Fabrication of α-Fe₂O₃–γ-Al₂O₃ core–shell nanofibers and their Cr(VI) adsorptive properties

Abstract

α-Fe₂O₃–γ-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 α-Fe₂O₃–γ-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 α-Fe₂O₃–γ-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 α-Fe₂O₃–γ-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 α-Fe₂O₃–γ-Al₂O₃ core–shell nanofibers are an attractive adsorbent for the removal of heavy metal ions from water.

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Introduction

The increasing worldwide contamination of freshwater systems has become one of the key environmental problems facing humanity. Heavy metal ion pollution is regarded as the most severe environmental contamination today. Various kinds of methods have been carried out for heavy metal ion removal such as precipitation, membrane filtration, reverse osmosis, solvent extraction and electrolysis, biological treatment, ion-exchange processes, and adsorption. Among them, adsorption technique has advantage of high efficiency, cost effectiveness and eco-friendly materials as adsorbents.^1,2

Iron or aluminum oxides and hydroxides are well known to show high uptake of cations and anions³ in natural environments, such as chromium,⁴ arsenate⁵ and phosphate.⁶ α-Fe₂O₃ and γ-Al₂O₃ are more suitable for the removal of toxic heavy metal ions and organic pollutants from wastewater because of their high thermal stabilities, large surface areas, low cost and abundant availability.^7,8 Recently, Chubar et　al. prepared Fe₂O₃·Al₂O₃·xH₂O with high specific surface area by a sol–gel method, and the Fe₂O₃·Al₂O₃·xH₂O adsorbent showed good adsorption ability for phosphate ions.⁹ Gulshan et　al. synthesized alumina–iron oxide compounds by a gel evaporation method and investigated its simultaneous uptake properties for Ni²⁺, NH₄⁺ and H₂PO₄⁻.¹⁰ Li et　al. prepared three types of alumina-coupled iron oxides by coprecipitation approach and investigated their photocatalytic activity for bisphenol A degradation.¹¹ Cao et　al. synthesized flowerlike α-Fe₂O₃ nanostructures with maximum capacities of 51 and 30 mg g⁻¹ for As(V) and Cr(VI) removal.¹² Wei et　al. obtained hollow nestlike α-Fe₂O₃ nanostructures with maximum removal capacities of 75.3, 58.5, and 160 mg g⁻¹ for As(V), Cr(VI), and Congo red.¹³ Ge et　al. prepared the hierarchical γ-Al₂O₃ with a facile hydrothermal method and this product exhibited fast and high adsorption capacities towards Cr(VI) and CO₂.¹⁴ Mahapatra et　al. investigated Fe₂O₃–Al₂O₃ nanocomposite fibers removal behavior of heavy metal ions from aqueous solution, the removal percentage was in the order of Cu²⁺< Pb²⁺< Ni²⁺< Hg²⁺.¹⁵

Currently, nanomaterials and nanotechnology have garnered worldwide attention for their application in environmental remediation and pollution control, because nanostructured surfaces offer large surface areas and rich valence states that provide enhanced affinity and adsorption capability toward pollutants.¹⁵ Electrospinning technique has been known to be a simple and versatile method to generate organic or inorganic nanofibers. This technique involves applying a high voltage on a viscous solution to fabricate ultrathin fibers with diameters in the range of 20–2000 nm. These nanofibers show a number of interesting characteristics such as large surface area per unit mass, high gas permeability, high porosity and small pore size. Therefore in recent studies, electrospun fibers have been widely used as matrixes or templates to fabricate functional materials with tunable structures, such as core–shell nanofibers,¹⁶ molecular sieve fibers,¹⁷ nanotubes,¹⁸ necklace-like nanostructures,¹⁹ hierarchically nanofibrous membrane,²⁰ functional-group chelating nanofiber membranes,²¹ and polymer-supported nanocomposite.²²

In this work, we have reported the preparation of a novel fiber-in-tube nanofibers containing of α-Fe₂O₃ nanofiber core and γ-Al₂O₃ shell via an electrospinning technique combining with vapor deposition and heat treatment techniques. The α-Fe₂O₃–γ-Al₂O₃ core–shell nanofibers exhibited ferromagnetic property and Cr(VI) removal performance. Therefore, this work provides a facile and effective approach for the fabrication of α-Fe₂O₃–γ-Al₂O₃ core–shell nanofibers, which can be potentially applied in heavy metal ions removal from wastewater effluents.

