water emulsion separation

Journal Pre-proofs Dual Superlyophobic Zeolitic Imidazolate Framework-8 Modified Membrane for Controllable Oil/Water Emulsion Separation Atian Xie, Jiuyun Cui, Jin Yang, Yangyang Chen, Jihui Lang, Chunxiang Li, Yongsheng Yan, Jiangdong Dai PII: DOI: Reference:

S1383-5866(19)33158-2 https://doi.org/10.1016/j.seppur.2019.116273 SEPPUR 116273

To appear in:

Separation and Purification Technology

Received Date: Revised Date: Accepted Date:

19 July 2019 30 October 2019 30 October 2019

Please cite this article as: A. Xie, J. Cui, J. Yang, Y. Chen, J. Lang, C. Li, Y. Yan, J. Dai, Dual Superlyophobic Zeolitic Imidazolate Framework-8 Modified Membrane for Controllable Oil/Water Emulsion Separation, Separation and Purification Technology (2019), doi: https://doi.org/10.1016/j.seppur.2019.116273

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1

Dual Superlyophobic Zeolitic Imidazolate Framework-8 Modified Membrane for

2

Controllable Oil/Water Emulsion Separation

3 4

Atian Xiea,b, Jiuyun Cuic, Jin Yangb, Yangyang Chenb, Jihui Langd, Chunxiang Lib,

5

Yongsheng Yanb*, Jiangdong Daib*

6 7

aSchool

8

212013, China

9

bInstitute

of the Environment and Safety Engineering, Jiangsu University, Zhenjiang

of Green Chemistry and Chemical Technology, School of Chemistry and

10

Chemical Engineering, Jiangsu University, Zhenjiang 212013, China

11

cSchool

12

China

13

d

14

Education, Jilin Normal University, Siping 136000, P. R. China

15

*Corresponding Author

of Materials Science and Engineering, Jiangsu University, Zhenjiang 212013,

Key Laboratory of Functional Materials Physics and Chemistry of the Ministry of

16 17

E-mail: [email protected], [email protected]

18

Tel: +86 0511-88790683; Fax: +86 0511-88791800

1

1

Abstract

2

The oil/water emulsion separation has become a thorny problem worldwide. Membrane

3

separation based on superwetting materials has attracted immense attention in the domain of

4

oil/water separation, while the single wettability severely limits their applications for treatment of

5

different types oily wastewater. In this work, ZIF-8 modified membrane (RC@PDA/ZIF-8) is

6

prepared via coordination-driven in situ self-assembly ZIF-8 on polydopamine (PDA) coated

7

regenerated cellulose membrane (RC). The surface wettability of resultant membrane can be

8

switched between underwater superoleophobicity and underoil (super)hydrophobicity by water or

9

oils prewetted without additional external stimuli, which is suitable for controllable oil/water

10

emulsion separation. Moreover, this membrane also displays superior separation capacity as high

11

as above 99% and good flux for various oil-in-water and water-in-oil emulsions. Furthermore, the

12

membrane shows good recyclability that it can retain high separation efficiency (98.5%) and flux

13

after at least 10 times cycles. Also, the controllable emulsion separation mechanisms are analyzed

14

and elucidated in detail. This ZIF-8 modified membrane with easy preparation and simple

15

operation exhibits excellent separation efficiency and recyclability for controllable oil/water

16

emulsion separation. Our work demonstrates MOFs-based membrane has potential applications

17

including controllable oil/water separation and fuel purification.

18 19

Keywords:

ZIF-8;

membrane

20

superhydrophobic; emulsion separation

separation;

2

underwater

superoleophobic;

underoil

1

1. Introduction

2

Water pollution and water ecological deterioration have attracted considerable worldwide

3

attention[1-4]. Oily wastewater from oil spills, discharge of industrial and urban sewage has

4

become one of the most urgent global environmental issues due to adverse environmental

5

impacts[5-6]. Especially the oil/water emulsion separation has become a thorny problem because

6

of the small oil droplets sizes and stable system. Much efforts have been made for resolving oil

7

pollution, such as gravity separation, centrifugal separation, floatation, electrocoagulation,

8

adsorption and membrane separation[7-10]. Therein, membrane separation, especially membrane

9

separation technology based on superwetting materials has been identified to be a more attractive

10

approach for oil/water separation for its cost-effective, highly efficiency, accessibility and energy

11

saving[11-15].

12

Recently, the preparation of superwetting materials and their application in oil/water

13

separation has achieved explosive progresses. All membranes show a trade-off effect between

14

permeability and selectivity[16]. While superwetting membranes show excellent selectivity in

15

oil/water separation due to preferred affinity to water or oil compared to traditional membranes.

