Highly permeable nanofibrous composite microfiltration membranes for removal of nanoparticles and heavy metal ions

Highly permeable nanofibrous composite microfiltration membranes for removal of nanoparticles and heavy metal ions

Separation and Purification Technology 233 (2020) 115976 Contents lists available at ScienceDirect Separation and Purification Technology journal hom...

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Separation and Purification Technology 233 (2020) 115976

Contents lists available at ScienceDirect

Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur

Highly permeable nanofibrous composite microfiltration membranes for removal of nanoparticles and heavy metal ions ⁎

Xiangxiang Liua, Bingyin Jiangb, Xing Yina, Hongyang Maa,c, , Benjamin S. Hsiaoc,

T



a

State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China State Key Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China c Department of Chemistry, Stony Brook University, Stony Brook, NY 11794-3400, USA b

A R T I C LE I N FO

A B S T R A C T

Keywords: Nanofibrous composite membrane Two-nozzle electrospinning Heavy metal ion Filtration Adsorption

Nanofibrous composite membranes using polyvinyl alcohol (PVA) and polyacrylonitrile (PAN) were fabricated by the two-nozzle electrospinning approach, where the PAN nanofibrous component was modified selectively, and the composite membranes were employed for removal of both nanoparticles and heavy metal ions such as chromium (VI) and cadmium (II) from contaminated water. The chemical compositions and morphologies of the membranes were determined by FTIR, TGA, elemental analysis, SEM, and porometer measurements. It can be seen that the two nanofibrous components were integrated together, where the PVA nanofibrous component was cross-linked by glutaraldehyde (GA) working as a mechanical support of the membrane, and the PAN nanofibrous component could be further functionalized independently. The PAN component was further modified by surface grafting or hydrolysis to fabricate composite microfiltration membranes which exhibited high separation efficiency and selectivity for particular sized-microspheres, while positively charged or negatively charged nature enabled impressive adsorption capability, as demonstrated by the removal of chromium (VI) and cadmium (II) ions from contaminated water, respectively.

1. Introduction Chromium (VI) and cadmium (II) ions, both exhibit high toxicity and carcinogenicity, are commonly found in water sources contaminated by industrial pollutions from mineral, textile, and paint industries, which rises an urgent problem to human’s health that needed to be addressed in priority [1–5]. Nanofibrous microfiltration membranes with high porosity, high surface area, and high functionalizability were fabricated by electrospinning technique and employed to get rid of heavy metal ions from contaminated water, where the purification efficiency in terms of adsorption capacity and permeation flux exhibited 2–10 times higher than that of commercially available absorbents [6–12]. Moreover, functional additives (e.g., metal-organic frameworks) integrated with nanofibers, which offer even higher heavy metal ion-adsorption to microfiltration membranes [13–16]. However, there are some concerns associated with the direct usage of electrospun nanofibrous membranes for practical filtration applications. One is that the mechanical properties of typical electrospun membranes are usually weak, and need to be enhanced for filtration under operating pressure [7,17]. Typically, non-woven substrates, such as polyethylene terephthalate (PET) or polypropylene (PP) microfibrous



mats, were used to mechanically support the electrospun layer, forming an asymmetrical membrane structure [18–20]. Unfortunately, the adhesion between the non-woven substrate and the electrospun nanofibrous scaffold can be poor, as the physical properties of the two layers (e.g. expansion coefficient) are quite different, which would drastically decrease the durability and robust of the membrane [21–23]. One way to address the issue of adhesion strength is by constructing symmetrical free-standing electrospun nanofibrous membranes with adequate strengths and without the non-woven support. Another concern is that the adsorption capability of the nanofibrous membrane per unit mass needs to be increased substantially, to meet the requirement of the filtration membrane for long-term use [24–27]. Typical approaches to improve the adsorption capacity of nanofibrous membranes include: (1) increase in the surface area by reducing the fiber diameter; (2) increase in the density of the surface functional groups (adsorption sites) by grafting branched species; and (3) increase in durability of the membrane by recycling and reusing the material many times over. On the other hand, a free-standing electrospun nanofibrous membrane without an additional substrate can decrease the total mass of the membrane, which is equal to increase the adsorption capacity of the membrane per unit mass. Therefore, two-nozzle

Corresponding authors at: State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China (H. Ma). E-mail addresses: [email protected] (H. Ma), [email protected] (B.S. Hsiao).

https://doi.org/10.1016/j.seppur.2019.115976 Received 4 July 2019; Received in revised form 24 August 2019; Accepted 24 August 2019 Available online 26 August 2019 1383-5866/ © 2019 Elsevier B.V. All rights reserved.

