UNIVERSITY OF CALGARY. Light Emitting Diode Based Photochemical Treatment of Contaminants in Aqueous Phase. Linlong Yu A THESIS

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1 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

2 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 TiO 2 photocatalysis. They are completely mineralized upon prolonged irradiation. Our results indicate that LEDs are a better light source than the mercury lamps. ii

3 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 TiO 2 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 (D L-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 D L-P was determined for the reactor. Enhanced performance of the reactor was observed where electron scavengers were introduced. iii

4 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. iv

5 Dedication This thesis is dedicated to my beloved parents. v

6 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 Background Photochemical Treatment Processes Light emitting diode (LED) in photocatalytic reactors Research Objectives and Scopes Thesis Overview...6 CHAPTER TWO: LITERATURE REVIEW Principle of Photochemistry Light and photon The electronic excited states Quantum yield Direct photolysis Photosensitized degradation Photocatalysis Advanced Oxidation Processes (AOPs) TiO 2 Photocatalysis TiO 2 as a photocatalyst Mechanism of TiO 2 photocatalysis The kinetics of photocatalytic degradation Factors affecting the photocatalytic degradation kinetics TiO 2 loading Light intensity ph Electron acceptor Hole/hydroxyl radicals scavenger Contaminants (Pesticides and PCBs) Pesticides ,4-dichlorophenoxyacetic acid (2,4-D) methyl-4-chlorophenoxyacetic acid (MCPA) Chlorophenols PCBs Photochemical treatment of pesticides and PCBs Direct photolytic degradation of pesticides and PCBs Photosensitized degradation of pesticides and PCBs Photocatalytic degradation of pesticides and PCBs...37 vi

7 2.5 Design of a photocatalytic reactor State of Photocatalyst in the Reactor Slurry photocatalytic reactor vs immobilized photocatalytic reactor TiO 2 immobilization through electrochemical anodization Light source Sunlight Mercury lamps Light emitting diode Artificially illuminated photocatalytic reactors Solar photocatalytic reactors: Radiation-field modelling The Radiation transport equation (RTE) Numerical methods to solve the RTE Radiation source models...61 CHAPTER THREE: ELECTRON TRANSFER SENSITIZED PHOTODECHLORINATION OF SURFACTANT SOLUBILIZED PCB Introduction Materials and methods Materials Methods PCB 138 solubilization with surfactants Photochemical reaction Sampling, extraction and GC analysis Results and Discussion Selectivity of surfactants Dechlorination of PCBs in TEA and NaBH4 systems MB and TEA MB and NaBH Photodegradation of Aroclor 1254 with NaBH4 and TEA The dechlorination pathways of PCB 138 using CTAB and TWEEN Conclusions...77 CHAPTER FOUR: LED-BASED PHOTOCATALYTIC TREATMENT OF PESTICIDES AND CHLOROPHENOLS Introduction Methods and Materials Photoreactor Chemicals Photocatalytic degradation Actinometric Experiment Analysis of sample HPLC Analysis TOC Analysis: Results and Discussions Photocatalytic degradation of pesticides and chlorophenols Photocatalytic degradation of pesticides mixtures...89 vii

8 4.3.3 Effect of Photocatalyst Loading Effect of Light Intensity Comparison between LED and Mercury Lamp Irradiation Conclusions...99 CHAPTER FIVE: DESIGN A HOMOGENEOUS RADIATION FIELD IN A UV- LED BASED PHOTOCATALYTIC REACTOR Introduction Advantage of homogeneous radiation field in a photocatalytic reactor Development of radiation field model UV-LED array and photocatalyst plate Radiation field model without shielding glass plate Radiation field model with a shielding glass plate Calibration and validation of the radiation field model Light intensity measurement Model light intensities vs measured light intensities Design of a homogenous radiation filed The effect of ID on the homogeneity of radiation field for a fixed gap Optimal combination of ID and gap Selection of the output of the UV-LED Conclusions CHAPTER SIX: A NOVEL LIGHT EMITTING DIODE BASED PHOTOCATALYTIC REACTOR FOR WATER TREATMENT Introduction Experimental details Chemicals Design and fabrication of an LED based photocatalytic reactor Preparation of anodized TiO 2 photocatalytic plate UV-LEDs module Photocatalytic system Radiation field and light intensity estimation Experimental set-up and sample analysis Result and discussion Degradation of phenoxy pesticides and chlorophenols in a flow-through LED based photocatalytic reactor Degradation of 2,4-D with different combination of (UV, TiO 2 photocatalyst plate, H 2 O 2 and O 2 ) in the UV-LED photoreactor Effect of D L-P Effect of flow rates on the photocatalytic degradation of 2,4-D Effect of UV light intensity Comparison of three different photocatalyst configurations Conclusions CHAPTER SEVEN: CONCLUSION AND RECOMMENDATION FOR FUTURE RESEARCH Conclusions viii

9 7.1.1 Photosensitized dechlorination of PCBs solubized in surfactant solution LED based photocatalytic treatment of pesticides and chlorophenols Design a homogenous radiation field model for photocatalytic reactor A novel light emitting diode photocatalytic reactor for water treatment Recommendations for Future Research Incorporating PCBs extraction using surfactants and PCBs photodechlorination using sensitized visible light UVC-LED The decay of photocatalytic activity and its life time Hollow microsphere coated with TiO 2 (HGMT) Scale-up of the reactor REFERENCES APPENDIX A: INVESTIGATION OF ULTRTRASONIC EXTRACTION OF POLYCHORINATED BIPHENYLS FROM SOIL A.1. Experimental A.1.1. Chemicals A.1.2. Pre-Processing of contaminated soil A.1.3. Ultrasonic extraction of PCBs A.1.4. Soxhlet extraction of the remaining PCBs in soil : A.1.5. Calculation of ultrasonic extraction efficiency A.2. Results and Discussions A.3. Reference APPENDIX B: INVESTIGATION OF PHOTODEGRADATION OF BIPHENYL IN ULTRAVIOLET WATER PURIFICATION SYSTEMS B.1. Experimental B.1.1. Chemicals B.1.2. Photoreactor B.1.3. Photodegradation of biphenyl in IPA B.2. Results and discussions APPENDIX C: UV VIS ABSORPTION SPECTRUM OF DIFFERENT PESTICIDES APPENDIX D: THE CALCULATION OF PERCENTAGE OF AVAILABLE PHOTONIC ENERGY FOR PHOTOCATALYTIC REACTION ix

