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University of Calgary
PRISM: University of Calgary's Digital Repository
Graduate Studies The Vault: Electronic Theses and Dissertations
2014-04-28
Light Emitting Diode Based Photochemical Treatment
of Contaminants in Aqueous Phase
Yu, Linlong
Yu, L. (2014). Light Emitting Diode Based Photochemical Treatment of Contaminants in Aqueous
Phase (Unpublished doctoral thesis). University of Calgary, Calgary, AB.
doi:10.11575/PRISM/26762
http://hdl.handle.net/11023/1447
doctoral thesis
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UNIVERSITY OF CALGARY
Light Emitting Diode Based Photochemical Treatment of Contaminants in Aqueous Phase
by
Linlong Yu
A THESIS
SUBMITTED TO THE FACULTY OF GRADUATE STUDIES
IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE
DEGREE OF DOCTOR OF PHILOSOPHY
DEPARTMENT OF CIVIL ENGINEERING
CALGARY, ALBERTA
April, 2014
© Linlong Yu 2014
ii
Abstract
In this research, photochemical treatment of pesticides and polychlorinated biphenyls
(PCBs) in aqueous medium were investigated. The studies on photochemical treatment
of these two groups of compounds, along with radiation field modelling, further, led to
the design of an efficient light emitting diode (LED) based flow-through photocatalytic
reactor.
Sensitized photodechlorination of PCBs in surfactant solutions was studied. Three types
of surfactants at different concentrations were investigated. The neutral and cationic
surfactants were found to be more effective than the anionic one. In each case the
surfactant concentration was found to play a significant role in the rate of dechlorination.
LED based photocatalytic degradation of pesticides and chlorophenols, namely 2,4-
dichlorophenoxyacetic acid (2,4-D), 2-methyl-4-chlorophenoxyacetic acid (MCPA) , 4-
chlorophenol (4-CP) and 2,4-dichlorophenol (2,4-DCP) was studied. Further, the impact
of photocatalyst loading and light intensity on the degradation rate was evaluated. The
degradation of 2,4-D under LED irradiation was compared to that with mercury discharge
lamp irradiation. The results show these compounds can be efficiently degraded using
LED based TiO2 photocatalysis. They are completely mineralized upon prolonged
irradiation. Our results indicate that LEDs are a better light source than the mercury
lamps.
iii
To design an efficient LED based photocatalytic reactor, a radiation field model was
developed in this research. The model was tested with experimental data and good
agreement between two was noted. The model can be used to optimize the photoreactor
and chose the optimal gap between adjacent LEDs, the irradiated distance and the light
output of LEDs for a homogenous radiation field.
Finally, an LED based photocatalytic reactor was designed and fabricated. The reactor
uses anodized TiO2 nanostructure as a photocatalyst. The performance of reactor was
evaluated and optimized by studying the degradation of 2,4-D. The effect of different
operational parameters on the reactor performance were investigated, including light
intensity, distance between the LED module and photocatalytic plate (DL-P), the flow rate
through the reactor, presence of external electron scavengers and photocatalyst
configuration. A power law relationship was observed between the light intensity (2.2
mW cm-2~17.3 mW cm-2) and the first order degradation rate constant for 2,4-D. A
suitable flow rate and DL-P was determined for the reactor. Enhanced performance of the
reactor was observed where electron scavengers were introduced.
iv
Acknowledgements
I would like to express my sincerest appreciation and gratitude to my supervisor Dr.
Gopal Achari and my co-supervisor Dr. Cooper H. Langford for their continuous
encouragement, intellectual advice, precious guidance and enthusiastic supports
throughout my doctoral program.
It is my fortune to have friendly colleagues, Dr. Jyoti Ghosh, Dr. Maryam Izadifard,
Jiansong Kong, Chien-Kai Kenneth Wang, Upasana Chamoli and Mitra mehrabani. I
greatly appreciate their helps. My gratitude is also extended to Mr. Daniel Larson for his
assistance with instruments and laboratory facilities during my research. Thanks to Mr.
Edward C. Cairns, Mr. Andrew Read, Mr. Mark Toonen and Robert Thomson for their
help on fabricating LED reactors.
I gratefully acknowledge the financial support provided by Samuel Hanen Foundation,
RES'EAU WaterNet Strategric Research Network, Natural Science and Engineering
Council of Canada and Department of Civil Engineering.
Finally, I would like to show my gratitude to my sister, my uncles and my aunties for
their supports in the past five years.
v
Dedication
This thesis is dedicated to my beloved parents.
vi
Table of Contents
Abstract ............................................................................................................................... ii Acknowledgements ............................................................................................................ iv Dedication ............................................................................................................................v Table of Contents ............................................................................................................... vi List of Tables .......................................................................................................................x List of Figures and Illustrations ......................................................................................... xi List of Symbols, Abbreviations and Nomenclature ...........................................................xv
CHAPTER ONE: INTRODUCTION ..................................................................................1 1.1 Background ................................................................................................................1 1.2 Photochemical Treatment Processes ..........................................................................3 1.3 Light emitting diode (LED) in photocatalytic reactors ..............................................4 1.4 Research Objectives and Scopes ................................................................................5 1.5 Thesis Overview ........................................................................................................6
CHAPTER TWO: LITERATURE REVIEW ......................................................................9 2.1 Principle of Photochemistry.......................................................................................9
2.1.1 Light and photon ................................................................................................9 2.1.2 The electronic excited states ............................................................................11 2.1.3 Quantum yield .................................................................................................11 2.1.4 Direct photolysis ..............................................................................................12 2.1.5 Photosensitized degradation ............................................................................13 2.1.6 Photocatalysis ..................................................................................................13 2.1.7 Advanced Oxidation Processes (AOPs) ..........................................................14
2.2 TiO2 Photocatalysis .................................................................................................14 2.2.1 TiO2 as a photocatalyst....................................................................................14 2.2.2 Mechanism of TiO2 photocatalysis .................................................................17 2.2.3 The kinetics of photocatalytic degradation ......................................................20 2.2.4 Factors affecting the photocatalytic degradation kinetics ...............................21
2.2.4.1 TiO2 loading ..........................................................................................21 2.2.4.2 Light intensity ........................................................................................22 2.2.4.3 pH ...........................................................................................................23 2.2.4.4 Electron acceptor ...................................................................................24 2.2.4.5 Hole/hydroxyl radicals scavenger ..........................................................26
2.3 Contaminants (Pesticides and PCBs) .......................................................................27 2.3.1 Pesticides .........................................................................................................27
2.3.1.1 2,4-dichlorophenoxyacetic acid (2,4-D) ...............................................29 2.3.1.2 2-methyl-4-chlorophenoxyacetic acid (MCPA) ....................................30 2.3.1.3 Chlorophenols ........................................................................................32
2.3.2 PCBs ................................................................................................................33 2.4 Photochemical treatment of pesticides and PCBs ....................................................36
2.4.1 Direct photolytic degradation of pesticides and PCBs ....................................36 2.4.2 Photosensitized degradation of pesticides and PCBs ......................................37 2.4.3 Photocatalytic degradation of pesticides and PCBs ........................................37
vii
2.5 Design of a photocatalytic reactor ...........................................................................38 2.5.1 State of Photocatalyst in the Reactor ...............................................................38
2.5.1.1 Slurry photocatalytic reactor vs immobilized photocatalytic reactor ....38 2.5.1.2 TiO2 immobilization through electrochemical anodization ..................40
2.5.2 Light source .....................................................................................................41 2.5.2.1 Sunlight ..................................................................................................42 2.5.2.2 Mercury lamps .......................................................................................44 2.5.2.3 Light emitting diode ...............................................................................45
2.5.3 Artificially illuminated photocatalytic reactors ...............................................47 2.5.4 Solar photocatalytic reactors: ..........................................................................53
2.6 Radiation-field modelling ........................................................................................56 2.6.1 The Radiation transport equation (RTE) .........................................................57 2.6.2 Numerical methods to solve the RTE ..............................................................60 2.6.3 Radiation source models ..................................................................................61
CHAPTER THREE: ELECTRON TRANSFER SENSITIZED PHOTODECHLORINATION OF SURFACTANT SOLUBILIZED PCB138 .......63
3.1 Introduction ..............................................................................................................63 3.2 Materials and methods .............................................................................................64
3.2.1 Materials ..........................................................................................................64 3.2.2 Methods ...........................................................................................................65
3.2.2.1 PCB 138 solubilization with surfactants ................................................65 3.2.2.2 Photochemical reaction ..........................................................................65 3.2.2.3 Sampling, extraction and GC analysis ...................................................66
3.3 Results and Discussion ............................................................................................67 3.3.1 Selectivity of surfactants .................................................................................67 3.3.2 Dechlorination of PCBs in TEA and NaBH4 systems ....................................72
3.3.2.1 MB and TEA ..........................................................................................72 3.3.2.2 MB and NaBH4 .....................................................................................73 3.3.2.3 Photodegradation of Aroclor 1254 with NaBH4 and TEA ....................74
3.3.3 The dechlorination pathways of PCB 138 using CTAB and TWEEN 80 .......75 3.4 Conclusions ..............................................................................................................77
CHAPTER FOUR: LED-BASED PHOTOCATALYTIC TREATMENT OF PESTICIDES AND CHLOROPHENOLS ...............................................................79
4.1 Introduction ..............................................................................................................79 4.2 Methods and Materials .............................................................................................81
4.2.1 Photoreactor .....................................................................................................81 4.2.2 Chemicals ........................................................................................................83 4.2.3 Photocatalytic degradation ..............................................................................83 4.2.4 Actinometric Experiment ................................................................................84 4.2.5 Analysis of sample ..........................................................................................85
4.2.5.1 HPLC Analysis ......................................................................................85 4.2.5.2 TOC Analysis: .......................................................................................85
4.3 Results and Discussions ...........................................................................................86 4.3.1 Photocatalytic degradation of pesticides and chlorophenols ...........................86 4.3.2 Photocatalytic degradation of pesticides mixtures ..........................................89
viii
4.3.3 Effect of Photocatalyst Loading ......................................................................93 4.3.4 Effect of Light Intensity ..................................................................................95 4.3.5 Comparison between LED and Mercury Lamp Irradiation .............................97
4.4 Conclusions ..............................................................................................................99
CHAPTER FIVE: DESIGN A HOMOGENEOUS RADIATION FIELD IN A UV-LED BASED PHOTOCATALYTIC REACTOR ..................................................100
5.1 Introduction ............................................................................................................100 5.2 Advantage of homogeneous radiation field in a photocatalytic reactor ................101 5.3 Development of radiation field model ...................................................................103
5.3.1 UV-LED array and photocatalyst plate .........................................................103 5.3.2 Radiation field model without shielding glass plate ......................................104 5.3.3 Radiation field model with a shielding glass plate ........................................109
5.4 Calibration and validation of the radiation field model .........................................110 5.4.1 Light intensity measurement .........................................................................110 5.4.2 Model light intensities vs measured light intensities .....................................112
5.5 Design of a homogenous radiation filed ................................................................114 5.5.1 The effect of ID on the homogeneity of radiation field for a fixed gap ........114 5.5.2 Optimal combination of ID and gap ..............................................................116 5.5.3 Selection of the output of the UV-LED .........................................................117
5.6 Conclusions ............................................................................................................118
CHAPTER SIX: A NOVEL LIGHT EMITTING DIODE BASED PHOTOCATALYTIC REACTOR FOR WATER TREATMENT ........................119
6.1 Introduction ............................................................................................................119 6.2 Experimental details ..............................................................................................121
6.2.1 Chemicals ......................................................................................................121 6.2.2 Design and fabrication of an LED based photocatalytic reactor ...................121
6.2.2.1 Preparation of anodized TiO2 photocatalytic plate. .............................121 6.2.2.2 UV-LEDs module ................................................................................122 6.2.2.3 Photocatalytic system ..........................................................................123
6.2.3 Radiation field and light intensity estimation ................................................124 6.2.4 Experimental set-up and sample analysis ......................................................126
6.3 Result and discussion .............................................................................................127 6.3.1 Degradation of phenoxy pesticides and chlorophenols in a flow-through
LED based photocatalytic reactor ..................................................................127 6.3.2 Degradation of 2,4-D with different combination of (UV, TiO2
photocatalyst plate, H2O2 and O2) in the UV-LED photoreactor . ...............128 6.3.3 Effect of DL-P .................................................................................................130 6.3.4 Effect of flow rates on the photocatalytic degradation of 2,4-D. ..................131 6.3.5 Effect of UV light intensity ...........................................................................133 6.3.6 Comparison of three different photocatalyst configurations .........................135
6.4 Conclusions ............................................................................................................136
CHAPTER SEVEN: CONCLUSION AND RECOMMENDATION FOR FUTURE RESEARCH ............................................................................................................137
7.1 Conclusions ............................................................................................................137
ix
7.1.1 Photosensitized dechlorination of PCBs solubized in surfactant solution ....137 7.1.2 LED based photocatalytic treatment of pesticides and chlorophenols ..........138 7.1.3 Design a homogenous radiation field model for photocatalytic reactor ........138 7.1.4 A novel light emitting diode photocatalytic reactor for water treatment ......139
7.2 Recommendations for Future Research .................................................................140 7.2.1 Incorporating PCBs extraction using surfactants and PCBs
photodechlorination using sensitized visible light .........................................140 7.2.2 UVC-LED ......................................................................................................140 7.2.3 The decay of photocatalytic activity and its life time ....................................140 7.2.4 Hollow microsphere coated with TiO2 (HGMT)...........................................140 7.2.5 Scale-up of the reactor ...................................................................................141
REFERENCES ................................................................................................................142
APPENDIX A: INVESTIGATION OF ULTRTRASONIC EXTRACTION OF POLYCHORINATED BIPHENYLS FROM SOIL ...............................................177
A.1. Experimental ........................................................................................................177 A.1.1. Chemicals ....................................................................................................177 A.1.2. Pre-Processing of contaminated soil ............................................................177 A.1.3. Ultrasonic extraction of PCBs .....................................................................177 A.1.4. Soxhlet extraction of the remaining PCBs in soil : ....................................178 A.1.5. Calculation of ultrasonic extraction efficiency ............................................178 A.2. Results and Discussions ..................................................................................179
A.3. Reference .............................................................................................................180
APPENDIX B: INVESTIGATION OF PHOTODEGRADATION OF BIPHENYL IN ULTRAVIOLET WATER PURIFICATION SYSTEMS.................................181
B.1. Experimental ........................................................................................................181 B.1.1. Chemicals .....................................................................................................181 B.1.2. Photoreactor .................................................................................................181 B.1.3. Photodegradation of biphenyl in IPA ..........................................................181
B.2. Results and discussions ........................................................................................182
APPENDIX C: UV VIS ABSORPTION SPECTRUM OF DIFFERENT PESTICIDES ..........................................................................................................184
APPENDIX D: THE CALCULATION OF PERCENTAGE OF AVAILABLE PHOTONIC ENERGY FOR PHOTOCATALYTIC REACTION ........................186
x
List of Tables
Table 2-1: Definitions of the quantum yield (Oppenlander, 2003). ................................. 12
Table 2-2: Generation of hydroxyl radicals for different AOPs. ...................................... 15
Table 2-3: Sales/use of the top 20 pesticide active ingredient in Canada (Brimble et al., 2005). .................................................................................................................. 28
Table 2-4: Slurry versus Immobilized Photocatalytic Systems (Lasa et al., 2005). ........ 39
Table 3-1: First order rate coefficients (K) for PCB138 dechlorination in TEA-MB system with different concentration of surfactants. .................................................. 71
Table 3-2: First order rate coefficients (K) for PCB138 dechlorination in NaBH4-MB system. ...................................................................................................................... 74
Table 4-1: Light intensity of different photoreactors. ....................................................... 84
Table 4-2: First order rate coefficients (K) for photocatalytic pegradation of different pesticides. .................................................................................................................. 89
Table 4-3: Mixtures of Pesticides. .................................................................................... 89
Table 4-4: Percentage removal of pesticides at 0.028 kJ energy dosage. ......................... 91
Table 4-5: Percentage of pesticides adsorbed on the surface of TiO2 after 30 minutes of stirring in the dark. ................................................................................................ 93
Table 5-1: Parameters used for radiation field model calculation. ................................. 113
Table 5-2: Comparison of modeled light intensity and measured light intensity. .......... 113
Table 6-1: Average light intensity received by the photocatalytic plate. ....................... 124
Table 6-2: First order kinetic rate constants for different photocatalyst configurations . 135
xi
List of Figures and Illustrations
Figure 2-1: Classification of electromagnetic radiation in the wavelength range below 1200 nm. [Reproduced from (Oppenlander, 2003) with the permission]. ................ 10
Figure 2-2: Phenomenological subdivision of ultraviolet radiation into four sub-bands and their characteristic effects. [Reproduced from (Oppenlander, 2003) with the permission]. ............................................................................................................... 10
Figure 2-3: Photochemical activation of TiO2. ................................................................. 19
Figure 2-4: Molecular structure of 2,4-dichlorophenoxyacetic acid. ............................... 29
Figure 2-5: Molecular structure of 2-methyl-4-chlorophenoxyacetic acid. ...................... 31
Figure 2-6: Molecular structure of chlorophenols. ........................................................... 32
Figure 2-7: Molecular structure of polychlorinated biphenyls. ........................................ 34
Figure 2-8: Solar spectral irradiance distribution on the surface of earth. [Reproduced from (Hulstrom et al., 1985) with permission] ......................................................... 42
Figure 2-9: Fractional cumulative integrated irradiance vs. wavelength. [Reproduced from (Hulstrom et al., 1985) with permission] ......................................................... 43
Figure 2-10: An inner working on an LED. [Adapted from (Wikipedia, 2011)] ............. 46
Figure 2-11: Various photochemical reactor configurations. [Reproduced from (Pareek et al., 2008) with permission] ...................................................................... 48
Figure 2-12: Scheme of a multiple tube reactor. [Reproduced from (Ray and Beenackers, 1998) with permission] ......................................................................... 50
Figure 2-13: Scheme of an optical fibre photocatalytic reactor. [Reproduced from (Nguyen and Wu, 2008) with the permission] .......................................................... 51
Figure 2-14: Scheme of a rotating disk reactor. [Reproduced from (Hamill et al., 2001) with the permission] ....................................................................................... 51
Figure 2-15: Top view of a distributive photocatalytic reactor. [Reproduced from (Ray and Beenackers, 1998) with permission] ......................................................... 52
Figure 2-16: Experimental setup of taylor vortex photocatalytic reactor:(1) motor, (2) speed controller, (3) gear coupling, (4) UV lamp (5) sample collection point (6) lamp holder (7) outer cylinder and (8) catalyst-coated inner cylinder. [Reproduced from (Dutta and Ray, 2004) with the permission]............................... 52
xii
Figure 2-17: Scheme of a fluidized photocatalytic reactor. [Reproduced from (Vaisman et al., 2005) with permission] ................................................................... 53
Figure 2-18: Solar photocatalytic reactor: (a) parabolic trough reactor (PTR) (b) compound parabolic collector (CPC). [Reproduced from (Braham and Harris, 2009) with permission] ............................................................................................. 54
Figure 2-19. Typical reactor layout for an (a) inclined plate collector and (b) double skin sheet photoreactor. [Reproduced from (Braham and Harris, 2009) with permission] ................................................................................................................ 55
Figure 2-20: Typical reactor layout for (a) horizontal rotating disk reactor and (b) water bell reactor. [Reproduced from (Braham and Harris, 2009) with permission] ................................................................................................................ 56
Figure 2-21: Schematic for photon transport. .................................................................. 57
Figure 3-1: Reductive dechlorination of PCB 138 using LMB with TEA as the reducing agent; [PCB 138] = 6.6 mg L-1, [SDS] = [CTAB] = [TWEEN 80] = 3.2 g L-1, [MB] = 750 mg L-1, [TEA] = 68 g L-1, Io = 5.2×1016 photon s-1 . ................... 68
Figure 3-2: Reductive dechlorination of PCB 138 using LMB with NaBH4 as the reducing agent; [PCB 138] = 20 mg L-1, [SDS] = [CTAB] = [TWEEN 80] = 3.2 g L-1; [MB] = 600 mg L-1; [NaBH4] = 20 g L-1, Io = 3.0×1016 photon s-1
. ................ 69
Figure 3-4: Dechlorination of Aroclor1254 solubilized with TWEEN 80 in the presence of MB and TEA or NaBH4: [Aroclor1254] = 10 mg L-1, [MB] = 600 mg L-1, [TWEEN80] = 1.6 g L-1, [TEA] = 108 g L-1, [NaBH4] = 20 g L-1, Io = 3.0 ×1016 photon s-1. ........................................................................................................ 75
Figure 3-5: The product distribution (after six minutes irradiation) for dechlorination of PCB 138 solubilized by TWEEN 80 (1.6 g L-1) or CTAB (1.6 g L-1) in the presence of MB (600 mg L-1) and TEA (68 g L-1), Io = 3.0 ×1016 photon/s, P: peak area of each congener from GC, Po: the peak area of initial PCB 138. ........... 76
Figure 3-6: The product distribution (after six minutes irradiation) for dechlorination of PCB 138 solubilized by TWEEN 80 (1.6 g L-1) or CTAB (1.6 g L-1) in the presence of MB (600 mg L-1) and NaBH4 (10 g L-1); Io = 3.0 ×1016 photon s-1, P: peak area of each congener from GC, Po: the peak area of initial PCB 138. ........... 77
Figure 4-1: LED photoreactor and insert. ......................................................................... 82
Figure 4-2: Photocatalytic degradation of different pesticides with UV-LED photoreactor (Io = 8.55×1016 photon s-1, CTiO2=2.0 g L-1, Co=20 mg L-1): (a) loss of parent pesticides; and (b) loss of total organic carbon. ........................................ 87
Figure 4-3: Photocatalytic degradation of pesticides mixture with UV-LED photoreactor based on the loss of pesticides detected by HPLC (Io =8.55×1016
xiii
photon s-1, CTiO2=2 g L-1, Co=20 mg L-1): (a) mixture containing 4-CP and 2,4-DCP; (b) mixture containing 4-CP and 2,4-D; (c) mixture containing 2,4-DCP and 2,4-D. .................................................................................................................. 90
Figure 4-4: Photocatalytic degradation of 2,4-D with different TiO2 loadings and LED irradiation (Io =8.55×1016 photon s-1, Co=20 mg L-1). ..................................... 94
Figure 4-5: Photocatalytic degradation of 2,4-D with UV-LED photoreactor under different light conditions (Co=20 mg L-1, CTiO2=2 g L-1). ......................................... 96
Figure 4-6: Photocatalytic degradation of 2,4-D in the two photoreactors: Co=20 mg L-1, CTiO2=2 g L-1. (a): LED reactor, Io =8.55×1016 photon s-1; (b): Rayonet reactor, Io =8.25×1016 photon s-1. ............................................................................. 98
Figure 5-1: UV-LED array and photocatalyst plate. ....................................................... 103
Figure 5-2: Directivity of radiation (NICHIA, 2013). ................................................... 105
Figure 5-3: Cartesian and polar coordinates in radiation system. ................................... 107
Figure 5-4: Scheme of UV-LED radiation. .................................................................... 109
Figure 5-5: Geometry of sensor. ..................................................................................... 111
Figure 5-6: The radiation field with different ID: (a) ID=0.01 m, gap=0.025 m; (b) ID= 0.04 m, gap=0.025 m. ...................................................................................... 115
Figure 5-7: The effect of irradiated distance (ID) on Maximum Error........................... 116
Figure 5-8: Optimal combination of ID and gap. ........................................................... 116
Figure 5-9: Selection of light output of UV-LED. .......................................................... 117
Figure 6- 1: SEM image of anodized TiO2 nanostructrure. ............................................ 122
Figure 6-2: Scheme of an LED based photocatalytic reactor. ........................................ 123
Figure 6-3: Radiation field on a photocatalyst plate under different conditions; (a) DL-
P = 0.014 m, 4 by 4 LEDs panel; (b) DL-P = 0.034 m, 4 by 4 LEDs panel; (c) DL-P = 0.054 m, 4 by 4 LEDs panel. ............................................................................... 125
Figure 6-4: Photodegradation of MCPA, 2,4-D, 2,4-DCP and 4-CP in a UV-LED photoreactor: flow rate = 2.03 L min-1; DL-P = 0.54 cm; Ia=17.3 mW cm-2. .......... 128
Figure 6-5: Photodegradation of 2,4-D in a flow-through UV-LED photoreactor: flow rate = 2.03 L min-1; DL-P = 0.54 cm ; Ia=17.3 mW cm-2. ........................................ 129
Figure 6- 6: The effect of DL-P on 2,4-D degradation: flow rate =2.03 L min-1, Ia=17.3 mW cm-2. ................................................................................................................. 131
xiv
Figure 6-7: The effect of flow rate on degradation of 2,4-D: DL-P = 5.4 cm, Iaverage=17.3 mW cm-2. ............................................................................................ 132
Figure 6-8: The effect of light intensity on degradation of 2,4-D: DL-P = 5.4 cm, Flow rate=2.03 L min-1. ................................................................................................... 134
Figure 7-1: Scheme of a scale-up LED based photocatalytic reactor. ............................ 141
Figure A-A-1: Ultrasonic extraction efficiency of PCBs from 10 g soil under different experimental conditions. ......................................................................................... 180
Figure A-B-1: Ultraviolet water purification system. ..................................................... 181
Figure A-B-2: Degradation of biphenyl under different flow rate. ................................ 183
Figure A-B-3: The pseudo first order kinetics of biphenyl degradation under different flow rate. ................................................................................................................. 183
Figure A-C-1: UV-Vis absorption spectra of 40 mg/L of 2,4-D in water. ..................... 184
Figure A-C-2: UV-Vis absorption spectra of 40 mg/L of 2,4-DCP in water. ................ 184
Figure A-C-3: UV-Vis absorption spectra of 40 mg/L of 4-CP in water. ...................... 185
Figure A-C-4: UV-Vis absorption spectra of 40 mg/L of MCPA in water. ................... 185
Figure A-D-1: The emission spectrum and TiO2 band edge. ......................................... 187
xv
List of Symbols, Abbreviations and Nomenclature
Symbol Definition
A Cross surface area, m2
Ap Area of photocatalyst plate, m2
c Light speed in vacuum, m s-1
C Concentration of substrate, mole L-1
Co Initial concentration of parent compound, mg L-1
d Distance, m
Ds-p The distance between shielding glass and photocatalyst, cm
Dl-p The distance between LEDs and photocatalyst, cm
do Specific distance, m
E Energy of photon, J
g Distance between point (x, y) and point (xo, yo), m
Gv Incident light intensity, photon s-1 m-2
h Plank constant, 6.62*10-34 J s
I Light intensity, mw cm-2
Ia Average light intensity, mw cm-2
Imax Maximum of light intensity, mw cm-2
Imeasured Light intensity measured by UV meter, mw cm-2
Imin Minimum of light intensity, mw cm-2
Imodel Light intensity calculated by model, mw cm-2
Io Light intensity measured by actinometry, photon s-1
It Light output of an LED lamp, mw
Iλ Specific light intensity, photon s-1 m-2
k Apparent reaction rate constant, mol L-1 s-1
K First order rate coefficient, s-1
Ka The average of first order rate coefficient, s-1
Kad Adsorption coefficient, L mol-1
lg The thickness of glass plate, m
Me Max error
xvi
p(Ω-Ω') Phase function for scattering in RTE
Q Number of photons
qa Rate of photon absorption, photon s-1
qe Rate of photon emission, photon s-1
qin Rate of photon in-scattered, photon s-1
qout Rate of photon out-scattered, photon s-1
r Radius, m
R Radial distance, m
Re Radiation directivity function
Re' Modified radiation directivity function
rp Kinetic reaction rate, mole L-1 s-1
rs Radius of the sensor, m
s Surface area, m2
S Direction vector, m
t Time, s
T Transmittance
v Frequency, s-1
V Elementary control volume, m3
Wa Volumetric rate of photon absorption, photon s-1 m-3
We Volumetric rate of photon emission, photon s-1 m-3
Win Volumetric rate of photon in-scattered, photon s-1 m-3
Wout Volumetric rate of photon out-scattered, photon s-1 m-3
x x-coordinates
xo x-coordinates of LED position
y y-coordinates
yo y-coordinates of LED position
z z-coordinates
α Volumetric absorption coefficient, m-1
β Extinction coefficient, m-1
γ Constant
η Constant
xvii
θ View angle, radian
λ Wavelength
λmax Wavelength of maximum emission, nm
σ Volumetric scattering coefficient, m-1
τ Asymmetry factor
ψ Scattering angle, radian
Ω Solid angle, steradian
Abbreviations Definition
2,4-D 2,4-Dichlorophenoxyacetic Acid
2,4-DCP 2,4-Dichlorophenol
4-CP 4-Chlorophenol
AOPs Advanced Oxidation Processes
ARPs Advanced Reduction Processes
CCA Chromated Copper Arsenate
CMC Critical Micelle Concentration
CTAB Cetyltrimethylammonium Bromide
DDT Dichlorodiphenyltrichloroethane
ECD Electron Captured Detector
EQE External Quantum Efficiency
GC Gas Chromatography
HGMT Hollow Glass Microspheres Coated with Anatase TiO2
HPLC High Performance Liquid Chromatography
IARC International Agency for Research Cancer
ID Irradiated Distance
LED Light Emitting Diode
LMB Leuco-methylene Blue
LVREA Local Volumetric Rate of Energy Absorption
MB Methylene Blue
MC Monte Carlo
MCPA 2-methyl-4-chlorophenoxyacetic acid
xviii
PCB138 2,2',3,4,4',5'-Hexachlorobiphenyl
PCBs Polychlorinated biphenyls
PEPO Photon Energy per Order
PFP Pentafluorophenyl
PVC Polyvinyl Chloride
RTE Radiation Transport Equation
SDS Sodium Dodecyl Sulfate
SEM Scan Electron Microscopy
TEA Triethylamine
TOC Total Organic Carbon
TWEEN80 Polyoxyethylene (80) Sorbitanmonooleate
USEPA United States Environmental Protection Agency
UV Ultraviolet
UVA Ultraviolet, subtype A
UVB Ultraviolet, subtype B
UVC Ultraviolet, subtype C
UV-Vis Ultraviolet-visible
VUV Vacuum Ultraviolet
1
Chapter One: INTRODUCTION
1.1 Background
In the past several decades, increased population, industrialization and agricultural
activities have led to an increase in the level of water contamination of receiving water
bodies. Pesticides, a major category of pollutants causing water contamination, pose a
potential threat to human health and the environment. Using pesticides is almost a
necessary way to maintain and improve the food production for an ever increasing world
population. However, extensive use of pesticides has resulted in water pollution in
different ways such as runoffs, run-ins and leaching (Polyrakis, 2009). The primary focus
of this thesis is to study the photochemical treatment of pesticides in water and to design
an efficient light emitting diode (LED) based photocatalytic reactor. Polychlorinated
biphenyls (PCBs) form a secondary interest in this thesis.
