Renewable Energies series - UNESCOunesdoc.unesco.org/images/0013/001332/133252e.pdfPart A addresses the theory and fundamentals of water decontamination by means of solar energy. The

Renewable Energies series

Title & copyright.qxd 11/10/03 4:39 PM Page 1

Solar detoxification

Edited byJulin Blanco GlvezSixto Malato Rodrguez


In the same series:

Geothermal energySolar photovoltaic project developmentSolar photovoltaic systems: technical training manual

The designations employed and the presentation of material throughout this

publication do not imply the expression of any opinion whatsoever on the

part of UNESCO concerning the legal status of any country, territory, city or

area or of its authorities, or concerning the delimitation of its frontiers or

boundaries.

The authors are responsible for the choice and the presentation of the facts

contained in this book and for the opinions expressed therein, which are not

necessarily those of UNESCO and do not commit the Organization.

Published in 2003 by the United Nations Educational,

Scientific and Cultural Organization

7, place de Fontenoy F-75352 Paris 07 SP

Typeset by S R Nova Pvt. Ltd., Bangalore, India

Printed by Jouve

ISBN 92-3-103916-4

UNESCO 2003

All rights reserved

Printed in France


Everyone is aware of the strong capacity of sunlight to degradeall kinds of objects. It is also well known that, to preventdegradation and deterioration, objects should be shelteredfrom sunlight or light irradiation in general. This characteris-tic of sunlight is based on the property of solar photons todirectly degrade and break down organic molecules, an effectthat it is known as photolysis.

In the 1970s, it was discovered that this effect could be sub-stantially enhanced with the help of a catalyst, opening up thefield of solar photocatalysis. But it was not until the mid-1980s(coinciding with a growing environmental consciousnessacross the world) that this process was given environmentalapplication, and its use in the treatment and degradation ofhazardous substances in water, air and soil began to be appre-ciated as a very attractive proposition. Of these three potentialapplications (water, air and soil), the treatment of water con-taminants using sunlight is considered of special relevancebecause of the magnitude of water-contamination problemsand its potential for worldwide application.

Solar photocatalytic degradation of water contaminants (a process also known as solar detoxification) is an outstand-ing example of how well solar applications and environmen-tal problems fit together. This remarkable application iscurrently the focus of many research institutions, and since1990 it has also been the main research and developmentobject of the Solar Chemistry group at Plataforma Solar deAlmera (PSA, the largest European facility for solar energyapplications) and the solar centre of CIEMAT, a public insti-tution in Spain devoted to research and technological devel-opment in the areas of energy and the environment.

The Solar Chemistry group at PSA have been participatingin the chemical application of water solar detoxification since

1990, in particular in projects researching photocatalyticdecontamination of used water at both national and interna-tional levels. The experience they acquired with solar detoxifi-cation systems at the engineering level led to the developmentand the installation at PSA of Europes largest solar detoxi-fication facility at pilot-plant scale, and this has been suc-cessfully used by many European research institutions. The authors hope that this book will illustrate the cross-linkedsynergistic relationships that have been developed amongdifferent European research groups involved in the PSAphotocatalysis research program over the last twelve years.

This book summarizes most of the scientific and techno-logical research performed at PSA in this area since 1990,through close collaboration with many scientific colleaguesfrom a large number of universities, research institutions andprivate companies and through many collaborative efforts,networking activities and joint projects.

The book is divided into two parts of five chapters each.Part A addresses the theory and fundamentals of waterdecontamination by means of solar energy. The objective ofthis part is to provide the reader with a background in readi-ness for Part B, which addresses the practical applicationsand systems engineering of the process. Our main motiva-tion in writing this book was to collect, in a comprehensiveand extensive way, all the work performed at PSA and makeit accessible not only to people interested in solar and photo-catalytic applications, but also to all those who are interestedin learning how environmental technology could help tosolve environmental problems in general.

Julin Blanco GlvezSixto Malato Rodrguez

Preface

Preface.qxd 11/10/03 4:40 PM Page v

Our deep gratitude is owed to our colleagues Dr ManuelRomero, Dr Benigno Snchez, Dr Ana Isabel Cardona andAlfonso Vidal from our mother institution CIEMAT, and toDr Christoph Richter (DLR-PSA), who has been deeplyinvolved in most of our projects. This gratitude is alsoextended to all CIEMAT-PSA staff for their great helpfulnessand high skill, which made possible the design, constructionand operation of the photocatalysis pilot plants that havebeen the bases of the success of all the projects we have beeninvolved with. The management and execution of theseprojects is directly related to the knowledge we havegathered from them, most of it included in this book. Wewould also like to give special recognition to Juan AntonioCamacho, Gins Garca, Jose Manuel Molina, Jaime Aranda,Bernab Calatrava and Angel Soler, together with theOperation and Maintenance team of the PSA.

We are grateful to the directors of the Plataforma Solar de Almera and Centro de Investigaciones Energticas,Medioambientales y Tecnolgicas (CIEMAT) for the supportthey gave us in producing this book.

In addition, this book would not have been possible with-out the help, contributions and assistance of many peoplearound the world who either provided text and graphicmaterial, reviewed the manuscript or simply providedvaluable suggestions.

For these reasons, we want to acknowledge the contribu-tions of Professor Jaime Gimnez and Dr David Curc(University of Barcelona), Professor Xavier Domnech andDr Jos Peral (University Autnoma de Barcelona), Dr DanielBlake and Alan Lewandoswky (NREL, United States),

Mike Prairie and Jim Pacheco (Sandia National Labs, United States), Dr Detleft Bahneman and Dr Roland Goslich(ISFH, Germany), Professor Ezzio Pelizzetti and ProfessorClaudio Minero (University of Torino, Italy), Professor Jean-Marie Herrmann (CNRS, France), Dr Csar Pulgarn(Ecole Politechnique Federale de Lausanne, Switzerland), Dr Manuel Collares-Pereira and Joao Farinha Mendes (INETI,Portugal), Dr Karl-Heinz Funken and Dr Christian Sattler(DLR, Germany), Professor Marc Anderson (University of WisconsinMadison, United States), Dr Yogi Goswami(Solar Energy and Energy Conversion Laboratory, Universityof Florida, United States), Professor Leonardo Palmisano (University of Palermo, Italy), Dr Charles Giannotti (CNRS,France), Dr Mirella Musci and Maria Cristina Casalle (CESI,Italy), Professor Rupert Bauer (Technical University of Vienna,Austria) and Professor Octav Enea (University of Poitiers,France).

Our acknowledgement and gratitude are also due to Martin Vincent (Ecosystem S.A., Spain), Joao Oliveira (Ao Sol, Portugal), Alex Ryer (International Light, Inc.), Dr Dieter Donitz (Schott-Rohrglas GmbH, Germany) andDr V.H. Kuester (ALANOD Aluminium Veredlung GmbHand Co, Germany).

For reasons of space we cannot mention all the peoplewho have made a contribution to the content of this bookbut, finally, we cannot forget the crucial help of CarmenMontesinos and Deborah Fuldauer, who devoted many long hours to the revision and correction of the variousmanuscripts.

To our families, for their patience and moral support.

Acknowledgements

acknowledgement.qxd 11/10/03 4:47 PM Page vii

List of Figures xiiList of Tables xviiNotes on the Editors xix

1. Introduction

Aims and objectives 1

Solar chemistry 2Water contaminants 3Photodegradation principles 6

Definitions 6Heterogeneous photocatalysis 6Homogeneous photodegradation 7

Application to water treatment 9Gas-phase detoxification 10

Summary 13Bibliography 14Self-assessment questions 15

2. Solar Irradiation


The power of light 18Ultraviolet light 18Visible light 19Infrared light 19

The solar spectrum 19Solar ultraviolet irradiation 21Atmospheric attenuation of solar radiation 22

Annual available ultraviolet radiation 24Solar radiation measurement 24

Detectors 25Filters 26Input optics 26


3. Experimental Systems


Laboratory systems 32Solar detoxification pilot plants 34Operation of pilot plants 36

Once-through operation 36Batch operation 37Modelling once-through and batch operation 37

Evaluation of solar UV radiation inside photoreactors 39Calibration of radiometers 39Correlation between radiometric and spectroradiometric

data 40

Contents

1

23

Contents.qxd 11/10/03 4:45 PM Page ix

Collector efficiency 41Actinometric experiments 42

Simplified method for the evaluation of solar UV radiationinside photoreactors 43


4. Fundamental Parametersin Photocatalysis


Direct photolysis 50Influence of oxygen 51pH influence 51Influence of catalyst concentration 52Initial contaminant concentration influence 54Radiant flux influence 56Temperature effect 57Quantum yield 57


5. Water Decontamination by Solar Detoxification


Detoxification of pollutants 64Total mineralization 65Degradation pathways 67Toxicity reduction 68Detoxification of inorganic pollutants 69

Quantum yield improvement through additional oxidants 69Hydrogen peroxide 70Persulfate 71Other oxidants 71

Catalyst modification 71Metal semiconductor modification 72

Composite semiconductors 73Surface sensitization 73

Recommended analytical methods 73Original contaminants 73Mineralization measurements (TOC) 73Analysis of intermediates 74Extraction methods 76Toxicity analysis 76


6. Solar Detoxification Technology


Solar collector technology generalities 82Collectors for solar water detoxification: features 85

Specific features of solar UV light utilization 85Parabolic trough collectors 86One-sun (non-concentrating) collectors 87Compound parabolic concentrator (CPC) 89Holographic collectors 91

Concentrated versus non-concentrated sunlight 91Technical issues 93

Reflective surfaces 93Photocatalytic reactor 95

Catalyst issues 98Slurry versus supported catalyst 98Catalyst recuperation and reuse 99


7. Solar Detoxification Applications

Aim and objectives 105

Introduction 106Industrial wastewater treatment 108

Contentsx

4

5

6

7

Contents.qxd 11/10/03 4:45 PM Page x

Phenols 108Agrochemical compounds 109Halogenated hydrocarbons 110Antibiotics, antineoplastics and other

pharmaceutical biocide compounds 111Wood preservative waste 111Removal of hazardous metal ions from water 112Other applications 112

