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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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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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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
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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.
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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
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with UV irradiation, and its concentration in the solution increases with the increase
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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
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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)
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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
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S4) would have an adverse effect on the U(VI) adsorption.
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When the systems were subjected to UV irradiation, the residual amounts of U(VI)
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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
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negative according to the equation of ECB (V)=-0.1-0.059 pH (25 oC). However, the
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transfer of interior excited e- of TiO2 to the catalyst surface can be suppressed under
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alkaline conditions by negative surface charges and a high concentration of OH- ions
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in solution, which facilitates the e--h+ recombination and therefore decreases the
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efficiency of U(VI) photoreduction.20,37 After the photocatalysis, the reaction systems
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at pH 3.4, 5.3, and 9.3 were further stirred in air atmosphere, and the release of
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deposited uranium was determined to be 38.3%, 1.1%, and nearly zero, respectively.
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This suggests that the U(IV) percentage among deposited uranium decreased with the
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increase of pH if the influence of pH on the desorption efficiency of re-oxidized U(VI)
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was not taken into account.
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Effect of ionic strength. NaClO4 was used as a background electrolyte due to its
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weak complexation ability to U(VI) ions. As illustrated in Figure 3(c), the coexistence
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of 0.1 M NaClO4 with U(VI) at pH 4.0, 6.2, and 8.2 all promoted the adsorption of
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U(VI) in the dark significantly. Actually, similar results had been addressed in
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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
296
(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
298
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
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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
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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
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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
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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
Residual uranium (%)
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
Residual uranium (%)
0.28 80 60
Dark
100
0
374
(b)
27.7%
Residual uranium (%)
Residual uranium (%)
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
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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
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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
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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
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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
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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
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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
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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
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Project
(No.
Page 25 of 30
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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
ACS Paragon Plus Environment
Environmental Science & Technology
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
ACS Paragon Plus Environment
Page 26 of 30
Page 27 of 30
Environmental Science & Technology
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
ACS Paragon Plus Environment
Environmental Science & Technology
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
ACS Paragon Plus Environment
Page 29 of 30
Environmental Science & Technology
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
ACS Paragon Plus Environment
Environmental Science & Technology
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.
30
ACS Paragon Plus Environment
Page 30 of 30