Journal of the Taiwan Institute of Chemical Engineers 45 (2014) 2176–2186

Surface modification of PVDF ultrafiltration membranes by remoteargon/methane gas mixture plasma for fouling reduction

Ruey-Shin Juang a,*, Chun Huang b, Chao-Lin Hsieh b

a Department of Chemical and Materials Engineering, Chang Gung University, Kwei-Shan 33302, Taoyuan, Taiwanb Department of Chemical Engineering and Materials Science, Yuan Ze University, Chung-Li 32003, Taiwan

A R T I C L E I N F O

Article history:

Received 3 April 2014

Received in revised form 26 June 2014

Accepted 29 June 2014

Available online 22 July 2014

Keywords:

Surface modification

PVDF membranes

Remote plasma

Ultrafiltration

Fouling reduction

A B S T R A C T

Ultrafiltration (UF) is considered as a potential alternative to gravity settling in liquid surfactant

membrane process for fast separation of water-in-oil (W/O) emulsions from the external aqueous phase.

Flux behavior was studied in this work during batch UF of W/O/W solutions by poly(vinylidene fluoride)

(PVDF) membrane with a molecular weight cut-off of 30 kDa after the membrane was modified by

remote cyclonic atmospheric-pressure plasma with argon gas and methane/argon gas mixture.

Physicochemical properties of the membrane surfaces including hydrophilicity, functional group

concentrations, and pore size distribution before and after plasma modifications were determined by

static contact angle measurements, X-ray photoelectron spectroscopy, and capillary flow porometry,

respectively. Higher surface concentrations of oxygen functional groups for plasma-functionalized

membranes were observed compared to the unmodified membranes. It was shown that UF flux was

significantly enhanced with the plasma-modified membranes under the conditions studied (feed, 5 vol%

W/O emulsions; stirring, 300 rpm; pressure, 35–138 kPa). Resistance-in-series analysis of the flux data

indicated that the proposed remote plasma treatment, particularly with methane/argon gas mixture,

could considerably reduce both resistances due to solute adsorption and cake layer formation on the

membrane surface, although the above two resistances always contributed more than 71% of the total

filtration resistance.

� 2014 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

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Journal of the Taiwan Institute of Chemical Engineers

jou r nal h o mep age: w ww.els evier . co m/lo c ate / j t i c e

1. Introduction

In the past 30 years, liquid membrane process has shown to be anovel method for selective recovery and separation of species fromdilute solutions, including metals, weak acids/bases, hydrocarbonsor biologically important compounds, and gaseous mixtures [1,2].The so-called uphill transport or pumping effect has received muchattention as a promising alternative to common solvent extractionbecause liquid membrane combines extraction and back-extrac-tion steps in one unit. Liquid surfactant membrane (LSM, alsoknown as emulsion liquid membrane), in which the carriersolution is reformed as small spherical shells to separate aqueousfeed and strip phases, represents one of the feasible types of liquidmembranes [1]. This method leads to a membrane area of 1000 to3000 m2/m3 of the equipment volume [2], which satisfies the

* Corresponding author. Tel.: +886 3 2118800x5702; fax: +886 3 2118668.

E-mail addresses: rsjuang@mail.cgu.edu.tw, rsjuang2312@gmail.com

(R.-S. Juang).

http://dx.doi.org/10.1016/j.jtice.2014.06.025

1876-1070/� 2014 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V.

wishes of process engineers to achieve large mass-transfer area.However, the separation of W/O emulsions from the externalaqueous phase by gravity settling is frequently not so effective andtime consuming [3] making the LSM processes limited for practicalapplications.

Low-pressure ultrafiltration (UF) and microfiltration (MF)processes have been widely applied in various chemical andbiochemical processes for the separation of dissolved andsuspended matter according to their sizes and molecular scales[4]. This is because UF and MF processes involve no phase changeand no need of extra chemical agents, which are more environ-mentally friendly and economic [5]. Also, the characteristics ofthese processes that make them excellent for many applicationsinclude low energy requirements, high recovery efficiency, andhigh throughput. To ensure nearly complete rejection of W/Oemulsion droplets by the membrane and the avoidance of possiblepore blocking of W/O emulsions, UF process appears to be apotential alternative to gravity settling unit because the size of W/O emulsion droplets, expressed as the Sauter diameter d32, is in therange 0.6–2.4 mm in common LSM process depending on agitator

All rights reserved.

Notation

A membrane filtration area (m2)

C solute concentration in the retentate (g/L)

C0 solute concentration in the feed (g/L)

Cm solute concentrations at the membrane surface (g/

L)

D shear-induced diffusivity (m2/s)

J UF flux of the actual feed solution (L/m2/h)

Jw1 pure water flux with fresh membrane defined in

Eq. (2) (L/m2/h)

Jw2 pure water flux with the used membrane defined in

Eq. (3) (L/m2/h)

m mass of solute deposited on the membrane (kg)

n compressibility index of the cake defined in Eq. (9)

DP applied pressure (Pa)

Ra resistance due to solute adsorption defined in

Eq. (1) (m�1)

Rc resistance of cake layer defined in Eq. (1) (m�1)

Rm intrinsic membrane resistance defined in Eq. (1)

(m�1)

Rp resistance due to concentration polarization layer

defined in Eq. (1) (m�1)

t filtration time (s)

V cumulative volume of the permeate (m3)

V0 initial volume of the feed (m3)

Greek letters

a specific resistance of the cake defined in Eq. (8) (m/

kg)

a0 resistance coefficient of the cake defined in Eq. (9)

(m/kg/Pan)

d cake thickness (m)

mp viscosity of the permeate (Pa s)

Subscript

ss steady-state value

R.-S. Juang et al. / Journal of the Taiwan Institute of Chemical Engineers 45 (2014) 2176–2186 2177

type and agitation intensity [6]. Like most of UF operations,however, membrane fouling will be serious during the process dueto the hydrophobic nature of W/O emulsions and membrane itself,making this UF process more critical in industrial applications.

