Environ. Sci. Technol. 2001, 35, 2388-2394
Hybrid Organic/Inorganic Reverse Osmosis (RO) Membrane for Bactericidal Anti-Fouling. 1. Preparation and Characterization of TiO2 Nanoparticle Self-Assembled Aromatic Polyamide Thin-Film-Composite (TFC) Membrane SEUNG-YEOP KWAK* AND SUNG HO KIM Hyperstructured Organic Materials Research Center (HOMRC) and School of Materials Science and Engineering, Seoul National University, San 56-1, Shinlim-dong, Kwanak-ku, Seoul 151-744, Korea SOON SIK KIM Saehan Industries Incorporated, #14, Nongseo-Ri, Kiheung-Eub, Yongin-City,Kyunggi-Do 449-900, Korea
Hybrid organic/inorganic reverse osmosis (RO) membranes composed of aromatic polyamide thin films underneath titanium dioxide (TiO2) nanosized particles have been fabricated by a self-assembly process, aiming at breakthrough of biofouling problems. First, positively charged particles of the colloidal TiO2 were synthesized by a sol-gel process, and the diameter of the resulting particles in acidic aqueous solution was estimated to be ≈2 nm by analyzing the UV-visible absorption characteristics with a quantum mechanical model developed by Brus. Transmission electron microscopy (TEM) further confirmed the formation of the quantum-sized TiO2 particles (∼10 nm or less). The TiO2 particles appeared to exist in the crystallographic form of anatase as observed with the X-ray diffraction (XRD) pattern in comparison with those of commercial 100% rutile and commercial 70:30% anatase-to-rutile mixture. The hybrid thin-film-composite (TFC) aromatic polyamide membranes were prepared by self-assembly of the TiO2 nanoparticles on the polymer chains with COOH groups along the surface. They showed improved RO performance in which the water flux even increased, though slightly. Field-emission scanning electron microscopy (FESEM) exhibited the TiO2 nanoparticles well adsorbed onto the surface. X-ray photoelectron spectroscopy (XPS) demonstrated quantitatively that a considerable amount of the adsorbed particles were tightly self-assembled at the expense of the initial loss of those that were loosely bound, and became stabilized even after exposure to the various washing and harsh RO operating conditions. The antibacterial fouling potential of the TiO2 hybrid membrane was examined and verified by measuring the viable numbers and determining the survival ratios of the Escherichia coli (E. coli) as a model bacterium, both with and without UV light illumination. The photocatalytic bactericidal efficiency was remarkably higher for the TiO2 hybrid 2388
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membrane under UV illumination, compared to that of the same membrane in darkness, as well as those for the neat membranes under either light condition.
Introduction One of the goals of research and industry in the reverse osmosis (RO) membrane fields has been to enhance, or at least maintain, water flux without sacrificing salt rejection over a long period, in order to increase efficiency and reduce the cost of operation. Nevertheless, the main difficulty in accomplishing this goal is fouling (1, 2), where a serious flux decline occurs as the actual operation time elapses. A detailed assessment of the costs of biofouling was made for the RO plant at Water Factory 21 in Orange County, CA (3). According to the report, the membranes, owing to the additional hydraulic resistance of the biofouling layer, operate at about 150% of their initial operating pressure over 80% of their life. A regular amount of chlorine is added continuously to the feedwater and membranes are cleaned periodically. The bottom line is $727,816 spent each year to control membrane biofouling. That represents about 30% of the total operating costs for the facility. The principal types of fouling are crystalline fouling (mineral scaling, or deposit of minerals due to an excess of the solution product), organic fouling (deposition of dissolved humic acids, oil, grease, etc.), particle and colloid fouling (deposition of clay, silt, particulate humic substances, debris, and silica), and microbial fouling (biofouling, adhesion and accumulation of microorganisms, and forming biofilms) (3). Various approaches to reducing fouling have been used, which