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water separation and water purification
Accepted Manuscript Full Length Article Facile way in fabricating a cotton fabric membrane for switchable oil/water separation and water purification Yubin Li, Ziliang Feng, Yi He, Yi Fan, Jing Ma, Xiangying Yin PII: DOI: Reference:
S0169-4332(18)30405-7 https://doi.org/10.1016/j.apsusc.2018.02.060 APSUSC 38518
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
27 May 2017 29 January 2018 5 February 2018
Please cite this article as: Y. Li, Z. Feng, Y. He, Y. Fan, J. Ma, X. Yin, Facile way in fabricating a cotton fabric membrane for switchable oil/water separation and water purification, Applied Surface Science (2018), doi: https:// doi.org/10.1016/j.apsusc.2018.02.060
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Facile way in fabricating a cotton fabric membrane for switchable oil/water separation and water purification Yubin Lia,c, Ziliang Fenga, Yi Hea,b,c*, Yi Fana,b,c*, Jing Maa,c, Xiangying Yina,c a. School of Chemistry and Chemical Engineering, Southwest Petroleum University, Chengdu City, Sichuan Province, P. R. China, 610500 b. State Key Lab of Oil and Gas Reservoir Geology and Exploitation (Southwest Petroleum University), Rd. 8, Xindu District, Chengdu City, Sichuan Province, P. R. China, 610500 c. Oil & Gas Field Applied Chemistry Key Laboratory of Sichuan Province (Southwest Petroleum University), Chengdu, Sichuan, China, 610500. *Address correspondence to this author. E-mail: [email protected], [email protected]. Phone and Fax: +86 02883037315
1
ABSTRACT: With dopamine and NiFe2O4 particles, a novel modified cotton fabric (PDA-NiFe2O4@CF) was prepared by one-pot method. Surface morphology, composition of the PDA-NiFe2O4@CF were investigated with SEM, EDX, XRD and FT-IR, respectively. According to the results, the cotton fiber surface was well coated with NiFe2O4 particles. Subsequently, wetting behavior of the modified cotton fabric was determined. The PDA-NiFe2O4@CF is superamphiphilic in air, and a dual lyophobic behavior was indicated with an oil contact angle (OCA) of 153° under water and a water contact angle (WCA) of 145° under oil. The rough micro-nano scale
surface
structure
and
high-surface-energy
compositions
of
the
PDA-NiFe2O4@CF makes the surface to be easily covered by one medium and enables it to repel other unmixable medium simultaneously. Therefore, water-oil mixtures can be separated on demand. Besides, with the unusual dual lyophobic surface of PDA-NiFe2O4@CF, both two types of emulsions were separated by gravity driven. On the other hand, it was also found that the as-prepared PDA-NiFe2O4@CF had good adsorption performance for methylene blue. Keywords: Cotton fabric, Emulsion, Water-oil separation, Membrane, Adsorption 1. Introduction With frequent oil-spill accidentals and increasing amount of industrial wastewater bringing severe environmental issues, separation of oil/water mixtures has always been a hot topic of research [1-3]. Among these, emulsions are still a tricky problem for separation, especially in the presence of stable surfactants [4-8]. Traditionally, polymer membranes with small sized porous structures have focused on separating 2
oil/water emulsions. However, the fabrication methods required tedious steps, precious materials and elaborate design of pore sizes [9-11]. Furthermore, wastewater in production and life is often a complex system [12-14], and organic pollutants exist simultaneously except emulsions [15-18]. Therefore, it is of great significance to design multifunctional materials, and the demand for developing more facile and effective techniques for water treatment has never stopped. In current works, inspired by the wetting behavior of lotus, researchers have developed a variety of bioinspired superwetting surfaces and interfaces [19-28]. Encouragingly, the superwetting surface is considered as a promising candidate for oil-water separation [29-30]. Thus, with the more common supplies, such as cotton fabric and stainless steel mesh, some simple and scalable approaches were founded for separation of emulsions. Liu et al. [31] prepared a Janus cotton fabric which can be used for rapid and efficient separation of oil from oil-in-water emulsion, and Lu et al. [32] developed a facile method to fabricate double-layer stainless steel mesh for effective separation of water-in-oil emulsion with high flux. However, most of these materials only able to separate a particular type of emulsion (water-in-oil or oil-in-water). Obviously, materials that can be employed to separate both two types of emulsions simultaneously are very attractive and promising. While, the relevant report is seldom meet. Besides, Ding et al. [16] provided a strategy for separating oil-in-water emulsion and adsorbing organic dyes with a same nanofibrous membrane, which was a good reference of the design of multifunctional materials. In our work, we built an unusual dual lyophobic surface in oil–water systems. The 3
PDA-NiFe2O4@CF was prepared via one-pot method with a commercial cotton fabric, NiFe2O4 ferrites and dopamine [33]. Surface of the PDA-NiFe2O4@CF is both superoleophobic under water and hydrophobic under oil. Therefore, water-oil mixtures can be separated on demand, and both two types of emulsions can be separate with the PDA-NiFe2O4@CF by gravity driven. In addition, the modified cotton fabric exhibited an excellent adsorption capacity to methylene blue (MB). Due to the simplicity of preparation and the effectiveness of separation and adsorption, it is believed that the as-prepared cotton fabric would be a good case for practical applications. 2. Experimental 2.1 Materials and fabrication of the PDA-NiFe2O4@CF A series of chemicals, containing Ni(NO3)2.6H2O, Fe(NO3)3.9H2O, ammonia solution, dopamine, Tris-buffer, sodium hydroxide, toluene, dichloromethane, hexadecane, chloroform, hexane and methylene blue are all analytical grade reagents. Deionized water with a resistance of 18.0 MΩ was obtained from the water purification system of UPC-Ⅲ. Cotton fabrics were purchased from the store and were cut into 2.0×2.0 cm2. 4.85 g Fe(NO3)3.9H2O and 1.75 g Ni(NO3)2.6H2O were dissolved in 100 mL deionized water, and the solution was stirred for 2 h. Under continuous stirring, the pH of the solution was adjusted to 8 by ammonia solution. Following, the mixture was transferred into a Teflon lined stainless autoclave which was then maintained at 180℃ for 8 h [34]. Before it was cooled to room temperature. NiFe2O4 precipitates were 4
rinsed with deionized water and ethanol for several times. The as-obtained products were further annealed at 550℃ for 2 h with a heating rate of 2℃/min. The preparation and application process of PDA-NiFe2O4@CF is presented in scheme 1.
