water separation and excellent antifouling properties

Journal Pre-proof Dual Super-Amphiphilic Modified Cellulose Acetate Nanofiber Membranes with Highly Efficient Oil/Water Separation and Excellent Antifouling Properties Weiwen Wang, Jixin Lin, Jiaqi Cheng, Zhixiang Cui, Junhui Si, Qianting Wang, Xiangfang Peng, Lih-Sheng Turng

PII:

S0304-3894(19)31536-5

DOI:

https://doi.org/10.1016/j.jhazmat.2019.121582

Reference:

HAZMAT 121582

To appear in:

Journal of Hazardous Materials

Received Date:

2 September 2019

Revised Date:

15 October 2019

Accepted Date:

31 October 2019

Please cite this article as: Wang W, Lin J, Cheng J, Cui Z, Si J, Wang Q, Peng X, Turng L-Sheng, Dual Super-Amphiphilic Modified Cellulose Acetate Nanofiber Membranes with Highly Efficient Oil/Water Separation and Excellent Antifouling Properties, Journal of Hazardous Materials (2019), doi: https://doi.org/10.1016/j.jhazmat.2019.121582

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Dual Super-Amphiphilic Modified Cellulose Acetate Nanofiber Membranes with Highly Efficient Oil/Water Separation and Excellent Antifouling Properties Weiwen Wang1, 2, Jixin Lin1, 2, Jiaqi Cheng1, 2, Zhixiang Cui1, 2, Junhui Si1, 2, Qianting Wang1, 2, Xiangfang Peng1, 2, and Lih-Sheng Turng3, 4* 1

Authors to whom correspondence should be addressed.

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School of Materials Science and Engineering, Fujian University of Technology, Fujian, 350118, China 2 Fujian Provincial Key Laboratory in the Universities of Polymer Materials and Production, Fujian, 350118, China 3 Wisconsin Institutes for Discovery, University of Wisconsin–Madison, Madison, Wisconsin 53715, United States 4 Department of Mechanical Engineering, University of Wisconsin–Madison, Madison, Wisconsin 53706, United States

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

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Graphical abstract

Highlights

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

A for

new

type

of

multifunctional

d-CA nanofiber

membranes

oil/water separation was successfully fabricated by electrospinning.



It can be used as water-removing and emulsion separation substance under harsh environment only by gravity driving force.



It possesses the highest separation flux of 38,000 L/m2·h, and the highest

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separation efficiency of 99.97%. The excellent anti-pollution and self-cleaning abilities endow the

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membrane with powerful cyclic stability and reusability.

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Abstract

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Along with increasing oily, industrial wastewater and seawater pollution, oil spills—and their clean-up via the separation of oil and water—are still a worldwide

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challenge. Aiming to fabricate an oil/water separation membrane with excellent comprehensive performance, we report here a new type of multifunctional

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deacetylated cellulose acetate (d-CA) membrane. The cellulose acetate (CA)

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nanofiber membranes are fabricated by electrospinning and then deacetylated to obtain the d-CA nanofiber membranes, which are super-amphiphilic in air, oleophobic in water, and super-hydrophilic in oil. The multifunctional d-CA nanofiber membranes can be used as water-removal substances for oil/water mixtures, as well as emulsified oil/water and oil/corrosive aqueous systems, with gravity as the only needed driving force. The d-CA nanofiber membranes possess the highest separation

flux, reaching up to 38,000 L/m2·h, and the highest separation efficiency, reaching up to 99.97% for chloroform/water mixtures under the force of gravity. In fact, the separation flux was several times higher than that of commercial CA (c-CA) membranes. The excellent anti-pollution and self-cleaning abilities endow the membranes with powerful cyclic stability and reusability. The d-CA nanofiber membranes show great application prospects in chemical plants, textile mills, and the

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food industry, as well as offshore oil spills, to separate oil from water.

Keywords: Oil/water separation; Deacetylated cellulose acetate; Nanofiber

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membrane; Antifouling; Superamphiphilic

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1. Introduction

With our fast-developing economy, chemical plants, textile mills, and food

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companies have increased in number dramatically. Likewise, the environmental

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pollution that comes with them has intensified. In particular, the problem of oily industrial wastewater and seawater pollution is becoming more and more serious,

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making the separation of immiscible solutions represented by oil/water separation a major challenge worldwide [1]. Therefore, it is particularly important to develop

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effective and low-cost solutions to remedy the large issue of oil/water contamination.

