be largely alleviated compared to that of the

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TiO2/Fe3O4, rendering this ternary composite a much higher stability. In addition,

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scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray absorption near

31

edge spectroscopy (XANES), and X-ray photoelectron spectroscopy (XPS) were used

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to explore the reaction mechanisms.

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KEYWORDS:

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photocatalysis

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TOC/Abstract art:

Uranium,

radioactive

wastewater,

TiO2,

Fe3O4,

graphene,

36 37

■ INTRODUCTION

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Large quantities of radioactive wastewater containing uranium have been released

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into the environment with the rapid development of nuclear energy since uranium is

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the current major fuel in most commercial reactors. In light of the long-term threats of

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uranium induced by its chemical and radioactive toxicity to the human being and

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environment,1 the separation and recovery of uranium from wastewater becomes an

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extremely imperative issue. Currently, the reduction of highly mobile hexavalent

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uranium (U(VI)) to sparingly soluble tetravalent uranium (U(IV)) oxides via different 2


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technologies has been regarded to be a plausible approach to eliminate uranium

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contamination,2 among which, the semiconductor photocatalysis has been particularly

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highlighted.

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Among a variety of photocatalysts,3 TiO2 has attracted increasing attention since

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the discovery of photocatalytic splitting of water under UV irradiation in 1972 due to

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its low cost, high photoactivity, and chemical stability.4 Under UV illumination, the

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electrons of TiO2 are excited from the valence band (VB) to conduction band (CB)

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and form electron (e-)-hole (h+) pairs. Most of e- and h+ will recombine and release

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energy as heat, which is undesirable for the efficient photocatalysis. Nevertheless, as

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the valence and conduction band potentials of TiO2 are around +3.1 and -0.1 V vs.

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SHE,5 respectively, oxidation and reduction reactions take place on the surface of

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TiO2. For instance, positive h+ oxidizes adsorbed water molecules and surface

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hydroxyl groups to create hydroxyl radicals (⋅OH). h+/⋅OH can oxidize most organic

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contaminants in wastewater into CO2, H2O, and other mineralization.6 Toxic metal

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ions such as Ag+, Cr(VI), Hg2+, Fe3+, Cu+, and Cu2+ could be efficiently eliminated

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from wastewater through the TiO2 photocatalysis induced reduction and deposition.7,8

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As the reduction potentials of UO22+/U4+ and UO22+/UO2 are 0.327 and 0.411 V vs.

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SHE, respectively,5,9 the reduction of U(VI) by using TiO2 as a photocatalyst should

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be thermodynamically feasible. In fact, relevant studies have been reported

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particularly in the absence of dissolved O2 (O2/H2O, 1.23 V vs. SHE),5,10-12 which

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competes with U(VI) for the consumption of TiO2 e- and decreases the reduction

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efficiency of U(VI).5,7 However, after wastewater treatments, the used TiO2

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nanoparticles are often recovered through tedious filtration or high speed

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centrifugation to avoid the secondary contamination, which is rather inconvenient and

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not economic. 3



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Combining TiO2 nanoparticles with magnetic substrates such as Fe3O4

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nanoparticles represents another useful approach for dealing with this problem, which

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makes the material recovery from suspension systems very easy under an external

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magnetic field.13 Moreover, magnetic TiO2/Fe2O3 has displayed superior activity over

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pure TiO2 towards Cr(VI) photoreduction.14 Xu et al. also observed an enhanced

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photocatalytic removal of Cr(VI) by TiO2/FeO composites and Fe(II)-doped TiO2

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spherical shells and attributed it to the additional chemical reduction of Cr(VI) by

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structural Fe(II) ions and newly formed Fe0 from the TiO2 e- induced reduction of

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Fe(II).15,16 Considering that Fe0 and Fe(II) are capable of reducing U(VI) to U(IV),17,18

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it is reasonable to believe that a TiO2/Fe3O4 composite could work for the efficient

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photocatalytic removal of U(VI). Whereas, the designed TiO2/Fe3O4 composite

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consists of two semiconductors in tight contact and the e- and h+ transfer from TiO2 to

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Fe3O4 will occur since the conduction and valence bands of Fe3O4 are all lower than

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those of TiO2 in energy.13,19 The transferred e- subsequently can be captured by

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structural Fe(III) ions to produce Fe(II), which tends to dissolve into solution phase

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(Fe3O4/Fe2+, 1.23 V vs. SHE).19 Such a photochemical dissolution of Fe3O4 might

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deteriorate photocatalytic property of the TiO2/Fe3O4 composite. From this point of

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view, a novel TiO2/Fe3O4 composite with the third material incorporated, which can

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decrease the Fe(III) reduction, is quite expected. On the other hand, graphene, a

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superb two-dimensional catalyst support, is well known to have an extremely high

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specific surface area and excellent electron transport property.6 It is reasonable to

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believe that the introduction of graphene may create an innovative solution for the

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further improvement of the TiO2/Fe3O4 photocatalysis system. Actually, recently

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synthesized TiO2/reduced graphene oxide (RGO)/Fe3O4 composites have proven

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much higher stabilities in photocatalysis mainly owing to the efficient transfer of TiO2 4


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e- to graphene (graphene/graphene•-, -0.08 V vs. SHE).20-22 In the current work, we

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will focus on the synthesis of magnetic TiO2/Fe3O4 and TiO2/RGO/Fe3O4

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photocatalysts and their applicability for the efficient photoreduction of U(VI) from

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various aqueous solutions. Furthermore, photocatalytic properties and mechanisms of

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these two composites were systematically studied and compared, and the results

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clearly manifest the enormous potential of the as-synthesized composites in the

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treatment of radioactive wastewater and will pave the way for further developing

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more efficient photocatalysis system.

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■ EXPERIMENTAL METHODS

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Reagents. All common chemicals (Aladdin Co., Shanghai, China) are of analytical

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grade. A 10 mM uranium stock solution was prepared by dissolving an appropriate

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amount of UO2(NO3)2⋅6H2O (Sigma-Aldrich Co.) in Milli-Q water (18.2 MΩcm,

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Millipore Co.).

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Preparation of TiO2, TiO2/Fe3O4, and TiO2/RGO/Fe3O4. Anatase TiO2 was

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prepared via a two-step method according to the literature.23 Typically, a solution of

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tetrabutyl titanate (Ti(BuO)4, 0.9 mL) with concentrated H2SO4 (0.375 mL) in 25 mL

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of ethanol was firstly added into a ethanol and water mixed solvent (350 mL/25 mL).

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After stirring for 0.5 h, the mixture was refluxed at 80oC for 24 under continuous stir

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to allow the slow hydrolysis of Ti(BuO)4. The formed amorphous TiO2 was collected

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by centrifugation and washed with ethanol and water successively. In the second step,

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the TiO2 was dispersed in water/DMF (10 mL/0.2 mL) and the suspension was

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hydrothermally treated in a 15 mL Teflon-lined autoclave at 200oC for 20 h, leading to

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a transformation of the amorphous state to crystalline anatase. After washing with

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water, the final white product was freeze-dried and stored in a desiccator. The

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procedure for the synthesis of TiO2/RGO is identical to that of pure TiO2 except that a 5



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single-layered graphene oxide (GO) solution (0.47-25 mL, 6.0 mg/mL) was added in

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the system in the first step. The preparation of GO by the Hummers’ method has been

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described in a previous publication.24

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In the synthesis of TiO2/Fe3O4, acid-resistant Fe3O4 nanoparticles25 were adopted to

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decrease the incidence of Fe(II) leaching during photocatalysis (Figure S1(a) of

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Supporting Information (SI)). A typical synthetic route was as follows: a portion of

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TiO2 (61.2 mg) was firstly transferred into a mixed solution containing KOH (0.909

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mL, 0.5 M), KNO3 (0.909 mL, 2.0 M), and water (4.909 mL). After purge with N2 for

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1 h, a freshly prepared FeSO4 solution (2.36 mL, 0.1 M) was added, followed by

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another 10 min N2 purge. Finally, the reaction mixture was statically aged at 90oC for

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2 h. The final black product was thoroughly washed by ethanol via magnetic

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separation and dried in vacuum at 50oC for 4 h. By stirring the composite in 6 M HCl

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overnight and measuring the dissolved iron ion concentration, the Fe3O4 loading was

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calculated to be 18.4 wt%. The optimization of Fe3O4 ratio in TiO2/Fe3O4 can be

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found in SI Figure S1(b). TiO2/RGO/Fe3O4 was prepared in the same way except that

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TiO2/RGO, instead of pure TiO2, was used as the starting material.

