Underwater superoleophobic modified polysulfone electrospun membrane with efficient antifouling for ultrafast gravitational oil-water separation

Accepted Manuscript Underwater superoleophobic modified polysulfone electrospun membrane with efficient antifouling for ultrafast gravitational oil-water separation M. Obaid, Euntae Yang, Dong-Hee Kang, Myung-Han Yoon, In S. Kim PII: DOI: Reference:

S1383-5866(17)33319-1 https://doi.org/10.1016/j.seppur.2018.02.043 SEPPUR 14407

To appear in:

Separation and Purification Technology

Received Date: Revised Date: Accepted Date:

12 October 2017 13 February 2018 23 February 2018

Please cite this article as: M. Obaid, E. Yang, D-H. Kang, M-H. Yoon, I.S. Kim, Underwater superoleophobic modified polysulfone electrospun membrane with efficient antifouling for ultrafast gravitational oil-water separation, Separation and Purification Technology (2018), doi: https://doi.org/10.1016/j.seppur.2018.02.043

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Underwater superoleophobic modified polysulfone electrospun membrane with efficient antifouling for ultrafast gravitational oil-water separation M. Obaid1, Euntae Yang1, Dong-Hee Kang2, Myung-Han Yoon2, and In S. Kim1,* 1

Global Desalination Research Center (GDRC), School of Earth Sciences and Environmental

Engineering, Gwangju Institute of Science and Technology (GIST), 123 Cheomdangwagi-ro, Buk-gu, Gwangju 61005, South Korea. 2

School of Materials Science and Engineering, Gwangju Institute of Science and Technology (GIST), 123 Cheomdangwagi-ro, Buk-gu, Gwangju 61005, Republic of Korea.

Corresponding author: Tel: +82-62-715- 2436, Fax: +82-62-715-2434 E-mail: [email protected]

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Abstract A superoleophobic underwater modified electrospun membrane is produced via a novel effective modification method, to overcome the hydrophobicity and fouling propensity of the electrospun polysulfone (PSf) membrane. In which the fabricated PSf electrospun membranes were immersed in different NaOH concentrations (0, 0.1, 0.5, and 1M) at different temperatures (25, 50, 75, and 100oC) for different time (1, 3, 5, 7h) to produce 64 modified membranes. Based on the characterization of the modified membranes, the best-modified membrane was chosen and was investigated in the oil-water separation system. More importantly, the best-modified membrane (M64) exhibits underwater superoleophobic feature and it could effectively separate soybean oil and hexane-water mixtures with a superior water flux of 11,865 and 14,016 LMH after the 5th cycle under a gravity-driven process, respectively. Moreover, the membrane possesses remarkable oil-water separation efficiency with more than 99.99% of oil rejection, where soybean oil and hexane content in the filtrate after the 5th cycle were less than 3 and 1 ppm, respectively, which is lower than the acceptable limit of 10 ppm. Also, the modified membrane exhibited antifouling properties with a water flux recovery ratio of 94 and 96.8% after the 5 th filtration cycles of soybean oil and hexanewater mixtures. Overall, this study introduces a simple and effective method to overcome the hydrophobicity of the PSf electrospun membrane and enhance the antifouling property.

Keywords: Underwater superoleophobic; oil-water separation; nanofiber membrane; modification method; PSf. -2-

