Journal of Membrane Science 211 (2003) 157–165

Design of TiO2 nanoparticle self-assembled aromatic polyamidethin-film-composite (TFC) membrane as an approach

to solve biofouling problem

Sung Ho Kima, Seung-Yeop Kwaka,∗, Byeong-Hyeok Sohnb, Tai Hyun Parkca Hyperstructured Organic Materials Research Center (HOMRC), School of Materials Science and Engineering, Seoul National University,

San 56-1, Shinlim-dong, Kwanak-ku, Seoul 151-744, South Koreab Department of Materials Science and Engineering, Pohang University of Science and Technology, San 31, Hyoja-dong,

Nam-ku, Pohang, Kyungbuk 790-784, South Koreac Department of Chemical Engineering, Seoul National University, San 56-1, Shinlim-dong, Kwanak-ku, Seoul 151-744, South Korea

Received 15 April 2002; accepted 16 September 2002

Abstract

Microbial biofouling is one of the major obstacles for reaching the ultimate goal to realize high permeability over a prolongedperiod of reverse osmosis operation. In this study, the hybrid thin-film-composite (TFC) membrane consisted of self-assemblyof TiO2 nanoparticles with photocatalytic destructive capability on microorganisms was devised as a novel means to reducemembrane biofouling. Then, the anti-fouling and fouling mitigation on the actual commercialized TFC was verified.

TiO2 nanoparticles of a quantum size (∼10 nm or less) in anatase crystal structure were prepared from the controlledhydrolysis of titanium tetraisopropoxide and characterized by X-ray diffraction (XRD) analysis and transmission electronmicroscopy (TEM). Hybrid thin-film-composite (TFC) membrane was prepared by self-assembly of the TiO2 nanoparticlesthrough coordination and H-bonding interaction with the COOH functional group of aromatic polyamide thin-film layer,which was ascertained by X-ray photoelectron spectroscopy (XPS). The hybrid membrane was shown to possess the dramaticphotobactericidal effect onEscherchia coli (E. coli) under UV light illumination. Finally, introduction of TiO2 nanoparticleson the actual commercial TFC membrane and application of RO field test after exposure to microbial cells verified a substantialprevention against the microbial fouling by showing less loss of RO permeability, offering a strong potential for possible useas a new type of anti-biofouling TFC membrane.© 2002 Elsevier Science B.V. All rights reserved.

Keywords: TiO2 hybrid membrane; Anti-fouling membrane; Photocatalytic bactericidal effect; TiO2 nanoparticles; Biofouling

1. Introduction

Reverse osmosis (RO) thin-film-composite (TFC)membranes are receiving the increased attention for avariety of applications in water desalination, ultrapurewater production, waste water treatment, and so on.

∗ Corresponding author.E-mail address: sykwak-p@gong.snu.ac.kr (S.-Y. Kwak).

However, a major obstacle to further use in industrialoperations is flux decline resulting from fouling[1,2].Several types of fouling can occur in the membranesystem, e.g. crystalline fouling, organic fouling, par-ticulate and colloidal fouling, and microbial fouling[3]. Various approaches to reduce fouling have beenperformed, which generally involve pretreatment ofthe feed solution, modification of the membrane sur-face properties (like hydrophobic or hydrophilic and

0376-7388/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved.PII: S0376-7388(02)00418-0

158 S.H. Kim et al. / Journal of Membrane Science 211 (2003) 157–165

electronegative or electropositive), optimization ofmodule arrangement and process conditions, and pe-riodic cleaning[2,4]. Even after long periods of thedevelopments, biofouling caused by microorganismsand biofilm accumulation on membrane still remainsthe main reason for flux decline[5,6].

It has been standard practice to control biologicalgrowth in the feedwater by the use of chlorine. Nobiofilm was formed from the disinfectant-treated wa-ter like chlorinated water[7]. However, chlorinationgenerates harmful byproducts such as trihalomethanesand other carcinogens although effective for the de-struction of bacteria and most viruses.