Pesticides exposure can cause different acute and chronic effects on human health
(Younes and Galal-Gorchev, 2000). A large number of pesticides, such as mancozeb,
dithiocarbamate and organophosphorus compound, manifest their toxicity through
functional and biochemical action in the central and peripheral nervous system (Kimura
et al., 2005). Several chronic diseases have been linked to the long-term exposure to
pesticides. Examples include porphyria following exposure to hexachlorobenzene,
delayed neuropathy from exposure to organophosphates and chloracne due to long-term
exposure to chlorophenoxy acid derivatives and chrolophenols (Younes and Galal-
Gorchev, 2000). Besides, cancers of the soft tissue, lung, gonads, liver, brain, the urinary
2
tract and the digestive system have been associated with long-term exposure to some
pesticides, although the association is not firm (Younes and Galal-Gorchev, 2000).
Polychlorinated biphenyls (PCBs) are toxic contaminants, which are less soluble in
water, but can bind to sediments of aquatic systems or adsorb on suspended particulates
(Sullivan et al., 1983, Manchester-Neesvig et al., 1996, Bergen et al., 1998). Once they
are released to the environment, they are difficult to remediate. The occurrence of water
contamination by PCBs is due to desorption from sediments or leaching from landfills
and contaminated soil. PCBs have been demonstrated to cause a variety of adverse health
effects. Data on animal experiments have provided conclusive evidence that PCBs are
carcinogenic to animals and can cause a number of non carcinogenic health effects,
including effects on the immune system, nervous system, endocrine system, reproductive
system and others (USEPA, 2013a). The studies also support that PCBs can cause
potential carcinogenic and non-carcinogenic effects to human beings (USEPA, 2013a).
Long term exposure to PCBs can cause damages to heart, kidney, liver and central
nervous systems (Erickson, 1997).
To alleviate water pollution with these two categories of pollutants, a variety of
techniques has been developed: bio-treatment (Hussain et al., 2009, Portier et al., 1990,
Zhang et al., 2004, Natarajan et al., 1996), membrane separation (Bhattacharya, 2006,
Boussahel et al., 2000), activated carbon adsorption (Foo and Hameed, 2010, Sotelo et
al., 2002), coagulation followed by settling (Dempsey and O'Melia, 1984) and others.
3
Although these methods, to some extent, can reduce the water contamination, several
drawbacks limit them from wider applications. Bio-technologies may require specific
pollutant resistant microbes as well as appropriate environmental conditions such as pH,
nutrients and temperature. Physical separation can remove contaminants from water and
transfer them to other phases. Nevertheless, the disposal of the concentrate or sludge can
be a serious problem. Besides, regeneration of adsorbents and fouling of membranes limit
the application of these techniques. To detoxify and degrade these compounds,
technologies based on photochemical processes are considered as good choices
(Devipriya and Yesodharan, 2005, Ollis et al., 1991, Parsons, 2004, Izadifard et al.,
2010a, Achari et al., 2003, Chu et al., 2005). These methods are quite fast and mostly
lead to complete degradation of the contaminants.
1.2 Photochemical Treatment Processes
Most photochemical treatment processes are based on advanced oxidation processes
(AOPs), using the generated hydroxyl radicals, positive holes, oxygen species and other
strong oxidants to degrade the organic compounds. They have been widely used in
removing organic contaminants in water and wastewater such as disinfection by-
products, pesticides, endocrine disruptors and so on (Parsons, 2004). Besides, during
some photochemical processes, highly reactive reducing radicals, such as free electrons,
may be formed. These strong reducing agents can be used to degrade the oxidized
contaminants such as nitrate, perchlorate, dichlorophenols and perfluorooctanoic acid
(Vellanki et al., 2013). Such photochemical treatment techniques are called the advanced
reduction processes (ARPs).
4
In this research, the major interest is focused on the removal of aqueous contaminants
such as pesticides. The most commonly used pesticides such as 2,4-D, MCPA and
chlorophenols were selected as the studied compounds. TiO2 photocatalysis based on
AOPs was chosen for degrading these compounds, as it does not consume large amount
of chemicals and is able to use the longer wavelength domain of ultraviolet light, which is
a part of UV region in the solar spectrum received on the surface of earth. Application of
TiO2 photocatalysis can lead to usage of longer (less energy) light sources. To treat PCBs
in aqueous medium, photosensitization based on ARPs are used.
1.3 Light emitting diode (LED) in photocatalytic reactors
To apply TiO2 photocatalysis in water and wastewater treatment, an efficient
photocatalytic reactor need to be designed and fabricated. The rapid development of LED
technology has made it a promising light source in photochemical applications. This
mercury-free light source is able to provide monochromatic light, has a longer lifetime,
and efficient electricity to light conversion (Würtele et al., 2011). Furthermore, the small
size of LEDs does not limit the geometry of the reactor. All these advantages have made
LEDs favourable in photocatalytic reactor designs. The application of LEDs has been
reported in photochemical treatment of air and water by several researchers (Huang et al.,
2009, Shie et al., 2008, Chen et al., 2005, Wang and Ku, 2006, Ghosh et al., 2008, Ghosh
et al., 2009). In this research, ultraviolet-light emitting diodes (UV- LEDs) are selected
for reactor design and fabrication.
5
1.4 Research Objectives and Scopes
The goal of this research is twofold: (1) develop efficient photochemical technologies to
treat contaminants (e.g. pesticides) in aqueous medium and design an efficient LED
based photocatalytic reactor; (2) study photochemical treatment of PCBs in aqueous
medium. To achieve this goal, four objectives are defined:
Dechlorinate PCBs in aqueous surfactant solutions using photosensitized visible
light irradiation.
o Investigate the photodechlorination of PCBs using Leuco-methylene blue
as a photosensitizer and cool white lamps as a light source
o Determine the effect of the type and the concentration of surfactants on the
photodechlorination of PCBs.
o Optimize the PCBs dechlorination conditions.
Investigate the photocatalytic degradation of certain pesticides in a batch UV-
LED photoreactor.
o Design a batch UVA-LED based photoreactor.
o Investigate the photocatalytic degradation of pesticides mixtures.
o Study the effect of photocatalyst loading and light intensity on the
photocatalytic degradation rate.
o Compare the photocatalytic degradation of pesticides in the LED
photoreactor with the mercury lamps.
Develop a radiation field model for a UV-LED photocatalytic reactor and design a
homogenous radiation field.
o Determine the most efficient radiation field for a photocatalytic reactor.
6
o Develop and validate a mathematical radiation field model for LED
arrays.
o Develop a method for designing a homogenous radiation field generated
by LED arrays.
Design, fabricate and test an efficient UV-LED based photocatalytic reactor, as
well as optimize the reactor performance and provide useful information for the
scale-up of the reactor.
o Design a novel photocatalytic reactor using UV-LED and TiO2 nanotubes.
o Evaluate the performance of the photocatalytic reactor by studying the
degradation of pesticides.
o Optimize the photocatalytic reactor performance through the study of the
effect of different operational parameters on the photocatalytic
degradation rate of pesticides.
1.5 Thesis Overview
This thesis contains seven chapters as outlined here.
Chapter one provides the general background information, research scope and objectives,
and an outline of the dissertation.
Chapter two provides a review of principles of photochemistry, photocatalysis,
photochemical treatment of PCBs and pesticides, designs of photocatalytic reactors and
radiation field modelling.
7
Chapter three describes a study of dechlorination of PCBs in surfactant solution with
visible light irradiation using a photosensitizer-Leuco methylene blue (LMB). In this
chapter, the generation of LMB through two different ways are studied. The impact of
surfactant type and surfactant concentration on PCBs photodechlorination efficiency is
investigated.
Chapter four describes photocatalytic degradation of phenoxy herbicides and
chlorophenols with a UV-LED light source in a TiO2 slurry system. During this research,
a batch UV-LED photoreactor is fabricated. The impact of light intensity and TiO2
loading on photocatalytic degradation is investigated.
Chapter five describes the development of a radiation field model for a LED based
photocatalytic reactor and the design of a homogenous radiation field.
Chapter six describes the design, fabrication and optimization of a flow-through LED
based photocatalytic reactor. Parameters such as flow rate, light intensity, and
photocatalyst configuration are studied.
Chapter seven provides a summary of research results as well as recommendations for
future research.
This thesis is written in a paper format where chapter 3, 4, 5 and 6 comprise separate
papers. Chapter 3 and 4 have been published as "Yu, Linlong; Izadifard Maryam; Achari,
8
Gopal; Langford, Cooper H., 2013. Electron transfer sensitized photodechlorination of
surfactant solubilized PCB 138. Chemosphere, 90, 2347-2351." and "Yu, Linlong;
Achari, Gopal; Langford, Cooper H., 2013. LED-Based Photocatalytic Treatment of
Pesticides and Chlorophenols. Journal of Environmental Engineering, 139, 1146-1151.",
respectively. Chapter 5 has been submitted to Journal of Environmental Engineering and
Science.
9
Chapter Two: LITERATURE REVIEW
2.1 Principle of Photochemistry
2.1.1 Light and photon
Photochemistry is the science of light-induced chemical reactions. The modern theory of
quantum mechanics considers light beam as consisted a number of photons which possess
the property of both waves and particles (Turro, 1991). Each photon has energy related to
its wavelength (Plank's Equation, Equation [2-1]). The shorter the wavelength the higher
the energy it carries.
E = hν =hcλ
[2-1]
Where E is the radiant energy of the photon (J), h is Plank constant (6.62*10-34 J·s), ν is
the frequency of photon (s-1), λ is the wavelength of photon (m) and c is the velocity of
photon travelling in vacuum (m s-1).
The wavelength range generally utilized in photochemistry lies between 170 nm and 1000
nm (Figure 2-1) (Oppenlander, 2003), which is divided into five sub region: the vacuum-
UV or VUV (below 200 nm), UV-C (200-280 nm), UV-B (280-315 nm), UV-A (315-380
nm), VIS (380-850 nm) and infrared (800-1000nm) The subdivisions of the UV spectral
domain are related to physical, chemical, biological or biochemical effects showed in
Figure 2-2.
10
Figure 2-1: Classification of electromagnetic radiation in the wavelength range below 1200 nm. [Reproduced from (Oppenlander, 2003) with the permission].
Figure 2-2: Phenomenological subdivision of ultraviolet radiation into four sub-bands and their characteristic effects. [Reproduced from (Oppenlander, 2003) with the permission].
Photoionization
M→M++e
0 200 400 600 800 1000
Wavelength (nm)
IR
Visible light UVA
UVB
UVC
VUV X-ray
γ-ray
Vibrational excitation
M→Mvib
Photoexcitation
M→M*
400 100 150 200 250 300 350
Wavelength (nm)
UVA (315-380 nm)
UVB (280-315 nm)
UVC(200-280 nm)
VUV (100-200nm)
Absorbed by Organic Chromophores
Absorbed by all substances including H2O,O2,CO2
Absorbed by all Proteins, DNA, RNA,O2
Sunburn Skin Cancer
Sun Tanning
11
2.1.2 The electronic excited states
In photochemistry, only the absorbed photon can cause a photochemical reaction, and
each photon is absorbed by a single molecule to initiate the reaction (Turro, 1991).
Absorption in the wavelength region of photochemical interest promotes the absorber
from its ground state to its excited state. Absorption at longer wavelengths (infra-red)
usually leads to the excitation of vibrations or rotations of a molecule in its ground state;
generally, only electronically excited states are involved in photochemical processes
(Wayne, 1988). The fates of excited species 'A*' are shown as below (Wayne, 1988) :
The excited species 'A*' can lose its energy by emitting a photon, which gives rise
to the phenomenon of luminescence.
The excess energy of 'A*' can also be relieved by an atom or molecule 'M' in the
form of physical quenching. Normally, in this process, the excess energy of 'A*' is
converted to translational or vibrational excitation of 'M*' at lower energy.
The excited species A* can transfer energy to other molecules to generate other
excited species, which can then participate in any of the processes including
relaxation to the ground state (radiationless decay);
The excited species A* may undergo dissociation, direct reaction, ionization or
spontaneous isomerization.
2.1.3 Quantum yield
The absorption of photons can cause other processes rather than the desired reaction. To
determine the efficiency of the photochemical reaction, the concept of quantum yield was
12
developed. Four commonly used definitions of quantum yield are shown in Table 2-1.
The quantum yield is a unitless constant, usually ranging from zero to one; the value of
quantum yield larger than one indicate a photo-induced chain reaction involving radicals
species or photo-generated catalysis (Oppenlander, 2003).
Table 2-1: Definitions of the quantum yield (Oppenlander, 2003).
Mathematical
expression
Definition
𝜙𝜆 =dn(event)/dt
Φpabs
Universally valid: Number n of events per unit time divided by the
number of photons absorbed during this period
𝜙𝜆 =dn(M)/dt
Φpabs
Number n of reactant molecules M consumed per unit time divided
by the number of photons absorbed during this period
𝜙𝜆 =dn(P′)/dt
Φpabs
Number n of photoproduct molecules P' formed per unit time
divided by the number of photons absorbed during this period
𝜙λ1−λ2
=dn(P′)/dt
Φpabs(λ1−λ2) ≠ 𝜙𝜆
Ratio of the number m of photoproduct molecules formed per unit
time to the total number of photons absorbed in the spectral region
λ1- λ2 during this period
note: Φpabs and Φp
abs(λ1−λ2)are the absorption rates of photons.
2.1.4 Direct photolysis
Direct photolysis involves the transformation of a chemical resulting from the direct
absorption of a photon. Absorption of photons with high energy can promote the
contaminants (e.g.2-chloro-N-methylacetanilide ) to their excited singlet states from
13
electronic ground state. This excited state can then undergo, among other processes; (i)
homolysis (ii) heterolysis or (iii) photoionization (Burrows et al., 2002). Most organic
compound show absorption bands at relatively short UV wavelengths capable of
producing direct photolytic degradation of these compounds.
2.1.5 Photosensitized degradation
Photosensitization is the process of initiating a reaction through the use of a
photosensitizer capable of absorbing light and transferring the energy or exchanging
electrons with the reactants (Burrows et al., 2002). The major advantage of
photosensitized photodegradation is its possibility to use light of wavelengths longer than
those corresponding to the absorption characteristics of the pollutants.
2.1.6 Photocatalysis
Photocatalysis is a chemical reaction induced by absorption light by a photocatalyst
(Ohtani, 2008a). With solid photocatalyst, the reaction is activated by absorption of a
photon with sufficient energy, i.e. equal or higher than the band-gap energy of the
photocatalysts (Fox and Dulay, 1993, Herrmann, 2005, Hoffmann et al., 1995). The
band-gap energy is the energy difference between the bottom of conduction band (lowest
unoccupied molecular orbital) and the top of the valance band (highest occupied
molecular orbital) related to the electronic structures of semiconducting materials.
Various semiconductors such as TiO2, CdO, ZnO, WO3, CdS, CdSe, GaP, GaAs, ZnS,
SnO2, Fe2O3, SrTiO3, BaTiO3 etc, have been used as photocatalysts. Generally, the best
photocatalytic performances are obtained with titanium dioxide as catalyst (Herrmann,
14
2005). The details of TiO2 photocatalysis fundamental and mechanism will be described
in the section 2.2.
2.1.7 Advanced Oxidation Processes (AOPs)
AOPs are processes designed to degrade recalcitrant organic compounds using chemical
oxidants. Most organic contaminants can be completely mineralized or partially
mineralized to innocuous compounds using appropriate AOPs. Currently, the major light
induced AOPs include UV&H2O2, Ozone&UV, vacuum UV, TiO2 photocatalysis, and
others. Besides, there are several non-light induced AOPs such as H2O2, Fenton’s reagent
and ozonation. Although different AOPs make use of different reaction systems (Table 2-
2), the chemistry of these reaction systems are similar: generation of highly reactive
oxidative species, such as hydroxyl radicals (OH•), positive holes and singlet oxygen
(Andreozzi et al., 1999). The oxidation potential of hydroxyl radicals are greater than that
of most conventional oxidants such as chlorine, oxygen, ozone, etc. (Parsons, 2004).
2.2 TiO2 Photocatalysis
2.2.1 TiO2 as a photocatalyst
TiO2 is considered to be the most successful photocatalyst as it has several advantages
such as: (1) photo active (2) low toxicity (3) biologically and chemically stable (4) able to
utilize near UV light and (5) economic (Bhatkhande et al., 2002, Linsebigler et al., 1995,
Hoffmann et al., 1995). Titanium dioxide naturally exists in three crystal forms: anatase,
rutile and brookite. Brookite is extremely difficult to synthesize, while anatase and rutile
15
Table 2-2: Generation of hydroxyl radicals for different AOPs.
Type of AOPs Spectral domain Reactions
Vacuum UV VUV 𝐻2𝑂ℎ𝑣��𝐻𝑂 ∙ +𝐻 ∙
UV/H2O2 UVC 𝐻2𝑂2ℎ𝑣��𝐻𝑂 ∙ +𝐻𝑂 ∙
TiO2/UV UVA-UVC
𝑇𝑖𝑂2ℎ𝑣�� 𝑒− + ℎ+
ℎ+ + 𝐻2𝑂 → 𝐻𝑂 ∙ +𝐻+
ℎ+ + 𝑂𝐻− → 𝐻𝑂 ∙
O3/UV UVC
𝑂3ℎ𝑣��𝑂(𝐷) + 𝑂2
𝑂(𝐷) + 𝐻2𝑂 → 𝐻2𝑂2
𝐻2𝑂2ℎ𝑣��𝐻𝑂 ∙ +𝐻𝑂 ∙
Ozonation No UV
𝐻𝑂− + 𝑂3 → 𝑂2 + 𝐻𝑂2−
𝐻𝑂2− + 𝑂3 ⇄ 𝐻𝑂2 ∙ +𝑂3 ∙−
𝐻𝑂2 ∙⇄ 𝐻+ + 𝑂2 ∙−
𝑂2 ∙−+ 𝑂3 → 𝑂2 + 𝑂3 ∙−
𝑂3 ∙−+ 𝐻+ → 𝐻𝑂3 ∙
𝐻𝑂3 ∙→ 𝐻𝑂 ∙ +𝑂2
𝐻𝑂 ∙ +𝑂3 → 𝐻𝑂2 ∙ +𝑂2
Fenton process No UV
𝐹𝑒2+ + 𝐻2𝑂2 ⇄ 𝐹𝑒𝑂2+ + 𝐻2𝑂
𝐹𝑒2+ + 𝐻2𝑂2 → 𝐹𝑒3+ + 𝑂𝐻− + 𝑂𝐻 ∙
𝐹𝑒3+ + 𝐻2𝑂2 ⇄ 𝐹𝑒𝑂𝑂𝐻2+ + 𝐻+
𝐹𝑒𝑂𝑂𝐻2+ ⟶ 𝐻𝑂2 ∙ +𝐹𝑒2+
16
can be produced easily in the laboratory (Bickley et al., 1991). Among these three crystal
forms, anatase and rutile are the two most commonly used types and have been employed
most in the photocatalytic study. The band-gap energy are, respectively, 3.0 eV, 3.2 eV,
for rutile phase and anatase phase, and its amorphous form is reported to have the band-
gap energy varying from 3.2 to 3.5 eV (Roy et al., 2011). Even though the most active
form of titanium dioxide is believed to be anatase, a mixed phase of anatase and rutile
appears to achieve better photocatalytic efficiency (Bickley et al., 1991). The co-presence
of anatase and rutile phase introduce mesoporosity and a wider pore size distribution,
which may be responsible for the high level of photocatalytic activity (Thiruvenkatachari
et al., 2008). Hurum et al. (2003) proposed three possible reasons for the greater
photocatalytic activity of TiO2 mixed phase: (1) the band-gap of rutile is smaller than that
of anatase and extends the useful wavelength range of photoactivity; (2) the transfer of
photoexcited electrons between rutile/anatase phase enhance the charge separation and
slows down electron-hole recombination; (3) the small size of the rutile crystallites
enhance the photocatalyst activity.
Degussa (Evonik) P25, Aeroxide TiO2 P25, via the chloride technology method is
currently the de-facto commercial reference TiO2 photocatalyst (Alonso-Tellez et al.,
2012). It is widely used in potocatalytic reaction systems because of its high
photocatalytic activity, and has been reported in more than one thousand papers since
1900 (Ohtani et al., 2010). P25 has a large surface area (50 m2 g-1) (Zertal et al., 2004)
and small crystal size (20 nm). Theoretically, a photocatalyst with larger surface area and
smaller particle size can provide more active sites for illumination and adsorption of the
17
reactants, leading to a higher expected photocatalytic activity. The composition of P25 is
reported to be 70% anatase and 30% rutile or 80% anatase and 20% of rutile, however,
the exact crystalline composition seems to be unknown, presumably due to a lack of
determination techniques for crystalline contents in nano-sized particle samples (Ohtani
et al., 2010, Ohtani, 2008b). Except Degussa P25, other commercial TiO2, such as
products from Millennium and Hombikat also show their high photocatalytic activities
(Zertal et al., 2004, Alonso-Tellez et al., 2012) .
2.2.2 Mechanism of TiO2 photocatalysis
The fundamentals and mechanism of TiO2 photocatalysis have been intensively reported
in many literatures (Fujishima et al., 2000, Gaya and Abdullah, 2008, Fox and Dulay,
1993, Herrmann, 1999). The overall process of TiO2 photocatalysis can be broken into
five independent steps (Herrmann, 2005, Mozia, 2010) :
Transfer of the reactants in the bulk solution to the TiO2 surface;
Adsorption of the reactants on the surface of TiO2;
Reaction in the adsorbed phase;
Desorption of the products;
Removal of by-products from the interface region.
