2011 - Fabrication and Characterization of Novel Antifouling Nanofiltration Membrane Prepared From Oxidized Multiwalled Carbon Nanotube_polyethersulfone Nanocomposite

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Journal of Membrane Science 375 (2011) 284–294

Contents lists available at ScienceDirect

Journal of Membrane Science

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / m e m s c i

Fabrication and characterization of novel antifouling nanofiltration membraneprepared from oxidized multiwalled carbon nanotube/polyethersulfonenanocomposite

Vahid Vatanpoura, Sayed Siavash Madaeni a,∗, Rostam Moradian b, Sirus Zinadini a, Bandar Astinchapb

a Membrane Research Center, Department of Chemical Engineering, Razi University, Kermanshah, Iranb Nano Science and Technology Research Center, Physics Department, Razi University, Kermanshah, Iran

a r t i c l e i n f o

Article history:

Received 24 January 2011Received in revised form 18 March 2011Accepted 28 March 2011Available online 6 April 2011

Keywords:

Functionalized CNTHydrophilicityRoughnessAntibiofouling

a b s t r a c t

This study describes the preparation, characterization and evaluation of performance and antifoulingproperties of mixed matrix nanofiltration membranes. The membranes were prepared by acid oxi-dized multiwalled carbon nanotubes (MWCNTs) embedded in polyethersulfone as matrix polymer. Thehydrophilicity of the membrane was enhanced by blending MWCNTs due to migration of functional-ized MWCNTs to membrane surface during the phase inversion process. The morphology studies of the prepared NF membranes by scanning electron microscopy (SEM) showed that very large macro-voids appeared in sub-layer by addition of low amount of functionalized MWCNT leading to increaseof pure water flux. By using the proper amount of modified MWCNTs, it was possible to increase boththe flux and the salt rejection of the membranes. In this work, the effect of CNT/polymer membrane forfouling minimization is presented. The antifouling performance of membranes fouled by bovine serumalbumin (BSA) was characterized by means of measuring the pure water flux recovery. The results indi-cate that the surface roughness of membranes play an important role in antibiofouling resistance of MWCNT membranes. The membrane with lower roughness (0.04 wt% MWCNT/PES) represented thesuperior antifouling property. The salt retention by the negatively charged MWCNT embedded mem-

brane indicated Donnan exclusion mechanism. The salt retention sequence for 0.04 wt% MWCNT wasNa2SO4 (75%)>MgSO4 (42%)> NaCl (17%) after 60 min filtration.© 2011 Elsevier B.V. All rights reserved.

1. Introduction

In the last several years, considerable attempts have been madeto develop new composite polymeric materials to improve bothperformance and antifouling properties of nanofiltration mem-branes. The main effort in optimal design of membrane processesis to establish maximum permeate flow while having maximumsolute rejection, with minimum capital and operating costs [1,2].This means that it is desirable to have a membrane lifetime aslong as possible [3]. Membrane lifetime and permeate fluxes are

primarily affected by the fouling phenomena (e.g., microbial adhe-sion, gel layer formation and solute adhesion) and concentrationpolarization (i.e., solute build-up) at the membrane surface [4].Selection of an appropriate membrane, pretreatment of the processfluid, adjustment of operating design, and conditions are all rec-ognized to control fouling to some extent. It is generally acceptedthatan increase in hydrophilicityprovidesbetter fouling resistancebecause protein andmany other foulantsare hydrophobicin nature

∗ Corresponding author. Tel.: +98 831 4274530; fax: +98 831 4274542.E-mail address: [email protected] (S.S. Madaeni).

[5]. Most nanofiltration membranes are electrically charged, whichsignificantly reduces the scale-formation.

Although many assays have been made to modify membranesurfaces by chemical modification such as by grafting hydrophilicmonomers on the membrane top layer, the effect is still toosmall to obtain adequate diminish of membrane fouling. Recentadvances in nanotechnology have expanded the range of appli-cations to membrane technologies to improve their synergisticeffects on water and wastewater treatment. Especially, valuableeffects of nanoparticles mixed matrix membranes (MMMs) on the

alleviation of membrane fouling have been reported recently frommany research groups [6–9].

Nanocomposites, in which polymersserveas hostsfor inorganicparticles of nanoscale dimensions, have attracted scientific andtechnologicalinterestin membranepreparation [10,11]. Nanocom-posites can exhibit properties that considerably differ from thoseof the bare polymer. Nanocomposite membranes canbe developedby assembling engineered nanoparticles into porous membranes[8,12,13] or blending them with polymeric or inorganic mem-branes [14,15]. Many membranes were fabricated with silica,zeolite, graphite, metal oxide nanoparticles, or carbon nanotubesto increase the novelty of membrane materials, permeability, and

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V. Vatanpour et al. / Journal of Membrane Science 375 (2011) 284–294 285

fouling-resistance as well as permeate quality [8,11–17]. Recently,there hasbeengrowinginterest in usingnew applications of porouscarbons because of their ability to interact with molecules notonlyat their surfaces but also within the bulk of the material [18]. Con-sidering the superior properties such as high flexibility, low massdensity,plustheeffective– stackinginteractionbetween carbonnanotube and aromatic compounds [19], carbon nanotube (CNT)and considerable nanochannels is considered to be excellent can-didate for substituting or complementing conventional nanofillersin the fabrication of nanocomposite membranes [20–22].