Titanium dioxide (TiO2) has been the focus of nu-merous investigations in recent years, because of itsphotocatalytic effects that decompose organic chem-icals and kill bacteria[8]. Therefore, it has beenapplied to a variety of problems of environmentalinterest in addition to water and air purification[9].Most of the works carried out in these areas havefocused on the use of TiO2 powders suspended inthe water as a catalyst[10]. However, there is thenecessity of separating the semiconductor particlesfrom the microbial cell suspension in a practical ster-ilization. The thin-film layer of aromatic polyamideTFC membranes is composed of the crosslinked formof three amide linkages and the linear form withpendant free carboxylic acid and two amide linkages[11,12]. The method of self-assembly of TiO2 onthe surfaces with the terminal functional groups (forexample, single-crystal silicon, quartz, and glass sub-strates) has been used to fabricate multilayer ultrathinfilms without the problems such as high temperature,solvent involvement, costly fabrication, and complexprocess control[13–15]. The self-assembly behaviorof TiO2 on polymer with COOH group is explainedby two different adsorption schemes. One scheme wasthat TiO2 was bound with two oxygen atoms of car-boxylate group via a bidentate coordination to Ti4+cations. The other scheme was to form a H-bond be-tween carbonyl group and the surface hydroxyl groupof TiO2 [16]. Thus, it is probable to self-assemble theTiO2 nanoparticles on TFC membrane surface.

TiO2 nanoparticle self-assembled aromatic poly-amide TFC membrane was prepared and charac-terized in our previous paper[17]. Fouling of ROmembranes is markedly influenced by membrane sur-face morphology. The rougher surface and the larger

surface area of TFC membranes make it possible tohave contact with more water, which attribute to thehigher permeability[18]. However, surface roughnessincreases membrane fouling by increasing the rateof foulant attachment onto membrane surface[19].Anti-fouling and fouling mitigation is essential to theflux-enhanced RO membrane. Thus, TiO2 nanoparti-cle was introduced on the flux-enhanced RO mem-brane by the method studied before and anti-foulingproperty of the hybrid membrane was focused on thispaper.

TiO2 nanoparticles are prepared from the controlledhydrolysis of titanium tetraisopropoxide[20]. The par-ticle structure and size are characterized by X-raydiffraction (XRD) analysis and transmission electronmicroscopy (TEM). The hybrid TFC membrane wasprepared by self-assembly of TiO2 on the membranesurface. X-ray photoelectron spectroscopy (XPS) isperformed with the RO-tested membrane after the ac-tual RO operation conditions. Although TiO2 photo-catalysis is reported to be effective in the photokillingof a wide variety of bacteria and virus[8], our studyhas been performed only forEscherichia coli (E. coli)because it is generally accepted as a primary, universaltarget bacterium for the researchers of such studies.The photocatalytic bactericidal ability of the hybridmembrane is evaluated by counting the viable numberof E. coli cells. Then, the anti-fouling and fouling mit-igation on the actual commercialized TFC membranewas examined and verified.

2. Experimental

2.1. Preparation and characterization of thenanosized TiO2 particles

TiO2 nanoparticles were prepared from thecontrolled hydrolysis of titanium tetraisopropox-ide at acidic condition[20]. A 1.25 ml sample ofTi(OCH(CH3)2)4 (Aldrich, 97%) dissolved in 25 mlof absolute ethanol by injection was dropped undervigorous stirring to 250 ml of distilled water (4◦C)adjusted to pH 1.5 with nitric acid. After this mixturewas stirred overnight, a transparent colloidal suspen-sion was resulted. Powdered sample was obtainedby evaporating (35◦C) using a rotavapor and drying(50◦C) under vacuum.

S.H. Kim et al. / Journal of Membrane Science 211 (2003) 157–165 159

The crystal structure of TiO2 nanoparticle wascharacterized by a MAC Science X-ray diffractometer(XRD, MXP18X–MF22–SRA) using 18 kW Cu K�(λ = 1.5418 Å) radiation. For comparison purposes,XRD analysis was perform on other commercialTiO2 particles such as Sigma–Aldrich rutile TiO2 andDegussa–Hüls P25 TiO2. The particle size was deter-mined by a JEOL transmission electron microscope(TEM, JEOL JEM-200CX) at 120 kV. For the TEMobservation, TiO2 powder in distilled water solution(0.5 g/l) was dropped on a carbon-coated grid andthen dried at room temperature.

2.2. Preparation and characterization of TiO2hybrid TFC membranes

Thin-film-composite (TFC) membrane was madevia interfacial polymerization ofm-phenylenediamine(MPD) in the aqueous phase (2 wt.%) and trimesoylchloride (TMC) in the organic phase (0.1 wt.%) onthe nonwoven fabric-reinforced polysulfone supports.The resulting TFC membrane was rinsed in a sodiumcarbonate solution (0.2 wt.%) and then washed withdistilled water. The neat TFC membrane with an areaof ca. 50.0 cm2 was dipped in the transparent TiO2colloidal solution for 1 h to deposit TiO2 nanoparti-cles on the membrane surface and then washed withwater.

