Journal of Membrane Science 640 (2021) 119836

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Journal of Membrane Science

Robust PVA-GO-TiO2 composite membrane for efficient separation

A R T I C L E I N F O A B S T R A C T

Keywords: Most super-wetting membrane materials are susceptible to oil pollution after separation process, causing a
Composite membrane serious degradation of the separation ability. The rough structure of underwater superoleophobic membrane can
Underwater superoleophobicity effectively improve anti-fouling property, thus achieving stable separation. Herein, a composite polyvinyl
Crude oil-in-water emulsion separation
alcohol-graphene oxide-titanium dioxide (PVA-GO-TiO2, PGT) membrane with rough structure is prepared
Stable flux
through a simple hydrothermal and suction-filtration method. The as-prepared PGT membrane displays
outstanding underwater superoleophobicity even for high-adhesion crude oil. Moreover, the as-fabricated
membrane exhibits efficient oil-in-water emulsions separation performance with separation efficiency of
larger than 99.1% and permeation flux of over than 762 L m− 2 h− 1 even after 10 min. Besides, the PGT mem­
brane could separate various oil-in-water emulsions in corrosive environments with separation efficiency of
higher than 99.3% and permeation flux of more than 760 L m− 2 h− 1. More importantly, the as-prepared
membrane could keep the underwater superoleophobicity after sanding test (200 times) and bending test (500
times). The composite membrane with excellent oil-in-water emulsions separation performance, corrosion
resistance and mechanical stability has broad prospects in the field of water purification.

1. Introduction industrial oily wastewater, and have been widely used to treat oil-water
mixtures because of their high efficiency, time-saving and operation-
Accidental oil leakage and frequent oily sewage emission during oil- easy characteristics [13–15]. The systematic manufacturing of super­
exploitation and transportation processes seriously threaten human wetting membranes usually depends on two important principles:
health and economic development [1,2]. Therefore, the effective tech­ micro-nano-scale rough structure and appropriate surface energy. Based
nologies for treating oily wastewater have attracted widespread atten­ on the difference in the wettability of the membrane surface to water
tion recently. According to the size dimension of the oil particle, oil in and oil, the superwetting membranes are divided into “oil-removing”
the oil/water mixture usually classify to free oil (diameter >150 μm), membrane (superoleophilic/superhydrophobic) and “water-removing”
dispersed oil (20 μm < diameter <150 μm) and emulsified oil (diameter membrane (superhydrophilic/underwater superoleophobic) [16]. Jiang
<20 μm) [3–5]. Traditional technologies, such as combustion [6], et al. prepared a crater-like superhydrophobic/superoleophobic film
centrifugation [7], gravity-driven separation [8], can effectively sepa­ with low surface energy and nanostructure, which could prevent water
rate free oil and dispersed oil, but they are ineffective for separating droplets from wetting the material while permitting oil to pass through
surfactant-stabilized oil/water emulsions [9,10]. It is still a giant chal­ readily [17,18]. Until now, many researchers have attached great
lenging for separation oil/water emulsions due to the thermodynamic emphasis on the preparation of “oil-removing” materials [19,20].
and kinetic stability of emulsified oils with tiny droplet sizes. Therefore, However, most “oil-removing” membranes are generally insufficient to
it is an urgent to develop an effective separation technology to achieve separate industrial oil/water mixtures, because plenty of industrial oils
effective separation oil/water emulsions [2,11,12]. are light oil. Moreover, drawing inspiration from the underwater
The superwetting membranes are suitable for treating a lot of superoleophobicity of fish scale [21], “water-removing” membrane have

* Corresponding author. Key Laboratory of Eco-functional Polymer Materials of the Ministry of Education, College of Chemistry and Chemical Engineering,
Northwest Normal University, Lanzhou, 730070, PR China.
E-mail address: jianli83@126.com (J. Li).