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(3.0%) containing 0.3 g of anhydrous aluminum chloride. The system was heated at 90 °C for 4 h to complete the reaction. The PVAm-grafted membrane was dried in an oven for 24 h after washing with distilled water for 5 times.

electrospinning process was employed to address those issues where nanofibrous composite membranes with integrated structure were prepared for water purification [28–31]. In this study, electrospun PAN and PVA nanofibers were fabricated and integrated together to create nanofibrous composite membranes by two-nozzle electrospinning technology. The composition of nanofibrous membranes was tuned by adjusting conditions including flow rate, weight ratio of PAN and PVA, and electrospinning parameters. In the composite membrane, PVA nanofibers with thicker fiber diameter were cross-linked to guarantee the adequate mechanical properties of the free-standing membrane, while finer PAN nanofibers were further modified by either surface-grafting or surface-hydrolysis to provide positively charged or negatively charged functional groups, respectively, to achieve the high adsorption capability. The adsorption capability of the membranes for chromium (Cr (VI)) and cadmium (Cd (II)) ions, with opposite charges, were evaluated, in addition to the demonstration of filtration performance for nanoparticle aqueous suspensions.

2.5. Surface hydrolysis of PAN nanofiber Sodium hydroxide aqueous solution (0.5 mol/L) was used to hydrolyze PAN nanofibers in the composite PAN-PVA membrane. In this treatment, 0.4 g of nanofibrous composite membrane was immersed in a 500 mL of NaOH aqueous solution and the system was equilibrated at 70 °C for 3 h. The resulting membrane was rinsed with distilled water thoroughly and dried in an oven at 40 °C for 24 h. 2.6. Surface charge evaluation The surface charge density of the modified nanofibrous composite membranes was evaluated with the conductometric titration method. In brief, 10 mg of the membrane was placed in a 50 mL of acidic aqueous solution with pH = 2.5–2.8. Sodium hydroxide aqueous solution (0.01 mol/L) was used to conduct the titration experiment. The content of either amino or carboxyl groups of the membrane was evaluated by this approach, which was synonymous to the surface charge density of the membrane.

2. Experimental 2.1. Materials Polyvinyl alcohol (PVA) (Mw = 7.3 × 104 Da, 98% hydrolyzed) and polyacrylonitrile (PAN) (Mw = 1.5 × 105 Da) were purchased from Aladdin Industrial Corp. and Macklin Inc. (China), respectively, without further treatment. Glutaraldehyde (GA, 50 wt% in water), potassium dichromate (K2Cr2O7), and cadmium nitrate tetrahydrate (Cd (NO3)2·4H2O) were bought also from Macklin Inc. Polyvinylamine (PVAm, Mw = 3.4 × 104, 90% hydrolyzed) was obtained from Jinjile Chem, Ltd. (Shanghai). Polystyrene microspheres suspension (2.5 w/v %, particle sizes of 0.3 and 0.5-μm, respectively) were purchased from Tianjin BaseLine ChromTech Research Centre (China) and diluted to 200 ppm before use. Other chemicals, including acetone, N,N-dimethyl formamide (DMF), and hydrochloric acid (36.5% aqueous solution), were purchased from Beijing Chemical Works. All chemicals were used as received without purification unless noted.

2.7. Microfiltration performance of nanofibrous composite membranes The filtration efficiency of nanofibrous composite membranes including cross-linked PAN-PVA, hydrolyzed PAN-PVA, and PVAm-gPAN-PVA were evaluated by microfiltration of microsphere suspension with particle sizes of 0.3 and 0.5 μm, respectively. A dead-end filtration cell with effective filtration area of 3.8 cm2 was employed and the flow rate was remained at 2.0 mL/min. The rejection ratio (R%) of the membrane was calculated based on the following equation.

Cp ⎞ R% = ⎜⎛1 − ⎟ × 100% Cf ⎠ ⎝

2.2. Fabrication of electrospinning PAN-PVA nanofibrous composite membranes

where Cp and Cf are the concentration of permeate and feed solutions determined by an UV spectrometer.

Electrospun PAN-PVA composite membranes were prepared by a custom built two-nozzle electrospinning setup [32]. Two polymer solutions, PAN with concentration of 10 wt% and PVA with concentration of 12 wt% (0.5 v/w% of Triton-100), were loaded into two syringe pumps on opposite sides of the collecting drum wrapped with aluminum foil. The typical electrospinning conditions for either side are: flow rate of 20 μL/min, electrospinning voltage of 20 kV, and distance between the spinneret and drum of 12 cm. The rotation speed of the drum remained at 60 rpm and the traveling distance on the drum was 12 cm. A closed chamber was used to control the electrospinning environment.

