Facile and tunable synthesis of carbon–γ-Fe₂O₃ submicron spheres through an aerosol-assisted technology and their application in oil spill recovery

Abstract

Magnetic carbon submicron spheres possessing good sorption abilities and easy collection are of interest in environmental engineering including oil spill cleanup. Here we describe a facile one-step process for the preparation of spherical submicron carbon–γ-Fe₂O₃ composites with controllable magnetic susceptibility and strong adsorption capability. Based on an aerosol-assisted technology, common precursors including sucrose, iron sulfate and a tiny amount of concentrated sulfuric acid were distributed and confined to numerous aerosol droplets, where the concurrence of sucrose carbonization and phase transition in iron species rapidly led to the formation of spherical carbon–γ-Fe₂O₃ composites. Meanwhile, magnetic saturation (M_s) values and the surface properties of the obtained composites could be controlled by simply adjusting the heating temperature, and the highest M_s at 44.83 emu g⁻¹ is much greater than those of samples obtained using other common methods. In the application of oil spill cleanup including diesel, gasoline and crude oil, these magnetic carbon submicron spheres exhibit high adsorption ability, easy separation and good recyclability. Such a simple technology may serve as a generalizable process to synthesize other types of magnetic composites such as titanium dioxide, silica and aluminum dioxide for broad applications.

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1. Introduction

Nowadays, crude oil and its petroleum products including diesel and gasoline are primary energy resources playing a vital role in the global economy.^1,2 However, the frequent occurrence of accidental oil spillages not only causes great economic losses, but also significant environmental issues. For example, it was reported that 4.9 million barrels of oil was spilled in the U.S. Deepwater Horizon oil platform explosion in 2010, damaging about 900 km of beach and 800 km of marsh along the Gulf of Mexico.³

All the time, sorption technology is regarded as a usual but competitive method in the field of environmental engineering, because this method holds a promising to provide the function of resources utilization in addition to environmental remediation. In view of this point, a variety of sorbents have been used and investigated in oil spill cleanup, such as clays,⁴ cellulosic fibers,⁵ polyurethane sponge,⁶ metal–organic frameworks (MOFs)⁷ and carbonaceous materials.⁸ Although all those materials exhibit great abilities in adsorbing spilled oil, the main difficulty was the separation and recycle of sorbents from their working systems. As is known to all that magnetically modified sorbents could offer an advantage of easy collection and recovery by simply applying an external magnetic field,^9,10 however, the challenge is how to efficiently embed a magnetic component into sorbents.

A number of studies have shown that carbon-based materials are good sorbents owing to their general applicability and high capacity in uptake of pollutants.^11,12 Accordingly, magnetic carbon composites are largely required in order to facilitate the separation step in the cleanup process. Hence, many efficient methods to synthesize magnetic carbon composites have been reported during the past years.^13,14 Basically, these methods could be divided into two categories: post-synthesis and in situ synthesis. The former includes incipient-wetness impregnation procedure,¹⁵ chemical vapor deposition,¹⁶ template method,¹⁷ etc., and the latter contains hydrothermal/solvothermal method,¹⁸ evaporation-induced self-assembly method,¹³ direct pyrolysis procedure,¹⁹ and so on. Generally, in situ synthesis is simpler and more preferable than post-synthesis, because complex and time-consuming steps such as repeated impregnation or template removal are not necessary any more. For example, He et al. have prepared magnetic carbon nanoparticles by hydrothermal dehydration of an aqueous glucose solution containing Fe–Au.¹⁸ Zhao et al. have reported that co-assembly of resols, nickel nitrate and the triblock copolymer F127 caused the formation of carbon with magnetic frameworks.²⁰ However, expensive precursors, strict experiment conditions, low magnetization and uncontrolled morphology still hamper the wide application of current in situ synthesis methods.

