Environ. Sci. Technol. 2009, 43, 4606–4611
Photocatalytic Oxidation of Escherischia coli, Aspergillus niger, and Formaldehyde under Different Ultraviolet Irradiation Conditions F E N G N A C H E N , † X U D O N G Y A N G , * ,† A N D QIONG WU‡ Department of Building Science, Tsinghua University, Beijing 100084, China, and Department of Biological Science and Biotechnology, Tsinghua University, Beijing 100084, China
Received February 17, 2009. Revised manuscript received April 29, 2009. Accepted April 29, 2009.
Ultraviolet (UV) light irradiation, including the type of light source, light intensity, and irradiation dosage, directly affects the photocatalytic reaction rate and energy consumption. In this study, we investigated the photocatalysis effect of decomposing organic matter and inactivation bacteria and fungi under various conditions of UV sources (UVA and UVC) and light intensities (from 0.01 to 10 W/m2). The effect of light intensity was evaluated by photocatalytic reaction rate and UV dosage defined as a product of light intensity and irradiation time necessary to achieve a certain reduction. The results confirmed the positive effect of increased light intensity on photocatalytic reactions and suggested that within the light intensity range applied in this study low light intensity with long exposure time has higher light utilization efficiency compared to that of high light intensity with short exposure time. A conception for selection of the appropriate light intensity and dosage for effective degradation of pollutants, while saving energy, was provided.
Introduction Volatile organic compounds (VOCs) and microbes are the omnipresent pollutants in buildings and present significant health effects. Both pollutants can be decomposed by photocatalytic oxidation (PCO) under ultraviolet (UV) illumination (1-6). However, most studies so far focused on one type of pollutant only (3-5). While in indoor air, different pollutants could coexist and require simultaneous removal by photocatalytic oxidation (1, 2, 6). In the photocatalytic oxidation process, UV light is the driving force of reactions by generating highly reactive oxygen species (ROS) such as the hydroxyl radical (•OH), H2O2, and O2-1 (7). Influence of light intensity has been widely evaluated on photocatalytic bactericidal action, primarily against Escherischia coli (E. coli) in TiO2 suspension (8, 9) and VOC degradation (7, 10) with PCO reactors. The inactivation rate constants for E. coli changed linearly with the light intensities, and there was no clear conclusion yet for mold fungi. For VOC degradation, the influence of irradiation intensity * Corresponding author phone: +86 10 6278 8845; fax: +86 10 6277 3461; e-mail:
[email protected]. † Department of Building Science. ‡ Department of Biological Science and Biotechnology. 4606
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depends on the range of light intensities so that the oxidation rate increasing with the light intensity followed a 0.5 order or 1 order when the illumination was above or below 10-20 W/m2. Existing studies showed that with higher intensity the light energy was utilized with lower efficiency because of the electron-hole recombination (10). The illumination time is also an important factor in influencing the PCO effect and is highly dependent on the light intensity. The extent of the illumination time to ensure sufficient E. coli inactivation shortened with an increase in light intensity. Sommer et al. (11) suggested that the same UV dosage with different light intensities led to different survival fractions for water disinfection. With a lower air flow rate, a higher VOC reaction rate was achieved due to longer contact and UV exposure time. Therefore, the illumination dosage is an important physical parameter influencing not only the oxidation effect, but also the energy consumption. The main purpose of this study is to evaluate the influence of light intensity with UVA and UVC on the PCO effect and identify the difference in UV dosage necessary for pollutant decomposition under different light intensities. E. coli, Aspergillus niger (A. niger), and formaldehyde were chosen as test pollutants. On the basis of this study, we attempt to provide a conception for the selection of appropriate light intensities to ensure effective photocatalytic reactions and enhance the effective utilization of light energy.
