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Electrospun Fibrous Membranes with Dual-scaled Porous Structure: Super hydrophobicity, Super lipophilicity, Excellent Water Adhesion, Anti-icing for Highly Efficient Oil Adsorption/Separation Di Zhang, Xin-zheng Jin, Ting Huang, Nan Zhang, Xiaodong Qi, Jing-hui Yang, Zuo-wan Zhou, and Yong Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b19523 • Publication Date (Web): 14 Jan 2019 Downloaded from http://pubs.acs.org on January 24, 2019

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ACS Applied Materials & Interfaces

Electrospun Fibrous Membranes with Dual-scaled Porous Structure: Super hydrophobicity, Super lipophilicity, Excellent Water Adhesion, Anti-icing for Highly Efficient Oil Adsorption/Separation Di Zhang, Xin-Zheng Jin, Ting Huang, Nan Zhang, Xiao-dong Qi, Jing-hui Yang, Zuo-wan Zhou, Yong Wang* School of Materials Science & Engineering, Key Laboratory of Advanced Technologies of Materials (Ministry of Education), Southwest Jiaotong University, Chengdu, 610031, China ABSTRACT: Developing highly efficient and multi-functional membranes toward oil adsorption and oil/water separation is of significance in oily wastewater treatment. Herein, a novel electrospun composite membrane with dual-scaled porous structure and nano-raised structure on each fiber was fabricated through electrospinning using biodegradable polylactide (PLA) and magnetic γ-Fe2O3 nanoparticles. The PLA/γ-Fe2O3 composite membranes show high porosity (>90%), superhydrophobic and superlipophilic performances with CH2I2 contact angle of 0o, good water adhesion ability like water droplet on petal surface, excellent anti-icing performance and good mechanical properties with a tensile strength of 1.31 MPa and a tensile modulus of 11.65 MPa. The superlipophilicity performance and dual-scaled porous structure endow the composite membranes with ultrahigh oil adsorption capacity up to 268.6 g/g toward motor oil. Furthermore, the composite membranes also show high oil permeation flux up to 2925 L/m2 h under the driving of gravity. Even for the oil/water emulsion, the composite membranes have high separation efficiency. We expect that the PLA/γ-Fe2O3 composite membranes can be used in oily wastewater treatment under various conditions through one-off adsorption or continuous oil/water separation, especially at low environmental temperature condition. KEYWORDS Electrospun fibrous membrane; dual-scaled porous structure; oil adsorption; oil/water separation; anti-icing

INTRODUCTION Oil pollution has already been one of the most severe environmental problems that greatly affect the activities of human and animals. Oil pollution can be induced through various scenarios, 1

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such as crude oil leakage during the storage and transportation, oil release during the machinery manufacturing and service, edible fat and oils in sanitary sewage, etc. To date, there are mainly three different strategies to remove oil pollution, including burning, filtering and collecting. Burning is inadvisable because it may induce another environmental pollution such as haze and toxic gas. Filtering and collecting have been thought as the most efficient and economical treatment ways and the collected oil can be used again. The ideal adsorbents may have high adsorption capacity, high adsorption efficiency and high regeneration ability, while the ideal separating materials toward continuous oil/water separation may have high separating efficiency and high flux.1 The first prerequisite for the highly efficient oil adsorbents or separating materials is that the materials must have the hydrophobic/lipophilic surface feature.2 Although some materials have the inherent

hydrophobic/lipophilic

characteristics,

such

as

hydrophobic

silica

(h-SiO2),3

polydimethylsioxane (PDMS)4 and poly(vinylidene fluoride) (PVDF),5 the acquirement of the hydrophobic/lipophilic surface is mainly through surface modification by grafting and/or coating the hydrophobic/lipophilic materials. Inspired from the surface structure of lotus leaf, Jiang et al6 proposed that the hydrophobic surface could be constructed by increasing the surface roughness of the material. After that, many strategies have been developed to construct the rough surface to improve the hydrophobic/lipophilic properties of the surface, such as constructing nanostructured papillae on mesh through coating hydrogel,7 constructing hierarchical roughness and micro/nanostructures on the textile surface through vapor-liquid sol-gel approach,8 etc. Porous structure with high porosity is believed to be another prerequisite for the highly efficient adsorbents or separating materials. Aerogels9-17 are typical porous materials with noteworthy advantages, such as light weight, extremely high porosity and large surface area and therefore, aerogels are thought to be the ideal candidates of the oil adsorbents. Generally, the maximum oil adsorption capacities of the aerogel-based adsorbents are hundreds of times of their own weights. However, most of aerogels cannot be used as the filter material to achieve the continuous oil/water separation because of the relatively small pore size and low mechanical and dimensional stability. Foams18 and sponges19,20 are the other kinds of porous materials with relatively large pore size. The framework, which may be constructed by polymers21-23 or 2