10 List of Tables Table 2-1: Definitions of the quantum yield (Oppenlander, 2003) Table 2-2: Generation of hydroxyl radicals for different AOPs Table 2-3: Sales/use of the top 20 pesticide active ingredient in Canada (Brimble et al., 2005) Table 2-4: Slurry versus Immobilized Photocatalytic Systems (Lasa et al., 2005) Table 3-1: First order rate coefficients (K) for PCB138 dechlorination in TEA-MB system with different concentration of surfactants Table 3-2: First order rate coefficients (K) for PCB138 dechlorination in NaBH 4 -MB system Table 4-1: Light intensity of different photoreactors Table 4-2: First order rate coefficients (K) for photocatalytic pegradation of different pesticides Table 4-3: Mixtures of Pesticides Table 4-4: Percentage removal of pesticides at kj energy dosage Table 4-5: Percentage of pesticides adsorbed on the surface of TiO 2 after 30 minutes of stirring in the dark Table 5-1: Parameters used for radiation field model calculation Table 5-2: Comparison of modeled light intensity and measured light intensity Table 6-1: Average light intensity received by the photocatalytic plate Table 6-2: First order kinetic rate constants for different photocatalyst configurations. 135 x

11 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] Figure 2-2: Phenomenological subdivision of ultraviolet radiation into four sub-bands and their characteristic effects. [Reproduced from (Oppenlander, 2003) with the permission] Figure 2-3: Photochemical activation of TiO Figure 2-4: Molecular structure of 2,4-dichlorophenoxyacetic acid Figure 2-5: Molecular structure of 2-methyl-4-chlorophenoxyacetic acid Figure 2-6: Molecular structure of chlorophenols Figure 2-7: Molecular structure of polychlorinated biphenyls Figure 2-8: Solar spectral irradiance distribution on the surface of earth. [Reproduced from (Hulstrom et al., 1985) with permission] Figure 2-9: Fractional cumulative integrated irradiance vs. wavelength. [Reproduced from (Hulstrom et al., 1985) with permission] Figure 2-10: An inner working on an LED. [Adapted from (Wikipedia, 2011)] Figure 2-11: Various photochemical reactor configurations. [Reproduced from (Pareek et al., 2008) with permission] Figure 2-12: Scheme of a multiple tube reactor. [Reproduced from (Ray and Beenackers, 1998) with permission] 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] 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] xi

12 Figure 2-17: Scheme of a fluidized photocatalytic reactor. [Reproduced from (Vaisman et al., 2005) with permission] Figure 2-18: Solar photocatalytic reactor: (a) parabolic trough reactor (PTR) (b) compound parabolic collector (CPC). [Reproduced from (Braham and Harris, 2009) with permission] Figure Typical reactor layout for an (a) inclined plate collector and (b) double skin sheet photoreactor. [Reproduced from (Braham and Harris, 2009) with permission] 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] Figure 2-21: Schematic for photon transport 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, I o = photon s 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 ; [NaBH 4 ] = 20 g L -1, I o = photon s Figure 3-4: Dechlorination of Aroclor1254 solubilized with TWEEN 80 in the presence of MB and TEA or NaBH 4 : [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, I o = photon s 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 ), I o = photon/s, P: peak area of each congener from GC, P o : the peak area of initial PCB 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 ); I o = photon s -1, P: peak area of each congener from GC, P o : the peak area of initial PCB Figure 4-1: LED photoreactor and insert Figure 4-2: Photocatalytic degradation of different pesticides with UV-LED photoreactor (I o = photon s -1, C TiO2 =2.0 g L -1, C o =20 mg L -1 ): (a) loss of parent pesticides; and (b) loss of total organic carbon Figure 4-3: Photocatalytic degradation of pesticides mixture with UV-LED photoreactor based on the loss of pesticides detected by HPLC (I o = xii

13 photon s -1, C TiO2 =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 Figure 4-4: Photocatalytic degradation of 2,4-D with different TiO 2 loadings and LED irradiation (I o = photon s -1, C o =20 mg L -1 ) Figure 4-5: Photocatalytic degradation of 2,4-D with UV-LED photoreactor under different light conditions (C o =20 mg L -1, C TiO2 =2 g L -1 ) Figure 4-6: Photocatalytic degradation of 2,4-D in the two photoreactors: Co=20 mg L -1, C TiO2 =2 g L -1. (a): LED reactor, I o = photon s -1 ; (b): Rayonet reactor, I o = photon s Figure 5-1: UV-LED array and photocatalyst plate Figure 5-2: Directivity of radiation (NICHIA, 2013) Figure 5-3: Cartesian and polar coordinates in radiation system Figure 5-4: Scheme of UV-LED radiation Figure 5-5: Geometry of sensor 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 Figure 5-7: The effect of irradiated distance (ID) on Maximum Error Figure 5-8: Optimal combination of ID and gap Figure 5-9: Selection of light output of UV-LED Figure 6-1: SEM image of anodized TiO 2 nanostructrure Figure 6-2: Scheme of an LED based photocatalytic reactor Figure 6-3: Radiation field on a photocatalyst plate under different conditions; (a) D L- P = m, 4 by 4 LEDs panel; (b) D L-P = m, 4 by 4 LEDs panel; (c) D L-P = m, 4 by 4 LEDs panel 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 ; D L-P = 0.54 cm; I a =17.3 mw cm Figure 6-5: Photodegradation of 2,4-D in a flow-through UV-LED photoreactor: flow rate = 2.03 L min -1 ; D L-P = 0.54 cm ; I a =17.3 mw cm Figure 6-6: The effect of D L-P on 2,4-D degradation: flow rate =2.03 L min -1, I a =17.3 mw cm xiii

14 Figure 6-7: The effect of flow rate on degradation of 2,4-D: D L-P = 5.4 cm, I average =17.3 mw cm Figure 6-8: The effect of light intensity on degradation of 2,4-D: D L-P = 5.4 cm, Flow rate=2.03 L min Figure 7-1: Scheme of a scale-up LED based photocatalytic reactor Figure A-A-1: Ultrasonic extraction efficiency of PCBs from 10 g soil under different experimental conditions Figure A-B-1: Ultraviolet water purification system 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 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 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 Figure A-D-1: The emission spectrum and TiO 2 band edge xiv

15 List of Symbols, Abbreviations and Nomenclature Symbol Definition A Cross surface area, m 2 A p Area of photocatalyst plate, m 2 c Light speed in vacuum, m s -1 C Concentration of substrate, mole L -1 C o Initial concentration of parent compound, mg L -1 d Distance, m D s-p D l-p d o E g The distance between shielding glass and photocatalyst, cm The distance between LEDs and photocatalyst, cm Specific distance, m Energy of photon, J Distance between point (x, y) and point (x o, y o ), m G v Incident light intensity, photon s -1 m -2 h Plank constant, 6.62*10-34 J s I Light intensity, mw cm -2 I a Average light intensity, mw cm -2 I max Maximum of light intensity, mw cm -2 I measured Light intensity measured by UV meter, mw cm -2 I min Minimum of light intensity, mw cm -2 I model Light intensity calculated by model, mw cm -2 I o Light intensity measured by actinometry, photon s -1 I t 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 K a The average of first order rate coefficient, s -1 K ad Adsorption coefficient, L mol -1 l g M e The thickness of glass plate, m Max error xv

