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Fabrication of a-

Alan G. MacDiarmid Institute, Jilin Unive

E-mail: xiangli@jlu.edu.cn; cwang@jlu.edu

431-85168292

† Electronic supplementary informa10.1039/c4ra03692a

Cite this: RSC Adv., 2014, 4, 42376

Received 23rd April 2014Accepted 1st September 2014

DOI: 10.1039/c4ra03692a

www.rsc.org/advances

42376 | RSC Adv., 2014, 4, 42376–4238

Fe2O3–g-Al2O3 core–shellnanofibers and their Cr(VI) adsorptive properties†

Xiang Li, Rui Zhao, Bolun Sun, Xiaofeng Lu, Chengcheng Zhang, Zhaojie Wangand Ce Wang*

a-Fe2O3–g-Al2O3 core–shell nanofibers have been synthesized via an electrospinning process combined

with vapor deposition and heat treatment techniques. The composite nanofibers exhibited ferromagnetic

properties and Cr(VI) removal performance. The Freundlich adsorption isotherm was applied to describe

the adsorption process. Kinetics of the Cr(VI) ion adsorption were found to follow a pseudo-second-

order rate equation. The obtained a-Fe2O3–g-Al2O3 core–shell nanofibers were carefully examined by

scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD),

Fourier transform infrared spectroscopy (FT-IR) and vibrating sample magnetometry (VSM). The

adsorption mechanism for Cr(VI) onto a-Fe2O3–g-Al2O3 core–shell nanofibers was elucidated by X-ray

photoelectron spectroscopy (XPS). The results suggested that the electrostatic adsorption between the

positively charged surface of a-Fe2O3–g-Al2O3 nanofibers and Cr(VI) species, and the electron–hole pair

provided by Fe2O3 induced the Cr(VI) reduction to Cr(III). It is anticipated that the a-Fe2O3–g-Al2O3 core–

shell nanofibers are an attractive adsorbent for the removal of heavy metal ions from water.

Introduction

The increasing worldwide contamination of freshwater systemshas become one of the key environmental problems facinghumanity. Heavy metal ion pollution is regarded as the mostsevere environmental contamination today. Various kinds ofmethods have been carried out for heavy metal ion removalsuch as precipitation, membrane ltration, reverse osmosis,solvent extraction and electrolysis, biological treatment, ion-exchange processes, and adsorption. Among them, adsorptiontechnique has advantage of high efficiency, cost effectivenessand eco-friendly materials as adsorbents.1,2

Iron or aluminum oxides and hydroxides are well known toshow high uptake of cations and anions3 in natural environ-ments, such as chromium,4 arsenate5 and phosphate.6 a-Fe2O3

and g-Al2O3 are more suitable for the removal of toxic heavymetal ions and organic pollutants from wastewater because oftheir high thermal stabilities, large surface areas, low cost andabundant availability.7,8 Recently, Chubar et al. preparedFe2O3$Al2O3$xH2O with high specic surface area by a sol–gelmethod, and the Fe2O3$Al2O3$xH2O adsorbent showed goodadsorption ability for phosphate ions.9 Gulshan et al. synthe-sized alumina–iron oxide compounds by a gel evaporationmethod and investigated its simultaneous uptake properties for

rsity, Changchun 130012, P. R. China.

.cn; Fax: +86-431-85168292; Tel: +86-

tion (ESI) available. See DOI:

2

Ni2+, NH4+ and H2PO4

�.10 Li et al. prepared three types ofalumina-coupled iron oxides by coprecipitation approach andinvestigated their photocatalytic activity for bisphenol Adegradation.11 Cao et al. synthesized owerlike a-Fe2O3 nano-structures with maximum capacities of 51 and 30 mg g�1 forAs(V) and Cr(VI) removal.12 Wei et al. obtained hollow nestlike a-Fe2O3 nanostructures with maximum removal capacities of75.3, 58.5, and 160 mg g�1 for As(V), Cr(VI), and Congo red.13 Geet al. prepared the hierarchical g-Al2O3 with a facile hydro-thermal method and this product exhibited fast and highadsorption capacities towards Cr(VI) and CO2.14 Mahapatra et al.investigated Fe2O3–Al2O3 nanocomposite bers removalbehavior of heavy metal ions from aqueous solution, theremoval percentage was in the order of Cu2+< Pb2+< Ni2+<Hg2+.15