Seaport tank terminals 113Groundwater decontamination 114Contaminated landfill cleaning 115Water disinfection 116Gas-phase treatments 117


8. Economic Assessment


Photochemical and biological reactors coupling 127

Cost calculations 129Example A: TiO2-based solar detoxification

plant 131Example B: Photo-Fenton based solar detoxification

plant 131Solar or electric photons? 133Solar resources assessment 139Comparison with other technologies 142

Thermal oxidation 142Catalytic oxidation 142Air stripping 143Adsorption 143Membrane technology 144Wet oxidation 145Ozone oxidation 145Advanced oxidation processes 145


9. Project Engineering


Feasibility study 152Identification of target recalcitrant

hazardous compounds 153Identification of possible pre-treatments 153Identification of most suitable photocatalytic process 154Determination of the optimum process parameters 154Post-treatment process identification 155Determination of treatment factors 155

Feasibility study example 156Background 156Experimentation: TiO2-persulfate tests 157Photo-Fenton tests 158Conclusions and treatment factors 159

Preliminary design 160Preliminary design example 161Final design and project implementation 163Example of final design and project

implementation 165


10. International Collaboration


International Energy Agency: the SolarPACES programme 176

The European Union 178The CYTED programme 180Main research activities 181

United States 181Spain 182

Recommendations for successful water treatment projects in developing countries 183


xiContents

8

9

10

Contents.qxd 11/10/03 4:45 PM Page xi

1.1 Schematic view of solar chemical applications 31.2 Furfural photo-oxidation and pentachlorophenol

mineralization (photochemical processes) 31.3 Behaviour of electrons and holes within a particle

of illuminated semiconductor in contact with anelectrolyte 7

1.4 UV spectra of acrinathrin and of sunlight between200 and 400 nm 9

1.5 Effect of UV radiation on a TiO2 particle dispersed in water 9

1.6 Diagram of an experimental system with a cross-section view of a gas-phase monolithicphotoreactor 11

1.7 Schematics and photo of a tubular gas-phase photoreactor 12

1.8 Diagram of the experimental system and photo of a tubular gas-phase photoreactor(dismounted) 13

2.1 The optical portion of the electromagnetic spectrum a), and a light wave front modelled as astraight line b) 19

2.2 Visible-light colour distribution 192.3a World solar irradiance, MWh m2 year1 202.3b Spectral solar radiation plotted from 0.2

to 4.5 m 202.4 Air mass and solar components 212.5 Ultraviolet spectra at the Earths surface

(standard ASTM) 222.6 Solar spectra at the Earths surface between

300 and 1,100 nm 23

2.7 Normalized solar UV spectra shown in Figure 2.6 24

2.8 TBDUV at different periods of the year at PlataformaSolar de Almera (37 N) 24

2.9 Responsivities of three detectors. In the inset is shown a schematic of the effect of a filter on detector responsivity 26

2.10 Relative spatial response of an ideal cosine diffuser(up) and a radiance lens barrel (down) 27

2.11 A solar global UV detector (tilted 37 and facingsouth) with a cosine diffuser 27

2.12 A solar direct UV detector installed on a solartracking system 27

3.1 Typical stirred-tank laboratory reactor 333.2 Typical recirculating laboratory system 343.3 a) Photocatalytic detoxification pilot-plant scheme.

b) CPCs at Plataforma Solar de Almera. c) Double-skin sheet collector at Plataforma Solar deAlmera. d) Two types of non-concentrating solarcollectors at the solar detoxification test stand at theUniversity of Florida 35

3.4 Schematic of two pilot plant operation concepts: a once-through operation and a batch operation 37

3.5 Procedure used to calculate the photon flux inside asolar reactor 40

3.6 A diagram of the various loss factors affecting thephoton flux inside a photoreactor 42

3.7 Typical photocatalytic degradation (rDCA) in a solar pilot plant under different UV solar light intensities 44

List of Figures

List of figures.qxd 11/10/03 4:44 PM Page xii

xiiiList of Figures

3.8 Photocatalytic degradation in a solar pilot plant. Concentration is plotted as a function of experimenttime (top) and accumulated energy (bottom). Solar UV power throughout the experiment is also shown 44

4.1 Effect of the concentration of dissolved oxygen on photocatalytic mineralization. [O2]0 = 8.5 mg L

1. In the inset, the usual effect of partial pressure ofoxygen on the photocatalytic reaction rate is shown 51

4.2 Mean particle size of TiO2 (P-25) suspended in waterversus pH [TiO2] = 0.2 g/L 52

4.3 Different laboratory photoreactor designs and zones of radiation penetration when illuminated indifferent ways 53

4.4 Influence of catalyst concentration on the rate of photocatalysis (normalized rates have been used to make it more easily understood) in differentreactors. In the inset, relative reaction rates of degradation (two different photoreactors) and mineralization (TOC) of the same contaminant are shown 53

4.5 Typical photocatalytic degradation. The insert showsdata adjusted to Equation 4.6 55

4.6 Graphics related to the adjustment of data to an LH type kinetic model 55

4.7 Initial degradation rate as function of the initialsubstrate concentration. The insert shows the lineartransformation of Equation 4.4, from which the rate constant and the adsorption coefficient can beestimated from the intercept and the slope,respectively 56

4.8 The relation between the photocatalytic reaction rate and the intensity of the radiationreceived 57

4.9 Plots of normalized concentration as a function ofaccumulated energy for the photodegradation and mineralization of phenol. C0 = 20 mg/L, TiO2 = 200 mg/L, pH0 = 5 59

5.1 General processes for the photo-oxidative or photo-reduction degradation of organic compoundsin aqueous solution sensitized by semiconductorparticles. Examples of photo-oxidation (PCP) andphotoreduction (CCl4) are shown 65

5.2 HPLC-UV chromatograms of photodegradation of oxamyl before photocatalytic treatment a) and when the oxamyl has completely disappeared b) 66

5.3 Evolution of H+ and Cl during pentachlorophenoldegradation. To more clearly demonstrate thatreaction 5.4 is completed, the concentration of TOCin mM is calculated assuming 1 mMol TOC =6 mMol of C = 72 mg of C 67

5.4 Chemical structures of pyrimethanil and its degradation products obtained during a photocatalytic treatment with TiO2 68

5.5 Electron capture by a metal in contact with asemiconductor surface 72

5.6 Concentrating solar reactor with a platinum/titaniumdioxide catalyst on ceramic saddles. Tested on aircontaminated by spray paint at Fort Carson ArmyBase in Colorado, USA 72

5.7 The excitation process in a semiconductor-semiconductor photocatalyst 73

5.8 Steps of excitation with a sensitizer in the presence of an adsorbed organic electron acceptor 73

5.9 Degradation pathway proposed for pirimiphos-methyl dissolved in water whenilluminated in the presence of TiO2 75

6.1 Non-concentrating solar collectors for domesticwater-heating applications 83

6.2 Medium concentrating solar collector: recirculatingparabolic trough reactor for water purification usingtitanium dioxide slurry at NREL 83

6.3 A high-concentration solar collector: a fixed-focus solar reactor 83

6.4 Yearly efficiency of solar collectors in an idealcloudless year: a PTC-one axis with differentorientations 84

6.5 Yearly efficiency of solar collectors in ideal cloudlessyear: flat plate with different inclinations 84

6.6 The first engineering-scale outdoor solardetoxification reactor using a one-axis parabolictrough collector. Part of the 465 m2 parabolic troughsystem at Sandia National Laboratory 85

6.7 A CIEMAT 384 m2 solar detoxification facility usingtwo-axis parabolic trough collectors, at PlataformaSolar de Almera 86

List of figures.qxd 11/10/03 4:44 PM Page xiii

List of Figuresxiv

6.8 Solar ray reflection on a one-axis parabolic troughcollector 87

6.9 Experimental set-up of a thin-film fixed-bed reactortested by ISFH at PSA installations. The housing ismade of Plexiglas and the catalyst is fixed on a flatglass plate 88

6.10 One-sun water treatment reactors with PTFE tubes at NREL: early reactor a) and reactor withtitanium dioxide immobilized on fibreglass bundles b) 88

6.11 Solar reflection on a CPC collector 896.12 Obtention of CPC involute 906.13 View of CPC shape a) and CPC photoreactor

array b) 906.14 Holographic concentration of solar light 916.15 Solar detoxification pilot plant in Cologne 926.16 Flux distribution on an absorber in the course of the

solar day (6:00 to 18:00). Simulation of CPC behaviour with the following data: collector orientation eastwest; semi-acceptance angle 60;truncation angle 80; absorber radius 13.6 mm; optical gap 2 mm; concentration ratio 1.17 92

6.17 Specular, diffuse and spread reflection from a surface 93

6.18 Reflectivity of fresh metal coatings for mirrors 946.19 Reflectivity of different aluminium and plastic film

surfaces 956.20 Transmittance of different materials suitable for the

manufacture of photoreactor tubes 966.21 Glass-tube manufacture. Different compositions

mean that the glass can be used for a wide variety ofapplications 97

6.22 Influence of iron on borosilicate glass light transmission (oxidative conditions). Samples: flat glass 3-mm in thickness 97

6.23 Zone of tubular reactor where light penetrates if thecatalyst concentration is 1 g L1 (TiO2 heterogeneousphotocatalysis) 98

6.24 a) Experimental concentrating solar reactor using titanium dioxide immobilized on glass wool for treating contaminated air streams. b) Parabolic trough reactor for water purification with immobilized titanium dioxide 99

6.25 Sedimentation experiments at different pH. [TiO2] = 0,2 g/L; [NaCl] = 0 M. The Y axis shows

the absorbance of the solution at 800 nm; noabsorbance means absence of TiO2 100

7.1 Complete mineralization of a complex mixture of organic contaminants containing phenols usingTiO2 (200 mg/L) and persulfate as an electron scavenger 108

