Î²-FeOOH nanofibrous membrane for

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Surfaces, Interfaces, and Applications

Hierarchical stabilized PAN/#-FeOOH nanofibrous membrane for efficient water purification with excellent anti-fouling performance and robust solvent-resistance Liyun Zhang, Yi He, Lan Ma, Jingyu Chen, Yi Fan, Shihong Zhang, Heng Shi, Zhenyu Li, and Pingya Luo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b12855 • Publication Date (Web): 29 Aug 2019 Downloaded from pubs.acs.org on August 30, 2019

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ACS Applied Materials & Interfaces

Hierarchical stabilized PAN/β-FeOOH nanofibrous membrane for efficient water purification with excellent anti-fouling performance and robust solvent-resistance Liyun Zhanga,b, Yi He*a,b, Lan Mac, Jingyu Chen*d, Yi Fan*a,e, Shihong Zhanga,b, Heng shia,b , Zhenyu Li a,f, and Pingya Luoa,b a. State Key Lab of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu, 610500, P. R. China. b. College of Chemistry and Chemical Engineering, Southwest Petroleum University, Chengdu, 610500, P. R. China. c. School of Science, Xihua University, Jinzhou Road, Chengdu, 610039, P. R. China. d. Institute for Frontier Materials, Deakin University, Geelong/Melbourne, Australia e. Chengdu Graphene Application Institute of Industrial Technology, Chengdu, 611130, P. R. China f. School of Materials Science and Engineering, Southwest Petroleum University, Chengdu, 610500, P. R. China *Address

correspondence

to

these

authors.

Email:

[email protected],

[email protected] & [email protected]: +86 02883037315.

1

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 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

ABSTRACT: Filtration membranes, with good anti-fouling performance and robust solvents resistance (e.g., organic solvents or highly acidic/alkaline/saline solvents), can effectively purify complex polluted water systems is specially demanded in practice but exists a challenge to be conquered. Herein, a simple method has been demonstrated to address the obstacles, applying the stabilized polyacrylonitrile (PAN) nanofibers/βFeOOH nanorods composite membrane as model. In this work, simply stabilizing PAN nanofibers in air can achieve robust solvent-resistance against organic solvents and strong inorganic acidic/alkaline/saline solutions. Hydrophilic β-FeOOH nanorods follow were anchored onto SPAN nanofibers of our electrospun membrane, and achieves superhydrophilicity (0°)/underwater superoleophobicity (>155°) for various oils. More importantly, SPAN/β-FeOOH nanofibrous membrane exhibits robust mechanical strength (274 MPa of Young modulus), excellent chemical stability, fast separation fluxes (2532 - 10146 L m-2 h-1) and satisfying removal ratios (>98.2%) against insoluble oils and soluble cationic dyes. In addition, good photocatalytic activity against organic pollutants provides our membranes with excellent flux restorability and endows a long-term use capacity. These outstanding performances endow our membrane a great potential application in purifying polluted aquatic system in worldly harsh conditions. Keywords: nanofibrous membrane, superwettability, dual-functional wastewater purification, visible-light degradation, solvent-resistant.

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1. INTRODUCTION The fresh water crisis, caused by the increasing oily water discharging from industrial effluent, the frequent oil spill accidents and domestic activities, has been a most critical worldwide challenge for the sustainable development of our world.1-3 To tackle this issue, diverse methods have been explored for oily water purification including air flotation, membrane separation, and skimmers, absorption and so on.4-6 Among those methods, polymeric filtration membranes (PFMs) technology have been considered as a great promise route for oily waste purification due to the high efficiency, low cost, flexibility, and simple operation process.7-10 However, the natural defect (fouled easily, low permeation ﬂux, etc.) of PFMs became a great obstacle for their development in practical applications.11-13 Currently,

bioinspired

superwetting

surfaces

have

made

amazing

development. Combined the high-surface-energy and surface roughness to design and construct the superhydrophilic and underwater superoleophobic surface, such special surficial wettability is widely believed to effectively prevent the fouling issues originated from oil phase within the oil-water mixture.14-19 However, faced complex sewage system contains insoluble oil and soluble dyes, such PFMs usually can’t achieve effective simultaneous removal for oil and dye. Besides, eliminating the fouling issue originated from adsorbing dye of membrane surface during the filtration, which will reduce their permeation flux, shorten the membrane lifespan and maybe cause secondary pollution, also is an 3