16

Furthermore, the coordination of hierarchical nanostructures interferes with the flow regime near

17

the water/membrane interface, resulting in the nonequilibrium adhesion of pollutants to enhance

18

the permeability of the membrane. In view of these merits, a multitude of superwetting materials

19

(mainly

20

superoleophobic etc.) based on various substrate materials (such as meshes[17-18], textiles[19],

21

polymer membranes[20]) have been designed and fabricated for oil/water separation. Although the

22

superwetting materials show a higher separation efficiency, the monotonous wettability of

23

membranes has severely limited their applications for treatment of different types oily wastewater.

24

To resolve this problem, smart materials with switchable wettability under external stimuli (such

25

as temperature[21], solvent[22], gas[23], pH[24], electricity[25], ion[26] and light[27]) have been

26

designed to realize on-demand oil/water separation. All these materials can obtain satisfactory

27

results in on-demand oil/water separation, but somewhat disadvantages that complex operation,

28

energy consumption (e.g., thermal, electrical and light energy) and secondary pollution (acidic and

29

alkaline solutions, toxic solvents and ions etc.) still need to be addressed. Consequently, greater

including

superhydrophobic/superoleophilic,

3

superhydrophilic/underwater

1

efforts should be devoted to develop membrane with switchable wettability for on-demand

2

oil/water separation.

3

Metal-organic frameworks (MOFs), an emerging class of crystalline porous materials has

4

received extensive research in different fields owing to its porosity and high surface area[28].

5

Among them, zeolitic imidazolate framework-8 (ZIF-8) has been widely used in sensing, gas

6

separation and adsorption because of low-costs, easy preparation and comparable high

7

thermal/chemical stability[29]. Indeed, MOFs-based membranes have been very recently

8

developed and applied in oil/water separation. For instance, Zhang at el.[30] reported

9

hydrophilic/underwater superoleophobic UiO-66-coated mesh membrane via a simple immersion

10

method. The mesh membrane exhibited excellent separation capacity and highly permeation flux

11

for oil/water mixtures. Ma at el.[31] prepared underwater oleophobic ZIF-8 modified stainless

12

steel meshes by seeding method possessing outstanding stability and well separation ability for

13

different oil/water mixtures. Li at el.[32] fabricated Cu(OH)2@ZIF-8 nanowire membranes with

14

switchable wettability between superhydrophobicity and superoleophobicity underwater by dried

15

and ethanol modified for oil/water separation and adsorption heavy-metal ions. Cao at el.[33]

16

fabricated MOF membranes by assembling poly(acrylic acid) modified UiO-66-NH2

17

(UiO-66-NH2@PAA) on substrate via vacuum-assisted method. The high hydrophilic/underwater

18

superoleophobic MOF membrane could separate oil/water emulsion with excellent separation

19

capacity. Cai at el.[34] demonstrated superhydrophobic MOF membrane with self-repairing by

20

wet-chemistry coating strategy for oil/water emulsion separation. Admirably, these pioneering

21

works are creative and encouraging. However, almost all the above MOF-based membranes are

22

unsuitable for either emulsion separation or versatile oil/water separation. Thus, the development

23

of MOF-based membranes with switchable wettability toward controllable oil/water emulsion

24

separation is extremely essential but still challenging.

25

In this research, ZIF-8 was selected as research object because of its low-costs, easy

26

preparation and comparable high thermal/chemical stability. Regenerated cellulose membrane (RC)

27

was used as a substrate membrane due to its biodegradable, high mechanical strength and

28

large-scale availability. Polydopamine (PDA) has been proved to be a secondary versatile

29

platform for immobilizing inorganic nanomaterials[35-36], thus PDA was introduced to enhance

30

dispersion and stability of modified ZIF-8 nanoparticles on membrane surfaces. ZIF-8 modified 4

1

membrane (RC@PDA/ZIF-8) was prepared via coordination-driven in situ self-assembly strategy

2

for controllable oil/water emulsion separation. The RC@PDA/ZIF-8 membrane showed

3

underwater superoleophobicity or underoil (super)hydrophobicity when prewetted by water or oil.

4

Thus, as-prepared membrane was capable of separating various oil-in-water and water-in-oil

5

emulsions with high separation efficiency and high fluxes. The mechanisms of wettability

6

switching of membrane surface was also elaborated by Young’s equation and two principal

7

criteria of dual superlyophobic in oil-water systems. This work demonstrated a ZIF-8 modified

8

membrane with prewetted-induced switchable wettability was suitable for controllable oil/water

9

emulsion separation. Furthermore, our work expanded the application of MOFs-based membrane

10

in controllable oily wastewater treatment.

11

2. Materials and Methods

12

2.1 Materials

13

Regenerated cellulose membranes (pore size: 0.45 μm, thickness: 170 μm, diameter: 25 mm)

14

were obtained from Sartorius (Goettingen, Germany). Soybean oil was obtained from Kaiyuan

15

Supermarket. Tween 80, Span 80, Zinc nitrate hexahydrate (Zn(NO3)2·6H2O), dopamine

16

hydrochloride, 2-Methylimidazole, tris(hydroxymethyl) aminomethane (Tris) and petroleum ether

17

were received from Aladdin Reagent Co., Ltd. (Shanghai, China). Methanol, toluene, anhydrous

18

ethanol, n-hexane, hydrochloric acid and 1,2-Dichloroethane were purchased from Sinopharm

19

Chemical Reagent Co., Ltd. (Shanghai, China).