https://doi.org/10.1016/j.memsci.2021.119836
Received 4 July 2021; Received in revised form 13 August 2021; Accepted 1 September 2021
Available online 4 September 2021
0376-7388/© 2021 Elsevier B.V. All rights reserved.
Q. Zhong et al. Journal of Membrane Science 640 (2021) 119836

been successfully manufactured by importing high surface energy ma­ polyvinylidene fluoride (PVDF)/GO membrane via crosslinking process
terials [22,23], which could effectively separate light oil/water mix­ realizing effective separation diverse oil-in-water emulsions with
tures. Inspired by the “water-removing” model, some scholars have put a permeation flux superior to 133 L m− 2 h− 1 and rejection efficiency
lot of effort into fabricating underwater superoleophobic membrane to exceed 99% [36]. However, as-mentioned membranes generally suffer
efficiently separate oil/water mixtures [24,25]. Nevertheless, these from low emulsion permeability and contamination by high viscosity
membranes also could not separate oil-in-water emulsions due to the oil, which hinder their practical application prospects. Therefore, it is of
minor diameter of emulsified oil droplets. Recently, a variety of vital importance to prepare a membrane with strong stability and
“water-removing” materials with suitable pore sizes have been engi­ excellent anti-high-viscosity oil fouling ability to realize efficient and
neered to tackle oil-in-water emulsions. For instance, Ge and co-workers stable emulsion separation.
fabricated a microspheres/nanofibers composite membrane through In this work, a composite PVA-GO-TiO2 membrane was developed
electrospinning and electrospraying, which realized the efficient through simple hydrothermal method and suction-filtration. As shown
microscale oil-in-water emulsions separation [26]. Shao and co-workers in Scheme 1, the fabrication of the composite membrane should first
prepared a PAN based nanofiber membranes with a multi-hydrophilic construct the PVA-GO porous structure via hydrothermal stir, and then
cross-linked network for fast separating oil/water emulsions [27]. TiO2 nanoparticles are introduced on the PVA-GO porous structure
However, the practical applications of these membranes are hindered by through simple ultrasound method. Thanks to the self-gathering of TiO2
undesirable mechanical properties. As an excellent and convenient nanoparticles and its combination with GO in water, thereby forming a
method, coatings have been broadly utilized to obtain oil-in-water rough structure, which was more conducive to improve the underwater
emulsion separation membranes [28]. For example, Dai et al. pre­ superoleophobicity of membrane. Interestingly, the PGT membrane
pared a robust TA-Fe@PVDF nanocomposite membrane with efficient displays outstanding separation performance towards various oil-in-
oil-in-water emulsions separation performance via one-step soaking water emulsions. Moreover, the as-prepared membrane can maintain
technology [29], Xu et al. reported a stable CNTs/MPPM membrane underwater superoleophobicity after sanding test (200 cyclic times),
through vacuum filtration for separation oil/water emulsions [30]. folding test (500 times). More importantly, the PGT membrane can
However, these membranes easily lose their underwater super­ efficiently separate oil-in-water emulsions in corrosive environments.
oleophobicity due to oil contamination after separation. The stable composite membrane with efficient emulsion performance
Polyvinyl alcohol (PVA), a polymer with excellent water solubility shows a competitive advantage in the field of oily wastewater
and plasticity, contains a large number of hydroxyl groups, which can be remediation.
used to build a stable and desired porous structure [31]. In addition,
two-dimensional graphene oxide (GO) sheets with excellent mechanical 2. Experimental section
properties and chemical inertness have attracted continuous attention
[32,33]. Various oxygen-containing functional groups are located on its 2.1. Materials
extended layered structure, which make GO an interesting candidate for
fabricating underwater superoleophobic surfaces by combing with other Polyvinyl alcohol (PVA) (1750 polymerization degree) was pur­
materials [34]. For instance, Sun and co-workers prepared a photoin­ chased from Tianjin Guangfu Fine Chemical Research Institute. The TiO2
duced ZIF-8/GO membrane through layer-by-layer route, which showed nanoparticle was purchased from Shanghai Xingya Co., Ltd. Moreover,
permeation flux of 110 ± 6 L m− 2 h− 1 and separation efficiency excee­ the synthesis of the GO according to the Hummers’ method and pro­
ded 93% in emulsions separation [35]. Yan and co-workers developed a cessed in our lab [34]. In the process, natural graphite was the raw

Scheme 1. Fabrication and emulsions separation processes of PGT membrane.

