MgAl-Layered Double

Article Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Fabrication of Magnetite-Graphene Oxide/MgAl-Layered Double Hydroxide Composites for Efficient Removal of Emulsified Oils from Various Oil-in-Water Emulsions Bo Zhang,† Runtao Hu,‡ Dejun Sun,‡ Tao Wu,*,‡ and Yujiang Li*,† †

J. Chem. Eng. Data Downloaded from pubs.acs.org by UNIV OF SUNDERLAND on 11/06/18. For personal use only.

Shandong Provincial Research Center for Water Pollution Control, School of Environmental Science and Engineering, Shandong University, Jinan, 250100, P. R. China ‡ Key Laboratory of Colloid and Interface Science of Education Ministry, Shandong University, Jinan, 250100, P. R. China S Supporting Information *

ABSTRACT: The oil contamination caused by surfactant-stabilized oil-in-water (O/W) emulsions are of increasing concern because of their persistent stability and high transportability. In this study, magnetic three-dimensional composites, which contain magnetite (Fe3O4) particles, grapheme oxide (GO), and MgAl-layered double hydroxides (MgAl-LDHs), were fabricated and applied as adsorbents to remove emulsified oils from various oil-in-water (O/W) emulsions. Transmission and electron scanning microscopies, X-ray diffraction, Fourier transform infrared, X-ray photoelectron spectroscopy, vibrating sample magnetometer, and ζ-potential analysis techniques were utilized to characterize MGO/MgAl-LDH composites. The particle−interface interaction was characterized by interfacial rheometer. Oil removal study results showed that the maximum removal efficiency for the crude oil was a mass ratio of LDHs/MGO = 1:1, while the maximum removal efficiencies for decane and white oil were a mass ratio of LDHs/MGO = 1:3. Under a suitable mass ratio of LDHs to MGO, 98−99% of emulsified oils could be removed by MGO/MgAl-LDH composites. Experimental data were best fitted to the Langmuir isotherm model. Thermodynamic analysis indicated that the adsorption process was both exothermic and spontaneous. The emulsified oil removal efficiencies were increased with increasing ionic strength. In addition, MGO/MgAl-LDH composites could maintain good oil removal efficiency after reusing over five cycles, indicating their great potential for use in an oily wastewater treatment. of surfactant molecules owing to their strong bioreactivity.5,6 In addition, emulsified oil droplets (EOs) could interact with other harmful chemicals, further threatening the natural environment. Therefore, before oily wastewaters can be discharged into water bodies, EOs or other petroleum hydrocarbons must be appropriately removed to meet environmental and health requirements. To date, various techniques, such as flotation technologies, oil skimmers, coalescers, settling links, and membranes have been used for removing free oil and dispersed oil.7−9 Recently, numerous investigations have demonstrated that the adsorption process could be considered as one of the most promising methods for emulsified oil removal due to its safe and easy operation, acceptable discharge quantity, and cost-effectiveness. Various adsorbent materials, including zeolites, clay minerals, metal-hydroxides, and carbon-based materials have been utilized for removing emulsified oil.10−13 Nevertheless,

1. INTRODUCTION As an important source of energy and raw materials, oils are being widely used in our daily lives and industrial processes. Oil contamination is one of the main sources of water pollution. With rapid industrial development and economic growth globally, large quantities of oily wastewater are produced by many industrial processes and human activities, including oil recovery, routine chemical activities, food processing, mining, petrochemical refineries, and steel and metal finishing industries.1−4 In modern society, the discharge amounts of oily wastewaters have increased rapidly due to the geometrical growth in population, rapid development of industry, expansion of oil production and utilization, as well as the frequent occurrence of oil spill accidents. To kinetically stabilize various emulsions, surfactants are often added in considerable amounts in emulsions during industrial processing. Such wastewaters that contain surfactant-stabilized oil-inwater (O/W) emulsions are of increasing concern and constitute a great potential hazard to the environment because of the persistent stability and high transportability of small-size emulsified oil droplets (EOs), as well as the serious eco-toxicity © XXXX American Chemical Society

Received: August 20, 2018 Accepted: October 22, 2018

A

DOI: 10.1021/acs.jced.8b00739 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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water treatment, hybrid materials, sensors, batteries, etc.26−28 However, when referring to oil removal, LDH itself shows poor abilities. As an amphiphilic particle, the GO has proven its ability in demulsification of oil-in-water emulsion.8 The LDH nanoplates attached on GO surfaces and/or intercalated into adjacent GO sheets are expected to create a 3D hierarchical network, which can reduce the charge barrier between GO and the oil interface; moreover, it also can improve the adsorbent surface’s roughness, dispersion, and interfacial activity at the oil−water interfaces, and enhances the adsorption capacity of EOs on the adsorbent. The objectives of this work were (1) to fabricate MGO/ MgAl-LDH composites with a fixed Fe3O4 concentration and to characterize the composites using scanning and transmission electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, ζpotential analyzer, and magnetometer; (2) to study the adsorption of EOs on the MGO/MgAl-LDH composites and elucidate the interactions between MGO/MgAl-LDH composites and EOs; (3) to investigate the effects of various operating factors, including the mass ratio of LDHs to MGO, adsorbent dosage, temperature, and ionic strength on the adsorption performance for various O/W emulsions; and (4) to explore the reusability of the MGO/MgAl-LDH composites.

