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Photoinduced superwetting membranes for separation of oil-in-water emulsions
Journal Pre-proofs Photoinduced superwetting membranes for Separation of Oil-in-Water Emulsions Reng-Yu Yue, Jing Guan, Chun-Miao Zhang, Peng-Cheng Yuan, Lin-Na Liu, Muhammad Zaheer Afzal, Shu-Guang Wang, Xue-Fei Sun PII: DOI: Reference:
S1383-5866(19)35374-2 https://doi.org/10.1016/j.seppur.2020.116536 SEPPUR 116536
To appear in:
Separation and Purification Technology
Received Date: Revised Date: Accepted Date:
28 November 2019 7 January 2020 7 January 2020
Please cite this article as: R-Y. Yue, J. Guan, C-M. Zhang, P-C. Yuan, L-N. Liu, M. Zaheer Afzal, S-G. Wang, X-F. Sun, Photoinduced superwetting membranes for Separation of Oil-in-Water Emulsions, Separation and Purification Technology (2020), doi: https://doi.org/10.1016/j.seppur.2020.116536
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© 2020 Published by Elsevier B.V.
Photoinduced superwetting membranes for Separation of Oil-in-Water Emulsions
Reng-Yu Yue, Jing Guan, Chun-Miao Zhang, Peng-Cheng Yuan, Lin-Na Liu, Muhammad Zaheer Afzal, Shu-Guang Wang, Xue-Fei Sun*
Shandong Key Laboratory of Water Pollution Control and Resource Reuse, School of Environmental Science and Engineering, Shandong University, Qingdao 266237, China
*Corresponding author: Fax: +86-532-58630936; E-mail: [email protected] (Xuefei Sun)
1
Abstract: The separation of oil/water emulsion has become a pressing worldwide
2
problem. In this study, a novel type of photoinduced separation membrane was prepared
3
via layer-by-layer (LBL) method of ZIF-8/GO composite. By integrating the unique
4
physical and chemical properities of ZIF-8 and GO, the ZIF-8/GO membrane exhibits
5
superior superoleophobicity under visible light and underwater anti-fouling for efficient
6
oil rejection from water. We found that the ZIF-8/GO membrane permeation flux was
7
110 ± 6 L m-2 h-1 under light irradiation, which was higher than the permeation flux of
8
membranes without irradiation (50 ± 6 L m-2 h-1). For toluene, the removal rate was
9
about 100 % at the first time, with new benzyl alcohol and benzaldehyde products
10
detected in the filtrate, indicating that toluene was degraded on the ZIF-8/GO
11
membrane surface. Continuous separation performed using filtration apparatus
12
demonstrates a high separation capacity, with long-term membrane stability. The high
13
water flux, high separation capacity and capacity for large-scale preparation of the ZIF-
14
8/GO membrane, shows great potential for industial application in the treatment of oil-
15
containing wastewater.
16
Keywords: Ultrafiltration, ZIF-8/GO composite, oil/water separation, photoinduced,
17
visible light.
18
1. Introduction
19
Oil-water separation has aroused widespread attention of the last few decades
20
owing to water pollution caused by industrial sewage and crude oil leakage, which
21
threaten aqueous habitats and people's health [1]. Separating oil from oily water is
22
becoming more and more urgent. The oil phase in wastewater can exist in three shapes
23
and is classified according to the size of the oil droplets as free oil (>150 μm), dispersed
24
oil (20–150 μm) and emulsified oil (<20 μm) [2]. Among them, the emulsified droplets
25
are generally highly stable because of the existence of a strong oil/ water interface film
26
and the adsorption of active components on the interface. [3]. Therefore, traditional
27
treatment methods, like gravity separation and skimming techniques are unable to
28
achieve desired effect. Membrane technologies are promising for the separation of oil
29
droplets smaller than ∼10 μm, because they are simple, compact, modular, relatively
30
inexpensive and highly efficient [4, 5]. However, traditional filtration membranes can
31
suffer from rapid membrane fouling during membrane separation processes, which was
32
caused by the cumulative deposition of original components (such as oil) on the
33
membrane surface and within membrane internal pores [6, 7]. Therefore, it is critical to
34
develop a novel oil/water separation membrane with excellent separation performance,
35
anti-oil-fouling and the ability to be recycled.
36
Many efforts have been dedicated to seek approaches to effectively weaken
37
membrane fouling. A large number of techniques have been applied to enhance the
38
hydrophilicity of membranes by either mixing common membrane polymers with
39
hydrophilic additives or modifying the membrane surface through physicochemical
40
post-modification. However, the existence of chain flexibility of the functionalized
41
polymer allows oil droplets to pass through the hydrophilic barrier, causing the
42
oil/water separation efficiency is still weakened during long-term operation. Recently,
43
it has been reported that the change of surface wettability of TiO2 induced by UV light
44
can realize the light driven oil-water separation [8, 9]. Optically driven control of liquid
45
motion on the TiO2 is highly promising since it can eliminate the need of direct
46
electrical contact with liquid or complex electronic circuit [10]. Nevertheless, an
47
insensitivity to respond to visible light spectrum of natural sunlight, slow kinetics and
48
a requirement for special environments to recover the original wetting state restrict the
49
practical applications of TiO2 surfaces. By contrast, visible light is non-invasive and it
50
is highly essential to construct responsive surfaces that are controllable by visible light
51
[11]. Therefore, advanced researches should be focused on developing powerful
52
antifouling membranes to achieve highly efficient and stable oil-water separation.
53
Here, we developed a novel photoresponsive and photocatalytic membrane made
54
from Zeolitic imidazolate frameworks/Graphene (ZIF-8/GO) nanocomposites for in
55
situ simultaneous emulsion separation and water purification under visible light. ZIFs
56
consist of transition metal ions and imidazolate/imidazolate type linkers which cover a
57
much wide range of pore sizes (0.2-1.5 nm) [12]. Therefore, water molecules (about
58
0.3-0.4 nm) can traverse the ZIF-8 micropore system (0.34 nm), while the emulsion
59
(100~500 nm) cannot [13]. However, the ZIF-8 structure is not stable in water under
60
ambient conditions [14]. It has been reported that by wrapping ZIF-8 with a layer of
61
GO, electronic conductivity and stability can be significantly improved [15, 16]. The
62
GO nanosheets first adsorbs water molecules and forms a hydration layer on the
63
membrane surface, which increases the space hindrance and can effectively prevent the
64
affinity between oil droplets and the membrane surface [6]. In addition, the structural
65
features of tunable active sites within ZIF lead to more powerful solar harnessing.
66
While graphene can efficiently weaken the recombination rate of photo-generated
67
electron-hole pairs when coupled with semiconductors, it could increase the
68
membranes photocatalytic performance. In this study, the oil degradation and
69
mineralization performance of the ZIF-8/GO nanocomposite membrane filtration
70
system were explored. We tested the antifouling properties and the operation stability
71
of this system, which further verify the practicability of this integrated membrane
72
catalysis process. Moreover, the toluene degradation pathway in the photocatalytic
73
membrane was explored by testing the reaction byproducts.
74
2. Materials and methods
75
2.1. Fabrication of ZIF-8/GO membrane
76
The ZIF-8/GO membrane was synthesized via the Layer-by-layer assembly of
77
ZIF-8/GO and TMC on the poly dopamine (PDA)-support substrate. First, a porous
78
PVDF membrane was used as an original support and dip-coated in a dopamine solution,
79
which is composed of 2 g L-1 dopamine and 10 mM Tris buffer solution (pH= 8.5).