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Q. Zhong et al. Journal of Membrane Science 640 (2021) 119836

material and purchased from Aladdin. The organic filter polyvinylidene the glass sheet in water. Sequentially, oil droplets (3 μL) were dropped
fluoride (PVDF) membrane (0.45 μm in aperture and 2.5 cm in radius) carefully from the bottom onto the membrane, respectively, and the
was obtained from Aladdin. Furthermore, A variety of oils involving final result was the average of three measurements. Crystal phase was
petroleum ether, n-hexane and kerosene were gained from Guangdong determined through X-ray diffraction meter (XRD, Rigaku Corp, D/max-
Guanghua Sci-Tech Co., Ltd. Crude oil was procured from Gansu Yu men 2400). The oil content was detected by infrared oil meter (SN-OIL480).
Oilfield Company. NaCl, NaOH and HCl were purchased from Shanghai The particle size distribution was measured by dynamic light scattering
Zhongqin Chemical Reagent Co., Ltd. Sodium dodecyl sulfate (SDS) was (DLS) measurement (Malvern Zetasizer Nano Zs).
originated from Shanghai Sinopharm Group Chemical Reagent Co, Ltd.
Deionized water was utilized as type of solvent for all solutions and 3. Result and discussion
above reagents were of analytical grade.
3.1. Surface morphology and chemical composition of PGT membrane
2.2. Fabrication of PGT membrane
As shown in Scheme 1, the PGT membrane is fabricated via simple
Firstly, the PVA homogeneous suspension was prepared through hydrothermal method and suction filtration. In the process, the intro­
adding 0.2 g PVA in 20 mL deionized water with stirring 5 h at 88 ◦ C, duction of TiO2 and GO was the key step, which formed the rough
and then the as-prepared PVA suspension was poured in 80 mL deion­ structure of the composite membrane for improving the underwater
ized water and continuously stirring 5 h at 88 ◦ C. Sequentially, 60 μL of superoleophobicity and fouling resistance.
10 mg/mL GO dispersion solution was dropped in as-prepared PVA Because the emulsified oil droplets possess a small particle size,
suspension, adding 20 mg of TiO2 nanoparticles in the mixed PVA/GO proper pore size distribution structure is the key for achieving effective
dispersion and then ultrasonically treated it for 1 h in the ultrasound emulsion separation. Herein, the corresponding microstructures of
system at lab temperature to form homogeneous PVA-GO-TiO2 suspen­ various membranes were obtained by FE-SEM. As shown in Fig. S2, the
sion. Afterwards, 3 mL of as-prepared PVA-GO-TiO2 suspension was surface pore size distribution of pristine PVDF membrane was uniform.
coated onto a PVDF membrane by a suction-filtration device under the Moreover, as shown in Fig. 1a1 and 1a2, on the surface of the PVA
pressure of 0.08 MPa, and the radius of coating was near 1.95 cm. In the membrane, the PVDF membrane was completely covered by a porous
end, this coated membrane has been maintained at lab temperature 24 h network structure, and its average pore size was about 1500 nm
to form steadily dried membrane. In addition, the PVA membrane and (Fig. S3). However, after coating with PVA-GO suspension, the porous
the PG membrane were prepared by coating 3 mL of PVA suspension and structure of PVA is wrapped by a thin coating, and the average pore size
3 mL of PVA-GO suspension on the PVDF membrane, respectively. of PG membrane surface is reduced to approximate 600 nm (Fig. S4).
The above phenomenon is caused by the strong hydrogen bond between
2.3. Preparation and separation of oil-in-water emulsions PVA and GO, which allows GO nanosheets to be evenly dispersed on the
PVA network structure (Fig. 1b1 and 1b2). [37]. Moreover, rough
Firstly, the volume ratio of water to various oils was maintained at micro-nanoscale structure is benefiting for improving the underwater
