- Email: [email protected]
glass fiber photocatalytic reactors in the removal of bioaerosols
Surface & Coatings Technology 205 (2010) S341–S344
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Novel TiO2 thin films/glass fiber photocatalytic reactors in the removal of bioaerosols Ching-Hsing Lin a,⁎, Jyh-Wei Lee b,c, Chen-Yu Chang d, Yu-Jie Chang e, Yao-Chuan Lee a, Mei-Yin Hwa a a
Dept. of Safety, Health and Environmental Engineering, Tungnan University, Taipei, Taiwan Dept. of Materials Engineering, Mingchi University of Technology, Taipei, Taiwan c Center for Thin Film Technologies and Applications, Mingchi University of Technology, Taipei, Taiwan d Center of General Education, National Taitung Junior College, Taitung, Taiwan e Graduate School of Environmental Education & Resources, Taipei Municipal University of Education, Taipei, Taiwan b
a r t i c l e
i n f o
Available online 12 August 2010 Keywords: Escherichia coli Sol–gel method Photocatalytic Disinfection tests
a b s t r a c t The photocatalytic disinfection of aerosolized Escherichia coli, a pathogenic microorganism was investigated. The air disinfection system is a novel TiO2 thin films/glass reactor filter and ultraviolet (UV) radiation air purification system, operating at a flow rate of 20 L/min. A special glass fiber reactor design allowed the UV lamp to be located in the center of the reactor. The photocatalyst reactor was then filled with TiO2 coated glass fiber substrates to examine the photocatalytic efficiency of the TiO2 filled reactor against aerosolized E. coli in cell concentrations of 105 CFU/mL. A slow sol–gel technique hydrolysis was used in this study to acquire fine and uniform TiO2 nanoparticles of 10–30 nm. The TiO2 thin films were obtained on glass fiber substrates by dipping method followed by thermal treatment. In an attempt to understand the structure and the morphology of TiO2 sol–gel thin films, analyses by X-ray diffraction (XRD) and scanning electron microscopy (SEM) were employed. The SEM images showed that uniform anatase TiO2 thin films were coated on fiber surfaces. A neublizer was applied to yield E. coli bioaerosol at a controlled humidity of 72%. Disinfection of E. coli under 254 and 365 nm light illumination was conducted to evaluate the photocatalytic ability of the TiO2 thin films. The nano-TiO2/glass fibers gel catalyst prepared in the laboratory showed good photocatalytic performance with the high degradation efficiency above 95% at both wavelengths. With the UV illumination switched off, the degradation efficiency dropped to below 60%. The improvement of the photocatalytic activity was ascribed to the fibers-based reactor with a screen mechanism which provided a huge surface area of TiO2 thin films. These results will be useful and assist engineers to design photocatalyst reactors for the industrial applications of bioaerosol removal. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved.
1. Introduction The decomposition of environmental pollutants and the disinfection of microorganisms using photocatalytic TiO2 have been widely studied during the past several decades due to its good environmental stability and excellent photocatalytic activity [1–3]. The photokilling of bacterial by active anatase TiO2 occurs via the oxidation of intracellular coenzyme A [4], peroxidation of the polyunsaturated phospholipid component of the lipid membrane [5], damage incurred to the cytoplasmic membrane [6] and destruction of cell wall [7,8], thereby causing cell death. The synthesis of anatase TiO2 thin films by various deposition techniques, such as sol–gel, chemical vapor deposition, impregnation and thermal evaporation has been reported [9–11]. The sol–gel method of anatase TiO2 fabrication is simple and low cost. It can yield high uniformity and fine particle size of the
⁎ Corresponding author. Tel.: + 886 2 86625935x119; fax: + 886 2 86625934. E-mail address: [email protected] (C.-H. Lin).
catalyst. This method can be operated at room temperature and is easy to control with chemical dosage and can be easily manipulated to produce different catalytic properties. The sol–gel method was usually used to deposit TiO2 films on the substrate in order to eliminate the bioaerosols by photocatalyst reactions. The synthetic fibers [12] and molecular sieves [13] have been used as substrates to control bioaerosols, due to the possibility of killing microorganisms with a lower pressure drop [14]. In previous work [15], the glass fibers covered with TiO2 thin films by sol–gel method were successfully employed as a photocatalyst reactor to degrade toluene. In this study, we have mainly focused on the disinfection of Escherichia coli under 254 and 365 nm light illumination with a novel TiO2 thin films/glass fiber reactor. For this investigation, TiO2 nano powders were synthesized by sol–gel. TiO2 thin films were further deposited on glass fiber substrates by dipping method followed by thermal treatment. In this study we ascribe the improvement of the high disinfection efficiency against aerosolized E. coli to the novel fibers-based reactor with a screen mechanism, which provides a huge effective and reactive surface area of TiO2 thin films for the photocatalytic reaction.
