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Journal of Membrane Science 392– 393 (2012) 167– 174

Contents lists available at SciVerse ScienceDirect

Journal of Membrane Science

jo u rn al hom epa ge: www.elsev ier .com/ locate /memsci

lectrospun nanofibrous membranes for high flux microfiltration

an Wanga, Yang Liua, Brandon Lia,b, Benjamin S. Hsiaoa,∗, Benjamin Chua,∗∗

Department of Chemistry, Stony Brook University, Stony Brook, NY 11794-3400, United StatesYale University, New Haven, CT 06511, United States

r t i c l e i n f o

rticle history:eceived 13 September 2011eceived in revised form5 December 2011ccepted 16 December 2011vailable online 26 December 2011

eywords:lectrospinningicrofiltration

a b s t r a c t

Highly porous electrospun nanofibrous membranes have gained considerable interest in water filtrationapplications. To understand the effects of electrospun nanofibrous structures on the filtration perfor-mance, a series of nanofibrous membranes with different fiber diameters, diameter distributions andmembrane thicknesses were prepared and studied. The results indicate a strong correlation between thephysical parameters of the membrane and the filtration performance. For example, a thicker membranewith a smaller average fiber diameter greatly favors the formation of a smaller pore size and narrowerpore size distribution, although the influence of the membrane thickness is relatively limited. Basedon successful control of the total composite structure (electrospun polyacrylonitrile (PAN)/non-wovenpolyethylene terephthalate (PET)) containing the electrospun layer thickness of 200 ± 10 �m and a mean

orometryacteria removal

fiber diameter of 100 ± 20 nm, a high flux microfiltration (MF) membrane with a maximum pore size of0.62 ± 0.03 �m and a mean pore size of 0.22 ± 0.01 �m was obtained. The PAN/PET nanofibrous MF mem-branes performed significantly better than the commercial MF membrane of the same mean pore size(0.22 �m), with two to three times higher flux (∼1.5 L/m2 h). The nanofibrous MF filter could maintaina very high rejection ratio of micro-particle and bacteria (LRV = 6). The results suggest that electrospun

are ex

nanofibrous membranes

. Introduction

Water is one of the most important resources on our planet.ecently, fresh water supplies have been depleting at an alarmingate. Over one billion people do not have access to clean water andver two billion people live in water-stressed areas [1]. In view ofhe rising global population and the growing concern of environ-

ental pollution, water purification has to be made more efficientnd more cost-effective. Among the varying water purificationechnologies, the pressure-driven membrane filtration processesincluding microfiltration (MF), ultrafiltration (UF), nanofiltrationNF) and reverse osmosis (RO)) are among the most energy-efficientnes. Unlike distillation, which consumes more energy, the mem-rane filtration processes are relatively fast, efficient and practical2]. MF is the lowest energy consuming filtration process, whichrimarily removes contaminants such as particles and bacteriay passing dirty water through a porous membrane with pore

izes from 0.1 to 10 �m [3,4]. For example, effective removal ofaterborne diseases such as cryptosporidium and Escherichia coli

t low pressures has been demonstrated using both polymeric MF

∗ Corresponding author. Tel.: +1 631 632 7793; fax: +1 631 632 6518.∗∗ Corresponding author. Tel.: +1 631 632 7928; fax: +1 631 632 6518.

E-mail addresses: bhsiao@notes.cc.sunysb.edu (B.S. Hsiao),chu@notes.cc.sunysb.edu (B. Chu).

376-7388/$ – see front matter © 2012 Elsevier B.V. All rights reserved.oi:10.1016/j.memsci.2011.12.019

cellent materials for high-flux MF applications.© 2012 Elsevier B.V. All rights reserved.

membranes and inorganic MF membranes (e.g. ceramic) [5–7]. Bothtypes of such MF membranes have their strengths and weaknesses.Ceramic MF membranes possess excellent thermal stability anddurability, but they have lower permeability, less flexibility and aremore expensive [8–10]. Polymeric membranes, such as the onesfabricated by the phase inversion method, are relatively cheap,but they have an asymmetric structure that leads to low surfaceporosity and also low permeability.

