Ultralong MnO2 nanowires enhanced multi-wall carbon nanotubes

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Materials and Interfaces

Ultralong MnO2 nanowires enhanced multi-wall carbon nanotubes hybrid membrane with underwater superoleophobicity for efficient oil-in-water emulsions separation Xuejie Yue, Tao Zhang, Dongya Yang, Fengxian Qiu, and Zhangdi Li Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b02577 • Publication Date (Web): 22 Jul 2018 Downloaded from http://pubs.acs.org on July 28, 2018

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Industrial & Engineering Chemistry Research

Ultralong MnO2 nanowires enhanced multi-wall carbon nanotubes hybrid membrane with underwater superoleophobicity for efficient oil-in-water emulsions separation Xuejie Yue a, Tao Zhang a,b*, Dongya Yang a, Fengxian Qiu a*, Zhangdi Li a a

School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, Jiangsu Province, China. b

Institute of Green Chemistry and Chemical Technology, Jiangsu University, Zhenjiang 212013, Jiangsu Province, China.

*Corresponding authors: Tel./fax: +86 0511 88791800. E-mail: [email protected] (T. zhang); fxqiu@ ujs.edu.cn (F. Qiu) 1

ACS Paragon Plus Environment

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Abstract Carbon nanotubes-based membranes with special wettability are ideal candidates to handle oily wastewater. However, their applications are limited owing to the low film-forming ability, unsatisfactory mechanical stability and insufficient reusability. Herein, this work presents an effective strategy for fabricating a flexible multi-wall carbon nanotubes (MWCNTs)/ultralong MnO2 nanowires (UL-MnO2-NWs) hybrid membrane, as well as its application in emulsion separation. In this strategy, the dispersed carbon nanotubes were obtained by acid treatment of agglomerate MWCNTs and the UL-MnO2-NWs were prepared by a hydrothermal route. Then, the MWCNTs/UL-MnO2-NWs hybrid membrane with an improved mechanical property and excellent reusability was fabricated via vacuum-filtration of MWCNTs and UL-MnO2-NWs

suspension.

superhydrophilicity

with

a

The water

obtained contact

hybrid

angle

of

membrane 0°

and

shows

underwater

superoleophobicity with an oil contact angle of 152°. It can effectively separate both surfactant-free and surfactant-stabilized oil-in-water emulsions with permeances up to 4900 L m-2 h-1 bar-1 and high separation efficiency of greater than 99.7%. More importantly, the as-prepared sample shows superior recyclability, antifouling property, and excellent mechanical and chemical stability, which match well with the requirements for treating the real emulsions. As a result, this hybrid membrane can be a potential candidate for practical application in water-in-oil emulsion separation.

Keywords:

Carbon nanotubes; MnO2 nanowires; emulsion separation;

superhydrophilicity; underwater superoleophobicity 2


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1. Introduction Recently, oil/water separation has been a world-wide problem as a result of the increasing release of oily industrial wastewater as well as the high-frequency oily spills.1-3 Driven by economic and environmental aspects, recovery of oils from oily wastewater is critical and the advanced technologies for oil/water separation are urgently needed.4, 5 Various methods, such as mechanical methods (e.g., by use of oil skimmers),6 chemical methods (e.g., by use of dispersants and in situ burning)7 and biological methods (e.g., by use of marine microorganisms)8 have been presented to treat oily wastewater. Although these techniques can deal with the immiscible oil/water mixtures, it is still a great challenge for the separation of oil/water emulsions, especially the surfactant-stabilized microemulsions with a droplet size below 20 µm.9 It is urgent to develop an efficient and broadly applicable approach to separate oil/water mixtures, especially surfactant-free and surfactant-stabilized emulsions.10, 11 Due to the obvious different interfacial effect of oil and water, filtration membranes with special wettability have been acknowledged as an advanced technology for the separation of surfactant-stabilized emulsions with a relatively simple process from an operational point of view.12-14 The popularly used membranes are the so-called “oil-removing” membranes with both hydrophobicity and oleophilicity, which could filtrate or absorb oil from water selectively.15-17 However, the “oil-removing” membranes are easily fouled and even blocked by absorbed oils, resulting in a quick decline of separation efficiency, as well as second pollution.18, 19 Inspired by the fish scales, “water-removing” membranes with hydrophilicity and 3