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Photocatalytic experiments. A photocatalytic apparatus used in the experiments

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was illustrated in SI Figure S2. A 100 mL jacketed quartz beaker cooled by circulation

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water was used as a photoreactor. In a typical photocatalytic experiment, TiO2,

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TiO2/Fe3O4, TiO2/RGO, or TiO2/RGO/Fe3O4 each containing 15.4 mg of TiO2 was

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firstly suspended in a 50 mL aqueous solution already containing U(VI) (0.1-0.4 mM)

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and/or other substrates. The pH of the mixture was adjusted to be in the range of 3.5

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to 9.3 by adding small volumes of NaOH and H2SO4 solutions (~0.1 M). The

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suspension was magnetically stirred in dark for 2 h to allow for the achievement of

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adsorption-desorption equilibrium. Then, the stirred suspension was illuminated with 6


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a high-pressure mercury lamp (100 W, the principal wavelength of 365 nm) mounted

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right above the photoreactor. During the adsorption and illumination period, the

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suspension was purged by N2 to ensure the reaction was under anaerobic condition. At

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given times, aliquots (0.7 mL) of the suspension were pipetted and filtered through

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0.45-µm Nylon syringe filters. Concentrations of metal ions in the filtrate were

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measured by Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES,

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Horiba JY2000-2, Japan), whereas Rhodamine B (RhB) was analyzed by UV-vis

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absorption measurement of its characteristic peak at 554 nm. Residual amount (%) of

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adsorbate in solution was defined as residual amount (%)=Ct*100/C0, where C0 and Ct

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are the concentrations (mM) of certain adsorbate in solution phase at initial and

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contact time t (min), respectively. Kinetic data after the irradiation were fitted by

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using the pseudo-first order model26 to obtain photoreaction rate constants (k, min-1)

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and evaluate photocatalytic activities of the photocatalysts quantitatively. Finally, after

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the photoreaction, residual photocatalysts were soon recovered by vacuum filtration or

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magnetic separation, washed thoroughly with ethanol, and vacuum dried for further

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characterizations. In some cases, the suspension was continuously stirred in dark

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under air atmosphere so as to observe the re-oxidation of immobilized U(IV) by

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dissolved O2 and the release of uranium from the catalyst surface.5 Additionally,

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aerobic experiments without N2 purge were also carried out and typical results are

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shown in SI Figure S3. A significantly inhibitory effect on the photocatalytic removal

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of U(VI) by pure TiO2 and TiO2/RGO/Fe3O4 was observed, whereas the photoactivity

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of TiO2/Fe3O4 towards U(VI) removal was little affected, highlighting its high

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reactivity and advantage in practical application.

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Material characterization. Microcosmic morphologies of the composites before

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and after the photocatalytic reaction were examined by SEM (S-4800, Hitachi) at an 7



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accelerating voltage of 10 kV. X-ray diffraction was carried out on a Bruker D8

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Advance instrument (Cu Kα, λ=1.5406 Å) with a step size of 0.02o. XANES data of U

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LIII-edge and Fe K-edge were collected at the beamline 1W1B of Beijing Synchrotron

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Radiation Facility. The XPS data were obtained by AXIS Ultra/Supra instrument

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(Kratos Analytical Ltd.), converted into VAMAS file format and processed by using

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CasaXPS software as well as curve-fitting.

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■ RESULTS AND DISCUSSION

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Characterization of TiO2, TiO2/Fe3O4, and TiO2/RGO/Fe3O4. SEM and XRD

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results of the pure TiO2, TiO2/18.4%Fe3O4, and TiO2/33.2%RGO/12.4%Fe3O4 are

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shown in Figure 1. The pure TiO2 contains irregularly spherical particles with

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diameters ~20 nm, and its XRD pattern presents an intensive peak at 2θ=25.28o (101)

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that coincides well with the pattern of the anatase phase of TiO2. For the TiO2/Fe3O4

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composite, it can be seen that Fe3O4 nanoparticles are uniformly dispersed in the TiO2

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matrix with the size mostly in the range of 100 to 300 nm, and portions of TiO2 adhere

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directly to the surface of Fe3O4. The diffraction peaks in the XRD pattern besides the

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anatase’s ones could be perfectly indexed to γ-Fe2O3/Fe3O4, suggesting the presence

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of the iron oxide phase. In the case of TiO2/RGO/Fe3O4, TiO2 and Fe3O4 nanoparticles

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are successively grown on the RGO sheets, and the interactions between the

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components (i.e., Ti-O-C,27 Fe-O-Ti, and Fe-O-C) were found to be strong that a

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ultrasonic treatment could not dissociate them. Its XRD pattern is similar to that of

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TiO2/Fe3O4 with additional small peaks at ~30o, probably originating from

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non-magnetic iron oxides adhered on RGO. The average crystal size of TiO2 and

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Fe3O4 was calculated to be around 17 and 47 nm from the full width at half-maximum

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of the (101) and (311) reflection, respectively, using Debye-Scherrer’s equation. The

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large crystalline grain size of Fe3O4 may be the reason for its acid resistance. 8


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Figure 1. SEM images and XRD patterns of (a) TiO2, (b) TiO2/18.4%Fe3O4, and (c)

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TiO2/33.2%RGO/12.4%Fe3O4, respectively.

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Enhanced photocatalytic removal of U(VI) by TiO2/Fe3O4. For comparison, the

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removal efficiencies of U(VI) by the TiO2/Fe3O4 composite, pure TiO2, pure Fe3O4,

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and a TiO2-Fe3O4 mixture (obtained by physically mixing the two semiconductors)

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were examined in a 0.1 mM U(VI) solution at pH 4.0, respectively, the results are

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shown in Figure 2. Under the current experimental conditions, no uranium removal

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was observed in the absence of photocatalysts, indicating that U(VI) photolysis is

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negligible.5 With the photocatalysts and during the equilibrium period in dark, the

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TiO2/Fe3O4, pure TiO2, and TiO2-Fe3O4 mixture shows comparable U(VI) removal

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(~14%), superior to the pure Fe3O4, which was considered mainly due to the U(VI)

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adsorption on TiO2 surface. Upon UV illumination and in the case of TiO2/Fe3O4,

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residual uranium (%) in the solution declined rapidly and after the irradiation for 30

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min, a complete removal of U(VI) could be achieved. The release of Fe2+ was also

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observed. The photoreaction rate constant (0.147 min-1) calculated after the irradiation

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was found to be 19.3 times higher than that in the pure TiO2 catalyzed system. The

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white pure TiO2 turned dark grey after the photocatalysis, suggesting the reductive 9



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deposition of U(VI) occurred on the surface.12 The pure Fe3O4 has little reactivity

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towards U(VI) photoreduction and Fe2+ release. A negligible amount of ⋅OH

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production had been reported for the UV-excited Fe3O4.28 The weak photoresponse of

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Fe3O4 can be explained in terms of a very narrow band gap (0.1 eV) and the existence

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of a favorable environment for e--h+ recombination due to the continuous e- hopping

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between structural Fe(II) and Fe(III).19 Additionally, the photocatalytic performance

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of the TiO2 and Fe3O4 mixed material is similar to that of the pure TiO2 even though a

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significant amount of Fe2+ released.

