Porous Aromatic Framework Modified Electrospun Fiber Membrane as

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Porous Aromatic Framework Modified Electrospun Fiber Membrane as a Highly Efficient and Reusable Adsorbent for Pharmaceuticals and Personal Care Products Removal Rui Zhao, Tingting Ma, Shuying Li, Yuyang Tian, and Guangshan Zhu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b04326 • Publication Date (Web): 22 Apr 2019 Downloaded from http://pubs.acs.org on April 22, 2019

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ACS Applied Materials & Interfaces

Porous

Aromatic

Framework

Modified

Electrospun

Fiber

Membrane as a Highly Efficient and Reusable Adsorbent for Pharmaceuticals and Personal Care Products Removal

Rui Zhao, Tingting Ma, Shuying Li, Yuyang Tian, Guangshan Zhu*

Key Laboratory of Polyoxometalate Science of the Ministry of Education, Faculty of Chemistry, Northeast Normal University, Changchun 130024, PR China

*Corresponding

authors：

Email address: [email protected].

1

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Abstract Water contamination by emerging organic pollutants such as pharmaceuticals and personal care products (PPCPs) is becoming more and more serious. Porous aromatic frameworks (PAFs) are considered as promising adsorbents to remove the PPCPs. In order to overcome the limitation of PAFs in their powder forms for large-scale applications, herein we proposed a strategy to covalently anchor PAFs onto electrospun polymer fiber membranes. Polyaniline (PANI) played the role of aromatic seed layer, which was coated on the electrospun polyacrylonitrile (PAN) fiber membrane firstly. Then, PAF-45 modification was in-situ synthesized in the presence of the PANI coated electrospun PAN fiber membrane. This study could make the PAF based materials be handled more easily and improve the surface area of electrospun fiber membrane. The obtained composite adsorbent (PAF-45-PP FM) was applied for the adsorption of three PPCPs: ibuprofen (IBPF), chloroxylenol (CLXN) and N,Ndiethyl-meta-toluamide (DEET), which exhibited high adsorption capacity and good recycling ability. According to the Langmuir model, the maximum adsorption capacities of PAF-45-PP FM toward IBPF, CLXN and DEET were 613.50, 429.18 and 384.61 mg/g, respectively. In addition, after ten adsorption-desorption cycles, the adsorption capacities toward the three PPCPs decreased slightly. Through an adsorption comparison test, the adsorption capacity of PAF-45-PP FM almost attributed to the loading PAF-45. The adsorption mechanism analysis illustrated that there were pore capture, hydrophobic interaction and π-π interaction between PPCPs and PAF-45-PP FM. Therefore, the PAF-45-PP FM can be potential adsorbents to purify water contaminated with PPCPs.

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Keywords: electrospun fiber membranes; porous aromatic frameworks; adsorption; PPCPs

Introduction Water pollution is an increasingly serious environmental problem around the world. As one group of emerging organic contaminants in water environment, pharmaceuticals and personal care products (PPCPs) have attracted much attention in recent years.1 PPCPs are widely used to increase living standards, especially in developed countries. It is reported that PPCPs could disturb hormonal actions and interfere with the endocrine system.2 Due to the extensive applications and poor elimination of PPCPs, they have been found in water resources (surface water and groundwater) and even in the tissues of fish and vegetables, which is a serious threat toward environment and human health.3 To treat this serious problem, many methods, including advanced oxidation,4 chlorination,5 photodegradation,6 biodegradation,7 membrane separation8 and adsorption,9 have been applied. Among them, adsorption is considered as an effective way to remove PPCPs due to its easy operation, low cost and versatile property. As an important part of adsorption method, adsorbents determine the feasibility and efficiency of the adsorption processes. Up to now, several adsorbents such as activated carbon, graphene, carbon nanotube, zeolite, mesoporous silica, etc., have been developed to eliminate PPCPs from water.10 However, novel and nonconventional adsorbents with high adsorption performance still need to be explored and studied. Recently, new porous material adsorbents from metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) have been used for PPCPs removal due to their appealing porous structural features and properties. For 3



instance, Jhung’s group used porous metal-organic framework (MOF) MIL-101 without and with –OH, –(OH)2, –NH2, and –NO2 groups for the adsorptive removal of PPCPs such as naproxen, ibuprofen, and oxybenzone from the aqueous solution.11 MIL-101-OH and MIL-101-(OH)2 showed the highest PPCPs adsorption capacities, suggesting that H-bonding was an important mechanism. Moreover, Mellah and coworkers studied the capture of pharmaceutical pollutants from water by a novel fluorine-bearing covalent organic framework (COF) TpBD-(CF3)2.12 This COF showed ibuprofen adsorption capacity of 119 mg/g at neutral pH. Though much attention has been paid to MOFs and COFs, the exploitation of other new porous materials with intrinsic stability and good performance for PPCPs removal is still necessary. Porous aromatic frameworks (PAFs), one promising family of porous framework materials, have been successfully synthesized in our previous study,13 which attracts increasing interest.14 PAFs possess high stability, high surface area, tunable pore size and low framework density, making them a wide range of potential applications in gas adsorption, separation, catalysis, etc.15-17 Recently, PAF based materials have also been applied as adsorbents to remove pollutants from wastewater. Ma’s group prepared amidoxime-grafted PAF-1 for uranium extraction from water.18 The PAF adsorbent showed a uranium uptake capacity of over 300 mg/g and effectively reduced the uranyl concentration from 4.1 ppm to less than 1.0 ppb in aqueous solutions within 90 min. Recently, our group combined PAF material with molecular imprinting technology.19 Two functional groups (pyridine ring and carboxyl group) were incorporated together into the hierarchical porous aromatic framework material. Due to a synergistic effect, the PAFs exhibited a good adsorption capacity toward Pb2+ ions (90.36 mg/g) and high selectivity. However, PAFs are insoluble in most of 4