2.8. Static adsorption of nanofibrous composite PAN-PVA membrane The adsorption capacity of the nanofibrous composite membrane was determined by a batch approach. In specific, 0.01 g of the membrane was placed in 20 mL of K2Cr2O7 or Cd(NO3)2 aqueous feed solution with the following concentrations: 40, 60, 80, 100, 120 mg/L for Cr (VI), in the form of CrO42−, and 20, 30, 40, 50, 60 mg/L for Cd (II), in the form of Cd2+, respectively. The results were used to determine the isothermal adsorption. In this test, the pH values of Cr (VI) and Cd (II) solutions were kept constant at 2.0 and 6.0, respectively, which was adjusted with 1.0 mol/L HCl solution at 25 °C under stirring. The equilibrium and original concentrations of Cr (VI) were determined by UV measurement (JINGHUA 759, Shanghai) after complexion with DPC (diphenyl carbazide) using the standard procedure; the concentrations of Cd (II) were investigated with ICP instrument (ICAP6300, Thermo Fisher Scientific).

2.3. Crosslinking PVA nanofibrous component in acetone with GA GA aqueous solution was employed to cross-link PVA to form cyclic acetal structure. In details, 0.3 g of nanofibrous composite membrane was immersed in 100 mL of acetone solution containing GA (0.03 mol/ L) and HCl (0.01 mol/L). The membrane was kept in the reaction system for 24 h. The cross-linked PAN-PVA membrane was washed with pure water for 3 times followed by drying in an oven at 40 °C for 24 h.

2.9. Recycling and reuse evaluation of nanofibrous composite PAN-PVA membrane The electrospun nanofibrous composite PAN-PVA membrane was recycled after Cr (VI) and Cd (II) adsorption using 0.1 mol/L of sodium hydroxide solution and 1 mol/L of nitric acid, respectively. The used membrane was stirred in 50 mL of sodium hydroxide solution or nitric acid solution at room temperature for 1 h. The membrane was then

2.4. Grafting of PVAm onto PAN nanofiber surface A pre-determined amount of nanofibrous composite membrane (0.3 g), after GA-crosslinking, was placed in a 300 mL of PVAm solution 2

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washed with distilled water for 3 times and dried in an oven at 40 °C for 24 h. The reuse of the membrane was demonstrated by repeating the experiment as described in Section 2.7 for 3 times. The usability of the recycled membrane was evaluated again using the adsorption capability test by calculating the regeneration efficiency defined by the ratio (in percentage) of adsorption capacities between the original membrane and the used membrane. 2.10. Membrane characterization Scanning electron microscopy (SEM) measurements were carried out using a JEOL JSM-7800F microscope (JEOL, Japan) to determine the surface and cross-sectional morphologies of the samples (liquid nitrogen was used to help the fracture of the water-wetted samples). A pore-size distribution analyzer (JW-PD200, JWGB Sci. &Tech.) was employed to determine the pore size distribution, where a wetting agent GQ-16 with a surface tension of 16 dynes/cm was used. The composition of the nanofibrous composite PAN-PVA membrane before and after the modification was calculated using an element analyzer (Vario EL cube, Elementar, Germany) to confirm the composition of the membrane through different two-nozzle electrospinning conditions. The surface grafting density of the nanofibrous composite membrane was also evaluated from the elemental analysis. Fourier Transform infrared spectroscopy (FTIR) spectra of nanofibrous composite membranes were achieved using the Tensor 27 instrument (Bruker Inc.). The surface chemistry of the membrane was qualitatively characterized by FTIR measurements before and after chemical modifications. Thermal gravimetric analysis (TGA) of the samples was performed on the TGA/ DSC1 instrument from Mettler-Toledo. The temperature scans were conducted at 10 °C/min from 50 °C to 600 °C under a nitrogen flow of 50 mL/min. Tensile test was conducted by using a tensile apparatus (UTM5205XHD, SUNS, Shenzhen) under asymmetric deformation. All membrane samples were cut into a dumbbell shape with dimensions of 50 mm × 10 mm. The initial length between the Instron clamps was 30 mm and the stretching rate was maintained at 5 mm/min.