Experimental

Materials

Poly(vinyl alcohol) (PVA, M_w = 75 000–80 000) was purchased from Beijing Chemical Factory. Triton-X 100 (C₃₄H₆₂O₁₁) was purchased from Aldrich. Ammonium ferric citrate [Fe(NH₄)₃(C₆H₅O₇)₂] was purchased from Shantou Xilong Chemical Corporation. Potassium dichromate (K₂Cr₂O₇) was purchased from Beijing Chemical Factory. All the reagents were used without further purification.

Characterization

The morphology of the nanofibers was characterized by scanning electron microscopy (SEM, Shimadzu SSX-550). Transmission electron microscope (TEM) measurements were conducted on a JEOL JEM 1200EX at 100 kV. HRTEM imaging was performed on a FEI Tecnai G2 F20 high-resolution transmission electron microscope operating at 200 kV. FT-IR spectra were recorded on a Bruker Vector-22 FT-IR spectrometer from 4000 to 400 cm⁻¹ using powder-pressed KBr pellets at room temperature. The amounts of initial Cr(VI) ions were determined using an inductive coupled plasma emission spectrometer (ICP, PerkinElmer OPTIMA 3300DV). The pH values of the reaction solutions 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-vis spectrophotometer. Analysis of the X-ray photoelectron spectra (XPS) was performed on Thermo ESCALAB 250 spectrometer with a Mg–K (1253.6 eV) achromatic X-ray source. The static magnetic properties of the α-Fe₂O₃–γ-Al₂O₃ core–shell nanofibers were measured using a vibrating sample magnetometer (VSM 7410) at a temperature of 300 K.

Electrospinning of ammonium ferric citrate/PVA composite nanofibers

PVA aqueous solution was prepared by dissolving 0.65 g of PVA powder in 9.35 g of deionized water, heating to 80 °C under stirring for 2 h, and then cooling down to room temperature. Afterward, 0.385 g of ammonium ferric citrate was added into the PVA aqueous solution and stirred for 3 h. Then the solution was loaded into a glass syringe and connected to high-voltage power supply (a Gamma High Voltage Research ORMOND BEACH.FL32174 dc power supply). 18 kV was provided between the cathode and anode at a distance of 15 cm to prepare ammonium ferric citrate/PVA composite nanofibers, and several silicon slices (size: 3 cm × 3 cm) were used as the collector, which was sticked on the surface of Al foil.

Preparation of α-Fe₂O₃–γ-Al₂O₃ core–shell nanofibers

Preparation of the aluminum phase. The Al vapor deposition on the ammonium ferric citrate/PVA composite nanofibers was performed in a commercial thermal evaporation system at a pressure of 5 × 10⁻⁴ Pa (KYKY Technology Development Co. Ltd. China). The depositing rate was 20 Å s⁻¹ and the thickness of the Al film was 150–190 nm. The ammonium ferric citrate/PVA–Al composite nanofibers were calcined in air with programmable heating process. The heating rate is 10 °C min⁻¹, the samples first were annealed at 600 °C for 2 h, and then heating up to 800 °C for 2 h. After the fibers were naturally cooled to room temperature, the α-Fe₂O₃–γ-Al₂O₃ core–shell nanofibers were obtained.

Adsorption of Cr(VI) ions

The Cr(VI) solution was prepared by dissolving potassium dichromate (K₂Cr₂O₇) in aqueous solution at pH = 2.0. The adsorption isotherm of Cr(VI) ions on α-Fe₂O₃–γ-Al₂O₃ core–shell nanofibers was studied by dispersing the samples (2 mg) in a series of flasks containing 45.0 mL Cr(VI) ions solution of varying initial concentrations, ranging from 9.0 to 62.0 mg L⁻¹. The flasks were placed on a shaker and kept at room temperature. After shaking for a certain time the solution was separated for concentration analysis, the initial concentration of the Cr(VI) ions in solution was determined by ICP spectrometer. The equilibrium concentration of Cr(VI) ions in the solution was measured by UV-vis absorption spectroscopy. The amount of the Cr(VI) ions adsorbed onto each sample (the adsorption capacity (q, in mg g⁻¹)) was calculated on the basis of the following equation:²³where C₀ and C_e are the initial and the equilibrium concentrations of the metal ions in the test solution (mg L⁻¹), V is the volume of the testing solution (L), and W is the weight of the adsorbent (g).