1. Introduction Every day massive quantities of oily wastewater are discharged from many industries, such as mining, textiles, food processing, petrochemicals, metal finishing, oil and pharmaceutical [13]. These oily wastewaters are a potential catastrophe to the environment and the necessity to treat these polluted wastewaters is an inevitable challenge for the protection of the environment [4]. Therefore, the development of highly efficient materials or technologies with cost-effective, low energy and practical for effective oily wastewater separation is highly demanded by the governments and enterprises [5]. So far, a variety of oily wastewater treatment technologies and materials have been introduced. Instead of the traditional separation methods, the membrane technologies were preferred due to their unique advantages such as; efficient, ease of operation, cost-effective and no by-product [6, 7]. In industries, pressure driven membranes are widely used for oily wastewater separation [8]. Nevertheless, the fouling is a major disadvantage for the membrane-based separation under pressure [9, 10]. Where the oil droplets are deposited on the surface of the membrane, then they are squeezed through the pores due to the high trans-membrane pressure difference across the membrane, leading to block the pores, decrease the purity of filtrate and decline the water flux [11-13]. Therefore, a membrane separation under gravity or at low pressure will be a suitable solution for overcoming this drawback. Furthermore, several approaches were introduced to improve the antifouling and hydrophilicity features of the membranes, such as membrane surface functionalization [14], surface modification [15], coating on the surface [16], and blending with hydrophilic materials [17]. However, a variety of polymers has been used for the fabrication of membranes, such as polyacrylonitrile (PAN), polysulfone (PSf), cellulose acetate (CA), polystyrene (PS), polyvinylidene fluoride (PVDF), …etc [15, 18-21]. -3-

Polysulfone (PSf) is one of the most widely used polymers to fabricate microfiltration, ultrafiltration, nanofiltration, and substrates for reverse osmosis membranes due to its excellent thermal, mechanical and chemical stabilities [22]. But, the hydrophobic nature of polysulfone membranes causes a terrible fouling during the separation process. To enhance the hydrophilicity, antifouling and performance of the PSf-based membranes, several studies have been reported including surface and structure modification. Where the hydrophilicity of the PSf membrane was increased by the oxygen plasma treatment by K. Kim et.al [22]. Moreover, the polymeric additives such as; polyethylene glycol (PEG), polyetherimide (PEI), polyethersulfone (PES), and polyvinylpyrrolidone (PVP) were blended with PSf to increase the fouling resistance and enhance the membrane performance [23]. Also, silica nanoparticles, grafted with a hydrophilic polymer, was added to enhance the wettability and water flux of the PSf [24]. In addition to blending the hydrophilic inorganic fillers to produce hydrophilic PSf membrane [25, 26]. Among the reported membrane fabrication techniques for PSf is an electrospinning technique, which is currently used in the manufacture of microfiltration membranes for several applications [27-30]. Electrospinning is a fabrication method that can produce continuous polymer fibers with submicron or nano-sized diameters [31, 32]. Which is useful to form electrospun nanofiber membrane for water purification [33-35]. Moreover, the electrospun membrane for microfiltration can overcome the low flux limitation of the membrane fabricated by phase inversion technique [36]. This attributed to their unique properties such as a high porosity, high surface area-to-volume ratio, interconnected open pores, low weight and good interconnectivity of pores [27]. Herein, a simple, novel and an effective modification technique to overcome the hydrophobicity and fouling tendency of the PSf electrospun membrane was introduced. -4-

Moreover, the modification was carried out by immersing 64 samples of PSf electrospun membrane in 4 different concentrations of NaOH at 4 different temperatures and for 4 different modification time. Furthermore, this innovative technique is very interesting because it was implemented in one step and considers a cost-effect technique. The results were very remarkable as the best modified PSf electrospun membrane showed very high water flux and low fouling during oil-water separation under gravity with separation efficiency more than 99.99%.

2. Experimental Section 2.1 Materials Polysulfone (PSf) (average Mw 60,000) and N,N-dimethylformamide (DMF, >99.8%, Sigma-Aldrich) were used to prepare the electrospun membrane. Sodium hydroxide (NaOH, OCI Company Ltd.) and deionized water (DW), obtained from a Milli-Q ultrapure water purification system, were applied for the surface modification. For oil-in-water separation experiments; soybean oil from the local market, hexane (>99.8%, Sigma-Aldrich) and DW were used.

2.2 Preparation of PSf electrospun nanofiber membrane. A desired amount of PSf was dissolved in DMF at 55 oC with vigorous stirring overnight. The produced solution (20 wt.% PSf) was transferred to three plastic syringes bearing a metal capillary. Typically, electrospinning was conducted at a 20 kV voltage, with 15 cm distance between the capillary tip and the rotating drum collector. The solution was pumped using a syringe pump at a constant flow rate of 0.7ml/h. The nanofibers were collected on a rotating

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drum collector (200 rpm) covered with a thin aluminum foil sheet. The electrospun membrane was dried at 110 ◦C for 2 days before further use.

