Desalination 330 (2013) 49–60

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Desalination

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Study on the interfacial bonding state and fouling phenomena ofpolyvinylidene fluoride matrix-reinforced hollow fiber membranesduring microfiltration

Xuliang Zhang a, Changfa Xiao a,⁎, Xiaoyu Hu b, Xin Jin c, Qianqian Bai a

a State Key Laboratory of Hollow Fiber Membrane Materials and Processes, Tianjin Polytechnic University, Tianjin 300387, Chinab Tianjin Motian Membrane Technology Co., Ltd., Tianjin 300457, Chinac School of Materials Science and Engineering, Tianjin Polytechnic University, Tianjin 300387, China

H I G H L I G H T S G R A P H I C A L A B S T R A C T

• The homogeneous-reinforced (HMR)PVDF membranes were firstly preparedthrough the dry–wet spinning process.

• The interfacial bonding state is detectedby the dynamic mechanical analysis in-directly.

• The membranes (both HMR and HTR)fouling of ink solution was attributedto initial pore blocking followed by cakeformation.

• HMR membranes have a favorable in-terface bonding state than HTR mem-branes.

⁎ Corresponding author at: State Key Laboratory of Hoand Processes, Tianjin Polytechnic University, No. 399, WDistrict, Tianjin 300387, China. Tel.: +86 22 83955138.

E-mail address: xiaotjpu@163.com (C. Xiao).

0011-9164/$ – see front matter. Crown Copyright © 2013http://dx.doi.org/10.1016/j.desal.2013.09.022

a b s t r a c t

a r t i c l e i n f o

Article history:Received 28 March 2013Received in revised form 22 September 2013Accepted 25 September 2013Available online 18 October 2013

Keywords:PVDFPANDMAReinforcedFouling resistance

Reinforced polyvinylidene fluoride (PVDF) and polyacrylonitrile (PAN) hollow fiber membranes were preparedthrough dry–wet spinning process and included PVDF or PAN polymer solutions (coating layer) andmatrix PVDFmembrane. The performance of the membranes varies with different polymer solutions and is characterized interms of ink solution and activated sludge flux, dynamic mechanical analysis (DMA), mean pore size, a mechan-ical strength test, and morphology observations by a scanning electron microscope (SEM). The results of thisstudy indicate that PAN coating layer is easy to peel off from the matrix membrane. Interfacial bonding strengthis also detected bydynamicmechanical analysis indirectly. The reinforcedmembranes showa narrower pore sizedistribution and smaller mean pore size than the matrix membrane. The elongation at break increases muchmore and the membranes are endowed with better flexibility performance. The homogeneous-reinforced(HMR) and heterogeneous-reinforced (HTR)membranes have a lower rate of flux decline comparedwithmatrixmembrane during ink solution filtration. The main types of all membranes fouling in microfiltration of ink solu-tion were attributed to initial pore blocking followed by cake formation. Then, in the activated sludge solution,the reinforced membranes have stable efficiency in antifouling property for the real MBR system.

Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved.

llow Fiber Membrane Materialsest of Binghshui Road, Xiqing

Published by Elsevier B.V. All rights

1. Introduction

In membrane bioreactor (MBR) system, high mechanical strengthand antifouling property are very important for application of hollow

reserved.

50 X. Zhang et al. / Desalination 330 (2013) 49–60

fiber membrane. Nowadays, the most popularly used hollow fibermembranes are prepared by wet-spinning method [1]. However, themembranes possess low tensile strength and are easily damaged inthe state of serviceability.

A braid-reinforced composite hollow fiber membrane comprising areinforcing material of a tubular braid and a polymer solution coatedon the surface of the tubular braid has been further studied to solvethese problems [2]. Such braid-reinforcedmembranes possess highme-chanical strength due to the high strength of the reinforced fiber; how-ever, the coating layer can be easily peeled off from the braid in theservice because the coating layer and reinforced fiber were not thesame compound and resulted in incompatible thermodynamics of theheterogeneous-reinforced (HTR) membranes. Such problems restrictedthe applications of these HTR membranes in engineering practice. Therequirement of the interfacial bonding strength between the coatinglayer and the reinforced fiber is higher than the tensile strength due tothe techniques of aeration and antiwash in the filtration process. Theinterfacial bonding state between the fiber and matrix plays a majorrole in themechanical behavior of a composite material. Good interfacebonding state that ensures the load transfer from the matrix to thereinforcement is a primary requirement for the effective use of rein-forcement properties [3].

Based on the results of the traditional reinforcement to the hollowfiber membranes, the membranes were prepared by the homogenousreinforced (HMR) method which can overcome those defects of similarHTR (fiber- or braid-reinforced)membranes. Homogenous reinforced isdescribed as the samematerial of the coating layer and reinforcedmem-brane in the coating process [4]. A fundamental understanding of theinterfacial region and a quantitative characterization of the level of in-terfacial bonding state are necessary [3]. In fact, both the transverseand the longitudinal strengths of a composite material depend heavilyon the strength of the fiber and matrix bond. Many scholars havedone researches on the characterization of the interfacial bondingstrength for the composite materials. These tests can be divided intolarge specimen macromechanical tests such as interlaminar shearstrength (ILSS) [5], and single fibermicromechanical tests such as singlefiber pull-out test [6]. However, there are many difficulties to do thesame tests for the HMR membranes. So it is necessary to characterizethe interfacial bonding strength of suchHMRmembranes indirectly. Dy-namic mechanical analysis (DMA) is a sensitive and versatile thermalanalysis technique, which has the potential to accurately characterizethe interfacial properties between the matrix and fiber for the compos-ite materials [7]. The interface contribution to the DMA results can beseparated from thematrix and fiber contributions due to their differentdegrees of crystallinity and tensile modulus.