0257-8972/$ – see front matter. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2010.08.009
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C.-H. Lin et al. / Surface & Coatings Technology 205 (2010) S341–S344
2. Materials and methods 2.1. Testing microorganisms The bacterial strain used herein was Gram-negative E. coli, which is often selected as a challenge bioaerosol for antimicrobial tests [16,17]. The bacteria stocks were kept on tryptone soya agar (TSA, Oxoid) plate at − 4 °C. Before antimicrobial tests, the bacteria cells were reactivated by culturing a loop of inoculum in 10 mL sterile nutrient broth under gentle shaking for 16–24 h in an incubator at 36 ± 1 °C and 80 rpm. The viable cell count was determined by plate counting technique on TSA plate after serial dilution. All specimens and test apparatuses were sterilized using pressurized steam autoclave prior to the test. Antimicrobial tests were performed under aseptic conditions. 2.2. Preparation of bioaerosols The bioaerosols used in the experiments were generated by a Collison six-jet nebulizer (BGI Inc., Waltham, MA, USA) with an air flow of 20 L/min with 40 psig. A diffusion dryer and active carbon was used to remove the water and impurities of the generated bioaerosols to form droplet particles. The suspension concentration of the cell was about 105 CFU/mL. 2.3. Sampling of pathogenic microbes AGI-30 impingers (Model 7540, ACE GLASS Inc., NJ, USA) were used for the bioaerosol sampling. This impinger has been recommended by the American Conference of Governmental Industrial Hygienists and the International Aerobiology Symposium for sampling viable microorganisms [18]. The sampling flow rate was 12.5 L/ min and the sampling time was 5 min. Sterilized buffer solution of 50 mL was used as the sampling medium for the aerosolized E. coli. The viable cell count was determined by plate counting technique on TSA plate after serial dilution. 2.4. TiO2 thin films preparation The deposition of TiO2 films on glass fiber (Glass wool 18421 from Sigma-Aldrich Fluka) substrates was accomplished by modified sol– gel technique at ambient room temperature [15]. These films were prepared by dissolving the titanium alkoxide precursor (Titanium (IV) n-propoxide, Ti (OC3H7)4, 98%+, Alfa Aesar) in an alcoholic bath (IPA, isopropanol, C3H7OH). Acetic acid was used to avoid the early
precipitation of the oxides. After addition of the acetic acid, the solution was stirred vigorously up to 24 h with a magnetic stirrer, till the hydrolysis was complete. After a gel formed, the solution was kept in an airtight beaker to preclude formation of gel. Then, 95% ethanol solution was added to the gel to form the final coating solution. The glass fibers were dipped into the solution to form TiO2 thin films on surfaces. The coated glass fiber substrates were preheated in air at 60 °C for 4 h, and then calcined at 200 °C for 2 h in air. 2.5. Disinfection system Fig. 1 is a schematic diagram showing the proposed continuous TiO2 thin films/glass fibers system. A special design of glass reactor of 10 cm in diameter and 20 cm in length allowed the UV lamp to be located in the center of the reactor. The photocatalyst reactor was then filled with TiO2 coated glass fibers to examine the photocatalytic activities against aerosolized E. coli. A total weight of 20 g of glass fibers with TiO2 thin film coatings were used in the photocatalyst reactor. It is noticed that the effectiveness of this novel reactor to degrade toluene was observed in previous works [15]. The disinfection of aerosolized E. coli was conducted using three different irradiation methods. Method A: UV light switched on only and no glass fibers were used; method B: UV light switched off and filled with glass fibers; method C: UV light switched on and filled with glass fibers. The disinfection efficiency against aerosolized E. coli in the absence and in the presence of UV 254 nm and 365 nm light irradiation has been studied. Each test was carried out 3 times and during the course of the study the reactor was kept at 25 ± 1 °C in a fume hood. 3. Results and discussion 3.1. Microstructure of the deposited photocatalyst The photocatalyst powders prepared by modified sol–gel method described above were analyzed by the X-ray diffractometer to examine the crystallinity of TiO2. As illustrated in Fig. 2(a), three major diffraction peaks of the anatase TiO2 crystal structure appeared at the 2θ values of 25.4, 37.9 and 48.2. Compared with the JCPDS database, the crystal structure of the photocatalyst prepared in the experiment was of anatase form [19]. On the other hand, only the most intensive peak of anatase phase was visible in the XRD pattern of TiO2 coated glass fibers as shown in Fig. 2(b). Other peaks of anatase TiO2 phases were covered by the amorphous pattern of glass fiber.