Electrospinning is a fabrication method that can create a non-woven nanofibrous structure that is useful in water purification[4,11–14]. This is because electrospun membranes can overcomethe flux limitations of polymeric and ceramic membranes. Specif-ically, electrospun membranes usually possess higher porosity(typically around 80%, but there is no upper limit), lower baseweight, larger effective surface area (up to 40 m2/g depending onthe fiber diameter) and continuously interconnected pores, whencompared to conventional polymer and ceramic membranes. Theseattributes can directly improve the flux performance without sac-rificing the contaminant rejection ratio [15]. The unique structurein nanofibrous membranes can benefit not only water filtrationbut also chemical separations in liquids in general [16]. For waterfiltration, the applications of electrospun membranes have been

successfully demonstrated in microfiltration [3,11], ultrafiltration[14] and nanofiltration [17].

The electrospun nanofibrous membrane structure could be con-trolled through the adjustment of various processing and solution

1 rane Science 392– 393 (2012) 167– 174

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2

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Table 1Optimized parameters for electrospinning of 4–10 wt% PAN solutions in this study.

PANconcentration(wt%)

Flow rate(�L/min)

Tip tocollectordistance(cm)

Voltage(kV)

Temperature(◦C)

Humidity (%)

4 20 6 12 20 504 20 10 13 20 504 22 19 30 20 506 20 6 14 20 506 20 10 16 20 506 22 19 25 20 508 20 6 13 20 508 20 10 20 20 508 22 19 23 20 50

10 20 6 12 20 50

68 R. Wang et al. / Journal of Memb

arameters, such as solution composition and concentration, elec-ric field strength, solution flow rate, environmental conditionse.g. humidity and solvent evaporation rate), collection orientation,tc. [18–21]. Recently, several research groups have prepared elec-rospun nanofibers with diameters ranging from 100 to 1000 nmor MF applications. One group reported that the desired nanofi-rous scaffold for an effective depth filter should have a uniformber diameter (i.e., a narrow fiber diameter distribution) and

suitable thickness [22]. Other research groups demonstratedhe use of nanofibrous membranes based on electrospun PAN,oly(vinylidene fluoride), polysulfone and grafted polymers toemove micron-sized particles [3,15,23]. They discovered that elec-rospun membranes with 160–380 nm fiber diameter, exhibitedood ability that could remove more than 90% of micron sized parti-les (diameter larger than 1 �m) from water, and very high flux rateseveral times higher than the commercial membrane at the samepplied pressure). However, the challenge of filtering sub-micronized particles using electrospun membranes was not successful.esearchers also correlated the fiber diameter and pore size in elec-rospun membranes [24,25]. Both values of bubble-point pore sizend mean flow pore size were found to decrease with decreasingber diameter.

The goal of the present study was to further the understandingf relationships between the structural parameters of electrospunanofibrous membranes (i.e., average fiber diameter, fiber diameterize distribution, membrane thickness and porosity) and micro-ltration performance for water purification. Based on the abovetudy, a nanofibrous membrane based on an electrospun polyacry-onitrile (PAN) nanofibrous scaffold and a non-woven polyethyleneerephthalate (PET) substrate was demonstrated and tested on theemoval of the micron-sized particles and E. coli from contaminatedater with a side-by-side challenge with the commercial micro-ltration membrane (GSWP, Millipore) of nominal 0.22 �m poreize, which has been widely used for bacteria removal and otherater filtration applications [26,27]. PAN and PET were chosen as

he polymers for the barrier layer and substrate, respectively, dueo their good mechanical and thermal stability as well as excellentolvent resistance.

. Experimental

.1. Materials

Polyacrylonitrile (PAN) with an average molecular weight (Mw)f 1.5 × 105 g/mol was purchased from Scientific Polymer Prod-cts; 99.8% N,N-dimethylformamide (DMF) was purchased fromigma–Aldrich; and polyethylene terephthalate (PET) non-wovenembrane was obtained from Sanko, Japan (No. 16-1). 0.20, 0.50

nd 1.0 �m polybead carboxylate microspheres were obtainedrom Polysciences, Inc. E. coli (ATCC 15597), a non-pathogenic bac-erium, was purchased and prepared in broth medium (ATCC 271).

commercial microfiltration membrane (Model GSWP, Millipore)ith a nominal pore size of 0.22 �m was also tested for comparisonurposes.