generally involve pretreatment of the feed solution, modification of the membrane surface properties (such as hydrophobic or hydrophilic and electronegative or electropositive), optimization of module arrangement and process conditions, and periodic cleaning (4). However, these methods vary widely in applicability and efficiency, thereby requiring a breakthrough to solve fouling problems. The most common RO membrane used for water treatment is the thin-film-composite (TFC) type composed of aromatic polyamide. Particularly for such aromatic polyamide TFC membranes fouling from the formation of biofilm on the surface caused by microorganisms has been regarded as of the uppermost importance (5). Microorganisms such as bacteria and viruses in water adhere to membrane surfaces and grow at the expense of nutrients accumulated from the water phase. The attached microorganisms excrete extracellular polymeric substances (EPS) and, thus, form biofilms (6). It has been reported that biofilm formation was related to the depletion of residual disinfectant concentration, and that no biofilm was formed from disinfectant-treated water, such as chlorinated water containing a residual of 0.04-0.05 mg/L free chlorine (7, 8). However, it is noted that chlorination, although effective for the destruction of microorganisms, generates harmful byproducts such as trihalomethanes and other carcinogens. Motivated by these results, the present paper is aimed at developing and characterizing a hybrid membrane possessing inorganic nanoparticles capable of killing the microbes without forming unwanted byproducts as a means of precluding the formation of biofilms and, hence, reducing fouling. Titanium dioxide (TiO2) has been the focus of numerous investigations in recent years, particularly because of its * Corresponding author phone: +82-2 880-6082; fax: +82-2 8766086; e-mail:
[email protected]. 10.1021/es0017099 CCC: $20.00
2001 American Chemical Society Published on Web 05/04/2001
FIGURE 1. Schematic of the thin-film-composite (TFC) reverse osmosis (RO) membrane and the chemical structure of the aromatic polyamide thin-film layer. photocatalytic effects that decompose organic chemicals and kill bacteria (9). TiO2 photocatalysis is known to generate various active oxygen species, such as hydroxyl radical, hydrogen peroxide, etc., by reductive reactions or oxidative reactions under light (10, 11). These active oxygen species further destroy the outer membrane of the bacterium cells and decompose the endotoxin from them. Nanoscale technology manipulates things on the nanoscale (generally regarded as 1-100 nm) which makes it possible to arrive at fundamentally new types of devices with much improved properties and/or novel functionality (12). Nanosized TiO2 particles, from the viewpoint of their photocatalytic capability to break down bacteria and organisms, will also be very useful because of their high surface area per unit volume and high abrasive resistance when coated on the target materials (9). Several different strategies to integrate TiO2 with target materials have been reported, which include self-assembly monolayer adsorption on functionalized surfaces, sol-gel synthesis, vacuum vaporization, sputtering, metal organic chemical vapor deposition (MOCVD), chemical vapor deposition (CVD), and the Langmuir-Blodgett (LB) method. Among them, the method of self-assembly of TiO2 on surfaces (for example, single-crystal silicon, quartz, and glass substrates), employing polymers with -CO2H or -SO3H functional groups, can be performed to fabricate multilayer ultrathin films, overcoming the limitations (such as high temperature, solvent involvement, costly fabrication, and complex process control) inherently associated with other methods (13-15). It is worth noting that the thin-film active layer of aromatic polyamide TFC RO membranes is composed of the cross-linked form of three amide linkages and the linear form with pendant free carboxylic acid as shown in Figure 1. The fraction of the linear carboxylic acid form has been estimated to be 30% to 50%, depending on the investigators (16-18). Thus, it is probable to self-assemble the TiO2 nanoparticles on the aromatic polyamide TFC membrane surface, thereafter expecting the appearance of a novel organic/inorganic hybrid TFC membrane for the photocatalytic bactericidal anti-fouling. In this study, quantum-sized TiO2 particles are prepared from the controlled hydrolysis of titanium tetraisopropoxide (19) and then characterized. The particle size and crystal structure of the resulting TiO2 nanoparticles are characterized by UV-visible absorption spectroscopy, transmission elec-