1. Air compressor
6. TiO2 thin films/glass fibers reactor
2. HEPA
7. UV light illumination power
3. Mass flow meter
modulator
4. Nebulizer
8. AGI-30 impingers
5. Humidity Conditioning device
9. Air pump
Fig. 1. Schematic diagram of continuous TiO2 thin films/glass fibers disinfection system.
C.-H. Lin et al. / Surface & Coatings Technology 205 (2010) S341–S344
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Fig. 2. XRD patterns of TiO2 sol–gel powders (a) after calcination and (b) coated on glass fibers.
SEM micrographs of TiO2 thin films/glass fiber composites are shown in Fig. 3(a) and (b). TiO2 nanoparticles are uniformly distributed on the surface of glass fibers, as shown in Fig. 3(a). The particle diameter of nano-TiO2 is around 10–30 nm. It is obvious that the grain size distribution of TiO2 nanoparticles produced by slow hydrolysis is uniform. The elemental compositions of the TiO2 deposits on the glass fibers by EDS analysis are 55% Ti, 41% O, and 3% Si in wt.%. As compared with the chemical composition of the glass fibers, 39% Si, 37% O, 7% Na, 6% Ti, 5% Ca and 3% K in wt.%, it is believed that 3% Si of TiO2 deposits is attributed to the glass fiber substrates. In addition, the EDX analysis of TiO2 deposits shows no significant levels of impurities from the deposition process. Fig. 4 illustrates the cross-sectional morphology of TiO2 coated fiber glass. The image of fiber glass in lower magnification is shown in the upper left portion (inset). It is obvious that the thickness of the TiO2 thin film is around 0.5 μm as indicated by arrows. Meanwhile, the continuity and uniformity of dip-coated TiO2 thin films on the glass fiber surfaces are also observed. 3.2. Disinfection performance with different operation patterns From the result shown in Fig. 5, when the reactor operated in the A condition, average disinfection efficiencies against aerosolized E. coli at UV 254 nm and 365 nm were only 39.4 and 31.1%, respectively. The
relatively low disinfection efficiencies were due to short residence time of the airborne flow when no fibers were present. In the B condition, aerosolized E. coli have been captured by the glass fibers, the screen effects were the main action mechanism of the glass fibers reactor. The average disinfection efficiencies of the two reactors were 49.6 and 54.3%, respectively. As the photocatalytic reaction started (condition C), it was clearly observed from the experimental data, that the disinfection rate of aerosolized E. coli became significant. Fig. 5 shows the effect of 254 nm and 365 nm UV light on E. coli, in the TiO2 thin film/glass fiber reactor system. As can be seen, E. coli were destroyed by the photocatalytic reaction at average rate of 97.7% for 254 nm light and 94.6% for 365 nm light, respectively. The destruction mechanisms of the aerosolized E. coli were ascribed to both the screen effects and the photocatalytic reaction, as revealed herein. 3.3. Disinfection performance with different irradiation lights Fig. 6 depicts the residual relative concentration of aerosolized E. coli after exposure to UV light of 254 nm, 365 nm wavelengths and with UV light switched off. It is obvious that the photoactivity of UV 254 nm and 365 nm light started after 5 min and the photocatalytic reactions became stable after 20 and 30 min, respectively. Apparently, the aerosolized E. coli disinfection efficiency of the UV 365 nm was lower than the UV 254 nm. This is because the E. coli cell was more
Fig. 3. SEM microphotographs of TiO2 thin films deposition on glass fibers at (a) low magnification and (b) in higher magnification to reveal the nanoparticles of TiO2.
