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Porous PVdF/GO Nanofibrous Membranes for Selective Separation and Recycling of Charged Organic Dyes from Water Abdul Ghaffar, Lina Zhang, Xiaoying Zhu, and Baoliang Chen Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b06081 • Publication Date (Web): 28 Feb 2018 Downloaded from http://pubs.acs.org on February 28, 2018

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Environmental Science & Technology

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Porous PVdF/GO Nanofibrous Membranes for Selective Separation

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and Recycling of Charged Organic Dyes from Water

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Abdul Ghaffar 1, 2, Lina Zhang 1,2, Xiaoying Zhu 1, 2 and Baoliang Chen* 1, 2

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1. Department of Environmental Science, Zhejiang University, Hangzhou, Zhejiang 310058,

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China.

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2. Zhejiang Provincial Key Laboratory of Organic Pollution Process and Control, Hangzhou

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310058, China.

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*Corresponding authors email:

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Dr. Chen Baoliang

[email protected]

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Tel:

+86 – 571 – 8898 – 2587

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Fax:

+86 – 571 – 8898 – 2587

13

16 17 18

Coauthor emails

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Abdul Ghaffar:

[email protected]

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Lina Zhang:

[email protected]

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Dr. Xiaoying Zhu:

[email protected]

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1 ACS Paragon Plus Environment


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Abstract

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Graphene oxide (GO) membranes are robust and continue to attract great attention due to their

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fascinating properties, despite their potential issues regarding stability and selectivity in aqueous-

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phase processing. That being said, however, the functional moieties of GO could be used for

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membrane surface modification, while ensuring simultaneous removal and recycling of industrial

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organic dyes. Herein, we present a versatile porous structured polyvinylidene fluoride-graphene

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oxide (PVdF-GO) nanofibrous membranes (NFMs), prepared by using simple and

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straightforward electrospinning approach for selective separation and filtration. The GO

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nanosheets were distributed homogenously throughout the PVdF nanofiber, regulating the

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surface morphology and performance of PVdF-GO NFM. The PVdF-GO NFMs possesses high

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mechanical strength and surface free energy (SFE), consequently resulting high permeation and

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filtration efficiency as compared to PVdF NFM. The selectivity towards positively charged dyes

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(99%) based on electrostatic attraction, while maintaining rejection (100%) for negatively

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charged dye from mixed solutions highlight the role of GO in PVdF-GO NFM, owing to uniform

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pores and negatively charged surface. In addition, the actual efficiency of NFMs could be

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recovered easily up to three consecutive filtration cycles by regeneration, thereby assuring high

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stability. The high permeation, purification and filtration efficiency, good stability and recycling

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of PVdF-GO NFMs are promising for use in practical water purification and applications,

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particularly for selective filtration and recycling of dyes.

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Keywords: PVdF-GO nanofibrous membrane, selectivity, molecular filtration, dye pollutant,

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regeneration.

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1. Introduction

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GO as verified to be two-dimensional and one atom thick sp2 carbon lattice, has attracted

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great attention in recent years because of its superior properties such as extremely large surface

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area with chemical inertness and strong mechanical properties.1–3 Membranes based on GO has

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great potential for separation and purification with promising flux, antifouling and high

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selectivity for gases and liquids.4–9 Unfortunately, the negatively charged functional moieties on

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GO causes repulsion in aqueous solution, resulting disintegration and destabilization but also

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decrease the selectivity of GO membranes.2,

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achieve asymmetric structure of GO membranes, the transfer process, large volume of liquid, GO

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alignment, layer-by-layer (LbL) assembly and dip coating, following CVD or restacking process

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arguably limits the scalability of GO membranes with rapid productivity issues for industrial

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applications.5, 13–15 Nonetheless, in practice, the abundant oxygen containing functional groups

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on GO can be used for surface modification of membranes and charge based separation. In

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addition, organic pollutants were mainly adsorbed on basal plane of GO nanosheets, while water

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enters in GO modified membrane surface primarily around the oxidized edges of GO

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nanosheets.7 Consequently, it is more important to recycle organic pollutants rather than

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degradation in order to minimize their environmental impact. For example, environmental

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problems associated with dye baths are related to wide variety of different components, often in

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relatively high concentrations. In future, many of dye processing units will face the requirement

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of reuse and recycling of industrial wastewater, because traditionally used methods are

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insufficient for obtaining the required water quality. So far, simultaneous removal and recycling

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has been paid very little attention.

10–12

Despite all this considerable progress to



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The separation and filtration technology play crucial roles in industrial water treatment,

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biomedical engineering, food processing, organic pollutants recovery, textile processing and

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pharmaceutical purification.16–20 The conventional separation technology like distillation and

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evaporation require immense space and are energy intensive, which makes the industrial

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processing costly in terms of operation, energy and resources.18 To date, the membranes made up

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of flexible polymers such as polysulfone, polyamide and cellulose are being utilized on large

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scale because of simple and easy processing despite their limitations like swelling,

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decomposition beyond critical temperature and pH, poor tolerance to corrosive solvents, oxidants,

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or strong acidic/alkaline reagents under mild operative conditions.21–23 In addition, the routine

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polymeric membranes can barely filter the aqueous mixtures and later are hard to recycle with

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prolonged durability.13–24 Nonetheless, charge based selectivity and separation for similar

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organic molecules with identical properties demand less energy and space, while being

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productive, durable and are considered to be environmental friendly and cheaper.25 Therefore,

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facile fabrication of membranes for charge based filtration with adequate thermal or chemical

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stability and effective processing under different environments are challenging. Besides, the

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composite membranes may overcome the drawbacks of polymeric membranes and could be good

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candidates for charge based filtration and selectivity under critical environmental conditions.

