Reusable and Recyclable Superhydrophilic Electrospun Nanofibrous

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Reusable and Recyclable Superhydrophilic Electrospun Nanofibrous Membranes with in-situ Co-crosslinked Polymer-Chitin Nanowhisker Network for Robust Oil-in-Water Emulsion Separation Jian-Xiang Wu, Jie Zhang, Yan-Li Kang, Gang Wu, Si-Chong Chen, and Yu-Zhong Wang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03102 • Publication Date (Web): 14 Dec 2017 Downloaded from http://pubs.acs.org on December 20, 2017

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ACS Sustainable Chemistry & Engineering

Reusable and Recyclable Superhydrophilic Electrospun Nanofibrous Membranes with in-situ Co-crosslinked Polymer-Chitin Nanowhisker Network for Robust Oil-in-Water Emulsion Separation Jian-Xiang Wu, Jie Zhang, Yan-Li Kang, Gang Wu, Si-Chong Chen,* Yu-Zhong Wang National Engineering Laboratory of Eco-Friendly Polymeric Materials (Sichuan), State Key Laboratory of Polymer Materials Engineering, College of Chemistry, Sichuan University, 29 Wangjiang Road, Chengdu, 610064, China. E-mail: [email protected] (Si-Chong Chen); ABSTRACT The structural stability and mechanical strength of hydrophilic polymer nanofiber membranes are relatively low due to the creep effect of polymer chain when swelling in water for a long time, which lead to a poor reusability in the fields of water treatment. In this work, we demonstrated a facile and green strategy for preparing eco-friendly hydrophilic nanofibrous membranes with excellent reusability and recyclability which could separate oil-in-water emulsion in practical

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conditions by using biobased chitin nanowhiskers (ChNWs) as reinforcement and “cocrosslinking hub” for hydrophilic nanofiber matrix. The as-prepared membranes exhibited very good separation performance with separation flux of 1100-1300 L·m-2·h-1 and separation efficiency over 99.5%. The reusability and recyclability of the membranes were evaluated and had proved to be robust enough to effectively separate emulsions after at least 5 times of recycling. Moreover, the NF membranes were capable of effectively separating oil-in-water emulsions not only a wide range of pH but also hyper-saline conditions. This work as well as cocrosslinked strategy enrich the exploration of superhydrophilic nanofibrous membranes for its significant potential in the field of oil spills and industrial oily wastewater treatments. KEYWORDS: Superhydrophilic nanofiber membrane, Co-crosslinked networks, Structural stability, Emulsion separation INTRODUCTION The oil-contaminated water pollution, one of the most serious problems resulting from ever-increasing industrial oily waste water and sewage, is an increasing danger to the health of the planet and poses a great threat to the life quality of human beings [1-3]. Nowadays, developing an effective treatment of water pollution, especially for the oil-in-water emulsion, is a significantly arduous task complying with the society and environmental demands [4-7]. Conventional chemical separation techniques such as electro-coagulation, air rotation, centrifugation, chemical demulsiers and biological treatment, suffer from numerous shortcomings including complex operational processes, low separation efficiency, high operation cost, highly energy-consuming and secondary pollution [8-12].


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Membranes from electrospun nanofibers, which have high surface area-to-volume ratio and complex pore structure, are widely made for filtration materials and considered as the one of most promising approach to separation [13-19]. In general, treatment of oily wastewater is required ‘water-removing’ type separation membranes with superhydrophilicity and high oleophobicity which are very suitable for oil-in-water emulsions and endow such materials with favorable antifouling properties and easily achieving regeneration [20-26]. However, the structural stability and mechanical strength of hydrophilic polymer nanofiber membranes are relatively low due to the creep effect of polymer chain when swelling in water for a long time, which lead to a poor reusability and applicability of the membranes. Therefore, a very few works were reported about hydrophilic polymer nanofiber membranes for emulsion separation. To achieve the hydrophilicity and structural stability of the electrospun membranes simultaneously, almost all researches are focus on the surface hydrophilic modification of hydropobic membranes, which involving physical methods such as blending with hydrophilic components [27-29], surface coating [30-33] or chemical methods [34-38] such as in-situ polymerization. Constructing hydrophilic surface layer based on noncovalent interaction can be very easily achieved by various methods such as co-deposition process [39], spraying [24], directly coating water-soluble polymers or amphiphiles onto membrane surfaces [30-33]. Although these physical methods have enhanced the membrane permeability and antifouling property to some extent, the stability and reusability of hydrophilic components on the membranes still remains an important issue that needs to be addressed. Because of the weak noncovalent interaction between the hydrophilic modified layer and the hydrophobic matrix, the hydrophilic components tend to release from the membranes during long-term application [40]. Chemical bonding can provide good stability of hydrophilic components on the surface of hydrophobic membrane, however, the