80
Then the PDA-support was placed in an oven at 65 °C followed by immersed in 0.15%
81
TMC solution dissolved by Isopar G for 15 min. Using Isopar G to rinse the support to
82
remove excess TMC and then immersed in 1.5 g L-1 ZIF-8/GO solution for 30 min to
83
allow attachment of nanocomposite layers of ZIF-8/GO. Finally, the ZIF-8/GO
84
membrane was dried at 50 °C under vacuum.
85
2.2. Membrane Characterization
86
For SEM image (S-4800, Hitachi Limited Inc., Japan), samples were sputtered
87
with gold for observation. The membrane surface roughness was characterized in
88
tapping mode by AFM (Dimension Icon, Veeco Instruments Inc., US). The static
89
contact angle representing the hydrophilicity of membrane surface was determined by
90
a contact angle analyzer.
91
2.3. Membrane Performance Evaluation
92
Water permeation and separation efficiency of the ZIF-8/GO membrane were
93
tested by using a dead-end filtration system at 25 ±0.50C. Prior to the flux test, the
94
membrane was placed in the filtration tank and stabilized overnight at a transmembrane
95
pressure of 20psi (0.14 MPa), using a blank PVDF membrane as a control.
96
To evaluate the ZIF-8/GO membrane rejection performance and its reusability, the
97
ZIF-8/GO membrane was tested with toluene/water emulsions, with each cycle
98
followed by filtering and cleaning of the membrane with ethanol under sonication. The
99
photocatalytic activities of ZIF-8/GO membranes were evaluated by the
100
photodegradation of toluene under 500W Xe lamp irradiation in ambient air and
101
temperature conditions. During the photocatalytic reaction, solutions were continually
102
stirred to maintain homogeneous suspensions. Then collecting the filtrate, and the oil
103
constituent within the filtrate and the photodegradation products were determined
104
byGas chromatography-mass spectrometry (GC-MS, Agilent 7010B, Agilent
105
Technologies Inc., US).
106
3. Results and discussion
107
3.1. Membrane Characterization
108
LBL deposition of PDA, TMC and ZIF-8/GO constructs a thin, nanostructured
109
and well-porous membrane surface. These membranes were characterized by SEM and
110
AFM were shown in Fig. 1. The PVDF membrane has typical spongy-like pores formed
111
by phase inversion with a roughness of 50.6 nm (Fig. 1a, c). After deposition of the
112
ZIF-8/GO layer, the membrane demonstrates a corrugated and curled layered structure,
113
with pores becoming less visible while the surface maintains particle aggregation with
114
thin layer covering. The surface roughness decreased to 26.0 nm (Fig. 1b, d). Smooth
115
membranes tend to form a hydration layer on the surface of the membrane, which
116
increases the space hindrance and can effectively prevent the adhesion between oil
117
droplets and membrane [17]. Water contact angle (CA) measurement confirms that the
118
original membrane is hydrophobic, with a water CA of nearly 70° (Fig. 1e). While after
119
modification, the CA declines steadily from ~600 to ~250 as the number of layers
120
increases. The decline in water contact can help improve the ZIF-8/GO membrane anti-
121
pollution properties, thereby increasing the number of reuse cycles of the ZIF-8/GO
122
membrane [3].
123
Water fluxes of the modified membranes significantly decreased from 572 ± 26 L
124
m-2 h-1 for control membranes, to 53 ± 5 L m-2 h-1 for PDA membranes and 68 ± 4 L
125
m-2 h-1 for PDA/TMC membranes (Fig. 1f). With the addition of cross-linked ZIF-8/GO
126
nanocomposites on top of the PVDF support, a water flux of 188 ± 8 L m-2 h-1 was
127
measured, which was greater than that of the PDA and PDA/TMC membranes. The
128
increased water flux was probably caused by the presence of hydrophilic functional-
129
groups in GO nanosheets, according with the contact angle results (Fig. 1e).
130
3.2. Separation Performance of oil-in-water emulsions
131
Fig. 2a shows the results regarding toluene separation from water emulsions. Under
132
light irradiation, the permeate flux presented higher values of about 130 L m-2 h-1 and
133
stabilized at 95 L m-2 h-1, while without light the flux stabilized at 65 L m-2 h-1. The
134
achievement of complete toluene removal (100%) illustrates the high separation
135
efficiency of ZIF-8/GO membranes with irradiation (Fig. 2b, Fig. S4). The high
136
removal percentage was mainly due to toluene being excluded and photocatalyzed by
137
the ZIF-8/GO membrane. DLS measurements established the precise difference in
138
droplet size pre- and post-separation (Fig. 2c). The feed liquid contained a wide droplet
139
size distribution in the range of 8 to 250 nm, while the maximum size of the ZIF-8/GO
140
membrane filtrate was less than 40 nm, indicating the ZIF-8/GO membrane can
141
effectively separate emulsions containing droplet sizes greater than 20 nm. Following
142
the irradiation process, the size of the droplet is similar to that of the filtrate without
143
irradiation. The higher rate of oil rejection can be attributed to nanopores on the GO
144
nanosheets the ZIF-8 layer.
145
3.3. Photoinduced wettability transition
146
In order to explore the photo-induced wettability change of the ZIF-8/GO
147
membrane, the water CA was measured under Xe lamp irradiation. As shown in Fig.
148
3a, the water CA changes in air with irradiation time. The ZIF-8/GO exhibited a water
149
CA of 450, which reduces rapidly upon irradiation. After 1.5 h, a CA of nearly 250 was
150
obtained, with the CA remaining stable from this point. Similar CA saturation has also
151
been observed in traditional electrowetting-on-dielectric applications [18]. The
152
movement of droplets can be controlled by adjusting the surface wettability that was
153
induced by light [19, 20].
154
It is generally recognized that photo-generated electrons and holes are able to
155
change the surface chemistry of membranes, which is conducive to contact with
156
liquid diffusion, whether through photocatalytic oxidation of organic species
157
adsorbed on the surface, or the increase of hydroxyl species owing to dissociated water
158
adsorption [21, 22]. Light-induced hydrophilicity of the ZIF-8/GO membrane is caused
159
by light-induced hydroxyl groups on the surface of graphene oxide and ZIF-8 captured
160
from water molecules, which was oxygenated into the •OH active species [23-25]. As
161
shown in Fig. 3b, the hydrophilicity of the ZIF-8/GO membrane is switchable. When
162
the membrane is placed in the dark after irradiation, its water CA recovers to 450,
163
becoming more hydrophobic than before. This cyclic process could be repeated many
164
times. When an oil droplet with same volume is placed on the membrane surface, its
165
underwater oil CA decreases slightly from 1510to ~900, while it remained unchanged
166
under irradiation, indicating the ZIF-8/GO membrane becomes oleophobic and
167
hydrophilic with irradiation (Fig. 3c).
168
3.4. Self-cleaning ability of the ZIF-8/GO membrane
169
In the application of oil-water separation, the traditional filter membrane has the
170
problem of pollution due to sticking oil, which seriously restricts the reuse and long-
171
term use of the membrane. [26, 27]. In order to understand the photocatalysis membrane
172
properties, a Xe lamp was used to irradiate the ZIF-8/GO membrane during the
173
filtration process, with GC-MS used to analyze the photocatalysis products. Fig. S5
174
shows the results of filtration in the presence and absence of irradiation, showing that
175
no other product was present except for toluene in the filtrate without irradiation.
176
Conversely, under illumination two new products were formed and detected in the
177
filtrate, benzyl alcohol and benzaldehyde (Fig. 4a and S5). This indicates that toluene
178
was degraded on the ZIF-8/GO membrane surface, with benzyl alcohol and
179
benzaldehyde concentrations of 1.176 and 1.249 ppm, respectively. In the presence of
180
light irradiation, one electron was captured from water molecules, which was
181
oxygenated into the •OH active species. Then, the •OH could decompose Toluene
182
efficiently to complete the photocatalytic process [28].
