Fabrication of γ-Fe 2 O 3 Nanoparticles by Solid-State Thermolysis of a Metal-Organic Framework , MIL-100 ( Fe ) , for Heavy Metal Ions Removal

Porous γ-Fe 2 O 3 nanoparticles were prepared via a solid-state conversion process of a mesoporous iron(III) carboxylate crystal, MIL-100(Fe). First, the MIL-100(Fe) crystal that served as the template of the metal oxide was synthesized by a low-temperature (<100C) synthesis route. Subsequently, the porous γ-Fe 2 O 3 nanoparticles were fabricated by facile thermolysis of the MIL-100(Fe) powders via a two-step calcination treatment.The obtained γ-Fe 2 O 3 was characterized by X-ray diffraction (XRD), N 2 adsorption, X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM) techniques, and then used as an adsorbent for heavymetal ions removal inwater treatment.This study illustrates that themetal-organic frameworksmay be suitable precursors for the fabrication of metal oxides nanomaterials with large specific surface area, and the prepared porous γ-Fe 2 O 3 exhibits a superior adsorption performance for As(V) and As(III) ions removal in water treatment.


Introduction
As(V) and As(III) are considered as primarily highly toxic pollutants in water resources due to their wide spread use in industrial processes, and their efficient removal from water is of great importance.A number of strategies have been applied in order to remove these contaminants from wastewater [1].Among them, adsorption-based removal technique is one of the most widely investigated methods because of its availability, profitability, ease of operation, and efficiency in comparison with other methods [2].So far, a variety of arsenic adsorbents have been evaluated, among which the granules of iron oxides or hydroxides exhibit high adsorption capacities and have made them the most commonly utilized adsorbents for the removal of the arsenic contaminants .However, fabrication of the porous -Fe 2 O 3 nanoparticles with a high specific surface area that favors a high adsorption capacity, as well as the separation of the saturated adsorbents so as to prevent secondary pollution during their usage, is still a challenging topic.
Recently, metal-organic frameworks (MOFs) with diverse architectures and morphologies have been recognized as promising precursors/templates to develop the porous iron oxides [25][26][27][28][29][30][31][32].For example, Xu and coworkers reported the fabrication of spindle-like mesoporous -Fe 2 O 3 using MOF MIL-88(Fe) as template [25].Their prepared spindlelike mesoporous -Fe 2 O 3 was composed of clustered Fe 2 O 3 nanoparticles with size of <20 nm.Zhang and coworkers demonstrated a facile synthesis of porous Fe 2 O 3 nanocubes by simultaneous oxidative decomposition of Prussian blue nanocubes [32].The derived porous Fe 2 O 3 nanocubes are composed of very fine Fe 2 O 3 nanoparticles with size of several nanometers.It should be stressed that, MOFs show distinct advantages due to their well-ordered crystalline structure, high porosity, large surface area, and tunable pore size, in comparison with other templates.However, facile and controlled synthesis of magnetic nanoparticles -Fe 2 O 3 from the thermolysis of Fe-based MOFs, is still rarely reported.
MIL-100(Fe), a novel mesoporous MOF, has a large BET specific surface area and pore volume with the presence of a significant amount of accessible Lewis acid metal sites upon dehydration [33,34].Considering that the synthesis and application of MIL-100(Fe) is an area of much interest and still in its infancy, in the current work, we synthesized the MIL-100(Fe) crystals at low temperature and atmospheric pressure conditions, and then fabricated the porous -Fe 2 O 3 nanoparticles by simple thermolysis of the obtained MIL-100(Fe) powders via a two-step calcination treatment.Compared to the ordinary -Fe 2 O 3 materials obtained by a solvothermal process and subsequent calcination, this strategy is simple, inexpensive, and scalable.When the prepared porous -Fe 2 O 3 nanoparticles were used as adsorbent in water treatment, this magnetic material showed an excellent performance for heavy metal ion removal.

