Preparation of Superhydrophobic Magnetic Cellulose Sponge for

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Preparation of Superhydrophobic Magnetic Cellulose Sponge for Removing Oil from Water Huili Peng,†,‡ Hao Wang,†,‡ Jianning Wu,†,‡ Guihua Meng,†,‡ Yixi Wang,†,‡ Yulin Shi,†,‡ Zhiyong Liu,*,†,‡ and Xuhong Guo†,‡ †

School of Chemistry and Chemical Engineering, Shihezi University, Shihezi, Xinjiang 832003, PR China Key Laboratory for Green Processing of Chemical Engineering of Xinjiang Bingtuan/Key Laboratory of Materials-Oriented Chemical Engineering of Xinjiang Uygur Autonomous Region/Engineering Research Center of Materials-Oriented Chemical Engineering of Xinjiang Bingtuan, Shihezi, Xinjiang 832003, PR China

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S Supporting Information *

ABSTRACT: It is still a challenging global task for oil/water separation. Here we fabricate superhydrophobic magnetic cellulose sponge (SMCE) that can be used to separate free oil/water mixtures and surfactant-stabilized W/O emulsions. The simple modification includes only two steps: a thin layer of ferroferric oxide (Fe3O4) was coated on cellulose sponge surface via codeposition method, and subsequently magnetic cellulose sponge was modified with hexadecyltrimethoxysilane, which could react with Fe3O4 or hydroxyl groups of cellulose. The purpose of coating Fe3O4 is to increase the roughness of the surface and recycle the sample by magnetic force. SMCE could separate oil−water mixtures with a high separation efficiency and good reusability. The sample is green, low cost, and environmental friendly, which makes it a promising candidate to be used in oil− water separation.

1. INTRODUCTION The development of industry and economy is accompanied by oily wastewater, which is a big problem in the world. Leaking oil brings many toxic compounds to the water, which causes severe environmental and ecological problems and also threatens the life of human beings.1 In the world, many oil spill accidents have occurred and never ended, such as the 2011 Bohai Bay oil spill, causing huge losses. Therefore the problem of oil/water separation has aroused considerable attention and has become a big challenge.2 Traditional approaches to treat oily wastewater include gravity separation, air flotation, an electrochemistry method, adsorption separation, ultrasonic wave and biochemistry methods, etc.3,4 Those conventional methods can separate immiscible oil/water mixtures effectively, but they are not useful for emulsified oil/water mixtures, especially not useful for surfactant-stabilized emulsions (droplet sizes < 20 μm). Currently ultrafiltration membrane based on the size of the pore structure can selectively allow materials of certain sizes to pass through and it can separate various industrial emulsions effectively.5,6 But the two-dimensional membrane structure with low porosity and short permeation channels usually causes serious fouling and degradation. In the process of membrane separation surfactant adsorption and pore plugging are likely to occur, which lead to a quick decline of the separation performance.7 In addition, ultrafiltration membrane is expensive and the preparation process is complicated. Therefore, it is necessary to research environmentally friendly materials with low prices, high oil−water separation efficiency, and outstanding reusability efficient. In recent years, researchers have found that three-dimensional (3D) porous materials are promising and attractive because of their versatile porosity, low density, large surface area, and surface roughness.8−10 Aerogel © XXXX American Chemical Society

and sponge are typical polymer materials, which are prepared by replacing the liquid component of the gel with the air and keep their network structure or the volume. Melamine sponges10 and polyurethane sponge11 have been used in oil/ water mixture separation. Celluose is a natural polymeric material, which has attracted much attention on wastewater treatment because of its attractive features such as low cost, renewability, biocompatibility, nontoxicity, and biodegradability.12 However, because of the high hydrophilicity of cellulose, cellulose cannot selectively remove oils and organic solvents from water, and this problem greatly influences their actual oil− water separation efficiency. So it is necessary to modify cellulose with low surface energy materials.13 Recently, various hydrophobic cellulose aerogels have been prepared for oil− water separation. For example, Son T. Nguyen14 prepared cellulose aerogels from paper waste and modified the aerogel with methyltrimethoxysilane to improve its hydrophobicity. Results showed the aerogel achieved high absorption capacities of 18.4, 18.5, and 20.5 g/g for three different crude oils at 25 °C, respectively. Chunde Jin15 prepared cellulose aerogel from waste newspaper (WNP), and then treated it with trimethylchlorosilane (TMCS) via a simple thermal chemical vapor deposition process, the results showed CBAs were rendered both hydrophobic and oleophilic and they exhibited good absorption performance for oils. However, previous efforts mainly focused on the oil absorption ability of cellulose aerogels, ignoring the recycle number and cost of oil recovery Received: October 14, 2015 Revised: December 25, 2015 Accepted: January 11, 2016