183
To further verify the impact of irradiation on the ZIF-8/GO membrane, following
184
a 30-minute filtration process without irradiation, the ZIF-8/GO membrane was
185
subsequently irradiated for 30 minutes (Fig. 4b). During the irradiation process,
186
permeation of the oil/water emulsion increased initially before declining steadily, after
187
which the level of permeation maintained a constant level. The final ZIF-8/GO
188
membrane permeation outcome following this process, was lower than under continual
189
irradiation conditions. This may be due to toluene fouling of the membrane during the
190
30-minute-long process, with toluene adhering to the membrane surface, causing
191
irreversible damage to the membrane. In the Fig.4(c) and (d), we use Benzyl alcohol
192
and Benz aldehyde as feed respectively to explore the effect of irradiation. We use the
193
Xe lamp to illuminate the ZIF-8/GO membrane surface after 30 mins, the permeate flux
194
of Benzyl alcohol and Benz aldehyde are not obviously fluctuant, they are continuous
195
and keep falling (Fig.4c and d).
196
To evaluate recyclability of the composite membrane, nine cycles of repeated use
197
experiments were performed. In each cycle, the ZIF-8/GO membrane was washed using
198
ethanol after filtration of the emulsion, then dried at 65 °C. Throughout the nine
199
repetitions, the membrane continued to exhibit a stable performance (Fig. 4e and f).
200
Fig. 4e shows the change in emulsion removal rate, in the presence or absence of light
201
irradiation. At the first cycle, the toluene removal efficiency reached 100 %, with a high
202
flux of >170 L·m-2·h-1 with irradiation. By contrast, it shows a relatively low permeation
203
flux of 70 L·m-2·h-1 when used for separation without irradiation. The separation
204
efficiency of the ZIF-8/GO membrane was ~93% and the permeation flux remained
205
stable after nine cycles of reuse. In the membrane recycling process in the absence of
206
light irradiation, the removal rate was higher after each cycle, as the water solubility of
207
benzyl alcohol and benzaldehyde make it easier to remove toluene from the membrane
208
surface after each cycle. Therefore, deposition of the ZIF-8/GO composite not only
209
strengthens the anti-fouling capability of membranes, but also enhances membrane
210
recyclability under light irradiation.
211
In order to test the applicability of the ZIF-8/GO membrane, the separation of five
212
types of oil emulsions were compared (Fig. 5). The ZIF-8/GO membrane exhibiting
213
excellent separation performance and removal efficiency, with a large handling
214
capacity and long-time operation stability. These results indicate that the composite
215
photocatalytic ZIF-8/GO membrane is highly promising for the separation and
216
treatment of oil/water emulsions.
217
4. Conclusion
218
We have fabricated a high-performance ZIF-8/GO ultrafiltration membranes for the
219
separation and degradation of oil from oil-water suspensions. With light irradiation, the
220
flux rate was 1.5-times of magnitude higher than membranes without light irradiation,
221
allowing an ultra-high separation efficiency of 99.99% to be achieved. Designing such
222
an ZIF-8/GO membrane provides an effective, economical and favorable tool in various
223
practical fields such as water treatment, fuel purification and the removal of commercial
224
emulsions.
225
Acknowledgements
226
We thank the National Natural Science Foundation of China (21576157) for the support
227
to this study.
228
References
229
[1] J. Gu, H. Fan, C. Li, J. Caro, H. Meng, Robust Superhydrophobic/Superoleophilic
230
Wrinkled Microspherical MOF@rGO Composites for Efficient Oil-Water
231
Separation, Angew. Chem. 58 (2019) 1-6.
232
[2] H.J. Tanudjaja, C.A. Hejase, V.V. Tarabara, A.G. Fane, J.W. Chew, Membrane-
233
based separation for oily wastewater: A practical perspective, Water. Res. 156
234
(2019) 347-365.
235
[3] C.L. Chen, D. Weng, A. Mahmood, S. Chen, J.D. Wang, Separation Mechanism
236
and Construction of Surfaces with Special Wettability for Oil/Water Separation,
237
ACS. Appl. Mater. Inter. 11 (2019) 11006-11027.
238
[4] M. Padaki, R. Surya Murali, M.S. Abdullah, N. Misdan, A. Moslehyani, M.A.
239
Kassim, N. Hilal, A.F. Ismail, Membrane technology enhancement in oil–water
240
separation. A review, Desalination. 357 (2015) 197-207.
241
[5] Y. Sun, Y. Li, L. Fang, L. Zhang, L. Cheng, T. Yoshioka, H. Matsuyama, Facile
242
development of poly(tetrafluoride ethylene-r-vinylpyrrolidone) modified PVDF
243
membrane with comprehensive antifouling property for highly-efficient
244
challenging oil-in-water emulsions separation, J. Membr. Sci. 584 (2019) 161-172.
245
[6] S.X. Zhang, G.S. Jiang, S.J. Gao, H.L. Jin, Y.Z. Zhu, F. Zhang, J. Jin, Cupric
246
Phosphate Nanosheets-Wrapped Inorganic Membranes with Superhydrophilic and
247
Outstanding Anticrude Oil-Fouling Property for Oil/Water Separation, ACS. Nano.
248
12 (2018) 795-803.
249
[7] Z. Pan, S. Cao, J. Li, Z. Du, F. Cheng, Anti-fouling TiO2 nanowires membrane for
250
oil/water separation: Synergetic effects of wettability and pore size, J. Membr. Sci.
251
572 (2019) 596-606.
252
[8] N. Ohtsu, N. Masahashi, Y. Mizukoshi, K. Wagatsuma, Hydrocarbon
253
Decomposition on a Hydrophilic TiO2 Surface by UV Irradiation: Spectral and
254
Quantitative Analysis Using in-Situ XPS Technique, Langmuir. 25 (2009) 11586-
255
11591.
256
[9] T. Zubkov, D. Stahl, T.L. Thompson, D. Panayotov, O. Diwald, J.T. Yates,
257
Ultraviolet light-induced hydrophilicity effect on TiO2(110)(1x1). Dominant role
258
of the photooxidation of adsorbed hydrocarbons causing wetting by water droplets,
259
J. Phys. Chem. B. 109 (2005) 15454-15462.
260
[10] G. Kwon, D. Panchanathan, S.R. Mahmoudi, M.A. Gondal, G.H. Mckinley, K.K.
261
Varanasi, Visible light guided manipulation of liquid wettability on
262
photoresponsive surfaces, Nat. Commun. 8 (2017) 14968.
263
[11] C.M. Xie, W. Sun, H. Lu, A. Kretzschmann, J.H. Liu, M. Wagner, H.J. Butt, X.
264
Deng, S. Wu, Reconfiguring surface functions using visible-light-controlled
265
metal-ligand coordination, Nat. Commun. 9 (2018).
266 267
[12] B. Khezri, M. Pumera, Metal-Organic Frameworks Based Nano/Micro/MillimeterSized Self-Propelled Autonomous Machines, Adv. Mater. 31 (2019) 11.
268
[13] F. Cacho-Bailo, B. Seoane, C. Tellez, J. Coronas, ZIF-8 continuous membrane on
269
porous polysulfone for hydrogen separation, J. Membr. Sci. 464 (2014) 119-126.
270
[14] H. Zhang, M. Zhao, Y.S. Lin, Stability of ZIF-8 in water under ambient conditions,
271
Micropor. Mesopor. Mat. 279 (2019) 201-210.
272
[15] T. Gan, J. Li, H. Li, Y. Liu, Z. Xu, Synthesis of Au nanorod-embedded and
273
graphene oxide-wrapped microporous ZIF-8 with high electrocatalytic activity for
274
the sensing of pesticides, Nanoscale. 11 (2019) 7839-7849.