underwater superoleophobicity have attracted widespread attentions.12,

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20, 21

The

“water-removing” membrane can remain clean as long as they are water-wetted.22 Various hydrophilic materials, such as hydrogel,23 cellulose,24 ZnO,25 and TiO226, have been studied to prepare “water-removing” membranes. Generally, these hydrophilic materials fall into two categories: polymers and inorganic metal oxide. Polymer like hydrogel usually shows weak environmental adaptability due to its water solubility, metamorphosis and degradation.23 Metal oxides may result in serious damage in the complex environments due to the poor corrosion resistance, which is unfavorable for oil/water separation. Due to the excellent thermal, mechanical and electrical properties, carbon nanotubes (CNTs) with three-dimensional network structure show many potential applications including energy storage and energy conversion devices, sensors, high-strength composites, and catalyst supports.27 Moreover, their low density and high porosity make them attractive candidates for treatment of oil-polluted water.28 Due to the good film-forming property, and excellent processing performance, highly dispersed single-walled CNTs are widely used to fabricate separation membrane. For example, Shi et al.29 successfully prepared ultrathin free-standing single-walled CNT network films by vacuum-filtering a single-walled CNTs suspension, which was used for the ultrafast separation of water-in-oil emulsions. Unfortunately, the expensive price of single-walled CNTs could hinder their large-scale application to separate oil/water mixtures. The widely available CNTs, such as MWCNTs tend to agglomerate with each other to reduce their surface energy, which is harmful to the 4


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film formation of CNTs.30 Moreover, the intrinsic hydrophobic property of CNTs can cause membrane fouling and clogging. The unsatisfactory mechanical property of CNTs-based membranes limits their practical applications for emulsion separation. Thus, it is of great significant to effectively disperse widely available MWCNTs using a simple and scalable approach to develop MWCNTs membrane with stable underwater superoleophobic property and high mechanical property for emulsion separation. In

this

work,

a

flexible

and

underwater

superoleophobic

MWCNTs/UL-MnO2-NWs hybrid membrane was fabricated by vacuum-filtering dispersed MWCNTs and ultralong MnO2 nanowires suspension through a sand-core filtration system. In order to obtain highly dispersed MWCNTs, MWCNTs was dispersed by the treatment with acid and subsequent sonification, which effectively improves the dispersion and film-forming ability of MWCNTs. UL-MnO2-NWs that obtained by a hydrothermal route were introduced into the hybrid membrane system, enhancing the mechanical stability of the hybrid membranes. Moreover, the MWCNTs/UL-MnO2-NWs membranes exhibited the characteristics of high film-forming ability, satisfactory mechanical and chemical stability and excellent reusability. In addition, comprehensive characterization of emulsion separation performances of the obtained MWCNTs/UL-MnO2-NWs membranes for various surfactant-free and surfactant-stabilized oil-in-water emulsions was carefully investigated. The obtained hybrid membranes will have great potential for treating real emulsions. 5