tron microscopy (TEM), and X-ray diffraction (XRD). Introduction of the TiO2 nanoparticles within the aromatic polyamide TFC membrane is performed by way of selfassembly of the nanoparticles through ionic interaction and H-bonding force with -COOH functional groups of the aromatic polyamide. The morphological structures of the resulting TiO2 self-assembled TFC membrane are investigated by field-emission scanning electron microscopy (FE-SEM), as well as by X-ray photoelectron spectroscopy (XPS), which analyzes the atomic concentrations of titanium. The RO performance test is then carried out to see whether any variation of water flux and salt rejection occurs in the presence of the TiO2 nanoparticles on top of the TFC membrane surface. Thereafter, XPS analysis of atomic concentrations of Ti is also performed with the RO-tested membrane to evaluate the binding durability of the self-assembled TiO2 nanoparticles, even after the harsh, actual operation conditions. Finally, to verify the photocatalytic bactericidal capability of this TiO2 nanoparticle self-assembled TFC membrane, the membrane surface is covered by a model suspension of Escherichia coli (E. coli) bacterium cells grown aerobically in the nutrient broth, which is in turn illuminated by UV radiation. Then, the bactericidal effect is observed and confirmed by counting the viable number of E. coli cells.
Experimental Section Synthesis and Characterization of the Nanosized TiO2 Particles. Nanosized TiO2 colloids were prepared from the controlled hydrolysis of titanium tetraisopropoxide, Ti(OCH(CH3)2)4, by following procedures in the literature (19). A 1.25-mL sample of Ti(OCH(CH3)2)4 (Aldrich, 97%) dissolved in 25 mL of absolute ethanol (J. T. Baker) by injection was added drop by drop under vigorous stirring to 250 mL of distilled water (4 °C) adjusted to pH 1.5 with nitric acid. This mixture was stirred overnight until it was clear and the transparent colloidal suspension (1.34 g/L) resulted. TiO2 surfaces dissociatively absorb water to form surface hydroxyl groups, which are believed to be the active sites for the adsorption of reactants. The surface properties of TiO2 particles are described by acid-base equilibria involving surface hydroxyl groups. As the isoelectric point of titanium dioxide corresponded to pH ) 4.5-6.8 (20), the resulting colloids would take the shape of stable cationic TiO2 complex at pH 1.5. VOL. 35, NO. 11, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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The UV-visible spectrum of the transparent TiO2 colloidal suspension was recorded with a Hewlett-Packard HP8452 diode array spectrophotometer to analyze the optical absorption characteristics of TiO2 and, thus, to determine the particle sizes theoretically. The particle sizes were also determined by a JEOL JEM-200CX transmission electron microscope (TEM) at 120 kV. For the TEM observation, the TiO2 colloidal suspension was dropped on a carbon-coated grid and then dried at room temperature. The crystal structure of TiO2 was characterized by X-ray diffraction (XRD). XRD analysis was performed on TiO2 powder samples with a MAC Science X-ray diffractometer (MXP18X-MF22-SRA), operating in the theta-theta geometry using 18kW Cu KR (λ ) 1.5418 Å) radiation. For comparison purposes, other commercial TiO2 particles such as Sigma-Aldrich rutile TiO2 and DegussaHu ¨ ls P25 TiO2 were also analyzed by XRD. Preparation of TiO2 Nanoparticle Self-Assembled ThinFilm-Composite (TFC) Membranes and Measurement of Transport Characteristics. The thin-film-composite (TFC) membranes were prepared via interfacial polymerization of m-phenylenediamine (MPD) in the aqueous phase (2 wt %) and trimesoyl chloride (TMC) in the organic phase (0.1 wt %) on the polysulfone supports reinforced by the