7.2 Formation of phenolic resins 1097.3 Reactor condensation wastewater from the

manufacture of phenolic resins 1097.4 Photocatalytic degradation of dichloromethane,

chloroform, trichloroethylene andtetrachloroethylene using a titania catalystmanufactured by ENEL (Italy) 110

7.5 Degradation of PCP at a PSA solar detoxificationfacility (Heliomans collectors loop) 112

7.6 Cr+6 to Cr+3 solar photocatalytic reduction at a PSAsolar detoxification facility (CPCs collectors loop).See also Figure 7.7 112

7.7 Simultaneous oxidation of phenol and reduction of Cr+6 to Cr+3 using solar detoxification technology 113

7.8 Degradation of metham sodium wastewater from atank-cleaning process. Catalyst deactivation isobserved after partial degradation of the initial TOC content 114

7.9 Generic concept of contaminated groundwatertreatment 114

7.10 Partial view of the 156 m2 parabolic-trough water treatment system tested on contaminated groundwater at a site at LLNL (United States) in 1992 115

7.11 One-sun reactor built by American EnergyTechnology, Inc. for treating contaminatedgroundwater in Florida (United States) in 1992 115

7.12 Non-concentrating solar detoxification system for BTEX-contaminated groundwater at a commercial site in Gainesville, Florida (UnitedStates), 1996 116

7.13 Simulation of contaminated landfill treatment usingsolar detoxification: mineralization of lindane 117

7.14 Common ultraviolet band designations based onbiological effects 117

7.15 Scheme of TCE gas-phase mineralization with PCO and a monolithic catalyst based onsepiolite/TiO2/Pt 118

List of figures.qxd 11/10/03 4:44 PM Page xiv

xvList of Figures

7.16 Destruction efficiencies of 400 ppm of TCE using aMgSiO4/TiO2/Pt monolithic catalyst: thermocatalyticand photocatalytic processes at differenttemperatures 118

7.17 Flat-plate reactor for treating contaminated airexiting from air-stripping units at McClelland AirForce Base, California, 1997 120

7.18 Self-cleaning windows. Evolution of photocatalytictreatment processes on glasses coated with titaniumdioxide 121

8.1 Conceptual scheme of photocatalytic + biologicaltechnologies coupling 127

8.2 Elimination of tensioactive compounds fromagrochemical industrial wastewater after a fewminutes of photocatalytic treatment 128

8.3 Photodegradation process: usual relationshipbetween the average oxidation state and time. Points A and B are also related to Figure 8.4 129

8.4 Photodegradation process: normal relationshipsbetween toxicity and time. Points A and B are alsorelated with Figure 8.3 129

8.5 Ultraviolet spectra: solar (standard and PSAmeasured spectrum) and two low-pressure 40 Wmercury lamps (QUV fluorescent lamps: UVB-313and UVA-340) 134

8.6 Comparative cost of UV photon collection/generationwith solar technology (CPCs) and electric lamps(electricity cost = 0.15/kWh), respectively. Datafrom tables 8.6 to 8.11. (Costs are indicated in euros, 1999) 139

8.7 Comparative cost of UV photon collection/generationwith solar technology (CPCs) and electric lamps(electricity cost = 0.05/kWh), respectively. Datafrom tables 8.6 to 8.11. (Costs are indicated in euros, 1999) 139

8.8 Average direct and global UV irradiance (sunrise tosunset) at PSA (Almera, Spain). Meteorological datarecords from 1991 to 1995 139

8.9 Average cloud factor for global UV irradiance(sunrise to sunset) at PSA (Almera, Spain).Meteorological data records from 1991 to 1995 140

8.10 Technology fit map: range of application of differentwater treatment technologies 142

8.11 Optimization of solar detoxification and GACtechnologies to PCP degradation 146

9.1 Experimental mobile pilot plant for solar waterdetoxification 153

9.2 Treating wastewater from the painting section at a carassembly factory (ultrafiltration from cataphoresisprocess) 154

9.3 Catalyst mixing system for the LLNL water treatmentsystem 155

9.4 Tonnes of pesticides used in the Almeria region (1995 data) 156

9.5 a) Solar mineralization of TOC from the insecticideabamectin; test performed in a parabolic troughsystem; average direct UV light: 38.1 watts m2. b) Samples of photocatalytic degradation in CPCsystem; average global UV solar radiation was:26.2 watts m2 (acrinatrin test); 33.6 watts m2

(methamidophos) and 33.7 watts m2 (lufenurontest). TiO2 (Degussa P25): 200 mg L

1 1579.6 TOC mineralization of a mixture of ten selected

pesticides (parabolic troughs); direct UV sunlight:36.3 watts/m2; TiO2 (Degussa P25) concentration:200 mg L1; persulfate addition: 0.01 molar; treatedvolume: 250 l. EC50 toxicity measured byMicrotox 158

9.7 TOC mineralization of a mixture of ten selectedpesticides (CPCs). TiO2 (Degussa P25)concentration: 200 mg L1, slurry 158

9.8 Pesticide degradation by Photo-Fenton process:comparison of different iron concentrations for100 ppm of pesticides in wastewater 159

9.9 TOC degradation of a mixture of ten selectedpesticides by TiO2 (Degussa P25) persulfate process. Degradation in function of UV collectedenergy (300 to 400 nm) 161

9.10 TOC degradation of a mixture of ten selectedpesticides by Photo-Fenton process and withdifferent iron concentrations. Degradation infunction of UV collected energy (300 to 400 nm) 161

9.11 Conceptual design of a solar detoxification plant for treatment and recycling of pesticide bottles 162

9.12 Layout design of solar detoxification plant fortreatment and recycling of pesticide bottles 163

9.13 Laser device for manufacturing TiO2 catalyst powder 165

9.14 Modular CPC solar collector 165

List of figures.qxd 11/10/03 4:44 PM Page xv

List of Figuresxvi

9.15 Manufacturing CPC reflectors 1669.16 Solar detoxification plant layout 1679.17 Solar detoxification plant design, isometric

drawing 1679.18 Solar detoxification plant construction, showing

lateral wall and pit to collect and contain anypotential leaks 168

9.19 Installation of supporting structure and CPC units 168

9.20 Photoreactor array input water manifold system 168

9.21 a) Installation of the main tanks in a solardetoxification facility. From left to right: storage tank, catalyst separation tank, buffer tank. b) Installation of tank fluid-level sensors. c) Installation of catalyst separation system 169

9.22 a) Installation and testing of the UV-A sensor device (front left). b) Testing the PLC and electronicequipment 170

9.23 a) and b) Two views of the completed solar detoxification treatment plant 170

List of figures.qxd 11/10/03 4:44 PM Page xvi

1.1 Representative chemical reactions that can store solarenergy (thermochemical processes) 3

1.2 Some of the organic compounds included in various lists of hazardous substances identified by the US EPA 4

1.3 Oxidation potentials of common substances andagents for pollution abatement. The more positivethe potential, the better the species is as an oxidizing agent 5

1.4 Photocatalytic properties of selected semiconductors 7

1.5 VOCs amenable to treatment via photocatalyticoxidation 12

3.1 Radiometric and spectroradiometric UVmeasurements at different times of the day (n 400 nm) 40

3.2 Radiometric and spectroradiometric UVmeasurements at different times of the day (n > 400 nm) 40

4.1 Relative photonic efficiencies for the two pesticidestreated, with phenol as the standard reference (C0 20 mg/L) 59

5.1 Some examples of TiO2-sensitized photodegradationof organic substrates 65

7.1 Example of processes producing phenol residues inwastewater 109

7.2 Examples of processes that are potentially producers of wastewater containing agrochemicalresidues 110

7.3 Examples of processes that are potentially producers of antibiotic and antineoplastic wastewater 111

7.4 Qualitative degradation rates of different compoundsscreened for photocatalytic activity: slow (s), medium(m) and fast (f ) 119

8.1 Estimated cost of typical sizes of solar detoxificationfacility (euros, 1999) 130

8.2 Estimated operating cost of a TiO2-persulfate solar detoxification plant (euros, 1999) 132

8.3 Estimated annual treatment cost of a TiO2-persulfatesolar detoxification plant (euros, 1999) 132

8.4 Estimated operating cost of a Photo-Fenton solardetoxification plant (euros, 1999) 133

8.5 Estimated annual treatment cost of a Photo-Fentonsolar detoxification plant (euros, 1999) 133

8.6 Estimated yearly cost of collecting 1.E+28 solar UV-photons at different yearly average UV globalirradiation (Costs are indicated in euros, 1999.) 135

8.7 Estimated yearly cost of collecting 5.E+28 solar UV photons at different yearly average UV globalirradiation (Costs are indicated in euros, 1999.) 136

8.8 Estimated yearly cost of collecting 1.E+29 solar UV photons at different yearly average UV globalirradiation (euros, 1999) 136

8.9 Estimated yearly cost collecting 5.E+29 solar UVphotons at different yearly average UV globalirradiation (euros, 1999) 136

8.10 Estimated yearly cost of collecting 1.E+30 solar UV photons at different yearly average UV globalirradiation (euros, 1999) 137

List of Tables

List of Tables.qxd 11/10/03 4:43 PM Page xvii

8.11 Estimated yearly cost of UV photon generation withelectric lamps. Electricity cost 0.15 Euros/kWh;FCR 17 per cent. In Table 8.11A the cost per lampis 100; in Table 8.11B the cost per lamp is 200.(euros, 1999) 138

8.12 Extraterrestrial solar irradiation 141

9.1 Selected pesticides for the feasibility studyassessment, CIEMAT (Spain), 1996 156

9.2 Degradation of pesticide mixture: mass treatmentfactor (Tfm) and volumetric treatment factor (Tfv)obtained for TiO2-persulfate and Photo-Fentonprocesses, CIEMAT (Spain), 1998 159

List of Tablesxviii

List of Tables.qxd 11/10/03 4:43 PM Page xviii

Julin Blanco Glvez

Julin Blanco Glvez obtained a Diploma in IndustrialEngineering from Seville University and a Masters inEnvironmental Sciences from the International OpenUniversity. He has sixteen years of experience working in different industrial sectors and during 1989 also workedas a consultant for the Spanish Normalization InstitutionAENOR.