.2. Preparation of nanofibrous membranes

The preparation procedures to fabricate nanofibrous mem-ranes are as follows. PAN solutions of 2–10 wt% were prepared byissolving polymer powder in DMF and stirring the solution at 60 ◦C

or 24 h until the solutions were homogeneous. Viscosities and con-uctivities of these solutions were measured by a Brookfield Digitaliscometer (Model LVTDCP) and an Oakton Eutech Conductivityeter (Con 11 Series), respectively.

10 20 10 25 20 5010 22 19 21 20 50

The PAN/DMF solution was electrospun directly onto a PET non-woven substrate using an automatic electrospinning instrument,consisting of a metallic spinneret, a syringe pump, a high-voltagepower supply and a translational collector. The electrospinninginstrument was operated within an environmental chamber, wherethe humidity was adjusted by a compressed air flush system. Allsamples were electrospun using a spinneret with a diameter of1.0 mm, a solution flow rate of 20–25 �L/min, ambient temperatureof 22–24 ◦C, and humidity of 40–50%, and the spinneret-to-collector distances from 6 to 24 cm were systematically varied. Theeffect of the solution concentration on the fiber diameter was alsoevaluated. In this study, the voltages that yielded the most sta-ble Taylor cones or stable jets were chosen, which ranged from 14to 30 kV [13]. The chosen electrospinning parameters of 4–10 wt%PAN solutions are illustrated in Table 1. The effect of the electro-spun PAN layer thickness on the filtration performance was alsosystematically investigated.

2.3. Characterization of nanofibrous membranes

The following parameters of the electrospun layer in nanofi-brous membranes, such as average fiber diameter, fiber orientationand scaffold thickness, were analyzed using the scanning elec-tron microscope (SEM, FEI Phenom) equipped with a Schottky fieldemission gun (10 kV) and a Robinson backscatter detector. All sam-ples were gold-coated for 30 s before SEM imaging in order toreduce the effect of charging. Cross-sectioned images were takenfrom the cleavage of the membranes after the liquid nitrogen freez-ing treatment. The Leica Microscopy Imaging software was used tomeasure the average fiber diameter from more than 40 randomlyselected fibers. The thickness of the electrospun scaffold layer wasdetermined from the combination of SEM cross-section images andthe micrometer.

Porosity, also known as packing density, defined the ratio of thevoid volume to the bulk volume. The porosity of the nanofibrousmembrane was estimated using the following expression:

porosity =(

1 − �

�0

)× 100%

where � is the density of the electrospun PAN scaffold and �0 is thedensity of PAN bulk polymer [28]. The density of the electrospunPAN scaffold layer was determined from the average of ten samples.

The bubble-point and mean flow pore sizes of the nanofibrousmembrane were determined by the capillary flow porometer (PMI,

Inc.). The maximum pore size in the membrane was measured bythe bubble-point test, which is based on the following principle. Bygradually increasing the non-reacting air pressure, the wetting liq-uid (Galwick, PMI) in the membrane is displaced by the air bubble

rane Science 392– 393 (2012) 167– 174 169

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2

2utwptnflbppfctder

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r

wts

mEahwamutcowf

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1086420

500

1000

1500

2000

Vis

cost

iy (P

aS)

Concentration (wt%)

A

1086420

50

100

150

Con

duct

ivity

(uS

/cm

)

Concentration(wt%)

B

Fig. 1. Relationships of (A) viscosity and (B) conductivity vs. PAN concentration forelectrospinning.

