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phenomena+of+polyvinylidene+fluoride+matrix-reinforced+hollow+fiber+membranes+during+microfiltration

Study on the interfacial bonding state and fouling phenomena of polyvinylidene ?uoride matrix-reinforced hollow ?ber membranes during micro ?ltration

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,China

b Tianjin Motian Membrane Technology Co.,Ltd.,Tianjin 300457,China

School of Materials Science and Engineering,Tianjin Polytechnic University,Tianjin 300387,China

H I G H L I G H T S

?The homogeneous-reinforced (HMR)PVDF membranes were ?rstly prepared through the dry –wet spinning process.?The interfacial bonding state is detected by the dynamic mechanical analysis in-directly.

?The membranes (both HMR and HTR)fouling of ink solution was attributed to initial pore blocking followed by cake formation.

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

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

a b s t r a c t

a r t i c l e i n f o Article history:

Received 28March 2013

Received in revised form 22September 2013Accepted 25September 2013Available online 18October 2013Keywords:PVDF PAN DMA

Reinforced

Fouling resistance

Reinforced polyvinylidene ?uoride (PVDF)and polyacrylonitrile (PAN)hollow ?ber membranes were prepared through dry –wet spinning process and included PVDF or PAN polymer solutions (coating layer)and matrix PVDF membrane.The performance of the membranes varies with different polymer solutions and is characterized in terms of ink solution and activated sludge ?ux,dynamic mechanical analysis (DMA),mean pore size,a mechan-ical strength test,and morphology observations by a scanning electron microscope (SEM).The results of this study indicate that PAN coating layer is easy to peel off from the matrix membrane.Interfacial bonding strength is also detected by dynamic mechanical analysis indirectly.The reinforced membranes show a narrower pore size distribution and smaller mean pore size than the matrix membrane.The elongation at break increases much more and the membranes are endowed with better ?exibility performance.The homogeneous-reinforced (HMR)and heterogeneous-reinforced (HTR)membranes have a lower rate of ?ux decline compared with matrix membrane during ink solution ?ltration.The main types of all membranes fouling in micro ?ltration 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 ef ?ciency in antifouling property for the real MBR system.

1.Introduction

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

Desalination 330(2013)49–60

?Corresponding author at:State Key Laboratory of Hollow Fiber Membrane Materials and Processes,Tianjin Polytechnic University,No.399,West of Binghshui Road,Xiqing District,Tianjin 300387,China.Tel.:+862283955138.

E-mail address:xiaotjpu@https://www.docsj.com/doc/f713503026.html, (C.

Xiao).

https://www.docsj.com/doc/f713503026.html,/10.1016/j.desal.2013.09.022

Contents lists available at ScienceDirect

Desalination

j o u r n a l h o m e pa ge :ww w.e l s e v i e r.c o m /l oc a t e /d e s a l

?ber membrane.Nowadays,the most popularly used hollow?ber membranes are prepared by wet-spinning method[1].However,the membranes possess low tensile strength and are easily damaged in the state of serviceability.

A braid-reinforced composite hollow?ber membrane comprising a reinforcing material of a tubular braid and a polymer solution coated on the surface of the tubular braid has been further studied to solve these problems[2].Such braid-reinforced membranes possess high me-chanical strength due to the high strength of the reinforced?ber;how-ever,the coating layer can be easily peeled off from the braid in the service because the coating layer and reinforced?ber were not the same compound and resulted in incompatible thermodynamics of the heterogeneous-reinforced(HTR)membranes.Such problems restricted the applications of these HTR membranes in engineering practice.The requirement of the interfacial bonding strength between the coating layer and the reinforced?ber is higher than the tensile strength due to the techniques of aeration and antiwash in the?ltration process.The interfacial bonding state between the?ber and matrix plays a major role in the mechanical behavior of a composite material.Good interface bonding state that ensures the load transfer from the matrix to the reinforcement is a primary requirement for the effective use of rein-forcement properties[3].

Based on the results of the traditional reinforcement to the hollow ?ber membranes,the membranes were prepared by the homogenous reinforced(HMR)method which can overcome those defects of similar HTR(?ber-or braid-reinforced)membranes.Homogenous reinforced is described as the same material of the coating layer and reinforced mem-brane in the coating process[4].A fundamental understanding of the interfacial region and a quantitative characterization of the level of in-terfacial bonding state are necessary[3].In fact,both the transverse and the longitudinal strengths of a composite material depend heavily on the strength of the?ber and matrix bond.Many scholars have done researches on the characterization of the interfacial bonding strength for the composite materials.These tests can be divided into large specimen macromechanical tests such as interlaminar shear strength(ILSS)[5],and single?ber micromechanical tests such as single ?ber pull-out test[6].However,there are many dif?culties to do the same tests for the HMR membranes.So it is necessary to characterize the interfacial bonding strength of such HMR membranes indirectly.Dy-namic mechanical analysis(DMA)is a sensitive and versatile thermal analysis technique,which has the potential to accurately characterize the interfacial properties between the matrix and?ber for the compos-ite materials[7].The interface contribution to the DMA results can be separated from the matrix and?ber contributions due to their different degrees of crystallinity and tensile modulus.