It is important to characterize the interfacial bonding state for theHMR membrane, which relates to the service life of the membrane inthe MBR system. It is also necessary to study the antifouling propertyin the applications. The advantages of microfiltration include superiorwater quality, easier control of operation, lower maintenance, andreduced sludge production in wastewater treatment. However, themembrane fouling inducing the rapid decline in the water flux andthe irreversible loss of productivity over time restricted its application[8]. Flux decline in membrane filtration is the result of the increase inmembrane resistance and the development of another resistancelayer, which can be elucidated in terms of pore blockage and cakeformation, respectively [9,10]. The effects of each fouling mechanismson the flux decline depend on factors such as membrane pore size,feed solution, membrane material, and operating conditions. Substan-tial experimental effort has been made to investigate the effects of var-ious parameters on flux decline and the mechanisms of membranefouling [11]. Particle sizes of sludge flocs, colloids and solutes in mixedliquor may strongly affect foulingmechanisms in a membrane filtrationsystem. In the membrane filtration process, if foulants are comparablewith themembrane pores (i.e., colloids), or smaller than themembranepores (i.e., solutes), adsorption on pore wall and pore-blocking will

easily occur. However, if foulants (i.e., sludge flocs and colloids) aremuch larger than the membrane pores, they tend to form a cake layeron the membrane surface [12,13]. The main difference between the ma-trix membrane and the reinforced membranes is the mean pore size;therefore, it is necessary to study the membrane-fouling factors that arecaused by the contaminants, in which particle size is between the meanpore size of matrix and reinforced membranes. In fact, the complex na-ture ofmembrane foulants and activated sludge has restricted us to clear-ly investigate the MBR fouling. A small amount of the sludge flocs andcolloids definitely exists in the MBR system, and their size distributionis between mean pore size of the matrix membrane and the reinforcedmembranes. Carbon ink solution is a colloidal solution and usually con-sists of carbon particles, a polymeric binder and a solvent [14] and allthese dispersoids are dispersed in water as colloidal particles. Carbonink solution is simpler and more stable compared to the activated sludgein the MBR system, and membrane resistance can be better analyzed toprovide guidance for the reason formembrane fouling in theMBR system.

In this study, the HMR and HTR hollow fiber membranes were pre-pared through dry–wet spinning process and included PVDF or PANcasting solution (coating layer) and the matrix membrane (preparationof porous PVDF hollow fiber membrane through melt spinning andstretching process). The effects of the polymer concentrations in thepolymer solutions on the reinforced membrane pore distribution werestudied. The type of fouling mechanisms prevalent in membranemicrofiltration treating ink solution was studied and themembrane foul-ing resistancedistribution after thefiltrationprocesswas examined. Then,themembrane fouling and cleaning processes for the activated sludge so-lution that is obtained from the real MBR system were also studied.

2. Experimental

2.1. Materials

PVDF (W no. 1300 powders, Tm = 170 °C) was purchased fromKureha Chemical Industrial Co. Ltd (Tokyo, Japan) and PAN (molecularweight of 50,000)was purchased from theQilu Petrochemical acrylic fac-tory. N, N-dimethylacetimide (DMAc, N99%) and Polyvinylpyrrolidone(PVP, K30, Mw = 30,000) were obtained from Tiantai Fine ChemicalCo. Ltd (Tianjin, China). Tween 80 (Tw-80) and sodium hypochlorite(NaClO) solution were purchased from Tianjin Fengchuan ChemicalReagent Science and Technology Co., Ltd. (Tianjin, China). The PVDFmatrix membrane (the PVDF membrane prepared by melting spinning)was supplied by Tianjin Motian Membrane Engineering & TechnologyCo., Ltd (Tianjin, China).

2.2. Preparation of HMR and HTR PVDF hollow fiber membranes

The PVDFmatrix reinforced hollow fibermembranes were preparedby using the dry–wet spinningmethod. Fig. 1 shows the spinning appa-ratus. According to the described method, the PVDF matrix membranewas coated with the polymer solutions (PVDF and PAN polymer solu-tions) and guided through a precipitation bath, in which the polymersolutions were converted into a microporous layer. The polymersolutions were prepared by the blending of different compositionsconsisting of PVDF or PAN, PVP K30, Tw-80, and DMAc (Table 1),under constant mechanical stirring in a three-necked round-bottomflask for 4 h at 70 °C. The dope solution temperature was 60 °C. Theextra coagulation was water and the temperature was 20 °C. The airgap was 10 cm and the take up speed was 15 cm/(r/min). All themembranes were prepared under an environmental humidity of 60%and at a temperature of 20 °C.

The HMRmembranes with the different PVDF concentrations, i.e., 6,10, 14 and 18 weight % (wt.%) were labeled M6, M10, M14, and M18,respectively. The HTR membrane with 10 wt.% PAN concentration waslabeled MN. The PVDF matrix membrane was designated as M0. Beforethe scanning electron microscope (SEM) tests, we put the resulting

Matrix membrane

Coating solution

Co-extrusion device

Coagulation bath

Matrix membraneInterface layer Coating layer

The cross-section model of theHMR membrane

WaterCirculating

Fig. 1. Schematic diagram describing the reinforced membrane induced by the dry–wet spinning process.