2.3 Modification of the surface of PSf electrospun nanofiber membrane To enhance the performance of the membrane and increase the membrane fouling resistance, the produced electrospun nanofiber PSf membranes were modified by soaking in sodium hydroxide solutions. Typically, 64 electrospun PSf samples were modified using NaOH solutions in different concentrations (0, 0.1, 0.5, and 1M) at different temperatures (25, 50, 75, and 100oC) for different time (1, 3, 5, and 7h) as shown in Table S1. In more details, the membrane samples were immersed in a known concentration of NaOH solution, where, the modification time was fixed at 1h and the influence of different temperatures were investigated (25- 100oC). Then, other samples were soaked in another concertation of NaOH solution, while the time was 1h and temperatures were investigated from (25-100oC). After investigating all the NaOH concentrations and the temperatures at a fixed time (1h), the above steps were repeated for different times; 3, 5, and 7h. Finally, the 64 treated membranes were rinsed thoroughly with distilled water before use.

2.4 Characterization of the electrospun nanofiber membranes Field emission scanning electron microscopy (FE-SEM; S-4700, Hitachi, Japan) was used to observe the morphologies of electrospun nanofibers. The goniometer (Phoenix 300, Surface Electro-Optics Co., South Korea) was used to measure the water contact angle on the membranes. The composition of the membrane was characterized using Fourier-transform infrared (FT-IR; Frontier FT-IR/NIR, Perkin Elmer, USA) spectroscopy. The oil concentration in the filtrate was analyzed by using a total organic carbon analyzer (TOC-L, SHIMADZU).

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The porosity ε of the electrospun membrane was determined by the gravimetric method. Firstly, the membrane samples were weighed as the wet weight (m1) after wiping the excess hexane from the surface. Then the wet samples were dried at 110 ◦C for 1 day to measure the dry weight (m2). Finally, the porosity of the electrospun membranes was calculated by Eq. 1 based on the average weight of three wet and dry membranes [37]. Eq.1 where m1, m2,

and

are the weight of wet membrane, the weight of dry membrane, the

density of hexane and the density of the membrane, respectively. The mechanical properties in term of stress-strain and tensile strength was measured for the pristine (PSf) and modified (M64) membranes using

an universal mechanical tester (BONGSHIN, Model DB35-10,

South Korea) at a speed of 20 mm/min.

2.5 Evaluation of the membrane performance for separation of oil-in-water mixture Two different oil-water mixtures namely; soybean oil-in-water and hexane-in-water mixtures at 400 g/l were prepared and used individually in the evaluation of the performance of the membrane. A glass filter holder in a dead-end mode was used to perform the separation experiments where the total membrane effective area was 12.56 cm2. The electrospun membrane was fixed in the glass filter holder, then oil-in-water mixtures were filtrated under gravity, where the water across the membrane and the oil remained on the top. Where, the water flux (J, LMH) of the membranes was calculated using Eq. (2); Eq.2 Where Am is effective membrane area, and V is the filtrate volume, and t is filtration time.

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To determine the separation efficiency or oil rejection, the oil content in collected water, after the separation process, was measured using the TOC analyzer. And the rejection (R) of the membranes were calculated using Eq. (3); Eq.3 Where Cp and Cf (ppm) are the concentration of the feed and permeate, respectively. 2.6 Evaluation of the antifouling of the membrane The antifouling properties of the modified membrane were estimated by measuring the flux recovery rate (FRR). To evaluate the reusability and the antifouling property of the modified membrane, the FRR was tested during the 5 filtration cycles of soybean-water and hexanewater mixtures. Firstly, the DW was permeated under gravity through the membrane where the water column was fixed for 5 minutes and the pure water flux (PWF) was recorded as Jw1. Then, separation cell was refilled with the oil-water mixture and the permeation flux (Jp) was measured. After each filtration cycle, the membrane was washed and cleaned. Finally, the recovered pure water flux (Jw2) was calculated using DW. The aforementioned steps were repeated for 5 Cycles and the FRR, as a suitable index of antifouling, was calculated for each cycle using eq.4. Eq.4 Furthermore, to study the organic fouling process in detail, the reversible (R r), irreversible (Rir) and the total (Rt) fouling ratios were calculated according to Eqs. 5 to 7 [38]; Eq.5 Eq.6 Eq.7 -8-