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the common solvent and thus face the problem of processability. PAFs in their powder form are difficult to recycle and can lead to the secondary pollution, which shows weak handleability and restricts their practical application. Usually, the blending of powdery PAF materials with flexible polymer membranes is an effective approach to overcome the mentioned shortcoming.20 However, these compact membranes could make PAF materials unable to fully contact with pollutants in wastewater treatment adsorption process. In the past decade, fiber membranes from electrospinning technique have received intensive interest from the researchers in diverse fields.21,22 Due to their easy preparation, high porosity and good membrane-forming properties, electrospun polymeric fiber membranes are often employed as flexible skeletons to load functional materials, which show great potential in catalysis, adsorption, biomedicine, sensor and energy fields.23-27 Single electrospun polymeric fiber membranes or their composition with functional materials have shown good adsorption capacities toward various pollutants from water, such as dyes, heavy metal ions, pesticides, radioactive element, inorganic phosphorus, etc.28-30 However, the adsorption toward PPCPs by electrospun fiber membranes is rarely reported. This is because that the adsorption capacities of electrospun fiber membranes toward common pollutants mainly attribute to the charged functional groups on the fibers via electrostatic or chelating interactions.31,32 Most of the PPCPs are lipophilic with a high octanol-water distribution coefficient (Kow) values.33 Adsorbents based on electrostatic or chelating interactions are usually not ideal for the adsorption of these PPCPs. Thus, for electrospun fiber adsorbents, the adsorption capacity should derive from the small porous structures of fibers. However, the specific surface area of electrospun fiber membrane is not high enough to obtain satisfying adsorption capacity toward PPCPs. 5



Moreover, the macroporous structure among the electrospun fibers cannot capture the PPCPs effectively. In order to improve the adsorption efficiency of electrospun fiber membranes, coating their surface by PAFs is considered to be a promising strategy. Meanwhile the processability of PAFs could be significantly improved. Inspired by the synthesis of zeolite membrane,34 a seeding growth strategy has been proposed. In the present study, PAF material was modified onto electrospun fibers to obtain flexible composite adsorbent for PPCPs removal from wastewater. Electrospun polyacrylonitrile (PAN) fiber membrane acted as the skeleton. PAF-45, which shows easy preparation and low-cost properties, was used as the PAF model. PAF-45 was modified onto polyaniline (PANI, as the aromatic seed layer which is necessary for PAFs loading onto substrate’s surface) coated electropun PAN fiber membrane. The as-prepared composite adsorbent was applied for the adsorption of three typical PPCPs: ibuprofen (IBPF), chloroxylenol (CLXN) and N,N-diethyl-meta-toluamide (DEET). Their molecular structures and physical properties are shown in Table 1. Furthermore, the adsorption performance and adsorption mechanism toward the three PPCPs by prepared composite adsorbent were investigated in detail. Most importantly, this study may provide a universal method for PAFs loading onto different material’s surface, which can improve the PAFs’ practicability.

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Table 1 Properties of three selected PPCPs under study. PPCPs

Molecular weight (g/mol)

LogKow

Dimension* (nm)

Common usage

IBPF

206.3

3.97

1.06×0.47

Non-steroidal anti-inflammatory drug

35

CLXN

156.6

3.37

0.62×0.54

Antimildew and antibacterial agent

36

DEET

191.3

2.02

1.08×0.55

Insect repellent

36

*Molecule

Molecular structure

Ref .

size was measured by Materials Studio 8.0.

Experimental Materials Polyacrylonitrile (PAN, Mw=150,000) was purchased from Shanghai Macklin Biochemical Co., Ltd. Aniline and ammonium persulfate ((NH4)2S2O8, APS) were purchased from Tiantai Chemical Corp. N,N-Dimethylformamide (DMF) was purchased from Beijing Chemical Factory. Biphenyl (99%) and anhydrous AlCl3 were received from Aladdin. Chloroform from Beijing Chemical Factory was dried over 4 Å molecule sieves before being used. IBPF (C13H18O2, 99%), CLXN (C8H9ClO, 98%) and DEET (C12H17NO, 99%) were purchased from Energy Chemical. Granular activated carbon was purchased from Tianjin Guangfu Fine Chemical Research Institute. Other inorganic reagents were purchased from Beijing Chemical Factory. Except for chloroform, other reagents were used as received without further treatment. Preparation of PAF-45 modified electrospun fiber membrane 7