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metals,18,24,25 endows the foams/sponges with relatively high mechanical stability. Through surface modification to achieve the hydrophobic/lipophilic surface, the foams/sponges may exhibit high oil/water separation efficiency with large flux,19 which ensures the continuous oil/water separation in the actual application process. However, it is worth noting that the oil adsorption capacity of the modified foam/sponges is relatively low. For example, Lu et al20 reported the superhydrophobic/superoleophilic ethyl cellulose (SEC) sponges that were reinforced by carbon nanotubes (CNTs) and the sponges exhibited the oil adsorption capacity of only 32-64 times of their own weight. Du et al23 reported an oil adsorbent prepared from commercial melamine foam (MF) with relatively high adsorption capacity, about 50-145 times of its own weight, however they did not report the flux and separation efficiency of the adsorbent during the continuous oil/water separation. Membranes are another kind of porous materials with relatively high porosity. Membrane separation technology usually has the apparent advantages, such as high separation efficiency, high flux, low fabrication cost and easy operation during the oil/water separation process and therefore, membrane separation technologies attract much attention of researchers and various kinds of membranes have been developed.26-29 Electrospun fibrous membranes, which are fabricated through the stacking of electrospun fibers, are suggested to be the ideal candidate for the continuous oil/water separation.30 To further enhance the oil/water separation efficiency and flux, surface modification has also been applied, but almost modification methods are mainly related to improving the superhydrophilicity of the electrospun membranes31-34 rather than enhancing the hydrophobic and/or lipophilicity. On the other hand, the electrospun membranes reported in literature only exhibit the oil/water separation ability and very low oil adsorption ability, which undoubtedly limits the application of the membranes in more fields. In addition, many researches have also demonstrated that incorporating nanoparticles is an efficient strategy to tailor the oil/waster separation efficiency of the membranes.35-38 Furthermore, with increasing concerns about reducing the possible environmental pollution induced by adsorbents or separating materials, degradable membranes that can be fully degraded in water possibly have greater potential applications in wastewater treatment. Polylactide (PLA) is a well-known biodegradable and biocompatible polymer and it can be fabricated from natural resources and therefore, PLA-based membranes have been demonstrated a good candidate of 3



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hemodialysis membrane.39,40 Recently, interests of researchers have been transferred to investigate the probability of the PLA-based membranes in wastewater treatment. For example, Gu et al41 reported a PLA-based nonwoven fabric with superoleophilic and superhydrophobic features. The organic solvent permeate flux was higher than 12000 L/m2 h but the oil adsorption capacity was only about 30 g/g. Xiong et al42 reported a PLA-based membrane with a TiO2 nano-particle inlaid surface toward oil/water separation and the water flux was above 950 L/m2 h under 0.1 MPa. However, no evidence could show the oil adsorption ability of the membranes. Similarly, Jing et al43 reported a PLA-based membrane with a large number of stereocomplex (sc) crystallites and the maximum oil permeate flux was up to 11000 L/m2 h. However, it is worth noting that the oil adsorption ability of the above PLA-based membranes is extremely low possibly due to that the pore size provided by the irregular stacking of electrospun fibers is too large to be used for oil adsorption. Maghemite nanoparticles γ-Fe2O3 recently attract much attention of researchers due to their promising properties in wastewater treatment,44-46 such as high stability, easy fabrication, good saturation magnetization, higher acid-resistance compared with magnetic Fe3O4, excellent adsorption ability toward heavy metal ions and organic pollutants, etc. For example, Afkhami et al 44

reported that the γ-Fe2O3 showed the maximum adsorption capacity of 208.33 mg/g toward

Congo red (CR) at pH value of 5.9. Wang P et al 45 found that the mesoporous magnetic γ-Fe2O3 exhibited a highly pH-dependent behavior, and the Cr (VI) adsorption capacity increasing with decreasing pH of solution. However, to date, there are not researches that report the application of γ-Fe2O3 in oil/water separation. Herein, we report a PLA-based electrospun membrane with homogeneous dispersion of maghemite γ-Fe2O3 nanoparticles on the fiber surface and dual-scaled micro/nano-pores in membrane. γ-Fe2O3 may enhance the roughness of the fiber surface, which facilitates the enhancement of the hydrophobicity and lipophilicity. Nanopores on each fiber provide the space for oil adsorption while micropores between fibers provide channel for oil permeation during the oil/water separation process. Thanks to the enhanced roughness and the presence of dual-scaled micro/nano-pores,

the

PLA/γ-Fe2O3

composite

membranes

exhibit

superhydrophobic/superlipophilic features, extremely high oil adsorption capacity (268.6 g/g toward motor oil) and high oil permeation flux (2925 L/m2 h), which endow the composite 4


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membrane with huge application in oily wastewater treatment. Another interesting result is that the membrane also exhibits excellent water adhesion, anti-icing performance and good mechanical properties, which may enlarge the application in rigorous gelid conditions.