Results and discussion

Morphologies

The morphologies of the as-electrospun ammonium ferric citrate/PVA composite nanofibers and α-Fe₂O₃–γ-Al₂O₃ core–shell nanofibers were observed by SEM and TEM measurements. As shown in Fig.　1a, the obtained ammonium ferric citrate/PVA composite nanofibers have a basic smooth and uniform structure, with the diameter in range of 300–650 nm (Fig.　1d). After vapor deposition and annealing, the composite nanofibers keep the smooth fiber morphology (Fig.　1b), with an average diameter of ∼330 nm (Fig.　1e). Fig.　1c shows the TEM image of the obtained nanofibers. The core–shell structure can been clearly seen. The core is α-Fe₂O₃ nanofibers derived from ammonium ferric citrate/PVA composite nanofibers and the shell is γ-Al₂O₃. After calcination (Fig.　1c and f), it can also be clearly seen that the α-Fe₂O₃ core fibers have rough surface, and the diameter is in the range of 53–76 nm, due to the crystallization related to the formation of particulate morphology. A HRTEM image taken from the edge of (Fig.　2a) was shown in Fig.　2b. The typical lattice fringes were clearly visible, with a spacing of 0.37 and 0.27 nm, corresponding to the (012) and (104) planes of α-Fe₂O₃, respectively. The spacing of 0.20 nm was corresponded to the (400) planes of γ-Al₂O₃, which further indicates that the nanofibers consisted of α-Fe₂O₃ core and γ-Al₂O₃ shell.

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

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

Structural properties

The chemical composition of the as-prepared product was analyzed using powder XRD measurements. As shown in Fig.　3, the diffraction angles at 2θ = 24.33°, 33.27°, 35.79°, 41.00°, 49.60°, 54.20°, 57.76°, 62.63°, 64.10° and 72.17° can be assigned to (012), (104), (110), (113), (024), (116), (018), (214), (300) and (119) planes of α-Fe₂O₃, which are in good agreement with the recorded values of JCPDS File Card no. 33-0664. The pronounced peaks at (012), (104), and (110) suggest that the obtained α-Fe₂O₃ was well-crystallized. The peaks at 38.55°, 46.03° and 67.14° degrees are assigned to the (222), (400) and (440) reflection of γ-Al₂O₃ (JCPDS File Card no. 10-0425). The results indicate that α-Fe₂O₃–γ-Al₂O₃ nanofibers have been successfully prepared.

Fig. 3 XRD patterns of α-Fe₂O₃–γ-Al₂O₃ core–shell nanofibers.

The FT-IR spectra shown in Fig.　4 were used to get information for the structural changes related to the pure PVA nanofibers, the ammonium ferric citrate/PVA composite nanofibers and the annealed α-Fe₂O₃–γ-Al₂O₃ core–shell nanofibers. For pure PVA nanofibers (Fig.　4a), a broad peak at about 3420 cm⁻¹ corresponds to H–OH stretching vibration, the peak at 2950 cm⁻¹ is related to –CH stretching vibrations, the peaks in the range of 1450–1300 cm⁻¹ and 1000–1160 cm⁻¹ could be ascribed to the –OH bending vibrations and C–O stretching vibrations, respectively. For the ammonium ferric citrate/PVA composite nanofibers (Fig.　4b), the peaks at 1400 cm⁻¹ corresponds to the COO– stretching vibrations. As shown in Fig.　4c, these characteristic peaks of PVA were almost removed from the nanofibers after calcination at 800 °C, illustrating the decomposition of organic matter. Furthermore, two peaks around 541 and 463 cm⁻¹ were observed, which can be ascribed to the metal–oxygen vibration of the α-Fe₂O₃ and γ-Al₂O₃.^24,25