these traditional adsorbent materials still possess some problems that limit the practical application in wastewater treatment, such as low selectivity, poor extraction capability, adsorbent materials regeneration difficulty, etc. As a result, the design and development of advanced adsorbent materials for cost-effective treatment of oily wastewaters are urgently needed. Owing to their attractive physical and chemical properties, nanomaterials such as boron nitride-based material,14 polymer-based nanocomposites,15 metal−organic framework-based materials,16 graphene oxide,17 and layered double hydroxide-based nanomaterials18 have been explored for many potential application, especially in environmental restoration. Moreover, the recovery of adsorbents is particularly critical for economical and effective treatment of oily wastewaters. It is still a vital issue that an adsorbent is difficult to separate from aqueous solution, and thus it is hard to be recycled. In recent years, magnetic adsorbent materials have provided an alternative approach to solve the challenge in separation and recovery of adsorbent materials from aqueous solution.12,17 Graphene is an excellent two-dimensional (2D) material that is atomically thin. It consists of sp2-hydridized carbons, and exhibits some unique physico-chemical properties, such as superior thermal conductivity, extremely large theoretical specific surface area, high flexibility, and mechanical strength.19 Consequently, graphene is applied in diverse fields, including nanoelectronics, field-effect transistors, optical devices, energy storage and conversion, and ultrasensitive sensors.20,21 Unfortunately, graphene is hydrophobic and tends to aggregate due to van der Waals forces existing between neighboring sheets, resulting in poor dispersibility in water. This significantly reduces the surface area and active sites, and thus is not favorable for the adsorption of pollutants. Graphene oxide (GO) is an oxidative derivative of graphene, containing oxygen-rich functional groups, such as epoxy (C−O−C), hydroxyl (−OH), carboxyl (−COOH), and carbonyl (−C O) groups on its basal planes and at the edges.22,23 Taking into account these oxygen-rich functional groups, GO can not only be well dispersed in water, but also be potentially used as nanoscale 2D building blocks for hybridization with other nanomaterials.17,24 Since magnetite (Fe3O4) nanoparticles possess good magnetic properties, Fe3O4 nanoparticles could constitute a viable choice to fabricate magnetic graphene oxide (MGO) hybrid materials. MGO can be easily achieved by chemical deposition of Fe3O4 nanoparticles onto GO sheets. The magnetic property, laminated structure, and existence of active sites make MGO a good substrate for the fabrication of magnetic graphene-based materials. Layered double hydroxides (LDHs), as hydrotalcite-like compounds or anionic clays, are a large class of layered inorganic materials that can be represented by the general 2+ X+ m− X− 2+ formula [M1−X M3+ X (OH)2] [(A )X/m] ·nH2O, where M 3+ and M are divalent and trivalent metal cations, respectively; Am− is the interlayer anion of charge m; X is the molar ratio of M3+/(M3+ + M2+) and generally has a value ranging from 0.17 to 0.33; and n is the molar amount of cointercalated water molecules.25 The characteristics of LDHs, including unique microstructure, tunable compositions, exchangeable interlayer anions, high structural positive charge density, and relatively high redox activity, lead to wide adaptability. In recent years, assembling low-dimensional nanostructures into three-dimensional (3D) architectures has been recognized as one of the most promising strategies for the fabrication of new multifunctional composites for potential applications in the fields of

2. EXPERIMENTAL SECTION 2.1. Materials. Graphite powder (average particle diameter < 45 μm, 99.99%) and decane (>99%) were purchased from Sigma-Aldrich Co., Ltd. (U.S.A.). MgCl2·6H2O (>98%), AlCl3· 6H2O (>97%), NH3·H2O (30%), FeCl3·6H2O (>98%), FeCl2· 4H2O (>99%), and sodium dodecyl benzenesulfonate (SDBS, > 98%) were purchased from the Sinopharm Chemical Reagent Co., Ltd. (China). White oil (Macol 52) was purchased from the Exxon Mobil Corporation (U.S.A.). In this study, a crude oil sample was provided by a local oilfield in Shengli, which has a water content of less than 0.5%, density of 854 kg/m3, and viscosity of 61 mPa·S at 45 °C. The main components of the crude oil, such as saturates, aromatics, resins, and asphaltenes were separated by a classical chromatography separation method based on ASTM D 2007.29,30 The results were listed in Table S1. All other chemicals were analytically pure, purchased from the Sinopharm Chemical Reagent Co., Ltd. (China), and used without further purification. Ultrapure water (Purkinje General, China) was used throughout the experimental process. 2.2. Preparation of GO. GO was synthesized from graphite powder by a modified Hummers and Offeman’s method.31 In the current synthesis, 98% H2SO4 (45 mL) and H3PO4 (5 mL) were added to a 250 mL three-neck flask containing graphite powder (2 g). Then, KMnO4 (6 g) was slowly added at 0 °C (using an ice bath) with continuous vigorous stirring for 30 min. The rate of adding KMnO4 was controlled to maintain the reaction temperature below 20 °C. Subsequently, the ice bath was removed and replaced by an oil bath, and the reaction mixture was heated to 35 °C and stirred for 1 h, at which time the mixture was diluted with ultrapure water (98 mL) and the temperature was rapidly increased up to 98 °C. The suspension was maintained at 98 °C for 30 min. Then, 280 mL of ultrapure water, 10 mL of H2O2 (30%) and 20 mL of HCl (5%) were added to the mixture and stirred at 60 °C for 30 min. After cooling at room temperature, the yellow product was separated by centrifugation at 8000 rpm, B



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various pH values were measured by a ZetaPALS (Brookhaven, U.S.A.) system. The magnetic properties of the samples were measured using a vibrating sample magnetometer (PPMS-9, Quantum Design, CA, USA) at room temperature under a maximum field of 30 kOe. The dynamic interfacial tension (IFT) was measured in 40 mg/L of SDBS aqueous solution using a Tracker surface rheometer (Tracker, France). 2.7. Preparation of Emulsified O/W Emulsions. Crude oil, white oil, and decane were used as model oil phases. Surfactant-stabilized O/W emulsions were prepared by mixing 2000 mg of oil, 1 L of ultrapure water, and 40 mg of SDBS. The oil/water mixtures were emulsified by a homogenizer (HG-15D, Dahan Scientific Co., Ltd., Korea) at a speed of 13000 rpm for 20 min. Each emulsion, with an oil initial concertation of 2000 mg/L, was stable and homogeneous within 48 h at room temperature. After 5 h, the oil content in the emulsions was still above 95% compared with the initial oil content. 2.8. Removal of Emulsified Oil and Recycling Test. Batch adsorption experiments were carried out at natural pH of the emulsions and performed on a TP-350s magnetic heating agitator (MIU, China) with a stirring speed of 400 rpm. The initial oil content of surfactant-stabilized O/W emulsions was 2000 mg/L. The effects of adsorbent dosage (0.1, 0.2, 0.3, and 0.4 g/L), mass ratio of LDHs to MGO (5:1, 3:1, 1:1, 1:3, and 1:5), temperature (283, 293, 303, and 313 K), and ionic strength on the removal efficiency were investigated. To study the influence of ionic strength, the adsorption experiments were performed by varying NaCl concentrations of 0.001, 0.01, 0.02, 0.05, and 0.1 M at 293 K. Moreover, to investigate the effect of pH on the charge density of MGO/MgAl-LDH composites, the desired initial pH (5, 6, 7, 8, 9, and 10) of the emulsions was adjusted by adding a negligible volume of 0.01 or 0.1 M HCl and NaOH solution using a PB-10 model pH meter (Sartorius, Germany). For EOs adsorption, the MGO/ MgAl-LDH composites, 150 mL of surfactant-stabilized O/W emulsions was added to 250 mL glass bottles to achieve the desired concentrations of the different components at a constant ionic strength, pH, and temperature. The glass bottles were stirred for at least 15 min at constant temperature to ensure equilibrium conditions. After adsorption equilibrium was achieved, the suspension was separated by a magnet. The content of the residual oil in the aqueous medium was determined by an infrared spectrometer oil content analyzer (Oil 460, China), and CCl4 was utilized as an extracting agent to extract oil from the aqueous medium. Desorption and regeneration of saturated MGO/MgAl-LDH composites were performed based on the recovery oil from the EOs-adsorbed MGO/MgAl-LDH composites with petroleum ether. The separation of the solid−liquid two phases was accomplished by an external magnet. The recovered adsorbent particles were again added to the O/W emulsion for subsequent adsorption experiments. Five cycles were performed to evaluate the reusability of the MGO/MgAl-LDH composites. The oil removal efficiency (%R) and adsorption capacity (q) were determined by the following equation:

