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Thermosensitive poly(N-isopropylacrylamide)-grafted magnetic nanoparticles for efficient treatment of emulsified oily wastewater
Journal of Alloys and Compounds 688 (2016) 513e520
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Thermosensitive poly(N-isopropylacrylamide)-grafted magnetic nanoparticles for efficient treatment of emulsified oily wastewater Ting Lü a, Shuang Zhang a, Dongming Qi 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 for Eco-Dyeing & Finishing of Textiles, 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 4 April 2016 Received in revised form 13 July 2016 Accepted 25 July 2016 Available online 28 July 2016
A class of thermosensitive and reusable magnetic nanoparticles (MNPs) was developed for removing emulsified oil droplets from aqueous media. Magnetic iron oxide (MIO) nanoparticles were prepared with a coprecipitation method, followed by coating with silica layer and further modification with gmethacryloxypropyl triisopropoxidesilane (MPS). Poly(N-isopropylacrylamide) (PNIPAM) molecular chains were then anchored onto the surfaces of MNPs via a “grafting through” reaction. The efficiency of synthetic MNPs for treating emulsified oily wastewater was evaluated as a function of dosage, pH, temperature and reusability. PNIPAM-grafted MNPs exhibited high demulsification efficiency at lower temperature, while decreased dramatically when the temperature exceeded the lower critical solution temperature (LCST) of PNIPAM (~33 C); moreover, MNPs could be facilely regenerated by rinsing with hot water and could be reused up to 7 cycles. The results indicated that PNIPAM-grafted MNPs could potentially be a new promising and environmentally friendly approach for effectively treating emulsified oil in wastewater. © 2016 Elsevier B.V. All rights reserved.
Keywords: Composite materials Chemical synthesis Magnetization Polymers Surfaces and interfaces
1. Introduction At present, oily wastewater causes severe environmental and ecological problems and also threatens the life of human beings [1]. The oily wastewaters are generally arisen from oil industry and oil spill accidents, and the oil often occurs as emulsified form, making it extremely difficult to separate the oil from the water phase [2]. Traditional treatment approaches for emulsified oil wastewaters [3,4], such as air flotation, electrochemistry method, adsorption separation and biochemistry methods, generally are not efficient enough to achieve satisfactory cleanup [5]. As a consequence, it is necessary to develop new cost-effective technologies that are able to efficiently remove emulsified oil from aqueous environment. In recent years, magnetic oil-water multiphase separation technique has attracted considerable attention, in which surfacemodified magnetic nanoparticles (MNPs) are used as the demulsifier [6e11]. Magnetic iron oxide (MIO) nanoparticles, such as maghemite (g-Fe2O3) and magnetite (Fe3O4), are often used as the magnetic substrate, because of their low cost, low cytotoxicity and
* Corresponding author. E-mail address: [email protected] (H. Zhao). http://dx.doi.org/10.1016/j.jallcom.2016.07.262 0925-8388/© 2016 Elsevier B.V. All rights reserved.
good biocompatibility [12,13]. Generally, coprecipitation method was commonly used to prepare MIO nanoparticles. Moreover, MIO nanoparticles could also be continuously produced by using coprecipitation method in an impinging stream-rotating packed bed reactor; in such a reactor, MIO nanoparticles could be prepared at a production rate of 2.23 kg/hour [14]. Meanwhile, some physical techniques [15,16], such as laser target evaporation and electric explosion of wires, were also suggested to be used to produce large amounts of MIO nanoparticles. However, MIO nanoparticles generally should be further modified with hydrophobic or amphiphilic substances in order to improve its hydrophobicity or interfacial activity. As a result, the modified MNPs can rapidly accumulate inside the oil droplet or at the oil droplet surface, thereby imparting the emulsified oil droplets with magnetic responsiveness for easy removal under external magnetic field [17,18]. Hou et al. successfully prepared bilayer oleic acid (OA)-coated MNPs by coprecipitation method in the presence of varying contents of OA and achieved a maximum demulsification efficiency of ~98% [19,20]. Zhou et al. reported that much higher oil removal rate could be achieved after anchoring the typical demulsifier 5010 on the MNPs surface [21]. Schmidt et al. reported the use of polystyrene coated-MNPs as the stabilizers for the
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preparation of cyclohexane-in-water Pickering emulsions, and found that they could be easily destabilized under magnetic field [22]. Lead et al. prepared polyvinylpyrrolidone (PVP)-coated MNPs via one-step modified polyol method, and found that the oil removal rate could reach nearly up to 100% under optimized conditions [23]. However, the above-mentioned oil-water separation processes were still time-consuming and not environmentalfriendly. It required several hours of mixing and even heating to obtain a satisfactory demulsification effect; moreover, organic solvents is needed for regeneration of the spent MNPs. Recently, an advanced stimuli-responsive hybrid MNPs consist of a magnetic core and an outmost thermosensitive polymer brushes were developed via surface-initiated atom transfer radical polymerization (SI-ATRP) and it was found that the synthetic MNPs could not only rapidly remove the toluene droplet from toluene-inwater emulsion under magnetic field [24], but also could be easily desorbed from toluene droplet surface via thermal stimulus, and hence the regeneration process is facile and eco-friendly. However, this SI-ATRP technology is complicated and expensive. As compared to SI-ATRP strategy, “grafting through” reactions could be a more economic and convenient synthetic method. Moreover, the layer formation process of “grafting through” reactions is somehow self-limiting and hence the resulting polymer layers are not sensitive to the reaction conditions [25]. Thus, “grafting through” method is more suitable to the industrialized production. Although the “grafting through” method would result in low grafting density [25], it was reported that the particles with lower grafting densities would favor the accumulation of nanoparticles at oil-water interface [26,27]. Therefore, the main objective of this study was to facilely develop advanced thermoresponsive MNPs for treating emulsified oily wastewaters by employing the convenient“grafting through” reaction for anchoring poly (N-isopropylacrylamide) (PNIPAM) molecular chains onto the surfaces of MNPs. The synthesized PNIPAM-grafted MNPs were characterized by various technologies and their performance and in treating the diesel-in-water emulsion was investigated in detail as a function of MNPs dosage, temperature and pH. Moreover, reusability of the MNPs was also evaluated.
solution which contained 15 g NaOH and 1 g PAA at 80 C with vigorous stirring under the protection of nitrogen. Once the solution was completely added to the reactor, the reaction mixture was heated to 90 C and the reaction lasted for 1 h additionally. The resulting magnetic nanoparticles was collected with a magnet and washed with water for several times. Finally, the magnetic nanoparticles were re-dispersed in water for further use. 2.3. Oxidation-reduction titration MIO nanoparticles (0.15 g) were firstly dissolved in aqueous H2SO4 solution (1.0 mol/L), and the resulting solution was then titrated with aqueous KMnO4 solution (0.01 mol/L). The titrating end point was considered to be reached when pale pink color appeared and did not fade in 1 min. The titration experiment was repeated for three times to determine the average value. 2.4. Synthesis of silica-coated MNPs 1 g of MIO nanoparticles were dispersed in 200 mL of water and the aqueous dispersion was then heated to 80 C under the protection of nitrogen. Thereafter, 20 mL of 1.0 mol/L sodium silicate solution was added dropwise to the aqueous MIO dispersion within 2 h with vigorous stirring and the pH was controlled and kept at 6.0 with 2 mol/L HCl solution. The reaction mixture was sequentially stirred for 3 h and the silica coated MIO (MIO@SiO2) nanoparticles was collected with a magnet. The MIO@SiO2 nanoparticles washed with deionized water for several times and dried in a vacuum oven at 50 C for 24 h. 2.5. Modification of silica-coated MNPs
2. Materials and methods
0.5 g of MIO@SiO2 nanoparticles were dispersed in 20 g of ethanol, then 0.05 g of MPS and 0.05 g of water were injected to this dispersion with vigorous stirring. Subsequently, the mixture was heated to 40 C under the protection of nitrogen. After 48 h, the MPS modified MNPs (MIO@SiO2-MPS) were collected with a magnet, followed by thorough washing with ethanol and water for 3 times in each case. The MPS modified MNPs was then redispersed in water for further use.