51X. Zhang et al. / Desalination 330 (2013) 49–60

membranes in glycerol water solutions (three parts glycerol to twoparts water) and then dried them in the air, so as to retain the porousstructure.

2.3. Membrane characterizations

2.3.1. Morphology examinationThe morphology of the membranes was observed by using SEM

(Quanta 800, FEI, Netherlands). The samples were frozen in liquid N2,followed by fracturing to expose their cross-sectional areas. Thereafter,they were sputtered with gold and recorded through SEM.

2.3.2. Membrane permeability

2.3.2.1. Carbon ink solution filtration. The filtration experiments werethen carried out by using carbon ink solution. The ink solution flux(ISF) of the membranes was measured with a 0.5 g/L ink solution andcalculated by using Eq. (1). The particle distribution of the ink solutionis shown in Fig. 2.

Each membrane was initially pressurized for 20 min at 0.15 MPa.Then pure water flux (PWF), Jw0, through the membranes was mea-sured by using a cross out-feeding system at a transmembrane pressure

Table 1Spinning parameters of reinforced membranes with different dope compositions.

Preparedmembrane

Polymerconcentration

PVP(wt.%)

Tw-80(wt.%)

DMAc(wt.%)

Matrixmembrane

M0 0 0 0 0 PVDF hollowfiber membraneM6 PVDF (wt.%) 6 7 3 84

M10 10 7 3 80M14 14 7 3 76M18 18 7 3 72MN PAN (wt.%) 10 7 3 80

of 0.1 MPa and at a linear velocity of 1.25 l/min. Then, the filtrationexperiments were carried out by using the ink solution. Fig. 3 showsthe PWF and ISF measurement apparatus. The membrane modulecontained two hollow fiber membranes with an effective length of19–20 cm. PWF and ISF were calculated as follows:

J ¼ VS� t

ð1Þ

where J is the PWF or ISF (m3·m−2·s−1),V is the quantity of the perme-ate (m3), S is the membrane area (m2), and t is the testing time (s).

Fig. 2. The particle distribution of ink solutions.

Manometer

Membrane model

Pump

Permeate

Feed tank

Vane

Fig. 3. Schematic diagram for filtration experiments.

52 X. Zhang et al. / Desalination 330 (2013) 49–60

The rejection was calculated by using Eq. (2):

R %ð Þ ¼ 1−Cp

C f

!� 100% ð2Þ

whereCp andCf are the concentrations at permeate and feed, respectively.After filtration of the ink solution, the membranes were removed

from the ink cake layer by using a sponge and washed with de-ionized water, then immersed in the de-ionized water for 30 min.Then, pure water flux of the cleaned membranes, Jw1, was measuredby using Eq. (1). Flux recovery ratio (FRR) was calculated by using thefollowing Eq. (3) [15]:

FRR %ð Þ ¼ Jw1

Jw0

� �� 100 ð3Þ

2.3.2.2. Activated sludge solution filtration. Activated sludgewas obtainedfrom the real MBR system. The concentration of the mixed liquorsuspended solids (MLSS) was approximately 3500 mg/L and the influ-ent chemical oxygen demand (COD) concentration of the MBR was300 ± 50 mg/L. The sludge supernatantwas prepared through the cen-trifugal method. The COD concentrations of the water samples wereanalyzed according to standard methods [16].

The testmethod of activated sludgeflux is the sameas the carbon inksolution filtration experiments. The filtration process includes threesteps: (1) First: purewater fluxmeasurement, Js1; activated sludge solu-tion filtration; 30 min of physical cleaning (the fouling membraneswere removed the cake layer by using a sponge and washed with de-ionized water, then immersed in the de-ionized water for 30 min.).(2) Second: pure water flux measurement, Js2; activated sludge solutionfiltration; 30 min of chemical cleaning (the fouling membranes wereimmersed in the 0.5 wt.% NaClO solution for 30 min, and then washedwith the de-ionized water). (3) Third: pure water flux measurement,Js3, followed by activated sludge solution filtration.

After physical cleaning, the flux recovery ratio (FRR) was calculatedby using the following Eq. (4):

FRR %ð Þ ¼ Js2Js1

� �� 100 ð4Þ

After chemical cleaning, the flux recovery ratio (FRR) was calculatedby using the following Eq. (5):

FRR %ð Þ ¼ Js3Js2

� �� 100 ð5Þ

2.3.3. Dynamic mechanical analysis (DMA)The mechanical properties were determined by using DMA

(DMA242C, Netzsch, Germany). The samples were measured at an op-erating frequency of 1 Hz and a heating rate of 5 °C/min. The sampleswere evaluated in the temperature range from−80 to 50 °C.

2.3.4. Tensile break strength measurementsThe tensile strength and elongation at break of the hollow fiber

membranes were determined at room temperature by using a YG061Felectronic single yarn tensile tester (Shandong, China). The tensile ratewas 10 mm/min. Five runs were performed for each specimen.

2.3.5. Determination of pore size and its distributionThepore size and its distribution of each samplewere determined by

using the capillary flow porometer (Porous Materials Inc., USA), andvalues were calculated from the pressure of the gas flow.