urgent required.20-23 On the other hand, filter membrane coupled photocatalytic technology for water purification, utilizing visible and UV light to catalyze eliminate organic pollutants on membrane surface, has been attested to be a facile and sustainable strategy to clean fouled membrane.24-25 For example, Feng et al. 26

prepared PDA layer /amine-functionalized TiO2 nanoparticles decorated the

polysulfone (PSf) membrane, which displays a good UV-induced self-cleaning performance. Li et al.27 used iron (II) phthalocyanine (FePc) to add the PVDF membrane matrix and endowed modified membrane the robust fouling resistance and degradation capability for organic contaminants under visible light radiation. Thus, integrating membrane separation and photocatalytic technology to improve anti-oilfouling performance, design a dual-functional wastewater purification and endow self-cleaning ability for separating complex waster system is very essential. Polymeric electrospun nanofibrous membranes (PENFMs) have aroused greatly interesting in the oil/water separation field due to their high permeation fluxes, large specific surface area and adjustable functionality. Ge et al.28 constructed superwettable polyacrylonitrile (PAN) nanofibrous membrane with lotus-leaf-like nanofibrous skin via electrospray and electrospinning. The asprepared PAN nanofibrous membrane displayed not only an ultrahigh permeation ﬂux (5152 L m−2 h−1) and removal efficiency (99.93%) for oil/water emulsions under 1 KPa, but also significant anti-oilfouling performance and robust long-term separation capacity. Zhu et al.29 utilized special engineering 4


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plastics (polysulfonamide, PSF) to prepare an outstanding thermal stability electrospun nanofibrous membrane. This calcinable polymer membrane achieves robust reclamation after fouled by polar pollutants. Nevertheless, these PENFMs were fabricated from precursor solutions via high voltage electric filed, which made these PENFMs naturally dissolve in these solvents (such as N, Ndimethylformamide, dimethyl sulfoxide, etc.). This character further turned into an obstacle and impaired the use of PENFMs in practical oily water purification from the industrial effluent for the large amount of organic/acidic/alkaline/saline solutions used in industries.30-31 Thus, rational design of PENFMs with high purification efficiency against emulsified oily wastewater, strong anti-fouling performance, and robust solvents resistance is highly required for oily-water separation from complex polluted aquatic system. Herein, we demonstrate a simple route to overcome mentioned above by anchoring β-FeOOH nanorods onto stabilized polyacrylonitrile nanofibrous membrane (SPAN NFM). During the stabilization of PAN, cyclization and dehydrogenation happened,32-33 enabling the PAN NFM with robust solvent resistance without destroy its flexibility. Through decorating hydrophilic βFeOOH nanorods onto SPAN nanofibers surface to construct micro/nanohierarchical

structures

on

SPAN

NFM,

which

further

provide

superhydrophilicity and underwater superoleophobicity, and good photocatalytic capacity for SPAN/β-FeOOH NFM. Importantly, the as-prepared membrane also can effectively remove insoluble oil and soluble dye from complex oily waste 5



system and can efficiently recovered its state under visible radiation. Moreover, the membrane displays fast flux (2532 - 10146 L m-2 h-1) and satisfying removal efficiency (>98.2%) under low pressure (≈0.2 bar) for various wastewater. Meanwhile, our SPAN/β-FeOOH NFM is robust enough to resist diverse organic solvents (e.g., DMF, NMP, DMAc and DMSO), and acidic/alkaline/saline solutions (1M HCl, 1M NaOH, 3.5 wt% NaCl) for 5 days.

