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Full-biobased Nanofiber Membranes towards Decontamination of Waste-water Containing Multiple Pollutants Yan-Li Kang, Jie Zhang, Gang Wu, Ming xuan Zhang, Si-Chong Chen, and Yu-Zhong Wang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.8b01996 • Publication Date (Web): 16 Jul 2018 Downloaded from http://pubs.acs.org on July 21, 2018

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ACS Sustainable Chemistry & Engineering

Full-biobased Nanofiber Membranes towards Decontamination of Waste-water Containing Multiple Pollutants

Yan-Li Kang, Jie Zhang, Gang Wu, Ming-Xuan Zhang, Si-Chong Chen,* Yu-Zhong Wang National Engineering Laboratory of Eco-Friendly Polymeric Materials (Sichuan), College of Chemistry, State Key Laboratory of Polymer Materials Engineering, Sichuan University, 29 Wangjiang Road, Chengdu, 610064, China. E-mail: [email protected] KEYWORDS: Full-biobased nanofiber membrane, Waste-water decontamination, Emulsion separation, Water soluble organic dyes ABSTRACT: Herein, we developed a full-biobased nanofiber (NF) membrane for decontaminating water from multiple pollutants simultaneously by using electrospun poly(Llactic acid) (PLLA) nanofibers as scaffold and β-cyclodextrins (β-CD) decorated polydopamine (PDA) as functional coating. The as-prepared β-CD-PDA@PLA NF membranes have good hydrophilicity, underwater oleophobicity and recyclability, which therefore endows the membrane with effective separation performance for oil-in-water emulsion. The flux and

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separation efficiency of the membranes are higher than 1500 L·m-2·h-1 and 99.5%, respectively. The β-CD-PDA@PLA NF membranes also exhibited good adsorbability (over 95%) to positive charged water-soluble organic pollutant during filtration owing to their negatively charged nature and high specific surface area. The used membranes, which could be easily recovered by washing with a small amount of solvents and used for next filtration cycle, have very good durability to decontamination the waste-water containing toluene emulsion and methylene blue simultaneously at least 10 cycles. INTRODUCTION The increasing wastewater discharges from the development of industries and frequent oil spills critically threatens the sustainable development of our planet.1-5 The influence of wastewater on the environment is long term and difficult to repair.6 Traditional techniques, including air flotation, oil-absorbing, gravitational separation, flocculation and coagulation for separating oil/water mixtures are not effective for separation of emulsified oil/water mixtures.7 Membrane technologies, which based on the size-sieving effect, is one of the most efficient method to separate industrial wastewater with various pollutants. Particularly, electrospun nanofiber (NF) membranes with special wettability have attracted considerable attention as an advanced technology for separation of surfactant-stabilized emulsion owing to their high specific area, large porosity and ease of scalable production from various materials.8-15 The elecrospun membranes applied in oil/water separation can generally be divided into two main types, i.e. superoleophilic/superhydrophobic membranes16 and underwater superoleophobic membranes.17 Comparatively speaking, underwater superoleophobic membrane is well suited to oil-in-water emulsions separation due to their antifouling properties against oil.18


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To meet the requirement of practical application, consideration must be given to both surface hydrophilicity and stable mechanical properties when preparing underwater superoleophobic membranes. By modifying the basic hydrophobic skeleton with hydrophilic component, tailored nanocomposite hybrid fibers with both hydrophilicity and mechanical properties could be easily obtained. Wang et al.19 reported preparation of hybrid coatings adhered firmly on polyvinylidene fluoride substrate in a one-step process by synergy of dopamine polymerization and tetraethoxysilane hydrolysis. Ge et al.20 prepared a NF membrane with hierarchical morphology and superhydrophilicity/underwater superoleophobicity by electrospraying polyacrylonitrile (PAN)/SiO2 nanoparticles onto electrospun PAN nanofibers for oil-in-water emulsions separation. Co-crosslinking hydrophilic nano-reinforcement with hydrophilic electrospun skeleton is another effective way to prepare underwater superoleophobic nanofiber membranes with excellent mechanical stability.21 Although underwater superoleophobic membrane shows very promising prospect for oil/water mixture separation, there remain a few unaddressed vital problems in practical applications. Besides insoluble oils, water-soluble organic pollutants such as organic dyes and surfactant are also typical pollutants in wastewater,22,23 which cannot be effectively removed by common superhydrophilic membranes. Moreover, in some cases, these water-soluble organic pollutants may also stabilize the oil-in-water emulsion and therefore lead to a substantial increase of difficulty in oil-water separation. Up to now, a very few researches have reported the removal of both oil and water-soluble organic pollutants.24-27 Chen and Huang et al.24,25 developed a novel hierarchical composite membrane based on carbon nanotube to separate oil-in-water emulsion and degrade toxic water-soluble organics simultaneously. This composite membrane is a promising candidate for waste-water decontamination because it can continuously dispose oily