2.4. Membrane-fouling analysis

The permeation flux of pure water across a clean membrane can bedescribed by Darcy's law as:

J ¼ ΔpμRm

ð6Þ

where J (m3m−2 s−1) is the permeation flux, Δp (Pa) the transmem-brane pressure (TMP), μ (Pa s) the absolute viscosity of the water, andRm (m−1) the clean membrane resistance.

For suspension filtration, flux decline is a result of the increase ofmembrane resistance and themechanisms ofmembrane foulingusuallyinclude pore blocking, concentration polarization and cake formation.So the degree of membrane fouling was quantitatively calculated,using the resistance in the following model [17]:

J ¼ ΔpμRt

ð7Þ

Rt ¼ Rm þ Rp þ Rc ð8Þ

where Rt is the total filtration resistance (m−1), Rm is the membraneresistance (m−1), Rp (m−1) is the resistance due to pore blocking, andRc (m−1) is the resistance arising from cake formation.

The intrinsic membrane resistance (Rm), the fouling resistancecaused by pore plugging and irreversible adsorption of foulants ontothe membrane pore wall or surface (Rp) and the cake resistance bycake layer formed on the membrane surface (Rc) can be calculated byusing the following equations [18]:

Rc ¼Δpμ Js

−Rm−Rp ð9Þ

Rp ¼ Δpμ Jw1

−Rm ð10Þ

Rc ¼Δpμ Js

−Rm−Rp ð11Þ

where Js is the flux of ink solution at steady state, Jw0 is the initial waterflux and Jw1 is the final water flux after removing the cake layer byflushing with de-ionized water.

53X. Zhang et al. / Desalination 330 (2013) 49–60

In the microfiltration of ink solution, pore blocking and cake forma-tion simultaneously existed in the membrane-fouling process. Such aprocess can be described by the standard pore blocking filtrationmodel and cake filtration law as follows. Then by fitting the models inEqs. (14) or (15) to the experimental history data of permeation flux,the predominant membrane-fouling mechanism at different stages ofthe microfiltration operation may be identified if the experimentaldata can be described by one of these models [19].

Pore-blocking resistance:

J ¼ J0e−Kpt ð12Þ

Cake resistance:

J2 ¼ J201þ J20Kct

ð13Þ

Fig. 4. Cross-sectional SEM morphology of hollow

Eqs. (11) and (12) can be rewritten in a linearized form as:Pore-blocking resistance:

ln J ¼ −Kpt þ ln J0 ð14Þ

Cake resistance:

1J2

¼ 1J20

þ Kct ð15Þ

where Kp, and Kc are system parameters relating to pore-blocking resis-tance, and cake formation resistance, respectively.

3. Results and discussion

3.1. Morphologies of PVDF hollow fiber membranes

The SEM images of the matrix membrane (M0), HMR membrane(M10), and HTR membrane (MN) are shown in Fig. 4. The matrix

fiber membranes (a): M0; (b): M10; (c) MN.

54 X. Zhang et al. / Desalination 330 (2013) 49–60

membrane is a kind of homogeneous membrane and there is no skinlayer at the cross section near the outer surface as shown in Fig. 4(a2).After modification, the PVDF and PAN coating layers can be clearly ob-served on the outer surface of the matrix membrane. The outermostlayer is the PVDF coating layer and the innermost layer is the PVDFma-trix membrane. The middle layer between the PVDF coating layer andthe PVDF matrix membrane are the interface layers, which endow theHMR membranes with high interfacial bonding as shown in Fig. 4(b2).Also, the coating layer prepared through the wet phase separation hasa denser pore structure compared with the matrix membrane. M10membrane shows typical finger-like pore structure near the outer sur-face. However, there were obvious changes in the HTR membrane(MN) after fracturing in the liquid N2 and then recorded through SEM.As can be seen in Fig. 4(c1), the PAN coating layer is easy to peel offfrom the matrix membrane and the interface structure is clearer thanM10, which makes the HTR membrane have bad interfacial bondingstrength. Similar to the PVDF coating layer, PAN coating layer has adense sponge-like and long finger-like pore structures, and the latterpore structure endows the membranes with good permeability in thefiltration process.

Fig. 5 shows the SEM results of the outer surfaces of the hollow fibermembranes. As can be seen, the matrix membrane (M0) has a rougherouter surface within obvious big pores, which was prepared by meltspinning and stretching process. The outer surface of the HMR mem-brane (M10) and HTR membrane (MN) appear denser and smootherthan those of M0.

Fig. 5. Outer surface SEM morphology of hollow fiber membranes (a): M0; (b): M10;(c) MN.

3.2. Tensile strength of the hollow fiber membranes

Fig. 6 shows the results of the tensile strength and elongationat break of the reinforced membranes. The tensile strength of thereinforcedmembranes, except for that of theM10membrane, decreasesslightly compared with the matrix membrane. As we know, the tensilestrength of the coating layer (PVDF or PAN) is smaller than the matrixmembrane. The erosion effect of the polymer solutions may havesome bad influence on the matrix surface, which provides the mainlytensile strength of the membrane. The main purpose of the experimentis to improve the tensile strength of the PVDF or PAN membrane pre-pared through the wet spinning method. The tensile strength of thereinforced membranes may have reduced compared with the matrix,butmore over than the conventional PVDF [20] or PAN [21]membranesprepared through the wet-spinningmethod. A low tensile strength in aPVDF or PAN membrane may easily cause fiber breakage in the state ofserviceability. The PVDF hollow fiber membranes prepared by the HMRmethod have highermechanical properties rather than those by thewetphase inversion method. The tensile strength of the HMR PVDF mem-branes is nearly 10 MPa, which is adequate for MBR application. Theelongation at break increased much more, from 50% (M0) to 99%(M14). The elongation at break then decreases as the PVDF concentra-tion increases at 18%. When the PVDF concentration increases to acertain level, the polymer gradually shows the unique elastic nature,and the rigidity of the molecular chains increases to some extent. Theelongation at break of all the reinforced membranes including MNmembrane increases much more than the matrix membrane, and thehollow fiber membranes are endowed with better flexibility perfor-mance, which is in accord with the DMA results.