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preparation processes of chemical modifications are relatively complex and difficult to control. Moreover, both physical and chemical modification tend to impair the intrinsic porosity of the electrospun membranes and lead to serious flux decrease unless a very fine controlling on the preparation process was achieved. Another important issue involved in the practical application of oil-water separation is that the oily waste water, especially in emulsified level, is usually highly alkaline, acidic and/or salty, which has posed serious challenges to the applicability of the separation membranes [41-42]. Therefore, it is imperative to develop novel electrospun separation materials with not only good separation properties, but also environmental friendliness, easiness of production, stability and reusability in complex practical environments [43]. Hence, in this work, we have designed a novel eco-friendly hydrophilic polymer filtration membranes with in-situ co-crosslinked polymer-nanoparticle network by using hydrophilic Poly N-isopropylacrylamide-co-N-methylolacrylamide

(PNIPAm-co-NMA)

as

the

polymeric

nanofibers matrix, and biobased nanoparticles, i.e. chitin nanowisker (ChNWs) as the reinforcement and co-crosslinked hub, which have dramatically improved reusability and recyclability for oil-water emulsion separation. Our idea is that: (i) ChNWs can reinforce the nanofibers without impairing the intrinsic hydrophilicity of nanofiber owing to its excellent rigidity, high strength and water affinity. Moreover, there are large number of hydroxyl groups on the surface of ChNWs which can serve as co-crosslinked hub to react with the Nmethylolacrylamide (NMA) units of PNIPAm-co-NMA, and therefore improve the structural stability of the nanofiber membrane to meet the actual separation environment. (ii) It is fabricated in a green and versatile way by traditional single spinneret electrospinning using water/THF as solvent followed by co-crosslinking at elevated temperature. The as prepared


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nanofiber membranes exhibited not only good separation performance, but also excellent stability, reusability and recyclability in complex oil-in-water emulsions within wide pH range from 1 to 13 or highly salty condition stabilized by surfactant. EXPERIMENTAL SECTION 2.1. Materials. N, N-isopropyl acrylamide (NIPAAm) was supplied by Kohjin Co. Ltd (Japan) and purified by recrystallization from n-hexane. NMA was purchased from Energy Chemical. 2, 2’-azodiisobutyronitrile (AIBN) was purchased from OMNICAL (Tianjing) and purified from ethanol. Chitin nanowhiskers (ChNWs) were produced by hydrochloric acid hydrolysis of Chitin. Tween-80, ethanol, dimethylformamide (DMF) and tetrahydrofuran (THF) were purchased from Kelong chemical reagent (Chengdu). 2.2. Preparation of P(NIPAAm-co-NMA)/ChNWs Nanofiber Membrane. The copolymer P(NIPAAm-co-NMA) was synthesized by free-radical polymerization and shown in the supporting information following the methods reported in literatures [44-47]. P(NIPAAm-coNMA)/ChNWs nanofiber membranes were prepared on a stainless steel mesh by electrospinning at room temperature. P(NIPAAm-co-NMA) and ChNWs were dissolved in water/THF mixture (at the ratio of 1:2) with a concentration of 15 wt%. The solution of P(NIPAAm-coNMA)/ChNWs was then stirred continuously for 12 hours prior to electrospinning. During electrospinning, the flow rate of the precursor solution was set at 0.15 mm/min via syringe pump to yield a stable Taylor cone. The needle used an inner diameter of 0.19 mm. A voltage supply wire was attached to the charging needle, and the substrate was grounded. A voltage of 20.0 kV was applied to the needle to yield nanofibers. The electrospinning time was kept constantly at 5 hours. Then cross-linking reaction of the nanofibrous membranes was carried at 130 °C.


