Magnetic adsorbents based on the composite of expanded graphite and NiFe2O4 for removal of heavy oils - Pham Van Thinh

Vietnam Journal of Science and Technology 56 (1A) (2018) 152-158 MAGNETIC ADSORBENTS BASED ON THE COMPOSITE OF EXPANDED GRAPHITE AND NiFe2O4 FOR REMOVAL OF HEAVY OILS Pham Van Thinh 1, 2, 3 , Hoang Ngoc Bich 1 , Bui Thi Phuong Quynh 4 , Nguyen Duy Trinh 1 , Long Giang Bach 1 , Nguyen Thi Thuong 1, * 1 NTT Institute of High Technology, Nguyen Tat Thanh University, No 300A Nguyen Tat Thanh, District 4, Ho Chi Minh City 2 Graduate University of Science and Technology, Vietnam Academy of Science and Technology, No 18 Hoang Quoc Viet, Cay Giay, Ha Noi 3 Faculty of Food – Environment – Nursing, Dong Nai Technology University, Bien Hoa City, Dong Nai Province 4 Faculty of Chemical Technology, Ho Chi Minh City University of Food Industry, 140 Le Trong Tan, Tay Thanh, Tan Phu, Ho Chi Minh City * Email: nthithuong@ntt.edu.vn Received: 15 August 2017; Accepted for publication: 25 February 2018 ABSTRACT The oil spill has severely caused environmental damage to ocean water resources resulting in significant negative influence on the ecosystem and putting the life of human at high risk. Hence, the development of efficient and low-cost adsorbents is an instrumental and essential task to purify oil-contaminated water. The corporation of magnetic particles into the adsorbents has recently considered as an innovative approach which facilitate phase separation under a magnetic field thus allowing easy and efficient recovery of the used adsorbents from polluted water in large scale. In this study, the expanded graphite (EG) and magnetic NiFe2O4 based composite prepared from available graphite source of Vietnam has been found as a potential adsorbent to remove crude oil (CO) and diesel oil (DO). The EG/NiFe2O4 was fabricated via a facile two-step process: (1) the expansion of graphite and (2) the decoration of magnetic NiFe2O4 on EG. The physical properties of as-obtained EG/NiFe2O4 were characterized by using Scanning Electron Microscope (SEM), Fourier-Transform Infrared Spectroscopy (FTIR), X-Ray Powder Diffraction (XRD), Vibrating-Sample Magnetometer (VSM) and nitrogen adsorption/desorption analysis. The effect of processing conditions including contact time, dosage of heavy oil and salinity of water on the adsorption capacity of the as-synthesized EG/NiFe2O4 composite for CO and DO was also investigated. Keywords: adsorbent, oil spills, magnetic expanded graphite (MEG), NiFe2O4. 1. INTRODUCTION Magnetic adsorbents based on the composite of expanded graphite and NiFe2O4 153 In the past few decades, petroleum products have gained currency in human life and industry. In details, the energy consumed from petroleum accounted about 65 to 70 percent, only about 20 to 22 percent from coal, 5 to 6 percent from water and 8 to 12 percent from nuclear energy. As oil consumption has increased in the parts of the world, a large amount of oil has been released into the environment during transportation and storage. The contamination of fuel oils in soil and water bodies has caused adverse impacts on marine ecosystem and living life. Therefore, the efficient treatment and recovery of leaked oils always draw attention of extensive global concern over the years, as a crucial task in environmental protection [1, 2]. Among many kinds of adsorbents available for oil sorption, expanded graphite (EG) has been considered as an instrumental material in removing efficiently contaminated oils from aqueous solution due to large specific surface area and low density [3-7]. Due to the fascinating porous, worm-like morphology, the adsorption of oil onto graphite layers could be facilitated via various kinds of pores formed during the exfoliation process, such as large spaces connecting the worm-like