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water separation
Accepted Manuscript Fabrication of oriented metal-organic framework nanosheet membrane coated stainless steel meshes for highly efficient oil/water separation Changchang Ma, Yujia Li, Pei Nian, Haiou Liu, Jieshan Qiu, Xiongfu Zhang PII: DOI: Article Number: Reference:
S1383-5866(19)31851-9 https://doi.org/10.1016/j.seppur.2019.115835 115835 SEPPUR 115835
To appear in:
Separation and Purification Technology
Received Date: Revised Date: Accepted Date:
10 May 2019 19 July 2019 19 July 2019
Please cite this article as: C. Ma, Y. Li, P. Nian, H. Liu, J. Qiu, X. Zhang, Fabrication of oriented metal-organic framework nanosheet membrane coated stainless steel meshes for highly efficient oil/water separation, Separation and Purification Technology (2019), doi: https://doi.org/10.1016/j.seppur.2019.115835
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Fabrication of oriented metal-organic framework nanosheet membrane coated stainless steel meshes for highly efficient oil/water separation Changchang Ma, Yujia Li, Pei Nian, Haiou Liu, Jieshan Qiu, Xiongfu Zhang1* State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, Dalian, 116024, China
Abstract: Due to frequent oil spill accidents, the efficient separation of oil/water mixtures is attracting one’s attention. In this work, we adopt a direct growth strategy for preparing continuous and highly oriented Zn2(bIm)4 (bIm=benzimidazole) ZIF nanosheet membranes on stainless steel meshes (SSMs) as substrate by the selfconversion of vertically arranged ZnO nanorods grown on the SSMs. This kind of perpendicularly oriented nanosheet membrane has superhigh hydrophobicity and special microstructure and thus is very promising for fabricating membrane separators and application in oil removal. On the SSMs with different meshes (500 mesh and 2000 mesh), the corresponding ZIF nanosheet membranes were successfully fabricated by the oriented growth of nanosheets to form different separators called as ZNM-SSM-500 and ZNM-SSM-2000, respectively. They were applied in the separation of different oil/water mixtures by using dichloromethane, petroleum ether, n-hexane, toluene, nitrobenzene and cyclohexane as model oils, and could demonstrate excellent separation performance. The ZNM-SSM-500 separator shows a separation efficiency of 99.8% with high flux of over 100000 L·m-2·h-1 for free oil/water mixtures containing 50 wt% water, while the ZNM-SSM-2000 separator * Corresponding Author: E-mail: [email protected] (X. Zhang) 1
exhibits a water rejection of as high as 99.95% for water-in-oil emulsions containing 2 wt% water. Moreover, the resulting nanosheet membrane could also demonstrate excellent thermal stability and recyclability. Therefore, the oriented nanosheet membrane is of great application potential in practical oil/water separation. Keywords: MOFs modified membrane; nanosheet membrane; oil/water mixture separation; oil/water emulsion separation 1. Introduction In recent years, worldwide oil pollution problems caused by frequent oil spill accidents, the rapid development of petroleum industry and the discharge of oily wastewater, have received extensive attention [1-3]. Oil-polluted water usually contains toxic chemicals, which may harm people's health and affect the balance of ecosystems [4].
Thus, the separation of oil and organic solvents from oily wastewater is becoming
a serious issue and an important global challenge for environmental and ecological protection. The conventional oil/water separation techniques, such as gravity separation, magnetic separation and flotation technologies, usually suffer from the high operational costs and low separation efficiency
[5].
Especially, in a sudden oil spill
accident, a large number of porous materials (e.g., foams, sponges and textiles) are usually used to absorb oil. However, unfortunately, such porous materials can often absorb both oil and water simultaneously, as a result, the oil separation efficiency for the process is quite low. Furthermore, these materials already used are commonly burned or buried, since their recycling is quite difficult, thus unavoidably resulting in a secondary environmental pollution
[6, 7].
2
Therefore, it is urgent to find highly
efficient, environment-friendly and recyclable materials or techniques for the separation of oil/water mixtures. Membrane technology with low energy consumption and simple separation equipment, as an emerging efficient separation technology, is considered a suitable method for separating oil/water mixtures in industry
[10].
[8, 9],
especially for the abundant emulsions
But the core of membrane technology depends on highly efficient
membrane materials. To date, considerable efforts have been devoted to the research on polymeric membranes
[11, 12],
ceramic membranes
[13],
zeolite membranes
[14],
and
carbon-based membranes [15] for oil/water separation. Despite great progress has been made, some issues such as separation efficiency, filtration flux, oil-fouling, etc. still exist in these kinds of membranes due to the intrinsical properties of the membrane materials, the small pore size of the membrane supports and the viscous oil. Actually, it is well known that the surface wettability of separation membranes plays a crucial role in separating oil/water mixtures because it can greatly improve the water flux and anti-oil-fouling ability
[16, 17].
Specifically, hydrophilic/oleophobic
materials and hydrophobic/oleophilic materials with special wettability are considered to be the most promising materials for the separation of oil/water mixtures
[18, 19].
Moreover, it is also very important to select some special membrane substrates with a pore size of tens to hundreds of microns, such as stainless steel mesh, copper mesh, etc.
[20-22],
since these can remarkably promote membrane filtration efficiency for the
separation of oil/water mixtures. For instance, Zhang et al.
[23]
prepared a Cu(OH)2
nanowired-haired membrane by surface oxidation of a copper mesh in an alkaline aqueous solution with (NH4)2S2O8 with a water contact angle of nearly 0° and oil contact angle of about 155°, exhibiting its superhydrophilic and underwater 3
superoleophobic property. And this membrane showed an efficient separation performance with high separation efficiency and separation capacity for both immiscible oil/water mixtures and oil-in-water emulsions. Yue et al.
[24]
fabricated a
hierarchical layer double hydroxide (LDH) membrane on a cellulose support with superhydrophobicity and superoleophilicity, showing excellent separation properties for free oil/water mixtures and surfactant-stabilized water-in-oil emulsions. Moreover, the LDH/cellulose membrane displayed stable recyclability and anti-corrosion properties in harsh conditions. Wang et al.[25], for the first time, synthesized a ZIF-8 membrane on stainless steel meshes for efficient oil/water separation. Owing its superhydrophilicity in air and superoleophobicity under water after prewetting, the membrane gave a high flux of as high as 50 L∙ m-2∙ s-1 and separation efficiency of over 99.9% with excellent stability and recyclability under a gravity-driven force. Jiang et al.[26] fabricated a Co3O4 nano-needle coated stainless steel mesh by a simple hydrothermal synthesis and subsequent calcination method. The mesh successfully accomplished the separation of alkaline and saline oil-in-water emulsions with a high efficiency of above 99% and a high flux of 2000 L∙m-2∙h-1, and may be an excellent membrane material for oil/water emulsion separation in a harsh environment. Cheng et
al.[27]
fabricated
Cu@Ag
film
with
superhydrophilic
and
underwater
superoleophobic properties on porous copper substrate by a simple acid etching and replacement reaction. Interestingly, the film became hydrophobic and superoleophilic after modified with n-dodecanethiol (DDT), denoted as Cu@Ag@DDT, realizing both oil-in-water and water-in-oil emulsion separation by utilizing the different surface wettability of the films. In addition, Guo et al.[28] coated HKUST-1 on the surface of stainless steel meshes by using polydopamine as a binder to obtain the 4
separators and applied them in both oil-in-water and water-in-oil emulsions via selective water filtration and adsorption, respectively, achieving an excellent separation performance. Therefore, the above investigation results have shown that the separators fabricated with appropriate membrane/film materials can greatly improve the separation performance for oil/water mixtures. Nevertheless， the design and fabrication of a high-performance membrane for highly efficient separation of oil/water mixtures is still a crucial challenge to membrane experts
[29].
Recently, the fabrication of two-dimensional (2D) metal-
organic frameworks (MOFs) nanosheets as a new class of 2D materials into a membrane/film supported on a porous ceramic substrate has demonstrated superhigh gas separation properties and has potential applications due to both the exceptional characteristics of the nanosheets and special microstructure of the membrane
[30-32].
However, it is still rather difficult to prepare such structured membranes fabricated with MOF nanosheets. The only method adopted is a two-step process so far, called as “top-down” method, including first the exfoliation of bulk laminar MOFs to obtain single-layer dispersed nanosheets and then the coating of the exfoliated nanosheets on the substrate to form nanosheet membranes
[33, 34].
Unfortunately, this “top-down”
method easily causes both some morphological damage, structural deterioration of the detached nanosheets during the exfoliation process and the pore interleaving of the achieved nanosheet membrane during the coating step, thus easily resulting in a lowperformance membrane. Moreover, it is also rather arduous, time-consuming and costly. In this report, based on our previous work
[35-37],
we use a bottom-up strategy to
directly prepare highly oriented Zn2(bIm)4 ZIF nanosheet membranes on SSMs for 5
fabricating hydrophobic filtrators and attempt to apply them in efficiently separating oil/water mixtures. ZIFs, as a subclass of MOFs, are constructed by bridging metal (Zn, Co) tetrahedra with imidazolate (or imidazolate derivative) ligands, which possess large surface areas, microporosity, and relatively high thermal and chemical stabilities. The Zn2(bIm)4 units are considered as subunits of 2D structure, in which the apices are occupied by Zn atoms and the sides are formed by benzimidazole ligands[36]. The fabrication strategy is based on the self-conversion of a layer of ZnO nanorods into a ZIF nanosheet membrane layer, as shown in the preparation scheme of Fig. 1. In this method, the ZnO nanorods layer on the SSM can act as a multifunctional role including the metal source, nucleation sites for the growth of ZIF nanosheet membrane and the anchoring sites between the membrane and SSM substrate, thus favoring the formation of a stable and continuous oriented nanosheet membrane. Based on the SSMs of different meshes, two nanosheet membrane separators, called as ZNM-SSM-500 and ZNM-SSM-2000, were fabricated and applied in separating different kinds of oil/water mixtures.
