Article pubs.acs.org/IECR
Preparation of Porous Polysulfone Microspheres and Their Application in Removal of Oil from Water Qingbo Yu,†,§ Yulun Tao,† Yiping Huang,† Zhongqing Lin,‡ Yonglong Zhuang,‡ Lanlan Ge,† Yuhua Shen,*,† Miao Hong,† and Anjian Xie*,† †
School of Chemistry and Chemical Engineering and ‡Modern Experiment Technology Center, Anhui University, Hefei 230039, PR China § Department of Materials Science and Engineering, Anhui University of Science and Technology, Huainan 232001, PR China S Supporting Information *
ABSTRACT: The monodisperse porous polysulfone (PSF) microspheres with hollow core/porous shell structure were prepared by a water-in-oil-in-water (W/O/W) emulsion solvent evaporation method. The morphology of PSF is investigated by using three different surfactants such as oleic acid, polyvinylpyrrolidone and polyoxyethylen(20)-sorbitanmonooleat. The prepared microspheres are developed as sorbents to remove oil from water due to their highly hydrophobic and superoleophilic properties. The PSF microspheres synthesized in the presence of oleic acid exhibit the best separation efficiency, which is 44.8 times higher than that of the pristine PSF powder. The microspheres with appropriate size, unsinkable properties, and excellent reproducibility can be quickly distributed and collected in seconds on the surface of water. The pore structure of PSF microspheres and interaction between oil and PSF are proposed to explain the high efficiency. technique20−23 has been reported extensively for the preparation of polymer microspheres. However, it has not been used to prepare porous mocrospheres. Here we report the fabrication of porous microspheres through the emulsion solvent evaporation technique. And, the oil sorption and oil−water separation of the microspheres are evaluated.
1. INTRODUCTION With increasing oil spill accidents, there is a growing demand for the materials capable of removing various forms of oil spills from water. The widely used absorbent materials1−7 include high-surface-area carbons, membranes, highly porous materials, superhydrophobic and uperoleophilic materials, etc. Among all those conventional materials, activated carbon is one of the most efficient materials because of its high surface area, abundant porosity, and sustainability.8 However, its development has disadvantages such as difficulty in collection owing to small particle size, high regeneration temperatures, and low separation efficiency caused by the coadsorption of water.9 Some membranes show a high adsorption capacity, but they are unsuitable for the cleanup of large-area oil spills because of poor distribution ability.3 Therefore, there is still an urgent call for a novel and efficient material with an appropriate size, high porosity, and superhydrophobic and superoleophilic properties. As a promising candidate, polysulfone (PSF) has superhydrophobic and superoleophilic properties. Our aim is to synthesize micrometer-sized PSF spheres with a hollow core/ porous shell structure. Because of their low density, appropriate size, and high specific surface, such materials are useful in removing oil from water. Hollow spherical materials have attracted a considerable amount of attention. Various synthetic approaches,10−15 such as hard and soft colloidal templating, layer-by-layer deposition, template-free synthesis and more recently microfluidics and particle-stabilized emulsions, have been developed to fabricate them. Recently, tailor-made hollow spheres, including tin oxide nanoparticle microshells,16 silica hollow spheres and microballons,17 silica nanocages18 and poly(methyl methacrylate) (PMMA) microspheres19 with hollow core/porous shell structure, have been developed with these techniques. As a convenient procedure, the emulsion solvent evaporation © 2012 American Chemical Society
2. EXPERIMENTAL SECTION 2.1. Materials. PSF was provided by Solvay Advanced Polymers. Poly(vinyl pyrollidone) (PVP), poly(vinyl alcohol) (PVA), oleic acid, polyoxyethylen(20)-sorbitanmonooleat, methylene chloride, and 1-methyl-2-pyrrolidinone (NMP) were all analytical-grade reagents (Shanghai Chemical Reagent Co. Ltd., China). The other reagents were used as received without further purification. Deionized water was used throughout the experiment to prepare solution. 2.2. Preparation of Three Kinds of PSF Microspheres. The oleic acid (0.1 g) was dissolved in 5 mL of methylene chloride. To that mixture, 0.3 mL of PVA (0.1%) solution was added under stirring for 5 min at room temperature (20 °C) to form a pre-emulsion. To this pre-emulsion was slowly added 0.1 g of PSF dissolved in 1 mL of NMP. This mixture was stirred for 2 h at room temperature (20 °C) and then slowly added into 50 mL of PVA (0.1%) solution. The system was warmed for 2 h at 35 °C. After solvent evaporation, the precipitate was filtered and washed using deionized water and Received: Revised: Accepted: Published: 8117
December 26, 2011 May 4, 2012 May 15, 2012 May 15, 2012 dx.doi.org/10.1021/ie203028h | Ind. Eng. Chem. Res. 2012, 51, 8117−8122
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Figure 1. SEM (a,b) and TEM (c,d) images of PSF-1 microspheres, inset shown in Figure 1b is a broken PSF-1.