20

2.2 Fabrication of RC@PDA/ZIF-8 Membrane

21

Dopamine hydrochloride (200 mg) was first added in Tris-HCl buffer solution (100 mL, 10

22

mM, pH=8.5) to forming dopamine solution. Then, several RC membranes were immersed in the

23

above solution for 6 h at 25 oC, the PDA coated RC membranes (RC@PDA) were rinsed with

24

deionized water and dried at 40 oC for 12 h.

25

Several RC@PDA membranes were immersed in 20 mL Zn(NO3)2·6H2O methanol solution

26

(0.05, 0.1, 0.2 M) for about 30 min, then 20 mL 2-Methylimidazole methanol solution (0.1, 0.2,

27

0.4 M) was poured into the above solution at ambient temperature for 1 h with gentle stirring (The

28

molar ratio of Zn(NO3)2·6H2O to 2-Methylimidazole is fixed at 1:2). The ZIF-8 modified 5

1

RC@PDA membranes (RC@PDA/ZIF-8) were obtained after washing with methanol and natural

2

drying at ambient temperature. The prepared membranes were labeled as RC@PDA/ZIF-8(0.05),

3

RC@PDA/ZIF-8(0.1) and RC@PDA/ZIF-8(0.2), respectively. If not specified, RC@PDA/ZIF-8

4

represented RC@PDA/ZIF-8(0.1).

5

2.3 Preparation of Oil/Water Emulsions

6

In this work, five types of surfactant-stabilized water-in-oil and oil-in-water emulsions were

7

prepared for evaluating separation performance of membrane. For water-in-oil emulsion, 1 mL of

8

water was added in 99 mL oil containing 100 mg of Span 80 and stirred for 12 h to form stable

9

water-in-oil emulsions. Moreover, 1 mL of oil was added in 99 mL water containing 10 mg of

10

Tween 80 and stirred for 12 h to form stable oil-in-water emulsions. The oils included

11

1,2-dichloroethane, petroleum ether, toluene, soybean oil and n-hexane.

12

2.4 Oil/Water Emulsion Separation

13

The membrane was equipped into a vacuum-driven filtration system with effective filtration

14

area of 2.0 cm2 (radius of membrane cell is 0.8 cm, see Fig. S1) under negative pressure (the

15

pressure can be regulated by a three-way valve), and the emulsion was poured onto the prewetted

16

(by oil or water) membrane surface. The oil-in-water and water-in-oil emulsions separation

17

process was carried out at 0.01 MPa and 0.02 MPa, respectively. The separation efficiency can be

18

calculated by the following equation:

19

R = (1 -

Cf

) × 100％

C0

(1)

20

herein Cf and C0 are the oil (or water) concentration of the filtrate and the original emulsion,

21

respectively. The oil concentration in the filtrate and the original emulsion was detected by a

22

UV-Vis spectrophotometer. Water concentration in the filtrate and the original emulsion was

23

detected using the Karl Fischer titration method.

24

The flux of the membrane can be calculated by the following equation:

25

J=

V A × ∆t

(2)

26

where J (L m-2 h-1) is flux, V (L) is the volume of filtrate, A (m2) is valid area of membrane, ∆t (h)

27

is the separation time.

28

2.5 Characterizations 6

1

The morphologies of different membranes and cross-sectional structure were observed by

2

Scanning Electron Microscopy (SEM, JSM-7001F, JEOL, Japan) and Field Emission Scanning

3

Electron Microscope (FESEM, FEI 400FEG, America). The roughness of membranes was

4

measured by Atomic Force Microscopy (AFM, MFP-3D, America). X-ray diffraction was

5

obtained by an X-ray diffractometer (XRD, Bruker D8 Advance, Bruker AXS, Germany). The

6

chemical composition of membranes was analyzed using attenuated total reflection-Fourier

7

transform infrared spectroscopy (ATR-FTIR, Nicolet560, America) and X-ray photoelectron

8

spectroscopy (XPS, Axis Ultra DLD, Kratos, Kyoto, Japan). The contact angles measurement of

9

membranes was conducted by a contact angle meter (OSA60, LAUDA Scientific, Germany). The

10

mechanical strength of different membranes was tested by Tensile Strength Test Machine

11

(WJ-LL-200, HANG ZHOU WU JIA MACHINE CO., LTD, China). The droplet sizes of the

12

emulsions were measured by dynamic light scattering (DLS) using a particle size analyzer (90Plus

13

PALS, Brookhaven Instruments Corporation, America).