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DOI: 10.1021/acs.iecr.5b03862 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research by squeezing or distillation. Therefore, it is important to research materials both used in oil spill and emulsified oil/water mixtures. Furthermore, superhydrophobic materials have attracted worldwide attention, such as antifouling, energy conversion, and water treatment. The roughness of the surface is one of the major characteristics to control the superhydrophobic of the surface.16 In this research, we choose Fe3O4 particles to fabricate micro/nanostructured surface. Herein, a novel simple superhydrophobic cellulose sponge was fabricated for oil spill and emulsified oil/water separation via a green method. Cellulose sponge (CE) was made by the sol−gel method and freeze-drying. Then sponge was coated with Fe3O4 nanoparticles via an in situ synthesis process, and subsequently it was modified with hexadecyltrimethoxysilane through a simple solution-immersion step. Before the modification of cellulose sponge, it showed a strong hydrophilic nature. After surface-modified cellulose sponge, it is superhydrophobic and had the capability for oil/water separation. The purpose of coating Fe3O4 on a cellulose sponge is to increase the roughness of the surface as well as the sample can be recycled by using magnetic force. SMCE can separate free oil/water mixture excellently, and it is worth noting that it can separate surfactant-stabilized W/O emulsions with high efficiency.

Figure 1. Schematic illustration of cellulose sponge preparation.

Multiple coatings of Fe3O4 were added to the surface of cellulose by following the same steps as mentioned above. 2.4. Modification of Magnetic Cellulose Sponge. The surface modification of the magnetic cellulose sponge was performed by a self-assembly of hexadecyltrimethoxysilane monolayer. The magnetic cellulose sponge was immersed into the hexadecyltrimethoxysilane ethanol solution. The modification was maintained at room temperature for 7 h. The resulting sponge was washed several times with anhydrous ethanol and dried in a vacuum oven at 60 °C. Scheme 1 shows the schematic illustration of the fabrication of SMCE.

2. EXPERIMENTAL SECTION 2.1. Materials. Medical absorbent cotton was provided by HeNan Piaoan Group. Ferric chloride (FeCl3·6H2O), (FeCl2 4H2O), epichlorohydrin (ECH), and hexadecyltrimethoxysilane were purchased from Tianjin Fuchen Chemical Research Institute. Ammonia solution (NH3·H2O, 25%) was produced by Tianjin Fuyu Fine Chemical Co., Ltd. Sodium hydroxide, urea, calcium carbonate, and ethanol were of analytical grade and purchased from Aladdin. All the solutions were made with deionized (DI) water. All chemicals are analytical grade and were used without further purification. 2.2. Preparation of Cellulose Sponge. Cellulose solution was prepared according to the adapted method.17 Cellulose was dissolved in 7% NaOH/12% urea/81% H2O solution at low temperature to obtain a 2 wt % solution. The cellulose solution was centrifuged at 5000 rpm for 10 min to remove some impurities. Subsequently, the desired amount of ECH and CaCO3 were added into the cellulose solution to cross-link the cellulose into hydrogel and CaCO3 as pore-foaming agent. To make a morphology of the sponge, we used a beaker as a mold. Cellulose solution was poured into a beaker to control the specimen thickness with 1 cm and diameter with 3.8 cm. After the solution had gelled, it was immersed into 1% HCl for coagulation and replacement of CaCO3. After coagulation, the hydrogel was washed by deionized water until neutral. At last, freeze-drying (Christ Germany) was carried out for the sample for 24 h at −50 °C. The schematic illustration of cellulose sponge preparation can be seen in Figure 1. 2.3. Preparation of Fe3O4 Nanoparticles on Cellulose Sponge. Fe3O4 nanoparticles were prepared on the surface of cellulose sponge by the codeposition method. In detail, the cellulose sponge was immersed into the mixture solution of FeCl3·6H2O and FeCl2·4H2O. Then, the obtained cellulose sponge containing Fe3+/Fe2+ was dipped in excess 1% NH3· H2O to synthesize the magnetic cellulose sponge. After that, the magnetic cellulose sponge was washed with ethanol and deionized water to remove the residual chemical regents and dried in a vacuum oven at 50 °C under vacuum for 24 h.

