Fabrication of 3D porous Mn doped α-Fe₂O₃ nanostructures for the removal of heavy metals from wastewater

Abstract

Three-dimensional porous Mn doped α-Fe₂O₃ nanostructures are successfully fabricated by calcined carbon spheres containing Fe(II) and Mn(II) ions, which were obtained by hydrothermal treatment of glucose, Fe(II), and Mn(II) mixed solutions. The obtained nanostructures exhibit excellent abilities for the removal of Pb(II), Cr(VI), and As(III) ions from wastewater with easy magnetic separation.

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The removal of heavy metal ions from wastewater has attracted intensive attention recently due to the high toxic risk of these metals to the environment and public health.¹ Several techniques including chemical precipitation, ion exchange, adsorption, membrane filtration, and electrodialysis have been developed for the removal of heavy metal ions from industrial wastewater.² Among them, adsorption is the most popular due to its high efficiency, easy operation and low cost.³ As adsorbents, iron oxide nanomaterials have been extensively used for the removal of heavy metal ions from wastewater because of their effective performance, low cost and environment-friendly properties.⁴ Various iron oxide nanostructures with controllable sizes and tailored shapes have been successfully fabricated by different methods to enhance their adsorption performances.^5–10 Among the various morphologies of iron oxide nanomaterials, three-dimensional (3D) hierarchically nanostructured materials exhibit enhanced adsorption performances for heavy metal ions compared to their bulk counterparts.^11,12 Although various 3D hierarchically nanostructured iron oxides including α-Fe₂O₃, Fe₃O₄, γ-Fe₂O₃, and α-FOOH have been successfully prepared by different methods,^7,9,11,13 the fabrication of 3D iron oxides with a strong affinity and high adsorption capacity for heavy metal ions still remains a big challenge.

Doping of various cations into iron oxides can result in improved performances for specific applications.^14–16 For example, Sn doped hematite nanostructures show remarkable improvement in photoelectrochemical performance,¹⁵ and Ni and Co doping at the hematite surface may reduce the overpotential for water oxidation on hematite photoanodes.¹⁶ It has also been reported that Mn doping could improve the magnetic properties of α-Fe₂O₃.^17,18 Moreover, a Mn dopant could increase the affinity and adsorption capacity of iron oxide nanoparticles for heavy metals.¹⁹ In previous research, we found that porous ferrite nanowires containing Mn exhibit an excellent ability to remove heavy metal ions.²⁰ Inspired by these results, we are interested to incorporate Mn into α-Fe₂O₃, the most stable iron oxide under ambient conditions, to prepare Mn doped 3D porous α-Fe₂O₃ nanostructures for the removal of heavy metals from solutions. Herein, 3D porous Mn doped α-Fe₂O₃ nanostructures were successfully prepared by calcined carbon spheres containing Fe(II) and Mn(II) ions for the removal of heavy metals from wastewater. As expected, the as-prepared 3D porous Mn doped α-Fe₂O₃ products exhibit excellent abilities in the removal of Pb(II), Cr(VI), and As(III) ions from solutions. It was feasible to use these adsorbents for fast recyclable treatment via simple magnetic separation as they enhance the magnetic properties of α-Fe₂O₃ nanostructures through the doping of Mn. To the best of our knowledge, this kind of magnetic 3D porous Mn doped α-Fe₂O₃ nanostructure has not been reported before. This material could be used as an absorbent for effectively removing pollutants from wastewater.