Electrospun PAN fiber membrane preparation: 10 wt% PAN solution (dissolving PAN powder in DMF at 90 °C for 6 h) was loaded into a 10 mL syringe to conduct the electrospinning process using an electrospinning apparatus. 18 kV was provided between the cathode and anode at a distance of 20 cm with a flow rate of 0.5 mL/h. PAN fiber membrane was collected by a metallic rotating roller drum. PANI coating onto PAN fiber membrane: The PANI coating was synthesized via in-situ chemical oxidative polymerization technique. In detail, 0.9 g APS and 500 L aniline were dissolved in 10 mL and 40 mL 1 M HCl respectively. 300 mg PAN fiber membranes were added into the aniline HCl solution, and then the (NH4)2S2O8 solution was added into to start the polymerization reaction. After stirring for 1.5 h at room temperature, the resultant fiber membrane was washed with 1M HCL, deionized water and ethanol three times, respectively; and then dried in a vacuum oven at 80 °C for 24 h. PAF-45 modification: 300 mg PANI coated electrospun PAN fiber membrane and 500 mg anhydrous AlCl3 were added into a 100 ml round-shaped flask, and this flask was firstly evacuated and then inflated with N2 for 3 times. 40 ml dried CHCl3 was injected into the flask via a syringe and the mixture was heated to 60 °C for 3 h. Another solution containing 200 mg biphenyl in 20 ml dried CHCl3 was added into the above solution and the mixture was kept under stirring at 60 °C for 10 h. After reaction, the resultant fiber membrane was collected and washing with 1.0 M HCl, CH3OH, and acetone. The fiber membrane was further purified by Soxhlet extraction with methanol for 48 h. The final product was obtained after drying at 100 °C for 12 h in vacuum. Characterization

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The morphology and microstructure of obtained fiber membranes were observed with field emission scanning electron microscopy (SEM, Hitachi SU8010) and transmission electron microscopy (TEM, JEOL JEM-3010). The chemical compositions of the fiber membranes were investigated using an energy dispersive spectrometer (EDS) attached to the Hitachi SU8010 SEM. The mean diameter of the fibers was calculated from measuring the different parts of the fibers at 100 different fibers from the SEM images using the commercial software package Image-Pro Plus. The surface roughness of single fiber was measured by an atomic force microscope (AFM, Cypher ES, Asylum Research Oxford Instruments). The mechanical properties of the ﬁber membranes were performed by assembling the membranes (dimensions: length = 40 mm, width = 20 mm) between two stainless steel clamps with a tensile speed of 10 mm·min−1 on a mechanical strength microtest device (410R250, Test Resources, Shakopee, MN, USA). Fourier-Transform infrared (FT-IR) spectra were acquired using a Nicolet iS50 Fourier transform infrared spectrometer. Solid-state 13C cross polarization magic angle spinning nuclear magnetic resonance (CP/MAS NMR) measurement was performed on a Bruker Avance III model 400 MHz NMR spectrometer at a MAS rate of 5 kHz. The electrical conductivity of obtained fiber membranes was determined using a classical four-point probe (RTS-2, Guangzhou Four Probe Technology Co., Ltd.) with a SourceMeter (Keithley 2400, United States). The detailed process for electrical conductivity measurement was shown in the Supporting Information. Thermogravimetric analysis (TGA) data were obtained on a Mettler toledo thermal analyzer at heating rate of 10 °C/min under air atmosphere. N2 adsorption-desorption isotherms were conducted on a Quantachrome Autosorb iQ SN analyzer at -196 °C. Analysis of the X-ray photoelectron spectra (XPS) was

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performed on Thermo ESCALAB 250 spectrometer with a Mg-K (1253.6 eV) achromatic X-ray source. Adsorption of PPCPs from water Stock solutions of the three PPCPs (500 mg/L) were prepared by dissolving them in water/methanol (9/1 v/v). The stock solutions were further diluted with deionized water to get the desired solution concentration. The pH of the PCPs solutions was adjusted to 7.0 ± 0.2 (considering the general pH of river and rain water37) by adding NaOH solution (0.1 M) or HCl solution (0.1 M). Kinetic experiments were performed by mixing 13 mg of adsorbent into 80 mL of PPCP solution with the initial concentration of 50 mg/L. The adsorption isotherms were investigated by initial concentrations ranging from 50 to 400 mg/L with the adsorbent dosage of 0.1 g/L. For the desorption-readsorption experiment, 3 mg adsorbent was added into 15 mL each PPCP solution (50 mg/L) for 6 h. The PPCP-adsorbed adsorbent was regenerated by immersing into ethanol at 60 °C for 6 h. After washing thoroughly with deionized water and ethanol, the adsorbent was reused in adsorption experiments and the process was repeated ten times. All the adsorption experiments were performed at 20±2 °C. IBPF, CLXN and DEET concentrations were determined by measuring the absorbances of the solutions using a UV spectrometer at 220, 279 and 265 nm, respectively. The adsorption capacity (q) of PPCPs onto each adsorbent was calculated on the basis of the following equation: q (mg g) =

(C0 - Ce)V

(1)

W

where C0 and Ce (mg/L) are the initial and the equilibrium concentration of PPCP in the aqueous solution, respectively. V (L) is the volume of the solution, and W (g) is the mass of the dry adsorbent.

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Each adsorption experiment is conducted in triplicates to obtain reproductive results with standard deviation < 5%. In the case of deviation larger than 5%, more tests are carried out.