Traditional oil/water separation methods, such as gravity, coalescence,

centrifugation, flotation, and so on, have the disadvantages of poor separation efficiency, high energy costs, and being time-consuming [2-3]. In contrast, membrane separation technologies possess the advantages of low energy consumption and

excellent separation efficiency. Therefore, they have received increased attention and have been widely used to treat oil-contaminated wastewater in recent years. Inspired by the "self-cleaning lotus leaf" [4-7], numerous oil/water separation functional membranes with super-hydrophobic surfaces were prepared and studied. Most of them showed hydrophobic and lipophilic properties and could be used as oil-removing materials [8-13]. However, these types of membranes have some

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obvious drawbacks. First, due to their lipophilic nature, their pores are easily polluted

or even blocked by oil, which affects their separation ability and limits their reusability. Second, since the densities of most oily substances are less than that of

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water, water is easily deposited on the surface of the membranes to form a water

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membrane between the oil phase and the membranes during the separation process, thereby blocking contact between the membranes and the oil phase such that

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separation cannot continue. To overcome this water membrane limitation, additional

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pressure is required, resulting in higher energy consumption.

Due to the defects of hydrophobic/lipophilic membranes, researchers have

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developed water-removal types of materials through reverse thinking inspired by the

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"fish scale effect" [14]. This type of material is hydrophilic in air, and oleophobic underwater [15-19], thereby allowing the aqueous phase to pass through freely without obstructing the oil phase. These materials have solved various defects of oil-removing materials and have attracted wide attention in the oil/water separation field. To date, various materials have been developed such as hydrogels [18, 20], polymer membranes [21-25], nitrocellulose [26], zeolite molecular sieves [27], silica

[28], TiO2 [29], ZnO [30], Cu(OH)2 [31, 32], and graphene oxide [33, 34]. However, the membranes with underwater oleophobic properties are not the ideal option for heavy oil/water separation because the heavy oil is easily deposited on the membrane surface to form a barrier during the separation process, thereby preventing water penetration. Therefore, it is well-known that membranes with a single wetting property can only be used to separate a defined oil/water mixture. As mentioned

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above, super-hydrophobicity/super-oleophilicity membranes are only appropriate for

separating heavy oil/water mixtures, while the super-hydrophilicity/oleophobicity membranes and underwater oleophobicity membranes are only appropriate for

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separating light oil/water mixtures.

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Dual functional membranes able to continually separate both heavy oil/water and

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light oil/water mixtures can be achieved by considering the existence of materials that are simultaneously super-amphiphilic in air, oleophobic underwater, and hydrophobic

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under oil. Inspired by the pitcher plant [35], the researchers proposed a material that was super-amphiphilic in air with dual super-lyophobic action in oil/water systems

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[36, 37]. As far as we know, the currently reported articles on such materials have

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rarely been used for emulsion separation driven solely by gravity. Meanwhile, their preparation methods are often complicated. In addition, most of the studies focused on separating a pure oil/water mixture without consideration of the harsh environmental conditions. In fact, most oil/water mixtures generated from the chemical industry are corrosive aqueous liquids including strong acids or strong alkalis. If the separation membranes could withstand harsh environmental conditions, their reusability and

service life would be significantly enhanced. To date, only a few reports about the separation of oil/corrosive aqueous mixtures have been issued. Therefore, it has become an urgent goal to develop a dually functional ideal material that can efficiently separate heavy oil/water mixtures, light oil/water mixtures, oil/corrosive aqueous solutions, and emulsified oil/water systems via a simple and economic

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method.

In recent years, cellulose acetate (CA) has been wildly used in the oil/water separation field because of its abundant sources, environment friendliness, and

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biodegradability [38-41]. Aiming to fabricate an oil/water separation membrane with

excellent comprehensive performance, we report here a deacetylated cellulose acetate

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(d-CA ) membrane that has the characteristics of being super-amphiphilic in air,

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oleophobic underwater, and super-hydrophilic under oil. The d-CA membrane cannot only separate pure oil/water and emulsified oil/water mixtures, it can also separate

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oil/corrosive aqueous systems under the driving force of gravity alone. Moreover, the d-CA membrane has the advantages of a high separation flux, high separation

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efficiency, simple preparation process, abundant resources, and good chemical

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stability, as well as being environmentally friendly and inexpensive. Due to its under-oil super-hydrophilic property, it has strong anti-pollution and excellent self-cleaning abilities, resulting in powerful reusability. Therefore, the d-CA membrane has broad and diverse application prospects for chemical plants, textile mills, and the food industry, among others, to separate oil/water mixtures.

2. Experimental Methods

2.1 Materials

Cellulose diacetate (CA) with 39.8 wt% acetyl and 3.5 wt% hydroxyl was obtained from Shanghai Aladdin Biochemical Technology Co., Ltd., China. N, N-dimethylacetamide (DMAC, ≥99.5%) and acetone (CH3COCH3, ≥99.5%) were obtained from Tianjin Zhiyuan Chemical Reagent Co., Ltd., China. Sodium hydroxide

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(NaOH, ≥96%) was purchased from Xiqiao Chemical Co., Ltd, China. All chemical solvents were analytical reagents and were used as received.