Materials and Methods Materials. The photocatalyst applied in this study was a pure anatase phase with average particle size of 6 nm. The material was prepared on clear glass plates (for E. coli inactivation and VOC degradation) or wood boards (for A. niger inactivation) using the wash-coated method (12). The volume of the TiO2 coating applied to each base plate was approximately 1.5 mg/cm2. Test Pollutants. E. coli was used as one of the model bacteria in this study, which is the most widely used bacteria in existing photocatalytic degradation studies. A. niger, which is prominent among indoor molds reported in moistureattacked buildings (13, 14) in several parts of the world, was also chosen to evaluate the antifungal effect. Formaldehyde was also selected as a target VOC pollutant, which is commonly found in indoor environments. The use of E. coli, A. niger, and formaldehyde as model organisms allowed us to make systematic and comparative studies on the photocatalytic oxidation effect. Light Sources. Mercury lamps (emission 254 nm peak) and black light lamps (emission 365 nm peak) were applied as the UVC light source and UVA light source, respectively. The light intensity was modified by adjusting the distance between the light and reaction surfaces and was measured with a radiometer equipped with 365 and 254 nm captors. In the existing photocatalytic studies (6-9, 15), the light intensities applied differed with a wide range of 1-1000 W/m2. In a typical office room, the intensity of artificial light is about 1-2 W/m2 for visible light of which the wavelengths range from 390 to 780 nm (15). The sun emits about 10-20W/m2 of UV light with wavelengths below 350 and 400 nm, which is defined as one sun equivalent (10). To cover the broad and realistic light intensity range, we used an intensity in this study of 1-10W/m2 for UVA and 0.01-1W/m2 for UVC with the microbes and 1-10 W/m2 for UVA and UVC with the formaldehyde. 10.1021/es900505h CCC: $40.75
2009 American Chemical Society
Published on Web 05/14/2009
FIGURE 1. Schematic of the photokilling test system. FIGURE 3. Log plot of the survival of E. coli cells versus reaction time. Initial number of 2.5 × 104 CFU; UVA light intensity of 1, 5, and 10 W/m2.
Results and Discussion
FIGURE 2. Schematic of the glass plate reactor. (1) Compressed synthetic air. (2) Pressure regulator. (3) Needle value. (4) Filter. (5) Humidifier. (6) Mass flow controller. (7) Mixing box. (8) Formaldehyde source. (9) Stainless steel. (10) UV lamps. (11) TiO2-coated glass plate. (12) Gas analyzer. (13) Quartz window. (14) Thermocouple.
Experimental Procedures One hundred microliters of E. coli aqua with a concentration of 1 × 105 (CFU)/mL (identified by UV-vis spectrophotometer) was piped onto sterilized glass plates coated with TiO2 film and uncoated; the plates were then placed in an airtight illumination chamber (Figure 1). Each case was performed in triplicate for parallel analysis. After being irradiated, the cell suspension was collected by washing the glass plate with 10 mL 0.9% saline solution at certain intervals. The 100 µL suspension was spread on the nutrient broth agar plate and incubated at 37 °C for 12 h before counting the survival bacteria number. For the antifungal effect of A. niger, a 20 µL spore suspension with a concentration of 1 × 106 spores/mL (decided by a blood counting chamber and microscope) was inoculated onto wood surfaces coated with TiO2. Uncoated surfaces were also used as a control. The illumination experiment for mold fungi was also conducted in the experiment apparatus showed in Figure 1. After being irradiated, the wood surfaces were cultured in the dark at 28 °C for 36 h. Survival spores on the surfaces grew to a feasible degree that the spore colony could be identified by a stereomicroscope and counted. For degradation of formaldehyde, the effect of organic compound removal was measured using a glass plate reactor (Figure 2). The synthetic air stream with a flow rate of 1 L/min mixed with the supplied formaldehyde in the mixing box, and the mixed gas was supplied through a mass flow controller to the photoreactor. In this study, the inlet formaldehyde concentration was conditioned to 1-3 ppm, which is the common level in “sick buildings” (7). The photoreactor effluent formaldehyde concentration was measured by a photoacoustic multigas monitor 1312. The test temperature surrounding the reaction surface was 29 ( 0.5 °C, and the relative humidity (RH) was 45 ( 5%.