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2.3. Oil-in-Water Emulsion Separation Test. Thirty milliliters of toluene was added into 970 mL of deionized water with 10 mg of Tween-80 as emulsifier. The solution was then stirred under 1500 rpm for 3 hours. The membrane was fixed between two vertical glass devices with the inner diameter of 16 mm. Upon conducting the emulsion separation the membranes were prewetted at first, then the as-prepared emulsions were feed directly onto the membrane and the water spontaneously permeated. The whole separation process was driven under the pressure of 0.3 bar, the fluxes for different membranes were determined by calculating the volume of filtrate after permeating 1 minute. After recording the separation performance, the membrane was washed by water to remove the oil, and then reused for next separation cycle. For recycle test, the membrane was cryo-dried after each 10 cycles of separation and then reused for separation. 2.4. Characterization of P(NIPAAm-co-NMA)/ChNWs Nanofiber Membrane. 1H-NMR data was recorded at room temperature by using a Bruker AV 400 (400 MHz) spectrometer and deuterated dimethyl sulfoxide. The morphology and fiber diameters of NF membranes were characterized by high-resolution scanning electron microscopy (JSM-5900LV). The average diameters were calculated from 20 fibers of the SEM images. The water and underwater-oil (dichloromethane) contact angles of the nanofiber membranes were determined by static watercontact-angle measurements in open air. A goniometer was used to measure and record the static contact angle on the nanofiber membranes. The oil-adhesion forces were measured using a highsensitivity microelectromechanical balance system (Data-Physics DCAT25, Germany) underwater. An oil droplet (5 µ L) was suspended with a metal ring and controlled to contact with the nanofiber membranes surface and then to leave. The forces were recorded during the entire time, and dichloroethane was used as detecting oils. The oil content in the filtrate after separation containing hexadecane, petroleum ether, isooctane, gasoline, edible


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oil were detected on a total organic carbon analyzer (SHIMADZU Japan). All measurements and experiments were performed at room temperature. The swelling properties of P(NIPAAmco-NMA)/ChNWs of membranes were tested. The swelling ratio was calculated on the basis of the P(NIPAAm-co-NMA)/ChNWs hydrogel amount of the composite membrane, but not the composite membrane. FT-IR spectroscopy was carried out using a Nicolet 6700 FT-IR spectrometer. The oil concentration in collected water was determined using a UV-vis (VARIAN Cary 50 from America). The separation efficiency (R (%)) is calculated according to eq 1: (%) = 1 −

CP × 100% C0

Where C0 and Cp are the oil content of the emulsions before and after separation. 1. RESULTS AND DISCUSSION 3.1. Preparation of P(NIPAAm-co-NMA)/ChNWs Nanofibrous Membranes. Figure 1 described the fabrication process of the nanofiber membranes for oil-water separation via a conventional single-spinneret electrospinning method. To achieve effective emulsion separation, the nanofiber (NF) membranes require dual wetting behavior (hydrophilicity and underwater high oleophobicity) and comparable mechanical strength. However, in most cases, the mechanical strength and structural stability of typical hydrophilic polymer nanofiber cannot match the requirement of long-term oil-water separation because of their swelling behavior in aqueous medium. To address these issues, biobased ChNWs (Figure1A), which have good hydrophilicity, rigidity, and abundant reactive hydroxyl or amino groups on surface, were designed as reinforcement and crosslinked hub for the hydrophilic nanofiber matrix.