Fig. 7 shows the significant differences in morphology between theHMR membrane (M10) and the HTR membrane (MN) after the tensilestrength test. It is obviously found that the PAN coating layer had beeneasy to peel off from the matrix membrane after the tensile strengthtest. However, the HMR membrane (M10) remains as a whole afterthe test, which is constituted of PVDF coating layer and matrix mem-brane. The HMR membrane shows higher bonding strength than theHTR membrane in the drawing process, which can indirectly improvethe service life of the membrane in the practical operation.

3.3. Dynamic mechanical analysis

Fig. 8 presents the results of DMA measurement on the matrixmembrane and reinforced membranes at 1 Hz. DMA is a technique inwhich an oscillatory force at a set frequency of the sample is appliedand reports change in stiffness and damping. It was used to assess theinterfacial adhesion between the coating layer and matrix membrane.Estimation of DMA will also enable us to quantify the coating layer/matrix interfacial bonding [22]. This approach can be used to locatethe glass transition temperature of the material, as well as to identifytransitions corresponding to other molecular motions. Fig. 4 shows

Fig. 6. The mechanical properties of hollow fiber membranes.Fig. 8. DMA thermograms of the hollow fiber membranes.

55X. Zhang et al. / Desalination 330 (2013) 49–60

the differences existing in the interface obviously between the coatinglayer (PVDF coating layer) and the matrix membrane. The aggregationstructures of the coating layer and thematrixmembrane have great dif-ferences because of the different preparation methods, although therawmaterials for them are the same. So the characteristics of reinforcedPVDF membranes would have some changes due to the interfacialbonding and combining effects in the DMA test process. The dynamicmechanical relaxation spectrum of normal PVDF presents up to four dif-ferent relaxations. The two major relaxations: β relaxation (−40 °C)

Fig. 7. Photos of the membranes (M10 and MN) after the tensile strength test.

corresponds to glass transition and hence to Brownian micromotion inthe backbone in the amorphous zones and α relaxation (+100 °C) isassigned to the liberation of polymer chains in the crystalline regions[23]. The tan δ curve in Fig. 8 and Table 2 shows a peak at about −12 °C for the matrix membrane, which relates to the β relaxation andcorresponds to glass transition temperature (Tg) of PVDF. The shift ofthe tan δ peak to a higher temperature than that of the normal PVDFmay be ascribed to hindered motion of polymer chains with goodPVDF crystalline structure by themelt-spinning and stretchingmethod.

After coating with PVDF or PAN polymer solutions, the Tg of thema-trixmembrane is lower than the reinforced PVDFmembranes as shownin Fig. 8 and Table 2. The peaks of Tg for the PVDF coating layer that wasprepared by thewet methodmay be covered by the curve of thematrixmembrane and cannot show clearly during the process of temperaturerise. As shown in Fig. 8, a shoulder peak can be seen at nearly −34.5 °C for M18 membrane. Therefore, properties of Tg observed arenot appreciably sensitive to variations of the coating layer [22]. Thepresence of a high temperature tan δ peak (Tg) was assigned to theglass transition of regions containing chains of reduced mobility [24].During deformation, good interfacial bonding strength will be easy toresult in the entanglement of PVDF chains between the coating layerand the matrix membrane, which would bind the movement of thePVDF main chains to some extent. Poor interfacial bonding strength,on the contrary, of the matrix membrane and the coating layer willshow their own Tg, respectively, such as M18 membrane, so that theglass transition peaks of PVDF change to a high temperature and the in-terfacial bonding strength increases with an increase in Tg. The differ-ences of Tg compared with M10 and M14 to M0 membrane havingsome significant changes are shown in Table 2. According to the abovetheory, M10 and M14 membranes possess better interfacial bondingstate than M18 and MN membrane. The erosion effect of the low poly-mer solutions (6%PVDF) is more remarkable than that of the high poly-mer solution, although it results in some bad influence on the matrixmembrane structure; it still makes the Tg of PVDF to increase a little.

Table 2The glass transition temperature and its differences of the hollow fiber membranes.

Membrane ID PVDFTg (°C)

Tg difference(°C)(Tgi-Tg0,i = 6,10,14,18,N)

E′(MPa)at 25 °C

M0 −12.3 0 296.7M6 −11.7 0.6 190.3M10 −9.9 2.4 241.9M14 −9.1 3.2 303.5M18 −12.6 −0.3 312.6MN −11.8 0.5 277.8

56 X. Zhang et al. / Desalination 330 (2013) 49–60

The apparent viscosity of the polymer solutions containing 18 wt.%PVDF is obviously greater than that of the low PVDF concentration,resulting in poor infiltration between the polymer solutions andthe matrix membrane [4]. Also, favorable infiltration can improveon the interfacial bonding strength greatly. So the change rates ofTg of M18 are lower than M10 and M14 membranes. PVDF and PANare thermodynamically partially compatible polymers [21], and noobvious diffusion phenomenon occurs at the interface betweenthe PAN coating layer and the PVDF matrix membrane. So the Tg ofPVDF in the MN membrane just has a little change compared withthe M0 membrane.

