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Treatment of emulsified oil wastewaters by using chitosan grafted magnetic nanoparticles
Journal of Alloys and Compounds 696 (2017) 1205e1212
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Treatment of emulsified oil wastewaters by using chitosan grafted magnetic nanoparticles Ting Lü a, Yi Chen a, Dongming Qi b, Zhihai Cao b, Dong Zhang a, Hongting Zhao a, * a
Institute of Environmental Materials and Applications, College of Materials and Environmental Engineering, Hangzhou Dianzi University, 310018 Hangzhou, China b Engineering Research Center of Eco-Dyeing and Finishing of Textiles of Ministry of Education, Zhejiang Sci-Tech University, Hangzhou 310018, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 August 2016 Received in revised form 30 November 2016 Accepted 9 December 2016 Available online 10 December 2016
In this study, a class of chitosan (CS)-grafted magnetic nanoparticles (MNPs) was synthesized for treating emulsified oil wastewaters. Fe3O4 MNPs were synthesized by using a solvothermal method, followed by coating with aminopropyl-functionalized silica (APFS) to form a surface for further grafting of CS molecular chains. The synthesized MNPs were characterized by various technologies and their demulsification performances were evaluated under various conditions. Results showed that synthetic Fe3O4 MNPs showed negligible demulsification effect, however, under both acidic and neutral conditions, APFScoated MNPs (Fe3O4@APFS MNPs) exhibited good demulsification effect via electrostatic attraction. It was also found that demulsification performance could be further enhanced upon CS grafting, especially under alkaline condition. Results showed that, under both acidic and neutral conditions, CS-grafted MNPs (Fe3O4@APFS-G-CS MNPs) could efficiently flocculate oil droplets mainly via electrostatic attraction, thereby facilitating magnetic separation; while under alkaline condition, MNPs could overcome electrostatic repulsion and be absorbed onto oil droplet surface via hydrophobic interaction, thereby exhibiting certain demulsification effect under magnetic field. Moreover, under both acidic and neutral conditions, synthesized MNPs could be recycled up to 7 cycles without showing significant loss in demulsification efficiency. In conclusion, CS-grafted MNPs can be easily synthesized and recycled, providing a potential promising approach for efficient treatment of emulsified oil wastewater. © 2016 Elsevier B.V. All rights reserved.
Keywords: Composite materials Magnetization Chemical synthesis Polymers Surfaces and interfaces
1. Introduction Currently, oil pollution has become a serious environmental problem and has attracted global concern. Oily wastewaters find their way into aquatic systems as a result of rapid development of oil exploitation and processing, food industry and mechanical processing. Although qualitative and quantitative composition of oily wastewaters may vary from one industrial field to another, significant part of oil occurs as emulsified form, which is extremely difficult to separate from the water phase. Therefore, it is imperative to develop cost-effective materials or technologies for treating emulsified oil wastewaters. In recent years, flocculation has been reported to be a useful and simple method to remove emulsified oil droplets from oily wastewaters. For example, Gao et al. and Lü et al. successfully synthesized
* Corresponding author. E-mail address: [email protected] (H. Zhao). http://dx.doi.org/10.1016/j.jallcom.2016.12.118 0925-8388/© 2016 Elsevier B.V. All rights reserved.
hydrophobically modified cationic polyacrylamide for effectively flocculating oil droplets [1,2]. In order to minimize secondary pollution risk, Ahmad et al. studied using natural and biodegradable flocculants, such as chitosan (CS), for treating emulsified oily wastewater [3,4]. Bratskaya et al. [5] and Lü et al. [6] developed and studied hydrophobically modified CS and CS-derived graft copolymer flocculants for enhancing demulsification efficiency. However, previously reported flocculation windows in oily wastewater treatment were kind of narrow; for example, the flocculation window width of CS and hydrophobically modified-CS was reported to be only about 1 mg/L, and its demulsification effect decreased significantly once the flocculant dosage exceeded optimal dosage [5]. Moreover, the flocculation process is quite time-consuming and the resulting flocs tend to float, thus leading to low demulsification efficiency. One approach to overcome these drawbacks is to covalently bind the flocculants molecules onto the surface of magnetic nanoparticles (MNPs), thereby imparting magnetic properties to the flocculants [7e11]. Accordingly, the resulting flocs or MNPs coated oil droplets could be more easily
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collected via magnetic separation [12e15]. CS is a natural linear biopolyaminosaccharide derived from alkaline deacetylation of chitin, the second most abundant natural biopolymer found in nature after cellulose [16,17]. CS has received increasing attention as a renewable polymeric material with many useful features such as nontoxicity, biocompatibility, biodegradability and antibacterial properties [18,19]. Due to its hydrophobic domains, CS molecule can exhibit excellent ability to interact with hydrophobic substances in aqueous solution [20]. Previous studies have also shown that CS molecular chains could be successfully adsorbed at oil-water interface to stabilize or flocculate oil droplets [21e23]. Therefore, it is assumed that CS-coated MNPs may also have the potentials to be absorbed onto emulsified oil droplets, thereby facilitating the removal of emulsified oil via magnetic separation. However, to the best of our knowledge, there are few reports regarding the use of CS-coated MNPs for treating emulsified oil wastewaters. Therefore, the main objective of this study was to develop CS-grafted MNPs for oil-water separation. Specifically, magnetically recoverable MNPs were fabricated by the following four steps: (1) Fe3O4 MNPs were synthesized by using a solvothermal method; (2) aminopropyl-functionalized MNPs (Fe3O4@APFS MNPs) were prepared via surface coating with aminopropyl-functionalized silica (APFS); (3) aldehydefunctionalized MNPs (Fe3O4@APFS-G MNPs) were synthesized via Schiff base reaction between aminopropyl-functionalized MNPs and glutaraldehyde; (4) CS-grafted MNPs (Fe3O4@APFS-G-CS MNPs) were finally prepared via Schiff base reaction between aldehyde-functionalized MNPs and CS. The synthesized CS-grafted MNPs were characterized by various technologies, and their demulsification performances, affecting factors (i.e. pH and salinity), demulsification mechanisms, as well as recyclability were evaluated.
2. Materials and methods 2.1. Materials Iron chloride hexahydrate (FeCl3$6H2O), anhydrous sodium acetate (NaAc), ethylene glycol, trisodium citrate dihydrate, tetraethoxysilane (TEOS), sodium hydroxide (NaOH), (3-aminopropyl) triethoxysilane (APTES, 97%), glutaraldehyde (aqueous solution, 50%) and chitosan (deacetylation degree ¼ 95% and viscosity ranging from 100 to 200 mPa s) were purchased from Aladdin Chemistry (Shanghai, China). Hydrochloric acid (HCl) was supplied by Zhejiang Sanying chemical reagent Co. Ltd. Ethanol, ammonium hydroxide and sodium chloride (NaCl) were purchased from Hangzhou Gaojing fine chemical Co. Ltd. A commercially available diesel was obtained from Sinopec. Deionized water was used throughout the experiment. All chemicals were of analytical grade and used as received.
