Environ. Sci. Technol. 2009, 43, 1180–1184
Correlation of Photocatalytic Bactericidal Effect and Organic Matter Degradation of TiO2 Part I: Observation of Phenomena F E N G N A C H E N , † X U D O N G Y A N G , * ,† FENGFEI XU,† QIONG WU,‡ AND YINPING ZHANG† Department of Building Science, Tsinghua University, Beijing 100084, China, and Department of Biological Science and Biotechnology, Tsinghua University, Beijing 100084, China
Received September 4, 2008. Accepted December 15, 2008.
This study aims to investigate the correlation of the photocatalytic oxidation effect of decomposing organic matter and inactivating bacteria using two different TiO2 materials: a Degussa P25 powder film and a commercial TiO2 thin film. The destructed organic matter was formaldehyde and the test bacterium was E. coli (JM 109 strain). The decomposition tests and the bactericidal tests were carried out in a plate reactor and on the TiO2 surface, respectively. Observations indicate that there exists an apparent correlation between the two photocatalytic processes of decomposing formaldehyde and inactivating E. coli. However, it is essential to distinguish the exact driver for microbe inactivation, in which both UV light irradiation and reactive oxygen species reaction are direct factors of disinfection, and for organic matter, in which only reactive oxygen species reaction contributes to degradation. Observations from this study would make it possible to use analogy as a potential method to evaluate the antimicrobial effect based on the organic compound degradation effect, whereby the latter is much easier to measure quantitatively.
Introduction Purification of environmental toxic substance in water and air using TiO2 photocatalysts has been extensively studied (1–3). Numerous reports described the application of PCO for conversion of volatile organic compounds (VOC) (4) or inactivation of microorganisms: bacteria, virus, and tumor cells (5). VOC and microbes mainly due to indoor moisture problems are the two omnipresent pollutants in buildings (6). Most studies investigating the mineralization efficiency and mechanisms so far have focus on one type of pollutant only, and the methodology of study is quite different. This is understandable because several important differences between microorganisms and organic compounds can be underlined, such as size, composition, viabilities, and inactivation process (1). To determine the efficiency of the photocatalytic process, we follow the disappearance of organic molecules due to even only one modification in the structure of organic molecules in case of VOC, whereas for degradation microorganism, the cell should be inactivated * 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. 1180
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to cellular death due to a loss of the integrity of the bacterial wall and to the oxidation of intracellular components (1). However, the process of decomposing organic matters and photo killing microbes is perceived to follow the similar mechanisms, and the reaction steps of degradation can be divided into excitation, the recombination, and the trapping of electron-hole pairs and the attack of reactive oxygen species (ROS) (7). The oxidative species OH• radical produced by irradiated TiO2 has been considered to be the dominant oxidative species contributing to the mineralization process of organic chemicals (7, 8) and biological cells (9, 10). For microorganism inactivation, the action of the radicals on the bacterial cell membrane leads to the perturbation of the different cellular processes and finally to the bacterial death (9). The reaction of other oxygen species can be negligible and the photo-oxidation reaction effect is closely relative to the amount of OH• radical which evidently depends on various parameters such as light intensity, nature of photocatalyst, and photocatalysis reaction time (7, 8, 11). Based on these similar mechanisms, it is reasonable to assume that a correlative effect on organic matter and microbes might exist in the photocatalytic process, and as such, it might be possible to evaluate the inactivation of microbes indirectly through the degradation effect on organic matters. The objective of this research was to study the possible association of photo mineralization phenomena against VOC and microorganisms. The degradation capabilities of formaldehyde and inactivation of E. coli under various light intensities and initial pollutant concentration were compared and analyzed.
Materials and Methods TiO2 Film Preparation. Two different nanophase types of TiO2 were applied in this study. TiO2 no. 1, a commercial product provided by a Japanese company, is a pure anatase phase with average particle size of 6nm. TiO2 no. 2, or Degussa P25, is another commercial photocatalyst with a mixed phase 80% anatase and 20% rutile. Both TiO2 materials were, respectively, prepared on the clear glass plate using the washcoated method (12). The sizes of glass plate used for the degradation test and photocatalytic inactivation test were 76 × 25 mm, and 20 × 25 mm, respectively. Experiment I: VOC Degradation Test Equipment and Procedures. Formaldehyde was selected as the target VOC pollutant which is commonly found in indoor environments. The degradation effect was measured using a glass-plate reactor consisting of a clean air supply unit, an organic air injection unit, chamber reactor assemblies, and a data acquisition system (Figure 1). Compressed synthetic air (mixture of high-purity nitrogen and oxygen with volume ratio 79:21) was supplied from a compressed cylinder. The air passed through a filter and was then divided into two streams. One stream passed through a flow meter while the other passed through a humidifier to control the humidity level at 45 ( 5% RH. 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 formaldehyde concentration was conditioned to 1-3 ppm, which is the common levels in “sick buildings” (7).The TiO2coated glass plates were placed in the reactor to test its photocatalytic effect. UVA light was provided by black-light lamp whose peak intensity was 365 nm. The distance between the UV lamps and quartz window was adjustable (from 0 to 90 cm) to vary the light intensity. The light intensity of reaction was measured using a UV power meter. The photoreactor 10.1021/es802499t CCC: $40.75
2009 American Chemical Society
Published on Web 01/21/2009
FIGURE 3. Degradation of formaldehyde (TiO2 no. 1 material; light intensity ) 5 W/m2; inlet concentration ) 2 ppm). FIGURE 1. Schematic of the glass-plate photoreactor. (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) ultraviolet germicidal lamps; (11) TiO2-coated glass-plate; (12) gas analyzer; (13) quartz window; (14) thermocouple.