Since 1990 he has worked at the Plataforma Solar deAlmera (PSA) on R&D projects linked to solar and environ-mental technology. From 1990 to 1993, he was the projectleader for the design and construction of the first Europeanpilot plant for experimentation in solar detoxification of industrial wastewaters. In 1993 he became the head ofPSAs Solar Chemistry department, which is devoted to the development of technology required for the utilization of solar radiation in photochemical processes. Since 1995 hehas also headed the CIEMAT Solar Chemistry department ofPSA, directing staff and scientific installations in Almera and Madrid on many national and international projects. In the same year, he was appointed Spanish NationalRepresentative for the Task II group of International EnergyAgency SolarPACES (Solar Power and Chemical EnergySystems), with the objective of worldwide coordination andknowledge sharing of international solar research activities.

He has given multiple lectures in courses, conferencesand seminars, including EURESCO conferences and is theco-author of 4 books, more than 30 international publica-tions and about 70 contributions to International Congressand Symposiums.

Sixto Malato Rodrguez

Sixto Malato Rodrguez obtained a Diploma in ChemicalEngineering from the Facultad de Ciencias of University of Granada, a Masters in Environmental Sciences from the Instituto de Investigaciones Ecolgicas, and a PhD inChemical Engineering from the University of Almera.

His professional career began in 1987 as JuniorResearcher in the Chemical Engineering Department of theUniversity of Almera. In 1988, he joined the ProductionDepartment in an oil refinery (REPSOL S.A.) in Puertollano.Since 1990 he has worked at the Plataforma Solar deAlmera. He participated in the development of all solarphotochemistry activities and all projects linked to the solar detoxification of water that took place in the early1990s at the Plataforma Solar de Almera. At present, he hasa permanent position as Senior Researcher of the SpanishMinistry of Science and Technology.

He has taken part in nine European Union FrameworkProgrammes, nine National R&D Projects, and five R&DContracts related to the development of solar technologiesfor wastewater treatment. He has been involved in the designand construction of all the European pilot plants for experi-mentation in solar detoxification of industrial wastewaters.

He is the author and co-author of several books as well ascontributing chapters and articles to more than 50 publica-tions. He has co-authored 14 articles in technical journalsand 85 contributions to different International Congressesand Symposiums. He has assisted in 21 Workshops andConferences relating to water treatment and has taughtcourses related to advanced wastewater treatment.

Notes on the Editors

Notes on the editors.qxd 11/10/03 4:41 PM Page xix

Aims

This chapter describes an alternative sourceof energy that combines sunlight andchemistry to produce chemical reactions. It outlines the basic chemical and physicalphenomena involved in solar chemistry. It reviews approaches that have been takenand progress that has been made, and givessome projections for the short- and long-termprospects for the commercialization of solarphotochemistry. It also introduces the focusof this book: solar detoxification.

Objectives

By the end of this chapter you willunderstand the main factors that causephotochemical reactions and you will be able to do five things:

1. Distinguish perfectly betweenthermochemical and photochemicalprocesses.

2. Understand the impact of pollutants onthe environment.

3. Calculate the energy flux of a light sourceand its relationship with semiconductorexcitation.

4. Understand the basic principles ofadvanced oxidation processes.

5. Describe the most important features of heterogeneous photocatalysis and theirapplication in the treatment of contaminatedaqueous effluents.

1C H A P T E R 1IntroductionChapter1.qxd 11/10/03 9:54 AM Page 1

1.1. Solar chemistry

The dramatic increases in the cost of oil beginning in 1974focused attention on the need to develop alternative sourcesof energy. It has long been recognized that the sunlightfalling on the Earths surface is more than adequate to supplyall the energy that human activity requires. The challenge is to collect this dilute and intermittent energy and to convertit to forms that are convenient and economical, and to usesolar photons in place of those from lamps. It must be keptin mind that today there is clear worldwide consensusregarding the need to find a long-term replacement for fossilfuels (which were produced millions of years ago and todayare merely consumed) by identifying other inexhaustible orrenewable energy sources. Under these circumstances, thegrowth and development of solar chemical applications canbe of special relevance. These technologies can be divided intwo main groups (Figure 1.1):

Thermochemical processes: solar radiation is converted into thermal energy, which causes a chemical reaction.Thermochemical processes produce a chemical reaction

from the thermal energy obtained from the Sun for thegeneral purpose of substituting for fossil fuels.

Photochemical processes: solar photons are directly absorbedby reactants and/or a catalyst, causing a reaction. Photo-chemical processes produce a chemical reaction from theenergy of the Suns photons for the general purpose ofcarrying out new processes.

It should be emphasized, as a general principle, that the firstcase is associated with processes that are feasible with con-ventional sources of energy. The second is related only tocompletely new processes or reactions that are presently car-ried out with electric arc lamps, fluorescent lamps or lasers.

From the outset, it was recognized that direct conversionof light into chemical energy held promise for the productionof fuels and chemical feedstock and the storage of solarenergy. Production of chemicals by reactions that arethermodynamically uphill can transform solar energy andstore it in forms that can be used in a variety of ways. Wideranges of such chemical transformations have been pro-posed. A few representative examples are given in Table 1.1to illustrate the concept.

Solar Detoxification2

NOTATION AND UNITS

Symbol Description Units

A Absorbance at wavelength

AOPs Advanced Oxidation Processes

c Speed of light nm/s, m/s

ci Concentration of component i mol

EG Semiconductor band-gap energy eV, J

E Spectral irradiance W m2 nm1

Eo Spectral irradiances incident into the medium W m2 nm1

El Spectral irradiances at a distance l W m2 nm1

EC50 Concentration that produce an effect in 50% of a population mg/L, mg/kg

GAC Granulated activated carbon

h Plancks constant J s

LC50 Concentration that produces death in 50% of a population mg/L, mg/kg

NOEL No observed effect level mg/kg/day

pi Partial pressure of component i atm

U Energy of a photon eV, J

Absorption coefficient cm1 atm1

Extinction coefficient mol1 cm1

Quantum yield

Wavelength nm, m

Chapter1.qxd 11/10/03 9:54 AM Page 2

These processes generally start with substances in low-energy, highly-oxidized forms. The essential feature is thatthese reactions increase the energy content of the chemicalsusing solar energy. For such processes to be viable, theymust fulfil the following requirements, as outlined by theNational Renewable Energy Laboratory (NREL) (1995) andslightly modified by the authors:

The thermochemical reaction must be endothermic. The process must be cyclic and have no side reactions

that could degrade the photochemical reactants. The reaction should use as much of the solar spectrum as

possible. The back reaction should be very slow, to allow storage of

the products, but rapid when triggered to recover theenergy content.

The products of the photochemical reaction should beeasy to store and transport.

The other way that sunlight is used in photochemistry is touse solar photons as replacements for those from artificialsources. The goal in this case is to provide a cost-effective andenergy-saving source of light to drive photochemical reactions

that give useful products. Photochemical reactions can beused to carry out a wide variety of chemical syntheses, rang-ing from the simple to the complex. Processes of this type maystart with more complex compounds than fuel-producing or energy-storage reactions and convert them to substancesfor which the photochemical step provides additional value or destroys harmful by-products. The principles of photo-chemistry are well understood and there is a wide range ofknown types of synthetic transformations (Figure 1.2). Giventhis, the problem becomes one of identifying applications inwhich the use of solar photons is possible and economicallyfeasible. The processes of interest here are photochemical;processes in which some component of the reacting system iscapable of absorbing photons in the solar spectrum. As pho-tons should be treated like any other chemical reagent in the process, their number is a critical element in solarphotochemistry (see Chapter 2).

Because they are very attractive technologically and envi-ronmentally, solar chemical processes have seen spectaculardevelopment in recent years. In the beginning, research insolar chemistry was centred only on converting the solarenergy into chemical energy, which could then be stored andtransferred over long distances. But together with this impor-tant application, other environmental uses have since beendeveloped, so that today the entire range of known solarchemical applications has a promising future. In principle,any reaction or process requiring an energy source can utilizesolar energy.

1.2. Water contaminants

Environmental pollution is a pervasive problem with wide-spread ecological consequences. Recent decades havewitnessed increased contamination of the Earths drinking-water reserves. The inventory of priority pollutants compiled

3Introduction

Heat Photons

ThermochemicalSteam reforming of methane

CH4 + H2O CO + 3H2 206 kJ/mol600850C

Photochemical processExcitation of a semiconductor

h + SC e + p+h E

Gof SC

Increase oftemperature

Modification ofchemical bonds

process

Modification ofchemical bonds

FIGURE 1.1. Schematic view of solar chemical applications.

TABLE 1.1. Representative chemical reactions that can storesolar energy (thermochemical processes).

H (kJ/mol)

CO2(g) CO(g) 1/2O2 286

CO2(g) 2H2O(g) CH3OH(l) 3/2O2 727

H2O(l) H2(g) 1/2O2 286

CO2(g) 2H2O(l) 1/6C6H12O6(s) O2 467

OCHO

OCHO

O O

h < 700 nm

Methylene blue/O2

C6Cl5OH + 9/2O2+ 2H2OTiO2

6CO2 + 5HCl

OCHO

O O

+ Oh < 390 nm

FIGURE 1.2. Furfural photo-oxidation and pentachloro-phenol mineralization (photochemical processes).


by the United States Environmental Protection Agency (USEPA) provides a convenient frame of reference for under-standing the importance of removing such contaminationfrom the Earth (Table 1.2).