Fig. 1. Scheme of the self-conversion of ZnO nanorods into oriented Zn2(bIm)4 nanosheet membranes supported on the SSMs. 6
2. Experimental section 2.1. Materials and chemicals Stainless steel meshes (SSMs: 500 mesh size and 2000 mesh size) as substrates (called as SSM-500 and SSM-2000) were purchased from the local market (Dalian, China). The SSMs were cut into small square blocks (3 cm × 3 cm) and then ultrasonically cleaned using acetone, distilled water and ethanol for twenty minutes, respectively, and dried at 80 ℃ for 2 h for later use. The chemicals used for the membrane preparation include zinc acetate (Zn(Ac)2·2H2O,
98.0%),
monoethanolamine
(C2H7NO,
MEA,
99.0%),
hexamethylenetetramine (C6H12N4, ≥99.0%), ammonium hydroxide (NH4OH, 28–30% aqueous
solution),
toluene
(PhMe,
≥ 99.0%),
zinc
nitrate
hexahydrate
(Zn(NO3)2·6H2O, ≥ 99.0%), ethylene glycol monomethyl ether (C3H8O2, EGME, 99.0%), anhydrous methanol (MeOH, ≥ 99.5%), acetone (≥ 99.5%), ethanol (≥ 99.7%), methyl orange and span 80 purchased from Sinopharm Chemical Reagent Co., Ltd. Benzimidazole (bIm, ≥ 99.0%) was purchased from Sigma-Aldrich Chemical Co., Ltd. All the above chemicals were used directly without further purifications. 2.2. Preparation of Zn2(bIm)4 nanosheet membranes for fabricating ZNM-SSM-500 and ZNM-SSM-2000 separators 2.2.1. Growth of a layer of ZnO nanorods on the SSMs A layer of ZnO nanorods was grown on the SSM by a simple hydrothermal process according to our previous report [39, 40], with minor modification. Brifly, the SSM was first coated using a stable Zn-based sol containing 14.3% Zn by dip-coating technique 7
before calcination at 400 ℃ for 3 h to get a thin layer of ZnO nanoparticles as seeds on the SSM for growing ZnO nanorods. Then, the SSM with ZnO nanoparticles was vertically placed in a synthesis solution in a 100 mL Teflon-lined autoclave for crystallization at 100 °C for 6 h to get a layer of vertically aligned ZnO nanorods for the preparation of ZNM-SSM-500 and ZNM-SSM-2000, respectively. The synthesis solution was obtained by 2.38 g Zn(NO3)2·6H2O and 1.12 g hexamethylenetetramine dissolved in 80 mL distilled water for stirring at room temperature for 0.5 h. 2.2.2 Preparation of ZNM-SSM-500 and ZNM-SSM-2000 Zn2(bIm)4 nanosheet membranes were grown on the ZnO nanorods layer-coated SSMs with different meshes (500 and 2000 mesh) in a Zn-free synthesis solution with a molar ratio of 1bIm:1NH4OH:45MeOH:45PhMe for achieving ZNM-SSM-500 and ZNM-SSM-2000, respectively. The solution was prepared as follows: 19.0 mL MeOH, 18.0 mL PhMe and 3.0 mL NH4OH were mixed together before adding 1.18 g bIm, and stirred around 5 min to get a clear solution. The SSM (500 mesh) with the ZnO nanorods was placed vertically in the above solution in an autoclave for selfconversion at 100 ºC for 12 h to obtain the ZNM-SSM-500. The SSM with 2000 mesh was used under the same condition for the growth to achieve the ZNM-SSM-2000. After the growth, the ZNM-SSM-500 and ZNM-SSM-2000 were carefully taken out and then soaked in methanol solution for two days to remove impurities. Finally, they were dried at room temperature for later use. 2.3. Separation experiments for free oil/water mixtures The separation experiment of free oil/water mixtures in which oil and water are not immiscible was implemented at room temperature by a homemade separation device with an efficient separation area of 3.14 cm2 with gravity as driving force. Six 8
different organic solvents, including dichloromethane, petroleum ether, n-hexane, toluene, nitrobenzene and cyclohexane, were chosen as the model oils. The ZNMSSM-500 was fixed on the separation device. Mixtures of water (dyed with methyl orange) and oil phase (v/v, 1/1) were poured into the device. The oil phase could pass though the membrane, but water was intercepted due to the hydrophobic/oleophilic properties of the membrane. The separation efficiency (η) can be calculated by the following equation. η=(m1/m0) ×100% where m0 is initial mass of water, m1 is the mass of the water that had been intercepted after separation. The separation flux (F) can be calculated by the following equation. F= V/(S×t) Where V is the volume of oil that permeates through the membrane, S is the efficient separation area, and t is the filtering time. 2.4. Separation experiments for emulsified oil/water mixtures The ZNM-SSM-2000 was used to separate emulsified oil/water mixtures in which the mixtures are not stratified. The surfactant-stabilized water-in-oil emulsion was prepared as reported in the literature with minor modification
[16].
Briefly, 0.3 g
of span 80 was added into 50 mL of dichloromethane and 1 mL of water with vigorous stirring for 6 h at room temperature to obtain a milk white solution. The separation process was similar to the above separation of free oil/water mixtures. The water rejection (R) was then calculated by the following equation. 9
(
R= 1‒
)
Cfiltrate Cfeed
× 100%
Where Cfiltrate and Cfeed are the water concentrations in the original emulsions and the filtrate, respectively. 2.5. Reusability and thermal stability tests for the membranes A repeatable separation experiment for oil/water mixtures was carried out to assess the reusability of the nanosheet membrane grown on the SSM. The used membrane was washed with ethanol and dried at 60℃ for 2 h. Then the oil separation efficiency for the used membrane was tested under the same condition as described above. To investigate the thermal stability of the as-prepared membrane in a high temperature, the membrane was calcined for 1 h in 100℃, 200℃, 300 ℃, and 400℃, respectively, before a separation experiment for oil/water mixtures was carried out. 2.6. Samples characterization The surface morphologies of the nanosheet membranes coated SSMs were recorded by scanning electron microscopy (SEM, FEI Nova NanoSEM 450 and Quanta 450 operating at 3 and 20 keV, respectively) and a Tecnai F30 transmission electron microscope (TEM). X-ray diffraction (XRD) analysis was recorded on a D/max-2400 X-ray diffractometer with Cu Kα radiation in the 2θ range of 3−60° operating at 40 kV/100 mA. The water contact angle instrument (OCAH200, Data physics, Germany) was utilized to measure the wettability of the samples. The concentrations of the water in original emulsions and filtrate were analyzed by using a Karl Fischer moisture titrator (831KF Coulometer). The optical microscopy images
10
were taken on Olympus BX53 by dropping emulsion solution on biological counting board.