14

3. Results and Discussion

15

3.1 Preparation and Characterization of Membranes

16

Fig. 1a illustrated the schematic diagram on the preparation of RC@PDA/ZIF-8 membrane.

17

RC membrane was coated by PDA to provide a secondary versatile platform for immobilizing

18

ZIF-8 nanoparticles. The Zn2+ ions were first immobilized on the surface of the PDA coated RC

19

membrane via coordination with amino groups. After that, organic ligands of 2-Methylimidazole

20

were added in solution for coordination assembly of Zn2+ ions to generate ZIF-8 seeds. As time

21

goes by, 2-Methylimidazole and Zn2+ ions further coordinate to form ZIF-8 nanoparticles on

22

membrane surface in situ. Finally, ZIF-8 modified membrane (RC@PDA/ZIF-8) with micro/nano

23

structures was prepared via coordination-driven in situ self-assembly strategy. As shown in Fig.

24

1b, a large number of ZIF-8 nanocrystal seeds with an average diameter of ca. 50 nm are attached

25

to the RC@PDA membrane surface although only 5 min immersing. Additionally, the mapping

26

images (Fig. 1c) of the RC@PDA/ZIF-8 nanocrystal seeds membrane displays the uniform

27

distribution of C, O, N and Zn elements, which indicate that the seeds attached evenly on the

28

surface of the membrane is beneficial to the secondary growth.

7

1 2

Fig. 1 (a) Schematic diagram on the preparation of RC@PDA/ZIF-8 membrane, the SEM images

3

(b) and mapping images (c) of RC@PDA/ZIF-8 nanocrystal seeds membrane

4

The morphological evolution of membranes was observed using SEM. The original RC

5

membrane displays a laminated porous structure (Fig. 2a1). A high-magnification SEM image of

6

RC membrane shows a smooth surface (Fig. 2a2). The PDA coated RC membrane (RC@PDA)

7

also displays a porous structure without obvious changes (Fig. 2b1). A high-magnification SEM

8

image reveals that rough PDA coating covered on the surface of RC membrane (Fig. 2b2). From

9

Fig. 2c1, abundant ZIF-8 nanoparticles attach on the surface of RC@PDA membrane. The ZIF-8

10

displays the typical regular dodecahedron with an average diameter of ca. 130 nm (Fig. 2c2). The

11

nano-scale ZIF-8 and intrinsic porous structure constructed multi-scale structures, which is vital

12

for achieving superwettability. It is very important to control the size of ZIF-8 nanoparticles for

13

desired surface wettability. Thus, we regulate particle size by simply adjusting the concentration

14

of Zn(NO3)2 and 2-Methylimidazole. The SEM images (Fig. S2a) of RC@PDA/ZIF-8(0.05)

15

membrane reveal sparse ZIF-8 particles with an average diameter of ca. 500 nm are distributed on

16

the surface of the membrane. When the Zn(NO3)2 concentration is 0.2 M, dense ZIF-8 particles

17

with an average diameter of ca. 50 nm adhered on surface of the membrane (Fig. S2b). The results

18

indicate the micro- and nanostructures on the surface of membrane can be flexibly regulated by

19

adjusting precursor concentration. 8

1 2

Fig. 2 Surface and cross section SEM images of RC (a), RC@PDA (b) and RC@PDA/ZIF-8 (c)

3

membranes

4

The cross-sectional SEM images of RC, RC@PDA and RC@PDA/ZIF-8 membranes are

5

shown in Fig. 2. From Fig. 2a3-4, the original RC membrane displays a lamellar porous structure

6

with a smooth surface. Fig. 2b3-4 indicates the RC@PDA membrane has a lamellar porous

7

structure similar to RC membrane, but the internal surface becomes rough attributing to PDA

8

coating. From Fig. 2c3, it can be apparently observed that the ZIF-8 nanoparticles uniformly

9

deposit on the internal surface of the RC@PDA membrane after in situ self-assembly, proving a

10

holistic surface modification. A high-magnification cross-sectional SEM image indicates the

11

RC@PDA/ZIF-8 membrane possess well-defined micro/nano hierarchical structures (Fig. 2c4),

12

which is benefit for enhancing the stability of superwettability.

13

The roughness of the membranes was investigated by AFM, as presented in Fig. 3. The

14

detected results indicate the surface roughness parameter Ra values of RC, RC@PDA and

15

RC@PDA/ZIF-8 membranes are 106.1, 123.7 and 110.6 nm, respectively. After PDA coating, the

16

surface of the membrane (Fig. 3b1-2) becomes rougher relative to the pristine RC membrane (Fig. 9

1

3a1-2). When ZIF-8 nanoparticles were assembled on the surface of RC@PDA membrane, the

2

surface roughness decreases again (Fig. 3c1-2). As known, the Ra value is obtained by calculating

3

the highest and lowest points of membrane surface. Thus, the assembly of ZIF-8 may decrease the

4

height differences of membrane surface resulting a lower Ra value. From 2D profiles of given

5

locations, serrate structure is becoming more and more prominent originating from the assembly

6

of ZIF-8 nanoparticles (Fig. 3a3-c3), indicating a well micro/nano hierarchical structures.