2.2. Synthesis of Fe3O4 MNPs Fe3O4 MNPs were prepared through a solvothermal method. Briefly, 1.62 g of FeCl3$6H2O, 4.32 g of NaAc and 0.48 g of trisodium citrate dihydrate were added to 60 mL of ethylene glycol to yield a transparent solution via vigorous stirring. This mixture was then transferred to a Teflon-lined autoclave (100 mL) and kept at 200 C for 12 h. The products were collected using a magnet, and washed with ethanol and deionized water three times. Finally, the Fe3O4 MNPs were re-dispersed in deionized water for further use.
2.3. Synthesis of Fe3O4@APFS MNPs € ber method. Fe3O4@APFS MNPs were prepared by a modified sto Typically, 0.2 g of Fe3O4 nanoparticles were dispersed in a mixture of 160 mL of ethanol and 40 mL of water by ultrasonication for 10 min. 0.6 mL of TEOS and 0.6 mL of APTES were then added to the mixture under continuous mechanical stirring, followed by the addition of 6 mL of ammonia solution. The reaction was performed at 32 C for 6 h under an atmosphere of nitrogen. The resulting Fe3O4@APFS MNPs were collected using an external magnet, rinsed with water and ethanol, and re-dispersed in water for further use. 2.4. Synthesis of CS-grafted MNPs CS-grafted MNPs were prepared via a “grafting to” reaction. Typically, 3 mL of aqueous glutaraldehyde solution (50 wt%) was added to above-mentioned Fe3O4@APFS MNPs dispersion. This mixture was then treated by powerful ultrasonic wave for 10 min to yield aldehyde-functionalized MNPs. The powerful ultrasonic wave was performed by applying a pulsed sequence (work 10 s, break 5 s), using a sonifier (Scientz JY92-II DN) at 400 W. The resulting MNPs were collected with the help of an external magnet and washed with water three times. Subsequently, Fe3O4@APFS-G-CS MNPs were obtained by adding 0.5 g of aldehyde-functionalized MNPs to 50 mL of CS solution (1 wt%) with intensive stirring for 3 h at room temperature. Fe3O4@APFS-G-CS MNPs were collected with a magnet and washed several times, and re-dispersed in water for further use. The synthesis procedure is illustrated in Fig. 1. 2.5. Demulsification test Diesel-in-water emulsion containing 0.2 wt% of diesel was prepared by powerful sonication, which was performed by applying a pulsed sequence (work 10 s, break 5 s) for 5 min, using a sonifier (Scientz JY92-II DN) at 400 W. Salinity of emulsion was regulated by using NaCl, while pH values were adjusted by using 0.1 mol/L HCl or NaOH. NaCl concentration and pH value of the emulsion were kept at 0.01 mol/L and 7.0, respectively, unless stated otherwise. Demulsification tests were performed at room temperature. A certain amount of MNPs were added to diesel-inwater emulsion and the mixture was then shaken by hand for 30 s. Thereafter, emulsified oil droplets were removed by applying an external magnetic field. Water transmittance after oil separation was determined to assess the demulsification effect. For CS flocculant, emulsion/CS mixture was shaken for 30 s, followed by quiescent settling for 30 min, and the supernate was then used to determine the transmittance. Initial transmittance of the emulsified oil wastewaters was measured to be close to zero. 2.6. Recycle tests After demulsification test, spent MNPs were washed with ethanol three times to remove the attached oil, followed by washing with water three times. Subsequently, the MNPs were redispersed in water by shaking by hand for 30s and reused in the next cycle of demulsifiaction test. Recycling procedure was totally performed for 8 rounds. 2.7. Characterization Morphologies of synthesized MNPs were examined by using transmission electron microscopy (TEM, JSM-1200EX, Japan) with an 80-kv acceleration voltage. Number-average particle size and size distribution were estimated by counting at least 200 particles in TEM images. X-ray powder diffraction patterns (XRD) were
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Fig. 1. Synthetic scheme of the CS-grafted MNPs.
obtained with an X-ray diffractometer (D8 Discover) using Cu Ka radiation in the 2q range of 10e80 . Fourier transform infrared spectra (FTIR) were recorded with a fourier transform infrared spectrometer (Nicolet 6700, Thermo Fisher Scientific, USA). Zeta potential was measured by using a zeta potential analyzer (Nano ZS90, Malvern Instruments Company, UK). Thermal properties were measured by a thermogravimetric analysis (TGA/DSC 1, Mettler Toledo, Swit) under nitrogen atmosphere with a heating rate of 10 C/min to 800 C. Magnetic properties were measured using a physical property measurement system (PPMS-9, Quantum Design, USA) at room temperature. Water transmittance was recorded by a UVevis spectrometer (UV-2600, Shimadzu, Japan) at a wavelength of 610 nm. Microscopic image was observed by a digital optical microscope (KH-7700, Hirox, Japan). 3. Results and discussions 3.1. Characterization of MNPs TEM images of Fe3O4, Fe3O4@APFS, and Fe3O4@APFS-G-CS MNPs, as well as Fe3O4 particle size distribution are shown in Fig. 2. The Fe3O4 nanoparticles were nearly spherical in shape (Fig. 2a), and its number-average size was estimated to be about 250 nm (Fig. 2b). In this study, the Fe3O4 nanoparticles were further coated with an APFS layer for CS grafting. As shown in Fig. 2c, for Fe3O4@APFS MNPs, a gray layer with a thickness of ~50 nm was successfully coated onto Fe3O4 nanoparticles, resulting in a coreshell structure. However, there was no significant difference between Fe3O4@APFS MNPs and Fe3O4@APFS-G-CS MNPs (Fig. 2d), suggesting that only a small amount of CS were successfully grafted onto the MNPs. Presumably, the initial surface-attached polymer layer will result in an efficient steric hindrance effect, thereby preventing further penetration of polymer chains from solution during the “grafting to” reaction [24,25]; in other word, the “grafting to” reaction is sterically hindered. Successful preparation of Fe3O4@APFS-G-CS MNPs was also confirmed by FTIR spectra (Fig. 3). For Fe3O4 MNPs, the absorption peak around 587 cm1 was assigned to Fe-O vibration, while the peak around 1620 cm1 was attributed to COO vibration [26], indicating the presence of carboxylate groups on Fe3O4 particles