Currently, nanomaterials and nanotechnology havegarnered worldwide attention for their application in environ-mental remediation and pollution control, because nano-structured surfaces offer large surface areas and rich valencestates that provide enhanced affinity and adsorption capabilitytoward pollutants.15 Electrospinning technique has been knownto be a simple and versatile method to generate organic orinorganic nanobers. This technique involves applying a highvoltage on a viscous solution to fabricate ultrathin bers withdiameters in the range of 20–2000 nm. These nanobers show anumber of interesting characteristics such as large surface areaper unit mass, high gas permeability, high porosity and smallpore size. Therefore in recent studies, electrospun bers havebeen widely used as matrixes or templates to fabricate

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functional materials with tunable structures, such as core–shellnanobers,16 molecular sieve bers,17 nanotubes,18 necklace-like nanostructures,19 hierarchically nanobrous membrane,20

functional-group chelating nanober membranes,21 andpolymer-supported nanocomposite.22

In this work, we have reported the preparation of a novelber-in-tube nanobers containing of a-Fe2O3 nanober coreand g-Al2O3 shell via an electrospinning technique combiningwith vapor deposition and heat treatment techniques. Thea-Fe2O3–g-Al2O3 core–shell nanobers exhibited ferromagneticproperty and Cr(VI) removal performance. Therefore, this workprovides a facile and effective approach for the fabrication ofa-Fe2O3–g-Al2O3 core–shell nanobers, which can be potentiallyapplied in heavy metal ions removal from wastewater effluents.

ExperimentalMaterials

Poly(vinyl alcohol) (PVA, Mw ¼ 75 000–80 000) was purchasedfrom Beijing Chemical Factory. Triton-X 100 (C34H62O11) waspurchased from Aldrich. Ammonium ferric citrate [Fe(NH4)3-(C6H5O7)2] was purchased from Shantou Xilong ChemicalCorporation. Potassium dichromate (K2Cr2O7) was purchasedfrom Beijing Chemical Factory. All the reagents were usedwithout further purication.

Characterization

The morphology of the nanobers was characterized by scan-ning electron microscopy (SEM, Shimadzu SSX-550). Trans-mission electron microscope (TEM) measurements wereconducted on a JEOL JEM 1200EX at 100 kV. HRTEM imagingwas performed on a FEI Tecnai G2 F20 high-resolution trans-mission electron microscope operating at 200 kV. FT-IR spectrawere recorded on a Bruker Vector-22 FT-IR spectrometer from4000 to 400 cm�1 using powder-pressed KBr pellets at roomtemperature. The amounts of initial Cr(VI) ions were determinedusing an inductive coupled plasma emission spectrometer (ICP,PerkinElmer OPTIMA 3300DV). The pH values of the reactionsolutions were measured by using a pH meter (STARTER 2100).The ultraviolet-visible (UV-vis) absorption spectroscopy of Cr(VI)ions solution were recorded using SHIMADZU UV-2501 UV-visspectrophotometer. Analysis of the X-ray photoelectronspectra (XPS) was performed on Thermo ESCALAB 250 spec-trometer with a Mg–K (1253.6 eV) achromatic X-ray source. Thestatic magnetic properties of the a-Fe2O3–g-Al2O3 core–shellnanobers were measured using a vibrating sample magne-tometer (VSM 7410) at a temperature of 300 K.

Electrospinning of ammonium ferric citrate/PVA compositenanobers

PVA aqueous solution was prepared by dissolving 0.65 g of PVApowder in 9.35 g of deionized water, heating to 80 �C understirring for 2 h, and then cooling down to room temperature.Aerward, 0.385 g of ammonium ferric citrate was added intothe PVA aqueous solution and stirred for 3 h. Then the solutionwas loaded into a glass syringe and connected to high-voltage

This journal is © The Royal Society of Chemistry 2014

power supply (a Gamma High Voltage Research ORMONDBEACH.FL32174 dc power supply). 18 kV was provided betweenthe cathode and anode at a distance of 15 cm to prepareammonium ferric citrate/PVA composite nanobers, andseveral silicon slices (size: 3 cm � 3 cm) were used as thecollector, which was sticked on the surface of Al foil.