2 EXPERIMENTAL SECTION 2.1 Materials The used polyacrylonitrile (PAN, Mw=150000) in this experiment can be available from Macklin Biochemical Co. Ltd., shanghai, China. N, Ndimethylformamide (DMF, 99.9%), N-methyl-2-pyrrolidone (NMP, 99%), N, Ndimethylacetamide (DMAc, 99.9%), dimethyl sulfoxide (DMSO, 99%) were obtained from Aladdin Chemical Co. Ltd., Shanghai, China. Methylene blue (MB), Iron (III) chloride hexahydrate (FeCl3. 6H2O), sodium dodecyl sulfate (SDS), hydrochloric acid (HCl, 35% - 37%), toluene, petroleum ether, n-hexane, and 1, 2-dichloroethane were supplied from Kelong Chemical Co. Ltd., Chengdu, China. All materials are of analytical grade and used without further treatment. 2.2 Fabrication of PAN nanofibrous membrane (PAN NFM) The PAN electrospinning solution (12%) was prepared by dissolving 1.2 g PAN powders in 10 mL DMF and continuously stirring at 500 rpm and 70 °C for 12 h. During the electrospinning process, The PAN solution was put into the 10 6


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mL syringe, pumped injection speed is 0.098 mm min-1 and the metal needle type is 21 gauge. 19 KV of positive voltage was put the needle tip, -1 KV of negative voltage was put the collecting substrate. The distance of collecting and translation of the needle tip was 20 cm and 15 cm, respectively. To ensure a similar thickness of nanofibrous membrane, electrospinning time was set as 8 h. In addition, the ambient temperature and humidity were maintained at 35 ± 5 °C and 45 ± 5%, respectively. Then, the collected membranes on the aluminum foil was put into vacuum oven to remove the residual solvent at 60 °C for night. 2.3 Stabilization of PAN nanofibrous membrane According to previous studies,33-35 the PAN molecular chains start occurring cyclization, dehydrogenation and oxidation reaction above 180°C. However, main reaction is cyclization below 238°C, and its speed will reach to its maximum value with increasing temperature to 230°C-238°C. Then increase 248°C, the main reaction translate into dehydrogenation and oxidation. Further reached 260°C, oxidation sharply occurs and -C≡N start decomposing for PAN molecular chains. But high temperature treatment for a long time would make fibers break. Thus, the stabilized PAN nanofibrous membrane (SPAN NFM) was fabricated by a staged thermal treatment method at different temperatures. In brief, PAN NFM was cut to 7 cm × 7 cm, and held it on a custom-made metal clip to prevent the PAN NFM from shrinking and make surface smooth at a high temperature. Put the clip into an oven, and the temperature gradually went up to

7



238 °C, 248 °C and 260 °C and maintained 30 min at each stage. Then, naturally cooled down to room temperature and obtained the SPAN NFM. 2.4 Mineralization of the SPAN NFM Mineral layer of stabilized PAN nanofibers surface was prepared according to previously published methods.36-37 In brief, dissolving 1.08 g FeCl3. H2O into 60 mL deionized water, then adding the 30 mL as-preparation HCl (10 mM) into the FeCl3 solution to form a uniform mixed solution under stirring. The prepared SPAN NFM was cut to 5 × 5 cm2 and immersed into the above solution at 60 °C for definite times. Finally, the mineralized membrane was taken out, deionized water cleaned it three times and dried it at 60 °C for 3 h. the final membranes were denoted as SPAN/β-FeOOHx NFMs, where x (1, 3, 6, 12) is the mineralized time, respectively. 2.5 Preparation and separation oil/water emulsion In general, using several oils (including petroleum ether, hexane, toluene and diesel) to prepare the surfactant-free emulsions (SFEs) and surfactantstabilized emulsions(SFEs).14, 28 The milky SFEs were prepared by mixing the oils (1 mL) and deionized water (99 mL), and placed them into ultrasonic cleaner (KQ-100DE) for 30 min at 25°C. In addition, for SSE, 10 mg SDS was dissolved in 99 mL water and 1 mL oils were added into it, the mixed solutions were ultrasonic treatment for 30 min to obtain stabilized oil-in-water emulsions. The oil/water separation property for as-prepared membrane was test under the dead-end filtration apparatus with permeation area of 1.767 cm2. The 8


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prewetted membranes with water were fixed in the apparatus, and various emulsions directly poured onto the prewetted membrane under 0.2 bar pressure. The permeation flux (J, L m-2 h-1) and the removal efficiency (RE, %) were calculate using eq. (1) and eq. (2), respectively. J = V/ (A. t)

(1)

RE (%) = (1 – Cf /Co) × 100%

(2)