Membrane fouling is often characterized by an irreversibledecline in flux in membrane filtration systems [7]. Considerableprogress has been made in understanding the interactions amongthe foulants, membranes, and operating parameters. Change inhydrophobicity of the membranes through surface modificationsappears to be relatively simple and economic although manymethods have been developed to overcome membrane fouling [8].It is known that low-temperature plasmas is a potential techniquebecause it enhances surface hydrophilicity of polymer membranewithout altering its bulk properties and unfavorable thermaleffects; i.e., the favorable bulk attributes can be maintained [9]. Inspite of excellent surface modification efficiency, the currentlyavailable low-temperature plasma processes are mainly limited tothe restricted volume of plasma reactor and the requirement forone or more vacuum/chemical cycles [10,11]. To overcome suchrestrictions, atmospheric-pressure plasma can be seen as apromising candidate. Atmospheric-pressure plasma obtains sev-eral benefits such as vacuum free, chamberless surface treatment,and dry process. However, despite the wide-ranging use of

atmospheric-pressure plasma process, the modification of mem-brane surface by atmospheric-pressure plasma has not beenextensively examined and reported. Depending on the input gasused for plasma formation and the operating parameters, it can beused to activate polymeric membrane surface by inserting activespecies and/or a cross-linking process.

In this work, flux characteristics during batch UF of W/O/Wsolutions were studied using the membranes before and aftersurface modifications by cyclonic atmospheric-pressure plasmawith argon and methane/argon mixture. Poly(vinylidene fluoride)(PVDF) membrane was adopted here due to its versatileapplicability for UF operations and its superior chemical resis-tances [12] although it has comparatively low intrinsic surfaceenergy. Therefore, PVDF membranes were modified by cyclonicatmospheric-pressure plasma with various operating parameters.Changes in surface hydrophilicity and chemical compositions ofthe plasma-modified PVDF membrane were characterized by staticcontact angle measurements and X-ray photoelectron spectrosco-py (XPS), respectively. This work is the first step in exploring thepotential of cyclonic atmospheric-pressure plasma modification asa mean of enhancing the flux of UF membranes.

Surfactant sorbitan monooleate (Span 80) and acidic carrierdi(2-ethylhexyl)phosphoric acid (D2EHPA) were added in theorganic solvent (kerosene) to formulate W/O emulsions, in whichthe internal water phase contained mineral acid. An equimolarmixture of Cu(II) and Zn(II) was chosen as the model externalaqueous phase. This is merely because the practical and stablecompositions of W/O/W solution can be determined easily basedon their effective separation [13]. All UF experiments wereconducted at a stirring speed of 300 rpm and different appliedpressures (35–138 kPa). Finally, the fouling resistances wereanalyzed using the resistance-in-series model to clarify the roleof the change in surface hydrophilicity of the membranes on fluxenhancement. Such a deeper analysis is of practical importance,which allows us to evaluate application potential of membranesurface modifications by plasma in chemical and biochemicalprocess industries.

2. Resistance-in-series model

Flux decline in batch or cross-flow UF processes can be causedby several factors including concentration polarization, cakeformation, solute adsorption, and pore plugging [7]. All thesefactors will introduce additional resistances on the feed side to thetransport of solvent across the membrane. The resistance-in-seriesmodel that considers the resistances due to the membrane itself,solute adsorption, concentration polarization, and cake formationhas been often applied to describe such processes. This model isparticularly applicable for analysis of fouling resistances in the UFof W/O/W solutions because the present W/O emulsion dropletscan be treated as macromolecular solutes compared to the poresize of UF membranes [14].

In this regard, the permeate flux of UF process (J) can beexpressed as [15,16]:

J ¼ DP

m pRt¼ DP

mp Rm þ Rc þ Ra þ R p

� � (1)

where DP is the applied pressure (Pa), mp is the viscosity ofpermeate (Pa s), Rt is the total filtration resistance (m�1), Rm is thehydraulic resistance of membrane itself (m�1), Rc is the resistanceof cake layer (m�1), Ra is the resistance due to the adsorption offoulants and/or possibly membrane plugging (m�1), and Rp is theresistance due to concentration polarization layer (m�1).

Among all these resistances, Rp can be removed by rinsing withdeionized water and Rc can be removed by cleaning the membranesurface with sponge [17]. Therefore, Rm and Ra can be determined

R.-S. Juang et al. / Journal of the Taiwan Institute of Chemical Engineers 45 (2014) 2176–21862178

as follows:

Rm ¼DP

mpJw1

(2)

Rm þ Ra ¼DP

m pJw2

(3)

where Jw1 and Jw2 are the pure water fluxes with fresh membraneand with the used membrane after removing cake layer usingsponge, followed by rinsing with deionized water (L/m2/h),respectively.

Because the compositions of the retentate varied with time inbatch UF system, particularly at the beginning of the process, theresistances Rc and Rp were calculated only when the flux reachedpseudo-steady state. In fact, Juang and Lin [18] reported thefeasibility of UF process for the separation of W/O emulsionsfrom the external aqueous phase. W/O emulsions were preparedwith ultrasound by mixing an organic phase containing 6 vol%D2EHPA and 3 vol% Span 80 in kerosene and an internal aqueousphase consisting of 7.8 mmol/L Cu(II)-EDTA chelated anions atpH 4. These emulsions were then suspended in deionized water,the external aqueous phase, to form W/O/W solution. In thatstudy, the chelated anions were acted as tracers to determinethe extent of destruction of W/O emulsions during UF processbecause such anions cannot be transported using acidic carriersto the external aqueous phase [13]. Under the applied pressurerange of 35–104 kPa, Cu(II) was undetectable in the permeateduring batch UF of W/O/W solutions using Amicon regeneratedcellulose YM membranes (MWCO = 10 and 30 kDa). It istherefore assumed that demulsification does not occur duringUF process and the rejection of W/O emulsions equals 1 in thepresent system.

The variation of solute concentration in the retentate at eachtime interval can be calculated from Eq. (4) in a batch system whenthe flux J and the solute rejection of unity are given [19]:

C ¼ C0V0

V0 � AR t

0 Jdt� � (4)

where C is the solute concentration in the retentate (g/L), V0 is theinitial volume of the feed (m3), and A is the membrane area (m2).