275
[16] H.Y. Yang, N.X. Wang, L. Wang, H.X. Liu, Q.F. An, S.L. Ji, Vacuum-assisted
276
assembly of ZIF-8@GO composite membranes on ceramic tube with enhanced
277
organic solvent nanofiltration performance, J. Membr. Sci, 545 (2018) 158-166.
278
[17] R. Xing, R. Huang, W. Qi, R. Su, Z. He, Three-dimensionally printed bioinspired
279
superhydrophobic PLA membrane for oil-water separation, AIChE. J. 64 (2018)
280
3700-3708.
281 282
[18] F. Mugele, Fundamental challenges in electrowetting: from equilibrium shapes to contact angle saturation and drop dynamics, Soft Matter. 5 (2009) 3377-3384.
283
[19] G. Kwon, D. Panchanathan, S.R. Mahmoudi, M.A. Gondal, G.H. McKinley, K.K.
284
Varanasi, Visible light guided manipulation of liquid wettability on
285
photoresponsive surfaces, Nat. Commun. 8 (2017).
286
[20] D. Tian, Z. Guo, Y. Wang, W. Li, X. Zhang, J. Zhai, L. Jiang, Phototunable
287
Underwater Oil Adhesion of Micro/Nanoscale Hierarchical-Structured ZnO Mesh
288
Films with Switchable Contact Mode, Adv. Funct. Mater. 24 (2014) 536-542.
289
[21] S.J. Gao, Z. Shi, W.B. Zhang, F. Zhang, J. Jin, Photoinduced Superwetting Single-
290
Walled Carbon Nanotube/TiO2 Ultrathin Network Films for Ultrafast Separation
291
of Oil-in-Water Emulsions, ACS. Nano. 8 (2014) 6344-6352.
292
[22] H. Kang, Y. Liu, H. Lai, X. Yu, Z. Cheng, L. Jiang, Under-Oil Switchable
293
Superhydrophobicity to Superhydrophilicity Transition on TiO2 Nanotube Arrays,
294
ACS. Nano. 12 (2018) 1074-1082.
295
[23] R. Wang, K. Hashimoto, A. Fujishima, M. Chikuni, E. Kojima, A. Kitamura, M.
296
Shimohigoshi, T. Watanabe, Light-induced amphiphilic surfaces, Nature. 388
297
(1997) 431-432.
298
[24] M.A. Nasalevich, M. van der Veen, F. Kapteijn, J. Gascon, Metal–organic
299
frameworks as heterogeneous photocatalysts: advantages and challenges,
300
CrystEngComm. 16 (2014) 4919-4926.
301
[25] Y.F. Zhi, Z.P. Li, X. Feng, H. Xia, Y.M. Zhang, Z. Shi, Y. Mu, X.M. Liu, Covalent
302
organic frameworks as metal-free heterogeneous photocatalysts for organic
303
transformations, J. Mater. Chem. A. 5 (2017) 22933-22938.
304
[26] G. Fux, G.Z. Ramon, Microscale Dynamics of Oil Droplets at a Membrane Surface:
305
Deformation, Reversibility, and Implications for Fouling, Environ. Sci. Technol.
306
51 (2017) 13842-13849.
307
[27] X. Lin, M. Yang, H. Jeong, M. Chang, J. Hong, Durable superhydrophilic coatings
308
formed for anti-biofouling and oil-water separation, J. Membr. Sci. 506 (2016)
309
22-30.
310
[28] M.A. Nasalevich, M.V.D. F. Veen, Kapteijn, J. Gascon, Metal–organic
311
frameworks as heterogeneous photocatalysts: advantages and challenges.
312
Crystengcomm, 16 (2014) 4919-4926.
(b)
(a)
2μm
2μm
(c)
(d)
(e)
(f) 600
Water flux (LMH)
Contact angle (0)
80 60 40 20 0
Blank
PDA
PDA/TMC ZIF-8/GO
400
200
0
Blank
PDA PDA/TMC ZIF-8/GO
Fig.1. SEM characterization of (a) original membrane and (b) ZIF-8/GO membrane; AFM image of (c) original membrane and (d) ZIF-8/GO membrane; (e) The water contact angles of membranes; (f) Water flux of membrane with different layers.
(a) 160
Blank membrane ZIF-8/GO membrane without irradiaion ZIF-8/GO membrane with irradiaion
Flux (LMH)
140 120 100 80 60 20
40
60
80
100
120
Filtraion time (min)
Absorbance
0.7 0.6 0.5
Emulsifier aqueous (diluted 500 times) Blank membrane (diluted 500 times) ZIF-8/GO without irradiation (diluted 50 times) ZIF-8/GO with irradiation (diluted 50 times)
characteristic peak
0.4 0.3
(c)16 14
Intensity (%)
(b) 0.8
12
6 4 2
220
230
240
Wavenumber (nm)
250
Feed
8
0.1 210
ZIF-8/GO with irradiation
10
0.2
0.0 200
ZIF-8/GO without irradiation Blank membrane
0
50
100
150
200
Size (nm)
250
300
Fig.2. (a) Permeation flux of membrane with and without irradiation; (b) The UV-VIS analysis of oil/water feed before and after separation; (c) The dynamic light scattering (DLS) analysis of feed and filtration.
Water Contact Angle (0)
(a)
(c)
45 40
1 min
5 min
Air Water
With irradiation
Air Water
Without irradiation
35 30 25
(b)
0.0
0.5
1.0
1.5
2.0
2.5
Oil
Irradiation time (h)
70
Water Contact Angle (0)
0 min
Water
Irradiation
With irradiation
Dark
60
Oil
50
Water
Without irradiation
40
Oil
30 0
1
2
3
4
5
6
Air
Without irradiation
Cycle Number
Fig.3. (a) Variation of water CA of ZIF-8/GO membrane in air (after irradiation) as a function of time. (b) Cycles of variation of water CA in the process of being irradiated by light and placed in the dark. (c) Dynamic spreading behavior of water in air, oil in air, and underwater on ZIF-8/GO membrane after irradiation
1.4 1.2
20
1.0
15
0.8 0.6
10
0.4
5
0.2
0
Flux (LMH)
25
(b)120
1.6
Toluene Benzyl alcohol Benz aldehyde
Concentration (ppm)
Concentration (ppm)
(a)30
110 without irradiation 100 90 80 70 60
0.0 0
20
40
60
80
100 120
0
Filtration time (min)
150
150
Flux (LMH)
(d)160
Flux (LMH)
(c)160 140 without irradiation 130
with irradiation
110
20
40
60
80
Filtraion time (min)
(e) 120
100
120
with irradiation without irradiation
100
40
60
80
Filtraion time (min)
140 without irradiation 130
110
100
120
with irradiation
20
40
60
80
100
Filtraion time (min)
120
(f)180 160
140
Flux (LMH)
Rejection (%)
20
120
120
80
120
60
With irradiation Without irradiation
100
40 20 0
with irradiation
80 60
1
2
3
4
5
6
7
8
Cycle number (times)
9
10
1
2
3
4
5
6
7
8
Cycle munber (times)
9 10
Fig.4. (a) the concentration of three different substances in filtration. The permeation flux of (b) oil/water feed, (c) Benzyl alcohol feed and (d) Benz aldehyde feed. (e) The rejection and (f) the recyclability of the ZIF-8/GO membrane with and without irradiation.
Fig. 5. Oil rejection efficiency of oil-in-water emulsions separated by ZOF-8/GO film under light.
Highlights (1) Developing a photoinduced superwetting membranes via layer-by-layer deposition of ZIF-8/GO composite. (2) The ZIF-8/GO membrane exhibits superior superoleophobicity under visible light and underwater anti-oil-fouling for efficient oil/water separation. (3) The high water flux, high separation capacity and capacity for large-scale preparation of the ZIF-8/GO membrane shows great potential for practical application in oil-containing wastewater treatment.