Scheme 1. Schematic Illustration of the Fabrication of SMCE

2.5. Characterization. The morphologies of cellulose sponges were determined by scanning electron microscope (JSM-6490LV, Japan) with an accelerating voltage of 15 kV. Before observation, the cellulose sponges were sputtered with gold. X-ray diffraction (XRD) measurements for the cellulose sponges were carried out on the diffractometer (D8-Advance, Bruker, Germany) with Cu Kα radiation (λ = 1.542 nm) at 40 kV and 40 mA in the range 5−60° at room temperature. The surface composition of the samples was investigated by X-ray photoelectron spectroscopy (XPS) (AMICUS ESCA3200). The water contact angles were measured by a contact angle meter (DSA100, Kruss). A 3 μL water sample was dropped onto the surface of the sponges and five locations were tested in order to get the average value as the final contact angle. A microscope (BX53, Olympus) was used to observe droplets in emulsions. 2.6. Oil/Water Separation Experiments. The oil/water separation schematic illustration is shown in Figure 8a. Oil/ water mixtures were prepared through mixing 10 mL of oil and 10 mL of water under shaking.18 Water-in-oil emulsions were prepared through surfactant (0.1 wt %) and water (1 wt %) mixed with oil, and subsequently the mixture was emulsified by high-speed mixing.19 The cylindrical cellulose sponge (3 cm in diameter and 0.5 cm in thickness) was fixed between two glass tubes. The whole separation process was driven by a vacuum B

DOI: 10.1021/acs.iecr.5b03862 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research driven filtration system with the vacuum degree at 0.02 MPa. The separation efficiency was calculated according to eq 1.20 The flux of cellulose sponge was determined by calculating the volume of oil permeated in unit time using eq 2.21 η% = m1/m0 × 100

(1)

flux = m /(AΔPt )

(2)

Here, m0 and m1 are the mass of the water (or the solvents) before and after separation process, respectively, m is the mass of the oil filtered (kg), A is the effective filtration sponge area (m2), t is the filtration time (h), and ΔP is the suction pressure across the sponge (bar).

3. RESULTS AND DISCUSSION 3.1. Structure and Morphology Analysis. FT-IR spectras of CE and SMCE are shown in Figure 2. The peaks Figure 4. XRD spectra of cellulose, RCE, and SMCE.

