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Robust magnetic polystyrene foam for high efficiency and removal oil from water surface
Accepted Manuscript Robust magnetic polystyrene foam for high efficiency and removal oil from water surface Liuhua Yu, Gazi Hao, Lei Xiao, Qiushi Yin, Mengting Xia, Wei Jiang PII: DOI: Reference:
S1383-5866(16)31713-0 http://dx.doi.org/10.1016/j.seppur.2016.09.022 SEPPUR 13238
To appear in:
Separation and Purification Technology
Received Date: Revised Date: Accepted Date:
5 January 2016 13 June 2016 14 September 2016
Please cite this article as: L. Yu, G. Hao, L. Xiao, Q. Yin, M. Xia, W. Jiang, Robust magnetic polystyrene foam for high efficiency and removal oil from water surface, Separation and Purification Technology (2016), doi: http:// dx.doi.org/10.1016/j.seppur.2016.09.022
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Robust magnetic polystyrene foam for high efficiency and removal oil from water surface Liuhua Yu, Gazi Hao, Lei, Xiao, Qiushi Yin, Mengting Xia, and Wei Jiang* National Special Superfine Powder Engineering Research Center of China, Nanjing University of Science and Technology, Nanjing 210094, PR China
Abstract In this study, a robust magnetic polystyrene foam with selective absorption ability was successfully fabricated via a low-cost method. Owing to its highly hydrophobic and superoleophilic properties, it could selectively absorb a variety of oils and organic solvents from a polluted water surface. The robust magnetic polystyrene foam had a large absorption capacity up to 17.83 times of its own weight and could be separated from the surface of water by magnetic technology. More importantly, the magnetic polystyrene foam could still retain a high absorption capacity and water contact angle for oil-water separation after the 10th cycle. In addition, the magnetic polystyrene foam could be reused through a simple squeeze treatment. High absorption capacity, special selectivity and excellent recyclability promise the material great potential for the cleanup of oils and organic solvents from water surface.
Keywords: PS, Selectivity, Magnetic, Absorption, Environment friendly
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1. Introduction With the acceleration of global economic integration, the development of water transportation and the continuous development of shallow marine oil, the oil spills exhibit a rising tendency [1]. Oil leakage accidents bring us great economic loss and have devastating effects on the marine ecological environment. What’s worse, it also has adverse effects on human health [2,3]. Therefore, the oil pollution becomes an urgent problem to be solved, which has important significance to the sustainable development of human and society existence. From the last century, people have attached great importance to the oil spill accidents, and taken a series of methods to remove oils from waste water [4-7]. Due to use safety, low cost and environmental friendliness, adding absorbents is considered to be an effective way [8]. To our best knowledge, oil absorption material can be divided into two kinds, namely traditional oil absorption materials and novel oil absorption materials. Traditional oil absorption materials such as zeolites [9], graphite [10], and straw [11], etc. [12] have low oil absorption capacity, poor selectivity, and non-recyclability, which hardly meet the requirement for disposing the large-scale oil spills. Therefore, it is necessary to invent new oil absorption materials, which contain nano composites of powder [13], two-dimensional structure of fiber and film [14-16], as well as the three-dimensional network structure of the bulk materials [17-24]. Three-dimensional porous materials have great application prospect in oil absorption, due to their high porosity, large specific surface area and adjustable surface properties [25-31]. However, most of these bulk absorbents were prepared 2
with expensive/toxic raw materials or consumed much time/energy via sophisticated procedures, making their large-scale production difficult [19,22-24,32]. Therefore, it’s highly imperative to develop absorbents with low-cost fabricating procedure, available raw materials and excellent cyclicity. Commercial foam is an available raw material, which endows some specific properties to the substrate, enhancing its absorption property. Considering that magnetic materials were easily manipulated under the magnetic field, many researchers magnetized materials by deliberate incorporation of suitable magnetic particles. Fe3O4 particles are greatly attractive as magnetic recycling agent due to their inherent low toxicity and strong magnetism [25,26]. As a kind of typical polymer adsorption material, polystyrene has caught large interest of researchers on account of its low-cost, stable chemical property, highly hydrophobicity and superoleophilicity [13,33,34]. The introduction of PS into magnetic foam with a three dimensional network structure can not only enhance the thermal stability of the foam but also improve absorption capacity. In the current study, we successfully produced a robust magnetic polystyrene foam with anti-compressible, oil absorptive and reusable characteristics. In this paper, the preparation of magnetic Fe3O4 particles coated by oleic acid was reported in our previous work [35]. The magnetic particles were introduced into the foam through ultrasonic dispersion method, and then the highly hydrophobic magnetic polystyrene foam was synthesized by emulsion polymerization, which is one of the most widely applied industrial polymerization processes around the world [36]. The fabrication method was simple and low-cost without using expensive reaction apparatus, high 3
temperature and high pressure, and long reaction time. Furthermore, the as-prepared magnetic polystyrene foam had the following advantages: first, the robust foam had a high absorption capacity for several kinds of oils and organic solvents. Second, the robust foam could be recycled under the action of the magnetic field, the absorption of oils and organic solvents could be easily collected through a simple mechanical squeeze. Third, the robust foam could retain high absorption capacity and water contact angle for oil-water separation after the 10th cycle. All the results reveal that the robust magnetic polystyrene foam has high potential application in oil removal from water surface.
2. Experimental Section
2.1 Materials and Chemicals The following chemicals were used in the experimental work: shock absorption foam with density of 0.045g cm-3 and a void volume of about 94.2% (Aladdin), divinylbenzene (Aladdin), FeCl3·6H2O (Sinopharm Chemical Reagent Co. Ltd., Shanghai), NaAc (Sinopharm Chemical Reagent Co. Ltd., Shanghai), styrene (Sinopharm Chemical Reagent Co. Ltd., Shanghai), sodium dodecyl sulfonate (Sinopharm Chemical Reagent Co. Ltd., Shanghai), absolute ethanol (Nanjing Chemical Reagent Co. Ltd., Nanjing), oleic acid (Shanghai No.4 Reagent & HV Chemical Co. Ltd., Shanghai), ethylene glycol (Shanghai No.4 Reagent & HV Chemical Co. Ltd., Shanghai), azobisisobutyronitrile (Shanghai No.4 Reagent & HV 4
Chemical Co. Ltd., Shanghai), polyethylene glycol 4000 (Xilong Chemical Reagent Co. Ltd., Shantou).