The mass of W/O emulsions deposited m can be obtained byintegrating the mass balance equation [19]:

dm

dt¼ J � C � D

dCm � Cð Þ

� �A (5)

where Cm is the concentration of W/O emulsions at the membranesurface (g/L), d is the cake thickness (m), and D is the shear-induceddiffusivity (m2/s). In Eq. (5), J � C � A denotes convective flowtoward the membrane and D(Cm � C)A/d is the opposite diffusiveflow. It was reported that back diffusion is mainly due to shear-induced diffusion for particles with diameters in the range of0.5 mm up to 30 mm [20]. Shear-induced diffusion represents therandom interactions between one particle and the others, whichinduce random displacements of the particles within the streamlines. In such cases, D is proportional to the wall shear rate. BecauseCm� C, Eq. (5) can be simplified as Eq. (6) when the flux reachespseudo-steady state [21]:

JssC ¼ D

d

� �Cm (6)

Substitution of Eq. (6) into Eq. (5) yields:

dm

dt¼ J � Jssð Þ � C � A (7)

It is accepted that the resistance of cake layer formed at themembrane surface by suspended matter (e.g., W/O emulsions), Rc,is proportional to the thickness of the cake and applied pressure[22]. Thus, we have

Rc ¼m

A

� �a (8)

where a is specific resistance of the cake. The pressure dependenceof specific resistance is proposed by [22]:

a ¼ a0 DP� �n

(9)

where n is the cake compressibility index and a0 is the resistancecoefficient (i.e., the specific cake resistance at unit pressure drop)depending on particle size and shape. The time-dependent value of(m/A) is obtained here by simultaneously solving the set of Eqs. (4)and (7)–(9) using the METLAB DOFTlab software. The resistance Rp

ascribable to accumulation of solutes near the membrane/solutioninterface under steady state can be finally obtained, provided thatthe total resistance Rt was calculated from Eq. (1).

3. Materials and methods

3.1. Reagents and solutions

Span 80 (density, 0.986 g/cm3 at 25 8C; HLB = 4) and organicsolvent kerosene, obtained from Wako Pure Chemical Industries,Ltd., Japan and Union Chemical Works Ltd., Taiwan, respectively,were used as received. D2EHPA (purity, 98.5%) was purchased fromMerck Co. and was used without further purification. The waterused here was deionized by a Millipore Milli-Q system. Theexternal aqueous phase consisted of equimolar Cu(NO3)2 andZn(NO3)2 (0.015 mol/L) in deionized water, whereas the internalaqueous phase contained 0.1 mol/L HNO3. The pH value of theexternal aqueous phase was adjusted in the range 2.0–6.0,measured using a pH meter (Horiba, Model F-23), by adding asmall amount of 0.1 mol/L HNO3 or NaOH. The organic phasecontained 0.1 mol/L D2EHPA but the concentration of Span 80 wasvaried from 3 vol% to 7 vol% to seek for optimal composition toformulate stable W/O emulsions.

3.2. Preparation of W/O emulsions

In a 0.5-dm3 beaker (90 mm I.D., 150 mm height, and fitted withfour glass baffles 15 mm wide), the W/O emulsions were preparedby mixing an aqueous internal phase (150 cm3) and an organicphase (150 cm3) using a homogenizer equipped with an emulsorscreen (Silverson, Model L4R, UK) at 3000 rpm for 6 min.

The stability of W/O emulsions during LSM process was firsttested by detecting the leakage of Cu(II) from internal aqueousphase. Here, a glass vessel of 120 mm I.D. and 180 mm height,fitted with 4 glass baffles, 20 mm wide was used. An amount ofW/O emulsions was mixed with an aqueous external phase(500 cm3) in the stirred vessel using a Cole-Parmer Servodyneagitator with 6-paddle impeller at 300 rpm. Experiment wasstarted at the end of the pouring of W/O emulsions. At a presettime intervals, the external aqueous phase sample was takenafter it was separated from W/O emulsions by passing W/O/Wsolution through a syringe PVDF filter (Whatman, 0.2 mm). Eachexperiment was duplicated under identical conditions. Theconcentration of Cu(II) in the sample was then analyzed by anatomic absorption spectrophotometer (Varian FS220). Thereproducibility of concentration measurements was within 5%(mostly, 2%).

R.-S. Juang et al. / Journal of the Taiwan Institute of Chemical Engineers 45 (2014) 2176–2186 2179

3.3. Membrane and atmospheric-pressure plasma deposition system

PVDF membrane (Model prefix JW), purchased from GEOsmonics, USA, with a MWCO of 30 kDa, was selected becausetypical W/O emulsion droplets prepared here had a mean size of1.62 mm measured with a Particle Analyzer PN A54412 (BeckmanCoulter, Inc.) (not shown). In addition, JW designation is commonlyused for cell harvesting, oil/water separations, and suspended solidremoval. The disc membrane had a diameter of 76 mm with ageometric (flat surface) area of 41.8 cm2.

The cyclonic atmospheric-pressure plasma system used tomodify PVDF membranes was shown in Fig. 1a. The aim of this typeof plasma is based on the use of two rotating double-pipe typeplasma jets to create the discharge cyclone to enlarge the treatedsample size. This atmospheric-pressure plasma cyclone obtains agas compartment and two discharge jets placed certain distanceapart inside gas compartment [11]. The high argon gas flow rate(10 slm, standard liter per minute) is introduced from the upside ofthe plasma system and passes through gas compartment as theionization gas. An electrical field is applied to ignite plasma glowdischarge by a 13.56 MHz radio frequency (RF) power supply. Thesample is mounted on an X–Y movable table in order to simulatein-line processing at variable line speeds. Fig. 1b shows theinfrared thermal imaging of cyclonic plasma with touching thesubstrate. The sensitivity of infrared thermal measuring techniqueis 150 mK at 25 8C scene temperature. The radiant temperatureprofile of the cyclonic atmospheric-pressure plasma measured byinfrared thermal analysis was similar to those by thermocouple

Fig. 1. (a) Schematic diagram of cyclonic atmospheric-pressure plasma system and

(b) the temperature profile of infrared thermal imaging.

thermal analysis in Fig. 1b at low temperature (30–87 8C)measured with a thermocouple thermometer.