washed several times with ultrapure water and alcohol, and freeze-dried for 24 h in order to obtain graphite oxide powder. 2.3. Preparation of MGO Hybrids. The MGO hybrids were prepared by coprecipitation of FeSO4·7H2O and FeCl3· 6H2O in the existence of GO. A 0.15 g sampling of GO was first ultrasonicated in 200 mL of ultrapure water to form a homogeneous dispersion. Then, the GO dispersion was transferred to a 250 mL three-neck flask under N2 atmosphere. A mixed aqueous solution of FeCl3·6H2O (5.2 g) and FeSO4· 7H2O (4.4 g) in 20 mL of ultrapure water was added to the GO suspension. The mixture was heated to 90 °C and stirred under N2 atmosphere. Then, NaOH solution (1.5 mol/L) was added in order to adjust the pH of the mixture to 11, and the reaction mixture was stirred for 45 min. The resulting product was separated by using a permanent magnet and washed five times with ultrapure water/ethanol. A portion of the MGO was dried in vacuum at 70 °C, the other part was mixed with ultrapure water to obtain a suspension of MGO. 2.4. Preparation of the MgAl-LDHs Sol. A chloride anion intercalated MgAl-LDHs precursor with an Mg/Al molar ratio of 3:1 was prepared by a coprecipitation method.32 Briefly, to 2000 mL of mixed metal salt solution containing 0.3 M MgCl2·6H2O and 0.1 M AlCl3·6H2O was added diluted NH3·H2O (ultrapure water/NH3·H2O = 5:1, v/v) to adjust the pH to 10.3 ± 0.2. The reaction process was carried out at room temperature (25 °C). The resulting slurry was aged at 25 °C for 1 h. The precipitate was filtered and washed well with ultrapure water. The MgAl-LDHs filter cake was collected and dried at 80 °C for 6 h in an oven to convert the filter cake into MgAl-LDHs sol. 2.5. Preparation of MGO/MgAl-LDH Composites. The synthesis of the MGO/MgAl-LDH composites was carried out in two sets. In the first set, the concentration of the MGO was fixed at 10 g/L, and the concentration of the LDHs was fixed at 20 g/L. In the second set, the MGO and LDHs were mixed in different proportions to achieve the desired adsorbent dosage. In addition, five samples with LDHs:MGO mass ratios of 5:1, 3:1, and 1:1, 1:3, and 1:5 were produced. After adding the LDHs to the as-prepared MGO suspension, the mixture was sonicated for 10 min to permit the LDHs as cross-linked agent molecules to join MGO sheets through electrostatic attraction and hydrogen bonding, and then the mixture was separated by a magnet. The obtained products were directly used to remove EOs from the O/W emulsions. The resulting MGO/MgAlLDH composites were defined as MGO/MgAl-LDH 1, MGO/ MgAl-LDH 2, MGO/MgAl-LDH 3, MGO/MgAl-LDH 4, and MGO/MgAl-LDH 5, corresponding to LDHs:MGO mass ratios of 5:1, 3:1, 1:1, 1:3, and 1:5, respectively. 2.6. Characterization. The structural features and surface morphologies of GO, Fe3O4, MGO, MgAl-LDHs, and MGO/ MgAl-LDHs were observed on a JEOL1200 transmission electron microscope (TEM, JEOL, Japan) and a Zeiss Supra 55 scanning electron microscope (SEM, Zeiss, Germany). Powder X-ray diffraction measurements were collected on a X’Pert Super X-ray diffractometer (XRD, Philips, Holland) with Cu Kα radiation (λ = 1.54178 Å) at a scan rate of 0.02 deg/s, 2θ varying between 8°−80°. Fourier transformed infrared spectra were collected using KBr discs on a Vector 22 infrared spectrometer (FT-IR, Bruker, Germany) in the spectral range of 400−4000 cm−1 with a resolution of 2 cm−1. X-ray photoelectron spectroscopy measurements were performed on a ESCALAB 250XI spectrometer (XPS, Thermo Fisher Scientific, U.S.A.). The ζ-potentials of the samples at

%R =

qe = C

C0 − Ce 100 C0

(C0 − Ce)V W

(1)

(2) DOI: 10.1021/acs.jced.8b00739 J. Chem. Eng. Data XXXX, XXX, XXX−XXX


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Figure 1. TEM images of (a) GO, (b) Fe3O4, (c) MGO, (d) MgAl-LDHs, (e) MGO/MgAl-LDH 3, and (f) MGO/MgAl-LDH 4.

where C0 (mg/L) and Ce (mg/L) are the concentration of EOs in solution at initial and equilibrium time; V (mL) and W (g) are the volume of the O/W emulsions and the mass of the adsorbent.