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

Magnetic behavior

The magnetic behaviors of the samples were conducted by isothermal magnetic hysteresis measurements at room temperature (300 K) with the field sweeping from −20 to 20 kOe. As shown in Fig.　5, the loop suggests that the α-Fe₂O₃–γ-Al₂O₃ core–shell nanofibers are ferromagnetic at room temperature, mainly because of the α-Fe₂O₃ contents display ferromagnetic behavior. The coercivity force (H_c), remnant magnetization (M_r) and saturated magnetization (M_s) of the samples are estimated to be 2.24 kOe, 0.0585 emu g⁻¹ and 0.111 emu g⁻¹, respectively. The observed coercivity of the α-Fe₂O₃–γ-Al₂O₃ core–shell nanofibers is larger than that of the rod,²⁶ nanofiber,²⁷ shuttle-like α-Fe₂O₃,²⁸ Al³⁺ doped α-Fe₂O₃ nanoplates,²⁹ and Fe–SiO₂ core–shell nanostructures,³⁰ indicating that the magnetization of ferromagnetic materials is sensitive to microstructural characteristics such as size, shape and defects in crystal structure.³¹

Fig. 5 Magnetic hysteresis loops of α-Fe₂O₃–γ-Al₂O₃ core–shell nanofibers at room temperature.

Adsorption kinetics

The core–shell structures of the obtained α-Fe₂O₃–γ-Al₂O₃ nanofibers afford its potential applications for waste adsorption. In this study, the feasibility of α-Fe₂O₃–γ-Al₂O₃ core–shell nanofibers as Cr(VI) ion remover was explored. Fig.　6 shows the adsorption capacity of Cr(VI) ions in the aqueous solution at different times. Clearly, the adsorption for the Cr(VI) ions capacity increased with the increasing of the contact time, and reached equilibria at about 120 min. Here, we use the pseudo-second-order model to evaluate the kinetics of Cr(VI) removal on α-Fe₂O₃–γ-Al₂O₃ core–shell nanofibers (Fig.　7). The linearized form of the pseudo-second-order equation is as follows:²¹where k₂ (g mg⁻¹ min⁻¹) is the pseudo-second-order rate constant, q_e (mg g⁻¹) is the adsorption capacity at adsorption equilibrium and q_t (mg g⁻¹) is the adsorption at time t. The kinetics of the removal process was shown in Fig.　7. Table 1 summarizes the calculated q_e values, pseudo-second-order rate constants k₂, and correlation coefficient values (R²). The calculated q_e is calculated from the slope and intercept of the plots of t/q_t versus t.

Fig. 6 The adsorption capacity of α-Fe₂O₃–γ-Al₂O₃ core–shell nanofibers for different concentrations of Cr(VI) ions as a function of time.

Fig. 7 The pseudo-second-order model for adsorption of Cr(VI) ions by α-Fe₂O₃–γ-Al₂O₃ core–shell nanofibers.

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

Concentration of Cr(VI) ions (mg L^−1)	qe (mg g^−1)	k2 (g mg^−1 min^−1)	R^2
10.7	13.45	0.00314	0.9930
19.7	18.82	0.00167	0.9915
32.4	34.83	0.00048	0.9646
41.8	39.00	0.00054	0.9835
61.7	57.34	0.00046	0.9779

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The Freundlich isotherm is an empirical equation that is used to describe adsorption at multilayer and adsorption on a heterogeneous surface. It is expressed by the following linear form.²⁰

K_f and n are Freundlich isotherm constants that are related to the adsorption capacity and the adsorption strength of the adsorbent, respectively. K_f and n can be obtained from the intercept and the slope of the linear plot of log(q_e) versus log(C_e), as shown in Fig.　8, are summarized in Table 2. According to the obtained results, the adsorption data of the Cr(VI) ions on the α-Fe₂O₃–γ-Al₂O₃ core–shell nanofibers are fitted particularly well with the Freundlich model, as indicated by the very high values of the correlation coefficient (R² over 0.95). It is usually accepted that a value of 2 ≤ n < 10 represents an easy adsorption, a value of 1 ≤ n < 2 represents a moderate adsorption, while a value of n　< 1 represents a difficult adsorption.³² For the obtained core–shell structure nanofibers, the value of n is 1.254. This indicates that the adsorption is not very strong.