Where A (m2) is the membrane filtration area, V (L) is the filtrates volume, the whole separation time (t) is set 1 min. Cf and Co are the oil concentration in the filtrates and the feed solution, respectively. 2.6 Evaluation of Photocatalytic Activity The degradation of methylene blue (MB) was used to evaluate the photocatalytic performance of SPAN/β-FeOOH NFM. The whole degradation process was carried out in 100 mL glass device with recycled water to maintain a room temperature. A 300 W xenon lamp with a 420 nm optical filter (PLSSXE300/300UV, Perfect-Light, China) was regard as a typical visible-light source. Typically, the as-fabricated membranes (2.5×2.5 cm2 , weight : 5 ~ 7 mg) and 50 μL H2O2 were simultaneously added into 50 mL MB solution (10 ppm, pH =7) , stirring at 200 rpm for 60 min in the dark to reach the adsorption equilibrium. After a fixed time interval, taking out 2 mL dyes from solutions to monitor the concentration of remnant dyes by UV-vis spectrophotometer. 2.7 Apparatus and Characterization

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The electrospinning process was completed by the electrospinning instrument (SS-2535H, Ucalery, Beijing). The X-ray diffraction (XRD) (X'Pert PRO MPD, Holland) and Fourier transform infrared spectroscopy (FT-IR) (Thermo Fisher Nicolet iS10) were characterized the chemical components of membrane surface, respectively. Morphologies of the membranes surface and the distribution of β-FeOOH nanorods on the fibres surface were observed by field emission scanning electron microscopy with X-ray energy dispersive spectrometry. (FE-SEM, JSM-7500F, JEOL, Tokyo, Japan). The water contact angles (WCAs) in air and underwater oil contact angles (UWOCAs) of the membranes were evaluated by a contact angle measuring instrument (KRUSS DSA30S, Hake, Germany). The diffuse reflectance spectra (DRS) were obtained by an UV–vis spectrophotometer (PerkinEImer, Lambda850, America, BaSO4 used as the reference). Surface Zeta potentials of membranes were measured on an electrokinetic analyzer (Anton Paar SurPASS2, Austria). The size of oil droplets of oil-in-water emulsions were observed by optical microscopy (Nikon Digital Sight DS-F11, Japan). The TOC content of filtrates were measured by a total

organic

carbon

analyzer

(Shimadzu

TOC-VCPH,

Japan).

The

concentrations of dyes were determined by a UV-vis spectroscopy (Shimadzu, UV-1800, Japan).

3. RESULTS AND DISCUSSION 3.1 Preparation and characterization of membranes 10


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To clearly see the whole procedure, all the steps have been illustrated in Figure 1a. Firstly, The PAN NFM is prepared by electrospinning. Then, the obtained membrane is stabilized in air to obtain a strong stability in complex environment. At this stage, with complicated chemical reactions occurring, PAN molecules has been transformed into a non-melttable ladder structure.32 At last, biomimetic mineralization is used to construct β-FeOOH nanorods on SPAN fibers surface to enhance surficial roughness, strengthen adsorption and photocatalytic capacity towards organic pollutants. During in-situ growth βFeOOH process, it can be ascribed to eq. (3) and (4). Firstly, SPAN nanofibers rapidly adsorbed Fe3+ due to abundant of O- and N-containing functional groups on the surface. Obviously, Fe3+ easily hydrolyzed and formed Fe(OH)3 in the aqueous solution. Subsequently, the low pH ( 99.7 % and 99.1 %, respectively. These results of low TOC contents of filtrates and high separation efficiency of SPAN/β-FeOOH12 membrane for various emulsions verified the oil droplets in water phase were effectively removed.

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Figure 4. (a) Photographs and optical micrographs of SFE (top) and SSE (bottom) for toluene, respectively. (b) Permeation fluxes of various oil/water emulsions permeated through the SPAN/β-FeOOH12 membrane under 0.2 bar. (c) TOC contents in filtrate of different emulsions and corresponding separation efficiency. 3.4 Photocatalytic activity Based semiconductor-β-FeOOH nanorods with 2.06 eV band gap decorated PANbased electrospun membrane, the light adsorption properties of pure PAN and βFeOOH modified PAN NFMs were further explored by UV-vis diffuse reflectance spectroscopy (DRS). Figure 5a showed a stronger and broader absorption in the visiblelight range (λ > 400 nm) of SPAN/β-FeOOH12 membrane compared to only a very narrow adsorption in UV light (λ < 400 nm) of PAN NFM. This result possesses outstanding visible-light response and trap capacity for SPAN/β-FeOOH12 NFM, it implied that the membrane can become a potential visible-light derived photocatalyst membrane. In this work, we chose methylene blue (MB) as representative organic contaminant to evaluate the photodegradation performance and the results were shown in Figure 5b - 5d. Figure 5b showed UV–vis absorption spectra of MB solutions which undergo different illumination times with SPAN/β-FeOOH12 NFM. It can be clearly see that the absorbance at 664.8 of MB become weaken gradually and almost disappear for 40 min irradiation. Besides, Figure 5c demonstrated relative concentrations of MB solutions at different illumination times, which implied the conjugated chromophore structure of MB molecules were rapidly destroyed in the presence of visible-light 20


