Photocatalytic Inactivation of Airborne Bacteria in a Continuous-Flow

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Ind. Eng. Chem. Res. 2008, 47, 7580–7585

Photocatalytic Inactivation of Airborne Bacteria in a Continuous-Flow Reactor Amrita Pal,† Simo O. Pehkonen,‡ Liya E. Yu,‡ and Madhumita B. Ray*,§ Department of Chemical and Biomolecular Engineering, National UniVersity of Singapore, Singapore, DiVision of EnVironmental Science & Engineering, National UniVersity of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Department of Chemical and Biochemical Engineering, Thompson Engineering Building, UniVersity of Western Ontario, London, Ontario, Canada, N6A 5B9

In this study, a continuous annular reactor was used to characterize the TiO2-mediated inactivation of an aerosolized Gram-negative bacterium, Escherichia coli K-12 (ATCC 10798), by varying UV-A intensity (0.5-3.4 mW/cm2), relative humidity (RH) (from 51 ( 0.61 to 85 ( 4.7%), and photocatalyst loading (960 and 1516 mg/m2) at an air flow rate of 1 L/min. Inactivation rate of E. coli K-12 increased with an increase in TiO2 loading, UV-intensity, and RH. A UV-A dose of 0.03-0.204 J/cm2 at an average UV-A intensity of 0.5-3.4 mW/cm2, at a residence time of 1.1 min, is sufficient to fully and continuously inactivate E. coli K-12 passing through the reactor. The photocatalytic inactivation rates obtained in the continuous flow reactor compared well with our earlier batch inactivation rates conducted at a UV-A intensity of 0.015 mW/cm2 and a TiO2 loading of 1516 mg/m2. This demonstrates the possibility of scaling up of the photocatalytic inactivation process for bioaerosol based on batch kinetic data. 1. Introduction Many studies have shown the association between indoor bioaerosols and health problems such as sick building syndromes,1 irritant responses, infectious diseases, respiratory problems, hypersensitive reactions,2 airborne transmission of tuberculosis,3 and asthma.4 Heating, ventilation, and air conditioning (HVAC) systems are commonly used in offices, houses, factories, and large enclosed buildings. Due to poor maintenance in many cases, excessive dirt may be collected in HVAC systems and serve as a breeding ground for microorganisms.5 In addition, bacteria and fungi can also be disseminated from contaminated humidifiers to affect particularly immunosuppressed patients in hospitals.6 Since most people spend 80-90% of their time indoors,7 economical and effective inactivation devices that can be easily retrofitted with existing HVAC systems are needed. Since the pioneering work in 1985,8 sterilization of bacteria through photocatalysis using titanium dioxide (TiO2) has been examined mainly in aqueous environments. For example, Matsunaga et al.9 reported a >99% sterilization efficiency for a system in which an aqueous suspension of E. coli underwent TiO2-assisted photochemical inactivation for 16 min of irradiation using a mercury lamp with an intensity of 1800 µeinsteins/ m2 s. To improve the water quality on a village-scale, Gill and McLoughlin10 investigated disinfection kinetics of E. coli K-12 in a continuous (aqueous) flow reactor coupling UV with TiO2. Unlike photocatalytic disinfection of bacteria in aqueous systems, information on the inactivation of bacteria in air remains limited. Goswami et al.5 used a recirculating duct system incorporating Degussa P25 TiO2 and UV-A at intensity of 10 mW/cm2 [at 350 nm wavelength and at a relative humidity (RH) of 50%] to inactivate Gram-negative bacterium Serratia marcescens. After 8 h of photocatalysis at air velocity of 0.376 * To whom all correspondence should be addressed. Tel: 1-519661-2111 ext 81273. Fax: 519-661-3498. E-mail: [email protected]. † Department of Chemical and Biomolecular Engineering, National University of Singapore. ‡ Division of Environmental Science & Engineering, National University of Singapore. § University of Western Ontario.