nonwoven fabric as schematically depicted in Figure 1. The polysulfone layer acts as the support to give membranes the mechanical strength to resist RO pressure. The thin-film layer governs the actual separation of the solute and the passage of the solvent. The resulting aromatic polyamide TFC membrane was rinsed in a sodium carbonate solution (0.2 wt %) and then washed with distilled water. The final membrane, with an area of ca. 50.0 cm2, was dipped in the transparent TiO2 colloidal solution for 1 h to deposit TiO2 nanoparticles on the membrane surface, then rinsed extensively with water. Reverse osmosis (RO) performance tests were conducted at 225 psi using 2000 ppm NaCl solution at 25 °C with the apparatus of a continuous-flow type. The water flux was determined by direct measurement of the permeate flow:
Flux (gfd) )
permeate (gallon) membrane area (ft2)‚time (day)
(1)
The salt rejection was measured by the salt concentration in the permeate obtained through measurements of the electrical conductance of the permeate and the feed using a conductance meter (Orion model 162):
(
Rejection (%) ) 1 -
permeate conductance × 100 (2) feed conductance
)
Characterization of Morphological and Chemical Structures of Membrane Surface. The surface topologies of the TiO2 nanoparticle-introduced aromatic-polyamide-TFC membrane were investigated with a Philips XL30 FEG field emission scanning electron microscope (FESEM). The surface morphology of the neat aromatic-polyamide-TFC membrane was also examined and was compared with that of the TiO2 self-assembled version. For the FESEM observation, the membrane samples were cut into appropriate sizes and the surfaces were coated with platinum or gold by a sputtercoating machine. X-ray photoelectron spectroscopy (XPS) experiments were carried out with a Kratos AXIS HS spectrometer using a Mg KR X-ray source (1253.6 eV). The X-ray gun was operated at 10 kV and 1 mA, and the charge neutralization system was used to obtain high-resolution spectra for the insulating materials, such as polymers, by reducing the surface charge. The spectrum was obtained at the photoelectron takeoff angles (defined as the angle between the detected photoelectron beam and the membrane surfaces) of 30° and 90° to give sampling depths of ca. 23 Å and ca. 45 Å, respectively. 2390
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FIGURE 2. UV-visible absorption spectrum of the dilute nanosized TiO2 colloidal suspension. The elemental composition analysis was performed on carbon, nitrogen, oxygen, and titanium, which constituted the hybrid membrane as well as the neat membrane. The sensitivity factors of individual elements were taken with the values from the standard vision library provided by the manufacturer, which were based on a combination of photoelectric cross-section, transmission function, and inelastic mean free path. Evaluation of Photocatalytic Bactericidal Effect of TiO2 Self-Assembled TFC Membrane. Escherichia coli (E. coli) bacterium cells (DH5R strain) were grown aerobically in 10 mL of nutrient broth (Luria-Bertani medium) at 37 °C for 12-16 h. The Luria-Bertani (LB) medium was prepared with 1 wt % Bacto-tryptone, 0.5 wt % yeast, and 1 wt % NaCl. The grown cells were centrifuged at 10000 rpm for 1 min and diluted to an appropriate concentration with sterilized water. The E. coli cell dilution (150 µL, total 1.0 × 104 cells) was pipetted onto either a TiO2 hybrid TFC membrane or a neat membrane, which were placed in an incubator at a constant temperature of 37 °C. Some of the individual membranes were illuminated with an 8-W black light (VWR UVLS-28) and some were not. The light intensity at the peak of 365 nm was 500 µW cm-2 at 3 in., which was determined by the procedure provided by the manufacturer. After illumination up to the intended exposure time, the cells were pipetted out and collected in 1.0 wt % aqueous sodium chloride solution. The collected solutions were spread onto a LB agar plate and incubated for 12-16 h to determine the number of viable cells in terms of colony-forming units (CFU) as a function of time. The initial cell number was determined to be 9420 in 150 µL of cell dilution suspension spread onto a LB plate without illumination.