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Electrospinning technology allows a promising and straightforward approach to fabricate

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consecutive and perpetual nonwoven NFMs containing organic and inorganic fibers or both with

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proportions ranging from micro to nanoscale.26–29 Although, the NFMs exhibit porous structure

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and high surface area under different compositions, which have been reported earlier for

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separation and purification. Currently, some NFMs such as silica-titanium dioxide, carbon-silica,

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cyclodextrin, polyethersulfone, biochar-polyvinylidene fluoride and hollow zinc oxide were used


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in separation, filtration and photocatalysis.24, 30–35 However, despite all this considerable research

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and progress regarding NFMs, the existing NFMs are mostly tenuous resulting easy breakage,

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which may put possible difficulties during practical applications.13, 24, 30, 36, 37 Therefore, the facile

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fabrication of GO into PVdF NFM would be of both technological and scientific importance. To

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date, the NFMs containing PVdF-GO has not been utilized for charge based separation and

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filtration of complex mixtures, which would be of great interest for industrial applications.

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Herein, we report a porous structured PVdF-GO NFM fabricated by electrospinning

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technology. The premise of our strategy is that GO is sustainably incorporated into NFMs for

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desirable recycling of organic dyes rather than degradation from mixed organic dye solutions,

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based on electrostatic attraction/repulsion besides molecular size. More importantly, the knotty

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porous structure in between twisted fibers, GO embedded NFMs significantly gives rise to high

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permeation while simultaneously maintaining filtration efficiency even after three filtration

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cycles. The selective separation and recycling based on NFMs may provide a new approach to

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manufacture molecular filters for selectivity, filtration efficiency and high solvent permeate flux

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toward mixed organic molecules within fields of practical molecular purification and separation

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science.

107 108

2. Materials and Methods

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2.1. Materials.

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Natural graphite flake (325 mesh, 99.8%) was purchased from Alfa Aesar. PVdF (Mw ~

111

180000 by GPC) was obtained from Sigma-Aldrich. Analytical grade dimethylacetamide

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(DMAc >99%), formamide, diiodomethane, ethylene glycol, glycerol, methylene blue (MB),

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neutral red (NR), basic fuchsin (BF), rhodamine B (RB) and methyl orange (MO) were



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purchased from Aladdin chemicals reagent company (Shanghai, China). HPLC grade acetone

115

(>99.5%) was obtained from Tedia, U.S.A. Ultrapure water was produced by CascadaTM IX-

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water purification system, PALL Corporation. All chemicals were used as obtained without

117

further refinement.

118

2.2. Synthesis of GO and Preparation of PVdF-GO NFMs.

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GO was synthesized from natural Graphite flake (325 mesh, 99.8%, Alfa Aesar) by modified

120

Hummers method.38, 39 The detailed process regarding GO synthesis can be found in supporting

121

information (SI).1,

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straightforward electrospinning technique with subsequent PVdF and GO mixtures. The PVdF-

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GO precursor solution was obtained by adding PVdF and GO into DMAc and acetone under

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sonication for 1h and stirring at 70°C for 12h. The composition of electrospinning solution was

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20% PVdF, 64% DMAc and 16% acetone for PVdF NFM, 17.5% PVdF, 2.5% GO, 64% DMAc

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and 16% acetone for PVdF-GO 2.5% NFM, and 15% PVdF, 5% GO, 64% DMAc and 16%

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acetone for PVdF-GO 5.0% NFMs, respectively. The resultant homogeneous solutions were then

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filled into 10mL syringe with 18 gauge needle. The syringe was positioned horizontally for 30

129

min and air was removed completely. The solutions were fed at controllable propulsion velocity

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of 0.5mL h−1 with an automatic pump. The electrospinning was performed by using QZNT–E01

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spinning equipment with a voltage supply of 15kV at 8cm distance. The entire spinning process

132

was performed under ambient operating temperature and relative humidity of 25°C and 40%,

133

respectively. The NFMs were then thermally treated in between glass plates at 120°C for 8h in

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dry-oven to assure pore diameter and preserve physical property. Finally, the NFMs were rinsed

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with methanol and ultrapure water to remove any remaining impurities completely.

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2.3. Characterization of NFMs

39

The PVdF and PVdF-GO NFMs were prepared by combining the


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The surface morphologies of NFMs were examined by field emission scanning electron

138

microscopy (FE-SEM, S-4800, Hitachi, Tokyo). Transmission electron microscopy (JEM-1230,

139

JEOL Ltd., Japan) and energy dispersive X-ray spectroscopy (EDX) were used to get insight into

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the microstructure and composition of NFMs. Raman spectra were collected at room temperature

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with LabRamHRUV Raman spectrometer (JDbin-yvon, FR); equipped with thermoelectrically

142

cooled charge-coupled device (CCD), with an excitation source by Ar+ laser at 514 nm

143

wavelength. The crystal phase of PVdF and PVdF-GO NFMs were analyzed by X-ray diffraction

144

(XRD, Rigaku D/MAX/PC 2550, Japan) equipped with Cu Kα radiation source. The mechanical

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properties in terms of tensile stress and strain were obtained by Universal material experiment

146

machine (Zwick/Reoll Z020). The surface area (SA) was measured via Brunauer, Emmett and

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Teller (BET) method by N2 (0.162 nm2) gas sorption at 77 K, on NOVA-2000E surface area

148

analyzer (Quantachrome Instruments). The nonlocal density functional theory (NLDFT) model

149

was applied to analyze pore size distribution (PSD). An electro-kinetic analyzer Anton Paar

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SurPASS (GmbH, Austria) was used to evaluate streaming potentials of NFMs. The

151

measurements were carried out using 200mL of 0.1 mM KCl solution as electrolyte and pH was

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adjusted by 0.1M HCl or 0.1M NaOH. The water contact angles (CA) of NFMs were obtained

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by OSA200–T Optical Contact Angle analyzer following air drop (~10 µL) method with a

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micro-syringe to the targeted NFM surface. Attenuated total reflectance fourier transformed

155

infrared (ATR-FTIR) spectra were obtained using a Thermo-Fisher Scientific; with 64 scans in

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range of 4000-400 cm-1 at resolution of 4 cm-1 for each NFM.