Figure 2. SEM images of PSF-2 (a,b) and PSF-3 (c,d).
ethanol (40 °C), respectively, and finally dried under vacuum. The synthesized products were designated as PSF-1. To understand the influence of different surfactants on the morphology of PSF, a set of experiments were performed with another two types of surfactants such as PVP and polyoxyethylen(20)-sorbitanmonooleat, while the other synthetic parameters were unchanged. The synthesized products were designated as PSF-2 and PSF-3, respectively.
2.3. Procedure for Oil Absorption Tests. All tests were performed at room temperature (20 °C). To analyze the oil sorption capacity of PSF microspheres, they were dispersed into the motor oil, and the microspheres absorbed the oil quickly. The oil-absorbed microspheres were then separated from oil. The motor oil was removed from the surface of the microspheres by ultrasonically washing in ethanol for 10 min. After being dried at vacuum, the microspheres could be reused. 8118
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Figure 3. Shape of a water drop on the surfaces of PSF-3 (a), PSF-2 (b) or PSF-1 (c).
Figure 4. An oil drop on the surfaces of various samples: PSF-3 (a), PSF-2 (b), and PSF-1 (c).
Figure 5. (a) Maximum oil−adsorption capacity of various samples (the pristine PSF powder was denoted by PSF-0); (b) SEM image of PSF-1 after absorbing motor oil.
measurements were taken with a Hitachi S4800 scanning electron microscope. Transmission electronic microscopy (TEM) was performed by using a JEM-2100F instrument with a field emission gun operating at 200 kV. Fourier transformation infrared (FTIR) spectra were obtained using a NEXUS-870 spectrophotometer.
The oil-adsorption capacity of the microspheres was calculated by the formula as follows: k = (m2 − m1)/(m1)
(1)
where k is the sorption capacity (g/g), and m1 and m2 are the weight of the microspheres before and after oil absorbance, respectively. For determination of the oil/water selectivity, the highly hydrophobic microspheres were scattered on a layer of superfluous motor oil on water surface. After sorption, the oil-absorbed microspheres were then separated from the water surface using a homemade net. The oil-absorption capacity was determined by eq 1. 2.4. Characterization. Water contact angles (CAs) measurements were performed using an optical video contact angle instrument (model OCA 40, Dataphysics, Germany) at room temperature. Before measurements, the microspheres were placed on a slide and pressed into a flat film. A deionized water droplet or a motor oil droplet was used as the indicators. Field emission scanning electron microscopy (FESEM)
3. RESULTS AND DISCUSSION Figure 1 shows the SEM and TEM images of the PSF microspheres fabricated by W/O/W. From Figures 1a,b, we can see clearly that the microspheres are monodisperse with the diameter ranging from 22 to 31 μm when oleic acid is used as the surfactant. In addition, the shell of microspheres is porous with diameters in the range of 1−3 μm. As shown in the inset of Figure 1b, the PSF microspheres possess not only interconnected pores but also hollow core structure. Figures 1 panels c and d depict the TEM images of the PSF-1 at the edge and center section. The results further confirmed the hollow core/porous shell structure. 8119
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To further clarify the effects of surfactants on the formation of microspheres, a set of experiments were performed by using PVP and polyoxyethylen(20)-sorbitanmonooleat as surfactants. In the presence of PVP, PSF microspheres (PSF-2) have a smooth surface with a few pores (Figure 2a). Moreover, the average pore diameters decrease to about 900 nm (see Figure 2b). When polyoxyethylen(20)-sorbitanmonooleat is used, only a few PSF microspheres (PSF-3) are obtained (Figure 2c), and the surface of the microspheres is rugged (Figure 2d). It is obvious that the types of surfactants have significant impacts upon the morphology. Similar phenomenon was reported in the preparation of poly(lactic-co-glycolic acid) (PLGA) microspheres.24 The surface porosity could be controlled by the used surfactants. The hollow core structure may be attributed to the stability of surfactant. Nihant25 et al. found that the internal structure of microparticles could be changed from a multivesicular to a matrix-like structure depending on the stability of surfactant. So the morphology of PSF microspheres with hollow core/porous shell structure can be controlled by varying the types of surfactants. Figure 3 displays water contact angles on the surfaces of various products. There is no obvious