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irradiation. Moreover, photographs for before and after visible-light degradation of the MB solution also intuitively recorded the every change of blue MB to transparent solution at different time interval, (Figure 5d) which demonstrated SPAN/β-FeOOH12 NFM has excellent photo-Fenton catalytic activity.

Figure 5. (a) UV-vis diffuse reflection spectra of PAN and SPAN/β-FeOOH12 NFM. (b) UV-vis adsorption spectral and (c) Photocatalytic activity curve of MB at different illumination time. through photocatalytic degradation. (d) photographs of undergoing different visible-light degradation time of MB solutions, respectively. 3.5 Simultaneously removal of oil and dyes in water As everyone knows, oily wastewater is complex system containing insoluble oils and soluble organic matter (eg. dyes, etc.). The removal of these watersoluble contaminants have always been a difficult problem for the oil/water 21



filtration membrane. In order to evaluate simultaneous removal of insoluble oils (diesel) and soluble dyes (MB) removal ability of SPAN/β-FeOOH12 membrane. Figure 6a exhibites the separation device for oil/dye mixed emulsion. In detail, 10 mL initial concentration of 5 ppm methylene blue (MB) solution or 1 % oily emulsion with 5 ppm MB solution were poured onto membrane surface, rapidly through SPAN/β-FeOOH12 membrane under 0.2 bar driven pressure and transparent, colorless filtrate were collected, respectively. Moreover, from Figure 6b and Figure S9, the disappeared absorption bands of MB (664.8 nm) and turned into flat of filtrate in UV–vis spectrum hint the oil and dye were removed simultaneously, and the separation efficiency of oil and dye both reached above 99%. This result could be attributed to oxidation of PAN fibers endows abundant N, O-containing groups and β-FeOOH nanorods with natural negative charge on the high porous nanofibrous membrane, which make the SPAN and SPAN/βFeOOH12 NFMs surface show -71.01 mV and -117.39 mV zeta potential, respectively. (Table S2, ESI†) The negative charges provide strong the electrostatic interaction between SPAN/β-FeOOH12 NFM and cationic MB dye and selected capture cationic dye during oily emulsion separation. (Figure 6c) 5152

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Figure 6. (a) Photograph of the oil/dye mixture emulsion separation apparatus. (b) UV-vis spectra of diesel-in-dye/water emulsion and corresponding filtrate. (c) Removal mechanism of cationic dyes for SPAN/β-FeOOH12 membrane. (d) - (e)The changes of permeation fluxes and removal efficiency of diesel/MB emulsions in cycle experiment by using different methods to clean the fouled SPAN/β-FeOOH12 membranes, respectively, including (d) hydraulic cleaning and (e) visible-light induction self-cleaning. For continuously treating complex oily wastewater system with organic contaminants, fouling of membrane is an inevitable question because of the accumulation of organic. Figure 6d and 6e recorded the changes of permeation flux and separation efficiency of cyclic treating of MB/diesel emulsions, and hydraulic or photo-induction cleaned the membrane surface after finishing every cyclic experiment. For hydraulic cleaning method (Figure 6d), the permeation flux irreversibly reduced to 1875 from 2556 L m-2 h-1 and removal efficiency also 23