Conflict of interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
S1383-5866(19)35374-2 https://doi.org/10.1016/j.seppur.2020.116536 SEPPUR 116536
To appear in:
Separation and Purification Technology
Received Date: Revised Date: Accepted Date:
28 November 2019 7 January 2020 7 January 2020
Please cite this article as: R-Y. Yue, J. Guan, C-M. Zhang, P-C. Yuan, L-N. Liu, M. Zaheer Afzal, S-G. Wang, X-F. Sun, Photoinduced superwetting membranes for Separation of Oil-in-Water Emulsions, Separation and Purification Technology (2020), doi: https://doi.org/10.1016/j.seppur.2020.116536
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© 2020 Published by Elsevier B.V.
Photoinduced superwetting membranes for Separation of Oil-in-Water Emulsions
Reng-Yu Yue, Jing Guan, Chun-Miao Zhang, Peng-Cheng Yuan, Lin-Na Liu, Muhammad Zaheer Afzal, Shu-Guang Wang, Xue-Fei Sun*
Shandong Key Laboratory of Water Pollution Control and Resource Reuse, School of Environmental Science and Engineering, Shandong University, Qingdao 266237, China
*Corresponding author: Fax: +86-532-58630936; E-mail: [email protected] (Xuefei Sun)
1
Abstract: The separation of oil/water emulsion has become a pressing worldwide
2
problem. In this study, a novel type of photoinduced separation membrane was prepared
3
via layer-by-layer (LBL) method of ZIF-8/GO composite. By integrating the unique
4
physical and chemical properities of ZIF-8 and GO, the ZIF-8/GO membrane exhibits
5
superior superoleophobicity under visible light and underwater anti-fouling for efficient
6
oil rejection from water. We found that the ZIF-8/GO membrane permeation flux was
7
110 ± 6 L m-2 h-1 under light irradiation, which was higher than the permeation flux of
8
membranes without irradiation (50 ± 6 L m-2 h-1). For toluene, the removal rate was
9
about 100 % at the first time, with new benzyl alcohol and benzaldehyde products
10
detected in the filtrate, indicating that toluene was degraded on the ZIF-8/GO
11
membrane surface. Continuous separation performed using filtration apparatus
12
demonstrates a high separation capacity, with long-term membrane stability. The high
13
water flux, high separation capacity and capacity for large-scale preparation of the ZIF-
14
8/GO membrane, shows great potential for industial application in the treatment of oil-
15
containing wastewater.
16
Keywords: Ultrafiltration, ZIF-8/GO composite, oil/water separation, photoinduced,
17
visible light.
18
1. Introduction
19
Oil-water separation has aroused widespread attention of the last few decades
20
owing to water pollution caused by industrial sewage and crude oil leakage, which
21
threaten aqueous habitats and people's health [1]. Separating oil from oily water is
22
becoming more and more urgent. The oil phase in wastewater can exist in three shapes
23
and is classified according to the size of the oil droplets as free oil (>150 μm), dispersed
24
oil (20–150 μm) and emulsified oil (<20 μm) [2]. Among them, the emulsified droplets
25
are generally highly stable because of the existence of a strong oil/ water interface film
26
and the adsorption of active components on the interface. [3]. Therefore, traditional
27
treatment methods, like gravity separation and skimming techniques are unable to
28
achieve desired effect. Membrane technologies are promising for the separation of oil
29
droplets smaller than ∼10 μm, because they are simple, compact, modular, relatively
30
inexpensive and highly efficient [4, 5]. However, traditional filtration membranes can
31
suffer from rapid membrane fouling during membrane separation processes, which was
32
caused by the cumulative deposition of original components (such as oil) on the
33
membrane surface and within membrane internal pores [6, 7]. Therefore, it is critical to
34
develop a novel oil/water separation membrane with excellent separation performance,
35
anti-oil-fouling and the ability to be recycled.
36
Many efforts have been dedicated to seek approaches to effectively weaken
37
membrane fouling. A large number of techniques have been applied to enhance the
38
hydrophilicity of membranes by either mixing common membrane polymers with
39
hydrophilic additives or modifying the membrane surface through physicochemical
40
post-modification. However, the existence of chain flexibility of the functionalized
41
polymer allows oil droplets to pass through the hydrophilic barrier, causing the
42
oil/water separation efficiency is still weakened during long-term operation. Recently,
43
it has been reported that the change of surface wettability of TiO2 induced by UV light
44
can realize the light driven oil-water separation [8, 9]. Optically driven control of liquid
45
motion on the TiO2 is highly promising since it can eliminate the need of direct
46
electrical contact with liquid or complex electronic circuit [10]. Nevertheless, an
47
insensitivity to respond to visible light spectrum of natural sunlight, slow kinetics and
48
a requirement for special environments to recover the original wetting state restrict the
49
practical applications of TiO2 surfaces. By contrast, visible light is non-invasive and it
50
is highly essential to construct responsive surfaces that are controllable by visible light
51
[11]. Therefore, advanced researches should be focused on developing powerful
52
antifouling membranes to achieve highly efficient and stable oil-water separation.
53
Here, we developed a novel photoresponsive and photocatalytic membrane made
54
from Zeolitic imidazolate frameworks/Graphene (ZIF-8/GO) nanocomposites for in
55
situ simultaneous emulsion separation and water purification under visible light. ZIFs
56
consist of transition metal ions and imidazolate/imidazolate type linkers which cover a
57
much wide range of pore sizes (0.2-1.5 nm) [12]. Therefore, water molecules (about
58
0.3-0.4 nm) can traverse the ZIF-8 micropore system (0.34 nm), while the emulsion
59
(100~500 nm) cannot [13]. However, the ZIF-8 structure is not stable in water under
60
ambient conditions [14]. It has been reported that by wrapping ZIF-8 with a layer of
61
GO, electronic conductivity and stability can be significantly improved [15, 16]. The
62
GO nanosheets first adsorbs water molecules and forms a hydration layer on the
63
membrane surface, which increases the space hindrance and can effectively prevent the
64
affinity between oil droplets and the membrane surface [6]. In addition, the structural
65
features of tunable active sites within ZIF lead to more powerful solar harnessing.
66
While graphene can efficiently weaken the recombination rate of photo-generated
67
electron-hole pairs when coupled with semiconductors, it could increase the
68
membranes photocatalytic performance. In this study, the oil degradation and
69
mineralization performance of the ZIF-8/GO nanocomposite membrane filtration
70
system were explored. We tested the antifouling properties and the operation stability
71
of this system, which further verify the practicability of this integrated membrane
72
catalysis process. Moreover, the toluene degradation pathway in the photocatalytic
73
membrane was explored by testing the reaction byproducts.
74
2. Materials and methods
75
2.1. Fabrication of ZIF-8/GO membrane
76
The ZIF-8/GO membrane was synthesized via the Layer-by-layer assembly of
77
ZIF-8/GO and TMC on the poly dopamine (PDA)-support substrate. First, a porous
78
PVDF membrane was used as an original support and dip-coated in a dopamine solution,
79
which is composed of 2 g L-1 dopamine and 10 mM Tris buffer solution (pH= 8.5).
80
Then the PDA-support was placed in an oven at 65 °C followed by immersed in 0.15%
81
TMC solution dissolved by Isopar G for 15 min. Using Isopar G to rinse the support to
82
remove excess TMC and then immersed in 1.5 g L-1 ZIF-8/GO solution for 30 min to
83
allow attachment of nanocomposite layers of ZIF-8/GO. Finally, the ZIF-8/GO
84
membrane was dried at 50 °C under vacuum.