Fig. 1. ATR spectra of (a) PVA membrane, (b) PAN membrane, (c) PAN-PVA membrane, (d) PAN-GA crosslinked PVA membrane, (e) PVAm-grafted-PANPVA membrane, and (f) hydrolyzed PAN-PVA membrane.

In the first modification scheme, polyvinylamine (PVAm) was used to functionalize the PAN surface by addition reaction of the amino group (in PVAm) and cyano group (in PAN) to produced an amidine linkage (the reaction was catalyzed with AlCl3). This scheme effectively grafted PVAm onto the PAN nanofiber surface (reaction (I) in Scheme 1) [37,38]. The amino groups in the grafted chains should work as adsorption sites to attract negatively charged heavy metal ions, such as chromium (VI). In the second modification scheme, PAN was hydrolyzed with a strong base, such as sodium hydroxide aqueous solution, and formed polyacrylate acid sodium salt (reaction II in Scheme 1) [39,40]. The resulting carboxylate groups should coordinate with positively charged heavy metal ions [8,41,42], such as cadmium (Cd (II)).

3.2. Chemical compositions of nanofibrous composite membranes 3. Results and discussion

The surface functionality of the initial PAN-PVA nanofibrous composite membranes, PVAm-grafted-PAN-PVA and hydrolyzed PAN-PVA membranes was determined with FTIR-ATR instrument, and the results are shown in Fig. 1. The ATR spectrum of the PAN-PVA composite membrane exhibited a vibration band of the cyano group located at 2243.3 cm−1, which matched well with the signal of 2243.5 cm−1 in single PAN membrane. Meanwhile, the peaks at 3328.9 and 1092.9 cm−1 in PAN-PVA could be assigned to the stretching vibration of hydroxyl group and C-O bond, respectively. These signals also matched well with those in single PVA membrane (3323.4 and 1091.6 cm−1, respectively). The use of glutaraldehyde (GA) to crosslink the PVA component was also effective, as evidenced by the appearance of stretching vibration of the acetal groups in the spectrum of PAN-GA crosslinked PVA membrane at 1136.1 cm−1 [43]. In PVAm-grafted-PAN-PVA membrane, the bending vibration at 1578.3 cm−1 could be assigned to N-H of PVAm [41], and the signal at 2244.1 cm−1 could be assigned to the cyano group (it became relatively weak in the grafted product), and the vibration at 1735.1 cm−1 could be assigned to the carbonyl group [41]. These results confirmed the amino-grafting involving the addition reaction of amino group and cyano group. Finally, the hydrolysis of the PAN components in the composite membrane was also confirmed by the vibration at 1736.7 cm−1, which indicated the production of carbonyl groups by the hydrolysis of cyano under basic conditions [40]. The chemical compositions of PAN-PVA nanofibrous composite membranes before and after surface grafting or hydrolysis were also determined by elemental analysis. The results are shown in Table 1. In this test, the percentage of nitrogen should decrease after the GAcrosslinking of PVA due to the formation of acetal group, where the

3.1. Surface modification of electrospun nanofibrous composite membranes The electrospun nanofibrous composite PAN-PVA membrane, as well as nanofibrous PAN and PVA membranes were fabricated through a two-nozzle electrospinning process, where the concentrations of PAN and PVA solutions were 10 wt% and 12 wt%, respectively. It should be noted that the PVA nanofibrous component must be chemically crosslinked to issue water stability to the composite membrane for water purification [33]. Typically, GA was selected as the cross-linker and cyclic acetal structure was formed [32,34–36]. Two different modification strategies were then applied to functionalize the PAN component of the PAN-PVA nanofibrous composite membrane, aiming to enhance function for water purification. The chosen PAN modification schemes are illustrated in Scheme 1.

Scheme 1. Surface modification routes of PAN nanofibrous component. 3

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3.4. Morphology of electrospun nanofibrous composite membranes

Table 1 Elemental analysis data of PAN-PVA nanofibrous composite membranes before and after surface modifications. Samples

C (%)

H (%)

N (%)

PAN-PVA membrane PAN-GA crosslinked PVA membrane PVAm-grafted-PAN-PVA membrane Hydrolyzed PAN-PVA membrane