7 8

Fig. 3 AMF images of (a) RC, (b) RC@PDA and (c) RC@PDA/ZIF-8 membranes

9

The phase structure of as-prepared ZIF-8 powder, RC@PDA and RC@PDA/ZIF-8

10

membranes was characterized using XRD. From Fig. 4, the typical diffraction peaks of

11

as-prepared ZIF-8 powder were matched well with the reported crystal structure data of ZIF-8[37].

12

The several main diffraction peaks of ZIF-8 crystals can be observed in the XRD pattern of

13

RC@PDA/ZIF-8 membrane, indicated the successful assembly of ZIF-8 on membrane surface.

10

1 2

Fig. 4 XRD pattern of ZIF-8 powder, RC@PDA and RC@PDA/ZIF-8 membranes

3

The surface functional groups of RC, RC@PDA and RC@PDA/ZIF-8 membranes were

4

investigated by ATR-FTIR. As shown in Fig. 5a, pristine RC membrane shows specific absorption

5

peaks at 3362, 2918 and 1062 cm−1, which belong to the stretching vibrations of -OH, C-H and

6

O-C-O in cellulose[38]. The spectrum of RC@PDA membrane exhibits similar absorption peaks

7

relative to the spectrum of RC membrane, which may be attributed to the coincidence of

8

absorption peaks of -NH2 and -OH. In the spectrum of RC@PDA/ZIF-8 membrane, several new

9

peaks at 421, 759 and 1147 cm−1 are attributed to the stretching vibration of Zn-N, Zn-O and C-N,

10

respectively[39-40]. The results indicate the ZIF-8 crystals have successfully assembled on

11

membrane surface. The chemical composition of RC, RC@PDA and RC@PDA/ZIF-8

12

membranes was further studied by XPS. Figure 5b presents the XPS survey spectra, the signals of

13

C 1s and O 1s can be seen in RC membrane, a new signal of N 1s emerges in RC@PDA

14

membrane compared to pristine RC membrane. After ZIF-8 assembly, a conspicuous new signal

15

of Zn 2p is observed in RC@PDA/ZIF-8 membrane. The detailed elements percentage of RC,

16

RC@PDA and RC@PDA/ZIF-8 membranes was recorded in Table S1. The core-level XPS

17

spectrum of N 1s for RC@PDA membrane (Fig. 5c) displays three fitting peaks at 398.9, 399.7

18

eV and 400.5 eV assigning to C-N, C-NH-C and -NH2, respectively[41], which prove the PDA has

19

successfully deposited on the surface of RC membrane. Additionally, the core-level XPS spectrum

20

of Zn 2p for RC@PDA/ZIF-8 membrane (Fig. 5d) reveals two peaks at 1021.48, and 1044.58 eV

21

attributing to the Zn 2p3/2 and Zn 2p1/2, respectively, which further confirm the successful 11

1

assembly of ZIF-8 on membrane surface.

2 3

Fig. 5 (a) ATR-FTIR spectra of RC, RC@PDA and RC@PDA/ZIF-8 membranes, (b) XPS survey

4

spectra of RC, RC@PDA and RC@PDA/ZIF-8 membranes, (c) N 1s and (d) Zn 2p core-level

5

signals of RC@PDA/ZIF-8 membrane

6

3.2 Wettability of Membranes

7

The water contact angle (WCA) and oil contact angle (OCA) were measured to determine

8

surface wettability of membranes. As shown in Fig. 6a, the RC and RC@PDA membranes show a

9

WCA of 0°, which is mainly attributed to the good hydrophilicity of cellulose and PDA coating.

10

Interestingly, all RC@PDA/ZIF-8 membranes display a WCA of 0°, which is different from

11

previous reports due to ZIF-8 is usually hydrophobic[42]. In order to find out the reasons, the

12

WCA of vacuum-dried ZIF-8 powder (at 80 °C) and naturally dried ZIF-8 powder (at ambient

13

temperature) were measured. From Fig. S3, the vacuum-dried ZIF-8 powder shows high

14

hydrophobicity with WCA of 147±2o, while the naturally dried ZIF-8 powder displays moderate

15

wettability with WCA of 59.2±0.7o. The results indicate ZIF-8 powder under different drying

16

conditions have different surface energy. We speculate that a large number of methanol molecules

17

remaining in the micropores of ZIF-8 reduce the surface energy of ZIF-8, the similar result was

18

demonstrated by Li and co-workers[32]. Furthermore, Tian et al.[43] proposed two design criteria

19

of underoil superhydrophobic and underwater superoleophobic surface: re-entrant structures and 12

1

an appropriate surface chemistry (moderate WCA on a flat surface, theoretically between 56° and

2

74°) Thus, dual superlyophobic surfaces can be constructed by combining the micro- and

3

nanostructures and appropriate surface energy of ZIF-8 (natural drying at ambient temperature).