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Figure 1. (A) SEM images of ChNWs. (B) Illustration of the preparation of P(NIPAAm-coNMA)/ChNWs nanofibrous membranes. (C) 1H-NMR spectra of P(NIPAAm-co-NMA). (D) Illustration of the cross-linked nanofibrous membranes. (E) Optical photographs showing the robust flexibility of ChNWs-10% membranes. (F) FT-IR spectrum of NF membranes before and after crosslinking. On the other hand, a superhydrophilic and eco-friendly copolymer, P(NIPAAm-co-NMA), which containing about 10 mol% of NMA units, was used as fiber matrix. NIPAAm and NMA units endowed the copolymer with excellent hydrophilicity, while the NMA unit can serve as the crosslinking site. The obtained samples with the ChNWs contents of x % were denoted as ChNWs-x%, and the pristine NF membrane without ChNWs was denoted as ChNWs-0%. As shown in Figure 1(C), P(NIPAAm-co-NMA) has been successfully prepared according to scheme S1 [44-47]. The spinning solution was obtained by directly mixing the water solution of


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P(NIPAAm-co-NMA) and water dispersion of ChNWs with certain ratio, and therefore used for electrospinning to prepare P(NIPAAm-co-NMA)/ChNWs composite nanofibers (Figure 1B). In order to prevent the creep deformation of hydrophilic P(NIPAAm-co-NMA) nanofiber during swelling in water, crosslinking of P(NIPAAm-co-NMA)/ChNWs composite nanofibers was performed at 130 °C. Since the crosslinking reaction mainly occurred within the inner part of nanofibers, it had no obvious influence on the appearance and properties of the nanofibers. The electrospun membranes exhibited very good toughness and flexibility as shown in the optical photograph (Figure 1E). The co-crosslinking process was also characterized by FTIR spectroscopy, as illustrated in Figure 1F. The crosslinking reaction of P(NIPAAm-co-NMA)/ChNWs generally occurred through the formation of -C-O-C- bond between the -OH group of NMA units and ChNWs or also within one of the components itself, and of methylene bridge by consuming water and formaldehyde molecules, as shown in Figure 1D. As illustrated in Figure 1F, the transmittance bands at ~1030 cm-1of NF membranes, indicative of C-OH bonding, decreased in intensity after crosslinking. Furthermore, the bands at 1096.10 cm-1 and 661.37 cm-1, indicative of stretching vibration and bending vibration of C-OH bonding from P(NIPAAm-co-NMA) were not observed after crosslinking. Especially, it was worth noting that a peak (1073 cm-1) gradually increased indicative of -C-O-C- bonds which were formed during the crosslinking reaction. The signal intensity of C-OH (1270 cm-1) decreased after crosslinking, indicating formation of methylene bridges (Figure 1D). Figure 2 recorded the microscopic morphology of P(NIPAAm-co-NMA)/ChNWs nanofibers with different ChNWs contents.

It could be seen that the NF membranes were revealing

uniform, smooth, continuous and bead-free nanofibers without any adhesion among the adjacent


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nanofibers, indicating the ChNWs were capable of well dispersed among nanofibers. This high dense entanglement one another led to the formation of a randomly oriented 3D nonwoven porous network for further separation. As presented in the table S1, the average diameters had a small increase due to the increase of concentration and viscosity of spinning solution after adding ChNWs. Figure 2 A2-D2 presented morphology of the NF membranes after crosslinking at 130 °C for 12 h, which almost had no difference as those corresponding samples before crosslinking (Figure 2, A1-D1). The average diameters of the samples (Table S1) also confirmed this result. Although the as-prepared nanofibers from electrospinning have desired porosity, high surface area-to-mass ratios, their long-term structural stability and practical applicability when swelling in oil-contaminated waste-water are still critical issues need to be addressed. Creep of the hydrophilic polymer inevitably occurred and caused serious deformation of the nanofiber even after crosslinking (Figure 2, A3), which may therefore impair the separation performance of NF membrane. With the addition of ChNWs, the creep deformation of nanofibers was effectively prevented in a certain extent (Figure 2, B3-D3). For those nanofiber with relatively high ChNWs content (10-15 wt%), the morphology and diameter of freeze-dried P(NIPAAm-coNMA)/ChNWs nanofiber almost keep unchanged (Figure 2, C3-D3 and Table S1) comparing to corresponding samples before swelling in water. This phenomenon could be explained by the cocrosslinked structure of P(NIPAAm-co-NMA) and ChNWs. The ChNWs have much larger scale and higher weight than P(NIPAAm-co-NMA) molecular chains, the movement of polymer chains in a wide range may therefore effectively prohibit after co-crosslinking. As a consequence, the co-crosslinked P(NIPAAm-co-NMA)/ChNWs nanofiber maintained a very good structural stability. The creep deformation of P(NIPAAm-co-NMA) may have important effect not only on the microscopic morphology, but also on the macroscopic appearance of the