2. Experimental section 2.1 Materials MWCNTs was purchased from Suzhou TANFENG graphene Tech Co., Ltd., Suzhou, China. Potassium sulphate (K2SO4), nitric acid (HNO3, 68%), potassium persulphate (K2S2O8), manganese sulphate monohydrate (MnSO4·H2O), sodium dodecyl benzene sulfonate (SDBS), and ethanol (C2H5OH) were obtained from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China. Oils and organic solvents were obtained from the local market, Zhenjiang, China. All reagents were not further purified. 2.2 Preparation of highly dispersed MWCNTs The dispersed MWCNTs were obtained by the acid treatment and subsequent sonification. In a typical procedure, 1.5 g of MWCNTs and 5.0 mL of 68 wt% HNO3 were transferred into a Teflon-lined autoclave and sealed to heat at 140 °C for 4.5 h. Afterward, the autoclave was cooled to room temperature, and the obtained MWCNTs were thoroughly washed by water and dried in an oven at 60 °C for 8 h. Then, 10 mg of MWCNTs and 10 mg of SDBS was dissolved into 100 mL of water. The mixture was sonicated under 600 W for 10 h at room temperature. Finally, the obtained black mixture solution was centrifuged for 30 min at 12000 r/min to remove the undispersed MWCNTs. The supernatant solution was collected as the dispersed MWCNTs suspension. 2.3 Preparation of UL-MnO2-NWs The UL-MnO2-NWs were obtained by a hydrothermal route. Typically, 6.65 g of 6


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K2SO4, 20.5g of K2S2O8 and 6.5 g of MnSO4·H2O were dissolved in 160 mL of water. The mixture solution was transferred into a Teflon-lined stainless steel autoclave. The autoclave was then kept at 250 °C for 60 h in an oven and then cooled to room temperature. The obtained UL-MnO2-NWs was thoroughly washed using the deionized water to remove the residual salt. To obtain dispersed UL-MnO2-NWs, 1.0 g of UL-MnO2-NWs was resuspended in 250 mL of water and stirred vigorously for 1 days to yield a homogeneous suspension. 2.4 Preparation of MWCNTs/UL-MnO2-NWs membrane The flexible MWCNTs/UL-MnO2-NWs hybrid membrane was fabricated via a vacuum-filtration process. In a typical experiment, a mixture solution of 30 mL of the obtained MWCNTs suspension and 10 mL of the UL-MnO2-NWs suspension was stirred for 1 h. The mixture suspension was filtered on a vacuum-filtration setup with a cellulose ester filter membrane (4.5 µm) to form the supporting layer. The thickness of the MWCNTs/UL-MnO2-NWs membrane was tuned by adjusting the volume of the homogeneous suspension. Finally, the obtained hybrid membrane was washed by the water and ethanol several times to remove SDBS and the cellulose ester filter membrane was carefully peeled off from the supporting layer. The overall fabrication process of the hybrid membrane is illustrated in Fig. 1. 2.5 Preparation of oil-in-water emulsions The surfactant-free oil-in-water emulsions were prepared by mixing 196 mL of water and 4 mL of oil (namely toluene, isooctane, chloroform) and stirring vigorously for 12 h to product a milky solution. The surfactant-stabilized oil-in-water emulsions 7



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were obtained by full mixing of 1.2 g of span 80, 4 mL of oil (namely toluene, isooctane, chloroform), and 196 mL of water. The mixture was stirred vigorously for 12 h to obtain surfactant-stabilized oil-in-water emulsions. 2.6 Characterizations The surface morphology of the MWCNTs/UL-MnO2-NWs membrane was obtained on a field-emission scanning electron microscope (JSM-7001F, Japan). Contact angles (CAs) were measured on KSV CM200 at room temperature. FT-IR spectra were performed on a Nicolet Nexus 470 FT-IR spectrometer in the range of 4000–500 cm-1 via potassium bromide (KBr, optical grade) pellet. The phase structure of the MWCNTs/UL-MnO2-NWs membrane was analyzed by using an X-ray diffractometer (XRD) (Shimadzu XRD-6100) with Cu Kα radiation. The droplet sizes of emulsions were analyzed using dynamic light scattering (DLS, Zetasizer Nano ZS 90) and an optical microscopy (Olympus CX41). The oil content in the filtrate was measured by an Aurora 1030W total organic carbon analyzer. 2.7 Separation of oil-in-water emulsions The obtained MWCNTs/UL-MnO2-NWs membrane was immersed in the water for 30 s before and then fixed by a vacuum driven filtration system with the vacuum degree at -15 KPa. The effective diameter of the filter is 40 mm. The permeance (Q) for the emulsions was calculated by Eq. (1). The separation efficiency (R) characterized by oil rejection was calculated by Eq. (2). ܳ= ܴ = ൬1 −