2.1. Materials
2.6. Preparation of PNIPAM-grafted MNPs
Iron chloride hexahydrate (FeCl3$6H2O), Iron(II) chloride tetrahydrate (FeCl2$4H2O), Sodium metasilicate nonahydrate (Na2SiO3$9H2O), and N-isopropylacrylamide (NIPAM) were supplied by Shanghai Macklin Biochemical Co. Ltd. Water-soluble initiator 2,20 -Azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride (AIBI, 98.5 wt%) were purchased from DuPont Company. Polyacrylic acid (PAA), sodium hydroxide (NaOH) and g-methacryloxypropyl triisopropoxidesilane (MPS, 98 wt%) were purchased from Aladdin Chemistry (Shanghai, China). Hydrochloric acid (HCl), sulfuric acid (H2SO4) and potassium permanganate (KMnO4) were supplied by Zhejiang Sanying chemical reagent Co. Ltd. Ethanol was purchased from Hangzhou Gaojing fine chemical Co. Ltd. The deionized water was used throughout the experiment. All chemicals were of analytical grade and used without further purification.
The thermoresponsive MNPs was prepared via a “grafting through” reaction. Typically, 0.2 g of MPS modified MNPs and 1.0 g of NIPAM monomer were mixed with 38.8 g of water. The mixture was then heated to 28 C and purged with nitrogen for 30 min. Afterward, the initiator AIBI was introduced and the polymerization lasted for 24 h. The PNIPAM-grafted MNPs (MIO@SiO2-PNIPAM) were collected with the help of a magnet and was washed with water repeatedly. The synthetic MNPs were dispersed in water for further use.
2.2. Synthesis of MIO nanoparticles MIO nanoparticles were prepared through the conventional coprecipitation method. Briefly, 10.8 g of FeCl3$6H2O and 3.98 g of FeCl2$4H2O were dissolved in 50 mL of 0.5 mol/L HCl solution. Above solution was then added dropwise to 250 mL of aqueous
2.7. Demulsification test The diesel-in-water emulsion containing 0.2 wt% of diesel was prepared by powerful sonication for 5 min. The emulsion was found to be stable over 3 weeks without significant phase separation. The demulsification test was performed at 25 C except as otherwise indicated. A certain amount of MNPs were added to the disel-inwater emulsion and the mixture was shaken by hand for 20 s to let the MNPs assemble onto the diesel-water interface. Upon applying an external magnetic field, the MNPs coated diesel droplets were moved to the vial wall. The water transmittance after oil separation was determined to assess the demulsification effect of MNPs.
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Fig. 1. TEM images of the MIO (a) and MIO@SiO2 (b) nanoparticles. Insert shows the particle size distribution of MIO nanoparticles.
2.8. Recycle tests After the demulsification test, the spent MNPs were washed with hot water (60 C) for 3 times to remove the attached oil. The regenerated MNPs were then reused in the next cycle of oil-water separation. This recycling procedure was performed for 7 rounds to assess the reusability of the MNPs. 2.9. Characterization Size and morphology of the synthesized MNPs were examined
by transmission electron microscopy (TEM, JSM-1200EX, Japan) with an 80-kv acceleration voltage. The number-average particle size and size distribution were estimated by counting at least 500 particles in TEM images. X-ray powder diffraction patterns were obtained with an X-ray diffractometer (XRD, D8 Discover) sing Cu Ka radiation in the 2q range of 10e80 . Fourier transform infrared spectra (FTIR) of various samples were recorded with a fourier transform infrared spectrometer (Nicolet 6700, Thermo Fisher Scientific, USA) using KBr pellet technique. Surface chemical composition of the nanoparticles was investigated using x-ray photoelectron spectroscopy (XPS, K-Alpha, Thermo Fisher
Fig. 2. XPS spectra of MIO and MIO@SiO2 nanoparticles: (a) survey spectra of MIO and MIO@SiO2 nanoparticles, (b) high-resolution spectra of Fe2p for MIO nanoparticles, (c) highresolution spectra of Si2p for MIO@SiO2 nanoparticles, (d) high-resolution spectra of O1s for MIO@SiO2 nanoparticles.
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Fig. 3. FTIR spectra of MIO, MIO@SiO2, MIO@SiO2-MPS, and MIO@SiO2-PNIPAM nanoparticles.
Fig. 5. TGA curves of MIO, MIO@SiO2, MIO@SiO2-MPS, and MIO@SiO2-PNIPAM nanoparticles.
Scientific, USA). Zeta potential of the prepared nanoparticles was measured by using a zeta potential analyzer (Nano-Z Zetasizer, Malvern Instruments Company, UK). The content of organic compounds of the MNPs was measured by a thermal gravimetric analyzer (TGA/DSC 1, Mettler Toledo, Swit) under nitrogen atmosphere with a heating rate of 10 C/min to 700 C. The magnetic properties were measured using a physical property measurement system (PPMS-9, Quantum Design, USA) at room temperature. The water transmittance was recorded by a UV-vis spectrometer (UV2450, Shimadzu, Japan) at a wavelength of 610 nm. The microscopic image was observed by means of a digital optical microscope (KH7700, Hirox, Japan). The process of oil-water separation was photographed by a digital camera (SX200IS, Canon, Japan). 3. Results and discussions 3.1. Characterization of MNPs The TEM images of MIO and MIO@SiO2 nanoparticles are showed in Fig. 1. The MIO particles were non-spherical; the particle size ranged from 4 to 22 nm and its number-average size was
Fig. 6. Zeta potential of MIO, MIO@SiO2, and MIO@SiO2-PNIPAM nanoparticles at various pH levels.
Fig. 4. XRD patterns of MIO, MIO@SiO2, and MIO@SiO2-PNIPAM nanoparticles.
Fig. 7. Magnetization curves of MIO, MIO@SiO2, and MIO@SiO2-PNIPAM nanoparticles.
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Fig. 8. Digital pictures and microscope images of the oil-water separation process at various stages: (a) diesel-in-water emulsion, (b) after MNPs added, after introducing magnetic field: (c) 20 s, (d) 40 s, (e) 60 s.