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wastewater. Li et al.26 developed an double-layer polyester (DL-PET) membranes by surface modifying PET membranes with polydopamine (PDA) and Ag nanoparticles to remove watersoluble organic pollutants. Gao et al.27 reported a double layer mesh based on TiO2 for water purification. These methods were capable to effectively remove both insoluble oil and watersoluble organic pollutants. However, a two-steps process is necessary which including separation of oil/water mixture and photo catalyzed decomposition of water-soluble organics with relative long time or external source such as UV irradiation.28-30 Another important issue is that the raw materials for most of existing separation membranes are derived from non-biobased and nonbiodegradable chemical products,31 such as polyvinylidene fluoride (PVDF), polyurethane (PU), polyimide (PI). The postprocessing of the out-of-service membranes is still a critical issue since these materials are nondegradable. Discard or combustion of these nondegradable membranes inevitably leaded to secondary pollution to the environment.32-34 Therefore, developing a renewable sources-based and biodegradable material for decontaminate water, especially simultaneously separating insoluble oil and soluble organic pollutants, is of great significance. Herein, in this work, we developed a full-biobased and biodegradable nanofiber (NF) membrane for decontaminating water from multiple pollutants simultaneously by using electrospun poly(L-lactic acid) (PLLA) nanofibers as the scaffold, which were then surface modified with polydopamine (PDA) and β-cyclodextrins (β-CD) respectively. PLA owns advantages of good mechanical property, non-toxicity, broad sources35-40 and can be degraded into CO2 and water completely in nature,41,42 which are very suitable for using as the scaffold of nanofiber membranes. Hydrophilic modification of PLA NF membrane with dopamine (PDA@PLA) is based on an oxidative polymerization, resulting in PDA coatings and also endowing the membrane with a hierarchical rough surface. Moreover, the PDA surface


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layer, which exhibit negative charges owing to the N or O-containing groups, may also facilitated the adsorption of soluble organics with positive charges in water. Subsequently, biobased β-CD were used to modify the surface wettability of PDA@PLA NF owing to not only their hydrophilicity but also their ability to form host-guest inclusion complexes (ICs) with organics, which therefore further enhanced the separation efficiency of the NF membrane to pollutants. Our ideas are interpreted as below: (a) Benefited from the synergy of PLA basic NF scaffold and in-situ formed β-CD-PDA coating, the as-prepared superhydrophilic β-CD-PDA@PLA NF membranes exhibited a threedimensional (3D) structure with nano-micropore and rough surface, which allows the membrane to block the emulsified oil droplets and to adsorb positive charged organic pollutants in water simultaneously just though a simple one-step filtration. (b) All raw materials used for constructing the membrane, i.e. PLA, dopamine, and β-CD, are biobased and biodegradable. Therefore, the out-of-service membranes can be easily disposed without negative impact on environment. . EXPERIMENTAL SECTION Materials. Poly (L-lactide) (PLLA) was provided by Jinan Daigang Biomaterial Co., Ltd. Dopamine hydrochloride (98%), trifluoroethanol (TFE), tris-(hydroxymethyl) aminomethane (Tris) were provided by Energy Chemical Co., Ltd. β-cyclodextrin (β-CD), P-toluenesulfonyl chloride (TsCl), ethanol, dichloromethane, toluene, methylene blue (MB), methyl orange (OG), tween-80, acetonitrile and ethylenediamine (EDA) were obtained from Chengdu kelong chemical reagent Co., Ltd. Water utilized throughout the studies is distilled water.