The influence of the irradiation condition was investigated by comparing the survival number curve versus illumination time, reaction constant, and UV dosage necessary to achieve a certain reduction. Degradation Processes with Different Irradiation Conditions. E. coli Inactivation. The photocatalytic effect of different UV light irradiations on E. coli degradation was studied. Figure 3 shows the curves related to the semilog survival E. coli number on the substrates (with and without TiO2-coated film) as a function of time. UVA was applied as the irradiation source because UVC gave a strong disinfection effect, and the oxidation effect of TiO2 could not be clearly distinguished from the UVC germicidal effect. The UVA light intensities range from 1 to 10 W/m2. It can be observed from the results that the E. coli survival number did not follow a simple exponential decay with time. Rather, it consisted of two or three kinetic phases, namely, the induction phase, exponential decay phase, and finally, slow decay phase. For reactions with low light intensity on surfaces without the TiO2 film and with the TiO2 film, the induction phase with very low inactivation occurred at the beginning of irradiation. In this phase, the active species were generated, and UV deleterious effect began to attack the membrane but did not sufficiently overcome the selfdefense and autorepair mechanisms of the species or cause serious damage to the bacteria to cause cell death. In the next phase, massive generation rapidly overcame the selfprotection mechanisms, and as a result, the E. coli survival number decreased exponentially with time because the antistress enzymes were no longer able to protect the bacterial membrane against oxidation. In the last phase of photocatalytic treatment, inactivation of bacteria became extremely slow because the few active bacteria remaining were in competition for •OH, with inactivated bacteria and metabolites released during the process. Benabbou et al. (9) discussed the mechanism in detail. Not every curve contained these three phases, and the time duration of each phase depended on the oxidation rate. With an increase in light intensity, the induction period was shortened. When the light intensity was as high as 5 or 10 W/m2, the induction phase was not observed, and the photocatalytic kinetic process went into the exponential decay phase directly. The above results suggest that the self-defense and autorepair mechanisms for protecting the bacteria were more efficient at low light intensity, especially without TiO2. Light intensity also influenced the time it took to proceed to the last phase. Without the TiO2 material and with light intensity lower than 5 W/m2, the survival number slowly decreased within the experimental period of 90 min. With the TiO2 material and with a decrease in light intensity from 10 to 1 W/m2, the time necessary to 2 log inactivate E. coli increased from 20 to 60 min. VOL. 43, NO. 12, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 4. Log plot of the survival of A. niger spores versus reaction time. Initial number of 370 spores; UVA light intensity of 1 and 10 W/m2.
FIGURE 5. Log plot of the survival of A. niger spores versus reaction time. Initial number of 250 spores; UVC light intensity of 1, 0.1, and 0.01 W/m2. A. niger Spores Inactivation. It is clear that A. niger is much more resilient to the photolysis effect and photocatalysis effect than E. coli by comparing Figures 3 and 4. With UVA light intensity of 1 W/m2, the photolysis and photocatalysis cannot overcome the self-protection mechanism, and the viable spores number did not change in presence or absence of TiO2. Although with higher light intensity (10 W/m2), only 62% of the spores on the TiO2-coated film substrates were inactivated after 38 h irradiation. The photocatalytic effect improved mostly because of increase in ROS generation. With UVC light irradiation, there was an obvious change in the number of survival spores with illumination time as shown in Figure 5. Because of the strong disinfection effect of UVC light, the light intensities of UVC for A. niger was relatively low, ranging from 0.01 to 1 W/m2, in order to distinguish between the UVC inactivating effect and photocatalysis effect. The photokilling kinetic processes differs with various light intensities. With 0.01 W/m2 UVC irradiation, the induction period was between 30 and 15 min for reaction on blank surfaces and TiO2-coated surfaces, respectively. With UVC intensity of 1 W/m2, the lowest light intensity applied in the UVA experiment, the survival number declined exponentially from the beginning of irradiation and more than 80% of the spores were inactivated within 15 min. UVC was much more effective than UVA in inactivating E. coli and A. niger, which could be explained by the fact that UVC is implicated in DNA lesions that cause inhibition of DNA replication and bacterial mutation (10). The effect of TiO2-coated surfaces was little improved compared to that of the surfaces without TiO2, suggesting that the inactivation effect was primarily induced due to the strong bactericidal effect of UVC but not photocatalytic oxidation. Formaldehyde Degradation. The effects of UVA and UVC light irradiation on formaldehyde degradation was studied 4608