3. Results and Discussion

3.1 Influence of the modification process on the membrane morphology The influence of NaOH solutions on the electrospun membranes at different temperatures for various modification times on the surface morphology was investigated using FE-SEM (Fig.S1 and Fig.1). Figure 1 displays FE-SEM images of the pristine and modified PSf electrospun nanofiber membranes obtained from electrospun solutions having 20% PSf. As shown in Fig.1A the smooth and bead free nanofibers were observed for the pristine PSf nanofiber membrane. On the other hand, the FE-SEM images of the modified membranes were presented in Fig.S1 and Fig.1B-E.

As it can be seen in Fig.S1, When the PSf

membranes were exposed to DW, 0.1 and 0.5M of NaOH at all different temperatures (25 – 100oC) for different times (1 – 7h), there was no significant change in surface morphology in comparison to the pristine PSf. Also, the other modified membranes that were modified with 1M at temperature less than 100 oC were not affected and no change in the morphology was observed (Fig.S1).

Conversely, the modification using 1M NaOH at 100 oC showed a

significant altering in the morphology of the nanofibers starting from 3h modification time (Fig. 1B-E). From Fig.1C, it was noticed that the cross-linking between the nanofibers was observed as well as the change in the color was noticed. The change in the morphology was significantly increased by the modification time at 1M and 100 oC. From Fig.1E and F, It is worth mentioning that, M64 modified membrane has the highest change in the morphology.

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By the comparison in Fig.1 E and F, it is clear that the modified membrane no has uniform cross-linking. Fig.1:

3.2 Chemical composition of the pristine and modified membranes Figure 2 displays the full and specific FT-IR spectra for the pristine and modified PSf electrospun membranes. As shown in Fig. 2A and B, a series of peaks are appeared at 1585, 1487 cm-1 indicating the presence of aromatic C=C stretching groups of the PSf. The peak at 1237 cm-1 refers to asymmetric C–O–C stretching of aryl ether group while an asymmetric and symmetric O=S=O stretching groups appeared at 1150 and 1105 cm-1, respectively [39]. On the other hand, the FT-IR results of the modified membrane (M64) showed the presence of new peak of –OH stretching peak at range 3600 – 3200 cm-1 [24], as shown in Fig.2A and the inset. Fig. 2:

However, no new peaks were observed for other modified membranes (M16, M32, M48). Additionally, the FT-IR results of other modified membranes at different modification conditions were presented in Fig.2C and D. where no new peaks appeared for the modification with 0.5M NaOH solution at 100oC for either 5 or 7h, as shown in Fig.2C. Also, same results were obtained in the case of 1M NaOH for 7h at different temperature (25, 50, and 75). So from FT-IR results, it can be concluded that the high chemical stability of PSf resists the modification process at all conditions and the significant change was observed only

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for M64 that was modified in 1M at 100 oC for 7h. Based on the FT-IR results, we hypothesize that treatment of the PSf membrane with NaOH at a harsh condition (1M for 7h at 100oC) resulted in a nucleophilic attack of OH group on the benzene rings, which is adjacent to the sulfonyl group, because it is electron withdrawing [40]. This SO2 group which already is a positively polarized make the neighboring benzene ring more prone to attack by OH- [41].