Storage modulus is a measure of the maximum energy stored inthe material during one cycle of oscillation. It also gives an idea ofstiffness behavior and load bearing capability of the composite mate-rial [25]. In this case, the storage modulus of the reinforced mem-branes at 25 °C is lower than that of the matrix membrane, asshown in Table 2, but the results are contrary for M18 membrane.In the glassy region, the polymer chains are in a frozen state andhighly immobile which results in high storage modulus. As the tem-perature increases, the polymer chains becomemoremobile and losetheir close packing arrangement in the rubbery region. Coating with10 wt.% PVDF or 10 wt.% PAN on thematrix membrane surface, how-ever, results in a low decrease of stiffness of matrix membrane andhence low storage modulus, endowing the reinforced membranewith more flexibility and ductility properties during the techniquesof aeration and antiwash process.

Fig. 9. Pore size and distribution of the mem

3.4. Determination of pore size and distribution

Fig. 9 shows the pore size and distribution of the membranes. Thematrix membrane prepared through the melt-spinning and stretchingmethod shows a wider pore size distribution than M10, M14, and MNmembranes. It is difficult to possess small membrane pores and a nar-row pore size distribution in the melt-spinning and stretching process,so nearly 50% of the membrane pores are larger than 0.2 μm and somebig pores are nearly 1 μm. Such situation not onlymakes themembranewith low rejection in the filtration process, but also results in severemembrane fouling. The contaminated membrane will have a minorflux recovery rate in the backwash process because of the loosesponge-like pore structure. After coating with PVDF and PAN polymersolutions, themean pore size of the HMR and HTRmembranes is small-er than the matrix membrane as shown in Table 3. The coating layerthickness and porosity of the HMR membranes decrease with an in-crease in PVDF concentration. HTR membrane (MN) shows a thinnercoating layer and lower porosity compared with M10 membrane inthe same polymer solution. MN membrane seems denser than thatof M10. The smaller mean pore size changes the membrane frommicrofiltration levels to ultrafiltration levels in fact. Pore size distribu-tions of M10 and M14 are narrower than that of M0, and the scope ofthe pore size distribution of the membranes has become even moreconcentrated with an increase in the polymer solution. A number ofmembrane pores range from 0.08 to 0.2 μm and are more than 95% forM10, and for M14, 90% of membrane pores in the range from 0.08 to

branes (a: M0; b: M10; c: M14; d: MN).

Table 3Characterization of PVDF hollow fiber membranes.

Membrane ID M0 M6 M10 M14 M18 MN

ODa, IDb/mm 1.214, 0.729 1.410, 0.839 1.200, 0.721 1.237, 0.707 1.234, 0.716 1.234, 0.715Wall thickness(mm) 0.235 ± 0.009 0.283 ± 0.007 0.258 ± 0.024 0.281 ± 0.017 0.277 ± 0.023 0.267 ± 0.018Coating layer thickness(mm) 0 0.035 ± 0.003 0.040 ± 0.002 0.051 ± 0.005 0.068 ± 0.005 0.043 ± 0.004Porosity (%) 69.405 59.540 60.909 51.323 43.787 54.145Mean pore size (μm) 0.227 0.119 0.101 0.085 0.084 0.095

a Outer diameter.b Inner diameter.

57X. Zhang et al. / Desalination 330 (2013) 49–60

0.1 μm. Smaller pore size distribution is in favor of improved filteringaccuracy. HTR membrane (MN) has a denser pore structure than HMRmembrane (M10) at the same concentration. The results in Fig. 9(b, d)and Table 3 show that the mean pore size of MN membrane is smallerthan M10, and a number of membrane pores range from 0.08 to0.1 μm which is 63% for MN and 47% for M10.

3.5. Carbon ink solution filtration performance

The carbon ink solutionwasfilteredwith themembranes to evaluatethe filtration performance of membranes. Fig. 2 shows the distributionof particle size of the ink solution. As shown in Fig. 10(a), the steadyISF decreases 90.1%, 66.4%, 37.4%, 33.5%, 11.9% and 46.2% for M0, M6,M10, M14, M18 and MN membranes compared with the initial waterflux, respectively. The rates of the matrix membrane flux decay are

Fig. 10. Ink solution flux and rejection of the membranes depend on time.

very high during initial filtration, but decrease with time, finallyreaching the steady state. HMR and HTR membranes have lower ratesof flux decline during ink solution filtration compared with the matrixmembrane. M14 and M18 membranes exhibit no significant decline influx during the ink solution filtration. In the initial filtration stage, therejection increases much compared with the reinforced membranes.The rejection of M0 membrane is just 62% and is far below the M10(90.7%) or MN (92.9%) membrane because the biggest membranepores of M0 membrane are nearly 1 μm.