Although carbon nanotubes have very excellent separation,electrical and mechanical properties, preparation of MMMs usingthese materials for membrane fabrication face the problems [23],such as notsuitabledispersion anddissolutionof synthesizedCNTsin various organic solvents and different polymers and weak inter-action of the interface between the CNTs and the polymer matrix[24,25].

Introducing hydrophilic functional groups into the surface of the CNTs, functionalization by chemical agents and attaching polargroups to CNT sidewalls are the some methods, which have beenwidely used for uniform dispersion of nanotubes in a polymermatrix and enhanced CNT adhesion to the polymer [26–28]. Usu-ally, surface modification with chemical agents and acid treatment

can improve the uniform dispersion of CNTs [25,29].Carbon nanotubes weremostly used for preparation of gassepa-

ration polymer mixed matrix membranes to enhance both gas per-meability and selectivity [30–32]. However, there are some papersindicating that CNTs were applied in fabrication of pervaporation[22,33], polymer electrolyte membrane fuel cell [34] and ultrafil-tration [25,35–38]. To our knowledge, carbon nanotubes were notused for preparation of mixed matrix nanofiltration membranes.

From theoretical view, functionalized carbon nanotubes canenhance the properties of nanofiltration membranes by increasinghydrophilicity and surface charge of membrane top layer. Increasein hydrophilicity provides better fouling resistance and increasein surface charge raises the electrostatic interactions and Donnanexclusion effect leading to improve salt rejection [39,40].

In this work, we reported novel nanocomposite membranescomposed of polyethersulfone (PES) and acid treated functional-ized multiwalled carbon nanotubes for nanofiltration application.Functionalized MWCNT was embedded in PES matrix using phaseinversion method. The membrane structure and properties werecharacterized by AFM, SEM, FTIR andwater contact anglemeasure-ments. Fouling resistance of prepared mixed matrix membraneswas studied using BSA as foulant. Nanofiltration performance wastested by rejection of NaCl, Na2SO4 and MgSO4 salts.

2. Materials and methods

2.1. Materials

All chemicals used in the experiments were of reagent grade.The MWCNTs (95% of purity) with 10–30 nm in outer diameterwere obtained from Petroleum Research Center (Tehran, Iran).Polyethersulfone (PES ultrason E6020P with MW = 58,000 g/mol)and dimethylacetamide (DMAc) were supplied by BASF Company.Polyvinyl pyrrolidone (PVP) with 25,000 g/mol molecular weight,nitric acid (65%), sulfuric acid (98%), NaCl, Na2SO4 and MgSO4

were supplied from Merck Ind. Bovine serum albumin (BSA) wereobtained from Sigma.

2.2. Preparation of modified MWCNTs

In order to remove the impurities of raw multiwalled carbon

nanotubes (such as the metallic catalysts particles and amorphous

carbon) and graft functional groups on the surface of MWCNTs,1 g of raw carbon nanotube was soaked in 100 ml solution of 3 MHNO3/H2SO4 (1/3, v/v) and were sonicated for 1h. The solutionwas refluxed atabout 400 K for 12h [41] and then wasdiluted with2 l deionized water and filtered through a 0.45m membrane. Themodified CNTswere rinsed withdeionized water to reachto neutralpH.

2.3. Preparation of asymmetric oxidized MWCNT/PES

nanofiltration membranes

Asymmetric MWCNT/PES nanofiltration membranes were pre-pared via phase inversion induced by immersion precipitation [42]using casting solutions containing PES (18wt%), proper amountof modified MWCNT and polyvinyl pyrrolidone (PVP, 1wt%) inDMAc as solvent. Precise amount of modified MWCNT (0.04, 0.1and 0.4wt%) was dispersed into 81.6–82g of DMAc to prepare aMWCNT solution and sonicated for 30 min for an adequate disper-sion. Note that membranes marked as 0.04wt% MWCNT refer tomembranes prepared in a casting solution in which the content of the MWCNTs with respect to PES+ DMAC was 0.04 wt%. After dis-persing MWCNTs in solvent, PES and PVP were dissolved in thedope solution by continuous stirring for 24h. The resultant homo-

geneouspolymersolutionwasagainsonicated10mintoremoveairbubbles. Next, the solution was sprinkled and cast using self-madecasting knife with 100m thickness on glass plate. This wasimme-diately moved to the non-solvent bath (distilled water at 20±1 ◦C)for immersionwithout any evaporation. A thin polymeric film sep-arated from the glass within a few minutes. After primary phaseseparation and membrane formation, the membranes were storedin fresh distilled water for 24 h to guarantee the complete phaseseparation. Subsequently, membranes were sandwiched betweentwo sheets of filter papers for 24h at room temperature for drying.