2.2 Preparation of the CA Nanofiber Membrane

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The CA powder (3.4 g) was put into 20 mL of an acetone/DMAC (v/v, 2/1)

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mixed solvent and then oscillated for 24 h at 60°C using a water bath oscillator to obtain a homogeneous 17 wt% CA electrospinning solution. The CA nanofiber

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membranes were fabricated by an electrospinning instrument (Shenzhen Tongli Micro

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& Nano Technology Co. Ltd, China). During electrospinning, the needle inner diameter, electrospinning voltage, collection distance, injection rate, electrospinning

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time, environmental temperature, and humidity were 0.52 mm, 18 kV, 15 cm, 1 mL/h,

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7 h, 25°C, and 40%, respectively.

2.3 Preparation of the d-CA Nanofiber Membrane

The d-CA membranes were obtained by simply immersing CA nanofiber

membranes in 100 mL of a 0.5 M/L NaOH solution and deacetylating for 3 h. Then the d-CA membranes were taken out and washed 3 times using deionized water and dried by using vacuum oven.

2.4 Characterization

SEM images were obtained from field emission SEM (Nova Nano450, USA) at an accelerating voltage of 15 kV. The FTIR spectrum was measured by a Thermo Scientific NICOLET 6700. The water contact angle (WCA) and oil contact angle (OCA) in air and under liquid (water or oil) were measured on a DSA25 machine

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(Germany). The water and oil droplets were 5 μL. The separation efficiency was determined by a chemical oxygen demand (COD) digestion apparatus (DRB200,

HACH, USA) and a COD meter (DR900, HACH, USA). All macrograph images were

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obtained from a Canon camera (EOS-60D).

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2.5 Oil/Water Mixture Separation

The oil/water mixture separation was performed by a lab-scale cross-flow

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filtration instrument with a filtration area of 0.5 cm2. The effective filtration area of

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the nanofiber membrane was 3.14 cm2. Fifty mL of an oil/water mixture (v/v, 1:1) was obtained by mixing 25 mL of oil colored with Oil Red O and 25 mL of water colored

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with methylene blue. In this study, kerosene, n-hexane, petroleum ether, toluene, peanut oil, chloroform, and carbon tetrachloride were applied to evaluate the

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separation flux and separation efficiency of the d-CA nanofiber membranes. All of the filtration operations were carried out solely using the force of gravity.

2.6 Oil/water Emulsion Separation

A series of oil/water emulsions were used as feed solutions to assess the separation performance of the d-CA membrane. To prepare the emulsion, 0.1 mL of

span80 was added to 100 mL of an oil/water mixture (v/v, 100/1) under mechanical stirring for 2 h and then ultrasonicated for 2 h. The prepared oil/water emulsion was allowed to rest for 24 h, and no demulsification was observed. All of the filtration operations were carried out using only the driving force of gravity.

The separation flux (F) of the nanofiber membrane was calculated using

F 

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Equation (1),

V

(1)

ST

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where V was the volume of water or oil that passed through the membrane, S was the effective area of the membrane, and T was the time required for fixed liquid

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penetration.

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The separation efficiency (η) of the membrane was calculated according to Equation (2),

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𝐶

𝜂 = (1 − 𝐶1 ) × 100 0

(2)

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where C0 and C1 were the COD values of the repelling liquid phase before and after the separation process, respectively.

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2.7 Corrosive Solution Separation An oil/water corrosive solution was used to evaluate the chemical stability and

corrosion resistance properties of the d-CA nanofiber membrane. A quantity of 100 mL of corrosive solution were prepared by mixing 50 mL of petroleum ether with 50 mL of 1 mol/L HCl, 1 mol/L NaOH, and 1 mol/L NaCl. The separation was performed

by a lab-scale cross-flow filtration instrument. All of the filtration operations were carried out solely under the driving force of gravity.

3. Results and Discussions 3.1 Preparation of d-CA Nanofiber Membranes The CA nanofiber membranes were first fabricated using the electrospinning technique (Figure 1 (a)). Then they were simply treated by immersing them into a

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weak alkali-NaOH solution to obtain the d-CA nanofiber membranes (Figure 1 (b)). After deacetylation, most of the ester groups in the CA molecule (Figure 1 (a))

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transferred into the hydroxyl group of the d-CA molecule (Figure 1 (b)). The d-CA nanofiber membrane showed super-amphiphilic properties in air due to the

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co-existence of a hydrophilic group (hydroxyl) and a lipophilic group (ester) in air.