59.60 58.74 56.42 56.54

5.51 7.67 7.42 7.97

11.44 8.38 14.89 7.11

The surface morphology of the electrospun nanofibrous composite PAN-PVA membrane, pristine PAN and PVA nanofibrous membrane were observed through SEM measurements, and the results were shown in Fig. 3. The fiber diameters of single PVA and PAN nanofibrous membranes were 776 ± 88 nm and 145 ± 45 nm, respectively, as shown in Fig. 3(A) and (B), indicating that the mean diameter of PVA nanofibers is about 4-times higher than that of PAN nanofibers. PAN-PVA membrane could be distinguished based on the fiber diameter, as shown in Fig. 3(C). The thicker fiber diameter belongs to the PVA component measured at 824 ± 138 nm, and thinner one attributed to the PAN component measured at 217 ± 50 nm. GA was employed to cross-link the PVA nanofibrous component in the composite membrane for water purification application since the PVA nanofibers without cross-linking can be dissolved in water. The fiber diameters of PVA and PAN nanofibrous components in the crosslinked membrane were 866 ± 79 nm and 216 ± 33 nm respectively, a remaining 4-fold relationship between them. The PVA nanofibers covalently bonded together at the interconnects by the formation of acetal structure and served as a skeleton, as shown in Fig. 3(D), where PAN nanofibers also attached tightly on the surface of the PVA nanofibers and occupied the space defined by PVA scaffolds. SEM was also employed to further investigate the morphology of chemically modified nanofibrous composite membrane, where the results are shown in Fig. 4. Overall, the initial structure seemed to remain unchanged by the chosen modification schemes. However, the careful examination yielded some subtle feature changes. Fig. 4(A) and (B) illustrate the membrane structure after the PVAm modification. From Fig. 4(A), it appeared that the adhesion between the finer PAN nanofibers and thicker PVA nanofibers was more enhanced after the surface grafting. The cross-sectional view (Fig. 4(B)) of PVAm-grafted PAN-PVA composite membrane also indicated that the two nanofibrous components were interlocked tightly and the three dimensional nanofibrous network became much integrated. Fig. 4(C) and (D) are the SEM images of hydrolyzed PAN-PVA nanofibrous composite membrane. It was seen that the reaction of hydrolysis using sodium hydroxide did not really change the initial structure, where no enhanced adhesion between PAN and PVA was observed. The corresponding mechanical properties of the nanofibrous membranes after hydrolysis also supported these observations.

surface grafting of PVAm on PAN should increase the percentage of nitrogen. The above changes were seen in the elemental analysis result, where the percentage of nitrogen decreased to 8.38% in PAN-GA crosslinked PVA membrane from the initial 11.44% in PAN-PVA membrane, and increased to 14.89% in PVAm-grafted-PAN-PVA membrane, indicating that the nanofibrous composite membrane has been effectively modified with positively charged amino groups. The effectiveness of the hydrolysis of cyano groups was also observed, where negatively charged carboxylate groups could be obtained, evidenced by the decrease in percentage of nitrogen from 8.38% to 7.11% after 3-h hydrolysis of the PAN-GA crosslinked PVA membrane. These results were consistent with the observation from ATR spectra as shown in Fig. 1.

3.3. Thermal stability of nanofibrous composite membranes The surface grafting and hydrolysis reactions were further evidenced by thermal gravimetric analysis, and the corresponding TGA curves are shown in Fig. 2. It was clear to see that the thermal stability of all nanofibrous composite membranes including original, cross-linked, surface grafting, and surface hydrolyzed composite membrane are in between single PAN and PVA nanofibers, though the onset decomposition temperatures are quite different. Cross-linked PAN-PVA nanofibrous composite membrane exhibited highest onset decomposition temperature which was about 291.4 °C, indicated that cross-linking structure of PVA enhanced the thermal stability of the composite membrane. On the other hand, the surface grafting and hydrolysis decreased the thermal stability of the composite membrane, as indicated by TGA data, the onset decomposition temperature were 193.0 and 136.9 °C, respectively, and those results could be attributed to the introduction of active functional groups such as amino and carboxyl groups through surface modification of PAN nanofibrous component in the composite membrane.

3.5. Mechanical properties of nanofibrous composite membranes Using the surface grafting scheme, the PVAm-grafted PAN-PVA composite membrane exhibited impressive mechanical properties having the ultimate tensile strength and Young’s modulus as 11.0 ± 1.8 and 264.0 ± 11.7 MPa, respectively. The observed modulus was almost comparable with that of some PET non-woven mats, such as FO2413 (its ultimate tensile strength and Young’s modulus were 27.2 ± 5.3 and 276.2 ± 113.0 MPa, respectively) [44]. This was probably because PVAm contained multi-functional amino-groups, which could serve as a cross-linker to enhance the bonding among PAN nanofibers. The hydrolysis of PAN nanofibers also affected the strength of the PAN-PVA membrane, where higher Young’s modulus and higher ultimate tensile strength were also observed than the single PVA or PAN membranes. Based on the above results, it could be concluded that integrated PAN-PVA nanofibrous composite membranes could be used as free-standing membranes without the support of PET non-woven substrate. This conclusion was also supported by the study of dimensional changes of the composite membrane during the PAN-grafting process, as shown in Table 2. It was found that single PAN nanofibrous membrane could reduce 43.1% of its area due to shrinkage during the grafting process, which would lead to the debonding (or separation) between the PAN layer and