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2.4. Adsorption Kinetics and Filtration

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Five different organic dyes namely MB, NR, BF, RB and MO (purity > 98%) were selected

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as model representative pollutants to get insight into adsorption kinetics and dynamic molecular



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filtration. These five compounds were selected based on their variation in electrostatic charge

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and molecular size and detailed physicochemical properties are listed in Table S1. Adsorption

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kinetics of dyes were obtained using a batch equilibration experiment at an initial concentration

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of 5mg L-1. Briefly, the sorbates were dissolved in water and 5mg NFM samples were immersed

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in five kind of solutions to probe higher affinity and adsorption capacity toward organic dyes.

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Furthermore, the molecular filtration experiments were carried out by using Millipore filtration

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device equipped with N2 cylinder for pressure, a digital balance and computer. Prior to filtration

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process, the NFM samples were initially compacted for 15min at 1 bar to get steady flux. Four

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mixtures of organic dyes with opposite electrostatic charge (positive/negative), i.e., MB/MO,

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NR/MO, BF/MO and RB/MO were prepared, each dye with 40mg L−1 concentration (1:1

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volume), respectively. Three mixtures with same electrostatic charge (positive/positive), i.e.,

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MB/NR, MB/BF and MB/RB were also prepared to further evaluate the molecular filtration

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efficiency among positively charged molecules. Following, the mixed dye solutions were forced

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to flow through NFMs, at an average pressure of 1bar by N2 gas. The permeate concentration

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was measured by UV–Vis spectrophotometer (UV-2550, Shimadzu), at selected wavelengths

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(Table S1). The separation efficiency (η) of filtration process was calculated by the following

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equation:40

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𝜂% =

[𝑂𝑀]𝐹 × 100 [𝑀𝑂]𝐹 + [𝑂𝑀]𝐹

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where [OM] represents the organic molecules including MB, NR, BF, RB, MO and [MO] F and

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[OM]F are the concentrations of MO and the other four organic dyes in filtrate, respectively.

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After molecular filtration process of mixed dyes, the NFMs were washed thoroughly by mixed

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solution of hydrochloric acid and ethanol (1:1 by volume) as eluent. Subsequently, the washed

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NFMs were dried at 80°C to remove residual solvent, thus NFMs can be easily regenerated. The 8 ACS Paragon Plus Environment

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regenerated NFMs were recycled three times and used again for filtration up to three consecutive

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runs to examine the regeneration and reusability of as-prepared PVdF-GO 5.0% NFM.

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2.5. Data Analysis

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The fractal dimension, SFE, work of adhesion between solid-liquid surfaces was examined

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for characterization of NFMs. Pseudo-first-order and pseudo-second-order kinetic models were

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used to analyze the adsorption kinetics and capacity of NFMs. More details about data analysis

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equations for fractal dimension, SFE, work of adhesion, kinetic models, are presented in the SI. Table 1. The structural characteristics and properties of NFMs. Parameter

PVdF

PVdF-GO 2.5%

PVdF-GO 5.0%

60.71/39.14/0.15

67.44/30.81/1.75

69.02/28.76/2.22

127.56 ±7.23

149.12 ±12.45

174.68 ±9.87

ID/IG c

N/A

0.98

0.96

Tensile strength (MPa)

4.39 ±1.3

7.43 ±0.84

11.7 ±2.25

Tensile strain (%)

30.90 ±13.31

33.10 ±3.85

40.10 ±10.96

12.43

13.09

51.47

0.030

0.046

0.077

2.434

2.435

2.377

3.212

3.493

3.902

116.60 ±4.1

72.80 ±4.52

52.10 ±2.21

Surface free energy (mJ m )

51.50

57.36

88.50

Pure water flux (LMH)

193.02 ±6.95

439.00 ±10.23

663.54 ±54

Single fiber composition % (C/F/O) Mean fiber dia

a

b

2

-1 d

Surface area (m g )

Pore volume (cc g-1) e Fractal dimension (D)

f

Isoelectric point (pI) Water contact angle (°) -2 g

a

EDX, b Based on 10 measurements from SEM images, c Raman spectra, d BET method, e NLDFT, f from

N2 isotherms, g Neumann’s method.

190 191

3. Results and Discussions

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3.1. Morphologies and Structure of NFMs

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The facile fabrication of NFMs for effective adsorption of different organic molecules, while

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being selective requires intellectual functional design of filtration membrane, based on controlled 9 ACS Paragon Plus Environment


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relation between structural geometry and pore selectivity. We constructed NFMs for multilevel

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functionality based on the following criteria: (a) Effective pore geometry thereby assuring

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maximum adsorption for selected organic dyes while minimum sacrifice for pure water

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permeation, (b) porous structure with controlled thickness to facilitate the practical application,

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and (c) gradient selectivity for complex aqueous mixtures. The required criteria for NFMs were

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fulfilled by scalable and versatile strategy of high speed continuous stirring followed by

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advanced electrospinning approach. The homogenous solution and controlled ratio of GO can

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ensure an effective performance of NFMs. Thus, uniform synthesis and tunable geometry of

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NFMs can be alleviated.