estimated to be about 10 nm (Fig. 1a). After silica coating, the nanoparticles exhibited core-shell structures (Fig. 1b) and some aggregates were observed. XPS measurement was performed to examine the surface composition of nanoparticles (Fig. 2). The atomic content of Fe2p was 29.3% for MIO nanoparticles. However, the atomic content of Fe2p decreased to 2.53% after silica coating, while the atomic content of Si2p increased to 29.7% (Fig. 2a). In Fe2p spetra of MIO sample (Fig. 2b), the main peaks at 710.4 and 712.4 eV are due to Fe2þ and Fe3þ [29], respectively. In XPS spetra of MIO@SiO2 (Fig. 2c), the peak of Si2p is at 103.1 eV, in agreement with Si2p in SiO2 [30]; the O1s spectra consist of two peaks at 532.3 and 530.0 eV (Fig. 2d), corresponding to O1s in SiO2 and MIO [31], respectively. These results confirmed that MIO@SiO2 nanoparticles were successfully synthesized. Fig. 3 shows the FTIR spectra of the MIO, MIO@SiO2, MIO@SiO2MPS and PNIPAM-grafted MNPs. As compared with the spectra of MIO, an intense peak at 1099 cm1 was observed for MIO@SiO2, MIO@SiO2-MPS and PNIPAM-grafted MNPs, attributable to the SiO-Si antisymmetric stretching vibration. This result further confirmed the successful silica coating on the MIO surface. The surface modification with MPS was verified by the adsorption peak at 1732 cm1, attributed to the C]O stretching vibration. The absorption bands at 1370, 1393 and 1461 cm1 for the PNIPAMgrafted MNPs can be ascribed to the vibrations of isopropyl; while the bands at 1544 and 1649 cm1 can be assigned to the N-H bending vibration and C]O stretching vibration, respectively. All of these results suggested that the PNIPAM molecular chains had been covalently grafted to the surface of MIO@SiO2 nanoparticles. The crystal structure of magnetic nanoparticles was characterized by XRD. Fig. 4 shows the XRD patterns of MIO, MIO@SiO2 and PNIPAM-grafted MNPs. Diffraction peaks at 2q of 30.3, 35.6, 43.2, 57.3, and 63.1 were observed for MIO, indicating a cubic spinel structure. Moreover, titration experiment was carried out to examine the composition of MIO nanoparticles. After MIO nanoparticles (0.15 g) were dissolved in aqueous H2SO4 solution, KMnO4 was used to titrate the resulting solution. At first, KMnO4 solution was quickly decolorized once it was dropwise added to the solution, suggesting the existence of Fe2þ ion. It was found that a total 4.20 mL of KMnO4 solution was needed to oxidize all Fe2þ ion. The molar ratio of Fe3þ/Fe2þ was calculated to be approximatively 4:1, which is away from the theoretical ratio (2:1) in Fe3O4. This result indicated that a portion of Fe3O4 nanoparticles were oxidized,
suggesting the MIO nanoparticles were most possibly a mixture of maghemite (g-Fe2O3) and magnetite (Fe3O4). Similar XRD peaks were also observed for MIO@SiO2 and PNIPAM-grafted MNPs, indicating that both silica coating and PNIPAM grafting had no obvious effect on the structure of magnetic particles. Besides, for MIO@SiO2, the broad diffraction band between 19 and 28 could be attributed to the occurrence of amorphous SiO2, further confirming the formation of the silica layer around the MIO nanoparticles. TGA analyses were used to acquire a better understanding of the thermal stability and the content of organic compounds on the nanoparticles. Fig. 5 presents the TGA curves of bare MIO, MIO@SiO2 and PNIPAM-grafted MNPs. For bare MIO and MIO@SiO2 nanoparticles, negligible weight loss could be observed over the temperature range up to 700 C, indicating few organic compounds on the nanoparticle surface. However, about 2.8% weight loss was observed between 200 and 600 C, due to the decomposition of anchored MPS. Moreover, a significant weight loss (~20.5 wt%) is observed when PNIPAM-grafted MNPs was heated from 200 to 600 C attributable to the decomposition of PNIPAM grafted on
Fig. 9. Effect of PNIPAM-grafted MNPs dosage on the water transmittance after oil separation at pH ¼ 7.0.
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indicated that the surface properties changes with the surface modification process during the synthesis of PNIPAM-grafted MNPs. The magnetic properties of MIO, MIO@SiO2 and PNIPAM-grafted MNPs were examined and the results are shown in Fig. 7. The saturation magnetization was measured to be about 70, 45 and 33 emu/g for bare MIO, MIO@SiO2 and PNIPAM-grafted MNPs, respectively. The reduction in the saturation magnetization is most likely caused by the coating of nonmagnetic silica and PNIPAM layers. Despite the reduction in the saturation magnetization, the PNIPAM-grafted MNPs can still be rapidly collected with a magnet. More importantly, the MNPs remain superparamagnetic after grafting of PNIPAM. As a result, the PNIPAM-grafted MNPs can be rapidly redispersed after the removal of external magnetic field for regeneration and recycle. 3.2. Demulsification performance of MNPs Fig. 10. Effect of pH on the demulsification efficiency of PNIPAM-grafted MNPs.
MNPs. The amount of PNIPAM grafted on the MIO@SiO2 nanoparticles is estimated to be about 220 mg/g. Fig. 6 shows the zeta potential of MIO, MIO@SiO2 and PNIPAMgrafted MNPs. As compared with bare MIO, the zeta potentials decreased significantly after the surface coating of silica layer. Even at pH 4.0, the zeta potential of MIO@SiO2 became negative (10.1 mv). However, after the formation of non-ionic PNIPAM layer on the surface of MIO@SiO2 nanoparticles, the zeta potential of PNIPAM-grafted MNPs is close to 0 mv at pH 4.0 or pH 7.0. Even at pH 10.0, the zeta potential of PNIPAM-grafted MNPs is only 8.5 mv, remarkably higher than that of MIO@SiO2 nanoparticles. As anticipated, the zeta potential measurement results
In order to evaluate the demulsification performance of PNIPAM-grafted MNPs, the oil-water separation process was illustrated in Fig. 8. The diesel-in-water emulsion was milky and homodisperse (Fig. 8a), and the size of emulsified oil droplets ranged from about 300 nm to 4 mm as shown in microscopic image. Upon the introduction of PNIPAM-grafted MNPs, the emulsion remained homodisperse but its color immediately changed to black (Fig. 8b). Due to the excellent interfacial activity of PNIPAM [24,28], PNIPAM-grafted MNPs could quickly accumulate at the diesel oilwater interface as shown in microscopic image, thereby imparting the emulsified oil droplets with magnetic responsiveness. As a result, the emulsified oil droplets could be rapidly separated within 60 s by placing a magnet near the glass vial (Fig. 8cee). After the oilwater separation, the system became transparent (Fig. 8e) and nearly no oil droplets could be observed in microscopic image.
Fig. 11. Effect of temperature on the demulsification efficiency of PNIPAM-grafted MNPs (a), and microscopic images of the mixture of PNIPAM-grafted MNPs and emulsion at various temperatures: (b) 25 C, (c) 40 C, (d) 60 C.
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Fig. 12. Study of reusability of the PNIPAM-grafted MNPs in different cycles of oilwater separation.
In order to further assess the demulsification performance, the effect of MNPs dosage was investigated at pH 7.0. Fig. 9 clearly indicates that both of MIO and MIO@SiO2 exhibited negligible demulsification effect due to their hydrophilic characters. After the MPS anchoring, the demulsification effect was merely slightly improved. However, the PNIPAM-grafted MNPs exhibited superior demulsification performance. An increase in the dosage of PNIPAMgrafted MNPs led to an increase in the water transmittance, reaching as high as ~97% at the dosage of 100 mg/L. The effect of pH on the demulsification performance was also studied (Fig. 10), and the results indicated that pH has little influence on the demulsification efficiency. In summary, PNIPAM-grafted MNPs are capable of efficiently removing emulsified diesel oil from the wastewater
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(2.0 g/L oil) at a ratio of 20:1 (oil:MNPs) under various pH conditions. Since PNIPAM is a thermoresponsive polymer, having a lower critical solution temperature (LCST) ~33 C, therefore, the effect of temperature on demulsification effect of PNIPAM-grafted MNPs was investigated at the dosage of 100 mg/L as shown in Fig. 11a. Upon treatment, the water transmittance was measured to be around 96% at the temperature ranging from 10 to 30 C, while started to reduce significantly at the temperature around the LCST (33 C) of PNIPAM. In order to understand this phenomenon, microscopic images of the emulsion containing MNPs at various temperatures were examined. As described earlier, at 25 C, majority droplets were coated with MNPs and the droplets were taupe (Fig. 11b). However, at 40 C, the MNPs started to desorb from the oil droplet and hence part of oil droplets became transparent (Fig. 11c); when the temperature was increased up to 60 C, majority of the oil droplets were colorless and transparent (Fig. 11d), suggesting significant desorption of MNPs from oil droplets. The desorption of PNIPAM-grafted MNPs could be attributed to the coilto-globule transition of PNIPAM at the temperature above LCST [24,28]. As a result, the demulsification effect decreased when the temperature exceeded the LCST (33 C).