X-ray photoelectron spectroscopy (XPS) was per-formed with a Kratos AXIS HS spectrometer using MgK� X-ray (1253.6 eV). The X-ray gun was operated at10 kV and 1 mA. The spectra were taken at the takeoffangle (defined as the angle between the detected pho-toelectron beam and the membrane surfaces) of 30◦ togive a sampling depth of ca. 23 Å. The sensitivity fac-tors of individual elements for quantitative analyseswere taken with the values from the standard VisionLibrary provided by the manufacturer, which werebased on a combination of photoelectric cross-section,transmission function, and inelastic mean free path.

2.3. Evaluation of photocatalytic bactericidaleffect of hybrid TFC membranes

Escherichia coli (E. coli) cells (DH5�), preculturedin 10 ml of Luria-Bertani (LB) medium at 37◦C for16 h, were washed by centrifuging at 10,000 rpm for1 min and were suspended and diluted to an appro-

priate concentration with sterilized water. The LBmedium was prepared with 1 wt.% Bacto-tryptone,0.5 wt.% yeast, and 1 wt.% NaCl in distilled water.The cell dilution (150�l, total 1.0 × 104 cells) waspipetted onto both hybrid membranes and neat TFCmembranes and placed in an incubator at 37◦C.These systems were illuminated with an 8 W blackUV lamp (VWR UVLS-28). The light intensity atthe peak of 365 nm was approximately 500�W/cm2

at 3′′. A total of 150�l of E. coli dilution was col-lected. The appropriate dilutions were plated on LBagar medium and incubated for 16 h. The survivalratio of E. coli was determined by counting the num-ber of viable cells in terms of colony-forming units(CFU).

2.4. Introduction of TiO2 nanoparticles on theactual commercial TFC membrane

Commercial low pressure (LP) TFC membrane wasprovided from Saehan (Yongin-City, Korea). LP mem-brane is a newly invented high-flux membrane, whichproduces relatively high flux at lower feed pressureand saves energy and operating cost. Hybrid LP mem-branes were prepared by dipping neat LP membranesinto the TiO2 colloidal solution for 1 h.

Reverse osmosis (RO) transport characteristics weredetermined in laboratory at 225 psi, 2,000 ppm NaClat 25◦C for 30 min with the apparatus of a continuousflow type. The water flux was calculated by directmeasurement of the mass of the permeate flow:

flux (gfd) = permeate(gal)

membrane area(ft2)time(day)(1)

The salt rejection was measured by the salt concentra-tion in the permeate obtained through measurementsof the electrical conductance of the permeate and thefeed using a conductance meter:

rejection(%) =(

1 − permeate conductance

feed conductance

)× 100

(2)

2.5. Actual fouling test of hybrid membranes

E. coli cell dilution and nutrient broth were addedto distilled water for fouling test. Hybrid and neat LPmembranes were dipped in theE. coli cell dilution

160 S.H. Kim et al. / Journal of Membrane Science 211 (2003) 157–165

(500 ml, 1.7× 107 cells/ml), which were placed in anincubator at a constant temperature of 30◦C. Thesesystems were illuminated with the UV lamp for 4 hper day. Then, transport characteristics of the fouledmembranes were measured at an interval of 24 h. Fora comparison purpose, the fouling test was also per-formed on the hybrid and neat LP membrane withoutillumination.

3. Results and discussion

3.1. Crystal structure and size of synthesized TiO2nanoparticle

Fig. 1 shows the X-ray diffraction images of TiO2particles. The experimental spacing were comparedwith those reported for rutile (1 1 0) (2θ of 27.45◦)and anatase (1 0 1) (25.24◦) to identify the particlestructure[21]. The synthesized particles are composedentirely of anatase. The particle size is determinedby transmission electron microscopy (TEM) explicitly.The particles size is about 10 nm (Fig. 2).

Fig. 1. XRD images of the synthesized TiO2 by comparison withcommercial particles.

Fig. 2. TEM micrograph of the TiO2 nanoparticles.

