Asymmetric Aerogel Membranes with Ultrafast Water Permeation for

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Functional Nanostructured Materials (including low-D carbon)

Asymmetric Aerogel Membranes with Ultra-Fast Water Permeation for Separation of Oil-in-Water Emulsion Ya-nan Liu, Yanlei Su, Jingyuan Guan, Jialin Cao, Runnan Zhang, Mingrui He, and Zhongyi Jiang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b09362 • Publication Date (Web): 19 Jul 2018 Downloaded from http://pubs.acs.org on July 20, 2018

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ACS Applied Materials & Interfaces

Asymmetric Aerogel Membranes with Ultra-Fast Water Permeation for Separation of Oil-in-Water Emulsion Yanan Liuab, Yanlei Suab, Jingyuan Guanab, Jialin Caoab, Runnan Zhangab, Mingrui Heab, Zhongyi Jiangab∗ a Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China b Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, China

Keywords: aerogel membrane, asymmetric structure, ultra-fast water permeation, antifouling properties, oil/water separation

∗ Corresponding author. School of Chemical Engineering and Technology, Tianjin University, No. 92, Weijin Road, Nankai District, Tianjin 300072, China Tel: 86-22-27406646. Fax: 86-22-23500086. E-mail address: [email protected] (Z.Y. Jiang) 1 ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract: Owing to highly porous and low density attributes, aerogels have been actively utilized in catalysis and adsorption processes, but their great potential in filtration requires exploitation. In this study, an asymmetric aerogel membrane is fabricated via one-pot hydrothermal reaction induced self-crosslinking of poly(vinyl alcohol) (PVA), which exhibits ultra-fast permeation for separation of oil-in-water emulsion. Meanwhile, carbon nanotubes (CNT) were added to improve the mechanical strength of the aerogel membranes. The self-crosslinking of PVA forms the supporting layer, and the exchange of water and vapor at the interface of PVA solution and air generates the separating layer as well as abundant hydroxyl groups on membrane surface. The density, porosity, pore size and wettability of aerogel membrane can be tuned by PVA concentration. Owing to high porosity (>95%) and suitable pore size ( 95%) used in this study was decorated by carboxylic groups, which was commercial products provided by Nanjing XFNANO Materials Tech Co., Ltd. Deionized water (>18 MΩ/cm) used in all experiments was purified by Milli-Q system. And the other reagents were acquired from local reagent corporation and used without further purification. 2.2. Fabrication of PVA@CNT aerogel membranes PVA was added into deionized water with the concentration of 3 wt%, 4 wt% and 5 wt%, after dissolving, H2SO4 and CNT were added in the solution with the concentration of 0.2 mol/L and 0.25 wt%, respectively. The mixtures were treated with ultrasonication for 10 min to obtain intensive mixing of these reagents, then 3.5 mL above mixture was transferred into Teflon lined autoclave with a diameter of about 4.8 cm, and kept in 200oC for 24 h. All aerogel membranes were kept in deionized water to eliminate the residual ions before performance measurement. And freezing drying was used to dry the aerogel membrane before characterization of membrane. 2.3. Performance measurement of PVA@CNT aerogel membranes The separation performances were carried out with a filtration cell with a filtration area of 0.9 cm2 (Millipore Model 8003), and each membrane was evaluated more than three times to ensure high reproducibility of this measure. All the test proceeded at a stirring speed of 200 rpm and the feed side was driven by gravity of 5 cm liquid column (4.9*10-3 bar). The

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permeation flux J (L m-2 h-1 bar-1) of each membrane was calculated by filtration deionized water for 5 min according to the following equation: J=