50:1, and the concentration of SDS surfactant was controlled at 0.2 mg/ superoleophobicity. In order to obtain the rough structure, TiO2 nano­
mL. The oil-in-water emulsion was prepared by intensive stirring the particles was introduced onto the previous PVA-GO porous structure. As
above mixture at 2500 rpm for 3 h, respectively. Furthermore, the sta­ shown in Fig. 1c1 and 1c2, when the PVDF membrane is coated with
bilities of various oil-in-water emulsions have been tested through PVA-GO-TiO2 suspension, the TiO2 nanoparticles are uniformly
standing experiment. As shown in Fig. S1, various oil-water emulsions agglomerated on the surface of the PGT membrane, and the average pore
can still maintain an emulsified state after two weeks, indicating that the size is further reduced to about 150 nm (Fig. S5) [38]. Furthermore, the
emulsion has excellent stability. Moreover, the separation of emulsions both AFM images and root-mean-square roughness (Rq) of the PVA, PG,
were performed by suction-filtration equipment (suction-filtration and PGT membranes have been measured, respectively. As shown in
pressure was kept at 0.75 bar), in which the membrane was sandwiched Fig. S6, the Rq of the PVA, PG and PGT membranes were 137 nm, 168
between the wide-mouth flask and suction flask. Herein, permeation nm and 264 nm, respectively. Clearly, the surface roughness of mem­
flux (J) was defined as equation (1): branes were enlarged gradually with the introduction of GO and TiO2.
Moreover, Fig. 1d shown the cross-section of the PGT membrane, and its
J=
V
(1) thickness was about 57.4 μm.
St In addition, the crystal structure and chemical bond types were
verified by XRD and FT-IR spectroscopy, respectively. As shown in
Where V (L) is the permeating volume; S (m2) presents the contact area
Fig. 1e, the characteristic peaks of pure TiO2 and PVA are consistent
between emulsions and membranes, t is the separation time. Further­
with the peak patterns of TiO2 and PVA in the PGT membrane [39,40].
more, the separation efficiency was figured up as following:
( ) However, the characteristic peaks of GO disappeared in the X-ray
Cp diffraction pattern of the PGT membrane, which may be caused by that
R= 1− × 100% (2)
Co GO nanosheets are totally exfoliated and dispersed in suspension [41].
Furthermore, the FT-IR spectroscopy of the PG coating and PGT coating
Where CP (mg/L) and Co (mg/L) represent the oil content after and were shown in Fig. 1f. A wider and stronger characteristic peak at 3430
before emulsions separation, respectively. cm− 1 was attributed to the stretching vibration band of –OH. The peak at
1725 cm− 1 was assigned to the bending vibration of C– – O. The peak at
2.4. Characterization 1110 cm− 1 was originated from the vibration absorption of C–O–C.
Moreover, the peaks at 2924 cm− 1, 1463 cm− 1 and 2853 cm− 1 were
Field-emission scanning electron microscope (Zeiss) （FE-SEM） corresponded to the -C-H stretching vibration, and the peak at 689 cm− 1
was conducted to observe microstructure. Image-Pro Plus Version 6.0 belongs to the TiO2. The characteristic peaks of PVA and GO were
was utilized to obtain the pore size of membranes. The surface chemical discovered in the FT-IR spectra of PG coating. Moreover, the charac­
compositions were illustrated by energy-dispersive spectrometry (EDS). teristic peaks of PVA, GO and TiO2 were corresponding to the FT-IR
Above optical photographs were taken on a mobile phone named vivo spectra of PGT coating after introducing TiO2. Furthermore, the
x27. The Contact angles (CAs) were surveyed on a contact angle (CA) Raman spectra of materials also were performed to study molecular
apparatus (SL200KB). The underwater oil CA measurement process is as structure of materials. As shown in Fig. S7, the Raman peak of PVA and
following: the membrane was first adhered on a glass, and then inversed GO were evidently appeared in the Roman spectra of PG membrane.