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Therefore, when the water density is greater than the oil density (ρwater > ρoil), the d-CA nanofiber membrane can be prewetted by water to selectively remove water

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from the oil/water mixture (Figure 1 (c)). When the water density is less than the oil density (ρwater < ρoil), the d-CA nanofiber membrane can be prewetted by oil to

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selectively remove oil from the oil/water mixture (Figure 1 (c)). The d-CA nanofiber

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can also be used to separate emulsified oil/water (Figure 1 (d)). The separation process is completed solely under the force of gravity. To study the influence of the deacetylation time on the properties of the CA nanofiber membrane, different deacetylation times of 1, 2, and 3 h were used and are referred to as d-CA/1 h, d-CA/2 h and d-CA/3 h, respectively. If not otherwise specified, d-CA/3 h is referred to as d-CA in the following text.

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Figure 1. (a) Fabrication of a CA nanofiber membrane by electrospinning. (b)

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Fabrication of a d-CA nanofiber membrane by deacetylating. (c) Schematic diagram

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separation of emulsified oil/water.

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of the selective separation of an oil/water mixture. (d) Schematic diagram of the

The morphology and diameter distribution of CA nanofiber membranes before

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and after deacetylating for 3 h were studied using FESEM and are shown in Figure 2. Both CA and d-CA membranes show good nanofiber morphology, while the average

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diameter of d-CA membranes was slightly larger than that of CA membranes resulting

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from the swell of CA nanofibers during the deacetylation process. The average diameter, porosity, and water absorption of CA nanofiber membranes increased with increasing CA solution concentration (Figure S1 (a)–(f)). The FTIR spectra of CA and d-CA nanofiber membranes under different deacetylation times are illustrated in Figure 2 (e, f). For the CA nanofiber membrane, the peaks at 1740 cm-1 and 1234 cm-1 can be attributed to C=O and C–O–C, respectively. After deacetylation for 1, 2, and 3

h, for d-CA/1 h, d-CA/2 h, and d-CA/3 h nanofiber membranes, the peak at 1740 cm-1, which was attributed to C=O, became weak (Figure 2(f)). However, a new peak at 3394 cm-1 attributed to OH appeared, indicating that the CA nanofiber membranes were successfully deacetylated [42]. The deacetylation degree (DD%) of d-CA nanofiber membranes under different deacetylation times is provided in Figure S2. The DD% of nanofiber membrane deacetylation for 1, 2, and 3 h were 8.9%, 29.3%,

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and 34.8%, respectively, thereby indicating that the DD% increased with increasing deacetylation time. It is believed that the higher the DD% generated, the more

a

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b

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hydroxyl groups that are present.

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10 μm

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c

d

10 μm

f

e

Figure 2. (a, b) SEM images and (b, c) diameter distribution of CA nanofiber

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membranes (a, c) before and (b, d) after deacetylating for 3 h. (e) FTIR spectra of CA and d-CA nanofiber membranes under different deacetylation times. (f) Partial

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enlargement of (e).

3.2 Characterization of d-CA Nanofiber Membrane Wettability

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Figure 3 shows the wettability properties of the CA and d-CA nanofiber

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membranes. The water contact angle (WCA) and oil contact angle (OCA) in air and in liquid (water or oil) were evaluated. The CA nanofiber membrane exhibited

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hydrophobic (127°) and super-oleophilic properties, with the OCA sharply decreasing to 0° within 0.11 s in air, while showing amphiphobic properties with a WCA of 137°

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and OCA of 126° under water and oil (Figure 2 (a, b)). However, the d-CA nanofiber

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membrane exhibited super-amphiphilic (super-hydrophilic and super-oleophilic) properties when the WCA sharply decreased to 0° within 0.0019 s, and the OCA sharply decreased to 0° within 0.000396 s in air when water and petroleum droplets dripped onto the surface of the membrane (Figure 3 (c)). The WCA of water in toluene was 0° and the OCA of toluene in water was about 130° (Figure 3 (d)), thereby implying oleophobic properties under water and super-hydrophilic properties

under oil. Compared with the d-CA nanofiber membrane, the commercial CA membrane (c-CA, purchased from Shanghai Xingya Co., Ltd, China, diameter = 47 mm) with a porous structure (Figure S3 (a)) had lower super-amphiphilic properties, with the WCA sharply decreasing to 0° within 2.63 s and the OCA sharply decreasing to 0° within 0.38 s in air (Figure S3(c)), and also showed super-amphiphobic properties under water and oil (Figure S3 (d)). Figure 3 (e) provides the OCAs of the

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d-CA membrane to various oils in water. All OCAs in water were greater than 130°, with the peanut oil reaching up to 154°, further indicating its excellent oleophobic properties in water. For the same oil, the OCAs of the d-CA nanofiber membrane in

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water were higher than those of the c-CA membrane (Figure S3 (b)). These results

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indicate that the d-CA nanofiber membrane prepared in this study had better super-amphiphilic properties in air and had better anti-pollution due to its

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super-hydrophilic properties under oil in comparison with that of the c-CA membrane.