m/s, 82% inactivation of the bacteria was achieved. Keller et al.11 were the first to report a 99.1-99.8% removal of airborne nonpathogenic E. coli using UV-A (wavelength of 380 nm) and TiO2. Subsequently, Pal et al.12 also demonstrated the feasibility of using TiO2 photocatalysis for continuous inactivation of bioaerosls. By employing an advanced silver ion doped TiO2 catalyst along with UV-A (10 mW/cm2), Vohra et al.13 achieved complete inactivation of various microbes such as Bacillus cereus, Staphylococcus aureus, E. coli, Aspergillus niger, and MS2 bacteriophage in air. The pioneering work on photocatalytic oxidation of bacteria demonstrates vast potential for indoor air disinfection due to low power consumption, long service life, low maintenance requirement, and compatibility with HVAC system14 and warrants complete parametric investigation for scaling up and commercial utilization. In this work, a continuous-flow reactor for photocatalytic inactivation of bioaerosols is developed and its performance is characterized with respect to TiO2 loading, UV-A intensity, and RH. In addition, inactivation rates obtained from the continuous-flow reactor using a fluorescent light are compared with those from our earlier batch kinetic studies using the same light source.15 2. Materials and Methods 2.1. Materials. Nonporous titanium dioxide (TiO2, P25, Degussa AG) with a primary particle diameter of 21 nm, specific surface area of 50 ( 15 m2/g, and a crystal distribution of 80% anatase and 20% rutile was used as the photocatalyst in this study. Gram-negative bacterium E. coli K-12 (ATCC 10798) was used for the inactivation studies. E. coli was selected as the test species as its chromosome map is most comprehensively studied; this bacterium is most frequently selected for UV disinfection studies16 and is a representative species to assess inactivation efficiency. In addition, investigation of E. coli K-12 in this continuous reactor will allow us to compare the inactivation rates with our earlier work on batch inactivation of various bacteria, including E. coli K-12, in which a celluloseacetate filter membrane of pore size 0.45 µm was used for bacterial support. Two blacklight blue lamps (FL8BLB, Sanyo Denki) with a peak emission at 365 nm and an energy output of 8 W each,

10.1021/ie701739g CCC: $40.75  2008 American Chemical Society Published on Web 09/11/2008

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Figure 1. (a) Experimental setup for continuous inactivation, (b) annular reactor and stainless steel filter supports, and (c) cross-sectional view of the annular reactor.

were used as the light source for the photocatalytic reactor. The lamps were arranged in series and housed in quartz tubes inside the reactor. On the surface of the quartz tubes, a digital radiometer (VLX-3W, UVItec) was used to determine the intensity of UV-A light. 2.2. Continuous Flow Reactor. Figure 1a shows the schematic diagram of the continuous photocatalytic inactivation system. The main component, an annular reactor (Figure 1b), consisted of an inner quartz cylinder (length, 70 cm; outer diameter, 2.5 cm; inner diameter, 2.2 cm) and an outer cylinder made of stainless steel (length, 42 cm; outer diameter, 6.35 cm; thickness, 2.6 mm). A Teflon buffer tank containing a volume similar to the annular reactor was employed to premix bioaerosols and carrier air. A stainless steel mesh with a pore size of 55 µm was placed at the inlet of the reactor to ensure a uniform

flow of aerosolized bacteria in the reactor. Two hollow stainless steel supports were structured to hold catalyst-coated filters around the quartz tubes in the reactor, as shown in Figure 1c. 2.3. Dip Coating of Filters. To coat glass-fiber filters (pore size 1 µm, PALL, Gelman Laboratory) with TiO2, a dip-coating device was employed by following procedures reported by Ray and Beenackers.17 A dipping speed of 4 mm/s was performed followed by a 5-min drying using four infrared lamps (250 W each) to ensure even attachment of TiO2 onto filters. On the basis of five to six individual coating tests, an average TiO2 loading of approximately 168 ((10%) mg/m2 per dip was achieved (shown in Table 1). Inactivation experiments were carried out at two TiO2 loadings, 960 and 1516 mg/m2, corresponding to five and 10 dips, respectively. These loadings were selected on the basis of