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The great enhancement of the U(VI) photoreduction by the TiO2/Fe3O4 composite

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could be explained as follows: upon irradiated with the UV light, the photogenerated

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e- of TiO2 is transferred to Fe3O4 across their point of contact and then induces the

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reduction of Fe3O4, temporarily written as Fe(II)O+2e-+H2O→Fe0+2OH- and

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Fe(III)2O3+2e-+H2O→2Fe(II)O+2OH-. Objectively, these reactions hinder the

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recombination of transferred e- and h+ and produce reductive Fe0 and additional Fe(II)

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ions. However, the newly formed Fe(II) has a probability to dissolve into solution,

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therefore becoming an inactive aqueous Fe2+ ion, which may be a contributor to the

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observed lower photoactivities of TiO2/Fe2O315 and Fe(III)-doped titania29 towards

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Cr(VI) reduction. In the current case, the acid-resistant Fe3O4 guarantees a low level

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of Fe(II) leaching and therefore highly reactive Fe0 and remaining Fe(II) on the

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surface of Fe3O4 and survival photoelectrons of TiO2 are all involved in the U(VI)

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reduction, leading to the fast removal of U(VI) from aqueous solution.

10


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Dark

0.16

No catalyst TiO2/Fe3O4 TiO2 Fe3O4 TiO2-Fe3O4 TiO2/RGO TiO2/RGO/Fe3O4

40 20

0.08

0.04 pHf 5.3

0 0

50

100

2+/3+

0.12 60

(mM)

80

Fe

Residual uranium (%)

0.20

Light

100

150

200

pHf 4.3 pHf 5.1 250

0.00 300

Contact time (min) 234 235

Figure 2. Comparison of U(VI) removal among different photocatalysts.

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m(TiO2)=15.4 mg, m(Fe3O4)=3.5 mg, m(RGO)=9.4 mg, V=50 mL, C0(U)=0.1 mM,

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and pH=4.0. Closed and open symbols represent residual uranium (%) and

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corresponding Fe2+/Fe3+ concentration (mM) in solution at a given time, respectively.

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Solution pH after photocatalysis (pHf) was indicated in the figure.

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Systematical studies on photocatalytic performance of TiO2/Fe3O4. Herein, the

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effects of initial uranium concentration, solution pH, ionic strength, the composition

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of wastewater, and organic pollutants on the U(VI) removal by TiO2/Fe3O4 were

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studied in detail.

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Effect of initial uranium concentration. In order to evaluate the treatment capacity

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of TiO2/Fe3O4, the C0(U)-dependent removal kinetics of U(VI) was examined at pH

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4.0 and illustrated in Figure 3(a). With the increase of C0(U) from 0.05 to 0.4 mM, the

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photoreaction rate constants decrease from 0.15 to 0.0175 min-1. Nevertheless, the

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elimination of U(VI) at C0(U)=0.4 mM (corresponding to 252 mg U/g catalyst) was

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achieved at the irradiation time of 85 min, much faster than the corresponding system

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with pure TiO2 photocatalysis. The release of Fe2+ always occurred upon the system

251

with UV irradiation, and its concentration in the solution increases with the increase

252

of irradiation time, attaining the maximum at the time of U(VI) complete removal. 11



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Additionally, it is interesting to note that the maximum Fe2+ release is linearly

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correlated with C0(U) (R2=0.998) and a 27.7 wt% percentage of total Fe3O4 was

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dissolved at C0(U) 0.4 mM. It therefore appears that the U(VI) photoreduction

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promotes Fe2+ release from the TiO2/Fe3O4 composite, which is possibly through the

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redox reaction of Fe0+UO22+→Fe2++UO2↓ and/or an enhanced e- transfer from TiO2

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to Fe3O4. After the U(VI) elimination, the Fe2+ concentration decreases slightly with

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the irradiation time, which can be attributed to the re-oxidation of Fe2+ to Fe3+ by

260

h+/⋅OH19 and immediate adsorption of Fe3+ ions on the catalyst surface.7

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Effect of solution pH. The removal efficiency of U(VI) by TiO2/Fe3O4 was then

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examined at C0(U) 0.1 mM and pH ranging from 3.4 to 9.3. As shown in Figure 3(b),

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at pH 3.4, additional 25 min was required to completely remove U(VI) compared to

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the previous result at pH 4.0. In addition, the maximum dissolution of Fe3O4 was

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recorded to be 31.0%, confirming that high acidity is beneficial to Fe(II) leaching.30 In

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the range of pH from 5.3 to 8.2, a large fraction (~75%) of U(VI) was adsorbed on the

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TiO2/Fe3O4 during the equilibrium period in dark. In fact, TiO2 has been demonstrated

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to be a highly effective adsorbent for U(VI) mainly through the complexation of

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TiO2-O(H) with U(VI).31-33 Recently, Bonato et al. reported that the uptake of U(VI)

270

onto TiO2 was enhanced in particular at pH>6 when the dissolved CO2 was purged

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with N2.34 At pH 9.3, the adsorption of U(VI) in dark decreased to 65%. Under this

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condition, negatively charged (UO2)3(OH)7- and UO2(OH)3- species was the

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predominant in the solution,34,35 and electrostatic repulsion between them and

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negatively charged catalyst surface36 (pHPZC(TiO2/Fe3O4)=5.9 as shown in SI Figure

275

S4) would have an adverse effect on the U(VI) adsorption.

276

When the systems were subjected to UV irradiation, the residual amounts of U(VI)

277

in the solutions decreased rapidly. Nevertheless, at pH 8.2 and 9.3, a 4-5% proportion 12


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of U(VI) kept dissolved even after the experiments and no Fe2+ release was detected.

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The increase of pH can make the conduction band potential of TiO2 (ECB) more

280

negative according to the equation of ECB (V)=-0.1-0.059 pH (25 oC). However, the

281

transfer of interior excited e- of TiO2 to the catalyst surface can be suppressed under

282

alkaline conditions by negative surface charges and a high concentration of OH- ions

283

in solution, which facilitates the e--h+ recombination and therefore decreases the

284

efficiency of U(VI) photoreduction.20,37 After the photocatalysis, the reaction systems

285

at pH 3.4, 5.3, and 9.3 were further stirred in air atmosphere, and the release of

286

deposited uranium was determined to be 38.3%, 1.1%, and nearly zero, respectively.

287

This suggests that the U(IV) percentage among deposited uranium decreased with the

288

increase of pH if the influence of pH on the desorption efficiency of re-oxidized U(VI)

289

was not taken into account.