Preparation of a-Fe2O3–g-Al2O3 core–shell nanobers

Preparation of the aluminum phase. The Al vapor depositionon the ammonium ferric citrate/PVA composite nanobers wasperformed in a commercial thermal evaporation system at apressure of 5 � 10�4 Pa (KYKY Technology Development Co.Ltd. China). The depositing rate was 20 A s�1 and the thicknessof the Al lm was 150–190 nm. The ammonium ferric citrate/PVA–Al composite nanobers were calcined in air withprogrammable heating process. The heating rate is 10 �Cmin�1, the samples rst were annealed at 600 �C for 2 h, andthen heating up to 800 �C for 2 h. Aer the bers were naturallycooled to room temperature, the a-Fe2O3–g-Al2O3 core–shellnanobers were obtained.

Adsorption of Cr(VI) ions

The Cr(VI) solution was prepared by dissolving potassiumdichromate (K2Cr2O7) in aqueous solution at pH ¼ 2.0. Theadsorption isotherm of Cr(VI) ions on a-Fe2O3–g-Al2O3 core–shell nanobers was studied by dispersing the samples (2 mg)in a series of asks containing 45.0 mL Cr(VI) ions solution ofvarying initial concentrations, ranging from 9.0 to 62.0 mg L�1.The asks were placed on a shaker and kept at room tempera-ture. Aer shaking for a certain time the solution was separatedfor concentration analysis, the initial concentration of the Cr(VI)ions in solution was determined by ICP spectrometer. Theequilibrium concentration of Cr(VI) ions in the solution wasmeasured by UV-vis absorption spectroscopy. The amount ofthe Cr(VI) ions adsorbed onto each sample (the adsorptioncapacity (q, in mg g�1)) was calculated on the basis of thefollowing equation:23

q ¼ ðC0 � CeÞVW

(1)

where C0 and Ce are the initial and the equilibrium concentra-tions of the metal ions in the test solution (mg L�1), V is thevolume of the testing solution (L), and W is the weight of theadsorbent (g).

Results and discussionMorphologies

The morphologies of the as-electrospun ammonium ferriccitrate/PVA composite nanobers and a-Fe2O3–g-Al2O3 core–shell nanobers were observed by SEM and TEM measure-ments. As shown in Fig. 1a, the obtained ammonium ferriccitrate/PVA composite nanobers have a basic smooth anduniform structure, with the diameter in range of 300–650 nm(Fig. 1d). Aer vapor deposition and annealing, the compositenanobers keep the smooth ber morphology (Fig. 1b), with an

RSC Adv., 2014, 4, 42376–42382 | 42377

Fig. 1 SEM images of (a) ammonium ferric citrate/PVA composite nanofibers, (b) a-Fe2O3–g-Al2O3 core–shell nanofibers. TEM images of (c), (f)a-Fe2O3–g-Al2O3 core–shell nanofibers. The distribution of fiber diameters from (d) the ammonium ferric citrate/PVA composite nanofibers,and (e) a-Fe2O3–g-Al2O3 core–shell nanofibers.

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average diameter of �330 nm (Fig. 1e). Fig. 1c shows the TEMimage of the obtained nanobers. The core–shell structure canbeen clearly seen. The core is a-Fe2O3 nanobers derived fromammonium ferric citrate/PVA composite nanobers and theshell is g-Al2O3. Aer calcination (Fig. 1c and f), it can also beclearly seen that the a-Fe2O3 core bers have rough surface, andthe diameter is in the range of 53–76 nm, due to the crystalli-zation related to the formation of particulate morphology. AHRTEM image taken from the edge of (Fig. 2a) was shown inFig. 2b. The typical lattice fringes were clearly visible, with aspacing of 0.37 and 0.27 nm, corresponding to the (012) and(104) planes of a-Fe2O3, respectively. The spacing of 0.20 nmwas corresponded to the (400) planes of g-Al2O3, which furtherindicates that the nanobers consisted of a-Fe2O3 core andg-Al2O3 shell.