16 p(ω-ω') Phase function for scattering in RTE Q Number of photons q a Rate of photon absorption, photon s -1 q e Rate of photon emission, photon s -1 q in Rate of photon in-scattered, photon s -1 q out Rate of photon out-scattered, photon s -1 r Radius, m R Radial distance, m Re Radiation directivity function Re' Modified radiation directivity function r p Kinetic reaction rate, mole L -1 s -1 r s Radius of the sensor, m s Surface area, m 2 S Direction vector, m t Time, s T Transmittance v Frequency, s -1 V Elementary control volume, m 3 W a Volumetric rate of photon absorption, photon s -1 m -3 W e Volumetric rate of photon emission, photon s -1 m -3 W in Volumetric rate of photon in-scattered, photon s -1 m -3 W out Volumetric rate of photon out-scattered, photon s -1 m -3 x x-coordinates x o y y o z x-coordinates of LED position y-coordinates y-coordinates of LED position z-coordinates α Volumetric absorption coefficient, m -1 β Extinction coefficient, m -1 γ Constant η Constant xvi

17 θ View angle, radian λ Wavelength λ max Wavelength of maximum emission, nm σ Volumetric scattering coefficient, m -1 τ Asymmetry factor ψ Scattering angle, radian Ω Solid angle, steradian Abbreviations 2,4-D 2,4-DCP 4-CP AOPs ARPs CCA CMC CTAB DDT ECD EQE GC Definition 2,4-Dichlorophenoxyacetic Acid 2,4-Dichlorophenol 4-Chlorophenol Advanced Oxidation Processes Advanced Reduction Processes Chromated Copper Arsenate Critical Micelle Concentration Cetyltrimethylammonium Bromide Dichlorodiphenyltrichloroethane Electron Captured Detector External Quantum Efficiency Gas Chromatography HGMT Hollow Glass Microspheres Coated with Anatase TiO 2 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 xvii

18 PCB138 PCBs PEPO PFP PVC RTE SDS SEM TEA TOC TWEEN80 USEPA UV UVA UVB UVC UV-Vis VUV 2,2',3,4,4',5'-Hexachlorobiphenyl Polychlorinated biphenyls Photon Energy per Order Pentafluorophenyl Polyvinyl Chloride Radiation Transport Equation Sodium Dodecyl Sulfate Scan Electron Microscopy Triethylamine Total Organic Carbon Polyoxyethylene (80) Sorbitanmonooleate United States Environmental Protection Agency Ultraviolet Ultraviolet, subtype A Ultraviolet, subtype B Ultraviolet, subtype C Ultraviolet-visible Vacuum Ultraviolet xviii

19 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 1

20 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. 2

21 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 byproducts, 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). 3

22 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. TiO 2 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 TiO 2 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 TiO 2 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. 4

23 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. 5

24 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 TiO 2 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. 6

25 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 TiO 2 slurry system. During this research, a batch UV-LED photoreactor is fabricated. The impact of light intensity and TiO 2 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, 7

26 Gopal; Langford, Cooper H., Electron transfer sensitized photodechlorination of surfactant solubilized PCB 138. Chemosphere, 90, " and "Yu, Linlong; Achari, Gopal; Langford, Cooper H., LED-Based Photocatalytic Treatment of Pesticides and Chlorophenols. Journal of Environmental Engineering, 139, ", respectively. Chapter 5 has been submitted to Journal of Environmental Engineering and Science. 8

27 Chapter Two: LITERATURE REVIEW 2.1 Principle of Photochemistry 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 = hc λ [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 ( nm), UV-B ( nm), UV-A ( nm), VIS ( nm) and infrared ( nm) The subdivisions of the UV spectral domain are related to physical, chemical, biological or biochemical effects showed in Figure

28 Photoionization M M + +e γ-ray X-ray VUV UVC Photoexcitation M M * Vibrational excitation M M vib UVB UVA Visible light IR Wavelength (nm) Figure 2-1: Classification of electromagnetic radiation in the wavelength range below 1200 nm. [Reproduced from (Oppenlander, 2003) with the permission]. Absorbed by Organic Chromophores VUV ( nm) Absorbed by all substances including H 2 O,O 2,CO 2 UVC( nm) Absorbed by all Proteins, DNA, RNA,O 2 UVB ( nm) Sunburn Skin Cancer UVA ( nm) Sun Tanning Wavelength (nm) 400 Figure 2-2: Phenomenological subdivision of ultraviolet radiation into four subbands and their characteristic effects. [Reproduced from (Oppenlander, 2003) with the permission]. 10

29 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 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 11

30 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 Definition expression φ λ = dn(event)/dt Φ p abs φ λ = dn(m)/dt Φ p abs φ λ = dn(p )/dt Φ p abs φ λ1 λ 2 = dn(p )/dt Φ p abs(λ 1 λ 2 ) φ λ Universally valid: Number n of events per unit time divided by the number of photons absorbed during this period Number n of reactant molecules M consumed per unit time divided by the number of photons absorbed during this period Number n of photoproduct molecules P' formed per unit time divided by the number of photons absorbed during this period Ratio of the number m of photoproduct molecules formed per unit time to the total number of photons absorbed in the spectral region λ 1 - λ 2 during this period note: Φ p abs and Φ p abs(λ 1 λ 2 ) are the absorption rates of photons 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 12

31 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 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 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 TiO 2, CdO, ZnO, WO 3, CdS, CdSe, GaP, GaAs, ZnS, SnO 2, Fe 2 O 3, SrTiO 3, BaTiO 3 etc, have been used as photocatalysts. Generally, the best photocatalytic performances are obtained with titanium dioxide as catalyst (Herrmann, 13

32 2005). The details of TiO 2 photocatalysis fundamental and mechanism will be described in the section 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&H 2 O 2, Ozone&UV, vacuum UV, TiO 2 photocatalysis, and others. Besides, there are several non-light induced AOPs such as H 2 O 2, 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 TiO 2 Photocatalysis TiO 2 as a photocatalyst TiO 2 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 14

33 Table 2-2: Generation of hydroxyl radicals for different AOPs. Type of AOPs Spectral domain Reactions Vacuum UV VUV H 2 O hv HO +H UV/H 2 O 2 UVC H 2 O 2 hv HO +HO TiO 2 hv e + h + TiO 2 /UV O 3 /UV Ozonation UVA-UVC UVC No UV h + + H 2 O HO +H + h + + OH HO O 3 hv O(D) + O 2 O(D) + H 2 O H 2 O 2 H 2 O 2 hv HO +HO HO + O 3 O 2 + HO 2 HO 2 + O 3 HO 2 +O 3 HO 2 H + + O 2 O 2 + O 3 O 2 + O 3 O 3 + H + HO 3 HO 3 HO +O 2 Fenton process No UV HO +O 3 HO 2 +O 2 Fe 2+ + H 2 O 2 FeO 2+ + H 2 O Fe 2+ + H 2 O 2 Fe 3+ + OH + OH Fe 3+ + H 2 O 2 FeOOH 2+ + H + FeOOH 2+ HO 2 +Fe 2+ 15