3.3 Influence of the modification process on the porosity, wettability and mechanical properties. The porosity of electrospun membrane was calculated using Eq.1. As shown in Fig. 3, It can be seen that the pristine membrane owns the large porosity >95%. Moreover, all the modified membranes showed almost same porosity, except these membranes which modified in 1M at 100 oC for different time (M16, M32, M48, and M64). Where the porosities decreased by 1.7, 2, 8.6, and 10% for M16, M32, M48, and M64, respectively. Moreover, the decline in the porosity is because of the structure of PSf nanofiber after modification process, at the high NaOH concentration and high temperature became compacter than the pristine one. In addition to the cross-linking of these modified membranes. However, the FE-SEM images is an evidence of these finding as shown in Fig.1 and Fig.S1.

Fig. 3: The wettability of the pristine and modified electrospun nanofiber membranes was investigated by measuring the water contact angle in air (WCA) and oil contact angle under water and the results were presented in Fig.4, Fig.S2 and Fig.S3. Where, the effect of different NaOH concentrations at different temperatures on WCA of the membranes for 1, 3, 5, and 7h were shown in Fig. 4A, B, C, and D, respectively. As shown in Fig. 4A-D, the pristine PSf - 11 -

membrane displays a high hydrophobic performance as the corresponding WCA is ~125±2.6o. However, the modification process has no significant effect on the WCA of the membranes which modified at temperatures 25, 50, and 75oC in all NaOH concentrations and for all soaking times (Fig.4A-D and Fig.S2). Conversely, the modification at 100oC in 1M showed a decrease in the WCA of the modified membranes, where the WCA decline to 120, 119, 110, and 51o for M16, M32, M48, and M64, respectively. Interestingly, as is shown in Fig. 4E and Fig.S3, the modified membrane (M64) exhibits underwater superoleophobicity with an underwater oil contact angle of 158°±8 while the pristine membrane shows superoleophilicity with oil contact angle of 0o. Remarkably, the proposed modification technique altered the hydrophobicity of the PSf electrospun membrane to hydrophilicity in air and superoleophobicity under water. This change in the wettability is attributed to the presence of –OH group, as it was confirmed by FT-IR (Fig.2), which increases the surface energy of the modified membrane and increases the hydrophilicity. Fig. 4: The mechanical properties in terms of stress-strain and tensile strength of the pristine (PSf) and modified (M64) membranes were measured and the results were presented in Fig.5. From Fig.5A, it was noticed that the M64 membrane exhibited an increase in stress, indicating a stiffer and stronger membrane compared to the pristine membrane (PSf). On the other hand, its flexibility was significantly decreased. More interestingly, the tensile strength of M64 was enhanced by approximately 2.9 times compared to the pristine membrane, as shown in Fig.5B. This an increase in the stiffness and strength is recognized to the good cross-linking between the nanofibers of the M64 membrane as shown in Fig.1, which also decreases the degree of mobility and decreases the flexibility. Fig. 5: - 12 -

3.4 Evaluation of antifouling properties and performance of the membrane in separation of oil-in-water mixtures The pristine and the proposed modified membranes (M64) were examined in a dead end setup (batch mode) using oil-in-water mixtures under gravity to determine the water flux and the separation efficiency. To conduct the separation experiments, the soybean oil-in-water mixture was poured onto the modified membrane, while the height of oil-in-water mixture was fixed for 5 minutes by adding water continuously. Water speedily passed through the modified membrane while, the oil was retained above the modified membrane (Video S1, Supporting information). To make the separation process easy and to decrease the energy consumption, only the gravity was used for the separation. It is worth mentioning that the deionized water (DW) was first used and the pure water flux (PWF) was 13462 ± 840 LMH (Fig.6A and B). Then the soybean oil-in-water mixture was introduced for the first cycle, where the water flux was 12203 ± 633 LMH, then the membrane was washed and reused for the second cycle where it exhibited water flux of 11972±844 LMH. Moreover, the fluxes of the modified membrane (M64) for 5 separation cycles were comparatively stable and at the range of 13462 - 9673 (Fig. 6.A and B). However, the recyclability of the modified membrane (M64) was examined by calculating the change of flux with the time during the separation of oil-in-water for 5 cycles. From Fig.6A, it can be seen that, no observed changes in the flux of the modified membrane during 5 filtration cycles. Where the decrease in the flux in the 3rd and 4th cycles can be attributed to the change of column height of the soybean oil-in-water mixture during the experiments. And the flux of the 5th cycle is an evidence for this, where it was almost equal to the flux of the 1st and 2nd cycles.

