Functionalized Fe3O4@C Nanospheres with Adjustable Structure for

Research Article pubs.acs.org/journal/ascecg

Cite This: ACS Sustainable Chem. Eng. 2017, 5, 11042-11050

Functionalized Fe3O4@C Nanospheres with Adjustable Structure for Efficient Hexavalent Chromium Removal Linglin Zhou,†,‡ Guilong Zhang,†,∥ Jie Tian,‡ Dongfang Wang,†,‡ Dongqing Cai,*,†,∥ and Zhengyan Wu*,†,∥ Key Laboratory of High Magnetic Field and Ion Beam Physical Biology, and ∥Key Laboratory of Environmental Toxicology and Pollution Control Technology of Anhui Province, Hefei Institutes of Physical Science, Chinese Academy of Sciences, 350 Shushanhu Road, Hefei 230031, People’s Republic of China ‡ University of Science and Technology of China, No. 96 Jinzhai Road, Hefei 230026, People’s Republic of China Downloaded via FORDHAM UNIV on June 29, 2018 at 20:47:44 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

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S Supporting Information *

ABSTRACT: Core−shell magnetic Fe3O4@C nanospheres functionalized with active chemical groups were synthesized via a one-step template-free solvothermal process and employed to remove hexavalent chromium (Cr(VI)) in aqueous media. The structure of Fe3O4@C nanoparticles could be effectively adjusted by the concentration of precursor. The Fe3O4 core possessed a high magnetism for the convenient separation of Fe3O4@C from aqueous solution, and high porosity and abundant functional groups (−OH, −COOH, and CC) of carbon layer contributed to a superior performance on Cr(VI) removal. Therein, Cr(VI) ions could be efficiently adsorbed into the carbon layer and reduced to trivalent chromium (Cr(III)) mainly by CC, and the resulting Cr(III) were chelated by the −COOH group. Noteworthy, the carbon layer thickness, pore size, and amount of chemical groups significantly increased with the increase of Fe3O4@C particle size resulting in a rising removal efficiency on Cr(VI). This study provides a promising approach for removing Cr(VI) and may have a huge application potential. KEYWORDS: Core−shell nanospheres, Adjustable structure, Removal, Hexavalent chromium, Adsorption and reduction

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INTRODUCTION Water pollution from heavy metal ions has attracted considerable attention over recent years because of their carcinogenicity, mutagenicity, genotoxicity, and bioaccumulation through the food chain, causing critical health and environmental issues.1 Hexavalent chromium (Cr(VI)), one of the highly toxic heavy metal ions, was generated from industrial processes such as tanning, electroplating, wood preservation, water cooling, and so on.2 Chromium mainly exists as Cr(VI) and trivalent chromium (Cr(III)) in aqueous environment.3 Therein, Cr(III) shows a low toxicity even in a very high dosage, whereas Cr(VI) is more toxic and carcinogenic owing to its strong oxidizing properties.4,5 The Environmental Protection Agency of the United States has regulated that the maximum concentration of Cr(VI) discharged to surface water must be below 0.05 mg/L.6 Therefore, it is crucial to remove Cr(VI) from wastewater or reduce Cr(VI) to Cr(III) prior to discharge into the environment. A variety of nanomaterials have been fabricated and used in the environmental field,7−10 such as removing Cr(VI) from the environment.11−13 Therein, carbon nanomaterials, a type of important adsorbent with excellent chemical and mechanical stability, have a wide-range potential application.14 Because of © 2017 American Chemical Society

their large specific surface area, porous structure, and environmentally friendly properties, carbon nanomaterials have aroused much attention as a highly effective and economical adsorbent to remove heavy metals.15,16 For instance, Santhana Krishna Kumar et al. prepared tetra nheptylammonium bromide functionalized multiwalled carbon to effectively adsorb Cr(VI)/Cr(III) from aqueous solution.15 Wang et al. prepared amino-functionalized carbon spheres for enhancing Cr(VI) adsorption from water.16 Despite their high removal efficiency for heavy metals, time-consuming and complicated separation procedures from solution greatly limited their application. In order to solve these problems, magnetic functionalization of carbon nanomaterials was developed as a feasible approach.17 Qiu et al. reported the synthesis of magnetic mesoporous carbon nanocomposite and investigated its application for Cr(VI) removal.18 Nevertheless, these magnetic carbon nanomaterials mainly suffered from severe synthesis conditions and complicated synthesis process. Hence, several one-pot approaches have been developed to Received: August 27, 2017 Revised: September 29, 2017 Published: October 4, 2017 11042