2.4. Characterization of the prepared membranes

FTIR spectra were recorded by the attenuated total refection

(ATR) technique using Bruker-IFS 48 FTIR spectrometer (Ettlingen,Germany)withhorizontalATRdevice(Ge,45◦)tocharacterizepres-ence of functionalized groups in multiwalled carbon nanotubesstructure and presence of these groups in prepared MWCNT/PESmembranes.

Atomic force microscopy was used to analyze the surface mor-phology and roughness of the prepared membranes. The AFMdevice was Nanosurf ® Mobile S scanning probe-optical micro-scope (Switzerland) equipped with Nanosurf ® MobileS software(version 1.8). Small squares of the prepared membranes (approxi-mately1cm2)werecutandgluedonglasssubstrate.Themembranesurfaces were imaged in a scan size of 5m×5m. The surfaceroughness parameters of the membranes, which are expressed interms of the mean roughness (S a), the root mean square of the Z

data (S q) and the mean difference between the highest peaks andlowest valleys (S z) were obtained by Nanosurf ® MobileS software.

A scanning electron microscope directly provides the visualinformation of the cross-sectional morphology of the membrane.Philips-XL30 and Cambridge scanning electron microscope (SEM,CamScan MV2300) was utilized in this work. The membranes werecut into small pieces andcleaned with filter paper. The pieces wereimmersed in liquid nitrogen for 60–90 s and were frozen. Frozenfragments of the membranes were broken and kept in air for dry-ing. The dried samples were gold sputtered for producing electricconductivity. After sputtering withgold, theywere viewed withthemicroscope at 17 kV.

Membrane hydrophobicity was quantified by measuring thecontact angle that was formed between the membrane surface and

water.Contactanglesweremeasuredusingthesessiledropmethod



286 V. Vatanpour et al. / Journal of Membrane Science 375 (2011) 284–294

with a goniometer (G10, KRUSS, Germany). All contact angle mea-surements were made using 2l of deionized water. To minimizethe experimental error, the contact angle was measured at fiverandom locations for each sample and the average was reported.

2.5. Permeation test

Carbon nanotube embedded nanofiltration membranes were

characterized by measuring the pure water flux, salt rejection andBSA fouling tests. Experiments were carried out in a dead-end cell(125 ml volume) with a membrane surface area of 12.56 cm2. Themembrane was left on rigid sponge and placed in the cell. Thecell was fitted with a pressure gauge. Pressurized nitrogen gas wasused to force the liquid through the membrane. The feed solutionwas stirred at the rate of 300 rpm. Each membrane was initiallypressurized at 6 bar for 30 min, then the pressure was lowed tothe operating pressure of 4 bar. The water flux J w,1 (kg/m2 h) wascalculated by the following equation:

J w,1 =M

A t (1)

where M (kg) was the weight of permeated water, A (m2) was the

membrane area and t (h) was the permeation time. Permeateswere collected over a given period and weighed. The experimentswere carried out at 25±1 ◦C. The single salt rejection was deter-mined using conductometer. The rejection of salts was obtainedby:

R (%) =

1−C p

C f

× 100 (2)

where C p and C f areionconcentrationinpermeateandfeed,respec-tively.

2.6. Analysis of membrane fouling

After water flux tests, the stirred cell was rapidly refilled with

protein solution (150 mg/l BSA solution in pH = 7.0±

0.1). The fluxfor protein solution J p (kg/m2 h) was measured based on the waterquantity permeating the membranes at 4 bar for 2 h.

Therejections(R)ofBSAwerecalculatedbyEq. (2), where C p andC f were the protein concentration in permeate and feed solutions,respectively. After filtration of BSA solution, the membranes werewashed with distilled water. The membranes were initially rinsedand then were immersed in distilled water for 20min. This wasfollowed by estimation of water flux of cleaned membranes J w,2

(kg/m2 h). The flux recovery (FR) was calculated as follow:

FR (%) =

J w,2

J w,1

× 100 (3)

Generally, higher FR indicates better antifouling property of the

nanofiltration membrane.To analyze the fouling process in details, several ratios were

defined to describe the fouling resistance of the MWCNT/PESmembranes [43,44]. The total fouling ratio (Rt) was defined andcalculated as following:

Rt =

1−

J p

J w1

× 100% (4)

Here, Rt was the degree of total flux loss caused by total fouling.Reversible fouling ratio (Rr) and irreversible fouling ratio (Rir)

were also defined and calculated by following equations, respec-tively.