Results and Discussion Particle Size and Crystal Structure of Synthesized TiO2. Figure 2 shows the UV-visible spectrum of the TiO2 colloidal solution obtained via sol-gel synthesis of titanium tetraisoproxide. The onset of absorption (λos) and the corresponding band gap energy (Eg) of the bulk TiO2 have been determined to be λos ) 385 nm and Eg ) 3.2 eV for anatase (21), and λos ) 415 nm and Eg ) 3.0 eV for rutile (22), respectively. The band gap of the TiO2 colloidal solution is measured to be 3.44 eV (361 nm) according to the spectral analysis in the figure, which is in agreement with other research. This corresponds to the 0.24 eV blue shift from the bulk-phase band gap of anatase (3.2 eV), indicating that the ultra-small TiO2 particles are formed. Particle size can be further estimated according to a theoretical prediction proposed by Brus (23):
∆Eg )
h2 1.8e2 2 R 8R µ
(3)
where ∆Eg is the band gap shift, R is the radius of the particle, µ is the reduced mass of the exciton (µ ) 1.63 me; me is the electron rest mass) for TiO2, and is the dielectric constant of the semiconductor ( ) 184) for TiO2 (21). The band gap shift of 0.24 eV for the TiO2 colloids corresponds to a particle diameter of ca. 2.0 nm, which implies so-called quantum(Q) sized TiO2 particles. The particle size is also investigated by transmission electron microscopy (TEM) as shown in Figure 3, where black spots are the synthesized TiO2 particles and all of them measure less than 10 nm. The discrepancy between the particle sizes determined by TEM and UVvisible spectroscopy might ascribe to the agglomeration of the Q-sized TiO2 particles during the drying procedure to prepare TEM samples. X-ray diffraction (XRD) analysis is employed to characterize the crystal structure of the TiO2 nanoparticles. It is known that TiO2 particles are in two different crystal forms, i.e., anatase and rutile (24). In the anatase (Tio[O2]c) structure, the oxygens form a cubic closest packing, and the titanium atoms lie in octahedral voids. In the rutile (Tio[O2]h) form, the oxygens are arranged approximately in a hexagonal closest packing, and the titanium atoms occupy a row pattern. Many researchers claim that the anatase appears to be the most photoactive and stable nanoparticles for widespread practical
applications (9, 25, 26), whereas the rutile is photocatallytically inactive (27-29) or much less active (30-33), although it shows strong photoactivity selectively toward some cases (9, 34). Figure 4 compares the X-ray diffraction patterns of three types of TiO2 particles as designated in the figure. The 100% rutile TiO2, Figure 4(b), shows the characteristic peaks located at 2θ of 27.45°. As for the diffraction pattern of Degussa P25 TiO2, Figure 4(c), which is a nonporous 7:3 anatase-torutile mixture and is one of the most often used photocatalysts (9), the 2θ of eminent peaks are 25.24° for anatase and 27.46° for rutile. Comparing diffraction patterns of (a) with (b) and (c), it is confirmed that our TiO2 nanoparticles are composed entirely of anatase, which promises the highest photoreactivity and the best efficiency for destroying the microorganisms. Reverse Osmosis Performance and Surface Characterization of TiO2 Self-Assembled and Neat Membranes. As described earlier, the fabrication of TiO2 self-assembled thinfilm-composite (TFC) membranes was carried out by dipping the neat aromatic polyamide membrane into the solution of colloidal TiO2 particles followed by washing with water. According to the recent investigation of the adsorption behavior of carboxylic acid on TiO2 by virtue of diffuse reflectance infrared Fourier transform (DRIFT), the process of self-assembly between carboxylic acid and TiO2 was explained by two different adsorption schemes (35). One scheme was that TiO2 was bound with two oxygen atoms of
FIGURE 3. TEM micrograph of the TiO2 nanoparticles. VOL. 35, NO. 11, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 4. XRD images of the synthesized TiO2 (a), commercial rutile TiO2 (Sigma-Aldrich) (b), and commercial P25 TiO2 (DegussaHu1 ls) particles (c).