7582 Ind. Eng. Chem. Res., Vol. 47, No. 20, 2008 Table 1. Numbers of Dips and TiO2 Coatings on Filters thickness of the TiO2 average nmo. TiO2 of TiO2 loading coating on the loading per dip loading per (mg/m2) membrane filter (nm)a dip (mg/m2) dips (mg/m2) 5 7 10 12

960 1130 1516 2010

253 297 399 529

192 161 151 168

168 ( 10.3%

a The specific gravity of TiO2 has been taken as 3.8, assuming uniform coverage.

our previous batch photocatalytic study, showing that an optimum TiO2 loading, ranging from 511 to 1666 mg/m2, maximized inactivation of most Gram-positive and Gramnegative bacteria.15 According to the American type Culture Collection (ATCC) specifications, the E. coli K-12 has average size of 1 µm. The average particle diameter of TiO2 is 21 nm and the thickness of the TiO2 coating varies from 293 to 529 nm (Table 1), indicating that pores of the membrane are mostly covered, thus minimizing the penetration of the bacteria into the filter. 2.4. Preparation of Bacterial Strains. E. coli K-12 cells were inoculated in 10 mL of Luria-Bertani broth and incubated for 16 h at 121 rpm in a rotating water-bath shaker at 37 °C. The cultured bacteria were centrifuged at 4000 rpm for 5 min, washed with an autoclaved 0.9% sodium chloride solution twice, and resuspended in 50 mL of an autoclaved 0.9% sodium chloride solution. Initial experiments were carried out using undiluted bacteria. However, the control experiments (in the absence of light) using undiluted bacteria registered a huge number of colony growth on the agar plate. The colony growth was too high to count. Hence the bacteria had to be diluted for all the experiments, which led to countable colony growth on the agar plate. The bacterial solutions were then successively diluted up to 2000 times using 0.9% sodium chloride solution. Between each dilution, the bacterial suspension was well stirred to ensure uniform mixing. About 22 mL of diluted solution was used in a bioaerosol nebulizing generator (BANG) (CH Technologies) to generate bioaerosols in the reactor. Throughout each experiment, the bacterial solution in the BANG was maintained as a uniform suspension using a magnetic stirrer. 2.5. Experimental Procedure. To generate a continuous flow of bacterial aerosols in the reactor, purified air was blown through BANG at a rate of 1 lpm. The Teflon buffer tank was employed to premix the bioaerosol stream, additional carrier air, and moisture to control the RH in the annular reactor. Flows of bioaerosols and fresh air were adjusted using mass flow controllers to maintain a constant RH in the reactor. The reactor was operated at an air flow rate of 1 L/min. A thermohygrometer (37950-03, Cole Parmer) was used to monitor RH in the reactor and at the exit of the reactor during blank tests. Since absence of bacteria is the only difference between blank tests and actual inactivation experiments, RH measured during blank tests can represent the moisture content during photocatalytic disinfection. At the reactor outlet, for every 2 min, bacteria were collected onto eosin methylene blue (EMB) agar plates using a singlestage Anderson sampler (Andersen N6, CIH Equipment Co.) collecting particles of 0.65-1 µm. As a safety measure, effluent air was passed through an impinger containing bactericidal solution before it was released to the atmosphere. The entire experimental setup was placed inside a black box to minimize influence of external light sources. All sampling plates containing bacteria were carefully sealed and placed in an incubator at 37 °C. Bacteria colonies were counted after 24 h of incubation

Figure 2. Triplicate measurements of dark adsorption and loss of E. coli K-12 as a function of time (min) at TiO2 loading of 1516 mg/m2, relative humidity of 97 ( 2.5%, and air flow rate of 1 lpm.