3. Results and discussion 3.1. Formation and characterization of nanosheet membranes The like-fish-scale nanosheets grow outward on the SSM surface and form a layer of extremely coarse surface, which could greatly improves the hydrophobicity. According to our previous reports
[35],
a highly oriented (parallelled to the
surface of substrate) Zn2(bIm)4 nanosheet membrane for gas separation could be prepared by a direct growth strategy on a porous Al2O3 tube as substrate coated with a layer of ZnO nanoparticles (NPs). Based on the above work, here, a highly oriented Zn2(bIm)4 nanosheet membrane which is perpendicular to the metal rod surface of the SSM substrate was synthesized by the self-conversion of a layer of ZnO nanorods grown on the SSM. It is essential to pre-introduce a layer of ZnO nanorods on the SSM that plays three important roles: (1) connecting the membrane layer and the SSM substrate tightly to avoid the membrane from peeling off; (2) supplying the metal source for the growth of Zn2(bIm)4 nanosheet membrane and providing heterogenous nucleation sites for favoring the uniform growth of membrane on the surface of the substrate; (3) controlling the growth direction of nanosheets perpendicular to the metal rod surface of the SSM. Moreover, it should be emphasized that the synthesis was conducted in a Zn-free solution only containing bIm ligands, which makes the membrane favorably grow just on the SSM, avoiding the generation of nucleation and crystal growth in the bulk solution. Fig. 2 is the SEM images of the SSMs and membranes achieved. It is clearly seen that the grid pore diameter of the 11
bare SSM-500 is about 30-40 μm (Fig. 2a) and the bare SSM-2000 is around 7 μm (Fig. 2g) and their surface are extreme smooth (Fig. 2b, h). As shown in Fig. 2c, d, a layer of vertically arranged ZnO nanorods with a diameter of about 200 nm and length of around 2 μm was regularly grown on the surface of the SSM. This layer greatly favors the self-converted formation of a nanosheet membrane and also reduces the grid pore diameter of the SSM. After the self-conversion in the Zn-free ligands synthesis solution, a layer of membrane consisting of nanosheets was formed as shown in Fig. 2e-f, i-j. The ZIF sheets were closely arranged on the surface in parallel each other (perpendicular to the substrate surface), forming a continuous membrane layer. Every nanosheet had around 2-4 μm length, 2 μm width and about 20 nm in thickness, showing a high aspect ratio (Fig. 2f, j). The majority of the pore diameters for the SSM-500 are reduced to 10-15 μm after the growth of nanosheet membrane (Fig. 2e), except for some large pore sizes at individual locations, because the pore sizes of the SSM substrate are not uniform. The existence of these large pore sizes is conducive to increasing oil/water separation flux while ensuring a superb separation efficiency. Moreover, after the membrane growth, the surface roughness of the SSM was greatly increased compared with the bare SSM and its hydrophobicity was also improved as shown in Fig. 4, which is also beneficial to the separation of oil/water mixtures. It should be emphasized that for the ZNM-SSM-500 supported on the SSM500, the nanosheet membrane only grows on the surface of the metal rods and does not cover the meshes due to the larger grid holes. Actually, the grown nanosheet membrane mainly acts as a modification layer on the surface of the SSM and simultaneously changes the hydrophobicity and coarseness of the SSM surface. Therefore, the ZNM-SSM-500 separator is mainly applied in separating free oil/water mixtures, demonstrating its high filtration flux. However, this kind of separator is not 12
suitable for the separation of oil/water emulsion due to the existence of grid pores. While the SSM- 2000 possesses relatively small grid pore size, thus the ZIF nanosheet
Fig. 2. SEM images of the bare SSM-500 (a, b) and SSM-2000 (g, h), ZnO nanorods layer grown on the SSM-500 (c, d) and oriented Zn2(bIm)4 nanosheet membranes 13
self-converted on the SSM-500 (e, f), and on the SSM-2000 (i, j), respectively. The inset is the cross section of the ZnO rods grown on the SSM. membrane layer can completely cover the grid holes of the SSM-2000 and thus form a continuous nanosheet membrane on the surface of the SSM, acting as a separation function through both the pores of the nanosheet membrane and the interspaces among the oriented nanosheets. Therefore, the ZNM-SSM-2000 separator is mainly applied in the separation of oil/water emulsions, indicating its high selectivity for small oil droplets in emulsified oil/water mixtures. A series of experiments on different crystallization times of nanosheets were done on glass slides as substrate in view of its flat surface to further explore the details of the self-conversion of ZnO nanorods into the oriented Zn2(bIm)4 nanosheet membrane. As shown in Fig. 3a, b, in the initial stage of the reaction (2 h), the ZnO nanorods dissolved from the center section in the basic solution containing both bIm and ammonia and shaped into hollow morphology. Moreover, on the wall of the nanorods, a large number of tiny crystals appeared. From the cross-sectional SEM image, it could be also seen some precursors of Zn2(bIm)4 nanosheets produced by the coordination of the dissolved Zn2+ from the ZnO nanorods with bIm ligands in the solution. As the reaction time was prolonged to 7 h (Fig. 3c, d), the nanosheet precursors obviously increased and covered the most surface of the top of the ZnO nanorods. The cross-sectional SEM image indicates that the spaces between the nanorods were hardly observed by the filling of the nanosheet precursors generated. Actually, the precursors produced in the initial stage can act as a seed role and induce the formation of Zn2(bIm)4 nanosheets along the ZnO nanorods as shown in Fig.3e, f. When the crystallization time was increased to 12 h, a layer of nanosheets was formed 14
on the top of the ZnO nanorods in which the nanosheets grew upward in parallel (perpendicular to the substrate surface) on the top of the ZnO nanorods. Moreover, the nanosheets arranged closely and were highly oriented, thus forming an oriented nanosheet membrane. XRD patterns of Fig. 3g clearly indicate that the nanosheets belong to the typical Zn2(bIm)4 structure which is in accordance with simulated Zn2(bIm)4 crystal structure, and the membrane shows merely one reflection [223], indexed to the crystallographic plane of the Zn2(bIm)4 structure, except for ZnO characteristic peaks, thus confirming the highly orientation of the as-prepared nanosheet membrane (ref. no. 675375, Cambridge Crystallographic Data Centre).
15
Fig. 3. SEM images of the samples obtained on the ZnO nanorods by different growth times (a,b: 2h; c,d: 7h; e,f: 12h), XRD patterns of different samples (g), and Oil contact angle of bare SSM substrate and nanosheet membrane with 500 mesh size and 2000 mesh size (h), respectively. The wettability of the above highly oriented ZIF nanosheet membrane was monitored by measuring the water and oil contact angle, respectively. As shown in Fig.4, the water contact angles of the original bare SSM substrates for both 500 mesh and 2000 mesh were near 90° (86.5° and 88°), showing that the SSMs are slightly hydrophilic. While the water contact angle of the ZnO nanorods layers was nearly 0°, indicating that the ZnO nanorods layers are completely hydrophilic. Surprisedly, the water contact angles of the nanosheet membranes grown on the SSMs markedly rose to 140° for the ZSN-SSM-500 and 152.5° for the ZSN-SSM-2000, demonstrating the strong hydrophobicity due to the excellently hydrophobic characteristic of the ZIF nanosheets. From the Fig. 3h, it is clearly seen that the nanosheet membrane possesses an oil contact angle of near 0°. Once oil droplets reach the membrane surface, they immediately spread out and wet it, and then penetrate through the membrane. Thus, this further confirms the superoleophylic property of this kind of nanosheet membrane.
16
Fig. 4. Water contact angles of the bare SSM substrates, ZnO nanorods layers and nanosheet membranes with 500 mesh size and 2000 mesh size, respectively. Moreover, the nanosheet layer with a like-fish-scale structure grown on the SSMs also favors increasing the hydrophobicity of the membrane materials, since the roughness and chemical composition of a material surface greatly influences its wettability. Based on the above analysis, the separation mechanism can be summarized as the following two aspects: (1) the Zn2(bIm)4 nanosheet material has excellently hydrophobic property; (2) the nanosheet membrane layer with a like-fishscale structure favors increasing the surface roughness, thus greatly improving the hydrophobicity of the membrane. According to the Cassie model theory that roughness increases hydrophobicity, thus, water contact angle also rises
[40].
A model
equation can be used to describe the wetting behavior as follows: cosθ*=-1+(1+cosθ)f Where θ is the contact angle of the original substrate, and θ* is the contact angle of a modified rough surface, f presents the area fraction of the liquid that is in contact with air. 17
Therefore, based on the above theory, it is not difficult to understand that why the water contact angle of the highly oriented membrane grown on the SSM-2000 is bigger than that on the SSM-500. This result should be attributed to the following two reasons: 1) the increase in the number of the Zn2(bIm)4 nanosheets with intrinsic hydrophobicity per unit area enhanced the repellent of water; 2) the high orientation and continuity of the Zn2(bIm)4 nanosheets improved the surface roughness and microstructure of the SSM-2000 substrate. 3.2. Separation of free oil/water mixtures To evaluate the hydrophobic separation ability of the above as-prepared highly oriented membrane grown on the SSM with 500 mesh (ZNM-SSM-500), a gravitydriven separation device was used to separate different free oil/water mixtures. The ZNM-SSM-500 was first used to separate a dichloromethane/water mixture in which dichloromethane as oil phase, with a higher density, stayed at the bottom, while water dyed with methylene orange for convenient observation was in the upper part due to a lower density. As shown in Fig. 5a, it was clearly observed that when the dichloromethane/water mixture was quickly poured into the separation device, the oil phase could immediately wet and permeate the membrane and grid pores, whereas the water was held back by the membrane. The filtrate was also transparent and limpid, suggesting that no water permeated through the ZNM-SSM-500. Moreover, the separation time was very short, the flux was as high as 101400 L·m-2·h-1 and separation efficiency reached over 99.8％ for the dichloromethane/water mixture through the ZNM-SSM-500.
Therefore， the ZNM-SSM-500 (highly oriented
membrane grown on the SSM with 500 mesh) could efficiently separate free
18
dichloromethane/water mixtures due to its
special structure and strong
hydrophobicity. Actually, for oily wastewater, there usually exist two kinds of states: One is the oil sinking into the water for heavy oil wastewater; another is floating on the water for light oil wastewater. Thus, a series of heavy and light oils were selected as representatives, respectively, for this separation experiment to further check the separation properties of the ZNM-SSM-500. Fig. 5b shows the separation results for the application of the ZNM-SSM-500 in the different oil/water mixtures. The separation efficiencies of the ZNM-SSM-500 for both the heavy and light oil/water
Fig. 5. Gravity-driven separation device and process (a) and separation performance (b) for free oil/water mixtures of the nanosheet membrane with 500 mesh size.
19
mixtures exceeded over 99.6%, though the fluxes had some difference, showing that the ZNM-SSM-500 is of wide adaptability in separating free oil/water mixtures due to its strong hydrophobicity. It should be pointed out that the light oil/water mixtures must be poured into the separation device slowly in order that the oil rather than the water could first contacts the nanosheet membrane and thus a high separation efficiency could be achieved for light oil/water mixtures. In order to understand the separation mechanism of free oil/water mixtures through the as-prepared nanosheet membrane coated SSM, a model was designed to describe the separation process. As shown in Fig. 6, oil phase can easily pass through the ZNM-SSM-500 by gravity without other energy, however, the water phase is prevented from permeating through the ZNM-SSM-500 due to the repulsive forces. The highly oriented nanosheets with nanoscale spacing and perpendicular to the surface of the SSM effectively increase the roughness of the surface and thus the air is trapped between the nanosheets, forming a gas–liquid–solid three-phase interface. The gas-solid interface can prevent the water phase from wetting and permeating. The following equation can describe the separation mechanism of oil/water mixtures through the highly oriented nanosheet membrane grown on the SSM.