Rr = J w2 − J p

J w1

× 100 (5)

Table 1

Solubility parameters of different solvents.

Solvents Solubility parameters (MPa0.5)

ıd ıp ıh ıt

DMAc 16.8 11.5 10.2 22.7NMP 18 12.3 7.2 22.9DMSO 18.4 16.4 10.2 26.6DMF 17.4 13.7 11.3 24.8

Rir =

J w1 − J w2

J w1

× 100 (6)

Obviously, Rt was the sum of Rr and Rir.

3. Results and discussions

Dimethylacetamide was used as solvent in preparation of oxi-dized MWCNT blended PES membranes. The dispersion behaviorof the nanotubes in solvents can be explained with the solvent sol-ubility parameter. It is reported that the carbon nanotubes (CNT)disperse appropriately in the solvents with dispersive component(ıd) values of 17–18MPa0.5 [45]. However, they precipitate inthe solvents with high polar component (ıp) values or hydrogen-bonding component (ıh) values, because of their hydrophobicnature. Therefore, the used solvent for preparation of carbonnanotube mixed membrane has to be low polar. In addition,hydrogen-bonding component may be with dispersive componentvalue of about 17–18MPa0.5. Table 1 shows solubility parame-ters of various solvents, which commonly used in fabrication of polyethersulfone membranes. Dimethylacetamide, based on theabove explanation, was the best solvent for the dispersion of thesurface modified MWCNTs and selected for preparation of MWCNTblended membranes.

3.1. Characterization of oxidized MWCNTs

In order to fabricate the MWCNTs/PES blend membranes, it

is necessary to make a homogeneous MWCNTs solution in sol-vent before adding PES polymer. However, MWCNTs are stronglyhydrophobic and generally have very low solubility in all solvents.To overcome these problems, the surface of the MWCNTs wastreated withstrongacid i.e. concentrated H2SO4 andHNO3 to intro-ducing hydrophilic functional groups. Acid-treated MWCNTs areknown to have carboxyl groups on their surfaces showing gooddispersion in polar organic solvents [28].

Fourier-transform infrared spectroscopy (FTIR) was used toidentify introduced functional groups onto the surface of acidtreated MWCNT. A difference between FTIR spectra of raw andmodified MWCNTs was shown in Fig. 1. The new peaks emerg-ing at 1520, 1680 and 3300–3600 cm−1 after the acid treatmentcorresponded to C C, C O, and O–H bonds, respectively [41,46].

This confirms the attachment of the functional groups onto theMWCNTs, which can provide a large number of chemical adsorp-tion sites. Meanwhile, the hydrophilic properties of the functionalgroups improve the dispersibility of oxidized MWCNTs in aqueoussolution.

3.2. Characterization of prepared membranes

Fig. 2 exhibits the surface ATR–FTIR spectra of the PESmembrane and the blend membrane with 0.4 wt% of MWCNTs.According to Fig. 1, the functionalized MWCNTs exhibited threemain peaks: 1520, 1680 (C O) and 3300–3600 cm−1 (–OH). TheFTIR spectrum of the blend membrane (Fig. 2) depicts the samepeaks at 1666 and 3400–3600cm−1. Appearance of these peaks

approves the presence of oxidized multiwalled carbon nanotube




Fig. 1. The FTIR spectra of raw MWCNTs (a) and oxidized MWCNTs (b).

Fig. 2. The FT-IR spectra of the PES and 0.4% oxidized MWCNT/PES membranes.

in the surface of the membranes. The bands at1249 and 1134cm−1

can be attributed to the stretching vibrations of S O asymmet-ric and S O symmetric, respectively [25]. In addition, these peaksappear in the spectra of MWCNTs/PES blend membranes. However,the spectra of blend membranes are different from that of the vir-gin PES membrane, showing C O bond of carboxyl groups around1680cm−1 by the added MWCNTs.

The hydrophilicity of MWCNT/PES membrane surfaces wascharacterized by water contact angle. The contact angles of thesurfaces of the blend membranes determined by the sessile dropmethod. As shown in Fig. 3, the contact angles of the blend mem-branes gradually declined when the amount of MWCNTs wasincreased in the mixed matrix membranes. Lower water contact

40

45

50

55

60

65

70

PES only 0.04% MWCNT 0.2% MWCNT 0.4% MWCNT

C o n t a c t a n g

l e ( 0 )

Membrane type

Fig. 3. Static water contact angle of the MWCNT/PES mixed matrix membranes

(average contact angle and standard deviation of five replicates are reported).

angle means stronger hydrophilicity. The pristine PES membranehad the highest water contact angle of 66.3±2.2◦. This indicatedthe 0.4 wt% MWCNT membranes possessed lower water contactangles (59.6±1.7◦), which indicated the hydrophilicity of mem-branes was improved with an increase of MWCNT amount. Itcan be explained by the fact that during the phase inversionprocess, hydrophilicMWCNTsmigratesspontaneously to themem-brane/water interface to reduce the interface energy [38,47]. Themigrationof MWCNTsto thesurfaceof blendmembranes wasobvi-ous when the top and the bottom surface photographs of preparedmembranes were compared as presented in Fig. 4. By increasingamount of mixed multiwalled carbon nanotube in PES matrix, themembrane top surface became darker. Additionally, color of bot-tom surface of membrane (glass side) was very lighter than topsurface (water exposed side in phase inversion process), indicat-ing migration of functionalized MWCNTs to the top-layer surfaceof membrane.