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Effect of ionic strength. NaClO4 was used as a background electrolyte due to its

291

weak complexation ability to U(VI) ions. As illustrated in Figure 3(c), the coexistence

292

of 0.1 M NaClO4 with U(VI) at pH 4.0, 6.2, and 8.2 all promoted the adsorption of

293

U(VI) in the dark significantly. Actually, similar results had been addressed in

294

literatures. For instance, Guo et al. observed a promoting effect of 0.01-0.1 M NaNO3

295

on the U(VI) adsorption on TiO2 at pH 4.1-4.3.31 Moreover, hydrous TiO2

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(crypto-crystalline anatase) has been utilized to extract uranium from seawater

297

(typical pH 7.8-8.2) even though the salinity of seawater is up to ~3.5%.33 The

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possible explanations can be as follows: electrolyte ions effectively shield surface

299

charges of the adsorbents through electrostatic attraction, thereby driving charged

300

U(VI) species accessible to the material surface. In addition, both U(VI)

301

photoreduction and Fe3O4 photodissolution were not much affected, including the

302

incomplete removal of U(VI) at pH 8.2. 13



303

Treatment of mimic radioactive wastewater. Besides U(VI), mimic radioactive

304

wastewater (pH 4.0) contains Sr2+, Co2+, Zn2+, Ni2+, La3+, Nd3+, Sm3+, Gd3+, and Yb3+

305

in nitrate forms. From the results illustrated in Figure 3(d), one can see that the initial

306

adsorption of U(VI) was slightly inhibited by the coexistence of competitive metal

307

ions, the rate constant (0.0299 min-1) for the U(VI) photoreduction was lower than that

308

of the system in the absence of competitive metal ions, whereas after the irradiation

309

for 90 min, a small proportion (2.5%) of U(VI) was left in the solution. Additionally,

310

the maximum Fe2+ release in this system was only 34 µM. As for other metal ions,

311

only Sm(III) and Yb(III) concentrations in the solution decreased by 10.5 and 27.8%,

312

respectively, after the photocatalysis, suggesting their reductive deposition.

313

Energy-dispersive X-ray (EDX) analyses further confirmed the deposition of

314

samarium and ytterbium on the catalyst surface. Actually, the standard reduction

315

potentials of Sm3+/Sm2+ and Yb3+/Yb2+ are -1.55 and -1.15 V vs. SHE, respectively,

316

therefore the reduction of Sm3+/Yb3+ by TiO2 e- should be thermodynamically

317

unfeasible. Nevertheless, at pHf 5.6, newly formed Sm2+/Yb2+ ions could be

318

precipitated as (hydr)oxides, resulting in very low levels of free Sm2+/Yb2+ ions in the

319

solution, which may provide an important driving force for the Sm3+/Yb3+ reduction.

320

Synergistic effect of U(VI) and organic pollutant removal. Besides the competitive

321

metal ions, real radioactive wastewater often contains various complexants and

322

organic acids, which might coordinate with U(VI) and be potentially detrimental to its

323

removal efficiency.5 On the other hand, in a photocatalytic system, U(VI) and organic

324

substrates can consume e- and h+/⋅OH separately to proceed photocatalytic reduction

325

and oxidation processes, respectively, thereby facilitating the charge separation. As a

326

result, a synergistic effect can be expected. Herein, RhB, which is an important dye

327

and ethylenediaminetetraacetic acid (EDTA), extensively existing in radioactive 14


Page 14 of 30

Page 15 of 30


328

wastewater, were used as model organic molecules.

329

The simultaneous cleanup of U(VI) and RhB by TiO2/Fe3O4 at pH 4.0, 6.2, and 9.3

330

was illustrated in Figure 3(e). At pH 4.0, the influence of RhB on the U(VI) removal

331

and Fe3O4 photodissolution was not significant. While at pH 6.2 and 9.3, the presence

332

of RhB increased the adsorption of U(VI) in the dark by ~10%, which might be

333

attributed to the formation of U(VI)-RhB complexes. Actually, the presence of

334

carboxylic acid salts, e.g., formate and acetate has been reported to be able to enhance

335

the U(VI) adsorption on TiO2.10 After UV irradiation, residual U(VI) in the two

336

solutions could be completely eliminated, therefore solving the aforementioned

337

incomplete cleanup of U(VI) at pH 9.3. Moreover, the exposure of the suspension at

338

pH 9.3 to air released a significant percentage (6.6%) of deposited uranium, indicating

339

the increase of U(IV) ratio. The promoted U(VI) photoreduction can be ascribed to

340

the effective h+/⋅OH scavenge by RhB oxidation, thereby prolonging the e- life.

341

Additionally, the dye photosensitization mechanism, i.e., a direct reduction of U(VI)

342

by excited RhB (RhB*, RhB+•/RhB*, -1.4 V vs. SHE)38 was probably not significant

343

based on a control experiment without the photocatalyst.

344

On the other hand, the degradation of RhB was evidenced by the gradual loss of its

345

pinkish red color with the increase of irradiation time. In this regard, the TiO2/Fe3O4

346

composite should show an inferior photoactivity compared to the pure TiO2 since the

347

transferred h+ to Fe3O4 has a lowered oxidation ability.19 Figure 3(e) just shows this

348

tendency with the degradation rate constants catalyzed by TiO2 and TiO2/Fe3O4 to be

349

0.0936 and 0.0368 min-1, respectively, at pH 4.0. However, after the addition of U(VI),

350

the rate constant in the TiO2/Fe3O4 catalyzed system was 2.7 times higher, which

351

should be relative to the photoreduction of U(VI), consuming TiO2 e- and therefore

352

prolonging the life of h+. In contrast, when increasing solution pH to 6.2 and 9.3, the 15



353

RhB degradation was slowed with the rate constants of around 0.0478 min-1. It’s

354

known that the pH increase will make the valence band potential shift to a more

355

cathodic value (i.e., EVB (V)=+0.31-0.059pH (25oC)). Additionally, the oxidation

356

product of RhB would change from CO2 to bicarbonate at neutral pH, which is a free

357

radical scavenger39 and the aforementioned acceleration of e--h+ recombination under

358

alkaline conditions is also not beneficial for the RhB degradation.

359

The influence of EDTA on the U(VI) removal was examined at the molar ratio of

360

U(VI) to EDTA of 1:4 and at pH 4.0, 6.2, and 8.3. As shown in SI Figure S5, the

361

adsorption of U(VI) on TiO2/Fe3O4 in the dark similarly increased with the increase of

362

solution pH,5 suggesting that in adsorbed U(VI)-EDTA complexes, UO22+ should

363

interact with the composite surface.40 However, the adsorption percentage of U(VI)

364

was only 36.4% even at pH 8.3, the reason could be related with the strong

365

coordination affinity of EDTA with U(VI) which may weaken the interaction of the

366

material surface with U(VI).41 Upon UV irradiation, residual U(VI) (%) decreased

367

rapidly and after the photocatalysis, around 5.6% of the total uranium was left in the

368

solutions at all pH we have examined. The pHf was found to be approximately 8.5,

369

regardless of the initial solution pH, which is in good agreement with the results on

370

pure EDTA degradation photocatalyzed by P25 TiO2.42 Finally, the incomplete

371

removal of U(VI) was ascribed to the high pHf because the adsorption and oxidation

372

degradation of EDTA can be negligible at this pH7,40,42 and in the similar

373

EDTA-absent systems, similar levels of remaining uranium have been discussed.