∆𝑃 =
2γLV 𝑅
= ‒
CγLVcos θ A
Where γLV represents the surface intension, R is the radius of the meniscus, θ is the water or oil contact angle, C is the circumference of the mesh pore and A represents the cross-sectional area of the mesh pore. For the case of the strong hydrophobic membrane, the water contact angle θ is larger than 140°, so the theoretical intrusion pressure ∆𝑃>0, which explains why the water can’t pass through 20
the membrane. On the contrary, the oil contact angle of the membrane is nearly 0°, so the theoretical intrusion pressure ∆𝑃<0, thus, indicating that the oil phase can easily permeate the membrane. This formula well explains the reason that the highly oriented membrane can efficiently separate oil/water mixture in theory, which is completely in accordance with the above experiment results.
Fig. 6. Schematic diagrams of the wetting model of the nanosheet membrane. 3.3. Separation of oil/water emulsion In actual situations, besides free oil/water mixtures, there still exists a class of emulsied oily wastewater from many industrial processes, in which oil or water in the form of droplets is dispersed in the mixtures and there is no clear boundary between oil phase and water phase. Therefore, it is also necessary to separate these emulsied oil/water mixtures, but it is more difficult and challenging because the diameter of dispersed water/oil droplets in water-in-oil/oil-in-water emulsion is fairly small. So it is required that the pore size of the filter fabricated has to be small so enough as to have the opportunity to efficiently separate the emulsion. As discussed above in the 3.1 section, the ZNM-SSM-500 separator fabricated with the nanosheets still has large grid pore sizes of about 10-15 μm, while the water or oil droplets dispersed in the emulsion are usually very small and only several microns in diameter. Therefore, it is no doubt that the ZNM-SSM-500 separator is not suitable for this kind of the 21
separation of water-in-oil/oil-in-water emulsion, though it has high filtration flux. Consequently, we selected the SSM with 2000 mesh as substrate (SSM-2000) to support the nanosheet membrane because it possesses relatively small grid pore size. As a result, the oriented nanosheet membrane fully covered the grid pores of the SSM-2000 and thus a ZNM-SSM-2000 separator without any visible holes was achieved. Then we made attempt to apply the as-fabricated hydrophobic ZNM-SSM2000 in separating a 2 wt% water-in-oil emulsion formed by dispersion of a little water in much dichloromethane. The separation process of the water-in-oil emulsion was similar to that of the free oil/water mixtures. When the stable water-indichloromethane emulsion was poured onto the highly oriented nanosheet membrane supported on the SSM-2000, the oil immediately wet and permeated through the membrane with gravity as driving force without other energy, but the water was retained above, thus realizing the efficient separation of water-in-oil emulsion as shown in Fig. 7a, b. Optical microscopy was applied to inspect the separation performance of the emulsion through the ZNM-SSM-2000. A large amount of water droplets exist in the emulsion (Fig. 7a) and are less than 5μm in size. But after the treatment with the ZNM-SSM-2000, no obvious water droplets can be observed in the filtrate in the whole view (Fig. 7b), demonstrating the high separation efficiency of the ZNM-SSM-2000. Moreover, from the Fig. 7c, the original water-in-oil emulsion was white, milky and presented an evident Tyndall phenomenon with a clear red light beam, while the filtrate became transparent and also had no Tyndall effect. In the separation process, the maximum flux could reach about 636 L·m-2·h-1 and the water rejection was as high as 99.9%, which further confirmed the high separation performance of the ZNM-SSM-2000 separator in the water-in-oil emulsion.
22
Fig. 7. Optical micrographs (a, b) and images (c) of the water-in-oil emulsion before and after separation. 3.4. Recyclability and stability of the highly oriented nanosheet membrane Generally, in the separation process of oil/water mixtures, besides the separation efficiency and flux, the stability and durability of the membrane are also important due to the harsh environment in actual separation application of oil/water mixtures. Therefore, we conducted both the high-temperature stability and recyclability of the both ZNM-SSM-500 and ZNM-SSM-2000 separators. Especially, it is usually thought that ZIF-based materials are not stable enough. Thus we first investigated the thermal stability by means of the calcination for 1h at different temperatures, respectively, for the prepared nanosheet membrane, then a series of relative characterizations for the calcined samples were made, as shown in the Fig.8. It is clearly seen from the SEM images of Fig. 8a-d that the sheet-like morphologies of three samples obtained after the calcination at 100, 200 and 300 ℃ for 1h, respectively, were still kept intact, while the nanosheet structure of the sample 23
calcined at 400 ℃ for 1h was completely destroyed, showing that the nanosheet membrane has a considerable thermal stability within the range of no more than 300℃. From the Fig. 8e, the water contact angles of the samples were slightly decreased after the calcination, but the separation efficiencies of the calcined membrane for the oil/water mixtures were still as high as over 99%, further confirming that the calcined membranes are still perfect and can demonstrate excellent separation performance for the oil/water mixtures. However, for the sample calcined at 400℃ for 1h, its water contact angle was close to 0° due to the destruction of the nanosheet membrane and thus its wettability was also changed from hydrophobicility to hydrophilicity, thus resulting in the fact that there was no separation ability for the free oil/water mixtures. XRD pattern of Fig. 8f also indicated that there was not any characteristic peak of Zn2(bIm)4 nanosheets except for the peaks of ZnO for the sample calcined at 400 ℃ for 1h, which is in agreement with the above SEM and separation results.
24
Fig. 8. SEM images of the calcined samples (a-d, a: 100 ℃; b: 200 ℃; c: 300 ℃; d: 400 ℃ ) and the change in water contact angles and separation efficiencies of the different samples (e), and XRD patterns of Zn2(bIm)4 powders and the membrane calcined at 400 ℃ for 1 h (f). The recyclability of the fabricated separators was also evaluated for the ZNMSSM-500 using the free dichloromethane/water mixture and ZNM-SSM-2000 separators using the water-in-oil emulsion containing 2 wt% water. As shown in the Fig. 9a, the ZNM-SSM-500 separator gave a high oil/water separation efficiency of over 99% and there was no obvious reduction in oil/water separation performance within the cycles of 20 times. Meanwhile, the flux of the ZNM-SSM-500 separator 25
did not decrease significantly except for slight fluctuations. Moreover, the sheet-like morphology of the membrane was still kept intact after the separation cycles of 20 times (Fig. 9b). As for the ZNM-SSM-2000 separator, both the flux and the separation efficiency decreased slowly with the increasing of times (Fig. 9c, d). However, after washing the separator with ethanol and drying, the flux and separation efficiency returned to the original state, indicating that the ZNM-SSM-2000 separator was fouled during the cycle, but the separator could be restored to its original state after a simple washing process. Therefore, the strong hydrophobic/oleophilic ZNM-SSM500 and ZNM-SSM-2000 separators could demonstrate a high separation efficiency and flux for oil/water mixtures as well as excellent thermal stability and recyclability.
Fig. 9. Recyclability for the separation of oil/water mixtures of ZNM-SSM-500 (a, b) and ZNM-SSM-2000 (c, d). 4. Conclusions
26
In conclusion, we for the first time prepared highly oriented and continuous ZIF nanosheet membranes on the SSMs coated with a layer of ZnO nanorods as substrate by a ZnO-induced direct growth strategy. The ZnO nanorods grown on the SSMs can not only provide the zinc source and serve as nucleation and anchoring sites for the growth of the nanosheet membrane, but also control the growth direction of nanosheets. Hence, the grown nanosheets are perpendicular to the SSM surface and parallel to each other, forming a highly oriented and like-fish-scale membrane layer with strong hydrophobicity. Based on the SSMs with different meshes, two nanosheet membrane separators (ZNM-SSM-500 and ZNM-SSM-2000) were fabricated and employed to separate free oil/water mixtures and water-in-oil emulsion, respectively, using dichloromethane, petroleum ether, n-hexane, toluene, nitrobenzene and cyclohexane as model oils. The ZNM-SSM-500 separator unexpectedly gave a high separation efficiency of 99.8% and flux of 101400 L·m-2·h-1 for free oil/water mixtures containing 50 wt% water, while the ZNM-SSM-2000 separator exhibited water rejection as high as 99.9% and the flux of about 636 L·m-2·h-1 for water-in-oil emulsion dispersed with 2 wt% water. More importantly, the perpendicularly oriented nanosheet membrane also has excellent thermal stability and recyclability for the separation of oil/water mixtures, which are very important in practical industrial applications. Therefore, it is believed that it has great potential for the scalable preparation of the nanosheet membrane with unique oil/water separation properties. Acknowledgments The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (Nos. 21878032, 21476039). References 27
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Graphical abstract
34
Research Highlights：
1. A layer of highly oriented Zn2(bIm)4 nanosheet membrane was successfully grown on stainless steel meshes (SSMs). 2. The like-fish-scale structure of the nanosheet membrane increased the surface roughness and hydrophobicity of the SSMs. 3. The membrane exhibited excellent separation performance for both free oil/water mixtures and water-in-oil emulsion.