DOI: 10.1021/acssuschemeng.7b02983 ACS Sustainable Chem. Eng. 2017, 5, 11042−11050

Research Article

ACS Sustainable Chemistry & Engineering

Figure 1. TEM images of Fe3O4@C-4 (a, b, c) (insets of a and b are the HRTEM images of the shell and core of Fe3O4@C-4), Fe3O4@C-3 (d, e, f), Fe3O4@C-2 (g, h, i), Fe3O4@C-1 (j, k, l), and Fe3O4@C-5 (m, n, o) with different magnifications.

fabricate magnetic carbon nanomaterials.19−21 However, in previous research, thin carbon layer and lack of functional groups resulted in low amount of active adsorption sites and thus weak adsorption capacity, which was unfavorable for the application in heavy metal ions treatment. Therefore, it is important to develop an one-step approach to facilely adjust the structure of Fe3O4@carbon nanoparticles with thick carbon layer and abundance functional groups for efficient removal of Cr(VI). In this study, carboxyl-functionalized magnetic Fe3O4@C nanospheres with diameter of 40−250 nm and shell thickness of 20−100 nm were fabricated through one-step solvent thermal treatment. The concentration of precursor played a key role in controlling the morphology and structure of the Fe3O4@C nanospheres. Owing to the mesoporous thick carbon layer with abundant active chemical groups (−COOH and C C), Fe3O4@C nanospheres exhibited high removal capacity on Cr(VI) from aqueous solution through adsorption and reduction. More importantly, the Fe3O4@C nanospheres could be well dispersed in water and easily separated by an external magnet because of their excellent performance of hydrophilic and superparamagnetism. Thus, this work provides an economically viable and promising approach to fabricate

novel core−shell magnetic nanoparticles for Cr(VI)-contaminated water treatment.

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EXPERIMENTAL SECTION

Materials. Ferrocene (Fe(C5H5)2, purity of 99%) was purchased from Aladdin Co. (Shanghai, China). Polyethylene glycol (PEG) with a molecular weight of 1000, hydrogen peroxide (H2O2, purity of 30%), acetone (C3H6O, purity of 99%), potassium dichromate (K2Cr2O7), 1,5-diphenylcarbazide (DPC), ethylenediaminetetraacetic acid (EDTA), and other chemicals were purchased from Sinopharm Co. (Shanghai, China). All chemicals were used as received without further purification. Deionized water was used in all the experiments. Synthesis of Fe3O4@C Nanospheres. Ferrocene and PEG with mass ratios (Wferrocene:WPEG) of 0.2:0.2, 0.4:0.4, 0.6:0.6, 0.8:0.8, and 1.0:1.0 g were dissolved in 35 mL of acetone under magnetic stirring (120 rpm) for 30 min. H2O2 (2 mL) was slowly added to the solution and stirred for 2 h. After that, the resulting solution was transferred into a Teflon-lined stainless steel autoclave and maintained at 200 °C for 48 h. The precipitates were collected via magnetic separation and washed at least three times with alcohol. Finally, the black products were dried at 60 °C in a vacuum oven for 12 h. The resulting products based on Wferrocene:WPEG of 0.2:0.2, 0.4:0.4, 0.6:0.6, 0.8:0.8, and 1.0:1.0 g were designated as Fe3O4@C-1, Fe3O4@C-2, Fe3O4@C-3, Fe3O4@ C-4, and Fe3O4@C-5, respectively. Cr(VI) Removal Experiments. To investigate the removal behavior of Cr(VI), 10 mg of Fe3O4@C nanoparticles were added 11043


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Figure 2. EDX spectrum (a) and TEM image (b) of Fe3O4@C-4 and the corresponding elemental mapping images (c−e) of the rectangular region I.