The third step includes all the photochemical processes (Herrmann, 2005) and is
summarized in Equations [2-2]~[2-14] (Mozia, 2010) and Figure 2-3. The initial step of
photon-induced reaction is the excitation of TiO2 by absorbing photon with formation of
electron-hole pair. Once TiO2 absorbs photons with sufficient energy, i.e. equal or larger
than its band-gap energy, electrons are promoted from the valence band to the conduction
18
band, while positive holes (h+) are left in the valence band (Equation [2-2]). The electron
and the hole can migrate to the catalyst surface and participate in the redox reactions
(Equations [2-4] ~ [2-14]) with different species adsorbed on the catalyst surface. A
recombination of the electron and hole will occur if no suitable electron and hole
scavengers are present (Equation [2-3]). If oxygen is present in the water (e.g. water
open to air), it will capture the electron in the conduction band to form the superoxide
radical ion while the remaining hole can react with surface-bond H2O molecule or
hydroxide ion to produce hydroxyl radicals. Hydroxyl radicals can also be generated
following the pathways through reactions shown in Equations [2-7] ~ [2-11]. The
hydroxyl radicals generated on the surface of illuminated TiO2 are supposed to be the
primary oxidizing species in the photocatalytic oxidation processes, which are highly
reactive and can degrade most organic compound and eventually convert them to CO2,
H2O and other inorganic compounds.
𝑇𝑖𝑂2ℎ𝑣�� 𝑇𝑖𝑂2 (𝑒− + ℎ+) [2-2]
𝑒− + ℎ+ → heat [2-3]
ℎ+ + 𝐻2𝑂 → 𝑂𝐻 ∙ +𝐻+ [2-4]
ℎ+ + 𝑂𝐻− → 𝑂𝐻 ∙ [2-5]
𝑒− + 𝑂2 → 𝑂2− ∙ [2-6]
𝑂2−. +𝐻+ → 𝐻𝑂2 ∙ [2-7]
𝐻𝑂2 ∙ +𝐻𝑂2 ∙→ 𝐻2𝑂2 + 𝑂2 [2-8]
𝑒− + 𝐻2𝑂2 → 𝑂𝐻 ∙ +𝑂𝐻− [2-9]
19
𝑂2− ∙ +𝐻2𝑂2 → 𝑂𝐻 ∙ +𝑂𝐻− + 𝑂2 [2-10]
𝐻2𝑂2ℎ𝑣�� 2𝑂𝐻 ∙ [2-11]
𝑂𝑟𝑔𝑎𝑛𝑖𝑐 𝑐𝑜𝑚𝑝𝑜𝑢𝑛𝑑 + 𝑂𝐻 ∙→ 𝑑𝑒𝑔𝑟𝑎𝑑𝑎𝑡𝑖𝑜𝑛 𝑝𝑟𝑜𝑑𝑢𝑐𝑡𝑠 [2-12]
𝑂𝑟𝑔𝑎𝑛𝑖𝑐 𝑐𝑜𝑚𝑝𝑜𝑢𝑛𝑑 + ℎ+ → 𝑜𝑥𝑖𝑑𝑎𝑡𝑖𝑜𝑛 𝑝𝑟𝑜𝑑𝑢𝑐𝑡𝑠 [2-13]
𝑂𝑟𝑔𝑎𝑛𝑖𝑐 𝑐𝑜𝑚𝑝𝑜𝑢𝑛𝑑 + 𝑒− → 𝑟𝑒𝑑𝑢𝑐𝑡𝑖𝑜 𝑝𝑟𝑜𝑑𝑢𝑐𝑡𝑠 [2-14]
Figure 2-3: Photochemical activation of TiO2.
20
2.2.3 The kinetics of photocatalytic degradation
The Langmuir-Hinshelwood (LH) model proves to be the best for simulation of the
kinetic rate of initial photocatalytic degradation (Kumar et al., 2008, Matthews, 1988,
Mills and Morris, 1993). In the LH model, the rate of photocatalytic reaction is controlled
by the reaction of the adsorbed molecules. Firstly, the substrate adsorbs on the surface of
the photocatalyst and then undergoes photocatalytic degradation. This model is based on
several assumptions (Fox and Dulay, 1993):
Adsorption by the substrate is identical for each site and is independent of surface
coverage;
At equilibrium, the number of surface adsorption sites is fixed;
Each surface site is only combined with one substrate molecule;
The adjacent adsorbed molecules do not react with each other;
The rate of adsorption is greater than other chemical reactions;
No irreversible blocking of active sites by binding to product occurs.
Ollis (2005) has shown that the model can fit the data well even though the adsorption
process is not at equilibrium as is shown by the dependence of the adsorption coefficient
dependence on light intensity.
The Langmuir-Hinsheldwood Kinetic Expression for the photocatalytic degradation were
shown in Equation [2-15] (Fox and Dulay, 1993).
rp =𝑑𝑐𝑑𝑡
=𝐾𝑎𝑑𝑘𝐶
1 + 𝐾𝑎𝑑𝐶 [2-15]
21
Where rp is the reaction rate (mol L-1 s-1), C is the concentration of substrate (mole L-1),
Kad is adsorption coefficient for substrate (L mol-1), k is the apparent kinetics rate
constant occurring at the active site on the photocatalyst surface (mol L-1 s-1)
When the initial concentration contaminants is very high, 𝐾𝑎𝑑𝐶 ≫ 1, consequently,
Equation [2-15] can be simplified as zero order reaction kinetics:
𝑑𝑐𝑑𝑡
= k [2-16]
At highly diluted concentration, 𝐾𝑎𝑑𝐶 ≪ 1, the photocatalytic degradation become first
order reaction (Equation [2-17]) (Mozia, 2010, Herrmann, 2005).
𝑑𝑐𝑑𝑡
= 𝐾𝑎𝑑𝑘𝐶 [2-17]
2.2.4 Factors affecting the photocatalytic degradation kinetics
2.2.4.1 TiO2 loading
The effect of TiO2 loading on photocatalytic degradation of different contaminants in
aqueous solution has been widely investigated (Chen and Liu, 2007, Singh et al., 2007,
Kaneco et al., 2009, Liu et al., 2009, Wu et al., 2010, Muneer et al., 2005, Qamar et al.,
2006, Pizarro et al., 2005, Garcia and Takashima, 2003). Generally, at a low
photocatalyst loading range, the photocatalytic reaction rates were observed to be
proportional to the catalyst loading. As the photocatalyst loading reaches an optimal
22
value, the photocatalytic reaction rate becomes independent of photocatalyst amount and
becomes constant. A further increase of photocatalyst loading beyond the optimum can
even inhibit photocatalytic reaction. This phenomena is associated with the effect of
photocatalyst loading on active surface area of photocatalyst and the lack of light
penetration into solution (Ahmed et al., 2011, Mozia, 2010, Chen and Liu, 2007,
Adesina, 2004). An increase of TiO2 loading can enlarge the active surface area available
for reactant adsorption and photon absorption, hence a higher photocatalytic degradation
rate was expected. On the other hand, high loading of photocatalyst can cause light
scattering and screening effects, which impede the penetration of light into the solution
far from radiation source (Chen and Liu, 2007, Lea and Adesina, 1998, Singh et al., 2007,
Rahman and Muneer, 2005). Moreover, the agglomeration of photocatalyst at high solid
loading can result in a loss of active surface area (Chen and Liu, 2007, Lea and Adesina,
1998). The trade-off between these two opposite effects leads to an optimal photocatalyst
loading for the photocatalytic reaction.
2.2.4.2 Light intensity
Light intensity is another key parameter in the TiO2 photocatalysis. A power law
relationship (Equation [2-18] ) between the photocatalytic reaction rate (k) and light
intensity (I) was observed in a number of experimental studies (Wang et al., 2012, Choi
et al., 2000, Kim and Hong, 2002, Obee and Brown, 1995, Ohko et al., 1997). The
exponent (α) varies from one to zero as the light intensity increases.
23
𝐾 ∝ 𝐼𝛼 [2-18]
The effect of light intensity on the kinetics of the photocatalytic degradation process due
to the competition between electron-hole generation and electron-hole recombination
were summarized by Ollis et al.(1991) as follows:
At low light intensity range, electron-hole formation dominates and the apparent
photocatatytic reaction rate is proportional to the light intensity;
At intermediated light intensities, electron-hole pair generation competes with the
recombination of electron-hole and the photocatalytic reaction rate is linear to the
square root of light intensity;
At high light intensities, the photocatalyst loading become a limiting factor,
consequently, the increased light intensity does not improve the photocatalyic
reaction rate.
2.2.4.3 pH
The effect of pH on photocatalytic process is complicated (Fox and Dulay, 1993,
Konstantinou and Albanis, 2004, Mozia, 2010, Akpan and Hameed, 2009). Firstly, the
surface charge of TiO2 and ionization state of some contaminants is strongly influenced
by pH, which thus impacts the adsorption behaviour of contaminants. The surface charge
of TiO2 at different pH is determined by the following reactions.
𝑇𝑖𝑂𝐻 + 𝐻+ ⇔ 𝑇𝑖𝑂𝐻2+ [2-19]
24
𝑇𝑖𝑂𝐻 + 𝑂𝐻− ⇔ 𝑇𝑖𝑂− + 𝐻2𝑂 [2-20]
The isoelectric point of commercial available TiO2 (Degussa P25) is observed at pH=6.8
(Pelton et al., 2006, Mozia, 2010). When pH is lower than 6.8, the surface of P25 is
positively charged and the adsorption of negatively charged contaminants is favoured,
while at pH>6.8, the negatively charged P25 more likely attract the positively charged
contaminants. Secondly, at low pH condition, the TiO2 particles tend to agglomerate, as
a result, the available surface area for reactants adsorption and photon illumination is
reduced, which finally limit the photocatalytic reaction. Thirdly, the reaction between
hydroxide ions and positive holes can generate hydroxyl radicals. At low pH, the positive
holes are expected to be the major oxidation species whereas at high pH levels, the
predominant oxidation species are considered to be hydroxyl radicals. In alkaline
solution, the generation of hydroxyl radicals are easier through oxidizing more hydroxide
ions available on TiO2 surface, thus the efficiency of the process is logically enhanced
(Gonçalves et al., 1999, Shourong et al., 1997). Optimal pH for photocatalytic studies at
both low pH or at high pH have been observed. Higher photocatalytic degradation
efficiency for chlorophenols (Augugliaro et al., 1988), glyphosate (Muneer and Boxall,
2008) were observed at higher pH. However, some other contaminants like 2,4-D (Trillas
et al., 1995) and anionic dyes (Sakthivel et al., 2003) favor an acidic condition.
2.2.4.4 Electron acceptor
In the application of TiO2 photocatalysis, electron-hole recombination is the major step
of energy waste. Without suitable electron acceptors, the recombination step is
25
predominant and limits the photocatalytic reaction. The presence of electron acceptors in
solution can accelerate the photocatalytic degradation rate by (1) preventing the electron-
hole recombination (2) increasing the concentration of reactive oxygen species and
oxidation rate of intermediate compound. (Muruganandham and Swaminathan, 2006,
Singh et al., 2007, Ahmed et al., 2011). Usually, the dissolved oxygen in solution is used
as electron acceptor and promotes the photocatalytic reaction. Besides oxygen, other
oxidants such as H2O2, K2S2O8 KBrO3 can also act as electron acceptors. The effects of
these electron acceptors on photocatalytic degradation of pesticides has been investigated
by several researchers (Chen and Liu, 2007, Bahnemann et al., 2007, Singh et al., 2003,
Singh and Muneer, 2004, Rahman et al., 2006, Wei et al., 2009). All results indicate a
higher photocatalytic degradation rate when additional electron acceptors were
introduced.
Chen and Liu (2007) reported that adding a small amount of H2O2 (up to 0.1 mM) can
improve the efficiency of photocatalytic degradation of glyphosate. However, at high
concentration of H2O2 (larger than 0.1mM) the photocatalytic degradation of glyphosate
is retarded. Similar effects were also found in photocatalytic degradation of other
contaminants such as azo dyes (So et al., 2002), dicamba (Chu and Wong, 2004)
monochlorbenzene (Tseng et al., 2012) and triclosan (Yu et al., 2006). Hydrogen
peroxide is considered to be a better electron acceptor than oxygen (Equation [2-21]).
Moreover, under UV irradiation, hydrogen peroxide can also undergo direct photolysis
and generate hydroxyl radicals ( Equation [2-22]) (Ahmed et al., 2011).
26
𝐻2𝑂2 + 𝑒𝐶𝐵− → 𝑂𝐻 ∙ +𝑂𝐻− [2-21]
𝐻2𝑂2 + ℎ𝑣 → 2 𝑂𝐻 ∙ [2-22]
However, excess hydrogen peroxide can scavenge the generated hydroxyl radicals
(Equations [2-23]~[2-24]) and retard the photocatalytic degradation. Besides, the high
concentration of hydrogen peroxide can absorb and attenuate the UV light available for
TiO2 excitation (Muruganandham and Swaminathan, 2006, Chu and Wong, 2004).
𝐻2𝑂2 + 𝑂𝐻 ∙→ H2O + HO2 ∙ [2-23]
HO2 ∙ +𝑂𝐻 ∙→ H2O + O2 [2-24]
2.2.4.5 Hole/hydroxyl radicals scavenger
The photocatalytic degradation of pesticides occurs through reactions with the generated
holes or surface hydroxyl radicals. Some inorganic anions present in solution such as Cl-,
NO3- SO4
2- , CO32- and HCO3
- can act as hydroxyl radicals scavenger and inhibit the
photocatalytic oxidation (Konstantinou and Albanis, 2004, Wu et al., 2009, Chen et al.,
1997). Although the hydroxyl radical scavengers can react with hydroxyl radicals/holes
to form corresponding radicals, the reactivity of these radicals is lower than that of
hydroxyl radical or holes. Therefore, a decrease of photocatalytic degradation efficiency
in the presence of inorganic ions is usually observed. Wu et al. (2009) reported that the
presence of 0.05 mM Cl- and NO3- significantly inhibit photocatalytic degradation of
terbufos. However, the same phenomenon was not observed in some photocatalytic
studies. D'Oliveira et al. (1993) found that the presence of 0.1M inorganic anions (Cl-,
27
SO42-, and NO3
-) did not change the initial rate of photocatalytic degradation of 3-
chlorophenol. Mehrvar et al. (2001) studied the effect of hydroxyl radical scavengers on
photocatalytic degradation of 1,4-dioxane and tetrahydrofuran and found that bicarbonate
and carbonate ions slowed down the 1,4-dioxane degradation rate but did not
significantly affect the tetrahydrofuran degradation rate.
2.3 Contaminants (Pesticides and PCBs)
2.3.1 Pesticides
Pesticides are defined as any substance or mixture of substances intended to prevent,
destroy, repel, mitigate or attack any pest such as insects, weeds, microorganisms, fungi,
and others (USEPA, 2011). Pesticides are broadly classified into two groups: chemical
pesticides and bio-pesticides. Most conventional pesticides in large scale use are
chemically based. Pesticides are further classified as herbicides, fungicides, insecticides,
molluscicides, nematicides, plant growth regulators, pheromones, acaricides, repellents
and rodenticides (Tadeo, 2008). The active portion of a chemical pesticide is known as
the active ingredient (Kamrin, 2000).
Pesticides are very important in increasing food production and controlling weeds
(Polyrakis, 2009). Since 1950, pesticide usage has grown 50-fold to about 2.5 million
tons per year (Tadeo, 2008). The pesticides sold in Canada add up to more than 40
million kilograms, which represents approximately 3% of pesticide sale in the world. In
Canada, pesticides are regulated by the Pest Management Regulatory Agency (PMRA)
28
under the pest control products act. Currently, more than 7000 pesticide products are
registered for use in Canada (Brimble et al., 2005). Table 2-3 lists the top 20 pesticides
used in Canada.
Table 2-3: Sales/use of the top 20 pesticide active ingredient in Canada (Brimble et al., 2005).
RANK Active ingredient Type Amount (×106 kg)
1 Glyphosate H 4.608
2 Creosote AM 2.163
3 MCPA H 1.540
4 2,4-D H 1.490
5 CCA AM 0.824
6 Triallate H 0.707
7 Mancozeb F 0.655
8 Ethalfluralin H 0.598
9 Atrazine H 0.553
10 Brommoxynil H 0.544
11 Surfactant bend A 0.505
12 Mineral oil A/G/H/I 0.394
13 Petroleum Hydrocarbon Blend A 0.375
14 Trifluralin H 0.356
15 S-metolachlor H 0.293
16 Chlorothalonil F 0.265
17 Metolachlor H 0.261
18 Chlorpyrifos I 0.252
19 Mecoprop H 0.252
20 1,3-dichloroproprene I 0.248
A: adjuvant AM: anti-microbial F: fungicide G: growth regulator H: herbicide I:
insecticide
29
There is evidence that extensive usage of pesticides has an effect on water quality and is
associated with various environmental and human health problems. Persistent pesticides,
such as DDT and lindane, showing highly toxic effects and cause severe damage to
ecological system are forbidden or strictly controlled; consequently, the impact of these
pesticides become less and less. Currently, most of pesticides frequently used in Canada
are non-persistent and are considered less harmful to the environment and humans.
Nonetheless, from a long term perspective they may cause chronic effects which are
difficult to characterize with current technology. The pesticides studied in this thesis are
two commonly used phenoxy herbicides and chlorophenols.
2.3.1.1 2,4-dichlorophenoxyacetic acid (2,4-D)
2,4-D (Figure 2-4) is a systemic phenoxy herbicide, capable of controlling many types of
broadleaf weeds, e.g. dandelion (Humburg, 1989). It has been widely applied in forest
management, cultivated agriculture, pasture rangeland, and lawns and to control aquatic
vegetation. The name brands for 2,4-D related herbicides include Aqua-Kleen, Barrage,
Malerbane, Planotox, Lawn-keep, Salvo, Weedone , among others (Kamrin, 2000).
Figure 2-4: Molecular structure of 2,4-dichlorophenoxyacetic acid.
30
2,4-D is considered to be potentially harmful to both animals and humans and can cause
toxic effect on aquatic wildlife and aquatic ecosystem. The toxicity of 2,4-D in animals
has been studied extensively (USEPA, 2005): the acute oral LD50 varies from 638 mg/kg-
1250 mg/kg in rat; the acute dermal LD50 is higher than 2000 mg/kg in rabbits; the acute
inhalation LD50 is higher than 1.179mg/L in rats. Experiments on rats show that high
doses of 2,4-D may result in fetuses with abdominal cavity bleeding and increased
mortality (Kamrin, 2000). Long term exposure to 2,4-D may result in an increase of the
probability of malignant tumours (Kamrin, 2000). For humans, a high level of 2,4-D can
result in coughing, dizziness, burning and temporary loss of muscle coordination (Laws
and Hayes, 1991). Hardell (1981) suggested that 2,4-D is associated with Hodgkin’s
disease, non-Hodgkin’s lymphoma, and soft tissue sarcoma. Nevertheless, no evidence
from epidemiologic studies show that the exposure to 2.4-D can cause cancers (Garabrant
and Philbert, 2002). It is classified as Group D chemical (USEPA, 2005), one that is not
classifiable as to human carcinogenicity.
2.3.1.2 2-methyl-4-chlorophenoxyacetic acid (MCPA)
MCPA (Figure 2-5 ) is also a systemic post-emergence phenoxy herbicide used to control
a wide spectrum of broadleaf weeds (Kamrin, 2000). In Canada, it is registered for use on
agricultural sites, on fine turf (parks, golf courses, zoos, botanical gardens, athletic
playing fields and play ground) and lawns (residences public and commercial buildings)
and sod (grown in sod farms harvested for transplanting), in forestry (spruce seedlings for
reforestation) and at industrial sites (vegetation control) (Health Canada, 2010 ). There
31
are four MCPA related active ingredients: MCPA acid, MCPA dimethylamine salt,
MCPA sodium salt, MCPA and MCPA 2-ethylhexyl ester. Trade names for MCPA or
MCPA related product include Agroxone, Class MCPA, Agritox, Agroxone, Agronzone,
Class MCPA, Dakatota, Envoy, Gordon's Amine and among others (Kamrin, 2000).
Figure 2-5: Molecular structure of 2-methyl-4-chlorophenoxyacetic acid.
The toxicological experiments on rats and rabbits show that MCPA is a slightly toxic
compound (Kennepohl et al., 2010, Kamrin, 2000, USEPA, 2004). Symptoms in humans
due to very high acute exposure include twitching, drooling, low blood pressure, slurred
speech, jerking and spasms and unconsciousness (Kennepohl et al., 2010). Long-term
exposure to MCPA can result in reduced feeding rates and retarded growth rates in rates
(World Health Organization, 2004). MCPA has a moderate to low toxicity to birds, with
reported LC50 value of 377 mg/kg in bobwhite quail; and it is slightly toxic to freshwater
fish, with reported LC50 values ranging from 117 to 232 mg/L in rainbow trout (Kamrin,
2000, World Health Organization, 2004). All of the available evidence indicates that
MCPA does not cause cancer (Kennepohl et al., 2010).
32
2.3.1.3 Chlorophenols
Chlorinated phenols are a group of compounds consisting of phenol with substituted
chlorines (Figure 2-6). There are 19 chlorophenol congeners including three
monochlorophenols, six dichlorophenols, six trichlorophenols, three tetrachlorophenols
and one pentachlorophenol (Exon, 1984). Most of purified chlorinated phenols are
colorless crystalline solids; with an exception that 2-chlorophenol is a clear liquid.
(Health Canada, 2008). They have an unpleasant odor, which is medicinal, pungent,
phenolic, strong, or persistent (USEPA, 1980) .
Figure 2-6: Molecular structure of chlorophenols.
Chlorophenols can be formed by direct chlorination or the hydrolysis of the higher
chlorinated derivatives of benzene (USEPA, 1980). They can also be formed through
chlorination of water containing natural phenol or phenolic wastes. They have been
widely used in the production of dyes, pigments, phenolic resins, pesticides (USEPA,
1980). Certain chlorophenols are also used directly as pesticides such as fungicides, flea
repellents, wood preservatives, and so on. In Canada, chlorophenols are no longer in
production. However, they are continued to be imported. There are 110 chlorophenols
33
related pesticide products registered for used in Canada under the Pest Control Products
Act (Health Canada, 2008) .
The toxic effects of chlorophenols are related to the degree of chlorination. Generally
chlorophenols with higher degree of chlorination are more toxic. Acute exposure to lesser
chlorinated phenols in humans results in muscular twitching, tremors, spasms, ataxia,
weakness, convulsions and collapse (Health Canada, 2008). Acute poisoning by
pentachlorophenol can cause general weakness, anorexia, sweating, nausea, fatigue,
ataxia, headache, hyperpyrexia, vomiting, tachycardia, abdominal pain, terminal spasms
and death (Health Canada, 2008). Soft-tissue sarcomas, Hodgkin's disease and leukaemia
have been reported in epidemiological studies of occupational groups exposed to
chlorinated phenols and phenoxy acids. IARC (1987) identified chlorophenols as possible
humans carcinogens (Group 2B compound).
2.3.2 PCBs
PCBs are a class of nonpolar components which consist of 1 to 10 chlorine atoms on a
biphenyl ring (Figure 2-7). There are 209 different PCB configurations, commonly
referred as congeners, based on the number of chlorines and their positions on the
biphenyl ring (Erickson, 1997). PCBs were commercially produced as complex mixtures
containing multiple congeners, which were manufactured and sold under many different
names. In North America, Aroclor is the best known trade name for PCB mixtures. The
brand Aroclor is always followed with four digit suffix number: the first two of that
generally refers to the number of carbon atoms in the biphenyl ring and the last two of
34
that indicates the degree of chlorination (USEPA, 2012). The molecular structures of
PCBs lead to high chemical stability, low dielectric constants, high thermal conductivity
and low flammability. These properties led PCBs to be widely used in the manufacture of
hydraulic fluids, plasticizers, carbonless copy paper, fluorescent lamp ballasts, flame
retardants, ink, adhesives, and other consumer products (USEPA, 2000).
Figure 2-7: Molecular structure of polychlorinated biphenyls.
PCBs are not readily soluble in water, but soluble in organic solvents, oils and fats. PCBs
are highly stable under most environmental conditions, and can be bioaccumulated in
plants, fish and other living tissues (Erickson, 1997). A number of toxicological studies
have identified PCBs as toxic compounds to animals, humans and ecosystems (Erickson,
1997, USEPA, 2013a). Like any other toxic substance, the toxicological effect depends
on the exposure dosage, exposure duration and routes. The structure of PCBs also
determines its toxicological effect. Normally, those PCB structures that contain no ortho-
chlorine substituent or only a single ortho-chlorine substitute are more toxic. Studies in
35
animals provide conclusive evidence that PCBs can cause cancer in animals and a
number of non carcinogenic health effects, including effects on the immune system,
nervous system, endocrine system and reproductive system (USEPA, 2013a).
Furthermore, PCBs can cause potential carcinogenic and non-carcinogenic effects of
PCBs to human beings (USEPA, 2013a). Therefore, USEPA and IARC have classified
PCBs as probable human carcinogens (Group B2). The human non-carcinogenic health
effects associated with the exposure to PCBs include chloracne and rashes on the skin,
liver damage, dermal and ocular lesions, irregular menstrual cycles and lowered immune
responses, fatigues, headaches, coughs, and unusual skin sores, and among others
(Erickson, 1997).
Concern about the adverse effects of PCBs has caused the production of PCBs to be
banned in 1979 in US (USEPA, 2013b). The global production of PCBs has been banned
by the Stockholm Convention on Persistent Organic Pollutants in May 2004 (Fiedler,
2007). Today, PCBs previously introduced into the environment have become the major
source of PCB related problems. PCBs do not readily break down in the environment and
can remain for long periods of time cycling between water, soil and air (USEPA, 2013b).
They are released to water from contaminated soil, sediments and landfill. PCBs in the
water and soils can move into atmosphere through volatilization, and return back to the
soil and surface waters through wet and dry deposition (USEPA, 2000).
36
2.4 Photochemical treatment of pesticides and PCBs
2.4.1 Direct photolytic degradation of pesticides and PCBs
Most pesticides show UV-Vis absorption bands at relatively short UV wavelengths,
therefore, a short UV wavelength light source is required in direct photolysis of
pesticides. Direct photolysis using UVC (254nm) are reported to treat different pesticides
(Gal et al., 1992, Herweh and Hoyle, 1980, Zepp and Cline, 1977). As well, sunlight or
simulated sunlight for direct photodegradation of pesticides were also investigated
(Samanidou et al., 1988, Wilson and Mabury, 2000, Okamura et al., 1999, Miille and
Crosby, 1983, Ellis and Mabury, 2000, Ying and Williams, 1999). Since sunlight
reaching the earth's surface contain a very small fraction of short wavelength UV
radiation, the direct photolysis of pesticides under sunlight irradiation is not efficient.