Fig. 1a shows the power X-ray diffraction (XRD) pattern of the calcined products. All the diffraction peaks in the XRD pattern can be readily indexed as α-Fe₂O₃ by comparing with the standard pattern (JCPDS 33-0664), also shown in the figure, indicating the high purity of the products. Scanning electron microscopy (SEM) images show that the as-synthesized α-Fe₂O₃ materials maintain the morphology of flower-like nanostructures accompanying a few aggregates of nanoparticles (Fig. 1b). The high-magnification SEM image reveals that the flower-like nanostructures have a diameter of 0.5–1 μm, and are formed of irregular nanorods with diameters of 100–300 nm (inset of Fig. 1b). The transition electron microscopy (TEM) images (Fig. 1c) show that the flower-like nanostructures composed of numerous nanoparticles are closely packed and held together to form nanorods and are then self-assembled into flower-like structures. The high-resolution transition electron microscopy (HRTEM) images show that the nanocrystals have diameters of 5–10 nm (inset in Fig. 1c), and that the nanocrystals show interlayer spacings of 0.252 nm (Fig. 1d), which correspond to the (110) plane of the as-prepared α-Fe₂O₃.^21,22 Systematic investigation of different reaction conditions revealed that the amounts of Fe(II), Mn(II) and glucose, the Fe(II)/Mn(II) ratios, the calcination temperature and the reaction time obviously affect the morphology of the final products (Fig. S1–S5†).

Fig. 1 XRD pattern (a), SEM images (b), and TEM images (c and d) of the synthesized Mn doped α-Fe₂O₃ nanostructures (hydrothermal conditions: 48 mmol FeSO₄·7H₂O, 12 mmol MnSO₄·H₂O, 20 mmol glucose; carbon spheres containing Fe(II) and Mn(II) calcined at 500 °C for 3 h in air).

The energy dispersive X-ray spectrometry (EDX) spectrum of the as-synthesised α-Fe₂O₃ nanostructures confirms the presence of Mn, Fe, and O elements (Fig. S6†), validating the doping of Mn in α-Fe₂O₃. SEM EDX was utilized to further verify the elemental composition of the Mn doped α-Fe₂O₃, as well as the nanoscale spatial uniformity of the element distribution. All the elements are homogeneously distributed in the Mn doped α-Fe₂O₃ samples (Fig. 2). The form of the Fe present in the Mn doped α-Fe₂O₃ nanostructures was further confirmed by X-ray photoelectron spectroscopy (XPS) analysis due to its sensitivity to Fe(II) and Fe(III) cations. Fig. S7† indicates that a well-resolved satellite peak is found at 719.0 eV, which indicates the absence of Fe(II) ions in the samples.²³ It can be seen that the Mn 2p_3/2 peak is centered at about 642 eV while the Mn 2p_1/2 peak is at about 654 eV (Fig. S7†). However, the variation in XPS binding energies of Mn 2p from Mn(II) to Mn(IV) is too small (less than 1.0 eV) to precisely evaluate the Mn valence.

Fig. 2 Low-magnification SEM image of the Mn doped α-Fe₂O₃ nanostructures and SEM elemental distribution mapping for Fe, Mn, and O elements.

Magnetization curves were measured for the as-synthesized Mn doped α-Fe₂O₃ nanostructures and pure α-Fe₂O₃ at room temperature. As depicted in Fig. 3, it is easy to see an obvious hysteresis loop at the full scale for the two samples, indicating weak ferromagnetic properties at 300 K for the two sets of samples, which is in agreement with the observations previously reported.²⁴ It can be seen from Fig. 3 that the magnetization saturation value is about 6.2 emu g⁻¹ for the Mn doped α-Fe₂O₃ nanostructures, which is approximately two times higher than that of the pure α-Fe₂O₃ (3.2 emu g⁻¹). These results reveal that the doping of Mn can enhance the magnetic properties of the α-Fe₂O₃ crystal materials in our experiments. Despite the lower saturation magnetization, the porous Mn doped α-Fe₂O₃ nanostructures are still efficient in magnetic manipulation and recovery of the sorbent in water treatment, as shown in the inset of Fig. 3.

Fig. 3 The magnetization hysteresis of the as-synthesized Mn doped α-Fe₂O₃ and pure α-Fe₂O₃ nanostructures.

Fig. 4 N₂ adsorption and desorption isotherms and pore-size distribution (inset) for the porous Mn doped α-Fe₂O₃ nanostructures.