3.2. Surface characterization of hybrid membrane

Fig. 3 shows the XPS spectrum of hybrid mem-brane. The photoelectron peak for Ti atom appearsclearly at a binding energy,Eb = 456.2 eV for Ti2p3/2 and 461.9 eV for Ti 2p1/2. TiO2 nanoparticlesare self-assembled on the hybrid membrane surface.The relative concentrations are calculated with thefollowing equation in the analysis:

Ci = Ai/Si∑mj Aj/Sj

(3)

where Ai is the photoelectron peak area of the el-ement i, Si the sensitivity factor for the elementiincluding the photoelectron cross-section, andm thenumber of elements in the sample.Table 1shows theelemental composition for the hybrid membranes withvarious RO operational hours. Some of the adsorbedTiO2 particles are detached from the membranes afterRO-pressurized operation for 30 min. However, noadditional loss is progressed as the RO operationalhours are increased till after 7 days.

S.H. Kim et al. / Journal of Membrane Science 211 (2003) 157–165 161

Fig. 3. XPS peaks of TiO2 hybrid membranes.

Table 1Elemental compositions of the TiO2 hybrid TFC membranes byXPS analyses

Content Samples after varioustreatments

1 2 3

CarbonAtomic concentration (%) 60.0 60.9 59.5Mass concentration (%) 51.0 52.9 51.2

OxygenAtomic concentration (%) 29.4 26.8 30.7Mass concentration (%) 33.3 31.0 35.3

NitrogenAtomic concentration (%) 8.3 10.8 8.3Mass concentration (%) 8.2 10.9 8.3

TitaniumAtomic concentration (%) 2.2 1.5 1.5Mass concentration (%) 7.5 5.2 5.2

Analyses were performed for the TiO2 self-assembled TFC ROmembranes (1) just after prepared and washed with flowing water,(2) after RO operation with run time of 30 min, (3) after ROoperation for another 7 days.

3.3. Design of TiO2 nanoparticle self-assembledhybrid membrane

TiO2 nanoparticle in the anatase form is very pho-toactive and practical for the widespread environmen-tal applications such as water purification, wastewatertreatment, hazardous waste control, air purification,and water disinfection[9,20,22]. In this research, hy-brid TFC membrane is devised by the self-assemblybetween TiO2 nanoparticle and polymer with COOHgroups (Fig. 4). As RO process in this study is operatedin the cross-flow mode under high pressure, simplyadsorbed particles may be detached from membranesurface. XPS results inTable 1 indicate that someTiO2 particles in hybrid membrane have a sufficientbinding strength for the actual operation, which agreewith other researches on the adsorption behavior ofTiO2 nanoparticle[8,16]. It is concluded that a novelorganic–inorganic hybrid membrane is successfullyprepared by self-assembly process (Fig. 5).

3.4. Photocatalytic bactericidal effect of hybridmembrane

Fig. 6compares the survival ratios ofE. coli in boththe hybrid and the neat TFC membranes with and with-out light illumination. A 60% ofE. coli cells in neatTFC membrane survive under dark condition withoutUV illumination. In our experimental setup, cell solu-tion was used without nutrients to suppress the multi-plication of cell. Thus, the natural diminution of cellpopulation was unavoidable. TiO2 hybrid membranein the same condition shows slightly less survivalratio. UV light causes sterilization, and 37% of cellssurvived after 4 h UV illumination. TiO2 nanoparticlesaccelerate this sterilization effects. Hybrid membraneunder UV light reaches complete sterilization within4 h. The result of the survival ratios shows that thehybrid membrane under UV can eliminate bacteriaby the photocatalytic bactericidal effects of TiO2.

3.5. Reverse osmosis (RO) performance of hybridLP membrane

Table 2contains the RO performance data of waterflux and salt rejection for the neat low pressure (LP)and the hybrid LP membranes. Introduction of TiO2nanoparticle into membrane surface brings small RO

162 S.H. Kim et al. / Journal of Membrane Science 211 (2003) 157–165

Fig. 4. Mechanism of self-assembly of TiO2 nanoparticles.

performance changes of 3.8 gfd flux-loss and 1.3%rejection-enhancement.Fig. 7 shows the plot of thewater flux of the hybrid and the neat LP membraneswith and without UV light illumination after expo-sure to microbial cells. As shown in figure, waterflux decreases for all the membranes due to microbialfouling. The flux reduction should be attributed tomembrane biofouling. Although the flux for the neat

Fig. 5. Schematic drawing of hybrid membrane.