3.3. Reusability of MNPs High reusability of the MNPs is desired in order to reduce the operational cost of oil-water separation. As discussed above, the MNPs could desorb from oil droplet at the temperature above LCST, accordingly, reusability of the PNIPAM-grafted MNPs was evaluated and hot water (60 C) was used for the regeneration. Fig. 12 indicates that the PNIPAM-grafted MNPs still exhibited high demulsification effciency even after 7 use cycles. In light of above-mentioned, a schematic diagram of use of MNPs for in oil-water separation is shown in Fig. 13. After the PNIPAM-grafted MNPs were introduced into the diesel in water
Fig. 13. Schematic illustration of the recycle of PNIPAM-grafted MNPs via regulating the temperature during the oil-water separation.
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emulsion, the MNPs efficiently accumulated at the oil droplet surface due to the interfacial activity. Accordingly, these MNPs coated oil droplet could be rapidly removed from water with external magnetic field. Subsequently, the collected MNPs were washed with hot water and regenerated, ultimately reused in the next cycle. 4. Conclusion A class of thermosensitive and reusable MNPs was successfully synthesized via the “grafting through” method. The synthesized PNIPAM-grafted MNPs exhibited strong magnetic response properties as well as excellent interfacial activity at room temperature. In diesel-in-water emulsion, the synthetic MNPs could rapidly assemble to the oil-water interface, resulting in efficient removal of emulsified oil in the presence of magnetic field. Results showed that the demulsification performance was thermoresponsive while not pH-responsive. At higher temperature (above LCST of PNIPAM (~33 C)), the PNIPAM-grafted MNPs tended to desorb from the emulsified oil droplets, and therefore the MNPs could be facilely regenerated by just using hot water and be reused up to 7 cycles without showing significant decrease in its demulsification efficiency. In summary, this thermosensitive and reusable magnetic demulsifier could be potentially be a promising materials for the treatment of emulsified oily wastewater. Acknowledgments The authors wish to thank the research support from the National Natural Science Foundation of China (NNSFC) project (#21506045 and #41271249). References [1] B. Wang, W. Liang, Z. Guo, W. Liu, Biomimetic superlyophobic and superlyophilic materials applied for oil/water separation: a new strategy beyond nature, Chem. Soc. Rev. 44 (2015) 336e361. [2] S.Y. Bratskaya, V.A. Avramenko, S. Schwarz, I. Philippova, Enhanced flocculation of oil-in-water emulsions by hydrophobically modified chitosan derivatives, Colloids Surf. A 275 (2006) 168e176. [3] Q. Ke, Y. Jin, P. Jiang, J. Yu, Oil/water separation performances of superhydrophobic and superoleophilic sponges, Langmuir 30 (2014) 13137e13142. [4] F. Jiang, Y.L. Hsieh, Amphiphilic superabsorbent cellulose nanofibril aerogels, J. Mater. Chem. A 2 (2014) 6337e6342. [5] H.L. Peng, H. Wang, J.N. Wu, G.H. Meng, Y.X. Wang, Y.L. Shi, Z.Y. Liu, X.H. Guo, Preparation of superhydrophobic magnetic cellulose sponge for removing oil from water, Ind. Eng. Chem. Res. 55 (2016) 832e838. [6] M.D. Chen, W. Jiang, F.H. Wang, P. Shen, P.C. Ma, J.J. Gu, J.Y. Mao, F.S. Li, Synthesis of highly hydrophobic floating magnetic polymernanocomposites for the removal of oils from water surface, Appl. Surf. Sci. 286 (2013) 249e256. [7] L. Zhang, L.L. Li, Z.M. Dang, Bio-inspired durable, superhydrophobic magnetic particles for oil/water separation, J. Colloid Interface Sci. 463 (2016) 266e271. [8] Y.N. Chen, X. Lin, N. Liu, Y.Z. Cao, F. Lu, L.X. Xu, L. Feng, Magnetically recoverable efficient demulsifier for water-in-oil emulsions, ChemPhysChem 16 (2015) 595e600. [9] L. Zhang, J. Wu, Y. Wang, Y. Long, N. Zhao, J. Xu, Combination of bioinspiration: a general route to superhydrophobic particles, J. Am. Chem. Soc. 134 (2012)
9879e9881. [10] L. Wu, J.P. Zhang, B.C. Li, A.Q. Wang, Magnetically driven super durable superhydrophobic polyester materials for oil/water separation, Polym. Chem. 5 (2014) 2382e2390. [11] J.A. Flores, A. Pavía-Sanders, Y.C. Chen, D.J. Pochan, K.L. Wooley, Recyclable hybrid inorganic/organic magnetically active networks for the sequestration of crude oil from aqueous environments, Chem. Mater. 27 (2015) 3775e3782. [12] Y.Q. Yang, M.M. Kang, S.M. Fang, M.H. Wang, L.H. He, X.Z. Feng, J.H. Zhao, Z.H. Zhang, H.Z. Zhang, A feasible C-rich DNA electrochemical biosensor based on Fe3O4@3D-GO for sensitive and selective detection of Agþ, J. Alloy. Compd. 652 (2015) 225e233. [13] L.Y. Chen, Z. Lin, C.L. Zhao, Y.Y. Zheng, Y. Zhou, H. Peng, Direct synthesis and characterization of mesoporous Fe3O4 through pyrolysis of ferric nitrateethylene glycol gel, J. Alloy. Compd. 509 (2011) L1eL5. [14] H.L. Fan, S.F. Zhou, G.S. Qi, Y.Z. Liu, Continuous preparation of Fe3O4 nanoparticles using impinging stream-rotating packed bed reactor and magnetic property thereof, J. Alloy. Compd. 662 (2016) 497e504. [15] A.P. Safronov, I.V. Beketov, S.V. Komogortsev, G.V. Kurlyandskaya, ~ aga, S.M. Bhagat, Spherical magnetic A.I. Medvedev, D.V. Leiman, A. Larran nanoparticles fabricated by laser target evaporation, AIP Adv. 3 (2013) 052135. [16] Y.A. Kotov, Electric explosion of wires as a method for preparation of nanopowders, J. Nanoparticle Res. 5 (2003) 539e550. [17] N. Ali, B.L. Zhang, H.P. Zhang, W. Li, W. Zaman, L. Tian, Q.Y. Zhang, Novel janus magnetic micro particle synthesis and its applications as a demulsifier for breaking heavy crude oil and water emulsion, Fuel 141 (2015) 258e267. [18] P. Tempesti, M. Bonini, F. Ridi, P. Baglioni, Magnetic polystyrene nanocomposites for the separation of oil and water, J. Mater. Chem. A 2 (2014) 1980e1984. [19] J.L. Liang, H.P. Li, J.G. Yan, W.G. Hou, Demulsification of oleic-acid-coated magnetite nanoparticles for cyclohexane-in-water nanoemulsions, Energy Fuels 28 (2014) 6172e6178. [20] J.L. Liang, N. Du, S. Song, W.G. Hou, Magnetic demulsification of diluted crude oil-in-water nanoemulsions using oleic acid-coated magnetite nanoparticles, Colloids Surf. A 466 (2015) 197e202. [21] S.Q. Li, N.X. Li, S.B. Yang, F.Y. Liu, J.C. Zhou, The synthesis of a novel magnetic demulsifier and its application for the demulsification of oil-charged industrial wastewaters, J. Mater. Chem. A 2 (2014) 94e99. [22] A. Kaiser, T. Liu, W. Richtering, A.M. Schmidt, Magnetic Capsules and pickering emulsions stabilized by core-shell particles, Langmuir 25 (2009) 7335e7341. [23] S. Palchoudhury, J.R. Lead, A facile and cost-effective method for separation of oil-water mixtures using polymer-coated iron oxide nanoparticles, Environ. Sci. Technol. 48 (2014) 14558e14563. [24] Y. Chen, Y.K. Bai, S.B. Chen, J.P. Ju, Y.Q. Li, T.M. Wang, Q.H. Wang, Stimuliresponsive composite particles as solid-stabilizers for effective oil harvesting, ACS Appl. Mater. Interfaces 6 (2014) 13334e13338. €dge, O. Prucker, J. Rühe, “Grafting Through”: mechanistic [25] M. Henze, D. Ma aspects of radical polymerization reactions with surface-attached monomers, Macromolecules 47 (2014) 2929e2937. [26] T. Saigal, H.C. Dong, K. Matyjaszewski, R.D. Ti1ton, Pickering emulsions stabilized by nanoparticle with thermally responsive grafted polymer brushes, Langmuir 26 (2010) 15200e15209. [27] L.M. Foster, A.J. Worthen, E.L. Foster, J.N. Dong, C.M. Roach, A.E. Metaxas, C.D. Hardy, E.S. Larsen, J.A. Bollinger, T.M. Truskett, C.W. Bielawski, K.P. Johnston, High interfacial activity of polymers “grafted through” functionalized iron oxide nanoparticle clusters, Langmuir 30 (2014) 10188e10196. [28] J.T. Tang, P.J. Quinlan, K.C. Tam, Stimuli-responsive pickering emulsions: recent advances and potential applications, Soft Matter 11 (2015) 3512e3529. [29] F. Zhang, Y.J. Shi, Z.S. Zhao, B.L. Ma, L.S. Wei, L.P. Lu, Amino-functionalized Fe3O4/SiO2 magnetic submicron composites and In3þ ion adsorption properties, J. Mater. Sci. 49 (2014) 3478e3483. [30] Z.W. Li, Y.F. Zhu, Surface-modification of SiO2 nanoparticles with oleic acid, Appl. Surf. Sci. 211 (2003) 315e320. [31] C.Q. Yang, G. Wang, Z.Y. Lu, J. Sun, J.Q. Zhuang, W.S. Yang, Effect of ultrasonic treatment on dispersibility of Fe3O4 nanoparticles and synthesis of multi-core Fe3O4/SiO2 core/shell nanoparticles, J. Mater. Chem. 15 (2005) 4252e4257.