Results and discussion

Figure 1. Schematic representation of preparation of PAF-45-PP fiber membrane from electrospun PAN fiber membrane. The dashed box is the uncertain structures of PAF-45 (blue represents meta-substitution, while red and green stand for para-substitution and ortho-substitution, respectively).

Composite fiber membrane preparation and characterization In this study, PAFs are used to modify electrospun fiber membranes. Since PAF-1 was successfully synthesized by our group in 2009,13 various PAFs have been obtained through several coupling strategies to connect aromatic rings, such as Suzuki coupling reaction, Sonogashira-Hagihara coupling reaction, Yamamoto reaction, ethynyl trimerization reaction, carbazole-based coupling polymerization, etc.38 However, most of these reactions are complicated, which need expensive catalysts 11



and monomers. In consideration of practical industrial production, PAF-45 from Scholl reaction seems to be a good candidate due to its low-cost catalyst AlCl3 and monomer biphenyl.39 Polyacrylonitrile (PAN), a commonly used polymer, was electrospun into fiber membrane to act as the fibrous skeleton for PAF-45 loading. The schematic preparation process is displayed in Figure 1. To obtain stable and strong PAF-45 loading, electrospun PAN fibers were firstly coated with polyaniline (PANI) as the seed layer. PANI coated PAN (PANI-PAN) fiber membrane (FM) were placed into the liquid phase Scholl reaction for PAF material modification. During Scholl reaction, the coupling happened between the benzene rings from PANI and biphenyl; and then biphenyl further reacts with each other via C–C bonding to form PAF-45 (dashed box in Figure 1) which grows onto fibers to prepare PAF-45 coated PANI-PAN (PAF-45-PP) fiber membrane. The monomer and Scholl reaction for the construction of PAF-45 are shown in Figure S1.

Figure 2. SEM images and diameter distributions of PAN FM (a, b, c), PANI-PAN FM (d, e, f) and PAF-45-PP FM (g, h, i). 12


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The morphology of obtained fiber membranes was observed by SEM images (Figure 2). As shown in Figure 2a~2c, PAN fibers have smooth surfaces with an average diameter of 189 nm. PANI-PAN fibers show rough surface morphology due to the PANI coating and the average diameter increases to 268 nm (Figure 2d~2f). After PAF-45 modification, the fiber surface becomes rougher and obvious particles are observed, which load onto PAF-45-PP fibers uniformly (Figure 2g and 2h). For comparison, PAF-45 was grown onto PAN fibers without PANI seed layer. Its SEM image is shown in Figure S2. Few particles are coated onto PAN fiber surfaces, indicating that PANI seed layer is essential for PAF-45 loading. Moreover, the PAF45-PP fibers still show long and continuous morphology. The average diameter has a large increase to 313 nm (Figure 2i). The microstructures of obtained fibers were also investigated by TEM images and are shown in Figure S3. The morphology observed from TEM images is consistent with the SEM results. In addition, the surface morphology and surface roughness of single fiber at each stage were evaluated by AFM (Figure 3). The surface morphology obtained from AFM images is similar with SEM and TEM observation. The surface topography of PAN FM and PANI-PAN FM show smooth hills with Ra values of about 20.59 and 57.21 nm, which reflect the surface roughness. After PAF-45 modification, the Ra value obviously increases to 111.72 nm due to the larger PAF-45 particles loading onto the fiber surface. The morphology observation preliminarily confirms that PAF-45 is modified onto PANIPAN fibers. Due to the coating property of PANI onto different materials, we predict that this method can make PAFs load onto various substrates’ surfaces, such as coating polymer film, steel mesh, ceramic disk, etc.

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Figure 3. AFM images of PAN FM (a), PANI-PAN FM (b) and PAF-45-PP FM (c).

Figure 4. Stress-strain curves (a), FTIR spectra (b), EDS spectra (c) and TGA curves (d) of obtained fiber membranes.

Though PAF-45 is coated onto fiber surfaces, PAF-45-PP fiber membrane still keeps its good flexibility (Figure S4). Stress-strain curves were conducted to study the mechanical properties of obtained fiber membranes (Figure 4a). Comparing with PAN fiber membrane, the tensile strength of PANI-PAN fiber membrane increases to 8.11 MPa from 5.82 MPa and the elongation at break decreases to 10.85% from 18.86%. This is because that the coating PANI makes the fibers connect with each other more tightly. PAF-45 consists of rigid framework with low flexibility. After 14