34 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 bandgap 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 TiO 2 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 TiO 2 P25, via the chloride technology method is currently the de-facto commercial reference TiO 2 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 m 2 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 16

35 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 TiO 2, such as products from Millennium and Hombikat also show their high photocatalytic activities (Zertal et al., 2004, Alonso-Tellez et al., 2012) Mechanism of TiO 2 photocatalysis The fundamentals and mechanism of TiO 2 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 TiO 2 photocatalysis can be broken into five independent steps (Herrmann, 2005, Mozia, 2010) : Transfer of the reactants in the bulk solution to the TiO 2 surface; Adsorption of the reactants on the surface of TiO 2 ; 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 TiO 2 by absorbing photon with formation of electron-hole pair. Once TiO 2 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 17

36 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 H 2 O 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 TiO 2 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 CO 2, H 2 O and other inorganic compounds. TiO 2 hv TiO 2 (e + h + ) [2-2] e + h + heat [2-3] h + + H 2 O OH +H + [2-4] h + + OH OH [2-5] e + O 2 O 2 [2-6] O 2. +H + HO 2 [2-7] HO 2 +HO 2 H 2 O 2 + O 2 [2-8] e + H 2 O 2 OH +OH [2-9] 18

37 O 2 +H 2 O 2 OH +OH + O 2 [2-10] H 2 O 2 hv 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 TiO 2. 19

38 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). r p = dc dt = K adkc 1 + K ad C [2-15] 20

39 Where r p is the reaction rate (mol L -1 s -1 ), C is the concentration of substrate (mole L -1 ), K ad 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, K ad C 1, consequently, Equation [2-15] can be simplified as zero order reaction kinetics: dc dt = k [2-16] At highly diluted concentration, K ad C 1, the photocatalytic degradation become first order reaction (Equation [2-17]) (Mozia, 2010, Herrmann, 2005). dc dt = K adkc [2-17] Factors affecting the photocatalytic degradation kinetics TiO 2 loading The effect of TiO 2 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 21

40 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 TiO 2 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 Light intensity Light intensity is another key parameter in the TiO 2 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. 22

41 K I α [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 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 TiO 2 and ionization state of some contaminants is strongly influenced by ph, which thus impacts the adsorption behaviour of contaminants. The surface charge of TiO 2 at different ph is determined by the following reactions. TiOH + H + TiOH 2 + [2-19] 23

42 TiOH + OH TiO + H 2 O [2-20] The isoelectric point of commercial available TiO 2 (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 TiO 2 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 TiO 2 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 Electron acceptor In the application of TiO 2 photocatalysis, electron-hole recombination is the major step of energy waste. Without suitable electron acceptors, the recombination step is 24

43 predominant and limits the photocatalytic reaction. The presence of electron acceptors in solution can accelerate the photocatalytic degradation rate by (1) preventing the electronhole 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 H 2 O 2, K 2 S 2 O 8 KBrO 3 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 H 2 O 2 (up to 0.1 mm) can improve the efficiency of photocatalytic degradation of glyphosate. However, at high concentration of H 2 O 2 (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). 25

44 H 2 O 2 + e CB OH +OH [2-21] H 2 O 2 + 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 TiO 2 excitation (Muruganandham and Swaminathan, 2006, Chu and Wong, 2004). H 2 O 2 + OH H 2 O + HO 2 [2-23] HO 2 +OH H 2 O + O 2 [2-24] 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 -, NO - 3 SO , CO 3 and HCO - 3 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 NO - 3 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 -, 26

45 SO 2-4, and NO - 3 ) 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) 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) 27

46 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 ( 10 6 kg) 1 Glyphosate H Creosote AM MCPA H ,4-D H CCA AM Triallate H Mancozeb F Ethalfluralin H Atrazine H Brommoxynil H Surfactant bend A Mineral oil A/G/H/I Petroleum Hydrocarbon Blend A Trifluralin H S-metolachlor H Chlorothalonil F Metolachlor H Chlorpyrifos I Mecoprop H ,3-dichloroproprene I A: adjuvant AM: anti-microbial F: fungicide G: growth regulator H: herbicide I: insecticide 28

47 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 ,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. 29

48 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 LD 50 varies from 638 mg/kg mg/kg in rat; the acute dermal LD 50 is higher than 2000 mg/kg in rabbits; the acute inhalation LD 50 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 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 30

49 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). 31

50 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 32

51 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) 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 33

52 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 orthochlorine substituent or only a single ortho-chlorine substitute are more toxic. Studies in 34

53 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). 35

54 2.4 Photochemical treatment of pesticides and PCBs 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. 36

55 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 Photocatalytic degradation of pesticides and PCBs TiO 2 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 TiO 2 was reported using light ranging from 340 nm to 365 nm (Carey et al., 1976, Chiarenzelli et al., 1995, Wang and 37

56 Hong, 2000). Some researchers believe that hydroxyl radicals generated during TiO 2 photocatalysis oxidize PCBs molecules, leading to PCBs photodegradation and the eventual formation of CO 2 (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 State of Photocatalyst in the Reactor 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 TiO 2 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 38

57 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 Advantages Uniform catalyst distribution High surface area /volume ratio Limited mass transfer Minimum catalyst fouling effects Well mixed particle suspension Low pressure drop Immobilized reactor 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) 39

58 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, TiO 2 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) TiO 2 immobilization through electrochemical anodization Immobilized TiO 2 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 TiO 2 nanostructure using electrochemical anodization approach. After that, many studies on fabricating anodic 40

59 TiO 2 nanostructure have been reported. The anodization was carried out using a twoelectrode 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 V for non-aqueous electrolytes. The formation of TiO 2 nanostructure in these fluorine containing electrolytes is the result of competition between the electrochemical oxidation of Ti and electrical field induced etching of TiO 2 as well as chemical etching of TiO 2 by fluorine ions (Wang and Lin, 2009). The fluoride ion can promotes the growth of anatase TiO 2 with high reactive facets such as (001) facets (Yang et al., 2008). Usually, the TiO 2 structure obtained via anodization at room temperature are in an amorphous form. However, an amorphous form of TiO 2 does not show a good photocatalytic activity (Wu et al., 2011). To obtain TiO 2 nanostructure with high photocatalytic activity, the prepared TiO 2 should be converted to crystallized form (anatase/rutile) with high temperature annealing treatment (eg. 500 o C) (Roy et al., 2011) 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 TiO 2 photocatalysis, only photons with 41