In the present work, we describe an in situ method to entrap γ-Fe₂O₃ nanoparticles in spherical submicron carbon spheres but based on a novel one-step aerosol-assisted process, aiming to easily obtain magnetic carbon composites with controllable magnetic susceptibility and strong adsorption capability for the oil spill recovery. In this method, only common precursors comprising sugar, iron sulfate and tiny amount of concentrated sulfuric acid are used, and then are confined to small spherical aerosols as a result of aerosolization. Under the condition of heating, chemical reactions including carbonization of sugar to carbon, transition of iron salt to maghemite occur in each aerosol droplets. Herein, each droplet is analogous to a micro-reactor so that all reactions could occur despite in a very short time (∼10 s). Therefore, submicron magnetic carbon composites with well-defined spherical structure could be generated in a continuous process without any post-treatments. In the work, the influence of the heating temperature to magnetization parameters and the surface properties are well studied. In the investigation of oil spill recovery, these magnetic carbon submicron spheres show high adsorption ability and good recyclability in treatments of gasoline, diesel and crude oil. The potential distinct advantages of this preparation method could be summarized as the following: (1) this method is a facile one-step process, where the formation of carbon matrix and the entrapment of the magnetic component took place concurrently; (2) it is available to control the magnetic and surface properties of the resulting composites simply through modulating the operating temperature; (3) in principle, the aerosol-assisted technology relies on evaporation-induced interfacial self-assembly confined to spherical aerosol droplets, so that composites prepared by the aerosol process show a well-defined spherical shape; (4) this simple, generalizable process has the potential to be extended to synthesize other magnetic composites such as TiO₂–γ-Fe₂O₃, SiO₂–γ-Fe₂O₃ and Al₂O₃–γ-Fe₂O₃ for broad applications.

2. Experimental

2.1 Materials

All chemicals were used as received without further treatment. Sucrose was purchased from Tianjin Kemiou Chemical Reagent Co., Ltd. Ferrous sulfate heptahydrate (FeSO₄·7H₂O, AR) was purchased from Tianjin Damao Chemical Reagent Factory. Sulfuric acid were purchased from Sinopharm Chemical Reagent Co. Ltd. Crude oil, diesel and gasoline were supplied by Liaoning Baolai Petrochemical Group Co., Ltd. Deionized water was used throughout the experiments except the real seawater obtained from Bohai Bay, Panjin Liaodongwan sea area for oil spill recovery experiments.

2.2 Sample preparation

Typically, a precursor solution was prepared by dissolving 6 g of sucrose, 5 g of FeSO₄·7H₂O and 400 μL of concentrated H₂SO₄ in 50 mL of water. In the aerosol-assisted technology, the drying and heating processes took place in a 60 cm × 4.3 cm (length × inside diameter) glass-tube inside a tube furnace with a length of 34 cm. The entering pressure of carrier gas (N₂) was modulated at 0.6 MPa to ensure the residence time of about 10 s in the drying and heating zone. In the experiments, various temperatures at 400, 500, 600, 700, 800, 900 and 1000 °C in the heating zone were investigated. Finally, a 0.22 μm × 144 mm membrane was used in a stainless steel filter for the collection of the produced black particles.

2.3 Characterization

X-ray diffraction (XRD) spectra were recorded on a Shimadzu-7000S XRD equipped with a Cu-Kα radioactive source (λ = 0.154 nm) at 40 kV/20 mA. Each spectrum was collected in the 2θ range from 10° to 90° at a scanning speed of 5° min⁻¹. Magnetic measurements were performed using a vibrating sample magnetometer (VSM, MPMS, SQUID) with a maximum magnetic field of ±20 kOe at room temperature. The nitrogen adsorption/desorption isotherms were measured on an Autosorb-1 nitrogen adsorption apparatus. The pore size distribution plots were obtained by the Barret–Joyner–Halenda (BJH) model. Water contact angle measurement was carried out by a JC2000D2A instrument (Powereach Instruments, Shanghai, China) equipped with a high resolution camera and an automated water injector. Based on the correlation of the shape of the drop and its surface tension, the contact angle between water droplet and the analyzed surface was obtained following the Young–Laplace equation. The general morphological structures of the materials were characterized using a Hitachi-4800 field emission scanning electron microscope (FE-SEM) and a Tecnai-20 transmission electron microscope (TEM). Thermogravimetric analysis (TGA) was performed on a Mettler Toledo DSC/TGA1 instrument by heating samples between 30 and 900 °C at a 10 °C min⁻¹ heating rate. Fourier transform infrared (FT-IR) spectra were collected on a Thermo Fisher Nicolet iS10 FTIR spectrometer by the standard KBr pellets method over a frequency range of 500–4000 cm⁻¹. X-ray photoelectron spectroscopy (XPS) was conducted with a Scienta ESCA-300 high-solution X-ray photoelectron spectrometer (HR-XPS). A Kα X-ray beam at 3.8 kW was generated from an Al rotating anode.