As can be seen in Fig. 3, increasing the MWCNT amount tomore than 0.2 wt% did not result in further enhancement of thehydrophilicity. Celik et al. [38] also reported this behavior. Thismight be explained by the irregular positioning of MWCNTs in themembrane structure at over 0.4 wt% MWCNT content [35], whichlead to aggregation and reducing effective surface of nanotubes.

In order to understand the influence of MWCNT embedding onthefinal polyethersulfone membranestructure, thecross-sectionof the prepared MWCNT/PES blend membranes were observed usingscanning electron microscopy (SEM). The cross-section imagesof membranes prepared from 0, 0.04, 0.2 and 0.4wt% MWCNTare shown in Fig. 5. The membrane prepared from PES withoutMWCNT (Fig. 5a and b) exhibits a typical asymmetric structureand developed macro-voids and dense thick top-layer. Gener-ally, asymmetric structure of the membrane consists of a thindense top-layer and a porous sub-layer, which is filled up byclosed cells within polymer matrix. The membranes prepared by0.04wt% MWCNT/PES (Fig. 5c and d) and 0.2 wt% MWCNT/PES(Fig. 5e and f) demonstrated the strong change in sub-layer andskin layer morphology. The porosity of top-layer and sub-layer

were increased. Very large macro-voids appeared in sub-layerby addition of low amount of functionalized multiwalled carbonnanotubes. This result may be explained by the fast exchange of solvent and non-solvent in the phase inversion process due tothe hydrophilic MWCNTs [48], and occurring interactions betweencomponents in the casting solution and phase inversion kinetics.AT-IR spectroscopy allowed us to check the effectiveness of hydro-gen bond of functional groups of modified MWCNT with water asnon-solvent. It showed a stretch band at 3400–3600cm−1 (Fig. 2)for MWCNT/PES membranethat is thecharacteristicband of hydro-genation bond. This indicates the higher affinity of MWCNT/PESmembrane to water.

However, when the content of MWCNTs increased to 0.4wt%,the pore walls started to formation in sub-layer and macro-void

size reduced (see Fig. 5g). As a result, porosity of the 0.4 wt% mem-brane was decreased. This is maybe due to the increased viscosityof the MWCNTs/PES blend solution [49]. The viscosity of the blendsolutions increases along with the contents of MWCNTs. Increaseof the viscosity usually delays the exchange of solvent and non-solvent. Conclusively speaking, in this case, two factors (increasinghydrophilicity and increasing viscosity by the added hydrophilicMWCNTs) act at same time for the formation of the micro-porousblend membranes. When the content of the added MWCNTs is lessthan 0.2 wt%, increased hydrophilicity of the solution plays majorrole to form macro-voids and high porous structure. On the otherhand, blending more than 0.2wt%, increases viscosity of the solu-tion (the major factor affecting the pore size). This suppresses theformation of macro-voids and reduces the porosity of membrane.

Wu et al. [36] observed this behavior in preparation of ultrafiltra-




Fig. 4. Digital photographs of top and bottom surface of MWCNT embedded PES membranes with different concentrations.

tion membrane by incorporating carboxylated multiwalled carbonnanotubesintoamatrixofbrominatedpolyphenyleneoxide(BPPO)and Choi et al. [25] in fabrication of oxidized MWCNT/PSf mem-branes. Nevertheless, Celik et al. [38] reported the contrary aspecti.e. suppressing the formation of macro-pores by the addition of MWCNTs into the PES membrane structure.

Fig. 5b, d, f and h, which are SEM images of top-layer of pre-pared membranes, show at first by adding/increasing of oxidizedMWCNT, the size of top-layers pores enhanced until 0.2 wt% of MWCNT andagain reduced by further increase of MWCNT amount.This order of pore size and porosity change is well matched with

pure water flux of the membranes as shown in Fig. 6. Water per-

meation was increased up to 0.2wt% of MWCNT and reducedby further increase of MWCNT concentration in polymer matrix.The water permeation of membranes is usually controlled by twofactors; hydrophilicity and the pore size andstructure of the mem-brane. The hydrophilic groups of MWCNTs surface improve thehydrophilicity of membrane surface (see Fig. 3). This increasein hydrophilicity results an increase in water flux. In addition,the increase in pore size leads to increasing water permeationthrough the membranes. Increasing water flux up to 0.2 wt% of functionalized MWCNT content can be attributed to improvementof membrane hydrophilicity due to functional groups of carbon

nanotube and increase of the pore size of top-layer causing the




Fig. 5. SEM micrographs of cross-sections of membranes with different blend compositions (a, b) pristine PES, (c, d) 0.04% MWCNT/PES, (e, f) 0.2% MWCNT/PES and (g, h)0.4% MWCNT/PES.