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PAF-45 modification, PAF-45-PP fiber membrane shows a decreased tensile strength of 6.67 MP and elongation at break of 8.14%. However, the mechanical property of PAF-45-PP fiber membrane can make PAF based adsorbents be separated from adsorption solution easily, which satisfies its usage in practical adsorption application. The chemical structures of obtained fiber membranes were characterized by FT-IR spectra (Figure 4b). In the spectrum of PAN FM, peaks at 2241 cm-1 and 1736 cm-1, are attributed to the stretching vibrations of the nitrile group and the carbonyl group of the ester of the methylacrylate comonomer, respectively.32 Moreover, the peaks at 1455 cm-1 and 1378 cm-1 are assigned to C–H blending and CH3 symmetric blending, respectively.24 After PANI coating, a new peak at 1306 cm-1 is attributed to typical C– N stretching vibrations in aromatic amine.40 Other peaks at 1109, 795, and 504 cm-1 assigning to C=N stretching, 1,4-substituted phenyl ring stretching and deformation of benzenoid rings of PANI are also observed,41 indicating that PANI has been coated onto PAN FM. The spectrum of PAF-45-PP FM shows the difference in low wavenumber range (1000~500 cm-1). The peak of 795 cm-1 for 1,4-substituted phenyl ring stretching shifts to 811 cm-1, suggesting that benzene rings from PANI are reacted with biphenyl during Scholl reaction. The monomer biphenyl corresponds to five adjacent hydrogens in benzene rings. The peaks in the range of 850~700 cm-1 are ascribed to two, three and four adjacent hydrogen vibrations, respectively.39 In addition, the new peak at 908 cm-1 is assigned to vibration of isolated hydrogen atom.42 Solid-state 13C CP/MAS NMR spectrum of PAF-45-PP FM was conducted to verify the structure of networks and their corresponding building blocks (Figure S5). The peak at 29 ppm is attributed to the aliphatic carbons (–CH– or –CH2–) in the PAN polymer.43 The several broad signals in the region 110–155 ppm are corresponding to the aromatic carbon atoms in benzene rings. The peak at 125 ppm is 15



due to the non-substituted aromatic carbon from PANI and PAF-45.44,45 Moreover, the signal belonging to nitrile groups (–CN) from PAN may overlap with this peak (125 ppm).43 The peak at 147 ppm is assigned to the substituted aromatic carbon from PAF-45.44 Additionally, a shoulder peak at 153 ppm is observed, which belongs to the non-protonated carbon attached to the nitrogen in PANI.46 These results suggest that biphenyl reacts with each other via C–C bonding to form PAF-45. According to previous reports,39,42 PAF-45 is amorphous with uncertain structures. The main substitution ours at para, ortho, and meta positions (Figure 1), which is consistent with our FT-IR results. The surface element contents of obtained fiber membranes were further analyzed by the qualitative SEM-EDS (Figure 4c). As the results listed in Table 2, due to the introduction of aromatic amine, the C/N ratio increases to 3.10 from 2.93 after PANI coating. The C atomic fraction of PAF-45-PP FM has an obvious increase to 77.85 at.% and the N atomic fraction decreases to 12.67 at.%. The C/N ratio also increases to 6.14. This is because that the loaded PAF-45 is constructed from biphenyl without nitrogen element. The element content results correspond to the chemical reactions for fiber preparation and FT-IR results. The electrical conductivity of obtained fiber membranes was also studied. Owning to the coating of conducting polymer PANI, the electrical conductivity of PANI-PAN FM is 8.51×10-3 S/cm. However, PAF-45-PP FM shows almost no electrical conductivity property. This is because that the fibers are covered with non-conducting PAF-45. Moreover, the conjugated structure of PANI is also influenced by the bonding between aniline ring and biphenyl from PAF-45, which blocks the electron transfer. The above characterization results suggest that PAF-45 has been modified onto electrospun fibers successfully.

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Table 2 Element content from EDS spectra and BET analysis results of obtained fiber membranes. sample PAN FM PANI-PAN FM PAF-45-PP FM

Element content (at.%) C N C/N 72.28 24.68 2.93 71.63 23.09 3.10 77.85 12.67 6.14

SBET (m2/g) 9.2 11.3 262.4

BET results Pore size (nm) pore volume (cm3/g) 3.17 0.026 4.78 0.033 0.62 0.151

The thermal stability of the obtained fiber membranes was investigated by TGA analysis (Figure 4d). PAN FM shows a rapid and sharp weight loss of 24.6% in the temperature range from 264 to 304 °C, which attributes to the cyclization of nitrile groups on PAN.24 The following slow degradation is due to the decomposition of PAN polymer chain and release of the oxidation of carbon. After coating PANI, the rapid weight loss temperature becomes higher at 286 °C and the loss is 18.4%, suggesting that PANI coating can improve the thermal stability of PAN FM. Compared with PAN FM and PANI-PAN FM, PAF-45-PP FM shows higher initial degradation temperature at 302 °C with a weight loss of 15.5% due to the good thermal stability of PAF-45 (Figure S6, PAF-45 shows almost no weight loss before 325 °C). Moreover, the smaller weight loss 15.5% for PAF-45-PP FM can also confirm the modification by PAF-45.

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Figure 5. N2 adsorption-desorption isotherms (a) and pore size distributions (b) of obtained fiber membranes.

The nitrogen sorption isotherms at 77 K (Figure 5a) were further conducted to investigate the surface areas of the prepared membranes (Brunauer–Emmett–Teller (BET) surface area here), which are important factors for PPCPs adsorption applications. The BET surface area of PAN FM is only 9.2 m2/g due to its irregular macroporous structure and the compactness of polymer fiber. PANI-PAN FM shows a BET surface area of 11.3 m2/g, indicating that PANI has almost no influence on the surface area. After PAF-45 modification, the BET surface area could reach 262.4 m2/g, which is attributed to the high surface area of PAF-45 (780.2 m2/g). PAF-45 and PAF-45-PP FM exhibit sharp uptakes at low relative pressures, indicating the existence of micropores in their structures. The pore sizes of PAF-45 and PAF-45-PP FM distribute around at 0.60 nm and 0.62 nm (Figure 5b), respectively, suggesting that loading PAF-45 onto electrospun fibers shows little impact on PAF-45’s pore 18


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structure. The BET results confirm that PAF-45 loading can improve the surface area of electrospun fiber membrane, which will be beneﬁcial to the adsorption process. PPCPs adsorption performance

Figure 6. PPCPs adsorption by different fiber membrane adsorbents (PPCP concentration: 50 mg/L and adsorbent dosage: 0.2 g/L).