and then daily for four consecutive days to examine potential regrowth of the colonies. Freshly coated filters were used for every experiment. After each experiment, the reactor was cleaned with dry air and 70% ethanol to remove contamination from an earlier experiment. Triplicate tests using each inactivation condition were conducted with an average error of (5%. 3. Results and Discussion 3.1. Adsorption Equilibrium. Adsorption experiments were carried out in the absence of light. To determine the dark adsorption equilibrium of E. coli K-12 on a TiO2-coated filter, triplicate measurements were carried out using filters with a TiO2 loading of 1516 mg/m2 at RH of 94.5-99.5%. This RH is higher than normal in an air-conditioned indoor environment but was naturally obtained in the reactor by nebulization of a bacterial water suspension. However, there is always a drop in humidity in the reactor when the UV-A lamp was switched on due to the increase in temperature. At a flow rate of 1 lpm, Figure 2 shows that the adsorption equilibrium of airborne bacteria reached around 90 min. Although two additional flow rates of 1.2 and 1.5 lpm ((1%) were also tested (not shown in Figure 2), 1 lpm with a residence time of 1.1 min in the reactor was chosen to conduct subsequent experiments due to the satisfactory adsorption equilibrium. In absence of UV-A, ∼40% of bacteria is lost due to adsorption on TiO2-coated filters and inner walls of the reactor. Hence, for the following photocatalytic inactivation experiments, UV-A was switched on after 90 min of bacterial flow in the reactor. The increased level of humidity reduces the extent of dark adsorption, which was reflected in the raw data qualitatively but not presented here due to poor resolution of data. 3.2. Effects of UV-A Intensity on the Inactivation of E. coli K-12. As expected, Figure 3 shows that at constant TiO2 loading of 1516 mg/m2 and RH (85 ( 4.7%, n ) 6), inactivation rates of E. coli K-12 increased with increasing UV-A intensity varying from 0.5 to 3.4 mW/cm2. This is expected and is consistent with the work of Goswami et al.,5 who observed that stronger UV-A intensity led to higher destruction rates of Gramnegative bacterium S. marcescens in a recirculating duct system. Lin and Li26 also observed that higher black light intensity from 240 to 2100 µW/cm2 also increased the inactivation rates of Bacillus subtilis and Penicillium citrinum in the presence of TiO2. Interestingly, instead of exhibiting the characteristics

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Figure 3. Effect of UV-A intensities on the outlet concentration of E. coli K-12 as a function of irradiation time (min) at a TiO2 loading of 1516 mg/m2, relative humidity of 85 ( 4.7%, and flow rate of 1 lpm.

Figure 4. Effect of TiO2 loading on the outlet concentration of E. coli K-12 at UV-A intensity of 3.4 mW/cm2, relative humidity of 85 ( 4.7%, and flow rate of 1 lpm.

of continuous flow reactors, i.e., a steady concentration at the reactor outlet, the concentration profiles from the reactor in all experiments showed an exponential decay for initially 12-15 min of irradiation. In fact, for low intensities of 0.5-2.5 mW/ cm2, the outlet concentration increased immediately after the UV light was switched on. This could be due to desorption and resuspension of deposited bacteria with an increase in temperature with radiation source turned on. The initial increase in concentration due to desorption in photocatalysis is also present in chemical systems.18 In addition, the radiation source takes about 10 min to reach its steady output. These two factors can cause an apparent retardation in degradation kinetics, which is evident from the relatively shallow slopes of the curves seen in Figure 3 at lower intensity of 0.5 and 1.83 mW/cm2 and at 2-5 min after the UV light was switched on. This shallowness of the slope in the initial 2-5 min can also be seen in Figures 4 and 5. In the absence of light, a steady state count of bacteria at the reactor outlet was reached after 90 min when the adsorption equilibrium for the bacterial concentration in the reactor was achieved (Figure 2). However, with photocatalysis, the outlet concentration reached steady-state (≈zero concentration) after 12-15 min of irradiation in all experiments. At a higher intensity of 3.4 mW/cm2, the reactor concentration reached steady state (zero outlet concentration) within 10 min. At this