60 energy equal or larger than band-gap energy of TiO 2 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 Sunlight Irradiance ( W m -2 μm -1 ) Wavelength (nm) Figure 2-8: Solar spectral irradiance distribution on the surface of earth. [Reproduced from (Hulstrom et al., 1985) with permission] 42

61 The sun is a spherical UV/VIS source with a radiant power of 3.842*10 26 W (Oppenlander, 2003). Only a small part of its radiation can reach the earth through travelling the *10 11 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). Fraction Wavelength (nm) Figure 2-9: Fractional cumulative integrated irradiance vs. wavelength. [Reproduced from (Hulstrom et al., 1985) with permission] 43

62 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) 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 ( 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 nm with an ordinary quartz envelope. The 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 44

63 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) 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 to 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 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 45

64 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, 46

65 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) 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), 47

66 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 48

67 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 TiO 2. The UV light can reach the supported TiO 2 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. 49

68 Taylor vortex reactor (Figure 2-16) consisting of two coaxial cylinders. The inner cylinder coated with TiO 2 is rotated to generate a vortex-induced fluid instability. Fluidized bed photocatalytic reactor (Figure 2-17) which has a fluidized bed consisting of small TiO 2 -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] 50

69 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] 51

70 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] 52

71 Figure 2-17: Scheme of a fluidized photocatalytic reactor. [Reproduced from (Vaisman et al., 2005) with permission] 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 concentratingtype 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 53

72 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 54

73 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 Typical reactor layout for an (a) inclined plate collector and (b) double skin sheet photoreactor. [Reproduced from (Braham and Harris, 2009) with permission] 55

74 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). 56

75 2.6.1 The Radiation transport equation (RTE) In heterogeneous media (slurry TiO 2 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 (m 3 ), the photon balance equation can be described as (Cassano et al., 1995): dq dt + A di λ = q a + q e + q in q out [2-25] Where is the number of photon in control volume (photon), t is time (s), A is the cross section surface area (m 2 ), is the spectral light intensity (photon m -2 s -1 ), is rate of 57

76 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 di λ = q a + q e + q in q out [2-26] Equation [2-27] is obtained by dividing Equation [2-26] with V; V=A*ds, AdI λ V = qa /V + q e /v + q in /v q out /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). di λ (s, Ω) ds = W a + W e + W in W out [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: W e = 0 [2-29] 58

77 In general, linear isotropic constitutive equations (Equation [2-30] &[2-31])can be used to characterize absorption and out-scattering (Cassano et al., 1995), W a = α λ I λ (s, Ω) [2-30] W out = σ λ I λ (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: β λ = α λ + σ λ [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. 4π W in = 1 4π σ λ p(ω Ω)I λ (s, Ω )dω 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]: 1 4π p(ω Ω)dΩ = 1 0 4π [2-34] 59

78 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). P HG (φ) = 1 4π 1 τ 2 [2-35] (1 + τ 2 2τ cos(φ)) 3/2 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 di λ (s, Ω) ds 4π = β λ I λ (s, Ω) + 1 4π σ λ p(ω Ω)I λ (s, Ω )dω 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: 4π G v (s) = I λ (s, Ω)dΩ 0 [2-37] Where is the incident light intensity (photon m -2 s -1 ). And the LVREA at any point is given by LVREA= α λ G v (S) [2-38] 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 60

79 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 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 61

80 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. 62

81 Chapter Three: ELECTRON TRANSFER SENSITIZED PHOTODECHLORINATION OF SURFACTANT SOLUBILIZED PCB 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. 63

82 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 (NaBH 4 ). Both approaches are studied in this paper. Air oxidation of LMB closes a catalytic cycle that consumes the reductant. 3.2 Materials and methods 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. 64

83 3.2.2 Methods 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 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 NaBH 4. 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 (I o ) was measured using ferrioxalate actinometry 65

84 that monitored wavelengths from 254 to 500 nm, covering the region of LMB absorption (Calvert and Pitts, 1966). Values were photon s -1 for 14 lamps and 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 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 o C. The GC/ECD temperature programming was set up as follow: The initial temperature for each run was set at 80 o C, which was 66

85 ramped up to 180 o C at a rate of 10 o C min -1 ; the temperature beyond 180 o C was ramped at a rate of 3 o C until it reached 270 o C 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 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 NaBH 4 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 NaBH 4 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 67

86 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 NaBH 4 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 NaBH 4 reduction SDS CTAB TWEEN C/Co Irradiation time (min) 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, I o = photon s

87 1.2 1 SDS CTAB TWEEN C/Co Irradiation time (min) 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 ; [NaBH 4 ] = 20 g L -1, I o = 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 69

88 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 70

89 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] Type of [Surfactant] [MB] [TEA] I o K mg L -1 Surfactant mg L -1 mg L -1 g L -1 photon s -1 s SDS SDS CTAB CTAB CTAB TWEEN TWEEN TWEEN TWEEN

90 3.3.2 Dechlorination of PCBs in TEA and NaBH4 systems As mentioned before, both the TEA and NaBH 4 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 NaBH 4 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 NaBH 4 is unavoidable MB and TEA TEA₁ TEA₂ TEA₃ TEA₄ 0.8 C/Co Irradiation time (min) 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, I o = photon s

91 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 MB and NaBH4 For this system, effects of changing the concentrations of both MB and NaBH 4 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 NaBH 4, a higher concentration of NaBH 4 led to a higher reaction rate, however, an experiment where the concentration of NaBH 4 was doubled did not contribute to a significant improvement in photodechlorination rate once the concentration of NaBH 4 exceeded 10 g L

92 Table 3-2: First order rate coefficients (K) for PCB138 dechlorination in NaBH 4 - MB system. [PCB138] Type of [Surfactant] [MB] [NaBH4] I o K mg L -1 Surfactant mg L -1 mg L -1 g L -1 photon s -1 s TWEEN TWEEN TWEEN TWEEN TWEEN TWEEN TWEEN 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 NaBH 4 systems. The results indicate that Aroclor 1254 was > 95% dechlorinated within 10 minutes in both TEA and NaBH 4 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). 74

93 1.2 1 TEA NaBH₄ 0.8 C/Co Irradiation time (min) Figure 3-4: Dechlorination of Aroclor1254 solubilized with TWEEN 80 in the presence of MB and TEA or NaBH 4 : [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, I o = photon s 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 75

94 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-NaBH 4 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 BH - 4 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. 45% CTAB TWEEN80 30% P/Po 15% 0% 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 ), I o = photon/s, P: peak area of each congener from GC, P o : the peak area of initial PCB

95 45% CTAB TWEEN80 30% P/Po 15% 0% 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 ); I o = photon s -1, P: peak area of each congener from GC, P o : the peak area of initial PCB 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 77