V A∆∆ tp

(1)

where V (L) was the volume of permeated water, A (m2) was the efficient area of membrane, ∆t (h) was the recorded time and ∆p (bar) was the pressure drop. To evaluate the antifouling properties of PVA@CNT aerogel membranes in oil/water separation, three kinds of surfactant-stabilized oil-in-water emulsions were prepared: 100 mg pump, 100 mg silicone oil or 100 mg hexadecane and 10 mg sodium dodecyl sulfate were added into 100 mL deionized water and were treated by ultrasonic bath for 20 min, and the oil-in-water emulsions could last more than one month without any phase separation. And the surface zeta potential and oil droplet distribution in oil-in-water emulsions were characterized using particle size/zeta potential analyzer (Zetasizer nano ZS90) (Figure S1). The surfactant-stabilized oil-in-water emulsions were filtrated by as-prepared membranes for 10 min, and a UV-spectrophotometer (UV-9200) was utilized to measure the content of oil droplets in the filtrate and feed solution. A three stage filtration process of water-oil-water proceeded to evaluate the antifouling properties of as-prepared membranes. The initial permeation flux of deionized water (Jwater), the permeation flux of surfactant-stabilized oil-in-water emulsion (Joil), the recovered permeation flux of deionized water (Jrecovery) after water rinsing for 1 min were collected. And the higher flux recovery ratio (FRR = Jrecovery / Jwater) indicated better antifouling properties. 2.4. Characterization Scanning electron microscopy (SEM) recording on a Nova Nanosem 430 field-emission scanning electron microscopy and atomic force microscope (AFM) performing on a Bruker multimode 8 atomic force microscope system were used to measure the surface morphology of PVA@CNT aerogel membranes. Fourier transform infrared spectroscopy (FTIR) (VERTEX70) and X-ray photoelectron spectroscopy (XPS) operating on Perkin Elmer Phi 6 ACS Paragon Plus Environment

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1600 ESCA system with Mg Kα (1254.0 eV) as the radiation source were used to analyze the surface and inside compositions of PVA@CNT aerogel membranes. A contact angle goniometer (JC2000D2 M Contact Angle Meter) was utilized to measure the water contact angle of membrane surface and membrane inside and oil contact angle underwater of membrane surface. The characterization of inside of PVA@CNT aerogel membrane proceeded through wiping off the surface and measuring the middle of as-prepared membranes. The density of PVA@CNT aerogel membrane (ρa=M/V, M and V were the mass and volume of the cuboids, respectively) was measured by cutting into several regular cuboids after freeze-drying, and the porosity of as-prepared membranes was calculated following the Equation:15 porosity ሺ%ሻ= ൬1- a ൰ ×100% ρ ρ

(2)

p

where ρp was the solid density of PVA, which was 1325 kg m-3 according to the data from manufacturer. 3. Results and Discussion 3.1. Preparation and structure features of PVA@CNT aerogel membranes The PVA@CNT aerogel membranes were fabricated by hydrothermal reaction induced self-crosslinking of PVA, as shown in Figure 1a. A certain amount of PVA and CNT were added into a Teflon-lined autoclave, and after adding H2SO4 as a catalyst, 24 h hydrothermal process proceeded at 200oC. During the hydrothermal process, the –OH groups in PVA were activated by Brønsted acid, converting to –OH2+ groups, which were better leaving groups (Figure S2). The acid-functionalized dehydration of PVA was conducted by two means of crosslinking, ether linkages and double-bond through β-elimination.15 Fourier transform infrared (FTIR) exhibited the chemical structure of PVA after the hydrothermal process, and the FTIR of the inside of PVA@CNT aerogel membrane was acquired by wiping off the surface of PVA@CNT aerogel membrane. As shown in FITR spectra, after hydrothermal 7 ACS Paragon Plus Environment