3.6. Membrane fouling during carbon ink solution filtration

Filtration resistances of the hollow fiber membranes are presentedin Table 4. As it can be seen, the total resistance of the M0 membraneis greater than the HMR and HTR membranes. As indicated by Eq. (7),the permeation flux reduction ofM0membrane during a constant pres-sure filtration is attributed to the increase of the total resistance (Rt).The Rt of M10 membrane is lower than one-third of theM0membrane.For the matrix membrane (M0), cake resistance (Rc) resistance isobserved tomake themost contribution (more than 71%) to theflux de-cline. After coating with PVDF or PAN polymer solutions on the matrixmembrane surface, the wall thickness of such HMR and HTR mem-branes increases with an increase in polymer solutions. The intrinsicmembrane resistances (Rm) of the above membranes increase with anincrease in PVDF concentration and get nearly the same values forM10 and MN membranes. With the increment of Rm, the pure waterflux of the HMR and HTR membranes is significantly decreased. In theprocess of the ink solution filtration, the fouling resistance (Rp) ofHMR and HTR membranes that was caused by pore blocking and irre-versible adsorption of foulants onto themembrane pore wall or surfacedecreases greatly. In particular, the Rp ofM0membrane is approximate-ly five times that of M10 membrane. The mean pore size of the HMR orHTR membranes is smaller than the matrix membrane. Because moresuspension particles in the ink solution approached the matrix mem-brane pores and resulted in pore plugging and irreversible adsorption.The FRR of M0 is lower than that of the HMR or HTR membranes.With more and more particles accumulated on the membrane, theprocess transited to a cake formation process, which dominates themost part of a microfiltration in the later stage for M0 membrane. TheRc of HMR or HTR membranes is cake formation, which does not seemto have as significant an impact on microfiltration flux decay as thepore-blocking mechanisms because the permeation decay is relativelylow during this period of time, in comparison with that in the pore-blocking stage.

Table 4Filtration resistance and FRR of the membranes after carbon ink solution filtration.

MembraneID

Rm × 1012/m−1 Rp × 1012/m−1 Rc × 1012/m−1 Rt × 1012/m−1 FRR

M0 0.38 0.72 2.71 3.81 34.1%M6 0.52 0.45 0.58 1.55 53.9%M10 0.77 0.14 0.32 1.23 85.1%M14 1.59 0.10 0.69 2.38 94.6%M18 2.29 0.17 0.18 2.64 93.1%MN 0.67 0.36 0.22 1.25 65.4%

58 X. Zhang et al. / Desalination 330 (2013) 49–60

Fig. 11 shows the determination of foulingmechanisms frommodelfitting to experimental data of M0, M10 andMNmembranes at the dif-ferent stage. Because carbon ink consists of a range of different particlesize powders, so pore blocking will coexist with the cake formation inthe process of membrane fouling. A change in the value of Kp, or Kc

obtained from the slope of the best-fitting straight lines for themicrofiltration data gives an indication that the corresponding type ofmembrane fouling is affected by the reinforcement method. In the ini-tial stage (t b 600 s), all the membranes (M0, M10 and MN) coincidewith the pore blocking law, and it means that themain source of pollut-ants comes from the small particles in the carbon ink solution. As ob-served in Fig. 10(a), the water flux of membranes is significantlydecreased at this stage due to the pore-blocking effect. The slopes ofthefitting lines in the Fig. 11(a) show that the Kp of the reinforcedmem-branes is far lower than the matrix membrane, and reveal that the de-gree of water flux decay is reduced due to the membrane poreblocking. The adsorption capacity of the particles in the ink solutionwith various materials for the reinforced membranes is different. Inthe later period of filtration (t N 1500 s), M0 membrane is preferablyfitting the cake layer model, which is the major pollution factor of themembrane filtration. Cake formation fouling dominates the most partof the microfiltration operation in the later stage after the initial pore-blocking mechanism, and results in slower permeation flux decay overtime. The changes of ISF for M0 membrane in the middle stage are

Fig. 11. Determination of fouling mechanisms from model fitting to experimental data(dotted line indicates thefitted straight line). (a) Fitting of pore blockingmodel, (b) fittingof cake fouling model.

attributed to the combination effect of pore blocking and cake forma-tion. The mean pore size of M10 and MN membranes is close to thesmallest particles in the ink solution, so the phenomenon of poreblocking that is owing to the small particles is weaker, then morelarge particles deposited on the membrane surface rapidly and formedthe cake layer, resulting in the lower degree of water flux decay.

Fig. 12 shows the differences of the membranes before and afterremoving the ink cake layer. The ink cake layer obviously appears ontheM0, M10 andMNmembrane surfaces after filtrating of ink solution.The photo of M0 membrane does not have obvious changes after re-moving the ink cake layer. There are so many particles in the ink solu-tion with a smaller size than the pores of M0 membrane, which can beeasily embedded in the membrane surface pores. The suspension inkparticles in the membrane pores cannot be washed by the pure waterin the clean process; it is irreversible fouling of the membrane. So thecolor of the fouling M0 membrane after removing the ink cake layerand washing with water is nearly the same as before. However, thecolors of the fouling M10 and MN membranes are distinguished fromM0membrane after removing the ink cake layer. Only 14.5% of the par-ticle size in the ink solution is smaller than 0.2 μm, and more than 95%pore sizes of M10 and MN membranes are also smaller than 0.2 μm.So most part of the particles in the ink solution cannot be easy toembed into the membrane pores and cause severe irreversible fouling.Most particles just accumulated on the membrane surface and formedthe cake layer, which is easily removed from the membrane surface.M10 and MNmembranes retain higher FRR than M0 membrane.