reduced to 45.6% from 98.2% for MB, respective. This result can attribute to adsorption equilibrium of dye and remnant oil droplets adhere on membrane surface under 0.2 bar pressure. A conventional hydraulic cleaning method (simply rinsed several times with water to clean the membrane) hardly wipe off these adsorbed dye and adherent residual oil droplets on the membrane surface. For comparison, after finishing every separation experiment, we used 40 min photo-induction self-cleaning method to remove the adsorbed organic matter. As show in Figure 6e, the flux and separation efficiency still completely maintained initial state after 5 cyclic experiments, which indicated the excellent self-cleaning and reusability capacity of SPAN/β-FeOOH12 membrane. Moreover, the changes of photographs, corresponding WCA and UWOCA of original, fouled and cleaned states were recorded to further characterize the photo-induction self-cleaning performance of SPAN/β-FeOOH12 NFM. As shown in Figure 7a, the initial SPAN/β-FeOOH12 membrane demonstrated superhydrophilicity (~0°) and underwater superoleophobicity (~157.8°). However, after treating diesel/MB mix emulsion for 1 min, the SPAN/βFeOOH12 NFM inevitably suffered serious pollution due to adsorbed organic matter. (Figure 7b) The CA simultaneously increased to 98.5° and UWOCA reduced to 137.8°. When the illumination was introduced onto the fouled membrane surface for 40 min under stirring, the superhydrophilicity/underwater superoleophobicity again was recovered, the CA again reduced to 0° and UWOCA increased to 156.4°, respectively. (Figure 7c) This result again 24


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confirmed that the SPAN/β-FeOOH12 NFM was endowed outstanding photoinduced self-cleaning performance.

Figure 7. Photographs of (a) initial, (b) fouled by MB and diesel and (c) Cleaned by under visible-light induction 40 min SPAN/β-FeOOH12 membranes and its CA in air and UWOCA, respectively. 3.6 Separation and self-cleaning mechanism Moreover, a plausible mechanism for separating diesel/water emulsion contain MB and the photo-induced self-cleaning process for SPAN/β-FeOOH12 NFM is displayed Figure 8. When oil/MB mixed emulsions are poured onto the prewetted SPAN/β-FeOOH12 membrane, the insoluble oil droplets first are rejected and freely roll on the SPAN/β-FeOOH12 NFM surface because of outstanding underwater repelling oil-adhere ability of membrane, then the adjacent oil droplets collide and coalesce to the larger diameter oil droplets under driving force. After that, large diameter oil droplets would spontaneously 25



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completely detach from the membrane surface due to surface oleophobicity and rise to the surface of the emulsion to form free oil because of ρH2O > ρdiesel.28 On the contrast, MB is soluble organic matter and it would permeate into the membrane together with water. But the β-FeOOH nanorods with negative charge on the nanofibers provide abundant adsorption site for cationic MB during the separation process, the MB molecules are extracted and fixed on the membrane. Thus, the SPAN/β-FeOOH12 NFM displays remarkable separation capacity for oil/dye mixed emulsions. For the photo-induced self-cleaning performance of SPAN/β-FeOOH12 NFM, the degradation process for organic pollutant (R) can be follow eq. (6)–(10) to further expound.36 hν β - FeOOH  e-  h +

(6)

e-  O 2  O-2

(7)

O-2  H 2O 2  OH  OH -  O 2

(8)

h +  H 2O 2  OH  OH 

(9)

OH  R   H 2O  CO 2

(10)

Under the visible light irradiation, β-FeOOH nanoparticles generate electron (e-)–hole (h+) pairs. Subsequently, excited electrons (e-) from value band (VB) transfer to the conduction band (CB) of β-FeOOH. After that, the e- in the CB of β-FeOOH combine with adsorbing O2 and generate superoxide radical (·O2ˉ) on the β-FeOOH surface, and sectional ·O2ˉ subsequently react with H2O2 further produce a hydroxyl radicals (·OH) with oxidation potential of 2.8 V. Moreover, photo-induced hole (h+) on the VB of β-FeOOH also easily react with H2O2 to 26


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generate more ·OH. During the whole self-cleaning process, ·O2ˉ and ·OH would directly oxidize the adsorbed organic molecular on the SPAN/β-FeOOH12 membrane into nontoxic product (CO2, H2O) due to their high oxidative capacity.53-54

Figure 8. Schematic illustration of separation and the photo-induction selfcleaning of the SPAN/β-FeOOH12 membrane 3.7 Stability in harsh environments