UVA were carried out simultaneously and each collection of cells were sampled in triplicate.
Results and Discussion Analysis of Catalytic Degradation Processes. Figure 3 shows one typical degradation test result. The distinction between the light on and light off is due to the photocatalytic degradation. A mass balance can be set up based on that the rate of disappearance of formaldehyde by reaction must equal the rate of transport of formaldehyde to the catalyst surface (14): φVkrCs ) Akm(C - Cs)
FIGURE 2. Schematic of the photokilling test system. effluent formaldehyde concentration was measured by INNOVA photoacoustic multigas monitor 1312 (S&V Samford Instruments Ltd., Denmark. Uncertainty is ( 2%). The test temperature was 29 ( 0.5 °C. Experiment II: Photokilling Experiment. E. coli was used as a model bacterium in this evaluation. E. coli cells were precultured in 5 mL Luria-Bertani (LB) nutrient broth (pH 7) at 37 °C for 12 h. To eliminate broth medium, the E. coli cultures in LB medium were washed by centrifuging at 4000 rpm The treated cells were then resuspended and diluted to different concentration of 1 × 105 to 11 × 108 colony forming units (CFU)/mL (identified by UV/vis spectrophotometer) of with sterilized 0.9% saline water. 100 µL of E. coli cell suspension was piped onto each TiO2-coated glass plate. There were three plates for each experimental condition, and the plates were placed in an airtight illumination chamber to prevent drying (Figure 2). Each case was performed in triplicate for parallel analysis. Prior to each experiment, the quartz window, the photoreactor and the TiO2-coated glass plates were autoclaved in 121 °C for 20 min to ensure that no other microorganisms affected the results. The photoreactor was illuminated with blacklight lamp, and the light intensity peaking at 365 nm was measured by UV power meter (UV-A type). After illumination, the cell suspension was collected by washing the glass plate with 0.9% saline solution at certain interval. Then the collected cells suspension were diluted appropriately, and 100 µL diluted suspension was spread on the nutrient broth agar plate and incubated at 37 °C for 12 h. Three replicate plates were used for each incubation. Counting of the colony forming units was done on plates containing between 30 and 300 colonies (13). During all the experiments, control tests in the conditions of dark and only
(1)
Where φ is the effectiveness factor (-); V the catalyst volume (m3); kr the intrinsic kinetic reaction constant (s-1); Cs the concentration of formaldehyde at the catalyst surface (ppm); A the geometric catalyst surface (m2); km the external mass transfer coefficient (m/s), C the bulk concentration of formaldehyde (ppm). The rate equation can be expressed in terms of catalyst volume and substance concentration: -R ) k0C ) φkrCs
(2)
With k0being the apparent reaction rate constant (s-1). The effective factor φ ) 1 unless the destruction efficiency is very low (14). The equation can hence be written as a power law expression of first order with respect of the bulk concentration of the formaldehyde: R)
krC dC )dt 1 + (krV ⁄ kmA)
(3)
After integration and combination with eq 2 and eq 3 results in C ) exp(-k0t) C0
(4)
With C0being the initial bulk concentration of formaldehyde (ppm); t the reaction time (s); The apparent reaction rate constant is a combination with reaction kinetics and mass transfer rate: δ 1 1 ) + k0 km kr
(5)
δ, the thickness of catalytic layer, can be calculated from the weight (0.005928 g),the absolute density of the catalyst coating (3.86 g/cm3), and the area of the wash coat (19 cm2), and also the porosity (0.8) (14). It is in order of 1 µm in this research. kmfor the reaction can be obtained using the correlation with Sherwood number (15): Sh )
kmd0 D
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(6)
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Sh ) 1.86
(
ReScd0 L
)
1⁄3
(7)
where D is the diffusivity of formaldehyde in air )18.6 × 10-6 (m2/s); d0 being the inner dimension of the reactor cell )0.0037 (m); Re is the Reynolds number)80; Sc is the Schmidt number (-) ) (ν)/(D) ) (15.6 × 10- 6)/(18.6 × 10- 6) ) 0.839; L is the length of the catalytic surface ()0.076 m). By incorporating eqs 3 and 4, km ) 1.38 × 10- 2(m/s). The results of k0 range from 0.5-6 (s-1). (δ)/(km)(in order of (10-4-10-5)) , (1)/(k0). It is reasonable that the mass transfer is negligible and that the intrinsic reaction rate constant is equal to the apparent reaction rate constant, thus ln
C ) -krt C0
(8)