Three typical PPCPs ibuprofen (IBPF), chloroxylenol (CLXN) and N,N-diethylmeta-toluamide (DEET) were selected as the pollutant models for adsorption study. A comparison of adsorption capacities for different fiber membranes was firstly studied to explore the effect of PAF-45. As shown in Figure 6, PAN FM has almost no adsorption capacities for the three PPCPs. After PANI coating, the adsorption capacities show few improvements, which are less than 15 mg/g. However, for PAF45-PP FM, the adsorption capacities toward three PPCPs increase significantly exceeding 110 mg/g. The results indicate that the adsorption capacities toward three PPCPs by PAF-45-PP FM almost attribute to PAF-45 materials. Furthermore, the adsorption performance of PAF-45-PP FM toward the three PPCPs was investigated in detail.

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Figure 7. Adsorption kinetic curves of IBPF (a-c), CLXN (d-f) and DEET (g-i) onto PAF-45-PP FM: experimental data (a, d and g), pseudo-first-order kinetic plots (b, e and h), and pseudo-second-order kinetic plots (c, f and i).

To know the adsorption rate and saturation adsorption time which can give us important information for wastewater treatment, the adsorption of PPCPs onto PAF45-PP FM in the initial concentration of 50 mg/L as functions of time is shown in Figure 7a, 7d and 7g. Fast adsorption processes could be obtained in the first 60 min for the three PPCPs due to the abundant available adsorption sites in initial stage. Then, the adsorption rate becomes slow until the adsorption equilibrium of 360 min. The kinetic data was further studied by two common kinetic models (the pseudo-firstorder kinetic model and the pseudo-second-order kinetic model). Their linear equations are expressed as follows:31 k1t

log(qe - qt) = logqe - 2.303 t qt

1

(2)

t

(3)

= k q2 + qe 2 e

20


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where qt and qe (mg•g-1) are the adsorption capacity at time t and equilibrium time, respectively. k1 (min-1) and k2 (g•min-1•mg-1) are the pseudo-first order model rate constant and the pseudo-second order model rate constant, respectively. Their fitting linear curves are displayed in Figure 7 (a, d and g) and Figure 7 (c, f and i). The calculated kinetic parameters are summarized in Table 3. By comparing the fitting curves and correlation coefficients (R2), pseudo-second-order model could better describe the adsorption kinetics of IBPF, CLXN and DEET onto PAF-45-PP FM than pseudo-first order model. In addition, the adsorption kinetics by commercial activated carbon (AC) was also studied for comparison. As shown in Figure S7 and Table S1, the adsorption capacities of AC are much lower than those of PAF-45-PP FM and the adsorption rates are also slower.

Table 3 The fitting parameters of pseudo-first-order and pseudo-second-order model. Pseudo-first-order model PPCPs IBPF CLXN DEET

qe (mg/g) 153.06±1.68 116.27±1.56 117.49±1.45

k1 (min-1) 0.021±0.03 0.026±0.02 0.033±0.02

Pseudo-second-order model R2 0.9037 0.9400 0.9724

qe (mg/g) 166.67±2.71 124.84±1.98 138.50±2.45

21


k2 (g/mg•min) (1.95±0.19)×10-4 (3.80±0.27)×10-4 (4.12±0.17)×10-4

R2 0.9982 0.9984 0.9977


Figure 8. Adsorption isotherms of IBPF (a), CLXN (b) and DEET (c) onto PAF-45-PP FM.

Adsorption isotherms were applied to investigate the adsorption performance of PAF-45-PP FM, which are useful for understanding the maximum adsorption capacity and the interaction between an adsorbate and an adsorbent. The adsorption isotherms 22


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for IBPF, CLXN and DEET onto PAF-45-PP FM are shown in Figure 8. The isotherm data were analyzed by two widely-used isotherm models, namely Langmuir and Freundlich. Their linear equations are as follows:47 Langmuir isotherm (homogeneous and monolayer adsorption): Ce qe

Ce

1

(4)