particles, wedge-shaped pores available on the external surface or ellipsoidal intra-particle pores [8, 9]. However, EG’s small particle size and high dispersion limit its recovery and recyclability in a large-scale water environment [7, 10, 11]. Recently, the combination of magnetic particles such as CoFe2O4 into graphite layers can yield new composites of high value in removing pollutants due to high adsorption capacity and easily handling to recovery adsorbed material [10- 12]. Herein, the aim of this study is to investigate the adsorption performance of the magnetic NiFe2O4 decorated expanded graphite (EG/NiFe2O4) for diesel oil (DO) and crude oil (CO) in aqueous solution. Nickel ferrite (NiFe2O4) with low magnetic coercivity, high electrical resistivity, low eddy current loss and excellent chemical stability is versatile magnetic material widely used in many fields of magnetic drug delivery, magnetic information storage device, ferrofluids, sensors, catalysis, microwave absorber [13, 14]. Thus, the nickel ferrite included composites provide an efficient platform for removal of heavy oils. The EG/NiFe2O4 was fabricated via facile two-step process including the exfoliation of graphite using chemical intercalation and the incorporation of NiFe2O4 on EG using the acid citric-based sol-gel route. The as-prepared composites were then applied to remove DO and CO with varying influential factors such as contact time, oil dosage and salt concentration. Simultaneously, the properties of as-obtained EG/NiFe2O4 were characterized by using Scanning Electron Microscope (SEM), Fourier-Transform Infrared Spectroscopy (FTIR), X-Ray Powder Diffraction (XRD), Vibrating- Sample Magnetometer (VSM) and nitrogen adsorption/desorption analysis. 2. MATERIALS AND METHODS 2.1. Materials Natural graphite powder (granule size > 1.25 mm and carbon content ≥ 85 %) was derived from Yen Bai province, Vietnam. H2SO4 (98 %) and H2O2 (30 %), FeCl3.9H2O, NiCl2.6H2O, C6H8O7.H2O, ammonia solution (25 %) (Xilong Chemical Co., Ltd) were used as received without further purification. 2.2. Preparation of samples EG was firstly synthesized by intercalation method using H2O2 (30 %) as an oxidizing agent and H2SO4 (96 %) as an intercalating agent. The natural graphite flake was added into the mixture of H2SO4 and H2O2. After intercalation time was 50 minutes, the mixture was Pham Van Thinh, et al. 154 continually washed with water on until pH value reached 5-6. The filtered solids were dried at 80 o C overnight and exfoliated in a microwave oven at 750 W for 30 s. In next step, EG/NiFe2O4 was prepared by adding magnetic NiFe2O4 to EG through the sol- gel process. In details, a mixture of Fe(NO3)3.9H2O, NiCl2.6H2O (with a molar ratio of Fe 3+ /Ni 2+ = 2:1) was homogeneously dissolved in citric acid solution. Afterwards, the EG was added into the mixture with a molar ratio of EG/NiFe2O4 = 4:1. In order to keep the sol stable, the pH of the solution was adjusted to about 9 using ammonia solution. Next, the mixture was heated at an 80°C under continuous stirring for 2 h or until it was concentrated followed by further drying at 80°C overnight in an oven. Finally, the obtained composites were calcined at 600 °C. 2.3. Characterization methods The morphology of EG/NiFe2O4 samples was studied by a S4800 SEM instrument (Japan). X-ray diffraction patterns were recorded in a Siemens D5000 Diffractometer with a CuK_radiation (1.5406 A) at a scan velocity of 2 o(2θ) min-1). The FT-IR spectra of sample mixed with KBr powder reference were obtained in the range of 4000−400 cm−1 using a Bruker Alpha FT-IR (Fourier transform infrared) spectrophotometer. Measurements of static magnetic moment were conducted on a GMW 3474-140 magnetometer equipped with a superconducting magnet to produce fields up to 16 kOe. To determine specific