[26]. For Fe3O4@APFS MNPs, the absorption peak of Si-O-Si appeared at 1054 cm1 whereas the peak at 1551 cm1 was assigned to the bending vibration of eNH2. For Fe3O4@APFS-G-CS MNPs, the characteristic absorption peak of at 1634 cm1 could be attributed to the stretching vibration of C]N [27], suggesting the occurrence of Schiff base reaction. Additional evidence of the Schiff base adduct is the obvious occurrence of peak at 1408 cm1, a signature peak for a Schiff base adduct. This vibrational band is analogous to the deformation of the C-H band observed for aldehydes [28]. All of these results confirmed that CS molecular chains have been successfully grafted onto Fe3O4@APFS MNPs. Crystal structures of the synthesized Fe3O4, Fe3O4@APFS, and Fe3O4@APFS-G-CS MNPs were investigated by XRD (Fig. 4). The diffraction peaks can be indexed to the cubic spinel structure. Six characteristic peaks (2q ¼ 30.3 , 35.6 , 43.2 , 53.5 , 57.3 , and 62.7 ) related to their corresponding indices ((220), (311), (400), (422), (511), and (440)) were observed in the case of Fe3O4 MNPs. Besides these peaks, XRD pattern of Fe3O4@APFS MNPs also shows a broad peak at 2q of 20e28 , corresponding to amorphous silica. Furthermore, another broad peak at 2q of 20.1, owing to CS [29], was observed in the XRD pattern of Fe3O4@APFS-G-CS MNPs, further confirming the successful coating of CS onto Fe3O4@APFS MNPs. In order to quantitatively determine the composition of MNPs, thermogravimetric analysis was conducted. Fig. 5 showed the TGA curves of CS and synthetic Fe3O4, Fe3O4@APFS, and Fe3O4@APFS-GCS MNPs in the range of 25e800 C. The TGA curve of Fe3O4 MNPs showed a weight loss of about 9.5 wt% from 150 to 800 C, which was ascribed to the degradation of stabilizer sodium citrate and sodium acetate [26]. This result suggested that the carboxyl groups were an essential component of Fe3O4 MNPs. For Fe3O4@APFS NMPs, a weight loss of 14.5 wt% was observed from 150 to 800 C; as compared with Fe3O4 MNPs, additional weight loss of ~5.0 wt % could be attributed to the decomposition of aminopropyl and the dehydration of APFS layer. After surface coating with CS, the weight loss of Fe3O4@APFS-G-CS was 15.4 wt% in the range of 150e800 C. Based upon the weight loss of CS (~80 wt%), the grafting ratio of CS was estimated to be 1.1 wt%. This grafting ratio was lower than previously reported value (around 5 wt%) through a similar “grafting to” reaction [30,31], which could be attributed to the
Fig. 2. TEM images of Fe3O4 (a) MNPs, particle size distribution of Fe3O4 (b), and TEM images of Fe3O4@APFS (c) and Fe3O4@APFS-G-CS (d) MNPs.
Fig. 3. FTIR spectra of CS and Fe3O4, Fe3O4@APFS, and Fe3O4@APFS-G-CS MNPs.
Fig. 4. XRD patterns of Fe3O4, Fe3O4@APFS, and Fe3O4@APFS-G-CS MNPs.
larger particle size of MNPs (i.e. lower specific surface area) in this study. Zeta potential of Fe3O4, Fe3O4@APFS, and Fe3O4@APFS-G-CS MNPs was examined at various pH levels (Fig. 6). It was reported that pure Fe3O4 particles has an isoelectric point around 6.5 [31], while the synthetic Fe3O4 particles were always negatively charged at pH 4.0, 7.0 and 10.0, primarily due to the existence of carboxylate groups on synthesized Fe3O4 particle, as confirmed by FTIR and TGA analyses. However, after surface coating with APFS layer, the zeta potential increased significantly and became positive at pH 4.0 and 7.0 (24.3 and 18.2 mv, respectively) due to the protonation of aminopropyl groups (pKa ~10.6) [32], consistent with that previously reported [33]. It was found that there was no significant difference between the zeta potentials of Fe3O4@APFS-G-CS MNPs
Fig. 5. TGA curves of CS and Fe3O4, Fe3O4@APFS, and Fe3O4@APFS-G-CS MNPs.
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and Fe3O4@APFS MNPs. The zeta potentials of Fe3O4@APFS-G-CS MNPs were 21.7, 17.6 and 21.8 mv at pH 4.0, 7.0 and 10.0, respectively. It should be noted that, at pH 7.0, Fe3O4@APFS-G-CS MNPs was positively charged, although the amino groups of CS (pKa 6.2e6.8) was not protonated [17], suggesting that many aminopropyl groups were retained on the surface after the “grafting to” reaction. 3.2. Magnetic property of MNPs
Fig. 6. Zeta potential of Fe3O4, Fe3O4@APFS, and Fe3O4@APFS-G-CS MNPs at various pH levels.
Magnetic properties of the synthesized MNPs were investigated by PPMS at room temperature. Fig. 7 shows the magnetic hysteresis loops of Fe3O4, Fe3O4@APFS, and Fe3O4@APFS-G-CS MNPs. It can be seen that no obvious remanence and coercivity were observed for all synthesized MNPs, indicating their superparamagnetic behavior. The magnetic saturation values of Fe3O4, Fe3O4@APFS, and Fe3O4@APFS-G-CS MNPs were measured to be 72.4, 33.0, and 30.5 emu/g, respectively. The reduction in magnetic saturation value is most likely caused by the coating of nonmagnetic silica and CS layers. However, it was found that Fe3O4@APFS-G-CS MNPs could still be rapidly collected with a magnet and be re-dispersed in aqueous media after the removal of external magnetic (Fig. 7), suggesting their suitability for magnetic separation. 3.3. Demulsification performance
Fig. 7. Magnetization curves of Fe3O4, Fe3O4@APFS, and Fe3O4@APFS-G-CS MNPs, insert shows the photographs of aqueous MNPs dispersion (a) and MNPs collection with a magnet (b).
Demulsification performance of Fe3O4, Fe3O4@APFS, and Fe3O4@APFS-G-CS MNPs, as well as CS flocculant, were investigated and compared under various conditions (Fig. 8). It can be seen from Fig. 8 (a, b, c) that both CS flocculant and Fe3O4 MNPs exhibited very poor demulsification performance at various pH levels. However, upon being coated with APFS layer, the demulsification performance of MNPs was significantly enhanced at pH 4.0 and 7.0. Generally, emulsified oil droplets are always negatively charged at
Fig. 8. Demulsification performance of Fe3O4, Fe3O4@APFS, and Fe3O4@APFS-G-CS MNPs and CS flocculant at various pH levels (aec), as well as the comparison of demulsification efficiency of Fe3O4@APFS-G-CS at various pH levels (d).