= qm + bqm

Freundlich isotherm (heterogeneous and multilayer adsorption): 1

(5)

logqe = logKF + nlogCe

where qe is the equilibrium adsorption capacity (mg/g), Ce is the equilibrium concentration (mg/L), and qm and b are Langmuir constants related to maximum adsorption capacity and binding energy, respectively; KF and n are empirical constants that indicate the Freundlich constant and heterogeneity factor, respectively. The linear fitting curves are shown in Figure S7. The obtained isotherm parameters are listed in Table 4. According to R2 and fitted curves, the adsorption isotherms of the three PPCPs by PAF-45-PP FM are all fitted better with Langmuir isotherm model, suggesting a possible monolayer adsorption and specific homogenous sites within the adsorbent. The maximum adsorption capacities for IBPF, CLXN and DEET from Langmuir models are 613.50, 429.18 and 384.61 mg/g, respectively. To further understand the adsorption performance of PAF-45-PP FM, maximum adsorption capacities were compared with other PPCP adsorbents. As summarized in Table S2, the adsorption capacity of PAF-45-PP FM is better than most of reported adsorbents. It is also necessary to explore the removal efficiency of PAF-45-PP FM toward the PPCPs in low concentration range. PAF-45-PP FM was used to remove the three PPCPs (IBPF, CLXN and DEET) in the initial concentration of 500 g/L with the adsorbent dosage of 0.1 g/L. The removal process was proven by the UV-Vis spectra 23



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(Figure S9). The absorption peaks belonging to IBPF, CLXN and DEET almost disappear after the adsorption process, confirming the good removal capability. The removal efficiencies toward the three PPCPs are all above 99%, suggesting that PAF45-PP FM can remove traces of PPCPs from water.

Table 4 The fitting parameters of Langmuir and Freundlich equations. PPCPs IBPF CLXN DEET

Langmuir isotherm qm b (mg/g) (L/mg) 613.50±21.73 0.0138±0.0017 429.18±15.83 0.0128±0.0014 384.61±11.79 0.0171±0.0015

R2 0.9942 0.9946 0.9964

Freundlich isotherm KF n 33.88±8.33 23.01±7.63 34.49±8.46

2.07±0.34 2.05±0.02 3.46±0.56

R2 0.9403 0.9740 0.9672

Figure 9. Coexisting ions effect (a) and adsorption–desorption cycles (b) of PPCPs adsorption by PAF-45-PP FM (PPCP concentration: 50 mg/L and adsorbent dosage: 0.2 g/L).

In real water environment, there are a variety of ionic species which may affect the adsorption capacity of adsorbent to some extent. The effect of coexisting ions (Na+, Mg2+, Ca2+, Cl-, NO3- and SO42-) on IBPF, CLXN and DEET adsorption by PAF-45PP FM was explored (Figure 9a). The concentration (10 mmol/L for each ion) of coexisting ions is much higher than that of PPCPs (50 mg/L). In comparison with control groups, the coexisting ions have no obvious influence on PPCPs adsorption, suggesting that PAF-45-PP FM can remove PPCPs from complex saline waste-water. Regeneration and reuse are of great significance in practical adsorption application. For powdery adsorbents, the recycle has to go through a high consumption 24


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centrifugation process. The loading of PAF-45 onto electrospun fibers could transform PAF based materials from powder to membrane, which can be easily separated from the adsorption solution by tweezers. In this study, ethanol is used as the desorption eluent to regenerate PPCPs-adsorbed PAF-45-PP FM. Through detecting the concentrations of PPCPs in the eluent, the desorption efficiencies are all over 99%. As shown in Figure 9b, the adsorption capacities for the three PPCPs decreased slightly after ten adsorption–desorption cycles. Moreover, the morphology of PAF-45-PP FM could be well maintained after ten cycles (Figure S10). The results indicate that PAF-45-PP FM has stability and reusability. PPCPs adsorption mechanism

Figure 10. (a) Effect of pH on IBPF adsorption by PAF-45-PP FM. (b) Plot of logKow value and adsorption capacity of PPCPs. (c) Adsorption comparison for PS FM and PAF-45-PP FM. (d) The proposed adsorption mechanism toward PPCPs by PAF-45-PP FM.

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It is important to understand the possible adsorption mechanism between PAF-45PP FM and PPCPs, which can give guidance for subsequent study. In general, several adsorption mechanisms such as hydrophobic, π-π interactions, H-bond, electrostatic and coordination have been used to explain the adsorption toward PPCPs.48-50 To have an insight into the adsorption mechanism in this study, effect of solution pH on PPCP adsorption (IBPF as the representative) was conducted and is shown in Figure 10a. Solution pH can change the surface charges of adsorbent and adsorbate, and then influence the adsorption process which is usually explained by electrostatic interactions.51 In the selected pH range, pH values show little influence on the adsorption toward IBPF. The result suggests that the adsorption by PAF-45-PP FM is independent of electrostatic interactions. PAF-45 is a hydrophobic material, due to its framework of benzene ring. IBPF, CLXN and DEET have high octanol-water distribution coefficients (Kow), which are also hydrophobic (Table 1). Thus, hydrophobic interaction may be considered as one possible adsorption mechanism.36 To account for hydrophobicity effects of PPCPs on adsorption process, log Kow was plotted against their adsorption capacity (qm). As shown in Figure 10b, PPCP with high logKow displays high adsorption capacity, suggesting that hydrophobic interaction could be applied to explain the adsorption between PAF-45-PP FM and the three PPCPs. To further explain the hydrophobic interaction, the solution pH effect on DEET with low hydrophobicity was conducted. The result is shown in Figure S11 and pH values also have little influence on the adsorption toward DEET, indicating that hydrophobic interaction is indeed one of the adsorption mechanisms. Moreover, according to the chemical structures (Table 1 and Figure 1), PAF-45 and the three PPCPs all have the aromatic ring. Thus, the π-π interaction can be a contribution to the adsorption mechanism.50 To further understand this adsorption 26