Scheme 1. Schematic illustration of preparation and application process for PDA-NiFe2O4@CF 300 mg NiFe2O4 particles, 40 mg dopamine and a piece of cotton fabric were added into 20 mL tris (10 mM) buffer solution whose pH had been adjusted to 8.5 by NaOH aqueous. After ultrasonic treatment for 30 min, the mixture was stirred for 24 h at 50℃, and the PDA-NiFe2O4 @CF was dried in a vacuum oven at 50℃ for 6 h. 2.2 Characterization Morphologies and compositions of the sample were analyzed by FE-SEM (JSM-7500F) and X-ray energy dispersive spectrometry (EDX, INCA). Chemical composition of the sample surface was characterized with FT-IR spectra (WQF520 spectrometer). The phase structure and crystals orientation was identified by X-ray diffraction (XRD, PANalytical X’Pert Pro diffractometer) with Cu Kα radiation. Contact angle (CA) of a 5μL droplet was measured by using the KRUSS DSA30S. TOC (total organic carbon) was measured using Shimadzu TOC-VCPH analyzer. 5
Optical microscope (Nikon Digital Sight DS-F11, Japan) was employed to analyze the emulsions before and after filtering separation. 2.3 Preparation of the emulsions and separation experiments Toluene-in-water (1:100) and water-in-dichloromethane (1:100) were made by mixing oil and water with addition of 0.2 g/L SDS and 2 g/L Span80 by sonicating for 10 min and sharply stirring for 6 h. Both two emulsions were highly stable for hours in
laboratory
environment.
Besides,
SDS
stabilized
diesel-in-water,
hexadecane-in-water, and Span80 stabilized water-in-chloroform, water-in-hexane were prepared as well. The as-prepared PDA-NiFe2O4@CF with diameter of 2.0 cm was sandwiched between two vertical glass tubes. The freshly prepared emulsions were poured into the upper tube, and the flux was determined by calculating the volume of liquid permeated within 5 min under gravity driven. The height of feed emulsions kept 10 cm. Before falling into the oil-in-water or water-in-oil emulsion, the membrane was wetted by water or oil. 2.4 Adsorption measurements The adsorption performance of PDA-NiFe2O4 @CF was tested by removing the typical organic dye of methylene blue (MB). The concentration of MB in the solution was 10 ppm and 30 ppm, respectively. In general, the modified fabric was immersed into the MB aqueous solution with stirring for a designated time, and then the resultant adsorption capacity was measured by a UV-Vis spectrometer (Shimadzu, UV-1800) at certain time intervals. 3. Results and discussion 6
Fig.1. SEM images of NiFe2O4 particles (a), pristine cotton fabric (b) and the PDA-NiFe2O4@CF (c) As indicated from the microstructure in Fig. 1, the pristine cotton fabric exhibits an obvious 3D opening geometry with fiber diameter of about 10-15 μm, and the cotton fibers are interlaced with each other. After one-pot treatment, NiFe2O4 particles were located on the fiber surface with a random homogeneous distribution. Dopamine which could self-polymerize and adhere on almost any substrates protected the particles from leaching. PDA ensured the bonding strength between NiFe2O4 and fibers. There are three major reasons for using NiFe2O4 particles. First, the metal oxide particles can improve the hydrophilicity of the cotton fabric surface. Then, when the fabric is filled, the pore size decreases, and it can be applied to the separation of oil-water mixtures. Besides, the NiFe2O4 ferrites itself has a good adsorption effect on the organic dyes. With energy dispersive X-ray (EDX) mapping, structure composition of the PDA-NiFe2O4@CF was further investigated. As shown in Fig. 2, the cotton fiber surface is uniformly coated with Ni, Fe and O elements. Besides, the corresponding X-ray diffraction (XRD) patterns of the as-prepared NiFe2O4 particles and PDA-NiFe2O4 @CF are presented in Fig. 3a. It reveals that the relevant lattice planes of the obtained PDA-NiFe2 O4@CF matched well with that of 7
the NiFe2O4 particles whose crystalline spinel structure is consistent with the XRD data in JPPDS No.10-0325 (NiFe2O4 phase) [16]. In combination of the figures, the cotton fibers were well coated with the NiFe2 O4 particles. The polydopamine of PDA-NiFe2O4@CF was proved by FT-IR in Fig. 3b. Compared with NiFe2O4 particles, in the case of the as-prepared sample, the adsorption band of 1080, 1510 and 3425 cm-1 are assigned to the stretching vibration of N-H, O-H and C-O-H [35,36], respectively. Other relative peaks around 670 and 587 cm-1 are caused by the inorganic particles [37,38].
Fig. 2. The corresponding elemental mapping images of C, O, Fe and Ni of the coated cotton fiber.