96 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. 78

97 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 TiO 2 -based photocatalysis, which is known to be broadly applicable. TiO 2 has low toxicity, is biologically and chemically stable, and is economical (Bhatkhande et al., 2002, 79

98 Linsebigler et al., 1995, Hoffmann et al., 1995). The bandgap energy of TiO 2 varies from ev based on its structure and size. Commercially used TiO 2 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 TiO 2 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 H 2 O 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 TiO 2 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, 80

99 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 TiO 2 -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 Photoreactor An LED-based photoreactor fitted with UV LED lamps (λ max = 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 81

100 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 (λ max = 350 nm, full width at half maximum = 50 nm, Hitachi FL8BL-B) was used. Figure 4-1: LED photoreactor and insert. 82

101 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; TiO 2 (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 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 TiO 2 photocatalyst. The amounts of TiO 2 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. 83

102 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 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 ( 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

103 4.2.5 Analysis of sample 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 TiO 2 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 o C. 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 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 85

104 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 Results and Discussions Photocatalytic degradation of pesticides and chlorophenols In the UV-Visible spectrum range from 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 TiO 2 photocatalyst. Consequently, direct photolysis does not play a role in the degradation of these chemicals. Any photodegradation that may occur is a consequence of TiO 2 photocatalysis. Figure 4-2 (a) plots the change in concentration of the four pesticides with 2 g L -1 TiO 2 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. 86

105 (a) MCPA 2,4-D C/Co-Pesticides ,4-DCP 4-CP Energy dosage (kj) (b) MCPA 2,4-D C/Co-TOC ,4-DCP 4-CP Energy dosage (kj) Figure 4-2: Photocatalytic degradation of different pesticides with UV-LED photoreactor (I o = photon s -1, C TiO2 =2.0 g L -1, C o =20 mg L -1 ): (a) loss of parent pesticides; and (b) loss of total organic carbon. 87

106 The results, obtained using a HPLC, show that all four compounds can be catalytically degraded in the LED reactor. For MCPA, with kj energy dosage, 90% was removed from solution, and when the energy dosage reached kj more than 99% was degraded. More than 70% of 2,4-D degraded within kj dosage and it became undetectable with a 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 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 TiO 2 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 88

107 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 ,4-D CP ,4-DCP 0.52 Note: Experiments with 2 g L -1 TiO 2 and a light intensity of photon s 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 A B C Composition 20 mg L -1 of 4-CP, 20 mg L -1 of 2,4-DCP 20 mg L -1 of 2,4-D, 20 mg L -1 of 4-CP 20 mg L -1 of 2,4-D, 20 mg L -1 of 2,4-DCP 89

108 (a) CP 2,4-DCP CP (mixture) 2,4-DCP (mixture) 0.5 C/Co Energy dosage (kj) (b) CP 2,4-D CP (mixture) 2,4-D (mixture) 0.5 C/Co Energy dosage (kj) (c) 1.5 2,4-D 2,4-DCP 1.0 2,4-D (mixture) 2,4-DCP (mixture) 0.5 C/Co Energy dosage (kj) Figure 4-3: Photocatalytic degradation of pesticides mixture with UV-LED photoreactor based on the loss of pesticides detected by HPLC (I o = photon s -1, C TiO2 =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

109 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 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 kj energy dosage; however, the percentage removal of Table 4-4: Percentage removal of pesticides at kj energy dosage. Percentage removal Case 2,4-D 4-CP 2,4-DCP Single pesticides Mixture A _ Mixture B _ Mixture C 72 _ -24 a Note: Energy dosage =0.028 kj. a in the mixture C, the concentration of 2,4-DCP is increased by 24% at the energy dosage of kj, reflecting production from 2,4-D. 91

110 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 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 TiO 2 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 TiO 2 in the solution, which indicates 2 g L -1 TiO 2 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 92

111 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 TiO 2 after 30 minutes of stirring in the dark. Percentage adsorption Case 2,4-D 4-CP 2,4-DCP Single pesticides Mixture A / 3 3 Mixture B 14 3 / Mixture C 14 / Effect of Photocatalyst Loading Five different TiO 2 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 TiO 2 loading 93

112 C/Co [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 Irradiation time (min) (a) r (min -1 ) [TiO 2 ] (g L -1 ) (b) Figure 4-4: Photocatalytic degradation of 2,4-D with different TiO 2 loadings and LED irradiation (I o = photon s -1, C o =20 mg L -1 ). appearing to approach a limiting value with increased TiO 2 loadings, as expected for light-limiting conditions (Augugliaro et al., 1988). The light-limiting condition is 94

113 confirmed by the dependence of first-order rate on light intensity. An increase of TiO 2 loading at lower levels can significantly improve the photocatalytic degradation rate. When the TiO 2 loading increased to 0.5 g L -1 from 0.2 g L -1, the rate constant doubled; however, at a high level, increasing TiO 2 loading does not contribute too much to the improvement of photocatalytic degradation rate. An increase in TiO 2 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 TiO 2 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 TiO 2, the efficiency of photocatalytic degradation depends on the surface of TiO 2 particles, which can simultaneously be in contact with the target contaminant and illuminated by light. At low TiO 2 loadings and unchanged light intensity the active absorbing surface area is a limiting factor, thus better degradation efficiency is observed as TiO 2 loading increased. Nevertheless, with increased TiO 2 loadings, the light available is utilized efficiently and light becomes the limiting factor Effect of Light Intensity Investigations were conducted with varying light intensities. With the optimal TiO 2 loading of 2.0 g L -1, four different light intensities ( , , and 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 95

114 Iₒ= ¹⁵ photon s ¹ Iₒ= ¹⁶ photon s ¹ Iₒ= ¹⁶ photon s ¹ Iₒ= ¹⁶ photon s ¹ C/Co Irradiation time (min) (a) r(min -1 ) I o (*10 16 photon s -1 ) (b) Figure 4-5: Photocatalytic degradation of 2,4-D with UV-LED photoreactor under different light conditions (C o =20 mg L -1, C TiO2 =2 g L -1 ). order rate constant was min -1 at a light intensity of photon s -1, and the largest rate constant was min -1 at light intensity of photon s -1. The 96

115 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 TiO 2 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, TiO 2 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 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 kj per order for the LED irradiation while it increased to 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 97

116 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 photons at a wavelength of 365 nm, or 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 TiO 2 and hence become useless. Our calculations based on the emission spectra of Mercury lamps LED 0.80 C/Co Energy dosage (kj) Figure 4-6: Photocatalytic degradation of 2,4-D in the two photoreactors: Co=20 mg L -1, C TiO2 =2 g L -1. (a): LED reactor, I o = photon s -1 ; (b): Rayonet reactor, I o = 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), 98