process, the stretching adsorption peak at 3000-3700 cm-1 weakened, because –OH groups were consumed. And there were more –OH groups remaining on the surface than inside of PVA@CNT aerogel membrane, demonstrating that dehydration of hydroxyl groups on membrane surface was suppressed. An obvious peak appeared at 1080 cm-1 for the stretching of the C-O-C bond, implying the formation of ether bond. The weakening of –OH groups and appearing of C-O-C groups implied the dehydration process between –OH groups on PVA chain leading to self-crosslinking of PVA. The attenuation of –OH groups and occurrence of – C-O-C- groups demonstrated that ether linkages dominated the whole reaction. Moreover, the self-crosslinking of PVA could be verified by the results of thermogravimetric analysis for the increasing of decomposition temperature, as shown in Figure S3. Meanwhile, the XPS spectrum of PVA@CNT aerogel membrane shown in Figure 1c manifested that the oxygen on membrane surface was higher than the oxygen inside membrane bulk, and thus the more hydroxyl groups resided on membrane surface than inside, which was in accordance with the results of FTIR spectra. The self-crosslinking of PVA was contributed to the network of PVA@CNT aerogel membranes and would influence the structure of as-prepared membranes. In this study, the concentrations of PVA were 3 wt%, 4 wt% and 5 wt%, which were denoted as PVA@CNT-3, PVA@CNT-4 and PVA@CNT-5, respectively. With the increase of PVA concentration, the density of PVA@CNT aerogel membrane increased and the porosity decreased, as shown in Figure 1d. And the porosity of PVA@CNT aerogel membrane was in the range of 92.0%-95.6%, which was conducive to high water permeation, because of low mass transfer resistance.22


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Figure 1. a) Schematic illustration for the fabrication process and network structure of PVA@CNT aerogel membranes, b) FTIR spectra of inside and outer surface of PVA@CNT-3 membrane and the mixture of PVA and CNT c) XPS spectra of inside and outer surface of PVA@CNT-3 membrane, d) density and porosity of PVA@CNT aerogel membrane after freeze-drying The aerogel membranes possessed asymmetric structures including separating layer and supporting layer, as shown in Figure 2 (Figure S4). The surface of as-prepared membrane was relatively smooth, and possessed obvious pores, which was beneficial to oil/water separation. And with the increase of PVA concentration, the pore size of as-prepared membrane decreased from 84±82 nm for PVA@CNT-3 membrane to 20±15 nm for PVA@CNT-5 membrane, which was measured by image analysis software,37 because more PVA would form more compact structure to decrease the membrane pore size. It should be mentioned that the distribution of PVA@CNT membrane pore size was rather large, most probably because of the flexibility and self-crosslinking of polymer as well as the fast exchange of water and vapor at the interface of PVA solution and air, however, the membrane exhibited superior performance in separating oil-in-water emulsion, because the droplets size 9 ACS Paragon Plus Environment


of most of oil-in-water emulsions was a few hundred nanometers.38-39 Meanwhile, the PVA@CNT aerogel membrane possessed hierarchical nanostructure, and the surface roughness of the as-prepared membranes decreased slightly because of the decrease of pore size (Figure S5). The surface roughness parameter Ra decreased from 25.54±2.63 nm for PVA@CNT-3 membrane to 24.91±1.95 nm for PVA@CNT-5 membrane, and Rq decreased from 35.91±3.21 nm for PVA@CNT-3 membrane to 33.19±2.68 nm for PVA@CNT-5 membrane. The inside of as-prepared membrane exhibited a 3D interconnecting network with uniform pores distributing in the dendritic arborization, which would contribute to the rapid transfer of water, leading to high water permeation. Also, with the increase of PVA concentration, the structure become denser and the pores distribution in the dendritic arborization decreased, resulting in low porosity, which was in accordance with calculation of porosity (Figure S6). The formation of asymmetric structure was attributed to the hydrothermal synthesis process, because the self-crosslinking of PVA would influence the structure of aerogels. During the hydrothermal synthesis process, the exchange of water and vapor took place at the interface of PVA aqueous solution and air, which would increase the concentration of PVA and suppress the self-crosslinking of PVA, resulting in dense surface structure and abundant hydroxyl groups stayed on membrane surface. And in the PVA solution, the self-crosslinking of PVA formed microspheres, which connected with each other to produce 3D interconnecting network with dendritic arborization, leading to high porous supporting layer.14 The thickness of as-prepared membrane was in the range of 648.9 µm to 749.3µm, and decreased with the increase of PVA concentration due to high self-crosslinking of PVA (Figure S7). Meanwhile, the mechanical strength of PVA@CNT aerogel membranes was increased with the increase of PVA concentration, and the existence of CNT was contributed to the enhancement of mechanical strength (Figure S8). In addition, due to the limitation of membrane mechanical strength, the PVA@CNT aerogel membranes with PVA concentration under 3 wt% could not be fabricated. 10 ACS Paragon Plus Environment