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The surface morphology of NFMs was examined thoroughly by SEM and representative

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images are shown in Figures 1a, 1b and 1c. The morphologies of NFMs were changed gradually

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with GO and became brownish in color with close distribution of GO throughout NFMs. Notably,

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the NFMs exhibited typically nonwoven structure with random and uniform arrangement of

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consistent fibers. The NFMs exhibited obvious porous geometry with three dimensional pores

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interconnected with each other through triangular junctions. The corresponding skeletonized

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images reveal the triangular junctions with numerous nano, meso and micropores (Fig. 1d, 1e

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and 1f). The significant difference between PVdF NFM and PVdF-GO NFMs is the unique

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surface of fibers that consist of GO nanosheets (~1µm). The NFMs with uniform interconnected

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pores endows the stable structure benefiting for high permeation.23 The morphology of pristine

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fibers was observed to be smoother without any apparent difference, while maintaining the

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porous geometry. The mean diameter of PVdF fiber was 127.56nm and gradually increased with

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GO (149.12 and 174.68nm, Table 1). This phenomenon could be ascribed to viscosity and


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electrical conductivity of spinning solution, resulting in irregular whipping and larger stretching

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of jet during electrospinning process.41

Figure 1. SEM images of electrospun NFMs (a) PVdF, (b) PVdF-GO 2.5%, and (c) PVdF-GO 5.0%. The corresponding skeletonized transformation of NFMs (d) PVdF, (e) PVdF-GO 2.5%, and (f) PVdF-GO 5.0%, respectively. 219 220

The representative TEM images provide spontaneous insight into microstructure of single

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PVdF and PVdF-GO fibers (Fig. 2a, 2b and 2c). Notably, the NFMs were composed of smooth

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fibers without any significant disordered geometry, which is also consistent with SEM images

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(Fig. 1a, 1b and 1c). As observed in Figures 2b, and 2c, GO nanosheets were dispersed

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uniformly, both internally and externally within fibers, endowing the PVdF-GO matrix with

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classical rough structure. The GO nanosheets were attached on fiber surface homogeneously at

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nanometer level, potentially as active sites. In order to confirm the existence of GO nanosheets

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on nanofiber surface, the EDX was performed on single fiber (Fig. 2d, 2e and 2f). As can be seen

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roughly from single fiber composition (Table 1), the molar ratio of oxygen increased with GO

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ratio. The EDX images verify the successful incorporation of GO nanosheets on fiber surface. 11 ACS Paragon Plus Environment


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(d)

(e)

(f)

Figure 2. TEM images of NFMs (a) PVdF, (b) PVdF-GO 2.5%, and (c) PVdF-GO 5.0%. Elemental mapping images of C, F and O of selected area (arrow) from single fiber at 25nm scale (d) PVdF, (e) PVdF-GO 2.5% and (f) PVdF-GO 5.0%. The symbols ,

and

represent

C, F and O elements, respectively. 231 232

The surface chemistry of NFMs was further examined by Raman spectroscopy and XRD

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patterns. In Raman spectra, two prominent peaks D-band (1349cm−1) and G-band (1582cm−1),

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were observed for GO (Fig. 3a). The D-band intensity originated from sp3 hybridization and G-

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band was induced by crystalline graphitic/sp2 carbon atoms, respectively. The Raman bands

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observed at 795cm-1 and 839cm-1 in PVdF NFM corresponds to α and β phases, which are often

237

used for phase identification in PVdF. The PVdF-GO 2.5% and PVdF-GO 5.0% NFMs clearly

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showed the presence of D-band and G-band (Fig. 3a), confirming the presence of adjacent GO

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nanosheets, thereby preserving the π-stacking on fiber surface. The intensity of D peak was

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comparable to G peak and ratio of D-band to G-band (ID/IG) provides a sensitive measure of

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disorder and crystallite size of graphitic layers. The D-band intensity was lower than G-band, 12 ACS Paragon Plus Environment

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indicating the structural defects and disorder, because of oxidation and reduction process. The

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intensity ratio of D-band to G-band (ID/IG) of GO (1.01) was larger than PVdF-GO NFMs (0.98

244

and 0.96, Table 1), suggesting the existence of deeper sp2 domains. A clear transition of PVdF

245

with GO could also be observed by SEM images of NFMs (Fig. 1b and 1c). In addition, XRD

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patterns of as-prepared PVdF and PVdF-GO NFMs are shown in Figure 3b. The XRD pattern of

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GO reveals an intense and sharp peak at 9.74°, related to the diffraction peak of GO. The XRD

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pattern of PVdF NFM consists predominantly α-phase (nonpolar) with characteristic (020), (110)

249

and (021) peaks at 2θ = 18.4°, 19.9° and 26.6°, respectively. The PVdF-GO NFMs show a broad

250

peak around 2θ = 9.76°, indicative of polar β-phase (200) and (110) peaks and progressively

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increased with GO content. However, the XRD peaks in PVdF-GO NFMs were broad with

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presence of semi crystalline and GO nanosheets implying α-, β- and γ-phases. The presence of

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GO in PVdF matrix does not change the structural crystal phase compared with PVdF, despite

254

different α-, β- and γ-phase compositions. As the maximum concentration of GO is 5wt% in

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NFMs, their corresponding peaks in XRD patterns were weaken, but progressively lead to peak

256

formation. Thus, the Raman and XRD analysis direct confirmation of graphitic structure,

257

anchored on PVdF fiber surfaces. The substantial arrangement of graphitic structure is promising

258

for efficient water permeation and better filtration performance.23



Figure 3. (a) Raman spectra of GO and PVdF-GO NFMs. (b) XRD patterns of GO, PVdF and PVdF-GO NFMs. (c) Tensile strength of PVdF and PVdF-GO NFMs. 259 260

3.2. Mechanical Properties of NFMs

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The mechanical properties of PVdF-GO NFMs in terms of tensile stress and strain were

262

measured to probe the ability to withstand the transmembrane pressure during filtration. The

263

corresponding tensile stress and strain were listed in Table 1 and Figure 3c, respectively. The

264

tensile stress of PVdF NFM was 4.39 MPa, with an elongation of 30.9%, both being the lowest

265

among prepared NFMs. The NFMs with GO i.e., PVdF-GO 2.5% and PVdF-GO 5.0% had

266

higher tensile strength (7.43 and 11.7MPa), which could be attributed to contribution of GO

267

within polymer matrix, thereby decreasing the slippage of fibers during testing process, in

268

comparison to PVdF-NFM. Besides the slippage, GO may cause higher load transfer efficiencies

269

at interface, thus possibly result in higher tensile stress. In addition, tensile strain of PVdF-GO

270

2.5% and PVdF-GO 5.0% NFMs at break (33.10 and 40.10%) was also higher than PVdF NFM

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(30.90%) (Fig. S1). The tensile stress and strain of NFMs increased with GO ratio, owing to the

272

transformation of fibrous structure. Nonetheless, the NFMs exhibited enhanced rigidity along

273

with stronger ductile properties following GO addition.