Figure 5. Effect of RH on inactivation of E. coli K-12 at UV-A intensity of 3.4 mW/cm2 with and without TiO2 loading of 1516 mg/m2 and flow rate of 1 lpm.

point, both the UVA intensity and humidity in the reactor also reached steady values. Even at a low intensity of 0.5 mW/cm2, complete inactivation of E. coli K-12 was possible at steady state, and the applied UV-A dose varied from 0.03 to 0.204 J/cm2. 3.3. Effect of TiO2 Loadings on the Inactivation of E. coli K-12. Figure 4 shows that an increase in the TiO2 loading from 0 to 1516 mg/m2 enhanced the inactivation of E. coli K-12. In the absence of TiO2, the observed decrease in bacterial count is attributed to direct inactivation due to UV-A. Oguma et al.19 reported that UV radiation at 320-400 nm damages organisms, mainly through exciting photosensitive molecules within cells, thus producing reactive oxygenated species (ROS) to adversely affect genome and other intracellular molecules. This can cause sublethal or lethal cell mutations, growth delay, etc. Relative to noncatalytic inactivation (without TiO2), the higher inactivation rates at TiO2 loadings of 960 and 1516 mg/m2 (Figure 4) are expected. As with UV-A irradiation, TiO2 generates a large amount of •OH, which is a potential biocide with strong oxidation potential and nonselective reactivity.20 Due to the large number of hydroxyl radicals generated at higher TiO2 loadings, the inactivation rate is faster and hence has a steeper slope compared to that in absence of TiO2, as can be seen in the experimental graph (Figure 4). Cho et al.21 measured photocatalytic degradation rates of E. coli during UV illumination of TiO2 suspensions (in liquid medium). Their results demonstrated a linear correlation between steady-state concentrations of •OH (or [•OH]ss) and inactivation rates of E. coli, indicating that •OH is the primary oxidant responsible for E. coli inactivation. Our batch kinetic studies with different loadings of TiO2 in the range of 230-1660 mg/m2 also indicated a linear relationship between inactivation rates of E. coli K-12 and TiO2 loading.15 3.4. Effect of Relative Humidity on the Inactivation of E. coli K-12. Figure 5 shows that, in absence of TiO2, three RH of 51 ( 0.61, 72 ( 2.5, and 85 ( 4.7% resulted in similar inactivation rates of E. coli K-12. This indicates that direct photolysis induced by UV-A for more than 11 min can fully inactivate E. coli K-12 over a range of RH. It is not surprising that at all RH, with a TiO2 loading of 1516 mg/m2 (in solid symbols), photocatalysis resulted in faster inactivation of bacteria compared to photolysis (in the absence of TiO2, shown by solid symbols). For TiO2-catalyzed photodegradation (hollow symbols in Figure 5), greater degradation rates occur at higher RH. The greater degradation rate at higher RH is possibly due to

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generation of a greater amount of reactive oxygen species (ROS) and •OH in the presence of TiO2 and higher RH. It is known that upon irradiation with photons of wavelength e 385 nm, TiO2 forms e- (electrons) and h+ (holes). The electrons react with oxygen to form ROS, such as O2•-, HO2•, and H2O2, and the holes react with water attached onto TiO2 to form •OH (eqs 1-4).22 O2 + e- f O2•-

(1)

O2•- + H+ f HO2•

(2)

2HO2• f O2+H2O2

(3)

H2O + h+ f •OH + H+

(4)