35
S1383-5866(19)31851-9 https://doi.org/10.1016/j.seppur.2019.115835 115835 SEPPUR 115835
To appear in:
Separation and Purification Technology
Received Date: Revised Date: Accepted Date:
10 May 2019 19 July 2019 19 July 2019
Please cite this article as: C. Ma, Y. Li, P. Nian, H. Liu, J. Qiu, X. Zhang, Fabrication of oriented metal-organic framework nanosheet membrane coated stainless steel meshes for highly efficient oil/water separation, Separation and Purification Technology (2019), doi: https://doi.org/10.1016/j.seppur.2019.115835
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Fabrication of oriented metal-organic framework nanosheet membrane coated stainless steel meshes for highly efficient oil/water separation Changchang Ma, Yujia Li, Pei Nian, Haiou Liu, Jieshan Qiu, Xiongfu Zhang1* State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, Dalian, 116024, China
Abstract: Due to frequent oil spill accidents, the efficient separation of oil/water mixtures is attracting one’s attention. In this work, we adopt a direct growth strategy for preparing continuous and highly oriented Zn2(bIm)4 (bIm=benzimidazole) ZIF nanosheet membranes on stainless steel meshes (SSMs) as substrate by the selfconversion of vertically arranged ZnO nanorods grown on the SSMs. This kind of perpendicularly oriented nanosheet membrane has superhigh hydrophobicity and special microstructure and thus is very promising for fabricating membrane separators and application in oil removal. On the SSMs with different meshes (500 mesh and 2000 mesh), the corresponding ZIF nanosheet membranes were successfully fabricated by the oriented growth of nanosheets to form different separators called as ZNM-SSM-500 and ZNM-SSM-2000, respectively. They were applied in the separation of different oil/water mixtures by using dichloromethane, petroleum ether, n-hexane, toluene, nitrobenzene and cyclohexane as model oils, and could demonstrate excellent separation performance. The ZNM-SSM-500 separator shows a separation efficiency of 99.8% with high flux of over 100000 L·m-2·h-1 for free oil/water mixtures containing 50 wt% water, while the ZNM-SSM-2000 separator * Corresponding Author: E-mail: [email protected] (X. Zhang) 1
exhibits a water rejection of as high as 99.95% for water-in-oil emulsions containing 2 wt% water. Moreover, the resulting nanosheet membrane could also demonstrate excellent thermal stability and recyclability. Therefore, the oriented nanosheet membrane is of great application potential in practical oil/water separation. Keywords: MOFs modified membrane; nanosheet membrane; oil/water mixture separation; oil/water emulsion separation 1. Introduction In recent years, worldwide oil pollution problems caused by frequent oil spill accidents, the rapid development of petroleum industry and the discharge of oily wastewater, have received extensive attention [1-3]. Oil-polluted water usually contains toxic chemicals, which may harm people's health and affect the balance of ecosystems [4].
Thus, the separation of oil and organic solvents from oily wastewater is becoming
a serious issue and an important global challenge for environmental and ecological protection. The conventional oil/water separation techniques, such as gravity separation, magnetic separation and flotation technologies, usually suffer from the high operational costs and low separation efficiency
[5].
Especially, in a sudden oil spill
accident, a large number of porous materials (e.g., foams, sponges and textiles) are usually used to absorb oil. However, unfortunately, such porous materials can often absorb both oil and water simultaneously, as a result, the oil separation efficiency for the process is quite low. Furthermore, these materials already used are commonly burned or buried, since their recycling is quite difficult, thus unavoidably resulting in a secondary environmental pollution
[6, 7].
2
Therefore, it is urgent to find highly
efficient, environment-friendly and recyclable materials or techniques for the separation of oil/water mixtures. Membrane technology with low energy consumption and simple separation equipment, as an emerging efficient separation technology, is considered a suitable method for separating oil/water mixtures in industry
[10].
[8, 9],
especially for the abundant emulsions
But the core of membrane technology depends on highly efficient
membrane materials. To date, considerable efforts have been devoted to the research on polymeric membranes
[11, 12],
ceramic membranes
[13],
zeolite membranes
[14],
and
carbon-based membranes [15] for oil/water separation. Despite great progress has been made, some issues such as separation efficiency, filtration flux, oil-fouling, etc. still exist in these kinds of membranes due to the intrinsical properties of the membrane materials, the small pore size of the membrane supports and the viscous oil. Actually, it is well known that the surface wettability of separation membranes plays a crucial role in separating oil/water mixtures because it can greatly improve the water flux and anti-oil-fouling ability
[16, 17].
Specifically, hydrophilic/oleophobic
materials and hydrophobic/oleophilic materials with special wettability are considered to be the most promising materials for the separation of oil/water mixtures
[18, 19].
Moreover, it is also very important to select some special membrane substrates with a pore size of tens to hundreds of microns, such as stainless steel mesh, copper mesh, etc.
[20-22],
since these can remarkably promote membrane filtration efficiency for the
separation of oil/water mixtures. For instance, Zhang et al.
[23]
prepared a Cu(OH)2
nanowired-haired membrane by surface oxidation of a copper mesh in an alkaline aqueous solution with (NH4)2S2O8 with a water contact angle of nearly 0° and oil contact angle of about 155°, exhibiting its superhydrophilic and underwater 3
superoleophobic property. And this membrane showed an efficient separation performance with high separation efficiency and separation capacity for both immiscible oil/water mixtures and oil-in-water emulsions. Yue et al.
[24]
fabricated a
hierarchical layer double hydroxide (LDH) membrane on a cellulose support with superhydrophobicity and superoleophilicity, showing excellent separation properties for free oil/water mixtures and surfactant-stabilized water-in-oil emulsions. Moreover, the LDH/cellulose membrane displayed stable recyclability and anti-corrosion properties in harsh conditions. Wang et al.[25], for the first time, synthesized a ZIF-8 membrane on stainless steel meshes for efficient oil/water separation. Owing its superhydrophilicity in air and superoleophobicity under water after prewetting, the membrane gave a high flux of as high as 50 L∙ m-2∙ s-1 and separation efficiency of over 99.9% with excellent stability and recyclability under a gravity-driven force. Jiang et al.[26] fabricated a Co3O4 nano-needle coated stainless steel mesh by a simple hydrothermal synthesis and subsequent calcination method. The mesh successfully accomplished the separation of alkaline and saline oil-in-water emulsions with a high efficiency of above 99% and a high flux of 2000 L∙m-2∙h-1, and may be an excellent membrane material for oil/water emulsion separation in a harsh environment. Cheng et
al.[27]
fabricated
Cu@Ag
film
with
superhydrophilic
and
underwater
superoleophobic properties on porous copper substrate by a simple acid etching and replacement reaction. Interestingly, the film became hydrophobic and superoleophilic after modified with n-dodecanethiol (DDT), denoted as Cu@Ag@DDT, realizing both oil-in-water and water-in-oil emulsion separation by utilizing the different surface wettability of the films. In addition, Guo et al.[28] coated HKUST-1 on the surface of stainless steel meshes by using polydopamine as a binder to obtain the 4
separators and applied them in both oil-in-water and water-in-oil emulsions via selective water filtration and adsorption, respectively, achieving an excellent separation performance. Therefore, the above investigation results have shown that the separators fabricated with appropriate membrane/film materials can greatly improve the separation performance for oil/water mixtures. Nevertheless， the design and fabrication of a high-performance membrane for highly efficient separation of oil/water mixtures is still a crucial challenge to membrane experts
[29].
Recently, the fabrication of two-dimensional (2D) metal-
organic frameworks (MOFs) nanosheets as a new class of 2D materials into a membrane/film supported on a porous ceramic substrate has demonstrated superhigh gas separation properties and has potential applications due to both the exceptional characteristics of the nanosheets and special microstructure of the membrane
[30-32].
However, it is still rather difficult to prepare such structured membranes fabricated with MOF nanosheets. The only method adopted is a two-step process so far, called as “top-down” method, including first the exfoliation of bulk laminar MOFs to obtain single-layer dispersed nanosheets and then the coating of the exfoliated nanosheets on the substrate to form nanosheet membranes
[33, 34].
Unfortunately, this “top-down”
method easily causes both some morphological damage, structural deterioration of the detached nanosheets during the exfoliation process and the pore interleaving of the achieved nanosheet membrane during the coating step, thus easily resulting in a lowperformance membrane. Moreover, it is also rather arduous, time-consuming and costly. In this report, based on our previous work
[35-37],
we use a bottom-up strategy to
directly prepare highly oriented Zn2(bIm)4 ZIF nanosheet membranes on SSMs for 5
fabricating hydrophobic filtrators and attempt to apply them in efficiently separating oil/water mixtures. ZIFs, as a subclass of MOFs, are constructed by bridging metal (Zn, Co) tetrahedra with imidazolate (or imidazolate derivative) ligands, which possess large surface areas, microporosity, and relatively high thermal and chemical stabilities. The Zn2(bIm)4 units are considered as subunits of 2D structure, in which the apices are occupied by Zn atoms and the sides are formed by benzimidazole ligands[36]. The fabrication strategy is based on the self-conversion of a layer of ZnO nanorods into a ZIF nanosheet membrane layer, as shown in the preparation scheme of Fig. 1. In this method, the ZnO nanorods layer on the SSM can act as a multifunctional role including the metal source, nucleation sites for the growth of ZIF nanosheet membrane and the anchoring sites between the membrane and SSM substrate, thus favoring the formation of a stable and continuous oriented nanosheet membrane. Based on the SSMs of different meshes, two nanosheet membrane separators, called as ZNM-SSM-500 and ZNM-SSM-2000, were fabricated and applied in separating different kinds of oil/water mixtures.