85
2.2. Membrane Characterization
86
For SEM image (S-4800, Hitachi Limited Inc., Japan), samples were sputtered
87
with gold for observation. The membrane surface roughness was characterized in
88
tapping mode by AFM (Dimension Icon, Veeco Instruments Inc., US). The static
89
contact angle representing the hydrophilicity of membrane surface was determined by
90
a contact angle analyzer.
91
2.3. Membrane Performance Evaluation
92
Water permeation and separation efficiency of the ZIF-8/GO membrane were
93
tested by using a dead-end filtration system at 25 ±0.50C. Prior to the flux test, the
94
membrane was placed in the filtration tank and stabilized overnight at a transmembrane
95
pressure of 20psi (0.14 MPa), using a blank PVDF membrane as a control.
96
To evaluate the ZIF-8/GO membrane rejection performance and its reusability, the
97
ZIF-8/GO membrane was tested with toluene/water emulsions, with each cycle
98
followed by filtering and cleaning of the membrane with ethanol under sonication. The
99
photocatalytic activities of ZIF-8/GO membranes were evaluated by the
100
photodegradation of toluene under 500W Xe lamp irradiation in ambient air and
101
temperature conditions. During the photocatalytic reaction, solutions were continually
102
stirred to maintain homogeneous suspensions. Then collecting the filtrate, and the oil
103
constituent within the filtrate and the photodegradation products were determined
104
byGas chromatography-mass spectrometry (GC-MS, Agilent 7010B, Agilent
105
Technologies Inc., US).
106
3. Results and discussion
107
3.1. Membrane Characterization
108
LBL deposition of PDA, TMC and ZIF-8/GO constructs a thin, nanostructured
109
and well-porous membrane surface. These membranes were characterized by SEM and
110
AFM were shown in Fig. 1. The PVDF membrane has typical spongy-like pores formed
111
by phase inversion with a roughness of 50.6 nm (Fig. 1a, c). After deposition of the
112
ZIF-8/GO layer, the membrane demonstrates a corrugated and curled layered structure,
113
with pores becoming less visible while the surface maintains particle aggregation with
114
thin layer covering. The surface roughness decreased to 26.0 nm (Fig. 1b, d). Smooth
115
membranes tend to form a hydration layer on the surface of the membrane, which
116
increases the space hindrance and can effectively prevent the adhesion between oil
117
droplets and membrane [17]. Water contact angle (CA) measurement confirms that the
118
original membrane is hydrophobic, with a water CA of nearly 70° (Fig. 1e). While after
119
modification, the CA declines steadily from ~600 to ~250 as the number of layers
120
increases. The decline in water contact can help improve the ZIF-8/GO membrane anti-
121
pollution properties, thereby increasing the number of reuse cycles of the ZIF-8/GO
122
membrane [3].
123
Water fluxes of the modified membranes significantly decreased from 572 ± 26 L
124
m-2 h-1 for control membranes, to 53 ± 5 L m-2 h-1 for PDA membranes and 68 ± 4 L
125
m-2 h-1 for PDA/TMC membranes (Fig. 1f). With the addition of cross-linked ZIF-8/GO
126
nanocomposites on top of the PVDF support, a water flux of 188 ± 8 L m-2 h-1 was
127
measured, which was greater than that of the PDA and PDA/TMC membranes. The
128
increased water flux was probably caused by the presence of hydrophilic functional-
129
groups in GO nanosheets, according with the contact angle results (Fig. 1e).
130
3.2. Separation Performance of oil-in-water emulsions
131
Fig. 2a shows the results regarding toluene separation from water emulsions. Under
132
light irradiation, the permeate flux presented higher values of about 130 L m-2 h-1 and
133
stabilized at 95 L m-2 h-1, while without light the flux stabilized at 65 L m-2 h-1. The
134
achievement of complete toluene removal (100%) illustrates the high separation
135
efficiency of ZIF-8/GO membranes with irradiation (Fig. 2b, Fig. S4). The high
136
removal percentage was mainly due to toluene being excluded and photocatalyzed by
137
the ZIF-8/GO membrane. DLS measurements established the precise difference in
138
droplet size pre- and post-separation (Fig. 2c). The feed liquid contained a wide droplet
139
size distribution in the range of 8 to 250 nm, while the maximum size of the ZIF-8/GO
140
membrane filtrate was less than 40 nm, indicating the ZIF-8/GO membrane can
141
effectively separate emulsions containing droplet sizes greater than 20 nm. Following
142
the irradiation process, the size of the droplet is similar to that of the filtrate without
143
irradiation. The higher rate of oil rejection can be attributed to nanopores on the GO
144
nanosheets the ZIF-8 layer.
145
3.3. Photoinduced wettability transition
146
In order to explore the photo-induced wettability change of the ZIF-8/GO
147
membrane, the water CA was measured under Xe lamp irradiation. As shown in Fig.
148
3a, the water CA changes in air with irradiation time. The ZIF-8/GO exhibited a water
149
CA of 450, which reduces rapidly upon irradiation. After 1.5 h, a CA of nearly 250 was
150
obtained, with the CA remaining stable from this point. Similar CA saturation has also
151
been observed in traditional electrowetting-on-dielectric applications [18]. The
152
movement of droplets can be controlled by adjusting the surface wettability that was
153
induced by light [19, 20].
154
It is generally recognized that photo-generated electrons and holes are able to
155
change the surface chemistry of membranes, which is conducive to contact with
156
liquid diffusion, whether through photocatalytic oxidation of organic species
157
adsorbed on the surface, or the increase of hydroxyl species owing to dissociated water
158
adsorption [21, 22]. Light-induced hydrophilicity of the ZIF-8/GO membrane is caused
159
by light-induced hydroxyl groups on the surface of graphene oxide and ZIF-8 captured
160
from water molecules, which was oxygenated into the •OH active species [23-25]. As
161
shown in Fig. 3b, the hydrophilicity of the ZIF-8/GO membrane is switchable. When
162
the membrane is placed in the dark after irradiation, its water CA recovers to 450,
163
becoming more hydrophobic than before. This cyclic process could be repeated many
164
times. When an oil droplet with same volume is placed on the membrane surface, its
165
underwater oil CA decreases slightly from 1510to ~900, while it remained unchanged
166
under irradiation, indicating the ZIF-8/GO membrane becomes oleophobic and
167
hydrophilic with irradiation (Fig. 3c).
168
3.4. Self-cleaning ability of the ZIF-8/GO membrane
169
In the application of oil-water separation, the traditional filter membrane has the
170
problem of pollution due to sticking oil, which seriously restricts the reuse and long-
171
term use of the membrane. [26, 27]. In order to understand the photocatalysis membrane
172
properties, a Xe lamp was used to irradiate the ZIF-8/GO membrane during the
173
filtration process, with GC-MS used to analyze the photocatalysis products. Fig. S5
174
shows the results of filtration in the presence and absence of irradiation, showing that
175
no other product was present except for toluene in the filtrate without irradiation.
176
Conversely, under illumination two new products were formed and detected in the
177
filtrate, benzyl alcohol and benzaldehyde (Fig. 4a and S5). This indicates that toluene
178
was degraded on the ZIF-8/GO membrane surface, with benzyl alcohol and
179
benzaldehyde concentrations of 1.176 and 1.249 ppm, respectively. In the presence of
180
light irradiation, one electron was captured from water molecules, which was
181
oxygenated into the •OH active species. Then, the •OH could decompose Toluene
182
efficiently to complete the photocatalytic process [28].