Since at a given temperature, higher RH indicates more water molecules in air, enhancing the production of •OH, RH of 85% is expected to achieve more rapid inactivation during TiO2 photocatalysis. More than 70% of incoming bacteria was inactivated at RH of 85%, while only about 40% was inactivated at RH of 51% after 4 min of radiation. This indicates that, at geographical locations with high humidity, TiO2 photocatalysis can be effective or energy efficient for inactivation of bioaerosols. However, Ko et al.23 suggested maximum inactivation of airborne bacteria (e.g., S. marcescens and Mycobacterium boVis BCG) below 75% RH, since higher RH could enlarge bioaerosol particles (including bacterial cells) due to greater retention of intracellular moisture, which subsequently reduced inactivation efficiencies under UV irradiation. Our data also show that photolysis rates were relatively insensitive to changes in RH, since enlarged bioaerosol particles with more water uptake at higher RH could shield microorganisms from damage caused by UV-A. Goswami et al.5 studied photocatalytic inactivation of S. marcescens in a recirculation duct using UV-A and TiO2 coated on fiberglass filters for air conditioning; they observed that resultant inactivation rates significantly enhanced by increasing RH from 30% to 50%. However, at a higher RH of 85%, around 10% of the microorganisms were still viable. It could be that S. marcescens prefers an environment with high RH.24 This also indicates that, depending on the species, above an optimal RH, the photocatalytic inactivation rate of bacteria may reduce dramatically. While such an optimal RH was not observed in our experiments, the RH range tested in this study is reflective of indoor conditions in tropical locations without HVAC. Without conducting a detailed biochemical mechanism study using photocatalysis, it can be inferred from the literature22 that inactivation process of bacteria in photocatalysis is possibly due to the chemical reaction of hydroxyl radical with cell materials. Thus, the effect of moisture was estimated using the LangmuirHinshelwood model (Figure 6) for heterogeneous catalysis assuming moisture was adsorbed onto TiO227 before reactions and the reaction with bacterial cell occurs on the surface. Since equation 4 shows that generated •OH concentration is proportional to the concentration of water molecules (i.e., RH) in the system, according to Langmuir-Hinshelwood equation, r)-

k ′ KC dC ) dt (1 + KC)

(5)

or 1 1 1 ) + r k ′ KC k′

(6)

where r is the initial inactivation rate, C is the relative humidity, k′ is the intrinsic rate constant, and K is the apparent adsorption

Figure 6. The Langmuir-Hinshelwood model for the effect of relative humidity on initial rate. r stands for initial inactivation rate of E. coli K-12 obtained from Figure 5 at three different RH (51, 72, and 85%).

equilibrium constant for water. On the basis of limited experimental data, Figure 6 shows a linear relationship between 1/r and 1/[RH] with an R2 value of 0.96. 3.5. Comparison between Batch and Continuous Studies. In our earlier batch kinetic study of inactivation of several bacteria involving TiO2 and UV-A light present in commercial fluorescent lamps, ≈97% inactivation of E. coli K-12 was obtained after 15 min of exposure to a fluorescent light (with UV-A intensity of 0.013 mW/cm2) and TiO2 catalysts (1666 mg/m2).14 In this work, using UV-A intensity of 0.015 mW/cm2 and 1516 mg/m2 TiO2, the continuous reaction system inactivated over 98% of E. coli K-12 after 15 min (before reaching steady state), which is comparable to the results obtained in our earlier batch reactor. As mentioned earlier, the continuous flow reactor data for initial 15 min of irradiation showed an Nt/N0 profile to be exponential, similar to batch inactivation. This comparison indicates that photocatalytic inactivation of bacteria is chemical in nature and can be scaled up accordingly. A kinetic model has been developed and is discussed in the following section. 3.6. Establishment of Rate Equation Using Nonlinear Regression Analysis. Using the experimental data, an empirical kinetic model incorporating UV-A intensity, TiO2 loading, and RH for the photocatalytic inactivation of E. coli K-12, based on nonlinear regression analysis,25 is developed. The generic kinetic model can be represented in eq 7. k3)f(I,[TiO2],[RH])