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3.3. Quantitative Analysis of Porous Structure

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The effective BET-SA and PSD of NFMs were systematically analyzed by N2 gas

276

adsorption-desorption, to probe molecular affinity and selectivity. The corresponding N2 gas

277

adsorption-desorption isotherms were typically type I (Fig. 4a), showing rapid condensation and

278

monolayer adsorption of N2 molecules inside porous NFMs. Generally, N2 adsorption was

279

increased following relative pressure (P/P0) approaching < 0.9, and central region of P/P0

280

indicated the existence of micropores in NFMs. The measured SA and PV of PVdF NFM was

281

12.43m2 g-1 and 0.03cc g-1, with maximum adsorption as high as 30.16cc g-1 STP while pressure

282

approaching saturation (P/P0 = 0.99), respectively (Table 1). The PVdF-GO 2.5% NFM had

283

almost similar SA as that of PVdF NFM, i.e., 13.09m2 g-1 with 0.046cc g-1 PV (Table 1). In

284

addition, PVdF-GO 5.0% NFM exhibited higher SA (51.47m2 g-1) and PV (0.077cc g-1) with

285

gradual increase in GO content with maximum adsorption for N2 as 62.94cc g-1 STP (Fig. 4a and

286

Table 1).

287

The geometric complexity and hierarchical textures of NFMs could be further analyzed by

288

fractal dimension (D) analysis following Frenkel-Halsey-Hill (FHH) theory based on N2

289

adsorption data. The plots of ln(V/Vmono) against ln(ln(P/P0)) were reconstructed from N2

290

isotherms for each NFM (Fig. 4b). The corresponding FHH plots had the same coverage region

291

for PVdF and PVdF-GO 2.5% NFMs, while a higher coverage region for PVdF-GO 5.0% was

292

observed comparatively. The calculated D was in order of PVdF (2.434), PVdF-GO 2.5% (2.435)

293

and PVdF-GO 5.0% (2.377), respectively, which further confirms the structural difference of

294

NFMs mainly composed of meso and micropores. The distinct PSD for PVdF-GO 5.0% NFM is

295

in good agreement with its higher SA and PV as compared to PVdF and PVdF-GO 2.5% NFM

296

(Fig. 4c and Table 1). Notably, most of the mesopores were observed within the range of



297

1−10nm, with narrow size distribution and PVdF-GO 5.0% had much different mesopores.

298

Hence, BET SA and PSD reveals the significance of GO, thereby increasing the N2 gas

299

penetration, with 3D hierarchical architectures providing active sites, and may serve as transport

300

path to accelerate mass diffusion.

Page 16 of 34

Figure 4. (a) N2 adsorption–desorption isotherms of NFMs. (b) Plots of ln(V/Vmono) against ln(ln(P/P0)) reconstructed from N2 isotherms. (c) NLDFT PSD curves of relevant NFMs. 301 302

3.4. Surface Composition and Properties of NFMs

303

The surface charge properties of NFMs were examined from streaming potential and current

304

measurements over a pH range of 3 to 11 (Fig. 5a). The PVdF NFM was found to be less

305

negative in comparison to PVdF-GO NFMs. The GO functionalized PVdF NFMs possessed

306

significantly different surface charge characteristics, and became more negative owing to the

307

increase in density of oxygen containing functional moieties. The overall zeta (ζ) potential values

308

progressively became less negative due to the deprotonation of surface functional groups (Fig.

309

5a). The ζ potential behavior is in accordance with surface functional groups present in both GO

310

and PVdF. The pI of PVdF NFM was approximately 3.01 as resulted from the adsorption of

311

hydroxide ions originating from the self-ionization of water (Table 1). The pI for PVdF-GO 2.5% 16 ACS Paragon Plus Environment

Page 17 of 34


312

and PVdF-GO 5.0% NFMs were found to be 3.63 and 3.68, following GO ratio (Table 1). The

313

surface charge of NFMs in presence of GO significantly changed, thereby indicating the

314

dominance of GO on NFM surface. Furthermore, the ζ potential values indirectly confirms the

315

presence of GO on NFM surface, thereby regulating the surface properties.

316

Figure 5. (a) Plots of ζ potential versus pH values of NFMs. (b) Plots of contact angle for different solvents. (c) ATR-FTIR spectra of GO and NFMs. 317 318

The surface wettability of NFMs were evaluated by SFE based on five probe liquids namely

319

water, formamide, diiodomethane, ethylene glycol and glycerol (Fig. 5b). The surface wettability

320

based on CA hysteresis for PVdF-GO NFMs were expected to vary as compared to PVdF NFM

321

due to the presence of hydrophilic functional groups in GO. The PVdF NFM exhibited

322

hydrophobic nature (CA > 90°) and had the highest average water CA of 116.5°(Table 1). PVdF

323

does not have significant polar groups; therefore it tends to be hydrophobic. The average water

324

CA of NFMs was decreased in presence of GO (72.5°and 52°, Table 1) and thus considered to

325

be hydrophilic. This trend is consistent with the presence of surface functional moieties such as –

326

OH and –CH on GO, as observed in ATR-FTIR spectra (Fig. 5c). Hence, NFMs were shifted to



327

hydrophilic surface following GO addition. In order to further observe the dynamic wettability of

328

NFMs, organic solvents with different polar group were used. It is noteworthy; that PVdF-NFM