௏

(1)

஺×௱௧×௉ ஼೑೔೗೟ೝೌ೟೐ ஼೑೐೐೏

൰ × 100%

8


(2)

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where V is the volume of filtrate, A is the effective area of the filter, ∆t was the time required for separation, and P is the vacuum pressure. Cfiltrate and Cfeed are the oil concentration of the collected filtrate and the original emulsion, respectively.

Fig. 1. Schematic illustrations of fabrication of MWCNTs/UL-MnO2-NWs hybrid membrane.

3. Result and discussion 3.1. Membrane morphology In order to achieve an excellent superhydrophilic/underwater superoleophobic surface, the appropriate surface roughness of the membrane is a prerequisite.22 The surface morphology and structural characterization of MWCNTs/UL-MnO2-NWs membrane are investigated by SEM and the results are shown in Fig. 2 and Fig. S1 (Supporting Information). As shown in Fig. 2A, the obtained products consist of 9



one-dimensional MnO2 nanowires with a relative smooth surface, and the nanowires have a wide diameter distribution varying from 30 to 210 nm. All the nanowires are evenly dispersed and intertwined each other to form 3D MnO2 networks formed through self-supporting forces. The low-magnification SEM image of MnO2 nanowires (Fig. S1A†) reveals that the lengths of the MnO2 nanowires can reach several tens to hundreds of micrometers. Due to the ultralong one-dimensional nanowires structure and high dispersion, the MnO2 nanowires can a promising support for the MWCNTs/UL-MnO2-NWs membrane to enhance mechanical property. The treatment with acid and subsequent sonification can be an effective means to interrupt the mutual entanglement of carbon nanotubes. Comparing the agglomerated MWCNTs block that closely entwined (Fig. S1B†), the MWCNTs after the treatment with acid and subsequent sonification were dispersed well, as shown in Fig. 2B. Moreover, the dispersed MWCNTs can contact with each other closely, indicating the good film-formation of the samples that is critical for the fabrication of MWCNTs/UL-MnO2-NWs membrane. After the vacuum-filtering the MWCNTs and UL-MnO2-NWs suspension, a flexible MWCNTs/UL-MnO2-NWs membrane was obtained (Fig. S2A†) and the membrane shows a rough surface (Fig. S2B†). SEM images of the as-prepared sample are shown in Fig. 2C and D. As shown in Fig. 2C, the UL-MnO2-NWs (marked by arrows) and the acid treated MWCNTs uniformly dispersed during the vacuum-filtering process, forming a dense hybrid membrane. The UL-MnO2-NWs are interlaced with each other and stretch across the entire membrane, forming a support 10


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network. The special network structure may be beneficial to enhance the mechanical stability of the membrane.12 The dispersed MWCNTs are filled with the entire membrane, and no crashes are observed. This may be due to the high dispersion of MWCNTs and the compatibility between two fibers. The high magnification SEM image of the MWCNTs/UL-MnO2-NWs membrane is shown in Fig. 2D. The hybrid membrane with a wide distribution varying from 59 to 92 nm (Fig. S3†) shows a three-dimensional porous structure. The three-dimensional porous structure can facilitate rapid water-spreading into the membrane by capillary effects. The dispersed MWCNTs not only entangle with each other, but also closely surround the ultralong MnO2 nanowires (as shown in Fig. 2D and E), implying the extreme compatibility between these two fibers.