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Preparation of PLA Nanofiber Membranes. PLA nanofiber membranes were prepared on an aluminium-foil paper by electrospinning at room temperature. Typically, spinning solution with a concentration of 5 wt% was prepared by dissolving a certain amount of PLA in trifluoroethanol (TFE). Before electrospinning, the PLA solution was continuously stirred for 24 h. the flow rate of the PLA solution was set at 0.2 mm/min with a syringe pump. The inner diameter of needle used is 0.21 mm, while the voltage applied to the needle was set at 18.0 kV. The electrospun PLA nanofibers were collected onto an aluminium-foil paper and vacuum dried at 50 °C. Preparation of β-CD-PDA@PLA Nanofiber Membranes. HCl-Tris buffer solution (pH 8.5, 50 mM) of dopamine hydrochloride with concentration of 2 g/L was prepared. PLA membrane (4.5 cm × 4.5 cm) was then immerged into the buffer solution for 24 h at 25 °C. The modified membrane was washed with distilled water and ethanol, respectively, before vacuum drying. The mono-6-deoxy-6-ethylenediamine-β-CD (β-CD-EDA) was prepared following a previously reported procedure (see also in Scheme S1).43 1H-NMR (Figure S1) spectrum of β-CD-EDA was shown in the Supporting Information. PDA@PLA NF membranes immersed into buffer solution (pH 8.5) of Tris (10 mM). Then, the β-CD-EDA were added and stirred at 50 °C for 24 h. Finally, the β-CD-PDA@PLA membranes were rinsed with distilled water and ethanol respectively before vacuum drying. Separation Test of Oil-in-Water Emulsion. 3 wt% toluene-in-water emulsions with high stability were prepared using a certain amount of Tween-80 as emulsifier; the stirring speed and time for emulsifying was 1500 rpm and 3h, respectively. The separation of oil-in-water emulsion as well as the antifouling evaluation of the membranes were conducted by a filtration system


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equipped with a vacuum pump at 30 KPa (vacuum degree -30 KPa). The flux of the different membranes was obtained by eq (1). Flux = V /At

(1)

Where flux is filtrate flux or the pure water (L·m-2·h-1), V refers to the permeate volume (L), A and t represent the NF membrane active area (m2) and the permeation time (h), respectively. The active areas of the different PLA NF membranes are 4.91 cm2. Separation Test of Water or Oil-in-Water Emulsion Containing Soluble Dyes. The water solution of MB or OG with concentration of 3 ppm were obtained by directly dissolving the dyes in distilled water. The wastewater with both emulsified toluene (3 vol%) and soluble MB (3 ppm) was prepared freshly before use. The wastewater was filtrated through the membrane at 10 KPa, and the filtrate was collected in the vial. UV-vis spectrophotometer (VARIAN Cary 50 from America) was employed to investigate the concentrations of oils and dyes in water before and after filtration at their respective maximum absorbance wavelengths. The following equation was used to calculate the separation efficiencies of oils or dyes: R % = − / × 100%

(2)

Where refers to the initial concentration of oils or dyes, while refers to the concentration of oils or dyes after filtration. Desorption and regeneration of membranes. After filtration, the β-CD-PDA@PLA NF membrane was washed with a small amount of HCl solution (0.1 M), ethanol and distilled water, respectively. Finally, the recovered membranes were dried at 50 °C under vacuum, and then used for the next purification cycle. Characterizations. 1H-NMR was recorded on a Bruker AV 400 (400 MHz) spectrometer at ambient temperature. A high-resolution scanning electron microscopy (SEM, JSM-5900LV) was


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used to investigate the morphology of pristine and modified PLA NF membranes. Static watercontact-angle measurements were performed to detect the water and underwater-oil (dichloromethane) contact angles of the NF membranes in open air. The static contact angle on the NF membranes was recorded on a goniometer. A high-sensitivity microelectromechanical balance system (Model DCAT25, Data-Physics, Germany) was used to measure the oil-adhesion forces underwater. X-ray photoelectron spectroscopy (XPS, Shimadzu Axis Ultradld spectroscope) was used to analyze the chemical composition of NF membranes. FTIR spectra in the range 400−4000 cm-1 were recorded on a Bruker IFS66 spectrometer. The pore size of the NF membranes were recorded based on the bubble point method with a Capillary Flow Porometer (Porolux 1000, Benelux Scientific, Belgium). The zeta potentials of samples were recorded on a Malvern Zetasizer Nano ZS90. Specific surface areas were calculated based on the Brunauer-Emmett-Teller (BET) method. All measurements were performed at room temperature. RESULTS AND DISCUSSION Preparation of β-CD-PDA@PLA Nanofibrous Membranes. To achieve both effective separation and ecological sustainability, the electrospun NF membrane demands not only enough mechanical strength and superhydrophilicity/underwater oleophobicity, but also good environmental friendliness such as biodegradability and biomass source. Among various biobased and biodegradable materials, PLA exhibits numerous advantages including biocompatibility, good mechanical properties and easy processability, which makes it a promising candidate for eletrospun scaffold. After dissolving PLA in a good solvent with certain concentration, it can be processed into nanofiber membrane easily with very good tunability through electrospinning (Figure 1A). The micro-morphology of pristine and modified PLA NF membranes were investigated by SEM. The pristine PLA NF membrane displays a 3D porous