So kr can be regressed with the inlet concentration C0, outlet concentration C and the reaction time t (0.076 s in the experiment). Figure 4 shows the survival of E. coli as a function of time. It should be emphasized that the survival curve does not follow a simple exponential decay process as a function of inactivation time (5). Three kinetic steps in the photocatalytic bactericidal effect of on E. coli have been observed in the absence or presence of TiO2. The self-defense mechanisms induced by UV in the stressed cells partially diminishes the effect of light during the first step of the phototreatment which makes antibacterial destruction slow in the first period. In the latter case, the massive generation rapidly overcomes the self-protection mechanisms of the bacterial, and as a result, the concentration decreases exponentially after illumination because the antistress enzymes are no longer able to protect the bacterial membrane against oxidation. In the last period of photo treatment, inactivation of bacteria becomes extremely slow because the few active bacteria remaining are in competition for OH• with both the inactivated bacteria and the metabolites released during the photoprocess (11). Not every curve contains these three steps and the time of each step depends on the oxidation rate. In order to compare the effects with variable conditions, the results were evaluated by inactivation rate constant k of the Click model, one of the most commonly used models for TiO2 inactivation kinetics of microbes (16–18). The model is expressed by the following equation: ln
Nt ) -kt + b N0
(9)
Where Nt is the remaining E. coli concentration at time t (CFU/mL); N0 is the initial E. coli concentration (CFU/mL); k is the photokilling constant (s-1); b is a constant (-). The correlative number is more than 0.9, suggesting that the model fits well to the measured decay curve.
FIGURE 5. The influence of light intensity on VOC degradation and bacteria inactivation. It should be emphasized that the antibacterial effect is not only due to the photocatalytic oxidation, but also the photosis effect. The correlation analysis between formaldehyde degradation and E. coli inactivation should be based on the similar mechanism that the amount of reactive oxygen species produced is the influencing factor. For organic matters, UVA light itself does not contribute directly to degradation. However, this is not true for microbes. In previous photocatalytic antibacterial studies, the bactericidal effect of UV light irradiation was not excluded from the integrated effect. In Figure 4, the unalloyed effect of photolysis (kUVA ) 0.203 × 10-3) was only fraction of the integrated effect (kUVA+TiO2)1.685 × 10-3 1/s). After eliminating the UVA disinfection effect kUVA, the photocatalytic oxidation effect can be evaluated by a net parameter k ) 1.482 × 10-3 1/s. Especially with different light intensities, only k reflects the amounts of reactive oxygen species produced and their effect. In this study, the net parameter k was applied to evaluate the inactivation effect by photocatalytic oxidation. The kr(s-1) in eq 8 and net parameter k (s-1) from eq 9 reflect the intrinsic oxidation capability for formaldehyde and E. coli,, respectively. Thus they can be applied as the index for analogy analysis. Analogy under Different Light Intensities. Irradiation light intensity is an important physical parameter influencing the photocatalytic oxidation effect. In this study, the influences of light intensity on both photocatalytic degradation of formaldehyde and inactivation of E. coli were investigated with the same range of light intensity. Figure 5 presents the influence of light intensity on degradation rate constant (kr) and inactivation rate constant (k). Through linear regression, both the two rate constants increase linearly with the UVA light intensity in the range of 1-10 w/m2 (R2 ) 0.96). Deactivation of E. coli as a function of light intensity presents a similar kinetic shape to the degradation of formaldehyde probably because the photochemical mechanisms are common to both processes. Both kr and k increase with increasing light intensity due to the amount increasing of reactive oxygen species generated on the TiO2 surface. On TiO2 thin film, the steady state concentration of OH• on the TiO2 surface ([OH•]s) can be expressed as [OH•]s )
FIGURE 4. Inactivation of E.coli (TiO2 no. 1 material; Light Intensity ) 5 W/m2; initial concentration)105 cuf/mL). 1182
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(1 - a)IaΦ kD
(10)
Where Ia is the number of light quanta absorbed per second, Φ is quantum yield, kD is the diffusion-limited rate constant, and R is parameter depending on the PH (10). Study conducted in TiO2 suspension showed that the OH• concentration is linearly correlative to the light intensity (19). Analogy under Different Initial Contaminant Concentrations. Previous studies which focused on the degradation of formaldehyde or microbes have shown that changes in
FIGURE 6. Dependence of degradation rate on different inlet contaminant concentrations. (With light intensities of 1 W/m2, 5 W/m2, 10 W/m2).