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Fig. 9. Effect of NaCl concentration on the demulsification performance of Fe3O4@APFS-G-CS MNPs.
exhibited better demulsification performance than that of Fe3O4@APFS MNPs, especially under alkaline condition (Fig. 8aec). Fig. 8d shows the effects of pHs on demulsification performances of Fe3O4@APFS-G-CS MNPs. At pH 10.0 and a dosage of 0.5 g/L, water transmittance nearly reached an equilibrium value (~76%); however, at pH 7.0, demulsification efficiency was enhanced and water transmittance could easily reach ~90%. At pH 4.0, Fe3O4@APFS-G-CS MNPs showed the best demulsification performance and only 0.4 g/ L of MNPs was needed to get a water transmittance of ~90%. Since oily wastewaters usually contain a certain amount of salts, the effects of NaCl on demulsification performance of Fe3O4@APFS-G-CS MNPs were also examined (Fig. 9). It was found that the addition of NaCl exhibited little effect on demulsification effect over the studied concentration range (0e0.1 mol/L), suggesting certain saltresistance of the synthesized MNPs. 3.4. Demulsification mechanism
various pH levels [5], while at pH 4.0 and 7.0, Fe3O4@APFS MNPs are positively charged (Fig. 6); as a result, Fe3O4@APFS MNPs could be easily absorbed onto oil droplet surface via electrostatic attraction, thereby imparting magnetic responsiveness to emulsified oil droplets. Consequently, under both acidic and neutral conditions, emulsified oil droplets could be effectively removed by Fe3O4@APFS MNPs in 20 s with the help of magnetic field. It can be seen in Fig. 8(a, b) that water transmittance increased with the increase of MNPs dosage. The maximum water transmittance (~88%) was achieved when the dosage of Fe3O4@APFS MNPs is about 0.5 g/L at pH 4.0 and 0.73 g/L at pH 7.0, respectively. However, under alkaline condition, Fe3O4@APFS MNPs were negatively charged; as a result, MNPs could not be favorably attached to emulsified oil droplets due to strong electrostatic repulsion, resulting in negligible demulsification effect (Fig. 8c). In an attempt to explore potential approaches to further improve the demulsification performance of MNPs, CS was grafted onto Fe3O4@APFS MNPs. It was found that Fe3O4@APFS-G-CS MNPs
Fig. 10 shows the microscopic images of emulsified oil wastewater, as well as the mixture of Fe3O4@APFS-G-CS MNPs and wastewater at various pH levels. The size of emulsified oil droplets seemed to be less than 6 mm (Fig. 10a). Since Fe3O4@APFS-G-CS MNPs is positively charged at pH 4.0 and 7.0, hence negativelycharged oil droplets could be effectively attached to MNPs and further aggregated with each other via electrostatic attraction; accordingly, bigger oil droplets and magnetic flocs with a size exceeding 100 mm were formed rapidly (Fig. 10bec). These big magnetic flocs could be easily collected by magnet in 20 s, and, due to gravity, could also easily settle down in the absence of magnet, resulting in clean water within 20 min. Furthermore, it was worthy noting that, at pH 4.0 and 7.0, Fe3O4@APFS-G-CS MNPs showed improved demulsification efficiency as compared with that of Fe3O4@APFS MNPs, although zeta potentials (i.e. positive charge intensity) of the former were slightly lower than the latter (Fig. 6), suggesting that hydrophobic interaction between grafted CS and oil droplets might also play an important role. This was further verified
Fig. 10. Microscopic images of emulsified oil wastewater (a), and the mixture of Fe3O4@APFS-G-CS MNPs and wastewater at various pH levels: (b) pH ¼ 4.0, (c) pH ¼ 7.0, (d) pH ¼ 10.0.
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Fig. 11. Schematic illustration of the demulsification mechanism of Fe3O4@APFS-G-CS MNPs at various pH levels.
by the fact that, at pH 10.0, Fe3O4@APFS-G-CS MNPs could still be attached onto oil droplet surface (Fig. 10d), despite strong electrostatic repulsion between MNPs and oil droplets at pH 10.0. As a result, under alkaline condition, majority of emulsified oil droplets could still be successfully removed by Fe3O4@APFS-G-CS MNPs with the help of external magnetic field. Accordingly, a possible demulsification mechanism was proposed and illustrated in Fig. 11. Under both acidic and neutral conditions, Fe3O4@APFS-G-CS MNPs acted like cationic magnetic flocculants. In diesel-in-water emulsion systems, MNPs could be adsorbed to the surfaces of negatively charged oil droplets and efficiently flocculate the oil droplets mainly via electrostatic interaction, thereby facilitating magnetic separation. However, under alkaline condition, Fe3O4@APFS-G-CS MNPs act like magnetic surfactant and tends to accumulated at the oil-water interface via hydrophobic interaction, thereby imparting magnetic properties on emulsified oil droplets for magnetic separation. 3.5. Recycling tests An advantage of magnetic demulsifiers over conventional chemical demulsifiers is their potentials to be recycled and reused. Fig. 12 shows the reusability of Fe3O4@APFS-G-CS MNPs at various
pH levels and it was found that Fe3O4@APFS-G-CS MNPs could be recycled up to 7 cycles at pH 4.0 and 7.0 without showing significant loss in demulsification efficiency, suggesting its excellent recyclability under acidic and neutral conditions. 4. Conclusions In this study, CS-grafted MNPs were successfully synthesized and their efficiencies in separating emulsified oil from aqueous environments were evaluated. It was found that CS-grafted MNPs exhibited strong magnetic response properties and superior demulsification performance under both acidic and neutral conditions, and that demulsification efficiency declined gradually with an increase in pH value. It was concluded that, under both acidic and neutral conditions, electrostatic interaction could be the dominant demulsification mechanism involved; while under alkaline conditions, hydrophobic interaction was the main mechanism. Moreover, it was found that the CS-grafted MNPs could be reused up to 7 cycles without significant loss in separation efficiency under both acidic and neutral conditions. CS-grafted MNPs could be easily synthesized and be potentially used as a class of promising materials for efficiently removing oil droplets from emulsified oil wastewaters. Acknowledgements The authors wish to thank the financial support from the National Natural Science Foundation of China project (#21506045 and #41271249), the project of Natural Science Foundation of Zhejiang Province (#LQ14B070006) and Innovative R&D Research Fund of Hangzhou Dianzi University (ZX140206318006). References
Fig. 12. Reusability of Fe3O4@APFS-G-CS MNPs in demulsification test at various pH levels.