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Fig. 4. The cross-sectional morphology of TiO2 coated fiber glass.
sensitive to UV 254 nm than UV 365 nm. When there is no light irradiation, the disinfection performance of the reactor becomes unstable. There is a gradual upward trend of the disinfection curve. That is to say, without the photocatalytic reaction more captured E. coli in the reactor will release to the air again as time passes. If the photocatalytic reaction starts, the captured E. coli in the reactor are thoroughly eliminated. After the photocatalytic tests exposure to UV 254 nm and 365 nm light was done, residual E. coli were washed out and cultured on TSA plates. The results produced no significant colony on the TSA plates. This proves that the captured E. coli in the reactor were thoroughly eliminated. Yu et al. [12] used a mechanical filter, which is made of synthetic fibers, coated with the Degussa P25 TiO2 photocatalyst to investigate the bioaerosols removal efficiency of the combination of negative air ionization and photocatalytic oxidation under different relative humidities. The most removal efficiencies for aerosolized E. coli were 0.304 ± 0.06 to 0.364 ± 0.008 [12]. The TiO2 thin film/glass fiber reactor system prepared in this study showed good photocatalytic performance with high degradation efficiency above 95% under 254 and 365 nm light illumination. Meanwhile, the total test time for this novel reactor was longer than 60 h indicating that the duration ability is promising. 4. Concluding remarks
Fig. 6. Residual relative concentration of Escherichia coli at UV light of 254 nm, 365 nm wavelength and UV light switched off.
glass fibers prepared in the laboratory showed good photocatalytic performance. A slow sol–gel technique hydrolysis used in this study produced fine and uniform TiO2 nanoparticles. The SEM images showed that fiber surface was coated with TiO2 thin films around 0.5 μm thick, which consisted of nanoparticles. The disinfection tests showed that photocatalytic activity against aerosolized E. coli had an effect at 5 min. The photocatalytic activity became stable with high efficiency above 95%, 20–30 min later. The improvement of the aerosolized E. coli destroyed was ascribed to the fibers-based reactor with a screen mechanism which trapped aerosolized E. coli and provided a huge surface area of TiO2 thin films for the photocatalytic reaction. Furthermore, since the photocatalytic TiO2 thin film/glass fibers were fabricated from the low-cost sol–gel and dipping methods, this process is promising for many practical photocatalyst applications, such as pathogen disinfection, purification of volatile organic compounds, and removal of odorous substances. References [1] [2] [3] [4] [5] [6]
This work clarifies the disinfection behavior of anatase TiO2 thin film/glass fibers exposed to various light sources. The TiO2 coated
[7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
Fig. 5. Disinfection efficiency of aerosolized Escherichia coli in the TiO2 thin films/glass fibers reactor with A: UV light switched on only and without glass fiber; B: UV light switched off and filled with glass fibers; C: UV light switched on and filled with glass fibers.
K. Hashimoto, H. Irie, A. Fujishima, Jpn. J. Appl. Phys. 44 (2005) 8269. M. Kitano, M. Matsuoka, M. Ueshima, M. Anpo, Appl. Catal., A 325 (2007) 1. H.J. Zhang, D.Z. Wen, Surf. Coat. Technol. 201 (2007) 5720. T. Matsunaga, R. Tomoda, T. Nakajima, H. Wake, FEMS Microbiol. Lett. 29 (1985) 211. V.A. Nadtochenko, A.G. Rincon, S.E. Stanca, J. Kiwi, J. Photochem Photobiol., A: Chem. 169 (2004) 131. P.C. Maness, S. Smolinski, D.M. Blake, Z. Huang, E.J. Wolfrum, W.A. Jacoby, Appl. Environ. Microbiol. 