Synthesis of Three-Dimensional Fe 3 O 4 / Graphene Aerogels for the Removal of Arsenic Ions from Water

We report the synthesis of three-dimensional Fe 3 O 4 /graphene aerogels (GAs) and their application for the removal of arsenic (As) ions from water. The morphology and properties of Fe 3 O 4 /GAs have been characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and superconducting quantum inference device. The 3D nanostructure shows that iron oxide nanoparticles are decorated on graphene with an interconnected network structure. It is found that Fe 3 O 4 /GAs own a capacity of As(V) ions adsorption up to 40.048mg/g due to their remarkable 3D structure and existence of magnetic Fe 3 O 4 nanoparticles for separation. The adsorption isotherm matches well with the Langmuir model and kinetic analysis suggests that the adsorption process is pseudo-second-ordered. In addition to the excellent adsorption capability, Fe 3 O 4 /GAs can be easily and effectively separated from water, indicating potential applications in water treatment.


Introduction
Arsenic's history in science, medicine, and technology has been overshadowed by its notoriety as a poison in homicides.Arsenate and arsenite contaminants in groundwater threaten ecological balance and human health and result in several diseases such as skin or lung cancer [1].What is worse, with the intensification of human activities, especially mining activities, combustion of fossil fuels has led to more arsenic species pollute groundwater [2].Thus the remediation of arsenic pollution by way of adsorption has attracted worldwide attention [3][4][5].Compared with other adsorbents, magnetic adsorbents such as Fe 3 O 4 exhibit unique advantages due to their quick and effective magnetic separation [6,7].However, the absorption capacity of most synthesized adsorbents and its efficient magnetic separation are difficult to balance as decreasing magnetic particles size enhances adsorption capacity which would undesirably decrease response to an external magnetic field [8,9].
Graphene, a two-dimensional atomically thick carbon atom arranged in a honeycomb lattice, has attracted much attention for its potential applications in sensors, catalysis, energy-storage devices, and environmental fields because of its excellent electronic, mechanical, and other properties [10][11][12][13][14][15].Generally, graphite can be oxidized by strong oxidants and easily exfoliated to the formation of graphene oxide (GO) and reduced graphene oxide (RGO) by reductants.The chemical oxidation modification methods generate plenty of oxygen-containing functional groups in GO and RGO, which offer a potential way to produce large scale of graphenebased materials with a low cost [16].The unique surface property enables GO as an ideal substrate to anchor inorganic nanoparticles (NPs) for many applications, such as lithium ion battery [17] and water purification [11].GO is employed as a scaffold for modifying metal oxide nanoparticles to improve their adsorption performance for its lateral dimension up to micrometers and thickness under several nanometers [18][19][20].Although GAs supported Fe 3 O 4 NPs (Fe 3 O 4 /GAs) have been employed for some applications, such as oxygen reduction reaction [21], there are limited reports on the fabrication of Fe 3 O 4 /GAs as absorbent for removing As ions from water up to now.
In this study, 3D Fe 3 O 4 /GAs have been fabricated through hydrothermal method, which have macroporous framework of graphene sheets with uniform deposition of Fe 3 O 4 NPs.Three-dimensional (3D) graphene aerogels (GAs) with interconnected mesoporous network, allowing access and diffusion of ions and molecules, seem to be a good candidate as support for iron oxide Fe 3 O 4 NPs.It was found that Fe 3 O 4 /GAs as self-supported structured adsorbent show excellent capability of removal of As(V) ions in water treatment.Additionally, our results have confirmed that Fe 3 O 4 /GAs can be easily removed from water by magnetic separation.

Experimental
GO was synthesized using the modified Hummers method [22].Briefly, 0.5 g of graphite powder and 3 g of potassium permanganate were placed in a 250 mL flask; 60 mL of concentrated sulfuric acid and 6.6 mL of concentrated nitric acid were slowly dropped into the flask under stirring for 12 h at 60 ∘ C. Then the mixture was diluted with 0.5 L of deions (DI) water, and excessive 15 mL of hydrogen peroxide (30 wt%) was dumped into the mixture to make bright yellow solution in an ice bath.After repeated centrifugation until neutral (pH = 7) with excessive deionized water (DI), graphite oxide was obtained.Exfoliation of graphite oxide to graphene oxide was achieved by ultrasonication.Then 150 mg of FeC 2 O 4 ⋅2H 2 O was added to 35 mL of 2.0 mg⋅mL −1 GO suspension under magnetic stirring for 0.5 h.The stable suspension was sealed in a 50 mL telfon-lined autoclave and hydrothermally treated at 180 ∘ C for 12 h and subsequently freeze-dried for 12 h.Finally, after thermal treatment at 600 ∘ C for 5 h in Ar gas with 400 sccm, 3D Fe 3 O 4 /GAs were obtained.The structure and surface morphology of Fe 3 O 4 /GAs were investigated by X-ray diffraction (XRD), FEI Quanta 200F scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS), and Tecnai G2 F20 transmission electron microscopy (TEM) equipped with selected area electron diffraction (SAED) patterns and scanning TEM (STEM).
The adsorption capability of Fe 3 O 4 /GAs for As(V) ions from water was performed at room temperature.Firstly, individual stock solutions of 1000 mg⋅L −1 and 100 mg⋅L −1   As(V) ions were prepared by dissolving Na 2 HAsO 4 ⋅7H 2 O in deionized water, respectively.Fe 3 O 4 /GAs equivalent to 2 mg of aerogels were added into 10 mL of As(V) containing solution with different concentrations (0.50, 1.0, 2.5, 5.0, 10.0, 15.0, 20.0, and 30.0 mg⋅L −1 ) in the presence of 0.01 M NaCl at a neutral pH.The temperature was maintained at 25 ∘ C and the adsorption equilibrium was obtained after 12 h.The remaining concentrations of As(V) ions were magnetically separated by a small magnet and measured by inductively coupled plasma-atomic emission spectroscopy (ICP-AES).