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Treatment of emulsified oil wastewaters by using chitosan grafted magnetic nanoparticles Ting Lü a, Yi Chen a, Dongming Qi b, Zhihai Cao b, Dong Zhang a, Hongting Zhao a, * a
Institute of Environmental Materials and Applications, College of Materials and Environmental Engineering, Hangzhou Dianzi University, 310018 Hangzhou, China b Engineering Research Center of Eco-Dyeing and Finishing of Textiles of Ministry of Education, Zhejiang Sci-Tech University, Hangzhou 310018, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 August 2016 Received in revised form 30 November 2016 Accepted 9 December 2016 Available online 10 December 2016
In this study, a class of chitosan (CS)-grafted magnetic nanoparticles (MNPs) was synthesized for treating emulsified oil wastewaters. Fe3O4 MNPs were synthesized by using a solvothermal method, followed by coating with aminopropyl-functionalized silica (APFS) to form a surface for further grafting of CS molecular chains. The synthesized MNPs were characterized by various technologies and their demulsification performances were evaluated under various conditions. Results showed that synthetic Fe3O4 MNPs showed negligible demulsification effect, however, under both acidic and neutral conditions, APFScoated MNPs (Fe3O4@APFS MNPs) exhibited good demulsification effect via electrostatic attraction. It was also found that demulsification performance could be further enhanced upon CS grafting, especially under alkaline condition. Results showed that, under both acidic and neutral conditions, CS-grafted MNPs (Fe3O4@APFS-G-CS MNPs) could efficiently flocculate oil droplets mainly via electrostatic attraction, thereby facilitating magnetic separation; while under alkaline condition, MNPs could overcome electrostatic repulsion and be absorbed onto oil droplet surface via hydrophobic interaction, thereby exhibiting certain demulsification effect under magnetic field. Moreover, under both acidic and neutral conditions, synthesized MNPs could be recycled up to 7 cycles without showing significant loss in demulsification efficiency. In conclusion, CS-grafted MNPs can be easily synthesized and recycled, providing a potential promising approach for efficient treatment of emulsified oil wastewater. © 2016 Elsevier B.V. All rights reserved.
Keywords: Composite materials Magnetization Chemical synthesis Polymers Surfaces and interfaces
1. Introduction Currently, oil pollution has become a serious environmental problem and has attracted global concern. Oily wastewaters find their way into aquatic systems as a result of rapid development of oil exploitation and processing, food industry and mechanical processing. Although qualitative and quantitative composition of oily wastewaters may vary from one industrial field to another, significant part of oil occurs as emulsified form, which is extremely difficult to separate from the water phase. Therefore, it is imperative to develop cost-effective materials or technologies for treating emulsified oil wastewaters. In recent years, flocculation has been reported to be a useful and simple method to remove emulsified oil droplets from oily wastewaters. For example, Gao et al. and Lü et al. successfully synthesized
* Corresponding author. E-mail address: [email protected] (H. Zhao). http://dx.doi.org/10.1016/j.jallcom.2016.12.118 0925-8388/© 2016 Elsevier B.V. All rights reserved.
hydrophobically modified cationic polyacrylamide for effectively flocculating oil droplets [1,2]. In order to minimize secondary pollution risk, Ahmad et al. studied using natural and biodegradable flocculants, such as chitosan (CS), for treating emulsified oily wastewater [3,4]. Bratskaya et al. [5] and Lü et al. [6] developed and studied hydrophobically modified CS and CS-derived graft copolymer flocculants for enhancing demulsification efficiency. However, previously reported flocculation windows in oily wastewater treatment were kind of narrow; for example, the flocculation window width of CS and hydrophobically modified-CS was reported to be only about 1 mg/L, and its demulsification effect decreased significantly once the flocculant dosage exceeded optimal dosage [5]. Moreover, the flocculation process is quite time-consuming and the resulting flocs tend to float, thus leading to low demulsification efficiency. One approach to overcome these drawbacks is to covalently bind the flocculants molecules onto the surface of magnetic nanoparticles (MNPs), thereby imparting magnetic properties to the flocculants [7e11]. Accordingly, the resulting flocs or MNPs coated oil droplets could be more easily
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collected via magnetic separation [12e15]. CS is a natural linear biopolyaminosaccharide derived from alkaline deacetylation of chitin, the second most abundant natural biopolymer found in nature after cellulose [16,17]. CS has received increasing attention as a renewable polymeric material with many useful features such as nontoxicity, biocompatibility, biodegradability and antibacterial properties [18,19]. Due to its hydrophobic domains, CS molecule can exhibit excellent ability to interact with hydrophobic substances in aqueous solution [20]. Previous studies have also shown that CS molecular chains could be successfully adsorbed at oil-water interface to stabilize or flocculate oil droplets [21e23]. Therefore, it is assumed that CS-coated MNPs may also have the potentials to be absorbed onto emulsified oil droplets, thereby facilitating the removal of emulsified oil via magnetic separation. However, to the best of our knowledge, there are few reports regarding the use of CS-coated MNPs for treating emulsified oil wastewaters. Therefore, the main objective of this study was to develop CS-grafted MNPs for oil-water separation. Specifically, magnetically recoverable MNPs were fabricated by the following four steps: (1) Fe3O4 MNPs were synthesized by using a solvothermal method; (2) aminopropyl-functionalized MNPs (Fe3O4@APFS MNPs) were prepared via surface coating with aminopropyl-functionalized silica (APFS); (3) aldehydefunctionalized MNPs (Fe3O4@APFS-G MNPs) were synthesized via Schiff base reaction between aminopropyl-functionalized MNPs and glutaraldehyde; (4) CS-grafted MNPs (Fe3O4@APFS-G-CS MNPs) were finally prepared via Schiff base reaction between aldehyde-functionalized MNPs and CS. The synthesized CS-grafted MNPs were characterized by various technologies, and their demulsification performances, affecting factors (i.e. pH and salinity), demulsification mechanisms, as well as recyclability were evaluated.
2. Materials and methods 2.1. Materials Iron chloride hexahydrate (FeCl3$6H2O), anhydrous sodium acetate (NaAc), ethylene glycol, trisodium citrate dihydrate, tetraethoxysilane (TEOS), sodium hydroxide (NaOH), (3-aminopropyl) triethoxysilane (APTES, 97%), glutaraldehyde (aqueous solution, 50%) and chitosan (deacetylation degree ¼ 95% and viscosity ranging from 100 to 200 mPa s) were purchased from Aladdin Chemistry (Shanghai, China). Hydrochloric acid (HCl) was supplied by Zhejiang Sanying chemical reagent Co. Ltd. Ethanol, ammonium hydroxide and sodium chloride (NaCl) were purchased from Hangzhou Gaojing fine chemical Co. Ltd. A commercially available diesel was obtained from Sinopec. Deionized water was used throughout the experiment. All chemicals were of analytical grade and used as received.