16


Page 16 of 30


13.1%

20

pHf=5.4 0

100

150

200

250

40

pHf=5.1

0

0.00 60

120

Dark

180

240

1000

Light

0.09

pH8.2, 0.1M NaClO4

20

0.03

80

120

150

180

210 0.16

Light

0.12

75 U, pH4.0 U+RhB, no cat., pH9.0

50

0.08

25

0.04

0

0.00

100

TiO2,RhB,pH4

75

RhB,pH4.0 U+RhB,pH4.0 U+RhB,pH6.2 U+RhB,pH9.3

50 25

(mM)

25

Dark

0.03

20 0.00

0

2+/3+

0

0.06

Gd Nd Sm Yb U

2+/3+

40

0.00

(e) 100

Co La Ni Sr Zn

60

(f)

100

0

20

Dark

Light

100

125

Dark

150

Light

175

200

Dark

Light

0.15

80 60

0.10 40

1st

20

3rd

2nd

0.05

0

0 0

25 100

125

150

175

200

0.20

2+/3+

0.06

pH6.2, 0.1M NaClO4

Residual element (%)

+0.1M NaClO4

2+/3+

+0.01M NaClO4 40

Fe

60

0.09

(mM)

0.12 pH4.0, only U +0.001M NaClO4

0.14

0.07

100

80

0.21

20

0

(mM)

pH 4.0 pH 6.2 pH 9.3

2+/3+

pH 3.4 pH 5.3 pH 8.2

60

(d)

0.15

0

U (%)

0.28

300

Light

Fe


50

Dark

100

0.35

80

0.00

(c)

Residual RhB (%)

(mM)

pHf=4.4 0.07

Dark, Air

Fe

0.14

Light 31.0%

(mM)

14.9%

Fe

40

0.21

17.6%

2+/3+

0.05mM 0.1mM 0.2mM 0.4mM TiO2, 0.4mM


0.28 80 60

Dark

100

0

374

(b)

27.7%



0.35

Light

Fe

Dark

100

225

0.00 0

Contact time (min)

(mM)

(a)

Fe

Page 17 of 30

100 200

0

100

200

0

100

200

Contact time (min)

375

Figure 3. Systematical studies on the photocatalytic removal of U(VI) by TiO2/Fe3O4,

376

m(TiO2/Fe3O4)/V=0.38 g/L. (a) effect of initial uranium concentration, pH=4.0; (b)

377

effect of solution pH, C0(U)=0.1 mM; (c) effect of ion strength, C0(U)=0.1 mM;

378

C0(NaClO4)=0-100 mM; (d) the treatment of mimic radioactive wastewater,

379

C0(metal)=0.1 mM, pH=4.0; (e) synergetic effect of U(VI) photoreduction and RhB

380

oxidation degradation, C0(U)=0.1 mM, C0(RhB)=0.05 mM; (f) recyclable

381

performance of TiO2/Fe3O4 (● and ○) and TiO2/RGO/Fe3O4 (■ and □) in three

382

successive runs, m(TiO2/RGO/Fe3O4)/V=0.57 g/L, C0(U)=0.1 mM, and pH=4.0.

383

Suppression

of

Fe3O4

photodissolution

in

TiO2/RGO/Fe3O4.

In

the

384

TiO2/RGO/Fe3O4, a direct contact between TiO2 and RGO will decrease the

385

probability of e- transfer to Fe3O4 as graphene is an excellent electron acceptor and 17



386

transporter,20,21 thereby inhibiting the Fe3O4 dissolution during the photocatalysis. The

387

RGO mass ratio in TiO2/RGO/Fe3O4 was optimized to be 33.2 wt% by considering

388

both aspects of photocatalytic activity and the extent of Fe3O4 photodissolution (SI

389

Figure S6). For comparison, the photocatalytic performance of the optimized ternary

390

composite was presented in Figure 2. It can be seen that the adsorption of U(VI) in the

391

dark increased to 35%, attributable to important roles played by residual

392

oxygen-containing functional groups on RGO sheets. The maximum photodissolution

393

of Fe3O4 in the ternary composite was drastically decreased by 76% compared to that

394

of the TiO2/Fe3O4 binary composite, whereas the photocatalytic removal of U(VI) was

395

also obviously slowed. In fact, TiO2/RGO composites have been extensively reported

396

to be able to enhance photocatalytic reduction of metal ions43,44 and oxidative

397

degradation of organic pollutants,6 which was ascribed to the e- transfer from TiO2 to

398

RGO and effective suppression of e--h+ recombination. However, regarding reaction

399

rate constants for U(VI) photoreduction, no improvement was achieved by the

400

TiO2/RGO composites with the RGO ratio ranging from 0.6 to 38.0 wt% (Figure S7 in

401

SI). Poor accessibility of U(VI) to RGO due to its coverage by TiO2 and the fact that

402

RGO becomes an e--h+ recombination center at a high RGO loading might be the

403

reason. Similarly, Chen et al. found that platinization of TiO2 had little effect on the

404

U(VI) photoreduction although accelerated deposition of some metals had been

405

reported.5,45 Therefore, the somewhat inhibition of TiO2/RGO/Fe3O4 photocatalytic

406

activity could be attributed to the decreased e- transfer to Fe3O4, accompanied by

407

lower levels of Fe0 and Fe(II) production and probable invalidation of TiO2 e- once

408

transferred to RGO.

409

Reusability of TiO2/Fe3O4 and TiO2/RGO/Fe3O4. Magnetic property facilitates

410

the recyclable utilization of the photocatalysts. Deposited uranium on the surface was 18


Page 18 of 30

Page 19 of 30


411

firstly desorbed by stirring the reacted photocatalysts in a 0.1 M (NH4)2CO3 solution

412

for 3 h in air. As shown in SI Figure S8, a 82.0% desorption of uranium was achieved,

413

very close to the results of Keen et al.,33 and no Fe3O4 loss occurred. The recyclable

414

performance of TiO2/Fe3O4 and TiO2/RGO/Fe3O4 was displayed in Figure 3(f). U(VI)

415

removal by the two photocatalysts nearly kept stable after three cycles. As for Fe2+

416

release, in the case of TiO2/Fe3O4, it decreased successively with the increase of

417

recycle time, which might be related with the detachment of TiO2 and Fe3O4

418

nanoparticles generated by the regular stirring in our experiments and/or the

419

accumulated Fe3O4 dissolution. Interestingly, in the case of the TiO2/RGO/Fe3O4

420

composite, low levels of Fe2+ release were maintained and effective magnetic

421

separation could be achieved owing to the strong ligation of TiO2, Fe3O4, and RGO

422

sheets. Hence, it can be safely concluded that the as-prepared photocatalysts possess

423

the potential to be recyclable and convenient materials for the efficient treatment of

424

radioactive wastewater. More meaningfully, we clearly demonstrated the successful

425

persistent hindrance for Fe2+ release via the incorporation of RGO.

426

Reaction mechanism. In order to deeply elucidate interaction mechanisms

427

between U(VI) and the photocatalysts, the selected TiO2, TiO2/Fe3O4, and

428

TiO2/RGO/Fe3O4 samples after the reaction with 0.1 mM U(VI) were characterized by

429

means of SEM, XRD, XANES, and XPS. SEM images and XRD patterns are

430

presented in SI Figure S9 and Figure 4(a), respectively. The microscopic morphology

431

of the TiO2/Fe3O4 composites with U(VI) accumulated at pH 4.0, 6.2, and 8.2 is

432

similar to that of the fresh one except that the SEM images become less conspicuous.