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Furthermore, the modified membrane (M64) was investigated for separation of hexane-inwater mixture and the results showed that the water flux for 5 cycles was almost stable in a range 13,110 to 12899 LMH, as shown in Fig.6A and B. By the comparison, the pristine PSf electrospun membrane was investigated at the same condition and it had no water flux under gravity (zero water flux) as shown in Video S2 and Fig.S4. This can be attributed to the high hydrophobicity of the pristine PSf membrane. The fouling significantly affects the membrane performance causing a flux decline and membrane deterioration. Herein, the membrane fouling was studied by measuring the fouling parameters; FRR, Rt, Rr and Rir during alternative filtration of DW and oil-water mixtures. The results of the FRR of 5 filtration cycles of soybean and hexane-water mixtures were presented in Fig.6C and D, respectively. As expected, the modified membrane (M64) that showed the underwater superoleophobicity (see Fig.4F and Fig.S3), had an excellent flux recovery ratio. From Fig.6C, it was found that the FRR of the modified membrane (M64) in 5 separation cycles of the soybean-water mixture were 97.5, 98.1, 76.8, 72.7 and 94%, respectively. Furthermore, the modified membrane showed higher FRR in case of hexane-water mixture numerically; 95, 91.8, 99.8, 98.5 and 96.9% for the 5 cycles, as shown in Fig.6D. To quantitatively study the antifouling behavior of the modified membrane, the fouling parameters (Rt, Rr and Rir) were calculated during each cycle, to point out the level of cleaning efficiency, and the results are also plotted in Fig. 6C and D. In general, a bigger value of Rt denotes a higher fouling and total flux loss. As it can be seen from Fig.6C, the Rt of M64 in first to fifth filtration cycles of soybean oilwater mixture were 9.4, 11.7, 26.6, 28.1 and 11.9%, respectively. It is clear that the reversible fouling (Rr) prevailed the total fouling for the first and second cycles, but the other cycles - 14 -

showed the irreversible fouling (Rir) is dominant. However, the noticed increase in the total fouling ratio (Rt) for the third and fourth cycles is attributed to the change of the head of the oil-water column on the membrane during these cycles. For Hexane-water mixture, the M64 maintained its high antifouling efficiency during five cycles with Rt values of 9.2, 13.5, 8.7, 2.7 and 10.6%, respectively, as shown in Fig.6D. Furthermore, the reversible fouling (Rr) was dominant for all the cycles excluding the first cycle. These results indicated that the oil foulant could be washed easily away by washing or physical rinsing of the modified membrane. However, the obtained results could be assigned to the underwater superoleophobicity property of M64 membrane which forms a water layer on its surface, hindering the oil adsorption into the membrane. Overall, the modified membrane exhibited an excellent flux recovery and an excellent organic fouling resistant, indicating an excellent reusability and durability of the modified membrane. Fig. 6:

The separation efficiency of the modified membrane was investigated and from Fig.7, it was observed that the oil was retained on the membrane and only the water can pass through the modified membrane. Moreover, oil was not observed in the water filtrate (colored blue), indicating a high separation performance. To detect the oil quantitatively in the filtrate, the oil content in the filtrate, after each cycle, was measured by TOC and the oil rejection was calculated according to Eq.3 and the results were presented in Fig.8. Fig.7: From Fig. 8, it was observed that the modified membrane showed extremely high oil rejection of 99.99% for all separation cycles for different oil-water mixtures. Furthermore, the modified membrane is successful diminished the soybean oil content in the filtrate from 400 g/l to less - 15 -

than 3 ppm which is less than the acceptable discharge limit of 10 ppm, as shown in Fig.8A. In addition, the modified membrane exhibited outstanding separation efficiency for the separation of the hexane-water mixture (Fig.8B). From the results, it was noticed that the hexane concentration in the filtrate does not exceed 1ppm with a rejection efficiency of >99.99%.