Fig. 2. SEM images of ultralong MnO2 nanowires (A) and dispersed MWCNTs (B); SEM images of MWCNTs/UL-MnO2-NWs membrane with different bar and the UL-MnO2-NWs is marked by the white line (C and D); structure schematic of the MWCNTs/UL-MnO2-NWs membrane (E). 11



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XRD was used to identify crystallographic phase and purity of the samples. As shown in Fig. 3, typical MWCNTs diffraction peaks at 25.58° and 43.53° can be assigned to the (0 0 2) and (1 0 1) planes, respectively. As expected, these two peaks are weaker and broader when compared with the original MWCNTs (Fig. S4†), indicating the low crystallinity of dispersed MWCNTs. This might be due to the chemical breaking by the nitric acid, resulting in the breaking and dispersion of MWCNTs. The diffraction peaks marked by black dots are attributed to the typical phase of MnO2 (JCPDS card No. 72-1982). No other impurity phases were detected, suggesting the high quality of MnO2 nanowires. For the MWCNTs/UL-MnO2-NWs membrane, the peaks marked by black dots are associated with MnO2 nanowires crystal. Meanwhile, the peak marked by purple diamond at 25.58° is attributed to the dispersed

MWCNTs.

Moreover,

the

intensity

of

the

peaks

in

the

MWCNTs/UL-MnO2-NWs membrane is lower than the peaks of UL-MnO2-NWs and MWCNTs, indicating the homogeneous mixing of UL-MnO2-NWs and MWCNTs, which is consistent with the results from SEM studies shown in Figure 1C.

12


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Fig.

3.

The

XRD

patterns

of

UL-MnO2-NWs,

MWCNTs,

and

MWCNTs/UL-MnO2-NWs membrane. To investigate the impacts of the treatment of MWCNTs in acid and subsequent sonification, the FT-IR spectra of the original MWCNTs and dispersed MWCNTs are acquired and compared as depicted in Fig. 4. Compared with the original MWCNTs, the dispersed MWCNTs shows a broad and strong characteristic peak at 3428 cm-1 that can be attributed to the O−H bond stretching of hydroxy, resulting from the functionalization process during the acid treatment. The bands around 2943 and 2820 cm-1 are attributed to the asymmetric and symmetric C-H stretching, respectively. The peaks around 1620 and 1340 cm-1 can be attributed to C=O band, and these peaks in the dispersed MWCNTs are obviously stronger than the peaks of original MWCNTs. Moreover, the dispersed MWCNTs displays a new peak at 1740 cm-1 that can be attributed to C=O band. The new peak and the stronger peaks at 1620 and 1340 cm-1 13



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can prove that the treatment of MWCNTs with acid and subsequent sonification not only effectively improves the dispersion of MWCNTs (Fig. S1† and Fig. 2A), but also realizes a surface functionalization process of carbon nanotubes to increase the surface energy of MWCNTs, which is extremely beneficial to enhance the hydrophilicity of MWCNTs.

Fig. 4. The FT-IR spectra of original MWCNTs and dispersed MWCNTs. 3.2. Mechanical and chemical stability The typical tensile stress-strain curves of MWCNTs membrane and MWCNTs/UL-MnO2-NWs membrane by dynamic mechanical analysis were shown in Fig. 5A. After introducing the UL-MnO2-NWs, the MWCNTs/UL-MnO2-NWs membrane shows a very high tensile stress of 24.9 MPa, which is 83% higher than that

(13.6

MPa)

of

pure

MWCNTs

membrane.