433

Worsened material conductivities could be the reason caused by the reductive

434

deposition of U(VI) and hydrolysis precipitation of Fe3+ on the surfaces at high pH. In

435

their XRD patterns, the characteristic peaks of anatase and γ-Fe2O3/Fe3O4 remain, 19



Page 20 of 30

436

suggesting that the transferred e- from TiO2 did not enter the interior of Fe3O4 deeply,

437

and at ~28o, a new weak peak appears, which can be assigned to the contribution of

438

U3O7,17 therefore clarifying the occurrence of U(VI) reduction. Amadelli et al. once

439

reported a compound with the structure close to that of U3O8 accumulated on TiO2

440

after UV irradiation by infrared analyses.10 In the XRD pattern of the

441

TiO2/RGO/Fe3O4 treated at pH 4.0, the U3O7 peaks cannot be observed, revealing a

442

limited conversion of U(VI) to U(IV) after the incorporation with RGO. The peaks

443

correlated with non-magnetic iron oxides disappear, thereby suggesting their

444

preferential dissolution during the photocatalysis reaction. After the three runs

445

(photocatalysis

446

TiO2/RGO/Fe3O4 still show similar morphologies and XRD patterns to those of the

447

corresponding fresh materials, proving the high stabilities of the photocatalysts under

448

the current conditions and well explaining the excellent reusability.

and

desorption),

both

the

regenerated

TiO2/Fe3O4

and

449

XANES can provide a fingerprinting method for judging oxidation states of metal

450

ions. Therefore, U LIII- and Fe K-edge XANES spectra of the samples were collected

451

and the normalized results are shown in SI Figure S10. It can be observed that U

452

LIII-absorption edges of the uranium-loaded TiO2 and TiO2/Fe3O4 obtained at various

453

pH are located between those of U(IV)O2 and U(VI)O2(OH)2, which act as references,

454

suggesting the uranium we measured should be a mixture of U(VI) and U(IV) species.

455

In addition, the absorption curve of the TiO2/RGO/Fe3O4 with U(VI) appears more in

456

accordance with that of U(VI)O2(OH)2, illustrating that the predominant valence state

457

of uranium in this sample is U(VI). As for the oxidation state of iron, the Fe K-edge

458

absorption curves of the TiO2/Fe3O4 and TiO2/RGO/Fe3O4 composites before and

459

after the photoreaction are very similar each other and more identical to that of the

460

reference Fe3O4 rather than α-Fe2O3, suggesting that Fe3O4 is the predominant iron 20


Page 21 of 30


461

phase. These results are in good agreement with the XRD characterizations.

462

Compared to the bulk analyzing techniques of XRD and XANES, XPS is more

463

surface sensitive and can afford direct information of oxidation states concerned.

464

Therefore, XPS spectra were recorded to further identify the oxidation states of

465

uranium and iron on the photocatalyst surface. U 4f7/2 and Fe 2p3/2 lines of the

466

samples are displayed in Figures 4(b) and 4(c), respectively. U 4f7/2 profiles are well

467

decomposed into two peaks at 380.0±0.1 and 381.6±0.3 eV, which represent the

468

binding energies of U(IV) and U(VI), respectively.12,46 Next, through peak resolution

469

and fitting of the spectra, the percentage of U(IV) in the TiO2 and TiO2/Fe3O4

470

composite after the treatments at pH 4.0 can be determined to be 53.1 and 80.9%,

471

respectively, clearly showing that the binary composite greatly promotes the

472

photoreduction of U(VI). Actually, an enhanced photoreduction of Cr(VI) to Cr(III)

473

has also been reported in the Fe(II)-doped TiO2 system compared to pure TiO2.16

474

Nevertheless, the percentage of U(IV) in the TiO2/Fe3O4 treated at pH 6.2 and 8.2

475

decreases to 40.3 and 28.6%, respectively. This decreasing tendency of U(IV)

476

percentage with the increase of pH is well in line with the aforementioned results of

477

deposited uranium release after the exposure to air. At pH 6.2 and 8.2, large

478

proportions of U(VI) had been adsorbed on the surfaces of TiO2 in the dark. However,

479

The Fe3O4 regions in the composite appear to possess higher photoreactivity based on

480

photo-deposition results of Ag (SI Figure S11), therefore leading to lower levels of

481

U(VI) photoreduction. At pH 8.2, the decrease of available e- number would further

482

suppress the U(VI) photoreduction. Additionally, the U(IV) percentage in the

483

TiO2/RGO/Fe3O4 is only 22.1%, well consistent with XRD and XANES results.

484

As shown in Figure 4(c), the broad Fe 2p3/2 peaks of the uranium-loaded

485

TiO2/Fe3O4 obtained at pH 4.0, 6.2, and 8.2 shift to high binding energy (~710.5 eV) 21



486

compared to that of the fresh TiO2/Fe3O4, clearly showing the increase of the Fe(III)

487

percentage on the surfaces. Fe0 with the binding energy of 706.7 eV1 was not be

488

identified probably due to its limited amount or it had been consumed by the U(VI)

489

reduction. Then, curve fitting was carried out to quantitatively determine the Fe(III)

490

percentage by using the parameters of Gupta and Sen multiplet peaks for Fe(II) and

491

Fe(III)47. On the surface of fresh TiO2/Fe3O4, the Fe(III) percentage is 62%, which is

492

near to the value in real Fe3O4. Whereas, after reaction with U(VI) at pH 4.0 and 6.2,

493

the Fe(III) percentage increases to around 86%, which could be the result of the

494

complicated surface reactions, e.g., Fe0 and Fe(II) oxidation by U(VI) and the

495

adsorption/hydrolysis precipitation of Fe3+ during the photocatalysis. On the

496

TiO2/Fe3O4 treated at pH 8.2, the percentage of Fe(III) is back to 76%, which is also

497

accompanied by the decrease of available e- and a low level of U(VI) photoreduction.

498

In addition, the satellite peak of Fe 2p3/2 at ~719 eV also evidences valence states of

499

iron.48 The satellite peak for the α-Fe2O3 reference is clearly distinguishable, while

500

that for the fresh TiO2/Fe3O4 is absent. The Fe 2p3/2 of the uranium-loaded TiO2/Fe3O4

501

treated at pH 4.0 and 6.2 has weak satellite peaks, whereas it is not conspicuous for

502

the TiO2/Fe3O4 reacted at pH 8.2. These results are in good accordance with the curve

503

fitting results. Similar transformation of Fe(II) to Fe(III) has also been observed in the

504

TiO2/FeO and Fe(II)-doped TiO2 photocatalysis for Cr(VI) removal.15,16 Nevertheless,

505

it appears that the inert Fe(III)-rich external layer of TiO2/Fe3O4 is able to be

506

effectively activated upon the UV illumination based on the excellent reusability of

507

the composite probably via the transferred e- from TiO2.

22


Page 22 of 30

Page 23 of 30


508 509

Figure 4. (a) XRD patterns of the TiO2/Fe3O4 and TiO2/RGO/Fe3O4 composites

510

treated with 0.1 mM U(VI) at pH 4.0, 6.2, and 8.2 and the recycled photocatalysts

511

after 3 runs; (b) narrow scan of U 4f7/2 XPS spectra with the curve fitting results; (c)

512

corresponding Fe 2p3/2 spectra of the TiO2, TiO2/Fe3O4 and TiO2/RGO/Fe3O4

513

composites reacted at C0(U)=0.1 mM, and (d) proposed mechanism for photocatalytic

514

reduction of U(VI) and organic molecule decomposition by the TiO2/Fe3O4 and

515

TiO2/RGO/Fe3O4 composites. Band diagrams of TiO2, Fe3O4, and graphene at pH 7.0

516

were calculated from data available in the literatures.19,22

517

Finally, the charge transfer in the composites and the proposed mechanism for U(VI)

518

photoreduction and organic pollutant decomposition are schematically shown in

519

Figure 4(d). Essentially, the transfer of e- from excited TiO2 to Fe3O4 induces the

520

reduction of structural Fe(II) and Fe(III) ions mainly located on the Fe3O4 surface,

521

creating reductive Fe0 and additional Fe(II), which are involved in the U(VI) 23



Page 24 of 30

522

reduction together with surviving photoelectrons on both TiO2 and Fe3O4. In the

523

TiO2/RGO/Fe3O4 composite, the RGO sheet attracts e- from TiO2, therefore

524

decreasing the Fe3O4 dissolution and rendering the ternary composite a much higher

525

stability. On the other hand, oxidative h+ and ⋅OH species effectively decomposes

526

organic pollutants and the synergistic treatment effect occurs when U(VI) and

527

organics coexists in the system. In summary, the developed TiO2/Fe3O4 and

528

TiO2/RGO/Fe3O4 photocatalysts here represent potentially suitable materials for the

529

efficient removal of uranium in complicated environmental pollution cleanup and

530

nuclear waste management.