3.4. Characterization and analysis of the plasma-modified membrane

surface

Prior to experiments, each PVDF membrane sample wascleaned in an ultrasonic water bath for 20 min, thoroughly rinsedwith deionized water for 30 min, and dried completely in air. Thestatic contact angle of PVDF membrane was measured byprojecting an image of an automatic sessile droplet resting on amembrane surface with a Magic Droplet Model 100SB videocontact angle system (Sindatek Instruments Co., Taiwan). Afteratmospheric-pressure plasma treatment, the unmodified andplasma-modified PVDF membranes were placed on a verticallyand horizontally adjustable sample stage. After 0.1-mL waterdroplet has made contact with the polymer surface, a snapshot ofthe image was taken. The captured image was saved and thecontact angle was measured at leisure. To understand the changeof surface characteristics of PVDF membranes, the dispersion andpolar interaction contributions to surface energy of the materialswere calculated by the Owens-Wendt model [23]. The liquids usedfor calculating surface energies of the unmodified and plasma-modified PVDF membranes were water and diiodomethane ofknown gp (polar component) and gd (disperse component). Thesurface energy of a solid (gs) has two components, namely, a polarcomponent and a disperse component. The polar and dispersecomponents are responsible for hydrophilic and hydrophobicproperties, respectively.

A VG Scientific Microlab 310F system equipped with Mg Ka X-ray source (1253.6 eV) and a concentric hemispherical analyzerwas used for surface analysis with X-ray photoelectron spectros-copy (XPS). Spectra were acquired with the angle between thedirection of the emitted photoelectrons and the surface of PVDFmembranes being equal to take-off analysis angle 70̊. For furtherevaluating the differences in pore properties caused by cyclonicatmospheric-pressure plasma, the pore size distribution and meanpore diameter of the unmodified and plasma-modified PVDFmembranes were measured with an advanced capillary flowporometry (CFP-1500AE, Porous Materials, Inc., USA).

3.5. UF apparatus and experiments

Batch UF experiments were conducted in a stirred cell with acapacity of 300 cm3 (Millipore, XFUF07601). The experimentalsetup was the same as that described previously [24]. Temperaturewas controlled at around 25 8C by air conditioner. The feed volumewas 280 cm3 and the cell was stirred at 300 rpm using a magneticmotor. This stirring speed was selected because it could lead toeffective agitation but prevent formation of a serious vortex in thecell. The applied pressure (DP) was monitored with pressurized N2

gas by means of a transducer. The permeate flux (J) was measuredby an electrical balance (Mettler AG204), connected to a personalcomputer. The viscosity of the permeate (the external aqueousphase) was measured using a Brookfield viscometer with ULadapter (Model LVDV-11+, USA).

The typical time profiles of the flux J over the entire process canbe empirically expressed in the following exponential dependence[24]:

J ¼Xr

i¼1

Ji�1 � Jið Þexp �kitð Þ þ Ji (10)

Here, the steady-state flux was obtained at t ! 1 through theselection of r such that the differences between the fitted andmeasured fluxes were less than 1% (in most cases, r = 2–4).

Distance from nozzle to substrate (mm)

15.012.510.07.55.0

Co

nta

ct a

ng

le (

de

gre

e)

0

20

40

60

80

100

120

Surfa

ce fre

e e

nerg

y (m

J m

-2)

0

20

40

60

80

water

diiodomethane

surface free energy(a) Ar plasma

Distance from nozzle to substrate (mm)

15.012.510.07.55.0

Co

nta

ct a

ng

le (

de

gre

e)

0

20

40

60

80

100

120

Su

rface

free

en

erg

y (m

J m

-2)

0

20

40

60

80

water

diiodomethane

surface free energy(b) Ar/CH

4 plasma

Fig. 2. The average contact angles and surface free energy changes of the plasma-

modified PVDF membranes with distance away from downstream (plasma

conditions: argon flow rate 10 slm, RF power input 100 W, and treatment time

150 s).

R.-S. Juang et al. / Journal of the Taiwan Institute of Chemical Engineers 45 (2014) 2176–21862180

The used membranes were immediately flushed with waterafter UF, and then regenerated in sequence by rinsing with 0.1 mol/L NaOH, 1.4 mmol/L NaOCl, and 10 mmol/L HCl in an ultrasoniccleaner (Brandson B20, USA) for 10–30 min each. Only the cleanedmembrane was repeatedly used here if the difference in pure waterflux with the cleaned and fresh membranes was smaller than 5%.

4. Results and discussion

4.1. Formulation and performance of LSM process

The stability of W/O emulsions is primarily determined by thetype and concentration of surfactant as well as the volume ratio oforganic to internal aqueous phases during the preparation of W/Oemulsions [2,3]. At a fixed volume ratio of organic to internalaqueous phases of unity, preliminary experiments have shownthat 3 vol% of Span 80 is enough to formulates stable emulsions bydetecting the leakage of Cu(II) from the internal aqueous phase ofW/O emulsions during LSM process. In this case, the mean size ofW/O emulsion droplets is measured to be 1.62 mm.

On the other hand, the pH of the external aqueous phase and thevolume ratio of external aqueous phase to W/O emulsions playcrucial roles in the separation of Cu(II) and Zn(II) by a LSM processcontaining D2EHPA as mobile carriers [13]. Separate experimentshave revealed satisfactory results under the conditions of anexternal aqueous pH of 4 and a volume ratio of 5. The separationfactor of Cu(II) to Zn(II), defined as the concentration ratio of Cu(II)to Zn(II) in internal aqueous phase divided by that in externalaqueous phase, is about 120 after 50-min LSM operation.Consequently, such compositions of the W/O/W solutions areselected for further UF experiments in this work.