R. Wang et al. / Journal of Memb

hrough the largest sized pore [29]. The relationship between theaximum pore size and corresponding pressure, as expressed by

he Young-Laplace equation, was used for this purpose. The mean-ow pore size was measured when 50% of flow passed through theores. This value differs from the mean pore size because the flowhrough large diameter pores can be disproportionately larger thanhe flow through small diameter pores. Typically, the mean flowore size is more useful in evaluation of water filtration character-

stics [30,31].

.4. Microfiltration evaluation

A dead-end filtration module (Microsyringe Filter Holder5 mm, Millipore) equipped with an air pressure control wassed to measure the pure water flux and retention ratio ofhe nanofibrous membrane (the tested sample had a disc shapeith a diameter of 25 mm) [32]. All chosen PAN/PET membranesossessed a thickness of 200 ± 10 �m (the thickness of the elec-rospun PAN layer was about 100 �m), where the electrospun PANanofiber had an average diameter of 100 ± 20 nm. The pure waterux was defined as the volume of water passing through the mem-rane per unit time, per unit area and per unit of transmembraneressure. This property reflects the energy required to generateermeate and is an easy way to compare the permeability of dif-erent membranes. In this measurement, PAN/PET membranes andommercial membranes (GSWP, Millipore) were completely wet-ed first, and then tested at ambient temperatures (22–24 ◦C) andifferent pressures ranging from 1 to 40 psi. The time taken forach flux collection was measured with a standard stopwatch andounded to the nearest second.

The micro-particle retention ratio of the nanofibrous membraneas tested using the following procedure. 100 ppm suspensions of

.20 �m, 0.50 �m and 1.0 �m polybead carboxylate microspheresere prepared in distilled water by ultrasonic dispersion for 30 min.fter the pre-treatment of aqueous compression at 15 psi, a 10 mLuspension was filtered through the membrane (PAN/PET mem-rane and commercial membrane, GSWP, Millipore) using theead-end cell at a constant flow rate of 192 L/m2 h. The carbon con-entrations in the feed solution and the permeate solution wereeasured using the total organic carbon analyzer (TOC, Shimadzu

OC-VCPN). The retention ratio (or rejection) of the carboxylateicro-particles was calculated by the following expression:

ejection =(

1 − Cp

Cf

)× 100%

here Cf represents the carbon concentration in the feed solu-ion and Cp represents the carbon concentration in the permeateolution.

The bacteria test was also carried out using a dead-end filtrationodule (Microsyringe Filter Holder 25 mm, Millipore) and 5 mL of

. coli suspensions (106 CFU/mL). Before testing, both filtration cellnd connectors were soaked in a concentrated solution of sodiumypochlorite (1000 ppm) for 30 min and then washed repeatedlyith distilled water, while all other pieces of glass equipment were

utoclaved (15 min at 120 ◦C). The bacteria retention measure-ents were performed at ambient temperatures (22–24 ◦C) and

nder a constant flow rate of 192 L/m2 h. The bacterial concentra-ion was determined by a colony assay procedure, where the cellulturing was standardized to minimize the effects of variationsf bacterial physiology on the results. The bacterial retention ratioas calculated in terms of LRV (Log Reduction Value) defined as

ollows:

og reduction value = log

(Cf

Cp

)

where Cf and Cp are the bacterial concentrations of the feed andpermeate solutions (CFU/mL), respectively.

3. Results and discussion

3.1. Probing of stable electrospinning conditions

For the given Mw = 1.5 × 105 g/mol of PAN, 2–10 wt% of polymercould be readily dissolved in DMF and form homogeneous solu-tions. Fig. 1 illustrates that both the dynamic viscosity and electricalconductivity of these solutions increased with increasing solutionconcentration. In particular, the viscosity had a profound effecton the electrospinning process and the resulting fiber morphol-ogy. When the viscosity was too low, e.g. 2 wt%, the phenomenonof electro-spraying occurred and no stable formation of fiber jetcould be formed. This might be because the 2 wt% polymer solu-tion was fairly close to the overlap concentration of PAN in DMF,which lacked sufficient polymer chain entanglements for stablejet formation [33]. At higher polymer concentrations, not only didthe viscosity increase, but the higher conductivity further facili-tated the stretching of entangled polymer chains. Driven by thecombined effect of the viscoelastic and the Coulomb interactions,the 4–10 wt% PAN/DMF solutions could be successfully electrospun

under a variety of operating conditions.