Experimental Section
2.1.Preparation of the Porous -Fe 2 O 3 .The template, MIL-100(Fe), was prepared by a facile low-temperature (<100 ∘ C) synthesis route according to the procedure described in our previous work [35].Afterwards, the MIL-100(Fe) powders were placed in a tube furnace under N 2 gas flow (1 ∘ min −1 ), heated to 450 ∘ C, and maintained at 450 ∘ C for 2 h.Then the resulting black powders were placed in a box furnace, and heated to 450 ∘ C (1 ∘ min −1 ), and maintained at 450 ∘ C for 2 h in air.This two-step calcination resulted in the final porous -Fe 2 O 3 nanoparticles.

Characterization. The X-ray powder diffraction (XRD)
patterns were obtained on a Philips PW3040/60 diffractometer, using CuK radiation ( = 0.1541 nm) in a scanning range of 3 to 50 ∘ at a scanning rate of 1 ∘ min −1 .N 2 adsorption isotherms were obtained at −196 ∘ C using a Micromeritics ASAP 2020 instrument.The samples were outgassed under vacuum at 150 ∘ C for 10 , prior to the adsorption measurements.The scanning electron microscope (SEM) observations were performed on a Hitachi S-4800 apparatus equipped with a field emission gun.The transmission electron microscopy (TEM) observations were carried out on a JEOL JEM-1200 working at 120 kV.The surface electronic states of the synthesized samples were investigated by X-ray photoelectron spectroscopy (XPS, Thermo Scientific EscaLab 250Xi using AlK radiation).The XPS data were internally calibrated fixing the binding energy of C 1s at 284.6 eV.

Water Treatment Experiment.
Solutions containing different concentrations of As(V) and As(III) were prepared using NaH 2 AsO 4 ⋅12H 2 O and As 2 O 3 as the sources of heavy metal ions, respectively.To obtain the adsorption isotherms, 10 mg of -Fe 2 O 3 was added to the above solution under stirring at room temperature.After the specified time, the solid and liquid were separated and an IRIS Intrepid IIXSP inductively coupling plasma-atomic emission spectrometer (ICP-AES) was used to measure the concentration of metal ions in the remaining solution.