RESULTS AND DISCUSSION Characterization of the PLA/γ-Fe2O3 composite membranes Fig. 1a shows the morphologies of the electrospun PLA and PLA/γ-Fe2O3 composite membranes. It is emphasized that in the main text, for the PLA/γ-Fe2O3 composite membrane, the content of γ-Fe2O3 was maintained at 10 wt% and γ-Fe2O3 was treated by 0.5 mL glycine aqueous solution if there is no additional illustration. The addition of glycine is aimed to promote the dispersion of γ-Fe2O3. It is clearly seen that pure PLA and PLA/γ-Fe2O3 composite membranes show the dual-scaled micro/nano-pores. The irregular stacking of electrospun fibers leads to the formation of the micropores between fibers while many nanopores are present on each fiber. The formation of the nanopores on fibers is mainly related to the electrospinning parameters,47,48 such as the relative humidity (RH) within the spinning unit, the solvent miscibility/interaction with water, the take-up velocity and crystallization ability of polymer, etc. The white dots observed on the fiber surface of the PLA/γ-Fe2O3 composite membrane represent the γ-Fe2O3 nanoparticles. It is clearly seen that γ-Fe2O3 nanoparticles exhibit homogeneous dispersion on electrospun fibers, which can be further confirmed through the homogeneous dispersion of Fe element characterized through the element energy dispersive spectroscopy (EDS) mapping as shown in Figure 1b. The crystalline structures and the chemical features of the pure PLA membrane, γ-Fe2O3 particles and the PLA/γ-Fe2O3 composite membrane were also characterized using wide angle x-ray diffraction (WAXD) and Fourier transform infrared spectroscope (FTIR) and the results are listed in Supporting Information Figure S1. The results further confirm that γ-Fe2O3 particles are successfully incorporated onto the electrospun PLA fibers. The surface roughness of the electrospun pure PLA fiber and PLA/γ-Fe2O3 composite fiber was characterized through using atomic force microscope (AFM) and the typical images are shown in Figure 1c. The presence of γ-Fe2O3 particles leads to the increase of the surface roughness to a certain extent, although the degree of the increase is small. Furthermore, the porosity of the composite membranes was also measured and the results are shown in Figure 1d and Figure 1e. 5



The electrospun PLA membrane shows the porosity of 87.7%. It can be seen that all the composite membranes show higher porosity compared with the pure electrospun PLA membrane. However, it is also found that the effects of γ-Fe2O3 and glycine contents on the porosity of the composite membrane are inconspicuous.

(a)

PLA

(c)

PLA/γ-Fe2O3-10

PLA

(d) 100

(e) 91.2

92.2

92

90.7

90.3

100 86.5

80

90.3

92

92.4

92.6

0.25

0.5

1

1.5

Porosity (%)

80

60 40 20 0

(b)

PLA

PLA/γ-Fe2O3-10

PLA/γ-Fe2O3-10

Porosity (%)

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

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60 40 20

6

8

10

12

0

14

0

Content of -Fe2O3 (wt%)

Content of glycine (mL)

Figure 1: (a) SEM images showing the morphologies of the electrospun PLA and PLA/γ-Fe2O3-10 membranes characterized at different magnifications, (b) showing the EDS analysis of element Fe, (c) showing the comparison of AFM images of the electrospun PLA and PLA/γ-Fe2O3-10 fiber surfaces, and (d) and (e) showing the effects of γ-Fe2O3 and glycine contents on the porosity of the electrospun PLA/γ-Fe2O3 composite membranes, respectively.

6


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Surface performances and mechanical properties measurements

(b)160 Contact angle of distilled water (o)

(a) H2O CH2I2 PLA H2O CH2I2

140 120 100 80 60 4

PLA/γ-Fe2O3-10

6

8

10

12

14

16

Content of -Fe2O3 (wt%)

(c)

(d)

(e) 1.2

Stress (MPa)

Ice accumulation

Increasing time

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


0.8

0.4

PLA/-Fe2O3-10 composite membrane 0.0

0

10

20

30

Strain (%)

Figure 2: (a) contact angle measurements of distilled water and CH2I2 on the electrospun PLA and PLA/γ-Fe2O3-10 membranes, (b) variation of contact angle (H2O) of the composite membrane with increasing γ-Fe2O3 content, (c) images showing that the composite membrane has the similar water adhesion to that of the petal surface, (d) images showing the formation of ice accumulations on glass slide and membrane surface with increasing time after being taken out from liquid nitrogen, and (e) showing the stress-strain curve of the PLA/γ-Fe2O3-10 membrane.

The hydrophobic and lipophilic performance of the electrospun membranes were measured using distilled water and CH2I2 as the probe liquids. As shown in Figure 2a, the pure PLA electrospun membrane shows the water contact angle of 133o, which is very similar to that reported in literature.43 However, for the PLA/γ-Fe2O3 composite membrane, the water contact angle is apparently enhanced up to 148o, indicating the superhydrophobic feature of the composite 7



membrane surface. In addition, the pure PLA and PLA/γ-Fe2O3 composite membranes show the excellent wettability by CH2I2 and the contact angle is 0o, indicating the superlipophilic feature of the membrane surface. Specifically, for the PLA/γ-Fe2O3 composite membrane, once the CH2I2 was dropped on the membrane surface, it was quickly adsorbed by the membrane in only one second as shown in Supporting Information Movie 1, indicating the superlipophilicity and excellent oil adsorption performances. The effect of γ-Fe2O3 content on the hydrophobic performance of the PLA/γ-Fe2O3 composite membranes were also measured and the results are shown in Figure 2b. All the composite membranes show the superhydrophobic feature and the maximum contact angle of the distilled water was achieved at 10 wt% γ-Fe2O3 (143.1o). However, further increasing γ-Fe2O3 content leads to the slight decrease of the contact angle. At extremely high γ-Fe2O3 content, the viscosity of the solution is greatly increased, which not only results in the relatively poor dispersion of γ-Fe2O3 in the solution but also results in the morphological change of the fiber surface as shown in Supporting Information Figure S2. Furthermore, at extremely high content, the electrospinning also becomes difficult. By the way, the effects of glycine content on the morphology and hydrophobicity of the PLA/γ-Fe2O3 composite membranes were also investigated and the results are shown in Supporting Information Figure S3 and S4. It can be seen that when the γ-Fe2O3 nanoparticles were treated by 0.5 mL glycine, the PLA/γ-Fe2O3 composite membrane shows the biggest contact angle, representing the best hydrophobicity of the composite membrane surface. Furthermore, the rough surface of the electrospun PLA/γ-Fe2O3 fibers with the nanopores and the nano-raised structure of γ-Fe2O3 on fiber surface shows a good water adhesion even if the membrane was tilted and inverted as shown in Figure 2c, very similar to the scenario of water droplet on the petal surface.49-51 In addition, the superhydrophobic characteristic endows the PLA/γ-Fe2O3 composite membrane with excellent anti-icing performance as shown in Figure 2d, and the detailed record of the anti-icing measurement can be seen in Supporting Information Movie 2. In this measurement, the composite membrane was fixed on a glass sheet and immersed into liquid nitrogen for several minutes, then it was taken out. Different from the formation of many tiny ice accumulations on the glass sheet due to the condensation of water in the air, there is not any ice accumulation can be seen on the surface of the composite membrane, indicating the excellent anti-icing performance. This indicates that the PLA/γ-Fe2O3 composite membrane may 8