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The super-amphiphilic properties in air of the d-CA nanofiber membrane improved with increasing deacetylation time (Figure S4 (a), (c)). Compared with the d-CA/3 h

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nanofiber membrane with oleophobic properties under water and super-hydrophilic properties under oil, the d-CA/1 h and d-CA/2 h nanofiber membranes showed

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amphiphobic properties under water and oil (Figure S4 (b), (d)), implying that the wettability properties of the CA nanofiber membrane could be easily controlled by simply adjusting the deacetylation time to meet different requirements for oil/water separation. The oleophobic properties under water of the d-CA nanofiber membrane improved with increasing deacetylation time (Figure S4 (e), (f)) due to the increase of

the hydroxyl group. The effect of the surface morphology of the membrane on its wettability properties were also evaluated based on the 3-D d-CA block with a porous structure (Figure S5 (a)) and the d-CA membrane with a smooth solid surface (Figure S5 (c)) fabricated by the solution casting method (see the detailed preparation process in the supporting materials). The 3-D porous d-CA block showed super-amphiphilic properties (Figure S5 (b)), while the solid d-CA membrane showed super-lipophilicity

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and hydrophilicity (Figure S5 (d)) in air, indicating that the wettability properties of

the materials were governed by the chemical structure, surface morphology, and

a water

b

oil

WCA in oil

e

137°

0.11s

126°

0.0019s

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d WCA in oil

c

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OCA in water

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internal structure [43].

0° OCA in water

130°

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0.000396s

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Figure 3. (a) WCA and OCA (petroleum ether) in air for the CA nanofiber membrane. (b) WCA in petroleum ether and OCA (petroleum ether) in water for the CA nanofiber

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membrane. (c) WCA and OCA (petroleum ether) in air for the d-CA nanofiber membrane. (d) WCA in petroleum ether and OCA (petroleum ether) in water for the d-CA nanofiber membrane. (e) OCA of various oils in water for the d-CA nanofiber membrane. 3.3 Oil/Water Mixture and Oil/Water Emulsion Separation

Figure 4 (a) provides the separation flux of d-CA and c-CA membranes for various oil/water mixtures under only the force of gravity. For the d-CA membrane, the water removal flux and oil removal flux reached up to 29,000 L/m2·h and 38,000 L/m2·h, respectively. The separation flux of the d-CA membrane increased significantly in comparison with that of the c-CA membrane. It is worth noting that the separation flux of the d-CA membrane was 12 times higher than that of the c-CA

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membrane for water/peanut oil separation. This result indicates that the d-CA nanofiber membrane had an excellent separation flux under only gravity. In addition,

the larger the solution concentration was, the higher the porosity of the d-CA

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nanofiber membrane was, resulting in a higher separation flux (Figure S6). Taking a

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petroleum ether/water mixture as an example, the separation flux at different separation cycles for c-CA and d-CA membranes are provided in Figure 4 (b). For the

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c-CA membrane, the separation flux sharply decreased from 1000 L/m2·h to nearly

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zero after being recycled 5 times due to oil pollution of the membrane. However, for the d-CA membrane, the separation flux showed no obvious changes and still kept a

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high-value of about 22,000 L/m2·h, even after being used 25 times. This indicates that the separation flux of the d-CA membrane can be 100% recovered by simply washing

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with water due to its remarkable self-cleaning ability and strong anti-pollution property; thus, it can be easily reused.

Compared with the c-CA membrane, the separation efficiency of the d-CA nanofiber membrane increased (Figure 4 (c)). The separation efficiency of the d-CA nanofiber membrane was more than 99.5% for both water and oil removal, and was

even as high as 99.97% for water/heavy oil mixtures, thus exhibiting prominent separation properties. Taking the petroleum ether/water mixture as an example, the separation cycle effects on the separation efficiency of the d-CA and c-CA membranes are shown in Figure 4 (d). The separation efficiency of c-CA was about 98.5% and showed no obvious changes with an increase of separation cycles. However, it is interesting to note that the separation efficiency of d-CA increased from 98.5% to

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99.8% with an increase in cycle times, and still retained a high separation efficiency of 99.8% after 50 cycles. This indicates that the d-CA nanofiber membrane had

remarkable separation efficiency and good recyclability. Moreover, the compre-

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hensive properties of the d-CA nanofiber membranes fabricated in this study for

50000

a

Water-removing 30000 20000

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10000 0

Oil-removing

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2 Flux (L/m ·h)

40000

c-CA d-CA

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membranes, as shown in Table 1.