4.2. Surface properties of the unmodified and plasma-modified

membranes

Surface modification of polymer membranes by plasmatechniques alters surface hydrophilicity; however, it more or lessenlarges the pores particularly for porous membranes, even if low-temperature plasma were used. To examine the effect of surfaceproperty changes on UF flux characteristics, the change ofmembrane pore size should be minimized during plasmaprocessing. In this regard, the so-called ‘‘remote’’ plasma (awayfrom the glow as far as possible) was applied in this work. Theremote plasma generally obtains mild treatment results so that it isfavorable for polymeric materials. In atmospheric-pressure plasmastate, the plasma species can lose its reactivity in the short timedue to much higher collision frequency among the plasma particlesin atmospheric environment [25]. As a result, plasma significantlyloses their reactivity in a remote position. To examine surfacemodification effects of remote plasma, PVDF membrane wassubjected to cyclonic atmospheric-pressure plasma exposure inthe downstream at a position 5–15 mm away from the glow. Thelong-life plasma species from the plasma are allowed to diffuse andget in contact with the membrane.

Fig. 2 shows the modification results on a plasma exposure ofcyclonic atmospheric-pressure plasma with argon and methane/argon mixture. It is noted that water contact angle of theunmodified PVDF membrane is about 98̊. As seen in Fig. 2, watercontact angle decreases and surface free energy increases by bothplasmas with argon and methane/argon mixture. It is found that10-mm plasma exposure distance causes superior surface modifi-cation effect on plasma-modified PVDF membranes from contactangle and surface energy data. However, the mean pore diametersignificantly changes form 0.11 mm to 0.29 mm (not shown) whenPVDF membrane is treated by such plasma. Hence, plasma with adistance of 15 mm is selected as the ‘‘remote’’ plasma here, which

still contains active species and radicals that obtain surfacemodification effects. The longer living active species generated inthis plasma are also effective for the membrane surface.

Fig. 3 shows the changes of water contact angles as a functionof treatment time, plasma power, and Ar gas flow rate for remoteplasma. The decreased contact angles represent improved surfacehydrophilicity feature of PVDF membrane. There is a possibleinterpretation: Plasma species generate active sites from theradicals released from cyclonic atmospheric pressure plasma;then these radicals could interact with the PVDF surface togenerate dangling bonds, leading to the formation of surfaceactive sites [26]. The use of conventional reactive plasmamodification promotes a considerable increase in surface hydro-philicity; however, as a result of high instability of the speciesgenerated during and after plasma modification, the hydrophilicproperties achieved by plasma modification are quickly lost. Fromthis approach, the use of a mixture of an organic monomer (CH4)can be recognized as a potential candidate that inducesfunctionalization and enhances the durability. Fig. 4 displaysthe changes of water contact angles as a function of CH4 flow rate,treatment time, plasma power, and Ar flow rate for remote plasmawith Ar/CH4 mixture. From serial contact angle analysis, thehydrophilicity of PVDF membrane is modulated by operatingparameters. The possible surface modification effect by Ar/CH4

plasma is controlled by the competition between deposition andsubstitution in the plasma state. After plasma treatment by Ar/CH4 mixture, plasma polymer layer changes the chemicalstructure of PVDF membrane surface. The mechanism is recog-nized as competitive ablation and polymerization principlerelating the ablation of materials in plasma to the deposition ofmaterials in plasma [27].

Treatment time (s)

1501209060300

Wate

r conta

ct angle

(degre

e)

50

60

70

80

90

100

110

Ar plasma-modified

unmodified PVDF

(a)

Plasma power (W)

1801501209060300

Wate

r conta

ct angle

(degre

e)

50

60

70

80

90

100

110

Ar plasma-modified

unmodified PVDF

(b)

Argon flow rate (slm)

1086420

Wa

ter

co

nta

ct a

ng

le (

de

gre

e)

50

60

70

80

90

100

110

Ar plasma-modified

unmodified PVDF

(c)

Fig. 3. Effect of plasma treatment parameters on the average contact angles of Ar

plasma-modified PVDF membranes (plasma conditions: argon flow rate, 10 slm, RF

power input, 150 W, or treatment time, 90 s, except for the varying parameter).

Methane flow rate (slm)

0.50.40.30.20.10.0

Wa

ter

co

nta

ct a

ng

le (

de

gre

e)

50

60

70

80

90

100

110

Ar/CH4 plasma-modified

unmodified PVDF

(a)

Treatment time (s)

1501209060300

Wa

ter

co

nta

ct

an

gle

(d

eg

ree

)

50

60

70

80

90

100

110

Ar/CH4 plasma-modified

unmodified PVDF

(b)

Plasma power (W)

1801501209060300

Wa

ter

co

nta

ct a

ng

le (

de

gre

e)

50

60

70

80

90

100

110

Ar/CH4 plasma-modified

unmodified PVDF

(c)

Argon flow rate (slm)

1086420

Wa

ter

co

nta

ct a

ng

le (

de

gre

e)

50

60

70

80

90

100

110

Ar/CH4 plasma-modified

unmodified PVDF

(d)

Fig. 4. Effect of plasma treatment parameters on the average contact angles of Ar/

CH4 plasma-modified PVDF membranes (plasma conditions: methane flow rate

0.2 slm, argon flow rate 10 slm, RF power input 150 W, or treatment time 90 s,

except for the varying parameter).

R.-S. Juang et al. / Journal of the Taiwan Institute of Chemical Engineers 45 (2014) 2176–2186 2181

In summary, the improved hydrophilicity of PVDF membranesis depicted by a decrease in water contact angles when operatingparameters are adjusted. On the other hand, CH4 flow rate is quiteinvariable. The higher plasma power and Ar flow rate, the moreimproved hydrophilicity of PVDF membrane is achieved. Despitethe relatively high water contact angles at longer treatment time,the plasma-modified PVDF membrane still shows improvedhydrophilicity. The following conditions for plasma modificationare applied in further studies: (1) Ar flow rate 10 slm, RF powerinput 150 W, and treatment time 90 s for remote Ar plasma and (2)CH4 flow rate 0.2 slm, Ar flow rate 10 slm, RF power input 150 W,and treatment time 90 s for remote Ar/CH4 plasma. It is found thatwater contact angles of the PVDF membrane decrease from 98̊ to71̊ and 67̊, respectively, after both types of plasma processing.