Direct photolysis of PCBs using short wavelength UV (254nm) has been reported to take
place in different organic solvents (Yao et al., 1997, Miao et al., 1999, Hawari et al.,
1992, Dhol, 2005). Direct photolysis of PCBs in alkaline isopropanol media were
observed as the most effective. Hawari et al. (1992) reported a high quantum yield (~30)
for direct photolysis of Aroclor1254 in alkaline isopropanol. Direct PCB photolysis
involving the use of sunlight irradiation has not shown to be effective, since PCBs do not
absorb light with wavelength above 300 nm.
37
2.4.2 Photosensitized degradation of pesticides and PCBs
Photosensitized degradation of pesticides have been successfully achieved using the
sensitizers like anthraquinone, N,N,N',N'-tetrarnethylbenzidine and humate (Galadi and
Julliard, 1996, Stangroom et al., 1998, Galadi et al., 1995, Crank and Mursyidi, 1992).
Photosensitization of PCBs using phenothiazine and hydroquinoes has also been studied
(Hawari et al., 1992, Chu and Kwan, 2002). A high quantum yield (2.33) was observed in
the photosensitized dechlorination of PCBs with phenothiazine in alkaline isopropanol
(Hawari et al., 1992). Izadifard et al. (2008) successfully used leuco-methylene blue
(LMB) as a photosensitizer to treat PCBs in acetonitrile/water mixture under visible light
irradiation.
2.4.3 Photocatalytic degradation of pesticides and PCBs
TiO2 photocatalysis has been applied to the treatment of various pesticides including
amide herbicides, bipyridium herbicides, carbamate insecticides, chloroniotinoid
insectids, chlorophenol pesticides, halobenzonitrile pesticides, organochlroine
insecticides, organophosphorus pesticides, phenol-based pesticides, pyrimidine
pesticides, thiocarbamate herbicides, micellaneous, etc. (Kamble et al., 2004, Echavia et
al., 2009, Herrmann et al., 1998, Trillas et al., 1995, Serra et al., 1994, Chen et al., 1999,
Muneer and Boxall, 2008, Topalov et al., 2001, Ormad et al., 2010, Gelover et al., 2004,
Bamba et al., 2008, Kim et al., 2006, Zaleska et al., 2000, Muszkat et al., 1992, Burrows
et al., 2002). Photocatalytic degradation of PCBs with TiO2 was reported using light
ranging from 340 nm to 365 nm (Carey et al., 1976, Chiarenzelli et al., 1995, Wang and
38
Hong, 2000). Some researchers believe that hydroxyl radicals generated during TiO2
photocatalysis oxidize PCBs molecules, leading to PCBs photodegradation and the
eventual formation of CO2 (Wang and Hong, 2000). However, there are also reports of
hydroxyl radicals leading to oxygenation of the PCB ring which produces more toxic
compounds (Safe, 1994, Gierthy et al., 1997) .
2.5 Design of a photocatalytic reactor
A good photocatalytic reactor should be an appropriate combination of photocatalyst,
light source and geometry. The photocatalyst should be easily separated or immobilized;
the light source should be energy efficient; the geometry of reactor should make the
photocatalyst, target compound and photons come together efficiently; and it should be
scalable.
2.5.1 State of Photocatalyst in the Reactor
2.5.1.1 Slurry photocatalytic reactor vs immobilized photocatalytic reactor
A variety of photocatalytic reactors have been designed in the past two decades.
Generally, photocatalytic reactors can be classified into two major groups: slurry reactors
and fixed film reactors. In slurry reactors, the nanoparticle TiO2 is dispersed in the
solution. In the immobilized reactors, the photocatalysts are immobilized on an inert
substrate such as alumina pellets, molecular sieve, glass wall, glass fibre or ceramic
membranes (Parsons, 2004). Table 2-4 compares the two categories of reactors and
summarizes their advantages and disadvantages. The slurry photocatalytic reactors are
39
very efficient in terms of photons due to high surface area to volume ratio. However, the
use of slurries requires further separation steps involving either filtration, centrifugation
or coagulation, which increases the complexity of the overall processes and the
operational cost (Denny et al., 2009). The immobilized photoreactors are less efficient
Table 2-4: Slurry versus Immobilized Photocatalytic Systems (Lasa et al., 2005).
Slurry reactor Immobilized reactor
Advantages
• Uniform catalyst distribution
• High surface area /volume ratio
• Limited mass transfer
• Minimum catalyst fouling effects
• Well mixed particle suspension
• Low pressure drop
Advantages
• Continuous operation
• Improved removal of organic material
from water phase while using a
support with adsorption properties
• No need for catalyst separation
operation
Disadvantages
• Requires post-process seperation
step
• Important light scattering and
adsorption in the particle suspended
medium (Ollis et al., 1991))
Disadvantages
• Low light utilization efficiencies
• Restricted processing capacities
(Turchi and Ollis, 1988, Matthews
and McEvoy, 1992, Matthews, 1991)
• Possible catalyst deactivation and
catalyst wash out (Serrano and de
Lasa, 1997)
40
systems for pollutant degradation due to its smaller availability of illuminated surface
area per mass and substrate mass transport issues but it offers an advantage as the
secondary separation of catalyst from the treated water is not needed. One way to
improve the surface area to reaction volume ratio in an immobilized photocatalytic
reactor is to use supported semiconductor photocatalysts. They are a form of slurry
reactor with improved separation. In this type of photocatalytic reactor, TiO2 is coated on
small particles which can be easily separated (Geng and Cui, 2010, Imoberdorf et al.,
2008a, Vaisman et al., 2005, Pozzo et al., 2005, Kanki et al., 2005, Chiovetta et al., 2001,
Haarstrick et al., 1996).
2.5.1.2 TiO2 immobilization through electrochemical anodization
Immobilized TiO2 can be prepared through electrochemical anodization (Gong et al.,
2001, Paulose et al., 2006, Macak et al., 2005, Wang and Lin, 2008), dipping-coating
(Mikula et al., 1995), sol-gel method (Negishi et al., 1998, Negishi and Takeuchi, 2001,
Yu et al., 2001, Watanabe et al., 2000), chemical vapour deposition (Nakamura et al.,
2001, Kaliwoh et al., 2002, Watanabe et al., 2002), pulsed laser deposition (Yamamoto et
al., 2001) and reactive evaporation (Zeman and Takabayashi, 2002, Mergel et al., 2000).
Among these methods, electrochemical anodization is considered to be superior as it is
economical, convenient, and produce highly photoactive and mechanically durable films
(Li et al., 2013, Natarajan et al., 2011a, Yu et al., 2010, Xie and Li, 2006).
Zwilling et al. (1999) reported the first self-organized anodic TiO2 nanostructure using
electrochemical anodization approach. After that, many studies on fabricating anodic
41
TiO2 nanostructure have been reported. The anodization was carried out using a two-
electrode cell with titanium metals as anode and different materials (Pt, Pd, Ni, Fe, Co,
Al, Carbon and other materials ) as the counter electrode (Allam and Grimes, 2008).
Aqueous/non-aqueous electrolytes (ethylene glycol, glycerol, DMSO or ionic liquids)
containing approximately 0.05 M-0.5 M fluoride ions are used during anodization (Roy et
al., 2011). The applied voltage is between 1-30V for aqueous electrolyte and 5-150 V for
non-aqueous electrolytes. The formation of TiO2 nanostructure in these fluorine
containing electrolytes is the result of competition between the electrochemical oxidation
of Ti and electrical field induced etching of TiO2 as well as chemical etching of TiO2 by
fluorine ions (Wang and Lin, 2009). The fluoride ion can promotes the growth of anatase
TiO2 with high reactive facets such as (001) facets (Yang et al., 2008).
Usually, the TiO2 structure obtained via anodization at room temperature are in an
amorphous form. However, an amorphous form of TiO2 does not show a good
photocatalytic activity (Wu et al., 2011). To obtain TiO2 nanostructure with high
photocatalytic activity, the prepared TiO2 should be converted to crystallized form
(anatase/rutile) with high temperature annealing treatment (eg. 500 oC) (Roy et al., 2011).
2.5.2 Light source
The light source is a key component for photoreactor design. To select the appropriate
lamps, technical and economic considerations should be taken into consideration
(Oppenlander, 2003): firstly, the radiation source should provide the photons that can be
directly or indirectly utilized by the reactant. In TiO2 photocatalysis, only photons with
42
energy equal or larger than band-gap energy of TiO2 can be used, therefore, the
wavelength of chosen lamp should be shorter than 385 nm. Secondly, a high efficiency
light source should be chosen to make sure that, most of input electric energy can be
converted to desired light energy. This requires that unusable wavelengths (emission
wavelengths not aligned with absorption wavelengths) of light should be as little as
possible and the energy loss due to dissipation as heat should be limited. Thirdly, the
geometry and the size of lamps should not limit reactor design.
2.5.2.1 Sunlight
Figure 2-8: Solar spectral irradiance distribution on the surface of earth. [Reproduced from (Hulstrom et al., 1985) with permission]
0
200
400
600
800
1000
1200
1400
1600
1800
0 500 1000 1500 2000 2500
Irra
dian
ce (
W m
-2 μ
m-1
)
Wavelength (nm)
43
The sun is a spherical UV/VIS source with a radiant power of 3.842*1026 W
(Oppenlander, 2003). Only a small part of its radiation can reach the earth through
travelling the 1.4957*1011 m distance. The radiation received on the top of the earth's
atmosphere is around 1400 W m-2 (Ryer and Light, 1997). After passing through the
atmosphere, the solar radiation is attenuated due to the light absorption by the molecules
in the atmosphere. The solar radiation on the earth's surface strongly depends on the
weather condition, e.g. cloud, fog etc. With clear skies, the solar radiation at the earth's
surface is around 1000 W m-2 (Oppenlander, 2003). Also, it can vary with latitude, the
time of day and the season. The average annual solar irradiation on the earth's surface
range from 100 W m-2 (polar region) to about 300 W m-2 (desert regions) (Bolton, 1989).
Figure 2-9: Fractional cumulative integrated irradiance vs. wavelength. [Reproduced from (Hulstrom et al., 1985) with permission]
0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00
0 500 1000 1500 2000 2500 3000 3500 4000 4500
Fra
ctio
n
Wavelength (nm)
44
The spectral distribution of solar radiation received at the earth's surface is shown in
Figure 2-8. At the earth's surface, sunlight contains no VUV and UV-C radiation because
of their efficient absorption by oxygen (absorption at wavelength below 200 nm) and by
ozone ( absorption at wavelength below 330 nm), respectively, in the upper atmosphere,
(Finlayson-Pitts and Pitts Jr, 1986). Less than 5 % of solar radiation reaching on the
earth's surface is UV radiation (below 400 nm) and more than 95% of that is visible or
infrared radiation (Figure 2-9).
2.5.2.2 Mercury lamps
Mercury-vapor lamps are gas discharge lamps that use mercury in an excited state to
produce light. There are two major mercury lamp types based on the mercury pressure
inside the lamp: low pressure (LP) mercury lamp (0.001-13 mbar) and medium pressure
(MP) mercury lamp (~1333 mbar) (Oppenlander, 2003). LP lamps are extensively used
in the field of UV disinfection, which provide almost monochromatic UV radiation at
253.7 nm with an ordinary quartz envelope. The 253.7 nm monochromatic UV radiation
produced by the low pressure mercury lamp can be fairly efficiently converted to broader
band emission at longer wavelengths by coating the lamp envelope with a suitable set of
fluorescent materials, as in the fluorescent lamps used for interior lighting. LP mercury
lamps can convert 40% to 60% of electrical energy into radiant energy (Altena et al.,
2001). To generate a constant radiation output, the maximum electric power input of LP
mercury lamps is usually less than 300 W. MP mercury lamps can be operated with much
higher electrical input power up to 30 kW, but with a reduced UV radiant power
efficiency ( 30%-40%). Instead of monochromatic radiation, MP mercury lamps generate
45
polychromatic emission ranging from the UV (UVC~15-23%, UVB~6-7%, UVA~8%)
over the VIS (~15%) to the IR (47-55%)) (Oppenlander, 2003).
2.5.2.3 Light emitting diode
A new generation of lamps with promising features for photochemical applications has
been developed, the so-called light-emitting diodes (LED). The LED technology shows
many advantages over conventional light sources including energy savings (higher
current to light conversion), less heat production, longer lifetime (up to100 000 hours),
improved robustness, smaller size, faster switching, greater durability and reliability,
besides, it is more environmental friendly as it does not use mercury.
LED is a semiconductor light source, which consists of a chip of semiconducting material
doped with impurities to create a p-n junction (Schubert, 2006). A p-n junction consists
of n-type and p-type semiconductors. P-type semiconductors are a type of semiconductor
which is capable of providing extra positive charge (holes). N-type semiconductors are a
type of semiconductor which can provide an excess of negative electron charge carriers
(electrons). Both types of semiconductor are obtained by doping or adding an electron
acceptor/donor to the semiconductor in order to increase the number of free charge
carriers. The LED working mechanism is shown in Figure 2-10. When a light-emitting
diode is forward biased, the electron in the N part of the junction will be ejected into the
p-part of junction. The combination of an electron-hole pair can then lead to an emission
of a photon. The wavelength of photons released depends on the band gap energy of the
materials forming the p-n junction. The visible spectrum LEDs can be fabricated using
46
GaAsP, GaP, AlGaAs, GaAs, AlGaInP, GaAs, GsAsN or other semiconductors
(Schubert, 2006). The UV LEDs are mainly made of the Group III-nitride-based
semiconductors such as Boron nitride (Mishima et al., 1988), AlGaN (Yasan et al., 2002,
Adivarahan et al., 2004, Yasan et al., 2003, Chitnis et al., 2003), AlN (Khan et al., 2008,
Taniyasu et al., 2006) and AlGaInN (Khan et al., 2008, Kipshidze et al., 2002, Kipshidze
et al., 2003).
Figure 2-10: An inner working on an LED. [Adapted from (Wikipedia, 2011)]
In past few years, the energy efficiency and the output power of UV LEDs have
improved significantly, and the average price has dropped. So far, LED of low intensity
associated with UVA/UVB applications represented 89% of the overall UV-LED market
(Semiconductor Today, 2013). The major UV-LEDs producers include Nichia
Corporation, Lumileds, Cree, Mitsubishi, PARC, NTT, RIKEN, Nitride Semiconductors,
47
SETI, SEMILEDS, etc. Their products mainly emit the photons with wavelengths equal
or above 365nm and with power output ranging from a few micro watts at short
wavelengths to several watts at wavelengths around 365nm.
External quantum efficiency (EQE) reflects the energy efficiency of LED, which is
defined as the ratio of the number of photons emitted from the LED to the number of
electrons passing through the device. An excellent EQE (up to 40%) has been obtained
for LEDs in the UVA part of the spectral range (LEDs Magazine, 2012). However, the
average EQE of UV LED shorter than 365nm is at least one order of magnitude below
the best devices in the near UV wavelength range (Kneissl et al., 2011). Therefore,
applications of UVC-LED are still in their infancy and mainly for R&D purposes and
analytic instruments (Semiconductor Today, 2013). Following the same development
trend of UVA-LED, the journey on the path to efficient UVC-LED has just begun and
there are many optimistic reasons to produce highly efficient UVC-LED in the near
future (Kneissl et al., 2011).
2.5.3 Artificially illuminated photocatalytic reactors
The photocatalytic reactors using conventional UV mercury lamps can be classified as
two major categories; slurry type photocatalytic reactors and immobilized photocatalytic
reactors. The major geometries of slurry type photocatalytic reactors include:
annular reactors (Figure 2-11 (a)) which consists of two co-axial cylinders that
define the reaction zone (Mo et al., 2008, Johnson and Mehrvar, 2008, Chong et
al., 2009, Behnajady et al., 2009, Imoberdorf et al., 2007, Tang and Chen, 2004),
48
immersion well reactors (Figure 2-11 (b) and (c)) where one or more lamps are
immersed in the well stirred reactors (Thiruvenkatachari et al., 2005, Saquib and
Muneer, 2003),
elliptical reactors (Figure-2-11 (d)) of which foci are occupied by lamps and the
tubular reactor (Jacob and Dranoff, 1969),
parabolic reactors (Figure-2-11 (e)) of which foci are occupied by lamps (Alfano
et al., 2000).
Figure 2-11: Various photochemical reactor configurations. [Reproduced from (Pareek et al., 2008) with permission]
All the slurry type photocatalytic reactors should be incorporated with the photocatalyst
separation systems such as membrane filtration. To avoid this process, the immobilized
49
photocatalytic reactor is developed. One of the earliest type of immobilized
photocatalytic reactor is a spiral photoreactor where the photocatalyst is coated onto the
inner walls of a glass spiral which has a UV lamp in the centre (Matthews, 1987). Since
then, immobilized photocatalytic reactors with various designs have been developed. A
variety of immobilized photocatalytic reactors are listed below:
• Thin film reactor (Roselin and Selvin, 2011) in which a thin reacting fluid film
flowing through a surface coated with photocatalyst. The UV irradiation
penetrates the fluid film to reach the photocatalytic surface.
• Multiple tube reactor (Figure 2-12) containing of a cylindrical vessel within
which a number of hollow quartz glass tubes externally coated with photocatalyst
were placed. The liquid flows through the shell-side over the outside surfaces of
the coated tubes while the light travels through the inside of hollow tubes via an
aluminum reflector.
• Fiber optic cable reactor (Figure 2-13) in which a number of fiber optic cables
were coated with TiO2. The UV light can reach the supported TiO2 along the
optic cable.
• Rotating disk reactor (Figure 2-14) composed of a rotating disk coated with
photocatalyst. A thin film of liquid becomes entrained on the disc from the bulk
solution during rotation. Reaction takes place in the head space due to the
illumination.
• Distributive photocatalytic reactor (Figure 2-15) in which light conductors coated
on its outside surface with catalysts are embedded vertically. This configuration
provides a higher illuminated catalyst area per volume.
50
• Taylor vortex reactor (Figure 2-16) consisting of two coaxial cylinders. The inner
cylinder coated with TiO2 is rotated to generate a vortex-induced fluid instability.
• Fluidized bed photocatalytic reactor (Figure 2-17) which has a fluidized bed
consisting of small TiO2-coated particles such as glass beads. This configuration
improves the surface area to reaction volume ratio and mass transfer condition.
Apart from these reactors, there are several novel designs which are useful for specialized
applications, however, two inadequacies limit their use (Pareek et al., 2008). Firstly,
complex mechanical designs of novel photoreactors make construction difficult and
hinder their routine maintenance and cleaning. Secondly, the processing capacity of the
novel reactors is limited.
Figure 2-12: Scheme of a multiple tube reactor. [Reproduced from (Ray and Beenackers, 1998) with permission]
51
Figure 2-13: Scheme of an optical fibre photocatalytic reactor. [Reproduced from (Nguyen and Wu, 2008) with the permission]
Figure 2-14: Scheme of a rotating disk reactor. [Reproduced from (Hamill et al., 2001) with the permission]
52
Figure 2-15: Top view of a distributive photocatalytic reactor. [Reproduced from (Ray and Beenackers, 1998) with permission]
Figure 2-16: Experimental setup of taylor vortex photocatalytic reactor:(1) motor, (2) speed controller, (3) gear coupling, (4) UV lamp (5) sample collection point (6) lamp holder (7) outer cylinder and (8) catalyst-coated inner cylinder. [Reproduced from (Dutta and Ray, 2004) with the permission]
53
Figure 2-17: Scheme of a fluidized photocatalytic reactor. [Reproduced from (Vaisman et al., 2005) with permission]
2.5.4 Solar photocatalytic reactors:
Major designs of solar photocatalyic reactors have been reviewed by several authors
(Braham and Harris, 2009, Lasa et al., 2005, Thiruvenkatachari et al., 2008), including
parabolic trough reactors, compound parabolic reactors, inclined plate photocatalytic
reactors, double-skin sheet photocatalytic reactors, horizontal rotating disk reactors and
water bell reactors. A parabolic trough reactor (Figure 2-18 a) is a light concentrating-
type unit, which uses a long parabolic reflecting trough to concentrate solar radiation on a
transparent tubular reactor placed on the parabolic focal line. Compound parabolic
reactors (Figure 2-18 b) are trough reactors without light concentrating devices. The
reflector in compound parabolic reactor is characterized with a two half-cylinders of
54
Figure 2-18: Solar photocatalytic reactor: (a) parabolic trough reactor (PTR) (b) compound parabolic collector (CPC). [Reproduced from (Braham and Harris, 2009) with permission]
parabolic profile which allow indirect light to be reflected onto the tubular reactor. An
inclined plate photocatalytic reactor (Figure 2-19 a) consists of an inclined surface coated
55
with photocatalyst. The reactant fluid flows through the inclined surface to form a thin
film. An double-skin sheet photoreactor (Figure 2-19 b) uses a double-skin transparent
plexiglass to construct a long, convoluted back and forth channel on a flat plane through
which the reactant fluid flow. A horizontal rotating disk reactor (Figure 2-20 a) has a
rotating disk of which the surface is exposed to sunlight. The reactant fluid is injected up
from the center of the disk and forms a fluid film on the surface of the disk. The rotation
of disk generates a turbulent flow regime in the fluid film. A water bell reactor (Figure 2-
20 b) features a nozzle which sprays a continuous and unsupported thin film of liquid
exposed to solar irradiation.
Figure 2-19. Typical reactor layout for an (a) inclined plate collector and (b) double skin sheet photoreactor. [Reproduced from (Braham and Harris, 2009) with permission]
56
Figure 2-20: Typical reactor layout for (a) horizontal rotating disk reactor and (b) water bell reactor. [Reproduced from (Braham and Harris, 2009) with permission]
2.6 Radiation-field modelling
A uniform distribution of light is necessary for an efficient photocatalytic treatment
system. In this context, a radiation field model is very useful. The model need to compute
the rate of photon absorption at any position within the reactor or 'local volumetric rate of
energy absorption' (LVREA) (Cassano et al., 1995). In homogenous media, the change of
the light intensity along the direction of photon propagation is due to absorption process
in the reaction media, while, in heterogeneous media scattering of radiation by particles
should also be accounted in the variation of light intensity. Various radiation field models
in homogenous or heterogeneous environments have been described in several papers
(Imoberdorf et al., 2008b, Jacob and Dranoff, 1969, Jacobm and Dranoff, 1970, Irazoqui
et al., 1973, Alfano et al., 1986a, Alfano et al., 1986b, Alfano et al., 1986c).
57
2.6.1 The Radiation transport equation (RTE)
In heterogeneous media (slurry TiO2 photocatalysis), the light intensity may change
along its light path due to photon absorption, scattering and emission. In-scattering and
emission from reacting mixture can increase the light intensity, while, the photon
absorption and out-scattering can reduce the light intensity (Figure 2-21).
Figure 2-21: Schematic for photon transport.
In a control volume V (m3), the photon balance equation can be described as (Cassano et
al., 1995):
𝑑𝑄𝑑𝑡
+ 𝐴 ∗ 𝑑𝐼𝜆 = −𝑞𝑎 + 𝑞𝑒 + 𝑞𝑖𝑛 − 𝑞𝑜𝑢𝑡 [2-25]
Where is the number of photon in control volume (photon), t is time (s), A is the cross
section surface area (m2), is the spectral light intensity (photon m-2 s-1), is rate of
58
photon absorption (photon s-1), is rate of photon emission (photon s-1), is rate of
photon in-scattered (photon s-1), is rate of photon out-scattered (photon s-1).
At steady state: =0, then the equation can be written as:
𝐴 ∗ 𝑑𝐼𝜆 = −𝑞𝑎 + 𝑞𝑒 + 𝑞𝑖𝑛 − 𝑞𝑜𝑢𝑡 [2-26]
Equation [2-27] is obtained by dividing Equation [2-26] with V; V=A*ds,
𝐴𝑑𝐼𝜆𝑉
= −𝑞𝑎/V + 𝑞𝑒/v + 𝑞𝑖𝑛/v − 𝑞𝑜𝑢𝑡/v [2-27]
Finally, the photon balance in a control volume can be described by Equation [2-28]
(Pareek et al., 2008, Cassano et al., 1995).
𝑑𝐼𝜆(𝑠,𝛺)𝑑𝑠
= −𝑊𝑎 + 𝑊𝑒 + 𝑊𝑖𝑛 −𝑊𝑜𝑢𝑡 [2-28]
Where is the solid angle (steradian), is volumetric rate of photon absorption
(photon m-3 s-1), is volumetric rate of photon emission (photon m-3 s-1), is
volumetric rate of photon in-scattered (photon m-3 s-1), is volumetric rate of photon
out-scattered (photon m-3 s-1).
At normal or low temperature, the spontaneous radiation emission can be neglected:
𝑊𝑒 = 0 [2-29]
59
In general, linear isotropic constitutive equations (Equation [2-30] &[2-31])can be used
to characterize absorption and out-scattering (Cassano et al., 1995),
𝑊𝑎 = 𝛼𝜆𝐼𝜆(𝑠,𝛺) [2-30]
𝑊𝑜𝑢𝑡 = 𝜎𝜆𝐼𝜆(𝑠,𝛺) [2-31]
Where is the volumetric absorption coefficient (m-1) and is the volumetric
scattering coefficient (m-1), usually, the combination of the absorption coefficient and the
scattering coefficient is defined as the extinction coefficient:
βλ = 𝛼𝜆 + 𝜎𝜆 [2-32]
Where is the extinction coefficient (m-1).
In the RTE, In-scattering is a more complicated term, which can be described by
Equation [2-33] (Cassano et al., 1995). Two assumptions were made here:
• Every scattering is independent of each other.
• The scattering is elastic, which means that the frequency of scattered radiation is
the same as incident radiation.
𝑊𝑖𝑛 =1
4𝜋𝜎𝜆 � 𝑝(𝛺′ ⟶ 𝛺)𝐼𝜆(𝑠,𝛺′)𝑑𝛺′
4𝜋
0 [2-33]
Where is the phase function for the in-scattering of photons, usually, the
phase function is normalized according to Equation [2-34]:
14𝜋
� 𝑝(𝛺′ ⟶ 𝛺)𝑑𝛺′ = 14𝜋
0 [2-34]
60
The phase function is the one that make the RTE difficult to solve. To simulate the model
more efficiently, the Henyey-Greenstein (HG) phase function (Equation [37]) can be an
appropriate choice (Marugán et al., 2006).
𝑃𝐻𝐺(𝜑) =1
4π∗
1 − τ2
(1 + τ2 − 2τ ∗ cos(φ))3/2 [2-35]
Where is the scattering angle and τ is the asymmetry factor of the scattered radiation
distribution. The value of τ varies smoothly from -1 to +1. When τ =0, it represents an
isotropic phase function.