more exchange rate between solvent and non-solvent during thephase inversion. However, when the MWCNTs amount exceeds0.2 wt%, the high density of MWCNTs in the casting solution leadsto an increase in the viscosity of solution [34,49]. This will hin-der the exchange between solvent and non-solvent during thephase inversionand slow down theprecipitation of the membrane.

Also, the density of MWCNTs in the membrane is so large that thesteric hindrance and electrostatic interactions among the MWC-NTs and between the MWCNTs and the polymer chains cause tocluster the MWCNTs during phase inversion [36]. Therefore, a lessporous membrane was created andthe flux decreased in 0.4wt% of MWCNT.




0

2

4

6

8

10

12

14

16

18

765432

P u r e

w a t e r fl u x ( k g / m 2 h )

Pressure (bar)

0.2MWCNT

0.4MWCNT

0.04MWCNT

PES

5

6

7

8

9

10

0.400.200.00

F l u x

MWCNT%

4 bar

Fig. 6. Effect of MWCNT concentrations on pure water flux.

The complementary information on the distribution of theMWCNTs within the PES matrix is obtained through the AFMtopography images for different compositions of MWCNT/PES asillustrated in Fig. 7. In these images, the brightest area presents

Table 2

Surface roughness parameters of functionalized MWCNT/PES membranes obtainedfrom analyzing three randomly chosen AFM images.

MWCNT/PES composition Roughness parameters

S a (nm) S q (nm) S z (nm)

Pristine PES 21.4 ± 1.8 27.1 ± 2.3 181.7 ± 35.20.04 w t% MWCNT/PES 10.5 ± 1.2 15.5 ± 1.8 96.2 ± 23.30.20 w t% MWCNT/PES 59.1 ± 4.6 74.3 ± 6.8 496.7 ± 62.80.40 w t% MWCNT/PES 16.1 ± 4.1 21.9 ± 6.4 161.3 ± 22.7

the highest point of the membrane surface and the dark regionsindicate valleys. The images indicate that the roughness is reducedby adding 0.04 wt% MWCNT to polymer matrix. Next, in 0.2 wt%roughness extremely increases and again reduces in 0.4 wt% of oxidized MWCNT. The roughness parameters are presented inTable 2, which obtained from probing three randomly chosen AFMimages. The mean roughness (S a) of the pristine PES membranesreducedfrom21.4nmto10.5nmfor0.04wt%blendmembraneandseverely increased to 59.1nm for 0.2 wt% MWCNT. In low concen-tration of modified carbon nanotubes, because of low electrostatic

Fig. 7. Surface AFM images of the MWCNT/PES membranes with different concentrations of oxidized carbon nanotube (a) PES only, (b) 0.04%, (c) 0.2% and (d) 0.4% MWCNT.




0

10

20

30

40

50

60

70

80

90

2001751501251007550250

N a 2 S

O 4

R e j e c Ɵ o n ( % )

Time (min)

0.04 MWCNT

0.2 MWCNT

0.4 MWCNT

PrisƟne PES

Fig. 8. Na2SO4 retention by oxidized MWCNT embedded membranes versus time(4bar, pH=7.0±0.1, 200ppm Na2SO4).

interactions among the MWCNTs, they are regularly collocated inmembrane and the surface of membrane becomes smooth [35].However, due to increase of pore size and agglomeration of car-bon nanotubes, roughness of membrane surface was increased by

mixing high concentration of MWCNTs.At values above 0.2wt% MWCNTs, due to combined effect of

increased viscosity and lowered thermodynamic stability of theblend solution by the addition of hydrophilic MWCNTs [25], thepore size reduced again and roughness of the membrane changedto be smooth.

3.3. Nanofiltration performance

Theusual method to testthe separation capabilityof thenanofil-tration membranes is the performance of rejection experimentsthat is very well comparable to the real separation process. Rejec-tion tests with model salt solutions of various compositions give ageneral representationof themembraneseparation characteristics.

Salt retention measurements withNaCl, Na2SO4 andMgSO4 at apressure difference of 4 barwere carried outwith the dead-end cellinpH7. Fig.8 presentsthevariationofNa2SO4 rejection of preparedmembranes. Therejection of Na2SO4 for thepristinePESmembranewas 20%. This was altered to about 80, 70, and 40% for 0.04, 0.2 and0.4 wt% of MWCNT embedded membranes, respectively.