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Thermosensitive poly(N-isopropylacrylamide)-grafted magnetic nanoparticles for efficient treatment of emulsified oily wastewater Ting Lü a, Shuang Zhang a, Dongming Qi 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 for Eco-Dyeing & Finishing of Textiles, 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 4 April 2016 Received in revised form 13 July 2016 Accepted 25 July 2016 Available online 28 July 2016
A class of thermosensitive and reusable magnetic nanoparticles (MNPs) was developed for removing emulsified oil droplets from aqueous media. Magnetic iron oxide (MIO) nanoparticles were prepared with a coprecipitation method, followed by coating with silica layer and further modification with gmethacryloxypropyl triisopropoxidesilane (MPS). Poly(N-isopropylacrylamide) (PNIPAM) molecular chains were then anchored onto the surfaces of MNPs via a “grafting through” reaction. The efficiency of synthetic MNPs for treating emulsified oily wastewater was evaluated as a function of dosage, pH, temperature and reusability. PNIPAM-grafted MNPs exhibited high demulsification efficiency at lower temperature, while decreased dramatically when the temperature exceeded the lower critical solution temperature (LCST) of PNIPAM (~33 C); moreover, MNPs could be facilely regenerated by rinsing with hot water and could be reused up to 7 cycles. The results indicated that PNIPAM-grafted MNPs could potentially be a new promising and environmentally friendly approach for effectively treating emulsified oil in wastewater. © 2016 Elsevier B.V. All rights reserved.
Keywords: Composite materials Chemical synthesis Magnetization Polymers Surfaces and interfaces
1. Introduction At present, oily wastewater causes severe environmental and ecological problems and also threatens the life of human beings [1]. The oily wastewaters are generally arisen from oil industry and oil spill accidents, and the oil often occurs as emulsified form, making it extremely difficult to separate the oil from the water phase [2]. Traditional treatment approaches for emulsified oil wastewaters [3,4], such as air flotation, electrochemistry method, adsorption separation and biochemistry methods, generally are not efficient enough to achieve satisfactory cleanup [5]. As a consequence, it is necessary to develop new cost-effective technologies that are able to efficiently remove emulsified oil from aqueous environment. In recent years, magnetic oil-water multiphase separation technique has attracted considerable attention, in which surfacemodified magnetic nanoparticles (MNPs) are used as the demulsifier [6e11]. Magnetic iron oxide (MIO) nanoparticles, such as maghemite (g-Fe2O3) and magnetite (Fe3O4), are often used as the magnetic substrate, because of their low cost, low cytotoxicity and
* Corresponding author. E-mail address: [email protected] (H. Zhao). http://dx.doi.org/10.1016/j.jallcom.2016.07.262 0925-8388/© 2016 Elsevier B.V. All rights reserved.
good biocompatibility [12,13]. Generally, coprecipitation method was commonly used to prepare MIO nanoparticles. Moreover, MIO nanoparticles could also be continuously produced by using coprecipitation method in an impinging stream-rotating packed bed reactor; in such a reactor, MIO nanoparticles could be prepared at a production rate of 2.23 kg/hour [14]. Meanwhile, some physical techniques [15,16], such as laser target evaporation and electric explosion of wires, were also suggested to be used to produce large amounts of MIO nanoparticles. However, MIO nanoparticles generally should be further modified with hydrophobic or amphiphilic substances in order to improve its hydrophobicity or interfacial activity. As a result, the modified MNPs can rapidly accumulate inside the oil droplet or at the oil droplet surface, thereby imparting the emulsified oil droplets with magnetic responsiveness for easy removal under external magnetic field [17,18]. Hou et al. successfully prepared bilayer oleic acid (OA)-coated MNPs by coprecipitation method in the presence of varying contents of OA and achieved a maximum demulsification efficiency of ~98% [19,20]. Zhou et al. reported that much higher oil removal rate could be achieved after anchoring the typical demulsifier 5010 on the MNPs surface [21]. Schmidt et al. reported the use of polystyrene coated-MNPs as the stabilizers for the
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preparation of cyclohexane-in-water Pickering emulsions, and found that they could be easily destabilized under magnetic field [22]. Lead et al. prepared polyvinylpyrrolidone (PVP)-coated MNPs via one-step modified polyol method, and found that the oil removal rate could reach nearly up to 100% under optimized conditions [23]. However, the above-mentioned oil-water separation processes were still time-consuming and not environmentalfriendly. It required several hours of mixing and even heating to obtain a satisfactory demulsification effect; moreover, organic solvents is needed for regeneration of the spent MNPs. Recently, an advanced stimuli-responsive hybrid MNPs consist of a magnetic core and an outmost thermosensitive polymer brushes were developed via surface-initiated atom transfer radical polymerization (SI-ATRP) and it was found that the synthetic MNPs could not only rapidly remove the toluene droplet from toluene-inwater emulsion under magnetic field [24], but also could be easily desorbed from toluene droplet surface via thermal stimulus, and hence the regeneration process is facile and eco-friendly. However, this SI-ATRP technology is complicated and expensive. As compared to SI-ATRP strategy, “grafting through” reactions could be a more economic and convenient synthetic method. Moreover, the layer formation process of “grafting through” reactions is somehow self-limiting and hence the resulting polymer layers are not sensitive to the reaction conditions [25]. Thus, “grafting through” method is more suitable to the industrialized production. Although the “grafting through” method would result in low grafting density [25], it was reported that the particles with lower grafting densities would favor the accumulation of nanoparticles at oil-water interface [26,27]. Therefore, the main objective of this study was to facilely develop advanced thermoresponsive MNPs for treating emulsified oily wastewaters by employing the convenient“grafting through” reaction for anchoring poly (N-isopropylacrylamide) (PNIPAM) molecular chains onto the surfaces of MNPs. The synthesized PNIPAM-grafted MNPs were characterized by various technologies and their performance and in treating the diesel-in-water emulsion was investigated in detail as a function of MNPs dosage, temperature and pH. Moreover, reusability of the MNPs was also evaluated.