It is important to characterize the interfacial bonding state for the HMR membrane,which relates to the service life of the membrane in the MBR system.It is also necessary to study the antifouling property in the applications.The advantages of micro?ltration include superior water quality,easier control of operation,lower maintenance,and reduced sludge production in wastewater treatment.However,the membrane fouling inducing the rapid decline in the water?ux and the irreversible loss of productivity over time restricted its application [8].Flux decline in membrane?ltration is the result of the increase in membrane resistance and the development of another resistance layer,which can be elucidated in terms of pore blockage and cake formation,respectively[9,10].The effects of each fouling mechanisms on the?ux 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?ux decline and the mechanisms of membrane fouling[11].Particle sizes of sludge?ocs,colloids and solutes in mixed liquor may strongly affect fouling mechanisms in a membrane?ltration system.In the membrane?ltration process,if foulants are comparable with the membrane pores(i.e.,colloids),or smaller than the membrane pores(i.e.,solutes),adsorption on pore wall and pore-blocking will easily occur.However,if foulants(i.e.,sludge?ocs and colloids)are much larger than the membrane pores,they tend to form a cake layer on 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 are caused by the contaminants,in which particle size is between the mean pore size of matrix and reinforced membranes.In fact,the complex na-ture of membrane foulants and activated sludge has restricted us to clear-ly investigate the MBR fouling.A small amount of the sludge?ocs and colloids de?nitely exists in the MBR system,and their size distribution is between mean pore size of the matrix membrane and the reinforced membranes.Carbon ink solution is a colloidal solution and usually con-sists of carbon particles,a polymeric binder and a solvent[14]and all these dispersoids are dispersed in water as colloidal particles.Carbon ink solution is simpler and more stable compared to the activated sludge in the MBR system,and membrane resistance can be better analyzed to provide guidance for the reason for membrane fouling in the MBR system.

In this study,the HMR and HTR hollow?ber membranes were pre-pared through dry–wet spinning process and included PVDF or PAN casting solution(coating layer)and the matrix membrane(preparation of porous PVDF hollow?ber membrane through melt spinning and stretching process).The effects of the polymer concentrations in the polymer solutions on the reinforced membrane pore distribution were studied.The type of fouling mechanisms prevalent in membrane micro?ltration treating ink solution was studied and the membrane foul-ing resistance distribution after the?ltration process was examined.Then, the membrane 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.1300powders,Tm=170°C)was purchased from Kureha Chemical Industrial Co.Ltd(Tokyo,Japan)and PAN(molecular weight of50,000)was purchased from the Qilu Petrochemical acrylic fac-tory.N,N-dimethylacetimide(DMAc,N99%)and Polyvinylpyrrolidone (PVP,K30,Mw=30,000)were obtained from Tiantai Fine Chemical Co.Ltd(Tianjin,China).Tween80(Tw-80)and sodium hypochlorite (NaClO)solution were purchased from Tianjin Fengchuan Chemical Reagent Science and Technology Co.,Ltd.(Tianjin,China).The PVDF matrix membrane(the PVDF membrane prepared by melting spinning) was supplied by Tianjin Motian Membrane Engineering&Technology Co.,Ltd(Tianjin,China).

2.2.Preparation of HMR and HTR PVDF hollow?ber membranes

The PVDF matrix reinforced hollow?ber membranes were prepared by using the dry–wet spinning method.Fig.1shows the spinning appa-ratus.According to the described method,the PVDF matrix membrane was coated with the polymer solutions(PVDF and PAN polymer solu-tions)and guided through a precipitation bath,in which the polymer solutions were converted into a microporous layer.The polymer solutions were prepared by the blending of different compositions consisting of PVDF or PAN,PVP K30,Tw-80,and DMAc(Table1), under constant mechanical stirring in a three-necked round-bottom ?ask for4h at70°C.The dope solution temperature was60°C.The extra coagulation was water and the temperature was20°C.The air gap was10cm and the take up speed was15cm/(r/min).All the membranes were prepared under an environmental humidity of60% and at a temperature of20°C.