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FIGURE 6. Influence of irradiation source on the formaldehyde degradation rate. Light intensity of 1, 5, and 10 W/m2; Inlet concentration of 1 ppm. under similar intensities of 1, 5, and 10 W/m2. Figure 6 shows that the degradation rate increased approximately linearly with an increase in light intensity by UVA and UVC. The UV light alone did not cause formaldehyde degradation in the absence of TiO2. The formaldehyde mineralization was all due to the increase in ROS produced on TiO2 surfaces. Determination of a Reaction Kinetic Model. The positive influence of higher light intensity can be observed from the degradation curves shown in Figure 6. A reaction rate constant was applied to quantitatively analyze the photocatalytic oxidation effect. The click model (ln (Nt/N0) ) -kt) is simple and easy to be applied as a disinfection kinetic model (16, 17). However, neither the shoulder for the induction phase, nor the tailing off of inactivation for the second slow decay phase mentioned previously can be explained using this model. The delay click model shown below was developed to describe the inactivation kinetics, including the induction phase (18, 19) ln
Nt ) N0
{
t e tc 0 )-k(t - tc) t g tc
(1)
where Nt (CFU) is the survival E. coli or A. niger spore number on a surface after a period of reaction time t (s), N0 is the initial E. coli or A. niger spore number inoculated on a surface (CFU), k is the inactivation rate constant (s-1), and tc is the time interval of the shoulder. The value can be determined by evaluating equation tc )
ln(Si) k
(2)
where Si, denotes the y-intercept of the shifted exponential portion of a survival curve with a shoulder, which is normally fixed as S ) 1, if the shoulder is unsuspected as is shown in Figure 7. By comparing the correlation coefficient (R2) between the logarithm of the predicted survival ratio and the logarithm of the observed survival ratio for the two kinetic models, we can indicate that the delayed click model better fits the experimental results. Detailed data are in the Supporting Information. Other sophisticated models such as the series event model, the Hom model, and rational model have better advantages for explaining the TiO2 activation kinetics of bacteria, especially E. coli (16, 17). However, because the delayed click model allows for a tc calculation and consequently makes it possible to compare the induction periods, this model was used in this study. The inactivation of bacteria and mold fungi is a synergistic bactericidal effect of UV light and oxidative radicals generated at the TiO2 illuminated surfaces, while the degradation of
FIGURE 7. Development of a shoulder curve showing the parameter Si and tc. VOC is caused by the latter effect alone. Microbial inactivation by photolysis and the integrated effect was judged by the two rate constants k(UV) and k(UV+TiO2) for each mechanism, respectively. A net parameter k was defined by deducting the UV disinfection effect from the total effect, which equals the difference between k(UV+TiO2) and k(UV). On the basis of our previous study (20), VOC degradation follows the first-order reaction kinetic as ln
C ) -krt C0
(3)
where C0 is the inlet concentration (mg/m3), C is the outlet concentration (mg/m3), t is the reaction time (s)and kr is the intrinsic kinetic reaction constant (s-1). Influence of Light Intensity on the Reaction Rate Constant. Graphs a and b of Figure 8 show the inactivation rate constants for E. coli and A. niger in the presence and absence of TiO2 with different light intensities. The constants increased with irradiation light intensity in the experimental ranges. With UVA and UVC irradiation on TiO2-coated surfaces, the formaldehyde degradation rate constant also increased along with the light intensity (Figure 8c). Influence of Light Intensity on UV Dosage Demand. As discussed previously, the time required for a certain number microbe cell inactivations and formaldehyde degradation is shortened with an increase in light intensity. UV dosage is an important parameter for estimating light energy consumption and energy utilization efficacy. In order to illustrate the photocatalytic process, the time dosage demand for the same inactivation percentage with or without a photocatalyst under different UV intensities was investigated. The results are presented in Table 1. In the photocatalytic experiments with light intensities of 1, 5, and 10 W/m2 for E. coli inactivation, the dose necessary to reach a 1 log reduction of cultivable bacteria increased from 1701 J/m2 to 8317 J/m2, which is several times lower than that for the degradation by irradiation alone. Compared with the reactions on surfaces without TiO2, the photocatalytic effect reduces light energy consumption. For inactivation of A. niger spores, the UV dosage demanded for a 50% reduction with lower UVC light intensity was less than that with higher UVC light intensity, when the light intensity was below 1 W/m2. Within the experimental range, the peak UV dose value for A. niger inactivation occurred at 0.5 W/m2 for surfaces with TiO2 and without TiO2. As the light intensity further increased to 1 W/m2, the required UV dose decreased. In the formaldehyde degradation reaction, the mass rate-averaged concentration on the cross section decreased with contact time, following an exponential trend (21). On the basis of the first-order reaction kinetic model presented previously, the reaction time demanded to reach a 50% reduction depended on the intrinsic kinetic reaction