(7)

where k3 (min-1), I (mW/cm2), [TiO2] (mg/m2), and [RH] represent inactivation rate constant, UV-A intensity, TiO2 loading, and relative humidity, respectively. Inactivation rate constants kap, resulting from the experiments by varying individual parameter one at a time such as I, [TiO2], and [RH], are calculated from Figures 3-5. Using the coefficients obtained from individual rate equations, the final form of the kinetic model is k ) 86.6177 × (0.0141I2 - 0.0352I + 0.051) × (8.9 × 10-8[TiO2]2 - 8.4 × 10-5[TiO2] + 0.031) × (0.1823[RH] - 0.0598)(8) The reaction rate constants estimated on the basis of the empirical eq 8 fit well with the experimentally obtained values, as shown in Figure 7 (R2 ) 0.9942); however, it should be noted

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Figure 7. Comparison of experimental and calculated rate constants for UV-A photocatalytic inactivation of E. coli K-12.

that this empirical model is only applicable to E. coli K-12 and in the range of parameters used in the study. 4. Conclusions A continuous reaction system was developed to examine the effects of light intensity, TiO2 loadings, and relative humidity (RH) on inactivation efficiencies of aerosolized E. coli K-12. Inactivation of E. coli K-12 increased with increasing UV-A intensity from 0.5 to 3.4 mW/cm2, increasing TiO2 loading from 0 to 1516 mg/m2, and an increase in RH from 51 to 85%. At steady state, which typically occurred after 15 min, this system would treat about 1.1 L/min of air with a residence time of 1.1 min with complete inactivation of the aerosolized E. coli K-12 using UV-A (3.4 mW/cm2) and a RH of 85%, with a TiO2 loading of 1516 mg/m2. An empirical model is developed to evaluate the effects of three parameters on the photocatalytic inactivation rate of E. coli K-12. Comparable inactivation rates obtained in batch and continuous systems suggest that the batch inactivation rates for bacteria can be used to scale up the photocatalytic inactivation systems. Acknowledgment The authors acknowledge the financial support from National University of Singapore (Grant Nos. RP-279-000-131-112 and RP-288-000-015-112). Literature Cited (1) Bholah, R.; Subratty, A. H. Indoor biological contaminants and symptoms of sick building syndrome in office buildings in Mauritius. Int. J. EnViron. Health Res. 2002, 12, 93. (2) Green, C. F.; Scarpino, P. V. The use of ultraviolet germicidal irradiation (UVGI) in disinfection of airborne bacteria. EnViron. Eng. Policy 2002, 3, 101. (3) Xu, P.; Peccia, J.; Fabian, P.; Martyny, J. W.; Fennelly, K. P.; Hernandez, M.; Miller, S. L. Efficacy of ultraviolet germicidal irradiation of upper-room air in inactivating airborne bacterial spores and mycobacteria in full scale studies. Atmos. EnViron. 2003, 37, 405.