According to Equations [2-25] ~ [2-35], the final RTE can be written as
𝑑𝐼𝜆(𝑠,𝛺)𝑑𝑠
= −βλ𝐼𝜆(𝑠,𝛺) +1
4𝜋𝜎𝜆 � 𝑝(𝛺′ ⟶ 𝛺)𝐼𝜆(𝑠,𝛺′)𝑑𝛺′
4𝜋
0 [2-36]
For homogenous media (when a photocatalyst is immobilized), the scattering term on
Equation [2-36] can be excluded.
Then the incident light intensity from all direction:
Gv(s) = � 𝐼𝜆(𝑠,𝛺)𝑑𝛺4𝜋
0 [2-37]
Where is the incident light intensity (photon m-2 s-1).
And the LVREA at any point is given by
LVREA= αλGv(S) [2-38]
2.6.2 Numerical methods to solve the RTE
The RTE is an integral-differential equation, an exact analytical solution is impossible
except for homogeneous photoreaction systems, where scattering phenomenon is not
taken into account (Pareek et al., 2008). Numerical method offers a viable alternative to
61
solve the RTE. Carvalho and Farias (1998) reviewed a variety of methods developed to
numerically solve the RTE, including Zone method, Monte Carlo (MC) method, flux
method and hybrid method. Among these methods, the MC method is been accepted as
efficient and reliable. (Pareek et al., 2008, Pasquali et al., 1996). MC is a statistical
method, which is based on following the probable trajectories and fates of photons inside
the reaction zone, until their final absorption in the system or existing out of the system.
Both the trajectories and fates are decided with the help of random numbers, which are
generated by a random function through computer (Pareek et al., 2008). Consider a
photon entering into the reaction zone, it may get absorbed by the particle and its
trajectory ends there or it may be scattered by the particle to a new direction and the
trajectory continues until being absorbed by other particles or exiting out of the reaction
zone (Pareek et al., 2008). Whether absorption or scattering is determined by a random
choice based on the absorption coefficient and the scattering coefficient. To solve the
RTE with the Monte Carlo method, the optical properties of the reaction medium like
absorption coefficient scattering coefficient and the phase function should be obtained.
Further, the boundary conditions are needed.
2.6.3 Radiation source models
Radiation source model are the functions predicting the light intensity emitted from the
lamps. When the scattering effect is negligible such as in homogeneous medium, the
radiation source model can be directly used as a radiation filed model. The radiation
source model also play an important role in solving the boundary condition of RTE.
Alfano et al. (1986b) reviewed a number of radiation source models and classified them
62
into two categories: incidence models and emission models. Three incidence models have
been proposed (Pareek et al., 2008, Alfano et al., 1986b), including the radial incidence
model, the partially diffuse incidence model and the diffuse incidence model. The
development of incidence models requires an existing specific radiant energy distribution
to be assumed in the reactor space. Besides, these incidence model always need one or
more experimentally adjustable parameters, which is dependent on the size and the
configuration of the reactor (Alfano et al., 1986b). To overcome this problem, emission
models based on the lamp emission are developed and regarded as a preferred choice for
radiation source modeling. A lamp may be regarded as a point (LED), a line, a surface, or
a volume source. Depending upon the nature of the lamp, different emission models such
as line source model, surface source model, volume source model have been developed
(Pareek et al., 2008, Alfano et al., 1986b). The line source models were considered as
appropriate methods to simulate the light intensity distribution over the photocatalytic
plate when the photocatalytic reactor is equipped with tubular lamps (the geometry of
conventional mercury lamp) (Salvadó-Estivill et al., 2007).
A good radiation field model can accurately predicted the light intensity distribution
within photoreactors. Such model can be used as a tool to figure out the optimal
arrangement of light source and reactor geometry. Furthermore, radiation field model is
very important to the mathematically simulating photochemical treatment processes.
63
Chapter Three: ELECTRON TRANSFER SENSITIZED PHOTODECHLORINATION OF SURFACTANT SOLUBILIZED PCB138
3.1 Introduction
Direct and sensitized photodechlorination of polychlorinated biphenyls (PCBs) dissolved
in organic or organic/water mixtures have been the focus of various investigations (Bunce
et al., 1978, Ruzo et al., 1974, Hawari et al., 1992, Hawari et al., 1991, Miao et al., 1996,
Izadifard et al., 2008, Lin et al., 1996, Jakher et al., 2007). The low solubility of PCBs in
water necessitates the presence of an organic solvent. Surfactants also have been used to
solubilize PCBs in water (Yao et al., 2000, Bunce et al., 1978, Hawari et al., 1992,
Hawari et al., 1991, Miao et al., 1999). Using a surfactant instead of an organic solvent is
advantageous for two reasons: lower cost and minimized side reactions (Chu et al., 1998).
To-date investigations on PCBs dissolved in water by surfactants has been restricted to
direct UV photolysis, which requires high energy photons with wavelengths less than 300
nm (Chu et al., 2005, Chu et al., 1998). The application of sensitized dechlorination can
make use of photons with longer wavelengths, eventually leading to using sunlight for
dechlorination. Besides, sensitized dechlorination of PCBs can also result in a high
photodegradation rate (Dhol, 2005). To the best of our knowledge, no research is reported
on sensitized dechlorination of PCBs dissolved in water using surfactants; though there
are a few reports on sensitized reaction, where an aliphatic amine which cannot function
as a sensitizer is used to enhance the efficiency of the reaction (Chu and Kwan, 2002,
Chu and Kwan, 2003). In this case, PCB itself must be excited so that an electron transfer
from the aliphatic amine to PCBs becomes favorable.
64
This paper presents the results of an investigation on sensitized dechlorination of a PCB
congener - 2,2',3,4,4',5'-hexachlorobiphenyl (PCB 138) and a commercial mixture of
PCBs (Aroclor 1254). PCBs were dissolved in water using an anionic surfactant (sodium
dodecyl sulfate, SDS), a nonionic surfactant (polyoxyethylene (80) sorbitan monooleate,
TWEEN 80) or a cationic surfactant (cetyltrimethylammonium bromide, CTAB). The
sensitizer of choice is leuco-methylene blue (LMB), which has been reported to
effectively dechlorinate PCBs under blue and near UV irradiation (Izadifard et al.,
2010a). LMB, while being stable in an oxygen devoid environment, is produced
efficiently upon reduction of methylene blue (MB). MB can be reduced under red light
and in the presence of an aliphatic amine (such as triethylamine, TEA), or by a thermal
reaction using sodium borohydride (NaBH4). Both approaches are studied in this paper.
Air oxidation of LMB closes a ‘catalytic’ cycle that consumes the reductant.
3.2 Materials and methods
3.2.1 Materials
All PCB congeners and Aroclor 1254 were purchased from Chromatographic Specialties
Inc.; MB, CTAB, and 99.5% pure TEA were obtained from Sigma; 99.8% pure hexane
and 98% pure sodium borohydride were procured from EMD; ultra pure SDS was
obtained from MP Biomedicals and TWEEN 80 was purchased from VWR. All reagents
were used as received. Milli-Q ultrapure water was used in the experiments.
65
3.2.2 Methods
3.2.2.1 PCB 138 solubilization with surfactants
For the selected surfactant (SDS, CTAB and TWEEN 80) a stock solution with 4 g L-1
concentration was prepared. Each surfactant stock solution was prepared by dissolving 4
g of pure surfactant in 1 L milli-Q water with the aid of sonication. The PCB stock
solution was prepared by dissolving 10 mg PCB138 or 10 mg Aroclor 1254 in 10 ml
acetonitrile. For each photolysis experiment, a certain amount of PCB stock solution,
PCB 138 or Aroclor 1254, in acetonitrile (1000 mg L-1) was pipetted into a 20 ml Pyrex
glass vial and, left at room temperature in a fume hood for one day to evaporate the
acetonitrile completely. Then a known amount of surfactant stock solution was added to
the vial and the mixture was left in the sonicator for 4 hours to prepare the PCB
surfactant solution.
3.2.2.2 Photochemical reaction
The sensitizer of choice, LMB, was generated in two ways: (1) reaction between MB and
TEA under visible light irradiation and (2) thermal reaction between MB and sodium
borohydride. In each case, to the prepared PCB surfactant mixture was added a MB
solution along with either TEA or NaBH4. The final solution volume was made equal to
20 ml with milli-Q water. Uniform mixing during irradiation was achieved by placing the
sample vial on a magnetic stirring plate. The samples (contained in a Pyrex glass vial)
were irradiated in a Rayonet photoreactor equipped with either 8 or 14 cool white
fluorescent lamps. The intensity of light (Io) was measured using ferrioxalate actinometry
66
that monitored wavelengths from 254 to 500 nm, covering the region of LMB absorption
(Calvert and Pitts, 1966). Values were 5.2×1016 photon s-1 for 14 lamps and 3.0×1016
photon s-1 for 8 lamps. This convenient actinometer measures the light intensity inside the
reaction vessel and provides an order of magnitude value of intensity in the region of
LMB absorbance. Our irradiation source, the cool white lamps emit light in visible range,
up to 700 nm. MB absorption peak is at 660 nm, so it can be effectively reduced to LMB
with the cool white lamps. It is the actinometry method used here that measures
wavelengths between 254 nm to 500 nm. Since our sensitizer is LMB, whose absorption
is within this range, we used this actinometry.
3.2.2.3 Sampling, extraction and GC analysis
Exactly 0.5 ml illuminated samples were taken at different irradiation times and
transferred into a small glass vial. To each aliquot was added 2 ml of hexane, the vial was
covered by aluminum foil and was left in the wrist shaker for 1 hour to extract PCBs
from the water mixture. Around 80% extraction efficiency was obtained following this
procedure.
The extracted PCBs were analyzed using an Agilent 6890 gas chromatograph equipped
with auto-sampler and electron capture detector (ECD), using a fused silica capillary
column DB608. Helium was used as the carrier gas with a flow rate of 1.5 ml min-1 and
the temperature of the injection port was 280 oC. The GC/ECD temperature programming
was set up as follow: The initial temperature for each run was set at 80 oC, which was
67
ramped up to 180 oC at a rate of 10 oC min-1; the temperature beyond 180 oC was ramped
at a rate of 3 oC until it reached 270 oC where it was held for 15 minutes (USEPA,
1996b). For Aroclor 1254, six major peaks were chosen and a multipoint calibration
curve was made by using those six peaks.
Each experiment was conducted in duplicate and results were reported as an error bar
along with the average.
3.3 Results and Discussion
3.3.1 Selectivity of surfactants
The critical micelle concentration (CMC) in water for SDS, CTAB and TWEEN 80 were,
respectively, 2300 mg L-1 (Mandal et al., 1988), 324 mg L-1 (Paredes et al., 1984) and 15
mg L-1 (Hillgren et al., 2002). The sensitized dechlorination of PCB 138 in these
surfactant solutions are shown in Figure 3-1 and Figure 3-2. Figure 3-1 presents results
where LMB was produced under irradiation, while Figure 3-2 where it was produced
thermally. Based on our previous experiments published elsewhere (Izadifard et al.
2010b), the NaBH4 based reactions are faster than TEA based reactions. Consequently, to
ensure that we can reliably measure the concentration of PCB (above the detection limit)
within the irradiation time, a higher concentration of PCB and a lower light intensity was
applied in the NaBH4 system. We are fully aware that two different initial concentrations
were used for the two experiments, that is why we are careful in the paper not to compare
68
the results of Figure 3-1 with those of Figure 3-2. We have instead compared the
performance of different surfactants amongst themselves, presented in either Figure 3-1
or Figure 3-2. Of the three surfactants investigated, the dechlorination efficiency of PCB
138 solubilized with TWEEN 80 or with CTAB are similar, when TEA is used. However,
in the NaBH4 system the performance of CTAB is better than that of TWEEN 80. In
addition, it takes only half the irradiation intensity to achieve similar results with NaBH4
reduction.
Figure 3-1: Reductive dechlorination of PCB 138 using LMB with TEA as the
reducing agent; [PCB 138] = 6.6 mg L-1, [SDS] = [CTAB] = [TWEEN 80] = 3.2 g L-1,
[MB] = 750 mg L-1, [TEA] = 68 g L-1, Io = 5.2×1016 photon s-1 .
0
0.2
0.4
0.6
0.8
1
1.2
0 5 10 15 20 25 30
C/C
o
Irradiation time (min)
SDS CTAB TWEEN80
69
Figure 3-2: Reductive dechlorination of PCB 138 using LMB with NaBH4 as the
reducing agent; [PCB 138] = 20 mg L-1, [SDS] = [CTAB] = [TWEEN 80] = 3.2 g L-1;
[MB] = 600 mg L-1; [NaBH4] = 20 g L-1, Io = 3.0×1016 photon s-1.
In both cases, dechlorination of PCB 138 is less efficient if SDS is used. It is worth
noting that the different surfactants were all used at the same concentration, which was
above the CMC of all of them. There are three possibilities for these results:
(1) CMC for SDS is much higher than that for CTAB and TWEEN 80, which correlate
to lesser solubilization of the PCBs. Possibly, the concentration of SDS used was
0
0.2
0.4
0.6
0.8
1
1.2
0 5 10 15 20 25 30
C/C
o
Irradiation time (min)
SDS CTAB TWEEN80
70
insufficient to completely solubilize PCB 138, and the extent of PCB 138 solubilization
in surfactant solution influenced the dechlorination efficiency. (This was investigated by
experimenting with higher concentrations of SDS, see below)
(2) The hydrophobic chain of surfactant is the hydrogen source for photodechlorination.
Possibly, CTAB and TWEEN 80, with longer hydrophobic chains than SDS, may orient
better with respect to the aryl radical and prevent aryl Cl⋅ recombination.
(3) Dye molecule incorporation in micelle species is more favorable for the cationic dye
and the anionic surfactant, affecting results (Aksu et al., 2010).
To evaluate the first possibility and study the effect of the concentration of surfactant on
the reaction rate, for the MB-TEA system SDS, CTAB and TWEEN 80 were tested at
higher concentrations than their corresponding CMCs. An apparent first order kinetic rate
coefficient for each situation appears in Table 3-1. As the concentration of SDS
increased, the extent of PCB138 solubilization increased and the declorination efficiency
improved. However, as the concentration of CTAB and TWEEN 80 becomes much
higher than CMC, the rate of dechlorination decreases. Possibly, at higher concentration
of the surfactants, LMB and PCB 138 are bound at sites distant from each other.
Since our ultimate objective is to develop a system that can photodegrade PCBs extracted
from soils and sediments and knowing that cationic surfactants are not good choices for a
surfactant-aided soil washing system as they can strongly adsorb onto soil (Wang and
71
Keller, 2009) and the anionic surfactant (SDS) did not give good results, the nonionic
surfactant (TWEEN 80) was chosen for further investigations except for the pathway
study.
Table 3-1: First order rate coefficients (K) for PCB138 dechlorination in TEA-MB
system with different concentration of surfactants.
[PCB138]
mg L-1
Type of
Surfactant
[Surfactant]
mg L-1
[MB]
mg L-1
[TEA]
g L-1
Io
photon s-1
K
s-1
6.6 SDS 3200 750 68 5.2 ×10-16 3.0 ×10-4
6.6 SDS 700 750 68 5.2 ×10-16 7.9 ×10-4
6.6 CTAB 914 750 68 5.2 ×10-16 3.9 ×10-3
6.6 CTAB 3430 750 68 5.2 ×10-16 1.2 ×10-3
6.6 CTAB 11500 750 68 5.2 ×10-16 9.4 ×10-4
6.6 TWEEN 80 45.7 750 68 5.2 ×10-16 3.0 ×10-3
6.6 TWEEN 80 228 750 68 5.2 ×10-16 4.3 ×10-3
6.6 TWEEN 80 914 750 68 5.2 ×10-16 2.3 ×10-3
6.6 TWEEN 80 2290 750 68 5.2 ×10-16 1.8 ×10-3
72
3.3.2 Dechlorination of PCBs in TEA and NaBH4 systems
As mentioned before, both the TEA and NaBH4 systems can be used to reduce MB to
LMB. Depending on the light sources available either one or both can be used. If using
sunlight is the final goal, both NaBH4 and TEA can be used. In case of using
approximately monochromatic light sources such as LEDs at 436 nm (Izadifard et al.,
2010b), using a strong reductant such as NaBH4 is unavoidable.
3.3.2.1 MB and TEA
Figure 3-3: Dechlorination of PCB 138 solubilized with TWEEN 80 in the presence of MB and different concentrations of TEA, [PCB 138] = 6.6 mg L-1, [MB] = 750 mg L-1, [TWEEN80] = 914 mg L-1, [TEA]1 = 22.6 g L-1 [TEA]2 = 31.5 g L-1, [TEA]3 = 45 g L-1, [TEA]4 = 68 g L-1, Io = 5.2×1016 photon s-1.
0
0.2
0.4
0.6
0.8
1
1.2
0 10 20 30 40 50 60
C/C
o
Irradiation time (min)
TEA₁ TEA₂ TEA₃ TEA₄
73
Figure 3-3 presents the results of an optimization study, wherein PCBs solubilised with
TWEEN 80 are dechlorinated in the presence of MB with varying concentrations of TEA.
The result indicates that when 23 g L-1 of TEA was used, there was no significant
dechlorination after a hour of irradiation; as the concentration of TEA increased to 31.5 g
L-1, more than 90% of PCB138 dechlorinated in 40 minutes, and it only took 20 minutes
to obtain the same removal efficiency at a concentration of 45 g L-1 of TEA. The
increased concentration of TEA in can improve the removal efficiency, however, the
limited solubility of TEA in water provides an upper limit to the TEA concentration that
can be used.
3.3.2.2 MB and NaBH4
For this system, effects of changing the concentrations of both MB and NaBH4 on the
reaction rate were studied (Table 3-2). In the chosen range of concentrations for MB, the
lowest concentration provided the best results, which can be due to the self quenching of
the dye at high concentrations (Turro, 1991). In the chosen range of concentrations for
NaBH4, a higher concentration of NaBH4 led to a higher reaction rate, however, an
experiment where the concentration of NaBH4 was doubled did not contribute to a
significant improvement in photodechlorination rate once the concentration of NaBH4
exceeded 10 g L-1.
74
Table 3-2: First order rate coefficients (K) for PCB138 dechlorination in NaBH4-
MB system.
[PCB138]
mg L-1
Type of
Surfactant
[Surfactant]
mg L-1
[MB]
mg L-1
[NaBH4]
g L-1
Io
photon s-1
K
s-1
20 TWEEN 80 1600 600 2 3.0 ×1016 2.1 ×10-3
20 TWEEN 80 1600 600 5 3.0 ×1016 2.5 ×10-3
20 TWEEN 80 1600 600 8 3.0 ×1016 4.0 ×10-3
20 TWEEN 80 1600 600 10 3.0 ×1016 5.0 ×10-3
20 TWEEN 80 1600 600 20 3.0 ×1016 5.1 ×10-3
20 TWEEN 80 1600 300 20 3.0 ×1016 6.0 ×10-3
20 TWEEN 80 1600 60 20 3.0 ×1016 6.5 ×10-3
3.3.2.3 Photodegradation of Aroclor 1254 with NaBH4 and TEA
A commercial mixure of PCBs (Aroclor 1254) was chosen to explore a practical case
using an LMB-surfactant system. To this end, a 10 mg L-1 Aroclor 1254 was studied
using both the TEA and the NaBH4 systems. The results indicate that Aroclor 1254 was >
95% dechlorinated within 10 minutes in both TEA and NaBH4 systems under the present
white light (see Figure 3-4). In this system, there is no evidence to support the chain
reaction mechanism advanced by Izadifard et al. (2010b).
75
Figure 3-4: Dechlorination of Aroclor1254 solubilized with TWEEN 80 in the
presence of MB and TEA or NaBH4: [Aroclor1254] = 10 mg L-1, [MB] = 600 mg L-1,
[TWEEN80] = 1.6 g L-1, [TEA] = 108 g L-1, [NaBH4] = 20 g L-1, Io = 3.0 ×1016 photon
s-1.
3.3.3 The dechlorination pathways of PCB 138 using CTAB and TWEEN 80
In all CTAB and TWEEN 80 cases, a stepwise dechlorination is evident. PCB 138 loss is
accompanied by appearance of lower chlorinated PCBs. Penta, tetra, tri, di and mono
chloro PCBs, initially increase as PCB138 decreases. These photoproducts were
identified by comparing the retention time of the photoproducts to those of available
standards. Whether pathways were influenced by surfactants was examined by comparing
product distributions after 6 minutes of irradiation (Figure 3-5 & Figure 3-6). The results
0
0.2
0.4
0.6
0.8
1
1.2
0 5 10 15 20 25 30
C/C
o
Irradiation time (min)
TEA NaBH₄
76
show that in MB-TEA case, for both surfactants, the primary photoproduct was PCB 118,
which is produced by losing an ortho chlorine. In case of MB-NaBH4 system, it was PCB
87 which appeared as the primary product by losing a chlorine at the meta position. This
may be attributable to nucleophilic attack by BH4- at the meta position as suggested by
the calculated charge distributions on carbons at different position in PCB congeners
(Chang et al., 2003): carbons at meta positions have lower charge density than those in
ortho positions.
Figure 3-5: The product distribution (after six minutes irradiation) for
dechlorination of PCB 138 solubilized by TWEEN 80 (1.6 g L-1) or CTAB (1.6 g L-1)
in the presence of MB (600 mg L-1) and TEA (68 g L-1), Io = 3.0 ×1016 photon/s, P:
peak area of each congener from GC, Po: the peak area of initial PCB 138.
0%
15%
30%
45%
P/Po
CTAB TWEEN80
77
Figure 3-6: The product distribution (after six minutes irradiation) for
dechlorination of PCB 138 solubilized by TWEEN 80 (1.6 g L-1) or CTAB (1.6 g L-1)
in the presence of MB (600 mg L-1) and NaBH4 (10 g L-1); Io = 3.0 ×1016 photon s-1, P:
peak area of each congener from GC, Po: the peak area of initial PCB 138.
3.4 Conclusions
There are two major conclusions that are drawn from this study: Firstly, it is shown that
PCBs can be dechlorinated in an aqueous medium using sensitized visible light. This
opens an opportunity to exploit sunlight for dechlorinating PCBs, thus significantly
lowering the energy costs. Of course in order to dissolve the PCBs certain surfactants are
necessary. Secondly, amongst the different kinds of surfactants, the non ionic (TWEEN
0%
15%
30%
45%
P/Po
CTAB TWEEN80
78
80) and the cationic (CTAB) surfactants work better than the anionic surfactant (SDS) for
dechlorination, even though the cationic surfactant is not preferred for PCB extraction
from soil. The concentration of the surfactant plays a role in the rate of dechlorination.
The results provides promise to develop a practical method to dechlorinate PCBs in
aqueous solution using surfactants and sensitized visible light.
79
Chapter Four: LED-BASED PHOTOCATALYTIC TREATMENT OF PESTICIDES AND CHLOROPHENOLS
4.1 Introduction
Extensive and sometimes excessive use of pesticides has led to surface and ground water
pollution. Whereas some pesticides may degrade in the environment, others may persist
and pose an ecological risk. This paper focuses on those pesticides that are commonly
used in North America, such as 2-Methyl-4-chlorophenoxyacetic acid (MCPA), 2,4-
Dichlorophenoxyacetic acid (2,4-D), 4-chlorophenol (4-CP) and 2,4-dichlorophenol (2,4-
DCP). Although these are considered to be less harmful than some other environmental
pollutants, their continued use may have long-term consequences. 2-Methyl-4-
chlorophenoxyacetic acid and 2,4-D rank third and fourth, respectively, in the amount of
pesticide used in Canada (Brimble et al., 2005). Chlorophenols are not only widely used
in pesticides, but they are also formed upon chlorination (during disinfection of water and
wastewater) of humic matter (Exon, 1984, Health Canada, 2008). Health Canada
recommends a maximum concentration of 0.1, 0.1 and 0.9 mg L-1, respectively, for 2,4-D,
MCPA and 2,4-DCP in drinking water (Health Canada, 2012).
Advanced oxidative processes (AOPs) have been shown to be successful in degrading
organic contaminants such as pesticides in water (Parsons, 2004, Andreozzi et al., 1999).
Whereas AOPs have a wide range of applications, our focus is to use TiO2-based
photocatalysis, which is known to be broadly applicable. TiO2 has low toxicity, is
biologically and chemically stable, and is economical (Bhatkhande et al., 2002,
80
Linsebigler et al., 1995, Hoffmann et al., 1995). The bandgap energy of TiO2 varies
from 3.0-3.2 eV based on its structure and size. Commercially used TiO2 powder (P25)
has a bandgap energy of 3.2 eV, equal to the energy of photons with wavelength of 385
nm. Photons with this energy or higher can promote electrons in the valence band of such
TiO2 to its conduction band, leaving a positive hole in the valence band (Fox and Dulay,
1993). An electron scavenger, such as oxygen, can capture the electron from the
conduction band and form a superoxide radical ion, whereas the remaining hole is able to
oxidize most organic molecules or oxidize H2O to surface hydroxyl radicals. The holes
and hydroxyl radicals in addition to the superoxide oxygen radicals are reactive species
and can initiate the degradation of pesticides.
Use of mercury discharge lamps to conduct irradiation is the conventional approach in
TiO2 photocatalysis. However, energy costs and lamp life are factors limiting
applications in photocatalysis. In addition, after their service life, the mercury in these
lamps poses an environmental hazard. A light emitted diode (LED), a recent and novel
light source, has a long lifespan, high energy efficiency and small size. LEDs are also
mercury-free and cost effective. These advantages render it an attractive light source for
investigating AOPs. So far, there are only a few papers on LED photocatalysis applied to
the field of environmental engineering. The combination of visible LED and
photocatalysts has been used to treat chlorophenol (Ghosh et al., 2008) and inactivate E.
coli (Chen et al., 2011) in aqueous media. Air purification using ultraviolet (UV) LED
photocatalysis has been reported in several papers (Huang et al., 2009, Shie et al., 2008,
81
Chen et al., 2005). Wang and Ku (2006) successfully used UV LED photocatalysis to
degrade dyes in water sample. To the best of our knowledge, no research focuses on
photocatalytic treatment of pesticides with UV-LED. In this paper, an investigation on
TiO2-based photocatalytic degradation of four pesticides in an LED photoreactor is
reported. Further, a comparison is made between the efficiency of degradation with UV
LED lamps and mercury discharge lamps (black lamps).
4.2 Methods and Materials
4.2.1 Photoreactor
An LED-based photoreactor fitted with UV LED lamps (λmax = 365 nm, full width at half
maximum = 15 nm, NSHU551B) was designed and fabricated. The LED lamps were
procured from Nichia Corporation (Japan). The outer body of the batch reactor is made of
PVC, whereas a reflective material is coated on the inside to minimize loss of light.