difference among the three kinds of microspheres. The water contact angles range from 150° to 160°(see Supporting Information, videos S1−S3. The oil contact angles of the three kinds of microspheres are close to zero (Figure 4). In particular, the video of a motor oil droplet on the surface of various products can be seen in Supporting Information, videos S4−S6. The motor oil quickly spread on the surfaces of the highly hydrophobic microspheres. The wetting time for an oil droplet added to the microspheres is found to be 4 s, further indicating their superoleophilic properties. The sorption test was performed in oil medium without any water to investigate the maximum oil sorption capacity of PSF microspheres. For comparison, the sorption of the pristine PSF powder was also studied. Figure 5a shows the oil-adsorption capacities of PSF-0, PSF-1, PSF-2, and PFS-3, calculated by eq 1, respectively. The sorption capacity of pristine PSF powder is 5.5 g/g. For the PSF-2 and PSF-3, the absorption capacities are higher (12.5 and 11.7 g/g, respectively). It should be noted that the sorption capacity of PSF-1 is 32.9 g/g which is six times that of the pristine PSF powder. Figure 5b shows the SEM image of the oil-saturated PSF-1. It is obvious that the pores are filled with oil, and the oil adheres to the surface of the micospheres. Figure 6 shows the FTIR spectra of PSF microspheres, motor oil, and the oil-absorbed microspheres. The spectrum of Figure 6a reveals the bands at 1321 and 1188 cm−1, attributed to symmetric and asymmetric stretching vibrations of SO bonds in the polysulfone backbone. The band corresponding to the stretching vibration of C−O−C structures appears around 1264 cm−1. In addition, the sharp bands at 1620, 1562, and 1403 cm−1 are attributed to a benzene ring. The characteristic peaks of motor oil are shown in Figure 6b and remain unchanged in Figure 6c. However, the characteristic peaks (ν(SO), ν(C− O−C)) of PSF shift slightly (Figure 6c). The band for the benzene ring, 1562 cm−1, has disappeared. A new band can be observed at 964 cm−1. The above changes of FTIR spectra indicate the interaction between PSF microspheres and motor oil. According to the above results, the highly hydrophobic and superoleophilic properties for the PSF microspheres with three kinds of morphologies are not much different. However,
Figure 6. FTIR spectra of PSF microspheres (a), motor oil (b), and the oil-absorbed microspheres (c).
experimental results in oil sorption indicate that the morphology affects oil sorption capacity. The hollow core/ pore shell structure with the width of interconnected pores changing in 1−3 μm is favorable in oil sorption. The same phenomena were observed by Zhu.1 High porosity and appropriate void size of sorbent provided large amounts of storage volume for oil, acting as an oil reservoir, which was the key for the high adsorption capacity. In addition, the hollow core/porous shell structure and the oleophilic-hydrophobic properties lead to higher buoyancy. In fact, it is an important parameter for the oil sorption and removing from the spilled area. As a kind of sorbent for cleaning oil from water surface, Figure 7 displays the process. An oil droplet labeled with dye was dropped onto the middle of the water surface (Figure 7a), and then PSF-1 was scattered on the surface. The motor oil was absorbed quickly, as shown in Figure 7b. In addition, PSF-1 floated over the water surface before and after the oil sorption (from the shadow in Figure 7b). More interesting, the oil-absorbed microspheres could be easily removed using a homemade net owing to the appropriate size (Figure 7c), and the oil-free water was obtained (Figure 7d). The oil-adsorption capacities of PSF-1 and pristine PSF powder were investigated in a layer of superfluous motor oil on the water surface; the maximum capacity of PSF-1 is 33.6 g/g. Considering the experimental error, there is no difference in absorption capacities for pure oil and oil/water mixture systems. However, for pristine PSF powder, the maximum absorption capacity is 0.75 g/g. Thus, compared with pristine PSF powder, PSF-1 presents excellent oil/water selectivity (44.8 times) due to its higher buoyancy. The recyclability of PSF-1 is shown in Figure 8a. After three cycles, the PSF-1 can absorb motor oil up to 14.2 times of selfweight. The SEM image of PSF-1 after ultrasonic washing in ethanol is shown in Figure 8b. It is clear that most of the pores are vacant, implying potential application in recycling. However, some pores still are adhered, which induce the lower absorption capacity after three cycles.