531

■ ASSOCIATED CONTENT

532

Supporting Information. Figures showing the ratio optimization of TiO2/Fe3O4,

533

TiO2/RGO, and TiO2/RGO/Fe3O4, photocatalytic apparatus, zeta potential and

534

photodissolution of TiO2/Fe3O4, photoactivity comparison in air and N2, EDTA effect

535

on U(VI) removal, effective desorption of deposited uranium, SEM and XANES

536

characterizations of reacted photocatalysts, photo-deposition of Ag, and reactivity of

537

TiO2/Fe0(FeO).

538

■ AUTHOR INFORMATION

539

Corresponding Author

540

* Tel.: +86-10-88233968; fax: +86-10-88235294; e-mail: [email protected] (W.Q.

541

Shi).

542

■ ACKNOELEDGMENTS

543

This work was supported by the Natural Science Foundation of China (Grants

544

11575213,

545

JCKY2016212A504).

546

■ REFERENCES

21577144,

11675192)

and

Science

Challenge

24


Project

(No.

Page 25 of 30


547 548

(1) Manos, M.J.; Kanatzidis, M.G. Layered metal sulfides capture uranium from seawater. J. Am. Chem. Soc. 2012, 134, 16441-16446.

549

(2) Yan, S.; Hua, B.; Bao Z.; Yang, J.; Liu, C.; Deng, B. Uranium(VI) removal by

550

nanoscale zerovalent iron in anoxic batch systems. Environ. Sci. Technol. 2010, 44,

551

7783-7789.

552

(3) Wang, H.; Yuan X.; Wu, Y.; Huang, H.; Peng, X.; Zeng, G.; Zhong, H.; Liang, J.;

553

Ren, M. Graphene-based materials: Fabrication, characterization and application

554

for

555

storage/generation. Adv. Colloid Interf. Sci. 2013, 195-196, 19-40.

the

decontamination

of

wastewater

and

wastegas

and

hydrogen

556

(4) Zhao, Y.; Tao, C.; Xiao, G.; Wei, G.; Li, L.; Liu, C.; Su, H. Controlled synthesis

557

and photocatalysis of sea urchin-like Fe3O4@TiO2@Ag nanocomposites.

558

Nanoscale 2016, 8, 5313-5326.

559

(5) Chen, J.; Ollis, D. F.; Rulkens, W. H.; Bruning, H. Photocatalyzed deposition and

560

concentration of soluble uranium(VI) from TiO2 suspensions. Colloids Surf., A

561

1999, 151, 339-349.

562 563 564 565

(6) Zhang, H.; Lv, X.; Li, Y.; Wang, Y.; Li, J. P25-graphene composite as a high performance photocatalyst. ACS Nano 2010, 4 (1), 380-386. (7) Chen, D.; Ray, A. K. Removal of toxic metal ions from wastewater by semiconductor photocatalysis. Chem. Eng. Sci. 2001, 56, 1561-1570.

566

(8) Zhang, Y.C.; Yang, M.; Zhang, G.; Dionysiou, D.D. HNO3-involved one-step low

567

temperature solvothermal synthesis of N-doped TiO2 nanocrystals for efficient

568

photocatalytic reduction of Cr(VI) in water. Appl. Catal. B 2013, 142-143,

569

249-258.

570

(9) Lu, C.; Zhang, P.; Jiang, S.; Wu, X.; Song, S.; Zhu, M.; Lou, Z.; Li, Z.; Liu, F.;

571

Liu, Y.; Wang, Y.; Le, Z. Photocatalytic reduction elimination of UO22+ pollutant 25



572

under visible light with metal-free sulfur doped g-C3N4 photocatalyst. Appl. Catal.,

573

B 2017, 200, 378-385.

574

(10) Amadelli, P.; Maldotti, A.; Sostero, S.; Carassiti, V. Photodeposition of uranium

575

oxides onto TiO2 from aqueous uranyl solutions. J. Chem. Soc. Faraday Trans.

576

1991, 87(19), 3267-3273.

577

(11) Selli, E.; Eliet, V.; Spini, M. R.; Bidoglio, G. Effects of humic acids on the

578

photoinduced reduction of U(VI) in the presence of semiconducting TiO2 particles.

579

Environ. Sci. Technol. 2000, 34, 3742-3748.

580 581

(12) Bonato, M.; Allen, G. C.; Scott, T. B. Reduction of U(VI) on the surface of TiO2 anatase nanotubes. Micro & Nano Lett. 2008, 3 (2), 57-61.

582

(13) Zhang, Q.; Meng, G.; Wu, J.; Li, D.; Liu, Z. Study on enhanced photocatalytic

583

activity of magnetically recoverable Fe3O4@C@TiO2 nanocomposites with

584

core-shell nanostructure. Optical Mater. 2015, 46, 52-58.

585

(14) Challagulla, S.; Nagarjuna, R.; Ganesan, R.; Roy, S. Acrylate-based

586

polymerizable sol-gel synthesis of magnetically recoverable TiO2 supported Fe3O4

587

for Cr(VI) photoreduction in aerobic atmosphere. ACS Sustainable Chem. Eng.

588

2016, 4, 974-982.

589

(15) Xu, S. C.; Zhang, Y. X.; Pan, S. S.; Ding, H. L. Li, G. H. Recycle magnetic

590

photocatalysts of Fe2+/TiO2 hierarchical architecture with effective removal of

591

Cr(VI) under UV light form water. J. Hazard. Mater. 2011, 196, 29-35.

592

(16) Xu, S. C.; Pan, S. S.; Xu, Y.; Luo, Y. Y.; Zhang, Y. X.; Li, G. H. Efficient

593

removal of Cr(VI) from wastewater under sunlight by Fe(II)-doped TiO2 spherical

594

shell. J. Hazard. Mater. 2015, 283, 7-13.

595

(17) Li, Z.; Wang, L.; Yuan, L.; Xiao, C.; Mei, L.; Zheng, L.; Zhang, J.; Yang, J.;

596

Zhao, Y.; Zhu, Z.; Chai, Z.; Shi, W. Efficient removal of uranium from aqueous 26


Page 26 of 30

Page 27 of 30


597

solution by zero-valent iron nanoparticle and its graphene composite. J. Hazard.

598

Mater. 2015, 290, 26-33.

599

(18) Scott, T. B.; Allen, G. C.; Heard, P. J.; Randell, M. G. Reduction of U(VI) to

600

U(IV) on the surface of magnetite. Geochim. Cosmochim. Acta 2005, 69,

601

5639-5646.

602

(19) Beydoun, D.; Amal, R.; Low, G. K.-C.; Mcevoy, S. Novel photocatalyst:

603

titania-coated magnetite. Activity and photodissolution. J. Phys. Chem., B 2000,

604

104, 4387-4396.

605

(20) Cheng, L.; Zhang, S.; Wang, Y.; Ding, G.; Jiao, Z. Ternary P25-graphene-Fe3O4

606

nanocomposite as a magnetically recyclable hybrid for photodegradation of dyes.

607

Mater. Res. Bull. 2016, 73, 77-83.

608

(21) Lin, Y.; Geng, Z.; Cai, H.; Ma, L.; Chen, J.; Zeng, J.; Pan, N.; Wang, X. Ternary

609

graphene-TiO2-Fe3O4 nanocomposite as a recollectable photocatalyst with

610

enhanced durability. Eur. J. Inorg. Chem. 2012, 4439-4444.