cm−1 were observed in SMCE which correspond to the bending of Si−O−Si and C−Si of hexadecyltrimethoxysilane.9,22 The new absorption peak at around 580 cm−1 was observed in SMCE which corresponds to the characteristic absorption peak of Fe3O4.23 The stretching vibrations of the hydroxyl groups of CE and SMCE were shifted from 3336 to 3456 cm−1, indicating that a strong interaction occurred between the hydroxyl groups of cellulose and hexadecyltrimethoxysilane (as shown in Scheme 1). However, there is a small decrease in −OH intensity because the amount of hexadecyltrimethoxysilane is much less than −OH. Figure 3 shows SEM images of the samples. As revealed in Figure 3a, the surface of cellulose sponge was smooth, almost nonporous. Figure 3b shows a cross-section of the sponge; numerous pores with pore sizes of 100−300 μm are randomly distributed internally. As is also evident from the SEM images of the modified cellulose sponge (Figure 3c,d), Fe 3 O 4 nanoparticles easily attach to a cellulose sponge, which has a nanostructure similar to lotus leaves.24 Supporting Information Figure S1 shows that Fe3O4 nanoparticles (20 nm) coat the surface and interior of the SMCE. Furthermore, Fe3O4 nanoparticles can tightly adhere to the cellulose sponge through two ways: first, the hydrogen-bond interaction between Fe3O4 and cellulose; second, the chemical bond interaction between the hydroxyl groups from cellulose and hydrolyzed silane. The XRD spectra of the cellulose, regenerated cellulose (RCE), and SMCE is shown in Figure 4. As observed, native cellulose had typical diffraction peaks at 2θ = 14.7°, 22.9°, 26.6°, and 34.4°.25 The regenerated cellulose sponge from NaOH/urea solution had diffraction peaks at 2θ = 11.9°, 19.9°, and 21.3°.26 The results indicated that an obvious conversion from native cellulose I to cellulose II occurred here.27 The XRD pattern of the Fe3O4 and the diffraction peaks at 30°, 35.3°, 42.9°, 57°, and 62.4° which corresponds to the (220), (311), (400), (511) and (440)23 lattice planes of the cubic phase of Fe3O4 (JCPDS 19-629) was observed for SMCE. The typical diffraction pattern of regenerated cellulose still existed in SMCE, only with lower intensity. It can be explained by the formation of hydrogen bonds between cellulose and Fe3O4. The surface composition and bonding environment of the samples were investigated by X-ray photoelectron spectroscopy (XPS). Figure 5a,b show the XPS of CE and SMCE. After

Figure 2. FTIR spectra of CE and SMCE.

Figure 3. (a) SEM image of the CE surface. (b) Cross-sectional SEM image of CE. (c) SEM image of SMCE surface. (d) Cross-sectional SEM image of SMCE.

at 3300−3450 cm−1 correspond to the stretching of −OH. The peaks at 2900 cm−1 is attributed to the stretching of a C−H bond present in cellulose. Small peaks at 800 cm−1 and 1270 C

DOI: 10.1021/acs.iecr.5b03862 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 5. XPS survey spectra before (a) and after (b) modified sponge, and the high-resolution XPS spectra of the Fe 2p peaks (c) and Si 2p (d).

Figure 7. (a) Contact angle measurement for SMCE. (b) Water droplet (colored with blue dye) on SMCE. (c) Photographs of the selective sorption of oil (colored with Sudan III) with SMCE.

3.2. Wettability and Separation. We also research the influence of Fe3O4 coating times on contact angle as showed in Figure 6. The analyses above have confirmed the deposition of Fe3O4 and C16H33O3Si on the surface of cellulose sponge. The water contact angle for the pristine cellulose sponge could not be recorded, because cellulose is highly hydrophilic and the water droplet penetrated quickly into the sponge. The contact angle results showed that the cellulose sponge was modified by C16H33O3Si without coating Fe3O4 displayed hydrophobicity. Coating Fe3O4 can increase the contact angle of the cellulose sponge, and the contact angle increases with the increase of coating times. After three times coating the sponge displayed

Figure 6. Influence of the coating times on contact angle.

modification, two new characteristic peak with binding energies of 710.5 and 100.2 can be seen from Figure 5b which are attributed to Fe 2p and Si 2p.28,29 The results indicated that Fe3O4 has been deposited on the surface of the sponge effectively and hexadecyltrimethoxysilane has modified the sponge successfully. It has also been noticed that the intensity of the characteristic peaks of Fe (2p) is relatively weak, which suggests the Fe3O4 layer deposited on the cellulose sponge is very thin. D

DOI: 10.1021/acs.iecr.5b03862 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 8. (a) Schematic illustration of the cellulose sponge for separating oil and water mixture. (b) Photograph of toluene (colored by Sudan III) floating on the water (c) after toluene separation from the water by filtration, and (d) temporal variation of oil (toluene) mass flux and toluene oil collection with time.

Figure 9. Separation ability of SMCE: (a) for different oils in the first cycle; (b) for toluene after different separation cycles.