2.4 Oil sorption and reusability of materials

To assess the performance of aerosol-assisted magnetic carbon submicron spheres in oil spill recovery, three kinds of oil including gasoline, diesel and crude oil under two kinds of conditions including fresh water (DI water) and seawater (from Panjin Liaodongwan sea area, China) were investigated. Due to its high viscosity and low mobility, the method to study crude oil sorption is different from that in gasoline and diesel. In the experiments, 10 mL of gasoline or diesel was first added to a 100 mL beaker containing 50 mL of DI water or seawater, followed by the addition of 0.03 g of the materials and stirring. At several time intervals (4, 8, 12, 20, 40, 60 and 90 min), the materials together with adsorbed oil were simply collected by an external magnet bar and weighted. The oil sorption capacity of materials was calculated by the following formula (eqn (1)):where Q_t (g g⁻¹) is the oil sorption capacity of aerosol-assisted magnetic carbon submicron spheres at time t, and m_w (g) is the materials weight after the oil sorption. To recycle the materials, ethanol was used to wash out adsorbed oil thoroughly, and then the products dried at 60 °C for next cycle.

The method to measure the sorbent capacity of the materials for crude oil was close to that in previous reported with a gentle modification.²¹ In detail, 0.03 g of the prepared materials was added to a 100 mL beaker containing 50 mL of DI water or seawater firstly, followed by dropwise adding crude oil and stirring. Then, an external magnetic field with the strength at 1.5 T was applied to separate absorbed oil from water and weighted. Once the addition of crude oil surpassed the sorption ability, the extra crude oil will not being trapped and could not move to the side wall of the beaker by the magnet bar. On the basis of this observation, crude oil sorption capacity was determined.

3. Results and discussion

3.1 Synthesis and characterization

Magnetic carbon submicron spheres were prepared using an aerosol setup as depicted in Fig. 1a, where a commercial atomizer (HRH WAG-3, Beijing Huironghe Company) was employed to generate aerosol droplets. Fig. 1b represents the formation route of magnetic carbon composites based on the aerosol-assisted technology. In the first stage, the homogeneous starting solution stored in a beaker was broke into aerosol droplets by the commercial atomizer. Hence, all precursors including sucrose, iron sulfate and H₂SO₄ were distributed and dispersed in a small and limited space. In the subsequent drying and heating stage, each of droplets with a relatively confined dimension acted as a micro-reactor, where possible reactions including water evaporation, carbonization of sucrose (eqn (2)) and phase transition of iron species (eqn (3)) occur, leading to the formation of carbon–γ-Fe₂O₃ composites.

Fig. 1 (a) Schematic of the aerosol set-up and (b) schematic of synthesis route of the magnetic carbon submicron spheres.

Undoubtedly, the phase transition of iron salt in the heating zone determined the magnetic properties of the synthesized composites. Therefore, the XRD patterns of aerosol-assisted samples prepared at the various heating temperatures were analyzed. As shown in Fig. 2a, there is an obvious change in the structure of iron species when aerosol droplets were heated up to 600 °C. At this point, all the diffraction peaks of the resulting composites are well indexed to the lattice of γ-Fe₂O₃ with correspondence planes of (220), (311), (400), (422), (511) and (440) according to JCPDS no. 39-1346, which means that 600 °C is the threshold for the formation of γ-Fe₂O₃ in the aerosol-assisted process. In addition, when the temperature in the heating zone was set to progressively higher values at 700, 800, 900 and 1000 °C, no further phase transformation was observed and the structure of γ-Fe₂O₃ was retained in the composites. Based on Scherrer's equation, the sizes of γ-Fe₂O₃ could be calculated as 8.7, 9.0, 10.2, 10.7, and 11.5 nm for the samples under 600, 700, 800, 900 and 1000 °C, respectively. However, when droplets were only heated to 400 or 500 °C, it was found that the iron salt could not be transformed to maghemite due to insufficient energy, implying the resulting composites under these conditions are non-magnetic.