Figure 3. (A−E) XRD patterns, FTIR spectra, TGA curves in oxygen, magnetic hysteresis loops, and zeta potentials of Fe3O4@C-1 (a), Fe3O4@C-2 (b), Fe3O4@C-3 (c), and Fe3O4@C-4 (d); (F) zeta potentials of Fe3O4@C-4 under different pH conditions. to 20 mL of Cr(VI) aqueous solution at pH 5.0. Then, the solid and liquid were magnetically separated at different interval times, and the residual concentrations of Cr(VI) were measured by DPC spectrophotometric method.22 To study the effect of pH on removal, the pH of the solution (with Cr(VI) concentration of 100 mg/L) was adjusted to 1.0, 2.0, 3.0, 5.0, and 7.0 using hydrochloric acid (HCl) and sodium hydroxide (NaOH), wherein the Cr(VI) concentrations in

solutions were measured with a contact time of 24 h. The effects of initial Cr(VI) concentrations (10, 25, 50, 75, 100, 150, and 200 mg/L) on Cr(VI) removal efficiency were also investigated with a contact time of 24 h. All the Cr(VI) removal tests were conducted at ambient temperature (25 ± 1 °C). The removal capacity (RC) and removal efficiency (RE) of Cr(VI) were calculated according to eqs 1 and 2 respectively 11044


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ACS Sustainable Chemistry & Engineering RC(mg/g) = (C0 − C t)/m × V

(1)

RE(%) = (C0 − C t)/C0 × 100%

(2)

latter two. These results indicated that the precursor concentration showed a critical influence on the structure of Fe3O4@C. Besides, the carbon layer displayed a porous structure with plenty of nanopores, which was beneficial for the adsorption of Cr(VI). Interestingly, a halo consisting of a number of satellite-like nanoparticles around the Fe3O4 core could be found within the carbon layer of Fe3O4@C-3 and Fe3O4@C-4, respectively, which was probably attributed to the Fe3O4 nanoparticles not engaged in the self-assembly of the Fe3O4 core. In addition, the insets of Figure 1a and b illustrated that the crystal layer distances of the carbon shell and Fe3O4 core (311) were approximately 0.361 and 0.251 nm, respectively. The composition of Fe3O4@C-4 was detected using EDX spectroscopy and elemental mapping. As displayed in Figure 2a, Fe, C, and O existed in Fe3O4@C-4, wherein the Cu was attributed to the copper net used as a substrate. Additionally, Figure 2b−e demonstrated that Fe atoms distributed predominately in the core and the halo layer, whereas C atoms distributed evenly throughout the Fe3O4@C-4 particle, suggesting that the core consisting of Fe3O4 was coated by the carbon shell. These results provided evidence for the welldefined core−shell nanostructure and the composition of Fe3O4@C. The crystalline structures of Fe3O4@C samples were determined by XRD to investigate their compositions. As shown in Figure 3A, the square-marked peaks corresponded to the (220), (311), (222), (400), (422), (511), and (440) crystal planes of face-centered-cubic Fe3O4.25 Additionally, the diffraction peak (2θ = 24.7°) increased from Fe3O4@C-1 to Fe3O4@C-4, suggesting that a part of the carbon layer existed in the form of crystalline carbon.26 The peak intensities of Fe3O4 increased significantly from Fe3O4@C-1 to Fe3O4@C-4, indicating that the crystallinity of Fe3O4 was enhanced with the increasing precursors concentration, which was probably because higher precursors concentrations could facilitate the self-assembly of Fe3O4/C nanoparticles to form denser cores as discussed in Figure 1. The composition of Fe3O4@C was further characterized by FTIR analysis. As illustrated in Figure 3B, the peak of 592 cm−1 was assigned to Fe−O vibration, further confirming the presence of Fe3O4.27 From Fe3O4@C-1 to Fe3O4@C-4, the strength of peaks at 1703 and 3419 cm−1 increased gradually, indicating the increase of −COOH amount.20,28 Meanwhile, the intensities of peaks at 804 and 1316 cm−1 increased with the precursors concentration, which was ascribed to the ringstretching of cyclopentadiene groups in the remaining ferrocene.29,30 According to the TG curve in Figure 3C, the weight loss before 200 °C was attributed to the removal of water. Additionally, the increasing weight loss ratios (23, 29.10, 31.37, and 47.19%) from Fe3O4@C-1 to Fe3O4@C-4 after 200 °C were probably because of the decomposition of their carbon shells with increasing thickness from 20 to 100 nm. Besides, the magnetic performance of Fe3O4@C samples was investigated. As shown in Figure 3D, the saturation magnetizations of Fe3O4@C-1, Fe3O4@C-2, Fe3O4@C-3, and Fe3O4@ C-4 were 24.28, 20.80, 16.18, and 18.62 emu/g, respectively, wherein the decreasing saturation magnetization from Fe3O4@ C-1 to Fe3O4@C-3 was mainly related to the increasing thickness of carbon shell. However, a slight increase of saturation magnetization was found from Fe3O4@C-3 to Fe3O4@C-4, which was probably because more satellite-like Fe3O4 nanoparticles existed in the carbon layer of Fe3O4@C-4.