FIGURE 8. Dependence of degradation rate constant on different inlet contaminant concentrations.(With light intensities of 1 W/m2, 5 W/m2, 10 W/m2).
FIGURE 9. Dependence of inactivation rate on different initial bacteria concentrations. (With light intensity of 10W/m2). FIGURE 7. Dependence of inactivation rate on different initial bacteria concentrations. (With light intensity of 10W/m2). the initial microbe concentrations and inlet formaldehyde concentrations can lead to different oxidation effects (4, 11). The studies found that the reaction rate of formaldehyde was enhanced with the increase of inlet concentration (20). The reaction rate was defined as R ) (Cin - Cout)Qf ⁄ A
(11)
Where Cin is the inlet concentration (ppm); Cout is the outlet concentration (ppm), Qf is the flow rate (L/min), A is the area of reaction surface (cm2). The results (Figure 6) in this study agree with those from previous studies. And for E. coli inactivation, the reaction rate was evaluated by the first 15 min of irradiation: n ) (C0 - C15)Qe ⁄ A
(12)
Where C0 is the initial concentration (cfu/mL), C15 is the concentration after 15 min of irradiation (cfu/mL), Qe is the amount of bacteria solution (mL). The functional dependence of the degradation rate on initial concentration for E. coli established in Figure 7 is similar to that for formaldehyde. In this study, we also evaluated the influence of initial concentration by reaction rate constants. Figures 8 and 9 present the effects of inlet formaldehyde concentration and initial cell concentration, respectively on degradation rate constant kr and inactivation rate constant k. With formaldehyde as the target pollutant (Figure 8), the degradation rate constant decreases linearly with the increase of initial concentration (1.2-2.8 ppm). And decreasing rate of the degradation rate constant k slowed down with lower light intensity. For E. coli (Figure 9), a first order of the inactivation rate constant with the initial concentration (105-108cfu/mL) was found. Apparent analogous trends were observed for each degradation process of formaldehyde and E. coli. Analogy For Different Photocatalytic Materials. The photocatalytic effect of a commercial TiO2 and P25 film was compared for destructing formaldehyde and inactivating E.
FIGURE 10. Dependence of reaction rate constants on different photocatalysts (With light intensity of 10 W/m2; inlet concentration of 2.8 ppm, initial concentration of 105 cfu/mL). coli cells under light intensity of 10 W/m2. Figure 10 shows that in cases 2.80 ppm of inlet concentration, the degradation rate on commercial TiO2 film was higher than that on P25 film. And a higher photocatalytic inactivation rate of E. coli was also observed on commercial TiO2 film that was twice as much as the one on P25 film (Figure 10) under initial concentration of 105 cfu/mL. With the same UVA light irradiation, the commercial TiO2 material applied seemed to be more active and produced more ROS with the same irradiation. However, the advantage of commercial TiO2 for inactivating E. coli was more prominent than that of formaldehyde degradation. On P25 film, the amount of the OH• roduced with the same irradiation was less. So the attack by OH• adical or other oxidation species on both the two target pollutants was alleviative. For formaldehyde, the molecules were photocatalytically oxidized to corresponding acids as the intermediate but not only the carbon dioxide and water (21). But both the reaction pathways lead to the disappearance of organic molecules so it did not make a large difference in degradation efficiency. Whereas for the degradation microorganism, the palliative attack may not overcome the bacteria protection or recovery mechanism, so the E. coli survived (1). That difference of viability between the two target pollutants can account for the contrast of the change extent. Considering the results, both of the photo-oxidation reactions against formaldehyde and E. coli follow exponentialVOL. 43, NO. 4, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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order kinetics. The degradation rate constant (kr) of decomposing formaldehyde and the inactivation rate constant (k) of inactivating E. coli change in an analogical trend with the condition of initial contaminant concentration or characters of photocatalytic material. The experimental observations from this study indicate that there exists an apparent correlation between the photocatalytic processes of decomposing formaldehyde and inactivating E. coli and that both processes depend on the amount of reactive oxygen species produced. Influences of key parameters, i.e., light intensity, initial concentration, and material character on the effect of formaldehyde degradation or E. coli inactivation also behave in analogous manner with similar trends. Analogy is a potential method to evaluate the antimicrobial effect based on the organic compound degradation effect, whereby the latter is much easier to measure quantitatively. Further research on different materials and pollutants under a wider range of conditions need to be conducted to understand this relationship fully.
Acknowledgments This study is financially supported by the National Natural Science Foundation of China Project No. 50436040, and the National “115” Key Support Project of China Project No. 2006BAJ02A08.
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