Fig.8: Overall, a comparison between the obtained flux and oil rejection from the modified membrane (M64) and numerous literature reports under gravity was presented in Fig. 9 and Table S.2. Herein, the obtained results were comparable to those reported in literature, as shown in Fig.9, and the modified membrane (M64) had the highest flux among these reported membranes. Fig. 9:

4. Conclusion In Summary, a new effective and simple modification method for PSf electrospun nanofiber membranes was introduced. The modification of the PSf electrospun membrane in 1M NaOH at 100oC for 7h strongly affected the properties of the membrane. Thus, a superoleophobic underwater modified membrane (M64) was successfully produced. Accordingly, the modified membrane (M64) exhibited superior water flux, excellent separation efficiency, and excellent antifouling properties. Conversely, the chemical stability of the PSf resists the modification process at a temperature <100o C in low NaOH concentrations. More interestingly, the ultrafast separation process under gravity makes the modified membrane a promising - 16 -

candidate for numerous practical applications, such as water purification and oil recovery. Overall, the introduced modified membrane can contribute to addressing the water shortage.

Acknowledgement This research was supported by a grant (code18IFIP-B087389-05) from Plant Research Program funded by Ministry of Land, Infrastructure, and Transport.

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List of Figures Fig.1: FE SEM images for the electrospun nanofiber membranes obtained from precursor solutions having 20 PSf. (A) pristine PSf, (B to F) modified membranes in 1M NaOH at 100 oC for 1, 3, 5, and 7h, respectively. (E and F images for the same membrane; M64) Fig. 2: FT-IR results for pristine and modified PSf electrospun membranes; (A) and (B) full spectra and the spectra at specific ranges (1700 -400 cm-1) of the pristine and modified membrane in 1M at 100oC for different times, respectively. (C) Full spectra of the pristine and modified membrane in 0.5M at 100oC for 5 and 7h, and (D) display the pristine and modified membrane in 1M for 7h at 25, 50, and 75 oC. Fig. 3: Average porosity of the pristine and modified membranes, where the x-axis from 1-64 represents the membrane code based on Table S1 and value 0 represents the pristine PSf. Fig. 4: Effect of NaOH concentrations at different temperatures on the water contact angle of the PSf membrane for different modification times 1h(A), 3h(B), 5h(C) and 7h(D) and wetting behavior of the modified and pristine membrane toward oil (hexane) underwater (E and F), respectively. Fig. 5: Mechanical properties of the pristine and the modified membrane (M64): (A) stressstrain curve and (B) tensile strength. Fig. 6: Water Flux of the modified membrane (M64) during 5 filtration cycles of soybean-water and hexane-water mixtures; (A) the water flux vs. time, (B) the average water flux, and (C &D) the summary of the corresponding fouling parameters (FRR, Rt, Rr and Rir) during 5 cycles of filtration of Soybean oil-water mixture and hexane-water mixture, respectively. Fig.7: Photo images for the soybean oil-water separation process for the modified membrane M64 under gravity, where the water was colored with methylene blue. Fig.8: Oil rejection and concentration of oil in the filtrate for 5 filtration cycles of soybean oilwater (A) and hexane-water (B) mixtures. Fig. 9: Comparison between the water flux and oil rejection in this study (M64; Ref. 0) and recently reported publications using gravity for the separation (the X-axis is the number

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of Refs. from Table S2 supporting information; Ref. 1-6 are for oil-water mixtures and the others are for separation of oil-water emulsions).

A

B

C

D

E

F

Fig. 1:

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A

C

B

D

Fig. 2:

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Fig. 3:

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B

A

C

D

E

F

Fig. 4:

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B

A

Fig.5:

A

B

D

C C

Fig. 6:

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A

B Fig. 7:

A C

B

Fig. 8:

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Fig. 9:

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Graphical abstract

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Highlights  Underwater superoleophobic PSf electrospun membrane is introduced. 

The effective modified electrospun PSf membrane attained ultrafast water flux.



The modified membrane exhibited improved antifouling properties (94% FRR).



Excellent oil rejection with more than 99.99% was achieved.

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