2.2. Synthesis of Fe3O4 MNPs Fe3O4 MNPs were prepared through a solvothermal method. Briefly, 1.62 g of FeCl3$6H2O, 4.32 g of NaAc and 0.48 g of trisodium citrate dihydrate were added to 60 mL of ethylene glycol to yield a transparent solution via vigorous stirring. This mixture was then transferred to a Teflon-lined autoclave (100 mL) and kept at 200 C for 12 h. The products were collected using a magnet, and washed with ethanol and deionized water three times. Finally, the Fe3O4 MNPs were re-dispersed in deionized water for further use.
2.3. Synthesis of Fe3O4@APFS MNPs € ber method. Fe3O4@APFS MNPs were prepared by a modified sto Typically, 0.2 g of Fe3O4 nanoparticles were dispersed in a mixture of 160 mL of ethanol and 40 mL of water by ultrasonication for 10 min. 0.6 mL of TEOS and 0.6 mL of APTES were then added to the mixture under continuous mechanical stirring, followed by the addition of 6 mL of ammonia solution. The reaction was performed at 32 C for 6 h under an atmosphere of nitrogen. The resulting Fe3O4@APFS MNPs were collected using an external magnet, rinsed with water and ethanol, and re-dispersed in water for further use. 2.4. Synthesis of CS-grafted MNPs CS-grafted MNPs were prepared via a “grafting to” reaction. Typically, 3 mL of aqueous glutaraldehyde solution (50 wt%) was added to above-mentioned Fe3O4@APFS MNPs dispersion. This mixture was then treated by powerful ultrasonic wave for 10 min to yield aldehyde-functionalized MNPs. The powerful ultrasonic wave was performed by applying a pulsed sequence (work 10 s, break 5 s), using a sonifier (Scientz JY92-II DN) at 400 W. The resulting MNPs were collected with the help of an external magnet and washed with water three times. Subsequently, Fe3O4@APFS-G-CS MNPs were obtained by adding 0.5 g of aldehyde-functionalized MNPs to 50 mL of CS solution (1 wt%) with intensive stirring for 3 h at room temperature. Fe3O4@APFS-G-CS MNPs were collected with a magnet and washed several times, and re-dispersed in water for further use. The synthesis procedure is illustrated in Fig. 1. 2.5. Demulsification test Diesel-in-water emulsion containing 0.2 wt% of diesel was prepared by powerful sonication, which was performed by applying a pulsed sequence (work 10 s, break 5 s) for 5 min, using a sonifier (Scientz JY92-II DN) at 400 W. Salinity of emulsion was regulated by using NaCl, while pH values were adjusted by using 0.1 mol/L HCl or NaOH. NaCl concentration and pH value of the emulsion were kept at 0.01 mol/L and 7.0, respectively, unless stated otherwise. Demulsification tests were performed at room temperature. A certain amount of MNPs were added to diesel-inwater emulsion and the mixture was then shaken by hand for 30 s. Thereafter, emulsified oil droplets were removed by applying an external magnetic field. Water transmittance after oil separation was determined to assess the demulsification effect. For CS flocculant, emulsion/CS mixture was shaken for 30 s, followed by quiescent settling for 30 min, and the supernate was then used to determine the transmittance. Initial transmittance of the emulsified oil wastewaters was measured to be close to zero. 2.6. Recycle tests After demulsification test, spent MNPs were washed with ethanol three times to remove the attached oil, followed by washing with water three times. Subsequently, the MNPs were redispersed in water by shaking by hand for 30s and reused in the next cycle of demulsifiaction test. Recycling procedure was totally performed for 8 rounds. 2.7. Characterization Morphologies of synthesized MNPs were examined by using transmission electron microscopy (TEM, JSM-1200EX, Japan) with an 80-kv acceleration voltage. Number-average particle size and size distribution were estimated by counting at least 200 particles in TEM images. X-ray powder diffraction patterns (XRD) were
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Fig. 1. Synthetic scheme of the CS-grafted MNPs.
obtained with an X-ray diffractometer (D8 Discover) using Cu Ka radiation in the 2q range of 10e80 . Fourier transform infrared spectra (FTIR) were recorded with a fourier transform infrared spectrometer (Nicolet 6700, Thermo Fisher Scientific, USA). Zeta potential was measured by using a zeta potential analyzer (Nano ZS90, Malvern Instruments Company, UK). Thermal properties were measured by a thermogravimetric analysis (TGA/DSC 1, Mettler Toledo, Swit) under nitrogen atmosphere with a heating rate of 10 C/min to 800 C. Magnetic properties were measured using a physical property measurement system (PPMS-9, Quantum Design, USA) at room temperature. Water transmittance was recorded by a UVevis spectrometer (UV-2600, Shimadzu, Japan) at a wavelength of 610 nm. Microscopic image was observed by a digital optical microscope (KH-7700, Hirox, Japan). 3. Results and discussions 3.1. Characterization of MNPs TEM images of Fe3O4, Fe3O4@APFS, and Fe3O4@APFS-G-CS MNPs, as well as Fe3O4 particle size distribution are shown in Fig. 2. The Fe3O4 nanoparticles were nearly spherical in shape (Fig. 2a), and its number-average size was estimated to be about 250 nm (Fig. 2b). In this study, the Fe3O4 nanoparticles were further coated with an APFS layer for CS grafting. As shown in Fig. 2c, for Fe3O4@APFS MNPs, a gray layer with a thickness of ~50 nm was successfully coated onto Fe3O4 nanoparticles, resulting in a coreshell structure. However, there was no significant difference between Fe3O4@APFS MNPs and Fe3O4@APFS-G-CS MNPs (Fig. 2d), suggesting that only a small amount of CS were successfully grafted onto the MNPs. Presumably, the initial surface-attached polymer layer will result in an efficient steric hindrance effect, thereby preventing further penetration of polymer chains from solution during the “grafting to” reaction [24,25]; in other word, the “grafting to” reaction is sterically hindered. Successful preparation of Fe3O4@APFS-G-CS MNPs was also confirmed by FTIR spectra (Fig. 3). For Fe3O4 MNPs, the absorption peak around 587 cm1 was assigned to Fe-O vibration, while the peak around 1620 cm1 was attributed to COO vibration [26], indicating the presence of carboxylate groups on Fe3O4 particles