65 (1999) 4094. Z. Huang, P.C. Maness, D.M. Blake, E.J. Wolfrum, S.L. Smolinski, J. Photochem Photobiol., A: Chem. 130 (2000) 163. K. Sunada, Y. Kikuchi, K. Hashimoto, J. Photochem Photobiol., A: Chem. 156 (2003) 227. S. K.r Sharma, M. Vishwas, K. Narasimha Rao, S. Mohan, D. Sreekantha Reddy, K.V. A. Gowda, J. Alloys Compd. 471 (2009) 244. D. Bhattacharyya, N.K. Sahoo, S. Thakur, N.C. Das, Thin Solid Films 360 (2000) 96. C.H. Hung, B.J. Marinas, Environ. Sci. Technol. 31 (1997) 562. K.P. Yu, G.W.-M. Lee, S.-Y. Lin, C.P. Huang, J. Aerosol Sci. 39 (2008) 377. T. Reztsova, C.H. Chang, J. Koresh, H. Idriss, J. Catal. 185 (1999) 223. D.F. Ollis, C.R. Acad, Sci. Paris - Series IIC - Chemistry 3 (2000) 405. C.H. Lin, C.Y. Chang, Y.J. Chang, J.W. Lee, M.Y. Hwa, Y.C. Lee, Adv. Mat. Res. 79–82 (2009) 927. V. Keller, N. Keller, M.J. Ledoux, M.C. Lett, Chem. Commun. 23 (2005) 2918. A. Vohra, D.Y. Goswami, D.A. Deshpande, S.S. Block, Appl. Catal., B: Environ. 64 (2006) 57. P.A. Jensen, W.F. Todd, G.N. Davis, P.V. Scarpino, Am. Ind. Hyg. Assoc. J. 53 (1992) 660. M. Wang, D.J. Guo, H.L. Lai, J. Solid State Chem. 178 (2005) 1996.
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Novel TiO2 thin films/glass fiber photocatalytic reactors in the removal of bioaerosols Ching-Hsing Lin a,⁎, Jyh-Wei Lee b,c, Chen-Yu Chang d, Yu-Jie Chang e, Yao-Chuan Lee a, Mei-Yin Hwa a a
Dept. of Safety, Health and Environmental Engineering, Tungnan University, Taipei, Taiwan Dept. of Materials Engineering, Mingchi University of Technology, Taipei, Taiwan c Center for Thin Film Technologies and Applications, Mingchi University of Technology, Taipei, Taiwan d Center of General Education, National Taitung Junior College, Taitung, Taiwan e Graduate School of Environmental Education & Resources, Taipei Municipal University of Education, Taipei, Taiwan b
a r t i c l e
i n f o
Available online 12 August 2010 Keywords: Escherichia coli Sol–gel method Photocatalytic Disinfection tests
a b s t r a c t The photocatalytic disinfection of aerosolized Escherichia coli, a pathogenic microorganism was investigated. The air disinfection system is a novel TiO2 thin films/glass reactor filter and ultraviolet (UV) radiation air purification system, operating at a flow rate of 20 L/min. A special glass fiber reactor design allowed the UV lamp to be located in the center of the reactor. The photocatalyst reactor was then filled with TiO2 coated glass fiber substrates to examine the photocatalytic efficiency of the TiO2 filled reactor against aerosolized E. coli in cell concentrations of 105 CFU/mL. A slow sol–gel technique hydrolysis was used in this study to acquire fine and uniform TiO2 nanoparticles of 10–30 nm. The TiO2 thin films were obtained on glass fiber substrates by dipping method followed by thermal treatment. In an attempt to understand the structure and the morphology of TiO2 sol–gel thin films, analyses by X-ray diffraction (XRD) and scanning electron microscopy (SEM) were employed. The SEM images showed that uniform anatase TiO2 thin films were coated on fiber surfaces. A neublizer was applied to yield E. coli bioaerosol at a controlled humidity of 72%. Disinfection of E. coli under 254 and 365 nm light illumination was conducted to evaluate the photocatalytic ability of the TiO2 thin films. The nano-TiO2/glass fibers gel catalyst prepared in the laboratory showed good photocatalytic performance with the high degradation efficiency above 95% at both wavelengths. With the UV illumination switched off, the degradation efficiency dropped to below 60%. The improvement of the photocatalytic activity was ascribed to the fibers-based reactor with a screen mechanism which provided a huge surface area of TiO2 thin films. These results will be useful and assist engineers to design photocatalyst reactors for the industrial applications of bioaerosol removal. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved.