Fig. 3. XRD patterns (a) and FT-IR (b) of the as-prepared NiFe2O4 particles and PDA-NiFe2O4@CF Wetting behavior of the PDA-NiFe2O4 @CF in different medium was investigated. 8
As illustrated in Fig. 4, due to the high-surface-energy chemical compositions and the rough micro-nano scale morphologies of the surface, PDA-NiFe2O4 @CF is superamphiphilic in air. When the droplets of water and oil contacted with the modified fabric, they spread rapidly. Interestingly, as presented in Fig. 5, after immersing the PDA-NiFe2O4@CF into water or petroleum ether, either the dichloromethane or water droplet sat on the treated cotton fabric. Underwater superoleophobic or underoil hydrophobic was observed with an underwater OCA of 153° and an underoil WCA of 145°. In addition, the contact angles of different oils underwater and water contact angles under different oils is displayed in Fig. 5(b, c). The PDA-NiFe2O4@CF presents underwater superoleophobicity for different types of oils with OCA larger than 150°. Correspondingly, the PDA-NiFe2O4@CF also show hydrophobicity under different types of oils with WCA larger than 140°. Consequently, the surface of PDA-NiFe2O4@CF exhibits an unusual dual lyophobic behavior.
Fig. 4. Process of water (upper) and oil (lower) droplets touching on the surface of the PDA-NiFe2O4@CF within 1 s (the interval of each image is 0.5 s)
9
Fig. 5. (a, c) Photographs of an underwater oil droplet (red) and an underoil water droplet (blue) on the PDA-NiFe2O4@CF. (b, d) The different types of OCA underwater and WCA under different types of oils on the PDA-NiFe2O4@CF. Why is it occurred to that the PDA-NiFe2O4 @CF has the wetting behavior of underwater superoleophobic and underoil hydrophobic? It could be deduced [40] that surface of the PDA-NiFe2O4 @CF contains not only numerous hydrophilic components which mainly come from the NiFe2O4 particles, but also a certain amount of oleophilic components from polydopamine. Besides, according to the Wenzel and Cassie-Baxter model [41], the amplification effect about liquid wettability on the rough surface could increase the hydrophilicity or oleophilicity, even to superwetting. Hence, when the rough PDA-NiFe2O4 @CF is in water, the hydrophilic components predominantly interact with water through building the intermolecular force and hydrogen bonds to form a stable water film, and this could lead to underwater 10
superoleophobic. Likewise, in oil, the dominant power of the rough membrane surface is the interaction between oil and the oleophilic components, forming a stable oil film to possess underoil hydrophobic [27,39]. In brief, the results can be attributed to the rough micro-nano scale surface structure and high-surface-energy compositions of the PDA-NiFe2O4@CF which makes the surface to be easily covered by one medium and enables it to repel other unmixable medium simultaneously [42]. The special wetting behavior of the interface is an effective means to induce water-oil separation. According to the dual lyophobic behavior, the modified fabric is expected to be useful for the separation of water-oil mixtures on demand. As demonstrated in Fig. 6(a), the PDA-NiFe2O4 @CF was sandwiched between two glass tubes and a mixture of hexadecane (red) and water (colorless) was poured into the upper tube. Due to the characteristic of underwater superoleophobic, water went through the fabric while hexadecane was kept in the upper. After wetting by water, the PDA-NiFe2O4@CF was not permeable to hexadecane. On the other hand, when the coated fabric was wetted by dichloromethane (colorless) prior to separation in Fig. 6(b), the PDA-NiFe2O4@CF was permeable to dichloromethane and water (blue) was retained on the upside. Only driven by gravity, the as-prepared treated cotton fabric exhibits a good water-oil separation capability.
11
Fig. 6. Mixtures separation of (a) hexadecane(red)-water(colorless) and (b) water(blue)- dichloromethane(colorless). Furthermore, we employed the as-prepared PDA-NiFe2O4@CF to separate two typical emulsions. As presented in Fig. 7, the PDA-NiFe2O4@CF was mounted between two vertical glass tubes, and the as-prepared emulsions were poured onto the PDA-NiFe2O4@CF, respectively. Consequently, only the external phase of the emulsion permeated through the PDA-NiFe2O4@CF, and emulsions (oil-in-water and water-in-oil) were separated barely under gravity (see movie S1 and S2). On the contrary, the internal phase of the emulsion was trapped in the upper tube. Moreover, as Fig. 7 shows, optical microscopic images of the emulsions and collected filtrates indicated that no droplets of the internal phase was observed in whole view. This novel modified cotton fabric exhibits a high performance of separation of both two types of emulsions. Additionaly, the PDA-NiFe2O4@CF membrane reached a separation flux of about 300 L.m-2.h-1 by solely gravity driven, and the rejection ratio is about 99% to oil or water. The separation process is energy conservation. Moreover, 12
similar separation flux and rejection ratio were obtained for other emulsions, such as SDS
stabilized
diesel-in-water,
hexadecane-in-water
and
Span80
stabilized
water-in-chloroform, water-in-hexane. Besides, the recyclability of the PDA-NiFe2O4@CF for emulsions separation under gravity was examined. Toluene-in-water and water-in-dichloromethane stabilized by SDS and Span80 were selected as feed emulsions. Each cycle test was conducted for 30 min, and flux changes were recorded every five minutes. The PDA-NiFe2O4@CF
was
water-in-dichloromethane
employed emulsion,
to
and
then
separate was
toluene-in-water washed
by
water
or or
dichloromethane after separating. As presented in Fig. 8, in each cycle, although both water and oil fluxes decreased with separation time increasing, which is resulted from the formation of oil or water cake on the surface, the separation ability of emulsions were highly stable with only washing after each cycle. Thus, the PDA-NiFe2O4 @CF shows a certain ability of antifouling characteristic. As far as the emulsion separation performance is concerned, the wetting behavior and pore size on the surface of a membrane are all very important. Micro-nano scale structures composed of particles on the cotton fiber surface could increase the roughness of the membrane surface to form a superwetting property. As a result, underwater oleophobic and underoil hydrophobic was realized. This wetting behavior of the membrane surface help to improve the antifouling effect of the membrane during the separation process. In addition, the adhesion of nano particles on the cotton fiber surface has effectively decreased the pore size [28] in the fabric membrane for 13
emulsion separation.