FIGURE 8. Influence of light intensity on (a) E. coli inactivation rate constant, (b) A. niger spores inactivation rate constant, and (c) formaldehyde degradation rate constant. constant, which is the function of intensity and other physical parameters (12). With a lower inlet concentration, the increase rate for the UV dosage was more remarkable than that with a higher inlet concentration. When the concentration of formaldehyde increased from 1.12 to 2.88 ppm, the increase rate for the UVA and UVC dosages decreased from 0.098 to 0.083 and 0.179 to 0.066, respectively (Table 2). More photons are produced when UV light intensity becomes stronger, and thus, the photocatalytic oxidation effects for bacteria, mold fungi, and formaldehyde are all improved. However, the increase in ROS is lower than the increase in light photons so that the amount of radical oxidation species is proportional to the square root of the light quanta absorbed but not linearly (22). In addition, after excitation, some of the electron-hole pairs take part in the reaction, while some others recombine together with the excessive •OH radical generated at high intensities (7, 10). The rate of the electron-hole recombination increases with an increase in light intensity, which leads to lower quantum efficiency (8). Part of the light energy was not utilized effectively but was lost as heat. In addition, the utilization efficiencies of light energy with different intensities were also conditioned by the pollutant conVOL. 43, NO. 12, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. UV Dosage Demand for the Same Reduction of Test Organisms at Different UV Light Intensities UV dose at light intensity (W/m2) and irradiation time surfaces without TiO2
surfaces with TiO2
test organism reduction (N/N0) UV type light intensity (W/m2) time (min) UV dose (J/m2) light intensity (W/m2) time (min) UV dose (J/m2) E. coli
0.1
UVA
A. niger
0.5
UVC
1 5 10 0.03 0.1 0.2 0.5 1
288 96 71 105 49 23 14 6
17295 28730 42564 239 341 319 423 380
1 5 10 0.03 0.1 0.2 0.5 1
28 24 13 32 26 17 8 3
1701 7470 8317 73 183 233 250 198
TABLE 2. UV Dosage Demand for the Same Degradation of Formaldehyde at Different UV Light Intensities UV dose at light intensity (W/m2) and irradiation time UVA test organism formaldehyde (1.12 ppm)
0.5
formaldehyde (2.88 ppm)
0.5
1.34 5.2 10.2 1.34 5.2 10.2
centration. The light energy was better utilized with a high target pollutant concentration. Viability is an important difference between microbes and VOCs. Therefore, the degradation processes followed different trends with varied light intensities. The light irradiation during the induction period did not make an obvious effect on the decrease in the survival number but acted as an accumulation for the cells destroyed. The light irradiation during the final slow decay phase was also not efficient for microbe cell inactivation. With a light intensity of 0.01-0.5 W/m2, A. niger degradation processes moved into the final slow decay phase before 50% inactivation. However, with light intensity of 1 W/m2, a relatively low survival number was obtained in the exponential decay phase. This can explain the appearance of inflection for a demanded UV dose in the A. niger experiment. According to this analysis, to achieve the same degradation level for a certain pollutant, the UV dosage demand is reduced with a decrease in light intensity. This suggests that, within some range of light intensity, low light intensity with long exposure time has a higher light utilization efficiency compared to that of high light intensity with short exposure time. The applicable ranges depend on reaction conditions such as the UV source, resistance of the pollutant to photolysis and photocatalysis, and so on. Further study is required to confirm light intensity ranges for certain conditions and supply more systemic conclusions. Light intensities with different UV sources directly influence the photocatalytic reaction effect and utilization efficiency of light energy. With different objectives, the reaction rate constant or light energy utilization efficiency remains an important factor for optimizing light intensity selection. For sterilizing surfaces with continual artificial UV irradiation, light energy utilization efficiency may be more important, and adequate contact time is essential to ensure sufficient reduction of pollutants under low light intensity. For air cleaners, the decrease in light intensity demands a longer period for adequate contact and reaction, and a reduced air flow and more reactive surfaces are demanded to reach the same disposing capability. Hence, 4610
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reduction (N/N0) light intensity (W/m2) time (min) UV dose (J/m2) light intensity (W/m2) time (min) UV dose (J/m2)
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0.87 0.27 0.20 1.31 0.36 0.24
1.17 1.41 2.02 1.75 1.87 2.47
1.34 5.2 10.2 1.34 5.2 10.2
1.13 0.48 0.31 1.56 0.43 0.26
1.51 2.51 3.13 2.09 2.24 2.67
light intensity should be optimized on the basis of specific decontamination purposes.
Acknowledgments This study is financially supported by the National Natural Science Foundation of China Project 50436040 and the National “115” Key Support Project of China Project 2006BAJ02A08.
Supporting Information Available Additional data on microbe survival number versus reaction time and kinetic model comparisons. Thisinformation is available free of charge via the Internet at http://pubs.acs.org.
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