Structural properties

The chemical composition of the as-prepared product wasanalyzed using powder XRD measurements. As shown in Fig. 3,

Fig. 2 TEM (a) and HRTEM (b) images of the as-prepared a-Fe2O3–g-Al2O3 core–shell nanofibers.

42378 | RSC Adv., 2014, 4, 42376–42382

the diffraction angles at 2q ¼ 24.33�, 33.27�, 35.79�, 41.00�,49.60�, 54.20�, 57.76�, 62.63�, 64.10� and 72.17� can be assignedto (012), (104), (110), (113), (024), (116), (018), (214), (300) and(119) planes of a-Fe2O3, which are in good agreement with therecorded values of JCPDS File Card no. 33-0664. Thepronounced peaks at (012), (104), and (110) suggest that theobtained a-Fe2O3 was well-crystallized. The peaks at 38.55�,46.03� and 67.14� degrees are assigned to the (222), (400) and(440) reection of g-Al2O3 (JCPDS File Card no. 10-0425). Theresults indicate that a-Fe2O3–g-Al2O3 nanobers have beensuccessfully prepared.

The FT-IR spectra shown in Fig. 4 were used to get infor-mation for the structural changes related to the pure PVAnanobers, the ammonium ferric citrate/PVA composite nano-bers and the annealed a-Fe2O3–g-Al2O3 core–shell nanobers.For pure PVA nanobers (Fig. 4a), a broad peak at about

Fig. 3 XRD patterns of a-Fe2O3–g-Al2O3 core–shell nanofibers.

This journal is © The Royal Society of Chemistry 2014

Fig. 4 FT-IR spectra of (a) PVA nanofibers, (b) the ammonium ferriccitrate/PVA composite nanofibers, and (c) a-Fe2O3–g-Al2O3 core–shell nanofibers.

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3420 cm�1 corresponds to H–OH stretching vibration, the peakat 2950 cm�1 is related to –CH stretching vibrations, the peaksin the range of 1450–1300 cm�1 and 1000–1160 cm�1 could beascribed to the –OH bending vibrations and C–O stretchingvibrations, respectively. For the ammonium ferric citrate/PVAcomposite nanobers (Fig. 4b), the peaks at 1400 cm�1 corre-sponds to the COO– stretching vibrations. As shown in Fig. 4c,these characteristic peaks of PVA were almost removed from thenanobers aer calcination at 800 �C, illustrating the decom-position of organic matter. Furthermore, two peaks around 541and 463 cm�1 were observed, which can be ascribed to themetal–oxygen vibration of the a-Fe2O3 and g-Al2O3.24,25

Magnetic behavior

The magnetic behaviors of the samples were conducted byisothermal magnetic hysteresis measurements at roomtemperature (300 K) with the eld sweeping from�20 to 20 kOe.As shown in Fig. 5, the loop suggests that the a-Fe2O3–g-Al2O3

core–shell nanobers are ferromagnetic at room temperature,

Fig. 5 Magnetic hysteresis loops of a-Fe2O3–g-Al2O3 core–shellnanofibers at room temperature.

This journal is © The Royal Society of Chemistry 2014

mainly because of the a-Fe2O3 contents display ferromagneticbehavior. The coercivity force (Hc), remnant magnetization (Mr)and saturated magnetization (Ms) of the samples are estimatedto be 2.24 kOe, 0.0585 emu g�1 and 0.111 emu g�1, respectively.The observed coercivity of the a-Fe2O3–g-Al2O3 core–shellnanobers is larger than that of the rod,26 nanober,27 shuttle-like a-Fe2O3,28 Al3+ doped a-Fe2O3 nanoplates,29 and Fe–SiO2

core–shell nanostructures,30 indicating that the magnetizationof ferromagnetic materials is sensitive to microstructuralcharacteristics such as size, shape and defects in crystalstructure.31