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NF membranes. As shown in Figure 2E and 2F, when immerging ChNWs-0% and ChNWs-10% NF membranes with same size into water, a very obvious contraction rather than swelling occurred for ChNWs-0%. This phenomenon can also be contributed to the creep deformation of P(NIPAAm-co-NMA). Since the hydrophilic P(NIPAAm-co-NMA) has very high chain mobility promoted by the interaction between water and polymer, the swelled nanofibers tend to adhere and merge together and therefore block the inner pores of the NF membrane. Comparatively, the NF membrane of ChNWs-10% can still maintain the original shape and size (Figure 2F) because of the prohibition effect of ChNWs on the chain motion of P(NIPAAm-co-NMA). The results were consistent with the following in table S2 the gelation ratio (G%) of crosslinked NF membranes increased while the swelling ratio (S%) decreased with ChNWs contents increasing. That is to say, the co-crosslinked ChNWs are very helpful for maintaining the porous structure of NF membranes even under swollen state.

Figure 2. SEM images of P(NIPAAm-co-NMA)/ChNWs with different ChNWs contents ChNWs-0% (A1-A3), ChNWs-5% (B1-B3), ChNWs-10% (C1-C3), ChNWs-15% (D1-D3) before (A1-D1) and after (A2-D2) crosslinking for 12 hours, and freeze-drying after gelation (A3-D3). The optical images of NF membranes with 0% and 10% ChNWs before (E) and after (F) swelling in water, respectively


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Generally speaking, crosslinking time has a significant influence on the co-crosslinked structure of the NF membranes. The diameter of freeze-dried P(NIPAAm-co-NMA)/ChNWs nanofibers decreased with crosslinking time increasing, and almost keep unchanged after 12 hours (Table S3), therefore, the stable pore structure could be achieved after 12 hours responding to adequate crosslinked structure. 3.2. Wettability of P(NIPAAm-co-NMA)/ChNWs Nanofibrous Membranes. To understand the influence of ChNWs addition on the wettability of those membranes, the contact angles including water contact angle (WCA) and underwater oil contact angle (UOCA) and their changes were investigated as shown in Figure 3. The water droplet on the surface of NF membrane without ChNWs could spread easily and permeate into the membrane within 2 seconds, and a water contact angle of about 0° was observed (Figure 3C). This should be derived from the inherent hydrophilic constituent of the NF membranes and nano-structured fibrous architecture [48]. For the ChNWs-10% NF membrane, WCA of 0° (Figure 3A) in air have also shown superamphiphilicity. In addition, with the increasing of ChNWs contents, water drops could still permeate into the membrane within 3 seconds and spread out completely after 4 seconds. For oleophobic characteristics, after being immersed in water (Figure 3B) 143° contact angles exhibited the underwater oleophobicity of the ChNWs-10% NF membranes owing to the inherent hydrophilicity of NF membranes and the micro/nanostructures of electrospinning nanofibers [48]. That is to say, when the as-prepared membrane was prewetted in water, water could be trapped into the hierarchical structure to then form a water-solid interface in the presence of oil [21, 24, 49]. The trapped water serves as a repulsive liquid phase for oils to contact with the NF membrane directly, yielding high oleophobic surfaces [21, 48]. With the increasing of ChNWs contents, a slight decrease in UOCA has been observed because ChNWs


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have a weaker hydrophilicity comparing to water soluble P(NIPAAm-co-NMA). Nevertheless, these NF membranes containing ChNWs still exhibited very high hydrophilicity and underwater oleophobicity.