[26]. For Fe3O4@APFS MNPs, the absorption peak of Si-O-Si appeared at 1054 cm1 whereas the peak at 1551 cm1 was assigned to the bending vibration of eNH2. For Fe3O4@APFS-G-CS MNPs, the characteristic absorption peak of at 1634 cm1 could be attributed to the stretching vibration of C]N [27], suggesting the occurrence of Schiff base reaction. Additional evidence of the Schiff base adduct is the obvious occurrence of peak at 1408 cm1, a signature peak for a Schiff base adduct. This vibrational band is analogous to the deformation of the C-H band observed for aldehydes [28]. All of these results confirmed that CS molecular chains have been successfully grafted onto Fe3O4@APFS MNPs. Crystal structures of the synthesized Fe3O4, Fe3O4@APFS, and Fe3O4@APFS-G-CS MNPs were investigated by XRD (Fig. 4). The diffraction peaks can be indexed to the cubic spinel structure. Six characteristic peaks (2q ¼ 30.3 , 35.6 , 43.2 , 53.5 , 57.3 , and 62.7 ) related to their corresponding indices ((220), (311), (400), (422), (511), and (440)) were observed in the case of Fe3O4 MNPs. Besides these peaks, XRD pattern of Fe3O4@APFS MNPs also shows a broad peak at 2q of 20e28 , corresponding to amorphous silica. Furthermore, another broad peak at 2q of 20.1, owing to CS [29], was observed in the XRD pattern of Fe3O4@APFS-G-CS MNPs, further confirming the successful coating of CS onto Fe3O4@APFS MNPs. In order to quantitatively determine the composition of MNPs, thermogravimetric analysis was conducted. Fig. 5 showed the TGA curves of CS and synthetic Fe3O4, Fe3O4@APFS, and Fe3O4@APFS-GCS MNPs in the range of 25e800 C. The TGA curve of Fe3O4 MNPs showed a weight loss of about 9.5 wt% from 150 to 800 C, which was ascribed to the degradation of stabilizer sodium citrate and sodium acetate [26]. This result suggested that the carboxyl groups were an essential component of Fe3O4 MNPs. For Fe3O4@APFS NMPs, a weight loss of 14.5 wt% was observed from 150 to 800 C; as compared with Fe3O4 MNPs, additional weight loss of ~5.0 wt % could be attributed to the decomposition of aminopropyl and the dehydration of APFS layer. After surface coating with CS, the weight loss of Fe3O4@APFS-G-CS was 15.4 wt% in the range of 150e800 C. Based upon the weight loss of CS (~80 wt%), the grafting ratio of CS was estimated to be 1.1 wt%. This grafting ratio was lower than previously reported value (around 5 wt%) through a similar “grafting to” reaction [30,31], which could be attributed to the
Fig. 2. TEM images of Fe3O4 (a) MNPs, particle size distribution of Fe3O4 (b), and TEM images of Fe3O4@APFS (c) and Fe3O4@APFS-G-CS (d) MNPs.
Fig. 3. FTIR spectra of CS and Fe3O4, Fe3O4@APFS, and Fe3O4@APFS-G-CS MNPs.
Fig. 4. XRD patterns of Fe3O4, Fe3O4@APFS, and Fe3O4@APFS-G-CS MNPs.
larger particle size of MNPs (i.e. lower specific surface area) in this study. Zeta potential of Fe3O4, Fe3O4@APFS, and Fe3O4@APFS-G-CS MNPs was examined at various pH levels (Fig. 6). It was reported that pure Fe3O4 particles has an isoelectric point around 6.5 [31], while the synthetic Fe3O4 particles were always negatively charged at pH 4.0, 7.0 and 10.0, primarily due to the existence of carboxylate groups on synthesized Fe3O4 particle, as confirmed by FTIR and TGA analyses. However, after surface coating with APFS layer, the zeta potential increased significantly and became positive at pH 4.0 and 7.0 (24.3 and 18.2 mv, respectively) due to the protonation of aminopropyl groups (pKa ~10.6) [32], consistent with that previously reported [33]. It was found that there was no significant difference between the zeta potentials of Fe3O4@APFS-G-CS MNPs
Fig. 5. TGA curves of CS and Fe3O4, Fe3O4@APFS, and Fe3O4@APFS-G-CS MNPs.
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and Fe3O4@APFS MNPs. The zeta potentials of Fe3O4@APFS-G-CS MNPs were 21.7, 17.6 and 21.8 mv at pH 4.0, 7.0 and 10.0, respectively. It should be noted that, at pH 7.0, Fe3O4@APFS-G-CS MNPs was positively charged, although the amino groups of CS (pKa 6.2e6.8) was not protonated [17], suggesting that many aminopropyl groups were retained on the surface after the “grafting to” reaction. 3.2. Magnetic property of MNPs
Fig. 6. Zeta potential of Fe3O4, Fe3O4@APFS, and Fe3O4@APFS-G-CS MNPs at various pH levels.
Magnetic properties of the synthesized MNPs were investigated by PPMS at room temperature. Fig. 7 shows the magnetic hysteresis loops of Fe3O4, Fe3O4@APFS, and Fe3O4@APFS-G-CS MNPs. It can be seen that no obvious remanence and coercivity were observed for all synthesized MNPs, indicating their superparamagnetic behavior. The magnetic saturation values of Fe3O4, Fe3O4@APFS, and Fe3O4@APFS-G-CS MNPs were measured to be 72.4, 33.0, and 30.5 emu/g, respectively. The reduction in magnetic saturation value is most likely caused by the coating of nonmagnetic silica and CS layers. However, it was found that Fe3O4@APFS-G-CS MNPs could still be rapidly collected with a magnet and be re-dispersed in aqueous media after the removal of external magnetic (Fig. 7), suggesting their suitability for magnetic separation. 3.3. Demulsification performance
Fig. 7. Magnetization curves of Fe3O4, Fe3O4@APFS, and Fe3O4@APFS-G-CS MNPs, insert shows the photographs of aqueous MNPs dispersion (a) and MNPs collection with a magnet (b).
Demulsification performance of Fe3O4, Fe3O4@APFS, and Fe3O4@APFS-G-CS MNPs, as well as CS flocculant, were investigated and compared under various conditions (Fig. 8). It can be seen from Fig. 8 (a, b, c) that both CS flocculant and Fe3O4 MNPs exhibited very poor demulsification performance at various pH levels. However, upon being coated with APFS layer, the demulsification performance of MNPs was significantly enhanced at pH 4.0 and 7.0. Generally, emulsified oil droplets are always negatively charged at
Fig. 8. Demulsification performance of Fe3O4, Fe3O4@APFS, and Fe3O4@APFS-G-CS MNPs and CS flocculant at various pH levels (aec), as well as the comparison of demulsification efficiency of Fe3O4@APFS-G-CS at various pH levels (d).