117 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 TiO 2 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 photon/s, a suitable loading of TiO 2 for 2,4-dichlorophenoxyacetic acid degradation was determined to be 2.0 g L -1. The rate constant at this loading was 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 TiO 2. If a solar reactor is considered as a competitor, the photon energy-use advantage of LEDs is even greater. 99

118 Chapter Five: DESIGN A HOMOGENEOUS RADIATION FIELD IN A UV-LED BASED PHOTOCATALYTIC REACTOR 5.1 Introduction TiO 2 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 (TiO 2 ) 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 100

119 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 = ηi γ [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 (K a ) occurring on the photocatalyst plate (Equation [5-2]). K a = KdA p A p = ηiγ da p A p [5-2] 101

120 Where A p is the area of the photocatalyst plate. Once the photocatalyst plate is homogenously irradiated, the average kinetics rate become: K a = ηi a γ da p A p [5-3] Where I a is the average light intensity for a homogenous radiation field. I a = IdA p A p [5-4] It is known that f(i) = ηi γ is a concave function. Hence, according to Jensen's inequality, ηi γ da p A p ηi a γ da p A p [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. 102

121 5.3 Development of radiation field model 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 (λ max =365nm, half width=9 103

122 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 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. 104

123 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 70 o 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) = θ 2 [5-7] When 1.22 θ < π/2, Re(θ)=0, so we have: Re(θ) = θ2, 0 < θ < , 1.22 θ < π/2 [5-8] Figure 5-2: Directivity of radiation (NICHIA, 2013). 105

124 (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 1 R 2 [5-9] (3) Develop a function between the light intensity [I(θ,R)] at any position and the referenced light intensity [I(0,d o )]. The referenced light intensity is the light intensity at a specific distance (d o ) 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,d o ) as: I(θ, R) = I(θ, d o ) d o 2 R 2 = I(0, d o) Re(θ) d 2 o [5-10] R 2 Considering the normal flux density, then: I n (θ, R) = I(θ, R) cos(θ) = I(0, d o ) Re(θ) d o 2 R 2 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. 106

125 Figure 5-3: Cartesian and polar coordinates in radiation system. cos (θ) = d R [5-12] R 2 = d 2 + g 2 = d 2 + (x x o ) 2 + (y y o ) 2 [5-13] θ = arctan ( (x x o )2 +(y y o ) 2 ) [5-14] d 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: 107

126 I n (x, y, d) = I(0, d o ) Re arctan ( (x x o) 2 + (y y o ) 2 ) d d 2 o d (d 2 + (x x o ) 2 + (y y o ) 2 ) 3/2 [5-15] Therefore, the normal light intensity at any point due to the contribution of multiple LED lamps is: n I sum (x, y, d) = I ni (x, y, d) i=1 n = (I(0, d o ) Re arctan (x x i) 2 + (y y i ) 2 d i=1 [5-16] d 2 o d (d 2 + (x x i ) 2 + (y y i ) 2 ) 3 ) 2 (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 (I t ) shown as Equations [5-17] ~ [5-20]. The light output of a UV-LED is the integral of radiation over the entire view angles: θ o I t = I(θ)ds 0 [5-17] The following relationships can be obtained from Figure 5-4. ds 2πr dl = 2πr R dθ = 2π R sinθ R dθ = 2πR 2 sinθ dθ [5-18] 108

127 Substitute Equation [5-6] and Equation [5-18] into Equation [5-17]: I t = I 0 2πR 2 I 0 = 2πR 2 θ o 0 θ o 0 I t Re(θ) sinθdθ Re(θ) sinθdθ [5-19] [5-20] Figure 5-4: Scheme of UV-LED radiation 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] 109

128 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) βlg cos (θ) 10 = Re(θ) = Re(θ) 10 βl g 10l g β (1 cos (θ) ) [5-22] 1 Where T(θ) is the ratio of light passing through the glass plate with a view angle ( θ), l g is the thickness of glass plate. Therefore, the modified radiation field model becomes: n I sum (x, y, d) = (I(0, d o ) Re arctan (x x i) 2 + (y y i ) 2 d i=1 d 2 o d (d 2 + (x x i ) 2 + (y y i ) 2 ) 3 ) 2 [5-23] Where d > l g. 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 Light intensity measurement The light intensity was measured using a Silver Line UV Radiometer (M007153, Geneq Inc. Canada). The measurements range from mw cm -2. The sensor of the 110

129 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]: I measured = Ids πr 2 = r s I 2πr dr 0 = s πr 2 s r s I 2r dr 0 [5-24] r 2 s Where r s is the radius of the sensor, I measured is the light intensity read from the UV radiometer, S is the area of the sensor. Figure 5-5: Geometry of sensor. 111

130 According to Equation [5-15] and [5-24], we have: 0 r s {I(0, d o ) Re arctan ( r d 2 o d o ) d o (d 2 2r dr = r 2 o + r 2 ) 3/2 s I measured [5-25] r 2 s I measured I(0, d o ) = 2d 3 r o {Re arctan ( r s r ) d o (d 2 dr 0 o + r 2 ) 3/2 [5-26] The average light intensity received by the sensor at 1 cm distance for a single UV-LED lamp is measured to be mw cm -2. Equation [5-26] provides the referenced light intensity to be 110 mw cm 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

131 Table 5-1: Parameters used for radiation field model calculation. Parameters Value d o 1 cm I(0, d o ) 110 mw cm -2 l g θ o cm 1.22 radian β cm -1 The difference between the model value and measured value was used to evaluate the relative errors as: Relative error = I model I measure I measure 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) I model (mw cm -2 ) I measure (mw cm -2 ) Relative Error I model (mw cm -2 ) I measure (mw cm -2 ) Relative Error % % % % % % % % % % % % % % 113

132 5.5 Design of a homogenous radiation filed The concept of Max Error (M e ) was extended to evaluate the homogeneity of the radiation field. M e = max [ I max I a 100%, I a I min 100%] [5-28] I a I a Where I max is the maximum light intensity, I min is the minimum light intensity and I a is the average light intensity. To achieve a high degree of homogeneity, the calculated M e should be small. If M e is zero, the radiation filed is completely homogenous. Here the radiation field is defined to be homogenous when M e is smaller than 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. 114

133 Therefore, the optimal "ID" is the smallest distance which generates a homogenous radiation field. (a) 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. (b) 115

134 250% 200% M e 150% 100% 50% 0% ID (m) Figure 5-7: The effect of irradiated distance (ID) on Maximum Error 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 The result (Figure 5-8) shows that the optimal ID is proportional to gap and the optimal ID is 1.26 times of gap ID = 1.26 *gap ID (m) gap (m) Figure 5-8: Optimal combination of ID and gap. 116

135 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. Light output per lamp (mw) 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 gap(m) Figure 5-9: Selection of light output of UV-LED. 117

136 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. 118

137 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. TiO 2 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 O 2 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 TiO 2 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, TiO 2 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 TiO 2 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 119