The HMR membranes with the different PVDF concentrations,i.e.,6, 10,14and18weight%(wt.%)were labeled M6,M10,M14,and M18, respectively.The HTR membrane with10wt.%PAN concentration was labeled MN.The PVDF matrix membrane was designated as M0.Before the scanning electron microscope(SEM)tests,we put the resulting

50X.Zhang et al./Desalination330(2013)49–60

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

2.3.Membrane characterizations

2.3.1.Morphology examination

The morphology of the membranes was observed by using SEM (Quanta800,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?ltration.The?ltration experiments were then carried out by using carbon ink solution.The ink solution?ux (ISF)of the membranes was measured with a0.5g/L ink solution and calculated by using Eq.(1).The particle distribution of the ink solution is shown in Fig.2.

Each membrane was initially pressurized for20min at0.15MPa. Then pure water?ux(PWF),J w0,through the membranes was mea-sured by using a cross out-feeding system at a transmembrane pressure of0.1MPa and at a linear velocity of1.25l/min.Then,the?ltration experiments were carried out by using the ink solution.Fig.3shows the PWF and ISF measurement apparatus.The membrane module contained two hollow?ber membranes with an effective length of 19–20cm.PWF and ISF were calculated as follows:

S?t

e1T

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).

The cross-section model of the

HMR membrane

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

Table1

Spinning parameters of reinforced membranes with different dope compositions.

Prepared membrane Polymer

concentration

PVP

(wt.%)

Tw-80

(wt.%)

DMAc

(wt.%)

Matrix

membrane

M00000PVDF hollow

?ber membrane M6PVDF(wt.%)67384

M10107380

M14147376

M18187372

MN PAN(wt.%)1073

Fig.2.The particle distribution of ink solutions.51

X.Zhang et al./Desalination330(2013)49–60

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

R %eT?

C p f

!?100%

e2T

where C p and C f are the concentrations at permeate and feed,respectively.

After ?ltration 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 30min.Then,pure water ?ux of the cleaned membranes,J w1,was measured by using Eq.(1).Flux recovery ratio (FRR)was calculated by using the following Eq.(3)[15]:

FRR %eT?

J w 1J w 0

?100

e3T

2.3.2.2.Activated sludge solution ?ltration.Activated sludge was obtained from the real MBR system.The concentration of the mixed liquor suspended solids (MLSS)was approximately 3500mg/L and the in ?u-ent chemical oxygen demand (COD)concentration of the MBR was 300±50mg/L.The sludge supernatant was prepared through the cen-trifugal method.The COD concentrations of the water samples were analyzed according to standard methods [16].

The test method of activated sludge ?ux is the same as the carbon ink solution ?ltration experiments.The ?ltration process includes three steps:(1)First:pure water ?ux measurement,J s1;activated sludge solu-tion ?ltration;30min of physical cleaning (the fouling membranes were removed the cake layer by using a sponge and washed with de-ionized water,then immersed in the de-ionized water for 30min.).(2)Second:pure water ?ux measurement,J s2;activated sludge solution ?ltration;30min of chemical cleaning (the fouling membranes were immersed in the 0.5wt.%NaClO solution for 30min,and then washed with the de-ionized water).(3)Third:pure water ?ux measurement,J s3,followed by activated sludge solution ?ltration.

After physical cleaning,the ?ux recovery ratio (FRR)was calculated by using the following Eq.(4):

FRR %eT?

J s 2J s 1

?100

e4T

After chemical cleaning,the ?ux recovery ratio (FRR)was calculated by using the following Eq.(5):

FRR %eT?

J s 3J s 2

?100

e5T

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 1Hz and a heating rate of 5°C/min.The samples were evaluated in the temperature range from ?80to 50°C.

2.3.4.Tensile break strength measurements

The tensile strength and elongation at break of the hollow ?ber membranes were determined at room temperature by using a YG061F electronic single yarn tensile tester (Shandong,China).The tensile rate was 10mm/min.Five runs were performed for each specimen.

2.3.5.Determination of pore size and its distribution

The pore size and its distribution of each sample were determined by using the capillary ?ow porometer (Porous Materials Inc.,USA),and values were calculated from the pressure of the gas ?ow.2.4.Membrane-fouling analysis

The permeation ?ux of pure water across a clean membrane can be described by Darcy's law as:J ?

Δp μR m

e6T

where J (m 3m ?2s ?1)is the permeation ?ux,Δp (Pa)the transmem-brane pressure (TMP),μ(Pa s)the absolute viscosity of the water,and R m (m ?1)the clean membrane resistance.

For suspension ?ltration,?ux decline is a result of the increase of membrane resistance and the mechanisms of membrane fouling usually include 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 ?

Δp μR t

e7TR t ?R m tR p tR c

e8T

where R t is the total ?ltration resistance (m ?1),R m is the membrane resistance (m ?1),R p (m ?1)is the resistance due to pore blocking,and R c (m ?1)is the resistance arising from cake formation.