In any case, a consensus exists that the environmentalimpact of a given contaminant depends on the degree ofexposure (its dispersion and the resulting concentration inthe environment) and on its toxicological properties.


Acetaldehyde

Acetamide

Acetone

Acetonitrile

Acetophenone

Acrolein

Acrylamide

Acrylic acid

Acrylonitrile

Aldrin

4-Aminoazobenzene

Aniline

o-Anisidine hydrochloride

Anthracene

Atrazine

Benzamide

Benzene

Benzidine

Benzo(a)pyrene

Benzyl chloride

Benzenehexachloride

Beta-Propoxur

Biphenyl

Bis(2-Chloroethoxy)methane

Bromoethane

Captan

Carbaryl

Carbon disulfide

Carbon tetrachloride

Catechol

Chlordane

Chloroacetic acid

Chlorobenzene

Chlorodibenzodioxins, various Chlorodibenzofurans

2-Chloroethyl vinyl ether

2-Chlorophenol

p-Chloro-m-cresol

o-,m-,p-Cresols

Cumene

Cyclohexane

2,4-Diaminoanisole

4,4-Diaminodiphenyl ether

Diazomethane

Dibenzofuran

1,2-Dibromoethane

1,2-Dichlorobenzene

1,1-Dichloroethane

2,4-Dichlorophenol

1,2-Dichloropropane

Dichlorvos

Dicofol

Diepoxybutane

Diethanolamine

Dimethyl phthalate

2,4-Dinitrophenol

1,2-Dinitrotoluene

2,4-Dinitrotoluene

1,4-Dioxane

1,2-Diphenylhydrazine

Disulfoton

Endosulfan

Epichlorohydrin

Ethylbenzene

Ethylene glycol

Ethylene thiourea

Fluometuron

Formaldehyde

Hexachlorobenzene

Hexachloroethane

Hexane

Hydroquinone

Isophorone

Isopropyl alcohol

Lindane

Malathion

Maneb

Mechlorethamine

Melamine

Methanol

Methoxychlor

Methyl acrylate

Methyl isocyanate

Methyl tert-butyl ether

Methylene bromide

4,4-Methylenedianiline

Methylhydrazine

4-Methylphenol

Mirex

Mustard gas

Nitrilotriacetic acid

o-Nitroaniline

Nitrobenzene

Nitrofen

Nitrogen mustard

Nitroglycerin

5-Nitro-o-anisidine

Nitrophenol

2-Nitropropane

n-Butyl alcohol

n-Dioctyl phthalate

N-Nitrosodiethylamine

N-Nitrosopiperidine

N-Nitroso-N-ethylurea

Octachloronaphthalene

Octane

Oxirane

Parathion (DNTP)

Pentachlorobenzene

Pentachlorophenol

Phenanthrene

Phosgene

Phthalic anhydride

Polybrominatedbiphenyls

Poly-chlorinated biphenyls(PCBs)

Pyrene

Quinone

Quintozene

Safrole

Set-Butyl alcohol

Sevin (carbaryl)

Styrene

Terephthalic acid

Tert-Butyl alcohol

Tetrachlorvinphos

1,1,2,2-Tetrachloroethane

Tetrahydrofuran

Thioacetamide

Thiourea

Toluene

2,4-Toluene diamine

Toluene diisocyanate

o-Toluidine hydrochloride

Total xylenes

Toxaphene

Triaziquone

Trichlorfon

2,4,6-Trichlorophenol

Trifluralin

1,2,4-Trimethylbenzene

2,2,4-Trimethylpentane

2,4,6-Trinitrotoluene

Urethane (ethyl carbamate)

Vinyl bromide

Vinyl chloride

Vinylidene chloride

Xylene (mixed isomers)

Zineb

TABLE 1.2. Some of the organic compounds included in various lists of hazardous substances identified by the US EPA.


The assessment of exposure involves an understanding ofthe dispersion of a chemical in the environment and estima-tion of the predicted concentration to which organisms will be exposed. For example, the pesticide fenaminphosoxidizes very quickly (a ten-day half-life) into sulfoxide andsulfone, while its pesticidal properties remain unaffected. Ahalf-life of seventy days has been found for degradation offenaminphos and its two metabolites. Furthermore, the twometabolites are more mobile (soluble) than fenaminphos(Hayo and Van Der Werf, 1996). Assessment of the contam-inants effect involves summarizing data on the effects thechemical has on selected representative organisms andusing these data to predict a no-effect concentration in aspecific niche.

Organisms may consume chemicals through ingestion offood and water, respiration or skin contact. When a chemicalcrosses the various barriers of the body, it reaches the meta-bolic tissue or a storage depot. The toxicity of a chemical isusually expressed as the effective concentration or dose ofthe material that would produce a specific effect in 50 percent of a large population of test species (EC50 or ED50). Ifthe effect recorded is lethal, the term LC50 (or LD50) is used.The no observed effect level (NOEL or NOEC) is the doseimmediately below the lowest level eliciting any type of tox-icological response in the study. For example, the pesticidemethamidophos, which has been classified as a Restricted-Use Pesticide (RUP) by US EPA, is highly toxic for mammals(acute oral LC50 16 mg/kg in rats and 30 to 50 mg/kg inguinea pigs), birds (bobwhite quail, CD50 8 to 11 mg/kg)and bees. The ninety-six-hour LC50 is 25 to 51 mg/L inrainbow trout, but concentrations as low as 0.22 ng/L arelethal to larval crustaceans in ninety-six-hour toxicity tests. A fifty-six-day rat-feeding study resulted in a NOEL of0.03 mg/kg/day (Tomlin, 1997).

Decontamination of drinking water is mainly by pro-cedures that combine flocculation, filtration, sterilizationand conservation, to which a limited number of chemicalsare added. Normal human sewage water can be efficientlytreated in conventional biological processing plants. But veryoften these methods are unable to reduce the power of thecontaminant. In these cases, some form of advanced biolog-ical processing is usually preferred in the treatment ofeffluents containing organic substances. Biological treatmenttechniques are well established and relatively cheap.However, these methods are susceptible to toxic com-pounds that inactivate the waste-degrading microorganisms.

To solve this problem, apart from reducing emissions, twomain water-treatment strategies are followed: first, chemicaltreatment of drinking water and contaminated surface andgroundwater, and second, chemical treatment of waste-waters containing non-biodegradable compounds.

Chemical treatment of polluted surface and groundwateror wastewater is part of a long-term strategy to improve thequality of water by eliminating toxic compounds of humanorigin before returning the water to its natural cycles. Thistype of treatment is suitable when a biological processingplant cannot be adapted to certain types of pollutants thatdid not exist when it was designed. In such cases, a poten-tially useful approach is to partially pre-treat the toxic wasteusing oxidation technologies to produce intermediates thatare more readily biodegradable. Light can be used, undercertain conditions, to encourage chemicals to break downthe pollutants into harmless by-products. Light can have adramatic effect on a molecule or solid because when itabsorbs light its ability to lose or gain electrons is oftenaltered. This electronically excited state is both a betteroxidizing and a better reducing agent than its counterpart inthe ground. Electron transfer processes involving excited-state electrons and the contact medium (for example water)can therefore generate highly reactive species like hydroxide(OH) and superoxide (O2

) radicals (see Table 1.3). Thesecan then be used to decompose a pollutant chemically intoharmless end-products. Alternatively, light can be useddirectly to break up pollutant molecule bonds photolytically.

5Introduction

TABLE 1.3. Oxidation potentials of common substances andagents for pollution abatement. The more positive thepotential, the better the species is as an oxidizing agent.

Oxidizing reagent Oxidation potential, V

Fluorine 3.06

Hydroxide radical (OH) 2.80

Ozone 2.07

Hydrogen peroxide 1.77

Chlorine dioxide 1.57

Chlorine gas 1.36

Oxygen 1.23

Hypochlorite 0.94

Iodine 0.54

Superoxide radical (O2) 0.33


These processes are called Advanced Oxidation Processes(AOPs). Many oxidation processes, such as TiO2/UV,H2O2/UV, Photo-Fenton and ozone processes (O3, O3/UV,O3/H2O2) are currently employed for this purpose.

1.3. Photodegradation principles

1.3.1. Definitions

For the benefit of those who may have a limited backgroundin photochemistry, a brief outline of some basic concepts of photochemistry is presented here. In order for photo-chemistry to take place, photons of light must be absorbed.The energy of a photon is given by

(1.1)

where h is Plancks constant (6.626 1034 J.s), c is the speed of light and is the wavelength. For a molecules bond to be broken, U must be greater than the energy of that bond.

When a given wavelength of light enters a medium, itsspectral irradiance E (W m

2 nm1) is attenuated accordingto the Lambert-Beer law, which is expressed in two ways,one for the gas phase and the other for the liquid phase:

gas phase (1.2)

liquid phase (1.3)

Eo and E

l are the incident spectral irradiance and at adistance l into the medium, is the absorption coefficient(cm1 atm1), pi is the partial pressure (atm) of component i, is the extinction coefficient (M

1 cm1), and ci is theconcentration (M) of component i. The absorbance A atwavelength is the product cil. The photochemicalquantum yield () is defined as the number of molecules of the target compound that react divided by the number ofphotons of light absorbed by the compound in a fixed periodof time. Normally, this unit is the maximum quantum yieldattainable.

The term photocatalysis implies the combination ofphotochemistry and catalysis. Both light and catalyst arenecessary to achieve or to accelerate a chemical reaction.Photocatalysis may be defined as the acceleration of a pho-toreaction by the presence of a catalyst. Heterogeneous

processes employ semiconductor slurries for catalysis,whereas homogeneous photochemistry is used in a single-phase system. Any mechanistic description of a photo-reaction begins with the absorption of a photon, sunlightbeing the source of photons in solar photocatalysis. In thecase of homogeneous photocatalytic processes, the interac-tion of a photon-absorbing species (transition metal com-plexes, organic dyes or metalloporphyrines), a substrate (e.g.the contaminant) and light can lead to chemical modificationof the substrate. The photon-absorbing species (C) is acti-vated and accelerates the process by interacting through astate of excitation (C*). In the case of heterogeneous photo-catalysis, the interaction of a photon produces the appear-ance of electron/hole (e and h) pairs, the catalyst being a semiconductor (e.g. TiO2 or ZnO). In this case, the excitedelectrons are transferred to the reducible specimen (Ox1)while concurrently the catalyst accepts electrons from theoxidizable specimen (Red2) that occupies the holes. In bothdirections, the net flow of electrons is null and the catalystremains unaltered.