Ninety UV LED lamps are arranged in 15 rows with each row having six lamps (Figure
4-1). Every six lamps in series are driven by an LED driver (LT3465). Series connection
of the LEDs can provide identical currents and eliminate the need for ballast resistors. To
minimize the rise of temperature, a small fan was fixed on the wall of the reactor. In
addition, a specific insert containing 15 holes was fabricated, which was used between
the lamps and the sample to partially block the light and consequently vary the intensity.
The number of holes of the insert, through which the light passed, was varied by covering
different number of holes with black tapes. The more holes were covered with black
82
tapes, the lower light intensity was obtained in the center of the insert. As the sample was
continuously stirred, the sample illumination was uniform.
For the irradiation experiments using mercury discharge lamps, a standard Rayonet
photoreactor equipped with 2 black lamps (λmax = 350 nm, full width at half maximum =
50 nm, Hitachi FL8BL-B) was used.
Figure 4-1: LED photoreactor and insert.
83
4.2.2 Chemicals
Ninety-eight percent pure MCPA, 99% pure 4-CP, 99% pure 2,4-D and 99% pure 2,4-
DCP, and 98% pure formic acid were procured from Sigma Aldrich; TiO2 (P25) was
bought from Degussa; and high-pressure liquid chromatography (HPLC)-grade
acetonitrile was obtained from VWR. All of the chemicals were used as received. Milli-Q
water was used in the experiments.
4.2.3 Photocatalytic degradation
Pesticide solutions containing either one pesticide or a pesticide mixture were prepared.
To prepare 20 mg L-1 of a single pesticide solution, 10 mg of its pure product was
dissolved in 500 ml of water using ultrasonication. For pesticide mixtures, 10 mg of each
pesticide was dissolved in 500 ml of water using ultrasonication. The solutions were kept
in dark and stored in a refrigerator. The irradiation experiments were conducted in a
small Pyrex glass vessel [an inner diameter of 2.8 cm], in which 20 ml of the solution was
placed. Different experiments were conducted by adding to the solution varying amounts
of TiO2 photocatalyst. The amounts of TiO2 for different experiments can be founded in
the caption of Figure 4-2 to Figure 4-6. Prior to irradiation, the solution that contained
photocatalyst was stirred for 30 minutes in the dark to ensure that the adsorption of the
pesticide onto the surface of photocatalyst reached equilibrium. During irradiation, a
magnetic stirrer was used to homogenize the solution, and 1-mL samples after different
irradiation periods were collected in a 1.5 ml centrifuge vial.
84
Experiments were conducted using both the LED photoreactor and the Rayonet
photoreactor. Each experiment was conducted in duplicate and results were reported as
an error bar along with the average.
4.2.4 Actinometric Experiment
The amount of radiation entering the reaction vessel was determined using ferrioxalate
actinometry which can monitor wavelengths from 254 nm to 500 nm (Calvert and Pitts,
1966). Table 4-1 shows the varying light intensities measured inside a 20 ml reaction
vessel for the LED and Rayonet photoreactor.
Table 4-1: Light intensity of different photoreactors.
Type of photoreactor Light intensity (×1016 photon s-1)
LED photoreactor 8.55
LED photoreactor with an insert 3.95
LED photoreactor with an insert, 10 holes covered 1.20
LED photoreactor with an insert, 14 holes covered 0.49
Rayonet photoreactor with two black lamps 8.25
85
4.2.5 Analysis of sample
4.2.5.1 HPLC Analysis
All collected samples were initially centrifuged (5,000 resolution min-1) for 5 minutes
with a Fisher Scientific Micro Centrifuge (Model 59A); the supernatant was then passed
through a 0.22 µm filter (Micro Separation) to remove all TiO2 particles. The filtrate was
then stored in a 2-ml amber glass vial in the refrigerator. 2,4-Dichlorophenoxyacetic acid,
MCPA, 2,4-DCP and 4-CP were identified and quantified using a Varian Prostar 210
HPLC instrument equipped with a 325-liquid chromatrography (LC) UV-Visible
detector. A Kinetex pentafluorophenyl (PFP) column (2.6-µM 100 Å) was used to
separate the parent compound and its byproducts. A 20 µL sample was injected and
isocratic elution with a 1.0 ml min-1 flow rate was used in analysis; the eluent was
comprised of 50% acetonitrile (0.1% w/v formic acid) and 50% water (0.1% w/v formic
acid). The UV-Visible detector wavelength was set at 280 nm and the temperature was
controlled at 25 oC. Identification of each compound was achieved by comparing their
retention time with known commercially procured standards. Each compound was
quantified using an external standard. For each compound, a calibration curve was
prepared by using seven different concentrations of standard solutions. The detection
limit for each pesticide is 0.1 mg L-1.
4.2.5.2 TOC Analysis:
Twenty milliliters of samples at different irradiation time were centrifuged at 2, 000
revolution min-1 for 10 minutes with a Centaur 2 centrifuge (Fisons). The supernatant was
86
then passed through a syringe filter of 0.44 µm pore size (Whatman Inc), and analyzed
using an Apollo 9000 Combustion total organic carbon (TOC) analyzer equipped with an
autosampler. The detection limit is 0.1 mg L-1.
4.3 Results and Discussions
4.3.1 Photocatalytic degradation of pesticides and chlorophenols
In the UV-Visible spectrum range from 250-700 nm, 2,4-D, in addition to the other three
pesticides (MCPA, 4-CP, and 2,4-DCP), have single peaks with a maximum at around
280 nm. None of them has absorption at either 350 nm (peak radiation for the mercury
discharge black lamps) or 365 nm (peak wavelength for the LEDs). Our control
experiments also showed that no degradation occurred by direct irradiation without TiO2
photocatalyst. Consequently, direct photolysis does not play a role in the degradation of
these chemicals. Any photodegradation that may occur is a consequence of TiO2
photocatalysis.
Figure 4-2 (a) plots the change in concentration of the four pesticides with 2 g L-1 TiO2
under UV-LED irradiation against the energy dosage. All the data reported here are
reliable and the errors for all duplicates are within 5%. Energy dosage, based on the
number of photons entering the solution, provides a scalable parameter and aids a
comparison between the mercury discharge lamps and LEDs. From a kinetic perspective,
it is linear with time and hence a surrogate for time is recognizing reaction orders.
87
Figure 4-2: Photocatalytic degradation of different pesticides with UV-LED
photoreactor (Io = 8.55×1016 photon s-1, CTiO2=2.0 g L-1, Co=20 mg L-1): (a) loss of
parent pesticides; and (b) loss of total organic carbon.
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0 0.05 0.1 0.15 0.2
C/C
o-Pe
stic
ides
Energy dosage (kJ)
(a) MCPA 2,4-D
2,4-DCP 4-CP
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0 0.1 0.2 0.3 0.4 0.5
C/C
o-TO
C
Energy dosage (kJ)
(b)
MCPA 2,4-D
2,4-DCP 4-CP
88
The results, obtained using a HPLC, show that all four compounds can be catalytically
degraded in the LED reactor. For MCPA, with 0.014 kJ energy dosage, 90% was
removed from solution, and when the energy dosage reached 0.041 kJ more than 99%
was degraded. More than 70% of 2,4-D degraded within 0.014 kJ dosage and it became
undetectable with a 0.041 kJ. Degradation of chlorophenols was slower than those of 2,4-
D and MCPA. To get about 50% degradation of 2,4-DCP and 4-CP, a 0.055 kJ energy
dosage was needed. When the energy dosage increased to 0.11 kJ, they achieved more
than 99% degradation. The first order rate coefficients (Table 4-2) showed that
photocatalytic degradation of MCPA is 10× faster than that of 4-CP. The photocatalytic
degradation of these compounds is caused by the attack of generated surface holes and/or
hydroxyl radicals. Possible degradation pathways attributable to hydroxyl radicals
attacking can be found in several papers (Theurich et al., 1996, Topalov et al., 2001,
Kwan and Chu, 2004). The major pathways of photocatalytic degradation of phenoxy
pesticides are through homolysis of carbon-oxygen bond on its aromatic ring. This step is
very fast and leads to higher disappearance rates for 2,4-D and MCPA than
chlorophenols. In adsorption experiment, respectively, 17%, 15%, 3% and 3% of MCPA,
2,4-D, 4-CP and 2,4-DCP adsorbed on the surface of TiO2 after 30 minutes dark. The
higher percentage of adsorption of MCPA and 2,4-D also contribute to higher
degradation rates based on Langmuir-Hinshelwood mechanism (Fox and Dulay, 1993).
Analysis of intermediates was not conducted, but measurement of TOC at different times
identifies the extent of mineralisation. Figure 4-2 (b) shows the TOC results for different
pesticides. For MCPA, 2,4-D, 4-CP and 2,4-DCP to be completely (not detectable)
mineralized, one needs less than 0.5 kJ energy. Complete mineralization shows that the
89
intermediates produced from the degradation of these four pesticides are not recalcitrant
and can be mineralised to carbon dioxide and other inorganic compounds.
Table 4-2: First order rate coefficients (K) for photocatalytic pegradation of
different pesticides.
Pesticide k (× 10-3 s-1)
MCPA 6.02
2,4-D 4.01
4-CP 0.63
2,4-DCP 0.52
Note: Experiments with 2 g L-1 TiO2 and a light intensity of 8.55×1016 photon s-1.
4.3.2 Photocatalytic degradation of pesticides mixtures
Most contaminated water contains a mixture of different compounds. To investigate how
pesticides compete with each other, three mixtures of pesticides were studied. Table 4-3
shows the composition of the mixtures.
Table 4-3: Mixtures of Pesticides.
Mixture Composition
A 20 mg L-1 of 4-CP, 20 mg L-1 of 2,4-DCP
B 20 mg L-1 of 2,4-D, 20 mg L-1 of 4-CP
C 20 mg L-1 of 2,4-D, 20 mg L-1 of 2,4-DCP
90
Figure 4-3: Photocatalytic degradation of pesticides mixture with UV-LED
photoreactor based on the loss of pesticides detected by HPLC (Io =8.55×1016 photon
s-1, CTiO2=2 g L-1, Co=20 mg L-1): (a) mixture containing 4-CP and 2,4-DCP; (b)
mixture containing 4-CP and 2,4-D; (c) mixture containing 2,4-DCP and 2,4-D.
0.0
0.5
1.0
1.5
0 0.03 0.06 0.09 0.12 0.15 0.18
C/C
o
Energy dosage (kJ)
(a) 4-CP 2,4-DCP 4-CP (mixture) 2,4-DCP (mixture)
0.0
0.5
1.0
1.5
0 0.03 0.06 0.09 0.12 0.15 0.18
C/C
o
Energy dosage (kJ)
(b) 4-CP 2,4-D 4-CP (mixture) 2,4-D (mixture)
0.0
0.5
1.0
1.5
0 0.03 0.06 0.09 0.12 0.15 0.18
C/C
o
Energy dosage (kJ)
(c) 2,4-D 2,4-DCP 2,4-D (mixture) 2,4-DCP (mixture)
91
Figure 4-3 presents the results for the photocatalytic degradation of pesticide mixtures. In
each case, the rate of degradation of a pesticide in the mixture was slower than that in
solution containing a single pesticide. To compare the mixture and solution containing
single pesticide quantitatively for each case, the percentage removal of pesticides at the
same energy dosage (0.028 kJ) for each case was summarized in Table 4-4. For Mixture
A, it was found that only 11% removal for 4-CP and 19% removal for 2,4-DCP were
achieved with 0.028 kJ energy dosage, however, more than 25% removal for 4-CP and
28% removal for 2,4-DCP was attained with this energy dosage when a single pesticide
solution was used. In Mixture B, the percentage removal of 2,4-D was similar to its single
pesticide solution with a 0.028 kJ energy dosage; however, the percentage removal of
Table 4-4: Percentage removal of pesticides at 0.028 kJ energy dosage.
Case
Percentage removal
2,4-D 4-CP 2,4-DCP
Single pesticides 90 25 28
Mixture A _ 11 19
Mixture B 88 11 _
Mixture C 72 _ -24a
Note: Energy dosage =0.028 kJ.
ain the mixture C, the concentration of 2,4-DCP is increased by 24% at the energy dosage
of 0.028 kJ, reflecting production from 2,4-D.
92
4-CP significantly decreased. In Mixture C, the removal rate of 2,4-D is apparently
smaller compared with its single pesticide solution with a 0.028 kJ energy dosage, and
the concentration of 2,4-DCP increased at this energy dosage as it was produced upon the
degradation of 2,4D. In Mixture C, the concentration of 2,4-DCP initially increased and
then decreased after reaching a maximum.
The previous results can possibly be explained by the competition between pesticides for
photons, adsorption sites, and the holes or hydroxyl radicals on the surfaces. In our case,
all of these pesticides neither absorb nor directly use a photon of wavelength of 350 or
365 nm; consequently, the competition for the photons can be neglected in this paper. The
results of pesticides adsorption on the surface of TiO2 after 30 minutes in the dark (Table
4-5) showed that the additional pesticides do not affect the adsorption of previous
existing pesticides on surface of TiO2 in the solution, which indicates 2 g L-1 TiO2 can
provide enough adsorption sites for pesticides in our experimental conditions and the
competition for the adsorption sites is trivial. Therefore, the competition for hydroxyl
radicals/holes may be the real reason causing the slower degradation rate in mixtures. In
mixture A, 4-CP and 2,4-DCP have similar properties, thus the competing effect for
hydroxyl radicals or holes is similar and their corresponding degradation rates were
significantly affected by each other. In mixture B, 2,4-D is a superior competitor than 4-
CP for harvesting hydroxyl radicals, because the carbon-oxygen bond on the aromatic
ring of 2,4-D is more easily broken down by hydroxyl radical. Thus, degradation of 2,4-D
is not affected too much but the degradation of 4-CP is significantly retarded. In mixture
93
C, hydroxyl radicals also favoured attacking 2,4-D, and lead to an accumulation of 2,4-
DCP at beginning and then decreased because of attacking by hydroxyl radicals. In this
case, the accumulation of 2,4-DCP can inversely retard the degradation of 2,4-D to form
2,4-DCP.
Table 4-5: Percentage of pesticides adsorbed on the surface of TiO2 after 30 minutes
of stirring in the dark.
Case
Percentage adsorption
2,4-D 4-CP 2,4-DCP
Single pesticides 15 3 3
Mixture A / 3 3
Mixture B 14 3 /
Mixture C 14 / 3
4.3.3 Effect of Photocatalyst Loading
Five different TiO2 loadings (0.2, 0.5, 1.0, 2.0 and 3.0 g L-1) were investigated for the
photocatalytic degradation of 2,4-D. Figure 4-4(a) shows the result of 2,4-D
concentration against irradiation time. The photocatalytic degradation of 2,4-D in our
system follows approximate first-order kinetics. Figure 4-4(b) plots the first-order rate
constants fitted over the first 15 minutes. The rate constants increased with TiO2 loading
94
(a)
(b)
Figure 4-4: Photocatalytic degradation of 2,4-D with different TiO2 loadings and
LED irradiation (Io =8.55×1016 photon s-1, Co=20 mg L-1).
appearing to approach a limiting value with increased TiO2 loadings, as expected for
light-limiting conditions (Augugliaro et al., 1988). The light-limiting condition is
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0 10 20 30 40 50 60
C/C
o
Irradiation time (min)
[TiO₂]=0.2 g/L
[TiO₂]=0.5 g/L
[TiO₂]=1.0 g/L
[TiO₂]=2.0 g/L
[TiO₂]=3.0 g/L
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0 0.5 1 1.5 2 2.5 3 3.5
r (m
in-1
)
[TiO2] (g L-1)
95
confirmed by the dependence of first-order rate on light intensity. An increase of TiO2
loading at lower levels can significantly improve the photocatalytic degradation rate.
When the TiO2 loading increased to 0.5 g L-1 from 0.2 g L-1, the rate constant doubled;
however, at a high level, increasing TiO2 loading does not contribute too much to the
improvement of photocatalytic degradation rate. An increase in TiO2 loading from 2.0 g
L-1 to 3.0 g L-1 only led to a 5% enhancement in the photocatalytic degradation rate.
Based on this result, a suitable loading of TiO2 was determined to be 2.0 g L-1, which had
a kinetic rate constant of 0.24 min-1.
As chemical reactions occur on the surface of TiO2, the efficiency of photocatalytic
degradation depends on the surface of TiO2 particles, which can simultaneously be in
contact with the target contaminant and illuminated by light. At low TiO2 loadings and
unchanged light intensity the active absorbing surface area is a limiting factor, thus better
degradation efficiency is observed as TiO2 loading increased. Nevertheless, with
increased TiO2 loadings, the light available is utilized efficiently and light becomes the
limiting factor.
4.3.4 Effect of Light Intensity
Investigations were conducted with varying light intensities. With the optimal TiO2
loading of 2.0 g L-1, four different light intensities (4.9×1015, 1.28×1016, 3.95×1016 and
8.55×1016 photon s-1) were investigated in 2,4-D photocatalytic degradation. Figure 4-5
shows the effect of different light intensities on 2,4-D degradation. The smallest first
96
(a)
(b)
Figure 4-5: Photocatalytic degradation of 2,4-D with UV-LED photoreactor under
different light conditions (Co=20 mg L-1, CTiO2=2 g L-1).
order rate constant was 0.0091 min-1 at a light intensity of 4.9×1015 photon s-1, and the
largest rate constant was 0.2404 min-1 at light intensity of 8.55×1016 photon s-1. The
0.00
0.20
0.40
0.60
0.80
1.00
1.20
0 10 20 30 40 50 60
C/C
o
Irradiation time (min)
Iₒ=4.90×10¹⁵ photon s⁻¹ Iₒ=1.28×10¹⁶ photon s⁻¹ Iₒ=3.95×10¹⁶ photon s⁻¹ Iₒ=8.55×10¹⁶ photon s⁻¹
0
0.05
0.1
0.15
0.2
0.25
0.3
0 1 2 3 4 5 6 7 8 9
r(m
in-1
)
Io (*1016 photon s-1)
97
percentage of 2,4-D eliminated was dependent on the light intensity, which is
proportional to the intensity of irradiation. Based on the previous results, the applied
TiO2 loading (2.0 g L-1) reported in this paper is not a limiting factor for the light
intensity investigated. Therefore, it was the light intensity that causes the different
degradation kinetics reported in this paper. Ollis et al. (1991) summarized the relationship
between light intensity and kinetics rate as follows: (1) at low light intensity electron-hole
formation dominates and rate is proportional to the light intensity; (2) as light intensity
increased, electron-hole pair generation competes with electron-hole recombination and
the kinetic rate is proportional to the square root of light intensity; (3) at an even higher
light intensity, TiO2 loading may become a limiting factor. The behaviour reported in this
paper indicates that the applied light intensity is relative low, so electron-hole formation
dominated in the photocatalytic processes.
4.3.5 Comparison between LED and Mercury Lamp Irradiation
Figure 4-6 shows the degradation of 2,4-D in both LED and the Rayonet photoreactor in
terms of concentration versus energy dosage. Photocatalytic degradation of 2,4-D with
LEDs as the light source has a higher energy-efficiency than that with mercury lamps. To
compare the efficiency of LED lamps and mercury lamps, the concept of photon energy
per order (PEPO) was developed and used. PEPO refers to the amount of photonic energy
required to reduce an order of magnitude of contaminant concentration. PEPO is 0.028 kJ
per order for the LED irradiation while it increased to 0.038 kJ per order for mercury
lamps. To understand this phenomenon, two possible hypotheses are provided. First,
photons with longer wavelengths have smaller energies; consequently, for the same
98
energy dosage, more photons enter the reaction vessel in the LED (365 nm) reactor than
the Rayonet reactor equipped with black lamps (350 nm). For example, a 1 J energy
dosage is equal to the total energy of 1.83×1018 photons at a wavelength of 365 nm, or
1.76×1018 photons at a wavelength of 350 nm, accounting for 4% excess photons for the
LED. Second, black lamps also emit more photons of longer wavelengths, which cannot
excite TiO2 and hence become useless. Our calculations based on the emission spectra of
Figure 4-6: Photocatalytic degradation of 2,4-D in the two photoreactors: Co=20 mg
L-1, CTiO2=2 g L-1. (a): LED reactor, Io =8.55×1016 photon s-1; (b): Rayonet reactor,
Io =8.25×1016 photon s-1.
lamps, show that, in the LED lamps, 99.9% of the energy is available for the reaction;
however, a smaller number (95.8%) was obtained for mercury lamps (see Appendix D),
0.00
0.20
0.40
0.60
0.80
1.00
1.20
0 0.02 0.04 0.06 0.08 0.1
C/C
o
Energy dosage (kJ)
Mercury lamps LED
99
another 4% advantage for LEDs. These approximate calculations can account, at least
qualitatively, for the 8% advantage of the LEDs. This shows the advantage of LEDs,
which will be even larger when compared to solar irradiation where the spectrum is broad
and limited in the ultraviolet B range at the Earth’s surface. The essential fact here is that
LEDs can be rendered monochromatic close to the band edge of the photocatalyt so that
photon loss does not occur from non-absorbed wavelengths, nor does energy above the
band edge get wasted as heat.
4.4 Conclusions
In this research, LEDs were shown to be a promising light source in photocatalytic
treatment of pesticides. For all four pesticides investigated, more than 99% degradation
was achieved within a short period of irradiation with 2.0 g L-1 of TiO2 and UV LED
(365 nm) irradiation. The degradation of pesticides in the mixture is slower than when
only one pesticide is investigated because of the competition for surface hydroxyl
radicals between different compounds. When this applied light intensity is 8.55×1016
photon/s, a suitable loading of TiO2 for 2,4-dichlorophenoxyacetic acid degradation was
determined to be 2.0 g L-1. The rate constant at this loading was 0.2404 min-1. The
relationship between light intensity and first order kinetic constants was linear.
Furthermore, the comparison between mercury lamps and LEDs show that LEDs can be
more energy-efficient, and the emission spectrum of LED lamps can be well-matched
with the absorption band of TiO2. If a solar reactor is considered as a competitor, the
photon energy-use advantage of LEDs is even greater.
100
Chapter Five: DESIGN A HOMOGENEOUS RADIATION FIELD IN A UV-LED BASED PHOTOCATALYTIC REACTOR
5.1 Introduction
TiO2 photocatalysis has been widely investigated for its use in water and wastewater
treatment and in air purification (Peral and Ollis, 1992, Hoffmann et al., 1995, Fujishima
et al., 2000). It is based on the principle that ultraviolet (UV) light when incident on the
photocatalyst (TiO2) leads to the generation of strong oxidants such as holes or hydroxyl
radicals. These oxidants are capable of oxidizing most organic compounds and eventually
leading to their mineralisation. Conventional UV light source for photocatalysis use
mercury lamps. With the advent of light emitting diodes (LEDs), they are increasingly
being considered as an attractive alternative to mercury lamps. The advantages of LEDs
include a longer life span, smaller size, higher energy efficiency where technology is
mature, and being mercury free. As LED technology advances further, it will become
more cost effective. The application of UV-LED in photocatalytic treatment of water and
wastewater is still novel and has only been reported by a few researchers (Natarajan et
al., 2011b, Wang and Ku, 2006, Yu et al., 2013).
As cost effectiveness these systems depend on the efficient use of light, an optimized
radiation field such that maximum photons are harvested, is necessary. Some researchers
(Imoberdorf et al., 2008b, Jacob and Dranoff, 1969, Jacobm and Dranoff, 1970, Irazoqui
et al., 1973, Alfano et al., 1986a) have described various radiation field models generated
by conventional light source both in homogenous and heterogeneous environments
101
involving different geometries. Wang et al (2012) studied the radiation field model
generated by the UV-LED array and showed that the homogeneity of the radiation is
affected by the distances between the UV-LED array and photocatalyst plate. However,
no discussion on how to develop an optimized homogenous radiation field was presented.
In this paper, a radiation field model is developed for a UV-LED array in a planar
photoreactor with the intent to use it for designing a homogenous radiation field such as
that photon harvesting can be maximized.
5.2 Advantage of homogeneous radiation field in a photocatalytic reactor
Several papers (Mehrotra et al., 2005, Ollis et al., 1991, Choi et al., 2000, Kim and Hong,
2002) have investigated the effect of light intensity on photocatatyic kinetics, showing
that first order photocatalytic kinetics rate (K) usually follows power-law dependence on
light intensity (I) ( Equation[5-1] ).
𝐾 = 𝜂𝐼𝛾 [5-1]
Where 𝜂, 𝛾 are constant, 𝜂 is a positive value and 𝛾 has a value between 0 and 1. The
performance of a photocatalytic reactor is determined by the average photocatalytic
kinetics rate (Ka) occurring on the photocatalyst plate (Equation [5-2]).
Ka = ∫KdApAp
= ∫𝜂𝐼𝛾dApAp
[5-2]
102
Where Ap is the area of the photocatalyst plate. Once the photocatalyst plate is
homogenously irradiated, the average kinetics rate become:
Ka = ∫𝜂𝐼𝑎𝛾dApAp
[5-3]
Where 𝐼𝑎 is the average light intensity for a homogenous radiation field.
𝐼𝑎 =∫ 𝐼dAp
Ap [5-4]
It is known that 𝑓(𝐼) = 𝜂𝐼𝛾 is a concave function. Hence, according to Jensen's
inequality,
∫𝜂𝐼𝛾dApAp
≤ ∫𝜂𝐼𝑎𝛾dApAp
[5-5]
Equation [5-5] revealed that the average photocatalytic kinetics rate in a homogeneous
radiation field is larger than that in a non-homogenous radiation field. Therefore, a
homogenous radiation field is superior for an ideal photocatalytic reactor. Besides, a
homogeneous radiation field can simplify the photocatalytic process model and require
less computation.
103
5.3 Development of radiation field model
5.3.1 UV-LED array and photocatalyst plate
Figure 5-1 provides the geometry of a UV-LED array and a fixed catalyst surface. The
UV-LED array panel is considered parallel to the photocatalyst plate. The UV-LEDs in
the panel were arranged as a regular array. The distance between UV-LEDs panel and
photocatalyst plate is known as the ''irradiated distance'' (ID). The distance between the
two adjacent LEDs is called as the ''gap''.
Figure 5-1: UV-LED array and photocatalyst plate.