2.2 Sample preparation 2.2.1 Preparation of Fe3O4 particles coated by oleic acid Fe3O4 particles were prepared by solvent thermal method which we reported previously [35]. A typical experiment for synthesis of Fe3O4 particles coated by oleic acid (OA-Fe3O4) was as follows: 30 mg of as-prepared Fe3O4 particles were dispersed into 10 mL of absolute enthanol by ultrasonication for 10 min until forming a black precursor solution. The precursor solution mixed with 1.2 mL of oleic acid was added to the three-neck flask with a mechanical stir for 2 h at room temperature. The prepared OA-Fe3O4 particles were purified by absolute ethanol and deionized water for three times in turn.
2.2.2 Preparation of the robust magnetic polystyrene foam The as-prepared magnetic polystyrene foams used Fe3O4 particles (MPF) and OA-Fe3O4 particles (OA-MPF) as magnetic recycling reagents. Fig. 1 is a schematic demonstration of preparation of MPF and OA-MPF. In a typical synthesis, shock absorption foam (3cm×1.5cm×1.5cm) was cleaned in deionized water and acetone with strong mechanical stirring at 50 °C for 4 h successively. Magnetic foam was obtained by immersing the pure foam in a homogeneous mixture of Fe3O4 particles or OA-Fe3O4 particles for 3 h under ultrasound (Fig. 1A, 1B). Then 600 mg of sodium 5
dodecyl sulfonate and magnetic foam were sequentially added into a three-neck flask stirring at the temperature of 75 °C in N2 atmosphere. Subsequently, 3 mL of styrene, 0.3 mL of divinylbenzene and 30 mg of azobisisobutyronitrile were slowly injected into the mixture solution and reacted for 6 h at the setting temperature. Finally, products were washed by absolute ethanol and deionized water to remove liquid and then dried at 60 °C to get the robust magnetic polystyrene foam (Fig. 1C).
Fig. 1 Illustration of the fabrication process of MPF and OA-MPF.
2.3 Characterization The chemical composition of the as-prepared materials was confirmed by X-ray diffraction (XRD, Bruker D8 Super Speed) with Kα radiation and the scanning angle ranged from 20° to 80° of 2θ at 40 kV. Field-emission scanning electron microscopy 6
(SEM, Model-S4800, Hitachi, Japan) were used to observe the surface morphology and structure of the magnetic polystyrene foam. Before SEM observation, all samples were fixed on aluminum stubs and coated with gold. The energy-dispersive X-ray spectroscopy (EDS) measurements were performed on an OXFORD INCA Energy Dispersive Spectrometer. The functional groups in the samples were examined by Fourier transform infrared spectroscopy (FTIR) in the range of 4000-500 cm-1 using KBr pellet technique on a Bruker Vector 22 spectrometer. Thermal gravimetric analysis (TGA) was carried out by Model TA2100 (TA Instruments, USA). The experiment was performed in the range from 50 °C to 650 °C at a heating rate of 10 °C/min under nitrogen atmosphere. Water contact angle (CAs) was determined by using a Drop Shape Analyzer SL200B (CAs, SL200B, Solon Tech. Co. Ltd., China) with a water droplet of 6 μL. The magnetic properties of the magnetic polystyrene foam was investigated in fields between ±8 kOe by a vibrating sample magnetometer (VSM, LakeShore 735). All the measurements were performed at room temperature.
2.4 Testing the effect of absorption capacity and reusability A piece of MPF and OA-MPF was immersed in oils and organic solvents at room temperature. Once equilibrium was attained, the sample was removed from the water surface using a magnet bar or a tweezer. The absorption capacity, K, of the magnetic polystyrene foam was calculated by weighing the sample before and after absorption and determined according to the following equation: K=(Ma-Mb)/Mb
(1) 7
where Ma and Mb represent the weights of the wet sample with oils or organic solvents (g) and the weight of the dry sample (g), respectively. The extrusion method was used to remove the absorbed oils and organic solvents. The magnetic polystyrene foam could be regenerated after drying in a vacuum oven directly. The reusability of magnetic polystyrene foam was evaluated by water contact angle measurements and measuring absorption capacity after many absorption and desorption cycles. All the experiments were repeated for five times.
3. Results and Discussion 3.1 Characterizations of the robust magnetic polystyrene foam The white pure foam turned into completely black after ultrasonication treatment, implying that the magnetic foam was successfully obtained. After a series of reaction, MPF and OA-MPF with three dimensional network structure were achieved, as shown in Fig. S1. Fe3O4 particles or OA-Fe3O4 particles in the foam could be tested by XRD measurements. All the diffraction peaks were indexed to the magnetic Fe3O4 phase. The reflections peaks of the as-prepared samples at 30.1°, 35.7°, 43.1°, 57.2° and 62.6° were ascribed to the (220), (311), (400), (511) and (440) planes of Fe3O4, which were similar to these characteristic peaks in the standard pattern (JCPDS file No. 19-0629). From the patterns, it was obvious that the peaks in curve a, curve b, curve c and curve d were similar, indicating that the crystal form of Fe3O4 particles in MPF and OA-MPF had not been changed, as shown in Fig. 2. Fe3O4 and PS were further characterized by energy-dispersive X-ray spectroscopy (EDS) to investigate the 8
surface of the existing elements. As it could be observed, the presence of Fe and O elements along with original C which were identified on pure foam were detected on MPF and OA-MPF (Fig. S2).
Fig. 2. XRD patterns for (a) Fe3O4, (b) OA-Fe3O4, (c) MPF, and (d) OA-MPF.