The intrinsic membrane resistance (R m ),the fouling resistance caused by pore plugging and irreversible adsorption of foulants onto the membrane pore wall or surface (R p )and the cake resistance by cake layer formed on the membrane surface (R c )can be calculated by using the following equations [18]:R c ?

Δp

μJ s

?R m ?R p e9T

R p ?Δp

μJ w 1

?R m

e10T

R c ?Δp

μJ s

?R m ?R p e11T

where Js is the ?ux of ink solution at steady state,J w0is the initial water ?ux and J w1is the ?nal water ?ux after removing the cake layer by ?ushing with de-ionized

water.

Permeate

Fig.3.Schematic diagram for ?ltration experiments.

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

In the micro?ltration of ink solution,pore blocking and cake forma-tion simultaneously existed in the membrane-fouling process.Such a process can be described by the standard pore blocking?ltration model and cake?ltration law as follows.Then by?tting the models in Eqs.(14)or(15)to the experimental history data of permeation?ux, the predominant membrane-fouling mechanism at different stages of the micro?ltration operation may be identi?ed if the experimental data can be described by one of these models[19].

Pore-blocking resistance:

J?J0e?K p te12TCake resistance:

J2?

J20

1tJ2

K c t

e13T

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

Pore-blocking resistance:

ln J??K p ttln J0e14T

Cake resistance:

tK c te15T

where K p,and K c are system parameters relating to pore-blocking resis-

tance,and cake formation resistance,respectively.

3.Results and discussion

3.1.Morphologies of PVDF hollow?ber membranes

The SEM images of the matrix membrane(M0),HMR membrane

(M10),and HTR membrane(MN)are shown in Fig.4.The

matrix Fig.4.Cross-sectional SEM morphology of hollow?ber membranes(a):M0;(b):M10;(c)MN.

X.Zhang et al./Desalination330(2013)49–60

membrane is a kind of homogeneous membrane and there is no skin layer at the cross section near the outer surface as shown in Fig.4(a2). After modi?cation,the PVDF and PAN coating layers can be clearly ob-served on the outer surface of the matrix membrane.The outermost layer is the PVDF coating layer and the innermost layer is the PVDF ma-trix membrane.The middle layer between the PVDF coating layer and the PVDF matrix membrane are the interface layers,which endow the HMR membranes with high interfacial bonding as shown in Fig.4(b2). Also,the coating layer prepared through the wet phase separation has a denser pore structure compared with the matrix membrane.M10 membrane shows typical?nger-like pore structure near the outer sur-face.However,there were obvious changes in the HTR membrane (MN)after fracturing in the liquid N2and then recorded through SEM. As can be seen in Fig.4(c1),the PAN coating layer is easy to peel off from the matrix membrane and the interface structure is clearer than M10,which makes the HTR membrane have bad interfacial bonding strength.Similar to the PVDF coating layer,PAN coating layer has a dense sponge-like and long?nger-like pore structures,and the latter pore structure endows the membranes with good permeability in the ?ltration process.

Fig.5shows the SEM results of the outer surfaces of the hollow?ber membranes.As can be seen,the matrix membrane(M0)has a rougher outer surface within obvious big pores,which was prepared by melt spinning and stretching process.The outer surface of the HMR mem-brane(M10)and HTR membrane(MN)appear denser and smoother than those of M0.

3.2.Tensile strength of the hollow?ber membranes

Fig.6shows the results of the tensile strength and elongation at break of the reinforced membranes.The tensile strength of the reinforced membranes,except for that of the M10membrane,decreases slightly compared with the matrix membrane.As we know,the tensile strength of the coating layer(PVDF or PAN)is smaller than the matrix membrane.The erosion effect of the polymer solutions may have some bad in?uence on the matrix surface,which provides the mainly tensile strength of the membrane.The main purpose of the experiment is to improve the tensile strength of the PVDF or PAN membrane pre-pared through the wet spinning method.The tensile strength of the reinforced membranes may have reduced compared with the matrix, but more over than the conventional PVDF[20]or PAN[21]membranes prepared through the wet-spinning method.A low tensile strength in a PVDF or PAN membrane may easily cause?ber breakage in the state of serviceability.The PVDF hollow?ber membranes prepared by the HMR method have higher mechanical properties rather than those by the wet phase inversion method.The tensile strength of the HMR PVDF mem-branes is nearly10MPa,which is adequate for MBR application.The elongation at break increased much more,from50%(M0)to99% (M14).The elongation at break then decreases as the PVDF concentra-tion increases at18%.When the PVDF concentration increases to a certain level,the polymer gradually shows the unique elastic nature, and the rigidity of the molecular chains increases to some extent.The elongation at break of all the reinforced membranes including MN membrane increases much more than the matrix membrane,and the

hollow?ber membranes are endowed with better?exibility perfor-mance,which is in accord with the DMA results.

Fig.7shows the signi?cant differences in morphology between the HMR membrane(M10)and the HTR membrane(MN)after the tensile strength test.It is obviously found that the PAN coating layer had been easy to peel off from the matrix membrane after the tensile strength test.However,the HMR membrane(M10)remains as a whole after the test,which is constituted of PVDF coating layer and matrix mem-brane.The HMR membrane shows higher bonding strength than the HTR membrane in the drawing process,which can indirectly improve the service life of the membrane in the practical operation.3.3.Dynamic mechanical analysis

Fig.8presents the results of DMA measurement on the matrix membrane and reinforced membranes at1Hz.DMA is a technique in which an oscillatory force at a set frequency of the sample is applied and reports change in stiffness and damping.It was used to assess the interfacial 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 locate the glass transition temperature of the material,as well as to identify transitions corresponding to other molecular motions.Fig.4

shows Fig.5.Outer surface SEM morphology of hollow?ber membranes(a):M0;(b):M10;

(c)MN.

54X.Zhang et al./Desalination330(2013)49–60

the differences existing in the interface obviously between the coating layer (PVDF coating layer)and the matrix membrane.The aggregation structures of the coating layer and the matrix membrane have great dif-ferences because of the different preparation methods,although the raw materials for them are the same.So the characteristics of reinforced PVDF membranes would have some changes due to the interfacial bonding and combining effects in the DMA test process.The dynamic mechanical relaxation spectrum of normal PVDF presents up to four dif-ferent relaxations.The two major relaxations:βrelaxation (?40°C)

corresponds to glass transition and hence to Brownian micromotion in the backbone in the amorphous zones and αrelaxation (+100°C)is assigned to the liberation of polymer chains in the crystalline regions [23].The tan δcurve in Fig.8and Table 2shows a peak at about ?12°C for the matrix membrane,which relates to the βrelaxation and corresponds to glass transition temperature (T g )of PVDF.The shift of the tan δpeak to a higher temperature than that of the normal PVDF may be ascribed to hindered motion of polymer chains with good PVDF crystalline structure by the melt-spinning and stretching method.

After coating with PVDF or PAN polymer solutions,the T g of the ma-trix membrane is lower than the reinforced PVDF membranes as shown in Fig.8and Table 2.The peaks of T g for the PVDF coating layer that was prepared by the wet method may be covered by the curve of the matrix membrane and cannot show clearly during the process of temperature rise.As shown in Fig.8,a shoulder peak can be seen at nearly ?34.5°C for M18membrane.Therefore,properties of T g observed are not appreciably sensitive to variations of the coating layer [22].The presence of a high temperature tan δpeak (T g )was assigned to the glass transition of regions containing chains of reduced mobility [24].During deformation,good interfacial bonding strength will be easy to result in the entanglement of PVDF chains between the coating layer and the matrix membrane,which would bind the movement of the PVDF main chains to some extent.Poor interfacial bonding strength,on the contrary,of the matrix membrane and the coating layer will show their own T g,respectively,such as M18membrane,so that the glass transition peaks of PVDF change to a high temperature and the in-terfacial bonding strength increases with an increase in T g .The differ-ences of T g compared with M10and M14to M0membrane having some signi ?cant changes are shown in Table 2.According to the above theory,M10and M14membranes possess better interfacial bonding state than M18and 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 in ?uence on the matrix membrane structure;it still makes the T g of PVDF to increase a

little.

Fig.6.The mechanical properties of hollow ?ber

membranes.

Fig.7.Photos of the membranes (M10and MN)after the tensile strength

test.

Fig.8.DMA thermograms of the hollow ?ber membranes.

Table 2

The glass transition temperature and its differences of the hollow ?ber membranes.Membrane ID PVDF T g (°C)T g difference(°C)

(T gi -T g0,i =6,10,14,18,N)E ′(MPa)at 25°C M0?12.30296.7M6?11.70.6190.3M10?9.9 2.4241.9M14?9.1 3.2303.5M18?12.6?0.3312.6MN

?11.8

0.5

277.8

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

The apparent viscosity of the polymer solutions containing18wt.% PVDF is obviously greater than that of the low PVDF concentration, resulting in poor in?ltration between the polymer solutions and the matrix membrane[4].Also,favorable in?ltration can improve on the interfacial bonding strength greatly.So the change rates of T g of M18are lower than M10and M14membranes.PVDF and PAN are thermodynamically partially compatible polymers[21],and no obvious diffusion phenomenon occurs at the interface between the PAN coating layer and the PVDF matrix membrane.So the T g of PVDF in the MN membrane just has a little change compared with the M0membrane.

Storage modulus is a measure of the maximum energy stored in the material during one cycle of oscillation.It also gives an idea of stiffness behavior and load bearing capability of the composite mate-rial[25].In this case,the storage modulus of the reinforced mem-branes at25°C is lower than that of the matrix membrane,as shown in Table2,but the results are contrary for M18membrane. In the glassy region,the polymer chains are in a frozen state and highly immobile which results in high storage modulus.As the tem-perature increases,the polymer chains become more mobile and lose their close packing arrangement in the rubbery region.Coating with 10wt.%PVDF or10wt.%PAN on the matrix membrane surface,how-ever,results in a low decrease of stiffness of matrix membrane and hence low storage modulus,endowing the reinforced membrane with more?exibility and ductility properties during the techniques of aeration and antiwash process.3.4.Determination of pore size and distribution

Fig.9shows the pore size and distribution of the membranes.The matrix membrane prepared through the melt-spinning and stretching method shows a wider pore size distribution than M10,M14,and MN membranes.It is dif?cult to possess small membrane pores and a nar-row pore size distribution in the melt-spinning and stretching process, so nearly50%of the membrane pores are larger than0.2μm and some big pores are nearly1μm.Such situation not only makes the membrane with low rejection in the?ltration process,but also results in severe membrane fouling.The contaminated membrane will have a minor ?ux recovery rate in the backwash process because of the loose sponge-like pore structure.After coating with PVDF and PAN polymer solutions,the mean pore size of the HMR and HTR membranes is small-er than the matrix membrane as shown in Table3.The coating layer thickness and porosity of the HMR membranes decrease with an in-crease in PVDF concentration.HTR membrane(MN)shows a thinner coating layer and lower porosity compared with M10membrane in the same polymer solution.MN membrane seems denser than that of M10.The smaller mean pore size changes the membrane from micro?ltration levels to ultra?ltration levels in fact.Pore size distribu-tions of M10and M14are narrower than that of M0,and the scope of the pore size distribution of the membranes has become even more concentrated with an increase in the polymer solution.A number of membrane pores range from0.08to0.2μm and are more than95%for M10,and for M14,90%of membrane pores in the range from0.08

Fig.9.Pore size and distribution of the membranes(a:M0;b:M10;c:M14;d:MN).

56X.Zhang et al./Desalination330(2013)49–60

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

3.5.Carbon ink solution ?ltration performance

The carbon ink solution was ?ltered with the membranes to evaluate the ?ltration performance of membranes.Fig.2shows the distribution of particle size of the ink solution.As shown in Fig.10(a),the steady ISF decreases 90.1%,66.4%,37.4%,33.5%,11.9%and 46.2%for M0,M6,M10,M14,M18and MN membranes compared with the initial water ?ux,respectively.The rates of the matrix membrane ?ux decay are

very high during initial ?ltration,but decrease with time,?nally reaching the steady state.HMR and HTR membranes have lower rates of ?ux decline during ink solution ?ltration compared with the matrix membrane.M14and M18membranes exhibit no signi ?cant decline in ?ux during the ink solution ?ltration.In the initial ?ltration stage,the rejection increases much compared with the reinforced membranes.The rejection of M0membrane is just 62%and is far below the M10(90.7%)or MN (92.9%)membrane because the biggest membrane pores of M0membrane are nearly 1μm.

3.6.Membrane fouling during carbon ink solution ?ltration

Filtration resistances of the hollow ?ber membranes are presented in Table 4.As it can be seen,the total resistance of the M0membrane is greater than the HMR and HTR membranes.As indicated by Eq.(7),the permeation ?ux reduction of M0membrane during a constant pres-sure ?ltration is attributed to the increase of the total resistance (R t ).The R t of M10membrane is lower than one-third of the M0membrane.For the matrix membrane (M0),cake resistance (R c )resistance is observed to make the most contribution (more than 71%)to the ?ux de-cline.After coating with PVDF or PAN polymer solutions on the matrix membrane surface,the wall thickness of such HMR and HTR mem-branes increases with an increase in polymer solutions.The intrinsic membrane resistances (R m )of the above membranes increase with an increase in PVDF concentration and get nearly the same values for M10and MN membranes.With the increment of R m ,the pure water ?ux of the HMR and HTR membranes is signi ?cantly decreased.In the process of the ink solution ?ltration,the fouling resistance (R p )of HMR and HTR membranes that was caused by pore blocking and irre-versible adsorption of foulants onto the membrane pore wall or surface decreases greatly.In particular,the R p of M0membrane is approximate-ly ?ve times that of M10membrane.The mean pore size of the HMR or HTR membranes is smaller than the matrix membrane.Because more suspension particles in the ink solution approached the matrix mem-brane pores and resulted in pore plugging and irreversible adsorption.The FRR of M0is lower than that of the HMR or HTR membranes.With more and more particles accumulated on the membrane,the process transited to a cake formation process,which dominates the most part of a micro ?ltration in the later stage for M0membrane.The R c of HMR or HTR membranes is cake formation,which does not seem to have as signi ?cant an impact on micro ?ltration ?ux decay as the pore-blocking mechanisms because the permeation decay is relatively low during this period of time,in comparison with that in the pore-blocking stage.

Table 3

Characterization of PVDF hollow ?ber membranes.Membrane ID

M10

M14

M18

OD a ,ID b /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±0.0090.283±0.0070.258±0.0240.281±0.0170.277±0.0230.267±0.018Coating layer thickness(mm)0

0.035±0.0030.040±0.0020.051±0.0050.068±0.0050.043±0.004Porosity (%)

69.40559.54060.90951.32343.78754.145Mean pore size (μm)

0.227

0.119

0.101

0.085

0.084

0.095

a Outer diameter.b

Inner

diameter.

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

Table 4

Filtration resistance and FRR of the membranes after carbon ink solution ?ltration.Membrane ID R m ×1012/m ?1R p ×1012/m ?1R c ×1012/m ?1R t ×1012/m ?1FRR M00.380.72 2.71 3.8134.1%M60.520.450.58 1.5553.9%M100.770.140.32 1.2385.1%M14 1.590.100.69 2.3894.6%M18 2.290.170.18 2.6493.1%MN

0.67

0.36

0.22

1.25

65.4%

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

Fig.11shows the determination of fouling mechanisms from model ?tting to experimental data of M0,M10and MN membranes at the dif-ferent stage.Because carbon ink consists of a range of different particle size powders,so pore blocking will coexist with the cake formation in the process of membrane fouling.A change in the value of K p ,or K c obtained from the slope of the best-?tting straight lines for the micro ?ltration data gives an indication that the corresponding type of membrane fouling is affected by the reinforcement method.In the ini-tial stage (t b 600s),all the membranes (M0,M10and MN)coincide with the pore blocking law,and it means that the main source of pollut-ants comes from the small particles in the carbon ink solution.As ob-served in Fig.10(a),the water ?ux of membranes is signi ?cantly decreased at this stage due to the pore-blocking effect.The slopes of the ?tting lines in the Fig.11(a)show that the K p of the reinforced mem-branes is far lower than the matrix membrane,and reveal that the de-gree of water ?ux decay is reduced due to the membrane pore blocking.The adsorption capacity of the particles in the ink solution with various materials for the reinforced membranes is different.In the later period of ?ltration (t N 1500s),M0membrane is preferably ?tting the cake layer model,which is the major pollution factor of the membrane ?ltration.Cake formation fouling dominates the most part of the micro ?ltration operation in the later stage after the initial pore-blocking mechanism,and results in slower permeation ?ux decay over time.The changes of ISF for M0membrane in the middle stage are

attributed to the combination effect of pore blocking and cake forma-tion.The mean pore size of M10and MN membranes is close to the smallest particles in the ink solution,so the phenomenon of pore blocking that is owing to the small particles is weaker,then more large particles deposited on the membrane surface rapidly and formed the cake layer,resulting in the lower degree of water ?ux decay.

Fig.12shows the differences of the membranes before and after removing the ink cake layer.The ink cake layer obviously appears on the M0,M10and MN membrane surfaces after ?ltrating of ink solution.The photo of M0membrane 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 M0membrane,which can be easily embedded in the membrane surface pores.The suspension ink particles in the membrane pores cannot be washed by the pure water in the clean process;it is irreversible fouling of the membrane.So the color of the fouling M0membrane after removing the ink cake layer and washing with water is nearly the same as before.However,the colors of the fouling M10and MN membranes are distinguished from M0membrane after removing the ink cake layer.Only 14.5%of the par-ticle size in the ink solution is smaller than 0.2μm,and more than 95%pore sizes of M10and MN membranes are also smaller than 0.2μm.So most part of the particles in the ink solution cannot be easy to embed into the membrane pores and cause severe irreversible fouling.Most particles just accumulated on the membrane surface and formed the cake layer,which is easily removed from the membrane surface.M10and MN membranes retain higher FRR than M0membrane.3.7.Activated sludge solution ?ltration and cleaning processes

The carbon ink solution causes severe pore-blocking and adsorption and lower FRR for the M0membrane through the rejection and the photos after removing the ink cake layer,in which the size distribution is between mean pore size of the matrix membrane and the reinforced membranes.Then,the activated sludge solution is provided to evaluate the membrane performance in the real MBR system.Fig.13presents the time-dependent ?ux of the membranes in operation.The ?ltration pro-cess includes three steps:(1)First:pure water ?ux measurement;acti-vated sludge solution ?ltration;20min of physical cleaning.(2)Second:pure water ?ux measurement;activated sludge solution ?ltration;30min of chemical cleaning.(3)Third:pure water ?ux measurement;activated sludge solution ?ltration.It can be seen that the ?ux decreased dramatically at the initial operation of activated sludge solution both for the three steps.Similar to the carbon ink solution ?ltration,the steady ?uxes of M10and MN membranes in the activated sludge solution are nearly 3–4times for the M0membrane in the ?rst step.With respect to activated sludge solution ?ltration,the membrane fouling occurs due to the following mechanisms:adsorption of solutes or colloids within/on membranes;then,deposition of sludge ?ocs onto the mem-brane surface and formation of a cake layer on the membrane surface and so on [12].It can be seen that the outer surface membrane pore of M0membrane is bigger than M10and MN membranes.The size of some small colloids is between the pore size of M0and M10or MN membranes that can easily plunge into the M0membrane pores but not to the M10and MN membranes.It makes the ?ux decay of M0membranes increase much.The sludge supernatant and ef ?uent COD concentration and its removal ef ?ciency are shown in Table 5.The ef ?u-ent COD concentration of the M10and MN membranes are lower than that of the sludge supernatant.The content of the organic pollutants in the activated sludge solution decreases so much by the biochemical effect of the sludge,and the COD removal ef ?ciency of sludge superna-tant reaches 87%.Then the ef ?uent COD concentration of reinforced membranes decreases much than the sludge supernatant and the COD removal ef ?ciency is more than 90%after the membrane ?ltration pro-cess.However,the ef ?uent COD concentration of M0membrane is high and greater than the sludge supernatant.Some colloidal and solute are easy to plunge into the big surface pores of the M0,then through

the

Fig.11.Determination of fouling mechanisms from model ?tting to experimental data (dotted line indicates the ?tted straight line).(a)Fitting of pore blocking model,(b)?tting of cake fouling model.

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

membrane and appear into the ef ?uent.Such phenomenon makes the quality of ef ?uent decrease and the COD concentration increase signi ?-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 carbon ink ?ltration.In the second step,the FRR of M0is lower than that of the HMR or HTR membranes after the physical cleaning process,which is the same as the results of carbon ink ?ltration.It is because some pollutants in the M0membrane pores cannot be effectively removed by the simple physical cleaning process and result in

lower

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 ?ux of membranes during the activated sludge ?ltration process for the matrix and reinforced membranes.

Table 5

Characterization of the ef ?uent COD and the ?ux recovery ratio after cleaning processes.Sample COD concentration (mg/L)COD removal ef ?ciency (%)FRR (%)Sludge

supernatant 36.587.8Physical cleaning Chemical cleaning M072.375.927.6106.1M1028.690.585.9141.4MN

16.4

94.5

70.1

94.4

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

FRR.The water?ux of the membranes in the third step is increased to some extent compared with that of the?rst step.The hydrophilicity of the fouling membrane surface is increased after chemical cleaning with NaClO solution.The steady?uxes in the activated sludge solution for M0,M10and MN after physical or chemical cleaning processes are close to that of the?rst step.It shows that the HMR and HTR membranes have stable ef?ciency in antifouling property for the real MBR system.

4.Conclusion

The reinforced hollow?ber membranes were prepared through the dry–wet spinning method of coating with PVDF or PAN polymer solu-tions on the PVDF matrix hollow?ber membrane surface.The HMR membranes have a favorable interfacial bonding state between the coating layer and the matrix membrane compared with the HTR mem-branes.PAN coating layer is easy to peel off from the matrix surface,as it is clearly seen from the SEM and photos.The glass transition peaks of PVDF in the reinforced membranes change to a high temperature and the interfacial bonding strength increases with an increase in T g.The elongation at break of HMR membranes increases much more,from 50%(M0)to99%(M14).After coating with PVDF and PAN polymer so-lutions,the mean pore size of the HMR and HTR membranes is smaller than that of the matrix membrane.

The HMR and HTR membranes have lower rates of?ux decline during ink solution?ltration compared with matrix membrane.The total resistance of the M0membrane is greater than the HMR and HTR membranes but in contrast to the results of FRR.In the initial stage (t b600s),all the membranes(M0,M10and MN)are coinciding with the pore blocking law and cake formation fouling dominates most part of the micro?ltration operation in the later stage.The steady?uxes of M10and MN membranes in the activated sludge solution are nearly 3–4times for the M0membrane.The FRR of M0is lower than that of the HMR or HTR membranes after the physical cleaning process. Acknowledgments

The authors thank the?nancial support of the National Basic Research Development Program of China(973Program,2012CB722706),the Na-tional Natural Science Foundation of China(51073120,21274109),the Science and Technology Plans of Tianjin(12JCZDJC26600),and the Basic Research Program of China National Textile and Apparel Council. References

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