4. CONCLUSION In summary, the porous PSF microspheres with hollow core/ porous shell structure have been prepared by the emulsion solvent evaporation technique. The porous PSF microspheres show high oil sorption ability and excellent oil−water 8120
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Figure 7. Removal of motor oil from water surface: (a) motor oil labeled by dye on the water surface; (b) PSF-1 sample on the surface of oil over the water; (c) the oil-saturated micospheres were moved away using a homemade net; (d) oil-free water was obtained.
Figure 8. (a) Oil-adsorption capacity of the PSF-1 treated by ultrasonic washing as three cycles; (b) SEM image of PSF-1 after ultrasonic washing. (2) Bayat, A.; Aghamiri, S. F.; Moheb, A.; Vakili-Nezhaad, G. R. Oil spill cleanup from sea water by sorbent materials. Chem. Eng. Technol. 2005, 28 (12), 1525. (3) Zhu, Q.; Tao, F.; Pan, Q. M. Fast and selective removal of oils from water surface via highly hydrophobic core−shell Fe2O3@C nanoparticles under magnetic field. ACS Appl. Mater. Interfaces 2010, 2 (11), 3141. (4) Zhang, Y. L.; Wei, S.; Liu, F. J.; Du, Y. C.; Liu, S.; Ji, Y. Y.; Yokoi, T.; Tatsumi, T.; Xiao, F. S. Superhydrophobic nanoporous polymers as efficient adsorbents for organic compounds. Nano Today 2009, 4 (2), 135. (5) Li, A.; Sun, H. X.; Tan, D. Z.; Fan, W. J.; Wen, S. H.; Qing, X. J.; Li, G. X.; Li, S. Y.; Deng, W. Q. Superhydrophobic conjugated microporous polymers for separation and adsorption. Energy Environ. Sci. 2011, 4 (6), 2062. (6) Adebajo, M. O.; Frost, R. L.; Kloprogge, J. T.; Carmody, O.; Kokot, S. Porous materials for oil spill cleanup: A review of synthesis and absorbing properties. J. Porous Mater. 2003, 10, 159. (7) Geise, G. M.; Lee, H. S.; Miller, D. J.; Freeman, B. D.; Mcgrath, J. E.; Paul, D. R. Water purification by membranes: The role of polymer science. J. Polym. Sci., Part B: Polym. Phys. 2010, 48, 1685. (8) Fuertes, A. B.; Marban, G.; Nevskaia, D. M. A desorption of volatile organic compounds by means of activated carbon fibre-based monoliths. Carbon 2003, 41, 87. (9) Sabio, E.; Gonzalez, E.; Gonzalez, J. F.; Gonzalez-Garcia, C. M.; Ramiro, A.; Ganan, J. Thermal regeneration of activated carbon saturated with p-nitrophenol. Carbon 2004, 42 (11), 2285. (10) Akartuna, I.; Tervoort, E.; Studart, A. R.; Gauckler, L. J. General route for the assembly of functional inorganic capsules. Langmuir 2009, 25 (21), 12419. (11) Xu, X. L.; Asher, S. A. Synthesis and utilization of monodisperse hollow polymeric particles in photonic crystals. J. Am. Chem. Soc. 2004, 126 (25), 7940. (12) Frank, C.; Rachel., A. C.; Helmuth, M. Nanoengineering of inorganic and hybrid hollow spheres by colloidal templating. Science 1998, 282, 1111.
separation efficiency. In particular, the microspheres exhibit unsinkable property, appropriate size, and recyclability, which are important in practical applications. We believe that the present work provides new insight into fabricating low-cost and highly efficient oil sorbents, which will offer important opportunities in the cleanup of oil spill and the removal of organic pollutants on water surface.