solution which contained 15 g NaOH and 1 g PAA at 80 C with vigorous stirring under the protection of nitrogen. Once the solution was completely added to the reactor, the reaction mixture was heated to 90 C and the reaction lasted for 1 h additionally. The resulting magnetic nanoparticles was collected with a magnet and washed with water for several times. Finally, the magnetic nanoparticles were re-dispersed in water for further use. 2.3. Oxidation-reduction titration MIO nanoparticles (0.15 g) were firstly dissolved in aqueous H2SO4 solution (1.0 mol/L), and the resulting solution was then titrated with aqueous KMnO4 solution (0.01 mol/L). The titrating end point was considered to be reached when pale pink color appeared and did not fade in 1 min. The titration experiment was repeated for three times to determine the average value. 2.4. Synthesis of silica-coated MNPs 1 g of MIO nanoparticles were dispersed in 200 mL of water and the aqueous dispersion was then heated to 80 C under the protection of nitrogen. Thereafter, 20 mL of 1.0 mol/L sodium silicate solution was added dropwise to the aqueous MIO dispersion within 2 h with vigorous stirring and the pH was controlled and kept at 6.0 with 2 mol/L HCl solution. The reaction mixture was sequentially stirred for 3 h and the silica coated MIO (MIO@SiO2) nanoparticles was collected with a magnet. The MIO@SiO2 nanoparticles washed with deionized water for several times and dried in a vacuum oven at 50 C for 24 h. 2.5. Modification of silica-coated MNPs
2. Materials and methods
0.5 g of MIO@SiO2 nanoparticles were dispersed in 20 g of ethanol, then 0.05 g of MPS and 0.05 g of water were injected to this dispersion with vigorous stirring. Subsequently, the mixture was heated to 40 C under the protection of nitrogen. After 48 h, the MPS modified MNPs (MIO@SiO2-MPS) were collected with a magnet, followed by thorough washing with ethanol and water for 3 times in each case. The MPS modified MNPs was then redispersed in water for further use.
2.1. Materials
2.6. Preparation of PNIPAM-grafted MNPs
Iron chloride hexahydrate (FeCl3$6H2O), Iron(II) chloride tetrahydrate (FeCl2$4H2O), Sodium metasilicate nonahydrate (Na2SiO3$9H2O), and N-isopropylacrylamide (NIPAM) were supplied by Shanghai Macklin Biochemical Co. Ltd. Water-soluble initiator 2,20 -Azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride (AIBI, 98.5 wt%) were purchased from DuPont Company. Polyacrylic acid (PAA), sodium hydroxide (NaOH) and g-methacryloxypropyl triisopropoxidesilane (MPS, 98 wt%) were purchased from Aladdin Chemistry (Shanghai, China). Hydrochloric acid (HCl), sulfuric acid (H2SO4) and potassium permanganate (KMnO4) were supplied by Zhejiang Sanying chemical reagent Co. Ltd. Ethanol was purchased from Hangzhou Gaojing fine chemical Co. Ltd. The deionized water was used throughout the experiment. All chemicals were of analytical grade and used without further purification.
The thermoresponsive MNPs was prepared via a “grafting through” reaction. Typically, 0.2 g of MPS modified MNPs and 1.0 g of NIPAM monomer were mixed with 38.8 g of water. The mixture was then heated to 28 C and purged with nitrogen for 30 min. Afterward, the initiator AIBI was introduced and the polymerization lasted for 24 h. The PNIPAM-grafted MNPs (MIO@SiO2-PNIPAM) were collected with the help of a magnet and was washed with water repeatedly. The synthetic MNPs were dispersed in water for further use.
2.2. Synthesis of MIO nanoparticles MIO nanoparticles were prepared through the conventional coprecipitation method. Briefly, 10.8 g of FeCl3$6H2O and 3.98 g of FeCl2$4H2O were dissolved in 50 mL of 0.5 mol/L HCl solution. Above solution was then added dropwise to 250 mL of aqueous
2.7. Demulsification test The diesel-in-water emulsion containing 0.2 wt% of diesel was prepared by powerful sonication for 5 min. The emulsion was found to be stable over 3 weeks without significant phase separation. The demulsification test was performed at 25 C except as otherwise indicated. A certain amount of MNPs were added to the disel-inwater emulsion and the mixture was shaken by hand for 20 s to let the MNPs assemble onto the diesel-water interface. Upon applying an external magnetic field, the MNPs coated diesel droplets were moved to the vial wall. The water transmittance after oil separation was determined to assess the demulsification effect of MNPs.
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Fig. 1. TEM images of the MIO (a) and MIO@SiO2 (b) nanoparticles. Insert shows the particle size distribution of MIO nanoparticles.
2.8. Recycle tests After the demulsification test, the spent MNPs were washed with hot water (60 C) for 3 times to remove the attached oil. The regenerated MNPs were then reused in the next cycle of oil-water separation. This recycling procedure was performed for 7 rounds to assess the reusability of the MNPs. 2.9. Characterization Size and morphology of the synthesized MNPs were examined
by transmission electron microscopy (TEM, JSM-1200EX, Japan) with an 80-kv acceleration voltage. The number-average particle size and size distribution were estimated by counting at least 500 particles in TEM images. X-ray powder diffraction patterns were obtained with an X-ray diffractometer (XRD, D8 Discover) sing Cu Ka radiation in the 2q range of 10e80 . Fourier transform infrared spectra (FTIR) of various samples were recorded with a fourier transform infrared spectrometer (Nicolet 6700, Thermo Fisher Scientific, USA) using KBr pellet technique. Surface chemical composition of the nanoparticles was investigated using x-ray photoelectron spectroscopy (XPS, K-Alpha, Thermo Fisher
Fig. 2. XPS spectra of MIO and MIO@SiO2 nanoparticles: (a) survey spectra of MIO and MIO@SiO2 nanoparticles, (b) high-resolution spectra of Fe2p for MIO nanoparticles, (c) highresolution spectra of Si2p for MIO@SiO2 nanoparticles, (d) high-resolution spectra of O1s for MIO@SiO2 nanoparticles.
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Fig. 3. FTIR spectra of MIO, MIO@SiO2, MIO@SiO2-MPS, and MIO@SiO2-PNIPAM nanoparticles.