The pure foam possessed a smooth surface and a three dimension porous structure, which could be proved from the SEM images in Fig. 3A. Clearly, the foam was composed of many interconnected micropores with a diameter of about 500 μm and the smooth surface which was changed to be very rough after modification with Fe3O4 particles and OA-Fe3O4 particles, as illustrated in Fig. 3B and 3D. The Fe3O4 particles and OA-Fe3O4 particles were anchored onto the surface of foam, which could be seen through magnification in Fig. 3C and 3E. While Fig. 3F and Fig. 3H exhibited that the PS layer was linked to the surface of Fe3O4 particles and OA-Fe3O4 9
particles through the Fe-O-C bond. The surface of MPF and OA-MPF showed hierarchical roughness (Fig. 3G and 3J). The PS layer not only made MPF or OA-MPF highly hydrophobic, but also further immobilized Fe3O4 particles or OA-Fe3O4 particles. None of the pores inside MPF and OA-MPF were blocked, which was helpful to the fast intake of oils and organic solvents. The SEM observations also displayed that a series of chemical reactions did not damage the foam. In order to investigate the stability of the magnetic polystyrene foam after modification with magnetic particles and polystyrene layer, we carried out the ultrasonic experiment and extrusion experiment. The magnetic polystyrene foam was ultrasonically treated in absolute ethanol and deionized water, and there were no particles and no color change in the solution (Fig. S3). Furthermore, the magnetic polystyrene foam could still remain its original shape after extrusion without any particles and PS layer exfoliation, as exhibited in Fig. 4(A-F).
10
Fig. 3. SEM images of pure foam, MPF and OA-MPF at different magnification.
Fig. 4. Compression recovery process of the magnetic polystyrene foam.
Fig. 5 showed FTIR spectra of pure foam, Fe3O4, OA-Fe3O4, MPF and OA-MPF. Obviously, peaks at 2984.4, 2926.2 and 1474.9 cm-1 should be attributed to C-H stretching of methyl or methylene, respectively (curve a). The absorption peak around 11
662.0 cm-1 could be assigned to the stretching vibration of the Fe-O bond (curve b). The peak which was exhibited in curve c at 1183.2 cm-1 could be ascribed to the presence of oleic acid and the absorption peaks belonged to PS at 879.8, 1236.4 and 3712.5 cm-1 [37]. However, in spite of absorption peaks of PS, two characteristic peaks at 1066.0 and 1405.9 cm-1 were corresponding to the characteristic of C-O-C bonds and stretching vibration of C=O bond, respectively (Fig. 5 curve c, curve d) [38]. All these characteristic peaks also appeared in OA-MPF (curve e), indicating that OA-Fe3O4 particles and PS layer were attached on the surface of pure foam.
Fig. 5. FTIR spectra for (a) Fe3O4, (b) OA-Fe3O4, (c) MPF, and (d) OA-MPF.
The thermal stability of pure foam, MPF and OA-MPF was investigated by thermogravimetic analysis. According to Fig. 6, the oleic acid coating ratio of Fe3O4 particles and PS layer coating ratio of the magnetic foam could also be examined 12
performed in a nitrogen flow. The difference in residue contents indicated that about 16.5 wt% and 20.3 wt% of the PS layer was introduced onto the surface of MPF and OA-MPF, respectively. The increase of PS coating ratio could be ascribed to the surface of OA-MPF which was modified by OA-Fe3O4 particles based on the theory of “similarity and intermiscibility” [39]. Identically, about 6.4 wt% of mass defect between 480 C and 615 C in curve b was attributed to the oleic acid coating ratio. Besides, MPF and OA-MPF could sustain a temperature up to 270 C, indicating that as-prepared foams with good thermal stability could be used for practical applications in the harsh environment with high temperature.
Fig. 6. TGA curves of (a) Pure foam, (b) OA-MPF and (c) MPF.
In addition, MPF and OA-MPF could be easily manipulated by using a magnet, as exhibited in the inset of Fig. 7 and Video S1. The magnetic property of the foam 13
after modified by Fe3O4 particles, OA-Fe3O4 particles and PS was recorded at room temperature. As was shown in Fig. 7, all the samples exhibited standard paramagnetic characteristic curves with no hysteresis due to the bulk foam embedded by superparamagnetic Fe3O4 particles. The pure foam became magnetic with a saturation magnetization of 29.1 emu/g and 24.3 emu/g after incorporation of Fe3O4 particles and OA-Fe3O4 particles, respectively. It could be obviously seen that the saturation magnetization of MPF and OA-MPF decreased to 18.4 emu/g and 16.3 emu/g, respectively. However, the decrease in the saturation magnetization of MPF and OA-MPF was reasonable. As was well known, the thicker coating layer was, the weaker magnetic strength became [35].
Fig. 7. Room-temperature magnetization curves of (a) Fe3O4, (b) OA-Fe3O4, (c) MPF, and (d) OA-MPF. The inset are two images showing the magnetic property of MPF and OA-MPF. 14
3.2 Wettability and absorption capacity of the robust magnetic polystyrene foam The wettability of the as-prepared material was carried out by measuring the contact angles of a water droplet and a lubricating oil droplet deposited on the surface of MPF and OA-MPF, as shown in Fig. 8. The apparent water contact angle of MPF and OA-MPF were 138.5°±1.5° and 141.5°±1.5°, respectively (Fig. 8A, 8B). It was obvious that the water contact angle of OA-MPF was higher than MPF, which may be attributed to the relatively lower surface energy of OA-Fe3O4 than Fe3O4. In contrast, when 6 μL lubricating oil droplet was added to the surface of MPF and OA-MPF, it was immediately absorbed with oil contact angle being about 0° (Fig. 8C, 8D), which confirmed that as-prepared foams absorbed oil quickly and repelled water completely. Interestingly, there was a thick layer of bubble on OA-MPF surface when it was immersed in water by external force, and the as-prepared foams could immediately floated on the water surface after release of the external force (Video S2). To our surprise, no water was found intaken by weighing method, demonstrating that the as-prepared foam was highly hydrophobic. What’s more, OA-MPF exhibited a highly hydrophobicity, even floating on 0.1 M NaCl solution and aqueous solutions with pH values ranging from 1 to 13 for 48 h, as shown in Fig. 9A.