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separating oil/water mixtures are better than most of the traditional nanofibrous

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ne ne ne oil rm ide her ose -hexa m et Tolue eanut lorofo chlor ker a N oleu h P r t C e r nt Pet rbo Ca

b

100

c-CA

102 Water-removing

d-CA

Oil-removing

99 98 97 96 95 94

d

Separation efficiency (%)

c

Separation efficiency (%)

101

c-CA Petroleum ether as an example d-CA

100

ne ne ne oil rm ide her ose -hexa m et Tolue eanut lorofo chlor u ker a N ole h P r t C e r nt Pet rbo Ca

98 96 94 92 90 1

5

10 15 20 25 30 35 40 45 50

Separation cycles

Figure 4. (a) Separation flux of c-CA and d-CA membranes for different oil/water

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mixtures. (b) Separation flux of petroleum ether/water at different separation cycles of c-CA and d-CA membranes. (c) Separation efficiency of c-CA and d-CA membranes for different oil/water mixtures. (d) The separation efficiency of petroleum ether/water

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at different separation cycles of c-CA and d-CA membranes.

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Figure 5 (a) provides the optical photographs and optical microscopic images of

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the emulsion (water/peanut oil) before and after passing through the d-CA nanofiber membrane. The emulsion looks like a milk-like solution and there were many oil

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droplets, with diameters ranging from hundreds of nanometers to several micrometers, that were clearly observable. After filtration, the oil droplets disappeared from the

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microscopic image and the corresponding filtrate became completely transparent. This

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result confirmed that the oil droplets in the emulsion were effectively retained by the d-CA nanofiber membrane. Figure 5 (b) and (c) show photographs of the emulsion filtrate and the separation efficiency of the d-CA nanofiber membrane for oil/water emulsion under different cycle times. It is interesting to note that the transparency of the filtrate improved as the number of cycles increased. Furthermore, the separation efficiency of the d-CA membrane for the oil/water emulsion increased from 86.3% to

96.9% as the number of cycles increased, and it continued to retain its high separation efficiency after 7 cycles. This may be attributed to the fact that small amounts of oil droplets stack on the surface of the nanofiber membrane in the initial period of the filtration cycle and act as a filter screen to increase the separation efficiency. In the meantime, the oil droplets attached to the membrane surface are repelled by water droplets, assembled together, and re-floated above the membrane due to the

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under-water oleophobicity and under-oil superhydrophilicity of the membrane. With the adhesion and de-adhesion of oil droplets to achieve dynamic equilibrium, the

separation efficiency of the nanofiber membrane remained stable. This indicates that

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the d-CA nanofiber membrane had excellent separation performance and self-cleaning

c

20μm

100

Separation efficiency (%)

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b b

na

20μm

after

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b efore

a

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abilities for oil/water emulsions.

80 60 40 20 0 1

2

3

4

5

6

7

8

9

10

Separation cycles

Figure 5. (a) Photographs of an oil/water emulsion and the corresponding filtrate as the emulsion passed through the d-CA nanofiber membrane. (b) Photographs of the

oil/water emulsion filtrate under different cycle times. (c) Separation efficiency of the d-CA nanofiber membrane for an oil/water emulsion with different cycle times.

The chemical durability and corrosion resistance properties of the d-CA nanofiber membrane were evaluated by separating petroleum ether/water mixtures with different corrosive solutions of 1 mol/L HCl, 1 mol/L NaOH, and 1 mol/L NaCl.

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After separating the oil/corrosive solution mixture over 10 cycles, although the average diameter of the d-CA nanofiber membrane slightly increased in comparison

with that of the original d-CA membranes (Figure S7), the fine nanofiber morphology

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still remained (Figure 6 (a)–(c)). Meanwhile, the FTIR spectra of the d-CA membrane

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showed no obvious changes after separating the oil/water corrosive solution after 10 cycles with HCl, NaOH, and NaCl (Figure 6 (d)). The separation efficiency and

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separation flux of the d-CA nanofiber membrane showed no obvious changes after 10 cycles of petroleum ether separation under different corrosive solutions (Figure 6 (e,

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f)). It was confirmed that the d-CA nanofiber membrane had excellent chemical

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resistance to corrosive acids, alkalines, and salts, thus showing promising application under extreme environmental conditions due to its inherently stable chemical struc-

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ture.