Fig. 1. Scheme of the self-conversion of ZnO nanorods into oriented Zn2(bIm)4 nanosheet membranes supported on the SSMs. 6
2. Experimental section 2.1. Materials and chemicals Stainless steel meshes (SSMs: 500 mesh size and 2000 mesh size) as substrates (called as SSM-500 and SSM-2000) were purchased from the local market (Dalian, China). The SSMs were cut into small square blocks (3 cm × 3 cm) and then ultrasonically cleaned using acetone, distilled water and ethanol for twenty minutes, respectively, and dried at 80 ℃ for 2 h for later use. The chemicals used for the membrane preparation include zinc acetate (Zn(Ac)2·2H2O,
98.0%),
monoethanolamine
(C2H7NO,
MEA,
99.0%),
hexamethylenetetramine (C6H12N4, ≥99.0%), ammonium hydroxide (NH4OH, 28–30% aqueous
solution),
toluene
(PhMe,
≥ 99.0%),
zinc
nitrate
hexahydrate
(Zn(NO3)2·6H2O, ≥ 99.0%), ethylene glycol monomethyl ether (C3H8O2, EGME, 99.0%), anhydrous methanol (MeOH, ≥ 99.5%), acetone (≥ 99.5%), ethanol (≥ 99.7%), methyl orange and span 80 purchased from Sinopharm Chemical Reagent Co., Ltd. Benzimidazole (bIm, ≥ 99.0%) was purchased from Sigma-Aldrich Chemical Co., Ltd. All the above chemicals were used directly without further purifications. 2.2. Preparation of Zn2(bIm)4 nanosheet membranes for fabricating ZNM-SSM-500 and ZNM-SSM-2000 separators 2.2.1. Growth of a layer of ZnO nanorods on the SSMs A layer of ZnO nanorods was grown on the SSM by a simple hydrothermal process according to our previous report [39, 40], with minor modification. Brifly, the SSM was first coated using a stable Zn-based sol containing 14.3% Zn by dip-coating technique 7
before calcination at 400 ℃ for 3 h to get a thin layer of ZnO nanoparticles as seeds on the SSM for growing ZnO nanorods. Then, the SSM with ZnO nanoparticles was vertically placed in a synthesis solution in a 100 mL Teflon-lined autoclave for crystallization at 100 °C for 6 h to get a layer of vertically aligned ZnO nanorods for the preparation of ZNM-SSM-500 and ZNM-SSM-2000, respectively. The synthesis solution was obtained by 2.38 g Zn(NO3)2·6H2O and 1.12 g hexamethylenetetramine dissolved in 80 mL distilled water for stirring at room temperature for 0.5 h. 2.2.2 Preparation of ZNM-SSM-500 and ZNM-SSM-2000 Zn2(bIm)4 nanosheet membranes were grown on the ZnO nanorods layer-coated SSMs with different meshes (500 and 2000 mesh) in a Zn-free synthesis solution with a molar ratio of 1bIm:1NH4OH:45MeOH:45PhMe for achieving ZNM-SSM-500 and ZNM-SSM-2000, respectively. The solution was prepared as follows: 19.0 mL MeOH, 18.0 mL PhMe and 3.0 mL NH4OH were mixed together before adding 1.18 g bIm, and stirred around 5 min to get a clear solution. The SSM (500 mesh) with the ZnO nanorods was placed vertically in the above solution in an autoclave for selfconversion at 100 ºC for 12 h to obtain the ZNM-SSM-500. The SSM with 2000 mesh was used under the same condition for the growth to achieve the ZNM-SSM-2000. After the growth, the ZNM-SSM-500 and ZNM-SSM-2000 were carefully taken out and then soaked in methanol solution for two days to remove impurities. Finally, they were dried at room temperature for later use. 2.3. Separation experiments for free oil/water mixtures The separation experiment of free oil/water mixtures in which oil and water are not immiscible was implemented at room temperature by a homemade separation device with an efficient separation area of 3.14 cm2 with gravity as driving force. Six 8
different organic solvents, including dichloromethane, petroleum ether, n-hexane, toluene, nitrobenzene and cyclohexane, were chosen as the model oils. The ZNMSSM-500 was fixed on the separation device. Mixtures of water (dyed with methyl orange) and oil phase (v/v, 1/1) were poured into the device. The oil phase could pass though the membrane, but water was intercepted due to the hydrophobic/oleophilic properties of the membrane. The separation efficiency (η) can be calculated by the following equation. η=(m1/m0) ×100% where m0 is initial mass of water, m1 is the mass of the water that had been intercepted after separation. The separation flux (F) can be calculated by the following equation. F= V/(S×t) Where V is the volume of oil that permeates through the membrane, S is the efficient separation area, and t is the filtering time. 2.4. Separation experiments for emulsified oil/water mixtures The ZNM-SSM-2000 was used to separate emulsified oil/water mixtures in which the mixtures are not stratified. The surfactant-stabilized water-in-oil emulsion was prepared as reported in the literature with minor modification
[16].
Briefly, 0.3 g
of span 80 was added into 50 mL of dichloromethane and 1 mL of water with vigorous stirring for 6 h at room temperature to obtain a milk white solution. The separation process was similar to the above separation of free oil/water mixtures. The water rejection (R) was then calculated by the following equation. 9
(
R= 1‒
)
Cfiltrate Cfeed
× 100%
Where Cfiltrate and Cfeed are the water concentrations in the original emulsions and the filtrate, respectively. 2.5. Reusability and thermal stability tests for the membranes A repeatable separation experiment for oil/water mixtures was carried out to assess the reusability of the nanosheet membrane grown on the SSM. The used membrane was washed with ethanol and dried at 60℃ for 2 h. Then the oil separation efficiency for the used membrane was tested under the same condition as described above. To investigate the thermal stability of the as-prepared membrane in a high temperature, the membrane was calcined for 1 h in 100℃, 200℃, 300 ℃, and 400℃, respectively, before a separation experiment for oil/water mixtures was carried out. 2.6. Samples characterization The surface morphologies of the nanosheet membranes coated SSMs were recorded by scanning electron microscopy (SEM, FEI Nova NanoSEM 450 and Quanta 450 operating at 3 and 20 keV, respectively) and a Tecnai F30 transmission electron microscope (TEM). X-ray diffraction (XRD) analysis was recorded on a D/max-2400 X-ray diffractometer with Cu Kα radiation in the 2θ range of 3−60° operating at 40 kV/100 mA. The water contact angle instrument (OCAH200, Data physics, Germany) was utilized to measure the wettability of the samples. The concentrations of the water in original emulsions and filtrate were analyzed by using a Karl Fischer moisture titrator (831KF Coulometer). The optical microscopy images
10
were taken on Olympus BX53 by dropping emulsion solution on biological counting board.