15
Fig. 8. Photograph of water droplets on the surface of (A) MPF and (B) OA-MPF. Photograph of lubricating oil droplets on the surface of (C) MPF and (D) OA-MPF.
Fig. 9. (A) Contact angles of OA-MPF after floating on 0.1 M NaCl solution and aqueous solutions with pH values ranging from 1 to 13 for 48 h. (B) Absorption rate curve of OA-MPF for lubricating oil. 16
MPF and OA-MPF could be used for absorbing oils and organic solvents, which was an important feature for practical applications [40,41]. Employing lubricating oil (labeled by Sudan I for clarity) in the selective absorption experiment, the lubricating oil was quickly absorbed by dipping the foams in 60 s (Fig. 9B). OA-MPF could float on the water surface after absorption of the lubricating oil. Subsequently, the lubricating oil was completely removed from the mixture and the regenerated foam still remained its original shape when taking the foam out of the water surface through applying an external magnetic field or external force, as illustrated in Fig. 10 and Video S3.
Fig. 10. Selective absorption and collection of lubricating oil (dyed with Sudan I) on water surface (colorless) by OA-MPF.
As illustrated in Fig. 11A, we could know that MPF intake capacity K of diesel oil, lubricating oil, DMF, and octane could reach 9.96, 12.51, 9.46 and 10.42 g/g, respectively. The absorption capacities of OA-MPF for these four kinds of oils and organic solvents were 14.41, 17.83, 13.62 and 14.40 g/g, respectively. The increase of the absorption capacity was attributed to the more PS layer of OA-MPF 17
than MPF, which agreed well with TGA analysis. As expected, the absorbed oils could be readily removed from the as-prepared OA-MPF through a simple squeezing process due to its excellent mechanical flexibility and water was not seen in the collected lubricating oil, as shown in Fig. 10E.
3.2 Reusability of the robust magnetic polystyrene foam It was satisfactory that the recycled foams could be reused for oil-water separation for many times and the regeneration capacity of MPF and OA-MPF was investigated as an important factor for a promising absorption material. As it could be clearly observed in Fig. 11, the absorption capacity of MPF and OA-MPF for four kinds of oils and organic solvents still kept larger than 8.5 g/g and 12.5 g/g of their own weight even after 10th cycle, respectively. Moreover, the robust magnetic polystyrene foam could still remain a high water contact angle after many absorption/desorption cycles, as exhibited in Fig. 12. Owing to its robust and stable porous structure, OA-MPF had still managed to have an absorption capacity than 90% of its initial value after the 10th cycle. The decrease of absorption capacity was caused by the residual oils in the pores of the foam. Compared with various novel oil absorption absorbents in Table 1, the robust magnetic polystyrene foam with excellent absorption capacity, low cost and high repeatability has the possibility of practical application in oils and organic solvents absorption.
18
Fig. 11. Absorption capacity for four kinds of oils and organic solvents after 10 cycles of (A) MPF and (B) OA-MPF.
Fig. 12. Water contact angle of OA-MPF after 10 cycles.
19
Table 1 Comparison of various novel oil absorbing absorbents. Absorbents
Absorption capacity (g/g)
Cost
Reuse efficiency
Reference
PDMS sponge
12
High
Low
28
Silicone sponge
3.8
Low
High
26
Graphene foam
12
Very high
Low
25
rGPU sponge
150
High
Very low
27
RGO sponge
45
Very high
Low
43
Spongy graphene
20
Very high
Low
45
PUR foam
4.5
Low
High
44
Foam graphene
15
Very high
High
42
OA-MPF
17.83
Very low
Very high
This work
4. Conclusions Robust
magnetic
polystyrene
foam
with
excellent
hydrophobicity,
superoleophilicity and magnetism has been successfully fabricated through a cost-effective and environment-friendly method. The incorporation of Fe3O4 particles or OA-Fe3O4 particles allowed the oil-absorbed foam to be easily collected by a magnet and the collected oils could be squeezed out by external force due to the foam’s compressibility. Furthermore, the lubricating oil intake capacity can be up to 17.83 and 16.21 times of its own weight at the first and tenth oil absorption, respectively. Interestingly, the robust magnetic polystyrene foam can absorb different kinds of oils and organic solvents with high absorption capacity and still keep a high water contact angle even after 10 water-oil separation cycles. With the advantages of compressibility, selective absorptivity and high stability, which are very important to the bulk oil absorption materials, the robust magnetic polystyrene foam may have 20
significant applications for removing oils and organic solvents from water surface.
Associated Content
Supporting information Information on digital images, EDS spectra and location of the sample after ultrasonic; three videos showing the magnetic property, unsinkable property and oil absorption process of the samples.
Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Project No. 41101287), the Scientific and Technical Supporting Programs of Jiangsu province (BE2012758) and Priority Academic Program Development of Jiangsu Higher Education Institutions.
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application for oil-water separation. ACS Nano. 7 (2013) 6875-6883. [33] N. Zhang, W. Jiang, T.H. Wang, J.J. Gu, S.T. Zhong, S. Zhou, T. Xie, J.J. Fu. Facile Preparation of Magnetic Poly(styrene-divinylbenzene) Foam and Its Application as an Oil Absorbent. Ind. Eng. Chem. Res. 54(2015) 11033-11039. [34] J.Y. Lin, B. Ding, J.M. Yang, J.Y. Yu, G. Sun, Subtle regulation of the microand nanostructures of electrospun polystyrene fibers and their application in oil absorption, Nanoscale, 4 (2012) 176-182. [35] L.H. Yu, G.Z. Hao, J.J. Gu, S. Zhou, N. Zhang, W. Jiang, Fe3O4/PS magnetic nanoparticles: Synthesis, characterization and their application as sorbents of oil from waste water. J. Magn. Magn. Mater. 394 (2015) 14-21. [36] P.A. Lovell, M.S. El-Aasser(Eds.), Emulsion polymerization and emulsion polymers, Wiley, Chichester, 1997. [37] D.Z. Wu, X.W. Ge, Z.C. Zhang, M.Z. Wang, S.L. Zhang, Novel One-Step Route for Synthesizing CdS/Polystyrene Nanocomposite Hollow Spheres, Langmuir 20 (2004) 5192-5195. [38] H. Lv, Q. Lin, K. Zhang, K. Yu, T.J. Yao, X.H. Zhang, J.H. Zhang, Yang, B. Facile Fabrication of Monodisperse Polymer Hollow Spheres, Langmuir, 24 (2008) 13736-13741. [39] Y.F. Zhang, L. Ding, W. Zhao, Y.H. Zhang, F. Han, Q.B. Feng, J.L. Song, M.H. Li, B. Dua, Q. Wei, Hydrophobic bifunctionalized hexagonal mesoporous silicas as efficient adsorbents for the removal of Orange IV, RSC Adv. 4 (2014) 49783-49789. 26
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27
Graphical Abstract:
Fig. Absorption and desorption of the magnetic polystyrene foam.