Fig. 5. TGA curves of MIO, MIO@SiO2, MIO@SiO2-MPS, and MIO@SiO2-PNIPAM nanoparticles.
Scientific, USA). Zeta potential of the prepared nanoparticles was measured by using a zeta potential analyzer (Nano-Z Zetasizer, Malvern Instruments Company, UK). The content of organic compounds of the MNPs was measured by a thermal gravimetric analyzer (TGA/DSC 1, Mettler Toledo, Swit) under nitrogen atmosphere with a heating rate of 10 C/min to 700 C. The magnetic properties were measured using a physical property measurement system (PPMS-9, Quantum Design, USA) at room temperature. The water transmittance was recorded by a UV-vis spectrometer (UV2450, Shimadzu, Japan) at a wavelength of 610 nm. The microscopic image was observed by means of a digital optical microscope (KH7700, Hirox, Japan). The process of oil-water separation was photographed by a digital camera (SX200IS, Canon, Japan). 3. Results and discussions 3.1. Characterization of MNPs The TEM images of MIO and MIO@SiO2 nanoparticles are showed in Fig. 1. The MIO particles were non-spherical; the particle size ranged from 4 to 22 nm and its number-average size was
Fig. 6. Zeta potential of MIO, MIO@SiO2, and MIO@SiO2-PNIPAM nanoparticles at various pH levels.
Fig. 4. XRD patterns of MIO, MIO@SiO2, and MIO@SiO2-PNIPAM nanoparticles.
Fig. 7. Magnetization curves of MIO, MIO@SiO2, and MIO@SiO2-PNIPAM nanoparticles.
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Fig. 8. Digital pictures and microscope images of the oil-water separation process at various stages: (a) diesel-in-water emulsion, (b) after MNPs added, after introducing magnetic field: (c) 20 s, (d) 40 s, (e) 60 s.
estimated to be about 10 nm (Fig. 1a). After silica coating, the nanoparticles exhibited core-shell structures (Fig. 1b) and some aggregates were observed. XPS measurement was performed to examine the surface composition of nanoparticles (Fig. 2). The atomic content of Fe2p was 29.3% for MIO nanoparticles. However, the atomic content of Fe2p decreased to 2.53% after silica coating, while the atomic content of Si2p increased to 29.7% (Fig. 2a). In Fe2p spetra of MIO sample (Fig. 2b), the main peaks at 710.4 and 712.4 eV are due to Fe2þ and Fe3þ [29], respectively. In XPS spetra of MIO@SiO2 (Fig. 2c), the peak of Si2p is at 103.1 eV, in agreement with Si2p in SiO2 [30]; the O1s spectra consist of two peaks at 532.3 and 530.0 eV (Fig. 2d), corresponding to O1s in SiO2 and MIO [31], respectively. These results confirmed that MIO@SiO2 nanoparticles were successfully synthesized. Fig. 3 shows the FTIR spectra of the MIO, MIO@SiO2, MIO@SiO2MPS and PNIPAM-grafted MNPs. As compared with the spectra of MIO, an intense peak at 1099 cm1 was observed for MIO@SiO2, MIO@SiO2-MPS and PNIPAM-grafted MNPs, attributable to the SiO-Si antisymmetric stretching vibration. This result further confirmed the successful silica coating on the MIO surface. The surface modification with MPS was verified by the adsorption peak at 1732 cm1, attributed to the C]O stretching vibration. The absorption bands at 1370, 1393 and 1461 cm1 for the PNIPAMgrafted MNPs can be ascribed to the vibrations of isopropyl; while the bands at 1544 and 1649 cm1 can be assigned to the N-H bending vibration and C]O stretching vibration, respectively. All of these results suggested that the PNIPAM molecular chains had been covalently grafted to the surface of MIO@SiO2 nanoparticles. The crystal structure of magnetic nanoparticles was characterized by XRD. Fig. 4 shows the XRD patterns of MIO, MIO@SiO2 and PNIPAM-grafted MNPs. Diffraction peaks at 2q of 30.3, 35.6, 43.2, 57.3, and 63.1 were observed for MIO, indicating a cubic spinel structure. Moreover, titration experiment was carried out to examine the composition of MIO nanoparticles. After MIO nanoparticles (0.15 g) were dissolved in aqueous H2SO4 solution, KMnO4 was used to titrate the resulting solution. At first, KMnO4 solution was quickly decolorized once it was dropwise added to the solution, suggesting the existence of Fe2þ ion. It was found that a total 4.20 mL of KMnO4 solution was needed to oxidize all Fe2þ ion. The molar ratio of Fe3þ/Fe2þ was calculated to be approximatively 4:1, which is away from the theoretical ratio (2:1) in Fe3O4. This result indicated that a portion of Fe3O4 nanoparticles were oxidized,
suggesting the MIO nanoparticles were most possibly a mixture of maghemite (g-Fe2O3) and magnetite (Fe3O4). Similar XRD peaks were also observed for MIO@SiO2 and PNIPAM-grafted MNPs, indicating that both silica coating and PNIPAM grafting had no obvious effect on the structure of magnetic particles. Besides, for MIO@SiO2, the broad diffraction band between 19 and 28 could be attributed to the occurrence of amorphous SiO2, further confirming the formation of the silica layer around the MIO nanoparticles. TGA analyses were used to acquire a better understanding of the thermal stability and the content of organic compounds on the nanoparticles. Fig. 5 presents the TGA curves of bare MIO, MIO@SiO2 and PNIPAM-grafted MNPs. For bare MIO and MIO@SiO2 nanoparticles, negligible weight loss could be observed over the temperature range up to 700 C, indicating few organic compounds on the nanoparticle surface. However, about 2.8% weight loss was observed between 200 and 600 C, due to the decomposition of anchored MPS. Moreover, a significant weight loss (~20.5 wt%) is observed when PNIPAM-grafted MNPs was heated from 200 to 600 C attributable to the decomposition of PNIPAM grafted on
Fig. 9. Effect of PNIPAM-grafted MNPs dosage on the water transmittance after oil separation at pH ¼ 7.0.
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indicated that the surface properties changes with the surface modification process during the synthesis of PNIPAM-grafted MNPs. The magnetic properties of MIO, MIO@SiO2 and PNIPAM-grafted MNPs were examined and the results are shown in Fig. 7. The saturation magnetization was measured to be about 70, 45 and 33 emu/g for bare MIO, MIO@SiO2 and PNIPAM-grafted MNPs, respectively. The reduction in the saturation magnetization is most likely caused by the coating of nonmagnetic silica and PNIPAM layers. Despite the reduction in the saturation magnetization, the PNIPAM-grafted MNPs can still be rapidly collected with a magnet. More importantly, the MNPs remain superparamagnetic after grafting of PNIPAM. As a result, the PNIPAM-grafted MNPs can be rapidly redispersed after the removal of external magnetic field for regeneration and recycle. 3.2. Demulsification performance of MNPs Fig. 10. Effect of pH on the demulsification efficiency of PNIPAM-grafted MNPs.