In order to examine the pore characteristics of the synthesized porous Mn doped α-Fe₂O₃ nanostructures, Brunauer–Emmett–Teller (BET) N₂ adsorption/desorption measurements were performed. The isothermal plots of N₂ adsorption/desorption for the Mn doped α-Fe₂O₃ nanostructures show type IV isotherms with an apparent hysteresis loop in the range 0.6–1.0 P/P₀ (Fig. 4), indicating the presence of mesopores in the composites. Based on the BET equation, the specific surface area of the Mn doped α-Fe₂O₃ nanostructures is 61.6 m² g⁻¹. The pore size distributions of the products, calculated from the desorption data using the Barrett–Joyner–Halenda (BJH) model, show a wide peak centered at ∼7.2 nm (inset in Fig. 4). However, the pore size distributions of the Mn doped α-Fe₂O₃ nanostructures are not limited to mesopores and clearly trespass in the domain of macroporosity. Compared with ordered mesoporous metal oxides, their pore size distributions are broader due to the irregular voids formed by the nanoparticles.

To further verify the advantage of porous Mn doped α-Fe₂O₃ nanostructures for water treatment, we evaluated their adsorption capabilities for toxic heavy metal ions at ambient temperature. The adsorption isotherms of Pb(II), Cr(VI) and As(III) were obtained with different initial concentrations as shown in Fig. 5. Experimental data were fitted well with the Langmuir adsorption model isotherm, and the maximal adsorption capacity of the porous Mn doped α-Fe₂O₃ nanostructures was about 306 mg g⁻¹ for Pb(II), 205 mg g⁻¹ for Cr(VI), and 358 mg g⁻¹ for As(III). It is worth mentioning that these values are much higher than those of previously reported 3D iron oxide nanostructures, such as flower-like α-Fe₂O₃ nanostructures (30 mg g⁻¹ for Cr(VI)),⁷ urchin-like α-FeOOH hollow spheres (80 mg g⁻¹ for Pb(II)),⁹ and mesoporous ferrite nanocrystal clusters (27 mg g⁻¹ for As(III)).²⁵ The high adsorption capacity for heavy metal ions may be due to the porous nanostructures,²⁶ small particles,²⁷ and the changes in surface chemistry caused by the doping of Mn.¹⁹ Heavy metal ion adsorption is likely based on the combination of electrostatic interactions between charged oxides and ions, and the ion exchange in the aqueous solution.

Fig. 5 Adsorption isotherms of Pb(II) at pH 5 (a), Cr(VI) at pH 3 (b), and As(III) at pH 7 (c) using the porous Mn doped α-Fe₂O₃ nanostructures (adsorbent dose: 1 g L⁻¹ for Pb(II) and Cr(VI), and 0.05 g L⁻¹ for As(III)).

Conclusions

In summary, 3D porous Mn doped α-Fe₂O₃ nanostructures have been successfully synthesized by calcined carbon spheres containing Fe(II) and Mn(II) ions. The Mn substitution induces an obvious increase in the magnetic properties of α-Fe₂O₃. The obtained products show high affinity and adsorption capacities for Pb(II), Cr(VI), and As(III) ions with easy magnetic separation, and the maximal adsorption is ca. 306 mg g⁻¹ for Pb(II), 205 mg g⁻¹ for Cr(VI) and 358 mg g⁻¹ for As(III). These results imply that this material could be used as an absorbent for effectively removing heavy metal ions from wastewater.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (no. 41371244, 41001139 and 51278481), the National High Technology Research and Development Program (“863” Program) of China (no. 2012AA062606), the International Science & Technology Cooperation Program of China (2011DFB91710) and the Open Fund of Key Laboratory of Urban Environment and Health, Institute of Urban Environment, Chinese Academy of Sciences (no. KLUEH201102).

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Footnote

† Electronic supplementary information (ESI) available: Details of experimental procedures, SEM images of the final products under different experimental conditions, EDX spectrum of the porous Mn doped α-Fe₂O₃ nanostructures, XPS spectra of Fe 2p (a) and Mn 2p (b) in the porous Mn doped α-Fe₂O₃ nanostructures. See DOI: 10.1039/c3ra46348f

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