Moreover,

the

MWCNTs/UL-MnO2-NWs membrane exhibited the strain of 1.12%, while the pure 14


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MWCNTs membrane just has the low strain of 0.49%. These results clearly suggest an improvement in the mechanical properties after introducing UL-MnO2-NWs. The chemical stability is another significant factor of the hybrid membrane for the practical oil/water separation.31 In order to investigate the chemical stability of the MWCNTs/UL-MnO2-NWs membrane, the hybrid membrane was soaked into solution with different pH values (1, 5 ,7, 9, and 14) for 48 h, as shown in Fig. 5B. The MWCNTs/UL-MnO2-NWs membrane soaked in the neutral water did not dissociate, indicating the extremely compatible entanglement between MWCNTs and UL-MnO2-NWs. More importantly, the hybrid membrane exhibits good chemical stability under strong acid and alkali solution, and no detachment was obtained, which can be proved by the SEM imagines of MWCNTs/UL-MnO2-NWs membranes after soaking in solution with different pH values (Fig. S5†). The high mechanical stability, extremely

compatible

entanglement,

and

chemical

stability

of

the

MWCNTs/UL-MnO2-NWs membrane indicate its promising application in the oil/water separation.

Fig.

5.

(A)

Stress-strain

MWCNTs/UL-MnO2-NWs

curves

membrane;

of (B)

MWCNTs Optical

15


membrane photos

of

and the


MWCNTs/UL-MnO2-NWs membrane before and after soaking in solution with different pH values. 3.3. Wettability The oil/water separation performance of the MWCNTs/UL-MnO2-NWs membrane is very dependent on the wetting behavior. In Fig. S6, both pure MWCNTs and UL-MnO2-NWs shows superhydrophilicity with a low water CA of 0° and 2o, indicating their high surface free energy. The wetting performance of the MWCNTs/UL-MnO2-NWs membrane is shown in Fig. 6. Due to the high surface free energy of the hybrid membrane, it shows superhydrophilic with a low water CA of ~ 0° and underwater superoleophobic with an oil CA of ~ 152° (Fig. 6A and B). The membrane also exhibits underoil high-hydrophilic behavior with a water CA of ~ 18°. As shown in Fig. 6C, when a water droplet was dropped into the membrane under oil environment (toluene), the water droplet was captured and finally absorbed by the MWCNTs/UL-MnO2-NWs membrane in 1.44 s. Due to the special superhydrophilic, underwater superolephobic, and underoil high-hydrophilic wetting properties, the hybrid membrane would prefer to absorb/penetrate water and repel oil in the oil/water environment, which is beneficial to capture water and repel the microsize oil droplets in water-rich environment. Meanwhile, the membrane with superhydrophilic and underwater superoleophobic properties tend to form water-layer on the surface of it, which can avoid the direct contact between oil and membrane during the oil/water separation.32 It could efficiently enhance the antifouling ability and ensure long-term stability of separation efficiency. 16


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Fig. 6. Wetting behavior of the MWCNTs/UL-MnO2-NWs membrane towards (A) water in air, (B) oil in water, and (C) water in oil; (D) underoil water wetting behavior of the MWCNTs/UL-MnO2-NWs membrane under oil environment. 3.4. Separation performance for oil-in-water emulsions Due to the porous structure, as well as the superior underwater superoleophobic, the MWCNTs/UL-MnO2-NWs membrane can selectively absorb and penetrate water from the water-rich environments, resulting in the rapid and efficient separating various types of oil-in-water emulsions. To evaluate the separation capacity of the MWCNTs/UL-MnO2-NWs membrane, the membrane with a thickness of 1 mm was fixed by a vacuum driven filtration system with an effective diameter of 40 mm with the vacuum degree at -15 KPa, as shown in Fig. S7. The surfactant-stabilized toluene-in-water emulsion was used as the model emulsion. As shown in Fig. S7, when the milky emulsion was poured into the filter cell, the oil droplets in the emulsion de-emulsified once touching the membrane, and the water was selectively absorbed and penetrated while the oil was retained above the composite membrane 17



(Movie S1†). The separation results are shown in Fig. 7A. In the digital photos of the emulsion before and after separation, the milky emulsion changes from opaque to transparent after the MWCNTs/UL-MnO2-NWs membrane filtration. The optical microscope images were used to detect microsize oil droplets of the emulsion before and after separation. It is clear that there are numerous oil droplets in the feed solution, while no oil droplets were detected in the filtrate, indicating the excellent separation property of the MWCNTs/UL-MnO2-NWs membrane. The dynamic light scattering is also used to evaluate the droplet size distributions of the emulsion before and after separation. The feed solution shows a wide diameter distribution from 425 nm to 3328 nm (Fig. 7B), while the filtrate exhibits a droplet size distribution ranging from 24 nm to 58 nm (Fig. 7C), suggesting that the MWCNTs/UL-MnO2-NWs membrane possesses good tiny oil droplets removal ability from surfactant-stabilized oil-in-water emulsion.