28
Highlight: NO.1 A low-cost method was applied to the fabrication of the magnetic polystyrene foam. NO.2 The absorbed oils and organic solvents could be collected through a simple squeeze treatment. NO.3 The as-obtained foam has the properties of highly hydrophobicity, and compressibility, NO.4 The sample could retain a high absorption capacity and water contact angle after the 10th cycle.
29
S1383-5866(16)31713-0 http://dx.doi.org/10.1016/j.seppur.2016.09.022 SEPPUR 13238
To appear in:
Separation and Purification Technology
Received Date: Revised Date: Accepted Date:
5 January 2016 13 June 2016 14 September 2016
Please cite this article as: L. Yu, G. Hao, L. Xiao, Q. Yin, M. Xia, W. Jiang, Robust magnetic polystyrene foam for high efficiency and removal oil from water surface, Separation and Purification Technology (2016), doi: http:// dx.doi.org/10.1016/j.seppur.2016.09.022
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Robust magnetic polystyrene foam for high efficiency and removal oil from water surface Liuhua Yu, Gazi Hao, Lei, Xiao, Qiushi Yin, Mengting Xia, and Wei Jiang* National Special Superfine Powder Engineering Research Center of China, Nanjing University of Science and Technology, Nanjing 210094, PR China
Abstract In this study, a robust magnetic polystyrene foam with selective absorption ability was successfully fabricated via a low-cost method. Owing to its highly hydrophobic and superoleophilic properties, it could selectively absorb a variety of oils and organic solvents from a polluted water surface. The robust magnetic polystyrene foam had a large absorption capacity up to 17.83 times of its own weight and could be separated from the surface of water by magnetic technology. More importantly, the magnetic polystyrene foam could still retain a high absorption capacity and water contact angle for oil-water separation after the 10th cycle. In addition, the magnetic polystyrene foam could be reused through a simple squeeze treatment. High absorption capacity, special selectivity and excellent recyclability promise the material great potential for the cleanup of oils and organic solvents from water surface.
Keywords: PS, Selectivity, Magnetic, Absorption, Environment friendly
1
1. Introduction With the acceleration of global economic integration, the development of water transportation and the continuous development of shallow marine oil, the oil spills exhibit a rising tendency [1]. Oil leakage accidents bring us great economic loss and have devastating effects on the marine ecological environment. What’s worse, it also has adverse effects on human health [2,3]. Therefore, the oil pollution becomes an urgent problem to be solved, which has important significance to the sustainable development of human and society existence. From the last century, people have attached great importance to the oil spill accidents, and taken a series of methods to remove oils from waste water [4-7]. Due to use safety, low cost and environmental friendliness, adding absorbents is considered to be an effective way [8]. To our best knowledge, oil absorption material can be divided into two kinds, namely traditional oil absorption materials and novel oil absorption materials. Traditional oil absorption materials such as zeolites [9], graphite [10], and straw [11], etc. [12] have low oil absorption capacity, poor selectivity, and non-recyclability, which hardly meet the requirement for disposing the large-scale oil spills. Therefore, it is necessary to invent new oil absorption materials, which contain nano composites of powder [13], two-dimensional structure of fiber and film [14-16], as well as the three-dimensional network structure of the bulk materials [17-24]. Three-dimensional porous materials have great application prospect in oil absorption, due to their high porosity, large specific surface area and adjustable surface properties [25-31]. However, most of these bulk absorbents were prepared 2
with expensive/toxic raw materials or consumed much time/energy via sophisticated procedures, making their large-scale production difficult [19,22-24,32]. Therefore, it’s highly imperative to develop absorbents with low-cost fabricating procedure, available raw materials and excellent cyclicity. Commercial foam is an available raw material, which endows some specific properties to the substrate, enhancing its absorption property. Considering that magnetic materials were easily manipulated under the magnetic field, many researchers magnetized materials by deliberate incorporation of suitable magnetic particles. Fe3O4 particles are greatly attractive as magnetic recycling agent due to their inherent low toxicity and strong magnetism [25,26]. As a kind of typical polymer adsorption material, polystyrene has caught large interest of researchers on account of its low-cost, stable chemical property, highly hydrophobicity and superoleophilicity [13,33,34]. The introduction of PS into magnetic foam with a three dimensional network structure can not only enhance the thermal stability of the foam but also improve absorption capacity. In the current study, we successfully produced a robust magnetic polystyrene foam with anti-compressible, oil absorptive and reusable characteristics. In this paper, the preparation of magnetic Fe3O4 particles coated by oleic acid was reported in our previous work [35]. The magnetic particles were introduced into the foam through ultrasonic dispersion method, and then the highly hydrophobic magnetic polystyrene foam was synthesized by emulsion polymerization, which is one of the most widely applied industrial polymerization processes around the world [36]. The fabrication method was simple and low-cost without using expensive reaction apparatus, high 3
temperature and high pressure, and long reaction time. Furthermore, the as-prepared magnetic polystyrene foam had the following advantages: first, the robust foam had a high absorption capacity for several kinds of oils and organic solvents. Second, the robust foam could be recycled under the action of the magnetic field, the absorption of oils and organic solvents could be easily collected through a simple mechanical squeeze. Third, the robust foam could retain high absorption capacity and water contact angle for oil-water separation after the 10th cycle. All the results reveal that the robust magnetic polystyrene foam has high potential application in oil removal from water surface.