The XPS survey scans and pore analysis for unmodified andremote plasma-modified PVDF membranes are shown in Fig. 5. Theenrichment of surface with oxygen-containing functionalities is

Binding energy (eV)

278280282284286288290292294296

C-C/C-HCF

2

(a) Unmodified PVDF

Average pore diameter (μm)

0.310.290.270.250.230.210.190.170.150.130.110.090.070.050.03

Po

re s

ize

dis

trib

utio

n (

%)

0

5

10

15

20

25

30

(d) Unmodified PVDF (MWCO = 30 kDa)

Binding energy (eV)

278280282284286288290292294296

C-C/C-H

C-OCF2

(b) Ar plasma-modified PVDF

Average pore diameter (μm)

0.310.290.270.250.230.210.190.170.150.130.110.090.070.050.03

Po

re s

ize

dis

trib

utio

n (

%)

0

5

10

15

20

25

30

(e) Ar plasma-modified PVDF

Binding energy (eV)

278280282284286288290292

C-C/C-HC-O

CF2

(c) Ar/CH4 plasma-modified PVDF

Average pore diameter (μm)

0.310.290.270.250.230.210.190.170.150.130.110.090.070.050.03

Po

re s

ize

dis

trib

utio

n (

%)

0

5

10

15

20

25

30

(f) Ar/CH4 plasma-modified PVDF

Fig. 5. XPS survey scans and pore size analysis for the unmodified and plasma-modified PVDF membranes.

R.-S. Juang et al. / Journal of the Taiwan Institute of Chemical Engineers 45 (2014) 2176–21862182

evident. It suggests that more free radicals are formed duringcyclonic atmospheric-pressure plasma treatment because air takespart in post-reactions. To identify major functional groupsintroduced into PVDF membrane surfaces after cyclonic atmo-spheric-pressure plasma modification, XPS deconvolution analysisof C1s peaks was performed. In addition, the narrow scans of theregion of C1s were analyzed to show the existence of various formsof carbons (Fig. 5). As shown in Fig. 5a, the C1s spectrum ofunmodified PVDF membrane contained two five-separated peaksat 284.6 eV and 289.5 eV corresponding to C–C/C–H and C–F2,respectively [28]. The spectra of plasma-modified PVDF mem-branes by Ar and Ar/CH4 mixture (Fig. 5b and c) also showed thesetwo peaks; at the same time, an additional peak at 286.8 eVappears and rises which could be attributable to C–O groups [29].The modification to the surface causes an additional increase of286.8 eV peaks for C–O bonds in this region. As a result of plasma

treatment, the percentages of CH and C–F2 decrease relative to theunmodified membrane. This decrease is counterbalanced by anincrease in the percentage of C–O groups. These results suggestthat remote plasma treatment cleaves CH and C-F2 bonds andintroduces oxygen-containing functional groups to molecularchain of the PVDF membrane surfaces.

Atmospheric pressure plasma is the promising technique forproviding surface modification with the controllable effect and isenvironmentally friendly. Table 1 shows that the high content of O isincreased after atmospheric pressure plasma surface modification.Thiscouldbeattributedtotheincrease inthenewlyformedfunctionalpolar (C–O) groups by the atmospheric pressure plasma treatment, aswillbediscussedfromtheXPSspectraofunmodifiedandatmosphericpressure plasma -modified PVDF membranes in Fig. 5.

Fig. 5 also shows the pore size distribution before and after remoteplasma surface modifications. The remote plasma modifications

Table 1Surface elemental analysis and chemical structure of various PVDF membranes by XPS.

PVDF C (%) F (%) O (%) O/C C–C, C–H (284.6 eV) C–F2 (289.5 eV) C–O (286.8 eV)

Unmodified 53.05 43.89 3.06 0.057 0.62 0.38 0

Ar plasma 49.05 41.35 9.06 0.196 0.46 0.43 0.109

Ar/CH4 plasma 50.91 39.05 10.04 0.197 0.61 0.27 0.12

R.-S. Juang et al. / Journal of the Taiwan Institute of Chemical Engineers 45 (2014) 2176–2186 2183

inevitably change the pore sizes and the number of larger surfacepores. The mean pore diameter slightly increases from0.11 � 0.02 mm (unmodified) to 0.12 � 0.04 mm and 0.12 � 0.05 mmafter both types of remote plasma modifications, as the distribution ofpore sizes are shifted to the lower values.

4.3. Flux decline and fouling analysis in batch UF process

Fig. 6 shows the variations of UF fluxes of W/O/W solutions withtime at different applied pressures. Here, the feed contains 5 vol%of W/O emulsions. Howell and Velicangil [30] divided UF processinto three time intervals: first few seconds (a quasi-steady-state

Time, t (s)

2000160012008004000

Flu

x, J

(L m

2 h

-1)

0

50

100

150

200

250

300

350

34.5 kPa

69.0 kPa

103.4 kPa

137.9 kPa

(a) Unmodified PVDF

Stirring: 300 rpm

Feed: 5 vol% of W/O emulsion

Time, t (s)

2000160012008004000

Flu

x,J

(L m

2 h

-1)

0

100

200

300

400

500

34.5 kPa

69.0 kPa

103.4 kPa

137.9 kPa

(b) Ar plasma-modified PVDF

Stirring: 300 rpm

Feed: 5 vol% of W/O emulsion

Time, t (s)

2000160012008004000

Flu

x,J

(L m

2 h

-1)

0

100

200

300

400

500

34.5 kPa

69.0 kPa

103.4 kPa

137.9 kPa

(c) Ar/CH4 plasma-modified PVDF

Stirring: 300 rpm

Feed: 5 vol% of W/O emulsion

Fig. 6. Effects of applied pressures on time-dependent fluxes during batch UF of W/

O/W solutions using the unmodified and plasma-modified PVDF membranes.

concentration polarization layer is set up), 1 to 10 min (soluteadsorption), and long term (cake formation). The gradual fluxdecline, instead of quick, is not purely a result of concentrationpolarization, particularly for unmodified PVDF membrane (Fig. 6a);other factors such as the adsorption of W/O emulsions on PVDFmembrane surface or cake formation will play a crucial role [14].For a solution containing various sizes of macromolecular solutes(e.g., W/O emulsion droplets in this case), it is also reported thatconcentration polarization can have a strong effect on soluteretention because the solutes that are completely retained willform a kind of second or dynamic membrane [7].