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be used in extremely low environmental temperature condition. The mechanical properties of the pure PLA and PLA/γ-Fe2O3 composite membrane were measured, and the typical stress-strain curve is illustrated in Figure 2e. Pure PLA membrane exhibits very poor mechanical properties and once the load is applied, the membrane is broken and therefore, it is very difficult to obtain the stress-strain curve. The PLA/γ-Fe2O3-10 composite membrane shows largely improved mechanical properties. It exhibits a tensile strength of 1.31 MPa and a tensile modulus of 11.65 MPa. The results show that the presence of γ-Fe2O3 nanoparticles also facilitates the improvement of the mechanical properties of the composite membrane. Obviously, the relatively good mechanical properties endow the PLA/γ-Fe2O3 composite membranes with greater application in wastewater treatment, especially they can be used in the continuous oil/water separation process. Oil adsorption measurement and evaluation Oil adsorption capacity was measured through qualitative evaluation and quantitative measurements to provide comprehensive understanding about the adsorption behavior of the composite membranes. In Figure 3a, the n-hexane was used as the probe oil and n-hexane, which was stained by Sudan III, was dropped on water. The stained n-hexane droplet floats on water due to its lower density compared with water. A piece of the PLA/γ-Fe2O3 composite membrane was clamped by tweezers and contacted the n-hexane droplet. In about 5 sec the n-hexane drop can be completely adsorbed by the PLA/γ-Fe2O3 composite membrane. If we use the carbon tetrachloride (CCl4) as the probe oil (Figure 3b), due to the larger density compared with the water, CCl4 locates at the bottle of the flask. Similarly, one can see that the PLA/γ-Fe2O3 composite membrane can easily adsorb CCl4 and exhibit the removal ability toward CCl4 as shown in Supporting Information Movie 3. In Figure 3c, the PLA/γ-Fe2O3 composite membrane was hanged between glasses and n-hexane was continuously dropped on the membrane. The record of the measurements can be seen in Supporting Information Movie 4. The composite membrane also exhibits the excellent adsorption ability and no n-hexane penetrate through the membrane. By the way, the PLA/γ-Fe2O3 composite membrane shows relatively good integrity and no breakage occurs due to its relatively good mechanical properties as shown in Figure 2e.

9



(b)

(a)

(c)

5 sec 320 280

240 Sorption Capacity (g/g)

Adsorption capacity (g/g)

280

200 160 120 80 40

ot

or

oi

l

S

n co ili

eo

il

s Ca

r to

l oi Co

rn

160 120 80

0

l l l il il ne oi oi oi ro eo g xa ar an liv he we tin ed be o a O C l y c N i nf So br Su Lu

l oi

(f) 300

N-Hexane Lubricating oil Sunflower oil Soybean oil Corn oil

250 200

Motor oil Silicon oil Castor oil Olives oil Cedar oil

100 50

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

12 8 10 Content of -Fe2O3 (wt%)

6

14

(g) 300

150

0

200

40

0 M

240 Corn oil Soybean oil

PLLA PLLA@γ-Fe2O3

Silicon oil Castor oil

(e)

320


(d)


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

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4.0

250 200 150 100

N-Hexane Lubricating oil Sunflower oil Soybean oil Corn oil

50 0

20

40

Motor oil Silicon oil Castor oil Olives oil Cedar oil

60

Surface tension (μN/m)

Viscosity of oil (Pa*S)

Figure 3：(a, b) Images showing the oil adsorption ability toward n-hexane (a) and CCl4 (b) of the PLA/γ-Fe2O3 composite membrane, (c) showing the adsorption behavior of the composite membrane when n-hexane was dropped on the membrane, (d) comparison of the oil adsorption capacity of the pure PLA membrane and PLA/γ-Fe2O3 composite membrane toward ten different oils, (e) variation of oil adsorption capacities of the composite membranes with increasing γ-Fe2O3 content, (f) and (g) showing the variations of oil adsorption capacities versus the viscosity and surface tension of oils, respectively.