Fig. 2 (a) XRD patterns of aerosol-assisted samples prepared at the heating temperature varied from 400 to 1000 °C, (b) the XPS of the sample (900 °C) and (c) high resolution spectra of Fe 2p region.

It is well known that XRD alone could not effectively identify the exact species of the iron oxide phase because the characteristic peaks of γ-Fe₂O₃ and Fe₃O₄ phases are very similar. Hence, XPS characterization was employed to further demonstrate the iron phase in the prepared magnetic carbon submicron spheres. Fig. 2b shows a full survey of the obtained magnetic carbon composites under 900 °C, the peaks centered at 284.7 eV and 530.4 eV represent the binding energy of C (1s) and O (1s), respectively.²² Importantly, as shown in the Fig. 2c, the photoelectron peaks at 711.9 eV and 725.7 eV represent the 2p_3/2 and 2p_1/2 binding energies of Fe in maghemite,²³ indicating the presence of γ-Fe₂O₃.

To further investigate the magnetic properties of the as-prepared aerosol-assisted composites, quantitative magnetic measurements were performed at room temperature with an applied magnetic field of ±20 kOe. Fig. 3 shows the magnetic hysteresis loops of all samples, which reflect their ferromagnetic behaviors. Clearly, different heating temperatures during the aerosol-assisted process led to distinct disparities in magnetic properties of the obtained composites. The magnetic saturation (M_s) values of the as-prepared composites at 600, 700, 800, 900 and 1000 °C are 5.16, 13.48, 27.40, 44.83 and 37.91 emu g⁻¹, respectively. From the overall trend, M_s values increase with the elevation of the heating temperature, which may be ascribed to the complete decomposition of iron salt and the formation of larger γ-Fe₂O₃ nanoparticles in virtue of interparticle sintering in the heating zone.²⁴ However, the highest M_s was obtained at 900 rather than 1000 °C. The reason for this phenomenon may be owing to the slight consumption of the magnetic component, because the redox reaction between γ-Fe₂O₃ and carbon could occur under the excessively high temperature. In addition, not surprisingly, we did not observe magnetic characteristics for the samples obtained at 400 and 500 °C because of the absence of magnetic iron oxides, in accordance with the previous XRD results.

Fig. 3 Effects of the heating temperature on the hysteresis loop of the as-prepared samples.

FT-IR spectra of a series of aerosol-assisted composites are presented in Fig. 4. For comparison, the spectrum of sucrose is also included. It was found that sucrose could be carbonized more or less during the aerosol-assisted process at all temperatures investigated. The absorption bands at 3346 cm⁻¹, 1647 cm⁻¹, 1093 cm⁻¹, 625 cm⁻¹ are attributed to the O–H stretching vibration, C=C stretching vibration, C–O stretching vibration and C–H deformation vibration, respectively.²⁵ Obviously, all these peaks gradually disappear with increasing temperatures of the aerosol-assisted operation, which means the degree of carbonization was getting higher and higher from 400 to 1000 °C, and hence less residual functional groups were remained on the surface. Meanwhile, the peak around 540 cm⁻¹ assigning to the Fe–O bond²⁶ becomes more intense after aerosol-assisted process, further confirming the formation of iron oxide γ-Fe₂O₃.

Fig. 4 FTIR spectra of sucrose and a series of aerosol-assisted samples prepared at different temperatures from 400 to 1000 °C.

From the above analyses, the aerosol-assisted carbon composite obtained at 900 °C exhibit an enhanced magnetization compared to those of magnetic carbon materials reported previously.^27,28 In addition, this composite was the product of sucrose with almost full carbonization, making it an ideal candidate as an adsorbent for the application of oil spill recovery benefiting from its more hydrophobic property.^6,29 Hence, the aerosol-assisted magnetic carbon composite prepared at 900 °C was chosen for our following studies.