MNPs. The amount of PNIPAM grafted on the MIO@SiO2 nanoparticles is estimated to be about 220 mg/g. Fig. 6 shows the zeta potential of MIO, MIO@SiO2 and PNIPAMgrafted MNPs. As compared with bare MIO, the zeta potentials decreased significantly after the surface coating of silica layer. Even at pH 4.0, the zeta potential of MIO@SiO2 became negative (10.1 mv). However, after the formation of non-ionic PNIPAM layer on the surface of MIO@SiO2 nanoparticles, the zeta potential of PNIPAM-grafted MNPs is close to 0 mv at pH 4.0 or pH 7.0. Even at pH 10.0, the zeta potential of PNIPAM-grafted MNPs is only 8.5 mv, remarkably higher than that of MIO@SiO2 nanoparticles. As anticipated, the zeta potential measurement results
In order to evaluate the demulsification performance of PNIPAM-grafted MNPs, the oil-water separation process was illustrated in Fig. 8. The diesel-in-water emulsion was milky and homodisperse (Fig. 8a), and the size of emulsified oil droplets ranged from about 300 nm to 4 mm as shown in microscopic image. Upon the introduction of PNIPAM-grafted MNPs, the emulsion remained homodisperse but its color immediately changed to black (Fig. 8b). Due to the excellent interfacial activity of PNIPAM [24,28], PNIPAM-grafted MNPs could quickly accumulate at the diesel oilwater interface as shown in microscopic image, thereby imparting the emulsified oil droplets with magnetic responsiveness. As a result, the emulsified oil droplets could be rapidly separated within 60 s by placing a magnet near the glass vial (Fig. 8cee). After the oilwater separation, the system became transparent (Fig. 8e) and nearly no oil droplets could be observed in microscopic image.
Fig. 11. Effect of temperature on the demulsification efficiency of PNIPAM-grafted MNPs (a), and microscopic images of the mixture of PNIPAM-grafted MNPs and emulsion at various temperatures: (b) 25 C, (c) 40 C, (d) 60 C.
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Fig. 12. Study of reusability of the PNIPAM-grafted MNPs in different cycles of oilwater separation.
In order to further assess the demulsification performance, the effect of MNPs dosage was investigated at pH 7.0. Fig. 9 clearly indicates that both of MIO and MIO@SiO2 exhibited negligible demulsification effect due to their hydrophilic characters. After the MPS anchoring, the demulsification effect was merely slightly improved. However, the PNIPAM-grafted MNPs exhibited superior demulsification performance. An increase in the dosage of PNIPAMgrafted MNPs led to an increase in the water transmittance, reaching as high as ~97% at the dosage of 100 mg/L. The effect of pH on the demulsification performance was also studied (Fig. 10), and the results indicated that pH has little influence on the demulsification efficiency. In summary, PNIPAM-grafted MNPs are capable of efficiently removing emulsified diesel oil from the wastewater
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(2.0 g/L oil) at a ratio of 20:1 (oil:MNPs) under various pH conditions. Since PNIPAM is a thermoresponsive polymer, having a lower critical solution temperature (LCST) ~33 C, therefore, the effect of temperature on demulsification effect of PNIPAM-grafted MNPs was investigated at the dosage of 100 mg/L as shown in Fig. 11a. Upon treatment, the water transmittance was measured to be around 96% at the temperature ranging from 10 to 30 C, while started to reduce significantly at the temperature around the LCST (33 C) of PNIPAM. In order to understand this phenomenon, microscopic images of the emulsion containing MNPs at various temperatures were examined. As described earlier, at 25 C, majority droplets were coated with MNPs and the droplets were taupe (Fig. 11b). However, at 40 C, the MNPs started to desorb from the oil droplet and hence part of oil droplets became transparent (Fig. 11c); when the temperature was increased up to 60 C, majority of the oil droplets were colorless and transparent (Fig. 11d), suggesting significant desorption of MNPs from oil droplets. The desorption of PNIPAM-grafted MNPs could be attributed to the coilto-globule transition of PNIPAM at the temperature above LCST [24,28]. As a result, the demulsification effect decreased when the temperature exceeded the LCST (33 C).
3.3. Reusability of MNPs High reusability of the MNPs is desired in order to reduce the operational cost of oil-water separation. As discussed above, the MNPs could desorb from oil droplet at the temperature above LCST, accordingly, reusability of the PNIPAM-grafted MNPs was evaluated and hot water (60 C) was used for the regeneration. Fig. 12 indicates that the PNIPAM-grafted MNPs still exhibited high demulsification effciency even after 7 use cycles. In light of above-mentioned, a schematic diagram of use of MNPs for in oil-water separation is shown in Fig. 13. After the PNIPAM-grafted MNPs were introduced into the diesel in water
Fig. 13. Schematic illustration of the recycle of PNIPAM-grafted MNPs via regulating the temperature during the oil-water separation.
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emulsion, the MNPs efficiently accumulated at the oil droplet surface due to the interfacial activity. Accordingly, these MNPs coated oil droplet could be rapidly removed from water with external magnetic field. Subsequently, the collected MNPs were washed with hot water and regenerated, ultimately reused in the next cycle. 4. Conclusion A class of thermosensitive and reusable MNPs was successfully synthesized via the “grafting through” method. The synthesized PNIPAM-grafted MNPs exhibited strong magnetic response properties as well as excellent interfacial activity at room temperature. In diesel-in-water emulsion, the synthetic MNPs could rapidly assemble to the oil-water interface, resulting in efficient removal of emulsified oil in the presence of magnetic field. Results showed that the demulsification performance was thermoresponsive while not pH-responsive. At higher temperature (above LCST of PNIPAM (~33 C)), the PNIPAM-grafted MNPs tended to desorb from the emulsified oil droplets, and therefore the MNPs could be facilely regenerated by just using hot water and be reused up to 7 cycles without showing significant decrease in its demulsification efficiency. In summary, this thermosensitive and reusable magnetic demulsifier could be potentially be a promising materials for the treatment of emulsified oily wastewater. Acknowledgments The authors wish to thank the research support from the National Natural Science Foundation of China (NNSFC) project (#21506045 and #41271249). References [1] B. Wang, W. Liang, Z. Guo, W. Liu, Biomimetic superlyophobic and superlyophilic materials applied for oil/water separation: a new strategy beyond nature, Chem. Soc. Rev. 44 (2015) 336e361. [2] S.Y. Bratskaya, V.A. Avramenko, S. Schwarz, I. Philippova, Enhanced flocculation of oil-in-water emulsions by hydrophobically modified chitosan derivatives, Colloids Surf. A 275 (2006) 168e176. [3] Q. Ke, Y. Jin, P. Jiang, J. Yu, Oil/water separation performances of superhydrophobic and superoleophilic sponges, Langmuir 30 (2014) 13137e13142. [4] F. Jiang, Y.L. Hsieh, Amphiphilic superabsorbent cellulose nanofibril aerogels, J. Mater. Chem. A 2 (2014) 6337e6342. [5] H.L. Peng, H. Wang, J.N. Wu, G.H. Meng, Y.X. Wang, Y.L. Shi, Z.Y. Liu, X.H. Guo, Preparation of superhydrophobic magnetic cellulose sponge for removing oil from water, Ind. Eng. Chem. Res. 55 (2016) 832e838. [6] M.D. Chen, W. Jiang, F.H. Wang, P. Shen, P.C. Ma, J.J. Gu, J.Y. Mao, F.S. Li, Synthesis of highly hydrophobic floating magnetic polymernanocomposites for the removal of oils from water surface, Appl. Surf. Sci. 286 (2013) 249e256. [7] L. Zhang, L.L. Li, Z.M. Dang, Bio-inspired durable, superhydrophobic magnetic particles for oil/water separation, J. Colloid Interface Sci. 463 (2016) 266e271. [8] Y.N. Chen, X. Lin, N. Liu, Y.Z. Cao, F. Lu, L.X. Xu, L. Feng, Magnetically recoverable efficient demulsifier for water-in-oil emulsions, ChemPhysChem 16 (2015) 595e600. [9] L. Zhang, J. Wu, Y. Wang, Y. Long, N. Zhao, J. Xu, Combination of bioinspiration: a general route to superhydrophobic particles, J. Am. Chem. Soc. 134 (2012)
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