Next, various filtration resistances were determined by theresistances-in-series model to clarify the roles of surface hydro-philicity changes of the UF membranes on their flux enhancementfrom a quantitative viewpoint. Fig. 7 shows the linear plots of 1/Jw1

and Jw2 against 1/DP with the modified and plasma-modified PVDFmembranes, respectively, indicating the validity of Eqs. (2) and (3)under the conditions studied. The values of Rm and Ra are thusobtained and listed in Tables 2–4, where the viscosity of thepermeate mp was measured to be 1.12 � 10�3 Pa s.

To obtain the resistance Rc, the values of n and a0 in Eq. (10)must be available. In principle, a has to be determined from theexperiments for irregular shaped solutes but can be obtained using

1/ΔP (Pa-1

)

3.5e-53.0e-52.5e-52.0e-51.5e-51.0e-55.0e-6

1/J

w1

(m

2 h

L-1

)

0.002

0.004

0.006

0.008

0.010(a) De ter mination of R

m

unmodified PVDF ( r2 = 0.992 )

Ar plas ma-modi fied ( r2 = 0.987)

Ar/C H4 plasma-modified (r2 = 0.990)

1/ΔP (P a-1

)

3.5e-53.0e-52.5e-52.0e-51.5e-51.0e-55.0e-6

1/J

w2

(m2 h

L-1

)

0.00

0.02

0.04

0.06

0.08

(b) Determin ati on of Ra

unmodi fied PVDF (r2 = 0. 991)

Ar plasma-mo difie d (r2 = 0 .988)

Ar/C H4 plasma-mo difie d (r2 = 0. 997)

Fig. 7. Determination of (a) the resistance of membrane itself (Rm) and (b) the

resistances due to solute adsorption (Ra) during batch UF of W/O/W solutions using

the unmodified and plasma-modified PVDF membranes.

Table 2Steady-state resistance in the UF of 5 vol% W/O emulsions using the unmodified PVDF membrane (average pore diameter 0.11 mm, water contact angle 98̊) at different applied

pressuresa.

DP (kPa) Rt (10�12 m�1) Rm (10�12 m�1) Ra (10�12 m�1) Rc (10�12 m�1) Rp (10�12 m�1)

34.5 11.90 0.97 (8.2%) 8.31 (69.8%) 1.73 (14.5%) 0.89 (7.5%)

69.0 15.10 0.97 (6.4%) 8.31 (55.0%) 4.30 (28.5%) 1.52 (10.1%)

103.4 18.40 0.97 (5.3%) 8.31 (45.2%) 7.07 (38.4%) 2.05 (11.1%)

137.9 26.10 0.97 (3.7%) 8.31 (31.8%) 13.80 (52.9%) 3.02 (11.6%)

a The number in the parenthesis indicates the percentage of each filtration resistance to the total resistance.

Table 3Steady-state resistance in the UF of 5 vol% W/O emulsions using the Ar plasma-modified PVDF membrane (average pore diameter 0.12 mm, water contact angle 71̊) at

different applied pressuresa.

DP (kPa) Rt (10�12 m�1) Rm (10�12 m�1) Ra (10�12 m�1) Rc (10�12 m�1) Rp (10�12 m�1)

34.5 6.43 0.71 (11.0%) 3.78 (58.9%) 1.49 (23.2%) 0.45 (6.9%)

69.0 8.72 0.71 (8.1%) 3.78 (43.3%) 3.55 (40.8%) 0.68 (7.8%)

103.4 8.95 0.71 (7.9%) 3.78 (42.2%) 3.70 (41.4%) 0.76 (8.5%)

137.9 9.21 0.71 (7.7%) 3.78 (41.0%) 3.91 (42.5%) 0.81 (8.8%)

a The number in the parenthesis indicates the percentage of each filtration resistance to the total resistance.

Table 4Steady-state resistance in the UF of 5 vol% W/O emulsions using the Ar/CH4 plasma-modified PVDF membrane (average pore diameter 0.12 mm, water contact angle 67̊) at

different applied pressuresa.

DP (kPa) Rt (10�12 m�1) Rm (10�12 m�1) Ra (10�12 m�1) Rc (10�12 m�1) Rp (10�12 m�1)

34.5 4.28 0.69 (16.1%) 1.84 (43.0%) 1.21 (28.3%) 0.54 (12.6%)

69.0 5.69 0.69 (12.1%) 1.84 (32.3%) 2.40 (42.2%) 0.76 (13.4%)

103.4 5.93 0.69 (11.6%) 1.84 (31.0%) 2.53 (42.7%) 0.87 (14.7%)

137.9 6.33 0.69 (10.9%) 1.84 (29.1%) 2.72 (43.0%) 1.08 (17.0%)

a The number in the parenthesis indicates the percentage of each filtration resistance to the total resistance.

log ΔP

5.25.15.04.94.84.74.64.5

log

α

11.6

11.7

11.8

11.9

12.0

12.1

12.2

slope = 0.59

correlation coefficient = 0.982

Unmodified PVDFStirring: 300 rpmFeed: 5 vol% of W/O emulsion

Fig. 8. Determination of compressibility index of the cake during batch UF of W/O/

W solutions using PVDF membrane.