where the C0 and Ct are the initial and resulting concentrations (mg/ g) respectively, m is the mass (g) of Fe3O4@C nanoparticles, and V is the volume (g/mL) of the solution. Reuse Performance of Fe3O4@C-4 Investigation. After Cr(VI) removal, the resulting Fe3O4@C-4-Cr was separated from the solution using a magnet with magnetic field intensity of 0.2 T. For desorption of Cr, Fe3O4@C-4-Cr was added to 50 mL of DPC acetone/aqueous solution (Vacetone/Vwater = 1:49, 1 g/L), and the resulting system was ultrasonically treated for 0.5 h. Then the resulting sample was collected using a magnet (0.2 T), washed with deionized water three times, and added to EDTA aqueous solution (1 g/L, 50 mL), and the resulting system was ultrasonically treated for 0.5 h. Finally, Fe3O4@C-4 was obtained after magnetic separation and washing with deionized water three times and reused to remove Cr(VI) according to the preceding method. Characterization. The morphologies of samples were observed on a transmission electron microscopy (TEM) (JEM-ARM200F, JEOL Co., Japan). The elemental mapping and the high-resolution TEM (HRTEM) images were measured with a scanning transmission electron microscope (STEM, JEM-ARM200F, JEOL Co., Japan). The structure and interactions were monitored using a Fourier transform infrared (FTIR) spectrometer (iS10, Nicolet Co., U.S.A.), a TTR-III X-ray diffractometer (XRD) (Rigaku Co., Japan), and an X-ray photoelectron spectroscope (XPS, ESCALAB 250, Thermo-VG Scientific Co., U.S.A.). The zeta potential of sample in distilled water was determined by a zetasizer 3000 (Malvern, U.K.) at ambient temperature. Magnetic properties were investigated using a vibrating sample magnetometer (Bruker Co., Germany) with an applied field between −10 000 and 10 000 Oe at room temperature. The thermal gravimetric (TG) analysis was performed by a thermogravimetric analyzer (Q5000IR, TA Co., U.S.A.) at a scan rate of 10 °C/min from room temperature to 800 °C in oxygen or nitrogen. The concentration of Cr(VI) was measured using an ultraviolet−visible (UV−vis) spectrophotometer (UV 2550, Shimadzu Co., Japan) at a wavelength of 540 nm.

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RESULTS AND DISCUSSION Structure Observation of Fe3O4@C. Fe3O4@C nanospheres were synthesized by thermal decomposition of precursors (ferrocene and PEG) in acetone with H2O2 as an oxidant. As shown by the TEM images in Figure 1, Fe3O4@C4, Fe3O4@C-3, Fe3O4@C-2, and Fe3O4@C-1 possessed distinct core−shell structures, wherein a carbon shell entirely wrapped around a cluster consisting of plenty of small Fe 3 O 4 nanoparticles, which was proved by the following analyses of Figures 2 and 3. From Fe3O4@C-1 to Fe3O4@C-4, both the sphere diameter (from 40 to 250 nm) and shell thickness (from 20 to 100 nm) increased with the concentration of precursors, especially from Fe3O4@C-2 to Fe3O4@C-3. Compared with other Fe3O4@C samples, Fe3O4@C-5 displayed an inhomogeneous morphology, wherein some Fe3O4@C-5 did not form regular nanospheres (shown by the arrow in Figure 1n), which was probably because of the rather high concentrations of precursors. Noteworthily, the thickness of the carbon layer in Fe3O4@C4 (100 nm) is greater than that reported in previous work;21,23,24 meanwhile, the Fe3O4/C core with such an integrated spheric morphology was rarely found in previous work. Additionally, the core densities of Fe3O4@C-3 and Fe3O4@C-4 were significantly higher compared with Fe3O4@ C-1 and Fe3O4@C-2; meanwhile, the cores and the whole particles of the former two samples exhibited more integrated spherical morphologies through self-assembly than those of the 11045


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Figure 4. Pore size distribution (a) and N2 adsorption−desorption isotherms (b) of different Fe3O4@C samples.