Fig. 2. TGA curves of electrospun PAN-PVA nanofibrous composite membranes. 4

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Fig. 3. SEM images of electrospun pristine PVA (A) and PAN (B) nanofibrous membranes, as well as PAN-PVA nanofibrous composite membranes before (C) and after (D) GA-crosslinking.

PAN-PVA membranes. It was found the PAN membrane suffered 53.9% bulk shrinkage was observed, while the PAN-PVA (PVA being crosslinked) composite membranes showed 3.0-times less bulk shrinkage than single PAN membrane.

the PET non-woven substrate. However, in the PAN-PVA composite membrane with PVA being crosslinked, only 7.6% area shrinkage was observed, that was about 5.7-times lower than that of single PAN membrane. The bulk shrinkage was also determined for both PAN and

Fig. 4. SEM images of nanofibrous composite membranes before and after the chemical modification of PAN. (A) Top and (B) cross-sectional views of PVAm-grafted PAN-PVA nanofibrous composite membrane; (C) top and (D) cross-sectional views of hydrolyzed PAN-PVA nanofibrous composite membrane. 5

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Table 2 Dimension changes of crosslinked PAN-PVA nanofibrous composite membrane as well as single PAN nanofibrous membrane during the modification. Sample discs

Diameter before grafting (mm)

Diameter after grafting (mm)

Thickness before grafting (µm)

Thickness after grafting (µm)

Area shrinkage (%)

Bulky shrinkage (%)

PAN nanofibers Cross-linked PAN-PVA

30.0 ± 0.0 30.0 ± 0.0

22.6 ± 0.5 28.8 ± 0.3

68.9 ± 0.9 91.0 ± 1.1

55.7 ± 1.7 80.5 ± 0.6

43.1 ± 2.5 7.6 ± 1.6

53.9 ± 2.8 18.2 ± 2.1

Fig. 6. Filtration performance of cross-linked PAN-PVA, PVAm-g-PAN-PVA and hydrolyzed PAN-PVA composite membranes against polystyrene microspheres. Fig. 5. Pore size and pore size distribution of original PAN-PVA composite membrane as well as PVA crosslinked, PVAm-grafted-PAN, and hydrolyzed PAN composite membranes.

in contaminated water, with high adsorption capacities.

3.6. Filtration performance of nanofibrous composite membranes

3.7. Adsorption capabilities of nanofibrous composite membranes

The structural characteristics (pore size and pore size distribution) relevant to the filtration performance of the varying composite membranes were investigated, as shown in Fig. 5. It was seen that the functionalization of nanofibrous composite membranes either by PVA cross-linking, PAN surface-grafting, or by PAN surface hydrolysis, all decreased the pore size (< 0.7 μm) as well as the pore size distribution. As a result, the water permeability of these membranes after chemical modifications was decreased when compared to that of the original membrane without modification, based on the Hagen-Poiseuille equation [45]. Furthermore, the pore size distribution also became narrower in modified membranes than that without modification (Fig. 5). It was noted that all membranes after chemical modifications exhibited still high water flux, e.g., 1030 ± 28 L/m2 h/psi for PVAm-gPAN composite membrane, indicating that they are good candidates for microfiltration, capable of removing any particulates with size larger than 0.7 µm [10]. As well-known the typical waterborne bacterial, E. coli, has a dimension of 0.5 μm × 2.0 μm, it would be expected that the nanofibrous composite membranes can get rid of bacterial such as E. coli with high rejection ratio from contaminated water. To verify this point, microfiltration performance of those composite membranes was demonstrated using polystyrene microsphere suspensions where the particle sizes are 0.3 and 0.5 μm, respectively. The flow rate of the filtration was remained at 2.0 mL/min and the operating pressure was about 1.0 psi throughout the filtration process, the results are shown in Fig. 6. It was interesting that all nanofibrous composite membranes exhibited higher than 99.3% of rejection ratios against 0.5-μm-particles. The PVAm-g-PAN-PVA membrane with lowest pore size showed even 100% of rejection ratio for 0.5-μm-particles, implying the high separation efficiency of the membrane for nano-sized particulates such as waterborne bacteria. Meanwhile, it was also noted that the hydrolyzed PAN-PVA membrane exhibited excellent selectivity against 0.3-μm and 0.5-μm-particles, evidenced by the rejection ratios which are 2.7% and 99.3%, respectively. In addition to high filtration efficiency, these membranes can also adsorb heavy metal ions, such as Cr (VI) or Cd (II)