surface area and porous structure, the nitrogen adsorption/desorption measurement of adsorbent was performed on Nova Station B. 2.4. Evaluation of adsorption capacity EG/NiFe2O4 was added directly into the mixture of oil and water contained in petri dishes at room temperature. After a certain contact time, the oil loaded EG/NiFe2O4 was collected using a magnet placed under the dish. The obtained data were used to calculate the adsorption capacities (g/g) using the following equation: 2 1 1 m m Adsorption capacity m where, m1 is the weight of EG/NiFe2O4 before oil adsorption (g), m2 is the weight of EG/NiFe2O4 after oil adsorption (g). 3. RESULTS AND DISCUSSION 3.1. Characterization The FTIR spectrum of EG and EG/NiFe2O4 composites in the range 400–4000 cm −1 is revealed in Figure 1A. The IR spectrum exhibits intense band at 3441 cm -1 ascribed to O-H stretching vibration. The peak appearing around 2931 cm -1 is due to stretching vibration of C-H. The absorbance peak at 1631 cm -1 is attributed to vibration of adsorbed water C=O. The band at IR spectrum also shows an absorption band at 1049 cm −1 corresponding to the C-O stretching mode, which belongs to graphite intercalation compound [14]. Figure 1B shows XRD patterns for EG and EG/NiFe2O4 composite. The main peaks at 23.2 o , 44.3 o , 54.8 o and 78.3 o are corresponding to (002), (101), (004) and (110) planes of graphite [15] and the characteristic peaks of NiFe2O4 include (220), (311), (222), (400), (422), (511) and (440) planes at 30.3 o , 35.7 o , 38.3 o , 43.3 o , 54.8 o , 57.4 o , 63.0 o , respectively [14, 16]. The Magnetic adsorbents based on the composite of expanded graphite and NiFe2O4 155 results demonstrate the successful incorporation of NiFe2O4 to expanded graphite structure without changing the typical morphology of graphite layers. The presence of NiFe2O4 particles in composite yielded magnetism on EG. The plot of magnetization (M) as a function of applied field (H) for EG and EG/NiFe2O4 is shown in Figure 1C. It can be observed that magnetiation of EG-NiFe2O4 reach at 14 emu/g and this result further demonstrates the deposition of NiFe2O4 in EG as well-agreed with the SEM and XRD result. Figure 1. FT-IR (A), XRD (B) and VSM (C) of EG and EG/NiFe2O4. Figure 2 presents the SEM image of EG and EG/NiFe2O4 revealing the typical layered morphology of exfoliated graphite with interconnected space between the graphite layers. The SEM image of EG (Figure 2a) displays a wrinkled and scrolled morphology and worm-like structure of the ultrathin graphite sheets and stacking of sheets. It can be seen that the EG sheets are densely decorated with nanosized NiFe2O4 without large aggregations (Figure 2b). The light nanoparticles of NiFe2O4 were found to effectively distribute over the wrinkled surface of EG leading to increase its surface roughness. According to the above analysis results, the NiFe2O4 nanoparticles were successfully introduced to the EG surface and provided it with magnetic property sufficient for further application. Figure 2. SEM of EG (a) and EG/NiFe2O4 (b). The pore volume and surface area of the as- synthesized EG/NiFe2O4 are 0.0097 cm 3 /g and 32.9 m 2 /g, respectively, which are higher than those of the unmodified EG (0.0053 cm 3 /g and 28.9 m 2 /g). It can be explained that graphite layers were re-expanded under the second heating Pham Van Thinh, et al. 156 process to incorporate NiFe2O4 on EG resulting to increased specific surface area. Another reason is given that acid citric molecules were decomposed in the sample calcination stage thus producing defects and large holes in the EG/NiFe2O4. However, the external surface area of the prepared EG/NiFe2O4 was calculated to be 7.0 m 2 /g, which decrease by half compared with EG of 15.0 m 2 /g. It is given that the external surface area here refers to the sum of area of the pore walls and outer surface of the expanded graphite flakes and therefore, the presence of NiFe2O4 on outer convex surface of EG’s pore will reduce the external surface area of EG/NiFe2O4 [17]. 