TiO2 MEDIATED PHOTOCATALYTIC INACTIVATION OF AIRBORNE BACTERIA

134 Pages·2008·2.09 MB·English

TiO MEDIATED PHOTOCATALYTIC INACTIVATION
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OF AIRBORNE BACTERIA

AMRITA PAL

NATIONAL UNIVERSITY OF SINGAPORE
2007
TiO MEDIATED PHOTOCATALYTIC INACTIVATION
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OF AIRBORNE BACTERIA

AMRITA PAL
B. ENGG. (HONS.), JADAVPUR UNIVERSITY, INDIA

A THESIS SUBMITTED
FOR THE DEGREE OF DOCTOR OF PHILOSOPHY
DEPARTMENT OF CHEMICAL AND
BIOMOLECULAR ENGINEERING
NATIONAL UNIVERSITY OF SINGAPORE
2007
Name: Amrita Pal.
Degree: PhD.
Dept.: Chemical and Biomolecular Engineering.
Thesis Title: TiO Mediated Photocatalytic Inactivation of Airborne Bacteria.
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Abstract
Microbial pollutants such as bacteria are one of the significant sources of indoor
air pollution. The first phase of this research aims to examine inactivation efficiencies of
eight bacterial species using a batch disinfection system with TiO assisted photocatalytic
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reactions coupled with a fluorescent light or UV-A lamp. The maximum inactivation of
most bacteria was achieved at an optimum TiO loading of 511-1666 mg/m2
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corresponding to a thickness of 294 - 438 nm of TiO layer on the surface. Gram-negative
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bacterium E. coli K-12 was most effectively inactivated with a rate constant of 0.2442
min-1, while Gram-positive Bacillus subtilis exhibited the most resistant response to the
photocatalytic treatment (0.0057 min-1). In the second phase, a continuous reaction
system was developed to examine effects of light intensity (0.5 to 3.4 mW/cm2), TiO
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loadings (960 and 1516 mg/m2) and relative humidity (RH) (51 ± 0.61 to 85 ± 4.7%) on
inactivation efficiencies of aerosolized Gram-negative bacterium E. coli K-12. Results
showed an increase in bacterial inactivation with increasing UV-A intensity, TiO loading
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and RH. E. coli K-12 is fully and continuously inactivated after 15 min of UV-A
exposure.

Keywords: TiO ; UV-A; E. coli K-12; Relative humidity; Inactivation efficiency;
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Continuous reaction system;
ACKNOWLEDGEMENTS
To begin with, I would like to express my sincere gratitude to my Supervisor Dr. Liya
E. Yu for her untiring and continuous guidance throughout my entire candidature. Her
advice along with constructive and critical evaluations helped me in invoking further
thinking in my research area and successful completion of my studies.
I would like to express my heartfelt gratitude to Dr. Madhumita B. Ray for
introducing me to my current research area of Photocatalytic inactivation of Bioaerosols
and guiding and supporting me throughout my research. I would also take this
opportunity to thank Dr. Simo O. Pehkonen for providing invaluable ideas and advice in
further enriching my research.
I am also grateful to other students in our research group specially, Dr. Yang Liming
and Mr. Gao Yonggang, Mr. Lim Jaehyun, Mr. Zhou Hu and Mr. Balasubramanian
Suresh Kumar, for their utmost help through valuable discussions. I wish to sincerely
thank our lab officers Mdm. Susan, Mdm. Li Xiang, Mdm. Sylvia, Mr. Boey, and Mdm.
Mary. I would also like to express my sincere gratitude to Mr. Ng for constructing the
continuous reactor.
I wish to express my deep gratitude to my family members and my husband for their
whole-hearted and unconditional support throughout my research.
Finally, I would like to acknowledge the financial support received from the National
University of Singapore, in the form of Research Scholarship.
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Table of Contents

Page
ACKNOWLEDGEMENTS i
TABLE OF CONTENTS ii
SUMMARY vi
LIST OF TABLES ix

LIST OF FIGURES x

NOMENCLATURE xii

CHAPTER 1 INTRODUCTION 1

1.1 Background 1

1.2 Objectives 5

1.3 Organization of thesis 7

CHAPTER 2 LITERATURE REVIEW 8

2.1 Indoor air pollution due to microbial pollutants 8

2.2 Effect of direct UV irradiation on bacteria 9

2.3 Effect of heterogeneous photocatalysis (UV-A and TiO ) on 11
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bacteria

2.3.1 Heterogeneous photocatalytic inactivation of bacteria in 13
aqueous phase

2.3.2 Heterogeneous photocatalytic inactivation of bacteria in 15
air phase

2.4 Effect of heterogeneous photocatalysis (fluorescent light and 16
TiO ) on bacteria
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2.5 Factors affecting the inactivation of bacteria 17

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2.5.1 Wavelength and light intensity 17
2.5.2 Type of bacteria 18

2.5.3 Concentration of TiO 19
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2.5.4 Crystal structure and loading of TiO 20
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2.5.5 Reaction time 21

2.5.6 Relative humidity 22
2.5.7 Temperature 24

2.5.8 Presence of interfering species 25

2.6 Current trends in Heating Ventilation and Air Conditioning 25
(HVAC) systems

CHAPTER 3 EXPERIMENTAL 29
3.1 Materials 29

3.1.1 Titanium dioxide 29

3.1.2 Bacteria strains 29

3.1.3 Irradiation source 32

3.2 Experimental setup 32

3.2.1 Batch inactivation system 32
3.2.2 Continuous inactivation system 33
3.3 Experimental procedure 35
3.3.1 Bacterial culture preparation 35
3.3.2 Coating of membrane filter 36
3.3.2.1 Batch studies 36