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Figure 2. a, b) The SEM and AFM images of PVA@CNT-3 membrane including the surface and inside, c) the surface SEM images of PVA@CNT-4 and PVA@CNT-5 membrane. The insets were SEM images of the cross-section of PVA@CNT aerogel membranes. 2.2. Wetting properties of PVA@CNT aerogel membranes The wetting properties of membranes including water contact angle in air and oil contact angle in water were critical properties for the membrane used in oil/water separation. Therefore, a series of contact angles were measured to evaluate the wetting properties of PVA@CNT aerogel membranes. The surface of PVA@CNT aerogel membranes was super-hydrophilic, and the water droplets could spread on the surface completely in hundred milliseconds, due to the residual hydroxyl groups and hierarchical nanostructure on membrane surface, as shown in Figure 1c. Such a rapid spreading of water on membrane surface could endow the as-prepared membranes with high water permeation, by the reason of the super-wetting properties.40 And with the PVA concentration increased, the time of water droplets spreading on the surface increased, which might be because that with the PVA concentration increased, the surface porosity decreased, as shown in Figure 2. Another reason 11 ACS Paragon Plus Environment


might be that with the PVA concentration increasing, the residual hydroxyl groups decreased, as shown in Figure 3b. A great deal of hydroxyl groups remaining on membrane surface endowed the PVA@CNT aerogel membrane with enhanced hydration capacity. Hence, the water could be entrapped on the membrane surface, leading to a robust hydration layer at the interface of water/membrane. As shown in Figure 3c, the as-prepared membrane possessed super-oleophobicity underwater for various oils including pump oil, silicone oil and hexadecane. A dynamic approach-compress-detach process was utilized to investigate the oil-adhesion experiment of PVA@CNT aerogel membranes underwater (Figure S9). During the process of oil contacting the membrane surface underwater, the trapped water on the hierarchical nanostructure of membrane surface would generate a hydration structure which could protect the membrane surface from being complete contact with the oil droplets. Hence, the effective contact area between membrane surface and oil droplet became discontinuous, which would prevent the oil from wetting membrane surface and keep a spherical droplet without deformation.6, 8, 41 It was noteworthy that the contact angle inside PVA@CNT aerogel membrane reached 148o for PVA@CNT-5 membrane, and with the increase of PVA concentration, the hydrophobicity was increased. According to the FTIR spectra, during the hydrothermal process, the self-crosslinking of PVA proceeded inside the aerogel membrane, consuming a large proportion of hydroxyl groups in PVA, leading to the increase of hydrophobicity. At the surface of aerogel membranes, the exchange of water and vapor took place at the interface of PVA aqueous solution and air, which would suppress the self-crosslinking of PVA, as a result, abundant hydroxyl groups in PVA were retained on membrane surface, ensuring the high hydrophilicity of membranes. Consequently, the PVA@CNT aerogel membranes possessed asymmetric wettability. In principle, the hydrophobicity of membrane would increase the transmembrane pressure to some extent, nevertheless, which could be influenced by pore size.42-43 When water transported through molecular-scale hydrophobic channels, a frictionless flow and high flow velocity took place, 12 ACS Paragon Plus Environment

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for the weak interfacial force between membrane and water molecules.44-45 And when the pores were large enough, the influence of hydrophobicity on water permeation flux could be ignored, because of the low intrusion pressure of water within hydrophobic pore.46 In this study, the pores inside the membrane were well connected and large enough, and the influence of hydrophobicity on water permeation flux was thus negligible. Meanwhile, the hydrophobicity of as-prepared membrane would increase the stability of membrane in water.

b

PVA@CNT-3 PVA@CNT-4 PVA@CNT-5

40

30

Atomic % C 1s 76.65 O 1s 23.35

O1s

50

C/S

o

Water contact angle( )

a

C1s

Hence, the PVA@CNT aerogel membranes could be used in practical oil/water separations.