Mixture

a

Petroleum ether

Mixture

b

Petroleum ether

1 mol/L NaOH

1 mol/L HCl

10 μm

10 μm

1065 1371

3394 2894

d

1 mol/L NaCl

d-CA HCl-10 cycles NaOH-10 cycles

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c

Petroleum ether

Transmittance (%)

Mixture

NaOH-10 cycles

10 μm

3500

3000

2500

2000

1500

1000

-1 Wavenumber (cm )

HCl

22000

f

-p

NaCl

20000 18000 16000

60

14000 12000 10000

4000 2000 0

NaOH

NaCl

HCl

2

re

80

Flux (L/m ·h)

NaOH

lP

Separation efficiency (%)

e 100

40 20 0

cle les cycle ycles cycle ycle 1 cy 10 cyc 1 1 10 c 10 c

8000 6000

cle les cycle ycles cycle ycles 1 cy 10 cyc 1 1 10 c 10 c

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ur

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Figure 6. Chemical durability of the d-CA nanofiber membrane for separation of petroleum ether under different corrosive solutions: (a) 1 mol/L HCl, (b) 1 mol/L NaOH, and (c) 1 mol/L NaCl. (d) FTIR spectra of the d-CA membrane before and after separating the water/petroleum corrosive solution for 10 cycles. (e) The separation efficiency and (f) separation flux of the d-CA nanofiber membrane before and after 10 cycles of petroleum ether separation in different corrosive solutions. 3.4 Antifouling Property of the d-CA Nanofiber Membrane Figure 7 shows the antifouling property (self-cleaning) of the d-CA nanofiber

membrane. The white, dry d-CA nanofiber membrane was completely absorbed by peanut oil colored with Oil Red O and became transparent when it was immersed into the oil bath (Figure 7 (a)). Then the oil-absorbing d-CA membrane was taken out and

immersed into a water bath. The peanut oil rapidly left the membrane and was replaced by water. Peanut oil droplets were found floating in the water and the d-CA membrane had returned to its original white color.

The injection needle of the contact angle measurement instrument was used to drip water droplets onto the surface of the oil-absorbing d-CA membrane (Figure 7

ro of

(b)). When the water droplet dripped onto the surface of the oil-absorbing d-CA membrane, the oil was immediately repelled and replaced by water droplets and left a white water spot, which restored the d-CA membrane to its clean state. These results

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indicate that the d-CA nanofiber membrane possessed excellent antifouling and

self-cleaning properties, making it easy to clean using water, even when polluted or

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blocked by oil. The d-CA nanofiber membrane can also effectively avoid secondary

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pollution in the application of oil/water separation and has great potential application

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as a reusable oil/water separation membrane.

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a

b

Figure 7. The antifouling property of the d-CA nanofiber membrane. (a) Oil-absorbing d-CA membrane immersed in a water bath. (b) Water droplets dripped

onto the oil-absorbing d-CA membrane. The density functional theory (DFT) simulation was applied to further study the antifouling property of the d-CA nanofiber membranes. The binding energy between the liquid molecules (water, toluene, carbon tetrachloride) and the polar hydroxyl groups from the d-CA nanofiber membrane was quantitatively calculated by DFT

ro of

[44].

Six cluster models representing the hydroxyl groups of water, toluene, and

carbon tetrachloride molecules on the d-CA membrane were considered (Figure 8).

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All dangling bonds were saturated with H atoms. All DFT calculations used the DMol3 package in Materials Studio (BIOVIA). The interactive Perdew–Burke–

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Ernzerhof (PBE) function was used under the generalized gradient approximation

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(GGA). The double numeric with polarization (DNP) method was used as the basis set for all atoms [45]. All model complexes and guest molecules were fully relaxed

na

before the energy calculation. The binding energy was calculated using Equation (3), (3)

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𝐸𝑏𝑖𝑛𝑑𝑖𝑛𝑔 = 𝐸3 − (𝐸1 + 𝐸2 )

where E3 was the bond energy of the d-CA molecule and the guest molecule after they

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were combined and optimized, E1 was the bond energy of the d-CA molecule, and E2 was the bond energy of the guest molecule.

For the d-CA nanofiber membrane, the competition relationship between the hydrophilic group and the lipophilic group existed due to the co-existence of ester groups and hydroxyl groups. When the deacetylation times were 1 and 2 h, the bond

energy between the hydroxyl group and the water molecule was in equilibrium with that between the ester groups and the oil molecule. Therefore, the d-CA/1 h and d-CA/2 h nanofiber membranes showed a super-amphiphobic property under water and oil. With an increase of deacetylation time, the hydroxyl group sharply increased, which resulted in the fact that the bond energy between the hydroxyl group and the water molecule was more dominant than that between the ester groups and the oil

ro of

molecule, thus showing its excellent hydrophilic property. Therefore, the d-CA/3 h

nanofiber membrane showed an oleophobic property under water and a superhydrophilic property under oil. Moreover, for the d-CA nanofiber membrane, the

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binding energy between the d-CA nanofiber membrane and the water molecule,

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toluene molecule, and carbon tetrachloride molecule were –63.051 kcal/mol, –9.314 kcal/mol, and 43.851 kcal/mol, respectively. The positive values indicate that the

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combination process is endothermic, resulting in an unstable structure. On the con-

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trary, the negative value indicates that the combination process is exothermic, reducing the overall energy needed to achieve a more stable structure. The more

ur

energy released, the more stable the structure is. Therefore, a combination structure between the d-CA nanofiber membrane and the water molecule is the most stable.