3. Results and discussion 3.1. Formation and characterization of nanosheet membranes The like-fish-scale nanosheets grow outward on the SSM surface and form a layer of extremely coarse surface, which could greatly improves the hydrophobicity. According to our previous reports
[35],
a highly oriented (parallelled to the
surface of substrate) Zn2(bIm)4 nanosheet membrane for gas separation could be prepared by a direct growth strategy on a porous Al2O3 tube as substrate coated with a layer of ZnO nanoparticles (NPs). Based on the above work, here, a highly oriented Zn2(bIm)4 nanosheet membrane which is perpendicular to the metal rod surface of the SSM substrate was synthesized by the self-conversion of a layer of ZnO nanorods grown on the SSM. It is essential to pre-introduce a layer of ZnO nanorods on the SSM that plays three important roles: (1) connecting the membrane layer and the SSM substrate tightly to avoid the membrane from peeling off; (2) supplying the metal source for the growth of Zn2(bIm)4 nanosheet membrane and providing heterogenous nucleation sites for favoring the uniform growth of membrane on the surface of the substrate; (3) controlling the growth direction of nanosheets perpendicular to the metal rod surface of the SSM. Moreover, it should be emphasized that the synthesis was conducted in a Zn-free solution only containing bIm ligands, which makes the membrane favorably grow just on the SSM, avoiding the generation of nucleation and crystal growth in the bulk solution. Fig. 2 is the SEM images of the SSMs and membranes achieved. It is clearly seen that the grid pore diameter of the 11
bare SSM-500 is about 30-40 μm (Fig. 2a) and the bare SSM-2000 is around 7 μm (Fig. 2g) and their surface are extreme smooth (Fig. 2b, h). As shown in Fig. 2c, d, a layer of vertically arranged ZnO nanorods with a diameter of about 200 nm and length of around 2 μm was regularly grown on the surface of the SSM. This layer greatly favors the self-converted formation of a nanosheet membrane and also reduces the grid pore diameter of the SSM. After the self-conversion in the Zn-free ligands synthesis solution, a layer of membrane consisting of nanosheets was formed as shown in Fig. 2e-f, i-j. The ZIF sheets were closely arranged on the surface in parallel each other (perpendicular to the substrate surface), forming a continuous membrane layer. Every nanosheet had around 2-4 μm length, 2 μm width and about 20 nm in thickness, showing a high aspect ratio (Fig. 2f, j). The majority of the pore diameters for the SSM-500 are reduced to 10-15 μm after the growth of nanosheet membrane (Fig. 2e), except for some large pore sizes at individual locations, because the pore sizes of the SSM substrate are not uniform. The existence of these large pore sizes is conducive to increasing oil/water separation flux while ensuring a superb separation efficiency. Moreover, after the membrane growth, the surface roughness of the SSM was greatly increased compared with the bare SSM and its hydrophobicity was also improved as shown in Fig. 4, which is also beneficial to the separation of oil/water mixtures. It should be emphasized that for the ZNM-SSM-500 supported on the SSM500, the nanosheet membrane only grows on the surface of the metal rods and does not cover the meshes due to the larger grid holes. Actually, the grown nanosheet membrane mainly acts as a modification layer on the surface of the SSM and simultaneously changes the hydrophobicity and coarseness of the SSM surface. Therefore, the ZNM-SSM-500 separator is mainly applied in separating free oil/water mixtures, demonstrating its high filtration flux. However, this kind of separator is not 12
suitable for the separation of oil/water emulsion due to the existence of grid pores. While the SSM- 2000 possesses relatively small grid pore size, thus the ZIF nanosheet
Fig. 2. SEM images of the bare SSM-500 (a, b) and SSM-2000 (g, h), ZnO nanorods layer grown on the SSM-500 (c, d) and oriented Zn2(bIm)4 nanosheet membranes 13
self-converted on the SSM-500 (e, f), and on the SSM-2000 (i, j), respectively. The inset is the cross section of the ZnO rods grown on the SSM. membrane layer can completely cover the grid holes of the SSM-2000 and thus form a continuous nanosheet membrane on the surface of the SSM, acting as a separation function through both the pores of the nanosheet membrane and the interspaces among the oriented nanosheets. Therefore, the ZNM-SSM-2000 separator is mainly applied in the separation of oil/water emulsions, indicating its high selectivity for small oil droplets in emulsified oil/water mixtures. A series of experiments on different crystallization times of nanosheets were done on glass slides as substrate in view of its flat surface to further explore the details of the self-conversion of ZnO nanorods into the oriented Zn2(bIm)4 nanosheet membrane. As shown in Fig. 3a, b, in the initial stage of the reaction (2 h), the ZnO nanorods dissolved from the center section in the basic solution containing both bIm and ammonia and shaped into hollow morphology. Moreover, on the wall of the nanorods, a large number of tiny crystals appeared. From the cross-sectional SEM image, it could be also seen some precursors of Zn2(bIm)4 nanosheets produced by the coordination of the dissolved Zn2+ from the ZnO nanorods with bIm ligands in the solution. As the reaction time was prolonged to 7 h (Fig. 3c, d), the nanosheet precursors obviously increased and covered the most surface of the top of the ZnO nanorods. The cross-sectional SEM image indicates that the spaces between the nanorods were hardly observed by the filling of the nanosheet precursors generated. Actually, the precursors produced in the initial stage can act as a seed role and induce the formation of Zn2(bIm)4 nanosheets along the ZnO nanorods as shown in Fig.3e, f. When the crystallization time was increased to 12 h, a layer of nanosheets was formed 14
on the top of the ZnO nanorods in which the nanosheets grew upward in parallel (perpendicular to the substrate surface) on the top of the ZnO nanorods. Moreover, the nanosheets arranged closely and were highly oriented, thus forming an oriented nanosheet membrane. XRD patterns of Fig. 3g clearly indicate that the nanosheets belong to the typical Zn2(bIm)4 structure which is in accordance with simulated Zn2(bIm)4 crystal structure, and the membrane shows merely one reflection [223], indexed to the crystallographic plane of the Zn2(bIm)4 structure, except for ZnO characteristic peaks, thus confirming the highly orientation of the as-prepared nanosheet membrane (ref. no. 675375, Cambridge Crystallographic Data Centre).
15
Fig. 3. SEM images of the samples obtained on the ZnO nanorods by different growth times (a,b: 2h; c,d: 7h; e,f: 12h), XRD patterns of different samples (g), and Oil contact angle of bare SSM substrate and nanosheet membrane with 500 mesh size and 2000 mesh size (h), respectively. The wettability of the above highly oriented ZIF nanosheet membrane was monitored by measuring the water and oil contact angle, respectively. As shown in Fig.4, the water contact angles of the original bare SSM substrates for both 500 mesh and 2000 mesh were near 90° (86.5° and 88°), showing that the SSMs are slightly hydrophilic. While the water contact angle of the ZnO nanorods layers was nearly 0°, indicating that the ZnO nanorods layers are completely hydrophilic. Surprisedly, the water contact angles of the nanosheet membranes grown on the SSMs markedly rose to 140° for the ZSN-SSM-500 and 152.5° for the ZSN-SSM-2000, demonstrating the strong hydrophobicity due to the excellently hydrophobic characteristic of the ZIF nanosheets. From the Fig. 3h, it is clearly seen that the nanosheet membrane possesses an oil contact angle of near 0°. Once oil droplets reach the membrane surface, they immediately spread out and wet it, and then penetrate through the membrane. Thus, this further confirms the superoleophylic property of this kind of nanosheet membrane.
16
Fig. 4. Water contact angles of the bare SSM substrates, ZnO nanorods layers and nanosheet membranes with 500 mesh size and 2000 mesh size, respectively. Moreover, the nanosheet layer with a like-fish-scale structure grown on the SSMs also favors increasing the hydrophobicity of the membrane materials, since the roughness and chemical composition of a material surface greatly influences its wettability. Based on the above analysis, the separation mechanism can be summarized as the following two aspects: (1) the Zn2(bIm)4 nanosheet material has excellently hydrophobic property; (2) the nanosheet membrane layer with a like-fishscale structure favors increasing the surface roughness, thus greatly improving the hydrophobicity of the membrane. According to the Cassie model theory that roughness increases hydrophobicity, thus, water contact angle also rises
[40].
A model
equation can be used to describe the wetting behavior as follows: cosθ*=-1+(1+cosθ)f Where θ is the contact angle of the original substrate, and θ* is the contact angle of a modified rough surface, f presents the area fraction of the liquid that is in contact with air. 17
Therefore, based on the above theory, it is not difficult to understand that why the water contact angle of the highly oriented membrane grown on the SSM-2000 is bigger than that on the SSM-500. This result should be attributed to the following two reasons: 1) the increase in the number of the Zn2(bIm)4 nanosheets with intrinsic hydrophobicity per unit area enhanced the repellent of water; 2) the high orientation and continuity of the Zn2(bIm)4 nanosheets improved the surface roughness and microstructure of the SSM-2000 substrate. 3.2. Separation of free oil/water mixtures To evaluate the hydrophobic separation ability of the above as-prepared highly oriented membrane grown on the SSM with 500 mesh (ZNM-SSM-500), a gravitydriven separation device was used to separate different free oil/water mixtures. The ZNM-SSM-500 was first used to separate a dichloromethane/water mixture in which dichloromethane as oil phase, with a higher density, stayed at the bottom, while water dyed with methylene orange for convenient observation was in the upper part due to a lower density. As shown in Fig. 5a, it was clearly observed that when the dichloromethane/water mixture was quickly poured into the separation device, the oil phase could immediately wet and permeate the membrane and grid pores, whereas the water was held back by the membrane. The filtrate was also transparent and limpid, suggesting that no water permeated through the ZNM-SSM-500. Moreover, the separation time was very short, the flux was as high as 101400 L·m-2·h-1 and separation efficiency reached over 99.8％ for the dichloromethane/water mixture through the ZNM-SSM-500.
Therefore， the ZNM-SSM-500 (highly oriented
membrane grown on the SSM with 500 mesh) could efficiently separate free
18
dichloromethane/water mixtures due to its
special structure and strong
hydrophobicity. Actually, for oily wastewater, there usually exist two kinds of states: One is the oil sinking into the water for heavy oil wastewater; another is floating on the water for light oil wastewater. Thus, a series of heavy and light oils were selected as representatives, respectively, for this separation experiment to further check the separation properties of the ZNM-SSM-500. Fig. 5b shows the separation results for the application of the ZNM-SSM-500 in the different oil/water mixtures. The separation efficiencies of the ZNM-SSM-500 for both the heavy and light oil/water
Fig. 5. Gravity-driven separation device and process (a) and separation performance (b) for free oil/water mixtures of the nanosheet membrane with 500 mesh size.
19
mixtures exceeded over 99.6%, though the fluxes had some difference, showing that the ZNM-SSM-500 is of wide adaptability in separating free oil/water mixtures due to its strong hydrophobicity. It should be pointed out that the light oil/water mixtures must be poured into the separation device slowly in order that the oil rather than the water could first contacts the nanosheet membrane and thus a high separation efficiency could be achieved for light oil/water mixtures. In order to understand the separation mechanism of free oil/water mixtures through the as-prepared nanosheet membrane coated SSM, a model was designed to describe the separation process. As shown in Fig. 6, oil phase can easily pass through the ZNM-SSM-500 by gravity without other energy, however, the water phase is prevented from permeating through the ZNM-SSM-500 due to the repulsive forces. The highly oriented nanosheets with nanoscale spacing and perpendicular to the surface of the SSM effectively increase the roughness of the surface and thus the air is trapped between the nanosheets, forming a gas–liquid–solid three-phase interface. The gas-solid interface can prevent the water phase from wetting and permeating. The following equation can describe the separation mechanism of oil/water mixtures through the highly oriented nanosheet membrane grown on the SSM.