Fig. 5a and b show its representative SEM and TEM images, respectively. Clearly, the obtained magnetic composites are monodispersed submicron-scale spheres and vary in size. The incorporation of magnetic γ-Fe₂O₃ nanoparticles in carbon matrix is more evident by a strong contrast in the TEM image, where iron oxides and carbon appeared as the black dots and the grey parts respectively due to their different electron densities. Therefore, hereafter we could simply use the abbreviation of MCSS (magnetic carbon submicron spheres) to denote these specific materials. In fact, the aerosol-assisted process for the fabrication of spherical silica or metal oxides has been reported previously.^30,31 However, to the best of our knowledge, this work is the first report discussing the controllable preparation of magnetic materials including magnetic carbon materials through the aerosol-assisted technology. Fig. 5c display the TGA curve of the MCSS, where a slight weight loss (around 3%) at temperature below 100 °C may be attributed to the adsorbed impurities during the sample transportation. Furthermore, there is a weight loss (∼31%) between 300 and 700 °C due to the burn-off of the carbon, which means the content of the Fe₂O₃ in the MCSS is around 66%. The N₂ adsorption isotherm of MCSS is shown in Fig. 5d, where the type-IV isotherms with type-H3 hysteresis are observed, implying the presence of asymmetric slit-shape mesopores based on the IUPAC classification.³² The surface area was calculated to be 194.9 m² g⁻¹ based on the Brunauer–Emmet–Teller (BET) method, and the corresponding Barret–Joyner–Halenda (BJH) desorption pore volume was 0.421 cm³ g⁻¹. Meanwhile, a narrow pore size distribution at a mean diameter of 3.9 nm (1.95 nm for mean radius) is illustrated in Fig. 5e, further proving the existence of mesopores. In fact, the porosity of adsorbents is desired in the sorption technology, which may ensure the convenient entry of adsorbates to contact adsorbents. In addition, as shown in the Fig. 5f, the apparent contact angle of a water drop placed on a bed of the obtained MCSS was 152 ± 2°, implying its high hydrophobicity.

Fig. 5 (a) SEM, (b) TEM, (c) TGA, (d) nitrogen adsorption–desorption isotherm and (e) the BJH pore size distribution and (f) water contact angle measurement of the aerosol-assisted carbon–γ-Fe₂O₃ prepared at 900 °C.

3.2 Applications in oil spill recovery

The effectiveness of MCSS in the oil spill recovery was first evaluated by qualitatively experiments as exemplified in Fig. 6. Herein three kinds of oil including gasoline, diesel and crude oil were examined. Apparently, compared to diesel and gasoline, crude oil (viscosity η = 11 Pa s) remains an exceedingly viscous fluid which was only fragmented into patches floating on the water surface rather than being spread to a thin layer (panel a1–a3). With the addition of MCSS materials (panel b1–b3), all kinds of oil were quickly gathered to the surrounding of particles in a few seconds (panel c1–c3). Stirring was employed to simulate a dynamic environment such as waves, currents and turbulence, which could facilitate the contacts between materials and oil. Eventually, MCSS together with the absorbed oil were simply collected by an external magnet bar (panel d1–d3). It is worth noting that the MCSS were floating in the spilled oil–water systems, which might be a consequence of highly hydrophobic property¹¹ created by the almost full carbonization of sucrose. In the practical application, the unsinkable property will make magnetic recovery more feasible and thus is certainly desirable. Therefore, the presented materials are holding the potential in oil spill cleanup processes including highly viscous oil.

Fig. 6 Removal of gasoline, diesel and crude oil from fresh water surface by carbon–γ-Fe₂O₃ under external magnet bar.