Adsorption kinetics

The core–shell structures of the obtained a-Fe2O3–g-Al2O3

nanobers afford its potential applications for waste adsorp-tion. In this study, the feasibility of a-Fe2O3–g-Al2O3 core–shellnanobers as Cr(VI) ion remover was explored. Fig. 6 shows theadsorption capacity of Cr(VI) ions in the aqueous solution atdifferent times. Clearly, the adsorption for the Cr(VI) ionscapacity increased with the increasing of the contact time, andreached equilibria at about 120 min. Here, we use the pseudo-second-order model to evaluate the kinetics of Cr(VI) removalon a-Fe2O3–g-Al2O3 core–shell nanobers (Fig. 7). The linear-ized form of the pseudo-second-order equation is as follows:21

t

qt¼ 1

k2qe2þ t

qe(2)

where k2 (g mg�1 min�1) is the pseudo-second-order rateconstant, qe (mg g�1) is the adsorption capacity at adsorptionequilibrium and qt (mg g�1) is the adsorption at time t. Thekinetics of the removal process was shown in Fig. 7. Table 1summarizes the calculated qe values, pseudo-second-order rateconstants k2, and correlation coefficient values (R2). The calcu-lated qe is calculated from the slope and intercept of the plots oft/qt versus t.

The Freundlich isotherm is an empirical equation that isused to describe adsorption at multilayer and adsorption on a

Fig. 6 The adsorption capacity of a-Fe2O3–g-Al2O3 core–shellnanofibers for different concentrations of Cr(VI) ions as a function oftime.

RSC Adv., 2014, 4, 42376–42382 | 42379

Fig. 7 The pseudo-second-order model for adsorption of Cr(VI) ionsby a-Fe2O3–g-Al2O3 core–shell nanofibers.

Table 1 Kinetics parameters for Cr(VI) adsorption onto a-Fe2O3–g-Al2O3 core–shell nanofibers with different concentrations

Concentration ofCr(VI) ions (mg L�1)

qe(mg g�1)

k2(g mg�1 min�1) R2

10.7 13.45 0.00314 0.993019.7 18.82 0.00167 0.991532.4 34.83 0.00048 0.964641.8 39.00 0.00054 0.983561.7 57.34 0.00046 0.9779

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heterogeneous surface. It is expressed by the following linearform.20

log qe ¼ log Kf þ 1

nlog Ce (3)

Kf and n are Freundlich isotherm constants that are related tothe adsorption capacity and the adsorption strength of theadsorbent, respectively. Kf and n can be obtained from the

Fig. 8 Freundlich adsorption isotherm of Cr(VI) onto a-Fe2O3–g-Al2O3 core–shell nanofibers at room temperature.

42380 | RSC Adv., 2014, 4, 42376–42382

intercept and the slope of the linear plot of log(qe) versus log(Ce),as shown in Fig. 8, are summarized in Table 2. According to theobtained results, the adsorption data of the Cr(VI) ions on the a-Fe2O3–g-Al2O3 core–shell nanobers are tted particularly wellwith the Freundlich model, as indicated by the very high valuesof the correlation coefficient (R2 over 0.95). It is usually acceptedthat a value of 2 # n < 10 represents an easy adsorption, a valueof 1 # n < 2 represents a moderate adsorption, while a value ofn < 1 represents a difficult adsorption.32 For the obtained core–shell structure nanobers, the value of n is 1.254. This indicatesthat the adsorption is not very strong.

The recyclable property of the obtained adsorbent is veryimportant for the materials used in the heavy metal ionsremoval. We repeated the Cr(VI) removal experiment for threetimes using the same absorbent aer the regeneration. 0.5 MNaOH eluent was used for the desorption process. Theadsorption capacity decreases for each new cycle aer desorp-tion. In the third cycle the adsorption capacity of the absorbentstill remains 81.3% of the rst cycle (ESI Fig. S1†).