To quantitatively evaluate the oil adsorption ability of the PLA/γ-Fe2O3 composite membrane, 10


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the representative PLA/γ-Fe2O3-10 composite membrane was used in the following adsorption measurements. First, the variation of adsorption amount with increasing adsorption time was investigated and the results are shown in Supporting Information Figure S5. It is clearly seen that the adsorption amount gradually increases with increasing adsorption time, and it maintains nearly invariant at adsorption time longer than 60 min due to the saturation effect. Therefore, in the subsequent adsorption measurements, the adsorption time was maintained at 60 min. Then, the adsorption ability of the PLA/γ-Fe2O3 composite membrane toward ten different oils and/or organic solvents were measured and the results are illustrated in Figure 3d. For comparison, the oil adsorption capacities of the pure PLA membrane are also provided. It is clearly seen that among the ten oils, the pure PLA membrane and PLA/γ-Fe2O3 composite membrane show the highest adsorption toward motor oil while the lowest adsorption toward n-hexane. For the PLA membrane, the adsorption capacities vary from 17.7 to 104.9 g/g, while for the PLA/γ-Fe2O3 composite membrane, the adsorption capacities vary from 23 to 268.6 g/g. Among all the selected oils, the PLA/γ-Fe2O3 composite membrane shows much larger adsorption capacity compared with the pure PLA membrane. Whatever, the oil adsorption measurements show that the PLA/γ-Fe2O3 composite membrane with dual-scaled porous structures has excellent oil adsorption ability. Furthermore, the effect of γ-Fe2O3 content on the oil adsorption capacity was also investigated and the results are shown in Figure 3e. Herein, four oils were selected. It is clearly seen that there is an appropriate γ-Fe2O3 content (10 wt%), at which the PLA/γ-Fe2O3 composite membrane show the higher adsorption capacity toward the selected oils. The effects of glycine content on the adsorption capacities of the PLA/γ-Fe2O3 composite membrane containing 10 wt% γ-Fe2O3 were also measured and the results are shown in Supporting Information Figure S6. The results show that when γ-Fe2O3 nanoparticles were treated by 0.5 mL glycine, the PLA/γ-Fe2O3 composite membrane shows the highest adsorption capacity, which agrees well with the best hydrophobicity of the composite membrane. To better understand the adsorption behaviors of the composite membranes, the viscosity and surface tension of the ten oils, which are cited from literature and listed in Supporting Information Table S1, versus the adsorption capacities obtained by the representative PLA/γ-Fe2O3 composite membrane are illustrated in Figure 3f and Figure 3g. It can be seen that the oils which have adsorption capacities higher than 200 g/g by the PLA/γ-Fe2O3 composite membrane, such as 11



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Table 1: Comparison of oil adsorption capacities of the adsorbents reported in literature and the PLA/γ-Fe2O3 composite membrane prepared in this work. Adsorbents

Type of oil


Ref.

Oils and organic solvents

2-5

[56]

Oils

10

[57]

Wood/epoxy biocomposite


6-20

[58]

Graphene-based sponges


54-165

[59]

Hierarchical dimensional MF


50-145

[23]

Carbon fiber aerogel


50-192

[60]

Carbon microbelt aerogel


56–188

[14]

Graphene/CNT foam


80-130

[61]

CNT sponge


80-180

[62]

Polyurethane/MnO2 foam


4.5-40

[63]

Biomass-derived banana peel/waste paper (BPWP) hybrid aerogel


35-115

[64]

Biomass carbon@SiO2@MnO2 aerogel


60-120

[65]

Porous Al2O3/acrylic resin composites


7-30

[66]

Carbon nanofiber (CNF) aerogels


106-312

[17]

Nitrogen-doped graphene foam


200-600

[16]

CNT/graphene aerogel


215-913

[15]

Cellulose hybrid membrane


5-20

[52]

Electrospun PSF/PGS membrane


54.7-61.4

[53]

Electrospun porous PS membrane

Oils

6.3-131.6

[54]

Electrospun PS/CNT membrane

Oils

111.5-122.8

[55]


23-268.6

This work

Metal-Organic Nanofibrous Gel Composites Hollow carbon spheres

PLA/γ-Fe2O3 composite membrane

motor oil, silicone oil, castor oil and corn oil, usually have relatively low viscosity and high surface tension. While the oils which have low viscosity and low surface tension, such as lubricating oil, sunflower oil and n-hexane or have relatively high viscosity and high surface tension, such as olives oil and cedar oil, have lower adsorption capacities by the PLA/γ-Fe2O3 12


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composite membrane. The adsorption capacities toward oils and organic solvents of the PLA/γ-Fe2O3 composite membrane are compared with the adsorption capacities of other oil adsorbents which have already been reported in literature14-17,23,52-66 and the results are listed in Table 1. It is clearly seen that the adsorption capacities of the PLA/γ-Fe2O3 composite membrane are much better than most of the other porous materials and only smaller than several graphene-based aerogels which exhibit extremely low density.15-17 Oil adsorbents based on electrospun fibrous membrane have been seldom reported. Most of the membranes are used as the filter rather than the adsorbent. Recently, Kollarigowda et al52 reported a membrane that was prepared based on cellulose and the membrane exhibited oil adsorption capacity of about 5-20 g/g. Lee et al53 reported a coaxially electrospun fibers containing polysulfone (PSF) and poly(glycerol sebacate) (PGS) and the fiber membrane showed oil adsorption capacity of 54.7-61.4 g/g. Wu and Wang et al54 reported the fabrication of the electrospun polystyrene (PS) membrane with porous structure on fiber surface and the results showed that the membrane had adsorption capacities of 6.3-131.6 g/g. Wu and An et al55 reported the hybrid PS/CNT fibrous membrane with oil adsorption capacities of 111.5-122.8 g/g. It is clearly seen that the PLA/γ-Fe2O3 composite membrane has the highest adsorption capacity (268.6 g/g) among the electrospun membranes as reported in literature.