As stated earlier, one of the applications of PVAm-g-PAN-PVA and hydrolyzed PAN-PVA composite membranes was the adsorption of heavy metal ions, such as Cr (VI) and Cd (II), respectively, for water purification. The adsorption capability of PVAm-g-PAN-PVA and hydrolyzed PAN-PVA composite membranes was investigated by batches of static adsorption experiments against Cr (VI) and Cd (II) ions, respectively. The adsorption of PVAm-g-PAN-PVA composite membrane for Cr (VI) was carried out at pH 2, and the major formats of Cr (VI) in the solution are Cr2O72− and HCrO4− [9,46,47], which are favorable for the interaction between the membrane and Cr (VI) ions (Fig. S1). Likewise, the optimum adsorption pH for Cd (II) was 6 [48,49]. The results of adsorption capacity (q) versus different adsorption time are illustrated in Fig. 7(A). It was found that the equilibrium of adsorption could be approached quickly for both of Cr (VI) and Cd (II) ions within 30 min, which indicated that the composite membranes possessed very good adsorption efficiency for heavy metal ions. The equilibrium adsorption capacity of the PVAm-g-PAN-PVA membrane (against Cr (VI)) was about 2-times higher than that of hydrolyzed PAN-PVA membrane which (against Cd (II)). This might be due to the higher density of amino-groups on the PAN surface than that of carboxylate groups. Theoretically, each reactive cyano-group located on the surface of PAN nanofibers could be converted to one carboxylate group after hydrolysis. However, it can also be grafted with a PVAm chain, which contained a larger amount of amino-groups available for adsorption. Two possible types of adsorption kinetics (i.e., pseudo-first order or pseudo-second order) were investigated for the Cr (VI) and Cd (II) adsorption study using PVAm-g-PAN-PVA membrane and hydrolyzed PAN-PVA membrane, respectively. The results are illustrated in Fig. 7(B). It was seen that using the first-order kinetics plot (i.e., log (qeqt) versus time, where qe and qt represent the amount of adsorption at equilibrium or time t, respectively), the R2 was found to be 96.9 and 89.7 for PVAm-g-PAN-PVA membrane and hydrolyzed PAN-PVA membrane, respectively; where using the second-order kinetics plot (i.e., t/qt versus time), the R2 was found to be 99.6 and 99.9, respectively. We thus concluded that the adsorption process obeyed the 6

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Fig. 7. (A) Increase in the adsorption capacity (q) against Cr (VI) and Cd (II) ions with time using PVAm-g-PAN-PVA and hydrolyzed PAN-PVA composite membranes, respectively, and (B) corresponding pseudo-first order (log qe-qt) and pseudo-second order (t/qt) kinetics plots of the adsorption processes.

Fig. 8. (A) Langmuir and (B) Freundlich models of isotherms of Cr (VI) and Cd (II) ions adsorption using PVAm-g-PAN-PVA membrane and hydrolyzed PAN-PVA membrane, respectively.

(VI) ions. The adsorption capacity of hydrolyzed PAN-PVA nanofibrous composite membrane also matched well with the surface charge density of 0.48 mmol/g (evaluated by conductometric titration [11,59,60]). This capacity was also higher than those of other electrospun nanofiberbased adsorbents [61–63]. Therefore, it is clear that the PAN-PVA composite membranes could be readily modified by different approaches for different adsorption applications, where better performance was obtained by the surface grafting of hyperbranched polymers on PAN component. Desorption performance was also carried out to demonstrate that the modified PAN-PVA nanofibrous composite membranes could be recycled and reused for many times. The results are shown in Fig. 9. In this test, three cycles of adsorption-desorption processes were conducted using the protocols outlined in the experimental section. In brief, 0.1 mol/L of sodium hydroxide and 1.0 mol/L of nitric acid aqueous solutions were employed to remove Cr (VI) and Cd (II) ions from the modified PAN-PVA nanofibrous composite membranes [8]. The regeneration efficiency was evaluated by the ratio of the adsorption capacities of recycled and original membranes. In Fig. 9, high regeneration efficiencies of 92 ± 2% and 90 ± 2% were achieved after 3 recycles of adsorption-desorption for PVAm-g-PAN-PVA and hydrolyzed PAN-PVA membranes, respectively. These results indicated that the modified PAN-PVA nanofibrous composite membranes are highly robust due to the integrated nanofibrous structure, and have great potential for practical applications in water purification.