Figure 5. (a and b) Effects of contact time and solution pH on Cr(VI) removal performance of different Fe3O4@C samples with initial Cr(VI) concentration of 100 mg/L. (c and d) Effects of initial Cr(VI) concentration on RC and RE of different Fe3O4@C samples at pH 5.

of nanopores in Fe3O4@C, which was in agreement with TEM images in Figure 1, and the pore diameter sizes were mainly in the scope of 2−20 nm. Besides, the number of pores with a certain diameter of Fe3O4@C-4 was obviously higher compared with other three samples, which was probably caused by the halo satellite-like distribution of Fe3O4 within the carbon layer of Fe3O4@C-4. These results indicated that the pore size of Fe3O4@C apparently increased with the increasing of the precursor contents, and the as-prepared Fe3O4@C with large surface area and porous structure could be used as ideal adsorbents for removing Cr(VI). Investigation on Cr(VI) Removal. The removal performance of Fe3O4@C samples was investigated. The effects of contact time on the removal performance of different Fe3O4@ C were investigated with initial Cr(VI) concentration of 100 mg/L. As shown in Figure 5a, the removal capacities of all Fe3O4@C samples increased rapidly during the initial stage and then became slow until equilibrium. Compared with other three samples, Fe3O4@C-4 possessed much faster removal rate of Cr(VI). Besides, from Fe3O4@C-1 to Fe3O4@C-4, the Cr(VI) removal capacities at equilibrium substantially increased from 30.78 to 80.89 mg/g. The increasing removal rate and capacity

The coercivity and remanence could not be detected, which proved the soft magnetic property of Fe3O4@C-4. Hence, this superior magnetic property guaranteed Fe3O4@C an excellent ability to be easily collected after a treatment on Cr(VI). Additionally, zeta potential results illustrated that more negative charges were obtained from Fe3O4@C-1 to Fe3O4@C4, demonstrating the increase of the amount of negatively charged −COOH in the carbon shell (Figure 3E), which was in accordance with the result of FTIR. The zeta potentials of Fe3O4@C-4 under different pH were investigated as shown in Figure 3F. It was clearly seen that the zeta potential of the Fe3O4@C-4 was positive at pH 1−3 and became negative at pH 5−7. This result indicated that the Fe3O4@C-4 surface was positively charged at pH lower than 3 and negatively charged at pH higher than 5, which might greatly influence the removal capacity for Cr(VI). N2 adsorption−desorption isotherm and pore size distribution curve of Fe3O4@C exhibited in Figure 4 revealed a large specific surface area of Fe3O4@C, ensuring Fe3O4@C could provide sufficient contact area and abundant active sites for Cr(VI) collection. What’s more, the pore size distributions of Fe3O4@C samples as shown in Figure 4a proved the existence 11046


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Figure 6. FTIR spectra (a) and XPS spectra (b−d: full range, Cr 2p, and C 1s); TGA curves in nitrogen (e) of Fe3O4@C-4 and Fe3O4@C-4/Cr; TEM images (f, g) of Fe3O4@C-4/Cr; and distribution maps of the merged (h), Fe (i), C (j), and Cr (k) of Fe3O4@C-4/Cr.

performance of Fe3O4@C was highly dependent on pH, wherein acidic condition greatly favored Cr(VI) removal. Therefore, Fe3O4@C had a great application potential in acid Cr(VI)-containing wastewater treatment. More importantly, the Cr(VI) removal capacities of Fe3O4@C-4 (156.56 mg/L) were much higher those of previously reported magnetic adsorbents, such as Fe3O4 micron-spheres (43.48 mg/g),32 polystyrene/Fe3O4/graphene (13.8 mg/g),33 and hydrophilic Fe3O4/carbon composites (43.86 mg/g).34 Noteworthy, the Cr(VI) removal capacity of the carbon in Fe3O4@C-4 was 331.8 mg/g, which was higher than amino-functionalized carbon spheres reported recently (∼240 mg/g).16 These results indicated that the satellite-like core−shell nanostructure played a key role for the higher porosity and removal capacity of Fe3O4@C-4 on Cr(VI) compared with bare Fe3O4 and nanocarbon spheres. As shown in Figure 5c, the removal capacities of Fe3O4@C samples increased with the increasing initial concentration of Cr(VI). However, the removal efficiencies of Fe3O4@C samples decreased with the increasing initial Cr(VI) concentrations owing to the decreasing ratio of active sites amounts of Fe3O4@C to Cr(VI) ions amounts. Importantly, the removal