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ASSOCIATED CONTENT
S Supporting Information *
Videos as described in the text. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +86-551-5108090. Fax: +86-551-5107342. E-mail: s_
[email protected];
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is supported by the National Science Foundation of China (91022032, 31070730, 21171001, 50973001 and 21173001), the Important Project of Anhui Provincial Education Department (ZD2007004-1), Key Laboratory of Environment-friendly Polymer Materials of Anhui Province.
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REFERENCES
(1) Zhu, H. T.; Qiu, S. S.; Jiang, W.; Wu, D. X.; Zhang, C. Y. Evaluation of electrospun polyvinyl chloride/polystyrene fibers as sorbent materials for oil spill cleanup. Environ. Sci. Technol. 2011, 45 (10), 4527. 8121
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(13) Lou, B. X. W.; Wang, Y.; Yuan, C. l.; Lee, J. Y.; Archer, L. A. Template-free synthesis of SnO2 hollow nanostructures with high lithium storage capacity. Adv. Mater. 2006, 18, 2325. (14) Nie, Z. H.; Xu, S. Q.; Seo, M.; Lewis, P. C.; Kumacheva, E. Polymer particles with various shapes and morphologies produced in continuous microfluidic reactors. J. Am. Chem. Soc. 2005, 127 (22), 8058. (15) Thomas, S.; Glenn, H.; Hafeli, U. O. Effects of chemical and physical parameters in the generation of microspheres by hydrodynamic flow focusing. Colloid Surf. B 2011, 87, 361. (16) Martinez, Carlos J.; Bernard, H.; Montgomery, Christopher B.; Steve, S. Porous tin oxide nanostructured microspheres for sensor applications. Langmuir 2005, 21, 7937. (17) Zoldesi, B. C. I.; Imhof, A. Synthesis of monodisperse colloidal spheres, capsules, and microballoons by emulsion templating. Adv. Mater. 2005, 17, 924. (18) Lou, X. W.; Yuan, C.; Zhang, Q.; A., L. Archer platinumfunctionalized octahedral silica nanocages: Synthesis and characterization. Angew. Chem. Int. Ed 2006, 45, 3825. (19) He, X. D.; Ge, X. W.; Liu, H. R.; Wang, M. Z.; Zhang, Z. C. Synthesis of cagelike polymer microspheres with hollow core/porous shell structures by self-assembly of latex particles at the emulsion droplet interface. Chem. Mater. 2005, 17 (24), 5891. (20) Zambaux, M. F.; Bonneaux, F.; Gref, R.; Maincent, P.; Dellacherie, E.; Alonso, M. J.; Labrude, P.; Vigneron, C. Influence of experimental parameters on the characteristics of poly(lactic acid) nanoparticles prepared by a double emulsion method. J. Controlled Release 1998, 50, 31. (21) Yang, Y. Y.; Chung, T. S.; Bai, X. L.; Chan, W. K. Effect of preparation conditions on morphology and release profiles of biodegradable polymeric microspheres containing protein fabricated by double-emulsion method. Chem. Eng. Sci. 2000, 55, 2223. (22) Chung, T. S. The limitations of using Flory−Huggins equation for the states of solutions during asymmetric hollow-fiber formation. J. Membr. Sci. 1997, 126, 19. (23) Jeyanthi, R.; Thanoo, B. C.; Metha, R. C.; Deluca, P. P. Effect of solvent removal technique on the matrix characteristics of polylactide/ glycolide microspheres for peptide delivery. J. Controlled Release 1996, 38, 235. (24) Bouissou, C.; Potter, U.; Altroff, H.; Mardon, H.; Christopher, V. D. W. Controlled release of the fibronectin central cell binding domain from polymeric microspheres. J. Controlled Release 2004, 95 (3), 557. (25) Nihant, N.; Schugens, C.; Grandfils, C.; Robert, J.; Philippe, T. Polylactide microparticles prepared by double emulsion/evaporation technique. I. Effect of primary emulsion stability. Pharm. Res. 1994, 11, 1479.
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