assembling LDHs and MGO, the LDHs nanoplatelets were randomly attached on the surface of MGO and intercalated into adjacent MGO sheets, which directly increased the roughness of the MGO sheets and reduced the aggregation of the MGO particles. The measured zeta potential of the MGO/ MgAl-LDH composites was positively charged in the pH range of 4.0−10.0, which indicated the successful assembly of LDHs and MGO (Figure S1). The XRD patterns of GO, Fe3O4, MGO, LDHs, and MGO/ MgAl-LDH composites were presented in Figure 3. GO exhibited the characteristic peaks at 11.58°, which corresponded to the (002) reflection with an interlayer spacing of 0.75 nm. Owing to the intercalation of oxygen-containing functional groups, the interlayer spacing of GO was larger than the d-spacing of graphite reported in the literature.33 The characteristic peaks of Fe3O4 appeared when 2θ was ≈30°(220), 36.1°(311), 43.7°(400), 57.4°(511), and 63.1°(440). The pattern of MGO showed weak and broad diffraction reflections, which were ascribed to the random arrangement of the GO nanosheets and the uneven interlayer spacing after deposition of magnetite particles. The characteristic peaks of (311) and (440) corresponded to the Fe3O4 nanoparticles, which suggested that the Fe3O4-phase had formed on the GO nanosheets. The MgAl-LDHs exhibited sharper and symmetric reflections. This behavior indicated a better crystallized LDHs structure. The XRD patterns of

3. RESULTS AND DISCUSSION 3.1. Characterization. The microstructures and morphology of the GO, Fe3O4, MGO, LDHs, and MGO/MgAl-LDH composites were characterized by TEM and SEM. As shown in Figure 1a and Figure 2a, some folding and wrinkles were presented in the GO sheet, which confirmed the corrugated structure of GO. Because of the large oxygen-rich functional groups, the zeta potential of GO was −34.96 mV. The Fe3O4 particles exhibited an average diameter of approximately 20 nm (Figure 1b and Figure 2b). From the TEM (Figure 1c) and SEM (Figure 2c) images of MGO, it can be seen that the Fe3O4 particles were anchored on the surface of graphene and tended to stack on the drapes and defects of the GO nanosheets. Moreover, the surfaces of MGO were rougher than those of the original GO. The measured zeta potential of MGO was −34.79 mV. The as-prepared LDHs show finely dispersed hexagonal platelets-like nanocrystals, and the average particle size was approximately 100 nm (Figure 1d and Figure 2d) with the zeta potential of +40.39 mV. Figure 1e,f and Figure 2e,f show the MGO/MgAl-LDH composites. After D



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Figure 2. SEM images of (a) GO, (b) Fe3O4, (c) MGO, (d) MgAl-LDHs, (e) MGO/MgAl-LDH 3, and (f) MGO/MgAl-LDH 4.

diffraction intensity of the characteristic peak was attributed to the disordered arrangement of LDHs nanoplatelets and MGO nanosheets. Moreover, the diffraction intensity of the characteristic peak decreased with the decreasing ratio of LDHs. FTIR was employed to characterize the functional groups and the bands formed by the groups. The FTIR spectra of GO, Fe3O4, MGO, LDHs, and MGO/MgAl-LDH composites were presented in Figure 4. The strong adsorption peaks located at 3431 cm−1 corresponded to the O−H stretching. The band at 1730 cm−1 was assigned to the CO stretching of carbonyl vibration, while another strong peak at 1622 cm−1 was attributed to CC vibration. In addition, the peak at 1056 cm−1 was due to the C−O vibration in the carboxyl group. The band at 834 cm−1 corresponded to the stretching vibrations of the epoxy group.34,35 In the FTIR spectra of Fe3O4 and MGO, the peaks at 580 cm−1 were attributed to Fe−O stretching. For MGO, the shifts of CC band (from 1622 to 1632 cm−1) and C−OH band (1056 to 1045 cm−1) indicated that the Fe3O4 nanoparticles were assembled with functional groups of GO. In the spectrum of the LDHs sample, the broad bands at 3461 and 1633 cm−1 represented the O−H stretching vibrations derived from the interlayer water molecules as well as the hydroxyl groups in the brucite-like layers.36 The band at 1373 cm−1 was due to contamination by CO32− during the LDHs

Figure 3. XRD patterns of (a) GO, (b) Fe3O4, (c) MGO, (d) MgAlLDHs, (e) MGO/MgAl-LDH 3, and (f) MGO/MgAl-LDH 4.

MGO/MgAl-LDH composites were shown in Figure 3 e,f, and the main characteristic peaks of LDHs and MGO could be found. These phenomena indicated that these two components existed in the MGO/MgAl-LDH composites. The weak E



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of Ms were 16.50, 20.05, and 23.34 emu/g, respectively. The Ms values of all magnetic composites were sufficiently strong to rapidly separate them in aqueous solution using a magnet and further used for recycling. XPS analysis was performed to elucidate the local chemical state of samples and binding interactions among GO, Fe3O4, and LDHs in the composites. As shown in Figure 6(A), C 1s (∼532.4 eV) and O 1s (∼284.6 eV) were found in the XPS spectra of GO (a) and MGO (c). Moreover, the peak at 711 and 725 eV presented in the spectra of Fe3O4 (b) and MGO (c) corresponded to Fe 2p1/2 and Fe 2p3/2, respectively, which indicated the presence of magnetite particles. In the spectra of MGO/MgAl-LDH composites (e,f), C 1s, O 1s, Fe 2p, Mg 1s, Al 2p, and Cl 2p could be observed. Moreover, the intensity of O 1s was stronger than the one in the spectrum of MGO. These phenomena verify the successful assembly of LDHs and MGO. In the high-resolution XPS spectrum of GO, the C 1s (Figure 6B) can be best fitted with four overlapped peaks at 284.8 (C−C/CC), 286.9 (C−O), 287.6 (CO), and 289.0 eV (O−CO).17,39 The spectrum of C 1s for MGO could be deconvoluted into three components of C−C/CC, C−O, and O−CO. Moreover, the peaks of the main oxygencontaining groups (C−O and O−CO) shifted to lower BE values, and the intensity of these peaks decreased after assembly with Fe3O4. The relative content of the carbon of MGO (28.30%) in C−O and O−CO was lower than GO (59.94%), and the relative content of the nonoxygenated carbon (C−C/CC) of MGO (71.69%) was higher than GO (40.05%). This phenomenon suggested the alteration of the local bonding environments after assembly with magnetite particles and that some interactions occurred between the Fe3O4 and GO. The high-resolution XPS spectrum of C 1s for MGO/MgAl-LDH composites could be deconvoluted into C− C/CC and O−CO bonds, and the peak of O−CO shifted to lower BE values, which suggested that some interaction, for example, ionic bond and hydrogen bonding, occurred between the LDHs and MGO nanosheets.40 The high-resolution spectra of O 1s was shown in Figure 6C. The O 1s spectrum of GO could be deconvoluted into two components: C−O (533.2 eV) and CO (532.4 eV). Compared with the spectrum of the as-prepared GO, the peak of lattice oxygen in Fe3O4 (Fe−O, 530.2 eV) could be observed in the O 1s spectrum of MGO. Additionally, the peak at 531.7 eV corresponded to the covalent Fe−O−C bond that was formed between Fe3O4 and the oxygen-containing groups of GO.41 In the Fe 2p spectrum (Figure 6D) of Fe3O4, the peaks at 711.3 and 724.8 eV were attributed to the Fe 2p1/2 and Fe 2p 3/2 spin−orbit peaks derived from Fe 3 O 4 , respectively.42 The same peaks were also observed in the spectra of MGO and MGO/MgAl-LDH composites, which suggested the formation of the Fe3O4 phase in the composites. The high-resolution XPS spectra of Mg 2p and Al 2p were shown in Figure 6E,F. For LDHs, the spectrum of Al 2p exhibited a peak located at 74.4 eV, indicating the presence of Al−OH. In addition, the Mg 2p peak was present at 49.9 eV, suggesting the existence of the Mg2+ species. Compared with that of the original LDHs, no BE shifts occurred in peaks of the Mg 2p and Al 2p, which indicated that the chemical valence and chemical microcircumstances of Mg and Al did not change after assembly with MGO. 3.2. Emulsified Oil Removal Studies. 3.2.1. Effect of Dosage and Mass Ratio of LDHs to MGO. The oil removal efficiency of crude oil, white oil, and decane O/W emulsions as