(1.4)

(1.5)

(1.6)

(1.7)

(1.8)

(1.9)

1.3.2. Heterogeneous photocatalysis

The concept of heterogeneous photocatalytic degradation issimple: a stable solid semiconductor is irradiated to stimulatea reaction at the solid/solution interface. By definition, thesolid can be recovered unchanged after many turnovers of theredox system. When a semiconductor is in contact with a liq-uid electrolyte solution containing a redox couple a chargetransfer occurs across the interface to balance the potentials ofthe two phases. An electric field is formed at the surface of thesemiconductor. The bands bend as the field forms from themass of the semiconductor towards the interface. During pho-toexcitation (a photon with appropriate energy is absorbed),band bending provides the conditions for carrier separation.In the case of semiconductor particles, there is no ohmic

e Ox Red + 1 1

h Red Ox+ + 2 2

C C e hh + +( )

R P*

C* R R* C*+ +C C*h

log( / )E E c lo l i =

ln( / )E E p lo l i =

Uhc=



contact to extract the majority carriers and to transfer them byan external conductor to a second electrode. This means that the two charge carriers should react at the semiconductor/ electrolyte interface with the species in solution. Understeady-state conditions, the amount of charge transferred tothe electrolyte must be equal and opposite for the two types of carriers. The semiconductor-mediated redox processesinvolve electron transfer across the interface. When electron/hole pairs are generated in a semiconductor particle, the elec-tron moves away from the surface to the mass of the semi-conductor as the hole migrates towards the surface (see Figure1.3). If these charge carriers are separated quickly enough theycan be used for chemical reactions at the surface of the photo-catalyst: that is, for the oxidation or reduction of pollutants.

Metal oxides and sulfides represent a large class of semi-conductor materials suitable for photocatalytic purposes.Table 1.4 lists a selection of semiconductor materials thathave been used for photocatalytic reactions, together withthe band-gap energy required to activate the catalyst. Thefinal column in the table indicates the wavelength of radia-tion required to activate the catalysts. According to Plancksequation, the radiation required to produce this gap must beof a wavelength () equal to or less than that calculated byequation 1.10.

(1.10)

where EG is the semiconductor band-gap energy, h is Plancksconstant and c is the speed of light.

Summarizing, a semiconductor particle is an ideal photo-catalyst for a specific reaction if:

The products formed are highly specific. The catalyst remains unaltered during the process. The formation of electron/hole pairs is required (gener-

ated by the absorption of photons with energy greaterthan that necessary to move an electron from the valenceband to the conduction band).

Photon energy is not stored in the final products, beingan exothermic reaction that is only retarded kinetically.

1.3.3. Homogeneous photodegradation

The use of homogeneous photodegradation (a single-phasesystem) to treat contaminated water dates back to the early 1970s. The first applications concerned the use ofUV/ozone and UV/H2O2. The use of UV light for thephotodegradation of pollutants can be classified into twoprincipal areas:

Photo-oxidation: light-driven oxidative processes princi-pally initiated by hydroxyl radicals.

Direct photodegradation: light-driven processes in whichdegradation proceeds following direct excitation of thepollutant by UV light.

= hcEG

7Introduction

Oxid1

Red1

Red2

Oxid2

h

recombination

recombination

FIGURE 1.3. Behaviour of electrons and holes within a particleof illuminated semiconductor in contact with an electrolyte.

TABLE 1.4. Photocatalytic properties of selected semi-conductors.

Band gap energy Corresponding Material required to activate wavelength

catalyst (eV) required (nm)

BaTiO3 3.3 375

CdO 2.1 590

CdS 2.5 497

CdSe 1.7 730

Fe2O3 2.2 565

GaAs 1.4 887

GaP 2.3 540

SnO2 3.9 318

SrTiO3 3.4 365

TiO2 3.0 390

WO3 2.8 443

ZnO 3.2 390

ZnS 3.7 336


Photo-oxidation involves the use of UV light plus an oxidantto generate radicals. The hydroxyl radicals then attack the organic pollutants to initiate oxidation. Three majoroxidants are used: hydrogen peroxide (H2O2), ozone and Photo-Fenton reaction. H2O2 absorbs photons fairlyweakly in the UV region, with absorption increasing as the wavelength decreases. At 254 nm, is 18 M

1 cm1,whereas at 200 nm it is 190 M1 cm1. The primary processfor the absorption of light below 365 nm is dissociation, toyield two hydroxyl radicals:

(1.11)

The use of hydrogen peroxide is now very common forthe treatment of contaminated water due to several practicaladvantages:

H2O2 is available as an easily handled solution that can be diluted in water to give a wide range ofconcentrations.

There are no air emissions. A high-quantum yield of hydroxyl radicals is generated

(0.5).

The major drawback is the low molar-extinction coefficient,which means that in water, when UV absorption is high, thefraction of light absorbed by H2O2 may be low unless verylarge concentrations are used. Furthermore, especially asconcerns the focus of this text, H2O2 photon absorption isvery low in the Solar UV range (up to 300 nm).

Ozone is generated as a gas in air or oxygen in concen-trations generally ranging from 1 to 8 per cent (v/v). It has a strong absorption band centred at 260 nm with 3,000 M

1 cm1. Absorption of light at this wavelengthleads to the formation of H2O2:

(1.12)

(1.13)

Hydroxyl radicals are then formed by the reaction of ozonewith the hydrogen peroxide conjugate base:

(1.14)

(1.15)

(1.16)

(1.17)

Since the net result of ozone photolysis is the conversionof ozone into hydrogen peroxide, UV-ozone would appear tobe only a rather expensive method of making hydrogen per-oxide. However, there are other oxidation-related processesthat occur in solution, such as the direct reaction of ozonewith a pollutant (see Table 1.3). Ozone may have advantagesin water, with high inherent UV absorbance, but it faces thesame problem as hydrogen peroxide as far as its use in solar-energy processes is concerned.

The essential process of the Fenton reaction (Safarzadeh-Amiri et al., 1996) is the same as for all AOPs. Highlyreactive radicals (such as HO and HO2

) oxidize nearly allorganic substances to yield CO2, water and inorganic salts.In the case of Photo-Fenton, Fe2 ions are oxidized by H2O2producing one OH ion (1.18), and the Fe3 or complexesobtained then act as the light-absorbing species producinganother radical while the initial Fe2 is recovered (1.19 and1.20).

(1.18)

(1.19)

(1.20)

Note that in Equation 1.20, the ligand R-COO can bereplaced by other organic groups (e.g. ROH or RNH2).Compared with other homogeneous photo-oxidationprocesses, the advantage of Photo-Fenton is the improvedlight sensitivity (up to a wavelength of 600 nm, correspon-ding to 35 per cent of the solar radiation). On the otherhand, disadvantages, such as the low pH values required(usually below pH 4) and the necessity of removing iron afterthe reaction, remain.

Some pollutants are able to dissociate only in the pres-ence of UV light. For this to happen, the pollutant mustabsorb light emitted by a lamp (or the Sun) and have areasonable quantum yield of photodissociation. Organicpollutants absorb light over a wide range of wavelengths, butgenerally absorb light more strongly at lower wavelengths,especially below 250 nm (Figure 1.4). In addition, thequantum yield of photodissociation tends to increase atlower wavelengths, since the photon energy is increasing(Eq. 1.1). The net chemical result of photodissociation isusually oxidation, since the free radicals generated can reactwith dissolved oxygen in the water. In practice, the range ofwastewaters that can be successfully treated by UV alone isvery limited. This defect is more relevant when solar energy

[ ( )]Fe OOC R Fe CO R + + ++ + 2 2 2h

Fe H O Fe H OH3 22+ + ++ + + + h

Fe H O Fe OH OH2 2 23+ + + + +

HO OH O3 2 +

O H O HO OH3 2 3 + +

HO O O HO2 3 3 2 + +

H O H O HO H O2 2 2 2 3+ + +

O D H O H O( )1 2 2 2+

O O D O31

2h +( )

H O OH2 2 2h



is used (see Figure 1.4) because only photons up to 300 nm are available.

1.4. Application to water treatment

As mentioned above, UV light can be used in several ways.However, direct photolysis can occur only when the contam-inant to be destroyed absorbs incident light efficiently. In thecase of UV/ozone and UV/hydrogen peroxide this does nothappen. Absorption by some sensitizer must initiate thereaction, and limited absorption by the solute or the additiverestricts efficiency. Furthermore, these mixtures often stillrequire large quantities of added oxidant. By contrast, in het-erogeneous photocatalysis dispersed solid particles absorblarger fractions of the UV spectrum efficiently and generatechemical oxidants in situ from dissolved oxygen or water (seeFigure 1.5). These advantages make heterogeneous photo-catalysis a particularly attractive method for environmentaldetoxification. The most important features of this processthat make it applicable to the treatment of contaminatedaqueous effluents are:

The process takes place at ambient temperature. Oxidation of the substances into CO2 is complete. The oxygen necessary for the reaction is obtained from

the atmosphere. The catalyst is cheap, innocuous and reusable. The catalyst can be attached to different types of inert

matrices.

For all these reasons, from now on only this method will bedealt with in this text.