∆𝑃 =
2γLV 𝑅
= ‒
CγLVcos θ A
Where γLV represents the surface intension, R is the radius of the meniscus, θ is the water or oil contact angle, C is the circumference of the mesh pore and A represents the cross-sectional area of the mesh pore. For the case of the strong hydrophobic membrane, the water contact angle θ is larger than 140°, so the theoretical intrusion pressure ∆𝑃>0, which explains why the water can’t pass through 20
the membrane. On the contrary, the oil contact angle of the membrane is nearly 0°, so the theoretical intrusion pressure ∆𝑃<0, thus, indicating that the oil phase can easily permeate the membrane. This formula well explains the reason that the highly oriented membrane can efficiently separate oil/water mixture in theory, which is completely in accordance with the above experiment results.
Fig. 6. Schematic diagrams of the wetting model of the nanosheet membrane. 3.3. Separation of oil/water emulsion In actual situations, besides free oil/water mixtures, there still exists a class of emulsied oily wastewater from many industrial processes, in which oil or water in the form of droplets is dispersed in the mixtures and there is no clear boundary between oil phase and water phase. Therefore, it is also necessary to separate these emulsied oil/water mixtures, but it is more difficult and challenging because the diameter of dispersed water/oil droplets in water-in-oil/oil-in-water emulsion is fairly small. So it is required that the pore size of the filter fabricated has to be small so enough as to have the opportunity to efficiently separate the emulsion. As discussed above in the 3.1 section, the ZNM-SSM-500 separator fabricated with the nanosheets still has large grid pore sizes of about 10-15 μm, while the water or oil droplets dispersed in the emulsion are usually very small and only several microns in diameter. Therefore, it is no doubt that the ZNM-SSM-500 separator is not suitable for this kind of the 21
separation of water-in-oil/oil-in-water emulsion, though it has high filtration flux. Consequently, we selected the SSM with 2000 mesh as substrate (SSM-2000) to support the nanosheet membrane because it possesses relatively small grid pore size. As a result, the oriented nanosheet membrane fully covered the grid pores of the SSM-2000 and thus a ZNM-SSM-2000 separator without any visible holes was achieved. Then we made attempt to apply the as-fabricated hydrophobic ZNM-SSM2000 in separating a 2 wt% water-in-oil emulsion formed by dispersion of a little water in much dichloromethane. The separation process of the water-in-oil emulsion was similar to that of the free oil/water mixtures. When the stable water-indichloromethane emulsion was poured onto the highly oriented nanosheet membrane supported on the SSM-2000, the oil immediately wet and permeated through the membrane with gravity as driving force without other energy, but the water was retained above, thus realizing the efficient separation of water-in-oil emulsion as shown in Fig. 7a, b. Optical microscopy was applied to inspect the separation performance of the emulsion through the ZNM-SSM-2000. A large amount of water droplets exist in the emulsion (Fig. 7a) and are less than 5μm in size. But after the treatment with the ZNM-SSM-2000, no obvious water droplets can be observed in the filtrate in the whole view (Fig. 7b), demonstrating the high separation efficiency of the ZNM-SSM-2000. Moreover, from the Fig. 7c, the original water-in-oil emulsion was white, milky and presented an evident Tyndall phenomenon with a clear red light beam, while the filtrate became transparent and also had no Tyndall effect. In the separation process, the maximum flux could reach about 636 L·m-2·h-1 and the water rejection was as high as 99.9%, which further confirmed the high separation performance of the ZNM-SSM-2000 separator in the water-in-oil emulsion.
22
Fig. 7. Optical micrographs (a, b) and images (c) of the water-in-oil emulsion before and after separation. 3.4. Recyclability and stability of the highly oriented nanosheet membrane Generally, in the separation process of oil/water mixtures, besides the separation efficiency and flux, the stability and durability of the membrane are also important due to the harsh environment in actual separation application of oil/water mixtures. Therefore, we conducted both the high-temperature stability and recyclability of the both ZNM-SSM-500 and ZNM-SSM-2000 separators. Especially, it is usually thought that ZIF-based materials are not stable enough. Thus we first investigated the thermal stability by means of the calcination for 1h at different temperatures, respectively, for the prepared nanosheet membrane, then a series of relative characterizations for the calcined samples were made, as shown in the Fig.8. It is clearly seen from the SEM images of Fig. 8a-d that the sheet-like morphologies of three samples obtained after the calcination at 100, 200 and 300 ℃ for 1h, respectively, were still kept intact, while the nanosheet structure of the sample 23
calcined at 400 ℃ for 1h was completely destroyed, showing that the nanosheet membrane has a considerable thermal stability within the range of no more than 300℃. From the Fig. 8e, the water contact angles of the samples were slightly decreased after the calcination, but the separation efficiencies of the calcined membrane for the oil/water mixtures were still as high as over 99%, further confirming that the calcined membranes are still perfect and can demonstrate excellent separation performance for the oil/water mixtures. However, for the sample calcined at 400℃ for 1h, its water contact angle was close to 0° due to the destruction of the nanosheet membrane and thus its wettability was also changed from hydrophobicility to hydrophilicity, thus resulting in the fact that there was no separation ability for the free oil/water mixtures. XRD pattern of Fig. 8f also indicated that there was not any characteristic peak of Zn2(bIm)4 nanosheets except for the peaks of ZnO for the sample calcined at 400 ℃ for 1h, which is in agreement with the above SEM and separation results.
24
Fig. 8. SEM images of the calcined samples (a-d, a: 100 ℃; b: 200 ℃; c: 300 ℃; d: 400 ℃ ) and the change in water contact angles and separation efficiencies of the different samples (e), and XRD patterns of Zn2(bIm)4 powders and the membrane calcined at 400 ℃ for 1 h (f). The recyclability of the fabricated separators was also evaluated for the ZNMSSM-500 using the free dichloromethane/water mixture and ZNM-SSM-2000 separators using the water-in-oil emulsion containing 2 wt% water. As shown in the Fig. 9a, the ZNM-SSM-500 separator gave a high oil/water separation efficiency of over 99% and there was no obvious reduction in oil/water separation performance within the cycles of 20 times. Meanwhile, the flux of the ZNM-SSM-500 separator 25
did not decrease significantly except for slight fluctuations. Moreover, the sheet-like morphology of the membrane was still kept intact after the separation cycles of 20 times (Fig. 9b). As for the ZNM-SSM-2000 separator, both the flux and the separation efficiency decreased slowly with the increasing of times (Fig. 9c, d). However, after washing the separator with ethanol and drying, the flux and separation efficiency returned to the original state, indicating that the ZNM-SSM-2000 separator was fouled during the cycle, but the separator could be restored to its original state after a simple washing process. Therefore, the strong hydrophobic/oleophilic ZNM-SSM500 and ZNM-SSM-2000 separators could demonstrate a high separation efficiency and flux for oil/water mixtures as well as excellent thermal stability and recyclability.
Fig. 9. Recyclability for the separation of oil/water mixtures of ZNM-SSM-500 (a, b) and ZNM-SSM-2000 (c, d). 4. Conclusions
26
In conclusion, we for the first time prepared highly oriented and continuous ZIF nanosheet membranes on the SSMs coated with a layer of ZnO nanorods as substrate by a ZnO-induced direct growth strategy. The ZnO nanorods grown on the SSMs can not only provide the zinc source and serve as nucleation and anchoring sites for the growth of the nanosheet membrane, but also control the growth direction of nanosheets. Hence, the grown nanosheets are perpendicular to the SSM surface and parallel to each other, forming a highly oriented and like-fish-scale membrane layer with strong hydrophobicity. Based on the SSMs with different meshes, two nanosheet membrane separators (ZNM-SSM-500 and ZNM-SSM-2000) were fabricated and employed to separate free oil/water mixtures and water-in-oil emulsion, respectively, using dichloromethane, petroleum ether, n-hexane, toluene, nitrobenzene and cyclohexane as model oils. The ZNM-SSM-500 separator unexpectedly gave a high separation efficiency of 99.8% and flux of 101400 L·m-2·h-1 for free oil/water mixtures containing 50 wt% water, while the ZNM-SSM-2000 separator exhibited water rejection as high as 99.9% and the flux of about 636 L·m-2·h-1 for water-in-oil emulsion dispersed with 2 wt% water. More importantly, the perpendicularly oriented nanosheet membrane also has excellent thermal stability and recyclability for the separation of oil/water mixtures, which are very important in practical industrial applications. Therefore, it is believed that it has great potential for the scalable preparation of the nanosheet membrane with unique oil/water separation properties. Acknowledgments The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (Nos. 21878032, 21476039). References 27
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Graphical abstract
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Research Highlights：
1. A layer of highly oriented Zn2(bIm)4 nanosheet membrane was successfully grown on stainless steel meshes (SSMs). 2. The like-fish-scale structure of the nanosheet membrane increased the surface roughness and hydrophobicity of the SSMs. 3. The membrane exhibited excellent separation performance for both free oil/water mixtures and water-in-oil emulsion.
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