170 R. Wang et al. / Journal of Membrane Science 392– 393 (2012) 167– 174

F N solm

3m

82hbhttcoabnfisaroRwch

ig. 2. SEM images of surfaces of PAN nanofibrous membranes electrospun from PAembrane electrospun from the 6 wt% PAN solution (D).

.2. Geometry examination of electrospun nanofibrousembranes

Fig. 2 illustrates that electrospun nanofibers prepared from wt% PAN/DMF solution had larger average fiber diameters of10 nm with no bead formation (Fig. 2A), while those at 4 wt%ad smaller average fiber diameters of 75 nm and non-uniformead formation within the web (Fig. 2C). Generally speaking, atigher polymer concentrations, higher conductivity could increasehe stretching force on the polymer chains, which should decreasehe fiber diameter. However, the increase in viscosity at high con-entrations could counteract the stretching of polymer chains. Inther words, polymer chains could be entangled to a greater extentt high polymer concentrations, thereby favoring the formation ofigger diameter fibers. Also, the polymer present in the beads can-ot be used to form fibers, so fewer polymers also results in thinnerbers in a diluted solution electrospinning. Fig. 2D shows the cross-ection view of a typical electrospun membrane, revealing the cleardvantage of using the electrospun nanofibrous scaffold as the bar-ier layer for microfiltration over the traditional barrier layer basedn porous membranes fabricated by the phase inversion method.

andomly deposited electrospun nanofibers clearly form a non-oven structure having a relatively larger porosity (78 ± 2% in this

ase) and three-dimensional inter-pore connectivity, resulting inigher flux or better permeability values.

utions at 8 wt% (A), 6 wt% (B) and 4 wt% (C), respectively, and the cross-section of a

3.3. Porometry evaluation of electrospun nanofibrous membranes

Pore size measurements were carried out on two series ofelectro-spun membranes, one with different average fiber diam-eters but the same barrier layer thickness (100 ± 10 �m), and theother with different layer thickness but the same average fiberdiameter (100 ± 20 nm). We note that the chosen PET non-wovensupport had a larger fiber diameter (30–50 �m), a larger pore size(100 �m) and a greater layer thickness (105 ± 5 �m) than those ofthe electrospun nanofibrous layer. Thus, the PET substrate was notthe limiting component in terms of the pore size and permeabilitymeasurements in the two-tier structure. Its role was only to providea good mechanical support.

The electrospun nanofibrous layers do not possess regular poreslike typical woven structures (e.g. screens). Instead, the transporta-tion through the non-woven structure reflects the formation of aninter-connected tortuous path for the liquids and particles to flowthrough [34]. In pore size measurements, one would assume equiv-alent cylindrical capillaries through the membrane and express thepore size in terms of the diameters of those capillaries. Such ameasurement on the nanofibrous membranes thus indicates the

effective pore size (e.g. the maximum pore size and the meanflow pore size) and pore size distributions. Fig. 3(A) illustrates theeffect of the average fiber diameter, controlled by the PAN con-centration and corresponding electrospinning conditions, on the

R. Wang et al. / Journal of Membrane Science 392– 393 (2012) 167– 174 171

50040030020010000

1

2

3

4

5A

B

Maximum pore sizeMean flow pore size

Por

e si

ze (m

icro

n)

PAN fiber average diameter (nm)

2.01.61.20.80.40.00

10

20

Dis

tribu

tion

(%)

(a) (b) (c)

Pore size diameter (micron)

Fig. 3. Effects of the nanofiber diameter on the maximum pore size and mean flowpore size (A), and pore size distribution (B) of the electrospun PAN membrane. Mem-b

mto4rmsMstdflcmditnoldtctbts

2001501005000.00

0.25

0.50

0.75

1.00A

B

Maximum pore sizeMean flow pore size

Por

e si

ze (m

icro

n)

PAN layer thickness (micron)