300

(b)

(a)

250


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


200 150 100 50 0

During adsorption

After desorption

1 2 3 4 5 6 7 8 9 10 11 12 Cycle times of adsorption/desorption measurements

Figure 4: (a) Images showing the appearance of the PLA/γ-Fe2O3 composite membrane during the oil adsorption process and after oil desorption, and (b) variation of motor oil adsorption capacity with increasing adsorption/desorption cycle times.

13



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The excellent oil adsorption capacities of the PLA/γ-Fe2O3 composite membrane may be attributed to the following mechanisms. First, the PLA/γ-Fe2O3 composite membrane show the dual-scaled porous structure, which provides more volume for the uptake of oils compared with the common fibrous membranes. Second, the dual-scaled porous structure provides more probability for the swelling of the composite membrane during the oil adsorption process, especially, the presence of the nanopores ensures that every PLA/γ-Fe2O3 fiber can be swelled, which further increases the volume for the uptake of the oils. By the way, the different degrees of swelling of the composite membrane in different oils may be one of the reasons that result in the different adsorption capacities of the PLA/γ-Fe2O3 composite membrane toward different oils. Third, the super lipophilicity of the fiber surface provides strong adhesion between PLA/γ-Fe2O3 composite membrane and oils. Furthermore, the adsorption stability of the PLA/γ-Fe2O3 composite membrane was also investigated and the results are shown in Figure 4. Herein, motor oil was selected as the probe oil. Figure 4a shows the photos of the composite membrane during adsorbing oil process and after oil desorption.

The

variation

of

adsorption

capacity

with

increasing

cycle

times

of

adsorption/desorption measurement is illustrated in Figure 4b. Although the adsorption capacity gradually decreases with increasing cycle times, the PLA/γ-Fe2O3 composite membrane still shows the high adsorption capacity (120.4 g/g) even after 10 cycle times, which is still better than most of the adsorbents as shown in Table 1, further indicating that the PLA/γ-Fe2O3 composite membrane is an excellent oil adsorbent. Oil/water emulsion separation performance and oil permeation evaluation The PLA/γ-Fe2O3 composite membranes show the superhydrophobic and superlipophilic performance and excellent oil adsorption ability. The presence of the large pores between electrospun fibers provides the permeation channel for oils, while the superhydrophobic and superlipophilic performance of the membrane surface and/or fiber surface may prevents the wetting of water. Furthermore, the adsorption of oil not only swells the PLA/γ-Fe2O3 fibers but also swells the fibrous membrane, which also facilitates the penetration of oils. This indicates that the PLA/γ-Fe2O3 composite membrane may be used for oil/water separation and in this process, only oils can penetrate the composite membranes while water is rejected. The oil/water emulsion separation performance of the PLA/γ-Fe2O3 composite membrane 14


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was measured through two different strategies, one-off separating oil from emulsified oil/water mixture and continuous separating oil through gravity-driven separation method. Herein, we use n-hexane to simulate oil and the detailed oil/water separation measurements can be seen in Supporting Information Movie 5. As shown in Figure 5a, the prepared n-hexane/water emulsion exhibits the white color and it is also very muddy and opaque. After being immersed the PLA/γ-Fe2O3 composite membrane and continuously stirred for several seconds, the solution becomes transparent and the membrane can be easily taken out from the solution. This indicates that most of n-hexane is adsorbed and removed from water by the PLA/γ-Fe2O3 composite membrane. This also indicates that the PLA/γ-Fe2O3 composite membrane may be a good water purification material if only a few oils are spilled into water.

(b)

(a)

2000

Oil flux (L/m2 h)

1500

Emulsion

1000

500

0

b2

b1

b3

(c)

0

1

2

3

4

5

6

7

8

9

10

11

10

11

Cycle times of oil/water separation

3000 2500 2000

2

Oil flux (L/m h)

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


1500 1000 500 0

c1

c1

c3

0

1

2

3

4

5

6

7

8

9

Cycle times of oil permeation

Figure 5: (a) One-off separating oil from oil/water emulsion by immersing the PLA/γ-Fe2O3 composite membrane, (b) continuously separating oil from oil/water emulsion (b1) using the PLA/γ-Fe2O3 composite membrane as the filter material (b2), and the variation of oil flux with increasing cycle times of oil/water separation measurements (b3), and (c) pure oil flux measurement using the n-hexane as the probe oil (c1) and the PLA/γ-Fe2O3 composite membrane as the filter material (c2), and the variation of oil flux with increasing the cycle times of oil permeation measurements.