1. Introduction The decomposition of environmental pollutants and the disinfection of microorganisms using photocatalytic TiO2 have been widely studied during the past several decades due to its good environmental stability and excellent photocatalytic activity [1–3]. The photokilling of bacterial by active anatase TiO2 occurs via the oxidation of intracellular coenzyme A [4], peroxidation of the polyunsaturated phospholipid component of the lipid membrane [5], damage incurred to the cytoplasmic membrane [6] and destruction of cell wall [7,8], thereby causing cell death. The synthesis of anatase TiO2 thin films by various deposition techniques, such as sol–gel, chemical vapor deposition, impregnation and thermal evaporation has been reported [9–11]. The sol–gel method of anatase TiO2 fabrication is simple and low cost. It can yield high uniformity and fine particle size of the
⁎ Corresponding author. Tel.: + 886 2 86625935x119; fax: + 886 2 86625934. E-mail address: [email protected] (C.-H. Lin).
catalyst. This method can be operated at room temperature and is easy to control with chemical dosage and can be easily manipulated to produce different catalytic properties. The sol–gel method was usually used to deposit TiO2 films on the substrate in order to eliminate the bioaerosols by photocatalyst reactions. The synthetic fibers [12] and molecular sieves [13] have been used as substrates to control bioaerosols, due to the possibility of killing microorganisms with a lower pressure drop [14]. In previous work [15], the glass fibers covered with TiO2 thin films by sol–gel method were successfully employed as a photocatalyst reactor to degrade toluene. In this study, we have mainly focused on the disinfection of Escherichia coli under 254 and 365 nm light illumination with a novel TiO2 thin films/glass fiber reactor. For this investigation, TiO2 nano powders were synthesized by sol–gel. TiO2 thin films were further deposited on glass fiber substrates by dipping method followed by thermal treatment. In this study we ascribe the improvement of the high disinfection efficiency against aerosolized E. coli to the novel fibers-based reactor with a screen mechanism, which provides a huge effective and reactive surface area of TiO2 thin films for the photocatalytic reaction.
0257-8972/$ – see front matter. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2010.08.009
S342
C.-H. Lin et al. / Surface & Coatings Technology 205 (2010) S341–S344
2. Materials and methods 2.1. Testing microorganisms The bacterial strain used herein was Gram-negative E. coli, which is often selected as a challenge bioaerosol for antimicrobial tests [16,17]. The bacteria stocks were kept on tryptone soya agar (TSA, Oxoid) plate at − 4 °C. Before antimicrobial tests, the bacteria cells were reactivated by culturing a loop of inoculum in 10 mL sterile nutrient broth under gentle shaking for 16–24 h in an incubator at 36 ± 1 °C and 80 rpm. The viable cell count was determined by plate counting technique on TSA plate after serial dilution. All specimens and test apparatuses were sterilized using pressurized steam autoclave prior to the test. Antimicrobial tests were performed under aseptic conditions. 2.2. Preparation of bioaerosols The bioaerosols used in the experiments were generated by a Collison six-jet nebulizer (BGI Inc., Waltham, MA, USA) with an air flow of 20 L/min with 40 psig. A diffusion dryer and active carbon was used to remove the water and impurities of the generated bioaerosols to form droplet particles. The suspension concentration of the cell was about 105 CFU/mL. 2.3. Sampling of pathogenic microbes AGI-30 impingers (Model 7540, ACE GLASS Inc., NJ, USA) were used for the bioaerosol sampling. This impinger has been recommended by the American Conference of Governmental Industrial Hygienists and the International Aerobiology Symposium for sampling viable microorganisms [18]. The sampling flow rate was 12.5 L/ min and the sampling time was 5 min. Sterilized buffer solution of 50 mL was used as the sampling medium for the aerosolized E. coli. The viable cell count was determined by plate counting technique on TSA plate after serial dilution. 2.4. TiO2 thin films preparation The deposition of TiO2 films on glass fiber (Glass wool 18421 from Sigma-Aldrich Fluka) substrates was accomplished by modified sol– gel technique at ambient room temperature [15]. These films were prepared by dissolving the titanium alkoxide precursor (Titanium (IV) n-propoxide, Ti (OC3H7)4, 98%+, Alfa Aesar) in an alcoholic bath (IPA, isopropanol, C3H7OH). Acetic acid was used to avoid the early