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Fig. 9. Effect of NaCl concentration on the demulsification performance of Fe3O4@APFS-G-CS MNPs.
exhibited better demulsification performance than that of Fe3O4@APFS MNPs, especially under alkaline condition (Fig. 8aec). Fig. 8d shows the effects of pHs on demulsification performances of Fe3O4@APFS-G-CS MNPs. At pH 10.0 and a dosage of 0.5 g/L, water transmittance nearly reached an equilibrium value (~76%); however, at pH 7.0, demulsification efficiency was enhanced and water transmittance could easily reach ~90%. At pH 4.0, Fe3O4@APFS-G-CS MNPs showed the best demulsification performance and only 0.4 g/ L of MNPs was needed to get a water transmittance of ~90%. Since oily wastewaters usually contain a certain amount of salts, the effects of NaCl on demulsification performance of Fe3O4@APFS-G-CS MNPs were also examined (Fig. 9). It was found that the addition of NaCl exhibited little effect on demulsification effect over the studied concentration range (0e0.1 mol/L), suggesting certain saltresistance of the synthesized MNPs. 3.4. Demulsification mechanism
various pH levels [5], while at pH 4.0 and 7.0, Fe3O4@APFS MNPs are positively charged (Fig. 6); as a result, Fe3O4@APFS MNPs could be easily absorbed onto oil droplet surface via electrostatic attraction, thereby imparting magnetic responsiveness to emulsified oil droplets. Consequently, under both acidic and neutral conditions, emulsified oil droplets could be effectively removed by Fe3O4@APFS MNPs in 20 s with the help of magnetic field. It can be seen in Fig. 8(a, b) that water transmittance increased with the increase of MNPs dosage. The maximum water transmittance (~88%) was achieved when the dosage of Fe3O4@APFS MNPs is about 0.5 g/L at pH 4.0 and 0.73 g/L at pH 7.0, respectively. However, under alkaline condition, Fe3O4@APFS MNPs were negatively charged; as a result, MNPs could not be favorably attached to emulsified oil droplets due to strong electrostatic repulsion, resulting in negligible demulsification effect (Fig. 8c). In an attempt to explore potential approaches to further improve the demulsification performance of MNPs, CS was grafted onto Fe3O4@APFS MNPs. It was found that Fe3O4@APFS-G-CS MNPs
Fig. 10 shows the microscopic images of emulsified oil wastewater, as well as the mixture of Fe3O4@APFS-G-CS MNPs and wastewater at various pH levels. The size of emulsified oil droplets seemed to be less than 6 mm (Fig. 10a). Since Fe3O4@APFS-G-CS MNPs is positively charged at pH 4.0 and 7.0, hence negativelycharged oil droplets could be effectively attached to MNPs and further aggregated with each other via electrostatic attraction; accordingly, bigger oil droplets and magnetic flocs with a size exceeding 100 mm were formed rapidly (Fig. 10bec). These big magnetic flocs could be easily collected by magnet in 20 s, and, due to gravity, could also easily settle down in the absence of magnet, resulting in clean water within 20 min. Furthermore, it was worthy noting that, at pH 4.0 and 7.0, Fe3O4@APFS-G-CS MNPs showed improved demulsification efficiency as compared with that of Fe3O4@APFS MNPs, although zeta potentials (i.e. positive charge intensity) of the former were slightly lower than the latter (Fig. 6), suggesting that hydrophobic interaction between grafted CS and oil droplets might also play an important role. This was further verified
Fig. 10. Microscopic images of emulsified oil wastewater (a), and the mixture of Fe3O4@APFS-G-CS MNPs and wastewater at various pH levels: (b) pH ¼ 4.0, (c) pH ¼ 7.0, (d) pH ¼ 10.0.
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Fig. 11. Schematic illustration of the demulsification mechanism of Fe3O4@APFS-G-CS MNPs at various pH levels.
by the fact that, at pH 10.0, Fe3O4@APFS-G-CS MNPs could still be attached onto oil droplet surface (Fig. 10d), despite strong electrostatic repulsion between MNPs and oil droplets at pH 10.0. As a result, under alkaline condition, majority of emulsified oil droplets could still be successfully removed by Fe3O4@APFS-G-CS MNPs with the help of external magnetic field. Accordingly, a possible demulsification mechanism was proposed and illustrated in Fig. 11. Under both acidic and neutral conditions, Fe3O4@APFS-G-CS MNPs acted like cationic magnetic flocculants. In diesel-in-water emulsion systems, MNPs could be adsorbed to the surfaces of negatively charged oil droplets and efficiently flocculate the oil droplets mainly via electrostatic interaction, thereby facilitating magnetic separation. However, under alkaline condition, Fe3O4@APFS-G-CS MNPs act like magnetic surfactant and tends to accumulated at the oil-water interface via hydrophobic interaction, thereby imparting magnetic properties on emulsified oil droplets for magnetic separation. 3.5. Recycling tests An advantage of magnetic demulsifiers over conventional chemical demulsifiers is their potentials to be recycled and reused. Fig. 12 shows the reusability of Fe3O4@APFS-G-CS MNPs at various
pH levels and it was found that Fe3O4@APFS-G-CS MNPs could be recycled up to 7 cycles at pH 4.0 and 7.0 without showing significant loss in demulsification efficiency, suggesting its excellent recyclability under acidic and neutral conditions. 4. Conclusions In this study, CS-grafted MNPs were successfully synthesized and their efficiencies in separating emulsified oil from aqueous environments were evaluated. It was found that CS-grafted MNPs exhibited strong magnetic response properties and superior demulsification performance under both acidic and neutral conditions, and that demulsification efficiency declined gradually with an increase in pH value. It was concluded that, under both acidic and neutral conditions, electrostatic interaction could be the dominant demulsification mechanism involved; while under alkaline conditions, hydrophobic interaction was the main mechanism. Moreover, it was found that the CS-grafted MNPs could be reused up to 7 cycles without significant loss in separation efficiency under both acidic and neutral conditions. CS-grafted MNPs could be easily synthesized and be potentially used as a class of promising materials for efficiently removing oil droplets from emulsified oil wastewaters. Acknowledgements The authors wish to thank the financial support from the National Natural Science Foundation of China project (#21506045 and #41271249), the project of Natural Science Foundation of Zhejiang Province (#LQ14B070006) and Innovative R&D Research Fund of Hangzhou Dianzi University (ZX140206318006). References
Fig. 12. Reusability of Fe3O4@APFS-G-CS MNPs in demulsification test at various pH levels.
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