183
To further verify the impact of irradiation on the ZIF-8/GO membrane, following
184
a 30-minute filtration process without irradiation, the ZIF-8/GO membrane was
185
subsequently irradiated for 30 minutes (Fig. 4b). During the irradiation process,
186
permeation of the oil/water emulsion increased initially before declining steadily, after
187
which the level of permeation maintained a constant level. The final ZIF-8/GO
188
membrane permeation outcome following this process, was lower than under continual
189
irradiation conditions. This may be due to toluene fouling of the membrane during the
190
30-minute-long process, with toluene adhering to the membrane surface, causing
191
irreversible damage to the membrane. In the Fig.4(c) and (d), we use Benzyl alcohol
192
and Benz aldehyde as feed respectively to explore the effect of irradiation. We use the
193
Xe lamp to illuminate the ZIF-8/GO membrane surface after 30 mins, the permeate flux
194
of Benzyl alcohol and Benz aldehyde are not obviously fluctuant, they are continuous
195
and keep falling (Fig.4c and d).
196
To evaluate recyclability of the composite membrane, nine cycles of repeated use
197
experiments were performed. In each cycle, the ZIF-8/GO membrane was washed using
198
ethanol after filtration of the emulsion, then dried at 65 °C. Throughout the nine
199
repetitions, the membrane continued to exhibit a stable performance (Fig. 4e and f).
200
Fig. 4e shows the change in emulsion removal rate, in the presence or absence of light
201
irradiation. At the first cycle, the toluene removal efficiency reached 100 %, with a high
202
flux of >170 L·m-2·h-1 with irradiation. By contrast, it shows a relatively low permeation
203
flux of 70 L·m-2·h-1 when used for separation without irradiation. The separation
204
efficiency of the ZIF-8/GO membrane was ~93% and the permeation flux remained
205
stable after nine cycles of reuse. In the membrane recycling process in the absence of
206
light irradiation, the removal rate was higher after each cycle, as the water solubility of
207
benzyl alcohol and benzaldehyde make it easier to remove toluene from the membrane
208
surface after each cycle. Therefore, deposition of the ZIF-8/GO composite not only
209
strengthens the anti-fouling capability of membranes, but also enhances membrane
210
recyclability under light irradiation.
211
In order to test the applicability of the ZIF-8/GO membrane, the separation of five
212
types of oil emulsions were compared (Fig. 5). The ZIF-8/GO membrane exhibiting
213
excellent separation performance and removal efficiency, with a large handling
214
capacity and long-time operation stability. These results indicate that the composite
215
photocatalytic ZIF-8/GO membrane is highly promising for the separation and
216
treatment of oil/water emulsions.
217
4. Conclusion
218
We have fabricated a high-performance ZIF-8/GO ultrafiltration membranes for the
219
separation and degradation of oil from oil-water suspensions. With light irradiation, the
220
flux rate was 1.5-times of magnitude higher than membranes without light irradiation,
221
allowing an ultra-high separation efficiency of 99.99% to be achieved. Designing such
222
an ZIF-8/GO membrane provides an effective, economical and favorable tool in various
223
practical fields such as water treatment, fuel purification and the removal of commercial
224
emulsions.
225
Acknowledgements
226
We thank the National Natural Science Foundation of China (21576157) for the support
227
to this study.
228
References
229
[1] J. Gu, H. Fan, C. Li, J. Caro, H. Meng, Robust Superhydrophobic/Superoleophilic
230
Wrinkled Microspherical MOF@rGO Composites for Efficient Oil-Water
231
Separation, Angew. Chem. 58 (2019) 1-6.
232
[2] H.J. Tanudjaja, C.A. Hejase, V.V. Tarabara, A.G. Fane, J.W. Chew, Membrane-
233
based separation for oily wastewater: A practical perspective, Water. Res. 156
234
(2019) 347-365.
235
[3] C.L. Chen, D. Weng, A. Mahmood, S. Chen, J.D. Wang, Separation Mechanism
236
and Construction of Surfaces with Special Wettability for Oil/Water Separation,
237
ACS. Appl. Mater. Inter. 11 (2019) 11006-11027.
238
[4] M. Padaki, R. Surya Murali, M.S. Abdullah, N. Misdan, A. Moslehyani, M.A.
239
Kassim, N. Hilal, A.F. Ismail, Membrane technology enhancement in oil–water
240
separation. A review, Desalination. 357 (2015) 197-207.
241
[5] Y. Sun, Y. Li, L. Fang, L. Zhang, L. Cheng, T. Yoshioka, H. Matsuyama, Facile
242
development of poly(tetrafluoride ethylene-r-vinylpyrrolidone) modified PVDF
243
membrane with comprehensive antifouling property for highly-efficient
244
challenging oil-in-water emulsions separation, J. Membr. Sci. 584 (2019) 161-172.
245
[6] S.X. Zhang, G.S. Jiang, S.J. Gao, H.L. Jin, Y.Z. Zhu, F. Zhang, J. Jin, Cupric
246
Phosphate Nanosheets-Wrapped Inorganic Membranes with Superhydrophilic and
247
Outstanding Anticrude Oil-Fouling Property for Oil/Water Separation, ACS. Nano.
248
12 (2018) 795-803.
249
[7] Z. Pan, S. Cao, J. Li, Z. Du, F. Cheng, Anti-fouling TiO2 nanowires membrane for
250
oil/water separation: Synergetic effects of wettability and pore size, J. Membr. Sci.
251
572 (2019) 596-606.
252
[8] N. Ohtsu, N. Masahashi, Y. Mizukoshi, K. Wagatsuma, Hydrocarbon
253
Decomposition on a Hydrophilic TiO2 Surface by UV Irradiation: Spectral and
254
Quantitative Analysis Using in-Situ XPS Technique, Langmuir. 25 (2009) 11586-
255
11591.
256
[9] T. Zubkov, D. Stahl, T.L. Thompson, D. Panayotov, O. Diwald, J.T. Yates,
257
Ultraviolet light-induced hydrophilicity effect on TiO2(110)(1x1). Dominant role
258
of the photooxidation of adsorbed hydrocarbons causing wetting by water droplets,
259
J. Phys. Chem. B. 109 (2005) 15454-15462.
260
[10] G. Kwon, D. Panchanathan, S.R. Mahmoudi, M.A. Gondal, G.H. Mckinley, K.K.
261
Varanasi, Visible light guided manipulation of liquid wettability on
262
photoresponsive surfaces, Nat. Commun. 8 (2017) 14968.
263
[11] C.M. Xie, W. Sun, H. Lu, A. Kretzschmann, J.H. Liu, M. Wagner, H.J. Butt, X.
264
Deng, S. Wu, Reconfiguring surface functions using visible-light-controlled
265
metal-ligand coordination, Nat. Commun. 9 (2018).
266 267
[12] B. Khezri, M. Pumera, Metal-Organic Frameworks Based Nano/Micro/MillimeterSized Self-Propelled Autonomous Machines, Adv. Mater. 31 (2019) 11.
268
[13] F. Cacho-Bailo, B. Seoane, C. Tellez, J. Coronas, ZIF-8 continuous membrane on
269
porous polysulfone for hydrogen separation, J. Membr. Sci. 464 (2014) 119-126.
270
[14] H. Zhang, M. Zhao, Y.S. Lin, Stability of ZIF-8 in water under ambient conditions,
271
Micropor. Mesopor. Mat. 279 (2019) 201-210.
272
[15] T. Gan, J. Li, H. Li, Y. Liu, Z. Xu, Synthesis of Au nanorod-embedded and
273
graphene oxide-wrapped microporous ZIF-8 with high electrocatalytic activity for
274
the sensing of pesticides, Nanoscale. 11 (2019) 7839-7849.
275
[16] H.Y. Yang, N.X. Wang, L. Wang, H.X. Liu, Q.F. An, S.L. Ji, Vacuum-assisted
276
assembly of ZIF-8@GO composite membranes on ceramic tube with enhanced
277
organic solvent nanofiltration performance, J. Membr. Sci, 545 (2018) 158-166.