Results and Discussion
The XRD patterns of the template MIL-100(Fe) (Figure S1 in Supplementary data available online at http://dx.doi.org/10.1155/2014/546956) are very similar with the simulated ones reported in literature [33,34], confirming the formation of the metal-organic framework.Figure 1 illustrates the XRD patterns of the sample derived from MIL-100(Fe) via the so-called two-step calcination treatment.It can be observed that all the diffraction peaks can be assigned to -Fe 2 O 3 with a cubic structure (JCPDS card no.39-1346) [32].No peak from any impurities can be observed in the XRD patterns, indicating high purity of -Fe 2 O 3 products, and also the template was completely transformed into -Fe 2 O 3 after the two-step calcination.Estimated grain size of the sample using Scherrer equation was about 30 nm.SEM and TEM images show that the -Fe 2 O 3 nanomaterial is made of nano-aggregates with a relatively homogeneous particle sized distribution between 20 and 50 nm (Figures S2 and S3 in Supplementary data), in accordance with the result calculated by Scherrer equation.The chemical composition of the template MIL-100(Fe) and the derived -Fe 2 O 3 were further characterized by using XPS.The XPS spectra of MIL-100(Fe) (Figure S4 in Supplementary data) are almost identical to those of Fe-BTC, the counterpart of MIL-100(Fe) [36].The XPS spectra of the derived -Fe 2 O 3 are shown in Figure 2(a).The predominant elements are Fe and O, and small amounts of residual C are also present.The peaks at binding energy of 56, 284.6, and 530.4 eV were designated for Fe3p3/2, C1s, and O1s, respectively.High resolution XPS of Fe2p is shown in Figure 2(b).Binding energy of Fe2p1/2 is 724.8 eV and binding energy of Fe2p3/2 is 711.1 eV.The presence of satellite peak at 719 eV is characteristic for maghemite.These results agree with literature values of the -Fe 2 O 3 particles [2].
For the template MIL-100(Fe), its specific surface area and total pore volume are 1800 m 2 g −1 and 1.0 cm 3 g −1 , respectively, estimated by N 2 adsorption and desorption (Figure S5(a) in Supplementary data).The pore size distribution curve of the MIL-100(Fe) sample displays two different pore sizes centered at about 1.9 and 2.2 nm (Figure S5(b) in Supplementary data) confirming the presence of the two types of mesoporous cages in MIL-100(Fe) [33].The N 2 analysis also demonstrates the mesoporous character of the derived -Fe 2 O 3 material, with type IV N 2 isotherm at 77 K (Figure 3(a)).This material has a relatively large specific surface area of 123.5 m 2 g −1 , which is triple as that of mixed Fe 2 O 3 prepared from Prussian blue [32].Such a high surface area value is presumed to be benefited from the two-step calcination of the template MIL-100(Fe).On the other hand, the pore size distribution (Figure 3(b)) reveals that most of the pores are focused on 10 nm, indicating the mesopores are effective formed for the -Fe 2 O 3 .
The large specific surface area, mesoporous characteristic and high pore volume may offer the obtained -Fe 2 O 3 as an efficient adsorbent for heavy metal ion removal in water treatment.Figure 4(a) shows a fast uptake rate of As(V) and As(III).It can be seen that the first 30 min corresponds to a rapid adsorption stage for both arsenite and arsenate.The residual concentration of arsenite was less than 40 mg L −1 and that of arsenate was less than 20 mg L −1 after this fast stage.Thereafter, the adsorption rates decreased and adsorption where Q max is the saturation adsorption capacity of the adsorbent (-Fe 2 O 3 ).It is observed that the adsorption could be well described by the Langmuir model ( 2 > 0.98) for both As(V) and As(III).The maximal adsorption capacity of the obtained -Fe 2 O 3 nanoparticles is ca.62.9 mg g −1 for As(III) and 90.6 mg g −1 for As(V).Compared to other typical adsorbents reported in the literature, as listed in Table 1, the obtained -Fe 2 O 3 showed significantly higher adsorption capacities for both of As(V) and As(III).Only Fe 2 O 3 CAHNs exhibited a higher adsorption capacity for As(V) than the prepared -Fe 2 O 3 .It should be mentioned that, most results reported in the literature were obtained at the optimized pH value range, thus those high removal capacities may not be achieved at real pH values under the practical conditions.However, the removal capacity values of the obtained -Fe 2 O 3 were measured without any pH value adjustment in the current work.Thus, the removal capacities obtained here are more indicative of the potential of this material in practical application.Heavy metal ion adsorption by metal oxide is likely due to the combination of static electrical attraction between oxides and heavy metal ions and ion exchange in the aqueous solution [23].The large specific surface area and porous structure may contribute to the superior performance of the obtained -Fe 2 O 3 .
In summary, porous -Fe 2 O 3 with large specific surface area, mesoporous characteristic and high pore volume was fabricated via facile solid-state conversion of the as-made MIL-100(Fe) crystal in the current work.When applied as an adsorbent for heavy metal ions removal in wastewater, the obtained -Fe 2 O 3 nanomaterials show an excellent adsorption performance for As(V) and As(III) ions removal in water treatment.

Figure 2 :Figure 3 :
Figure 2: XPS spectra of the prepared porous -Fe 2 O 3 : (a) survey spectrum and (b) high-resolution image of Fe spectrum.

Figure 4 :
Figure 4: Time-dependent concentration of As(III) (initial concentration of 100 mg L −1 ) and As(V) (initial concentration of 100 mg L −1 ) over -Fe 2 O 3 (a) and adsorption isotherms of As(III) and As(V) on -Fe 2 O 3 .

Table 1 :
The adsorption capacities of some typical adsorbents.