LP membrane is initially more than that for hybrid LPmembrane, it drops more rapidly. The flux of hybridLP membrane under UV is greater than that of neatLP membrane in the same condition after 1 day of thefouling. Within 3 days, the flux of neat LP membranein dark condition is decreased to 55.4 gfd, and that forhybrid membrane with UV illumination, however, is66.1 gfd. The ratios of flux to their initial values are

S.H. Kim et al. / Journal of Membrane Science 211 (2003) 157–165 163

Fig. 6. Cell number and survival ratio ofE. coli in the hybrid and the neat TFC membranes in the dark and with UV light illumination.

plotted inFig. 8. Neat LP membrane loses a 30% ofits original water permeability. However, there is onlyhalf of the loss in the hybrid LP membrane under UVillumination.

3.6. Anti-fouling effect of hybrid membrane

Membrane biofouling is initiated by the attachmentof a bacterial cell to the membrane surface and fol-lowed by cell growth and multiplication at the expenseof soluble feedwater nutrients[3]. The bacteria form-ing surface biofilm excrete extracellular polymericsubstances (EPS), which serves to firmly bind cells

Table 2Transport characteristics of low pressure (LP) RO membranes

Sample RO performance

Water flux(gfd)

Salt rejection(%)

No-TiO2 neat LP membrane 79.8 94.7TiO2 hybrid LP membrane 76.0 96.0

All the results were obtained with 2000 ppm NaCl in deionizedwater and at the operating pressure of 225 psi and the temperatureof 25◦C.

to the substratum and stimulate additional microbialcolonization. It was reported that the EPS materialswas associated with the final irreversible phase of bio-fouling [23]. Hybrid membrane has a photocatalyticbactericidal effect onE. coli. This result is due toTiO2 photocatalysis to generate various active oxygenspecies such as hydroxyl radical, hydrogen peroxide,

Fig. 7. Water flux of the hybrid and the neat LP membranes withand without UV light after exposure to microbial cells.

164 S.H. Kim et al. / Journal of Membrane Science 211 (2003) 157–165

Fig. 8. The ratio of flux to their initial values during foulingexperiment.

etc. by reductive reactions and oxidative reactions.These active oxygen species, anti-bacterial reagents,can inactivate cell viability by destroying the outermembrane of bacterium cells[24,25].

The anti-fouling of the hybrid LP membrane can beexplained from the mechanism of membrane biofoul-ing and the photocatalytic bactericidal effect of thehybrid membrane. Anti-bacterial species produced bythe hybrid membrane kill bacterial cells and preventbacterial attachment to membrane surface, which re-duce the membrane biofouling. Furthermore, thereare some reports that adhesion may be affected bythe growth cycle[26]. The number of cells attach-ing and the rate of attachment was greatest with logphase cultures and progressively decreased with sta-tionary and death phase cultures. From these facts,it is easily conjectured that the dead bacteria may bedetached from the membrane surface and swept offby concentrated water. Such photodisinfection prop-erty of hybrid membrane is expected to reduce themicrobiological fouling of RO membrane.

4. Conclusions

Biofouling by microorganisms has been knownto be the main cause to deteriorate the reverse os-mosis (RO) performance of the aromatic polyamidethin-film-composite (TFC) membranes. This studydevised a new type of hybrid TFC membrane as an ap-proach to solve biofouling problem and characterizedits photocatalytic bactericidal anti-fouling effect.

1. The anatase TiO2 nanoparticle of a quantum size(∼10 nm or less) was prepared by the controlledhydrolysis of titanium tetraisopropoxide. The par-ticle structure and size were characterized by X-raydiffraction analysis and transmission electron mi-croscopy (TEM).

2. The TiO2 nanoparticles were hybridized with thethin-film-composite (TFC) aromatic polyamidemembrane by self-assembly through coordinationand H-bond interaction with the COOH groupin membrane surface. X-ray photoelectron spec-troscopy (XPS) demonstrated quantitatively thatTiO2 particles were tightly self-assembled witha sufficient bonding strength for the actual ROprocess.

3. The photocatalytic bactericidal effect of the hy-brid TFC membrane was examined by determiningthe survival ratios of theEscherichia coli (E. coli)cell with and without UV light illumination. Thephotocatalytic bactericidal efficiency was remark-ably higher for the hybrid TFC membrane underUV light illumination than that without illumina-tion and the neat TFC membranes.

4. A new type of hybrid anti-fouling membrane wasdeveloped by introduction of TiO2 nanoparticles onthe actual commercial TFC membrane. The foul-ing experiment verified a substantial prevention ofthe hybrid membrane against the microbial foul-ing, suggesting a possible use as a new type ofanti-biofouling TFC membrane.

Acknowledgements

The authors are grateful to the Ministry of Environ-ment, Republic of Korea for their support of this studythrough Eco-Technopia 21 project.

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