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mechanism, XPS spectra of PAF-45-PP FM before and after IBPF adsorption were investigated.52 As shown in Figure S12a, the intensity of the O 1s binding energy peak at 532.7 eV increases obviously, confirming the IBPF adsorption. High resolution XPS spectra of C1s before and after IBPF adsorption were studied (Figure S12b). Before adsorption, the spectrum shows one peak at 284.9 eV assigning to C–C. After adsorption, the spectrum shows two peaks at 284.4 eV and 288.8 eV, which are assigned to C–C and C–O, respectively.53 In addition, the peak of C–C shifts to lower binding energy after the adsorption, which is ascribed to the π-π interactions.54 PAF-45 is a porous organic framework material with abundant micropore structures. The molecular size of three PPCPs at least one dimension (Figure S13) is less than or similar to the pore diameter of PAF-45. Pore-filling mechanism may be considered as a plausible mechanism. The pore-filling mechanism was studied by nitrogen adsorption isotherms. As shown in Figure S14, the BET surface area of PAF-45-PP FM decreases to 23.1 m2/g after adsorbing IBPF. This result indicates that the pore of PAF-45 is occupied and pore-filling mechanism can be applied to explain the adsorption of PPCPs. Moreover, the BET surface area of PAF-45-PP FM can recover to 259.1 m2/g after the regeneration suggesting the good recyclable ability. To further confirm this view, electrospun polystyrene (PS) fiber membrane was prepared. The preparation process is displayed in the Supporting Information and its SEM images are shown in Figure S15a. The structure of PS is similar to PAF-45 because they both have aromatic rings (Figure S15b). In addition, PS fiber membrane is also hydrophobic. The difference is that PS fiber membrane has no micropore structures. Thus, PS fiber membrane is a good control group for exploring the pore-filling mechanism of PAF-45-PP FM, which can eliminate the influence of hydrophobic interaction and π-π interaction. Their adsorption toward PPCPs is illustrated in Figure 27



10c. The adsorption capacities of PAF-45-PP FM (115~132 mg/g) were much higher than those of PS FM (39~43 mg/g). This can be seen that the micropore structures of PAF-45 make a major contribution to the adsorption of the three PPCPs. Therefore, on the basis of above investigation, it is proposed that PPCPS adsorption by PAF-45-PP FM involves the following mechanism: firstly, PPCPs are adsorbed by PAF-45-PP FM through hydrophobic interaction and π-π interaction. Subsequently, PPCPs are captured into the micropore of PAF-45. The proposed adsorption mechanism is schematically summarized in Figure 10d.

Conclusions In conclusion, PAF-45 was successfully modified onto the surfaces of electrospun PAN fibers with the assistance of seed layer PANI. The composition between PAF-45 and electrospun fiber membrane could increase the surface area of PAN fiber membrane to 262.4 m2/g from 9.2 m2/g and make PAF based adsorbent be separated from the adsorption solution easily. The prepared PAF-45 modified electrospun fiber membrane (PAF-45-PP FM) showed good adsorption performance toward three typical PPCPs: ibuprofen (IBPF), chloroxylenol (CLXN) and N,N-diethyl-metatoluamide (DEET). The adsorption comparison results indicated that the adsorption capacity by PAF-45-PP FM almost attributed to PAF-45 materials. The adsorption processes could all reach equilibrium at 360 min and the maximum adsorption capacities toward IBPF, CLXN and DEET from Langmuir models were 613.50, 429.18 and 384.61 mg/g, respectively. In addition, PAF-45-PP FM also showed excellent reusability. Through the mechanism analysis, hydrophobic interaction, π-π interaction and pore capture mechanism contributed to the adsorption processes. The present study provides the method for loading PAF materials onto electrospun fiber 28


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membranes and demonstrates their potential applications for the purification of water contaminated with PPCPs. Inspired by this work, further study will be conducted for loading PAFs on other substrates’ surfaces which may show broad applications, such as gas adsorption, gas separation, liquid separation, catalysis and so on.

ASSOCIATED CONTENT Supporting Information The supporting information is available free of charge on the ACS publications website at DOI: Details for electrical conductivity measurement, monomer and Scholl reaction for PAF-45, SEM images of PAF-45 onto PAN fibers without PANI seed layer, TEM images, optical image of PAF-45-PP FM, solid state 13C NMR spectrum, TGA curve of PAF-45 powder, Adsorption kinetic curves and parameters for activated carbon, Langmuir linear plots and Freundlich linear plots, comparison of the adsorption capacity, SEM image of PAF-45-PP FM after ten adsorption–desorption cycles, effect of solution pH on IBPF adsorption, XPS spectra of PAF-45-PP FM before and after IBPF adsorption, molecule sizes of three PPCPs, N2 adsorption isotherms of PAF-45PP FM after IBPF adsorption and after regeneration, SEM image of PS fiber membrane and chemical structure of PS.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. 29



Acknowledgements This work was supported by the National Natural Science Foundation of China (91622106, 21531003, 21601031 and 21802017) and the Fundamental Research Funds for the Central Universities (2412019QD005).

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Graphical Abstract

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Porous Aromatic Framework Modified Electrospun Fiber Membrane as

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