Figure 4. FT-IR spectra of (a) GO, (b) Fe3O4, (c) MGO, (d) MgAlLDHs, (e) MGO/MgAl-LDH 3, and (f) MGO/MgAl-LDH 4.

synthesis; the other peaks from 400 to 900 cm−1 were due to the M−O, M−OH, and O−M−O lattice vibrations of the LDHs cations.37 The spectra of MGO/MgAl-LDH composites mainly presented the same bands as LDHs. However, some characteristic peaks of GO and Fe3O4 were not clear, which was due to the overlaps of some peaks or the formation of chemical bonds between these components. Generally, the XRD and FTIR results indicated that the MGO/MgAl-LDH composites were successfully prepared. The magnetic hysteresis loops of the MGO and MGO/ MgAl-LDH composites that were obtained at 300 K and the magnetic field variation between −30 and 30 kOe were presented in Figure 5. The nature of the magnetic hysteresis

Figure 5. Magnetization curves of MGO, MGO/MgAl-LDH 3, and MGO/MgAl-LDH 4.

loops was S-like curves, indicative of the superparamagnetic characteristic of composites. Moreover, the hysteresis loops continued to rise without attaining saturation during the applied magnetic field. This behavior may due to the existence of α-Fe2O3 in magnetite particles.38 The Ms of the MGO/ MgAl-LDH 3, MGO/MgAl-LDH 4, and MGO were increased with the increasing loading amounts of MGO, and the values F



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Figure 6. (A) Wide scan XPS specra survey of (a) GO, (b) Fe3O4, (c) MGO, (d) MgAl-LDHs, (e) MGO/MgAl-LDH 3, and (f) MGO/MgAlLDH 4; narrow scan XPS spectra for (B) C 1s, (C) O 1s, (D) Fe 2p, (E) Mg 2p, and (F) Al 2p.

a function of MGO/MgAl-LDH composites dosage and mass ratio of LDHs to MGO was shown in Figure 7. It could be observed that the emulsified oil removal efficiencies of crude oil, white oil, and decane O/W emulsion by MGO/MgAl-LDH composites were all higher than 90% in the dosage range of 100 mg/L to 400 mg/L, which indicated the excellent oil/ water separation performance of the MGO/MgAl-LDH composites. For crude oil, the emulsified oil removal efficiencies increased with increasing dosage of MGO/MgAl-

LDH composites, while it no longer changed after reaching a plateau value (300 mg/L). In addition, at the same dosage, as the mass ratio of MGO increased, the crude oil removal efficiency increased. When the mass ratio of LDHs to MGO was 1:1 (MGO/MgAl-LDH 3), the crude oil removal efficiency reached the highest values (99.16%). The same trend could be observed at other dosages. For white oil and decane O/W emulsions, the oil removal efficiencies also increased with increasing dosage of MGO/MgAl-LDH G



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Figure 7. Effects of dosage and mass ratio of LDHs to MGO on the emulsified oil removal efficiencies of (a) crude oil, (b) white oil, and (c) decane (experimental conditions: initial oil concentration = 2000 mg/L, adsorbent dosage varied from 0.1 g/L to 0.4 g/L, mass ratio of LDHs to MGO varied from 5:1 to 1:5, pH = natural pH, and contact time = 15 min at 293 K).

Table 1. Application Cases of Different Adsorbents for the Removal of Emulsified Oils from Various O/W Emulsions adsorbents ferric oxidedoped carbon nanotubes magnetic carbon nanotubes

model oil

gasoline-in-water emulsion diesel-in-water emulsion powdered bentonite organoclay mineral oil-in-water emulsion OTAB modified sepiolite crude oil-in-water emulsion cork byproducts mineral oil-in-water emulsion zinc-based zeolitic imidazolate soybean oil-in-water framework emulsion MGO/MgAl-LDH composites crude oil-in-water emulsion white oil-in-water emulsion decane-in-water emulsion

initial oil concentration (mg/L)

adsorbent dosage (g/L)

adsorption equilibrium time (min)

oil removal efficiency (%)