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Q. Zhong et al. Journal of Membrane Science 640 (2021) 119836

Fig. 1. FE-SEM images of (a1, a2) PVA membrane, (b1, b2) PG membrane and (c1, c2) PGT membrane at low and high magnification, respectively. (d) Cross-sectional
of the PGT membrane. (e) XRD patterns of TiO2, PVA membrane, GO and PGT membrane, respectively. (f) FT-IR of the PVA, TiO2, GO, PG coating and PGT coating.

Moreover, after the introduction TiO2, the Roman peaks of these three the air. In addition, as shown in Fig. 2a, the underwater crude oil CAs of
materials were clearly discovered in the PGT membrane. The result the PVA membrane and the PG membrane are closed to 135 ± 5◦ and
consisted with the mentioned FT-IR spectrum characterization. Besides, 150 ± 2.5◦ , and the underwater crude oil CA of the PGT membrane are
the elemental distribution of PVA, PG and PGT membranes surface was close to 154 ± 1.5◦ , respectively, indicating the underwater oleopho­
surveyed by EDS. As illustrated in Fig. S8a and 8b, C, O elements were bicity was gradually increased with introducing GO and TiO2 nano­
evenly found on the PVA and PG membrane surface. After introduction particles. In addition, the other types of underwater oil CAs on the PGT
TiO2 nanoparticles, Ti element also was evenly distributed on the PGT membrane are shown in Fig. 2b, and they are all greater than 150◦ .
membrane surface (Fig. S8g), which confirmed that the TiO2 was suc­ Indeed, the change of underwater crude oil CAs and oil-adhesion from
cessfully uniformly distributed onto the PGT membrane. In addition, the PVA membrane to PGT membrane were corresponded to Wenzel equa­
cross-sectional EDS analysis of the PGT membrane was tested. As shown tion (3) and Cassie equation (4) [43]:
in Fig. S9, interestingly, the cross-sectional EDS mapping images of the
cosθw = r(γ SL − γ SW )/γWV (3)
PGT membrane were consistent with its surface EDS mapping images,
which demonstrate that the PVA, GO and TiO2 are uniformly distributed γ OV⋅ cosθO − γ WV ⋅cosθW
on the cross-sectional section of the PGT membrane. The results of cosθow = (4)
γOW
as-mentioned characterization proven that the PVA, PG and PGT
membranes have been fabricated successfully. Where θW and r represent the water CA value and surface roughness of
membrane surface, respectively. γSV, γSW, γLV, γOV, γOW were the surface
3.2. Wetting and anti-oil fouling abilities tensions between material surface and air, material surface and water,
water and air, oil and air, and oil and water, respectively. Firstly, the GO
Underwater superoleophobicity and weak oil adhesion of membrane expanded the proportion of the hydrophilic part of the membrane, thus
materials could efficiently prevent pore-plugging from oil fouling [42]. increasing the γSW value, which further increased the CA of underwater
The surface wettabilities of membranes were studied by testing the CAs crude oil compared with the PVA membrane. In addition, by introducing
of water and oil in different enviroments, respectively. The oil/water TiO2 nanoparticles, the amount of water captured on the membrane
CAs in the air and the underwater oil CAs are shown in Fig. 2a, surface increased, which further expands the CA value of underwater
respectively. The CAs of water and oil on PVA membrane, PG membrane crude oil. The above results provide a good precondition for the PGT
and PGT membrane are all close to 0◦ , showing they are amphiphilic in membrane to efficient emulsion separation.

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Q. Zhong et al. Journal of Membrane Science 640 (2021) 119836

Fig. 2. (a) CAs of kerosene and water in air and crude oil CAs underwater of PVA, PG and PGT membrane. (b) Underwater various oil CAs on the PGT membrane. (c)
Visible anti-fouling processes of PGT membrane. (d) Dynamic underwater crude oil contacting behavior with the PGT membrane.