Therejection capabilityof themembranewas initiallyincreasedby adding of low amount of 0.04 wt% functionalized multiwalledcarbon nanotube. Nevertheless, further increasing of MWCNTsdeclinedtheNa2SO4 saltretention. Thisbehavior wasalsoobservedfor NaCl and MgSO4 salt rejections (not presented).

Fig. 9 indicates the retention behavior of the 0.04 wt% MWCNTmembrane for three salt rejections (Na2SO4, MgSO4 and NaCl).Retention sequence was R (Na2SO4) > R (MgSO4) > R (NaCl). The

retention for the bivalent anion was the highest, whereas presenceof the bivalent cation(Mg2+) reduced theretention of sulfate anion.The retentionof the salt with a monovalent ionpair was the lowest.

The salt retention measurements showed that the behavior of the MWCNT blended membranes could be classified as Donnanexclusion mechanism, which are negatively charged. Acid-treatedMWCNTshavethehighnegativeZetapotentialinpH7.0 [37], whichlead to negative surface charge of the prepared membranes. Mem-branes in contact with an aqueous solution get an electric chargeby dissociation of surface functional groups or adsorption of ionsfrom the solutions [50], cause to electrostatic repulsion of chargedsolutes. The charge exclusion of ions is dependent on the charge of the membrane, the ionic strength, and the valence of the ions [51].

In a charged membrane in contact with an electrolyte solution,

the concentration of co-ions, i.e., ions with the same charge as the

0

10

20

30

40

50

60

70

80

90

100

2001751501251007550250

S a

l t R e t e n Ɵ o n

( % )

Time (min)

Na2SO4

MgSO4

NaCl

Fig. 9. Salt retention by 0.04wt% MWCNT/PES membrane (4bar, pH = 7.0±0.1,200ppm salt).

membrane, in the membrane will be lower than that in solution.Besides, the counter-ions, which have the opposite charge, have ahigherconcentrationinthemembranethaninthesolution.Becauseof this concentration difference of the ions, a potential differenceis generated at the interface between the membrane and the solu-

tion to provide an electrochemical equilibrium between solutionandmembrane. By this potential, whichis calledDonnan potential,co-ions are repelled by the membrane whereas counter-ions areattracted [52].

If it is assumed that the prepared MWCNT/PES membranesare negatively charged, the high retention for the sodium sulfate(bivalent co-ion, monovalent counter-ion) and the lower retentionfor sodium chloride (monovalent co-ion, monovalent counter-ion)agree to Donnan exclusion model.

Rejection of salts initially reduced by increasing the time of nanofiltrationand thenreached to constant value. Withan increasein salt filtration time, the concentration of the co-ion (same chargewith membrane)in the membrane increases causing to decrease of the fixed membrane charge. The decline of co-ion exclusion from

the membrane often leads to a lower rejection of the salt whereasthe rejection of the co-ion determines the rejection of the salt [53].

3.4. Antibiofouling and recycling properties of the membranes

The antifouling performance of the pristine polyethersulfoneand modified MWCNT/PES nanofiltration membranes was charac-terized by means of measuring water flux recovery after fouling byBSA solution. The results are shown in Figs. 10 and 11.

From the figures, it is clear that the flux recovery percentageof the MWCNT embedded membranes is higher than that of thepristine membrane. It means that the filtration performance of theprepared NF membranes wasenhancedwhen theywere exposed toprotein solution. The flux recovery value of pristine PES membrane

was only 29.7% implicating a poor antibiofouling property. In thebest case, related to 0.04 wt% MWCNT membrane,the fluxrecoverypercentage of the membrane was 87.7%.

It is generally accepted that there are four types of forces, whichis responsible for proteins adsorption to solid surfaces in aque-oussolution:electrostaticforce (Coulombforce),hydrogen bondingforce, hydrophobic force (entropy effect) and Van der Waals force[54].

The isoelectric point of BSA is about 4.9 [55], and the pHvalue of BSA solution was 7. Therefore, BSA molecules were nega-tively charged in this condition. Meanwhile, functionalized groupsof MWCNT existed in surface of membranes are also negativelycharged at pH 7 due to dissociation. Thus, a strong electrostaticrepulsion force between both negatively charged BSA molecules

and membrane surface is one important reason for improving




0

1

2

3

4

5

6

7

8

9

10

0

F l u x ( k g / m 2 h )

50 100 150 200

Time (min)

250 300

0.2 MWCNT

0.04 MWCNT

0.4 MWCNTPES only

350

Fig. 10. Flux versus time for functionalized MWCNT blended PES membranes at4 barduringthreesteps:water fluxfor120 min, BSAsolution(150ppm,pH =7 ±0.1)flux for 120 min, and water flux for 120min after 20 min washing with distilledwater.

the antifouling performance [55,56]. In addition, the increasing of hydrophilicity of membranes prepared from oxidized MWCNT (see

Fig. 3) resulted from bonding ionized carboxylic groups in MWCNTsurface with a water layer in aqueous solution is another reasonforpreventing proteins adsorption on the surfaces of MWCNT blendedmembranes.