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network of smooth nanofibers (Figure 1B). However, the intrinsic hydrophobicity and lipophilicity of PLA, to a large extent, restrict its direct application in emulsion separation. Therefore, hydrophilic surface modification was necessary for PLA NF membrane before further application. Dopamine, which is able to self-polymerize in an alkaline environment owing to aerobic auto-oxidation, was used for surface modification of PLA NF membrane.44,45 This PDA coating method has been proved to be an effective way to functionalize a wide range of material surfaces with hydrophilicity, because of its general applicability, economic practicality, and simplicity.46,47 The white PLA NF membranes (Figure 1B) became brownish black after modified with a thin layer of PDA (Figure 1C). SEM images suggest that the PDA micro/nanoparticles densely overlapped to form poly dispersive aggregates which uniformly coated on the PLA nanofibers, resulting in a core-shell structure with the PLA fibers as core and the coated PDA as shell. The formation of the PDA shell not only protected the PLA fiber inside from hydrolysis, but also endowed the nanofibers with a hierarchical rough surface structure, which is essential to build robust underwater superoleophobic surface. Besides hydrophilicity, hierarchical surface pattern is another crucial condition to achieve good performance for oil/water separation. Hydrophilic surface with nanoscale hierarchical structure can trap water on the rough surface to reduce the contact area between the membrane and oil droplet, which endows the membrane with oleophobic characteristic and low oil adhesion via the oil/water/solid system in the “Cassie-Baxter” state.18 Since the relatively low hydrophilicity of PDA limits its application for separating oil/water mixture,48-50 β-CD, which is a kind bio-based material and has high hydrophilicity, were then grafted on the surface of PDA@PLA NF for further improving the wettability and binding affinity. As shown in Figure 1D, the NF membranes modified with PDA and β-CD respectively did not show obvious differences in surface


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morphology (Figure 1D) comparing to that of PDA@PLA NF membrane, but the color of it changed from brownish black to yellowish brown because of the decoration of β-CD.

Figure 1. Schematic illustration of the electrospinning of β-CD-PDA@PLA NF membrane (A). Photographs and SEM images of (B) PLA NF membrane (C) PDA@PLA NF membrane (D) βCD-PDA@PLA NF membrane, the scale bars represent 2 µm. The inserts in (B, C, D) are digital photographs of corresponding NF membranes. (E) The distribution of pore size of NF membranes. (F) The graphical synthetic routes of β-CD-PDA@PLA NF membrane. To further investigate the influence of PDA and β-CD modification on the microstructures of membranes, bubble point method and N2 adsorption–desorption method were conducted. The corresponding results are summarized in Table S1 and Figure 1E. Consequently, the corresponding average pore diameters of the pristine PLA NF membranes decreased significantly from 2.79 µm to 1.30 µm after modifying with PDA, whereas BET surface area and


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cumulative pore volume showed obvious increase. This phenomenon could be attributed to accumulation of the PDA micro/nanoparticles coated on the PLA nanofibers, which enhanced adhesion structure in NF membranes and resulted in decreased pore size. Moreover, after further modifying the PDA@PLA NF membranes with β-CD, the membrane did not show evident change on the size, distribution and BET surface area of pores. These results are in good accordance with the SEM observations. The accumulation of PDA coating on PLA NF membrane can be attributed to the autoxidative polymerization process of DOPA (Figure 1F). Subsequently, aminated β-CD reacts with PDA through Schiff base reaction or Michael addition.44 As shown in FTIR spectra of sample modified with PDA (Figure 2A), the C=C resonance vibrations in the aromatic rings and N–H bending vibrations of PDA were observed at 1613 cm-1 and 1509 cm-1, respectively. Characteristic absorption band of β-CD decorated product was observed at 1632 cm-1 because of Schiff base reaction between β-CD and PDA which resulted in the C=N bonds and therefore confirmed the successful modification. XPS spectroscopy were used to reveal the chemical composition of NF membrane surfaces. The unmodified membrane only exhibits C 1s and O 1s peaks at 284.6 eV and 533.7 eV, respectively (Figure 2B). Peaks of N 1s appear after modifying with PDA or β-CD. The N 1s spectrum of PDA@PLA (Figure 2C) can be fitted into two peaks at 397.68 eV corresponding to -NHmoieties and 398.28 eV corresponding to -NH2 moieties, respectively. Although the polymerization mechanism of PDA is still ill-defined now, the presence of primary and secondary amine in PDA@PLA, which attributed to autoxidative polymerization of dopamine, confirmed the formation of PDA.45 The N1s spectrum of the membrane modified by β-CD-PDA can be deconvoluted into three component peaks that are correlated to the -C=N-, -NH- and -NH2


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moieties, respectively (Figure 2D). The signal of C=N bonds, which ascribed to Schiff base structure, thus indicated the reaction between β-CD and PDA (Figure 1F).