TABLE 1. Transport Characteristics of Aromatic TFC Membranes RO performance a sample neat TFC membrane TiO2 hybrid TFC membrane
water flux (gfd) salt rejection (%) 13.2 14.4
96.5 96.6
a All the results were obtained with 2000 ppm NaCl in deionized water and at the operating pressure of 225 psi and temperature of 25 °C.
a carboxylate group via a bidentate coordination to Ti4+ cations. The other scheme was to form a H bond between a carbonyl group and the surface hydroxyl group of TiO2. Although the majority of COOH groups are not dissolved into the form of free ion at pH 1.5, however, the distribution of electron density in a polar bond may be symbolized by partial charges: δ+ (partial positive) and δ- (partial negative). Two oxygen atoms of COOH groups are partial negative and have ionic character. These facts suppose that ionic interaction between the positive surface of the metal oxide particles and COOH groups with ionic character causes the adsorption, and that TiO2 particles are strongly bonded to the membrane surface by a bidentate coordination and a H bond. The basic requirement of such a hybrid TFC membrane is to preserve the reverse osmosis (RO) performance as much as possible before the integration of TiO2 nanoparticles. Table 1 contains the RO performance data of water flux and salt rejection for TiO2 self-assembled and neat aromatic polyamide TFC membranes; all the results were the arithmetic means of four replications. The TiO2 hybrid TFC membrane shows a slight increase in water flux as compared to that of the neat membrane. The observed flux increase upon integration of TiO2 nanoparticles can be explained by two facts. During self-assembly of TiO2 nanoparticles, the TFC membrane is exposed in the very-low-pH nitric acid, in which the acid has been found to cause partial hydrolysis on the membrane surface and increase hydrophilicity, and, hence, water flux (36). The other explanation may involve the water uptake characteristics of TiO2 particles (37), which are considered to be a further contribution to the increase of water flux. Shown in Figure 5 are the surface topologies of the TiO2 hybrid and the neat TFC membranes investigated by field-emission scanning electron microscopy (FESEM). The neat aromatic polyamide membrane has the typical surface morphology of a characteristic ridge-and-valley structure, Figure 5(a), which has been observed by many investigators as the analogous membrane under the trade 2392
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FIGURE 5. FESEM micrographs of the neat (a) and the TiO2 hybrid (b) aromatic polyamide TFC membranes. name of FT-30 (38). Figure 5(b) displays the surface image of the TiO2 self-assembled hybrid TFC membrane, where TiO2 nanoparticles appear to exist as nodular shapes of ca. 10 nm or less on the surfaces of the ridges and valleys. To confirm the self-assembly TiO2 nanoparticles within the hybrid membrane and further estimate their abrasive resistance to the washing steps involved in membrane preparation, as well as the actual operating conditions, X-ray photoelectron spectroscopic (XPS) analyses are carried out for the neat TFC membrane and the TiO2 hybrid TFC membrane treated under various conditions. The constituent elements of the thin-film layer of the neat TFC membranes are hydrogen, carbon, nitrogen, and oxygen, and additionally titanium for the hybrid TFC membrane. Thus, XPS analyses are performed on the elements of carbon, nitrogen, oxygen, and titanium, but not on hydrogen because its photoelectron cross-section is too small to be characterized by XPS. The core-electron binding energies of the constituent elements are typically 284.6 eV for C 1s, 397.9 eV for N 1s, 531.6 eV for O 1s, 453.8 eV for Ti 2p3/2, and 460.0 eV for Ti 2p1/2. Figure 6 (a-d) shows the resulting XPS spectra, in which all the photoelectron peaks appear at positions similar to the above values and the presence of Ti peaks (d). The hybrid membrane provides evidence of TiO2 self-assembling. On the basis of the observed photoelectron peaks and corresponding sensitivity factors, the relative atomic concentrations of the
FIGURE 6. XPS spectra of carbon (a), nitrogen (b), oxygen (c), and titanium (d) for the neat and the TiO2 hybrid aromatic polyamide TFC membranes. individual elements can be calculated:
Ci )
AiSi
(4)
m
∑A /S j