Fig. 7 presents the removal capacity and reusability of MCSS. In freshwater, the maximum capacities for the adsorption of gasoline, diesel and crude oil are 1.91 ± 0.55, 2.79 ± 0.65 and 7.25 ± 0.33 g g⁻¹, respectively. Considering that the densities of the used gasoline, diesel and crude oil are 0.73, 0.85 and 0.83 g cm⁻³, the volume absorption capacities (cm³ oil per g sorbents) are 2.62 ± 0.75, 3.28 ± 0.76 and 8.73 ± 0.40 cm³ g⁻¹, respectively. In seawater, these capacities are 1.76 ± 0.46 g g⁻¹ (2.41 ± 0.63 cm³ g⁻¹), 2.64 ± 0.33 g g⁻¹ (3.11 ± 0.39 cm³ g⁻¹) and 6.43 ± 0.60 g g⁻¹ (7.75 ± 0.72 cm³ g⁻¹), showing no obvious difference under oceanic conditions. For comparison, we examined the capability of MCSS in adsorbing water (without the addition of any oils) following the similar procedure and found that only negligible amount of water (approximately 0.07 g g⁻¹) was adsorbed due to the high hydrophobicity of the material. Furthermore, it is observed that there is a significant increase in crude oil sorption capacity compared to diesel and gasoline. This increase may be attributed to different sorption mechanisms, where accumulation of high viscous oil onto sorbent's surfaces and penetration of low viscous oil into porous spaces of the material are responsible for crude oil and gasoline or diesel sorption,³³ respectively. Meanwhile, the materials exhibit excellent recyclability that the capacities for oil sorption still arrive 1.42 ± 0.11 (gasoline–freshwater), 2.07 ± 0.43 (diesel–freshwater), 6.17 ± 0.35 (crude oil–freshwater), 1.49 ± 0.25 (gasoline–seawater), 2.47 ± 0.36 (diesel–seawater) and 5.89 ± 0.40 g g⁻¹ (crude oil–seawater) in the 5^th cycle, benefiting spilled oil recovery and operation costs.

Fig. 7 The maximum capacity of the aerosol-assisted carbon–γ-Fe₂O₃ for the separation of gasoline, diesel and crude oil from (a) fresh water and (b) seawater for five cycles.

Fig. 8 shows the kinetic analysis on the oil–water separation by MCSS. As mentioned above, crude oil is preferentially attracted to the external surface and unable to be incorporated into the body of MCSS materials. Hence, herein only diesel and gasoline sorption were investigated. Clearly, both gasoline and diesel sorption onto MCSS were fast in the initial stages owing to abundant available sorption sites. Meanwhile, linear regression analyses indicate the oil spill recovery by MCSS follows the Langmuir model, similar to the previously reported carbon materials.³⁴ Furthermore, a pseudo second-order rate law can be derived with respect to the sorption capacity q_t defined by eqn (4):³⁵

where q_e (g_oil g_MCSS⁻¹) is the equilibrium capacity, k_w (g_oil⁻¹ min⁻¹) is the sorption rate constant and m (g) is the MCSS dose. The values of k_w for gasoline–freshwater, diesel–freshwater, gasoline–seawater and diesel–seawater separation systems are 16.8, 25.8, 15.9 and 22.8 g_oil⁻¹ min⁻¹, respectively. These high values indicate the rapid kinetics in the processes of oil spill cleanup, implying that immediate capture of the materials to spilled oil.

Fig. 8 Kinetics of (a) gasoline–fresh water, (b) gasoline–seawater, (c) diesel–fresh water and (d) diesel–seawater separation using the aerosol-assisted carbon–γ-Fe₂O₃. Insets are linear regression plots based on the Langmuir model.

4. Conclusions

In this work, magnetic carbon submicron spheres were successfully synthesized in a facile one-step process based on an aerosol-assisted technology. In this technology, chemical reactions including sucrose carbonization and phase transition in iron species simultaneously occurred in aerosol droplets, which acted like spatially confined micro-reactors. Therefore, submicron carbon spheres with embedded magnetic component γ-Fe₂O₃ could be produced in a continuous process without any post-treatments. The magnetic properties of the obtained composites were controlled by the heating temperature, and the highest M_s at 44.83 emu g⁻¹ is much greater than those of samples obtained using other common methods. Meanwhile, the heating temperature made a great impact on the degree of sucrose carbonization, and thus affecting the surface properties of these magnetic carbon submicron spheres. Hence, these materials exhibited effective adsorption in the cleanup of oil spill in addition to easy separation and good recyclability.

Acknowledgements

Funding from the Fundamental Research Funds for the Central Universities (DUT16ZD226) is gratefully acknowledged.

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