Figure 3. (A) Droplets of oil (dyed red) and water (dyed blue) on the NF membranes in air. (B) Photograph of an underwater oil droplet (dyed red) on NF membranes. (C), (D) Variations of the water contact angles (WCA) and underwater oil contact angles (UOCA) of NF membranes with different ChNWs contents. (E) Photographs of dynamic measurements of underwater oilrepelling on the surface of NF membranes with 10% ChNWs. The underwater oil-adhesion test was conducted by an approach-compress-detach process (Figure 3E). An oil droplet (5 µL) was squeezed against ChNWs-10% NF membrane and then was lift up. When the oil droplet was gradually removed, it could overcome the adhesion force to


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detach from the membrane surface and no obvious deformation was observed, indicating very low oil-adhesiveness of the ChNWs-10% NF membrane. Meanwhile, the underwater oiladhesion forces of ChNWs-0% and ChNWs-10% NF membrane, recorded by a high-sensitivity microelectromechanical balance system, were 24±2.8 µN and 46±5.7 µN, respectively. These results suggested that although the addition of ChNWs slightly decreased the hydrophilicity of the membrane, it still had sufficiently low oil-adhesion characteristics and excellent oil-repelling property underwater for oil-in-water emulsion separation. The wettability of the different crosslinking time was also investigated (Figure S1 (A), (B)). The water drops spread out completely within 4 seconds, and UOCA were above 140°, which showed the crosslinking time almost had no influence on the wetting behavior. 3.3. Oil-in-Water Emulsion Separation. The high underwater oleophobicity and low oil adhesion of the membrane provided a good basis for oil-in-water separation. In this work, a surfactant-stabilized (Tween 80) oil-in-water emulsion was prepared as the model emulsions (Figure 4). As shown in Figure S2, optical images of toluene-in-water emulsion after were recorded with the storage time increasing for evaluating the stabilization of emulsion. There was no obvious increase in the size of the oil droplets even after 7 days at room temperature, and the emulsions still remained white milky and no stratification was observed. Figure 4A illustrated the equipment for oil-in-water separation, the freshly prepared oil emulsion was poured into a filter module (Figure 4B) equipped with the relevant separation membrane under the pressure of 0.3 bar. The white milky emulsion has become transparent in the collected filtrate (Figure 4C). As shown the optical microscopic images of the collected filtrate in Figure 4D, many oil droplets with diameter ranging from 4-25 µm were observed in emulsion before filtration, while the filtrate was transparent and no oil droplets were observed


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(Figure 4E), suggesting that oil had been successfully removed from the oily water. The separation performance of NF membranes with different ChNWs contents were evaluated by characterizing separation efficiency and flux. For NF membrane without ChNWs, it exhibited very high separation efficiency (99.8%) but relative low separation flux (268.7 L·m-2·h-1). This phenomenon was attributed to the contraction of P(NIPAAm-co-NMA) NF membrane during swelling in water, and the porosity of the membrane was decreased obviously because of the creeping deformation of P(NIPAAm-co-NMA) chains. With the addition of ChNWs, the separation efficiency of P(NIPAAm-co-NMA)/ChNWs NF membranes slightly declined to around 99.5-99.7% but the separation flux obviously increased to 1100-1300 L·m-2·h-1, which again proved that the introduction and co-crosslinking of ChNWs to P(NIPAAm-co-NMA) nanofiber made a very important contribution to the structural stability of the porous membrane. With the increasing of ChNWs content, the separation efficiency of NF membranes gradually decreased while the separation flux increased owing to the different crosslinking degree of these samples. The samples with higher crosslinking degree also have lower swelling ratio and therefore the higher porosity, which is in favor of passing through for both water and oil droplets. In consideration of balanced separation performance, the NF membrane with 10% of ChNWs was chosen for further investigation on reusability. The ChNWs-10% NF membrane was also used to separate oil-in-water emulsions containing various oils for evaluating its general applicability. The oil contents of the filtrates of these emulsions were measured by a total organic carbon analyzer, and the separation efficiencies are exhibited in Figure 4G. For oil/water mixtures containing hexadecane, petroleum ether, isooctane, gasoline and edible oil, the separation efficiencies were 99.85±0.01%, 99.90±0.02%,