probably resulted from the increasing thickness and porosity of the carbon shells. As displayed in Figure 5b, the pH effect on the removal capacity of Fe3O4@C was investigated with initial pH ranging from 1 to 7. It could be found that the removal capacity of Fe3O4@C nanoparticles increased with decreasing pH value. The maximum removal capacities of Fe3O4@C-1, Fe3O4@C-2, Fe3O4@C-3, and Fe3O4@C-4 were 98.52, 119.59, 121.53, and 156.56 mg/g at pH of 1.0, while the removal capacities decreased dramatically to 13.56, 30.28, 37.29, and 44.40 mg/g at pH 7.0, respectively. Normally, HCrO4− and Cr2O72− were the dominant species in aqueous solution at pH lower than 6, while CrO42− was the predominant form at pH above 6.31 The high removal efficiency at low pH was because the highly protonated −OH on Fe3O4@C surface promoted the adsorption of Cr(VI) ions through electrostatic attractions. The higher the pH, the weaker the protonation, the less positive the charges on Fe3O4@C surface, the weaker the electrostatic attractions between Fe3O4@C and negatively charged Cr(VI) ions, and the lower the removal capacity. Besides, the increasing competitive interaction between OH− and Cr(VI) ions with the increasing pH could probably be another reason. These results suggested that Cr(VI) removal 11047


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Figure 7. (a and b) Schematic illustration of the Cr(VI) removal mechanism of Fe3O4@C; (c) digital images of the solutions of Cr(VI) (I), Fe3O4@ C-4/Cr (II), and Fe3O4@C-4/Cr under a magnet (0.1 T) (III).

was in accordance with the XPS analysis in Figure 6d. At the same time, the highly toxic Cr(VI) was reduced to low toxic Cr(III), and the resulting Cr(III) was chelated by −COOH and adsorbed in the carbon shell. The element distribution maps in Figure 6f−k illustrated that Cr atoms mainly distributed in the carbon shell of Fe3O4@C-4, suggesting that the carbon shell played a major role in removal of Cr(VI). Additionally, after removal of Cr(VI) at pH of 1.0, no obvious change occurred to the structure of Fe3O4@C except the disappearance of satellitelike Fe3O4, indicating that Fe3O4@C was rather stable during the treatment process of Cr(VI) under acidic condition. Based on the aforementioned analysis and discussion, the mechanism of Cr(VI) removal by Fe3O4@C nanospheres was illustrated in Figure 7. First, the negatively charged Cr(VI) ions were adsorbed in the porous carbon shell of Fe3O4@C nanoparticles through electrostatic attractions. Second, the adsorbed Cr(VI) ions were transformed to Cr(III) via the reduction effect of CC in the cyclopentadiene groups of the remaining ferrocene. Third, Cr(III) ions were adsorbed in Fe3O4@C nanosheres through the chelation of −COOH. Finally, the resulting Fe3O4@C, Cr(VI), and Cr(III) mixtures were totally separated from the aqueous solution using a magnet attributed to the high magnetism of Fe3O4@C.