Furthermore, the anti-oil fouling abilities of PVA, PG and PGT of kerosene-in-water emulsion and crude oil-in-water emulsion of PGT
membrane were studied, respectively. As shown in Fig. S10, oil fouling membrane are shown in Video S4 (kerosene) and S5 (crude oil),
was obviously adhered on the surface of PVA membrane and PG mem­ respectively. The water can be fast permeated from the two kinds of
brane after above process. However, it is easy to remove the crude oil emulsions to the suction flask. Fig. 3a and d are the optical photographs
from the PGT membrane after the same process (Fig. 2c). Therefore, the of the separation process of above two kinds of emulsions. Moreover, as
PGT membrane has the weakest fouling propensity among all mem­ shown in Fig. 3c1 and 3f1, the filtrate completely turned into colorless
branes. Furthermore, the underwater crude oil dynamic contact be­ and transparent water from milk white and brown emulsions (Fig. 3b1,
haviors of PVA, PG and PGT membranes have been studied, respectively. 3e1), and there was no oil droplets in the filtrate (Fig. 3c2, 3f2). Inad­
As shown in Fig. 2d and Video S1, 3 μL of crude oil could be completely dition, the particle size distribution of emulsions before and after sep­
detached from the membrane after contacting with the PGT membrane. aration was also detected by DLS measurement. The particle size
Nevertheless, crude oil was adhered on PVA membrane or PG membrane distribution of the filtrate ranged from 400 to 1500 nm of the original
after contacting with underwater membrane surface (Fig. S11, Video S2 emulsion to 5–25 nm (Fig. 3c3, 3f3, 3b3, 3e3), which show that the oil
and S3). The above results demostrated that the PGT membrane has droplets were efficiently rejected by PGT membrane. For petroleum and
strongest oleophobicity underwater and lowest oil adhesion than the hexane-in-water emulsions, their oil droplets are also effectively rejec­
PVA and the PG membranes. ted by PGT membrane, respectively (Fig. S12). Furthermore, Fig. 4a and
Supplementary video related to this article can be found at https b showed the emulsions separation performance of PG membrane and
://doi.org/10.1016/j.memsci.2021.119836. PGT membrane, respectively. Interestingly, after introduction TiO2
nanoparticles, the separation efficiency of emulsions to PGT membrane
3.3. Emulsions separation performance are reached 99.3–99.7% (Fig. 4b), this result was obviously better than
that of PG membrane (Fig. 4a). This is because the PGT membrane has a
The above discussions show the PGT membrane has lowest oil suitable pore size and stronger underwater superoleophobicity, which
adhesion than PVA and PG membranes. Herein, the separation processes more effectively prevented oil from passing through the membrane [44].
of various oil-in-water emulsions have been performed on these three Importantly, the permeation flux on PGT membrane was 810–1083 L
kinds of membranes, respectively. In addition, the separation processes m− 2 h− 1, which closed to that of the PG membrane. Although the pore

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Q. Zhong et al. Journal of Membrane Science 640 (2021) 119836

Fig. 3. Separating processes photographs, optical photographs and particle size distribution of (a-c3) kerosene-in-water emulsion and (d-f3) crude oil-in-
water emulsion.

size of the PG membrane is bigger than that of PGT membrane, the shown in S13b, 13c and 13d, the whole separation performance of PVA
permeation flux decline of PG membrane for oil-in-water emulsions membrane was particularly poor than that of PG membrane, which show
separation was faster than latter. When PG membrane was used to the introduction of GO also enhanced the separation performance of
separate kerosene-in-water emulsion, although the PG membrane has a membrane.
bigger water flux in the first few seconds because of the larger pore size, Supplementary video related to this article can be found at https
its permeation trend was obviously slow down. However, the perme­ ://doi.org/10.1016/j.memsci.2021.119836.
ation flux was stable when separation process performed on the PGT
membrane. This phenomenon was attributed to the stronger anti-fouling 3.4. Chemical and mechanical stabilities
ability of the PGT membrane. To further verify above result, the
continuous separation process of kerosene-in-water emulsion was con­ Anti-corrosion ability of membrane plays decisive role in practical
ducted on the PG membrane and the PGT membrane, respectively. As application. Herein, the underwater oils CAs of various on the PGT
demonstrated in Fig. 4c, the flux decline rate in continuous separation membrane after immersion in corrosive environments have been
process of kerosene-in-water emulsion of PGT membrane was less than measured. As shown in Fig. 5a and b, the microstructure of the PGT
that of PG membrane, showing the emulsions separation performance membranes are almost unchanged after immersion it in 1 M HCl and
was improved by the introduction of TiO2 nanoparticles. As for the cy­ 3.5%wt NaCl solutions for 24 h, respectively. However, as shown in
clic stability of emulsions separation, as shown in Fig. 4d, the PGT Fig. 5c, after immersion in 1 M NaOH for 24 h, the microstructure dis­
membrane possess stable separation performance for kerosene-in-water appeared because TiO2 was corroded by NaOH. However, as shown in
emulsion. However, the cyclic stability of emulsions separation of PG Fig. S14, the surface of PGT membrane also has Ti element, indicating
membrane was obviously weak than that of PGT membrane (Fig. S13a). that the TiO2 was not completely corroded. Moreover, after the PGT
Furthermore, in order to study the role of GO on emulsions separation membranes were immersed in above-mentioned HCl and NaCl solution,
performance of membrane, the emulsions separation performance, respectively, its underwater oil CAs is kept above 150◦ (Fig. 5d and e),
continuous separation process and cyclic stability of kerosene-in-water while slightly small than 150◦ for 1 M NaOH (Fig. 5f). Although the
emulsion of PVA membrane has been performed, respectively. As underwater oil CAs of PGT membrane are slightly decreased, which