0.80.60.40.20.00

10

20

Dis

tribu

tion

(%)

(a) (b) (c)

Pore size diameter (micron)

Fig. 4. Effects of the electrospun layer thickness on the maximum pore size and

an electrospun layer thickness of 100 �m) with the mean flow pore

ranes a, b and c have average fiber diameters of 100, 210 and 320 nm, respectively.

aximum pore size and the mean flow pore size. By decreasinghe fiber diameter from 450 nm to 100 nm (without the formationf beads), the maximum pore size of the membrane changed from.0 ± 0.5 �m to 0.62 ± 0.03 �m, and the mean flow pore size waseduced from 1.8 ± 0.2 �m to 0.22 ± 0.01 �m. The latter values (theaximum pore size of 0.62 ± 0.03 �m, and the mean flow pore

ize of 0.22 ± 0.01 �m) fall within the requirements of effectiveF membranes to remove bacteria. Fig. 3(B) shows the relation-

hip between the fiber diameter and the pore size distribution ofhe membrane. It was seen that the membrane with smaller fiberiameter not only possessed smaller maximum pore size and meanow pore size, but also had a narrower pore size distribution. Theorrelations between the fiber diameter and the pore size are pri-arily determined by the density of fiber crossings, which in turn

efines the average length of a fiber segment between the cross-ngs; a quantity obviously linked to the pore size. In the limit ofhe planar filter approximation (i.e., infinitely small filter thick-ess) and the constant volume fraction of the polymer (equal tone minus the porosity), a smaller fiber diameter translates to aonger total fiber length per unit area, which would increase theensity of the crossings and reduce the average length betweenhe crossings. In the planar approximation, all these relationshipsan be easily quantified. For a thick 3D filter (as in our case), wherehe planar approximation breaks down, quantitative relationships

etween these quantities are much more difficult to describe, buthe qualitative behavior and trends found in the 2D planar case aretill applicable.

mean flow pore size (A), and the pore size distribution (B) of the electrospun PANmembrane. Membranes a, b and c have the PAN layer thickness of 30, 100 and180 �m, respectively.

Fig. 4 shows the effects of the electrospun layer thickness on themaximum pore size and mean flow pore size as well as the poresize distribution of the nanofibrous membrane. It was seen thatthe thicker electrospun layer generally resulted in smaller maxi-mum pore size, smaller mean flow pore size and narrower poresize distribution, but these parameters eventually reached plateauvalues at large layer thickness. This could be explained as follows.As the nanofiber was randomly deposited onto the PET substrateduring the electrospinning process, more deposition could effec-tively intersect and divide more pores, leading to smaller poresizes. However, this process to reduce pore size became ineffectivebeyond a certain electrospun layer thickness because the projectionof fiber crossings relevant to define the size of a given pore extendedonly over a finite range of the electrospun layer thickness. Oncethe total membrane thickness grew larger than this finite range,pore sizes would reach a plateau value and no longer decreasedupon further fiber deposition. For membranes with PAN nanofiberdiameters of 100 ± 20 nm and porosity of 78 ± 2%, the increasein the PAN layer thickness from 30 �m to 100 �m could lead toa shift of the pore size distribution to smaller values; the maxi-mum pore size was reduced from 0.74 ± 0.03 �m to 0.62 ± 0.03 �m;and the mean flow pore size was reduced from 0.45 ± 0.05 �mto 0.22 ± 0.01 �m. On the other hand, with the electrospun layerthickness further increased to 180 �m, the membrane only exhib-ited a maximum pore size of 0.61 ± 0.02 �m (vs. 0.62 ± 0.03 �m at

size of 0.22 ± 0.01 �m remaining essentially unchanged. Since thepermeability of the membrane is closely related to the fluid drivingpressure, the viscosity of the fluid and the thickness of the filter,

172 R. Wang et al. / Journal of Membrane Science 392– 393 (2012) 167– 174

Table 2The morphology of PAN/PET membrane and commercial GSWP membrane.