Fig. 7. Emulsion separation process of the PDA-NiFe2O4@CF, and the optical microscopic images of oil-in-water emulsion (upper) and water-in-oil emulsion (lower) before and after separation.
Fig. 8. Cycling performance of PDA-NiFe2O4 @CF. Emulsions separation of SDS stabilized toluene-in-water and Span80 stabilized water-in-dichloromethane We also demonstrate that the PDA-NiFe2O4@CF with porous structure has an efficient adsorption of organic dye methylene blue (MB) in water. With the UV-vis spectrometer, the performance of PDA-NiFe2O4 @CF was determined in a 20 ml of 30 ppm MB solution. As displayed in Fig. 9(a,b), the PDA-NiFe2O4@CF could adsorb 14
about all of the MB for about 100 min. While the adsorption capacity of pristine cotton fabric is less than 5% in 100 min. In addition, another key parameter, the responsive time to MB is rapid. The as-prepared coated fabric could adsorb about 72wt.% of MB in 30 min. On the other hand, from Fig. 9(c,d), at low MB concentration of 10 ppm, the response (10 min) of PDA-NiFe2O4@CF is also competitive under weak adsorption drive force. Owing to the synergetic effect of NiFe2O4 particles and polydopamine, adsorption capacity of the fabric membrane to organic dyes is improved significantly. The content of PDA-NiFe2O4 in the as-prepared coated fabric was calculated from the mass difference of the cotton fabric before and after treatment, and the adsorption capacity was calculated within the equation (1): (1)
Where
(mg/L) and
(mg/L) are the concentration at time t and the initial
concentration, respectively. m (g) is the mass of the adsorbents and V (L) is the volume of the tested solution.
(mg/g) is the adsorption capacity. In the 30 ppm
MB solution, the equilibrium capacity of PDA-NiFe2O4@CF for MB is about 95.5 mg/g. Two major reasons contribute to the improved adsorption capacity of the hierarchical structure fabric. The increased surface area, low diffusion resistance and electrostatic attraction which is resulted from the mixed metal oxide nanoparticles[38], and the electrostatic attraction, π-π stacking interactions and hydrogen bonding that is provided by aromatic rings of PDA layer on the particles and fibers surface [43]. Consequently, the as-prepared PDA-NiFe2O4 @CF has a strong affinity for MB in 15
water. Moreover, the modified cotton fabric can be recovered facilely, which avoids the tedious recycling process and secondary pollution of regular particles adsorbents.
Fig. 9. Adsorption ability and rate of the PDA-NiFe2O4@CF in 30 ppm (a,b) and 10 ppm (c,d) MB solution, respectively. 4. Conclusions In summary, we present a facile way to preparing a modified cotton fabric, which is superoleophobic in water and hydrophobic in oil. As a result, oil-water mixtures can be treated on demand. Moreover, both two types of emulsions can be separated with the PDA-NiFe2O4 @CF by gravity driven. This phenomenon is mainly ascribed to the rough
surface
structure
and
high-surface-energy
compositions
of
the
PDA-NiFe2O4@CF which makes the surface to be easily covered by one medium and enables it to repel other unmixable medium simultaneously. Furthermore, it was also found that the as-prepared PDA-NiFe2O4 @CF had good adsorption performance for 16
organic dyes. It is believed that this novel surface provides a reference for the development of water-oil separation and water treatment techniques. Acknowledgements The authors thank Xi Chen and Qiangbin Yang for the assistance with SEM and EDX measurements. This work was financially supported by Youth Science and Technology Creative Group Fund of Southwest Petroleum University (2015CXTD03) and the Majorly Cultivated Project of Sci-tech Achievements Transition (15CZ0005) from the Education Department in Sichuan Province. References [1] M.A. Shannon, P.W. Bohn, M. Elimelech, J.G. Georgiadis, B.J. Marinas, A.M. Mayes, Science and technology for water purification in the coming decades, Nature 452 (2008) 301-310. [2] S. Pezeshki, M. Hester, Q. Lin, J. Nyman, The effects of oil spill and clean-up on dominant US Gulf coast marsh macrophytes: a review, Environmental pollution 108 (2000) 129-139. [3] A. Jernelöv, How to defend against future oil spills, Nature 466 (2010) 182-183. [4] X. Lin, F. Lu, Y. Chen, N. Liu, Y. Cao, L. Xu, Y. Wei, L. Feng, One-step breaking and separating emulsion by tungsten oxide coated mesh, ACS applied materials & interfaces 7 (2015) 8108-8113. [5] B. Wang, W. Liang, Z. Guo, W. Liu, Biomimetic super-lyophobic and super-lyophilic materials applied for oil/water separation: a new strategy beyond nature, Chemical Society Reviews 44 (2015) 336-361. 17
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Highlights A novel modified cotton fabric (PDA-NiFe2O4@CF) was prepared by one-pot method. The PDA-NiFe2O4@CF presents superamphiphilicity in air and dual lyophobic behavior under water or oil. Both two types of emulsions can be separated with the PDA-NiFe2O4@CF. The PDA-NiFe2O4@CF had good adsorption performance for methylene blue.