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Q. Zhong et al. Journal of Membrane Science 640 (2021) 119836

Fig. 4. Emulsions separation performances of (a) PG membrane (b) PGT membrane. (c) The permeation flux variations on PG membrane and PGT membrane for
kerosene-in-water emulsion within 10 min, respectively. (d) Cyclic separating stability of PGT membrane about kerosene-in-water emulsion.

maintained the underwater oleophobicity. The result confirmed that the resistance. Moving it circularly on a sandpaper within 28 cm, surpris­
PGT membrane exhibits excellent corrosion resistance, thereby ensuring ingly, the PGT membrane can maintain the underwater super­
that it can be used in harsh corrosive environments. Therefore, several oleophobicity after 200 times, which obviously shown that the PGT
oil-in-water emulsions separation processes in corrosive environments membrane has excellent anti-abrasion ability. Moreover, due to the
were performed on PGT mmebrane. Herein, crude oil and kerosene are swelling effect, long-term stability of membrane in water is essential. As
used as the oil phases, and 1 M HCl, 1 M NaOH, and 3.5 wt% NaCl so­ illustrated in Fig. 7b, the PGT coating still did not fall off after being
lutions are taken as corrosive environments, respectively. As shown in immersed in water for one week. In addition, as displayed in Fig. 7c, no
Fig. 6, after separation, the oil was completely removed from milky detachment or cracking occurred on the surface of the membrane after
kerosene-in-HCl emulsion and brown crude-in-HCl emulsion (Fig. 6a3, being folded 500 times, which proved that the PGT membrane has
b3), and oil droplets cannot be detected in the filtrate (Fig. 6a4, 6b4). robust mechanical stability. Furthermore, a large amount of oily
Moreover, the particle size distribution was decreased from 500 to 2000 wastewater in life usually is not neutral, so there is a requisite to study
nm of kerosene/crude oil-in-HCl emulsion to 10–40 nm (Fig. 6a1, 6a5, the stability of the separation membranes in a corrosive environment
6b1 and 6b5). Similarly, the separation results of other emulsions (acid, alkali, and salt). In addition, abrasion resistance is also the key to
(kerosene in NaOH, kerosene in NaCl, crude oil in NaOH and crude oil in long-term separation of the separation membranes. As exhibited in
NaCl) are also shown in Fig. S15. Furthermore, as shown in Fig. 6c and d, Table 1, comparison with others separation membrane, the PGT mem­
for the corrosive kerosene-in-water emulsions, the separation efficiency brane exhibited excellent abrasion resistance and corrosion resistance,
was superior to 99.1% and permeation flux was more than 903 L h− 1 but most membranes cannot achieve the above two conditions at the
m− 2, respectively. For crude oil-in- corrosive water emulsions, the PGT same time. Even if there are membranes that have the above two
membrane showd rejection efficiency of larger than 99.3% and filtrate properties at the same time, their separation performances were not as
flux over than 760 L h− 1 m− 2, respectively. good as PGT membrane.
Mechanical stability of PGT membrane is important for withstand
harsh realistic environment. Therefore, the abrasion and folding ex­ 4. Conclusion
periments of PGT membrane have been carried out, respectively. As
shown in Fig. 7a. Putted the PGT membrane on 800 mesh sandpaper, In summary, a novel composite PGT membrane with underwater
and then used a weight of 200 g as an external force to test its abrasion superoleophobicity has been fabricated through simple hydrothermal

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Q. Zhong et al. Journal of Membrane Science 640 (2021) 119836

Fig. 5. SEM images of the PGT membrane after soaking in (a) 1 M HCl, (b) 3.5 % wt NaCl and (c) 1 M NaOH solutions. Underwater oil CAs of the PGT membrane
after soaking in (d) 1 M HCl, (e) 3.5 % wt NaCl and (f) 1 M NaOH solutions.