841

1.0

15

98.5

45

500

1.0

90

99.0

12

236

5.0

30

94.1

46

1800

7.0

180

99.0

47

150

1.0

200

90.0

48

450

0.1

100

66.7

49

2000

0.3

15

99.2

2000

0.3

15

99.7

2000

0.3

15

99.8

this work this work this work

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due to its amphiphilicity,8 it should overcome electrostatic repulsion to contact the oil surface. As the ratio of LDHs was loaded on the surface of MGO, the surface negative charge of GO was screened, which favored the contact of MGO/MgAlLDH composites and the oil surface. Furthermore, the phenomenon that maximum oil removal efficiencies of different O/W emulsions occurred at different mass ratios of LDHs/MGO was mainly due to the different properties of oil. Except for the alkanes, cycloalkanes, aromatic hydrocarbons, and alkenes, the crude oil also consisted of various interfacial active substance such as asphaltenes, resins, and naphthenic acids. Compared with the white oil and decane emulsions, the crude oil emulsion contains more negatively charged interfacial active substances. During the adsorption process, the composites that contain more positive charge density could more easily contact the EOs and interact with the interfacial active substance. Therefore, the maximum emulsified oil removal efficiency of the crude oil emulsion is achieved at a higher mass ratio of LDHs/MGO = 1:1. So far various adsorbent materials have been utilized for removing emulsified oil. As shown in Table 1, the present system has a higher oil removal efficiency than other systems under the condition of higher initial oil concentration and lower adsorbent dosage. Moreover, the present system required less time to complete the oil−water separation

composites, and reached the highest emulsified oil removal efficiency at the dosage of 300 mg/L. However, the maximum oil removal efficiencies for white oil (99.69%) and decane (99.84%) were found to be the mass ratio of LDHs/MGO = 1:3 (MGO/MGO/MgAl-LDH 4). This phenomenon could demonstrate that the oil removal behavior may be the same for the same types of oil. In addition, the oil removal process could be regarded as an adsorption process, as the dosage of adsorbent increased, and the unadsorbed oil droplets could thereby be adsorbed, resulting in an increase in oil removal efficiency. From Figure S2, it can be seen that the zeta potential decreased as the ratio of MGO increased. The zeta potential of pure GO was −34.96 mV. After Fe3O4 was loaded on the GO, there was almost no change in zeta potential. When the amounts of LDHs in MGO/MgAl-LDH composites increased, the general zeta potential of MGO/MgAl-LDH composites increased, as the LDHs were positively charged (+40.39 mV), which may screen the negative charge of MGO. The SDBS-stabilized O/W emulsion usually is negatively charged on its surface.43,44 The positively charged MGO/ MgAl-LDH composites can reduce the charge barrier between the GO and oil interface. We can suppose that the oil removal mechanism constitutes a process in which oil is adsorbed on MGO/MgAl-LDH composites. For negatively charged GO, although some studies have proven its affinity to the oil surface H



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Figure 8. (A) Images for the removal of EOs by MGO/MgAl-LDH composites using an external magnet of (a) pure crude oil-in-water emulsion, mixed aqueous medium of emulsion with MGO/MgAl-LDH 3 composite, and after oil−water separation; (b) pure white oil-in-water emulsion, mixed aqueous medium of emulsion with MGO/MgAl-LDH 4 composite, and after oil−water separation; (c) pure decane-in-water emulsion, mixed aqueous medium of emulsion with MGO/MgAl-LDH 4 composite, and after oil−water separation. (B) Optical microscope images of (d) before and after separation of emulsified crude oil droplets from O/W emulsion, (e) before and after separation of emulsified white oil droplets from O/W emulsion, and (f) before and after separation of emulsified decane oil droplets from O/W emulsion (experimental condition: initial oil concentration = 2000 mg/L, adsorbent dosage = 0.3 g/L, pH = natural pH, and contact time = 15 min at room temperature.

O/W emulsions was less than 20 μm, which suggested strong emulsion stability. The MGO/MgAl-LDH composites could be readily dispersed in the O/W emulsion and covered the EOs due to their amphiphilicity and positive charged surface. Moreover, the oil droplets coalesced during this process. When applying a magnetic field, the MGO/MgAl-LDH composites wrapped with oil droplets were easily separated from solution, which indicated strong magnetic sensitivity. 3.2.2. Adsorption Isotherms. The adsorption isotherms serve to study the adsorption capacity and reveal the adsorption behavior of the adsorption systems. In this study, the experimental equilibrium adsorption data were analyzed by Langmuir and Freundlich models and the results were displayed in Figure 9. The Langmuir model (eq 1) was used to describe the adsorption process that occurred at a homogeneous surface with no interaction existing between adsorbates when the adsorption reached equilibrium. The Freundlich model (eq 2) assumed that adsorption occurred on heterogeneous surfaces, and with the increasing degree of adsorption site occupation, the interaction strength between adsorbent and adsorbate decreased.50,51

process. In addition, we compared the emulsified oil removal efficiencies of various O/W emulsions by Fe3O4, GO, MGO, LDHs, and MGO/MgAl-LDH composites, individually. As shown in Figure S3, the Fe3O4 particle without any surface modification exhibited poor oil/water separation performance. For GO and MGO, the emulsified oil removal efficiencies of various O/W emulsions had a similar trend. As an amphiphilic particle, the GO and MGO could disperse on the oil−water interface. Assisted by the centrifugal operation or external magnetic field, the oil droplets were separated from the water. However, there is strong electrostatic repulsion between the negatively charged emulsified oil droplets (EOs) and GO or MGO, which is unfavorable for the contact with each other. It could be observed that the emulsified oil removal efficiencies of LDHs were higher than those of Fe3O4, GO, MGO. This phenomenon was attributed to the strong electrostatic attraction between LDHs and EOs. Once the LDH nanoplates were added into emulsions, they would fast interact with the SDBS on the surface of EOs, thus resulting in the destruction of the protective interfacial film and then decrease the stability of emulsions. However, due to the highly hydrophilic, LDHs could not disperse well either in oil or on the oil/water interface. Therefore, it was difficult for LDHs to completely remove the oil from the emulsions. Moreover, the highly dispersion-stable LDH nanoplates were also difficult to separate from aqueous solution. Compared with other adsorbents, the MGO/MgAl-LDH composites had the highest oil removal efficiencies for various emulsified oils. The high oil removal performance of MGO/MgAl-LDH composites is likely due to its amphiphilicity. The introduction of LDHs into the MGO matrix may significantly enhance the interfacial activities of GO through initiating a range of surface reactions and cause a charge inversion from negative to positive. The interfacial activities and positively charged surface make it capable of adsorbing on the oil−water interface and interacting with SDBS. As a result, the rate of the de-emulsion process was accelerated, and the MGO/MgAl-LDH composites exhibited high oil removal performance. Figure 8 presents the macroperformance of MGO/MgAlLDH composites in oil−water separation and optical microscope images of EOs before and after adsorption. The mean size of EOs in the as-prepared crude oil, white oil, and decane