Figure 2. FT-IR/ATR (A) and XPS spectra (B) of PLA, PDA@PLA and β-CD-PDA@PLA NF membranes. N1s XPS spectra of PDA@PLA (C) and β-CD-PDA@PLA (D) NF membranes. Wetting Behaviors of the NF membranes. The surface wettability plays key role in determining the separation performances of membranes for oil/water mixture. A water droplet was collected on the surface of the pristine PLA NF membrane, while an oil droplet spread out and permeated into the membrane very quickly suggesting a simultaneous hydrophobicity and oleophilicity (see Figure S2). The water contact angle (WCA) of the PLA NF membrane was 125° (Figure 3A), reflecting the hydrophobic nature of it. After modifying with PDA, the


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PDA@PLA NF membrane exhibited much improved hydrophilicity with a WCA of 26 ± 2° (Figure 3B) comparing to the PLA NF membrane, because of the large amount of hydroxyl and amino groups on the PDA surface. The PDA@PLA NF membranes further modified by β-CD exhibited superhydrophilicity with WCA of 0° (Figure 3C), which was beneficial to construct a stable hydration layer at the interface between oil and solid. The bonded water acts as a barrier against oil, resulting in a surface with very good underwater superoleophobicity. Under this condition, the water droplets spread out and permeated into the β-CD-PDA@PLA NF membrane (Figure 3C) immediately after it contacting with the surface of membrane. The underwater-oil droplets (dichloromethane, dyed red) were collected on the surface of β-CD-PDA@PLA NF membrane (Figure 3D). Theoretically, fabricating an underwater superoleophobic surface is closely dependent on the hydrophilicity and rough morphology of surface. According to the Wenzel model and Cassie-Baxter model, the underwater oleophobic properties of surface may enhanced with the hydrophilicity in air.18 In short, it demonstrated that the β-CD-PDA@PLA NF membrane exhibited superhydrophilicity and underwater high oleophobicity, which provided a solid foundation for separating oil/water mixture. To investigate the effect of β-CD content on the wettability of the NF membranes, a series of β-CD-PDA@PLA NF membranes were prepared by submerging PDA@PLA NF membrane into water solution of β-CD-EDA with different concentration, and the underwater oil contact angle (UOCA) of the obtained membranes were recorded. As shown in Figure 3E, the UOCA of β-CD-PDA@PLA increased first with the increasing concentration of β-CD-EDA and then reached equilibrium at a β-CDEDA concentration of 6 mg/mL. Due to the amine of β-CD reacting with PDA via Schiff base reaction or Michael addition,44 β-CD was chemical bonded to the surface of PDA shell. The βCD-PDA@PLA NF membranes with higher β-CD content exhibited higher oleophobicity.


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However, further increasing β-CD-EDA concentration in reaction solution have no help for increasing

oleophobicity

if

the

reaction

between

β-CD-EDA

and

PDA

surface

reached the saturation point. Therefore, a β-CD-EDA concentration of 6 mg/mL was chosen for preparing β-CD-PDA@PLA NF membrane for subsequent experiments, and the corresponding decorated NF membrane was expressed as xβ-CD-PDA@PLA, where x is the concentration of β-CD-EDA in mg/mL.

Figure 3. Photos of water droplets on (A) PLA, (B) PDA@PLA, and (C) β-CD-PDA@PLA NF membrane, the inserts are corresponding static WCA results. Photo of dichloromethane droplets (dyed red) on β-CD-PDA@PLA NF membranes underwater (D). Variations of UOCA of


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PDA@PLA NF membranes modified with different concentration of β-CD-EDA (E). Underwater oil-repelling behavior of the surface of β-CD-PDA@PLA NF membranes (F). The underwater oil adhesion test of β-CD-PDA@PLA NF membrane was evaluated by an approach-compress-detach test (Figure 3F). No obvious deformation of the oil droplet was observed during the lifting process, suggesting the low-oil adhesion of the β-CD-PDA@PLA NF membrane. Moreover, the underwater oil adhesion forces, measured by a high-sensitivity microelectromechanical balance system, of the β-CD-PDA@PLA NF membrane was 42.05 ± 7.3 µN. Comparatively, the PDA@PLA NF membrane exhibited much stronger oil-adhesion property than the membrane decorated with β-CD. Figure S3 demonstrated that the oil droplet stuck on the membrane surface of PDA@PLA even the inclination angle increased to 27°. After the hydrophilic modification by β-CD, the oil droplet on β-CD-PDA@PLA NF membrane could roll away underwater when the inclination angle of membrane surface increase to 18° (Figure S3), which again proved that the β-CD-PDA@PLA NF membrane had low oil-adhesion feature and good oil-repelling behaviour after the modification. These results suggested that the NF membrane was sufficiently qualified for separating oil-in-water emulsion after modifications with β-CD-PDA. Separation of Oil-in-Water Emulsion. The separation performances of NF membranes, such as water permeation fluxes and separation efficiency, were evaluated by using a vacuum filtration apparatus, and the effective separation area of the apparatus is 4.91 cm2 (Figure 4A). Significantly, the pure water flux of β-CD-PDA@PLA NF membrane at 10 KPa is 20000 L·m2