18


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Fig. 7. (A) Optical microscope images and digital photos of the surfactant-stabilized toluene-in-water emulsion; the oil droplets size distribution (B) before separation and (C) after separation. Only separating surfactant-stabilized toluene-in-water emulsion is not enough to face complex oil and water environment; therefore, separation experiments for separating various surfactant-stabilized oil-in-water emulsions and surfactant-free oil-in-water

emulsions

are

performed.

As

shown

in

Fig.

8A,

the

MWCNTs/UL-MnO2-NWs membrane shows a high separation efficiency of greater than 99.7% for surfactant-stabilized oil-in-water emulsions including toluene-in-water emulsion,

isooctane-in-water

emulsion,

petroleum-in-water

emulsion,

and

diesel-in-water emulsion, indicating an extremely high separation efficiency, while the oil contents in the filtrates are below 33 ppm. Moreover, the separation efficiency of four surfactant-free oil-in-water emulsions is up to 99.8% and the oil content in the filtrates is less than 25 ppm (Fig. 8B). The oil contents in filtrates are all below 33 ppm for both the surfactant-stabilized and surfactant-free oil-in-water emulsions, meeting

the

standards

for

wastewater

discharge.33

Moreover,

the

MWCNTs/UL-MnO2-NWs membrane also exhibits great permeance for both surfactant-stabilized and surfactant-free oil-in-water emulsions. As shown in Fig. 8C, the filtrate permeance for all the emulsions is up to 4900 L m-2 h-1 bar-1, which is significantly higher than of commercial ultrafiltration membrane (less than 300 L m-2 h-1 bar-1).19 The high filtrate permeance and low oil content exhibit superior oil-in-water emulsion separation capacity. 19



As shown in Fig. 8d, the filtrate permeance of the MWCNTs/UL-MnO2-NWs membrane decreases with the increase of membrane thickness from 1 mm to 5 mm. This change trend can be explained by Hagen-Poiseuille equation (the classical fluid dynamic theory): J = επrp2∆p/8µL, where J is the permeance of the membrane, which is described as the function of the surface porosity ε, the pore radius of rp, the pressure drop of the membrane ∆p, the oil viscosity µ and the effective filtrate distance L.34 Obviously, the permeance is inversely proportional to the effective filtrate distance L. The effective filtrate distance L of the membrane increases as the thickness of the membrane increases. Thus, the filtrate permeance of the membrane decreases with the increase in the thickness of the membrane. But for the change trend of separation efficiency, due to the longer effective distance of the liquid running through the membrane, more oil droplets can be separated from the emulsion, resulting in the increased separation efficiency with the increase of the thickness of membrane.33

20


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Fig. 8. The separation efficiency and oil content in the filtrate of (A) various surfactant-stabilized emulsions, and (B) various surfactant-free emulsions. (C) Permeance of various surfactant-stabilized water-in-oil emulsions and surfactant-free emulsions. (D) The impacts of the thickness of the membrane on permeance and separation efficiency. Due to the universality and intractability of oil fouling, an ideal oil/water filtration membrane should possess an extreme anti-fouling property. The anti-fouling property of the MWCNTs/UL-MnO2-NWs membrane was evaluated by detecting the change of permeance and oil content in the filtrate during several emulsion separation cycles. During each cycle test, 200 mL of emulsion was completely separated by the MWCNTs/UL-MnO2-NWs membrane and then the membrane was simply washed and soaked in the ethanol to remove the oil fouling in the membrane. The change of the permeance during the separation process was recorded, and the results are shown in Fig. 9A. As observed, the permeance gradually decreased in each separation cycle experiment, while the permeance can recover completely to its original permeance after a simply cleaning. Moreover, the oil contents in filtrate after separation are all less than 33 ppm, implying the separation efficiency of the membrane is not sacrificed. To further prove this point, the cycling separation efficiency of the membrane was also measured. As shown in Fig. 9B, the membrane presents high separation capacity even after 10 cycles, exhibiting the superior recyclability of the membrane.