Results and Discussion
The XRD patterns of Fe 3 O 4 /GAs and GAs are shown in Figure 1(a).The peaks at 2 values of 18.9 ∘ (111), 31.3 ∘ (220), 36.8 ∘ (311), 38.5 ∘ (222), 44.8 ∘ (400), 55.6 ∘ (422), 59.3 ∘ (511), 65.2 ∘ (440), and 77.3 ∘ (533) have confirmed the formation of Fe 3 O 4 (PDF No. 26-1136) in the hybrid [22].It is worth to note that no apparent diffraction peaks were found between 20 ∘ and 30 ∘ , indicating that Fe 3 O 4 NPs have been efficiently deposited on the graphene surface.The magnetic properties of Fe 3 O 4 /GAs were measured using a superconducting quantum inference device with an applied magnetic field of −20000 to 20000 Oe at room temperature.Figure 1(b) shows the curves of magnetization versus applied magnetic field, indicating ferromagnetic behavior of the Fe 3 O 4 /GAs.The saturation magnetization is 68.4 emu/g, which is lower than that of bulk Fe 3 O 4 (92 emu/g) due to the existence of reduced GO and the small size of Fe Figure 3 shows the XPS pattern of the Fe 3 O 4 /GAs, in which the peaks at 711 eV, 530 eV, and 284 eV are attributed to Fe 2p, O 1s, and C 1s, respectively [22].In the highresolution XPS spectrum of Fe 2p (Figure 3(b)), two broad peaks at 725 and 711 eV corresponding to Fe 2p 1/2 and Fe 2p 3/2 were found which are due to the spin orbit splitting [22].A comparison between the C 1s XPS spectrum of the hybrid (Figure 3(c)) and the C 1s XPS spectrum of GO (top inset in Figure 3(c)) indicates that the GO has been reduced to graphene by ferrous ions during the formation process, due to the significant intensity increase of sp 2 C=C bonds of graphene at 284.6 eV and intensity decrease of the oxygencontaining carbon bonds (epoxy C-O at 285.8 eV, carbonyl C=O at 287.3 eV, and carboxyl O=C-O at 289.8 eV [23]).Figure 3(d) shows the O 1s XPS spectrum of the hybrid, which is split into four peaks, and the max peak at 532.2 eV demonstrates the existence of Fe-OH that is favorable to removal of metal irons [14].
The adsorption isotherm of As ions onto Fe 3 O 4 /GAs (Figure 4(a)) shows the relationship between adsorbate and adsorbent.Description on the interaction between adsorption capacity and bond energy, adsorbents, and adsorbates,  0.982 can be determined from isotherm equilibrium models.The experimental data for As ions adsorption have been analyzed by the nonlinear forms of the Langmuir [24] (Figure 4(b)) and Freundlich [25] (Figure 4(c)) isotherm models.The Langmuir model can be expressed as The Freundlich model is given as follows: where   (mg⋅g −1 ) is the amount adsorbed at equilibrium and   (mg⋅L −1 ) is the equilibrium concentration of adsorbate.The Langmuir constant   (mg⋅g −1 ) represents the maximum adsorption capacity and  (L⋅mg −1 ) is related to the energy of adsorption.The Freundlich constant (  ) is the parameter and (1/) is the adsorption intensity (1 <  < 10).The values of parameters from these isotherm models are listed in Table 1.
The values of correlation coefficient  2 for the Langmuir (0.997) and Freundlich (0.982) models suggest that the adsorption data fit well with the Langmuir model.The maximum adsorption capacity (  ) and  are 40.048mg⋅g −1 and 0.124 L⋅mg −1 , respectively.The adsorption capacity of Fe 3 O 4 /GAs (  ) is larger than that of porous Fe 3 O 4 particles [26], magnetite-reduced GO composites [11], and other irongraphene hybrids [27].The  value is 1.359 in Freundlich mode, which indicates a favorable condition for adsorption.Considering that the adsorbent is environmental friendly and easily to be separated, Fe 3 O 4 /GAs can serve as a more efficient adsorbent in drinking water treatment in the future.

Conclusion
In summary, 3D Fe 3 O 4 /GAs have been fabricated by a combined method including hydrothermal reaction, freezedrying, and thermal treatment process.Owing to their 3D macroporous structure, the as-prepared Fe 3 O 4 /GAs have demonstrated excellent adsorptive property of 40.048 mg/g for removing As(V) ions from water and can be easily removed from water.It is believed that the present magnetic Fe 3 O 4 /GAs can be used for more applications, such as catalysis, sensors, and batteries.

3 O 4 Figure 2 :
Figure 2: (a and b) Typical SEM images of Fe 3 O 4 /GAs, (c and d) TEM images of Fe 3 O 4 /GAs, (e and f) HRTEM and SAED image of Fe 3 O 4 /GAs, and (g-j) STEM elemental mapping of Fe 3 O 4 /GAs.

Figure 3 :
Figure 3: (a) XPS spectrum of Fe 3 O 4 /GAs, (b) a detailed Fe 2p spectrum, (c) a C 1s spectrum of Fe 3 O 4 /GAs and the inset shows a C 1s spectrum of GO, and (d) a O 1s spectrum of Fe 3 O 4 /GAs.

Figure 4 :
Figure 4: (a) Adsorption isotherm of As on Fe 3 O 4 /GAs at room temperature, (b) Langmuir model, and (c) Freundlich model.

Table 1 :
Isotherm parameters for removal of As (V).