24
Graphical abstract
25
S0169-4332(18)30405-7 https://doi.org/10.1016/j.apsusc.2018.02.060 APSUSC 38518
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
27 May 2017 29 January 2018 5 February 2018
Please cite this article as: Y. Li, Z. Feng, Y. He, Y. Fan, J. Ma, X. Yin, Facile way in fabricating a cotton fabric membrane for switchable oil/water separation and water purification, Applied Surface Science (2018), doi: https:// doi.org/10.1016/j.apsusc.2018.02.060
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.
Facile way in fabricating a cotton fabric membrane for switchable oil/water separation and water purification Yubin Lia,c, Ziliang Fenga, Yi Hea,b,c*, Yi Fana,b,c*, Jing Maa,c, Xiangying Yina,c a. School of Chemistry and Chemical Engineering, Southwest Petroleum University, Chengdu City, Sichuan Province, P. R. China, 610500 b. State Key Lab of Oil and Gas Reservoir Geology and Exploitation (Southwest Petroleum University), Rd. 8, Xindu District, Chengdu City, Sichuan Province, P. R. China, 610500 c. Oil & Gas Field Applied Chemistry Key Laboratory of Sichuan Province (Southwest Petroleum University), Chengdu, Sichuan, China, 610500. *Address correspondence to this author. E-mail: [email protected], [email protected]. Phone and Fax: +86 02883037315
1
ABSTRACT: With dopamine and NiFe2O4 particles, a novel modified cotton fabric (PDA-NiFe2O4@CF) was prepared by one-pot method. Surface morphology, composition of the PDA-NiFe2O4@CF were investigated with SEM, EDX, XRD and FT-IR, respectively. According to the results, the cotton fiber surface was well coated with NiFe2O4 particles. Subsequently, wetting behavior of the modified cotton fabric was determined. The PDA-NiFe2O4@CF is superamphiphilic in air, and a dual lyophobic behavior was indicated with an oil contact angle (OCA) of 153° under water and a water contact angle (WCA) of 145° under oil. The rough micro-nano scale
surface
structure
and
high-surface-energy
compositions
of
the
PDA-NiFe2O4@CF makes the surface to be easily covered by one medium and enables it to repel other unmixable medium simultaneously. Therefore, water-oil mixtures can be separated on demand. Besides, with the unusual dual lyophobic surface of PDA-NiFe2O4@CF, both two types of emulsions were separated by gravity driven. On the other hand, it was also found that the as-prepared PDA-NiFe2O4@CF had good adsorption performance for methylene blue. Keywords: Cotton fabric, Emulsion, Water-oil separation, Membrane, Adsorption 1. Introduction With frequent oil-spill accidentals and increasing amount of industrial wastewater bringing severe environmental issues, separation of oil/water mixtures has always been a hot topic of research [1-3]. Among these, emulsions are still a tricky problem for separation, especially in the presence of stable surfactants [4-8]. Traditionally, polymer membranes with small sized porous structures have focused on separating 2
oil/water emulsions. However, the fabrication methods required tedious steps, precious materials and elaborate design of pore sizes [9-11]. Furthermore, wastewater in production and life is often a complex system [12-14], and organic pollutants exist simultaneously except emulsions [15-18]. Therefore, it is of great significance to design multifunctional materials, and the demand for developing more facile and effective techniques for water treatment has never stopped. In current works, inspired by the wetting behavior of lotus, researchers have developed a variety of bioinspired superwetting surfaces and interfaces [19-28]. Encouragingly, the superwetting surface is considered as a promising candidate for oil-water separation [29-30]. Thus, with the more common supplies, such as cotton fabric and stainless steel mesh, some simple and scalable approaches were founded for separation of emulsions. Liu et al. [31] prepared a Janus cotton fabric which can be used for rapid and efficient separation of oil from oil-in-water emulsion, and Lu et al. [32] developed a facile method to fabricate double-layer stainless steel mesh for effective separation of water-in-oil emulsion with high flux. However, most of these materials only able to separate a particular type of emulsion (water-in-oil or oil-in-water). Obviously, materials that can be employed to separate both two types of emulsions simultaneously are very attractive and promising. While, the relevant report is seldom meet. Besides, Ding et al. [16] provided a strategy for separating oil-in-water emulsion and adsorbing organic dyes with a same nanofibrous membrane, which was a good reference of the design of multifunctional materials. In our work, we built an unusual dual lyophobic surface in oil–water systems. The 3
PDA-NiFe2O4@CF was prepared via one-pot method with a commercial cotton fabric, NiFe2O4 ferrites and dopamine [33]. Surface of the PDA-NiFe2O4@CF is both superoleophobic under water and hydrophobic under oil. Therefore, water-oil mixtures can be separated on demand, and both two types of emulsions can be separate with the PDA-NiFe2O4@CF by gravity driven. In addition, the modified cotton fabric exhibited an excellent adsorption capacity to methylene blue (MB). Due to the simplicity of preparation and the effectiveness of separation and adsorption, it is believed that the as-prepared cotton fabric would be a good case for practical applications. 2. Experimental 2.1 Materials and fabrication of the PDA-NiFe2O4@CF A series of chemicals, containing Ni(NO3)2.6H2O, Fe(NO3)3.9H2O, ammonia solution, dopamine, Tris-buffer, sodium hydroxide, toluene, dichloromethane, hexadecane, chloroform, hexane and methylene blue are all analytical grade reagents. Deionized water with a resistance of 18.0 MΩ was obtained from the water purification system of UPC-Ⅲ. Cotton fabrics were purchased from the store and were cut into 2.0×2.0 cm2. 4.85 g Fe(NO3)3.9H2O and 1.75 g Ni(NO3)2.6H2O were dissolved in 100 mL deionized water, and the solution was stirred for 2 h. Under continuous stirring, the pH of the solution was adjusted to 8 by ammonia solution. Following, the mixture was transferred into a Teflon lined stainless autoclave which was then maintained at 180℃ for 8 h [34]. Before it was cooled to room temperature. NiFe2O4 precipitates were 4
rinsed with deionized water and ethanol for several times. The as-obtained products were further annealed at 550℃ for 2 h with a heating rate of 2℃/min. The preparation and application process of PDA-NiFe2O4@CF is presented in scheme 1.