Fig. 6. (a, b) Optical photographs and particle size distribution of two kinds of oil-in-HCl emulsions and filtrates. Separation performances of PGT membrane for (c)
crude oil-in-corrosive water emulsions and (d) kerosene-in-corrosive water emulsions, respectively.

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Q. Zhong et al. Journal of Membrane Science 640 (2021) 119836

Fig. 7. Optical photographs of (a) sanding test, (b) water-soaking test and (c) bending test for the PGT membrane, respectively.

Table 1
Comparison of various materials on crude oil-in-water emulsion separation.
Materials Fabricated method Droplets Size Corrosion Abrasion Efficiency Flux Ref.
(nm) resistance resistance (%) (L⋅m− 2⋅h− 1)

Brick powders coated PVDF Suction-filtration 100–600 Yes No 99.8 623 (0.85 bar) [45]
membrane
Egg shell coated PVDF Suction-filtration 200–1000 No No 99.7 229 (0.85 bar) [9]
membrane
Zwitterionic nanofibrous Electrospining, microemulsion 0–2800 No No 99.9 600 (Gravity) [46]
membrane polymerization
TiO2 coated stainless mesh Spraying 160–620 Yes Yes 99.8 159 (0.85 bar) [47]
Cu2þ/Alginate modified PVDF Layer-by-layer self-assembly 800–2000 Yes No 99.8 1230 (1 bar) [48]
membrane
PVA modified PVDF membrane Soaking, γ-ray irradiation 4–12 No No 94.2 360 (1 bar) [49]
Cupric phosphate modified Layer-by-layer self-assembly 100–1000 Yes No 95 1471 (1 bar) [50]
PVDF membrane
DA/APTES modified polymer Soaking 100 Yes No 99.8 71 (1 bar) [51]
membrane
PVA-GO-TiO2 coated PVDF Hydrothermal, Suction-filtration 0–2500 Yes Yes 99.7 810 (0.75 bar) This
membrane work

Abbreviation: DA: dopamine, APTES: aminopropyltriethoxysilane.

method and suction-filtration. The PGT membrane shows outstanding membrane has great advantages in crude oil emulsion separation. Based
separation performance for several kinds of oil-in-water emulsions on the as-mentioned preferred characteristics of the PGT membrane, it
under both neutral and corrosive environments. Moreover, the PGT has shown a perspective utility for oily-water remediation in practical
membrane has excellent mechanical stability, which is proven by the applications.
sanding tests (200 times) and the folding tests (500 times). Finally, we
made a comparison among the PGT membrane and others membranes Author statement
for separating crude oil emulsions in terms of mechanical stability,
corrosion resistance and separation performance. As a result, PGT Author contribution: Qi Zhong: Data curation, Investigation, Formal

9
Q. Zhong et al. Journal of Membrane Science 640 (2021) 119836

analysis, Methodology, Writing-original draft, Writing-review & editing; [15] H. Li, P. Mu, J. Li, Q. Wang, Inverse desert beetle-like ZIF-8/PAN composite
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Guogui Shi: Investigation, Formal analysis, Writing–review; Investiga­
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tion, Writing–review; Qing Sun:Investigation; Peng Mu: Formal analysis, [16] W. Liu, X. Bai, Y. Shen, P. Mu, Y. Yang, J. Li, Efficient separation of free organic
Supervision, Writing - review & editing. Jian Li: Formal analysis, liquid mixtures based on underliquid superlyophobic coconut shell coated meshes,
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China (161044), the Natural Science Foundation for Distinguished
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Research Fund of the Key Laboratory of Marine Materials and Related 795–803.
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