Totalthickness(�m)

Fiberdiameter(nm)

Maximumpore size(�m)

Mean flowpore size(�m)

ElectrospunPAN/PETmembrane

200 ± 10 100 ± 20 0.62 ± 0.03 0.22 ± 0.01

tsm

3

(fimflftm00paTovcrbbw

bpmcsmrttfts

4030201000

15

30

45

60

GSWP me mbra ne PAN/PET m embran e

Pur

e w

ater

Flu

x (1

03 L/m

2 h)

Press ure (psi)

Fig. 5. Pressure and pure water flux relations for electrospun PAN/PET membraneand Millipore GSWP membrane.

Table 3Retention of 1.0 �m, 0.50 �m, and 0.20 �m micro-particle suspensions using elec-trospun PAN/PET membrane and commercial GSWP membrane.

Micro-particles size 1.0 �m 0.50 �m 0.20 �m

GSWPmembrane

180 ± 10 – 0.55 ± 0.02 0.24 ± 0.01

he thinnest possible membrane (while maintaining a proper poreize for selectivity) is more preferred for the high-flux low-pressureicrofiltration.

.4. Microfiltration performance

The PAN/PET membrane with a total thickness of 200 ± 10 �mthe PAN electrospun layer thickness was 100 �m) and an averageber diameter of 100 ± 20 nm was used to evaluate the MF perfor-ance, listed in Table 2. The results were also compared with those

rom a 0.22 �m pore size rated commercial membrane (GSWP, Mil-ipore) with a similar thickness of 180 �m, which was designedor micro-organism removal in microfiltration applications. Thewo membranes (PAN/PET nanofibrous membrane and GSWP

embrane) have similar maximum pore sizes: 0.62 ± 0.03 �m vs..55 ± 0.02 �m and similar mean flow pore sizes: 0.22 ± 0.01 �m vs..24 ± 0.01 �m, respectively. Before the test, both membranes werere-washed using an aqueous pressure of 15 psi, in order to removell trapped air bubbles and open all the pores in the membranes.he structural advantage of electrospun nanofibrous membranesver traditional membranes was confirmed by the pure water fluxs. pressure test, as shown in Fig. 5. Three-dimensional inter-poreonnectivity and large porosity in the PAN nanofibrous membraneesulted in about 2–3 times higher flux than the commercial mem-rane over a range of pressures. It was noted from the results thatoth membranes had a low water entry pressure (lower than 2 psi),hich is favorable for low-pressure water purification.

To evaluate the selectivity of the electrospun PAN/PET mem-ranes, polybead carboxylate micro-particles of very narrowarticle-size distributions were used as a model contaminant. Thisethod could verify the pore sizes, as the particles were spheri-

al and the area of pore restriction could be assumed circular inhape. As shown in Table 3, we found that the electrospun PAN/PETembrane and the GSWP commercial membrane had a similar

etention ratio on 1.0 �m and 0.50 �m particle suspensions, andhe electrospun PAN/PET membrane performed better on the reten-

ion of the 0.20 �m particle suspension. This can be explained asollows. In comparison with the commercial membrane, the elec-rospun PAN/PET membrane had a similar maximum pore size,lightly larger than 0.50 �m, so most of the 1.0 �m and 0.50 �m

Fig. 6. SEM images of electrospun PAN/PET membrane after filtration using the

Electrospun PAN/PET membrane 99% 97% 93%GSWP membrane 99% 98% 90%

particles could be removed from water by size exclusion. However,the 93% removal of relatively smaller sized 0.20 �m particles wasnot achieved purely by the size exclusion, since the PAN/PET mem-branes had some larger pores. Fig. 6 illustrates the SEM imagesof the electrospun PAN/PET membrane after filtration using the0.20 �m particle suspension. In Fig. 6, it is seen that 0.20 �m par-ticles actually accumulated and formed a cake layer on the surfaceof the electrospun membrane, which blocked the open pores andrejected later particles from passing through, leading to a “higher”retention ratio. This fouling layer would decrease the permeabil-ity of the membrane, increase the transmembrane pressure andlimit the capacity of the filter. Similar behavior was also found inthe commercial membrane but to a slightly lower extent, under thedead-end filtration conditions. In the real-life filtration process, thefouling problem could be minimized and a much larger amount ofchallenge solutions could be used by modification of the filter sur-face, adding surfactant in the feed solution and using the cross-flowdesign, etc. [35–37].