precipitation of the oxides. After addition of the acetic acid, the solution was stirred vigorously up to 24 h with a magnetic stirrer, till the hydrolysis was complete. After a gel formed, the solution was kept in an airtight beaker to preclude formation of gel. Then, 95% ethanol solution was added to the gel to form the final coating solution. The glass fibers were dipped into the solution to form TiO2 thin films on surfaces. The coated glass fiber substrates were preheated in air at 60 °C for 4 h, and then calcined at 200 °C for 2 h in air. 2.5. Disinfection system Fig. 1 is a schematic diagram showing the proposed continuous TiO2 thin films/glass fibers system. A special design of glass reactor of 10 cm in diameter and 20 cm in length allowed the UV lamp to be located in the center of the reactor. The photocatalyst reactor was then filled with TiO2 coated glass fibers to examine the photocatalytic activities against aerosolized E. coli. A total weight of 20 g of glass fibers with TiO2 thin film coatings were used in the photocatalyst reactor. It is noticed that the effectiveness of this novel reactor to degrade toluene was observed in previous works [15]. The disinfection of aerosolized E. coli was conducted using three different irradiation methods. Method A: UV light switched on only and no glass fibers were used; method B: UV light switched off and filled with glass fibers; method C: UV light switched on and filled with glass fibers. The disinfection efficiency against aerosolized E. coli in the absence and in the presence of UV 254 nm and 365 nm light irradiation has been studied. Each test was carried out 3 times and during the course of the study the reactor was kept at 25 ± 1 °C in a fume hood. 3. Results and discussion 3.1. Microstructure of the deposited photocatalyst The photocatalyst powders prepared by modified sol–gel method described above were analyzed by the X-ray diffractometer to examine the crystallinity of TiO2. As illustrated in Fig. 2(a), three major diffraction peaks of the anatase TiO2 crystal structure appeared at the 2θ values of 25.4, 37.9 and 48.2. Compared with the JCPDS database, the crystal structure of the photocatalyst prepared in the experiment was of anatase form [19]. On the other hand, only the most intensive peak of anatase phase was visible in the XRD pattern of TiO2 coated glass fibers as shown in Fig. 2(b). Other peaks of anatase TiO2 phases were covered by the amorphous pattern of glass fiber.
1. Air compressor
6. TiO2 thin films/glass fibers reactor
2. HEPA
7. UV light illumination power
3. Mass flow meter
modulator
4. Nebulizer
8. AGI-30 impingers
5. Humidity Conditioning device
9. Air pump
Fig. 1. Schematic diagram of continuous TiO2 thin films/glass fibers disinfection system.
C.-H. Lin et al. / Surface & Coatings Technology 205 (2010) S341–S344
S343
Fig. 2. XRD patterns of TiO2 sol–gel powders (a) after calcination and (b) coated on glass fibers.
SEM micrographs of TiO2 thin films/glass fiber composites are shown in Fig. 3(a) and (b). TiO2 nanoparticles are uniformly distributed on the surface of glass fibers, as shown in Fig. 3(a). The particle diameter of nano-TiO2 is around 10–30 nm. It is obvious that the grain size distribution of TiO2 nanoparticles produced by slow hydrolysis is uniform. The elemental compositions of the TiO2 deposits on the glass fibers by EDS analysis are 55% Ti, 41% O, and 3% Si in wt.%. As compared with the chemical composition of the glass fibers, 39% Si, 37% O, 7% Na, 6% Ti, 5% Ca and 3% K in wt.%, it is believed that 3% Si of TiO2 deposits is attributed to the glass fiber substrates. In addition, the EDX analysis of TiO2 deposits shows no significant levels of impurities from the deposition process. Fig. 4 illustrates the cross-sectional morphology of TiO2 coated fiber glass. The image of fiber glass in lower magnification is shown in the upper left portion (inset). It is obvious that the thickness of the TiO2 thin film is around 0.5 μm as indicated by arrows. Meanwhile, the continuity and uniformity of dip-coated TiO2 thin films on the glass fiber surfaces are also observed. 3.2. Disinfection performance with different operation patterns From the result shown in Fig. 5, when the reactor operated in the A condition, average disinfection efficiencies against aerosolized E. coli at UV 254 nm and 365 nm were only 39.4 and 31.1%, respectively. The