329

was also non-wettable to glycerol, because CA was found to be higher than 90°(Table S2). In

330

addition, the NFMs with GO were wettable to all tested polar solvents (Table S2), suggesting

331

some polar groups were present on NFMs surface. The NFMs were less wettable to glycerol in

332

comparison to formamide, diiodomethane and ethylene glycol (Table S2). The surface wetting

333

behavior was further examined by solid-liquid interaction force depending on both polar and

334

dispersive parts. The PVdF NFM had the lowest SFE 51.50 mJ m-2, and SFE was increased with

335

GO following ratio up to 57.36 and 88.50mJ m-2, for PVdF-GO 2.5% and PVdF-GO 5.0% NFMs

336

(Table 1). The SFE of NFMs is consistent with work of adhesion (Table S3) for solid-liquid

337

interface. The higher SFE is attributed to increased surface polarity, thereby offering more polar

338

interactions for NFMs. The pure water flux of PVdF NFM was 193±6.95 LMH, which is

339

according to its hydrophobic nature (Table 1). The pure water flux of NFMs containing GO was

340

increased, i.e., 439±10.23 and 663±54 LMH for PVdF-GO 2.5% and PVdF-GO 5.0%. The

341

higher flux could be explained by the hydrophilic nature of PVdF-GO NFMs.

342

The changes in surface composition of NFMs were further analyzed by ATR-FTIR spectra

343

(Fig. 5c). For GO, several bands at 3371cm−1 for aliphatic –OH stretching, at 2850cm-1 for –CH

344

stretching, at 1719cm-1 and 1044cm-1 for C=C, C=O stretching were observed. The

345

representative peak in NFMs at 1400 cm-1 corresponds to –CH2 scissoring, wagging and

346

vibrating of vinyldine. The observed peak intensities at 1275, 1181, 1070, 883 and 840cm-1

347

represents –CF2 stretching, symmetrical stretching of –CF, bending of –CH bond, –CH2 wagging

348

of vinyldine and –CH2 rocking. The peak at 1070cm-1 became progressive due to crosslinking of

349

carboxylic acid functional group (–COOH) and asymmetric stretch of carboxylate (–COO–)


Page 18 of 34

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350

originated from GO. In addition, the band at 3300cm-1 was assigned to aliphatic –OH stretching,

351

corroboratively from GO.

352 353

3.5. Adsorption Kinetics of NFMs

354

The as-prepared NFMs had porous structure, large SA, enhanced mechanical strength with

355

prosperous flexibility as discussed above, thus could ensure promising results during molecular

356

filtration and selectivity process. Pseudo-first order and Pseudo-second order kinetic models

357

were used to investigate molecular adsorption and mass transfer on NFMs for selected organic

358

molecules. The NFMs were negatively charged and became more negative with GO (Fig. 5a),

359

because of oxygen containing functional moieties on surface of fiber along with pore orifice.

360

Therefore, we choose positively charged (MB, NR, BF and RB) and negatively charged (MO) as

361

representative organic dyes for selective adsorption and filtration experiments. The properties

362

and 3D molecular structure of organic dyes were presented in Table S1 and Figure S2,

363

respectively. The corresponding parameters and related fitting of kinetic models are listed in

364

Table S4 and Figure S3. The pseudo-second order model had a better fit to kinetics with higher

365

adsorption capacity as suggested by their correlation coefficients (r2) than pseudo-first order

366

model. The adsorption rate for positively charged molecules (MB, NR, BF and RB) increased

367

rapidly during first 24h and reached adsorption equilibrium in approximately 24h (Fig. S3a, S3b

368

and S3c), indicating physisorption process on heterogeneous fiber surface. In addition, GO

369

nanosheets provided additional active sites and contributed to adsorption uptake due to

370

compositional and structural difference in PVdF and PVdF-GO NFMs. These results are clearly

371

attributed to electrostatic attraction in between positively charged molecules and fiber matrix due

372

to improved ζ potential. The experimental adsorption capacity (Qe) followed the order of MB



373

(96.48) > NR (43.74) > BF (31.04) > RB (30.39), with maximum capacity for PVdF-GO 5.0%

374

NFM (Table S4), respectively. In contrast, the NFMs had negligible adsorption capacity for

375

negatively charged MO molecule i.e., 1.18, 1.33 and 1.56 mg g-1, respectively, for PVdF, PVdF-

376

GO 2.5% and PVdF-GO 5.0% (Table S4 and Fig. S3d), because of electrostatic repulsion.

377

Therefore, it is believed that the electrostatic interaction was the dominant force in between

378

positive charged organic molecules and negatively charged NFMs, since NFMs exhibited

379

negative as observed from ζ potentials.

380

To further evaluate the difference in adsorption capacities (Fig. 6a), three-dimensional (3D)

381

molecular volume was considered, which was in order of MB < NR < BF < RB.24, 42 The smaller

382

molecular volume may account in pore diffusion and will quickly adjust in nanopores to occupy

383

the adsorption sites, whereas, larger molecular volume would be sterically hindered for quick

384

adsorption, resulting in blockage of nanopores orifice, thus decreasing the adsorption uptake.1

385

Additionally, MB and NR have similar 3D molecular size with a difference in charged group,

386

which is smaller in NR, thus could result in decreased electrostatic attraction in between NFMs

387

and organic molecules.43 The negatively charged dye (MO), being the smallest one, could ingress

388

into the NFMs nanopores, thus resulted in smallest uptake.

389 390

3.6.Molecular Filtration Performance

391

The separation of organic molecules during dynamic purification process needs to be reliable

392

and predictable for practical applications featuring applied chemical purification based on

393

selectivity and molecular filtration. To check the practical performance and molecular selectivity

394

of PVdF-GO NFMs, we selected five different organic dyes with different electrostatic charge

395

and size to evaluate separation efficiency of NFMs as prototypical application.