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99.93±0.04%, 99.53±0.09%, 99.14±0.27%, respectively, indicating high capacity and general applicability of the ChNWs-10% NF membrane to separate oil-in-water emulsions.

Figure 4. (A) Photographic illustration of the separation device. The optical images of emulsion (B, C) and filtrate after separation (D, E) respectively. (F) The separation efficiency and flux of the P(NIPAAm-co-NMA)/ChNWs NF membranes with different ChNWs contents. (G) Flux and separation efficiency of ChNWs-10% NF membrane for separating oilin-water emulsions containing hexadecane, petroleum ether, isooctane, gasoline, edible oil, respectively.


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To test the reusability of the membranes, cyclic filtration tests were carried out by reusing the washed membranes for next cycle after recording the separation performance. As shown in Figure 5, the P(NIPAAm-co-NMA)/ChNWs-10% NF membrane exhibited outstanding reusability with no obvious decline of the permeation flux and separation efficiency even after 15 cycles. This phenomenon could be explained by the well-preserved micro- and macromorphology, as shown in the optical (Figure 5, A1-A2) and SEM (Figure 5, A3) photos of recycled P(NIPAAm-co-NMA)/ChNWs-10% NF membrane after 15 separation cycles by cryodrying. Therefore, benefiting from the formation of stable hydrophilic co-crosslinked network structure, the NF membranes possess both good robust reusability and separation performance. Comparatively, the P(NIPAAm-co-NMA) NF membrane without ChNWs can only experience 3 cycles of separation, the dried membrane exhibits very serious deformation and cannot be reused. The SEM image (Figure 5, B3) also suggests the structure of nanofiber network has already collapsed, which is even worse than the same sample just after soaking in pure water (Figure 2, A3) because of the vacuum filtration. Additionally, as shown in Figure S3, the P(NIPAAm-coNMA)/ChNWs-10% NF membrane also showed high retention of tensile strength and elongation at break after 15 cycles which further illustrated the excellent mechanical and chemical durability of this sample for oil-water emulsion separation. Besides the reusability (Figure 5C), we also evaluated the recyclability of the P(NIPAAm-co-NMA)/ChNWs NF membranes. The NF membrane was recycled by washing with water carefully and then drying after 10 cycles of separation. As shown in Figure 5D, the membrane maintained very good separation efficiency and flux after recycling at least 5 times. Considering the practical applications, emulsion separation membranes were often inevitably used in acidic, alkaline or saline environments. We have further studied the separation efficiency


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and flux over 15 cycles for acidic (A, D), alkaline (B, F) and salty (G, H) at 2 M NaCl emulsion by the same methods as the neutral emulsion (Figure 6). Change of the color of pH test paper from filtrates (C, E) demonstrated only oil drops were prevented on the NF membranes. The ChNWs-10% NF membranes tested in acidic, alkaline and salty conditions show steady separation performances as those in neutral circumstance. In a word, these NF membranes have potential applications in industrial oily wastewater treatment with broad pH range and hypersaline conditions.

Figure 5. The optical (A1-A2) and SEM (A3) images of recycled NF membrane with 10% of ChNWs after 15 cycles of separation. The optical (B1-B2) and SEM (B3) images of recycled NF membranes without ChNWs after 1 cycle of separation. Flux and separation efficiency of ChNWs-10% NF membranes after different separation cycles (C) and with different recycle times (D, during each recycle, 10 cycles of separation were performed).