·h-1, 4 times more than that of PDA@PLA NF membrane (Figure S4). Generally, the water flux

of NF membrane is primarily depended on the pore size and hydrophilicity. For PDA@PLA NF membranes, the pore size of membrane surface decorated with β-CD remains almost unchanged


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comparing to that of undecorated one. Therefore, in this case, the hydrophilicity of the NF membrane is responsible for deciding the water flux. The β-CD-PDA@PLA NF membrane exhibited much higher hydrophilicity than that of the PDA@PLA NF membrane, which can reduce the water permeation resistance sharply and lead to a much more rapid permeation of water through the membrane under same vacuum degree. In this work, the model toluene-in-water emulsions was prepared by mixing toluene and water using Tween 80 as the surfactant. To study the stability of the emulsion, the emulsion was placed in a sealed 20 mL small bottle after seven days. The emulsion was still milky white and the size of the oil droplets in water had no obvious change, illustrating that the emulsion has high stability and can be used for further separation (Figure S5). The separation performance for oilin-water emulsions of the NF membranes under different pressure was also evaluated. When an external driving pressure was loaded from 10 KPa to 50 KPa, the flux enhanced significantly from 810 to 2400 L·m-2·h-1. However, the separation efficiency in the filtrates decrease from 99.8% to 96% (Figure S6), because some tiny emulsion droplets may penetrate through the membranes under high external pressure. Considering the balance between flux and separation efficiency, 30 KPa was chosen as external driving pressure for subsequent experiments. Figure 4B and 4C are the optical microscopy photos of the feed emulsion and filtrate solution, respectively. A large amount of oil droplets with a diameter of 4-20 µm were observed in the emulsion. Meanwhile, for the transparent filtrate solution, no oil droplets could be observed, indicating that the oil had been successfully removed from the emulsion.


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Figure 4. Photograph of the filtration apparatus (A). The optical microscopy photos of (B) emulsion and (C) filtrate, respectively (Scale bar = 50 µm). The separation efficiency and flux of PDA@PLA membranes after modifying with different concentration of β-CD-EDA for surfactant-stabilized emulsions (D). The flux and separation efficiency of 6β-CD-PDA@PLA NF membrane for emulsions with difference surfactant concentration (E) and different oils (F). The surface wettability is a key point for separation performance of the NF membranes.51 As shown in Figure 4D, before decorated with β-CD, the PDA@PLA NF membrane displayed a high separation efficiency of 99. 5% but low flux of 990 L·m-2·h-1. With the decoration of β-CD, although the separation efficiency of β-CD-PDA@PLA NF membranes remained constant at about 99.5%, the separation flux significantly enhanced to 1500 L·m-2·h-1. Because the β-CD-


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PDA@PLA NF membrane exhibited much higher hydrophilicity than that of the PDA@PLA NF membrane, which endowed the membrane with a lower adhesive superoleophobic feature, resulting in increase in the permeation flux. Because of the small droplet size and good stability, the emulsions stabilized by emulsifier are difficult to separate. The droplets size of oil was obvious decrease with the increasing of surfactants contents, which resulted dramatically declined of flux (Figure 4E). Fortunately, the separation efficiency is extremely attractive even up to 99.5%, suggesting that the β-CD-PDA@PLA NF membrane has general applicability for emulsion with wide range of oil droplets size. Figure 4F demonstrated the separation performance of β-CD-PDA@PLA NF membranes for different oil-in-water emulsions. A total organic carbon (TOC) analyzer was used to determine the oil contents in the filtrates of these emulsions. The separation efficiencies of the β-CDPDA@PLA NF membrane for different emulsions derived from petroleum ether, isooctane, nhexane, and diesel oils were 99.73%, 99.88 %, 99.66%, 99.55%, respectively, suggesting universality and high capacity of the β-CD-PDA@PLA NF membrane for oil-in-water emulsions separation. Separation of Water Soluble Dyes. The polydopamine is a charged adhesive polymer which is able to functionalize almost all material surfaces.28,45 The negatively charged nature and the high specific surface area of PDA modified nanofibers make it a good candidate to adsorb watersoluble organic pollutants with opposite charges. In this work, the cationic methylene blue (MB) and anionic methyl orange (OG) with zeta potential of 1.55 mV and -2.37 mV, respectively, were used as the models to evaluate the adsorbability of modified NF membrane to water-soluble organics with different charges. As shown in Figure 5A, after filtrating 5 mL dye solution with concentration of 3 ppm through PDA@PLA NF membrane, the filtrate of MB solution became