Adsorption mechanism of Cr(VI) ions with a-Fe2O3–g-Al2O3

core–shell nanobers

XPS was further used to study the surface chemical composi-tions of the as-prepared and Cr(VI) ions adsorbed a-Fe2O3–g-Al2O3 core–shell nanobers. Fig. 9 shows the two survey spectrabefore and aer adsorption of Cr(VI) ions. Aer adsorption, thepresence of Cr on the surface of the a-Fe2O3–g-Al2O3 core–shellnanobers was obviously observed. High-resolution spectra ofCr 2p (Fig. 9b) could be deconvoluted into two components. Thepeaks at 576.6 eV (Cr 2p3/2) and 586.3 eV (Cr 2p1/2) are assignedto Cr(III) for Cr2O3. And the other peaks at 579.2 eV (Cr 2p3/2) and588.5 eV (Cr 2p1/2) are related to Cr(VI) for K2Cr2O7.33 Thisindicated that the adsorbed Cr on a-Fe2O3–g-Al2O3 core–shellnanobers existed two states, implying that the adsorptionprocess involved the some reduction of Cr(VI) to Cr(III). As shownIn Fig. 9c, before Cr(VI) adsorption two distinct peaks at bindingenergies of 710.6 eV for Fe 2p3/2 and 724.2 eV for Fe 2p1/2 with ashake-up satellite at 719.3 eV are observed. This is the charac-teristic of Fe(III) in Fe2O3.34 Then the binding energy of the Fe 2pshied to high energy aer Cr(VI) adsorption, due to the Fe2O3

as a semiconductor oxide was started by the adsorption of aphoton and generating electron–hole pairs during the adsorp-tion process. The peak at 74.7 eV (Al 2p) is assigned toaluminum atoms in Al2O3 (in Fig. 9d).

In this work, the Cr(VI) is removed through two processes.The rst is electrostatic adsorption. The adsorption property ofthe surface of Fe2O3 and Al2O3 strongly depends on pH insolution. When the pH is low, the Fe2O3 and Al2O3 can beprotonated. Its surface is charged positively, and electrostatic

Table 2 Freundlich parameters for adsorption of Cr(VI) onto a-Fe2O3–g-Al2O3 core–shell nanofibers

Metal ion Kf n R2

Cr(VI) 1.718 1.254 0.9739

This journal is © The Royal Society of Chemistry 2014

Fig. 9 XPS spectra of a-Fe2O3–g-Al2O3 core–shell nanofibers before (bottom curve) and after (top curve) adsorption of Cr(VI) ions.

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attraction between the charged surface and negatively chargedHCrO4

� and CrO42� in the acid solution (pH ¼ 2).14,20 The

second process is redox reaction. The Fe2O3 could produceelectron–hole pair under sunlight irradiation, which might beresponsible for the Cr(VI) reduction to Cr(III). Under sunlightirradiation and low pH value, electrons and holes wereproduced and the three-electron reaction of Cr(VI) reduced toCr(III) could happen.35,36

Conclusions

In this study, the a-Fe2O3–g-Al2O3 core–shell nanobers wereprepared via an electrospinning technique combining withvapor deposition and heat treatment techniques. Thecomposite nanobers exhibit ferromagnetic property and Cr(VI)removal performance. Kinetics of the Cr(VI) ions adsorption wasfound to follow pseudo-second-order rate equation. Theadsorption of Cr(VI) ions was tted well with the Freundlichisotherm equation. The regeneration studies demonstrated thatthe a-Fe2O3–g-Al2O3 core–shell nanobers can be recoveredeffectively. The adsorption mechanism for Cr(VI) on a-Fe2O3–g-Al2O3 core–shell nanobers was conrmed as electrostaticadsorption between hydroxyl on the a-Fe2O3–g-Al2O3 surfaceand Cr(VI) species, and the reduction of Cr(VI) to Cr(III). Based onthe obtained results, the novel a-Fe2O3–g-Al2O3 core–shell

This journal is © The Royal Society of Chemistry 2014

nanobers may be applied as a promising adsorbent in watertreatment.

Acknowledgements

This work was supported by the research Grants from theNational 973 Project (no. S2009061009), the National KeyTechnology Research and Development Program(2013BAC01B02), the National Natural Science Foundation ofChina (nos 21274052 and 51303060), Jilin Provincial Scienceand Technology Department Project (no. 20130206064GX), andResearch Fund for the Doctoral Program of Higher Education ofChina (no. 20120061120017).

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