Page 20 of 34

Page 21 of 34


396

Filtrate Filtrate

Filtrate

Filtrate

Filtrate

Filtrate

Filtrate

Filtrate

Filtrate

Figure 6. (a) Adsorption capacity of PVdF and PVdF-GO NFMs for five representative watersoluble dyes. The UV−Vis spectra of (b) MB (positive) and MO (negative) filtrate, oppositely charged dye mixtures (c) MB/MO, (d) NR/MO, (e) BF/MO (f) RB/MO, same charge dye mixtures (g) MB/NR, (h) MB/BF and (i) MB/RB; before and after filtration process. The inset photographs show the colors of aqueous solutions before and after (arrow) filtration process, respectively. 397 398

First, molecular filtration was executed with simple aqueous dye solutions at an average

399

pressure of 1bar. The colored feed solutions of positively charged organic dyes (MB, NR, BF

400

and RB) were turned into pure water color effectively, when passed through PVdF-GO 2.5% and

401

PVdF-GO 5.0% NFMs (Fig. 6b and S4). The PVdF-GO successfully adsorbed the positively

402

charged organic dyes efficiently and resulted permeate had 100% purity. In contrast, there was 21 ACS Paragon Plus Environment


403

negligible adsorption for negatively charged MO molecules with 100% permeation (Fig. 6b).

404

The UV-Vs spectra for all organic dyes are also consistent with efficient performance of NFMs;

405

as there was no peaks observed for positively charged dyes (Fig. 6b and S4); while same peak for

406

negatively charged feed solution (Fig. 6b) after permeation. In comparison to PVdF-GO NFMs,

407

PVdF NFM had obvious peaks for positively charged organic molecules, but concentration of

408

each dye came down. It means that PVdF NFM was not as efficient as PVdF-GO NFMs, and

409

thus permeated some of the positively charged organic molecules once saturated, but passed

410

negatively charged MO molecules thoroughly.

411

Second, the molecular filtration performance of as-prepared NFMs was further examined

412

with mixtures besides pure aqueous solution of various organic dyes. For mixed molecular

413

filtration, four mixtures based on opposite charge i.e., MB/MO, NR/MO, BF/MO and RB/MO

414

were filtered through each NFMs. The UV-Vis spectra of organic mixtures, before and after

415

filtration process are shown in Figures 6c, 6d, 6e and 6f. The colored feed solutions turned into

416

the original color of MO molecules after permeation and the color of NFMs changed to the color

417

of dye molecules being penetrated. This indicates that NFMs adsorbed positively charged

418

molecules quickly and allowed only MO molecules to pass through, highlighting the selective

419

affinity and separation from a mixture only for positively charged molecules. The average

420

concentration of MO after filtration through NFMs was approximately 98.37%, comparatively to

421

initial MO feed solution with simultaneous adsorption of positively charged molecules. The

422

concentration of positively charged molecules decreased absolutely from 40 mg L-1 to 0.16 (MB)

423

< 0.31 (NR) < 0.37 (BF) < 0.48 (RB) mg L-1, after permeation through PVdF-GO 5.0% NFMs,

424

because of prominent adsorption capacities during filtration process. The average filtration

425

efficiency for four mixed solutions (MB/MO, NR/MO, BF/MO and RB/MO) through PVdF-GO


Page 22 of 34

Page 23 of 34


426

5.0% NFM was 99.20%, 98.47%, 98.17% and 97.63%, respectively. The PVdF-GO 2.5% and

427

PVdF NFMs also captured positively charged molecules and residual concentration in permeate

428

was 3.44 (MB) < 3.47 (NR) < 3.61 (BF) < 3.72 (RB) mg L-1 and 5.52 (MB) < 5.72 (NR) < 5.94

429

(BF) < 6.21 (RB) mg L-1, respectively. The filtrates of PVdF-GO 5.0% and PVdF-GO 2.5%

430

NFMs had no sign of absorption peaks for MB, NR, BF and RB molecules but only the

431

representative peak of MO molecule in UV-Vis spectra for MB/MO, NR/MO, BF/MO and

432

RB/MO mixtures, whereas obvious peaks when passed through PVdF NFMs for both organic

433

molecules (Fig. 6c, 6d, 6e and 6f). The filtration efficiency of PVdF-GO 2.5% NFMs for mixed

434

solutions was 85.30%, 85.19%, 84.66% and 84.29% respectively and PVdF NFMs had the lower

435

filtration efficiency as 78.28%, 77.67% 76.96% and 76.15%, respectively, which suggests that

436

NFMs reached saturation capacity and no further adsorption was possible. Therefore, it is

437

reasonable to speculate that GO was actively involved to sorb positively charged molecules

438

being at PVdF fiber surface and thus exhibited better performance during filtration process, in

439

comparison to PVdF NFM. Filtration curves of dye solutions and their mixtures through NFMs

440

at 1 bar pressure are displayed in Fig. S5, respectively. The PVdF-GO NFM had lower

441

permeability for dye solutions and their mixtures (Fig. S5a) than that of PVdF-GO 2.5% and

442

PVdF-GO 5.0% NFMs (Fig. S5b and Fig. S5c), reasonably due to hydrophobic nature.

443

Finally, the molecular filtration was also performed by mixed organic molecules with same

444

electrostatic charge i.e., NR/MB, BF/MB and RB/MB. The NFMs exhibited almost 100%

445

efficiency during filtration process of NR/MB, BF/MB and RB/MB mixtures. The concentration

446

of MB was found to be slightly lower than NR, BF and RB for each NFMs filtrate following

447

PVdF-GO 5.0% < PVdF-GO 2.5% < PVdF NFMs, confirming the higher affinity of NFMs

448

towards MB molecules. The residual concentration in filtrate was in order of MB < NR < BF
NR > BF > RB, following GO addition, respectively.