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colorless, whereas that of OG still remained faint yellow. The zeta potentials of PDA@PLA NF membrane is -15.24 mV at the pH=7 due to the N or O-containing groups, which also discloses the reason for the selective adsorption behavior to cationic dyes than that of anionic dyes. Interestingly, the PDA@PLA NF membrane further decorated with β-CD can also adsorb a small amount of OG, and both MB and OG solutions became colorless after filtering (Figure 5A). The separation capacity of the β-CD-PDA@PLA NF membrane toward the cationic MB and anionic OG dyes were also studied as shown in Figure 5B. The separation efficiency of MB was constant above 95%, whereas the separation efficiency of OG decreased evidently from 97.6% to 18.3% with the permeation volume (PV) increasing to 30 mL. This phenomenon suggested that the βCD-PDA@PLA NF membrane has different adsorption mechanism toward water-soluble organics with different types of charges. The zeta potential of PDA@PLA NF membrane has slightly declined after β-CD modification (Table S2), which could be attribute to the reaction of β-CD-EDA with primary or secondary amine groups of PDA shell. Since the MB possessed obvious positive charge in water solution with zeta potential of 1.55 mV, β-CD-PDA@PLA NF membrane exhibited a strong affinity to MB. Thus, the zeta potential of β-CD-PDA@PLA NF membrane increased to -3.64 mV after adsorbing MB, suggesting that the electrostatic interaction between MB and β-CD-PDA@PLA is responsible for the removal of MB. Meanwhile, the host-guest inclusion with β-CD is responsible for the adsorption of OG. Since the adsorption capacity based on β-CD inclusion complex is limited by the degree of surface modification, the β-CD-PDA@PLA NF membrane is not a conducive system for adsorbing negative charged water-soluble organics.


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Figure 5. Images of the 3 ppm MB and OG solutions before and after being filtered by PDA@PLA or β-CD-PDA@PLA NF membrane (A). The separation efficiency of PDA@PLA and β-CD-PDA@PLA NF membrane for different types of dyes (B). Separation of MB/OG mixture solution: photographs of equipment and dynamic filtration (C). UV-vis spectra of MB/OG mixture solution before and after filtering (D). As shown in Figure 5C, 100 mL of green MB/OG mixture solution (3 ppm MB and 3 ppm OG) were filtered through the β-CD-PDA@PLA NF membrane at the gravity-driven with flux of 1770 L·m-2·h-1. After filtration, the colour of filtrate turned to yellow, suggesting that the MB had already been removed while the OG still remained in the filtrate. The concentrations of MB


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and OG in the solution before and after separation were determined by the UV-vis spectra (Figure 5D). The MB concentration decreased from 3.0 mg/L in initial solution to 0.009 mg/L in filtrate, while the OG concentration only exhibited little decrease after filtration. Thus, the separation efficiency was 99.70%, which also indicated that the β-CD-PDA@PLA NF membrane is able to selectively and effectively adsorb positively charged organic pollutant by dynamic filtration without interfering by the coexisting negatively charged organics. Recyclability of the NF membranes for oil-in-water emulsion separation and organic dye simultaneously. In practical application, the waste water mainly comes from discharging by many different resources.28 It is very valuable to effectively decontaminate wastewater containing mixed pollutants with one single procedure. In this work, the ability of β-CDPDA@PLA NF membrane for simultaneous and continuous removing oil and water-soluble organic contaminants was evaluated by treating with wastewater containing 3 ppm MB and 3 vol% toluene. As shown in Figure 6A, after filtrating 10 mL of wastewater through β-CDPDA@PLA NF membrane, the filtrate became colorless, illustrating that insoluble oils and MB had been successfully removed from the wastewater. Figure 6B suggested that the β-CDPDA@PLA NF membrane had a stable separation efficiency of above 99.5% for oil emulsion with the increase of permeation volume to 100 mL. Meanwhile, the separation efficiency of βCD-PDA@PLA NF membrane for MB exhibited no obvious decline with increasing PV to 50 mL. However, with further increasing PV to 100 mL, the separation efficiency decreased from 99.7% to 58.9%, because the adsorption capacity of β-CD-PDA@PLA NF membrane has reached the saturation point.