3.2. Adsorption of diesel oil (DO) and crude oil (CO) The adsorption capacity of as-prepared EG/NiFe2O4 for heavy oils was evaluated by using DO and CO. The adsorption rate is crucial factor to evaluate adsorption performance as well as feasibility for practical application of the adsorbents. The effect of contact time on DO and CO is presented in Figure 3A. It can be seen that the adsorbed DO and CO increased steadily and reached the peak of maximum capacity and then it decreased slightly. The results showed that the maximum adsorption capacity of EG/NiFe2O4 for DO of 30.2 g with only 5 min was higher than for CO of 28.9 g CO in 6 min. The main reason is that the glutinosity and density property of CO is larger than that of DO [10]. For EG, the adsorption for oils also showed in the same behavior with EG/NiFe2O4 that is the amount of sorbed DO higher than that of CO with faster adsorption rate due to higher glutinosity of CO; the sorption capacity were determined to be 47.7 g DO/gEG with 4 min and 39.8 gCO/gEG in 6 min. Figure 3. Influence of contact time (A), oil concentration (B), and NaCl concentration (C) on the adsorption capacity of EG/NiFe2O4 for DO and CO. Figure 3B reveals the influence of different oil dosage on the adsorption process of EG/NiFe2O4 for DO and CO. The adsorption capacity rose rapidly when increasing oil dosage and reached maximum capacity of 30.2 g at 37.5 g DO and 25.8 g for CO at 15 g CO using 0.2 g fixed EG/NiFe2O4. Then it dropped slightly given that EG/NiFe2O4 has adsorbed completely oil before reaching the maximum capacity, leading to low sorption capacity. Similarly, the maximum adsorption capacity of EG also gains 50.7 g/g at 37.5 g DO and 43.5 g/g at 15 g CO. Concerning with practical treatment of oil spills in marine environment, the interference of salt on adsorption needs to be tackled. Therefore, the effect of NaCl concentration was studied in the range of 0 to 3 % with oil dosage and EG/NiFe2O4 dosage fixed at 15 g and 0.2 g. A very slight increase in adsorption capacity occurred when rising salt concentration for DO whereas the adsorption capacity of EG/NiFe2O4 for CO decreased dramatically with increasing salinity to 3 % (Figure 3C). It is expected that the adsorption of DO and CO will raise in the increase of Magnetic adsorbents based on the composite of expanded graphite and NiFe2O4 157 salinity due to the “salting-out” effect which reduce aqueous solubility of oil in salt [18]. However, the adsorption of EG/NiFe2O4 for CO does act in opposite direction when increasing NaCl concentration on the basis of has the strongest influence of water cluster formation [19, 20]. Meanwhile, the interfering effect of NaCl concentration did not influence considerably the adsorption of DO onto the modified EG. This is important advantage of the as-synthesized EG/NiFe2O4 for application in treatment of real oil spills. Comparatively, salinity does slightly influence on the adsorption capacity of EG; for instance, the amount of oil sorbed by EG were 47.7, 47.8, 47.5, 49.2 and 52.7 g/g for DO and 39.8, 39.0, 37.9, 36.7 and 35.5 for CO obtained at 0 %, 0.5 %, 1 %, 2 %, 3 % NaCl, respectively. 4. CONCLUSIONS In brief, this study presents the fabrication, characterization of the magnetic NiFe2O4 decorated EG and its excellent performance in adsorptive sequestration of diesel oil (DO) and crude oil (CO) from aqueous solution. The maximum adsorption capacity for DO and CO was found to be about 30.2 g DO and 25.8 g CO per 1 g of adsorbent, occurring rapidly within 5-6 minutes. 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