Figure 9. (a) Solvent-resistance of PAN and SPAN/β-FeOOH12 membrane in DMF solvent. (b) The UWOCA and optical profiles of 1, 2-dichloroethane (3 27



μL) of SPAN/β-FeOOH12 membrane after immersing different organic solvent (DMF, DMAc, DMSO and NMP) and inorganic solutions (acid, alkali, saline) for 5 days. The chemical stability of SPAN/β-FeOOH12 NFM is significant feature to for treat emulsion in hard conditions. To test membrane stability, the obtained PAN, SPAN/β-FeOOH12 membrane were directly immersed in strong polar organic solvents or corrosion environments (such as 1M HCl, 3.5% NaCl, 1M NaOH solutions), respectively. As shown in Figure 9a, when original PAN membrane contacted with DMF (parameter polarity=6.4), which rapidly dissolved within 2 s completely. In contrast, the SPAN/β-FeOOH12 membrane was constructed by the stabilized PAN nanofibers and rod-like β-FeOOH nanoparticles hardly dissolved in DMF. As expected, SPAN/β-FeOOH12 NFM also can resist other strong polar solvents. (Figure S10 and Movie S1). Subsequently, the membrane was placed in these solutions for a period of time to investigate the robust solvent-resistance, we can find that the DMF and DMSO with membrane solution started changing into brown-yellow of membrane from transparent after 5 days, implying the SPAN/β-FeOOH12 NFM started to dissolve slowly. However, the color of DMAc and NMP solutions only have changed into faint yellow. Furthermore, the membrane also showed relatively stable in acid, alkali and saline environments. (Figure S11). Figure 9b demonstrated the UWOCA of SPAN/β-FeOOH12 after immersing various hard environment for 5 days, which showed that the membrane surface still maintain 28


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good underwater oleophobicity. Moreover, the SEM image of the after treatment membrane shows still retain its original surface morphology compared to asprepared SPAN/β-FeOOH12 (Figure S12), which further verify its strong resistsolvent property. These results all implying that the SPAN/β-FeOOH12 NFM can be used in a variety of complex environments.

Conclusions

In summary, a stable inorganic-organic electrospun PAN based nanofibrous membrane

with

superhydrophilicity/underwater

superoleophobicity

was

successfully prepared via a simple thermal treatment and mineralization process. With the synergistic effect of abundant N and O-containing groups and like-rod hierarchical structures on the surface of fibers, the obtained SPAN/β-FeOOH12 NFM exhibited excellent high underwater oil contact angle and fast water spreading.

Furthermore,

SPAN/β-FeOOH12

NFM

achieved

ultra-high

separation flux (≈8754 L m-2 h-1), outstanding removal efficiency (≈99.8%), excellent recyclability and removal of the soluble dyes and insoluble oils from wastewater simultaneously. Most importantly, the fouled membrane can reinstate original state and implement self-cleaning of membrane by visible-light degradation. At last, the SPAN/β-FeOOH12 NFM was immersed harsh environment (corrosion and polar organic solvent) and presented excellent stability. We believe that the rigid mineral SPAN NFM is very promising in wastewater purification field due to their facile fabricated procedure and can

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simultaneously and continuously remove insoluble oils and soluble dyes in water.

AUTHOR INFORMATION

Corresponding Author * Email: [email protected] * Email: [email protected] * Email: [email protected]

ORCID Liyun Zhang: 0000-0002-2282-5308 Lan Ma: 0000-0003-2845-3039 Yi Fan: 0000-0002-4532-6747 Shihong Zhang: 0000-0003-1871-3116 Pingya Luo: 0000-0003-1243-0545 Zhenyu Li: 0000-0003-0994-0405

Notes The authors declare no competing financial interests

ACKNOWLEDGEMENTS This work was financially supported by National Natural Science Foundation of China (51774245), Applied Basic Research Program of Science and Technology Department of Sichuan Province (No.2018JY0517), Open Fund (PLN161) of State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation (Southwest Petroleum University). We are grateful for experiment characterization test provided by “ceshigo” (www.ceshigo.com).

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Table of Contents

Superhydrophilic, underwater superoleophobic, photocatalytic and robust solvent-resistant stabilized PAN/β-FeOOH nanofibrous membrane for oily-water separation within complex polluted aquatic system.

39


Î²-FeOOH nanofibrous membrane for

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