21



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Fig. 9. (A) The permeance and oil content in filtrate during the antifouling test of the MWCNTs/UL-MnO2-NWs membrane; (B) Cycling separation performances of the MWCNTs/UL-MnO2-NWs

membrane

(surfactant-stabilized

toluene-in-water

emulsion as the model).

4. Conclusions In summary, we have demonstrated a reliable approach for fabrication of flexible MWCNTs/UL-MnO2-NWs hybrid membrane based on staggered structures via a vacuum-filtration of dispersed MWCNTs and UL-MnO2-NWs suspension. In this hybrid process, the one-dimensional MnO2 nanowires with lengths of up to hundreds of micrometers were prepared using a hydrothermal route, which provide the hierarchical skeletons and enhance the mechanical strength of hybrid membrane. The acid treatment of agglomerate MWCNTs result in the formation the dispersed building blocks of hybrid membrane, which also produces the hydrophilic functional group and enhances the surface wetting properties of hybrid membrane. Compared with the pure MWCNTs membrane, the MWCNTs/UL-MnO2-NWs hybrid membrane shows a high tensile stress (24.9 MPa), implying that the introduced ultralong MnO2 nanowires can effectively improve the mechanical stability of the hybrid membrane. More importantly, the hybrid membrane exhibits excellent chemical stability under 22


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strong acid and alkali solution for long times. It is worth mentioning that the membrane

can

effectively

separate

both

various

surfactant-stabilized

and

surfactant-free oil-in-water emulsions with high permeances up to 4900 L m-2 h-1 bar-1 and high separation efficiency of greater than 99.7%. Furthermore, the oil fouling in the membrane can be cleaned by a simple ethanol cleaning, indicating the excellent antifouling property of the hybrid membrane. In addition, hybrid membrane maintains high separation capacity even after 10 cycles and the excellent reusability is demonstrated. These attractive advantages endow the hybrid membrane to be a potential candidate for practical application in water-in-oil emulsion separation.

Supporting information SEM imagines of MnO2 nanowires and MWCNTs (Figure S1); photo of hybrid membrane and SEM image (Figure S2); SEM image with high magnification of the hybrid membrane (Figure S3); XRD patterns of original and dispersed MWCNTs (Figure S4); SEM images of hybrid membranes after soaking in solution with different pH values (Figure S5); Water CA of pure MWCNTs and pure UL-MnO2-NWs (Figure S6); Oil/water separation device (Figure S7); Separation of surfactant-stabilized toluene-in-water emulsion (AVI).

Acknowledgements This work was supported by Natural Science Foundation of Jiangsu Province (BK20161264, BK20160500 and BK20161362) and the National Nature Science Foundation of China (U1507115 and 21706100). This research was also supported by the High-Level Personnel Training Project of Jiangsu Province (BRA2016142), China 23



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Postdoctoral Science Foundation (2016M600373 and 2018T110452), China Postdoctoral Science Foundation of Jiangsu Province (1701067C, 1701073C and 1601016A), and Scientific Research Foundation for Advanced Talents, Jiangsu University (15JDG142), Key Research and Development Program of Jiangxi Province (20171BBH80008).

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Ultralong MnO2 nanowires enhanced multi-wall carbon nanotubes

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