(4) Ross, M. A.; Curtis, L.; Scheff, P. A.; Hryhorczuk, D. O.; Ramakrishnan, V.; Wadden, R. A.; Persky, V. W. Association of asthma symptoms and severity with indoor bioaerosols. Allergy 2000, 55, 705. (5) Goswami, D. Y.; Trivedi, D. M.; Block, S. S. Photocatalytic disinfection of indoor air. J. Sol. Energy Eng. 1997, 119, 92. (6) Arundel, A. V.; Sterling, E. M.; Biggin, J. H.; Sterling, T. D. Indirect health effects of relative humidity in indoor environments. EnViron. Health PerspectiVes. 1986, 65, 351. (7) American Thoracic Society. Environmental controls and lung disease. Am. ReV. Respir. Dis. 1990, 142, 915. (8) Matsunaga, T.; Tomoda, R.; Nakajima, T.; Wake, H. Photoelectrochemical sterilization of microbial cells by semiconductor powders. FEMS Microbiol. Lett. 1985, 29, 211. (9) Matsunaga, T.; Tomoda, R.; Nakajima, T.; Nakamura, N.; Komine, T. Continuous-sterilization system that uses photosemiconductor powders. Appl. EnViron. Microbiol. 1988, 54, 1330. (10) Gill, L. W.; McLoughlin, O. A. Solar disinfection kinetic design parameters for continuous flow reactors. J. Solar Energy Eng. 2007, 129, 111. (11) Keller, V.; Keller, N.; Ledoux, M. J.; Lett, M. Biological agent inactivation in a flowing air stream by photocatalysis. Chem. Commun. 2005, 2918. (12) Pal, A.; Min, X.; Yu, L. E.; Pehkonen, S. O.; Ray, M. B. TiO2 mediated inactivation of bioaerosols. Int. J. Chem. Reactor Eng. 2005, 3. (13) Vohra, A.; Goswami, D. Y.; Deshpande, D. A.; Block, S. S. Enhanced photocatalytic disinfection of indoor air. Appl. Catal. B: EnViron. 2006, 65, 57. (14) Yang, X.; Wang, Y. Photocatalytic effect on plasmid DNA damage under different UV irradiation time. Building EnViron. 2008, 43, 253–257. (15) Pal, A.; Pehkonen, S. O.; Yu, L. E.; Ray, M. B. Photocatalytic inactivation of Gram-positive and Gram-negative bacteria using fluorescent light. J. Photochem. Photobiol. A: Chem. 2007, 186, 335. (16) Harm, W. Biological Effects of UltraViolet Radiation; Cambridge University Press: Cambridge, 1980. (17) Ray, A. K.; Beenackers, A. A. C. M. Novel photocatalytic reactor for water purification. AIChE J. 1998, 44, 477. (18) Gang, Li.; Zhao, X.; Ray, M. B. Advanced oxidation of orange II using TiO2 supported on porous adsorbents: The role of pH, H2O2 and O3. Sep. Purif. Technol. 2007, 55, 91–97. (19) Oguma, K.; Katayama, H.; Ohgaki, S. Photoreactivation of Escherichia coli after low- or medium-pressure UV disinfection determined by an endonuclease sensitive site assay. Appl. EnViron. Microbiol. 2002, 68, 6029. (20) Ireland, J. C.; Klostermann, P.; Rice, E. W.; Clark, R. M. Inactivation of Escherichia coli by titanium dioxide photocatalytic oxidation. Appl. EnViron. Microbiol. 1993, 59, 1668. (21) Cho, M.; Chung, H.; Choi, W.; Yoon, J. Linear correlation between inactivation of E. coli and OH radical concentration in TiO2 photocatalytic disinfection. Water Res. 2004, 38, 1069. (22) Huang, N.; Xiao, Z.; Huang, D.; Yuan, C. Photochemical disinfection of Escherichia coli with a TiO2 colloid solution and a self-assembled TiO2 thin film. Supramol. Sci. 1998, 5, 559. (23) Ko, G.; First, M. W.; Burge, H. A. Influence of relative humidity on particle size and UV sensitivity of Serratia marcescens and Mycobacterium boVis BCG aerosols. Tubercle Lung Disease 2000, 80, 217. (24) Lighthart, B.; Hiatt, V. E.; Rossano, A. T., Jr. The survival of airborne Serratia marcescens in urban concentrations of sulfur dioxide. J. Air Pollut. Control Assoc. 1971, 21, 639. (25) Behnajady, M. A.; Modirshahla, N. Nonlinear regression analysis of kinetics of the photocatalytic decolorization of an azo dye in aqueous TiO2 slurry. Photochem. Photobiol. Sci. 2006, 5, 1078. (26) Lin, C. Y.; Lin, C. S. Inactivation of microorganisms on the photocatalytic surfaces in air. Aerosol Sci. Technol. 2003, 37, 939. (27) Rinco´n, A. G.; Pulgarin, C. Photocatalytical inactivation of E. coli: Effect of (continuous-intermittent) light intensity and of (suspended-fixed) TiO2 concentration. Appl. Catal. B EnViron. 2003, 44, 263.

ReceiVed for reView December 20, 2007 ReVised manuscript receiVed July 22, 2008 Accepted July 29, 2008 IE701739G