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This indicates that the interface that formed between the d-CA membrane and water was more stable than that formed between the d-CA nanofiber membrane and toluene or carbon tetrachloride (Figure 8). Therefore, the unstable interface that formed between d-CA and the low-polar liquid (oil) can easily be infiltrated by an immiscible high-polar liquid (water) and replaced by the stable interface formed between d-CA

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and water, showing that the oil was repelled from the d-CA membrane by water.

Figure 8. The optimized geometries and corresponding binding energies between

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guest molecules and the d-CA molecule as calculated by the density functional theory.

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4. Conclusions

In this study, a new type of multifunctional d-CA nanofiber membrane with

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superior comprehensive wettability properties for the effective separation of oil/water, emulsified oil/water, and oil/corrosive aqueous systems was successfully fabricated

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via simple electrospinning and deacetylation methods. The competition relationship

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between the hydroxyl groups and ester groups in the d-CA molecule endows the nanofiber membrane with the properties of being super-amphiphilic in air, oleophobic

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under water, and super-hydrophilic under oil. The d-CA nanofiber membrane can separate a pure oil/water mixture and an emulsified oil/water system, even in harsh environments such as those with strong acidity, alkalinity, or brine conditions, solely under the driving force of gravity. The highest water removal flux reached up to 29,000 L/m2·h , which is several times larger than that of the c-CA membrane. The

separation efficiency was above 99.97%. The recovery of the permeating flux was nearly 100%, even after 25 cycles of filtration using a simple water wash between cycles, due to its remarkable self-cleaning ability and strong anti-pollution property. Thus, the d-CA nanofiber membrane possesses great potential for use in chemical plants, textile mills, and the food industry, as well as in offshore oil spills, to separate

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oil/water mixtures.

Conflict of Interest

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The authors declare no conflict of interest.

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Declaration of interests

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☒ 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.

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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Acknowledgements

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The authors would like to acknowledge the financial support of the Outstanding

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Young Scientific Research Personnel Training Plan in Colleges and Universities of Fujian Province (Grant No. GY-Z160146), the Research Fund of Fujian University of Technology (Grant No. GY-Z15091, GY-Z160121), the Program of New Century Excellent Talents in the University of Fujian Province (Grant No. GY-Z17065), the External Cooperative Projects of Fujian Province (Grant No. 2018I0001), the Young Teachers Education Research Project (Grant No. JAT170377), the National Natural

Science Foundation of China (Grant No. 51303027, 61605027), the China Scholarship Council, and the Wisconsin Institute for Discovery (WID) at the University of Wisconsin–Madison.

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Table 1 Summary of nanofibrous membranes for oil/water separation applications.

TiO2

0°/ 0°

Water , oil

0°/

CA

PI/CA

15 5°/0° 16 3°/1°

Cu3(P O4)2

0°/

Water

15 3°/0°

Emuls ion

15 0°/0°

Emuls ion

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TiO2 @PPS

Oil

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PAAO

Oil

15 0 or 0°/-

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PVTF

Emuls ion

PS/Au Ag

15 3°/0°

Oil

PVTF

12 °/ 148°

Emuls ion

PSF PDMS

51 °/0° 9°/ 0°

-/4,00 0

lP

PTFE

Water

38,00 0 /29,00 0 16,95 4 /16,95 4 -/120, 000 1,215/ 3,106/ -

Water Water , oil,

99.97%

99.4%

99%

1,500 for emulsion 3,500 for emulsion < 4,000 for emulsion 2,843/ 1,400 for emulsion -/12,2 03 47, 000

Separ ation pressure

By gravity

Anti poll ution

-

99.5%

efs

T Yes

By gravity

1.3-1. 5 (KPa) By gravity By gravity

Yes

No No

Yes

99.4%

By gravity

Yes

99.98%

0.1 (MPa)

Yes

98.2%

0.09 (MPa)

Yes

> 97.5%

0.090 (MPa)

No

94% 99.2%

99.99% 97%

R

his work

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d-CA

0°/ 0°

Water , oil, emulsion

Materi als

Highest separation efficiency

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Type of removing

Oil flux /water flux (L/m2· h)

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W CA /O CA

By gravity 0.05 (MPa ) By gravity By gravity

[

1]

[

2]

[ 3] [ 4] [ 5] [ 6] [ 7] [ 8]

No

[ 9]

No

[ 10]

Yes Yes

[ 11] [ 12]

emulsion

0°/ -

Water , emulsion

491 for emulsion

99%

0.025 -0.07 (MPa )

No

[ 13]

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na

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PVAcoPE /SiO2

/34,50 0