pseudo-second order kinetics. The thermodynamics of the adsorption process was also explored using the Langmuir and Freundlich models to fit the adsorption isotherms [8], where the results are shown in Fig. 8. In Fig. 8, both Langmuir and Freundlich models were found to be capable of fitting the adsorption isotherm data (Ce represents the equilibrium concentration of the ion in solution when adsorption amount is qe). The Langmuir model gave a slightly higher R2 value (> 99.7). Consider the different physical base for the Langmuir and Freundlich models, we considered our data could be more suitably described by the Langmuir model. This implied that the heavy metal ions were covered on the surface of PVAm-g-PAN and hydrolyzed PAN nanofibers in composite membranes as monolayer, and the fiber surface could be considered as a homogeneous surface with identical adsorption sites. In this case, the maximum adsorption capacities of the composite membrane could be determined as 66.5 and 33.6 mg/g for Cr (VI) and Cd (II), respectively, based on the total weight of the composite membranes, where the PVA nanofibrous component works actually as a mechanical support. As a comparison, the maximum adsorption capacity of the single PAN nanofibrous membranes with PET non-woven mat (AWA-16-1, Japan) as a support, based on the total weight of the membranes, was only 22.2 and 11.2 mg/g for Cr (VI) and Cd (II), respectively, which are 3-times lower than that of the PAN-PVA composite membranes. In addition, the adsorption capacity of PVAm-g-PAN-PVA nanofibrous composite membrane was essentially higher than that of most reported adsorbents [50–55]. This high adsorption capacity could be attributed to the hyper-branched structure of PVAm grafted on the PAN surface. The surface charge density of PVAm-g-PAN-PVA membrane determined by conductometric titration was 0.70 mmol/g [9,56–58], which was consistent with the high adsorption capacity against the Cr

4. Conclusions Nanofibrous PAN-PVA composite membranes fabricated by twonozzle electrospinning approach were employed to remove heavy metal 7

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Fig. 9. Recyclability of modified PAN-PVA nanofibrous composite membranes for adsorption of Cr (VI) and Cd (II) ions.

ions with opposite charges: Cr (VI), in the form of CrO42−, and Cd (II), in the form of Cd2+, from contaminated water, after surface modifications. Two different modification strategies, surface grafting and hydrolysis, were conducted to issue functionality selectively to one of the nanofibrous components (PAN), which were confirmed to be successful by ATR, elemental analysis, and TGA measurements. The composite membranes with integrated nanofibrous network structures exhibited excellent mechanical properties, revealed by surface and crosssectional morphologies of the membranes. It is worth to note that the mechanical properties of the composite membrane, after surface crosslinking and grafting, were comparable with that of some PET nonwoven mats evidenced by tensile experiments and dimensional change determination. The application of the PAN modified composite membranes in microfiltration processes with both filtration and metal ion adsorption capability was demonstrated. The rejection ratios of the composite membranes were higher than 99.3% against 0.5-μm-particles and the high selectivity against 0.3-μm and 0.5-μm microspheres was observed from hydrolyzed PAN-PVA membrane. Moreover, the adsorption capacity of amino-groups modified PAN-PVA nanofibrous composite membrane was 66.5 mg/(g membrane) against Cr (VI) ions, which was 3-times higher than that of single PAN nanofibrous membrane with a PET substrate. The negatively charged hydrolyzed PANPVA membrane exhibited the adsorption capacity of 33.6 mg/(g membrane) against Cd (II) ions, which also demonstrated good adsorption efficiency. The modified PAN-PVA composite membranes could be recycled and reused for many times, as evidenced by the regeneration efficiency of above 90% after three recycle operations. Acknowledgements This work was supported by the National Natural Science Foundation of China (51673011), the State Key Laboratory of OrganicInorganic Composites at Beijing University of Chemical Technology (oic-201503004), the Fundamental Research Funds for the Central Universities (buctrc201501), and the National Science Foundation (DMR-1808690). Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.seppur.2019.115976. References [1] V. Thavasi, G. Singh, S. Ramakrishna, Electrospun nanofibers in energy and environmental applications, Energy Environ. Sci. 1 (2008) 205–221.

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