relatively low disinfection efficiencies were due to short residence time of the airborne flow when no fibers were present. In the B condition, aerosolized E. coli have been captured by the glass fibers, the screen effects were the main action mechanism of the glass fibers reactor. The average disinfection efficiencies of the two reactors were 49.6 and 54.3%, respectively. As the photocatalytic reaction started (condition C), it was clearly observed from the experimental data, that the disinfection rate of aerosolized E. coli became significant. Fig. 5 shows the effect of 254 nm and 365 nm UV light on E. coli, in the TiO2 thin film/glass fiber reactor system. As can be seen, E. coli were destroyed by the photocatalytic reaction at average rate of 97.7% for 254 nm light and 94.6% for 365 nm light, respectively. The destruction mechanisms of the aerosolized E. coli were ascribed to both the screen effects and the photocatalytic reaction, as revealed herein. 3.3. Disinfection performance with different irradiation lights Fig. 6 depicts the residual relative concentration of aerosolized E. coli after exposure to UV light of 254 nm, 365 nm wavelengths and with UV light switched off. It is obvious that the photoactivity of UV 254 nm and 365 nm light started after 5 min and the photocatalytic reactions became stable after 20 and 30 min, respectively. Apparently, the aerosolized E. coli disinfection efficiency of the UV 365 nm was lower than the UV 254 nm. This is because the E. coli cell was more
Fig. 3. SEM microphotographs of TiO2 thin films deposition on glass fibers at (a) low magnification and (b) in higher magnification to reveal the nanoparticles of TiO2.
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C.-H. Lin et al. / Surface & Coatings Technology 205 (2010) S341–S344
Fig. 4. The cross-sectional morphology of TiO2 coated fiber glass.
sensitive to UV 254 nm than UV 365 nm. When there is no light irradiation, the disinfection performance of the reactor becomes unstable. There is a gradual upward trend of the disinfection curve. That is to say, without the photocatalytic reaction more captured E. coli in the reactor will release to the air again as time passes. If the photocatalytic reaction starts, the captured E. coli in the reactor are thoroughly eliminated. After the photocatalytic tests exposure to UV 254 nm and 365 nm light was done, residual E. coli were washed out and cultured on TSA plates. The results produced no significant colony on the TSA plates. This proves that the captured E. coli in the reactor were thoroughly eliminated. Yu et al. [12] used a mechanical filter, which is made of synthetic fibers, coated with the Degussa P25 TiO2 photocatalyst to investigate the bioaerosols removal efficiency of the combination of negative air ionization and photocatalytic oxidation under different relative humidities. The most removal efficiencies for aerosolized E. coli were 0.304 ± 0.06 to 0.364 ± 0.008 [12]. The TiO2 thin film/glass fiber reactor system prepared in this study showed good photocatalytic performance with high degradation efficiency above 95% under 254 and 365 nm light illumination. Meanwhile, the total test time for this novel reactor was longer than 60 h indicating that the duration ability is promising. 4. Concluding remarks
Fig. 6. Residual relative concentration of Escherichia coli at UV light of 254 nm, 365 nm wavelength and UV light switched off.
glass fibers prepared in the laboratory showed good photocatalytic performance. A slow sol–gel technique hydrolysis used in this study produced fine and uniform TiO2 nanoparticles. The SEM images showed that fiber surface was coated with TiO2 thin films around 0.5 μm thick, which consisted of nanoparticles. The disinfection tests showed that photocatalytic activity against aerosolized E. coli had an effect at 5 min. The photocatalytic activity became stable with high efficiency above 95%, 20–30 min later. The improvement of the aerosolized E. coli destroyed was ascribed to the fibers-based reactor with a screen mechanism which trapped aerosolized E. coli and provided a huge surface area of TiO2 thin films for the photocatalytic reaction. Furthermore, since the photocatalytic TiO2 thin film/glass fibers were fabricated from the low-cost sol–gel and dipping methods, this process is promising for many practical photocatalyst applications, such as pathogen disinfection, purification of volatile organic compounds, and removal of odorous substances. References [1] [2] [3] [4] [5] [6]
This work clarifies the disinfection behavior of anatase TiO2 thin film/glass fibers exposed to various light sources. The TiO2 coated
[7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
Fig. 5. Disinfection efficiency of aerosolized Escherichia coli in the TiO2 thin films/glass fibers reactor with A: UV light switched on only and without glass fiber; B: UV light switched off and filled with glass fibers; C: UV light switched on and filled with glass fibers.
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