Scheme 1. Schematic illustration of preparation and application process for PDA-NiFe2O4@CF 300 mg NiFe2O4 particles, 40 mg dopamine and a piece of cotton fabric were added into 20 mL tris (10 mM) buffer solution whose pH had been adjusted to 8.5 by NaOH aqueous. After ultrasonic treatment for 30 min, the mixture was stirred for 24 h at 50℃, and the PDA-NiFe2O4 @CF was dried in a vacuum oven at 50℃ for 6 h. 2.2 Characterization Morphologies and compositions of the sample were analyzed by FE-SEM (JSM-7500F) and X-ray energy dispersive spectrometry (EDX, INCA). Chemical composition of the sample surface was characterized with FT-IR spectra (WQF520 spectrometer). The phase structure and crystals orientation was identified by X-ray diffraction (XRD, PANalytical X’Pert Pro diffractometer) with Cu Kα radiation. Contact angle (CA) of a 5μL droplet was measured by using the KRUSS DSA30S. TOC (total organic carbon) was measured using Shimadzu TOC-VCPH analyzer. 5
Optical microscope (Nikon Digital Sight DS-F11, Japan) was employed to analyze the emulsions before and after filtering separation. 2.3 Preparation of the emulsions and separation experiments Toluene-in-water (1:100) and water-in-dichloromethane (1:100) were made by mixing oil and water with addition of 0.2 g/L SDS and 2 g/L Span80 by sonicating for 10 min and sharply stirring for 6 h. Both two emulsions were highly stable for hours in
laboratory
environment.
Besides,
SDS
stabilized
diesel-in-water,
hexadecane-in-water, and Span80 stabilized water-in-chloroform, water-in-hexane were prepared as well. The as-prepared PDA-NiFe2O4@CF with diameter of 2.0 cm was sandwiched between two vertical glass tubes. The freshly prepared emulsions were poured into the upper tube, and the flux was determined by calculating the volume of liquid permeated within 5 min under gravity driven. The height of feed emulsions kept 10 cm. Before falling into the oil-in-water or water-in-oil emulsion, the membrane was wetted by water or oil. 2.4 Adsorption measurements The adsorption performance of PDA-NiFe2O4 @CF was tested by removing the typical organic dye of methylene blue (MB). The concentration of MB in the solution was 10 ppm and 30 ppm, respectively. In general, the modified fabric was immersed into the MB aqueous solution with stirring for a designated time, and then the resultant adsorption capacity was measured by a UV-Vis spectrometer (Shimadzu, UV-1800) at certain time intervals. 3. Results and discussion 6
Fig.1. SEM images of NiFe2O4 particles (a), pristine cotton fabric (b) and the PDA-NiFe2O4@CF (c) As indicated from the microstructure in Fig. 1, the pristine cotton fabric exhibits an obvious 3D opening geometry with fiber diameter of about 10-15 μm, and the cotton fibers are interlaced with each other. After one-pot treatment, NiFe2O4 particles were located on the fiber surface with a random homogeneous distribution. Dopamine which could self-polymerize and adhere on almost any substrates protected the particles from leaching. PDA ensured the bonding strength between NiFe2O4 and fibers. There are three major reasons for using NiFe2O4 particles. First, the metal oxide particles can improve the hydrophilicity of the cotton fabric surface. Then, when the fabric is filled, the pore size decreases, and it can be applied to the separation of oil-water mixtures. Besides, the NiFe2O4 ferrites itself has a good adsorption effect on the organic dyes. With energy dispersive X-ray (EDX) mapping, structure composition of the PDA-NiFe2O4@CF was further investigated. As shown in Fig. 2, the cotton fiber surface is uniformly coated with Ni, Fe and O elements. Besides, the corresponding X-ray diffraction (XRD) patterns of the as-prepared NiFe2O4 particles and PDA-NiFe2O4 @CF are presented in Fig. 3a. It reveals that the relevant lattice planes of the obtained PDA-NiFe2 O4@CF matched well with that of 7
the NiFe2O4 particles whose crystalline spinel structure is consistent with the XRD data in JPPDS No.10-0325 (NiFe2O4 phase) [16]. In combination of the figures, the cotton fibers were well coated with the NiFe2 O4 particles. The polydopamine of PDA-NiFe2O4@CF was proved by FT-IR in Fig. 3b. Compared with NiFe2O4 particles, in the case of the as-prepared sample, the adsorption band of 1080, 1510 and 3425 cm-1 are assigned to the stretching vibration of N-H, O-H and C-O-H [35,36], respectively. Other relative peaks around 670 and 587 cm-1 are caused by the inorganic particles [37,38].
Fig. 2. The corresponding elemental mapping images of C, O, Fe and Ni of the coated cotton fiber.