TABLE 2. Elemental Compositions of the TiO2 Hybrid TFC Membranes under Various Washing Conditions and RO Operational Hour relative atomic concentration (%)
j
j
where Ai is the photoelectron peak area of the element i, Si is the sensitivity factor for the element i, and m is the number of elements in the sample. In Table 2, the elemental compositions determined by an angle-resolved XPS analysis are summarized for the hybrid membranes with various washing conditions and RO operational hours. As seen in the table, there is an initial drop in the relative atomic concentration of Ti after washing the hybrid membrane which has been just formed from dipping into the TiO2 colloidal solution. An additional loss of TiO2 nanoparticles is observed upon further RO operation with run time of 30 min. Recognizing that the RO process in this study is operated in the cross-flow mode where the feed solution is pumped across the hybrid membrane parallel to its surface, it is thought that the loosely bound TiO2 particles cannot overcome the shear-flow force and are, thus, wiped out during the 30 min RO operation. However, the TiO2 loss does not continue to progress as the RO operational hours are increased, and the amount of TiO2 levels off even after 7 days of RO operation. This result indicates that a considerably substantial amount of TiO2 nanoparticles remains tightly bound on the surface of the membrane under actual RO running conditions, which
samplea
takeoff angle (°)
C
O
N
Ti
1
90 30 90 30 90 30 90 30
64.3 61.8 61.4 60.0 62.9 60.9 63.7 59.5
27.1 27.4 30.2 29.4 27.7 26.8 29.3 30.7
6.3 8.3 6.4 8.3 8.1 10.8 5.9 8.3
2.3 2.4 1.9 2.2 1.2 1.5 1.1 1.5
2 3 4
a Analyses were performed for the TiO self-assembled TFC RO 2 membranes (1) just after preparation, (2) after washing with flowing water, (3) after RO operation of 30 min, (4) after RO operation for another 7 days.
is expected to act as photocatalyst and reduce the biofouling as a result of destroying the bacteria on top of the membrane surface. Photocatalytic Bactericidal Effect of TiO2 Self-Assembled Membrane. Figure 7 shows the plots of the survival ratios of E. coli bacteria in both the TiO2 hybrid and the neat TFC membranes with and without UV light illumination as a function of time; the hybrid membrane was that of the 7-dayoperated RO. In the experimental setup, the natural diminution of cell population with time was unavoidable, probably VOL. 35, NO. 11, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 7. Photocatalytic bactericidal effects of the TiO2 hybrid and neat aromatic polyamide TFC membranes in the dark and with UV light illumination. due to an insufficient supply of the nutrients over a prolonged time interval, and the survival ratio of E. coli cells for the neat TFC membrane in the dark without illumination is inevitably decreased by ca. 40%. The TiO2 hybrid TFC membrane in the same dark condition is shown to affect and decrease slightly the survival ratio, implying that the TiO2 itself, even in the dark, may have minute photocatalysis on E. coli. For the neat TFC membrane under UV light illumination, the survival ratio of E. coli cells is reduced to ca. 40% within 3 h and ca. 37% within 4 h. The UV light causes more sterilization of the TiO2 hybrid membrane; the 10% of E. coli cells survived only after 3 h, reaching complete sterilization within 4 h in the presence of TiO2. The TiO2 self-assembled hybrid membrane under UV illumination possesses a remarkably higher photocatalytic bactericidal efficiency than that without illumination and the neat TFC membranes. The photocatalytic bactericidal capability demonstrated by the TiO2 selfassembled hybrid TFC membrane offers a strong potential for possible use as a new type of antifouling RO membrane. The bactericidal efficiency of the different species of bacteria and the actual antifouling performance of the hybrid membrane, as well as an idea concerning the light source inside the membrane module, will be revealed in a publication soon.
Acknowledgments The authors are grateful to the Korea Science and Engineering Foundation (KOSEF) for their support of this study through the Hyperstructured Organic Materials Research Center (HOMRC). They also express their appreciation to Saehan Industries Inc. for their permission to publish this work.
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Received for review September 28, 2000. Revised manuscript received February 12, 2001. Accepted March 1, 2001. ES0017099