4

The underwater OCAs (Take dichloroethane as an example) of all membranes are more than 150°,

5

and the underwater OCA gradually increases from 150.5° to 155.4° after PDA modification and

6

assembly of ZIF-8 attributing to established multi-scale structures. However, when the

7

concentration of precursor solution further increases, the underwater OCA slightly decreases to

8

152.5°, which may attribute to smaller ZIF-8 size reducing the surface roughness. The above

9

observations

indicate

RC@PDA/ZIF-8

membranes

show

superhydrophilicity/underwater

10

superoleophobicity. Moreover, Fig. 6b shows the underwater OCAs of RC@PDA/ZIF-8

11

membrane for different oils. It can be observed the RC@PDA/ZIF-8 membranes exhibit

12

underwater OCAs all above 150° demonstrating versatile underwater superoleophobic for

13

different oils. As shown in Fig 6c, all membranes display an OCA of 0°. Under oil (Take

14

petroleum ether as an example), water droplet can penetrate the surface of RC and RC@PDA

15

membranes owing to their flat surface and hydrophilicity. However, the underoil WCAs of

16

RC@PDA/ZIF-8(0.05), RC@PDA/ZIF-8(0.1) and RC@PDA/ZIF-8(0.2) membranes are 134.2°,

17

146.1° and 138.1°, respectively. The well-defined micro/nano hierarchical structures and

18

appropriate surface energy of ZIF-8 can trap the oil generating stable oil film, in which the oil film

19

keeps water from contacting membrane surface. Consequently, the water droplets preserve sphere

20

or ellipsoid to minimize surface tension underoil featuring high underoil WCA. Additionally, the

21

WCAs of the RC@PDA/ZIF-8 membrane under different oils were measured and shown in Fig 6d.

22

It can be observed the RC@PDA/ZIF-8 membrane shows high WCAs of all above 142.3° under

23

different oils indicating high underoil hydrophobicity.

13

1 2

Fig. 6 (a) Underwater OCA and WCA for different membranes, (b) underwater OCA of

3

RC@PDA/ZIF-8 membrane for different oils, (c) underoil WCA and OCA of the membranes, (d)

4

underoil (different oils) WCA of the RC@PDA/ZIF-8 membrane

5

3.3 Oil/Water Emulsion Separation Performance

6

The separation efficiency and flux of different membranes were first evaluated by separating

7

petroleum ether-in-water emulsion. From Fig. S4, the RC@PDA/ZIF-8(0.1) membrane shows

8

superior separation efficiency and good flux. Thus, considering the optimal wettability and

9

superior

separation

performance

of

membrane

RC@PDA/ZIF-8(0.1)

membrane,

the

10

RC@PDA/ZIF-8(0.1) membrane was selected for systematically controllable oil/water emulsion

11

separation. Digital photographs and optical microscopic photographs of feed emulsions and

12

filtrates are presented in Fig. 7. As we can see, the feed emulsions are milky, but all filtrates

13

become transparent. The optical microscopic photographs of feed emulsions reveal numerous oil

14

droplets are dispersed in water, but no oil droplets are observed in filtrates. Moreover, water-in-oil

15

emulsions separation also exhibits similar results. The droplet size of emulsions was analyzed by

16

dynamic light scattering measurement (Fig. S5). It is obvious that the average droplet sizes of all

17

emulsions are below 10 μm. Moreover, the droplet sizes of water-in-oil emulsions are smaller than

18

that of oil-in-water emulsions. As shown in Fig. S6, the feed emulsion is turbid with apparent

19

colloidal property, while the filtrate is totally transparent without Tyndall effect. We concluded 14

1

that the RC@PDA/ZIF-8 membrane possessed excellent separation effect for emulsions.

2 3

Fig. 7 Digital and optical microscopic photographs of emulsions before and after separation

4

The separation efficiency and flux of RC@PDA/ZIF-8 membrane was further investigated by

5

separating various types oil-in-water and water-in-oil emulsions. From Fig. 8a, the efficiencies of

6

RC@PDA/ZIF-8 membrane for all oil-in-water emulsions are as high as above 99%, and the

7

fluxes are 133.1–446.4 L m-2 h-1. From Fig. 8b, the efficiencies of RC@PDA/ZIF-8 membrane for

8

all water-in-oil emulsions are as high as above 99.5%. And the fluxes are 111.6 L m-2 h-1 for

9

water-in-petroleum ether, 210.6 L m-2 h-1 water-in-toluene, 108.7 L m-2 h-1 water-in-hexane, 20.4

10

L m-2 h-1 water-in-soybean oil, and 124.5 L m-2 h-1 water-in-dichloroethane, respectively. As

11

known, majority of the existing materials are only suitable for the controllable separation of