The bacteria removal test was carried out on the electro-

spun PAN/PET membrane with a total thickness of 200 ± 10 �mand an average fiber diameter of 100 ± 20 nm. The 10 mL E. coli(106 CFU/mL) aqueous suspension was used as the model con-taminant. After the filtration of the total volume, there was no

0.20 �m particle suspension from (A) surface and (B) cross-section views.

R. Wang et al. / Journal of Membrane Science 392– 393 (2012) 167– 174 173

oli sus

Etts(mcrcv

4

btacepimndFttW1dpsdiit

t1Wpotoi

[

[

[

Fig. 7. SEM images of electrospun PAN/PET membrane after E. c

. coli detectable from the permeate solution, which indicatedhe complete removal of E. coli, and a LRV of 6 was achieved forhe membrane. This result was expected, since the bacterial cellize (2 �m × 1 �m) was much larger than the maximum pore size0.62 �m). Fig. 7 illustrates the SEM images of electrospun PAN/PET

embrane after E. coli suspension filtration. The images furtheronfirmed the bacteria retention, where dead E. coli cells wereetained on the top of the membrane, and there was no sign of E. coliells leaking into the membrane structure from the cross-sectioniew.

. Conclusions

Based on a systematic study of the effects of electrospun mem-rane parameters, including mean fiber diameter and membranehickness on corresponding pore sizes (i.e., maximum pore sizend mean flow pore size), pore size distribution and filtration effi-iency, we demonstrate that nanofibrous membranes containing anlectrospun polyacrylonitrile (PAN) nanofibrous barrier layer and aolyethylene terephthalate (PET) non-woven support have signif-

cantly higher flux (2–3 times) than Millipore GSWP conventionalicrofiltration membranes for microfiltration. During electrospin-

ing, the fiber diameter and the degree of uniformity in fiberiameter decreased with decreasing PAN solution concentration.rom the tests at a constant total layer thickness (200 ± 10 �m) ofhe electrospun nanofibrous scaffold, a strong correlation betweenhe mean fiber diameter and the membrane pore size was observed.

hen the average fiber diameter decreased from 450 nm to00 nm, the maximum pore size of the electrospun membraneecreased from 4.0 ± 0.5 �m to 0.62 ± 0.03 �m, and the mean flowore size dropped from 1.8 ± 0.2 �m to 0.22 ± 0.01 �m. At a con-tant average PAN fiber diameter (100 ± 20 nm), membranes withifferent PAN layer thickness (30–100 �m) also showed a decrease

n pore size with increasing layer thickness, but the effect on max-mum pore size and mean flow pore size leveled off when thehickness was greater than 100 �m.

Based on these results, the PAN/PET membrane with a totalhickness of 200 ± 10 �m and an average fiber diameter of00 ± 20 nm was selected for the flux and retention evaluations.ith the high effective porosity (up to 80%), interconnected

ores and the optimized small pore size (maximum pore size

f 0.62 ± 0.03 �m and mean flow pore size of 0.22 ± 0.01 �m),he membrane performed significantly better in flux (2–3 times)ver the Millipore GSWP 0.22 �m membranes, while maintain-ng a high rejection level in the micro-particle retention test, and

[

pension filtration, from (A) surface and (B) cross-section views.

achieved complete bacteria removal. The electrospun membraneswill be useful for drinking water purification, as well as pre-filtersfor ultrafiltration, nanofiltration or reverse osmosis to minimizethe possible fouling and contamination from micro-organisms ormicro-particles.

Acknowledgements

Financial support of this work was provided by National ScienceFoundation (DMR-1019370). The authors wish to thank Dr. YimeiZhu of Brookhaven National Laboratory, and Drs. Dufei Fang andHongyang Ma from Stony Brook University for their assistance insome experiments.

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