3.3.2.2 Continuous studies 39

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3.3.3 Inactivation procedure 40

3.3.3.1 Batch studies 40

3.3.3.2 Continuous studies 41

3.3.4 Control experiments for batch photocatalytic studies 43

CHAPTER 4 RESULTS AND DISCUSSION 44

4.1 Batch photocatalysis 44
4.1.1 Bacteria inactivation under fluorescent irradiation 44
without TiO
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4.1.2 Bacteria inactivation under fluorescent irradiation and 46
TiO (Degussa P25)
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4.1.3 Inactivation of all bacteria 48

4.1.4 Comparison of fluorescent light inactivation with UV-A 52
inactivation

4.1.5 Comparison of photocatalytic inactivation between two 55
TiO catalysts, Degussa P25 and Hombikat UV-100
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4.2 Continuous photocatalytic studies 59
4.2.1 Establishment of adsorption equilibrium 59

4.2.2 Effect of UV-A intensity on the inactivation of E. coli K- 60
12

4.2.3 Effect of TiO loadings on the inactivation of E. coli K- 62
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12
4.2.4 Effect of relative humidity on the inactivation of E. coli 64
K-12

4.2.5 Comparison between batch and continuous reaction 69
studies

4.2.6 Establishment of rate equation using nonlinear regression 69
analysis

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4.2.7 Photocatalytic reactor- scale up from bench-scale to a 75
commercial unit

CHAPTER 5 CONCLUSIONS AND FUTURE WORK 79

5.1 Conclusions 79

5.1.1 Batch photocatalytic inactivation studies 80

5.1.2 Continuous photocatalytic inactivation studies 81

5.2 Future work 82

Photocatalytic inactivation effect on other strains of 82
bacteria

Mechanisms of TiO photocatalysis disinfection 83
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REFERENCES 84
APPENDICES

Appendix A 98
Appendix B 110
Appendix C 111
Appendix D 112
LIST OF PUBLICATIONS 118

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SUMMARY

Under a suitable environment, airborne bacteria can proliferate on various indoor
materials, such as in heating ventilation and air conditioning (HVAC) systems, leading to
undesired symptoms and responses associating with indoor air bioaerosols, such as sick
building syndromes, bronchitis, airborne transmission of tuberculosis, asthma, etc. This
can be particularly serious in tropical regions like Singapore due to high relative humidity
and warm temperatures throughout the year. While inactivation of bioaerosols using TiO
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photocatalytic reactions had not been studied in detail, TiO photocatalytic disinfection
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can be promising because irradiation of UV-A activates TiO reactions with water and
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oxygen forming OH radicals (·OH) and reactive oxygen species (ROS), which can
subsequently induce inactivation of bacteria. Hence, the objective of this work was to
systematically examine disinfection efficiencies of TiO photocatalytic disinfection
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systems by investigating effects of catalyst loadings, bacteria species, light intensity,
relative humidity, and/or system design.
The first phase of this study investigated the efficacy of TiO mediated
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inactivation of eight bacterial species in a batch system using two radiation sources, a
fluorescent (3900 Lux) light emitting a small amount of UV-A (0.013 mW/cm2 at 365
nm) and a UV-A lamp (4.28 mW/cm2 at wavelength 365 nm) in a batch disinfection
system. Loadings of TiO , varying from 234-8662 mg/m2, were impregnated on
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membrane filters and exposed to radiation source for the inactivation of eight bacteria (E.
coli K-12, Pseudomonas fluorescens, Bacillus subtilis, Microbacterium sp.,
Microbacteriaceae str. W7, Kocuria sp. KMM 3905, Paenibacillus sp. SAFN-007 and
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Gordonia terrae/ Actinomycetaceae). Overall, the inactivation rate increased with an
increase in the TiO loading, while the maximum inactivation of most bacteria was
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achieved at an optimum TiO loading of 511-1666 mg/m2, corresponding to a thickness
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of 294-438 nm of TiO coating over the filter surfaces. Gram-negative bacterium E. coli
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K-12 was most effectively inactivated (0.2442 min-1), while Gram-positive Bacillus
subtilis was most resistant to the batch photocatalytic treatment (0.0057 min-1). E. coli K-
12 was fully inactivated after 30 minutes of treatment at a TiO loading of 1666 mg/m2.
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Inactivation of 1 log was obtained for Microbacterium sp., Paenibacillus sp. SAFN-007
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and Microbacteriaceae str. W7 after 2 hours of illumination with a TiO loading of 1116
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mg/m2. The inactivation rates resulting from the irradiation of a UV-A lamp is
comparable with data available in literature, indicating consistency in the UV-A
photocatalytic inactivation of the microorganisms.
During the second phase of this research, a continuous annular reactor was
designed to characterize the TiO mediated inactivation of aerosolized Gram-negative
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bacterium E. coli K-12 (ATCC 10798) under various UV-A intensities (0.5 to 3.4
mW/cm2), relative humidities (RH) (51 ± 0.61 to 85 ± 4.7%), and two photocatalyst
loadings (960 and 1516 mg/m2) at an air flow rate of 1 l/min. Overall, inactivation of E.
coli K-12 increased with an increase in TiO loading, UV-intensity, or RH. An UV-A
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dose between 0.03 to 0.204 J/cm2/l under an average UV-A intensity of 0.5 to 3.4
mW/cm2, at a residence time of 1.1 min, is strong enough to fully and continuously
inactivate all incoming bacteria. Compared to the batch disinfection system, additional
experiments employing a low UV-A intensity of 0.015 mW/cm2 and a TiO loading of
2
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