Comparison of flux recovery of multiwalled carbon nanotubeembedded membranes shows that sequence of change in fluxrecovery are consistent with the membrane surface roughnessobserved by AFM images (see Fig. 7). The higher roughness of 0.2wt% MWCNT/PES cause to more decline in flux recovery. Thebestantifouling propertiesof 0.04wt% membranecan be attributedto lower roughness of its surface. It is believed that membranewith lower roughness and surface energy has stronger antifoul-ing abilities, and membrane fouling is enhanced by an increasein the surface roughness [5,57]. Furthermore, foulants are likely

to be absorbed in the valleys of membrane with coarser surfacesresulting in clogging of the valleys [58,59]. Therefore, it is impor-tant to fabricatemembrane with less surface energy and roughnessto improve antifouling ability and performance of the membrane.

The adsorption and deposition of proteins on the membranesurface and entrapment of proteins in the pores primarily causedmembrane fouling. Membrane fouling consisted of reversible foul-ing and irreversible fouling. Reversible protein adsorption led toreversible fouling which could be removed by simple hydrauliccleaning. On the contrary, irreversible fouling was caused by firmadsorption of protein molecules on the surface or entrapment of protein molecules in pores [43,60].

Fig. 11. Water flux recovery of MWCNT blended PES membranes after BSA fouling.

0

10

20

30

40

50

60

70

80

90

100

PrisƟne PES 0.04 0.2MWCNT 0.4MWCNT MWCNT

F o u l i n g R e s i s t a n c e ( % )

Total fouling resistance

reversible fouling

Irreversible fouling

Fig. 12. Fouling resistance ratio of MWCNT blended PES membranes.

0

1

2

3

4

5

6

7

8

9

9008007006005004003002001000

F l u x ( k g / m

2 h )

Time (min)

0.04 MWCNT

Fig. 13. Recycling properties of 0.04 MWCNT membranes during BSA filtration.

Fig. 12 shows total fouling ratio (Rt), reversible fouling ratio(Rr), and irreversible fouling ratio (Rir) values which calculated

from water flux before BSA fouling and after hydraulic cleaning toevaluate the antibiofouling properties. Reversible fouling of pris-tine and MWCNT modified PES membranes roughly was similar.However, the main difference in fouling resistance of membraneswas observed in irreversible fouling value. This implies the irre-versiblefouling dominates thetotal fouling.PristinePES membranehad high irreversible fouling ratio (70%, more than 77% of totalfouling) due to lower hydrophilicity and surface charge [54,56].Among the MWCNT blended membranes, 0.2 wt% membrane rep-resented highest irreversible fouling, which can be attributed tohigher roughness, as shown in Table 2. The 0.04 wt% MWCNT/PESmembrane had the highest FR value of 87.7% and the lowest Rir

value of 12%.The three cycle of BSA fouling experiments was performed to

evaluatethe durability of antibiofoulingproperty of 0.04wt% mem-brane. Fig. 13 demonstrates the flux recovery values in the threecycles are 87.7%, 89.6%, and 90.3%, respectively. Accordingly, thereversible fouling was dominant in total fouling and hydrauliccleaning still maintained high efficiency after multi-run.

4. Conclusions

The multiwalled carbon nanotubes (MWCNTs) were oxidizedby sulfuric acid and nitric acid to generate functionalized groupsin structure of MWCNTs in order to increasing dispersivity of car-bon nanotubes in polymer matrix. The treated MWCNTs exhibitedgood compatibility between MWCNTs and polyethersulfone com-ponents. The blending MWCNTs to PES matrix cause to increasing

hydrophilicity and water flux of pristine membrane. The MWCNTs




influenced mean pore size and porosity of membrane. Cross-sectional SEM images of membranes top-layer showed initiallyby adding/increasing of oxidized MWCNT, size of top-layers poresenhanced until 0.2 wt% of MWCNT and again reduced by furtherincreasing of MWCNT amount. In addition, fouling of membraneresulting from BSA filtration could be reduced by importing MWC-NTs to the blend membrane. The results confirmed that the surfaceroughness of membranes plays an important role in antibiofoul-ingresistance of MWCNT membranes, which 0.04wt% MWCNT/PESmembrane with lower roughness had the best antifouling prop-erties. The behavior of the MWCNT blended membranes in saltretention experiments indicated that the mechanism of salt rejec-tion is Donnanexclusion, which membranes surface are negativelycharged.

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2011 - Fabrication and Characterization of Novel Antifouling Nanofiltration Membrane Prepared From Oxidized Multiwalled Carbon Nanotube_polyethersulfone Nanocomposite