Eu

GD

a

ARRA

KOPMPM

1

mdpbal

rubphvt

NT

1d

Separation and Purification Technology 64 (2008) 38–47

Contents lists available at ScienceDirect

Separation and Purification Technology

journa l homepage: www.e lsev ier .com/ locate /seppur

ffect of silica particles on cellulose acetate blendltrafiltration membranes: Part I

. Arthanareeswaran ∗, T.K. Sriyamuna Devi, M. Raajenthirenepartment of Chemical Engineering, A. C. College of Technology, Anna University, Chennai 600025, India

r t i c l e i n f o

rticle history:eceived 26 April 2007eceived in revised form 22 July 2008ccepted 24 August 2008

eywords:rganic–inorganic membraneore statisticsembrane morphology

rotein separation

a b s t r a c t

This paper deals with the preparation of organic–inorganic ultrafiltration (UF) membranes by solutioncasting followed by phase separation method. The silica (SiO2) particles were added to the cellulose acetate(CA) polymer with the increment of 10 wt.% from 0 to 40% by weight using N,N-dimethyl formamide (DMF)as polar solvent. The prepared organic–inorganic membranes were characterized for UF performance suchas compaction, pure water flux, % water content, membrane hydraulic resistance, molecular weight cut-off (MWCO) and pore statistics. MWCO and pore statistics were investigated using protein solutions ofdifferent molecular weights. It is observed that by increasing the concentration of SiO2 in CA polymer, theMWCO, pore radius, surface porosity and pore density has been increased. The mechanical stability of theCA/SiO2 blend membranes increased initially and then declined with the addition of inorganic particle

embrane fouling-resistant ability above 10 wt.% to the casting solution. The morphological structure was changed with the addition of SiO2

particles in the casting solution. Further the permeate flux and % rejection of different molecular weight ofproteins were investigated which showed an increased protein permeate flux with the decreased soluterejection. Meanwhile, the effect of SiO2 content in the CA membranes on fouling-resistant ability wasstudied using BSA solution. The results indicated that increasing the SiO2 content in the casting solution,the reversible fouling resistance dominated the total fouling resistance thereby improving the fouling

end m

m(Ci[rmmofea

resistance ability of the bl

. Introduction

Membrane process at present time is extensively applied inany sectors of the manufacturing industries including water

esalination, ultra-pure water production, oil–water separation,roduction of beverages, electro-coat paint recovery, etc. [1]. Mem-rane processes have gained more importance than the other sep-ration processes due to low energy consumption, easy scale-up,ess or no use of chemicals and no harmful by-product formation.

Ultrafiltration (UF) is the most commonly used membrane sepa-ation method used to separate a solution containing desirable andndesirable components. Typical rejected species include sugars,io-molecules, polymers, and colloidal particles [2]. UF membrane

rocess depends on physical properties of the membrane, such asydraulic permeability, membrane thickness, process and systemariable like system pressure, filtration time and feed concentra-ion.

∗ Corresponding author. Present address: Department of Chemical Engineering,ational Institute of Technology, Tiruchirappalli 620015, India.el.: +91 431 2501811; fax: +91 431 2500133.

E-mail address: arthanaree10@yahoo.com (G. Arthanareeswaran).

r3g

phmbsn

383-5866/$ – see front matter © 2008 Elsevier B.V. All rights reserved.oi:10.1016/j.seppur.2008.08.010

embranes.© 2008 Elsevier B.V. All rights reserved.

In general, Cellulose acetate (CA) is one of the best membraneaterials and is very important in the field of Reverse osmosis

RO), UF, Gas permeation [3]. Asymmetric UF membranes based onA were prepared and studied extensively as a function of cast-

ng solution composition and membrane formation mechanism4]. CA membranes has been prepared by many of the membraneesearchers and characterized for their compaction, hydraulic per-eability and osmotic permeability [5]. CA and its derivativeembranes made them suitable as membrane materials because

f their moderate flux and moderate salt rejection, has good filmorming properties, they are relatively easy to manufacture, costffective, and highly non-toxic. However, CA is not suitable for moreggressive cleaning, has low oxidation, has poor resistance to chlo-ine, can be used only in the narrow temperature range (maximum0 ◦C) and has poor mechanical strength, the modification of CAains importance [6].

In practice, CA is blended with other polymers to improve theerformance of the membranes [7–12]. The blend membranes

ave shown improved permselectivity and permeability than thoseade of individual polymers. Few researchers have investigated the

lend of organic polymer with inorganic materials like alumina,ilica, zirconia, titania, etc. [13–18]. Bottino et al. [16] prepared theovel organic–inorganic membranes formed by uniform dispersion

and Pu

orStocptmtebatimiircbriotrdi

CbahpTmmrom

2

2

cforisastaoMg

2

i

tvc(tet4bwbtdr2cohmttctgtskwas

2

dw3i3ow

2

ptmMs

2

atte

2

G. Arthanareeswaran et al. / Separation

f fine silica particles in the porous matrix of polyvinylidenefluo-ide (PVDF) and observed that with the increasing concentration ofiO2 in PVDF, the permeate flux increased with the lowered pro-ein retention. Yan et al. [18] prepared blend membranes basedn PVDF and alumina (Al2O3) materials by phase inversion pro-esses and characteristic studies like membrane hydrophilicity,orosity, protein retention and surface morphologies were inves-igated. It was observed that the permeation flux increase of the

embrane is attributed to surface hydrophilicity increase due tohe hydrophilic inorganic nano-sized Al2O3 particles addition. Yangt al. [13] prepared PSf/TiO2 organic–inorganic composite mem-ranes via phase-inversion process and observed that with theddition of TiO2 particles to polymer casting solution resulted inhe increased pore numbers in the skin layer and the hydrophilic-ty also improved apparently that enhanced the pure water flux. The

echanical strength of membrane was enhanced through addingnorganic fillers, especially, at 2 wt.% filler concentration, the burst-ng strength and the breaking strength increased 50% and 26.7%,espectively. Wara et al. [14] reported the fabrication of nano-omposite membranes of Al2O3 particles in CA by using the solutionlending. We have examined consist of inorganic domains that areandomly dispersed with the polymer matrix. The presence of thenorganic phase can also serve to restrict the molecular motionsf the polymer chains and to induce an increase in the mean dis-ance (free volume) between polymer chains. We postulate that theestricted molecular motions and a favorable increase in the meanistance between chains can lead to a simultaneous improvement

n selectivity and permeability.Hence, for the present study, SiO2 particles are blended with

A with an increment of 10 wt.% from 0 to 40 wt.% to obtain mem-ranes with improved quality in terms of UF characteristics suchs compaction, pure water flux, percent water content, membraneydraulic resistance, and in terms of % solute rejection, MWCO,ore statistics, mechanical stability and membrane morphology.he fouling-resistance ability and the recycling of the CA/SiO2 blendembranes were evaluated through UF experiments using BSA asodel protein. Further, increase of SiO2 in CA is above 40 wt.%,

esulted in the turbid form and the polymer phase separationccurred. Hence, the threshold limit of SiO2 is maintained to aaximum of 40 wt.% in CA blend composition.

. Experimental procedure

.1. Materials

The CA used was obtained from Mysore Acetate and Chemi-als Company Limited, India, and was used, after recrystallatizationrom acetone. Analar grade N,N-dimethyl Formamide (DMF) wasbtained from was and sieved through molecular sieves foremoving moisture and stored in dried condition. Fumed sil-ca powders (hydrophilic silica powder, average primary particleize = 0.014 mm) was purchased from Central Drug House, Indiand dried at 80 C before use. Sodium lauryl sulfate (SLS) used asurfactant was obtained from Sisco Research Limited, India. Pro-eins like trypsin, pepsin, and bovine serum albumin (BSA) werelso obtained from Sisco Research Limited, India. Egg albumin wasbtained from Central Drug House, India. Phosphate buffers likeono-sodium di-hydrogen ortho phosphate and di-sodium hydro-

en ortho phosphate were procured from Central Drug House, India.

.2. Solution blending and membrane formation

CA–SiO2 flat sheet membranes were prepared by the phase-nversion method. A casting solution was prepared by dissolving

e[

J

rification Technology 64 (2008) 38–47 39

he CA (17.5 wt.%, by weight of the solution) in the DMF as sol-ent at room temperature and adding fumed SiO2 particles to theasting dopes while stirring. For the organic–inorganic membranesCA–SiO2), the CA polymer was first dissolved in the solvent andhen thoroughly mixed with the SiO2. Special care was taken tonsure homogeneous dispersion of the SiO2 particles. The concen-ration of SiO2 added to CA varied from 0 to 40 wt.%, beyond the0 wt.% resulted in the turbid form and did not form the mem-rane. This may due to concentration of CA in the casting solutionas reduced below 60 wt.%, in other words the polymer rich phaseecame very weak in the membrane formation, which in turn leado the inhibition of membrane formation. In order to obtain optimalispersions of the particles in the polymer solutions, agitation wasequired for at least 24 h. The casting solutions were then kept for4 h to remove air bubbles. The polymer solution with SiO2 parti-les was kept in dehumidifying chamber with a relative humidityf 60 ± 5%. The solution was then casted over a glass plate with theelp of doctor blade. The thickness of prepared membranes waseasured in different places of membrane, the average membrane

hickness was 0.2 ± 0.02 mm. This thickness is maintained for allhe membranes. The casted solution was then allowed to be in thehamber for 30 s for the evaporation of solvent from top surface ofhe membrane. The casted membrane was then immersed in theelation bath with water as non-solvent and DMF as solvent withhe addition of SLS as surfactant for 30 min for the leaching out ofolvent from the membrane surface [20]. This method is commonlynown as phase inversion technique [21]. The formed membranesere stored in the 0.1 wt.% formalin solution to avoid microbial

ttack. The stepwise preparation of the polymeric membrane ishown in Fig. 1.

.3. Experimental set up

The characterization methods were carried out in a batch type,ead end cell (ultrafiltration cell-S76-400-Model, Spectrum, USA)ith a diameter of 76 mm and effective membrane filtration area of

8.5 cm2 fitted with Teflon coated magnetic paddle. While carry-ng out the protein rejection studies, a constant agitation of speed00 rpm was used in to order to reduce concentration polarizationf the membranes. The cell was connected to a nitrogen cylinderith the pressure control valve and gauge through a feed reservoir.

.4. UF characterization

The prepared membranes were characterized for compaction,ure water flux, membrane hydraulic resistance, protein rejec-ion studies, fouling-resistant ability and recycling of the blend

embranes using dead end UF experiment. The % water content,WCO, pore statistics, mechanical stability and morphological

tudies were also investigated.

.4.1. CompactionThe thoroughly washed membrane was cut into desired shape

nd fitted in UF kit. The distilled water was fed into the UF kit fromhe pressure reservoir and the initial water flux was taken, 20 s afterhe pressurization at 414 kPa [22]. The permeate was collected forvery 1 h of time interval till it attains steady state.

.4.2. Pure water flux (PWF)After compaction, the experiment was carried out at a differ-

ntial system pressure of 345 kPa and the permeate was collected22]. The PWF was calculated using the equation:

w = Q

�t × A(1)

40 G. Arthanareeswaran et al. / Separation and Purification Technology 64 (2008) 38–47

wpm

2

mfwtw

%

2

ttm2tc

R

w�

Table 1Average solute radius of the proteins

Protein Molecular weight (kDa) Average solute radius (Å)a

Trypsin 20 21.5Pepsin 35 28.5EB

2

htet

%

waasfu(T(sabc(

2

lFpwc

R

waTcw

p

ε

w(pressure (dyn/cm2).

From the known values of ε and R (in cm), the pore density inthe membrane surface can be calculated using equation:

Fig. 1. Scheme showing the preparation method of CA/SiO2 UF membrane.

here Jw is the pure water flux (l m−2 h−1); Q is the amount ofermeates collected (l); �t is the sampling time (h) and A is theembrane area (m2).

.4.3. % Water contentWater content of the membranes was obtained after soaking

embranes in water for 24 h and the membranes were weighedollowed by mopping it with blotting paper. The wet membranesere placed in vacuum drier at 75 ◦C for 48 h and the dry weights of

he membranes were determined [22]. The percent water contentas calculated given by the equation:

Water content = (wet sample weight − dry sample weight)wet sample weight

×100 (2)

.4.4. Membrane hydraulic resistance (Rm)The membrane hydraulic resistance is the resistance offered by

he membrane to the flow of the feed. It is an indication of theolerance of the membrane towards hydraulic pressure and is deter-

ined by subjecting the membranes to varied pressure of 345, 275,07, 138, 69 kPa, respectively. It is evaluated from the inverse ofhe slope of transmembrane pressure vs. pure water flux. It is alsoalculated by the equation:

m = �p

Jw(3)

here Rm is the membrane hydraulic resistance (kPa/l m−2 h−1) andp is the pressure difference (kPa).

n

w

gg albumin 45 33.0SA 69 45.0

a Values given by Sarbolouki [24].

.4.5. Molecular weight cut-off (MWCO)MWCO characterization involves finding inert solutes, which

as the lowest molecular weight that is more than 80% and lesshan 100% retained by the membrane in a steady state RO or UFxperiments. Here the percent solute rejection, % SR is found usinghe equation:

SR = 1 − Cp

Cf× 100 (4)

here Cp and Cf are the concentrations of the solute in perme-te and feed solutions. This method is applied to the homogenousnd heterogeneous membranes, and the molecular weight of theolute retained depends only on the structure of the upstream sur-ace layer of the membrane [23]. The feed solutions were preparedsing the proteins of different molecular weight such as trypsin20 kDa), pepsin (35 kDa), egg albumin (45 kDa) and BSA (69 kDa).he protein solutions of 0.1 wt.% were prepared by dissolving it in0.5 M, pH 7.2) phosphate buffer that were used as standard feedolutions. The pH of the buffer solution was maintained at 7.2, sinceny change in the pH may lead to adsorptive fouling of the mem-rane surface [24]. The concentration of the feed and the permeateollected was found using UV–visible Double beam SpectrometerElica, SL 164, Double Beam) at �max = 280 nm.

.4.6. Pore statisticsFor studying the pore statistics, the proteins of different molecu-

ar weight such as trypsin, pepsin, egg albumin and BSA were used.rom the protein rejection studies described below, the averageore radius, surface porosity and pore density of the membraneere studied. The pores statistics [23] like average pore radius was

alculated using the equation:

= ˛

SR%× 100 (5)

here R is the average pore radius (Å) of the membrane; ˛ is theverage solute radius (Å) and is constant for each molecular weight.he average solute radii is known as “stoke radii” and the value of ˛an be found from the plot between the solute radius and moleculareight of the solute given by Sarbolouki which is shown in Table 1.

Assuming the membrane to be asymmetric skin type, the surfaceorosity of the membrane was found using the equation:

= 3��Jw

R × �P(6)

here ε is the surface porosity; � is the viscosity of the permeateg/cm s); Jw is the pure water flux (cm/s) and �P is the applied

= ε

� × R2

(7)

here n is number of pores/cm2.

and Pu

2

tblstsi

2

CMpippmuf

2

spisuhwiu

2

dbfre

%

rb

r

Hriirob

r

r

r

taifpeoflws

3

3

vogbpA2cfgiWf1frlar results were obtained by Yan et al. [18] for the alumina particlesin the PVDF blend membranes. The steady state flux of 100 wt.%CA and 40 wt.% SiO2 in the CA blend membrane resulted in anincreased value from 20.54 to 59.204 l m−2 h−1. Arthanareeswaranet al. [26] reported similar steady state observations for CA/PEG

G. Arthanareeswaran et al. / Separation

.5. Mechanical stability

The tensile intensity, tensile stress and break elongation ratio ofhe membranes were examined to investigate the mechanical sta-ility. Two dumbbell shaped specimen of 5 mm wide and 10 mm

ong, were punched out of the membrane film. Mechanical studiesuch as tensile strength, tensile stress and percentage break elonga-ion ratio were carried out using Instron 4500 model tensile testingystem at an extension rate of 2 mm/min, after fixing the samplesn the holders [13,18].

.6. Morphological studies

The top surface and cross-sectional morphology of theA/SiO2 blend membranes were studied using Scanning Electronicroscopy (SEM) (JEOL JSM-6360). The membranes were cut into

ieces of various sizes and mopped with filter papers and immersedn liquid nitrogen for few seconds and were frozen. The sam-les were mounted on the platinum sputtered sample holders torovide electrical conductivity to very thin layers of polymericembranes and photomicrographs were taken in very high vac-

um conditions operating at 20 kV with a magnification of 5000×or top surface and 250× for cross-section of the membranes.

.7. Protein separation studies

The protein rejection study was carried out using the proteinolutions of different molecular weight such as trypsin (20 kDa),epsin (35 kDa), egg albumin (45 kDa) and BSA (69 kDa). The exper-

mental pressure was maintained at 345 kPa to carry out the furthertudy. The protein of lowest molecular weight such as trypsin wassed first in order to reduce the fouling of the membranes withigher molecular weight protein molecules. The % solute rejectionas found for each protein solution using the prepared membranes

ndividually and was calculated using the Eq. (4) and their individ-al protein flux was also calculated using the Eq. (1).

.8. Fouling-resistant ability and recycling of blend membranes

After 4 h of ultrafiltration, the membranes were washed witheionized water for 20 min and the water flux of the cleaned mem-ranes was measured (Jw2) at 345 kPa. In order to evaluate theouling-resistant ability of the blend membranes, flux recoveryatio (FRR) was introduced [25] and calculated using the followingxpression:

FRR = Jw2

Jw1× 100 (8)

To analyze the fouling process in details, we defined severalatios to describe the fouling-resistant ability of the blend mem-rane. The first ratio was rt as in equation:

t = 1 − JpJw1

(9)

ere, rt was the degree of total flux loss caused by total fouling.r and rir were also defined to distinguish reversible fouling andrreversible fouling. Reversible fouling ratio (rr) describes the foul-ng caused by concentration polarization and irreversible foulingatio (rir) describes the fouling caused by adsorption or deposition

f protein molecules on the membrane surface. They are definedy equations:

r = (Jw2 − Jp)Jw1

(10)F

rification Technology 64 (2008) 38–47 41

ir = (Jw1 − Jw2)Jw1

(11)

Obviously, rt was the sum of rr and rir:

t = rr + rir (12)

In order to test the recycling potential of CA/SiO2 membranes,hree repetitive UF operations were carried out using BSA solutiont 345 kPa. The BSA solutions of 0.1 wt.% were prepared by dissolv-ng in (0.5 M, pH 7.2) phosphate buffer that were used as standardeed solutions. This recycling process includes four times run ofure water flux and three times run of BSA solution flux using deadnd UF experiment cell. The water cleaning processes were carriedut after every time of BSA solution flux, and again the pure waterux of cleaned membranes was measured. The process was stoppedith four runs, because the BSA molecules foul on the membrane

urface and the flux remained constant.

. Results and discussion

.1. Compaction

In this investigation, CA was modified by blending with SiO2 atarying composition from 0 to 40 wt.%. The effect of SiO2 particlef blend membranes can be visualized from Fig. 2 where theraph is plotted between compaction time vs. pure water flux oflend membranes. The initial flux that was collected after 20 s ofressurization, showed a value of 38.95 l m−2 h−1 for 100 wt.% CA.s the compaction time increased, the value of PWF decreased to0.54 l m−2 h−1 after 5 h and attained the steady state value, whichonfirms the completion of compaction process. This is due to theact that during compaction of membrane under pressure, the reor-anization of polymeric chains occurs which leads to the changen the membrane structure with the formation of uniform pores.

hen the concentration of SiO2 particle added to CA increasedrom 0 to 40 wt.%, the initial flux was increased from 38.95 to40.22 l m−2 h−1. This increase in trend may be due to the increasedree volume between CA and SiO2 in the polymer matrix whichesulted in the formation of pores on the membrane surface. Simi-

ig. 2. Effect of compaction on pure water flux of CA/SiO2 blend UF membrane.

42 G. Arthanareeswaran et al. / Separation and Purification Technology 64 (2008) 38–47

Table 2Effect of SiO2 concentration on CA blend membranes

Polymer (17.5 wt.%) Solvent (wt.%) PWF at 345 kPa (l m−2 h−1) Water content (%) Rm (kPa/l m−2 h−1) MWCO (kDa)

CA SiO2 DMF

100 0 82.5 15.58 76.59 21.65 2090 10 82.5 35.83 78.75 9.37 35

brobost

3

mT1attm4

stbi

3

m(TvttbwFcttsiPtabt

3

psi

m1lwtapvpudbibb8

3

stio1sfis[mb3ttas

3

fmsipaRb

80 20 82.5 42.0770 30 82.5 45.1860 40 82.5 46.74

lend ultrafiltration membranes. It is believed to form uniform,igid pores during compaction on the membrane surface and tobtain a steady state flux. Similar trend was obtained for the mem-ranes with varying composition of 90/10, 80/20 and 70/30 wt.%f CA/SiO2 blend composition. Further, if any trace quantity ofolvent present in the membrane surface, which is pass throughhe membrane along with the permeate during compaction.

.2. Pure water flux (PWF)

After completion of compaction, the pure water flux of blendembranes were studied at 345 kPa pressure and shown in Table 2.

he low pure water flux value of 15.58 l m−2 h−1 is obtained for00 wt.% CA membranes. This is because for the pure 100 wt.% CA,very tight polymer matrix is formed, thus the pores formed on

he membrane surface will be in smaller size. When the concentra-ion of SiO2 is increased from 10 to 40 wt.% in the pure CA blend

embrane, the PWF showed an increased value from 35.83 to6.74 l m−2 h−1.

The increase in the pure water flux may be due addition of fumedilica particles to CA has been confirmed to systematically increasehe average size of the free volume. Similar observations were madey He and Morisato [27], non-porous, nano-size fumed silica, wasncorporated into a PMP matrix.

.3. % Water content

Water content of the membrane relates hydrophilicity of theembrane [28]. The % water content was calculated using the Eq.

2) and the value of the membranes were shown in Table 2. Fromable 2, the % water content of 100 wt.% CA membrane showed aalue of 76.59%. In case of 60/40 wt.% of CA/SiO2 blend membrane,he percent water content was 82.56%. When the 40 wt.% SiO2 par-icles were present in the pure CA membrane, the water content oflend membrane was increased. Similar increase in water contentas interpreted by Lv et al. [12] when small amount of Pluronic

127 was incorporated into CA membranes. The increased waterontent may be due to the detachment of polymer chains fromhe silica surface, causing the interface voids. Further, this leadso increase in void volume resulting in the formation of biggerize pores on the membrane surface and increase the water uptaken the pores. Similar results were obtained by Yan et al. [18] forVDF/Al2O3 blend membranes which reveal that with the addi-ion of Al2O3 particle to the PVDF blend membrane, the contactngle decreased which showed an increase in hydrophilicity of thelend membranes which confirms an increased uptake of the waterhrough the membrane pores.

.4. Membrane hydraulic resistance (Rm)

In order to determine the membrane hydraulic resistance, theure water flux was measured by varying the transmembrane pres-ure of the system to 414, 345, 276, 207, 138, 69 kPa. The value of Rm

s calculated by Eq. (3), which reveals the resistance offered by the

iit

u

79.54 8.22 4581.32 7.54 4582.56 7.15 45

embrane to the feed flow. From Table 2, it is evident that the pure00 wt.% CA exhibited a higher value of 21.65 kPa/l m−2 h−1 due toow porosity of the CA membrane. In the CA/SiO2 blend membranes,

hen the concentration of SiO2 increased with 10 wt.% incrementso a maximum of 40 wt.% in the casting solution, Rm value gradu-lly decreased to 7.15 kPa/l m−2 h−1. The introducing of silica in theolymer chains is expected to be effective in promoting interfaceoid formation during the membrane preparation. The enhancedore size of the CA/SiO2 membrane due to the extended free vol-me between the polymer chains by addition SiO2 resulted in theecrease of Rm value. Yang et al. [13] studied PSf/TiO2 blend mem-ranes in terms of numerous pores on the membrane surface, which

s inversely proportional to the water flux of the respective mem-ranes. Similar decrease in Rm values was obtained for the otherlend membranes with varying composition of CA/SiO2 as 90/10,0/20, 70/30 wt.%.

.5. Molecular weight cut-off (MWCO)

The MWCO of a membrane corresponds to molecular weight ofolutes that has the rejection of more than 80% [23]. MWCO of allhe membranes of varying composition of CA/SiO2 was determinedndividually based on the study of protein rejection using proteinsf different molecular weight and tabulated in Table 2. MWCO of the00 wt.% CA is compared with the blend membranes with compo-ition 90/10, 80/20, 70/30 and 60/40 wt.% of CA/SiO2. It is evidentrom Table 2 that the MWCO of the pure 100 wt.% CA membranes 20 kDa. This is because the membrane formed of pure CA hasmaller pores size due to the tight polymer matrix. Malaisamy et al.30] obtained similar results for CA/Sulfonated polysulfone blend

embranes. When considering the MWCO of the newly formedlend membrane of 90/10 wt.% CA/SiO2 showed a higher value of5 kDa. With the further increase of 40 wt.% SiO2 to CA membrane,he MWCO further increased to 45 kDa. The increase in MWCO ofhe blend membranes may be due to the increase in free volume anddecrease in polymer chain segmental mobility by the presence of

ilica material in the polymer casting solution [31].

.6. Pore statistics

Pore statistics includes average membrane pore radius (Å), sur-ace porosity, and pore density. Average pore radius (R) of the

embrane is found from the % solute rejection using the proteinolutions prepared from trypsin, pepsin, EA, and BSA of 0.1 wt.%n phosphate buffers. The pure 100 wt.% CA membrane was com-ared with the CA/SiO2 blend composition of 90/10, 80/20, 70/30,nd 60/40 wt.%. It is observed that for pure 100% CA, the value ofis found to be 26.88 Å. The relation between the average mem-

rane pore radius and the concentration of silica in CA is plotted

n Fig. 3(a) and observed that the membrane average pore radiuss 41.25 Å when the concentration of SiO2 is increased to 40 wt.% inhe CA membrane [16].

Surface porosity (ε) gives the detail about total pore area pernit surface area of the membrane and calculated using Eq. (6).

G. Arthanareeswaran et al. / Separation and Pu

Fig. 3. (a) Effect of SiO2 concentration on average pore radius of CA/SiO2 blend UFmembranes; (b) effect of SiO2 concentration on surface porosity of CA/SiO2 blend UFmembranes; and (c) effect of SiO2 concentration on pore density of CA/SiO2 blendUF membranes.

ToT6pmoFv(CfmeaTcTs

3

iTb2wSa1ofliscuembmg

3

css8mptbSmnpm2Sbp

rification Technology 64 (2008) 38–47 43

he value of ε is found to be 5.72 × 10−5 when the concentrationf SiO2 is 0 wt.% in CA membrane which is observed from Fig. 3(b).he ε value showed comparatively a higher value of 10.89 × 10−5 for0/40 wt.% CA/SiO2 blend membrane. It was concluded that silicaarticles yield more polymer/particle interfacial area and provideore opportunity to disrupt polymer chain packing [29]. The result

n membrane pore density of CA/SiO2 blend membrane is shown inig. 3(c) where the graph is drawn between the silica concentrations. pore density (number of pores/cm2) and calculated using Eq.7). When 100 wt.% CA membrane is compared with the 60 wt.%A blend membranes, the pore density showed an increased valuerom 6.77 × 109 to of 8.4 × 109 which is apparent from the graph

entioned above. While adding silica (40%) to the casting solutionnhances the pore density (8.4 × 109) caused by the silica particlesggregate phenomenon, which leads to a pure water flux (Fig. 2).hese results demonstrate that adding if SiO2 particles to CA matrixan improve its porosity and increase the number of small pores.he effect of inorganic materials on polymer matrix in terms of poretatistics was investigated elsewhere [18,32].

.7. Mechanical stability

The testing results of mechanical stability including tensilentensity, tensile stress and break elongation ratio are listed inable 3. It is clear that the tensile intensity, tensile stress andreak elongation ratio of pure 100 wt.% CA membrane is 3.07 N,.05 N/mm2, 8.84%, respectively. The mechanical stability enhancesith the increase of the filler concentration, especially, at 10 wt.%

iO2 concentration to CA membrane, tensile intensity, tensile stressnd break elongation ratio reach a peak value of 3.79 N, 2.53 N/mm2,6.29%, respectively, and then decline with the further increasef filler concentration. These trend can be appreciated that theacts caused by addition of SiO2 particles such as the thicker skin-ayer, the suppression of macrovoids, and the interaction betweennorganic filler and polymers, result in the increase of mechanicaltrength of membrane. However, an excessive filler concentrationan cause the particles aggregation and make them not to disperseniformly in polymeric matrix, which will decline the membranelasticity, accordingly leading to weaken the mechanical stability ofembrane. Yang et al. [13] made similar observations for PSf/TiO2

lend UF membranes. Further, Yan et al. [18] also observed theechanical stability in terms of tensile intensity and break elon-

ation ratio for Al2O3/PVDF blend UF membranes.

.8. Morphological studies

To attain a better performance of membranes, the morphologi-al structure of the membrane has to be manipulated. Here in thistudy, the morphology of the membrane top surface and cross-ection of 100 wt.% CA is compared with the membranes made of0/20 and 60/40 wt.% of CA/SiO2 blend composition observed at aagnification of 5000× and 250× using SEM. The top surface of

ure 100 wt.% CA membrane is observed from Fig. 4(a) and seenhat with 0 wt.% of SiO2 in blend composition, the smooth mem-rane surface is formed and the smaller size pores are formed.imilar observations were made by Lv et al. [12] for novel blendembranes with CA and Pluronic F127 using phase separation tech-

ique. Fig. 4(b) and (c) shows that the aggregates SiO2 particles areacked by polymers or adsorbing on the surface of CA/SiO2 blend

embrane. When the concentration of SiO2 content increased to

0 wt.% in CA, the number of pores on the surface also increased.imilarly, with the further addition of 40 wt.% SiO2 to CA mem-rane, the membrane top surface is clearly visible with increasedore density. The higher silica concentration (40 wt.%) induces an

44 G. Arthanareeswaran et al. / Separation and Purification Technology 64 (2008) 38–47

Table 3Mechanical stability of the CA/SiO2 blend UF membranes

Composition (17.5 wt.%) Solvent (wt.%) Tensile intensity (N) Tensile stress (N/mm2) Break elongation ratio (%)

CA SiO2 DMF

100 0 82.5 3.07 2.05 8.8490 10 82.5 3.79 2.53 16.2980 20 82.5 3.3270 30 82.5 2.4560 40 82.5 2.01

Fig. 4. (a) SEM of the top surface view of pure CA membrane; (b) SEM of the topsurface view of 80/20 wt.% of CA/SiO2 blend UF membrane; and (c) SEM of the topsurface view of 60/40 wt.% of CA/SiO2 blend UF membrane.

asibdccfic

osSmitt(bwgwm

3

3

sptufipTa1cptstBts(Stawmmbp

2.21 13.251.63 10.161.38 8.01

pparent aggregate phenomenon, and produces a number of largeurface pores (see Fig. 3(b)). These facts indicate that there existnterfacial stresses between polymer and silica particles, whichuild up and finally are comfortable by forming interfacial poresue to the shrinkage of organic phase during the demixing pro-ess. Lin et al. [33] made similar observations when the lithiumhloride was added to the PVDF membrane. These facts can be con-rmed from the pore statistics of the blend membranes of varyingomposition of CA/SiO2 as described above.

The cross-section of the CA/SiO2 blend membranes wasbserved at a magnification of 250×. Fig. 5(a–c) shows the cross-ection of CA/SiO2 blend membranes that with the increase in theiO2 concentration, the asymmetric structure of the 100 wt.% CAembrane (Fig. 5(a)) becomes faint, the thickness of skin layer

ncreases with the addition of SiO2 of 20 wt.% in CA (Fig. 5(b)) andhe structure of sub-layer undergoes a transition from finger likeo sponge like structure for 60/40 wt.% of CA/SiO2 blend membraneFig. 5(c)). This shows the distribution of the SiO2 on the mem-rane surface that forms finger like pores linked by the sponge wallshich confirmed that a large number of micropores allow the fin-

er like pores to communicate with each other. The similar resultsere observed by Arthanareeswaran et al. [9] for CA/SPEEK blendembranes.

.9. Separation of proteins

.9.1. Rejection of proteinThe rejection of proteins like BSA, EA, pepsin and trypsin were

tudied separately with the blend membranes of CA/SiO2 of com-osition 100/0, 90/10, 80/20, 70/30 and 60/40 wt.%. The proteins forhe rejection study was used in the increasing order of their molec-lar weight hence, the lowest molecular weight trypsin was usedrst to avoid the blocking of the pores by higher molecular weightrotein molecules that might lead to fouling of the membranes.he protein rejection of blend composition of 90/10, 80/20, 70/30nd 60/40 wt.% of CA/SiO2 membranes were compared with pure00 wt.% CA membrane. The efficiency of protein rejection on blendomposition of membranes is shown in Fig. 6 where the graph islotted between silica concentration vs. % solute rejection. Fromhe graph it is evident that the rejection for 100 wt.% CA membranehowed a value of 94% for BSA and 80% for trypsin. The higher rejec-ion of protein may be due to the higher molecular weight of theSA solutes (69 kDa) when compared with lower molecular weightrypsin solutes (20 kDa). Malaisamy and Mohan [34] interpreted aimilar increase in protein rejection with higher molecular weightBSA) for CA/SPSf blend UF membranes. As the composition of theiO2 content increased from 10 to 40 wt.% in the casting solution,he % solute rejection of BSA decreased from 92 to 81%. Similarly,decreasing trend was obtained for all other blend membranes

ith other proteins like EA, pepsin, and trypsin, respectively. Thisay be due to the increased inhomogeneity between the polymeratrices, resulting in the formation of aggregate pores in the mem-

ranes. Bottino et al. [16] obtained a similar decreasing trend ofrotein rejection for PES blend membrane.

G. Arthanareeswaran et al. / Separation and Purification Technology 64 (2008) 38–47 45

Fct

3

fipctacf

Fb

mwct1mpiimiwpu

3.10. Analysis of membrane fouling during ultrafiltration

Under constant pressure, the effects of membrane fouling andconcentration polarization were usually observed by a considerabledecline in the flux with time. In the present work, the concentration

ig. 5. (a) SEM of the cross-sectional view of pure CA membrane; (b) SEM of theross-sectional view of 80/20 wt.% of CA/SiO2 blend UF membrane; and (c) SEM ofhe cross-sectional view of 60/40 wt.% of CA/SiO2 blend UF membrane.

.9.2. Permeate flux of proteinProtein flux is the measure of the product rate of the membrane

or given protein solution. The effect of varying SiO2 concentrationn the blend composition on the protein flux is shown in Fig. 7. Theermeate flux of pure 100 wt.% CA showed the lower value whenompared with the membranes made of increasing SiO2 concen-

ration in CA membrane. The permeate flux of 100 wt.% CA showedvalue of 5.19 l m−2 h−1 for BSA and with other proteins showed

omparatively higher permeate flux of 8.72, 11.95, 14.17 l m−2 h−1

or EA, pepsin and trypsin, respectively. The increase in the per-Fm

ig. 6. Effect of SiO2 concentration on % solute rejection of CA/SiO2 blend UF mem-ranes.

eate flux may be due to the fact that solutes of lower moleculareight pass through the smaller pore size membrane. When the

oncentration of the SiO2 is decreased to 40 wt.% in the casting solu-ion, the permeate flux for the given protein (e.g. BSA) increased to9.79 l m−2 h−1. The addition of fumed silica in CA pushes the per-eability of the CA membrane towards a bigger pore size with high

ermeability. Further incorporation of a small volume fraction ofnorganic fillers into the polymer matrix can result in a significantncrease in overall separation efficiency. The newly formed blend

embranes of 90/10, 80/20, 70/30 wt.% of CA/SiO2 showed a similarncrease in protein flux when other proteins like EA, pepsin, trypsin

ere used individually. Sivakumar et al. [28] obtained decreasedrotein flux when proteins of increasing molecular weights weresed for CA/PSf blend membrane.

ig. 7. Effect of SiO2 concentration on protein permeate flux of CA/SiO2 blend UFembranes.

46 G. Arthanareeswaran et al. / Separation and Purification Technology 64 (2008) 38–47

Table 4The total fouling resistance (rf), reversible resistance (rr), irreversible resistance (rr) and FRR value of different blend membranes using BSA solution

Composition (17.5 wt.%) Solvent (wt.%) Jw1 (l m−2 h−1) Jw2 (l m−2 h−1) Jp (l m−2 h−1) Fouling resistance ratio FRR %

CA SiO2 DMF rr rir rf

500206

poTtdflfsrttbCws

idTuFsohb

3

poCt

Fs

1AsttidtAtdmC6ohbmam

4

iS4s

100 0 82.5 15.58 9.80 20 82.5 42.07 29.60 40 82.5 46.74 35.

olarization was minimized because of the high molecular weightf BSA molecules and rigorous stirring near the membrane surface.herefore, the membrane fouling mostly caused the flux decline ofhe membranes. Membrane fouling is caused by the adsorption oreposition of the protein molecules on the membrane surface. Theux changes due to the fouling and cleaning of BSA were observed

or 100/0, 80/20 and 60/40 wt.% of CA/SiO2 blend membranes ishown in Table 4. As seen from Table 4, the irreversible foulingatio of 100 wt.% CA membrane is 0.39 and this value decreasedo 0.25 with 40 wt.% of SiO2 in CA membrane which indicates thathe fouling of BSA on the membrane surface decreased. This maye due to the fact that with the addition of inorganic particle toA membrane, the hydrophilicity of the CA membrane is enhancedhich reduces the interactions of the BSA particles on membrane

urface [19].Since long time operation with the product rate was of great

mportance for practical application of membranes, FRR was intro-uced to evaluate the recycling property of the blend membranes.he FRR values for the CA/SiO2 blend membranes were calculatedsing Eq. (8) and are presented in Table 4. It is observed that theRR values increased with the increase of SiO2 content in the castingolution, and the maximum FRR value reached 75% for 60/40 wt.%f CA/SiO2 blend UF membrane. This may be due to the increasedydrophilicity [18] with addition of SiO2 content in CA blend mem-rane.

.11. Recycling of the blend membranes

The excellent flux recovery property of blend membranes willrovide the long time membrane run without significant declinef separation performances. To study the recycling property of theA/SiO2 blend membranes, the graph is plotted between operationime vs. flux and the results are shown in Fig. 8. In case of pure

ig. 8. Flux variation of CA/SiO2 UF membranes during three times of BSA aqueousolution.

bpflhi

fgictmctiisdwmTptfltecv

5.19 0.37 0.39 0.76 6114.15 0.35 0.31 0.66 6919.79 0.38 0.25 0.63 75

00 wt.% CA membrane, the initial water flux was 15.58 l m−2 h−1.s the operation time increased, the water flux reached a steadytate value of 9.1 l m−2 h−1 after three runs of BS solution flux. Whenhe concentration of SiO2 content in the CA membrane is increasedo 40 wt.%, the initial water flux increased to 46.74 l m−2 h−1. Thencrease in permeation-flux of the CA/SiO2 blend membrane wasue to increase in the surface hydrophilicity and the efficient fil-ration area due to the addition of inorganic SiO2 particles [19].fter four runs of water flux and three runs of BSA solution flux,

he water flux reached a steady state value of 27 l m−2 h−1. Thisecrease in flux may be due to the fact that BSA particles foul theembrane surface. From the graph, it is evident that pure 100 wt.%

A membrane can be used for two runs of BSA solution flux and0/40 wt.% of CA/SiO2 blend membrane can be used for three runsf BSA solution flux. This increase may be due to the increasedydrophilicity of CA/SiO2 blend membranes that reduces the mem-rane fouling [18]. The outstanding flux recovery property of blendembranes indicates the long-run utilization and operation reli-

bility. Wang et al. [25] made similar observations for PES blendembranes.

. Conclusions

In this study, CA/SiO2 UF membranes were formed by phasenversion technique using DMF as solvent. The concentration ofiO2 particles increased from 0 to 40 wt.% to CA membrane, above0 wt.% did not form membrane. The prepared membranes weretudied for the UF performance such as compaction, PWF, mem-rane hydraulic resistance and percent water content. The UFerformance of CA/SiO2 membranes illustrates that the pure waterux and water content were increased, while the membraneydraulic resistance was decreased, as the concentration of SiO2

n the casting solution is increased.The MWCO was increased from 20 to 45 kDa with the increase

rom 0 to 40 wt.% SiO2 concentration in the casting solution. Theradual increase in average membranes pore radius, surface poros-ty and pore density of CA/SiO2 membranes were observed whenompared with pure CA membrane. The mechanical stability ofhe CA/SiO2 blend UF membranes were found using tensile testing

achine and observed that with the presence of inorganic parti-le of 10 wt.% in the casting solution, the mechanical stability inerms of tensile intensity, tensile stress and break elongation rationcreased, respectively. With the further increase of SiO2 particlesn the casting solution resulted in the decrease of the mechanicaltability of the UF membrane. The morphological studies were con-ucted using SEM. It is observed that the pore formation increasedith the addition of inorganic grains (SiO2) to the polymer (CA)embrane and change in morphological structure also observed.

he separation of protein studies also investigated using preparedure CA and CA/SiO2 UF membranes and observed that the rejec-ion of protein molecules decreased with the enhanced permeate

ux as the concentration of SiO2 particles in the casting solu-ion increased. The SiO2 content in CA/SiO2 blend membranesnhanced the fouling-resistant ability thereby improving the recy-ling ability of the CA/SiO2 blend membrane with increased FRRalue.

and Pu

R

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

G. Arthanareeswaran et al. / Separation

eferences

[1] M. Cheryan, Ultrafiltration, Microfiltration Handbook, Technomic Pub. Co. Inc.,Lancaster, PA, USA, 1998.

[2] R. Mahendran, R. Malaisamy, D. Mohan, Cellulose acetate and polyether sul-fone blend ultrafiltration membranes. Part I. Preparation and characterizations,Polym. Adv. Technol. 15 (2004) 149–157.

[3] O. Kuttowy, S. Sourirajan, Cellulose acetate UF membranes, J. Appl. Polym. Sci.19 (1975) 1449–1460.

[4] B. Kunst, D. Skevin, G. Dezolic, J.J. Peters, A light scattering and membraneformation study on concentrated CA solution, J. Appl. Polym. Sci. 20 (1976)1339–1353.

[5] S. Prabhakar, B.M. Misra, Studies on structural, kinetic and thermodynamicparameters of CA membranes, J. Membr. Sci. 29 (1986) 143–153.

[6] R.E. Kesting, Synthetic Polymeric Membranes, McGraw Hill, New York, 1971.[7] G.M. Shashidhara, K.H. Guruprasad, A. Varadarajulu, Miscibility studies on

blends of cellulose acetate and nylon 6, Eur. Polym. J. 38 (2002) 611–614.[8] C.J. Sajitha, R. Mahendran, D. Mohan, Studies on cellulose acetate–carboxylated

polysulfone blend ultrafiltration membranes––Part I, Eur. Polym. J. 38 (2002)2507–2511.

[9] G. Arthanareeswaran, K. Srinivasan, R. Mahendran, D. Mohan, M. Rajendran, V.Mohan, Studies on cellulose acetate and sulfonated poly (ether ether ketone)blend ultrafiltration membranes, Eur. Polym. J. 40 (2004) 751–762.

10] Z. Chen, M. Deng, Y. Chen, G. He, M. Wu, J. Wang, Preparation and performanceof cellulose acetate/polyethyleneimine blend microfiltration membranes andtheir applications, J. Membr. Sci. 235 (2004) 73–86.

11] R. Mahendran, R. Malaisamy, D. Mohan, Preparation, Characterization and effectof annealing on performance of cellulose acetate/sulfonated polysulfone andcellulose acetate/epoxy resin blend UF membranes, Eur. Polym. J. 40 (2004)623–633.

12] C. Lv, Y. Su, Y. Wang, X. Ma, Q. Sun, Z. Jiang, Enhanced permeation performanceof cellulose acetate ultrafiltration membrane by incorporation of Pluronic F127,J. Membr. Sci. 294 (2007) 68–74.

13] Y. Yang, H. Zhang, P. Wang, Q. Zhang, J. Li, The influence of nano-sized TiO2 fillerson the morphologies and properties of polysulfone UF membranes, J. Membr.Sci. 288 (2007) 231–238.

14] M.N. Wara, L.F. Francis, B.V. Velamakanni, Addition of alumina to celluloseacetate membranes, J. Membr. Sci. 104 (1995) 43–49.

15] R.P. Castro, Y. Cohen, H.G. Monbouette, Silica-supported PVP filtration mem-branes, J. Membr. Sci. 115 (1996) 179–190.

16] G. Bottino, V. Capannelli, P. D’Asti, Piaggio, preparation, and properties of novel

organic–inorganic porous membranes, Sep. Purif. Technol. 22 (2001) 269–275.

17] S. Liu, R. Highes, Preparation of porous aluminum oxide hallow fibre mem-branes by a combined phase-inversion and sintering method, Ceram. Int. 29(2003) 875–881.

18] L. Yan, Y.S. Li, C.B. Xiang, Preparation of PVDF UF membrane modified by nano-sized alumina and its antifouling research, Polymer 46 (2005) 7701–7706.

[

[

rification Technology 64 (2008) 38–47 47

19] L. Yan, Y.S. La, C.B. Xiang, S. Xianda, Effect of nano-sized alumina particleaddition of PVDF UF membrane performance, J. Membr. Sci. 276 (2006) 162–167.

20] M. Sivakumar, D. Mohan, R. Rangarajan, Preparation and performance of Cel-lulose acetate-polyurethane Blend membranes and their applications. Part I,Polym. Int. 47 (1998) 311–316.

21] P.V. Witte, P.J. Dijkstra, J.W.A. Vanden Berg, J. Feijen, Phase separation process inpolymer solution in relation to membrane formation, J. Membr. Sci. 117 (1996)1–31.

22] M. Sivakumar, A.K. Mohansundaram, D. Mohan, K. Balu, R. Rangarajan, Modifi-cation of CA: its characterization and application as an UF membranes, J. Appl.Polym. Sci. 67 (1998) 1939–1946.

23] M.N. Sarbolouki, General diagram for estimating pore size of UF and RO mem-branes, Sep. Purif. Technol. 17 (1982) 381–386.

24] J. Brinck, A.S. Jönsson, B. Jönsson, J. Lindau, Influence of pH on the adsorptivefouling of ultrafiltration membranes by fatty acid, J. Membr. Sci. 164 (2000)187–194.

25] Y.Q. Wang, Y.L. Su, X.L. Ma, Q. Sun, Z.Y. Jiang, Pluronic polymers andpolyethersulfone blend membranes with improved fouling-resistant abilityand ultrafiltration performance, J. Membr. Sci. 283 (2006) 440–447.

26] G. Arthanareeswaran, P. Thanikaivelan, J. Abdoul Rabuime, M. Raajenthiren, D.Mohan, Metal ion separation and protein removal from aqueous solutions usingmodified cellulose acetate membranes: role of polymeric additives, Sep. Purif.Technol. 55 (2007) 8–15.

27] P.Z. He, A. Morisato, Nanostructured poly (4-methyl-2-pentyne)/silica hybridmembranes for gas separation, Desalination 146 (2002) 11–15.

28] M. Sivakumar, D. Mohan, R. Rangarajan, Studies on cellulose acetate-polysulfone ultrafiltration membranes II. Effect of additive concentration, J.Membr. Sci. 268 (2006) 208–219.

29] S.B.T. Ersolmaz, Effect of zeolite particle size on the performance ofpolymer–zeolite mixed matrix membrane, J. Membr. Sci. 175 (2000) 285–288.

30] R. Malaisamy, R. Mahendran, D. Mohan, Cellulose acetate and sulfonatedpolysulfone blend ultrafiltration membranes. Part II. Pore statistics, molecu-lar weight cutoff, and morphological studies, J. Appl. Polym. Sci. 84 (2002)430–444.

31] C.J. Cornelius, E. Marand, Hybrid silica-polyimide composite membranes: gastransport properties, J. Membr. Sci. 202 (2002) 97–118.

32] C.D. Jones, M. Fidalgo, M.R. Wiesner, A.R. Barron, Alumina ultrafiltration mem-branes derived from carboxylate–alumoxane nanoparticles, J. Membr. Sci. 193(2001) 175–184.

33] D.-J. Lin, C.-L. Chang, F.-M. Huang, L.-P. Cheng, Effect of salt additive onthe formation of microporous PVDF membranes by phase inversion fromLiCl4/Water/DMF/PVDF system, Polymer 44 (2003) 413–422.

34] R. Malaisamy, D. Mohan, Cellulose acetate and sulfonated polysulfone blendultrafiltration membranes. Part III. Application studies, Ind. Eng. Chem. Res. 40(2001) 4815–4820.