3.7. Activated sludge solution filtration and cleaning processes

The carbon ink solution causes severe pore-blocking and adsorptionand lower FRR for the M0 membrane through the rejection and thephotos after removing the ink cake layer, in which the size distributionis between mean pore size of the matrix membrane and the reinforcedmembranes. Then, the activated sludge solution is provided to evaluatethemembrane performance in the realMBR system. Fig. 13 presents thetime-dependent flux of themembranes in operation. The filtration pro-cess includes three steps: (1) First: pure water flux measurement; acti-vated sludge solutionfiltration; 20 min of physical cleaning. (2) Second:pure water flux measurement; activated sludge solution filtration;30 min of chemical cleaning. (3) Third: pure water flux measurement;activated sludge solutionfiltration. It can be seen that the fluxdecreaseddramatically at the initial operation of activated sludge solution both forthe three steps. Similar to the carbon ink solution filtration, the steadyfluxes of M10 and MN membranes in the activated sludge solution arenearly 3–4 times for the M0 membrane in the first step. With respectto activated sludge solution filtration, the membrane fouling occursdue to the following mechanisms: adsorption of solutes or colloidswithin/on membranes; then, deposition of sludge flocs onto the mem-brane surface and formation of a cake layer on the membrane surfaceand so on [12]. It can be seen that the outer surface membrane pore ofM0 membrane is bigger than M10 and MN membranes. The size ofsome small colloids is between the pore size of M0 and M10 or MNmembranes that can easily plunge into the M0 membrane pores butnot to the M10 and MN membranes. It makes the flux decay of M0membranes increase much. The sludge supernatant and effluent CODconcentration and its removal efficiency are shown in Table 5. The efflu-ent COD concentration of the M10 and MN membranes are lower thanthat of the sludge supernatant. The content of the organic pollutantsin the activated sludge solution decreases so much by the biochemicaleffect of the sludge, and the COD removal efficiency of sludge superna-tant reaches 87%. Then the effluent COD concentration of reinforcedmembranes decreases much than the sludge supernatant and the CODremoval efficiency is more than 90% after the membrane filtration pro-cess. However, the effluent COD concentration of M0membrane is highand greater than the sludge supernatant. Some colloidal and solute areeasy to plunge into the big surface pores of the M0, then through the

Fig. 12. Photos of the membranes before and after removing the ink cake layer. (a: M0, b: M10, c: MN; 1: before removing the ink cake layer, 2: after removing the ink cake layer).

Fig. 13. Time-dependent flux of membranes during the activated sludge filtration processfor the matrix and reinforced membranes.

59X. Zhang et al. / Desalination 330 (2013) 49–60

membrane and appear into the effluent. Such phenomenon makes thequality of effluent decrease and the COD concentration increase signifi-cantly. It shows that the pore plunging has greater impact on the mem-brane fouling and consists of the results of the resistance in the carbonink filtration. In the second step, the FRR of M0 is lower than that ofthe HMR or HTR membranes after the physical cleaning process,which is the same as the results of carbon ink filtration. It is becausesome pollutants in the M0 membrane pores cannot be effectivelyremoved by the simple physical cleaning process and result in lower

Table 5Characterization of the effluent COD and the flux recovery ratio after cleaning processes.

Sample COD concentration(mg/L)

COD removalefficiency (%)

FRR (%)

Sludgesupernatant

36.5 87.8 Physicalcleaning

Chemicalcleaning

M0 72.3 75.9 27.6 106.1M10 28.6 90.5 85.9 141.4MN 16.4 94.5 70.1 94.4

60 X. Zhang et al. / Desalination 330 (2013) 49–60

FRR. The water flux of the membranes in the third step is increased tosome extent compared with that of the first step. The hydrophilicity ofthe fouling membrane surface is increased after chemical cleaningwith NaClO solution. The steady fluxes in the activated sludge solutionfor M0, M10 and MN after physical or chemical cleaning processes areclose to that of thefirst step. It shows that theHMRandHTRmembraneshave stable efficiency in antifouling property for the real MBR system.

4. Conclusion

The reinforced hollow fiber membranes were prepared through thedry–wet spinning method of coating with PVDF or PAN polymer solu-tions on the PVDF matrix hollow fiber membrane surface. The HMRmembranes have a favorable interfacial bonding state between thecoating layer and the matrix membrane compared with the HTR mem-branes. PAN coating layer is easy to peel off from thematrix surface, as itis clearly seen from the SEM and photos. The glass transition peaks ofPVDF in the reinforced membranes change to a high temperature andthe interfacial bonding strength increases with an increase in Tg. Theelongation at break of HMR membranes increases much more, from50% (M0) to 99% (M14). After coating with PVDF and PAN polymer so-lutions, the mean pore size of the HMR and HTR membranes is smallerthan that of the matrix membrane.

The HMR and HTR membranes have lower rates of flux declineduring ink solution filtration compared with matrix membrane. Thetotal resistance of the M0 membrane is greater than the HMR and HTRmembranes but in contrast to the results of FRR. In the initial stage(t b 600 s), all the membranes (M0, M10 and MN) are coinciding withthe pore blocking law and cake formation fouling dominates most partof the microfiltration operation in the later stage. The steady fluxes ofM10 and MN membranes in the activated sludge solution are nearly3–4 times for the M0 membrane. The FRR of M0 is lower than that ofthe HMR or HTR membranes after the physical cleaning process.

Acknowledgments

The authors thank the financial support of the National Basic ResearchDevelopment Program of China (973 Program, 2012CB722706), the Na-tional Natural Science Foundation of China (51073120, 21274109), theScience and Technology Plans of Tianjin (12JCZDJC26600), and the BasicResearch Program of China National Textile and Apparel Council.

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