2. Experimental Section
2.1 Materials and Chemicals The following chemicals were used in the experimental work: shock absorption foam with density of 0.045g cm-3 and a void volume of about 94.2% (Aladdin), divinylbenzene (Aladdin), FeCl3·6H2O (Sinopharm Chemical Reagent Co. Ltd., Shanghai), NaAc (Sinopharm Chemical Reagent Co. Ltd., Shanghai), styrene (Sinopharm Chemical Reagent Co. Ltd., Shanghai), sodium dodecyl sulfonate (Sinopharm Chemical Reagent Co. Ltd., Shanghai), absolute ethanol (Nanjing Chemical Reagent Co. Ltd., Nanjing), oleic acid (Shanghai No.4 Reagent & HV Chemical Co. Ltd., Shanghai), ethylene glycol (Shanghai No.4 Reagent & HV Chemical Co. Ltd., Shanghai), azobisisobutyronitrile (Shanghai No.4 Reagent & HV 4
Chemical Co. Ltd., Shanghai), polyethylene glycol 4000 (Xilong Chemical Reagent Co. Ltd., Shantou).
2.2 Sample preparation 2.2.1 Preparation of Fe3O4 particles coated by oleic acid Fe3O4 particles were prepared by solvent thermal method which we reported previously [35]. A typical experiment for synthesis of Fe3O4 particles coated by oleic acid (OA-Fe3O4) was as follows: 30 mg of as-prepared Fe3O4 particles were dispersed into 10 mL of absolute enthanol by ultrasonication for 10 min until forming a black precursor solution. The precursor solution mixed with 1.2 mL of oleic acid was added to the three-neck flask with a mechanical stir for 2 h at room temperature. The prepared OA-Fe3O4 particles were purified by absolute ethanol and deionized water for three times in turn.
2.2.2 Preparation of the robust magnetic polystyrene foam The as-prepared magnetic polystyrene foams used Fe3O4 particles (MPF) and OA-Fe3O4 particles (OA-MPF) as magnetic recycling reagents. Fig. 1 is a schematic demonstration of preparation of MPF and OA-MPF. In a typical synthesis, shock absorption foam (3cm×1.5cm×1.5cm) was cleaned in deionized water and acetone with strong mechanical stirring at 50 °C for 4 h successively. Magnetic foam was obtained by immersing the pure foam in a homogeneous mixture of Fe3O4 particles or OA-Fe3O4 particles for 3 h under ultrasound (Fig. 1A, 1B). Then 600 mg of sodium 5
dodecyl sulfonate and magnetic foam were sequentially added into a three-neck flask stirring at the temperature of 75 °C in N2 atmosphere. Subsequently, 3 mL of styrene, 0.3 mL of divinylbenzene and 30 mg of azobisisobutyronitrile were slowly injected into the mixture solution and reacted for 6 h at the setting temperature. Finally, products were washed by absolute ethanol and deionized water to remove liquid and then dried at 60 °C to get the robust magnetic polystyrene foam (Fig. 1C).
Fig. 1 Illustration of the fabrication process of MPF and OA-MPF.
2.3 Characterization The chemical composition of the as-prepared materials was confirmed by X-ray diffraction (XRD, Bruker D8 Super Speed) with Kα radiation and the scanning angle ranged from 20° to 80° of 2θ at 40 kV. Field-emission scanning electron microscopy 6
(SEM, Model-S4800, Hitachi, Japan) were used to observe the surface morphology and structure of the magnetic polystyrene foam. Before SEM observation, all samples were fixed on aluminum stubs and coated with gold. The energy-dispersive X-ray spectroscopy (EDS) measurements were performed on an OXFORD INCA Energy Dispersive Spectrometer. The functional groups in the samples were examined by Fourier transform infrared spectroscopy (FTIR) in the range of 4000-500 cm-1 using KBr pellet technique on a Bruker Vector 22 spectrometer. Thermal gravimetric analysis (TGA) was carried out by Model TA2100 (TA Instruments, USA). The experiment was performed in the range from 50 °C to 650 °C at a heating rate of 10 °C/min under nitrogen atmosphere. Water contact angle (CAs) was determined by using a Drop Shape Analyzer SL200B (CAs, SL200B, Solon Tech. Co. Ltd., China) with a water droplet of 6 μL. The magnetic properties of the magnetic polystyrene foam was investigated in fields between ±8 kOe by a vibrating sample magnetometer (VSM, LakeShore 735). All the measurements were performed at room temperature.
2.4 Testing the effect of absorption capacity and reusability A piece of MPF and OA-MPF was immersed in oils and organic solvents at room temperature. Once equilibrium was attained, the sample was removed from the water surface using a magnet bar or a tweezer. The absorption capacity, K, of the magnetic polystyrene foam was calculated by weighing the sample before and after absorption and determined according to the following equation: K=(Ma-Mb)/Mb
(1) 7
where Ma and Mb represent the weights of the wet sample with oils or organic solvents (g) and the weight of the dry sample (g), respectively. The extrusion method was used to remove the absorbed oils and organic solvents. The magnetic polystyrene foam could be regenerated after drying in a vacuum oven directly. The reusability of magnetic polystyrene foam was evaluated by water contact angle measurements and measuring absorption capacity after many absorption and desorption cycles. All the experiments were repeated for five times.
3. Results and Discussion 3.1 Characterizations of the robust magnetic polystyrene foam The white pure foam turned into completely black after ultrasonication treatment, implying that the magnetic foam was successfully obtained. After a series of reaction, MPF and OA-MPF with three dimensional network structure were achieved, as shown in Fig. S1. Fe3O4 particles or OA-Fe3O4 particles in the foam could be tested by XRD measurements. All the diffraction peaks were indexed to the magnetic Fe3O4 phase. The reflections peaks of the as-prepared samples at 30.1°, 35.7°, 43.1°, 57.2° and 62.6° were ascribed to the (220), (311), (400), (511) and (440) planes of Fe3O4, which were similar to these characteristic peaks in the standard pattern (JCPDS file No. 19-0629). From the patterns, it was obvious that the peaks in curve a, curve b, curve c and curve d were similar, indicating that the crystal form of Fe3O4 particles in MPF and OA-MPF had not been changed, as shown in Fig. 2. Fe3O4 and PS were further characterized by energy-dispersive X-ray spectroscopy (EDS) to investigate the 8
surface of the existing elements. As it could be observed, the presence of Fe and O elements along with original C which were identified on pure foam were detected on MPF and OA-MPF (Fig. S2).