Figure 8d shows the oil mass flux and oil separation efficiency with time. During 160 s the oil separation efficiency rose steadily and almost 100% of the oil had been collected from the oil−water mixture, indicating that the speed of the separation is fast. Furthermore, the oil mass flux decreased a little indicating there is no jam phenomenon in the process of separation. After 100 s, the oil mass flux decreased greatly because most of the oil was separated from mixture, decreasing the mass of oil. The oil separation efficiency for a series of oils is shown in Figure 9a. These separation efficiencies were consistently better than 95%, meaning that cellulose sponge is effective for different kinematic viscosities of oil. The mass flux is different with different kinematic viscosities of oil, and the mass flux of paraffin oil is lower than the others due to the high viscosity. From Figure 9b it can be seen after 5 times use, SMCE still maintains a high separation efficiency by taking the toluene/ water mixture as an example. As well the surface of the contact angle has not been decreased much. That shows superhydrophobic cellulose sponge is stable after being used many times. To investigate the emulsified oil separation capability, we prepared surfactant-stabilized water-in-oil through mixing water and oil in 1:100 (v/v) with the addition of 0.2 wt % Span80

superhydrophobicity about 156°. This might be attributed to the surface roughness of the cellulose sponge. It evidently confirms that this method has led to a major enhancement of hydrophobicity. Figure 7a,b shows SMCE has the property of water repellency, which is important for oil spill remediation in aqueous environments. As seen in Figure 7c, SMCE exhibited excellent selective sorption of oil on the surface of water. By dipping the SMCE into a mixture of oil and water, the oil was quickly absorbed into SMCE. SMCE continued to float on the water surface, and it was separated from solution by a magnet. Figure S2 shows the saturated magnetization (Ms) values of SMCE were 12.84 emu/g. The Ms value of SMCE is lower than that of Fe3O4, but SMCE still show strong magnetization. In this work, a small-scale oil−water separation device is shown in Figure 8a. SMCE was fixed between two glass tubes. To increase the contact area of the sample and oil the device must be tilted, because the density of oil is less than the density of water. The oil−water mixture was poured into the glass tube. The oil can permeate through the cellulose sponge, but the water droplets were repelled and remained in the top glass. Figure 8b,c correspond to the images of before and after separation; the oil−water mixture was separated completely. E

DOI: 10.1021/acs.iecr.5b03862 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported financially by funding from the National Natural Science Foundation of China (21367022 & 21467024) and Bingtuan Innovation Team in Key Areas (2015BD003).

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Figure 10. (a) Microscope image of the toluene-in-water emulsion before filtration. (b) Photograph of the toluene-in-water emulsion before (left) and after filtration (right). (c) Microscope image of the toluene-in-water emulsion after filtration, and (d) cycling performance of the cellulose sponge for emulsion separation.

under high stirring for 1.5 h.19 Before separation, the feed of surfactant-stabilized water-in-oil was a milky white liquid as shown in Figure 10b left. After separation, the collected filtrate was transparent compared to the feed. As shown in the optical microscopy images (Figure 10a), before separation, we can see compact droplets in the feeds of emulsions. However, in the corresponding filtrates no droplets are observed (Figure 10c). This shows that not only can the oil spill be separated but also the emulsion oil can be successfully separated by SMCE. Figure 10d shows mass flux of SMCE after five cycles use, there are no obvious changes in the water flux, which indicates the good reusability of the superhydrophobic cellulose sponge.

4. CONCLUSION SMCE with a high separation efficiency for free oil/water mixtures and surfactant-stabilized oil−water emulsions was prepared successfully via a facile approach. SMCE has a structure with a rough surface and micrometer pores cross section. The sample has a flux of 120 kg·m−2·h−1·bar−1 and a separation capacity above 98% for an oil−water mixture. This modification method is simple, and through this method the hydrophilic surface properties of other substrates can be easily converted into hydrophobic properties. Moreover, the superhydrophobic magnetic cellulose sponges have high potential for the treatment of oily wastewater in industry and daily life.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.5b03862. High-magnification SEM images, magnetization curves (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Tel: 0086-0993-2057276. Mobil Tel.: +86-13677533280. Email: [email protected]. F

DOI: 10.1021/acs.iecr.5b03862 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.iecr.5b03862 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX