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Journal of Membrane Science 330 (2009) 189204

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Journal of Membrane Science

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The effect of heat treatment of PES and PVDF ultrmorph n

A. Rahim nahMembrane Res

a r t i c l

Article history:Received 22 MReceived in revAccepted 27 DAvailable onlin

Keywords:Heat treatmenUltraltrationPolyethersulfonePoly(vinylidene uoride)Antifouling

and. The

estigalysis.eratuuxction

C coof PE

increases the protein rejection. This is due to slight decrease in membrane surface pore size. The treatmentof PES membrane with water at higher temperature results in a porous structure with superior perfor-mance. The fouling analysis of 20 min treated membrane indicates that the surface properties of 100 Cair treated and 95 C water treated PES membranes are improved compared to untreated membrane. TheSEM observation depicts that the morphology of air and water treated PVDF membranes was denser andsmoother with increasing the heat treatment temperature. The 20 min air treated PVDF membranes at

1. Introduc

Ultraltrmilk proteifor proteinpharmaceuasymmetricinduced phlm of thea suitable sby approprby several wtion, dry caprecipitatiosion precipis exchangedure results

Corresponcal Engineerin67149, Iran. Te

E-mail add

0376-7388/$ doi:10.1016/j.m100 C and water treated at 95 C exhibited the highest performance and antifouling properties. 2008 Elsevier B.V. All rights reserved.

tion

ation (UF) is widely used in the dairy industry forn standardization and in cheese manufacturing plantsconcentration in addition to water, waste water and

tical industry [1]. A common method to prepare thepolymeric ultraltration membranes is the diffusion-

ase separation technique [25]. In this process, a thinpolymer dissolved in an appropriate solvent is cast onupport by lmograph and phase separation is inducediate non-solvent. The phase inversion can be obtained

ays such as thermal and vapor-induced phase separa-sting and immersion precipitation [6]. The immersionn is the most efcient technique. During the immer-itation process, the solvent in the casting solution lmd with non-solvent in the coagulation bath. This proce-in asymmetric membrane exhibiting a dense top layer

ding author at: Membrane Research Center, Department of Chemi-g, Razi University, Faculty of Engineering, Tagh Bostan, Kermanshahl.: +98 912 2045410; fax: +98 831 4274542.ress: [email protected] (S.S. Madaeni).

and a porous sub-layer [7]. The polymer solution is thermodynam-ically unstable which is split into two liquid phases with differentcompositions: polymer-lean and polymer-rich. Liquidliquid phaseseparation is an important feature of the membrane formationprocess that occurs in the polymer solution after immersion in anon-solvent bath [6].

In the membrane ltration processes, the morphology of themembrane surface inuences the separation performance. Themembrane morphology can be affected by several parameters suchas composition of casting solution and coagulation bath, tempera-ture and post-treatment of the prepared membranes. Membranetreatment by hot air and water strongly changes the structure.Nouzaki et al. [8] found that the water ux was diminished andthe rejection was increased when the prepared polyacrylonitrilemembranes were treated in hot water. However the surface poresize was not changed. Tsai et al. [9] improved the pervaporationseparation of isopropanolwater mixtures using polyacrylonitrilehollow-ber membranes treated by hot air. The annealing effect ofasymmetric polyacrylonitrile membranes in hot water was inves-tigated by Jung et al. [10]. They showed that the size of pores andwater ux were reduced by annealing the membranes in hot water.Kim et al. [11] prepared the asymmetric polyacrylonitrile mem-brane with small pore size by phase inversion and post-treatment

see front matter 2008 Elsevier B.V. All rights reserved.emsci.2008.12.059ology and performance for milk ltratio

pour, S.S. Madaeni , M. Amirinejad, Y. Mansourpaearch Center, Department of Chemical Engineering, Razi University, Kermanshah, Iran

e i n f o

arch 2008ised form 11 December 2008

ecember 2008e 6 January 2009

t

a b s t r a c t

The at sheet polyethersulfone (PES)by immersion precipitation techniqueperformance of membranes were invow ltration of milk and fouling analayer by air treatment at various temp(PWF). However the milk permeation20 min with no change in protein rejefor air treated PES membrane at 100the membrane. The water treatment/ locate /memsci

altration membranes on

, S. Zereshki

poly(vinylidene uoride) (PVDF) membranes were preparedinuence of hot air and water treatment on morphology andted. The membranes were characterized by AFM, SEM, cross-The PES membrane turns to a denser structure with thick skinres during different times. This diminishes the pure water ux(MPF) was considerably improved at 100 C air treatment for. The smooth surface and slight decrease in surface pore sizempared to untreated membrane may cause this behavior forS membranes at 55 and 75 C declines the PWF and MPF and

190 A. Rahimpour et al. / Journal of Membrane Science 330 (2009) 189204

Fig. 1. Surface SEM images of untreated (a) PES and (b) PVDF membranes.

process. The effect of hot air treatment on performance of ultral-tration polyethersulfone hollow-ber membranes was studied byGholami et al. [12]. They showed that the pores of the PES hollow-ber membranes may be decreased by heat treatment. The soluteseparation was increased while the pure water permeation wasdecreased by improving the heat treatment temperature.

In this smorphologat sheet abranes werno report inmembranes

2. Experim

2.1. Materia

Polyetheand glassetamide (DGermany. T

tion. Polyvinylidenuride and polyvinylpirrolidone (PVP) with25,000 g/mol molecular weight as pore former were obtained fromAlfa-Aesar and Merck, respectively. 2-Propanol (IPA) was obtainedfrom Minko Company. Distilled water was used throughout thisstudy.

embr

anda iming Pat amogon oumatrnd 2

homubstr imn-sor (80tudy, the effects of hot air and hot water treatment ony and performance (PWF, MPF and protein rejection) ofsymmetric PES and polyvinylidenuride (PVDF) mem-e investigated. To the best of our knowledge, there is

open literature regarding the heat treatment of PVDF.

ental

ls

rsulfone (PES, Ultrason E 6020P, Mw = 58,000 g/moltransition temperature Tg = 225 C) and dimethylac-MAC) were obtained from BASF Aktiengesellschaft,he solvent, DMAC, was used without any purica-

2.2. M

PESsion vidissolvformerThe hoBasedbraneas 16 ausing aplate sbath foThe noof wateFig. 2. Cross-sectional SEM images of untreated (a) PES anane preparation

PVDF at membranes were prepared by phase inver-mersion precipitation. Dope solution was prepared byES and PVDF polymers in DMAC and adding PVP as pore

round 25 C with mechanical stirring at 200 rpm for 8 h.eneous polymer solution was kept to remove bubbles.r previous studies [1315], the concentration of mem-

ix (PES and PVDF) and pore former (PVP) were selectedwt.%, respectively. The solution was sprinkled and caste-made casting knife with 150m thickness on glass

rate. This was immediately moved to the non-solventmersion at room temperature without any evaporation.lvent for PES and PVDF were pure water and mixturevol.%) and 2-propanol (20 vol.%), respectively. The pre-d (b) PVDF membranes.

A. Rahimpour et al. / Journal of Membrane Science 330 (2009) 189204 191

Table

1Ef

fect

ofh

otai

rtr

eatm

ent

tem

per

atu

reon

PES

mem

bran

ep

erfo

rman

ce,s

urf

ace

rou

ghn

ess,

u

xlo

sses

,tot

al

ltra

tion

resi

stan

cean

d

ux

reco

very

(hea

ttr

eatm

ent

tim

e:20

min

).

Hea

ttr

eatm

ent

tem

per

atu

rePW

Fa

(kg/

(m2

h))

MPF

(kg/

(m2

h))

Prot

ein

reje

ctio

n(%

)R

ough

nes

sp

aram

eter

sC

onta

ctan

gle

()

Tota

lu

xlo

ssb

(J0

J p)/

J 0Ir

reve

rsib

le

ux

loss

c

(J0

J l)/

J 0To

talf

ouli

ng

resi

stan

ced

P/

J p

(m1

)(%

)FR

e(%

)

S a(n

m)

S q(n

m)

S z(n

m)

Un

trea

ted

PES

mem

bran

e39

429

977.

509.

3052

.76

8.1

0.93

0.83

4.9

10

1117

Hot

air

trea

ted

mem

bran

esat

:10

0 C

324

3397

4.56

5.84

42.5

68.

00.

890.

694.

3

1011

3413

0 C

151

3098

2.41

3.26

29.8

68.

40.

800.

664.

8

1011

3415

0 C

139

2698

6

8.2

0.80

0.70

5.3

10

1130

180

C10

415

991.

892.

2018

.7

0.86

0.63

9.6

10

1137

aO

per

atin

gco

nd

itio

ns

of

ltra

tion

:p

ress

ure

=3.

4ba

r,

owra

te=

5l/

min

,an

dte

mp

erat

ure

=30

C.

bTo

tal

ux

loss

=(J

0

J p)/

J 0;

J 0:

init

ialp

ure

wat

er

ux;

and

J p:

mil

kp

erm

eati

on

ux.

cIr

reve

rsib

le

ux

loss

=(J

0

J 1)/

J 0;

J 0:

init

ialp

ure

wat

er

ux;

and

J 1:

pu

rew

ater

u

xaf

ter

mil

k

ltra

tion

and

15m

inw

ater

clea

nin

g.d

Tota

lfou

lin

gre

sist

ance

=

P/

J p;

P:

tran

smem

bran

ep

ress

ure

;

:vi

scos

ity

ofp

erm

eate

;an

dJ p

:m

ilk

per

mea

tion

u

x.e

Flu

xre

cove

ry.

pared membranes were washed and stored in water for at least1 day to completely leach out the residual solvents and additives.The membranes were kept in aqueous 2-propanol solution with20 vol.% 2-propanol for 1 day. As the nal stage, membranes weredried by platemperatur

2.3. Membr

Treatmenheat treatedat 100, 120was selectefor differen

Treatmenglass ask wmembranesing. Then, thmembranes60 min).

2.4. Membr

2.4.1. ScannCambrid

MV2300) wof membrancleaned winitrogen fobroken andgold sputtewere takenmembraneimages (15,

2.4.2. AtomAtomic f

morphologDualScopeT

Denmark).mately 1 cmsurfaces weroughnessterms of the(Sq) and thevalleys (Sz)images.

2.4.3. FiltraThe perm

investigatedcell houses24 cm2. Thewith 3.2% oevaluationwater was ubranes. The345 kPa forat 345 kPafor 30 min.membraneusing the sttion, the me15 min andIn order tocing between two sheets of lter paper for 24 h at roome.

ane treatment procedure

t by hot air: the dry PES and PVDF membranes werein an oven (Parsian Teb Company) with air circulation

, 150 and 180 C for 20 min. Then, the best temperatured and the heat treatment of membranes was carried outt time periods (5, 20 and 60 min).t by hot water: PES and PVDF membranes were kept in aith hot water (55, 75 and 95 C) for 20 min. The treatedwere placed between two sheets of lter paper for dry-e best temperature was selected and heat treatment ofwas carried out for different time periods (5, 20 and

ane characterization

ing electron microscopy (SEM)ge scanning electron microscope (SEM, CamScanas used to investigate the morphology of cross-sectiones. The membranes were cut into the small pieces and

th lter paper. These pieces were immersed in liquidr 1015 s and were frozen. Frozen membranes were

kept in an air for drying. The dried samples werered for producing electric conductivity. The micrographs

in high vacuum conditions at 27 kV. The thickness ofskin layer was measured using high resolution SEM

000).

ic force microscopy (AFM)orce microscopy was employed to analyze the surface

y and roughness of membranes. The AFM apparatus wasM scanning probe-optical microscope (DME model C-21,Small squares of the prepared membranes (approxi-2) were cut and glued on glass substrate. The membranere imaged in a scan size of 2m 2m. The surface

parameters of the membranes which are expressed inmean roughness (Sa), the root mean square of the Z datamean difference between the highest peaks and lowest

were calculated by SPM DME software and surface AFM

tion performance and fouling analysiseation and separation properties of membranes werewith a cross-ow ltration rig at 30 C. The cross-ow

at sheet membrane pieces with an effective area ofpure water and pasteurized and homogenized milk

f protein and 1.5% of fat were employed as the feed forof membrane performance. In all experiments, distilledsed to characterize the PWF of fresh and treated mem-membranes were pre-compressed with pure water at

30 min. Then, the PWF (J0) and MPF (Jp) were evaluatedand at a ow rate of 5 l/min (or ow velocity of 2 m/s)The retention of protein was obtained for the preparedby measuring the amount of protein in the permeateandard Bradford method [16]. After 30 min of ultraltra-mbranes were washed with distilled water at 30 C for

the water ux of washed membranes was measured (J1).evaluate the fouling-resistant capability of membranes,


Fig. 3. SEM im

ux recover

FR (%) = J1J0

To analyzedened tobrane. Thes

total ux lo

irreversibleages of surfaces of hot air treated PES membranes at different temperatures during 20 m

y was calculated using the following expression:

100 (1)

the fouling process in details, several equations weredescribe the fouling-resistant capability of the mem-e equations are as follows:

ss = J0 JpJ0

(2)

ux loss = J0 J1J0

(3)

total foulin

P: transm

2.4.4. ContaIn order

acteristics ocontact angsured usingGermany] fionized waMembranein: (a) untreated, (b) 100 C, (c) 130 C, (d) 150 C, and (e) 180 C.

g resistance = PJp

(4)

embrane pressure and : viscosity of permeate.

ct angle measurementsto evaluate the variations in the surface wetting char-f the untreated and treated PES and PVDF membranes,les between water and membrane surface were mea-a contact angle measuring instrument [G10, KRUSS,

or the evaluation of the membrane hydrophilicity. De-ter was used as the probe liquid in all measurements.samples were cut to 5 cm 2 cm, then washed with de-


Fig. 4. Cross-s

ionized watattached toAfter a dropsurface, a vcomputer sangle was mthe average

2.4.5. Zeta pThe zeta

branes, canelectro-osmectional SEM images of hot air treated PES membranes at different temperatures during

er and dried at 30 C in vacuum oven. The samples werea smooth glass surface and placed on a black support.let of liquid was placed automatically onto the sample

ideo camera revealed the prole of the droplet on thecreen. To minimize the experimental error, the contact

easured at ve random locations for each sample andwas reported.

otentialpotential of at surfaces, such as ultraltration mem-be measured by either the streaming potential or

osis method. The streaming potential method is pre-

ferred overof at surfatrical potepotentials otro kinetica plated saoughly andsquarely inried out at(PMMA) aswas about 5were obtain20 min: (a) untreated, (b) 100 C, (c) 130 C, (d) 150 C, and (e) 180 C.

electro-osmosis when measuring the zeta potentialces. This is more convenient to measure small elec-

ntials rather than small rates of liquid ow. Zetaf the prepared membranes were measured by elec-

analyzer (EKA 1.00, Anton-Paar, Swiss) equipped withmple cell. The membrane samples were rinsed thor-

soaked in de-ionized water for 2 h and then cut2 cm 2 cm size plates. The measurements were car-25 C in KCl solution with poly(methyl methacrylate)

the reference plate (dimension of reference plate0 mm 38 mm 10 mm). Zeta potential measurementsed at pH 6.8.


Fig. 5. Three-dimensional AFM images of (a) untreated PES membrane and (b) treated PES membrane at 100 C for 20 min.

2.4.6. FTIR-ATR analysisFTIR spectra of PES and PES/PAI blend membranes were obtained

for spectroscopic investigation. All FTIR spectra were recorded bythe attenuated total refection (ATR) technique using Bruker-IFS 48FTIR spectrometer (Ettlingen, Germany) with horizontal ATR device(Ge, 45). 32 scans were taken with 4 cm1 resolution between 4000and 500 cm1.

3. Results and discussion

3.1. The morphology of untreated PES and PVDF membranes

It is well known that the performance of membranes is stronglydependent on the surface, sub-layer morphology and top layerthickness and compactness. Thus, the surface and sub-layer SEMmicrographs were employed to investigate the morphologies ofPES and PVDF membrane. The surface SEM images of the PESand PVDF

depicted in Fig. 1. The PES membrane represents numerous nod-ules in the surface. The shapes of nodules are regular with nosubstantial defects on the surface of membrane. The surface ofPVDF membrane is composed of a porous structure with dis-cernible spherical crystalline domains start to appear. In suchporous surfaces, both interconnected holes and networks areconstructed with small spherical particles connected with eachother.

The cross-sectional SEM micrographs of PES and PVDF mem-branes are represented in Fig. 2. The PES membrane exhibits atypical asymmetric structure composed of a thin and dense skinlayer and a porous bulk with nger-like structure. The skin layeris responsible for the permeation and retention of solutes whereasthe porous bulk acts as a mechanical support. For PVDF membranea thicker skin layer is established compared to PES membrane,with fewer nger-like pores in the support. Upon immersion of thepolymeric solution lm in the non-solvent bath containing large

t ofmembranes prepared via wet phase inversion are amounFig. 6. Surface SEM images of hot air treated PES membranes at 100 C during differentwater, the fast solventnon-solvent exchange occurstime: (a) 0 min, (b) 5 min, (c) 20 min, and (d) 60 min.


Fig differ

across thesive forcesnon-solvenpolymer atsub-layer aexempliedthin skin lamembrane.solvent andlead to thick(Fig. 2b).

3.2. Treatm

3.2.1. Hot aThe PES

and heated20 min. ThemembranesThe obtaine

t airajor

treateffec

e shod wi

ranesmor

Table 2Effect of hot ai

Heat treatmen

UntreatedPES membra

Hot air treated5 min20 min60 min

a Operatingb Total ux lc Irreversibld Total fouli. 7. Cross-sectional SEM images of hot air treated PES membranes at 100 C during

interface. This is combined with considerable repul-between PES and water (water is a very powerful

t for PES) leading to immediate precipitation of thethe interface. As a result, a thin skin layer and porouslong with nger-like pores are formed. This is clearly

by the image presented in Fig. 2a, which shows theyer and nger-like pores in the thick sub-layer of theOn the other hand, the slower penetration of non-

and hois no mthe un

Thegies archangemembThis iscomplete polymer segregation in PVDF solution lmer skin layer and denser sub-layer for PVDF membrane

ent of PES membranes

ir treatment of PES membranemembranes were placed in oven with air circulationat different temperatures (100, 130, 150 and 180 C) forhydrophilicity of the untreated and hot air treated PESwere elucidated by water contact angle measurement.

d results from contact angle measurements of untreated

layer of theheat treatmthe SEM mis formed w100 C. Thethe SEM im

To deterthe cross-sof membramembranesbranes at lowas strong

r treatment time on PES membrane performance, ux losses, total ltration resistance an

t time PWFa

(kg/(m2 h))MPF(kg/(m2 h)))

Proteinrejection (%)

Total ux lossb

(J0 Jp)/J0

ne 394 29 97 0.93

membranes for:351 28 97 0.92324 33 97 0.89

80 26 99 0.66

conditions of ltration: pressure = 3.4 bar, ow rate = 5 l/min, and temperature = 30 C.oss = (J0 Jp)/J0; J0: initial pure water ux; and Jp: milk permeation ux.e ux loss = (J0 J1)/J0; J0: initial pure water ux; and J1: pure water ux after milk ltratng resistance = P/Jp; P: transmembrane pressure; : viscosity of permeate; and Jp: ment times: (a) 0 min, (b) 5 min, (c) 20 min, and (d) 60 min.

treated PES membranes are presented in Table 1. Theredifference between contact angle and hydrophilicity ofed and hot air treated PES membranes.ts of heat treatment temperature on surface morpholo-wn in Fig. 3. The surfaces of PES membranes have beenth heat treatment. The nodule size of heat treated PES

are reduced compared to untreated PES membrane.e pronounced at higher temperatures. The porous sub-membranes (Fig. 4) turned to denser structures afterent being denser for higher temperatures. Moreover

icrographs reveal that a denser and thicker skin layerhen the heat treatment temperature was higher than

skin layer thicknesses were measured and indicated inages.mine the membrane shrinkage due to heat treatment,ectional SEM images were obtained. The shrinkagesnes were elucidated by measuring the thickness ofbefore and after heat treatment. The shrinkage of mem-wer temperature (100 and 130 C) was negligible. This

ly increased with hot air treatment at 150 and 180 C.

d ux recovery (heat treatment temperature: 100 C).

Irreversible uxlossc (J0 Jl)/J0

Total fouling resistanced

P/Jp (m1)Flux recovery(%)

0.83 4.9 1011 17

0.81 5.1 1011 190.69 4.3 1011 310.72 5.3 1011 28

ion and 15 min water cleaning.ilk permeation ux.


Fig. 8. FTIR-ATR spectra of (a) untreated PES membranes and (b) 95 C hot watertreated PES membranes.

The PES membranes were shrunk 18% and 40% with 150 and 180 Chot air treatments.

Table 1 reveals the inuences of heat treatment temperature onPWF and MPF along with protein rejection. The pure water uxesof membranes are diminished with increasing heat treatment tem-perature in the range of 100180 C. On the other hand, MPF ofmembranes is reasonably increased with heat treatment at 100 C.No strong change was appeared for 130 C heat treatment. The treat-ment at higher temperatures declined the MPF. The rejection ofproteins by membranes was slightly increased with heat treatment.

The decline in PWF of heat treated membranes can be attributedto the membrane morphology. The SEM images indicate that notonly the sizes of pores on the membrane surface are decreased

with hot air treatment but also the skin layer thicknesses of treatedmembranes are increased. Consequently, the pure water uxes ofmembranes were diminished with heat treatment. The changes inMPF of heat treated membranes are not similar to PWF. This may bedue to the surface properties and antifouling behavior of the treatedmembranes. The pure water uxes of membranes were decreasedby heat treatment for all cases. However the milk water perme-ation was rstly increased at low temperature. This was followedby a decline for heat treatment at high temperature. There is nostrong difference between surface pore size and skin layer thick-ness of untreated and treated PES membranes at 100 C. Thereforethe surface properties of these two membranes i.e. differences inmembrane fouling parameters such as total ux loss, irreversibleux loss and total fouling resistance may be considered as the mainfactor for MWP improvement.

To evaluate the inuence of heat treatment on membrane sur-face charge, the zeta potential of untreated and hot air treatedmembranes were measured. This is a good criterion for evaluationof membrane surface charge. The obtained results for zeta poten-tial of untreated membrane and hot air treated membrane at 130 Cwere 16.5 and 16.0 mV, respectively. This indicates negligible differ-ence between surface charges of two membranes i.e. heat treatmenthas no signicant effect on membrane surface charge.

The surface AFM images of untreated PES membrane and treatedmembranes at 100 C are depicted in Fig. 5. The images indicatethat the surface roughness of the PES membrane is decreased byheat treatment. The surface roughness parameters (Sa, Sq and Sz) foruntreated and treated membranes are depicted in Table 1. The dataindicate that the roughness parameters of the membranes weredecreased after heat treatment. It has been shown [17] that theselectivity of the membrane is increased with a decrease in surfaceroughness. Similar trend was observed in this study for milk proteinrejection. This may be explained on the basis of Kestings four tier

Fig. 9. Surface SEM images of water treated PES membranes at different temperatures during 20 min: (a) untreated, (b) 55 C, (c) 75 C, and (d) 95 C.


Fig. 10. tures

pore modeldefects for gtial void spaare compacdiminishedbility. On ththe nodulespactly. Thisof UF membthe surfaceimproves thparametersteins easilyThis increasthe membrtion of semloss, irreverPES membruntreated Pux recoverent temperaby air treat

t thactioopti

ed reof trhan

of trmem

raned 180se m

raturing tm te

Table 3Effect of hot w

Membrane

UntreatedPES membra

Hot water trea55 C75 C95 C

a Operatingb Total ux lc Irreversibld Total fouliCross-sectional SEM images of water treated PES membranes at different tempera

[18], in which pores of ultraltration membranes (oras separation membranes) are set equal to the intersti-ces between nodules. When spherical polymer nodulestly packed, the area of the inter-nodular void space isleading to an increase in ultraltration rejection capa-e other hand the depths of the crevices formed between

are decreased as the nodules are packed more com-results in a smoother surface [9,17]. Thus, the rejectionrane is increased with an increment in smoothness of

. The authors believe that the smooth surface not onlye milk protein rejection but also modies the foulingof membranes. The large amount of milk fat and pro-stick in void spaces between nodules in rough surfaces.es the membranes irreversible fouling. A summary of

suggesintrodu

Theobtainlosseslower ttancetreatedmemb150 anfor thetempeof heatoptimuane ux loss and total fouling resistance during ltra-i-skim milk is represented in Table 1. The total uxsible ux loss, total fouling resistance of heat treatedanes at 100 and 130 C are improved compared to theES membrane. These parameters increase the MPF. Theies for untreated and treated PES membranes at differ-ture are shown in Table 1. The ux recovery is improved

ment. The higher FR value for treated PES membranes

The SEMmembranesFig. 6. The crnodules andduring timeeffect of hetion is presdecreased d

ater treatment on PES membrane performance, total ux loss, irreversible ux loss and t

PWFa

(kg/(m2 h))MPF(kg/(m2 h))

Protein rejection(%)

Total ux lossb

(J0 Jp)/J0

ne 394 29 97 0.93

ted PES membranes at different temperature for 20 min:126 27 99 0.76

74 25 99 0.66340 31 98 0.90

conditions of ltration: pressure = 3.4 bar, ow rate = 5 l/min, and temperature = 30 C.oss = (J0 Jp)/J0; J0: initial pure water ux; and Jp: milk permeation ux.e ux loss = (J0 Jl)/J0; J0: initial pure water ux; and J1: pure water ux after milk ltrating resistance = P/Jp; P: transmembrane pressure; : viscosity of permeate; and Jp: mduring 20 min: (a) untreated, (b) 55 C, (c) 75 C, and (d) 95 C.

t the most protein fouling was reversible due to then of smooth surface.mum temperature was determined by comparing thesults in Table 1. Although the total and irreversible uxeated membranes at 130, 150 and 180 C are slightlythe treated membrane at 100 C, the total fouling resis-eated membrane at 100 C is low compared to other

branes. In the other hand, the PWF and MPF of treatedat 100 C is high compared to treated membranes at 130,

C. A negligible difference between protein rejectionsembranes was observed. In conclusion the optimum

e for hot air treatment is 100 C. Therefore the effectime on properties of PES membrane was performed atmperature.

micrographs of surfaces of unheated and heated PESwith air at 100 C for 5, 20 and 60 min are depicted in

oss-sectional images are shown in Fig. 7. The sizes of thethe porosities of the sub-layers are slightly decreased

. The skin layer thickness increases during time. Theating time (at 100 C) on PWF, MPF and protein rejec-ented in Table 2. The results indicate that the PWF isuring time from 5 to 60 min. This is due to formation of

otal ltration resistance.

Irreversible ux lossc

(J0 Jl)/J0Total fouling resistanced

P/Jp (m1)

0.83 4.9 1011

0.64 5.3 10110.41 5.8 10110.78 4.6 1011

on and 15 min water cleaning.ilk permeation ux.


Fig. 11. Surface SEM images of hot water treated PES membranes at 95 C during different times: (a) untreated, (b) 5 min, (c) 20 min, and (d) 60 min.

thicker skin layer and denser structure. The water ux is stronglydependentcomposed oThe MPF is

ux loss and ltration resistance data for untreated and treatedemble 2.negli

Fig. 12on the top layer resistance as long as the sub-layer isf the nger-like and open and cross-connected pores.

changed by changing the treatment time (Table 2). The

PES min Tabled to. Cross-sectional SEM images of hot water treated PES membranes at 95 C during differeranes at 100 C for 5, 20 and 60 min are summarizedTreatment of the PES membrane at 100 C for 5 mingible reduction of the both total ux loss and ux lossnt times: (a) untreated, (b) 5 min, (c) 20 min, and (d) 60 min.


due to irreversible fouling. Moreover the total fouling resistanceis increased. Further treatment of membrane to 20 min results insignicant improvement in total ux loss, irreversible ux loss andtotal fouling resistance. This justies the increment of MPF of 20 mintreated membrane. For 60 min treatment, although the total uxloss and irreversible ux loss are decreased, the total fouling resis-tance is increased which lead to reduction in MPF. The ux recoveryvalues for treated membranes at various times were higher com-pared to the ux recovery for untreated PES membrane (Table 2).

3.2.2. Hot water treatment of PES membraneThe membranes were kept in a glass ask with hot water at

55, 75 and 95 C for 20 min and in stress free conditions. To investi-gate the effect of pore former (PVP) on morphological change in hotwater treatment, the amount of remained PVP in the membranesduring formation by immersion precipitation should be monitored.The amount of residual PVP in the membrane structure in term ofmolecular weight was determined by Jung et al. [19] with IR spec-tra analyses. They showed when PVP with low molecular weight isemployed inis dissolvedand 20 minFig. 8. PVP wat around 1pared withThere is a vPES membrin the meming the memobserved betreated PESformer, PVPence on theeffects of wlayer morphsurface SEMthe surfacedecreased aperature. Mare stronglylayer with loto denser sThe thickneenhanced frtioned in thinuence on

The inuand proteinresults illustemperatur

Table 4Effect of hot water (95 C) treatment time on PES membrane performance(operating conditions of ltration: pressure = 3.4 bar, ow rate = 5 l/min, and tem-perature = 30 C).

Heat treatment time PWF(kg/(m2 h))

MWP(kg/(m2 h))


Untreated PES membrane 394 29 97

Treatment time5 min 150 28 9820 min 340 31 9860 min 410 33 96

higher temperature. The PWF for 95 C treated membrane is lowerthan untreated membrane. However the MPF is high compared tountreated membrane. Moreover the protein rejections of PES mem-branes are increased with hot water treatment. The deciency inPWF for trethe formati

ructuranendedtheme

f sux los

PESss anre loresi

rane.resi

to thMPFthe beatmtherper

olog60 m

er pent ad prme

emeabilitus seremerane

Table 5Effect of hot ai nd tot

Membrane tal u Jp)/J0 (J0 Jl)/J0 P/Jp (m )

UntreatedPVDF memb 4 0.78 20.5 1011

Hot air treated100 C 7 0.71 11.0 1011130 C 0 0.73 28.8 1011150 C 5 0.58 144 1011a Operating C.b Total ux lc Irreversibl ltration and 15 min water cleaning.d Total fouli d Jp: milk permeation ux.the casting solution; large amount of PVP (about 98%)in non-solvent bath. The FTIR-ATR spectra of untreated

hot water (95 C) treated PES membranes are depicted inith functional group of carbonyl (C O) exhibit a band

700 cm1. The FTIR-ATR spectrum of membrane pre-this additive indicates a symmetric band at this region.ery weak band in the area of 1700 cm1 for untreatedane. This is due to the small quantity of PVP remainedbrane i.e. the large amount of PVP is washed out dur-brane formation. Moreover no signicant changes are

tween FTIR-ATR spectra of untreated and hot air (95 C)membrane for 20 min. The small quantity of the pore, in the PES membrane structure has negligible inu-morphological changes during hot water treatment. Theater temperature on PES membranes surface and sub-ology are depicted in Figs. 9 and 10, respectively. Theimages of membranes treated in water indicate that

nodule size and porosity of membranes were initiallynd then increases with increment of hot water tem-oreover the sub-layer morphologies of the membranes

affected by water treatment. The highly porous sub-ng nger-like pores of untreated membrane is changed

tructure with thicker skin layer after heat treatment.sses of the skin layers of the treated membranes wereom 1.0 to 2.8m with heat treatment (Fig. 10). As men-e pervious section, the hot water treatment has servethe membrane shrinkage, especially at 55 C.

ences of water treatment temperature on PWF, MPFrejection are represented in Table 3. The obtained

trate that the PWF and MPF are declined for lowere treatment. This is followed by a considerable rise for

port stmembis ametion oftreatedment othe utreatedux lo75 C afoulingmembfoulingparedhigher

Onheat trFor furthe temmorph20 andsub-laytreatmMPF antreatedan incrpermeprevioan incmemb

r treatment on PVDF membrane performance, total ux loss, irreversible ux loss a

PWFa



Contact angle () To(J0

rane 112 7 98 92.5 0.9

PVDF membranes at different temperature for 20 min:104 12 98 92.4 0.8

54 5 100 92.2 0.919 2 100 92.5 0.9

conditions of ltration: pressure = 3.4 bar, ow rate = 5 l/min, and temperature = 30oss = (J0 Jp)/J0; J0: initial pure water ux; and Jp: milk permeation ux.e ux loss = (J0 Jl)/J0; J0: initial pure water ux; and J1: pure water ux after milk ng resistance = P/Jp; P: transmembrane pressure; : viscosity of permeate; anated membranes at 55 and 75 C can be attributed toon of thicker skin layer and denser surface and sup-re by treatment. Although the top layer of 95 C treatedis the thickest (see Fig. 10), the membrane performance

by higher surface porosity (see Fig. 9). The combina-two following factors results in higher MPF for 95 Cmbrane: (i) higher surface porosity and (ii) improve-rface properties by heat treatment. Table 3 representss and ltration resistance data for untreated and water

membranes at 55, 75 and 95 C for 20 min. The totald irreversible ux loss of treated membranes at 55 andw compared to untreated membrane while the totalstance of these membranes is higher than untreatedMoreover the total and irreversible ux losses and total

stance for membrane treated at 95 C were lower com-e untreated membrane. These parameters explain thefor 95 C treated membrane.asis of high MPF combined with high protein rejection,ent at 95 C was considered as the optimum condition.study, i.e. investigation of the effect of heating time,

ature was xed at 95 C. The surface and cross-sectionalies of untreated and treated membranes at 95 C for 5,

in are presented in Figs. 11 and 12. The surface andorosity of the membrane was decreased after 60 mint 95 C. The effect of heating time (at 95 C) on PWF,otein rejection is depicted in Table 4. The PWF of the

mbrane was initially decreased. This was followed bynt in water ux during time. Moreover the milk watery was improved by longer treating time. Similar to thection, these results can be explained by the fact thatnt in treatment time demonstrates positive effect onsurface properties.

al ltration resistance.

x lossb Irreversible ux lossc Total fouling resistanced1


Fig.

3.3. Treatm

3.3.1. Hot aThe effe

brane is prthe hot airmembranesPVDF memare depictesents a roua number oand 130 C,of crystallinface is intrmembrane.a dense sucating a fuuntreated aUntreated Pport which13. Surface SEM images of air treated PVDF membranes at different temperatures during

ent of PVDF membranes

ir treatment of PVDF membranect of hot air treatment on hydrophilicity of PVDF mem-esented in Table 5. The obtained results indicate that

treatment has no effect on hydrophilicity of PVDF. Surface SEM micrographs of untreated and treated

branes with hot air at 100, 130 and 150 C for 20 mind in Fig. 13. The untreated PVDF membrane repre-gh surface with noticeable pores which composed off crystalline domains. After membrane treatment at 100the membrane surface is still composed of a numbere domains but with smaller sizes. A smoother sur-

oduced by 100 and 130 C hot air treatment of PVDFTreatment at higher temperature (150 C) leads to

rface. The crystalline domains are disappeared indi-lly smooth surface. Cross-sectional SEM images ofnd treated PVDF membranes are shown in Fig. 14.VDF membrane contains nger-like pores in the sup-are not noticeably changed with air treatment at

100 C. Howmacro-voidwas carriedare formed(150 C).

The PWair treatedare represedeclined wperature. Thwith densethe membrment wasreduction aof untreatemay be dusurface bymembranethe smoothfouling.20 min: (a) untreated, (b) 100 C, (c) 130 C, and (d) 150 C.

ever the nger-like pores are transformed to smalls and a thick skin layer is appeared when treatmentout at 130 C. Moreover the inclined nger-like poresin the membrane sub-layer at higher temperatures

F, MPF and protein rejection of untreated and hotPVDF membranes at 100, 130 and 150 C for 20 minnted in Table 5. The PWF of PVDF membranes wasith heat treatment especially at higher treatment tem-is can be attributed to establishment a thick skin layersurface structure after heat treatment. The MPF of

ane increased from 7 to 12 kg/(m2 h) when heat treat-carried out at 100 C. This was followed by extremet higher temperatures. The difference between PWFd and treated membrane at 100 C is negligible. Thise to the minor alteration on pore size of membranetreating at 100 C. The higher MPF of 100 C treatedcompared to untreated membrane can be explained bysurface of treated membrane with lower tendency for


Fig. 14.

A summtance durinPVDF memversible uheat treatm100 C for vindicate tha20 min is th

3.3.2. Hot wFor hot

ask with hconditions.surface arewas slightlyerably imprmorphologihot water trCross-sectional SEM images of air treated PVDF membranes at different temperatures du

ary of the membrane ux loss and total fouling resis-g ltration of semi-skim milk for untreated and treatedbranes are represented in Table 5. The total and irre-x losses and total fouling resistance are diminished byent at 100 C. Thus, a further study was carried out atarious times (5, 20 and 60 min). The obtained resultst the air treatment of PVDF membrane at 100 C fore optimum condition (the data are not shown).

ater treatment of PVDF membranewater treatment, the membranes were kept in a glassot water at 75 and 95 C for 20 min and in stress freeThe effects of water temperature on PVDF membranesdepicted in Fig. 15. The surface porosity of membrane

decreased while the surface roughness was consid-oved with hot water treatment at 95 C. The sub-layeres of the membranes were not drastically changed byeatment (Fig. 16).

The obtasectional imaround 11%respectively

The perfrejection ofThe PWF oment. This cpore size anfrom 7 to 9any decreathe antifoulconcentratiand foulingmembranesPVDF membcomparisonux loss waring 20 min: (a) untreated, (b) 100 C, (c) 130 C, and (d) 150 C.

ined results for membrane shrinkage from SEM cross-ages indicate that the PVDF membranes were shrunkand 23% after hot water treatment at 75 and 95 C,

.ormance results obtained form PWF, MPF and proteintreated PVDF membranes are represented in Table 6.

f PVDF membrane was declined with hot water treat-an be attributed to the reduction in membrane surfaced porosity. The MPF of PVDF membrane was improvedand 10 kg/(m2 h) by 75 and 95 C treatments, without

se in protein rejection. Thus, it can be concluded thating properties of PVDF membrane is improved for milkon by hot water treatment. A comparison of ux losses

resistance of untreated and hot water treated PVDFat 75 and 95 C are represented in Table 6. Both treatedranes exhibited signicant decrease in total ux loss inwith the untreated membrane. Moreover irreversibles declined from 0.78 for the neat PVDF membrane to


0.56 and 0.95 C, respereduction in

The uxwhich indicin Table 6.compared t

Table 6Effect of hot w

Membrane

UntreatedPVDF memb

Hot water trea75 C95 C

a Operatingb Total ux lc Irreversibld Total fouliFig. 15. Surface SEM images of water treated PVDF membranes at different temperature

47 for the membrane treated with hot water at 75 andctively. Treatment of the PVDF membrane also led to a

total fouling resistance.recovery of untreated and treated PVDF membranesate the recycling property of the membrane is shownThe ux recoveries of treated membranes are highero untreated membrane. In other words the surface prop-

erty of PVDmembranesprotein agg

Further sand 60 minof PVDF me(the data ar

ater treatment on PVDF membrane performance, total ux loss, irreversible ux loss and

PWFa



Total ux(J0 Jp)/J0

rane 112 7 98 0.94

ted membranes at different temperature for 20 min:57 9 99 0.8458 10 100 0.82

conditions of ltration: pressure = 3.4 bar, ow rate = 5 l/min, and temperature = 30 C.oss = (J0 Jp)/J0; J0: initial pure water ux; and Jp: milk permeation ux.e ux loss = (J0 Jl)/J0; J0: initial pure water ux; and J1: pure water ux after milk ltrating resistance = P/Jp; P: transmembrane pressure; : viscosity of permeate; and Jp: ms during 20 min: (a) untreated, (b) 75 C, and (c) 95 C.

F membrane is modied by hot water treatment. Theseare capable to repulse the reversibly bound proteins or

regates from their surfaces.tudies were carried out at 95 C for different times (5, 20

). The obtained results indicate that the water treatmentmbrane at 95 C for 20 min was the optimum conditione not shown).

total ltration resistance.

lossb Irreversible uxlossc (J0 Jl)/J0

Total fouling resistanced

P/Jp (m1)

0.78 20.5 1011

0.56 16.0 10110.47 14.4 1011

on and 15 min water cleaning.ilk permeation ux.


4. Conclus

The PEShot water aresults were

1. Surface abranes wvarious tsurface w

2. The PWFand hotmembrandecreaseMPF of Plow andprotein rmembranbrane.

3. The optimare: 100Fig. 16. Surface SEM images of water treated PVDF membranes at different temperature

ion

and PVDF membranes were treated by hot air andt different conditions for milk ltration. The following

obtained from the treatment process:

nd cross-sectional morphologies of PES and PVDF mem-ere changed after hot air and hot water treatment atemperatures and different times. A dense and smoothith thick skin layer was established after treatment.of PES and PVDF membranes decreased with hot air

water treatment. However the MPF of PES and PVDFes were increased by air treatment at 100 C. This was

d for higher temperatures. For hot water treatment, theES and PVDF membranes were initially decreased forthen increased for higher temperatures. Moreover theejection by hot air and hot water treated PES and PVDFes were improves compared to the untreated mem-

um conditions for treating PES and PVDF membranesC in air and 95 C in water, both for 20 min.

References

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[19] B. Jung, J.K. Yoon, B. Kim, H.W. Rhee, Effect of molecular weight of polymericadditives on formation, permeation properties and hypochlorite treatment ofasymmetric polyacrylonitrile membranes, J. Membr. Sci. 243 (2004) 45.

The effect of heat treatment of PES and PVDF ultrafiltration membranes on morphology and performance for milk filtrationIntroductionExperimentalMaterialsMembrane preparationMembrane treatment procedureMembrane characterizationScanning electron microscopy (SEM)Atomic force microscopy (AFM)Filtration performance and fouling analysisContact angle measurementsZeta potentialFTIR-ATR analysis

Results and discussionThe morphology of untreated PES and PVDF membranesTreatment of PES membranesHot air treatment of PES membraneHot water treatment of PES membrane

Treatment of PVDF membranesHot air treatment of PVDF membraneHot water treatment of PVDF membrane

ConclusionReferences

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