Atomic % C 1s 82.27 O 1s 17.73

PVA@CNT-3 PVA@CNT-4 PVA@CNT-5

20

Atomic % C 1s 84.65 O 1s 15.35

10

0 100

200

300

400

500

700

600

600

190 180

400

300

200

180

Pump oil

Silicone oil

Hexadecane

100

800

d

750

160

700

170

140

160

650

o

Water contact angle( )

c

500

Bonding Energy (eV)

Time(ms)

150 140 130 120 110

120

600

Surface Inside

100

550 500

80

450

60

Time(ms)

0

Oil contact angle underwater(o)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


400 350

40

300

100

20

90

0

PVA@CNT-3

PVA@CNT-4

250 200

PVA@CNT-3

PVA@CNT-5

PVA@CNT-4

PVA@CNT-5

Figure 3. a) The dynamic water contact angle in air, b) the XPS spectrum of the surfaces PVA@CNT aerogel membrane c) the oil contact angle underwater of PVA@CNT aerogel membranes including pump oil, silicone oil and hexadecane, d) the instantaneous water contact angle of surface, water contact angle of inside and the time of surface water contact angle reaching to 0o. 3.3. Permeation performance of PVA@CNT aerogel membranes



The Hagen-Poiseuille equation was a classical fluid dynamic theory for the liquid permeation flux through an open pore. According to the equation (J=επrp2∆p/8µL), there were several parameters which could influence the permeation flux J including the porosity ε, the pore radius rp, the pressure drop ∆p, the liquid viscosity µ, and the total distance L traveled by the liquid passing through the membrane.7-9 And the porosity ε was in direct proportion to the permeation flux. As a high porosity material, aerogel had some intrinsic advantages in membrane technology for high water permeation flux. In our study, PVA@CNT aerogel membranes possessed high porosity, and with the increase of PVA concentration, the porosity decreased from 95.6% for PVA@CNT-3 membrane to 92.0% for PVA@CNT-5 membrane, as shown in Figure 1. Meanwhile, the surface pore and porosity also decreased with the increasing of PVA concentration in Figure 2. Therefore, the increased mass transfer resistance led to the decrease of water permeation, as shown in Figure 4. And the water permeation flux decreased from 135.5*103 Lm-2h-1bar-1 for PVA@CNT-3 membrane to 24761 Lm-2h-1bar-1 for PVA@CNT-5 membrane, under the transmembrane pressure of 4.9*10-3 bar (5 cm liquid column), which was 2 orders of magnitude higher than commercial filtration membranes. The permeation stability of the PVA@CNT aerogel membranes was evaluated by the dependence of water permeation flux on the applied transmembrane pressure in the range of 4.9*10-3 bar to 68.6*10-3 bar (5 cm liquid column to 70 cm liquid column) and the water permeation flux with water pH of 1.0-13.0. As shown in Figure 4b, the water permeability of PVA@CNT-3 membrane increased with the increase of the applied transmembrane pressure. Nevertheless, the water permeability cofficient was decreased with the increasing of applied pressure, demonstrating that the porous network structure of PVA@CNT aerogel membrane was compressed by the pressure, giving rise to the increase of mass transfer resistance. Simultaneously, it could be known from Figure 4c that the PVA@CNT aerogel membranes possessed stable water permeation flux in the pH range of 1.0-13.0. 14 ACS Paragon Plus Environment

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160000

a

b Water permeability(Lm-2h-1)

140000 120000

Flux(Lm-2h-1bar-1)

100000 80000 60000 40000 20000

1600

140000

1400

120000

1200

100000 1000 80000 800 60000

600 400

40000

200

20000

0

0

PVA@CNT-3

c

0 0

PVA@CNT-5

PVA@CNT-4

10

20

30

40

50

60

70

Pressure(10-3bar)

140000 120000

Flux(Lm-2h-1bar-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Water permeability coefficient(Lm-2h-1bar-1)

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100000 80000 60000 40000 20000 0

1

3

7

11

13

pH

Figure 4. a) Permeation flux of the PVA@CNT aerogel membranes, b) permeation flux of the PVA@CNT aerogel membranes under different applied transmembrane pressure and c) at different pH. 2.4. Oil/water separation performance of PVA@CNT aerogel membranes In this study, a three stage filtration experiment including the initial permeation flux of deionized water (Jwater), the permeation flux of surfactant-stabilized oil-in-water emulsion (Joil) and the recovered permeation flux of deionized water (Jrecovery) after water rinsing for 1 min, proceeded to evaluate the oil/water separation performance and antifouling properties of the PVA@CNT aerogel membranes. The PVA@CNT aerogel membranes possessed asymmetric structure including separating layer and supporting layer, which could maintain the high rejection and high water permeation flux synchronously. In this study, the rejection of PVA@CNT aerogel membranes was more than 99% for surfactant-stabilized pump oil-in-water emulsion, which was estimated by UV-vis spectrophotometer (Figure S10), and the water permeation flux could reach up to 135.5*103 Lm-2h-1bar-1 which was 2 orders of 15 ACS Paragon Plus Environment


magnitude higher than commercial filtration membranes with similar rejection. Due to the super-hydrophilicity of membrane surface, the PVA@CNT aerogel membranes possessed high antifouling properties, and the flux recovery was more than 99.6% for separation of surfactant-stabilized pump oil-in water emulsion. In particular, the initial water permeation flux of PVA@CNT-3 membrane was 135.5*103 Lm-2h-1bar-1, decreased to 48435 Lm-2h-1bar-1 during separation of oi-in-water emulsion, and after water rinsing for 1 min, the water permeation flux recovered to 134966 Lm-2h-1bar-1 which was 99.6% of the initial water permeation flux. As a result, PVA@CNT-3 membrane was selected for to the subsequent measurement of the membrane comprehensive performance. The long-term antifouling properties of PVA@CNT aerogel membranes were evaluated by cyclic separation experiments of PVA@CNT-3 membrane, containing surfactant-stabilized pump oil-in-water emulsion, surfactant-stabilized silicone oil-in-water emulsion and surfactant-stabilized hexadecane-in-water emulsion, as shown in Figure 5. After five cycles of separation experiments, the flux recovery ratio was more than 95% for surfactant-stabilized pump oil-in-water emulsion, 94% for surfactant-stabilized silicone oil-in-water emulsion and 93% for surfactant-stabilized hexadecane-in-water emulsion. Compared with those of the state-of-the-art membrane for oil/water separation (Table 1), the separation performances of PVA@CNT aerogel membranes were higher. All these results implied that the PVA@CNT aerogel membranes possessed excellent antifouling properties and could be used to separate surfactant-stabilized oil-in-water emulsion effectively. 100.2

Jwater

100.0

Joil 99.6

100000

99.4 99.2

80000

99.0 60000

98.8 98.6

40000

98.4 20000

140000

Jwater

120000

Separation efficiency(%)

Jrecovery

120000

b

99.8

Flux(Lm-2h-1bar-1)

a

140000

Flux(Lm-2h-1bar-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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100000 80000 60000 40000

Joil

20000

98.2

0 0

98.0

PVA@CNT-3

PVA@CNT-4

0

1

2

3

Cycle Number

PVA@CNT-5


4

5

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c

140000

d

140000

Jwater

120000

120000

100000

100000

Flux(Lm-2h-1bar-1)

Flux(Lm-2h-1bar-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


80000 60000 40000 20000

Joil

Jwater

80000 60000 40000

Joil

20000

0

0 0

1

2

3

4

5

0

1

Cycle Number

2

3

4

5

Cycle Number

Figure 5. a) The permeation fluxes (initial permeation flux of deionized water (Jwater), the permeation flux of surfactant-stabilized pump oil-in-water emulsion (Joil), the recovered permeation flux of deionized water (Jrecovery) and separation efficiency of the PVA@CNT aerogel membranes, b) the permeation flux of PVA@CNT-3 membrane for cyclic separation experiments containing permeation fluxes of deionized water and surfactant-stabilized pump oil-in-water emulsion, c) surfactant-stabilized silicone oil-in-water emulsion and d) surfactant-stabilized hexadecane-in-water emulsion.