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Figure 6. Flux and separation efficiency of ChNWs-10% NF membrane at different separation cycles with emulsion’s pH of 1 (A) and 13 (B). The optical images of filtrate after separation for emulsion with pH of 1 (C) and 13 (E), respectively. SEM images of cryo-dried ChWs-10% NF membranes after 15 cycles of separation for emulsion with pH of 1 (D) and 13 (F), respectively. (G) Flux and separation efficiency of ChNWs-10% NF membrane at different separation cycles for emulsion with 2M NaCl. SEM image (H) of ChNWs-10% NF membrane after 15 cycles of separation for emulsions with 2M NaCl.


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2. CONCLUSIONS In summary, we have developed a facile and green method to fabricate reusable and recyclable hydrophilic polymer nanofiber membranes for oil-in-water emulsion separation. The crosslinked composite NF membranes were prepared using P(NIPAAm-co-NMA) as the polymer matrix while ChNWs as the reinforcement and crosslinking hub. The co-crosslinked structure of P(NIPAAm-co-NMA)/ChNWs

NF

membranes

has

endowed

them

with

not

only

superhydrophilicity and high underwater oleophobicity, but also very good structural stability and resistance to creep deformation during swelling in water. They could effectively separate oilwater emulsion for many separation cycles and even after at least 5 times of recycling, indicating that the NF membrane possessed excellent reusability and recyclability. Moreover, the ChNWs10% NF membranes tested in acidic, alkaline and salty conditions also showed steady separation performances as those in neutral circumstance, indicating that which have very promising potential application in industrial oily water treatment with complex circumstance. AUTHOR INFORMATION Corresponding Author * [email protected] (S. C. Chen) ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: XXXXX/acssuschemeng.XXXXX, including: description of preparation of P(NIPAAm-coNMA) and ChNWs; tables listing the diameter and distribution of the NF membranes, swelling rate and gel rate of the NF membranes after crosslinking, diameter distribution of ChNWs-10%


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NF membrane at different crosslinking time; figures showing WCA and UOCA of the ChNWs10% NF membrane, optical images of toluene oil-in-water emulsion at room temperature, tensile strength retention and elongation at break retention of ChNWs-10% NF membrane after separating emulsions for 15 cycles. ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (No. 21474066 and No. 51421061). The Analytical and Testing Center of Sichuan University provided TEM analysis. REFERENCES (1) Zhang, W.; Liu, N.; Cao, Y.; Chen, Y.; Zhang, Q.; Lin, X.; Qu, R.; Li, H.; Lin, F. Polyacrylamide-Polydivinylbenzene Decorated Membrane for Sundry Ionic Stabilized Emulsions Separation via a Facile Solvothermal Method. ACS Appl. Mater. Interfaces 2016, 8, 21816-21823. (2) Wang, B.; Liang, W.; Guo, Z.; Liu, W.; Chem. Biomimetic super-lyophobic and superlyophilic materials applied for oil/water separation: a new strategy beyond nature. Chem. Soc. Rev. 2015, 44, 336-361. (3) Zhou, S.; Liu, P.; Wang, M.; Zhao, H.; Yang, J.; Xu, F. Sustainable, Reusable, and Superhydrophobic Aerogels from Microfibrillated Cellulose for Highly Effective Oil/Water Separation. ACS Sustainable Chem. Eng. 2016, 4, 6409-6416. (4) Sakthivel, T.; Reid, D. L.; Goldstein, I.; Hench, L.; Seal, S. Hydrophobic High Surface Area Zeolites Derived from Fly Ash for Oil Spill Remediation. Environ. Sci. Technol. 2013, 47, 5843-5850.


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For Table of Contents Use Only

Eco-friendly nanofibrous membranes with excellent reusability for effectively separating oil-inwater emulsion using biobased chitin nanowhiskers as reinforcement and “co-crosslinking hub”.


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Reusable and Recyclable Superhydrophilic Electrospun Nanofibrous

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