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

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

Metal ion	Kf	n	R^2
Cr(VI)	1.718	1.254	0.9739

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The recyclable property of the obtained adsorbent is very important for the materials used in the heavy metal ions removal. We repeated the Cr(VI) removal experiment for three times using the same absorbent after the regeneration. 0.5 M NaOH eluent was used for the desorption process. The adsorption capacity decreases for each new cycle after desorption. In the third cycle the adsorption capacity of the absorbent still remains 81.3% of the first cycle (ESI Fig.　S1†).

Adsorption mechanism of Cr(VI) ions with α-Fe₂O₃–γ-Al₂O₃ core–shell nanofibers

XPS was further used to study the surface chemical compositions of the as-prepared and Cr(VI) ions adsorbed α-Fe₂O₃–γ-Al₂O₃ core–shell nanofibers. Fig.　9 shows the two survey spectra before and after adsorption of Cr(VI) ions. After adsorption, the presence of Cr on the surface of the α-Fe₂O₃–γ-Al₂O₃ core–shell nanofibers was obviously observed. High-resolution spectra of Cr 2p (Fig.　9b) could be deconvoluted into two components. The peaks at 576.6 eV (Cr 2p_3/2) and 586.3 eV (Cr 2p_1/2) are assigned to Cr(III) for Cr₂O₃. And the other peaks at 579.2 eV (Cr 2p_3/2) and 588.5 eV (Cr 2p_1/2) are related to Cr(VI) for K₂Cr₂O₇.³³ This indicated that the adsorbed Cr on α-Fe₂O₃–γ-Al₂O₃ core–shell nanofibers existed two states, implying that the adsorption process involved the some reduction of Cr(VI) to Cr(III). As shown In Fig.　9c, before Cr(VI) adsorption two distinct peaks at binding energies of 710.6 eV for Fe 2p_3/2 and 724.2 eV for Fe 2p_1/2 with a shake-up satellite at 719.3 eV are observed. This is the characteristic of Fe(III) in Fe₂O₃.³⁴ Then the binding energy of the Fe 2p shifted to high energy after Cr(VI) adsorption, due to the Fe₂O₃ as a semiconductor oxide was started by the adsorption of a photon and generating electron–hole pairs during the adsorption process. The peak at 74.7 eV (Al 2p) is assigned to aluminum atoms in Al₂O₃ (in Fig.　9d).

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

In this work, the Cr(VI) is removed through two processes. The first is electrostatic adsorption. The adsorption property of the surface of Fe₂O₃ and Al₂O₃ strongly depends on pH in solution. When the pH is low, the Fe₂O₃ and Al₂O₃ can be protonated. Its surface is charged positively, and electrostatic attraction between the charged surface and negatively charged HCrO₄⁻ and CrO₄²⁻ in the acid solution (pH = 2).^14,20 The second process is redox reaction. The Fe₂O₃ could produce electron–hole pair under sunlight irradiation, which might be responsible for the Cr(VI) reduction to Cr(III). Under sunlight irradiation and low pH value, electrons and holes were produced and the three-electron reaction of Cr(VI) reduced to Cr(III) could happen.^35,36

Conclusions

In this study, the α-Fe₂O₃–γ-Al₂O₃ core–shell nanofibers were prepared via an electrospinning technique combining with vapor deposition and heat treatment techniques. The composite nanofibers exhibit ferromagnetic property and Cr(VI) removal performance. Kinetics of the Cr(VI) ions adsorption was found to follow pseudo-second-order rate equation. The adsorption of Cr(VI) ions was fitted well with the Freundlich isotherm equation. The regeneration studies demonstrated that the α-Fe₂O₃–γ-Al₂O₃ core–shell nanofibers can be recovered effectively. The adsorption mechanism for Cr(VI) on α-Fe₂O₃–γ-Al₂O₃ core–shell nanofibers was confirmed as electrostatic adsorption between hydroxyl on the α-Fe₂O₃–γ-Al₂O₃ surface and Cr(VI) species, and the reduction of Cr(VI) to Cr(III). Based on the obtained results, the novel α-Fe₂O₃–γ-Al₂O₃ core–shell nanofibers may be applied as a promising adsorbent in water treatment.

Acknowledgements

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

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra03692a

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