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Figure 6. Photographs of model waste-water containing 3 vol% toluene-in-water emulsion and 3 ppm MB before and after filtration (A). The separation efficiency of β-CD-PDA@PLA NF membranes for the different types of pollutants (B). Digital photograph and SEM image of βCD-PDA@PLA NF membrane after recycling (C). Separation efficiency of β-CD-PDA@PLA NF membrane in each separation cycles and the UOCA of β-CD-PDA@PLA NF membrane after every 5 cycles of separation (D). Figure S7 and Figure 6B suggested that the permeation flux of separation membrane may gradually decrease owing to the formation of filtration cake, which blocked the surface pores seriously and decreased the effective filtering area of the membrane, and therefore led to a quick


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decrease in the permeation flux.12 Meanwhile, with the increase of permeation volume, the absorption of organic dyes may also reach saturation. Therefore, recycling is necessary after filtration with a certain amount of waste water. To evaluate the recyclability, cyclic filtration tests using model wastewater containing oil-in-water emulsion and MB simultaneously were conducted by using the recycled membranes in new filtration cycle. The permeation fluxes and separation efficiency for toluene and MB were recorded during each cycle (10 min of filtration). The used membrane was recovered by washing it with a small amount of HCl water solution (0.1 M), ethanol and deionized water, respectively. Separation efficiency of β-CD-PDA@PLA NF membrane after every separation cycles and the UOCA of β-CD-PDA@PLA NF membrane after every 5 cycles of separation were recorded. As shown in Figure 6D, β-CD-PDA@PLA NF membrane was recycled and reused 30 cycles with stable separation efficiency for mixed pollutants. The UOCA of β-CD-PDA@PLA NF membrane was also no significant decline even after 30 cycles, which could be attributed to the stable micro- and macro-morphology of recycled β-CD-PDA@PLA membrane (Figure 6C). Thus, the β-CD-PDA@PLA membranes exhibited both good separation performance and robust recyclability benefiting from the stable hydrophilic structure. These results suggest that the βCD-PDA@PLA NF membrane is a promising candidate for waste-water decontamination. CONCLUSIONS In conclusion, a full biobased β-CD-PDA@PLA NF membranes for decontaminating water from multiple pollutants simultaneously by using electrospun PLA nanofibers as the scaffold and β-CD-PDA as functional coating was developed. After modifying with PDA and β-CD, the asprepared β-CD-PDA@PLA NF membranes exhibited superhydrophilicity and high underwater oleophobicity, which therefore endowed the membrane with effective separation performance for


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oil-in-water emulsion. The flux and separation efficiency of the membranes are higher than 1500 L·m-2·h-1 and 99.5%, respectively. The β-CD-PDA@PLA NF membranes also exhibited adsorbability (over 95%) to positive charged water-soluble organic pollutant during filtration owing to their negatively charged nature as well as the high specific surface area. This result suggesting that the β-CD-PDA@PLA NF membrane effectively decontaminates wastewater containing mixed pollutants with one single procedure. Moreover, the used membranes, which could be easily recovered by washing with a small amount of solvents and used for next filtration cycle, have very good durability to effectively decontaminate the waste-water containing toluene emulsion and methylene blue simultaneously at least 30 cycles. These results suggested that the β-CD-PDA@PLA NF membrane is a promising candidate for decontamination of the wastewater from emulsified oil and positive-charged water-soluble organics. A design idea was provided by this work to prepare separation materials in consideration of both separation performance and ecological sustainability. 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 preparation process of β-CD-EDA; Wettability of oil and water on pristine PLA membrane in air; underwater roll-off angles of PDA@PLA and β-CD-PDA@PLA NF membranes; the water flux of the β-CD-PDA@PLA NF


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membrane; the optical images of emulsion after placed for different times; the flux and separation efficiency of β-CD-PDA@PLA NF membrane for the emulsifier-stabilized emulsions under different vacuum degree; separation efficiency of β-CD-PDA@PLA NF membrane after different separation cycles; morphology characteristics of the various NF membranes obtained by BET analysis and bubble point method; the zeta potentials of the NF membranes and organic dyes solutions. ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (No. 21474066 and No. 51721091). The SEM analysis were provided by the Analytical and Testing Center of Sichuan University. REFERENCES

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Full-biobased β-CD-PDA@PLA nanofiber membranes for decontaminating waste-water from multiple pollutants.


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