278
[17] R. Xing, R. Huang, W. Qi, R. Su, Z. He, Three-dimensionally printed bioinspired
279
superhydrophobic PLA membrane for oil-water separation, AIChE. J. 64 (2018)
280
3700-3708.
281 282
[18] F. Mugele, Fundamental challenges in electrowetting: from equilibrium shapes to contact angle saturation and drop dynamics, Soft Matter. 5 (2009) 3377-3384.
283
[19] G. Kwon, D. Panchanathan, S.R. Mahmoudi, M.A. Gondal, G.H. McKinley, K.K.
284
Varanasi, Visible light guided manipulation of liquid wettability on
285
photoresponsive surfaces, Nat. Commun. 8 (2017).
286
[20] D. Tian, Z. Guo, Y. Wang, W. Li, X. Zhang, J. Zhai, L. Jiang, Phototunable
287
Underwater Oil Adhesion of Micro/Nanoscale Hierarchical-Structured ZnO Mesh
288
Films with Switchable Contact Mode, Adv. Funct. Mater. 24 (2014) 536-542.
289
[21] S.J. Gao, Z. Shi, W.B. Zhang, F. Zhang, J. Jin, Photoinduced Superwetting Single-
290
Walled Carbon Nanotube/TiO2 Ultrathin Network Films for Ultrafast Separation
291
of Oil-in-Water Emulsions, ACS. Nano. 8 (2014) 6344-6352.
292
[22] H. Kang, Y. Liu, H. Lai, X. Yu, Z. Cheng, L. Jiang, Under-Oil Switchable
293
Superhydrophobicity to Superhydrophilicity Transition on TiO2 Nanotube Arrays,
294
ACS. Nano. 12 (2018) 1074-1082.
295
[23] R. Wang, K. Hashimoto, A. Fujishima, M. Chikuni, E. Kojima, A. Kitamura, M.
296
Shimohigoshi, T. Watanabe, Light-induced amphiphilic surfaces, Nature. 388
297
(1997) 431-432.
298
[24] M.A. Nasalevich, M. van der Veen, F. Kapteijn, J. Gascon, Metal–organic
299
frameworks as heterogeneous photocatalysts: advantages and challenges,
300
CrystEngComm. 16 (2014) 4919-4926.
301
[25] Y.F. Zhi, Z.P. Li, X. Feng, H. Xia, Y.M. Zhang, Z. Shi, Y. Mu, X.M. Liu, Covalent
302
organic frameworks as metal-free heterogeneous photocatalysts for organic
303
transformations, J. Mater. Chem. A. 5 (2017) 22933-22938.
304
[26] G. Fux, G.Z. Ramon, Microscale Dynamics of Oil Droplets at a Membrane Surface:
305
Deformation, Reversibility, and Implications for Fouling, Environ. Sci. Technol.
306
51 (2017) 13842-13849.
307
[27] X. Lin, M. Yang, H. Jeong, M. Chang, J. Hong, Durable superhydrophilic coatings
308
formed for anti-biofouling and oil-water separation, J. Membr. Sci. 506 (2016)
309
22-30.
310
[28] M.A. Nasalevich, M.V.D. F. Veen, Kapteijn, J. Gascon, Metal–organic
311
frameworks as heterogeneous photocatalysts: advantages and challenges.
312
Crystengcomm, 16 (2014) 4919-4926.
(b)
(a)
2μm
2μm
(c)
(d)
(e)
(f) 600
Water flux (LMH)
Contact angle (0)
80 60 40 20 0
Blank
PDA
PDA/TMC ZIF-8/GO
400
200
0
Blank
PDA PDA/TMC ZIF-8/GO
Fig.1. SEM characterization of (a) original membrane and (b) ZIF-8/GO membrane; AFM image of (c) original membrane and (d) ZIF-8/GO membrane; (e) The water contact angles of membranes; (f) Water flux of membrane with different layers.
(a) 160
Blank membrane ZIF-8/GO membrane without irradiaion ZIF-8/GO membrane with irradiaion
Flux (LMH)
140 120 100 80 60 20
40
60
80
100
120
Filtraion time (min)
Absorbance
0.7 0.6 0.5
Emulsifier aqueous (diluted 500 times) Blank membrane (diluted 500 times) ZIF-8/GO without irradiation (diluted 50 times) ZIF-8/GO with irradiation (diluted 50 times)
characteristic peak
0.4 0.3
(c)16 14
Intensity (%)
(b) 0.8
12
6 4 2
220
230
240
Wavenumber (nm)
250
Feed
8
0.1 210
ZIF-8/GO with irradiation
10
0.2
0.0 200
ZIF-8/GO without irradiation Blank membrane
0
50
100
150
200
Size (nm)
250
300
Fig.2. (a) Permeation flux of membrane with and without irradiation; (b) The UV-VIS analysis of oil/water feed before and after separation; (c) The dynamic light scattering (DLS) analysis of feed and filtration.
Water Contact Angle (0)
(a)
(c)
45 40
1 min
5 min
Air Water
With irradiation
Air Water
Without irradiation
35 30 25
(b)
0.0
0.5
1.0
1.5
2.0
2.5
Oil
Irradiation time (h)
70
Water Contact Angle (0)
0 min
Water
Irradiation
With irradiation
Dark
60
Oil
50
Water
Without irradiation
40
Oil
30 0
1
2
3
4
5
6
Air
Without irradiation
Cycle Number
Fig.3. (a) Variation of water CA of ZIF-8/GO membrane in air (after irradiation) as a function of time. (b) Cycles of variation of water CA in the process of being irradiated by light and placed in the dark. (c) Dynamic spreading behavior of water in air, oil in air, and underwater on ZIF-8/GO membrane after irradiation
1.4 1.2
20
1.0
15
0.8 0.6
10
0.4
5
0.2
0
Flux (LMH)
25
(b)120
1.6
Toluene Benzyl alcohol Benz aldehyde
Concentration (ppm)
Concentration (ppm)
(a)30
110 without irradiation 100 90 80 70 60
0.0 0
20
40
60
80
100 120
0
Filtration time (min)
150
150
Flux (LMH)
(d)160
Flux (LMH)
(c)160 140 without irradiation 130
with irradiation
110
20
40
60
80
Filtraion time (min)
(e) 120
100
120
with irradiation without irradiation
100
40
60
80
Filtraion time (min)
140 without irradiation 130
110
100
120
with irradiation
20
40
60
80
100
Filtraion time (min)
120
(f)180 160
140
Flux (LMH)
Rejection (%)
20
120
120
80
120
60
With irradiation Without irradiation
100
40 20 0
with irradiation
80 60
1
2
3
4
5
6
7
8
Cycle number (times)
9
10
1
2
3
4
5
6
7
8
Cycle munber (times)
9 10
Fig.4. (a) the concentration of three different substances in filtration. The permeation flux of (b) oil/water feed, (c) Benzyl alcohol feed and (d) Benz aldehyde feed. (e) The rejection and (f) the recyclability of the ZIF-8/GO membrane with and without irradiation.
Fig. 5. Oil rejection efficiency of oil-in-water emulsions separated by ZOF-8/GO film under light.
Highlights (1) Developing a photoinduced superwetting membranes via layer-by-layer deposition of ZIF-8/GO composite. (2) The ZIF-8/GO membrane exhibits superior superoleophobicity under visible light and underwater anti-oil-fouling for efficient oil/water separation. (3) The high water flux, high separation capacity and capacity for large-scale preparation of the ZIF-8/GO membrane shows great potential for practical application in oil-containing wastewater treatment.
Conflict of interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.