performance of Fe3O4@C-4 was significantly higher than those of the other three samples, which was probably related to the significantly thicker carbon layer in Fe3O4@C-4. Additionally, the reuse property of Fe3O4@C-4 for removal of Cr(VI) was investigated. As shown in Figure S1, the removal efficiency slightly decreased with reuse time and could still reach 93% for the fifth time, indicating that Fe3O4@C-4 possessed excellent stability and reuse property, which was beneficial for lowering production cost and decreasing residual. Cr(VI) Removal Mechanisms. In order to reveal the removal mechanism of Cr(VI) by Fe3O4@C, FTIR and XPS measurements were carried out. As shown in Figure 6a, a new peak at 633 cm−1, ascribed to the stretching vibration of Cr(III)-O,35 appeared in the FTIR pattern of Fe3O4@C-4/Cr compared with Fe3O4@C-4, whereas the peaks at 804 and 1316 cm−1 for ring-stretching of cyclopentadiene group disappeared, indicating Cr(VI) was reduced to Cr(III) by cyclopentadiene groups (actually the CC on Fe3O4@C). Herein, the broadened and enhanced peak at 3419 cm−1 for Fe3O4@C-4/ Cr was probably because the cyclopentadiene groups were oxidized by Cr(VI). Furthermore, the strength of peaks at 1703 cm−1 declined greatly after Cr(VI) removal, mainly attributed to the chelation of −COOH with Cr(III) ions. Similar conclusions were obtained through XPS analysis. Figure 6b−d illustrated the XPS spectra of Fe3O4@C-4 before and after Cr(VI) removal. After Cr(VI) removal, typical peaks of Cr appeared in the XPS spectrum of Fe3O4@C-4/Cr (Figure 6b). As seen in the Cr 2p spectra of Fe3O4@C-4/Cr (Figure 6c), the significant peak at 577.54 eV could be attributed to Cr(III),14,36 proving the reduction of Cr(VI) to Cr(III) and the adsorption of Cr(III) in Fe3O4@C-4, which was in accordance with the results of FTIR analysis. As shown in Figure 6d, the main peak of sp2 CC at 284.6 eV was found in the spectra of Fe3O4@C-4,37 whereas several new peaks at 284.79, 286.56, and 288.5 eV assigned to C−C, C−O, and CO, respectively, appeared in the spectra of Fe3O4@C-4/Cr besides the main peak of CC.38−40 The results suggested that the CC bonds in Fe3O4@C-4 were oxidized to C−C, C−O, and CO by Cr(VI), which further confirmed the previous analysis. Furthermore, as shown in Figure 6e, the weight loss ratios of Fe3O4@C-4 and Fe3O4@C-4/Cr were 40.14 and 41.45% at temperature higher than 200 °C under nitrogen, respectively. Therein, the weight loss (49.13%) of Fe3O4@C-4/Cr except for Cr (156.56 mg/g) was higher than that of Fe3O4@C-4, displaying that Fe3O4@C-4/Cr possessed more oxygencontaining functional groups compared with Fe3O4@C-4. This result indicated that the CC bonds in Fe3O4@C-4 were probably oxidized to C−O and CO by Cr(VI), which

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CONCLUSIONS

In summary, a novel functionalized magnetic core−shell Fe3O4@C was fabricated through solvothermal decomposition of ferrocene with the presence of PEG. The particle diameter, thickness of carbon shell layer, pore size, and functional groups amounts of Fe3O4@C could be adjusted conveniently by the concentration of ferrocene and PEG, obtaining a spheric Fe3O4@C with carbon layer thickness of 100 nm which was significantly greater than that previously reported. The high porosity and abundant −OH endowed Fe3O4@C a superior adsorption performance on Cr(VI) through electrostatic attractions; meanwhile, the large number of CC therein contributed to a high reduction ability to transform highly toxic Cr(VI) to low toxic Cr(III), and then Cr(III) was chelated by −COOH. More importantly, these Fe3O4@C nanoparticles with high magnetization and superparamagnetism could be easily separated from solutions. Therefore, this work provides a facile approach to adjust the microstructure of Fe3O4@C with a porous thick carbon layer and thus excellent removal performance on Cr(VI) and may also have a tremendous application potential in the treatment of other pollutants. 11048


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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b02983. Reuse performance of Fe3O4@C-4 on Cr(VI) removal (Figure S1); pore volume and size of different samples (Table S1) (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: +86-551-65595143. Fax: +86-551-65595012. E-mail: [email protected]. *Tel.: +86-551-65595012. Fax: +86-551-65595012. E-mail: [email protected]. ORCID

Zhengyan Wu: 0000-0002-8142-1848 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge financial support from the National Natural Science Foundation of China (No. 21407151), the Youth Innovation Promotion Association of Chinese Academy of Sciences (No. 2015385), the Key Program of Chinese Academy of Sciences (No. KSZD-EW-Z-022-05), the Science and Technology Service Programs of Chinese Academy of Sciences (Nos. KFJ-STS-ZDTP-002 and KFJ-SW-STS-143), and the Grant of the President Foundation of Hefei Institutes of Physical Science of Chinese Academy of Sciences (No. YZJJ201502).

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Functionalized Fe3O4@C Nanospheres with Adjustable Structure for

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