qe =

qmKLCe 1 + KLCe

qe = KFCe1/n

(3) (4)

where qe (g/g) is the amount of EOs adsorbed onto the MGO/MgAl-LDH composites; qm (g/g) is the maximum monolayer adsorption capacity of MGO/MgAl-LDH composites; Ce (mg/L) is the EOs concentration in solution when adsorption reach equilibrium; KL (L/mg) is the Langmuir isotherm constant; KF ((g/g)/(L/mg)1/n) and n (dimensionless) are the Freundlich isotherm constants. The calculated values of Langmuir and Freundlich isotherm parameters were shown in Table 2. The results suggested that the experimental data conformed well to the Langmuir isotherm model at the temperature range of T = (283 to 313) K in term of the higher correlation coefficients values. The maximum adsorption capacities of MGO/MgAl-LDH composites for various EOs were all decreased with increasing temperature, which indicated that low temperature conditions I



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ΔG° = −RT ln Kd ln Kd =

ΔS° ΔH ° − R RT

(5)

(6)

where Kd is the constants for EOs distribution between the solid and liquid at the equilibrium, which were determined by the intercept of the plot of ln(qe/Ce) versus qe;53 the qe (mg/g) and Ce (mg/L) are defined as the amount of EOs adsorbed onto adsorbate and EOs equilibrium concentration in solution; R is the universal gas constant (8.314 J mol−1 K−1); and T is the adsorption temperature (K). ΔH0 and ΔS0 were calculated from the van’t Hoff equation, an intercept of the plot of ln Kd versus 1/T. As shown in Table 3, the negative values of ΔG° for the adsorption of various EOs onto MGO/MgAl-LDH composites at temperature range of T = (283 to 313) K indicated the spontaneous nature of the adsorption process. The negative values of ΔH° suggested that the adsorption of EOs onto MGO/MgAl-LDH composites was exothermic. As is wellknown, sorption can be considered as a dynamic equilibrium process between sorption and desorption. The adsorbate molecules could be attached on, and interact with the surface of adsorbents when they were close to the adsorbents. As the temperature increased, the movement of molecules increased, and the interactions between adsorbate molecules and adsorbents were more intense.54 Hence, the solubility of EOs in the O/W emulsion will increase with increasing temperature, and the EOs tended to remain in a liquid phase. Moreover, the viscosity of the liquid phase decreased as the temperature increased. With higher temperature, the surfactant molecules (SDBS) had a high possibility to adsorb and accumulate on the surface of EOs. Moreover, the adsorbed surfactant molecules formed a viscoelastic film on the surface of EOs, which improved the stability of the O/W emulsion.13 In addition, during the removal of emulsified oil, the −OH groups of LDHs might interact with SDBS through hydrogen bonding. As the temperature rises, these weak interactions might weaken, and SDBS tended to adsorb onto the oil−water interface. Consequently, the adsorption capacities of emulsified oil were increased with the increasing temperature. The ΔS° were found to be positive, indicating the increase randomness at the solid/liquid interface when EOs were adsorbed on the surface of MGO/MgAl-LDH composites. 3.2.4. Effect of Ionic Strength. NaCl was selected as a dissolved salt to investigate the effect of ionic strength on the removal of EOs by using MGO/MgAl-LDH composites. The results were shown in Figure 10. It can be seen that at a lower NaCl concentration (0.001−0.01 M), for all O/W emulsions, the emulsified oil removal efficiencies experienced a noticeable decrease compared with the O/W emulsions without NaCl (Figure 7). This phenomenon could be ascribed to the crystallization of NaCl on the surface of composites, thus sheltering some of the adsorption sites and reducing the surface area. As a result, the physicochemical properties of MGO/MgAl-LDH composites and their interaction with EOs might change. At a high NaCl concentration (0.02−0.1 M), the emulsified oil removal efficiency was increased with increasing NaCl concentration. This phenomenon indicated that the addition of salts had the effect of decreasing the stability of the O/W emulsions through the “salt out” effect. According to the extant literature, the addition of salts in aqueous solution could decrease the aqueous solubilities of

Figure 9. Adsorption isotherms of (a) crude oil onto MGO/MgAlLDH 3, (b) white oil, and (c) decane onto MGO/MgAl-LDH 4 (experimental conditions: initial oil concentration = 500−5000 mg/L, adsorbent dosage = 0.3 g/L, pH = natural pH, contact time = 15 min).

were more favorable for adsorption. The values of Langmuir constants (KL) and Freundlich parameters (n) were all in the range of 0 to 1 and 2 to 10, respectively, indicating the favorability of adsorption.52 3.2.3. Thermodynamic Parameters. Thermodynamic parameters, including Gibbs free energy (ΔG0), enthalpy (ΔH0), and entropy (ΔS0) play an important role in understanding the nature of the adsorption process. In this study, the thermodynamic parameters derived from the adsorption of EOs onto MGO/MgAl-LDH composites were calculated from the following equations:53 J



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Table 2. Langmuir and Freundlich Isotherms Parameters for the Adsorption of Various Oil-in-Water Emulsions onto MGO/ MgAl-LDH Composites Langumir isotherm

Freundlich isotherm 2

model oil

T (K)

qm (g/g)

KL (L/mg)

R

crude oil

283 293 303 313 283 293 303 313 283 293 303 313

16.91 15.96 15.76 15.54 18.03 17.47 16.93 16.47 17.62 17.26 16.61 16.47

0.02460 0.02191 0.01937 0.01804 0.03172 0.02457 0.02387 0.01448 0.03337 0.02726 0.02678 0.02248

0.9760 0.9834 0.9832 0.9838 0.9476 0.9409 0.9287 0.9859 0.9517 0.9813 0.9774 0.9456

white oil

decane

T (K)

ΔG° (kJ/mol)

ΔH° (kJ/mol)