Table 1. Comparison of the antifouling performance with the state-of-the-art oil/water separation membranes in the literature Flux Fabrication

FRR (Lm-2h-1ba

Membranes

Emulsion types

methods

Reference (%)

r) Hydrothermal PVA@CNT

GO/g-C3N4@Ti

Surfactant-stabil 3

135.5*10 method

ized oil-in-water

Vacuum-assisted

Surfactant-stabil 4536

O2

self - assembly

99.6

This work

99.9

8

95

47

ized oil-in-water

Non-solvent Surfactant-stabil PVDF/DA/TiO2

induced phase

600 ized oil-in-water

separation



SiO2-NP-decorat ed PVDF

delayed phase

surfactant-stabili 11.1*103

inversion

Vacuum-assisted

49

81.7

6

93

41

-

50

-

2

Surfactant-stabil

self - assembly

ized oil-in-water

Vacuum-assisted GO/palygorskite


2854 C

48


self - assembly PEI@CNT-TM

zed water-in-oil

Vacuum-assisted polymer@CNT

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self - assembly

ized oil-in-water

pNIPAm-co-AA Vacuum-assisted

Surfactant-stabil 35.9*103

m cohybrid self - assembly

ized oil-in-water

SWCNT

Non-solvent Surfactant-stabil PAA-g-PVDF

induced phase


separation

4. Conclusions An aerogel membrane with asymmetric structure was fabricated by one-pot hydrothermal reaction induced self-crosslinking of poly(vinyl alcohol) (PVA) to achieve ultra-fast permeation for separation of oil-in-water emulsion. The self-crosslinking of PVA endowed the aerogel membrane with high porosity (95.6%) which significantly decreased mass transfer resistance, leading to ultra-high water permeation flux of 135.5*103 Lm-2h-1bar-1 under gravity-driven flow, which was 2 orders of magnitude higher than commercial filtration membranes with similar rejection. Meanwhile, the asymmetric structure comprising separating layer and supporting layer afforded desirable rejection (99.0%) under high permeation flux. The residual hydroxyl groups on membrane surface rendered the aerogel membranes super-hydrophilicity and super-oleophobicity underwater. Hence, the as-prepared 18 ACS Paragon Plus Environment

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membranes exhibited excellent antifouling properties for various surfactant-stabilized oil-in-water emulsions including pump oil-in-water emulsion, silicone oil-in-water emulsion and hexadecane-in-water emulsion. And the flux recovery ratio of as-prepared membranes was preserved more than 93% for five cycle separation experiments of these three kinds of emulsions. The high water permeation flux originated from highly porous structure of aerogel, high rejection originated from asymmetric structure of aerogel and excellent antifouling properties originated from super-hydrophilicity and super-oleophobicity underwater of aerogel surface ensure the great application potential in oil/water separation, especially surfactant-stabilized oil-in-water emulsion separation. Hopefully, this study can stimulate the thinking about a new way to create high throughput membranes.

Acknowledgements: This study was supported by the National Natural Science Fundation of China (21621004), the National Key Research and Development Program-China (2016YFB0600503) and the National Science Fund for Distinguished Young Scholars (21125627).

Supporting Information Available: Characterization of surfactant-stabilized oil-in-water emulsions; The self-crosslinking mechanism of PVA leading to the network structure of PVA@CNT aerogel membranes; Thermogravimetric analysis (TGA) of PVA@CNT aerogel membranes; AFM images of PVA@CNT aerogel membranes; The SEM images of inside PVA@CNT aerogel membranes; The thickness of as-prepared membranes; The tensile strength of PVA@CNT and PVA-3 aerogel membranes; Dynamic approach-compress-detach oil-adhesion experiment of PVA@CNT aerogel membranes; The microscopic pictures of the emulsions before and after separation.



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The table of contents entry: PVA@CNT aerogel membrane is fabricated via one-pot hydrothermal reaction induced self-crosslinking of poly(vinyl alcohol) (PVA). The as-prepared membrane possesses high porosity and asymmetric structure, leading to ultrahigh permeation flux and good rejection for separation of oil-in-water emulsion. Meanwhile, the abundant hydroxyl groups on membrane surface afford excellent antifouling properties.


Asymmetric Aerogel Membranes with Ultrafast Water Permeation for

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