611

(22) Tay, Q.; Chen, Z. Effective charge separation towards enhanced photocatalytic

612

activity via compositing reduced graphene oxide with two-phase anatase/brookite

613

TiO2. Int. J. Hydrogen Energy 2016, 41, 10590-10597.

614

(23) Liang, Y.; Wang, H.; Casalongue, H. S.; Chen, Z.; Dai, H. TiO2 nanocrystals

615

grown on grapheme as advanced photocatalytic hybrid materials. Nano Res. 2010,

616

3 (10), 701-705.

617

(24) Li, Z.; Chen. F.; Yuan, L.; Liu, Y.; Zhao, Y.; Chai, Z.; Shi, W. Uranium(VI)

618

adsorption on graphene oxide nanosheets from aqueous solutions. Chem. Eng. J.

619

2012, 210, 539-546.

620

(25) Sugimoto, T.; Matijevic, E. Formation of uniform spherical magnetite particles

621

by crystallization from ferrous hydroxide gels. J. Colloid Interface Sci. 1980, 27



622 623

Page 28 of 30

74(1), 227-243. (26) Wang, X.H.; Li, J.G.; Kamiyama, H.; Moriyoshi,

Y.; Ishigaki, T.

624

Wavelength-sensitive photocatalytic degradation of methyl orange in aqueous

625

suspension over iron(III)-doped TiO2 nanopowders under UV and visible light

626

irradiation. J. Phys. Chem. B 2006, 110, 6804-6809.

627

(27) Zhao, Y.; Zhao, D.; Chen, C.; Wang, X. Enhanced photo-reduction and removal

628

of Cr(VI) on reduced graphene oxide decorated with TiO2 nanoparticles. J.

629

Colloid Interface Sci. 2013, 405, 211-217.

630

(28) Kalan, R. E.; Yaparatne, S.; Amirbahman, A.; Tripp, C. P. P25 titanium dioxide

631

coated magnetic particles: Preparation, characterization and photocatalytic activity.

632

Appl. Catal., B 2016, 187, 249-258.

633

(29) Navio, J.A.; Colon, G.; Trillas, M.; Peral, J.; Domenech, X.; Testa, J.J.; Padron,

634

J.; Rodriguez, D.; Litter, M.I. Heterogeneous photocatalytic reactions of nitrite

635

oxidation and Cr(VI) reduction on iron-doped titania prepared by the wet

636

impregnation method. Appl. Catal. B 1998, 16, 187-196.

637

(30) Huber, F.; Schild, D.; Vitova, T.; Rothe, J.; Kirsch, R.; Schafer, T. U(VI) removal

638

kinetics in presence of synthetic magnetite nanoparticles. Geochim. Cosmochim.

639

Acta 2012, 96, 154-173.

640

(31) Guo, Z.; Yan, Z.; Tao, Z. Sorption of uranyl ions on TiO2: effects of contact time,

641

ionic strength, concentration and humic substance. J. Radioanal. Nucl. Chem.

642

2004, 261(1), 157-162.

643

(32) Tsydenov, D. E.; Shutilov, A. A.; Zenkovets, G. A.; Vorontsov, A. V. Hydrous

644

TiO2 materials and their application for sorption of inorganic ions. Chem. Eng. J.

645

2014, 251, 131-137.

646

(33) Keen, N. J. Studies on the extraction of uranium from sea water. J. Br. Nucl. 28


Page 29 of 30


647

Energy Soc. 1968, 7, 178-183.

648

(34) Bonato, M.; Ragnarsdottir, K. V.; Allen, G. C. Removal of uranium(VI), lead(II)

649

at the surface of TiO2 nanotubes studied by X-ray photoelectron spectroscopy.

650

Water, Air, Soil Pollut. 2012, 223, 3845-3857.

651

(35) Sun, Y.; Shao, D.; Chen, C.; Yang, S.; Wang, X. Highly efficient enrichment of

652

radionuclides on graphene oxide-supported polyaniline. Environ. Sci. Technol.

653

2013, 47, 9904-9910.

654

(36) Nedoloujko, A.; Kiwi, J. TiO2 speciation precluding mineralization of

655

4-tert-butylpyridine accelerated mineralization via Fenton photo-assisted reaction.

656

Water Res. 2000, 34, 3247-3284.

657

(37) Li, Z.; Wang, H.; Zi, L.; Zhang, J.; Zhang, Y. Preparation and photocatalytic

658

performance of magnetic TiO2-Fe3O4/graphene (RGO) composites under

659

VIS-light irradiation. Ceram. Int. 2015, 41, 10634-10643.

660

(38) Wang, Q.; Chen, X.; Yu, K.; Zhang, Y.; Cong, Y. Synergistic photosensitized

661

removal of Cr(VI) and rhodamine B dye on amorphous TiO2 under visible light

662

irradiation. J. Hazard. Mater. 2013, 246-247, 135-144.

663 664 665 666 667

(39) Yang, J.; Davis, A. P. Photocatalytic oxidation of Cu(II)-EDTA with illuminated TiO2: kinetics. Environ. Sci. Technol. 2000, 34, 3789-3795. (40) Yang, J.; Davis, A. P. Competitive adsorption of Cu(II)-EDTA and Cd(II)-EDTA onto TiO2. J. Colloid Interface Sci. 1999, 216, 77-85. (41) Stefano, C. D.; Gianguzza, A.; Milea, D.; Pettignano, A.; Sammartano, S.

668

Sequestering

ability

of

polyaminopolycarboxylic

669

dioxouranium(VI) cation. J. Alloys Compd. 2006, 424, 93-104.

ligands

towards

670

(42) Alkaim, A. F.; Kandiel, T. A.; Hussein, F. H.; Dillert, R.; Bahnemann, D. W.

671

Enhancing the photocatalytic activity of TiO2 by pH control: a case study for the 29



672

degradation of EDTA. Catal. Sci. Technol. 2013, 3, 3216-3222.

673

(43) Lightcap, L. V.; Kosel, T. H.; Kamat, P. V. Anchoring semiconductor and metal

674

nanoparticles on a two-dimensional catalyst mat. storing and shuttling electrons

675

with reduced gaphene oxide. Nano Lett. 2010, 10, 577-583.

676

(44) Liu, X.; Pan, L.; Lv, T.; Zhu, G.; Lu, T.; Sun, Z.; Sun, C. Microwave-assisted

677

synthesis of TiO2-reduced grapheme oxide composites for the photocatalytic

678

reduction of Cr(VI). RSC Adv. 2011, 1, 1245-1249.

679 680

(45) Fan, J. W.; Liu, X. H.; Zhang, J. The synthesis of TiO2 and TiO2-Pt and their application in the removal of Cr(VI). Environ. Tech. 2011, 32(3-4), 427-437.

681

(46) Ding, C.; Cheng, W.; Sun, Y.; Wang, X. Effects of Bacillus subtilison the

682

reduction of U(VI) by nano-Fe0. Geochim. Cosmochim. Acta 2015, 165, 86-107.

683

(47) Grosvenor, A. P.; Kobe, B. A.; Biesinger, M. C.; Mclntyre, N. S. Investigation of

684

multiplet splitting of Fe 2p XPS spectra and bonding in iron compounds. Surf.

685

Interface Anal. 2004, 36, 1564-1574.

686 687

(48) Yamashita, T.; Hayes, P. Analysis of XPS spectra of Fe2+ and Fe3+ ions in oxide materials. Appl. Surf. Sci. 2008, 254, 2441-2449.

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