452

The corresponding UV-Vis absorption peaks of these three mixtures are shown in Figures 6g, 6h

453

and 6i, and the concentration of both positively charged organic dyes decreased. The filtration

454

processes of selected organic dyes by PVdF-GO NFMs indicate the acceptable filtration

455

efficiency and thus are of great potential for selective separation and purification for waste-water

456

treatment.

457

The proposed mechanism of filtration process for selected organic dyes is illustrated in

458

Figure 7. During molecular filtration, porous structure of NFMs, size exclusion and electrostatic

459

attraction-repulsion for charged organic dyes, simultaneously resulted for high efficient

460

performance.2,

461

linkers forming a continuous mask within PVdF network, thus acted as potential active sites for

462

interaction. The GO nanosheets also enabled pores affinity for charge selectivity within NFMs.

463

Presumably; the positively charged organic molecules were adsorbed quickly by electrostatic

464

forces during penetration within pores, whereas negatively charged organic molecules are

465

expectable to permeate through NFMs. In addition, the specific pore size of NFMs could

466

essentially stop or decelerate the permeation process of large size organic molecules gradually,

467

highlighting the size exclusion evident by hindered theory. Moreover, the stable pores of NFMs

468

consequently result efficient mass transport within inter-fiber network, empowering a powerful

469

permeation. Therefore, the open fibrous architecture and controlled PSD of NFMs regulated the

470

filtration performance and promoted high permeation.

13, 20

GO nanosheets were preserved well on surface of nanofibers like cross-


Page 24 of 34

Page 25 of 34


Figure 7. (a) Preparation of PVdF-GO NFMs. (b) Schematic illustration for distribution of GO nanosheets on PVdF nanofiber and proposed filtration process (before and after) towards the mixture of organic dyes by NFMs. 471 472

3.7. Regeneration of NFMs

473

The potential stability and reusability for molecular filtration process is essential for practical

474

application with long-term durability, whereas continuous recyclability and proper regeneration

475

remains challenging, thus are inevitable issues. The entire process of reusability and regeneration

476

can provide an insight into structural stability and long term reusability of as-prepared PVdF-GO

477

NFMs. The regeneration and recyclability performance of PVdF-GO 5.0% NFM was

478

systematically observed by ATR-FTIR and BET-N2 adsorption-desorption, before and after

479

filtration (Fig. S7a and S7b). ATR-FTIR spectra (Fig. S7a) of NFMs after permeation of MB and 25 ACS Paragon Plus Environment


480

NR at ~1400 and 1334 cm-1, were attributed to electrostatic attraction, and then completely

481

retrieved original spectra after regeneration by the eluent, confirming the total removal of MB

482

and NR molecules from the NFMs. Furthermore, the NFMs retains almost original filtration

483

efficiency after three consecutive regeneration cycles for further application, suggesting that the

484

NFM is effective and stable for further adsorption of MB or NR molecules from aqueous media.

485

In addition, the BET-N2 adsorption-desorption after regeneration cycles were almost the same as

486

original (Fig. S7b), suggesting the stable fibrous morphology and excellent reversibility of NFMs.

487

The inset of Fig. S7b show the permeate concentration of MB and NR from mixed solutions up

488

to three regeneration cycles, and the observed separation efficiency was almost the same as

489

original, highlighting the effective regeneration, stability and filtration performance. Table S5

490

lists the comparison of graphene-incorporated membranes for flux and dyes removal

491

performance from water. The separation efficiency with high permeation, simultaneous recycling,

492

while avoiding common problems associated with GO and common polymeric membranes are

493

important for industrial processing. The results achieved are desirable for large scale practical

494

applications, in industrial waste-water treatment plants. The stable performance throughout the

495

system indicates the effective role of GO nanosheets for environmental pollutant management,

496

thus have potential to be used for field specific applications.

497

GO can be used effectively for modification of membrane surface and structural geometry as

498

portrayed by electrospun PVdF-GO NFMs, thereby utilization the properties of GO, but

499

overwhelming the problems caused by functional moieties in aqueous-phase. The GO provided

500

prolonged durability and reusability for continuous regeneration cycles, which were hindered by

501

routine polymeric membranes for aqueous mixtures and are hard to recycle. Considering the

502

excellent chemical stability of GO, the reported membrane can be used for filtration of complex


Page 26 of 34

Page 27 of 34


503

organic mixtures with desirable separation and recycling, accompanied by high permeation, with

504

textile and pharmaceutical industries being the potential beneficiaries. The simple fabrication,

505

mimetic structure, unique properties and existence of GO, will enable more fundamental research

506

and real applications for selective filtration and recycling of environmental pollutants.

507 508

Appendix A. Supplementary data

509

The synthesis of GO and data analysis equations are detailed in SI. The physico-chemical

510

properties of selected organic dyes, average CA, work of adhesion, adsorption kinetics

511

parameters, comparison of graphene-incorporated membranes are listed in Tables S1 to S5,

512

respectively. The stress-strain curves of NFMs, 3D molecular structures of representative dyes,

513

adsorption kinetics graphs, UV-vis spectra (NR, BF and RB), filtration curves, removal

514

efficiency, ATR-FTIR and N2 isotherms of PVdF-GO 5.0% NFM before and after regeneration,

515

are presented in Figures S1 to S7, respectively. The supporting information is available free of

516

charge on the ACS Publications website.

517 518

Author Information

519

Corresponding Author

520

*Phone: 0086-571-88982587; fax: 0086-571-88982587; e-mail: [email protected].

521

Author disclosure statement

522

The authors declare that no competing financial conflicts exist.

523 524

Acknowledgments



525

This project was supported by the National Natural Science Foundation of China (21425730,

526

21537005, 21621005, and 21607124), the National Key Technology Support Program of China

527

(2015BAC02B01), and National Basic Research Program of China (2014CB441106). Abdul

528

Ghaffar also acknowledges the financial support for doctoral study from Zhejiang University.

529 530

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