Fig. 3. XRD patterns (a) and FT-IR (b) of the as-prepared NiFe2O4 particles and PDA-NiFe2O4@CF Wetting behavior of the PDA-NiFe2O4 @CF in different medium was investigated. 8
As illustrated in Fig. 4, due to the high-surface-energy chemical compositions and the rough micro-nano scale morphologies of the surface, PDA-NiFe2O4 @CF is superamphiphilic in air. When the droplets of water and oil contacted with the modified fabric, they spread rapidly. Interestingly, as presented in Fig. 5, after immersing the PDA-NiFe2O4@CF into water or petroleum ether, either the dichloromethane or water droplet sat on the treated cotton fabric. Underwater superoleophobic or underoil hydrophobic was observed with an underwater OCA of 153° and an underoil WCA of 145°. In addition, the contact angles of different oils underwater and water contact angles under different oils is displayed in Fig. 5(b, c). The PDA-NiFe2O4@CF presents underwater superoleophobicity for different types of oils with OCA larger than 150°. Correspondingly, the PDA-NiFe2O4@CF also show hydrophobicity under different types of oils with WCA larger than 140°. Consequently, the surface of PDA-NiFe2O4@CF exhibits an unusual dual lyophobic behavior.
Fig. 4. Process of water (upper) and oil (lower) droplets touching on the surface of the PDA-NiFe2O4@CF within 1 s (the interval of each image is 0.5 s)
9
Fig. 5. (a, c) Photographs of an underwater oil droplet (red) and an underoil water droplet (blue) on the PDA-NiFe2O4@CF. (b, d) The different types of OCA underwater and WCA under different types of oils on the PDA-NiFe2O4@CF. Why is it occurred to that the PDA-NiFe2O4 @CF has the wetting behavior of underwater superoleophobic and underoil hydrophobic? It could be deduced [40] that surface of the PDA-NiFe2O4 @CF contains not only numerous hydrophilic components which mainly come from the NiFe2O4 particles, but also a certain amount of oleophilic components from polydopamine. Besides, according to the Wenzel and Cassie-Baxter model [41], the amplification effect about liquid wettability on the rough surface could increase the hydrophilicity or oleophilicity, even to superwetting. Hence, when the rough PDA-NiFe2O4 @CF is in water, the hydrophilic components predominantly interact with water through building the intermolecular force and hydrogen bonds to form a stable water film, and this could lead to underwater 10
superoleophobic. Likewise, in oil, the dominant power of the rough membrane surface is the interaction between oil and the oleophilic components, forming a stable oil film to possess underoil hydrophobic [27,39]. In brief, the results can be attributed to the rough micro-nano scale surface structure and high-surface-energy compositions of the PDA-NiFe2O4@CF which makes the surface to be easily covered by one medium and enables it to repel other unmixable medium simultaneously [42]. The special wetting behavior of the interface is an effective means to induce water-oil separation. According to the dual lyophobic behavior, the modified fabric is expected to be useful for the separation of water-oil mixtures on demand. As demonstrated in Fig. 6(a), the PDA-NiFe2O4 @CF was sandwiched between two glass tubes and a mixture of hexadecane (red) and water (colorless) was poured into the upper tube. Due to the characteristic of underwater superoleophobic, water went through the fabric while hexadecane was kept in the upper. After wetting by water, the PDA-NiFe2O4@CF was not permeable to hexadecane. On the other hand, when the coated fabric was wetted by dichloromethane (colorless) prior to separation in Fig. 6(b), the PDA-NiFe2O4@CF was permeable to dichloromethane and water (blue) was retained on the upside. Only driven by gravity, the as-prepared treated cotton fabric exhibits a good water-oil separation capability.
11
Fig. 6. Mixtures separation of (a) hexadecane(red)-water(colorless) and (b) water(blue)- dichloromethane(colorless). Furthermore, we employed the as-prepared PDA-NiFe2O4@CF to separate two typical emulsions. As presented in Fig. 7, the PDA-NiFe2O4@CF was mounted between two vertical glass tubes, and the as-prepared emulsions were poured onto the PDA-NiFe2O4@CF, respectively. Consequently, only the external phase of the emulsion permeated through the PDA-NiFe2O4@CF, and emulsions (oil-in-water and water-in-oil) were separated barely under gravity (see movie S1 and S2). On the contrary, the internal phase of the emulsion was trapped in the upper tube. Moreover, as Fig. 7 shows, optical microscopic images of the emulsions and collected filtrates indicated that no droplets of the internal phase was observed in whole view. This novel modified cotton fabric exhibits a high performance of separation of both two types of emulsions. Additionaly, the PDA-NiFe2O4@CF membrane reached a separation flux of about 300 L.m-2.h-1 by solely gravity driven, and the rejection ratio is about 99% to oil or water. The separation process is energy conservation. Moreover, 12
similar separation flux and rejection ratio were obtained for other emulsions, such as SDS
stabilized
diesel-in-water,
hexadecane-in-water
and
Span80
stabilized
water-in-chloroform, water-in-hexane. Besides, the recyclability of the PDA-NiFe2O4@CF for emulsions separation under gravity was examined. Toluene-in-water and water-in-dichloromethane stabilized by SDS and Span80 were selected as feed emulsions. Each cycle test was conducted for 30 min, and flux changes were recorded every five minutes. The PDA-NiFe2O4@CF
was
water-in-dichloromethane
employed emulsion,
to
and
then
separate was
toluene-in-water washed
by
water
or or
dichloromethane after separating. As presented in Fig. 8, in each cycle, although both water and oil fluxes decreased with separation time increasing, which is resulted from the formation of oil or water cake on the surface, the separation ability of emulsions were highly stable with only washing after each cycle. Thus, the PDA-NiFe2O4 @CF shows a certain ability of antifouling characteristic. As far as the emulsion separation performance is concerned, the wetting behavior and pore size on the surface of a membrane are all very important. Micro-nano scale structures composed of particles on the cotton fiber surface could increase the roughness of the membrane surface to form a superwetting property. As a result, underwater oleophobic and underoil hydrophobic was realized. This wetting behavior of the membrane surface help to improve the antifouling effect of the membrane during the separation process. In addition, the adhesion of nano particles on the cotton fiber surface has effectively decreased the pore size [28] in the fabric membrane for 13
emulsion separation.