R.-S. Juang et al. / Journal of the Taiwan Institute of Chemical Engineers 45 (2014) 2176–21862184

the Carman–Kozeny equation for spherical particles such as latexparticles and bacterial cells [22,31]. As usual, the value of n can bedetermined from batch filtration experiments. In general, typicalfiltration has three regions where pore blocking, cake filtration,and cake filtration with compression, occur consecutively [32]. Inthe first region, the deposition of particles blocking the entry to apore or inside membrane pore causes a sharp increase in slope.This is followed by a minimum linear slope where particles depositon the membrane surface as a cake layer. The second region allowsus to determine the so-called modified fouling index [33]. The cakeadds additional resistance Rc to the resistance of membrane Rm,and flux decline under constant pressure filtration can bedescribed as follows [30,31]:

t

V¼

m pRm

ADPþ

m paC

2A2DP

� �V (11)

where V is the cumulative permeate volume over time t. On thebasis of the experimental data shown in Fig. 6, the plot of (t/V)versus V should obtain a from the slope of the resulting straightline. It is noted that the linear dependence is satisfactory under theconditions studied (correlation coefficient r2 > 0.9852). Thecompressibility index of the cake n during batch UF of the presentW/O/W solutions with PVDF membrane is therefore obtained to be0.59, as shown in Fig. 8. The value of a0 is 1.07 � 109 m/kg/Pa0.59

accordingly.In fact, Falahati and Tremblay [34] have used a tubular UF

ceramic membrane with a 300-kDa MWCO to separate highlyconcentrated and unstable oil-in-water (O/W) emulsions from oilywater with an oil content of 50–55 vol% at 30–50 8C. They reporteda cake compressibility index of 0.498–0.687. On the other hand, thecake layer formed has a compressibility index of 0.80 in cross-flow

MF of the fermentation broth of B. subtilis [30] and of 0.45–1.0 inbatch MF of various microbial suspensions [35].

4.4. Contribution of various filtration resistances

Once the steady-state resistance Rc is determined, the resis-tance Rp is obtained because the total resistance Rt can becalculated from Eq. (1). For comparison, Tables 2–4 compile theabove steady-state resistances in batch UF of W/O/W solutionswith the unmodified and plasma-modified PVDF membranes,respectively, at different applied pressures. As expected, the totalresistance Rt significantly decreases when PVDF membrane is

R.-S. Juang et al. / Journal of the Taiwan Institute of Chemical Engineers 45 (2014) 2176–2186 2185

modified with Ar or Ar/CH4 plasma. Such a drop primarily resultsfrom the decrease of Ra and Rc, irrespective of the use of remote Aror Ar/CH4 plasma [36]. This means that the combination of soluteadsorption (Ra) and cake formation (Rc) always plays a crucial rolein UF flux decline before and after the membranes are plasma-modified. Under the DP conditions studied, e.g., sum of thepercentages of Ra and Rc is in the range of 84.3–84.7%, 82.1–83.5%,and 71.3–72.1% using the unmodified, Ar plasma-, and Ar/CH4

plasma-modified membranes, respectively. The relatively largedecrease in Ra using Ar/CH4 plasma-modified membrane (Table 3),compared to with Ar plasma-modified one (Table 2), is probablydue to the less –CF2 group introduced during modification processas shown in Table 1 [28,29].

Masciola et al. [37] have predicted the flux in tubular UF of oil-in-water (O/W) emulsions with 120-kDa PVDF membrane at 30 8Cusing the resistance-in-series model. For the feed containing 0.85–25.5% oil, on the average, they found that solute adsorptionresistance is 63% of the total filtration resistance. The significantsuppression of bovine serum albumin adsorption on polypropyl-ene (PP) MF membrane was reported after the membrane wastreated by low-temperature NH3 plasma [36]. In submergedmembrane bioreactor system with PP microporous membrane, thereversible fouling resistance due to adsorbed layer decreased afterO2 or air plasma treatment; however, such a reversible foulingresistance (due to particle polarization and cake layer) was weaklydepended on membrane surface chemistry only [38,39].

The present analysis clearly indicates that remote plasmasurface modification of hydrophobic membranes such as PVDFenhances UF flux by reducing the resistances mainly resulting fromfoulant adsorption and cake formation. On the other hand, the keyrole of Ra and Rc on flux decline implies that flux cannot beenhanced by hydrodynamic methods only such as the use of cross-flow UF mode. To modify surface characteristics of the membranesor to select other types of membranes will be more practical.

5. Conclusions

The ability of using Ar and Ar/CH4 plasma-modified 30-kDaPVDF membranes for enhancing UF flux in phase separation of W/Oemulsions, along with the underlying mechanisms for fluxenhancement, was studied. Hydrophobic PVDF membrane surfacewas modified by remote cyclonic atmospheric-pressure plasma.The polar functional groups generated during plasma processingon PVDF membrane caused a decrease in water contact angle andan increase in surface energy. XPS results revealed that the surfaceconcentration of carbon decreased and of oxygen increased for theplasma-modified membranes compared to the virgin membrane.Chemical and surface morphological changes made on the surfaceof PVDF membranes after plasma treatment led to an increase inhydrophilic properties.

UF flux was enhanced with remote plasma-modified mem-branes under the conditions studied. Resistance-in-series analysisshowed that the total filtration resistance reduced by 46–65% and64–76% under steady state when PVDF membrane was modified byAr and Ar/CH4 plasmas, respectively. Such a drop mainly resultedfrom the decreased resistance due to solute adsorption and cakelayer. However, sum of these two resistances still contributed84.3–84.7%, 82.1–83.5%, and 71.3–72.1% of the total resistancewhen using unmodified, Ar plasma-, and Ar/CH4 plasma-modifiedmembranes, respectively. The key roles of solute adsorption andcake formation on flux decline implied that the flux could not beefficiently enhanced by hydrodynamic techniques only; surfacemodification of the membranes may be more practical. The presentresults demonstrated the promising potential of remote cyclonicatmospheric-pressure plasma as a surface modification techniquefor porous polymer membranes.

Acknowledgement

Financial support for this work by the National Science Council,R.O.C., under Grant number NSC99-2221-E-155-018-MY2 isgratefully appreciated.

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