12

oil/water mixtures due to their intrinsic properties. Although some works have been devoted to

13

controllable separating emulsions, certain disadvantages that complex operation, energy

14

consumption and secondary pollution still need to be addressed. The various membrane materials

15

for switchable emulsion separation are compared in Table 1. In this work, the surface wettability

16

can be switched by water or oils prewetted without additional external stimuli. Compared to the

17

existing stimuli-responsive materials, the RC@PDA/ZIF-8 membrane is easy to prepare and more

18

easily operated on the premise of comparable performance showing prospective application in

19

controllable oil/water emulsion separation. 15

1

Table 1 Comparison of various membrane materials for switchable emulsion separation Materials PNIPAAm-coated membrane switchable

PAAO

membrane Thermo-responsive Membrane Janus

Separation

Flux

Pressure

conditions

efficiency (%)

m-2

(bar)

Temperature

>99

500-1500

1

pH

>99.98

1000-1600

1

(L

h-1)

Methods Hydrothermal polymerization Surface modification

Ref.s [21] [44]

Electrospinning and Temperature

>98

60-120

0.1-0.3

radical

[45]

polymerization ceramic

membrane Janus F-TiO2@PPS Membrane Aliphatic Polyketone Membrane CC-coated membrane RC@PDA/ZIF-8 membrane

Control

Switching

Surface modification

>95

898-903

1-2

>98

640-950

0.9

/

>99.95

60-120

0.09

/

> 98.8

50-110

0.85

Leaching

/

>99

133-446

0.1-0.2

Self-assembly

surface Switching surface

and O2/N2 plasma Interfacial grafting Phase separation process

[46] [47] [48] [49] This work

2 3

Cyclic separation tests were carried out to evaluate the long-term separation performance of

4

the RC@PDA/ZIF-8 membrane, as illustrate in Fig. 8c, d. The efficiencies of RC@PDA/ZIF-8

5

membrane are more than 98.5% for regardless of oil-in-water or water-in-oil emulsions after 10

6

cycles separation, and the fluxes are slightly decreased after 10 cycles separation. Importantly, we

7

found that the ZIF-8 crystals were firmly attached onto RC@PDA membrane surface even 10

8

cycles separation, as proved by SEM images (Fig. 9a1-a2). These cyclic separation results

9

demonstrated that the RC@PDA/ZIF-8 membrane has excellent reusable performance and can be

10

applied in efficient oil/water emulsion separation. Additionally, to evaluate the stability of ZIF-8

11

nanoparticles in water, the morphology and crystal structure of ZIF-8 nanoparticles before and

12

after immersing in water for 24 h was studied by SEM and XRD. As shown in Fig. 9b, ZIF-8

13

nanoparticles are typical polyhedral structure. Importantly, the morphology of ZIF-8 nanoparticles

14

has no obvious change even immersing in water for 24 h (Fig. 9c). And the XRD patterns (Fig. 9d)

15

demonstrate the crystal phase of ZIF-8 nanoparticles has not changed after immersion. These

16

results indicate the good stability of ZIF-8 nanoparticles.

16

1 2

Fig. 8 Separation efficiency and fluxes of RC@PDA/ZIF-8 membrane for various oil-in-water (a)

3

and water-in-oil (b) emulsions, cyclic separation performance of RC@PDA/ZIF-8 membrane for

4

petroleum ether-in-water (c) and water-in-petroleum ether (d) emulsion

5 6

Fig. 9 SEM images of RC@PDA/ZIF-8 membrane after 10 cycles separation under different

7

magnification (a), SEM images of ZIF-8 particles before (b) and after (c) immersing in water for

8

24 h, XRD pattern of ZIF-8 nanoparticles before and after immersion (d)

9 10

3.4. Mechanical Properties The excellent mechanical strength of membranes is crucial for its real application. The 17

1

mechanical strength of RC, RC@PDA and RC@PDA/ZIF-8 membranes was investigated, and the

2

stress-strain curves was shown in Fig. 10. The tensile strength and elongation of primitive RC

3

membrane are 10.4 MPa and 9%, respectively. The mechanical strength was increased after PDA

4

coating from 10.4 MPa to 12.8 MPa for tensile strength and 9% to 10.7% for elongation. The

5

tensile strength of RC@PDA/ZIF-8 membrane was decreased from 12.8 MPa to 8.8 MPa with

6

similar elongation (10.3%). The results show that the RC@PDA/ZIF-8 membrane has good

7

mechanical properties and can meet the application requirements.

8 9 10

Fig. 10 The stress-strain curves of RC, RC@PDA and RC@PDA/ZIF-8 membranes 3.4 Controllable Emulsion Separation Mechanisms

11

Young’s equation was also applicable to liquid droplet on solid surface in a second liquid[50],

12

as illustrated in Fig. 11a. Thus, the wettability of RC@PDA/ZIF-8 membrane surface can be

13

clarified via Young’s equation:[51]

14

cosθ3 =

γl1 - gcosθ1 - γl2 - gcosθ2 γl1 - l2

(3)

15

herein, θ1, θ2 andθ3 represent CA of l1 in air, l2 in air and liquid1 (l1) in liquid2 (l2), respectively.