Fig. 2. XRD patterns for (a) Fe3O4, (b) OA-Fe3O4, (c) MPF, and (d) OA-MPF.
The pure foam possessed a smooth surface and a three dimension porous structure, which could be proved from the SEM images in Fig. 3A. Clearly, the foam was composed of many interconnected micropores with a diameter of about 500 μm and the smooth surface which was changed to be very rough after modification with Fe3O4 particles and OA-Fe3O4 particles, as illustrated in Fig. 3B and 3D. The Fe3O4 particles and OA-Fe3O4 particles were anchored onto the surface of foam, which could be seen through magnification in Fig. 3C and 3E. While Fig. 3F and Fig. 3H exhibited that the PS layer was linked to the surface of Fe3O4 particles and OA-Fe3O4 9
particles through the Fe-O-C bond. The surface of MPF and OA-MPF showed hierarchical roughness (Fig. 3G and 3J). The PS layer not only made MPF or OA-MPF highly hydrophobic, but also further immobilized Fe3O4 particles or OA-Fe3O4 particles. None of the pores inside MPF and OA-MPF were blocked, which was helpful to the fast intake of oils and organic solvents. The SEM observations also displayed that a series of chemical reactions did not damage the foam. In order to investigate the stability of the magnetic polystyrene foam after modification with magnetic particles and polystyrene layer, we carried out the ultrasonic experiment and extrusion experiment. The magnetic polystyrene foam was ultrasonically treated in absolute ethanol and deionized water, and there were no particles and no color change in the solution (Fig. S3). Furthermore, the magnetic polystyrene foam could still remain its original shape after extrusion without any particles and PS layer exfoliation, as exhibited in Fig. 4(A-F).
10
Fig. 3. SEM images of pure foam, MPF and OA-MPF at different magnification.
Fig. 4. Compression recovery process of the magnetic polystyrene foam.
Fig. 5 showed FTIR spectra of pure foam, Fe3O4, OA-Fe3O4, MPF and OA-MPF. Obviously, peaks at 2984.4, 2926.2 and 1474.9 cm-1 should be attributed to C-H stretching of methyl or methylene, respectively (curve a). The absorption peak around 11
662.0 cm-1 could be assigned to the stretching vibration of the Fe-O bond (curve b). The peak which was exhibited in curve c at 1183.2 cm-1 could be ascribed to the presence of oleic acid and the absorption peaks belonged to PS at 879.8, 1236.4 and 3712.5 cm-1 [37]. However, in spite of absorption peaks of PS, two characteristic peaks at 1066.0 and 1405.9 cm-1 were corresponding to the characteristic of C-O-C bonds and stretching vibration of C=O bond, respectively (Fig. 5 curve c, curve d) [38]. All these characteristic peaks also appeared in OA-MPF (curve e), indicating that OA-Fe3O4 particles and PS layer were attached on the surface of pure foam.
Fig. 5. FTIR spectra for (a) Fe3O4, (b) OA-Fe3O4, (c) MPF, and (d) OA-MPF.
The thermal stability of pure foam, MPF and OA-MPF was investigated by thermogravimetic analysis. According to Fig. 6, the oleic acid coating ratio of Fe3O4 particles and PS layer coating ratio of the magnetic foam could also be examined 12
performed in a nitrogen flow. The difference in residue contents indicated that about 16.5 wt% and 20.3 wt% of the PS layer was introduced onto the surface of MPF and OA-MPF, respectively. The increase of PS coating ratio could be ascribed to the surface of OA-MPF which was modified by OA-Fe3O4 particles based on the theory of “similarity and intermiscibility” [39]. Identically, about 6.4 wt% of mass defect between 480 C and 615 C in curve b was attributed to the oleic acid coating ratio. Besides, MPF and OA-MPF could sustain a temperature up to 270 C, indicating that as-prepared foams with good thermal stability could be used for practical applications in the harsh environment with high temperature.
Fig. 6. TGA curves of (a) Pure foam, (b) OA-MPF and (c) MPF.
In addition, MPF and OA-MPF could be easily manipulated by using a magnet, as exhibited in the inset of Fig. 7 and Video S1. The magnetic property of the foam 13
after modified by Fe3O4 particles, OA-Fe3O4 particles and PS was recorded at room temperature. As was shown in Fig. 7, all the samples exhibited standard paramagnetic characteristic curves with no hysteresis due to the bulk foam embedded by superparamagnetic Fe3O4 particles. The pure foam became magnetic with a saturation magnetization of 29.1 emu/g and 24.3 emu/g after incorporation of Fe3O4 particles and OA-Fe3O4 particles, respectively. It could be obviously seen that the saturation magnetization of MPF and OA-MPF decreased to 18.4 emu/g and 16.3 emu/g, respectively. However, the decrease in the saturation magnetization of MPF and OA-MPF was reasonable. As was well known, the thicker coating layer was, the weaker magnetic strength became [35].