The continuous separation of n-hexane/water separation was measured using the PLA/γ-Fe2O3 composite membrane as the filter material. As shown in Figure 5b, during the 15



n-hexane/water separating measurement, only n-hexane can penetrate the membrane and water is rejected. The maximum gravity-driven n-hexane flux during separating n-hexane/water emulsion is up to 1769 L/m2 h. Photographs of the emulsion before and after filtration are shown in Figure S7. It is clearly seen that most of n-hexane is separated from the emulsion. Although the n-hexane flux tends to decrease with increasing cycle times of oil/water separation measurements, it can be seen that after being measured for 10 times, the n-hexane flux is still 1184 L/m2 h. The decrease of n-hexane flux is possibly related to the excellent adsorption of the PLA/γ-Fe2O3 composite membrane. During the oil/water separation process, although the PLA/γ-Fe2O3 composite membrane was washed using ethyl alcohol, some pores were still possibly occupied by n-hexane, resulting in the decrease of the porosity. The pure oil flux was also measured using n-hexane as the probe oil and the results are shown in Figure 5c. It is clearly seen that the maximum n-hexane flux reaches up to 2925 L/m2 h. After being continuously measured for 10 cycles, the PLA/γ-Fe2O3 composite membrane still show the n-hexane flux of 1911 L/m2 h. The above oil/water separation and oil penetration measurements clearly show that the PLA/γ-Fe2O3 composite membrane can be used in continuous oil/water separation, which undoubtedly enlarge the application fields of the composite membrane. So far, many adsorbents and membranes have been fabricated to realize the oil/water separation. Most of adsorbents based on porous aerogels and/or foams exhibit excellent oil adsorption ability, but the desorption of oils is relatively difficult and it also takes time and considerable resources. The electrospun membranes usually exhibit extremely high flux, but these membranes usually exhibit no adsorption or very low adsorption capacity toward oils. The previous researches carried out by Xiong et al42 and Jing et al43 already demonstrated that the electrospun PLA fibrous membranes have great potential application in oil/water separation, but there was no evidence to show that their membranes had oil adsorption ability. Herein, we demonstrate that the PLA/γ-Fe2O3 composite membranes with dual-scaled porous structure not only show ultrahigh oil adsorption performance but also excellent oil/water separation ability. Obviously, the PLA/γ-Fe2O3 composite membrane fabricated in this work provides more chances and selection in the oily wastewater treatment through one-off adsorption or continuous oil/water separation, especially at low temperature condition. In addition, the composite membranes can be completely degraded in water and therefore, the wide application of such composite membrane does not bring 16


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the second-pollution to the environment. This is very significant for the sustainable development of the environment.

CONCLUSIONS In summary, we successfully fabricated a superhydrophobic and superlipophilic PLA/γ-Fe2O3 composite membrane through electrospinning technology. The composite membranes showed dual-scaled porous structure, including the micropores between fibers and nanopores on fibers. γ-Fe2O3 nanoparticles homogeneously dispersed on fibers and increased the roughness of the fiber surface. Besides the superhydrophobic and superlipophilic performances, the composite membrane also showed good water adhesion, anti-icing performances and good mechanical properties. The composite membranes showed ultrahigh oil adsorption ability and the maximum adsorption capacity reached up to 268.6 g/g when adsorbing motor oil, which was comparable to the common aerogel and/or foam-based adsorbents with extremely low density. Similar to the common electrospun fibrous membrane, the composite membrane also showed high oil permeation flux. The gravity-driven permeation flux was 1769 L/m2 h in separating oil/water emulsion while it reached up to 2925 L/m2 h for pure oil permeation. The ultrahigh oil adsorption capacity and high oil permeation flux endowed the composite membrane with great potential application in various conditions, especially at low temperature condition due to its excellent anti-icing performance.

EXPERIMENTAL SECTION Materials: PLA (2003D, with) was purchased from NatureWorks®, USA. It has a M w of about 2.53×105 g/mol and a D-isomer content of 4.3 wt%. All the chemical reagents, such as methylene chloride (CH2Cl2), glycine, N,N-dimethylformamide (DMF), n-hexane and dodecyltrimethylammonium bromide (DTAB), were purchased from Kelong Chemical Reagent Co. (Chengdu, China). γ-Fe2O3 (with diameter of about 20-30 nm) was purchased from Meklin company (Shanghai, China). Preparation of PLA/γ-Fe2O3 composite membrane: First, 1.5 g PLA was dissolved in the solution of 13.5 mL CH2Cl2 and 1.5 mL DMF, the mixture was continuously stirred at 36 °C for 3 17



h to ensure the complete dissolution of PLA and the concentration of PLA was 10% (w/v). The PLA solution (Solution A) was then used for the subsequent electrospinning. To prepare the PLA/γ-Fe2O3 composite membrane, 1.5 g PLA was dissolved in 13.5 mL CH2Cl2 to obtain the PLA solution (Solution B). 0.15 g γ-Fe2O3 was first mixed with 0.5 mL glycine aqueous solution (0.2 mol/L), then 1.5 mL DMF was added into the solution and the mixture was further stirred at 25 °C for 3 h to obtain the γ-Fe2O3 solution (Solution C). Subsequently, the PLA solution (Solution B) and γ-Fe2O3 solution (Solution C) were mixed together and the mixture was ultrasonically treated for 3 h to obtain the precursor solution (Solution D) for electrospinning. Then, the solution B and solution D were used to prepare the electrospun PLA and PLA/γ-Fe2O3 composite membranes, respectively. The electrospinning was carried out at an environmental temperature of 25 °C, a humidity of 40 %, a feeding rate of 0.03 mL/min, a voltage of 13 kV and a distance between syringe needle and collector of 15 cm. The diameter of syringe needle was 0.9 mm. The evaporation of solvent occurred in the atmosphere condition with common flow rate of air in room. Furthermore, 0.15 g γ-Fe2O3 was also treated by different amounts of glycine aqueous solution ranging from 0.25 to 1.5 mL and the content of γ-Fe2O3 was also varied in the range of 0.09-0.21 g. Correspondingly, the content of γ-Fe2O3 was 6, 8, 10, 12 and 14 wt% of PLA. Scanning electron microscopy (SEM): Morphologies of the electrospun PLA and PLA/γ-Fe2O3 composite membranes were characterized using a SEM Fei Inspect (FEI, the Netherlands) with energy dispersive spectroscopy (EDS) mapping. Sample was coated by a thin layer of gold and the characterization was carried out at an accelerating voltage of 20.0 kV. Atomic force microscopy (AFM): Surface morphologies of the electrospun PLA and PLA/γ-Fe2O3 fibers were further characterized using an AFM P/N: HE002-H (AIST-NT, Inc) (Veeco, USA) in tapping mode. Porosity Measurements: First, a piece of membrane (0.06 g) was immersed into ethanol for 6 h, then the membrane was taken out and the ethanol on the membrane surface was carefully wiped off by using filter paper. The porosity was calculated according to the following relation:

AK 

( w0  w)  100%  w0  (    ) w

(1)

Where w0 and w represent the weights of membrane before and after adsorbing ethanol, 18


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respectively, while

 (0.8 g/cm3) and  (varied in 1.248-1.807 g/cm3) are the densities of

ethanol and the PLA/γ-Fe2O3 composite membrane, respectively. Contact angle measurements: Contact angle measurements were conducted on a drop shape analysis system DSA 100 (KRṺSS, Germany) using double-distilled water (H2O) and diiodomethane (CH2I2) as the probe liquids at room temperature. All the measurements were repeated for at least five times. Anti-icing measurement ： The PLA/γ-Fe2O3 composite membrane was fixed on a glass slide and immersed into liquid nitrogen for 10 minutes. Then, the sample was taken out and placed in room temperature. The ice accumulation on the membrane surface and glass slide was recorded. Mechanical properties measurement: The mechanical properties of the membrane were measured using a Instron 5967 universal tester (USA). The measurements were carried out at a cross-head speed of 1 mm/min with a rectangular sample with a length of 17.5 mm and a width of 5 mm. Oil adsorption measurements: 0.1 g PLA/γ-Fe2O3 composite membrane was immersed into a glass beaker containing 100 mL oil for 60 min, then the membrane was carefully taken out and statically placed for 15 sec so that the oil on the membrane surface could be removed by gravity effect. Then, the weight of the membrane was carefully weighed. The adsorption capacity ( q , g g-1) toward oil was calculated through the following relation:

q

ww  w0 w0

(2)

Where, w0 and ww represent the weights of membrane before and after adsorbing oil, respectively. Here, 10 different oils were used to evaluate the adsorption selectivity of the PLA/γ-Fe2O3 composite membranes. The oil desorption was carried out through vacuum filtration. After that, the PLA/γ-Fe2O3 composite membrane was used to adsorb oil again. The adsorption/desorption measurements were repeated for 10 times to evaluate the adsorption stability of the composite membrane. Oil/water emulsion separation measurements: The oil/water separation measurements were carried out through one-off and continuous separating oil from the oil/water emulsion using n-hexane as the probe oil according to the following procedures. For the one-off separating 19



measurement, 10 mL n-hexane and 80 mL water were first mixed and emulsified by 0.14 g DTAB, then 10 mL emulsion was taken out for oil/water separation. Then, the PLA/γ-Fe2O3 composite membrane was immersed into the emulsion and stirred for several seconds. After that, the composite membrane was taken out and the residual solution was observed. For the continuous oil/water separating measurement, 0.14 g DTAB was added into the glass beaker containing 50 mL distilled water and 50 mL n-hexane and the mixture was stirred for 3 h to obtain the emulsified solution. The PLA/γ-Fe2O3 composite membrane was placed in a glass sand core filter and the emulsion was separated through the gravity-driven oil/water separation. The time for completely separating oil from the emulsified solution was recorded. The oil flux of the composite membrane was then deduced. In addition, the gravity-driven pure oil flux was also measured using pure n-hexane. The oil/water separation stability was also evaluated. After each measurement, the composite membrane was washed using ethyl alcohol and dried at 50 °C for 1 h, then it was used for next oil/water separation.



ASSOCIATED CONTENT

Supporting Information The Supporting Information is available free of charge on the ACS Publications Website at …. Experimental methods of FTIR and WAXD, FTIR and WAXD results, morphologies of the composite membranes with different γ-Fe2O3 contents, morphologies, hydrophobicity and adsorption capacities of the composite membranes with different glycine contents, photographs of the emulsion before and after filtration, viscosity and surface tension of oils, and description of Movies S1-S5.



AUTHOR INFORMATION

Corresponding Author Tel:

+86

28-87603042.

E-mail:

[email protected],

[email protected] ORCID: Yong Wang: 0000-0003-0655-7507 Notes 20


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The authors declare no competing financial interest.



ACKNOWLEDGEMENTS Authors express their sincere thanks to the National Natural Science Foundation of China

(51473137), the International Science and Technology Cooperation Project of Sichuan Province (2017HH0066) and the International Science and Technology Cooperation Project of Chengdu (2016-GH02-00097-HZ) for financial support.



REFERENCES

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We prepared a novel electrospun polylactide/γ-Fe2O3 fibrous membrane with dual-scaled porous structure. The membrane exhibited superhydrophobic and superlipophilic performances with ultrahigh oil adsorption capacity up to 268.6 g/g toward motor oil. 457x207mm (72 x 72 DPI)


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Electrospun Fibrous Membranes with Dual-Scaled ... - ACS Publications

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