For the experiments to calibrate and validate the model, a panel comprising of 16 UV-
LEDs arranged in a 4 by 4 array was fabricated. UV-LEDs were spaced 2.5 cm and
surface mounted on a 10 cm by 10 cm PCB board. UV-LEDs (λmax=365nm, half width=9
104
nm, NSCU033) were procured from Nichia Corporation. The UV-LEDs were driven by a
high current Quad output LED driver (LT3476). A Pyrex glass plate (thickness=0. 32 cm)
was obtained from Chemglass Life Sciences (Vineland, NJ, US) and used as a shielding
glass. The light transmittance of the glass plate is around 90% at 365 nm.
5.3.2 Radiation field model without shielding glass plate
In most photocatalytic reactors, the UV light source is protected by a glass shield (Figure
5-1.c), so the impact of shielding glass on radiation field model needs to be considered. In
this first instance, a radiation field model without shielding glass (Figure 5-1.b) is
developed. The optical effects such as scattering, reflection and refraction were assumed
to be negligible. All UV-LED lamps were assumed to be identical and considered to be
point light sources with a special directivity. The radiation field model was developed
using a similar method described in Wang et al (2012). Initially, a radiation field model
produced by a single UV-LED was established. Then the radiation field generated by
multiple UV-LEDs was developed by considering the sum of the contribution of each
UV-LED. A stepwise description of the development of the radiation field is given
below:
(1) Obtain the radiation directivity function [Re(θ)] for a single UV-LED. The radiation
directivity function (Equation [5-6]) is the ratio between light intensity [I(θ)] with view
angle θ and the light intensity with a zero view angle. View angle (θ) is defined as the
angle between the radiation direction and the direction perpendicular to the UV-LED.
105
𝑅𝑒(𝛉) = 𝐼(𝛉)𝐼(0)
[5-6]
For most UV-LEDs, the light emitted from UV-LED varies with the radiation directivity.
Figure 5-2 shows the radiation directivity of our UV-LED. The light intensity emitted
from zero view angle has the maximum value, and then it gradually decrease as the view
angle increases. The light intensity approach zero when the view angle is 70o or 1.22
radian. In this study a quadratic equation (Equation [5-7]) was used to simulate the
radiation directivity within 1.22 radian.
𝑅𝑒(θ) = 1 − 0.67θ2 [5-7]
When 1.22 ≤ θ < π/2, 𝑅𝑒(θ)=0, so we have:
𝑅𝑒(θ) = �1 − 0.67θ2, 0 < θ < 1.220, 1.22 ≤ θ < π/2
� [5-8]
Figure 5-2: Directivity of radiation (NICHIA, 2013).
106
(2) Determine the relationship between the light intensities at different radial distances
when view angle is the same. When the view angle is constant, the light intensity is
proportional to the inverse of the square of distance (Equation [5-9]).
𝐼 ∝ 1R2
[5-9]
(3) Develop a function between the light intensity [I(θ,R)] at any position and the
referenced light intensity [I(0,do)]. The referenced light intensity is the light intensity at a
specific distance (do) and with a zero view angle. Based on Equation [5-6] and [5-9], I(θ,
R) can be expressed as a function of I(0,do) as:
𝐼(θ, R) = 𝐼(θ, do) ∗do
2
R2 = 𝐼(0, do) ∗ 𝑅𝑒(θ) ∗do
2
R2 [5-10]
Considering the normal flux density, then:
𝐼𝑛(θ, R) = 𝐼(θ, R) ∗ cos(θ) = 𝐼(0, do) ∗ 𝑅𝑒(θ) ∗do
2
R2 ∗ cos(θ) [5-11]
(4) Convert the polar coordinates in Equation [5-11] to Cartesian coordinates. The
Cartesian coordinate system and the polar coordinate system are shown in Figure 5-3,
where the x-y plane is a photocatalyst plate.
107
Figure 5-3: Cartesian and polar coordinates in radiation system.
cos (θ) = dR [5-12]
R2 = d2 + g2 = d2 + (x − xo)2 + (y − yo)2 [5-13]
θ = arctan (�(x−xo)2+(y−yo)2
d) [5-14]
Incorporating Equations [5-12], [5-13] and [5-14] into Equation [5-11], the normal light
intensity at any point on a photocatalyst plate can be expressed as a function of d and its
x-y coordinates as:
108
In(x, y, d) = 𝐼(0, do) ∗ Re�arctan (�(x− xo)2 + (y − yo)2
d) �
∗do
2 ∗ d(d2 + (x − xo)2 + (y − yo)2)3/2
[5-15]
Therefore, the normal light intensity at any point due to the contribution of multiple LED
lamps is:
𝐼𝑠𝑢𝑚(x, y, d) = � Ini(x, y, d)n
i=1
= � (𝐼(0, do) ∗ Re�arctan��(x − xi)2 + (y − yi)2
d��
n
i=1
∗do
2 ∗ d
(d2 + (x − xi)2 + (y − yi)2)32
)
[5-16]
(5) Measure and estimate the referenced light intensity. The referenced light intensity
can be measured by UV radiometer or be estimated based on its relationship with the
light output of UV-LED (It) shown as Equations [5-17] ~ [5-20].
The light output of a UV-LED is the integral of radiation over the entire view angles:
It = � I(θ)dsθo
0 [5-17]
The following relationships can be obtained from Figure 5-4.
ds ≅ 2πr ∗ dl = 2πr ∗ R ∗ dθ = 2π ∗ R ∗ sinθ ∗ R ∗ dθ = 2πR2 ∗ sinθ ∗ dθ [5-18]
109
Substitute Equation [5-6] and Equation [5-18] into Equation [5-17]:
It = I0 ∗ 2πR2 ∫ Re(θ) ∗ sinθdθθo0 [5-19]
I0 =It
2πR2 ∫ Re(θ) ∗ sinθdθθo0
[5-20]
Figure 5-4: Scheme of UV-LED radiation.
5.3.3 Radiation field model with a shielding glass plate
The light intensity is attenuated due to the absorption by the shielding glass. The loss of
light intensity can be expressed by the Beer-Lambert law:
T = 10−β𝑙 [5-21]
110
Where T is the transmittance, 𝑙 is the light passing length in glass plate, and β is the
attenuation coefficient. After considering the light absorption by shielding glass plate, the
modified directivity function will be:
𝑅𝑒′(𝜃) = 𝑅𝑒(𝜃) ∗ T(𝜃)T(0) = 𝑅𝑒(𝜃) ∗ 10
−β𝑙𝑔𝑐𝑜𝑠 (𝜃)
10−β𝑙𝑔= 𝑅𝑒(𝜃) ∗ 10𝑙𝑔∗β∗(1− 1
𝑐𝑜𝑠 (𝜃)) [5-22]
Where T(𝜃) is the ratio of light passing through the glass plate with a view angle ( 𝜃), 𝑙𝑔
is the thickness of glass plate. Therefore, the modified radiation field model becomes:
𝐼𝑠𝑢𝑚(x, y, d) = � (𝐼(0, do) ∗ Re�arctan��(x − xi)2 + (y − yi)2
d��
n
i=1
∗do
2 ∗ d
(d2 + (x − xi)2 + (y − yi)2)32
)
[5-23]
Where d > 𝑙𝑔.
5.4 Calibration and validation of the radiation field model
For the purpose of model calibration, the light intensity at a distance of 1cm
perpendicular to a single UV-LED was used as referenced light intensity. The model was
validated using the light intensity generated by the UV-LED array at different distances
and locations.
5.4.1 Light intensity measurement
The light intensity was measured using a Silver Line UV Radiometer (M007153, Geneq
Inc. Canada). The measurements range from 0-2000 mW cm-2. The sensor of the
111
radiometer has a radius of 0.25 cm. The average light intensity received by the sensor
was reported in this UV radiometer reader. The sensor surface was kept parallel to the
UV-LED panel when the light intensity was measured.
The readout of the light intensity from the radiometer is the average light intensity
received by the sensor. A relationship between referenced light intensity and the UV
radiometer measured value need to be developed. The measured value received by the
sensor (Figure 5-5) can be expressed as Equation [5-24]:
Imeasured =∫ Idsπrs2
=∫ I ∗ 2πr ∗ drrs0
πrs2=∫ I ∗ 2r ∗ drrs0
rs2 [5-24]
Where rs is the radius of the sensor, Imeasured is the light intensity read from the UV
radiometer, S is the area of the sensor.
Figure 5-5: Geometry of sensor.
112
According to Equation [5-15] and [5-24], we have:
� {𝐼(0, do) ∗ Re �arctan (r
do) � ∗
do2 ∗ do
(do2 + r2)3/2
∗ 2r ∗ drrs
0= rs2 ∗ Imeasured [5-25]
𝐼(0, do) =rs2 ∗ Imeasured
2do3 ∗ ∫ {Re �arctan ( r
do) � ∗ r
(do2 + r2)3/2 ∗ drrs
0
[5-26]
The average light intensity received by the sensor at 1 cm distance for a single UV-LED
lamp is measured to be 103.9 mW cm-2. Equation [5-26] provides the referenced light
intensity to be 110 mW cm-2.
5.4.2 Model light intensities vs measured light intensities
To validate the model developed above, it was necessary to compare the theoretically
calculated values with measured ones. As the UV radiometer provides the average light
intensity received by the sensor, the average model light intensity received by the sensor
were calculated and compared. The radiation field model without shielding glass and
with shielding glass were respectively calculated using Equation [5-16], Equation [5-23]
and parameters provided in Table 5-1.
113
Table 5-1: Parameters used for radiation field model calculation. Parameters Value
do 1 cm
I(0, do ) 110 mW cm-2
lg 0. 32 cm
θo 1.22 radian
β 0.143 cm-1
The difference between the model value and measured value was used to evaluate the
relative errors as:
𝑅𝑒𝑙𝑎𝑡𝑖𝑣𝑒 𝑒𝑟𝑟𝑜𝑟 = | 𝐼𝑚𝑜𝑑𝑒𝑙−𝐼𝑚𝑒𝑎𝑠𝑢𝑟𝑒𝐼𝑚𝑒𝑎𝑠𝑢𝑟𝑒
| ∗ 100% [5-27]
Results presented in Table 5-2. show that the relative errors are small, which indicates
that the radiation filed model reliably captures the light intensity.
Table 5-2: Comparison of modeled light intensity and measured light intensity.
Position Without Shielding Glass With Shielding Glass
x (m) y (m) d (m) Imodel (mW cm-2)
Imeasure (mW cm-2)
Relative Error
Imodel (mW cm-2)
Imeasure (mW cm-2)
Relative Error
0.072 0.06 0.036 31.9 31.4 1.5% 27.9 26.2 6.6%
0.056 0.06 0.036 34.0 34.4 1.1% 29.8 27.8 7.1%
0.041 0.06 0.036 33.8 33.2 1.9% 29.6 28.6 3.6%
0.027 0.06 0.036 31.5 29.6 6.5% 27.6 28.0 1.3%
0.062 0.06 0.054 25.9 26.0 0.3% 22.9 22.0 4.0%
0.049 0.06 0.054 26.5 26.8 1.0% 23..4 23.2 1.0%
0.033 0.06 0.054 25.2 26.4 4.5% 22.3 22.6 1.4%
114
5.5 Design of a homogenous radiation filed
The concept of Max Error (Me) was extended to evaluate the homogeneity of the
radiation field.
𝑴𝒆 = max [ �Imax−IaIa
� ∗ 100%, �Ia−IminIa
� ∗ 100%] [5-28]
Where 𝐼𝑚𝑎𝑥 is the maximum light intensity, 𝐼𝑚𝑖𝑛 is the minimum light intensity and Ia is
the average light intensity.
To achieve a high degree of homogeneity, the calculated Me should be small. If Me is
zero, the radiation filed is completely homogenous. Here the radiation field is defined to
be homogenous when Me is smaller than 1%.
5.5.1 The effect of ID on the homogeneity of radiation field for a fixed gap
To investigate the impact of ID/gap ratio on the homogeneity of radiation field, a 2 m by
2 m square photocatalyst plate was studied. To ignore the edge effects, the degree of
homogeneity is only calculated within 10 cm by 10 cm square area. Equation [5-16] was
used to determine the radiation field without shielding glass. The gap between two
adjacent lamps is fixed at 2.5 cm, and the Max Errors of radiation field for different
distances were calculated. The radiation field results for different irradiated distance are
shown in Figure 5-6 and the Max Error results are provided in Figure 5-7. The results
show that the degree of homogeneity is low when distance between the UV-LED panel
and photocatalyst plate approaches zero and the degree of homogeneity is increased as
''ID'' increased. However, the light intensity decreases as the ID increases. An ideal
radiation field has a high degree of homogeneity and a minimal loss of light intensity.
115
Therefore, the optimal "ID" is the smallest distance which generates a homogenous
radiation field.
(a)
(b)
Figure 5-6: The radiation field with different ID: (a) ID=0.01 m, gap=0.025 m; (b) ID= 0.04 m, gap=0.025 m.
116
Figure 5-7: The effect of irradiated distance (ID) on Maximum Error.
5.5.2 Optimal combination of ID and gap
To determine the optimal combination of ID and gap, gap is varied from 0.01 m to 0.1
m, and the optimal ID for each gap is investigated using the method described in part
5.5.1. The result (Figure 5-8) shows that the optimal ID is proportional to gap and the
optimal ID is 1.26 times of gap.
Figure 5-8: Optimal combination of ID and gap.
0%
50%
100%
150%
200%
250%
0 0.02 0.04 0.06 0.08 0.1
Me
ID (m)
ID = 1.26 *gap
0
0.03
0.06
0.09
0.12
0.15
0 0.02 0.04 0.06 0.08 0.1
ID (m
)
gap (m)
117
5.5.3 Selection of the output of the UV-LED
In section 5.5.2, the optimal combination of ID and gap has been determined. The next
step is to choose the light output of LED lamps which generate a specific homogenous
radiation field. The selection of the light out of LED can use the following methods:
(1) Specify the dimension of the gap of UV-LEDs array and specify the designed light
intensity received by the photocatalyst plate.
(2) Calculate the irradiated distance based on the optimal ID/gap ratio to achieve a
homogenous radiation field.
(3) Calculate the required referenced light intensity based on the radiation field model.
(4) Calculate the light output of LED lamp based on Equation [5-19].
The required light out of LED for different gaps and different designed light intensity
were shown in Figure 5-9.
Figure 5-9: Selection of light output of UV-LED.
0 500
1000 1500 2000 2500 3000 3500 4000 4500 5000 5500
0 0.02 0.04 0.06 0.08 0.1
Ligh
t out
put p
er la
mp
(mW
)
gap(m)
Designed light intensity = 50 mW/ cm² Designed light intensity = 20 mW/ cm² Designed light intensity = 10 mW/ cm² Designed light intensity = 5 mW/ cm² Designed light intensity = 2 mW/ cm² Designed light intensity = 1 mW/ cm² Designed light intensity = 0.5 mW/ cm²
118
5.6 Conclusions
Radiation field model for a UV-LEDs array has been developed, which can predict the
normal light intensity at any location of a photocatalyst plate with any ID. Based on the
model, the degree of homogeneity of the radiation field is significantly affected by the ID
when the gap is fixed. Homogenous radiation field can be achieved by choosing an
optimal ID/gap ratio. The ratio is found to be 1.26 for Nichia UV-LED (NCSU033).
Besides, the method of selecting the light output of UV-LED for different gaps to achieve
a desired homogenous light intensity was developed and evaluated.
119
Chapter Six: A NOVEL LIGHT EMITTING DIODE BASED PHOTOCATALYTIC REACTOR FOR WATER TREATMENT
6.1 Introduction
In recent decades, "emerging contaminants" such as pharmaceuticals, personal care
products, pesticides have been frequently detected in domestic wastewater and surface
water (Petrovic et al., 2008). Conventional treatment processes do not specifically target
these emerging contaminants, and their presence in aquatic environments has led to
adverse ecological effects. TiO2 based photocatalysis has been shown to be an efficient
method in dealing with these contaminants (Herrmann, 1999, Miranda-García et al.,
2011, Belgiorno et al., 2007). This technology utilizes strong oxidants such as hydroxyl
radicals or holes or reactive oxygen species formed upon electron capture by O2 to
oxidize most organic compounds. Usually this leads to conversion of parent compounds
to harmless compounds and in many cases complete mineralisation.
The successful application of TiO2 photocatalysis in water treatment requires designing
simple and efficient photocatalytic reactors. Generally, photocatalytic reactors are
classified as (a) slurry reactors and (b) immobilized reactors. In slurry reactors, TiO2
powder is dispersed in water and continual mixing ensures it has good contact with light
source. Commercial Degussa P25 is acknowledged as the best readily available TiO2
powder with high photocatalytic activity. In contrast, in immobilized reactors,
photocatalysts are immobilized on inert substrates and become less photon-efficient due
to their low active surface area to volume ratio. However, immobilized reactors are more
120
practical as they do not require the secondary step of separation of TiO2 particles from
treated waters. Direct anodization of titanium is recognized as one of the better methods
to prepare immobilized TiO2 film due to its simplicity and being able to control the
thickness and morphology of a nanotubular TiO2 film (Li et al., 2009). One way to
improve the surface area to reaction volume ratio in an immobilized photocatalytic
reactor is to coat TiO2 on 250 micron to millimeter sized particles that can be easily
separated (Vega et al., Geng and Cui, 2010, Imoberdorf et al., 2008a, Vaisman et al.,
2005, Pozzo et al., 2005, Kanki et al., 2005, Chiovetta et al., 2001, Haarstrick et al.,
1996).
Most conventional photocatalytic reactors obtain their ultraviolet (UV) irradiation from
mercury arc lamps which are not environmental friendly and can be less energy efficient
than light emitting diodes (LEDs). Recently, the development of UV-LED technology
has made UV-LEDs lamps a promising replacement for mercury lamps in TiO2
photocatalysis application (Yu et al., 2013, Natarajan et al., 2011b). The advantages of
UV-LEDs include smaller size, higher durability, longer life, narrower spectrum, energy
efficiency, and fast switching.
In this paper, the details of design and fabrication of a novel immobilized UV-LED
photocatalytic reactor is presented. The reactor have been evaluated by testing two
phenoxy pesticides [2,4-dichlorophenoxyacetic acid (2,4-D), 2-methyl-4-
chlorophenoxyacetic acid (MCPA)] and chlorophenols. The operational parameters of
reactor were optimized using the 2,4-D degradation as an example.
121
6.2 Experimental details
6.2.1 Chemicals
Ninety-nine percent pure 2,4-D , 98% pure MCPA, 99% pure 4-CP, 99% pure 2,4-DCP,
98% pure formic acid, 98% pure ammonium fluoride as well as 99.7% pure Ti foil were
procured from Sigma Aldrich; HPLC grade acetonitrile and 99% pure ethylene glycol
were obtained from VWR. TiO2 (P25) was obtained from Degussa. Hollow glass
microspheres coated with anatase TiO2 (HGMT) were obtained from Cospheric, which
has a median diameter of 45 µm. All chemicals were used as received and Milli-Q water
was used in all experiments.
6.2.2 Design and fabrication of an LED based photocatalytic reactor
6.2.2.1 Preparation of anodized TiO2 photocatalytic plate.
In this research, the immobilized TiO2 photocatalytic plate was prepared by
electrochemical anodization. Prior to anodization, a 15 cm by 15 cm titanium foil was
dipped in a mixture of acetone, methanol and methylene chloride and sonicated for 30
minutes for degreasing (Wang and Lin, 2009). Then the titanium foil was rinsed and
cleaned with water and dried in fume hood. The anodization was conducted in a two
electrode cell setup with Ti foil as an anode and an aluminum foil as a cathode. The
electrolyte used was an ethylene glycol solution containing 2 % water and 0.5 %
ammonium fluoride. The anodization was performed with a static potential (30V) using a
Lambda Regulated Power supply (Model: LE 104-FM), at a room temperature for 24
hours. The anodized Ti foil was washed with water and dried in the fume hood.
122
Thereafter, the dried anodized Ti foil was annealed at 450oC in air for 3 hours to induce
crystallization to give a better photocatalytic ability (Chang et al., 2011) . The surface
structure of anodized TiO2 was examined using scan electron microscopy (SEM). The
SEM result (Figure 6-1) showed that TiO2 nanotubes were synthesized on the surface of
the titanium foil. The diameters of nanotubes were approximately 100 nm.
Figure 6- 1: SEM image of anodized TiO2 nanostructrure.
6.2.2.2 UV-LEDs module
The UV-LEDs module composed of 16 UV-LED lamps (NSCU330B, Nichia
Corporation, Japan). The lamp has a sharp peak at λ=365nm with the half width band of 9
123
nm. Individual lamps were assembled into a four by four array and mounted on a 10 cm
by 10 cm square circuit board. The distance between adjacent lamps was 2.5 cm. The
UV-LEDs were driven by a high current Quad output LED driver (LT3476). To generate
different light intensities a dual output power supply (Model TW5005D) was used to
provide direct currents ranging from 10 mA to 500 mA.
6.2.2.3 Photocatalytic system
The UV-LED module and photocatalytic plate were mounted parallel in a polyvinyl
chloride (PVC) based casing (Figure 6-2). A Hydro H55 CPU Cooler on the back of
LED module was used to control the temperature. A Pyrex borosilicate glass plate (CG-
1904-16, Chemglass life Sciences) was used to shield electronic parts. UVA light passes
efficiently through the Pyrex glass plate to reach the rectangular reaction zone.
Figure 6-2: Scheme of an LED based photocatalytic reactor.
124
The rectangular reaction zone has a width of 10 cm, a length of 10 cm and a depth of 5
cm. The distance between the shielding glass and the photocatalyst plate (Ds-p) can be
adjusted by inserting the photocatalyst plate in different slots. The distances between
different slots and the shielding glass plate were respectively 1 cm, 3 cm and 5 cm.
The inlets were on the sidewall perpendicular to the photocatalytic plate. The inlet
manifold system was composed of twelve small tubes. Openings on the side of each tube
face the photocatalytic plate. Such design enhanced the contact between influent
contaminants and the photocatalytic plate. Three small holes located at the top of the
opposite sidewall of reactor were used as outlets.
6.2.3 Radiation field and light intensity estimation
Table 6-1: Average light intensity received by the photocatalytic plate.
Input current (mA)
Ds-p (cm)
DL-P
(cm) Ia
(mw/cm2)
500 1 1.4 29.4 500 3 3.4 22.6 500 5 5.4 17.3 250 5 5.4 8.6 125 5 5.4 4.3 62 5 5.4 2.2
note: DL-P is larger than Ds-p due to the thickness of shielding glass plate and the gap
between lamps and the shielding glass.
125
Figure 6-3: Radiation field on a photocatalyst plate under different conditions; (a) DL-P = 0.014 m, 4 by 4 LEDs panel; (b) DL-P = 0.034 m, 4 by 4 LEDs panel; (c) DL-P = 0.054 m, 4 by 4 LEDs panel.
126
The average light intensity received by photocatalyst plate was estimated based on an
emission radiation field model developed for this design (see Chapter Five). The average
light intensities (Ia) for different situations are listed in Table 6-1. And the light intensity
distribution for different distance between the shielding glass and the photocatalyst plate
(DL-P ) with an input current of 500 mA is shown in Figure 6-3. Light intensity received
by the centre of photocatalyst plate was verified by a Silver Line UV radiometer
(M007153, Geneq Inc. Canada). When DL-P is 5.4 cm and the current is 500 mA, the
value reading from radiometer was 23 mW cm-2, which is in agreement with the model
data (Figure 6-3c).
6.2.4 Experimental set-up and sample analysis
A stock solution of 1.50 L of 20 mg L-1 pesticides or chlorophenol solution was prepared
by dissolving 30 mg of pure compound in water using ultrasonication. A peristaltic pump
(LaSalle Scientific Inc, Model: 400-205) circulated the solution containing the target
compounds between a reservoir covered with aluminum foil and the photocatalytic
reactor. The solution in the reservoir is continuously mixed with a magnetic bar.
Experiments were conducted at variable flow rate, variable light intensity, variable DL-P
and different photocatalyst configuration. All experimental conditions are described in
the captions of the figures. Prior to irradiation, the solution containing pesticides or
chlorophenols was circulated for 30 minutes to ensure that the adsorption of the
investigated compound onto the surface of photocatalyst reached equilibrium. After
different irradiation periods, a 1.0 mL sample from the reservoir was collected and
analyzed using a Varian Prostar 210 high performance liquid chromatography equipped
127
with a 325 LC UV-Vis detector (Yu et al., 2013). Variations in the obtained data in are
shown by error bars.
6.3 Result and discussion
6.3.1 Degradation of phenoxy pesticides and chlorophenols in a flow-through LED based photocatalytic reactor
To better understand the reactor performance from an energy perspective, the results are
expressed as the change of normalized concentration of parent compound versus the
energy dosage per unit volume. Energy dosage per unit volume is defined as the energy
of photons entering the reaction zone divided by the volume of sample treated (1.50 L).
The degradation of two phenoxy pesticides (2,4-D, MCPA) and two chlorophenols (4-
CP, 2,4-DCP) in the photocatalytic reactor are shown in Figure 6-4. The normalized
concentrations of parent compounds decreased as the energy dosage increased. With an
energy dosage of 25 kJ L-1, 91% of 2,4-D 85% of MCPA, 85% of 2,4-DCP and 81% of
4-CP were removed from the solution (approximately one log reduction). As reported in
Yu et al. (2013), these four compound can be efficiently photocatalytically decomposed
with slurry TiO2 in a UVA-LED batch reactor. In our immobilized photocatalytic reactor,
the TiO2 nanotubes growing on the titanium plate can also efficiently capture the UV
photons (365nm). The results show that both phenoxy pesticides and chlorophenol can be
degraded efficiently under the operational conditions shown in the caption of Figure 6-4.
128
Figure 6-4: Photodegradation of MCPA, 2,4-D, 2,4-DCP and 4-CP in a UV-LED photoreactor: flow rate = 2.03 L min-1; DL-P = 0.54 cm; Ia=17.3 mW cm-2.
6.3.2 Degradation of 2,4-D with different combination of (UV, TiO2 photocatalyst plate, H2O2 and O2) in the UV-LED photoreactor .
Photodegradation of 2,4-D under different experimental conditions is shown in Figure 6-
5. With only UV-LED irradiation, 13% of 2,4-D was removed at an energy dosage of
6.22 kJ L-1. In the UV-visible spectrum range from 250 nm-700 nm, 2,4-D has a single
peak with a maximum at around 280 nm and does not have an absorption at 365 nm
(peak wavelength for the LEDs). However, there is still a small amount of photons in the
UV-LED emission spectrum, leading to a direct photolysis of 2,4-D. The presence of
H2O2 (0.1%) in LED reactor did improve the degradation efficiency and 54% of 2,4-D
was removed from bulk solution with the same energy dosage. H2O2 has weak
absorption on the emission spectrum of LED, which may cause the photolysis of
hydrogen peroxide and generate hydroxyl radicals.