Whenever different semiconductor materials have beentested under comparable conditions for degradation of thesame compounds, TiO2 has generally been demonstrated tobe the most active, equalled only by ZnO. TiO2s strongresistance to chemical and photocorrosion, as well as itssafety and low cost, limits the choice of convenient alterna-tives (Herrmann, 1999). Furthermore, TiO2 is of specialinterest as it can use natural (solar) UV light. This is becauseit has an appropriate energetic separation between its valenceand conduction bands, which can be surpassed by theenergy content of a solar photon (see Table 1.4). Other semi-conductor particles, for example, CdS or GaP, absorb largerfractions of the solar spectrum and can form chemicallyactivated surface-bond intermediates, but unfortunatelythese photocatalysts are degraded during the repeatedcatalytic cycles involved in heterogeneous photocatalysis.Therefore, degradation of the organic pollutants present inwastewater using irradiated TiO2 suspensions is the mostpromising process, and R&D in this field has grown veryquickly during the last few years.

(1.21)

(1.22)

(1.23)

(1.24)

To date, evidence supports the idea that the hydroxyl radical(OH) is the main oxidizing specimen responsible for photo-oxidation of the majority of the organic compounds studied.The first effect, after absorption of near ultraviolet radiation,

O e Oads BC ads2 2( ) ( )+

( )

( )

TiO O Ti OH h

TiO O Ti OH H

IV IVBV+

IV IV

+ +

+

22

2

e h TiO TiO heat and/or ++ + +2 2 h

TiO e h TiO2 2h + + +

9Introduction

200 220 240 260 280 300 320 340 360 380 400

Wavelength, nm

UV solar spectrum

UV acrinathrin spectrum

Arb

rita

ry u

nits

FIGURE 1.4. UV spectra of acrinathrin and of sunlightbetween 200 and 400 nm.

h 3.0 eVO2

O2

WATER

H O2

OH + H+

h+

e

TiO2 Particle

Figure 1.5. Effect of UV radiation on a TiO2 particledispersed in water.


< 390 nm, is the generation of electron/hole pairs, whichare separated between the conduction and valence bands(Eq. 1.21). In order to avoid recombination of the pairsgenerated (Eq. 1.22), if the solvent is oxidoreductively active (i.e. water) it also acts as a donor and acceptor ofelectrons. Thus, on a hydrated and hydroxylated TiO2surface, the holes trap OH radicals linked to the surface (Eq. 1.23). In any case, it should be emphasized that eventrapped electrons and holes can rapidly recombine on thesurface of a particle (Eq. 1.22). This can be partially avoidedthrough the capture of the electron by pre-adsorbed molecu-lar oxygen, forming a superoxide radical (Eq. 1.24).

Whatever process the photo-oxidation takes, O2 andwater are essential for photo-oxidation with TiO2. There isno degradation in the absence of either. Furthermore, theoxidative species formed (in particular the hydroxyl radicals)react with the majority of organic substances. For example,in aromatic compounds, the aromatic part is hydroxylated,then successive steps in oxidation/addition lead to breakingof the rings. The resulting aldehydes and carboxylic acids aredecarboxylated and finally produce CO2. However, theimportant issue governing the efficiency of photocatalyticoxidative degradation is to minimize electron-hole recombi-nation by maximizing the rate of interfacial electron transferto capture the photogenerated electron and/or hole. Thisissue is discussed in more detail later.

Photocatalytic degradation has been most commonlyinvestigated with monoaromatics, and consequently thesepollutants appear as model compounds in dozens of scien-tific papers. Some monoaromatics investigated include:benzene, dimethoxybenzenes, halobenzenes, nitrobenzene,chlorophenols, nitrophenols, benzamide and aniline, mostof which are recognized as priority pollutants (see Table 1.2).In addition to these, several other types of molecules havealso been investigated as substrates for photocatalyticdegradation:

Haloaliphatics (e.g. trichloroethylene or tetrachloromethane):important because so many of these compounds havebeen released into the environment and contaminatewaters. Some also originate during water treatment bychlorination.

Water-miscible solvents (e.g. ethanol or alkoxyethanol):these compounds are very difficult to detoxify, as they are resistant to treatment and are poorly adsorbed on granulated activated carbon (GAC).

Pesticides: contaminate waters with a high level of agricul-tural runoff. Among the recently investigated compoundsare triazines, organophosphorous, carbamates, phenoxy-acids and organochlorines.

Surfactants: surface active agents are entering domesticand industrial wastewater in increasing amounts. Becausetheir biodegradability may be one of the more importantconstraints to their use, photocatalytic degradation hasreceived increasing attention.

Dyes: strongly coloured compounds can be removed by adsorption but it is always better to destroy them byoxidation.

Four exhaustive reviews by Blake (1994, 1995, 1997, 1999)describe almost 3,000 studies in this field carried out before1999.

Despite encouraging laboratory-scale data and someindustrial-scale tests, chemical oxidation detoxification isstill restricted to a few experimental plants (Dillert et al.,1999). The broader application of those technologiesrequires:

reactor optimization and modelling, and assessment of the efficiency of oxidation technology to

reduce the toxicity of effluents.

The following chapters of this book will attempt to highlightthese matters.

1.5. Gas-phase detoxification

Airborne pollutants (such as volatile compounds) can betreated during the gas phase with the UV/TiO2 process. Gas-phase treatment offers several advantages. In general,substrate mass-transport is an order of magnitude faster inthe gas phase than in the liquid phase. This in turn leads tomuch faster reaction rates. Oxidant starvation may be less ofa problem in the gas-phase. There is also no interference onthe photocatalytic surface from other species that are invari-ably present in aqueous treatment media (for example,anions). In addition, photocatalysis separation after use isnot a problem, unlike aqueous slurry suspensions. As solarenergy is used to drive the process, no fuel is required,gaseous affluent volume is reduced, no NOx is generated, noproducts of incomplete combustion are produced, CO2emanating from fuel burning is avoided, and substantial fuelsaving may be achieved. Since no burning takes place,



oxygen is only necessary at a stoichiometric ratio. Solar con-centrators enable small-size solar furnaces and even mobilesolar parabolic dishes to be sufficient for on-site destructionof low productions of highly toxic compounds.

On the other hand, there are indications that mineraliza-tion may not be complete with some organic substrates in the gas-phase. The TiO2 photocatalyst becomes less effective after prolonged use and must be reactivated withmoist air, which presumably restores the original degree of

hydroxylation on the oxide surface. There are also indicationsthat product (or intermediate) adsorption on the TiO2 surfacemay be problematic during the course of the reaction.

Pollutant substrates such as trichloroethylene, acetone,formaldehyde, m-xylene and NOx have been treated withTiO2/UV in the gas-phase in bench-scale tests. Field testshave also been conducted to treat effluent air emissions usingthis technique at different manufacturing plants in theUnited States (Rajeshwar, 1996).

11Introduction

CoANALYSER

2

VENT

VENT

FC

RELIEFVALVE

GC/MSSAMPLINGVALVE

DRYAIR

PC

MFC

MFC

TOL.OR XYL.

FILTER T

REACTORXeLAMP

HEATEXCHANGER

HEATTRACEREACTOR

T

FCH AIR2

VENT

VENT

FID

TCD

GC

He

DAS

INLET (REAR-FED)OUTLET (FRONT-FED)

REARTHERMOCOUPLES

MONOLITHICCATALYST

QUARTZWINDOW

FRONTTHERMOCOUPLE

OUTLET (REAR-FED)INLET (FRONT-FED)

FIGURE 1.6. Diagram of an experimental system with a cross-section view of a gas-phase monolithic photoreactor.


In figures 1.6, 1.7 and 1.8, different gas-phase photo-catalytic reactors are shown. Figure 1.6 presents a reactordesigned for use with monolithic catalysts (Avila et al.,1998). The reactor (40 mm inner diameter, 200 mm length)permits a change in the direction of gas flow from the rear tothe front and vice versa. The monolythic catalysts are placedinside the reactor so that light illuminates the internal sur-faces of the channels from the front. Gases are usually fed

from the front part of the reactor. A 4,000 W Xenon lamp,which supplies the photonic flux, has been adapted for thisreactor. A Pyrex window is used to filter wavelengths below300 nm (see Figure 1.4), a filter is used to remove IR radia-tion, and an electric heating jacket used to control tempera-ture. A series of thermocouples are installed to monitor thetemperature of the reactor inlet and outlet and, at variouspoints inside different channels of the monolith, to obtainaxial and radial temperature profiles. A reactant gas consist-ing of air with a flow of several litres per minute is mixedwith the VOC (hundreds of ppm) before entering the reac-tor. Contaminants are pumped through the monolith andoutlet gases, heat traced to avoid condensation, are sub-sequently analysed by direct sampling (on-line) using a gaschromatograph (GC) with a flame ionization detector (FID).Manual (off-line) samples that can be removed through asampling valve are Tenax adsorbent to trap volatilecompounds from the air. In addition, CO2 concentration is measured with an infrared gas analyser, to calculate thecarbon-mass balance.

In figures 1.7 and 1.8 a different type of gas-phasephotoreactor is shown. The tubular photoreactor consists of a Pyrex tube (Figure 1.7) containing a tubular matrix ofTiO2-based catalyst surrounding a UV lamp. The stream of gas flows between the inner wall of the catalyst and the surface of the UV lamp. The UV lamp irradiance is similar to the solar spectrum. A heat-trace box (Figure 1.8) thatcontrols process temperature encloses the photoreactor.


Outlet gases

Catalytic monolith

Inlet gases

Fluorescent lampThermocouple

FIGURE 1.7 Schematics and photo of a tubular gas-phase photoreactor.

TABLE 1.5. VOCs amenable to treatment via photocatalyticoxidation ( Jacoby et al., 1996).