16

γl1-g, γl2-g and γl1-l2 are interface tension of l1/gas, l2/gas and l1/l2, respectively.

17

The Eq. 3 can be used to explain why RC@PDA/ZIF-8 membrane becomes superoleophobic

18

underwater. The measurement results of CA indicate RC@PDA/ZIF-8 membrane is

19

superamphiphilic. That is to say, the θ1=θ2=0, thus cosθ1=cosθ2=1. When γl1-g < γl2-g, the Eq. 3 can

18

γl1 - gcosθ1 - γl2 - gcosθ2

1

be written as cosθ3 =

2

90°, proving the RC@PDA/ZIF-8 membrane can behave as an oleophobic surface underwater

3

(because γo-g < γw-g). Furthermore, Tian et al.[43] demonstrated intermediate surface chemistries

4

that satisfy re-entrant structures and an appropriate surface chemistry (moderate WCA on a flat

5

surface, theoretically between 56° and 74°) can guarantee Cassie-type wetting states for both

6

water-in-oil and oil-in-water cases, thus inducing both underwater superoleophobicity and

7

underoil superhydrophobicity. Here, the first criterion can ensure the formation of water or oil

8

layer in micro/nano hierarchical structures, and the second criterion guarantees the liquid layer not

9

to be displaced by the other suspended liquid. Thus, dual superlyophobic surfaces can be

10

constructed by combining the micro- and nanostructures and appropriate surface energy of ZIF-8

11

(natural drying at ambient temperature) (For a detailed discussion, see 3.2 section). When

12

prewetted by oils, the surface of RC@PDA/ZIF-8 membrane can form a stable repulsive layer

13

attributing appropriate surface chemistry of ZIF-8 and micro/nano hierarchical structures, which

14

prevents water from contacting membrane surface. Thus, the RC@PDA/ZIF-8 membrane can

15

repel water under oils (Fig. 11b).

γl1 - l2

<0 (γl1-l2>0), hence the values of θ3 must be greater than

16 17

Fig. 11 Schematic diagram of the controllable oil/water emulsion separation mechanism

18

As depicted in Fig. 11c, RC@PDA/ZIF-8 membrane is superoleophobic underwater when

19

prewetted by water due to the formation of stable water barrier layer. While the oil-in-water

20

emulsion was poured on the membrane surface, continuous phase (water) can penetrate through 19

1

the membrane, but dispersed phase (oil) is blocked on the surface of the membrane owing to the

2

difference of surface tension. And the intercepted dispersed phase tends to coalesce together to

3

achieve the separation of oil-water emulsion. Fig. 11d shows schematic diagram of the

4

water-in-oil emulsion separation by RC@PDA/ZIF-8 membrane. When prewetted by oil, the

5

stable oil barrier layer is generated to block water droplets, and oil phase can penetrate through the

6

membrane. In this work, the surface wettability can be switched by liquids prewetted without

7

additional external stimuli showing promising application in controllable oil/water emulsion

8

separation.

9

4. Conclusions

10

In

summary,

we

prepared

ZIF-8

modified

membrane

(RC@PDA/ZIF-8)

via

11

coordination-driven in situ self-assembly ZIF-8 on PDA coated RC membrane. By the

12

combination of micro/nano hierarchical structures and prewetted-induced wettability, the

13

membrane is capable of separating both stabilized oil-in-water and water-in-oil emulsions. More

14

importantly, the membrane also exhibits separation efficiency as high as above 99%, good flux

15

and recyclability. The merits of easy preparation and simple operation make the ZIF-8 modified

16

membrane a promising candidate separation material for controllable oil/water emulsion

17

separation without any additional external stimuli. Moreover, this work also expanded the

18

application range of MOFs-based membrane in on-demand oil/water emulsion separation.

19 20

Acknowledgements

21

The authors are grateful for financial support from the National Natural Science Foundation

22

of China (21776110 and 51608226), Natural Science Foundation of Jiangsu Province

23

(BK20170532, BK20181230, BK20181229 and BK20180192).

24 25

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14

24

Graphical abstract

1 2

ZIF-8 modified membrane (RC@PDA/ZIF-8) is prepared via coordination-driven in situ

3

self-assembly ZIF-8 on polydopamine (PDA) coated regenerated cellulose membrane (RC). The

4

surface wettability of resultant membrane can be switched between underwater superoleophobicity

5

and underoil (super)hydrophobicity by water or oils prewetted without additional external stimuli,

6

which is suitable for controllable oil/water emulsion separation. Our work demonstrates the

7

MOFs-based membrane has potential applications such as on-demand oil/water separation and

8

fuel purification.

9 10

25