0.00
0.20
0.40
0.60
0.80
1.00
1.20
0 5 10 15 20 25 30
C/C
o
Energy dosage per volume (kJ/L)
2,4-DCP 4-CP
MCPA 2,4-D
129
Figure 6-5: Photodegradation of 2,4-D in a flow-through UV-LED photoreactor: flow rate = 2.03 L min-1; DL-P = 0.54 cm ; Ia=17.3 mW cm-2.
The reactor was considered as a photocatalytic reactor while being mounted with a TiO2
photocatalyst plate. Such photocatalytic reactor can eliminate 40 % of 2,4-D at an energy
dosage of 6.22 kJ L-1. Bubbling oxygen in this photocatalytic reactor did not significantly
improve the degradation efficiency. In the experiments, the solution in reservoir is
thoroughly mixed using a magnetic stirrer. The aeration led to the sample getting
saturated with oxygen activity near 0.2 atm. This provided enough oxygen for
photocatalytic reactions. Therefore, further addition of oxygen into the system did not
boost the photocatalytic degradation. Apparently, the presence of 0.1% H2O2 in the
photocatalytic system resulted in a better degradation efficiency. At an energy dosage of
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
1.00
0 1 2 3 4 5 6 7
C/C
o
Energy dosage per volume (kJ/L)
UV only UV+H2O2 (0.1%) UV+TiO2 UV+TiO2+bubbling Oxygen UV+TiO2+ H2O2 (0.1%)
130
6.22 kJ L-1, 80% of 2,4-D is degraded. Hydrogen peroxide serves as a good electron
scavenger and accelerates the photocatalytic reaction.
6.3.3 Effect of DL-P
DL-P is a key factor for system scale-up. To study its effect on photocatalytic degradation,
experiments were conducted at three DL-P (1.4 cm, 3.4 cm and 5.4 cm). The results (see
Figure 6-6) show that the degradation of 2,4-D is relatively slow when DL-P is set to be
1.4 cm. The degradation efficiency improved as DL-P was increased to 3.4 cm, while
further increment of DL-P to 5.4 cm did not enhance the degradation efficiency. DL-P can
impact the photocatalytic degradation in two ways: (1) for the same photon energy input,
a uniform radiation field can result in more efficient distribution of activity over the
photocatalyst. In this reactor, a less uniform radiation field is obtained at shorter DL-P
(Figure 6-3); (2) for an immobilized photocatalytic reactor, the photocatalytic
degradation is limited by the mass transfer of the contaminants between the photocatalyst
surface and the bulk solution (Chen et al., 2001). At the same flow rate, the Reynolds
number decreases with DL-P, and hinders mass transfer. Therefore, from a kinetics
perspective, an optimal DL-P should make the light intensity received on the photocatalyst
plate uniform and not inhibit mass transfer.
One way to scale-up this system is to use a baffle reactor design which contains multiple
modules composed of UV-LED plate and photocatalytic plate. Each module has a
reaction zone and dead zone accommodating the electronics. The reaction zone volume
can be adjusted by changing DL-P, while the dead zone volume is limited to the electronic
131
part. The advantage of larger DL-P is that fewer modules are required for the same
reaction zone volume, and the total volume of reactor occupied is reduced. Therefore, in
this research, DL-P of 5.4 cm is superior to a DL-P of 3.4 cm.
Figure 6- 6: The effect of DL-P on 2,4-D degradation: flow rate =2.03 L min-1, Ia=17.3 mW cm-2.
6.3.4 Effect of flow rates on the photocatalytic degradation of 2,4-D.
To investigate the effect of flow rate on performance of LED photocatalytic reactor,
experiments were conducted at four different flow rates (0.72 L min-1, 1.50 L min-1, 2.03
L min-1 and 2.87 L min-1) and the results are shown in Figure 6-7. At the lowest flow rate
(0.72 L min-1), only 28 % 2,4-D removal was achieved with an energy dosage of 6.22
kJ/L. The removal percentage was improved as the flow rate increased and 40% of 2,4-D
0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00
0 2 4 6 8 10 12
C/C
o
Energy dosage per volume (kJ L-1)
D = 1.4 cm
D = 3.4 cm
D = 5.4 cm
132
was eliminated at a flow rate of 1.5 L min-1. The enhancement of degradation efficiency
due to the increase of flow rate was less significant at high flow rates.
Figure 6-7: The effect of flow rate on degradation of 2,4-D: DL-P = 5.4 cm, Iaverage=17.3 mW cm-2.
In a flow-through immobilized photocatalytic reactor, the flow rate impacts the mass
transfer of reactants between the photocatalyst surface and bulk solution. Higher flow
rate lead to a higher mass transfer rate and a faster overall reaction rate is expected. Flow
rate, along with reaction zone volume, determine the residence time of contaminants in
the reactor. In this study, the experiments were carried out in the circulated mode,
therefore, the residence time did not depend on flow rate but on the total operational time
and the ratio of reaction zone volume/total volume.
0.40
0.50
0.60
0.70
0.80
0.90
1.00
0 1 2 3 4 5 6 7
C/C
o
Energy dosage per volume (kJ L-1)
flow rate=0.72L/min
flow rate=1.50L/min
flow rate=2.03 L/min
flow rate=2.87 L/min
133
6.3.5 Effect of UV light intensity
The photocatalytic reactor performance at four different light intensities (2.2, 4.3, 8.6 and
17.3 mw cm-2) were investigated. Figure 6-8(a) summarized the first order kinetic rate
constants (K) at different light intensities. The lowest k (1.3×10-4 s-1) was obtained at a
light intensity of 2.2 mW cm-2 and the highest k (2.9×10-4 s-1) was obtained at a light
intensity of 17.3 mW cm-2. The first order kinetic rate constants increase with light
intensity fitting a power law relationship described by Equation [6-1].
𝑘 = 1.02 × 10−4 × 𝐼𝑎0.3753 [6-1]
The photocatalytic degradation rate kinetics depend on the efficiency of electron-hole
generation and recombination (Ollis et al., 1991). At lower light intensity range, the
electron-hole generation dominates and the reaction rate increases linearly with absorbed
irradiation intensity to a critical value. At a relatively higher light intensity, an increase of
the electron-hole recombination dominates and a power law relationship is obtained, as
observed in this study.
The power law relationship with an exponent less than one indicates a lower quantum
efficiency at a higher light intensity. Figure 6-8b reports results as a function of energy
dosage and showed that in our studied light intensity range, the low light intensity
condition is favored for energy efficiency.
134
(a)
(b)
Figure 6-8: The effect of light intensity on degradation of 2,4-D: DL-P = 5.4 cm, Flow rate=2.03 L min-1.
0.00E+00
1.00E-04
2.00E-04
3.00E-04
4.00E-04
0 5 10 15 20
k (s
-1)
Ia (mW cm-2)
0.00
0.20
0.40
0.60
0.80
1.00
1.20
0 1 2 3 4 5 6 7
C/C
o
Energy dosage per volume (kJ L-1)
I=17.3 mW/cm² I=8.6 mW/cm²
I=4.3 mW/cm² I=2.2 mW/cm²
135
6.3.6 Comparison of three different photocatalyst configurations
2,4-D photocatalytic degradation experiments were conducted with three different
photocatalyst configurations. They are: type (a), anodized TiO2 photocatalytic plate (10
cm by 10 cm); type (b), 2 g L-1 of P25, with an average diameter of 20 nm; type (c), 5 g
L-1 of hollow glass microspheres coated with anatase TiO2 (HGMT), of median particle
diameter of 45 µm.
Table 6-2: First order kinetic rate constants for different photocatalyst configurations
Photocatalyst configuration k (*10-4 s-1)
Type (a) 2.9 Type(b) 31.8 Type (c) 13.2
The first order rate constants for each case were reported in Table 6-2. The results show
that reaction rates in slurry type [type (b)] is ten times faster than that in immobilized
type [type (a)], and the performance of HGMT [type (c)] in removing 2,4-D is between
these two types. Note that the loading of HGMT is higher than that of P25.
The configuration of photocatalyst is a key factor for performance. The access to catalytic
surface by the photons and the reactants determines rate. Larger available catalytic
surface results in a higher rate. Among these three configurations, type (a) has the least
available surface area. Moreover, mass transfer of reactants becomes a limiting factor in
an immobilized catalyst type reactor. Type (b) is better than Type (c) possibly due to
136
higher photocatalyst loading, higher surface area and easier access to the surface of the
photocatalyst.
Kinetics in a slurry reactor is much superior to that in an immobilized type whereas the
operation of an immobilized reactor requires no further separation step. A modified
configuration-HGMT with a suitable concentration can result in a reaction rate
comparable to P25. Besides, the low density (0.22 g cm-3) of HGMT can make it float on
the surface of water and be conveniently recovered.
6.4 Conclusions
This paper presents the design and fabrication of a novel UV-LED based photocatalytic
reactor. The reactor shows its capability to decompose phenoxy pesticides and
chlorophenols. The study on different operational parameters, such as DL-P, flow rate,
light intensity and external electron scavenger provide useful information for system
scale-up. In this reactor, optimal DL-P was determined to be 5.4 cm and 1.5 L min-1 was
chose as a suitable flow rate. The power law relationship with an exponent 0.4 between
first order kinetics rate constants and the studied light intensities indicate increasing light
intensity to reduce reaction time is not energy efficiency at high power input. Adding
hydrogen peroxide is a good option to boost the reactor performance. Furthermore, a
modified photocatalyst (hollow glass microsphere coated with anatase TiO2) can be a
promising photocatalyst configuration, considering the reaction rate and operational
convenience.
137
Chapter Seven: CONCLUSION AND RECOMMENDATION FOR FUTURE RESEARCH
7.1 Conclusions
The photochemical technologies (photosensitization and TiO2 based photocatalysis)
developed in this research have successfully treated PCBs and pesticides in aqueous
medium. LEDs were shown to be a promising light source in TiO2 photocatalytic
application. As an important step for designing an efficient photo-reactor, a radiation
model was developed and validated. Finally, based on these work, an LED based flow-
through photocatalytic reactor were designed, fabricated and optimized. The reactor has
shown its capacity to efficiently treat water based contaminants like pesticides and
chlorophenols. The overall conclusion of this thesis can be further divided into four sub-
conclusions as given below:
7.1.1 Photosensitized dechlorination of PCBs solubized in surfactant solution
It is possible to dechlorinate PCBs in an aqueous medium using longer
wavelength ( visible light). The usage of visible light opens an opportunity to
utilize sunlight for PCBs treatment, thus significantly reducing the energy costs.
The types and the concentrations of surfactant can impact the PCBs
dechlorination rate. The cationic (CTAB) and non ionic (TWEEN 80) surfactants
work better than the anionic surfactant (SDS) for dechlorination, even though the
cationic surfactant is not preferred for PCB extraction from soil.
138
7.1.2 LED based photocatalytic treatment of pesticides and chlorophenols
Complete decomposition of the studied phenoxy pesticides and chlorophenols
was achieved within a short period of irradiation with a slurry TiO2 in a batch
UV-LED (365nm) reactor.
Due to the competition for surface hydroxyl radicals between different
compounds, the degradation rate of pesticides become slower as a second
pesticide is introduced into the solution.
A suitable loading of TiO2 (2g/L) for 2,4-dichlorophenoxyacetic acid degradation
was determined at the applied light intensity (8.55×1016 photon s-1). The rate
constant at this loading and this light intensity was found to be 0.2404 min-1.
The first order kinetic rate constants were proportional to the studied light
intensities.
The comparison between mercury lamps and UVA-LEDs show that UVA-LEDs
is more energy-efficient, since the emission spectrum of UVA-LED lamps can be
well-matched with the absorption band of TiO2. If a solar reactor is considered as
a competitor, the photon energy-use advantage of UVA-LEDs is even greater.
7.1.3 Design a homogenous radiation field model for photocatalytic reactor
Radiation field model for a UV-LEDs array has been developed, which can
predict the light intensity at any location of a photocatalyst plate with any ID.
Based on the model, the degree of homogeneity of the radiation field is
significantly affected by the ID when the gap is fixed.
139
A homogenous radiation field can be achieved by choosing an optimal ID/gap
ratio. The ratio is found to be 1.26 for Nichia UV-LED (NCSU033).
The method of selecting the light output of UV-LED for different gaps to achieve
a desired homogenous light intensity was developed and evaluated.
7.1.4 A novel light emitting diode photocatalytic reactor for water treatment
A novel LED based photocatalytic reactor was designed and fabricated. The novel
reactor is a combination of an environmental friendly light source and an
immobilized nanostructure photocatalyst.
The reactor shows its ability to treat water contaminated with phenoxy pesticides
and chlorophenols under different experimental conditions.
The study on the DL-P, flow rate, light intensity and external electron scavenger
provide useful information for reactor scale-up. Optimal DL-P was determined to
be 5 cm and 2 L/min was chose as a suitable flow rate for current reactor. The
power law relationship with a exponent 0.4 between kinetics and light intensity
was examined, indicating lower energy efficiency is reduced when the light
intensity is increased.
140
7.2 Recommendations for Future Research
7.2.1 Incorporating PCBs extraction using surfactants and PCBs photodechlorination using sensitized visible light
Our research shows that PCBs in surfactant solution can be dechlorinated using sensitized
visible light. Therefore, a study of PCBs extraction using surfactants followed by
dechlorination using sensitized visible light should be conducted in the future.
7.2.2 UVC-LED
Currently, the lower quantum efficiency and high price of UVC-LED limit its
application. Therefore, our research was focused on UVA-LED. Once high intensity
UVC-LED become commercially available as the development of the UVC-LED
technology, other AOPs requiring deep ultraviolet light, such as UV/H2O2, can be
investigated using UVC-LED.
7.2.3 The decay of photocatalytic activity and its life time
The activity of photocatalyst may decay with time, which can reduce the reactor
performance. Thus, factors causing the inactivation of photocatalyst and the methods for
regenerating the photocatalyst need to be studied. In addition, the decay of photocatalytic
activity can be used to predict the life time of photocatalyst.
7.2.4 Hollow microsphere coated with TiO2 (HGMT)
HGMT open a promising future for photocatalytic applications. Since the density of
HGMT is much lower than water, mixing of HGMT with water will become an issue. A
way with less energy to mix HGMT with water is needed to be studied.
141
7.2.5 Scale-up of the reactor
The reactor can be scaled up using a baffle design (Figure 7-1) with multiple LED panels
and photocatalyst plates. The hydraulic conditions and the mass transfer of reactants and
products in such system need to be studied.
Figure 7-1: Scheme of a scale-up LED based photocatalytic reactor.
142
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177
APPENDIX A: INVESTIGATION OF ULTRTRASONIC EXTRACTION OF
POLYCHORINATED BIPHENYLS FROM SOIL
A.1. Experimental
A.1.1. Chemicals
Aroclor1254 standard (1000 mg/L in hexane) was purchased from AccuStandard,
decachlorobiphenyl (200 mg/L in acetone) was procured from Sigma-aldrich, 99% purity
of 2-propanol (IPA), 99.9% purity of acetone and 99.9% purity of hexane were obtained
from EMD. The water used in this experiment is ultrapure water.
A.1.2. Pre-Processing of contaminated soil
500 g of wet contaminated soil was dried at 50 ℃ for two days. The dried contaminated
soil was then passed through a size-20 sieve and homogenized using a spatula, mortar
and pestle.
A.1.3. Ultrasonic extraction of PCBs
Ten gram of dried soil samples were placed in 40-ml glass vials. Each vial was filled with
a known amount of IPA and be sonicated for different time. After sonication, each vial
was centrifuged for 15 minutes at 1500 rpm. Then, one and half ml of the supernatant
was transferred to a 1.5-ml centrifuge vial for a second centrifugation (5 minutes at 1500
rpm) to remove the trace particles. After second centrifugation, 0.1 ml of secondary
178
centrifugation supernatant was diluted to 1 ml with IPA and analyzed using GC-ECD
(USEPA, 1996b).
A.1.4. Soxhlet extraction of the remaining PCBs in soil :
The remaining solid in part A.1.3 was holed with a Pasteur pipette and washed with 30 ml
water to remove residual IPA extract that might have lingered in the soil. The washing
was conducted by gently shaking the vials for ten seconds. After washing procedure, the
vials were again centrifuged for 5 minutes at 1500 rpm. The separated solid in the vial
was subjected to Soxhlet extraction (USEPA, 1996a). After Soxhlet extraction, the
concentrate extracts were carefully transferred into 250-ml beakers. The beakers
containing extract were then placed inside of a well-ventilated fume hood, at room
temperature, and evaporated to dryness. Ten ml of the hexane was used to redissolve the
dried solid in each beaker and was centrifuged for 5 minutes at 1500 rpm. 0.1-ml of the
supernatant was then diluted to 1ml with hexane and analyzed using GC-ECD. All the
experiments were conducted in duplicates
A.1.5. Calculation of ultrasonic extraction efficiency
The ultrasonic extraction efficiency (η) can be calculated using Equation [A-1]
η = ms−PCBsms−PCBs+ mr−PCBs
× 100% [A-1]
179
Where ms−PCBs is the mass of PCBs extracted using ultrasonication and is calculated
using Equation [A-2]; mr−PCBs is remaining PCBs in the soil after ultrasonication and is
calculated based on Equation [A-3]
ms−PCBs(mg) = C1 × V1 × 10 [A-2]
Where C1 (mg/L) is PCBs concentration obtained from GC equipment in part A.1.4,
V1(mL) is the volume of IPA used for ultrasonic extraction
mr−PCBs (mg) =C2 × 10 mL × 10
R
[A-3]
Where C2 (mg/L) is PCBs concentration obtained from GC equipment in part A.1.5, R is
the recovery rate of surrogate.
A.2. Results and Discussions
The ultrasonic extraction efficiencies of PCBs under different experimental conditions
were shown in Figure A-A-1. The ultrasonic extraction efficiency ranges from 15% to
30% under different conditions. When the IPA/soil ratio is 1:1, the increase of sonication
time from 15 min to 90 min did improve the extraction efficiency. Whereas, at higher
IPA/soil ratio (2:1 or 3:1), the extraction efficiency is not impacted by the investigated
180
Figure A-A-1: Ultrasonic extraction efficiency of PCBs from 10 g soil under different experimental conditions.
sonication time. For the same sonication time, the increase of IPA/soil ratio resulted in a
decrease of extraction efficiency. A possible reason is that at higher IPA/soil ratio
experiment, the volume of IPA is increased, therefore, the volumetric energy captured by
soil and surrounding IPA decrease, which leads to a lower extraction efficiency.
A.3. Reference
USEPA 1996a. USEPA Method 3540C soxhlet extraction. Washingtong, D.C.
USEPA 1996b. USEPA method 8082, polychlorinated biphenyls by gas chromatography.
Washingtong, D.C.
3:1
2:1
1:1
0
5
10
15
20
25
30
15 30
60 90
Extra
ctio
n ef
ficie
ncy
(%)
Sonication time (min)
181
APPENDIX B: INVESTIGATION OF PHOTODEGRADATION OF BIPHENYL
IN ULTRAVIOLET WATER PURIFICATION SYSTEMS
B.1. Experimental
B.1.1. Chemicals
Ninety nine percent purity of biphenyl was obtained from sigma aldrich, 99% purity of
isopropopanol (IPA) was procured from VWR.
B.1.2. Photoreactor
The testing UV water purification system (UVS238S) was purchased from Neotech Aqua
solutions. It is an annular photoreactor (Figure A-B-1) which is equipped with a medium
mercury lamp. The total power output of the medium mercury lamp is 150W and the
power output of that at 254 nm is 45 W. The capacity of the reactor is 6.5 L.
Figure A-B-1: Ultraviolet water purification system.
B.1.3. Photodegradation of biphenyl in IPA
500 mg/L biphenyl solution was prepared by dissolving 20 g biphenyl crystals in 40 L
IPA. The prepared biphenyl solution was then stored in a 250L reservoir. An air pump
182
was used to pump the biphenyl solution into photoreactor from the reservoir at a high
flow rate, and the exit of the photoreactor was connected to reservoir. The
photodegradation experiments were conducted in a circulated mode with three different
flow rates (6 gallon/min, 10 gallon/min, 14 gallon/min). One milliliter sample was
collected at different irradiation time. And the collected samples were analyzed using
GC/FID.
B.2. Results and discussions
The results were plotted as the change of biphenyl concentration verse the exposure
irradiation time in Figure A-B-2. The exposure irradiation equal to the total experimental
time multiplied by the ratio between the reaction zone volume and total treated volume
(6.5L/40L). It showed that biphenyl can be degraded in IPA solvent with a medium
mercury lamp. However, the direct photolysis of biphenyl in such system is relatively
slow, only 15% degradation was observed within 2 hour exposure irradiation time. To
complete remove the biphenyl from the system, a long exposure irradiation time is
required. Within the investigated flow rate range (6, 10, 14 gallon/min), the flow rate
does not impact the biphenyl degradation. To obtain a high degradation efficiency, flow
rate should be high enough to achieve complete mixing of solution in the reactor
chamber. The lowest flow rate studied is able to create enough mixing in the reactor
chamber. The plot of log of normalization of biphenyl concentration verse exposure
irradiation time in Figure A-B-3 indicate that direct photolysis of biphenyl in IPA follow
first order kinetics. The observed first order kinetics constant under different flow rate
varied from 0.075 h-1 to 0.077 h-1.
183
Figure A-B-2: Degradation of biphenyl under different flow rate.
Figure A-B-3: The pseudo first order kinetics of biphenyl degradation under different flow rate.
0.84
0.88
0.92
0.96
1
0 0.5 1 1.5 2 2.5
C/C
o
Exposure irradiation time (h)
Flow rate=6 gallon/min Flow rate=10 gallon/min Flow rate=14 gallon/min
-0.18
-0.16
-0.14
-0.12
-0.1
-0.08
-0.06
-0.04
-0.02
0 0 0.5 1 1.5 2 2.5
ln(C
/Co)
Exposure Irradation time ( h)
Low flow
medium flow
high flow
184
APPENDIX C: UV VIS ABSORPTION SPECTRUM OF DIFFERENT
PESTICIDES
Figure A-C-1: UV-Vis absorption spectra of 40 mg/L of 2,4-D in water.
Figure A-C-2: UV-Vis absorption spectra of 40 mg/L of 2,4-DCP in water.
0
0.1
0.2
0.3
0.4
0.5
0.6
250 300 350 400 450
Abs
orba
nce
Wavelength(nm)
0
0.1
0.2
0.3
0.4
0.5
0.6
250 300 350 400 450
Abs
orba
nce
Wavelength(nm)
185
Figure A-C-3: UV-Vis absorption spectra of 40 mg/L of 4-CP in water.
Figure A-C-4: UV-Vis absorption spectra of 40 mg/L of MCPA in water.
0
0.1
0.2
0.3
0.4
0.5
0.6
250 300 350 400 450
Abs
orba
nce
Wavelength(nm)
0
0.1
0.2
0.3
0.4
0.5
0.6
250 300 350 400 450
Abs
orba
nce
Wavelength(nm)
186
APPENDIX D: THE CALCULATION OF PERCENTAGE OF AVAILABLE
PHOTONIC ENERGY FOR PHOTOCATALYTIC REACTION
Assume the emission spectra of UVA-LED and mercury black lamps follow Gaussian
distribution as Equation [A-D-1].
𝑓(λ) = 𝑒−(λ−𝑏)2/𝑎 [A-D-1]
Where: a, b are constants; λ (nm) is the wavelength of photon, 𝑓(λ) is the relative light
intensity (the ratio between the emission light intensity at λ and the maximum emission
light intensity) .
For UVA-LED lamp (NSHU551B), wavelength corresponding to maximum emission
(λmax) is 365 nm, and the full width at half maximum of emission spectra is 15 nm; for
mercury black lamp (FL8BL-B), wavelength corresponding to maximum emission (λmax)
is 350 nm, and the full width at half maximum of emission spectra is 50 nm. These
conditions can be expressed as follow:
LED lamp �𝜆 = 365𝑛𝑚,𝑓(𝜆) = 1
𝜆 = 365 ± 152𝑛𝑚,𝑓(𝜆) = 0.5
�;
Mercury black lamp �𝜆 = 350 𝑛𝑚,𝑓(𝜆) = 1
𝜆 = 350 ± 502𝑛𝑚,𝑓(𝜆) = 0.5
�
Based on the conditions above, the spectral function for both UVA-LED and Mercury
black lamps were obtained as Equation [A-D-2] and [A-D-3]:
𝑓𝐿𝐸𝐷(𝜆) = 𝑒−(𝜆−365)2/81.5 [A-D-2]
187
𝑓𝑀𝑒𝑟𝑐𝑢𝑟𝑦(𝜆) = 𝑒−(𝜆−350)2/901 [A-D-3]
The energy of a single photon at wavelength λ can be calculated by Equation [A-D-4]
𝐸(𝜆) = hcλ
[A-D-4]
Where h is Planck`s constant (6.62× 10−34 J.s), c is the light speed in vacuum (3× 108
m/s), and E(λ) (J) is the energy of photon at wavelength λ (m).
Figure A-D-1: The emission spectrum and TiO2 band edge.
In TiO2 photocatalysis, the photon of wavelength above 385 nm is unavailable for the
reaction (Figure A-D-1). Therefore, the percentage of available energy for reaction in
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
250 300 350 400 450
Rel
ativ
e lig
ht in
tens
ity
Wavelength (nm)
UVA-LED Mercury black lamp TiO2 band edge
188
each case is equal to the sum of energy of photon (0 nm < λ<385nm) divided by the sum
of energy of photon (0 nm < λ ).
The percentage of available photonic energy for each case is calculated as follows:
𝑃𝐿𝐸𝐷 =∫ 𝑓𝐿𝐸𝐷(𝜆) ∗ 𝐸(𝜆)𝑑𝜆3850
∫ 𝑓𝐿𝐸𝐷(𝜆) ∗ 𝐸(𝜆)𝑑𝜆+∞0
= 99.9%
[A-D-3]
𝑃𝑀𝑒𝑟𝑐𝑢𝑟𝑦 =∫ 𝑓𝑚𝑒𝑟𝑐𝑢𝑟𝑦(𝜆) ∗ 𝐸(𝜆)𝑑𝜆3850
∫ 𝑓𝑚𝑒𝑟𝑐𝑢𝑟𝑦(𝜆) ∗ 𝐸(𝜆)𝑑𝜆+∞0
= 95.8%
[A-D-3]
Where PLED is the percentage of available photonic energy for LED; PMercury is the
percentage of available photonic energy for LED.