138 practical as they do not require the secondary step of separation of TiO 2 particles from treated waters. Direct anodization of titanium is recognized as one of the better methods to prepare immobilized TiO 2 film due to its simplicity and being able to control the thickness and morphology of a nanotubular TiO 2 film (Li et al., 2009). One way to improve the surface area to reaction volume ratio in an immobilized photocatalytic reactor is to coat TiO 2 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 TiO 2 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. 120

139 6.2 Experimental details 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. TiO 2 (P25) was obtained from Degussa. Hollow glass microspheres coated with anatase TiO 2 (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 Design and fabrication of an LED based photocatalytic reactor Preparation of anodized TiO 2 photocatalytic plate. In this research, the immobilized TiO 2 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. 121

140 Thereafter, the dried anodized Ti foil was annealed at 450 o C in air for 3 hours to induce crystallization to give a better photocatalytic ability (Chang et al., 2011). The surface structure of anodized TiO 2 was examined using scan electron microscopy (SEM). The SEM result (Figure 6-1) showed that TiO 2 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 TiO 2 nanostructrure 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 9 122

141 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 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 , 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. 123

142 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 (D s-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 Radiation field and light intensity estimation Table 6-1: Average light intensity received by the photocatalytic plate. Input current (ma) Ds-p (cm) D L-P (cm) I a (mw/cm 2 ) note: D L-P is larger than D s-p due to the thickness of shielding glass plate and the gap between lamps and the shielding glass. 124

143 Figure 6-3: Radiation field on a photocatalyst plate under different conditions; (a) D L-P = m, 4 by 4 LEDs panel; (b) D L-P = m, 4 by 4 LEDs panel; (c) D L-P = m, 4 by 4 LEDs panel. 125

144 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 (I a ) 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 (D L-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 D L-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) 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: ) 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 D L-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 126

145 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 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 TiO 2 in a UVA-LED batch reactor. In our immobilized photocatalytic reactor, the TiO 2 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

146 ,4-DCP MCPA 4-CP 2,4-D C/Co Energy dosage per volume (kj/l) 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 ; D L-P = 0.54 cm; I a =17.3 mw cm Degradation of 2,4-D with different combination of (UV, TiO 2 photocatalyst plate, H 2 O 2 and O 2 ) 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 H 2 O 2 (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. H 2 O 2 has weak absorption on the emission spectrum of LED, which may cause the photolysis of hydrogen peroxide and generate hydroxyl radicals. 128

147 C/Co UV only UV+H2O2 (0.1%) UV+TiO2 UV+TiO2+bubbling Oxygen UV+TiO2+ H2O2 (0.1%) Energy dosage per volume (kj/l) Figure 6-5: Photodegradation of 2,4-D in a flow-through UV-LED photoreactor: flow rate = 2.03 L min -1 ; D L-P = 0.54 cm ; I a =17.3 mw cm -2. The reactor was considered as a photocatalytic reactor while being mounted with a TiO 2 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% H 2 O 2 in the photocatalytic system resulted in a better degradation efficiency. At an energy dosage of 129

148 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 Effect of D L-P D L-P is a key factor for system scale-up. To study its effect on photocatalytic degradation, experiments were conducted at three D L-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 D L-P is set to be 1.4 cm. The degradation efficiency improved as D L-P was increased to 3.4 cm, while further increment of D L-P to 5.4 cm did not enhance the degradation efficiency. D L-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 D L-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 D L-P, and hinders mass transfer. Therefore, from a kinetics perspective, an optimal D L-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 D L-P, while the dead zone volume is limited to the electronic 130

149 part. The advantage of larger D L-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, D L-P of 5.4 cm is superior to a D L-P of 3.4 cm. C/Co D = 1.4 cm D = 3.4 cm D = 5.4 cm Energy dosage per volume (kj L -1 ) Figure 6-6: The effect of D L-P on 2,4-D degradation: flow rate =2.03 L min -1, I a =17.3 mw cm 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 131

150 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 C/Co flow rate=0.72l/min flow rate=1.50l/min flow rate=2.03 L/min flow rate=2.87 L/min Energy dosage per volume (kj L -1 ) Figure 6-7: The effect of flow rate on degradation of 2,4-D: D L-P = 5.4 cm, I average =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. 132

151 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 ( s -1 ) was obtained at a light intensity of 2.2 mw cm -2 and the highest k ( 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 = I a [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. 133

152 4.00E E-04 k (s -1 ) 2.00E E E I a (mw cm -2 ) (a) I=17.3 mw/cm² I=4.3 mw/cm² I=8.6 mw/cm² I=2.2 mw/cm² C/Co Energy dosage per volume (kj L -1 ) (b) Figure 6-8: The effect of light intensity on degradation of 2,4-D: D L-P = 5.4 cm, Flow rate=2.03 L min

153 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 TiO 2 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 TiO 2 (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 135

154 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 D L-P, flow rate, light intensity and external electron scavenger provide useful information for system scale-up. In this reactor, optimal D L-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 TiO 2 ) can be a promising photocatalyst configuration, considering the reaction rate and operational convenience. 136

155 Chapter Seven: CONCLUSION AND RECOMMENDATION FOR FUTURE RESEARCH 7.1 Conclusions The photochemical technologies (photosensitization and TiO 2 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 TiO 2 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 flowthrough 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 subconclusions as given below: 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. 137

156 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 TiO 2 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 TiO 2 (2g/L) for 2,4-dichlorophenoxyacetic acid degradation was determined at the applied light intensity ( photon s -1 ). The rate constant at this loading and this light intensity was found to be 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 TiO 2. If a solar reactor is considered as a competitor, the photon energy-use advantage of UVA-LEDs is even greater 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. 138

157 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 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 D L-P, flow rate, light intensity and external electron scavenger provide useful information for reactor scale-up. Optimal D L-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. 139

158 7.2 Recommendations for Future Research 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 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/H 2 O 2, can be investigated using UVC-LED 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 Hollow microsphere coated with TiO 2 (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. 140

159 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. 141

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195 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 177

196 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 = m s PCBs m s PCBs + m r PCBs 100% [A-1] 178

197 Where m s PCBs is the mass of PCBs extracted using ultrasonication and is calculated using Equation [A-2]; m r PCBs is remaining PCBs in the soil after ultrasonication and is calculated based on Equation [A-3] m s PCBs (mg) = C 1 V 1 10 [A-2] Where C 1 (mg/l) is PCBs concentration obtained from GC equipment in part A.1.4, V 1 (ml) is the volume of IPA used for ultrasonic extraction m r PCBs (mg) = C 2 10 ml 10 R [A-3] Where C 2 (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 179

198 30 Extraction efficiency (%) :1 0 2: Sonication time (min) 90 3:1 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. 180

199 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 181