Fig. 7. Emulsion separation process of the PDA-NiFe2O4@CF, and the optical microscopic images of oil-in-water emulsion (upper) and water-in-oil emulsion (lower) before and after separation.
Fig. 8. Cycling performance of PDA-NiFe2O4 @CF. Emulsions separation of SDS stabilized toluene-in-water and Span80 stabilized water-in-dichloromethane We also demonstrate that the PDA-NiFe2O4@CF with porous structure has an efficient adsorption of organic dye methylene blue (MB) in water. With the UV-vis spectrometer, the performance of PDA-NiFe2O4 @CF was determined in a 20 ml of 30 ppm MB solution. As displayed in Fig. 9(a,b), the PDA-NiFe2O4@CF could adsorb 14
about all of the MB for about 100 min. While the adsorption capacity of pristine cotton fabric is less than 5% in 100 min. In addition, another key parameter, the responsive time to MB is rapid. The as-prepared coated fabric could adsorb about 72wt.% of MB in 30 min. On the other hand, from Fig. 9(c,d), at low MB concentration of 10 ppm, the response (10 min) of PDA-NiFe2O4@CF is also competitive under weak adsorption drive force. Owing to the synergetic effect of NiFe2O4 particles and polydopamine, adsorption capacity of the fabric membrane to organic dyes is improved significantly. The content of PDA-NiFe2O4 in the as-prepared coated fabric was calculated from the mass difference of the cotton fabric before and after treatment, and the adsorption capacity was calculated within the equation (1): (1)
Where
(mg/L) and
(mg/L) are the concentration at time t and the initial
concentration, respectively. m (g) is the mass of the adsorbents and V (L) is the volume of the tested solution.
(mg/g) is the adsorption capacity. In the 30 ppm
MB solution, the equilibrium capacity of PDA-NiFe2O4@CF for MB is about 95.5 mg/g. Two major reasons contribute to the improved adsorption capacity of the hierarchical structure fabric. The increased surface area, low diffusion resistance and electrostatic attraction which is resulted from the mixed metal oxide nanoparticles[38], and the electrostatic attraction, π-π stacking interactions and hydrogen bonding that is provided by aromatic rings of PDA layer on the particles and fibers surface [43]. Consequently, the as-prepared PDA-NiFe2O4 @CF has a strong affinity for MB in 15
water. Moreover, the modified cotton fabric can be recovered facilely, which avoids the tedious recycling process and secondary pollution of regular particles adsorbents.
Fig. 9. Adsorption ability and rate of the PDA-NiFe2O4@CF in 30 ppm (a,b) and 10 ppm (c,d) MB solution, respectively. 4. Conclusions In summary, we present a facile way to preparing a modified cotton fabric, which is superoleophobic in water and hydrophobic in oil. As a result, oil-water mixtures can be treated on demand. Moreover, both two types of emulsions can be separated with the PDA-NiFe2O4 @CF by gravity driven. This phenomenon is mainly ascribed to the rough
surface
structure
and
high-surface-energy
compositions
of
the
PDA-NiFe2O4@CF which makes the surface to be easily covered by one medium and enables it to repel other unmixable medium simultaneously. Furthermore, it was also found that the as-prepared PDA-NiFe2O4 @CF had good adsorption performance for 16
organic dyes. It is believed that this novel surface provides a reference for the development of water-oil separation and water treatment techniques. Acknowledgements The authors thank Xi Chen and Qiangbin Yang for the assistance with SEM and EDX measurements. This work was financially supported by Youth Science and Technology Creative Group Fund of Southwest Petroleum University (2015CXTD03) and the Majorly Cultivated Project of Sci-tech Achievements Transition (15CZ0005) from the Education Department in Sichuan Province. References [1] M.A. Shannon, P.W. Bohn, M. Elimelech, J.G. Georgiadis, B.J. Marinas, A.M. Mayes, Science and technology for water purification in the coming decades, Nature 452 (2008) 301-310. [2] S. Pezeshki, M. Hester, Q. Lin, J. Nyman, The effects of oil spill and clean-up on dominant US Gulf coast marsh macrophytes: a review, Environmental pollution 108 (2000) 129-139. [3] A. Jernelöv, How to defend against future oil spills, Nature 466 (2010) 182-183. [4] X. Lin, F. Lu, Y. Chen, N. Liu, Y. Cao, L. Xu, Y. Wei, L. Feng, One-step breaking and separating emulsion by tungsten oxide coated mesh, ACS applied materials & interfaces 7 (2015) 8108-8113. [5] B. Wang, W. Liang, Z. Guo, W. Liu, Biomimetic super-lyophobic and super-lyophilic materials applied for oil/water separation: a new strategy beyond nature, Chemical Society Reviews 44 (2015) 336-361. 17
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Highlights A novel modified cotton fabric (PDA-NiFe2O4@CF) was prepared by one-pot method. The PDA-NiFe2O4@CF presents superamphiphilicity in air and dual lyophobic behavior under water or oil. Both two types of emulsions can be separated with the PDA-NiFe2O4@CF. The PDA-NiFe2O4@CF had good adsorption performance for methylene blue.
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Graphical abstract
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