Class of compound Chemicals tested

Aromatics Benzene, toluene

Nitrogen-containing ring Pyridine, picoline, nicotine

Aldehydes Acetaldehyde, formaldehyde

Ketones Acetone

Alcohols Methanol, ethanol, propanol

AlkanesEthylene, propene, tetramethyl ethylene

Terpenes -pinene

Sulfur-containing organics Methyl thiophene

Chlorinated ethylenesDichloroethylene, trichloroethylene,tetrachloroetylene

Acetyl chloridesDichloroacetyl chloride,tetrachloroacetyl chloride


Two thermocouples, located at the inlet and the outlet, givethe reaction temperature of gases in contact with the catalystsurface. In this case a stream of dry CO2-free air is deliveredfrom an air generator at flow rates of several litres perminute. The VOC is bubbled into the airstream and throughthe catalyst in the reactor. Outlet gases are analysed by on-line GC and manual samples are Tenax adsorbent.

Summary

A description is given of how solar chemistry could becomea significant segment of the chemical industry and how it can be used, under certain conditions, to provoke the chemical breakdown of pollutants into harmless by-products. The behaviour of contaminants in environmental

13Introduction

P GENERATORAIR

Peak

MFC

Checkvalve

MFC

VOCs

H O2

Secondary pollutant

O2

P

PHOTOREACTOR

HEATTRACE

REACTOR

Filter

Tenax

Venteo

Analizadorde CO2

C

CC

Vent

Relief valve

He

TCD

FID

GC (HP6890)

Vent

Vent

H2 Air He

DAS

FIGURE 1.8 Diagram of the experimental system and photo of a tubular gas-phase photoreactor (dismounted).


water is summarized. The basic concepts of photochemistryrelating to photolysis of chemical bonds, homogeneousphotodegradation and heterogeneous photocatalysis arereviewed. The use of semiconductors for wastewater treat-ment, with particular reference to TiO2, is discussed.Examples of the waste materials that have been treated suc-cessfully using TiO2 are presented. Gas-phase photocatalysisis also introduced.

Bibliography

AVILA, P.; BAHAMONDE, A.; BLANCO, J.; SNCHEZ, B.; CARDONA, A.I. AND ROMERO, M. 1998. Gas-phase photo-assisted mineralization of volatile compounds bymonolithic titania catalysts. In: Applied Catalysis B:Environmental, No. 17, pp. 7588.

BLAKE, D.M. 1994. Bibliography of Work on the Photocatalytic Removal of Hazardous Compounds from Water and Air, May. Springfield, Va., 22161, National Technical Information Service, US Depart. ofCommerce. Update Number 1: To June 1995, October 1995.Update Number 2: To October 1996, January 1997. UpdateNumber 3: To January 1999, May 1999.

DILLERT, R.; CASSANO, A.E.; GOSLICH, R. AND BAHNEMANN, D.1999. Large (1) scale studies in solar catalytic wastewatertreatment. Catalysis Today, No. 54, pp. 26782.

HAYO, M.G. AND VAN DER WERF, H. 1996. Assessing theimpact of pesticides on the environment. AgriculturalEcosystems and Environment, No. 60, pp. 8196.

HERRMANN, J.M. 1999. Heterogeneous photocatalysis:fundamentals and applications to the removal of varioustypes of aqueous pollutants. Catalysis Today, No. 53, pp. 11529.

JACOBY, W.A.; BLAKE, D.M.; FENNELL, J.A.; BOULTER, J.E.;VARGO, L.M.L.; GEORGE, M.C. AND DOLBERG, S.K. 1996.Heterogeneous photocatalysis for control of volatile organiccompounds in indoor air. Journal of the Air WasteManagement Association, Vol. 46, No. 9, pp. 8918.

NATIONAL RENEWABLE ENERGY LABORATORY. 1995. SolarPhotochemistry Twenty Years of Progress: Whats BeenAccomplished, and Where Does It Lead? Golden, Colo., NREL Report NREL/TP-433-7209.

RAJESHWAR, K. 1996. Photochemical strategies for abatingenvironmental pollution. Chemistry and Industry, No. 17,pp. 4548.

SAFARZADEH-AMIRI, A.; BOLTON, J.R. AND CATER, S.R. 1996. The use of iron in advanced oxidation processes. Journal ofAdvanced Oxidation Technology, Vol. 1, No. 1, pp. 1826.

TOMLIN, C.D.S. 1997. The Pesticide Manual. 11th ed.Farnham, Surrey, United Kingdom, British Crop Protection Council.



Self-assessment questions

Part A: True or false?

1. Solar energy is useful only to substitute for fossil fuels; itis converted into thermal energy, thus provoking chemicalreactions.

2. The toxicity of a chemical is the same for all species.

3. Biological treatment techniques are the cheapestwastewater treatment methods.

4. The energy of a photon depends on the ambienttemperature.

5. Heterogeneous photocatalysis employs liquid catalysts.

6. Light-driven oxidative processes are initiated by excitedelectrons on the catalyst surface.

7. Ozone can be produced from air.

8. The most important characteristics of photocatalysts are: resiliance against chemical and photocorrosion, safety,cost and band-gap.

9. The electron/hole recombination can be avoided byincreasing reaction temperature.

10. Heterogeneous photocatalysis can only be applied tomonoaromatics.

Part B

1. What is the most important difference betweenthermochemical and photochemical solar processes?

2. What is the usual way of expressing the toxicity of a chemical in the environment?

3. Why is biodegradation (a major mechanism inwastewater treatment) quite inefficient in treating certaintypes of wastewater?

4. What is the percentage of absorbed photons in asolution with the following characteristics: extinctioncoefficient 1327 cm1 M1, concentration of substrate0.01 M, illuminated pathlength 5.6 cm, with an extinctioncoefficient of 0.3?

5. What wavelength is required to excite a semiconductorwith a band-gap of 4.0 eV?

6. Name three important characteristics of heterogeneousphotocatalysis when used in a water-treatment process.

7. Why is TiO2 the most suitable photocatalyst forwastewater treatment?

8. What is the most important electron acceptor in water?

9. What is the most important product of photocatalyticdegradation of organic contaminants?

10. Why do hydroxyl radicals react with organicsubstances?

15Introduction


Answers

Part A

1) False. 2) False. 3) True. 4) False. 5) False. 6) False. 7) True. 8) True. 9) False. 10) False.

Part B

1. In thermochemical processes, solar radiation isconverted into thermal energy; in photochemical processesthe solar photons are absorbed directly by the reactants,giving rise to the reaction.

2. The toxicity of a chemical is usually expressed as theeffective concentration or dose of the material that wouldproduce a specific effect in 50 per cent of a large populationof test species (EC50 or ED50).

3. Because in the case of very toxic compounds,microorganisms need an extended period of adaptation,during which they are not completely non-viable.

4. 100 per cent and 3.8 per cent.

5. 310 nm.

6. The process takes place at ambient temperature, theoxygen necessary for the reaction is obtained from theatmosphere, and the catalyst is cheap, innocuous and canbe reused.

7. It has exhibited the highest activity. It is highly stable tochemical and photocorrosion. It can use natural UV.

8. Dissolved oxygen.

9. Carbon dioxide.

10. Because of its very high oxidation potential.



Aims

This chapter describes the power of light as a source of energy. It outlines the basicprinciples that are related to the light spectrumand specifically to the solar spectrum. It discusses solar UV radiation and its photonflux in more detail, because this part of thesolar spectrum is the most important fordriving chemical processes. The majoratmospheric variables determining theamount of UV solar radiation on the Earthssurface are discussed. A method forcalculating UV attenuation at a given site ispresented. Finally, solar radiationmeasurement systems are described.

Objectives

At the end of this chapter you willunderstand the main factors affecting solarradiation behaviour and you will be able to do six things:

1. Discriminate between the differentcomponents of solar radiation and theirprincipal characteristics.

2. Recognize typical solar spectra andunderstand the effect of Sun position on thesolar power reaching the Earths surface.

3. Find the photon flux of a polychromaticsource of energy with simple calculations.

4. Describe the most important componentsof the Earths atmosphere and their effect onpower and spectral distribution of the solarradiation.

5. Understand the procedures that permitsolar power to be calculated from availableradiation at any given site.

6. Comprehend the basic principles onwhich solar radiation measurement is based.

2C H A P T E R 2Solar IrradiationChapter2.qxd 11/10/03 9:57 AM Page 17

2.1. The power of light

Light is just one of various electromagnetic waves present inspace. The electromagnetic spectrum covers an extremelybroad range, from radio wavelengths of a metre or more,down to x-rays with wavelengths of less than one billionth ofa metre. Optical radiation lies between radio waves and x-rays on that spectrum and has a unique combination ofray, wave and quantum properties. At x-ray and shorterwavelengths, electromagnetic radiation tends to be quiteparticle-like in its behaviour, whereas toward the long wave-length end of the spectrum behaviour is mostly wavelike.The UV-visible portion occupies an intermediate position,having both wave and particle properties in varying degrees(See Figure 2.1a).

Like all electromagnetic waves, light waves can interferewith each other, become directionally polarized, and bendslightly when passing through an edge. These propertiesallow light to be filtered by wavelength or amplified coher-ently as in a laser. In radiometry, lights propagating wavefront is modelled as a ray travelling in a straight line (SeeFigure 2.1b). Lenses and mirrors redirect these rays alongpredictable paths. Wave effects are insignificant in a large-scale optical system, because the light waves are randomlydistributed and there are plenty of photons.

2.1.1. Ultraviolet light

Short wavelength UV-light exhibits more quantum proper-ties than its visible or infrared counterparts. Ultraviolet light


NOTATION AND UNITS

Symbol Description Units

AM Air mass ratio

fn Cloud factor

f Fraction of power associated with a wavelength nm1

H

Radiance exposure monthly average kJ m2

HTBD TBDUV radiance exposure kJ m2

I Photon flux density Einstein s1 m2

Na Quantity of photons absorbed by the system Photons s1

N0 Avogadros number, 6.023 1023 Photons mol1

N Number of photons supplied by a source of light of wavelength Photons s1

Q Energy of a monochromatic source of light of wavelength W m2 m1

T Transmittance

T Tra

Documents

Renewable Energies series - UNESCOunesdoc.unesco.org/images/0013/001332/133252e.pdfPart A addresses the theory and fundamentals of water decontamination by means of solar energy. The