ΔS° (J/mol)

crude oil

283 293 303 313 283 293 303 313 283 293 303 313

−4.26 −4.38 −4.45 −4.60 −4.32 −4.35 −4.46 −4.47 −4.31 −4.45 −4.58 −4.57

−1.16

10.95

−2.69

5.72

−1.65

9.49

white oil

decane

n

R2

2.19 1.99 1.83 1.75 2.05 1.91 1.94 1.52 2.10 1.94 2.01 2.01

3.034 3.052 2.992 2.972 2.576 2.694 2.813 2.717 2.657 2.679 2.850 2.994

0.8158 0.8546 0.8619 0.8781 0.8604 0.8358 0.8174 0.8834 0.8462 0.8886 0.8703 0.7981

less favorable, and the negative charge of EOs was shielded. For all of the above reasons, the stability of the O/W emulsion decreased with increasing NaCl concentrations, and the oil removal efficiencies were increased with increasing NaCl concentrations. 3.2.5. Desorption and Regeneration Study. The potential recyclability of MGO/MgAl-LDH composites is critical for the economical and effective treatment of oily wastewaters. The removal efficiency of emulsion oil by using the recycled composites was shown in Figure 11. The oil removal

Table 3. Thermodynamic Parameters Calculated for the Adsorption of Various Oil-in-Water Emulsions onto MGO/ MgAl-LDH Composites model oil

KF

Figure 11. Recyclability of MGO/MgAl-LDH 3 and MGO/MgAlLDH 4 for five cycles.

efficiencies of MGO/MgAl-LDH composites after the first cycle for crude oil, white oil, and decane were 99.11%, 99.69%, and 99.84%, respectively. Then, the oil removal efficiencies decreased with each cycle of reutilization. This suggested that the adsorption sites became heterogeneous after each sorption, and it was difficult for the composites to be used for another adsorption.42 However, the oil removal efficiencies still maintained 95.14%, 96.26%, and 97.88% after five cycles, which demonstrated that the MGO/MgAl-LDH composites possess good and stable adsorption−desorption efficiency for emulsion oil and good application potential. 3.2.6. The Possible Oil/Water Separation Mechanism. The surfactant-stabilized oil-in-water (O/W) emulsions were of

Figure 10. Effect of ionic strength on the emulsified oil removal efficiencies of crude oil by MGO/MgAl-LDH 3, white oil by MGO/ MgAl-LDH 4, and decane by MGO/MgAl-LDH 4.

various kinds of compounds, such as detergents, polymers, gases, etc., and influence intermolecular or intermicellar interactions.55,56 More importantly, in the presence of NaCl, the SDBS molecules could interact with NaCl through electrostatic attraction, which shielded the ionic headgroups. Hence, the SDBS molecules at the oil−water interface became K



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high stability due to the protective interfacial film between the emulsified oil droplets (EOs). In the emulsifying process, the surfactant molecules, which is SDBS, could adsorb on the oil/ water interface and form a negatively charged protective interfacial film, which acts as a physical barrier for preventing the coalescence of the EOs. As previous studied, the negatively charged MGO could disperse well on the surface of EOs due to its amphiphilicity. When the LDHs were loaded on the surface of MGO, a charge inversion from negative to positive occurred, which was favorable for the contact of MGO/MgAlLDH composites with EOs. As shown in Figure 12, once the

oil removal efficiencies were increased with increasing ionic strength. On the basis of these results, we found that the real oil removal process comprised two processes: (1) the oil droplets were adsorbed by MGO/MgAl-LDH composites and then the oil droplets coalesced in this process; and (2) the oil droplets adsorbed by MGO/MgAl-LDH composites were removed under a magnetic field, and thus the real oil removal mechanism could be interfacial covering combined adsorbate interfacial adsorption. Moreover, the MGO/MgAl-LDH composites had good cyclic oil removal performance, which indicates strong application potential for real oily wastewater treatment.

■

ASSOCIATED CONTENT

* Supporting Information S

. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.8b00739.

■

Figure 12. Schematic illustration of the adsorption of EOs onto MGO/MgAl-LDH composites.

Zeta potential values of MGO/MgAl-LDH composites materials as a function of pH; zeta potential of GO, LDHs, Fe3O4 and MGO/MgAl-LDH composites with different mixing ratios; comparison of the emulsified oil removal efficiencies of various O/W emulsions by Fe3O4, GO, MGO, LDHs, and MGO/MgAl-LDH composites; dynamic interfacial tensions of MGO/ MgAl-LDH composites plotted against time at different oil−water interfaces; and composition of the Shengli crude oil (PDF)

AUTHOR INFORMATION

Corresponding Authors

*Tel./Fax: +86-531-88365437. E-mail: [email protected]. *Tel./Fax: +86-531-88363358. E-mail: [email protected].

MGO/MgAl-LDH composites were added into the O/W emulsions and followed with vigorous stirring, the MGO/ MgAl-LDH composites would rapidly adsorb on the oil/water interface and interact with the protective interfacial film. Because of the strong electrostatic attraction, the protective interfacial film was broken and the finely dispersed EOs coalesced into bigger ones, which led to the increase in interfacial tension (Figure S4a−c) and decrease in stability of the emulsion. Then the MGO/MgAl-LDH composites wrapped with oil droplets were attracted and collected by an external magnetic field. As a result, the oils were successful separated from the solution.

ORCID

Dejun Sun: 0000-0003-0841-1501 Yujiang Li: 0000-0002-4970-9964 Funding

This work was supported by the National Natural Science Foundation of China (Grant No. 21677087) and the National Science and Technology Major Project of China (Grant No. 2016ZX05040−005). Notes

The authors declare no competing financial interest.

■ ■

4. CONCLUSION MGO/MgAl-LDH composites with different mass ratios of LDHs to MGO were fabricated by a facile three-pot reaction. GO was used as a matrix, the magnetite (Fe3O4) particles loaded on the surface of GO, and then MgAl-LDHs were assembled with MGO through electrostatic and hydrogenbond interactions. TEM, SEM, XRD, FT-IR, and XPS confirmed that the MGO/MgAl-LDH composites were successfully fabricated. The VSM analyzer proved that MGO/MgAl-LDH composites possessed good magnetic properties. The oil removal experiments demonstrated that when LDHs/MGO = 1:1, MGO/MgAl-LDH composites had the highest oil removal efficiency for crude oil. In addition, the maximum removal efficiencies for white oil and decane were achieved at the mass ratio of LDHs/MGO = 1:3. The adsorption data were well described by the Langmuir isotherm model. Adsorption thermodynamic results indicated that the adsorption process was both exothermic and spontaneous. The

ACKNOWLEDGMENTS The author expresses gratitude to Jake Carpenter from UCLA for linguistic assistance. REFERENCES

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