Fig. 7. Room-temperature magnetization curves of (a) Fe3O4, (b) OA-Fe3O4, (c) MPF, and (d) OA-MPF. The inset are two images showing the magnetic property of MPF and OA-MPF. 14
3.2 Wettability and absorption capacity of the robust magnetic polystyrene foam The wettability of the as-prepared material was carried out by measuring the contact angles of a water droplet and a lubricating oil droplet deposited on the surface of MPF and OA-MPF, as shown in Fig. 8. The apparent water contact angle of MPF and OA-MPF were 138.5°±1.5° and 141.5°±1.5°, respectively (Fig. 8A, 8B). It was obvious that the water contact angle of OA-MPF was higher than MPF, which may be attributed to the relatively lower surface energy of OA-Fe3O4 than Fe3O4. In contrast, when 6 μL lubricating oil droplet was added to the surface of MPF and OA-MPF, it was immediately absorbed with oil contact angle being about 0° (Fig. 8C, 8D), which confirmed that as-prepared foams absorbed oil quickly and repelled water completely. Interestingly, there was a thick layer of bubble on OA-MPF surface when it was immersed in water by external force, and the as-prepared foams could immediately floated on the water surface after release of the external force (Video S2). To our surprise, no water was found intaken by weighing method, demonstrating that the as-prepared foam was highly hydrophobic. What’s more, OA-MPF exhibited a highly hydrophobicity, even floating on 0.1 M NaCl solution and aqueous solutions with pH values ranging from 1 to 13 for 48 h, as shown in Fig. 9A.
15
Fig. 8. Photograph of water droplets on the surface of (A) MPF and (B) OA-MPF. Photograph of lubricating oil droplets on the surface of (C) MPF and (D) OA-MPF.
Fig. 9. (A) Contact angles of OA-MPF after floating on 0.1 M NaCl solution and aqueous solutions with pH values ranging from 1 to 13 for 48 h. (B) Absorption rate curve of OA-MPF for lubricating oil. 16
MPF and OA-MPF could be used for absorbing oils and organic solvents, which was an important feature for practical applications [40,41]. Employing lubricating oil (labeled by Sudan I for clarity) in the selective absorption experiment, the lubricating oil was quickly absorbed by dipping the foams in 60 s (Fig. 9B). OA-MPF could float on the water surface after absorption of the lubricating oil. Subsequently, the lubricating oil was completely removed from the mixture and the regenerated foam still remained its original shape when taking the foam out of the water surface through applying an external magnetic field or external force, as illustrated in Fig. 10 and Video S3.
Fig. 10. Selective absorption and collection of lubricating oil (dyed with Sudan I) on water surface (colorless) by OA-MPF.
As illustrated in Fig. 11A, we could know that MPF intake capacity K of diesel oil, lubricating oil, DMF, and octane could reach 9.96, 12.51, 9.46 and 10.42 g/g, respectively. The absorption capacities of OA-MPF for these four kinds of oils and organic solvents were 14.41, 17.83, 13.62 and 14.40 g/g, respectively. The increase of the absorption capacity was attributed to the more PS layer of OA-MPF 17
than MPF, which agreed well with TGA analysis. As expected, the absorbed oils could be readily removed from the as-prepared OA-MPF through a simple squeezing process due to its excellent mechanical flexibility and water was not seen in the collected lubricating oil, as shown in Fig. 10E.
3.2 Reusability of the robust magnetic polystyrene foam It was satisfactory that the recycled foams could be reused for oil-water separation for many times and the regeneration capacity of MPF and OA-MPF was investigated as an important factor for a promising absorption material. As it could be clearly observed in Fig. 11, the absorption capacity of MPF and OA-MPF for four kinds of oils and organic solvents still kept larger than 8.5 g/g and 12.5 g/g of their own weight even after 10th cycle, respectively. Moreover, the robust magnetic polystyrene foam could still remain a high water contact angle after many absorption/desorption cycles, as exhibited in Fig. 12. Owing to its robust and stable porous structure, OA-MPF had still managed to have an absorption capacity than 90% of its initial value after the 10th cycle. The decrease of absorption capacity was caused by the residual oils in the pores of the foam. Compared with various novel oil absorption absorbents in Table 1, the robust magnetic polystyrene foam with excellent absorption capacity, low cost and high repeatability has the possibility of practical application in oils and organic solvents absorption.
18
Fig. 11. Absorption capacity for four kinds of oils and organic solvents after 10 cycles of (A) MPF and (B) OA-MPF.
Fig. 12. Water contact angle of OA-MPF after 10 cycles.
19
Table 1 Comparison of various novel oil absorbing absorbents. Absorbents
Absorption capacity (g/g)
Cost
Reuse efficiency
Reference
PDMS sponge
12
High
Low
28
Silicone sponge
3.8
Low
High
26
Graphene foam
12
Very high
Low
25
rGPU sponge
150
High
Very low
27
RGO sponge
45
Very high
Low
43
Spongy graphene
20
Very high
Low
45
PUR foam
4.5
Low
High
44
Foam graphene
15
Very high
High
42
OA-MPF
17.83
Very low
Very high
This work
4. Conclusions Robust
magnetic
polystyrene
foam
with
excellent
hydrophobicity,
superoleophilicity and magnetism has been successfully fabricated through a cost-effective and environment-friendly method. The incorporation of Fe3O4 particles or OA-Fe3O4 particles allowed the oil-absorbed foam to be easily collected by a magnet and the collected oils could be squeezed out by external force due to the foam’s compressibility. Furthermore, the lubricating oil intake capacity can be up to 17.83 and 16.21 times of its own weight at the first and tenth oil absorption, respectively. Interestingly, the robust magnetic polystyrene foam can absorb different kinds of oils and organic solvents with high absorption capacity and still keep a high water contact angle even after 10 water-oil separation cycles. With the advantages of compressibility, selective absorptivity and high stability, which are very important to the bulk oil absorption materials, the robust magnetic polystyrene foam may have 20
significant applications for removing oils and organic solvents from water surface.
Associated Content
Supporting information Information on digital images, EDS spectra and location of the sample after ultrasonic; three videos showing the magnetic property, unsinkable property and oil absorption process of the samples.
Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Project No. 41101287), the Scientific and Technical Supporting Programs of Jiangsu province (BE2012758) and Priority Academic Program Development of Jiangsu Higher Education Institutions.
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Graphical Abstract:
Fig. Absorption and desorption of the magnetic polystyrene foam.
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Highlight: NO.1 A low-cost method was applied to the fabrication of the magnetic polystyrene foam. NO.2 The absorbed oils and organic solvents could be collected through a simple squeeze treatment. NO.3 The as-obtained foam has the properties of highly hydrophobicity, and compressibility, NO.4 The sample could retain a high absorption capacity and water contact angle after the 10th cycle.
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